Site-defined Cu-O ensembles enable hydrogen-conserving light-driven ethane upgrading

Domain: physics
Published: 2026-03-09
Editorial outcome: major_revision

Abstract
Upgrading light alkanes to value-added olefins is a long-standing challenge, owing to the high stability of C–H bonds and the tendency for overoxidation at elevated temperatures. Here, we introduce a light-driven strategy for ethane dehydrogenation using Cu-doped TiO , in which atomically dispersed Cu coordinated to bridging oxygen (O ) creates well-defined [Cu–O] ensembles that orchestrate site-specific, stepwise C–H activation. Photogenerated holes localize at O sites to initiate the first C–H cleavage, while adjacent Cu centers mediate β–H elimination and H evolution. In contrast, trace β–H activation at O sites generates *H species that cannot desorb due to a prohibitive coupling barrier with *H on O , leading to Cu reduction and progressive deactivation. Co-feeding CO restores the active Cu coordination environment and suppresses this deactivation process without perturbing the primary reaction pathway. This cooperative design achieves a C H production rate of 21.1 mmol g-1 h-1 with nearly stoichiometric H evolution and a quantum efficiency of 6.1% under 365 nm irradiation. These findings establish a site-defined, hydrogen-conserving route for photocatalytic alkane upgrading, offering a general blueprint for selective C–H bond transformations with long-term stability.

Introduction
With increasing demands for carbon neutrality and cleaner chemical production, the selective upgrading of light alkanes, key components of shale gas, into value-added chemicals has become a strategic goal in sustainable catalysis1-5. Among these alkanes, ethane (C H ), the second most abundant constituent (5–20 vol%) of shale gas, represents a promising feedstock for on-demand ethylene (C H ) production, given its availability, low cost, and the central role of C H in the global petrochemical supply chain6-9. However, catalytic ethane dehydrogenation (EDH) to ethylene remains fundamentally challenging due to the high bond dissociation energy of its C–H bonds (~415 kJ/mol) and the inherent difficulty in achieving high selectivity under practical conditions10-14.

In thermocatalytic systems, C–H activation typically proceeds via either noble metal nanoparticles (e.g., Pt, Pd) or via lattice oxygen species in reducible metal oxides such as ZnO, NiO, or CeO 13-17. These processes generally require elevated temperatures (600–800°C) to surmount the kinetic barrier for C–H cleavage18-20. Such harsh conditions often induce sintering of metal particles, excessive carbon deposition from deep dehydrogenation, and more critically, difficulties in regulating the properties, reactivity, and regeneration kinetics of active lattice oxygen21-25. The resulting active oxygen species are prone to uncontrolled spillover, which not only promotes nonselective overoxidation but also accelerates the consumption of valuable H during O - or CO - assisted regeneration via H O formation13,15,26. These issues fundamentally limit the atom- and energy-efficiency of thermocatalytic EDH (TEDH) processes and create a trade-off between activity, selectivity, and catalyst longevity.

Photocatalytic EDH (PEDH) has emerged as a promising low-temperature alternative that can harness photogenerated charge carriers to circumvent the thermodynamic and kinetic limitations of thermocatalysis12,27-38. Metal oxide systems such as LaMn Cu O and heterojunction catalysts like PdZn/ZnO have demonstrated light-assisted C–H activation through lattice oxygen participation or heterojunction-driven charge separation31,33. These successes underscore the viability of photocatalytic strategies in achieving energy-efficient alkane conversion. However, several key challenges remain unresolved. First, most photocatalytic systems suffer from low C H selectivity due to overoxidation by highly reactive oxygen species such as •OH or •O -, which indiscriminately cleave both C–H and C–C bonds12,34,39. Second, the formation, dynamics, and reactivity of surface hydrogen species (*H) remain poorly understood, especially their role in surface poisoning and deactivation16,40. Third, the lack of atomic-level control over the geometry of active sites and their redox cooperativity hampers the precise mediation of sequential hydrogen abstraction steps without undesired side reactions.

Herein, we address these challenges with a photocatalytic platform based on Cu-doped TiO , where atomically dispersed Cu bridged by oxygen (O ) forms site-defined [Cu–O] ensembles that serve as the primary centers for sequential C–H activation and H evolution. Photogenerated holes localize at O to trigger the first C–H cleavage in C H , while the adjacent Cu center promotes β–H elimination with concurrent H release. In contrast, β–H activation at Ti-bonded bridging oxygen (O ) generates *H species that are kinetically trapped in H evolution, leading to Cu reduction and catalyst deactivation. Co-feeding CO promotes the removal of *H from these non-cooperative O sites, preserving the active [Cu–O] ensembles without disturbing the primary dehydrogenation route. This light-driven, site-defined strategy achieves highly selective C H production with minimal H loss, offering an atom- and energy-efficient blueprint for sustainable alkane upgrading.

Results
Photocatalyst preparation and characterizations
Figure 1a illustrates the synthetic route for anchoring single Cu atoms on TiO (denoted as Cu /TiO ) via a previously reported MIL-125(Ti)-templated pre-encapsulation strategy41. Initially, MIL-125 with Ti vacancies (Ti v ) was synthesized, exhibiting a smooth disc-like morphology (Fig. 1b and Supplementary Fig. 1a). Upon impregnation with Cu2+, the Ti sites were occupied to form the Cu/MIL-125 intermediate, and a slight structural collapse occurred due to the polar solvent effect (Fig. 1c). Subsequent calcination of Cu/MIL-125 at 500 °C in air yielded the final Cu /TiO catalyst (Fig. 1d), with the (101) facet of anatase TiO 2 exposed (Supplementary Fig. 1b). Additionally, a slight increase in Ti3+, oxygen vacancies (V ), and surface hydroxyl groups (–OH) is observed after the introduction of lower-valent Cu (Supplementary Fig. 4).

Thermogravimetric analysis (TGA) confirmed the complete removal of organic linkers, as only adsorbed water loss below 200°C was observed with no additional weight loss up to 800°C (Supplementary Fig. 2). More detailed synthetic procedures are provided in the Methods.

Inductively coupled plasma-optical emission spectrometry determined a Cu loading of 1.4 wt% in Cu /TiO . Powder X-ray diffraction (XRD) patterns (Fig. 1e) confirm the formation of MOF intermediates and anatase TiO , while the absence of Cu-related diffraction peaks in both Cu/MIL-125 and Cu /TiO indicates that the MOF-based pre-encapsulation strategy effectively produces highly dispersed Cu species. Energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 1f) further showed the uniform distribution of Cu. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image reveals isolated bright spots corresponding to single Cu atoms (Fig. 1g). Atomic intensity profiles (Fig. 1h) from two representative regions show distinct contrast between Ti and Cu, and the equidistant arrangement (ca. 0.35 nm) of Ti and Cu atoms in region 2 provides direct evidence that individual Cu atoms are anchored on TiO by substituting Ti sites.

The oxidation states of Cu were analyzed using X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2a, a distinct satellite peak confirms the presence of Cu2+, which is further verified by electron paramagnetic resonance (EPR) spectroscopy (Supplementary Fig. 3)42. Deconvolution of the Cu 2p spectrum reveals two characteristic peaks at binding energy (B.E.) of 933.64 eV (62%) and 932.42 eV (38%). The former is assigned to Cu2+ and the latter to Cu+, as confirmed by the valence-sensitive Cu LMM Auger spectra. Specifically, the Cu LMM spectrum of Cu /TiO exhibits a kinetic energy shift toward the Cu+ position relative to the CuO reference (Fig. 2b).

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were performed to elucidate the local coordination environment of Cu sites in Cu /TiO . In the Cu K-edge XANES spectrum, the white-line peak intensity of Cu /TiO falls between those of CuO and Cu O (Fig. 2c), indicating an average Cu oxidation state between +1 and +2, consistent with the XPS results. Fourier-transformed EXAFS spectra in R-space (Fig. 2d) reveal a single peak corresponding to the first-shell Cu–O scattering, with no detectable Cu–Cu or Cu–(O)–Cu contributions. Wavelet transform analysis (Fig. 2e) further confirms this assignment, showing a dominant Cu–O feature at approximately 1.5 Å (R-space) and 4.4 Å⁻¹ (k-space), distinct from the Cu–Cu signature of metallic Cu foil at ~2.1 Å and 7.1 Å⁻¹. Furthermore, the O K-edge XANES spectrum of Cu /TiO retains the characteristic crystal field profile of TiO (Fig. 2f), exhibiting resolved absorption features at 529–536 eV and 536–548 eV regions that are assigned to electronic transitions from O 1s to O 2p–Ti/Cu 3d and O 2p–Ti/Cu 4s4p hybridized orbitals43,44, respectively. Within the 3d–hybridization region, crystal-field splitting yields well-defined t and e peaks, with the t /e ratio serving as a sensitive probe of electron occupancy in 3d orbitals45,46. As evidenced in Supplementary Figs. 5a and 5b, Cu /TiO exhibits a lower t /e ratio than undoped TiO , signifying enhanced electron accumulation in t orbitals. Considering the contrasting 3d electron configurations of Cu2+ (d9) and Ti4+ (d0) (Supplementary Fig. 5c), this reduction directly evidences Cu–O orbital hybridization. Concurrently, the overall attenuation of absorption intensity of Cu /TiO (Fig. 2f) further confirms the incorporation of Cu into the TiO lattice, rather than being surface-dangling, consistent with our HAADF-STEM observations (Fig. 1g). EXAFS fitting was used to further extract the quantitative spatial configuration of Cu atoms (Fig. 2g and Supplementary Table 1). The results indicate an average Cu coordination number of 3.6, suggesting the probable presence of 60% [Cu–O ] and 40% [Cu–O ] coordination units. These ratios correspond well with the concentrations of Cu2+ and Cu+ determined by XPS analysis, respectively. The quantitative agreement between XPS and XAS spectral analyses unambiguously confirms the near 100% accessibility of Cu active sites, representing exceptional atomic utilization efficiency. Taken together, we assign [Cu–O ] as the predominant configuration for Cu2+ species, while [Cu–O ] corresponds to Cu+ sites. To further evaluate the generality of this MOF-based pre-encapsulation strategy for synthesizing single-atom catalysts, we additionally prepared Ag/TiO under identical synthetic conditions. HAADF-STEM images of Ag/TiO display isolated bright spots that can be assigned to individual Ag atoms, and the corresponding Ag K-edge EXAFS spectra show Ag–O coordination without any detectable Ag–Ag or Ag–(O)–Ag paths (Supplementary Fig. 6). These results, together with the atomically dispersed Cu centers in Cu /TiO , strongly support that the present synthesis method is capable of stabilizing different metals predominantly in single-atom states.

Performance evaluation of PEDH
The PEDH experiments were carried out in a sealed quartz reactor under xenon lamp irradiation without extra heating. Infrared thermal imaging and thermocouple measurements indicated a catalyst surface temperature of approximately 108℃. (Supplementary Fig. 7). As shown in Fig. 3a, bare TiO exhibited negligible activity, whereas metal deposition (Cu, Au, Ag, Pd, and Pt) significantly altered the reaction pathway. Specifically, Au- and Ag-loaded TiO surfaces predominantly generated C H via C H coupling, while Pd- and Pt-loaded surfaces produced primarily H through deep dehydrogenation, accompanied by carbon deposition (Supplementary Fig. 8). In striking contrast, Cu /TiO demonstrated dramatically enhanced near-stoichiometric production of C H and H with 97% C H selectivity, highlighting efficient and controlled selective dehydrogenation to C H at the localized [Cu–O] sites. Switching the feed gas from C H to C H /H or C H /H (over-dehydrogenation product) yielded no significant products, further indicating that, under the present conditions, [Cu–O] sites show low intrinsic activity toward side reactions, including alkene hydrogenation and over-dehydrogenation (Supplementary Fig. 9). Furthermore, under a fixed coating area (~3.14 cm2), the mass-specific C H and H yields displayed pronounced catalyst dosage dependence (Fig. 3b), arising from limited photon penetration depth that restricts effective light absorption and prevents the lower catalyst layers from being fully utilized. Given that photothermal effects generally exhibit greater penetration depth than photoexcitation effects47,48, light-induced heating was consequently excluded as the primary performance contributor, supported by dark control experiments at the same catalyst surface temperatures (Supplementary Fig. 10a). However, when an ice bath was introduced under constant light intensity to attenuate the photothermal effect, the photocatalytic yield decreased from 8.4 to 3.8 mmol g-1 h-1 while the product selectivity remained unchanged, clearly demonstrating that the light-induced thermal effect (that is, phonon) plays an important synergistic role in the reaction. Moreover, the linear correlation of C H production with light intensity and wavelength-dependent PEDH performance further collectively establish the photon-driven and phonon-assisted mechanism of PEDH over a Cu /TiO (Supplementary Figs. 10b and 10c).

Remarkably, this system achieved an exceptionally high C H yield of 21.1 mmol g-1 h-1 (Fig. 3b) with an apparent quantum efficiency (AQY) of 6.1% at 365 nm (Supplementary Fig. 10d and Supplementary Table 2), surpassing state-of-the-art photothermal EDH systems at 650°C and achieving the highest reported efficiency among non-O PEDH systems to date (Fig. 3c and Supplementary Table 3)12,32-38,49-52.

The operational stability of the catalyst was evaluated by monitoring the time-dependent cumulative C H yield under prolonged reaction conditions. In a C H atmosphere, a nonlinear accumulation of C H over 240 min indicated gradual catalyst deactivation (Fig. 3d), despite the carbon balance remaining close to 1.0. This deactivation correlates with a macroscopic color change of the catalyst from light green to dark blue, as well as a decrease in the Cu2+ signal detected by in situ EPR (Supplementary Fig. 11a). Critically, this deactivation and the change in Cu2+ content could be effectively mitigated by CO co-feeding (Fig. 3d), indicating that the structural changes of the Cu active sites are reversible and that Cu2+ ([Cu-O ]) is likely a potential active center. However, the observed color change under reaction conditions cannot be simply attributed to the reduction of Cu2+ via photogenerated electron capture. As a critical control, under an Ar atmosphere, a comparable decrease in the Cu2+ signal is observed without any noticeable macroscopic color change (Supplementary Fig. 11b). This clear contrast indicates that the color evolution observed in reactive atmospheres does not arise solely from changes in the Cu valence state. The origin of these valence-state variations and their relationship to the reversible structural evolution of the Cu sites will be discussed in detail in the following sections. In addition, to clarify that the observed catalyst deactivation is not an artifact arising from the use of a sealed batch reactor, a series of supplementary control experiments were conducted. First, although a slight pressure increase is detected in the sealed system during the reaction, its magnitude consistently remains below 10% of the initial pressure, and its influence on the reaction kinetics can therefore be considered negligible (Supplementary Fig. 12). Second, the stabilizing role of CO and the intrinsic durability of the catalyst were further verified under continuous-flow conditions. While pronounced deactivation is observed in a C H /Ar atmosphere (Supplementary Fig. 13), the C H production rate is maintained at over 95% of its initial value during 20 h of operation in a C H /CO atmosphere (Fig. 3e). Meanwhile, HAADF-STEM analysis of the post-reaction catalyst (Supplementary Fig. 14) confirmed the preserved atomic dispersion of Cu species without detectable aggregation.

Temperature-programmed surface reaction (TPSR) studies in C H /CO and C H atmospheres elucidated thermal dehydrogenation pathways. Both systems exhibited identical reaction onset temperatures around 505°C (Fig. 3f and Supplementary Fig. 15a), consistent with TEDH activity profiles (Supplementary Fig. 15b), indicating that C H activation governs reaction initiation and serves as the rate-determining step. Stability evaluation of Cu /TiO for TEDH at 550°C revealed low initial activity (ca. 1.2 mmol g-1 h-1) with rapid deactivation to 20% of the initial activity within 10 h (Fig. 3g). Notably, compared with the aforementioned PEDH processes under continuous-flow conditions, this TEDH process exhibits higher H yields than C H yields alongside substantial carbon deposition (Fig. 3h), signifying the predominance of deep dehydrogenation side reactions. In contrast, the light-driven selectivity of PEDH enables targeted C–H activation while suppressing deep dehydrogenation and over-oxidation, highlighting the intrinsic advantage of photocatalysis for controlled C H -to-C H conversion.

Mechanistic investigations
A systematic Arrhenius analysis revealed a significant reduction in apparent activation energies (Ea) for photocatalytic and thermal C H activation. The photocatalytic Ea was 24.94 kJ mol-1, representing a 75% decrease compared to thermal catalysis (98.41 kJ mol-1, Fig. 4a), highlighting the enhanced ability of photocatalysis to facilitate C–H bond cleavage. Active species quenching experiments were conducted to further understand the mechanism of efficient C H activation under mild conditions (Fig. 4b). It should be noted that, to avoid interference arising from the quenchers themselves occupying catalyst surface sites or altering surface properties, two different quenchers were employed for each target species (h+ or e-) for cross-validation. The quenching efficiencies of all scavengers toward photogenerated electrons and holes were confirmed by photoluminescence spectroscopy (Supplementary Fig. 16). Specifically, when ferrocene (C H Fe) and sodium oxalate (Na C O ) were introduced as hole (h⁺) scavengers, the yields of C H and H decreased in an equimolar manner, indicating that h⁺ predominantly participates in the initial step, namely the cleavage of the first C–H bond of C H . In contrast, when potassium persulfate (K S O ) and potassium dichromate (K Cr O ) were introduced as electron (e⁻) scavengers, the C H /H ratio markedly deviated from the theoretical value of 1.0 and increased to 4.7 and 5.0, respectively, demonstrating that e⁻ plays a crucial role in promoting the kinetics of H formation. Meanwhile, quenching e⁻ led to a >90% decrease in the total product yield, which likely implies that the accumulation of *H species on the catalyst surface blocks active sites and ultimately causes catalyst deactivation. To further elucidate the role of Cu sites in the reaction, NH was introduced into the system to coordinate with Cu, forming Cu–NH complexes and thereby specifically blocking the Cu sites; this blocking effect was verified by NH –TPD measurements (Supplementary Fig. 17). The results show that the decrease in C H yield is more pronounced than that of H , and the additional decrease in C H (relative to H ) is stoichiometrically correlated with the formation of C H . This correspondence indicates that, once Cu sites are occupied, the reaction pathway leading to C H is effectively suppressed, and a fraction of the reaction intermediates is diverted toward a C–C coupling pathway to form C H . Accordingly, we propose that atomically dispersed Cu sites predominantly mediate β–H activation, thereby driving the dehydrogenation of the *C H intermediate to form C H . Once these Cu sites are blocked, the lack of accessible active sites for further dehydrogenation may lead to the accumulation and prolonged residence of *C H species on the catalyst surface, which in turn is more likely to promote C–C coupling pathways and ultimately yield C H . Highly sensitive synchrotron-based vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS) was employed to detect trace gaseous intermediates, observing signals at m/z = 28.031 and 29.037 under the C H atmosphere (Fig. 4c). Detailed analysis of the corresponding scanning photoionization efficiency (PIE) curve revealed well-defined ionization onsets at 10.40 eV and 8.18 eV (Supplementary Fig. 18), respectively, which unambiguously identified these species as C H (IE=10.40 eV) and •C H (IE=8.24 eV) based on their characteristic ionization energy (IE) thresholds53,54, with quantitative analysis indicating that 9.5% of the signal intensity at m/z=29.037 originated from C H isotopologues.

Further experiments were employed to elucidate the interaction between photogenerated carriers and reactant molecules. First, we compared in situ femtosecond transient absorption spectra (fs-TAS) of the Cu /TiO under different atmospheres using the 365 nm pump laser as the light source (Figs. 4d and 4e). Under Ar conditions, Cu /TiO presents TiO -like characteristic negative peaks at ca. 380 nm (Supplementary Fig. 19a), which are identified as the ground-state bleaching (GSBC) signals of TiO and provide insights into the population of photoelectrons in its conduction band (CB)55,56. Compared to pristine TiO , Cu /TiO exhibits a weaker GSB signal, indicating fewer electrons at its CB, likely due to Cu sites acting as shallow traps that capture photoexcited e-. By analyzing the recovery kinetics of the GSB signal, two distinct lifetimes were identified. The shorter lifetime (τ ) corresponds to the trapping of photogenerated e- from the CB of TiO into Cu sites, while the longer lifetime (τ ) is attributed to the recombination of electrons in the CB with holes in the valence band (VB) of TiO 57,58. The prolonged τ observed in Cu /TiO compared to pristine TiO indicates that electron transfer to Cu sites effectively delays charge recombination with VB holes, thereby extending the charge carrier lifetime (Supplementary Fig. 19b). Transient photocurrent measurements and electrochemical impedance spectroscopy (EIS) further confirm enhanced charge separation efficiency (Supplementary Fig. 20). Notably, under C H exposure, Cu /TiO exhibits enhanced GSB intensity with accelerated decay kinetics (Figs. 4d and 4e), indicating ethane's dual role as electron donor and acceptor. Specifically, adsorbed C H molecules consume photogenerated holes in the VB (i.e., injecting electrons into the VB). This process facilitates the transition from the ground to the excited state and increases the initial electron concentration in the CB, thereby enhancing the GSB signal. Meanwhile, the generated •C H promotes electron consumption by forming H through *H species, which arise from Cu-mediated β–H bond activation, further accelerating charge carrier dynamics. Besides, in situ time-resolved fluorescence spectroscopy (TRFL) was also conducted under different atmospheres. As shown in Supplementary Fig. 21 and Supplementary Table 4, the average carrier lifetime was extended from 2.09 ns to 6.07 ns after Cu loading, demonstrating effective suppression of carrier recombination. It is particularly noteworthy that upon switching the reaction atmosphere to C H , the carrier lifetime dramatically decreased to 4.12 ns. This observation directly verifies that the photogenerated carriers migrating to the catalyst surface undergo rapid redox reactions with C H molecules.

Subsequently, the distribution of photogenerated carriers under light irradiation was further investigated using in situ XPS. As shown in Supplementary Fig. 22, the Cu 2p peak is shifted toward lower B.E. while the O 1s peak is shifted toward higher B.E. under light irradiation, indicating that the e- and h+ tend to be localized in the Cu and O sites, respectively. Further theoretical analysis of the partial density of states (PDOS) for Cu /TiO and TiO reveals that Cu /TiO generates gap states originating from Cu d-orbitals compared to TiO , which theoretically confirms that the doped Cu sites act as shallow electron-trapping centers (Fig. 4f). Based on the above mechanistic evidence, we propose the reaction mechanism as follows. The C–H bond activation of C H through photogenerated holes constitutes the critical step for initiating the reaction, generating •C H . Subsequently, these •C H undergo dehydrogenation at adjacent Cu sites, releasing C H and adsorbed *H species. The accumulated *H species are then reduced via photogenerated electrons, culminating in the evolution of H . Notably, in the aforementioned in situ TAS and TRFL tests, the CO -induced kinetic decay of charge carriers was negligible versus the accelerated decay driven by C H (Supplementary Figs. 19 and 21). Simultaneously, •C H radical signals showed minimal perturbations under CO co-feeding (Fig. 4c), collectively confirming that C H non-oxidative dehydrogenation remains dominant in the C H /CO mixture. To elucidate the role of CO in stabilizing Cu /TiO described above, product distributions before and after CO introduction were analyzed. As shown in Fig. 4g, trace CO formation was detected only under C H /CO co-feeding, but was absent in the CO -free atmosphere. Control experiments with H /CO co-feeding also yielded no detectable CO, indicating that the catalyst is unable to effectively activate molecular H under the present conditions and thus cannot initiate a redox-mediated reverse water–gas shift (RWGS) pathway. These findings therefore rule out RWGS as the source of CO formation in the C H /CO co-feeding system. Furthermore, C18O isotope labeling experiments confirmed that the oxygen in CO originates from CO rather than from the lattice oxygen (Fig. 4h). To further elucidate the CO formation pathway, we set a baseline feed of C H : CO = 1:1 (20 mL each) and systematically varied the ratio by increasing either reactant. CO production showed a strong positive correlation with C H input (mirroring C H yield), but remained insensitive to CO concentration (Fig. 4i). These findings further support that CO yield is indirectly driven by C H conversion.

Subsequently, in situ near-ambient pressure XPS spectroscopy (NAP-XPS) was employed to monitor the chemical state evolution of Cu /TiO under C H and C H /CO atmospheres, and to elucidate the role of CO . The results revealed that the Cu2+ content decreased significantly under pure C H but was notably suppressed when CO was co-fed (Figs. 5a and 5b), corroborating prior in situ EPR observations (Supplementary Fig. 11) and further implying that Cu2+ ([Cu-O ]) species are closely associated with the catalytically active centers. However, it should be particularly noted that in a C H atmosphere, the transformation of Cu2+ to Cu⁺ is accompanied by an increase in –OH content. In contrast, under CO co-feeding conditions, –OH is significantly consumed. Accordingly, we speculate that under C H atmosphere, the deactivation of the catalyst may be attributed to the coverage of active sites by undissociated *H on lattice oxygen, which leads to the transformation of Cu2+ to Cu+. Besides, quasi-in situ Raman spectroscopy with stepwise feeding of C H and CO revealed that the Eg mode (~144 cm-1) gradually blueshifts with prolonged reaction time in C H atmosphere (Supplementary Fig. 23), indicating enhanced TiO unit cell expansion or strain42. Notably, this blueshift disappears upon switching the atmosphere to CO , further confirming that CO can reverse the microstructural changes induced in Cu /TiO under pure C H conditions.

Density functional theory (DFT) calculations on the electronic structure of the Cu /TiO further theoretically revealed the impact of *H adsorption on the Cu valence state. Specifically, adsorption of *H species on lattice oxygen increases the Cu–O bond length from 1.95 Å to 2.26 Å (Supplementary Fig. 24). The charge difference distribution (Supplementary Fig. 25) showed a strong interaction between *H adsorbed on lattice oxygen and the support, with electrons transferring from *H to the Cu site. Bader charge calculations further confirmed that *H coverage reduces the charge at Cu sites (Supplementary Fig. 24 and Supplementary Table 5). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to identify the key reaction intermediates on Cu /TiO under C H and C H /CO atmospheres. Before the DRIFTS measurements, C H temperature-programmed desorption (TPD) experiments were carried out to assess the adsorption behavior. The TPD profiles revealed a markedly enhanced C H adsorption on Cu /TiO compared to bare TiO (Supplementary Fig. 26), confirming [Cu–O] ensembles dominate C H adsorption. Under illumination in C H (Supplementary Fig. 27a), the immediate decay of C H adsorption peaks, accompanied by the simultaneous appearance of *CH CH species and gaseous C H , provided experimental evidence for the stepwise dehydrogenation of C H to C H . Concurrently, the observed generation of –OH indicated *H accumulation on the catalyst surface (Supplementary Fig. 27b). In contrast, CO co-feeding led to the formation of *COOH intermediates (Supplementary Fig. 27c) and the effective suppression of –OH generation (Supplementary Fig. 27d). These results indicate that CO promotes the removal of surface-adsorbed *H species, thereby regenerating the catalytic active sites and concurrently producing CO. This mechanistic insight further corroborates the intrinsic correlation between the CO formation rate and the ethane conversion behavior.

To further validate this mechanism, DFT simulations under the excited/ground-state were employed based on the potential active center model of [Cu–O ]. First, two potential surface bridging oxygen (O ) hole-trapping sites were investigated, which are bonded to Cu and Ti sites respectively and denoted as O and O sites. As revealed in Supplementary Figs. 28 and 29, the hole trapping energies of O and O relative to bulk self-trapped hole were -1.23 and 0.08 eV, confirming effective localization of photogenerated holes at the surface O site. Although Cu was previously confirmed as a shallow electron-trapping center, in subsequent simulations of O -initiated C–H bond activation we placed the photogenerated e- on a remote Ti site to avoid high-energy configurations arising from adjacent electron–hole localization, thereby better reflecting the realistic reaction environment. Critically, Fig. 5c demonstrates that upon hole capture by the O site, the C–H bond of C H undergoes facile cleavage via the hydrogen atom transfer (HAT) mechanism27, with a notably low barrier of 0.46 eV and a negative free energy change (ΔG) of -0.37 eV. This barrier and ΔG are 0.76 eV and 1.16 eV lower than those for ground-state C–H activation at the same O site, respectively (Supplementary Fig. 30), thus serving as a prerequisite for high PEDH activity under mild conditions. This process generates a free •C H radical and an adsorbed *H at the O site. The •C H radical then undergoes two successive mild endergonic steps to transfer a *H to the Cu site via HAT mechanism, involving adsorption at Cu site (0.29 eV) followed by β–H cleavage with a reaction energy of only 0.11 eV. In contrast, although the C–H activation ability of O site is also significantly enhanced after trapping photogenerated holes, with the corresponding barriers being comparable to that at O site, the resulting •C H radical exhibits an adsorption energy of 0.58 eV at the five-coordinated Ti site (Ti ), and the subsequent β–H bond cleavage has a reaction energy as high as 1.60 eV (Fig. 5c and Supplementary Fig. 31), both of which are significantly larger than those on the Cu site. This demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32). Finally, we calculated the generation of H at the [Cu–O] ensembles assisted by photogenerated e- transferred from nearby Ti sites to Cu, where *H–*H coupling energy barrier is 0.79 eV. Notably, •C H is also possible to continue to activate the β–H bond via hole-trapped O with an energy barrier of 0.07 eV (Supplementary Fig. 33), following the first C–H bond cleavage mechanism. The resulting *H is captured by the O adjacent to [Cu–O] ensembles. Consequently, the H formation is significantly hindered at the O and O sites (Fig. 5c and Supplementary Fig. 34), with an energy barrier as high as 2.79 eV. We also considered the possibility of *H migration from the O or O to Cu sites for *H–*H coupling, yet these processes remain challenging. For migration from the O to the Cu site, *H could first relocate to the O site (three-fold coordinated lattice oxygen) with an energy barrier of 1.48 eV, then proceeds from O to the Cu site with a barrier of 3.12 eV (Supplementary Fig. 35), while direct migration from O to the Cu site requires overcoming a 2.92 eV barrier (Supplementary Fig. 36). This indicates that consecutive dehydrogenation on two lattice oxygens leads to difficult desorption of *H, ultimately leading to deactivation of the catalyst due to *H coverage on the [Cu–O ] active sites. Further theoretical calculations (Supplementary Fig. 37) reveal that these two *H species can be transformed into H O via a low energy barrier of 0.40 eV, accompanied by a structural reconstruction from [Cu–O ] to [Cu–O ]. However, this process is still endergonic from a thermodynamic perspective, and the *H species preferentially remain adsorbed on the catalyst surface. Notably, in the presence of CO , CO can cooperatively interact with •C H to form a *COOH intermediate, which promotes the further transformation of the [Cu–O ] species generated during the dehydration process. This, in turn, drives the continuous consumption of the accumulated *H while simultaneously regenerating the [Cu–O ] sites.

Based on the foregoing findings, a light-driven EDH mechanism is delineated in Fig. 5d. Atomically dispersed Cu promotes the localization of photogenerated holes at directly bonded O sites, facilitating initial C–H activation in C H with the significantly reduced energy barriers. The resulting •C H species binds to Cu, where β–H elimination proceeds, and the *H generated is subsequently released as H through the cooperative action of site-defined [Cu–O] ensembles. This sequential dehydrogenation coupled with efficient H evolution accounts for the high activity and selectivity toward C H . In contrast, β–H activation at O sites adjacent to (but not coordinated with) Cu lacks Cu-mediated *H–*H coupling, preventing *H recombination and leading to *H accumulation that poisons active sites. Upon introducing CO , a self-cleaning cycle is established that promotes further transformation of surface *H, thereby removing the *H adlayer and regenerating active sites.

Methods
Preparation of catalysts
Synthesis of MIL-125(Tiv). The MIL-125 with Ti vacancies (Ti v ) precursor was synthesized according to the method described in previous literature41. First, 3.0 g of terephthalic acid (99%, Shanghai Aladdin Biochemical Technology Co., Ltd.) was dissolved in 54 mL of N, N-dimethylformamide (DMF, 99.9%, Shanghai Aladdin Biochemical Technology Co., Ltd.) with 30 min magnetic stirring at room temperature. Subsequently, 6 mL of methanol (99.9%, Shanghai Macklin Biochemical Technology Co., Ltd.) was added and stirred for 5 min, followed by the addition of 1.2 mL of tetrabutyl titanate (TBOT, 99%, Shanghai Aladdin Biochemical Technology Co., Ltd.) with additional 5 min stirring. The mixture was transferred to a sealed round-bottom flask and reacted at 130 °C for 20 h under Ar atmosphere using an oil bath. After cooling, the precipitate was collected by centrifugation, successively washed twice with DMF and twice with methanol, and vacuum-dried at 80 °C for 10 h to remove residual solvents, yielding MIL-125(Tiv).

Synthesis of Cu /TiO . 150 mg of MIL-125(Tiv) was dispersed in 50 mL of water/methanol solution (1:4 v/v). Under continuous stirring, a predetermined aliquot of 0.1 M CuCl (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd.) aqueous solution was added dropwise, followed by stirring for 10 h. The resulting mixture was centrifuged and washed thrice with water/methanol solution. After oven-drying at 80 °C, the solid was calcined in a muffle furnace at 500 °C for 4 h with a heating rate of 2 °C/min to obtain Cu /TiO .

Evaluation of catalytic performance
Batch-type performance test. First, the catalyst was uniformly dispersed in 1.0 mL deionized water and coated onto a glass slide (coating area: 3.14 cm2), followed by drying at 80 °C. The catalyst-coated glass slide was placed at the center of the bottom of a sealed 100 mL quartz reactor. To ensure complete replacement of residual air by ethane, the reactor was subjected to three evacuation/ethane-flushing cycles (1 atm, 99.9% C H per cycle) before performance test. A 300 W full-spectrum (250 ~1100 nm) Xe lamp (PLS-SXE300+) was used for vertical irradiation (4 h per reaction) without additional external heating, and the light intensity was monitored using a PL-MW2000 optical power meter. Pressure changes (defined as the difference between the instantaneous pressure and the initial pressure) during the reaction were continuously recorded with an AS511A digital differential manometer (SMART SENSOR). The reaction products were analyzed using a Shimadzu gas chromatograph (GC-2014C) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). For the C18O 2 isotope-labeled experiment, the gas-phase products were detected by a 7890A-5975C gas chromatography-mass spectrometer (Agilent Technologies Co., Ltd.).

The catalyst surface temperature was simultaneously monitored using a Uti160S infrared thermal imager (UNI-T) and a calibrated K-type thermocouple (diameter 0.25 mm, KAIPUSEN). The thermocouple probe maintained direct contact with the center of the catalyst bed, while the thermal imager was installed at an oblique angle above the reactor, oriented toward the top surface of the catalyst bed to capture thermal image data. The emissivity was established as ε = 0.95 based on an empirical calibration procedure performed with catalyst powder distributed uniformly on a pre-calibrated heating plate. During calibration, the heating plate temperature was systematically varied (50, 100, 150, and 200 °C) and the imager's emissivity setting was adjusted until its temperature readings concurred with the reference values at each point, thereby determining the optimal emissivity value for all subsequent experimental measurements.

Flow-type performance test. The performance tests of flow-type PEDH and TEDH were both evaluated in a PLS-LED 100 photothermal reactor (Beijing Perfectlight) equipped with external temperature control, where the catalyst-bed temperature was monitored by a KE-type thermocouple inserted into the center of the catalyst layer. For each test, 10 mg of catalyst was uniformly coated onto a heat-resistant quartz fiber filter membrane (fixed coating area: 3.14 cm2), and the flow rate of the feed gas (20 vol% C H /20 vol% CO /Ar) was set as 10 mL min-1. The apparent activation energy measurements were carried out in the same reactor, in which a UV light source with fixed intensity was employed to mitigate photothermal effects, and a relatively low temperature window of 60~140 ℃ was selected to ensure that the reaction proceeded in the kinetic regime and to minimize interference from internal and external mass transfer and thermal effects.

The corresponding product yield, selectivity, conversion, and carbon balance were calculated using the following equations:
Yield = [C H ] / (m*t) [1]
Selectivity = [C H ] / ([C H ]+0.5*[CH ]+2*[C H ]) *100% [2]
Conversion = ([C H ] - [C H ]) / [C H ] *100% [3]
Carbon balance = ([C H ]+0.5*[CH ]+2*[C H ]) / [C H ] [4]
where [C H ], [CH ], [C H ] and [C H ] represent the molar amounts of C H , CH , C H and C H , respectively. m and t represent the mass of the catalyst (g) and the reaction time (h), respectively.

Characterization
The morphological characteristics of the samples were characterized using scanning electron microscopy (SEM, Helios G4 UC), transmission electron microscopy (TEM, Talos F200X), and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Themis Z). Powder X-ray diffraction (XRD) patterns were acquired using a Rigaku D/MAX 2550 diffractometer with Cu Kα radiation (λ=1.5406 Å), scanning the 2θ range from 5° to 80° at a rate of 8° min-1. X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB 250Xi system (Thermo Fisher Scientific) with monochromatic Al Kα radiation (hν=1486.6 eV), and the binding energies were referenced and corrected using the C 1s peak at 284.8 eV. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were conducted on an Agilent 5800 VDV system. Thermogravimetric/differential thermal analysis (TG/DTA) was conducted using a Netzsch TGA209F1 instrument, with the temperature ranging from 50 to 800 °C. Raman spectroscopy measurements were performed using a Renishaw inVia Raman system, with a 532 nm laser serving as the excitation source. UV-Vis diffuse reflectance spectra of the dry-pressed disk samples were recorded on a Cary 500 spectrophotometer (Varian) with an integrating sphere, using BaSO as a reflectance reference standard. Electron paramagnetic resonance (EPR) measurements were performed using a 100G-18KG/EMX-8/2.7 instrument. Transient photocurrent response and electrochemical impedance spectroscopy (EIS) measurements were conducted using a ZAHNER PP211 electrochemical workstation with a standard three-electrode configuration. The working electrode consisted of fluorine-doped tin oxide (FTO) glass coated with the sample (5 mg, coating area: 1 cm2), with a platinum counter electrode and a saturated calomel electrode as a reference electrode. Transient photocurrent tests were performed in 0.5 M aqueous Na SO solution, while EIS measurements were carried out in an electrolyte system containing potassium ferricyanide. Ammonia temperature-programmed desorption (NH -TPD) measurements were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. Following saturation with 10% NH /He gas mixture at 50 °C, the sample was heated from 50 to 800 °C at a rate of 10 °C min-1 under a He flow, while the desorbed gases were monitored via a thermal conductivity detector (TCD). C H -TPD measurements followed an identical procedure to that of NH -TPD, with catalyst saturation achieved via pulsed injection of pure C H gas into the system. Temperature-programmed surface reaction (TPSR) experiments were performed on a TP5080 automatic multi-purpose adsorption analyzer, and the desorbed products were monitored in situ using a Hiden Analytical DECAR mass spectrometer during linear heating from 50 to 800 °C at 10 °C min-1. For C H -TPSR, the feed gas consisted of 20% C H balanced with Ar; for C H /CO -TPSR, the feed gas was composed of 20 vol% C H /20 vol% CO balanced with Ar.

XAFS measurement
Cu K-edge X-ray absorption spectroscopy (XAS) was collected in fluorescence mode at the XAFCA beamline of the Singapore Synchrotron Light Source (SSLS). Measurements employed a double-crystal Si (111) monochromator with energy calibration referenced to a Cu foil. O K-edge XAFS measurements were performed at the soft X-ray beamline of the National Synchrotron Radiation Laboratory (NSRL) (Hefei, China). The XAS data were processed using the Demeter software package (ATHENA and ARTEMIS)59.

In situ experiments
In situ synchrotron-based vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS). In situ SVUV-PIMS measurements were conducted at the Atomic and Molecular Physics beamline (BL09U) in the National Synchrotron Radiation Laboratory (NSRL) (Hefei, China)60. The catalyst was positioned at the center of a laminar flow reactor cell. Reaction products were sampled via molecular beam into the ionization chamber, ionized by synchrotron VUV light, and detected by a time-of-flight mass spectrometer. The experiment began by purging the reaction cell’s differential chamber (pressure stabilized at 20.0 Pa) with 50 vol% C H /Ar at a flow rate of 20 mL min-1 for 30 min to remove air. Illumination was provided by a 300 W xenon lamp (without external heating). Following 30 min of illumination, mass spectra were acquired at 10.5 eV with an integration time of 1000 s. Product photoionization efficiency (PIE) curves were scanned from 8.0 to 11.5 eV with an integration time of 240 s per point. The same procedure was repeated for data collection after switching the feed gas to 50 vol% C H /CO mixture.

In situ femtosecond transient absorption spectroscopy (fs-TAS). In situ fs-TAS measurements were performed using a Helios spectrometer (Ultrafast system) under pure Ar, 50 vol% C H /Ar, or 50 vol% C H /50 vol%CO . The amplified femtosecond Ti: sapphire laser system operated at a repetition rate of 1 kHz, and the probe light was collected via the lens of an InGaAs diode array.

In-situ time-resolved fluorescence spectroscopy (TRFL). In situ TRFL spectra were acquired on an Edinburgh Instruments FLS 980 spectrometer. Samples were sealed in 1 mm-pathlength quartz microcuvettes and measured under pure Ar, C H , or CO atmosphere. Excitation was provided by a 360 nm light-emitting diode (LED) source.

In situ NAP-XPS. In situ NAP-XPS was performed on a Thermo Scientific ESCALAB 250Xi system with Al Kα radiation (hν = 1486.6 eV). After Ar⁺ sputtering (3 keV, 40 eV pass energy, 0.1 eV step, 1 min), XPS data under vacuum were first collected, after which 50 vol% C H /Ar or 50 vol% C H /50 vol% CO were continuously introduced. Spectra were recorded sequentially under dark conditions, as well as 20 and 40 min after illumination with a 300 W xenon lamp. NAP-XPS spectra were averaged over 5 scans with charge referencing to C 1s (284.80 eV).

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In situ DRIFTS measurements were performed on a Thermo Fisher-Nicolet iS20 infrared spectrometer equipped with a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector, using a three-window diffuse-reflectance cell (Harrick). Before the test, samples were dehydrated at 120 °C for 1 h. Subsequently, 0.1 g of the dried catalyst was loaded into the sample cell and purged with Ar (99.9 vol%) for 30 min to eliminate surface-adsorbed species. Following Ar purging, a background spectrum was collected under an Ar atmosphere. Ar was then flushed out, and a continuous flow of reactant gas (either 20 vol% C H /Ar or 20 vol% C H /20 vol% CO /Ar) was introduced at a flow rate of 20 mL min-1. Spectra were collected at specified time intervals after initiating light irradiation. The spectral resolution was 4 cm-1, and the scanning range was 4000–600 cm-1.

Theoretical calculations
All spin-polarized calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation using the VASP 6.3.2 code61,62. The valence electronic states were expanded in plane-wave basis sets with an energy cutoff at 450 eV, and the occupancy of the one-electron states was calculated using the Gaussian smearing with SIGMA = 0.05 eV. The on-site Hubbard U term (PBE+U) was added for O orbitals and Ti orbitals with a value of 4.0 eV and 4.2 eV to facilitate the localization of trapped holes and electrons. In particular, the U correction on the O 2p orbitals helps to cancel self-interaction errors and enables a more realistic localization of photogenerated holes on surface O sites. In addition, to simplify the treatment of the photoexcited system by excluding the uncertain electron–hole interactions that depend on the specific trapping center, while simultaneously preserving the charge neutrality of the periodic slab and avoiding artificial background-charge corrections, an -OH group was introduced on the bottom surface as an electron acceptor63. To validate the accuracy of the DFT+U method64, complementary HSE06 hybrid functional calculations were systematically performed for all reaction steps, with corresponding energy values exclusively derived from the HSE0627. Structure optimizations were performed using the Quasi-Newton Broyden minimization scheme until the atomic forces on each ion were less than 0.05 eV/Å. The transition states (TSs) were searched using a constrained optimization scheme65. In this approach, we keep the C–H bond distance fixed at preselected values, and minimize the total energy with respect to all remaining orthogonal degrees of freedom. In practice, this allows the adsorbed molecules to rotate and translate under the imposed constraint, and the TiO surface is fully relaxed. A transition state is identified when the forces on all atoms vanish and the total energy exhibits a maximum along the chosen reaction coordinate while remaining a minimum with respect to all other degrees of freedom.

A 4-layer p (2×3) unit cell of 10.3 Å, 11.4 Å with a 20.0 Å vacuum between the slabs in the z direction was used for the construction of the anatase TiO (101) surface. The Brillouin zone was sampled with a 3×2×1 Monkhorst-Pack k-point mesh. The free energy calculations are based on Eq.5: ΔG=ΔE+ΔZPE-ΔTS [5] where E is the calculated total energy of the model, ZPE is the change of zero-point vibrational enthalpy, and TS is the entropy. For adsorbed intermediates, vibrational frequencies were calculated at the optimized geometries and used to obtain ΔZPE and the vibrational contribution to ΔS at 383 K. The ΔG corrections were then evaluated using the VASPKIT post-processing tool based on these vibrational frequencies66. For gas-phase species, translational, rotational and vibrational entropy contributions at 383 K were taken from standard thermochemical data (NIST).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors reported an investigation of synthesis of single atom Cu-loaded on TiO2 for photocatalytic ethane dehydrogenation with or without CO2. In the work, the authors carried out a comprehensive investigation on the catalytic performance and mechanism. From the experimental tests and DFT calculation, the authors have presented a clear mechanism on the photocatalysis. The work is innovation and provide new information on the catalytic system. It can be published after revision.\n1. The authors prepared several different metal-loaded catalysts and found that these catalysts presented various activity. However, they did not discuss the underline reason. It would be better for the authors to check if all the catalysts are single-atom catalysts.\n2. In the work, the authors suggested two Cu species in Cu/TiO2, Cu-O4 and Cu-O3. What are the detailed roles of these species in catalysis?\n3. In the tests, the authors found that CO production showed strong positive correlation with C2H6 but remained insensitive to CO2, suggesting CO yield is indirectly driven by C2H6 conversion. How?\n4. In DFT calculation, the authors did not check the CO2 involvement on catalysis. It is highly required for the authors to check the role of CO2 in photocatalysis.", "reviewer_2": "Photocatalytic ethane dehydrogenation is a promising route to value-added chemicals, but progress remains limited by low productivity. This study reports a Cu/TiO₂ catalyst with encouraging performance, supported by in-situ characterizations and DFT. The manuscript is clearly written and potentially insightful. At first glance, the work appears to deliver a major advance in activity; however, the activity is overstated and the interpretation is not sufficiently critical. The actual gains require more cautious evaluation. Substantial issues must be addressed before the manuscript can be considered for publication, with particular emphasis on significantly strengthening the mechanistic analysis. Below are the detailed concerns:\n1. From the XPS results, no Cu⁰ exists in the samples. The distributed Cu⁰ shown in Fig. 2A should be removed. The authors report 61% Cu²⁺ and 38% Cu⁺, which is not a minor Cu⁺ fraction. However, they state that “Cu species predominantly exists as Cu²⁺ with minor Cu⁺ states.”.\n2. The activity evaluation relies mainly on a batch reactor, yet the Batch-type performance test section does not report the pressure. In a sealed batch reactor the pressure evolves during reaction and this can inflate apparent activity. Please report initial and final pressures, and how pressure changes over time. In addition, surface temperature measurement needs clarification. Please specify the measurement method, the emissivity used for infrared measurements, and the calibration procedure. Different emissivity values can lead to large differences in the measured temperature.\n3. Batch and flow data in Table S3 should not be compared directly. Batch often overstates activity relative to flow, as your Fig. 3E shows. Please give apple to apple comparisons. Compare batch with batch and flow with flow, and test Cu/TiO₂ in a flow reactor under C₂H₆/Ar, including a stability test. Flow performance is the key metric for industrial relevance.\n4. The methods state that a 300 W Xe lamp (PLS-SXE300+) was used for vertical irradiation for 4 h without additional heating. Please clarify how activity was calculated for each rate. There is a discrepancy between Fig. 3B and Fig. 3D. In Fig. 3B, even with 10 mg catalyst, the reported rate is about 7 mmol h⁻¹ g⁻¹. In Fig. 3D under C₂H₆/Ar, the accumulated amount after 4 h is about 5.5 mmol g⁻¹, which corresponds to about 1.1 mmol h⁻¹ g⁻¹. In addition, please report the ethane conversion and the carbon balance under C2H6/Ar and C2H6/CO2/Ar.\n5. In Figure S9B, the activities were measured with a 777 mW cm⁻² xenon lamp and 10 mg catalyst. When the light intensity is about 310 mW cm⁻², the C₂H₄ yield in the figure is only around 1.1 mmol h⁻¹ g⁻¹. “yield” appears misspelled in the figure. In Figure S9C, the activity drops by only about 10% when wavelengths <400 nm are used, and there is no activity when wavelengths > 400 nm are used. However, in Table S2 at 365 nm with an intensity of only 54 mW cm⁻² and 50 mg catalyst, the reported C₂H₄ amount is 0.057 mmol h⁻¹. This corresponds to about 1.14 mmol h⁻¹ g⁻¹. Is this possible?\n6. The description of AQY conditions is inconsistent. The caption of Fig. S9A states “UV–visible diffuse reflectance spectra and corresponding AQY values of Cu₁/TiO₂ after 1 h of irradiation with different monochromatic lights. Default conditions: 10 mg catalyst, 1 atm pure C₂H₆, 777 mW cm⁻² xenon lamp, 4 h test, batch reactor.” In contrast, Table S2 states “The AQY measurements were conducted in a sealed quartz reactor using 50 mg of catalyst uniformly coated over a 1.0 cm radius circular area for a 1 h reaction.” Please ensure the accuracy of all data.\n7. The authors state that “these findings further support that CO yield is indirectly driven by C₂H₆ conversion, with CO₂ serving solely as the oxygen source.” The observed C₂H₄/H₂ ratio is close to 1, which is consistent with non-oxidative dehydrogenation. If CO₂ provides only oxygen, please clarify where the carbon in the produced CO comes from. Please calculate the full carbon balance. Conduct ¹³C labeling to confirm the source of carbon.\n8. What temperature was used for the Ea calculation under illumination? Was the photothermal effect excluded?\n9. Cu/TiO₂ is widely reported for RWGS. Why was no CO detected in the H₂/CO₂ control experiment?\n10. In the NAP-XPS results, what changes are observed in Ti?\n11. In the DFT section, the reported barrier reduction of 62.3% for C–H activation at the same Obr Cu site in Path 1 (1.22 eV to 0.46 eV) is clear. Please explain how the percentages 94.1%, 96.6%, and 71.7% were obtained for Paths 2, 3, and 4. For example, Path 4 is listed as 2.83 eV to 0.83 eV, which gives about 70.7% if computed as (2.83-0.83)/2.83, not 71.7%.\n12. In the dark-state DRIFTS experiments with C₂H₆ and CO₂, gas-phase C₂H₆ shows bands in the same region as surface species. The observed signals may therefore originate from gas-phase C₂H₆ rather than adsorbates. On this basis, the conclusion of preferential C₂H₆ adsorption over CO₂ is not supported. Please perform CO₂-DRIFTS and competitive C₂H₆/CO₂ adsorption.\n13. Figure S24 shows only the infrared spectrum of Cu₁/TiO₂. Please also include the spectrum of TiO₂ to enable comparison of the OH region. Use the same enlarged view and scaling of the OH region in Fig. S24a and Fig. S24b to allow a direct comparison.\n14. The authors state in line 157 that “[Cu–O] sites effectively suppress side reactions, including alkene hydrogenation and over-dehydrogenation.” This conclusion is not supported by comparative data. There is no direct comparison with TiO₂ or other controls that would demonstrate suppression rather than simply low intrinsic activity under the tested conditions. In addition, C₂H₆ is detected in reactions 2, 3, and 4, which indicates some hydrogenation occurs. Figure S8 supports only that Cu/TiO₂ is not efficient for C₂H₄ and C₂H₂ hydrogenation under the specific test conditions. Please provide additional evidence clarifying the role of Cu in the reaction and the role of CO₂.\n15. Figure S9A does not show a dark control at an equivalent catalyst surface temperature.\n16. How do C₁₀H₁₀Fe, K₂S₂O₈, (NH₄)₂C₂O₄, and NH₃ affect the surface properties, including the Cu valence and C₂H₆ adsorption? How to rule out residual effects of these reagents on the surface as a cause of the reduced activity?\n17. The way the ethyl is coordinated to the surface as shown in the *C2H5+*H panel in Fig. 5c seems odd for an intermediate (unless it’s a transition state), since based on coordination chemistry principles one would expect coordination through the carbon – is there experimental evidence supporting H-coordination to the surface?\n18. For Fig. 5c, what does each of the paths mean, and why do paths 1 and 2 become disconnected? I would avoid using percentages when comparing two different energies since it may be misleading and at times confusing to the reader (i.e. the 96.6% has a smaller gap than the 62.3%) but instead report the energy difference directly.\n19. For the computational methods, a +U correction is typically not applied to the oxygen but only the metal – if there’s any special reason for this it should be discussed; for the Ti, why was 4.2 eV chosen (is there a reference to it an experimental/computational benchmark?)\n20. The authors report delta G values and provide the equation associated with it, but there was no mention of how the entropy was obtained (i.e. were any vibrational/phonon calculations done, and how are the associated contributions to the vibrational part of the entropy obtained?)\n21. For the TS search, how the constrained optimization technique was used should be discussed.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "This manuscript by Lingzhi Wang and coworkers reported a site-defined photocatalytic strategy for the selective light-driven dehydrogenation of ethane (C₂H₆) to ethylene (C₂H₄) using atomically dispersed Cu–O ensembles on TiO₂ (Cu₁/TiO₂). The study highlights that Cu atoms coordinated with bridging oxygen form well-defined [Cu–O] motifs, which serve as active centers for stepwise C–H activation and H₂ evolution. Photogenerated holes localize at Obr–Cu sites to initiate the first C–H cleavage, while adjacent Cu centers mediate β–H elimination, resulting in efficient and hydrogen-conserving C₂H₆ upgrading. This catalyst achieved an impressive C₂H₄ yield of 21.1 mmol g⁻¹ h⁻¹, 97% C₂H₄ selectivity, and a quantum efficiency of 6.1% at 365 nm, outperforming state-of-the-art photocatalytic ethane dehydrogenation systems. The co-feeding of CO₂ was found to selectively scavenge surface *H species, thereby preventing catalyst deactivation and preserving long-term activity. The mechanism is well-supported by a combination of in situ EPR, DRIFTS, NAP-XPS, TRFL, fs-TAS, and DFT calculations, which collectively reveal that [Cu–O] ensembles orchestrate cooperative charge transfer and redox functionality, enabling controlled C–H activation and H₂ evolution under mild conditions. This paper provides atomic-level insight into the design of redox-cooperative single-atom photocatalysts for alkane upgrading and contributes significantly to the development of sustainable, low-temperature C–H activation chemistry. This paper will be potentially acceptable for publication in Nature Communications. However, I do not recommend publication in the present form because some questions need to be answered.\n1. The authors' characterization (XPS, XAS) clearly indicates that the as-prepared catalyst contains a substantial fraction of Cu+ (approx. 38% based on XPS deconvolution), coexisting with Cu2+. The manuscript's mechanistic discussion, however, focuses almost exclusively on the role of the [Cu2+−O 4] motif as the primary active site for C-H activation and H₂ evolution. The role of the significant initial population of [Cu+−O3] sites remains ambiguous. Are the Cu+ sites catalytically active for any step of the reaction, or are they merely spectators? Can these sites act as electron traps or play another role in the charge carrier dynamics that influence the overall efficiency? Is there any evidence for the interconversion between Cu+ and Cu2+ under reaction conditions, beyond the deactivation pathway identified?\n2. The authors report a catalyst surface temperature of ~108 °C under illumination. While the control experiments (wavelength dependence, dark controls, and a significantly lower activation energy under light) provide strong evidence for a predominantly photocatalytic mechanism, the potential contribution of a photothermal effect at this temperature warrants a more detailed discussion. A localized thermal effect could still play a role in overcoming certain reaction barriers. The authors should provide a more quantitative analysis or discussion to deconvolve the quantum effects from the light-induced thermal effects to strengthen their mechanistic claims further.\n3. The strategy of using CO2 to regenerate the catalyst by scavenging poisonous *H species is a highlight of this work. However, this process produces CO as a byproduct. The 20-hour stability test is promising, but for potential practical applications, the long-term consequences should be considered. Can the accumulation of CO in a continuous or recycle reactor lead to catalyst poisoning (e.g., via carbonyl formation on Cu sites) or promote side reactions like the reverse water-gas shift reaction with the co-produced H2?\n4. In the abstract, the phrase \"adjacent Cu centers mediate -H elimination\" should be specified as \"β-H elimination\" for clarity, consistent with the terminology used later in the manuscript.\n5. The observation that C4H10 is formed when Cu sites are blocked by NH3 is interesting. The authors state that this is because ∙CH5 radicals prefer coupling. A brief mechanistic explanation would be beneficial, clarifying why this occurs (e.g., due to increased surface concentration and lifetime of the radicals in the absence of a site for the second dehydrogenation step).\n6. The photocatalytic process is modeled under excited-state conditions, but the manuscript does not fully clarify how the \"excited\" surface states are constructed in DFT. Are the results from constrained hole localization sufficient to capture the photogenerated carrier distribution? A more detailed explanation of how the excited-state pathway is modeled would be better.\n7. The reported barrier for C–H activation at Obr–Cu under photoexcited conditions (0.46 eV) is impressively low compared to thermal activation (0.46 vs 1.2 eV+). However, the paper would benefit from additional comparison across different sites (Obr–Cu vs Obr–Ti vs terrace Ti sites) under the same computational framework, to confirm the relative preference quantitatively.\n8. The calculated barriers for *H migration (2.9–3.1 eV) are high, supporting the conclusion of *H accumulation. However, the authors could further discuss whether such high-energy events are still feasible under photon-driven conditions (where local hot carriers may contribute). The neglect of non-adiabatic effects may lead to an underestimation of possible *H dynamics.\n9. The proposed *H scavenging pathway via CO₂ → *HCOO → CO is strongly supported by DRIFTS and isotope labeling, but the DFT section only briefly covers CO₂ reactivity. It would strengthen the work if the energy profile for *HCOO formation and decomposition were presented explicitly, in parallel with the C₂H₆ dehydrogenation pathway.\n10. Following the first question, the experimental results show that the catalyst contains a significant amount of Cu+ (~38%). The current DFT model in Figure 5C focuses on what appears to be a Cu2+ site. Given the prevalence of Cu+, its potential role in the catalytic cycle cannot be ignored. Did the authors perform calculations on a model containing a Cu+ site (e.g., a [Cu+−O3] unit)? A comparative calculation of the C-H activation barrier on a modeled Cu+ site would be highly insightful. If the barrier is significantly higher, it would strengthen the claim that Cu2+ is the primary active state. If the barrier is comparable, it would suggest a more complex picture.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We sincerely thank all reviewers for their time and effort in evaluating our manuscript entitled \"Site-defined Cu-O ensembles enable hydrogen-conserving light-driven ethane upgrading\" (Manuscript Number: NCOMMS-25-73862-T). We are truly grateful for your constructive comments, which not only guide our research but also greatly help improve the manuscript quality. All comments have been carefully considered, and additional experiments have been performed where appropriate. Point-by-point responses are provided below, and the corresponding revisions have been incorporated into the revised manuscript. For clarity, reviewer comments are shown in blue, our responses in black, and changes in the manuscript are highlighted in yellow.\nResponse to Reviewer #1\nGeneral Comment: In this manuscript, the authors reported an investigation of synthesis of single atom Cu-loaded on TiO for photocatalytic ethane dehydrogenation with or without CO . In the work, the authors carried out a comprehensive investigation on the catalytic performance and mechanism. From the experimental tests and DFT calculation, the authors have presented a clear mechanism on the photocatalysis. The work is innovation and provide new information on the catalytic system. It can be published after revision.\nResponse: We sincerely thank you for your positive assessment of our work as well as your encouraging and valuable comments. We have carefully considered all of your comments and provide detailed, point-by-point responses below.\nComment 1: The authors prepared several different metal-loaded catalysts and found that these catalysts presented various activity. However, they did not discuss the underline reason. It would be better for the authors to check if all the catalysts are single-atom catalysts.\nResponse: We sincerely appreciate you for your valuable suggestions, which have significantly contributed to validating the reliability and universality of the single-atom catalyst synthesis method employed in our work. We fully understand the concern about confirming the structural characteristics of each metal catalyst. It should be emphasized that all metal loaded catalysts in this study were prepared using exactly the same synthetic protocol, in which key parameters such as precursor concentration, loading procedure, and calcination conditions were strictly kept identical. Therefore, taking comprehensive considerations into account, we selected Ag/TiO catalyst as a representative to supplement HAADF-STEM and EXAFS characterizations for further verifying the universality of the synthesis method. Isolated atomic bright spots of Ag species are observed in HAADF-STEM images, and EXAFS spectra show only Ag-O signals with no signals corresponding to Ag-Ag bonds or Ag-(O)-Ag bonds, clearly confirming that Ag is dispersed as single atoms (please see renumbered Supplementary Fig. 6). Together with the previously confirmed single-atom nature of Cu /TiO , these results strongly support the universality of this synthesis method in preparing single-atom active centers.\nFurthermore, we would like to clarify that the primary scientific objective of this work is to elucidate the reaction mechanism of Cu /TiO . The data for other metal single-atom catalysts are included mainly to place the performance of Cu /TiO in a broader catalytic context, rather than to provide a systematic, metal-by-metal interpretation of activity differences. The observed variations in activity are likely governed by a complex interplay of factors, including electronic structure, metal–support interaction strength, and the adsorption energetics of key reaction intermediates. A rigorous, quantitative analysis of these multidimensional structure–activity correlations would require a dedicated, systematic study and therefore lies beyond the scope of the present manuscript. We fully agree that this is an important topic and have identified it as a promising direction for future work, in which we plan to carry out comprehensive comparative investigations across different metal single-atom catalysts.\nThe characterization data for Ag/TiO and related discussions have been incorporated into the revised manuscript (Please see the revised manuscript, page 5, first paragraph), further strengthening the evidence for the generality of the synthesis method.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 8, Renumbered Supplementary Fig. 6)\nSupplementary Fig. 6. a HADDF-STEM image of Ag/TiO . b k3-weighted Fourier-transformed EXAFS spectra at Ag K-edge of Ag/TiO .\n(2) (Revised manuscript, page 5, first paragraph)\nTaken together, we assign [Cu–O ] as the predominant configuration for Cu2+ species, while [Cu–O ] corresponds to Cu+ sites. To further evaluate the generality of this MOF-based pre-encapsulation strategy for synthesizing single-atom catalysts, we additionally prepared Ag/TiO under identical synthetic conditions. HAADF-STEM images of Ag/TiO display isolated bright spots that can be assigned to individual Ag atoms, and the corresponding Ag K-edge EXAFS spectra show only Ag–O coordination without any detectable Ag–Ag or Ag–(O)–Ag paths (Supplementary Fig. 6). These results, together with the atomically dispersed Cu centers in Cu /TiO , strongly support that the present synthesis method is capable of stabilizing different metals predominantly in single-atom states.\nComment 2: In the work, the authors suggested two Cu species in Cu /TiO , Cu-O and Cu-O . What are the detailed roles of these species in catalysis?\nResponse: We sincerely thank you for raising this critical question regarding the catalytic roles of the two distinct copper species, [Cu–O ] and [Cu–O ], in the Cu /TiO catalyst. Our comprehensive analysis integrating in situ spectroscopic characterization with first-principles calculations unequivocally identifies the [Cu–O ] motif being identified as the primary active center for non-oxidative ethane dehydrogenation. This conclusion is robustly supported by quasi-in situ EPR (renumbered Supplementary Fig. 11) and in situ NAP-XPS (Figs. 5a and 5b) analyses, which consistently show that catalyst deactivation process is always accompanied by a pronounced decrease in the Cu2+ signal (associated with the [Cu–O ] motif). Under CO co-feeding conditions, the catalytic performance remains stable and the Cu2+ content is effectively maintained, further highlighting the central role of [Cu–O ] in sustaining activity.\nTo further clarify the role of [Cu–O ] and strengthen the assignment of [Cu–O ] as the main active site, we performed additional DFT calculations comparing β–H activation of the ·C H intermediate on these two motifs, guided by the active-species quenching experiments in Fig. 4b, which indicate that Cu sites are responsible for β–H activation. The results (please see renumbered Supplementary Fig. 32) reveal that on the most representative [Cu–O ] motif, the reaction energy for β–H bond cleavage in the ·C H intermediate is as high as 1.06 eV, indicating a strongly endergonic process. In sharp contrast, the same C–H activation step on the [Cu–O ] motif exhibits a much lower reaction energy of only 0.11 eV. This pronounced energy difference clearly demonstrates that [Cu–O ] ensembles as the key active centers for β–H activation, whereas [Cu–O ] ensembles make only a minor contribution to this crucial step.\nAdmittedly, due to the objective limitations of catalyst synthesis methods and the complexity of Cu valence state regulation, the existence of the [Cu–O ] structure is an intrinsic characteristic of the current system. Nevertheless, its presence does not affect the identification of the main active site. To further clarify the conclusion regarding [Cu–O ] as the primary active center, relevant insights have been comprehensively incorporated into the revised manuscript (Please see the revised manuscript, page 12, second paragraph), offering a more rigorous and coherent mechanistic explanation for the structure-activity relationship of the catalytic system.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 34, Renumbered Supplementary Fig. 32)\nSupplementary Fig. 32. Comparison of calculated energy diagrams for β–H activation at the Cu sites in [Cu-O ] and [Cu-O ] motifs.\n(2) (Revised manuscript, page 12, second paragraph)\nThis demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H br–Cu 2 5 activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32).\nComment 3: In the tests, the authors found that CO production showed strong positive correlation with C H but remained insensitive to CO , suggesting CO yield is indirectly driven by C H conversion. How?\nResponse: We greatly appreciate your valuable comments, which help us clarify the mechanism more clearly and facilitate readers' understanding. Our interpretation is that under C H /CO co-feeding, CO originates primarily from the reduction of CO driven by a small amount of surface *H generated during the selective dehydrogenation of ethane, followed by CO formation. Because the formation of *H is governed by the activation and conversion rate of C H , the CO yield increases with C H conversion. Under our experimental conditions, the CO supply is sufficient and its involvement does not constitute the rate-limiting step, which explains the observed insensitivity of CO formation to changes in CO concentration.\nThis mechanism is supported by two lines of evidence. First, no CO was observed in the H /CO co-feeding control experiment (Fig. 4g), excluding the dominant redox-type RWGS pathway driven by molecular H activation under the present conditions. Second, in situ NAP-XPS and in situ infrared results show that ethane dehydrogenation can generate detectable surface *H signals, and *H is consumed under CO atmosphere (Figs. 5a and 5b). To present the above mechanism more intuitively, we have strengthened the corresponding discussion in the revised manuscript and supplemented a detailed analysis of the CO formation pathway (Please see the revised manuscript, page 10, second paragraph and page 12, first paragraph). In addition, following your Comment 4, we have incorporated DFT results on the involvement of CO , providing a more systematic elucidation of the CO formation pathway (Please see Comment 4). These revisions are intended to help readers accurately understand the reaction process.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 10, second paragraph)\nAs shown in Fig. 4g, trace CO formation was detected only under C H /CO co-feeding, but was absent in the CO -free atmosphere. Control experiments with H /CO co-feeding also yielded no detectable CO, indicating that the catalyst is unable to effectively activate molecular H under the present conditions and thus cannot initiate a redox-mediated reverse water–gas shift (RWGS) pathway. These findings therefore rule out RWGS as the source of CO formation in the C H /CO co-feeding system.\n(2) (Revised manuscript, page 12, first paragraph)\nIn contrast, CO co-feeding led to the formation of *COOH intermediates (Supplementary Fig. 27c) and effective suppression of –OH generation (Supplementary Fig. 27d). These results indicate that CO promotes the removal of surface-adsorbed *H species, thereby regenerating the catalytic active sites and concurrently producing CO. This mechanistic insight further corroborates the intrinsic correlation between the CO formation rate and the ethane conversion behavior.\nComment 4: In DFT calculation, the authors did not check the CO involvement on catalysis. It is highly required for the authors to check the role of CO in photocatalysis.\nResponse: We sincerely appreciate your valuable comment. In response, we have now conducted additional DFT calculations to explicitly evaluate the involvement of CO in the photocatalytic process. These new results have been incorporated into the revised manuscript and Supplementary Information.\nAs outlined in the manuscript, the *H generated from β–H desorption at the O site does not readily desorb as H due to the high barrier of H–H coupling, leading to the deactivation of [Cu–O ] sites via *H coverage. Further calculations show that while direct reaction of *H with CO is thermodynamically unfavorable (∆G=3.5 eV), *H can combine with another *H at a neighboring O site to form H O with a low barrier of 0.40 eV, accompanied by the transformation of [Cu–O ] to [Cu–O ] (please see renumbered Supplementary Fig. 37). It should be noted that this dehydration step is still thermodynamically endergonic. This implies that, if the resulting [Cu–O ] structure cannot undergo further transformation, the *H species will thermodynamically tend to remain adsorbed, which is consistent with our experimental observation of *H buildup under a pure C H atmosphere. Under CO co‑feeding, CO cooperates with ·C H radicals from other sites to restore the [Cu–O ] structure via the *COOH pathway. As shown in Supplementary Fig. 37, adsorbed CO abstracts a β–H from ·C H to form *COOH (barrier: 1.32 eV), which then readily decomposes to adsorbed CO (barrier: 0.04 eV). After CO desorption, the [Cu–O ] site is regenerated and can continue to participate in β–H elimination, with the released *H subsequently coupling to form H . By contrast, direct CO dissociation via the MvK mechanism requires overcoming a much higher activation barrier (1.96 eV) and reaction energy (1.85 eV) (see Response Fig. R1), further corroborating the relative feasibility of the aforementioned cooperative pathway. These findings have been incorporated into the revised manuscript (Please see the revised manuscript, page 13, first paragraph), and we appreciate the reviewer’s insightful comments.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 39, Renumbered Supplementary Fig. 37)\nSupplementary Fig. 37. Reaction pathway for CO -assisted removal of surface *H species.\n(2) (Revised manuscript, page 13, first paragraph)\nThis indicates that consecutive dehydrogenation on two lattice oxygens leads to difficult desorption of *H, ultimately leading to deactivation of the catalyst due to *H coverage on the [Cu–O ] active sites. Further theoretical calculations (Supplementary Fig. 37) reveal that these two *H species can be transformed into H O via a low energy barrier of 0.40 eV, accompanied by a structural reconstruction from [Cu–O ] to [Cu–O ]. However, this process is still endergonic from a thermodynamic perspective, and the *H species preferentially remain adsorbed on the catalyst surface. Notably, in the presence of CO , CO can cooperatively interact with •C H to form a *COOH intermediate, which promotes the further transformation of the [Cu–O ] species generated during the dehydration process. This, in turn, drives the continuous consumption of the accumulated *H while simultaneously regenerating the [Cu–O ] sites.\nFig. R1. Reaction energy pathway for CO → CO on the [Cu–O ] site and corresponding DFT-optimized initial state (IS), transition state (TS), and final state (FS) structures.\nResponse to Reviewer #2\nGeneral Comment: Photocatalytic ethane dehydrogenation is a promising route to value-added chemicals, but progress remains limited by low productivity. This study reports a Cu/TiO catalyst with encouraging performance, supported by in-situ characterizations and DFT. The manuscript is clearly written and potentially insightful. At first glance, the work appears to deliver a major advance in activity; however, the activity is overstated and the interpretation is not sufficiently critical. The actual gains require more cautious evaluation. Substantial issues must be addressed before the manuscript can be considered for publication, with particular emphasis on significantly strengthening the mechanistic analysis. Below are the detailed concerns:\nResponse: We sincerely appreciate your comprehensive and insightful feedback on our manuscript. These valuable suggestions not only pinpoint the main shortcomings of the current version, but also provide a clear roadmap for enhancing the research content and the paper’s overall quality, which indeed provide crucial support for elevating the academic value of our work. In response, we have addressed all the points raised through careful revisions, ensuring the manuscript now fully meets publication standards in terms of data rigor and conclusion objectivity. Our detailed point-by-point responses follow.\nComment 1: From the XPS results, no Cu0 exists in the samples. The distributed Cu0 shown in Fig. 2A should be removed. The authors report 62% Cu2+ and 38% Cu+, which is not a minor Cu+ fraction. However, they state that “Cu species predominantly exists as Cu2+ with minor Cu+ states.”.\nResponse: We sincerely thank you for pointing out these inconsistencies. To address them, we have carefully revised the manuscript. Specifically, the distribution of Cu0 has been removed from Fig. 2a to align with the conclusions from the Cu LMM analysis in Fig. 2b (Please see the revised manuscript, page 4, Fig. 2a). Furthermore, the imprecise statement that \"Cu species predominantly exist as Cu2+ with minor Cu+ states\" has been deleted. The revised text now directly presents the quantitative results, stating that \"Deconvolution of the Cu 2p 3/2 spectrum reveals two characteristic peaks at binding energy (B.E.) of 933.64 eV (62%) and 932.42 eV (38%). The former is assigned to Cu2+ and the latter to Cu+, as confirmed by the valence-sensitive Cu LMM Auger spectra.\" (Please see the revised manuscript, page 4, second paragraph).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 4, second paragraph)\nDeconvolution of the Cu 2p spectrum reveals two characteristic peaks at binding energy (B.E.) of 933.64 eV 3/2 (62%) and 932.42 eV (38%). The former is assigned to Cu2+ and the latter to Cu+, as confirmed by the valence-sensitive Cu LMM Auger spectra. Specifically, the Cu LMM spectrum of Cu /TiO exhibits a kinetic energy shift toward the Cu+ position relative to the CuO reference (Fig. 2b).\n(2) (Revised manuscript, page 4, Fig. 2a)\nFig. 2. Electronic states and coordination of Cu in Cu /TiO . a High-resolution XPS (Cu 2p) and (b) Auger spectroscopy (Cu LMM) of Cu /TiO . c Normalized XANES, d k2-weighted Fourier-transformed EXAFS, and (e) Wavelet transform for the k2-weighted EXAFS spectra at Cu K-edge of Cu /TiO , Cu foil, Cu O and CuO. f Soft XANES spectra of Cu /TiO at O K-edge. g Corresponding R-space and k-space (the inset) fitting curves at the Cu K-edge for Cu /TiO .\nComment 2: The activity evaluation relies mainly on a batch reactor, yet the Batch-type performance test section does not report the pressure. In a sealed batch reactor the pressure evolves during reaction and this can inflate apparent activity. Please report initial and final pressures, and how pressure changes over time. In addition, surface temperature measurement needs clarification. Please specify the measurement method, the emissivity used for infrared measurements, and the calibration procedure. Different emissivity values can lead to large differences in the measured temperature.\nResponse: We sincerely appreciate your insightful comments, which have provided important guidance for improving our experimental design and data presentation. In accordance with your suggestions, we have revised the manuscript and supplemented the relevant descriptions as follows.\nFirst, regarding your concern that the time evolution of pressure in the sealed batch reactor may influence the apparent activity, we have added in situ records of the pressure difference, defined as the difference between the instantaneous pressure and the initial pressure, as a function of time, as well as the catalyst surface temperature as a function of time, under a reaction atmosphere of 20 vol% C H /20 vol% CO /Ar (please see renumbered Supplementary Fig. 12). In these experiments, the initial pressure in the reactor was 1.0 atm, and a AS511A digital differential manometer (SMART SENSOR) was used to continuously monitor the pressure during the reaction. The results show that, immediately after the introduction of light irradiation, the system pressure increases rapidly, which can be mainly attributed to enhanced gas molecular motion induced by the photothermal effect. This was followed by a slow and nearly linear pressure increase, corresponding to the ongoing C H dehydrogenation reaction. It is important to emphasize that throughout the entire reaction process, the maximum pressure increase is less than 10 kPa, which is below 10% of the initial pressure. Therefore, we consider that the pressure variation within this range has a negligible influence on the reaction rate and the evaluation of apparent activity and does not compromise the reliability of the conclusions. The descriptions of relevant data and corresponding measurement methods have been clearly integrated into the revised manuscript (Please see the revised manuscript, page 7, second paragraph and page 14, “Batch-type performance test” section).\nSecond, regarding the measurement of the catalyst surface temperature and the procedure for setting and calibrating the emissivity (ε), we have provided a detailed description in the “Methods” section of the revised manuscript. Specifically, the catalyst surface temperature was mainly monitored using a Uti160S infrared thermal imager (UNI-T). During the experiments, the camera was fixed at an oblique angle above the reactor and focused on the top surface of the catalyst bed in order to acquire thermal images of the catalyst layer. The emissivity of the camera was set to ε = 0.95. This value was obtained through an empirical calibration procedure in which a thin, uniformly spread layer of catalyst powder was placed at the center of a pre-calibrated heating plate. The temperature of the heating plate was successively set to 50, 100, 150, and 200°C, and the emissivity parameter of the thermal imager was adjusted until the infrared readings of the catalyst surface at each temperature matched the actual temperature of the heating plate as closely as possible. The resulting emissivity value was then used in all subsequent temperature measurements. In addition, to further verify the accuracy of the temperature measurements, we simultaneously recorded the catalyst surface temperature as a function of time using a K-type thermocouple under identical reaction conditions, as shown in renumbered Supplementary Fig. 12. The temperatures obtained from the thermocouple show excellent agreement with those measured by the infrared thermal camera, which provides additional confirmation of the reliability of the temperature measurement method. The complete calibration procedure of the thermal imager and the detailed protocol for surface temperature measurement have been systematically added to the “Methods” section of the revised manuscript (Please see the revised manuscript, page 14, “Batch-type performance test” section).\nWe again sincerely thank you for your careful review and valuable comments, which have significantly improved the completeness and transparency of the manuscript with respect to the description of experimental methods and the rigor of the data.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 7, second paragraph)\nThis clear contrast indicates that the color evolution observed in reactive atmospheres does not arise solely from changes in the Cu valence state. The origin of these valence-state variations and their relationship to the reversible structural evolution of the Cu sites will be discussed in detail in the following sections. In addition, to clarify that the observed catalyst deactivation is not an artifact arising from the use of a sealed batch reactor, a series of supplementary control experiments were conducted. First, although a slight pressure increase is detected in the sealed system during the reaction, its magnitude consistently remains below 10% of the initial pressure, and its influence on the reaction kinetics can therefore be considered negligible (Supplementary Fig. 12). Second, the stabilizing role of CO and the intrinsic durability of the catalyst were further verified under continuous-flow conditions. While pronounced deactivation is observed in a C H /Ar atmosphere (Supplementary Fig. 13), the C H production rate is maintained at over 95% of its initial value during 20 h of operation in a C H /CO₂ atmosphere (Fig. 3e).\n(2) (Revised manuscript, page 14, “Batch-type performance test” section)\nFirst, the catalyst was uniformly dispersed in 1.0 mL deionized water and coated onto a glass slide (coating area: 3.14 cm2), followed by drying at 80 °C. The catalyst-coated glass slide was placed at the center of the bottom of a sealed 100 mL quartz reactor. To ensure complete replacement of residual air by ethane, the reactor was subjected to three evacuation/ethane-flushing cycles (1 atm, 99.9% C2H6 per cycle) before performance test. A 300 W full-spectrum (250 ~1100 nm) Xe lamp (PLS-SXE300+) was used for vertical irradiation (4 h per reaction) without additional external heating, and the light intensity was monitored using a PL-MW2000 optical power meter. Pressure changes (defined as the difference between the instantaneous pressure and the initial pressure) during the reaction were continuously recorded with an AS511A digital differential manometer (SMART SENSOR). The reaction products were analyzed using a Shimadzu gas chromatograph (GC-2014C) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). For the C18O2 isotope-labeled experiment, the gas-phase products were detected by a 7890A-5975C gas chromatography-mass spectrometer (Agilent Technologies Co., Ltd.).\nThe catalyst surface temperature was simultaneously monitored using a Uti160S infrared thermal imager (UNI-T) and a calibrated K-type thermocouple (diameter 0.25 mm, KAIPUSEN). The thermocouple probe maintained direct contact with the center of the catalyst bed, while the thermal imager was installed at an oblique angle above the reactor, oriented toward the top surface of the catalyst bed to capture thermal image data. The emissivity was established as ε = 0.95 based on an empirical calibration procedure performed with catalyst powder distributed uniformly on a pre-calibrated heating plate. During calibration, the heating plate temperature was systematically varied (50, 100, 150, and 200 °C) and the imager's emissivity setting was adjusted until its temperature readings concurred with the reference values at each point, thereby determining the optimal emissivity value for all subsequent experimental measurements.\n(3) (Supplementary Information, Page 14, Renumbered Supplementary Fig. 12)\nSupplementary Fig. S12. Catalyst surface temperature and system differential pressure (pressure difference relative to initial) as a function of irradiation time over Cu /TiO . Standard conditions: 10 mg catalyst, 1 atm initial pressure 20 vol% C H /20 vol%CO /Ar, 777 mW cm-2 xenon lamp, sealed batch reactor.\nComment 3: Batch and flow data in Table S3 should not be compared directly. Batch often overstates activity relative to flow, as your Fig. 3E shows. Please give apple to apple comparisons. Compare batch with batch and flow with flow, and test Cu/TiO in a flow reactor under C H /Ar, including a stability test. Flow performance is the key metric for industrial relevance.\nResponse: Thank you for your valuable comments. First, we would like to clarify that under the condition of equivalent reactant concentrations in our work, the catalytic performance in the flow system (Fig. 3e) was observed to be higher than that in the batch system (Fig. 3d). This result corroborates previous findings in the literature, which report that flow activity is generally enhanced compared to batch activity (DOI: 10.1038/s41467-025-64389-9, 10.1038/s41467-025-61634-z). To prevent potential misunderstanding due to differing conditions, particularly regarding the batch data in Fig. 3b obtained under 99.9% C H , we have revised the caption of Fig. 3 to specify the test conditions for each dataset (Please see the revised manuscript, page 6, caption of Fig. 3). Regarding Table S3, originally intended as a comprehensive summary of reported data in this field, we have now added a \"Test Type\" column to clearly categorize each entry as Batch or Flow, as suggested (Please see Supplementary Table 3).\nAdditionally, we have supplemented the flow reaction tests under a 20 vol% C H /Ar atmosphere, with the results presented in renumbered Supplementary Fig. 13. The corresponding data analysis has also been incorporated into the revised manuscript (Please see the revised manuscript, page 7, second paragraph). Results from the 20-hour stability test demonstrate that the catalyst undergoes significant deactivation in the C H atmosphere, in clear contrast to its stable performance in the CO -containing atmosphere (Fig.3e).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 6, caption of Fig. 3)\nFig. 3. Catalytic performance. a Photocatalytic C H and H yields and C H selectivity over TiO and M/TiO (M=Cu, Au, Ag, Pd, Pt) under 1 atm 99.9 vol% C H . b Photocatalytic C H and H yields as a function of the catalyst dosage over Cu /TiO under 1 atm 99.9 vol% C H . c C H yield comparison with reported PEDH systems. d Time-dependent photocatalytic C H yield and C H conversion (%) over Cu /TiO in 20 vol% C H /Ar and 20 vol% C H /20 vol% CO /Ar. e Flow reactor product yields of Cu /TiO over a 20-h light-driven test under 20 vol% C H /20 vol% CO /Ar. f MS signals during C H /CO -TPSR test over Cu /TiO . g Product yield over a 10-h dark test at 550°C under 20 vol% C H /20 vol% CO /Ar. h Raman spectra of the spent catalysts after light/dark reactions. Standard conditions (a-e, g): 10 mg catalyst, 777 mW cm-2 xenon lamp, 4-h test, batch reactor. Error bars represent standard deviations obtained from three independent measurements.\n(2) (Supplementary Information, Page 15, Renumbered Supplementary Fig. 13)\nSupplementary Fig. S13. Flow reactor product yields of Cu /TiO over a 20-h light-driven test under 20 vol% C H /Ar.\n(3) (Revised manuscript, page 7, second paragraph)\nIn addition, to clarify that the observed catalyst deactivation is not an artifact arising from the use of a sealed batch reactor, a series of supplementary control experiments were conducted. First, although a slight pressure increase is detected in the sealed system during the reaction, its magnitude consistently remains below 10% of the initial pressure, and its influence on the reaction kinetics can therefore be considered negligible (Supplementary Fig. 12). Second, the stabilizing role of CO and the intrinsic durability of the catalyst were further verified under continuous-flow conditions. While pronounced deactivation is observed in a C H /Ar atmosphere (Supplementary Fig. 13), the C H production rate is maintained at over 95% of its initial value during 20 h of operation in a C H /CO₂ atmosphere (Fig. 3e).\nComment 4: The methods state that a 300 W Xe lamp (PLS-SXE300+) was used for vertical irradiation for 4 h without additional heating. Please clarify how activity was calculated for each rate. There is a discrepancy between Fig. 3B and Fig. 3D. In Fig. 3B, even with 10 mg catalyst, the reported rate is about 7 mmol h-1 g-1. In Fig. 3D under C H /Ar, the accumulated amount after 4 h is about 5.5 mmol g-1, which corresponds to about 1.1 mmol h-1 g-1. In addition, please report the ethane conversion and the carbon balance under C H /Ar and C H /CO /Ar.\nResponse: We greatly appreciate your questions and valuable suggestions. We address the relevant issues from the following three aspects.\nFirstly, regarding the calculation method of reaction rate, all reaction rates in this study are calculated based on the cumulative yield of the target product, which is specifically defined as the amount of ethylene generated per unit mass of catalyst per unit time. The corresponding calculation formula has been added to the experimental section as Eq.1. (Please see the revised manuscript, page 15)\nSecondly, the discrepancy between the data in Fig. 3b and Fig. 3d arises from differences in the feedstock gas concentration. Specifically, the performance tests in Fig. 2b were conducted in a 99.9 vol% C H atmosphere, whereas those in Fig. 3d were performed in either a 20 vol% C H /Ar or a 20 vol% C H /20 vol% CO /Ar atmosphere. To prevent any misunderstanding, we have revised the caption of Fig. 3 to clarify the respective reaction conditions for each panel. (Please see the revised manuscript, page 6, caption of Fig. 3)\nFinally, following your suggestion, we calculated the ethane conversion and carbon balance under C H /Ar and C H /CO /Ar atmospheres. The carbon balance results are all close to 1.0, which clearly rules out the possibility of carbon deposition. This result has been reported in the main text and labeled in Fig. 3d (Please see the revised manuscript, page 7, second paragraph ang Fig.3d), and the corresponding calculation formula for ethane conversion and carbon balance have also been included in the experimental section as Eq.3 and Eq.4. (Please see the revised manuscript, page 15).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 15, Eq.1, Eq.3 and Eq.4)\nYield = [C2H 4] / [C2H4 m*t] [1]\nConversion = [C2H6] inlet -[C2H6] outlet*100% / [C2H6] inlet [3]\nCarbon balance= [C2H4 ]+0.5*[CH4 ]+2*[C4H10] / [C2H6] consumed [4]\n(2) (Revised manuscript, page 6, caption of Fig. 3)\nFig. 3. Catalytic performance. a Photocatalytic C H and H yields and C H selectivity over TiO and M/TiO (M=Cu, Au, Ag, Pd, Pt) under 1 atm 99.9 vol% C H . b Photocatalytic C H and H yields as a function of the catalyst dosage over Cu /TiO under 1 atm 99.9 vol% C H . c C H yield comparison with reported PEDH systems. d Time-dependent photocatalytic C H yield and C H conversion (%) over Cu /TiO in 20 vol% C H /Ar and 20 vol% C H /20 vol% CO /Ar. e Flow reactor product yields of Cu /TiO over a 20-h light-driven test under 20 vol% C H /20 vol% CO /Ar. f MS signals during C H /CO -TPSR test over Cu /TiO . g Product yield over a 10-h dark test at 550°C under 20 vol% C H /20 vol% CO /Ar. h Raman spectra of the spent catalysts after light/dark reactions. Standard conditions (a-e, g): 10 mg catalyst, 777 mW cm-2 xenon lamp, 4-h test, batch reactor. Error bars represent standard deviations obtained from three independent measurements.\n(3) (Revised manuscript, page 7, second paragraph)\nIn a C H atmosphere, a nonlinear accumulation of C H over 240 min indicated gradual catalyst deactivation (Fig. 3d), despite the carbon balance remaining close to 1.0. This deactivation correlates with a macroscopic color change of the catalyst from light green to dark blue, as well as a decrease in the Cu2+ signal detected by in situ EPR (Supplementary Fig. 11a).\n(4) (Revised manuscript, page 7, Fig.3d)\nFig. 3. d Time-dependent photocatalytic C H yield and C H conversion (%) over Cu /TiO in 20 vol% C H /Ar and 20 vol% C H /20 vol% CO /Ar.\nComment 5: In Figure S9B, the activities were measured with a 777 mW cm-2 xenon lamp and 10 mg catalyst. When the light intensity is about 310 mW cm-2, the C H yield in the figure is only around 1.1 mmol h-1 g-1. “yield” appears misspelled in the figure. In Figure S9C, the activity drops by only about 10% when wavelengths <400 nm are used, and there is no activity when wavelengths > 400 nm are used. However, in Table S2 at 365 nm with an intensity of only 54 mW cm-2 and 50 mg catalyst, the reported C H amount is 0.057 mmol h-1. This corresponds to about 1.14 mmol h-1 g-1. Is this possible?\nResponse: We thank you for your careful comments. We have corrected the spelling error of \"Yield\" and conducted a systematic check of the entire manuscript to ensure no similar issues remain.\nRegarding the apparent contradiction between the C H yield under high light intensity (310 mW cm-2) in Fig. S9B (now renumbered Supplementary Fig. 10b) and the result under lower intensity in Supplementary Table 2 stems from the intrinsic difference between the light sources. As confirmed by the control experiments in Fig. S9C (now renumbered Supplementary Fig. 10c), the catalyst activity is mainly derived from the ultraviolet region, as this range can effectively excite the TiO . In the AQY test (Supplementary Table 2), a 365 nm monochromatic light source was used, which matches the catalyst's optimal absorption. This ensures highly efficient photon usage and a high quantum yield. By contrast, the test for Fig. S9B used a full-spectrum Xenon lamp (250~1100 nm, see spectral profile in Fig. R1 below). Although the total irradiance is higher, the spectrum contains a large fraction of visible and infrared light, to which the catalyst responds poorly. Therefore, the flux of effective UV photons under the full-spectrum 310 mW cm-2 condition is much lower than under the 54 mW cm-2 monochromatic light. The abundance of these \"ineffective\" photons dilutes the overall energy efficiency, explaining the lower yield despite the higher total light intensity. This phenomenon of efficiency differences caused by variations in light source spectral distribution has been widely reported in previous photocatalysis literature.\nTo avoid potential misunderstanding, we have supplemented the information regarding the wavelength(s) of the light source(s) in the \"Methods\" section (Please see the revised manuscript, page 14, “Batch-type performance test” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 14, “Batch-type performance test” section)\nA 300 W full-spectrum (250 ~1100 nm) Xe lamp (PLS-SXE300+) was used for vertical irradiation (4 h per reaction) without additional external heating, and the light intensity was monitored using a PL-MW2000 optical power meter.\nFig. R1. Spectral distribution of the 300 W full-spectrum xenon lamp.\nComment 6: The description of AQY conditions is inconsistent. The caption of Fig. S9A states “UV–visible diffuse reflectance spectra and corresponding AQY values of Cu /TiO after 1 h of irradiation with different monochromatic lights. Default conditions: 10 mg catalyst, 1 atm pure C H , 777 mW cm-2 xenon lamp, 4 h test, batch reactor.” In contrast, Table S2 states “The AQY measurements were conducted in a sealed quartz reactor using 50 mg of catalyst uniformly coated over a 1.0 cm radius circular area for a 1 h reaction.” Please ensure the accuracy of all data.\nResponse: We greatly appreciate your careful review and valuable reminder. The AQY data were exclusively obtained under the conditions described in Table S2, namely using 50 mg of catalyst uniformly coated in a sealed quartz reactor with a 1 h reaction time. The description in the original caption of Fig. S9A inadvertently included default reaction conditions that are not applicable to the AQY measurements. To eliminate any ambiguity, we have now revised the caption of Fig. S9 (now renumbered Supplementary Fig. 10) by removing irrelevant parameters (e.g., 777 mW cm-2 and 4-h test) and ensuring that the stated conditions are fully consistent with those used for AQY determination. Meanwhile, we have carefully checked the entire manuscript to ensure all key reaction parameters are now clearly stated across all figures and tables.\nComment 7: The authors state that “these findings further support that CO yield is indirectly driven by C H conversion, with CO serving solely as the oxygen source.” The observed C H /H ratio is close to 1, which is consistent with non-oxidative dehydrogenation. If CO provides only oxygen, please clarify where the carbon in the produced CO comes from. Please calculate the full carbon balance. Conduct ¹³C labeling to confirm the source of carbon.\nResponse: We sincerely thank you for your valuable suggestions and fully understand the concern regarding the origin of CO, and we apologize for any misunderstanding caused by our earlier imprecise wording. First, as you correctly noted, the CO yield detected in our system is extremely low (below 0.1 mmol g-1 h-1), which reflects the predominance of non-oxidative dehydrogenation in our system. This conclusion has also been corroborated by experiments using in situ TAS and in situ TRFL. Regarding the formation and origin of CO, the central conclusion of this study is not to simply emphasize CO as an oxygen donor, but rather to clarify that the oxygen atoms in the product CO originate entirely from gaseous CO , not from lattice oxygen involved in the deep oxidation of C H . This has been directly verified by isotopic labeling experiments using C18O . As for the source of the carbon atom in CO, based on the available experimental evidence, we maintain that it derives from CO . This conclusion is supported by two key observations: (1) The supplementary carbon‑balance data (Fig. 3d) show that the carbon balance approaches unity under a C H /Ar atmosphere. (2) The control experiment in Fig. 4g demonstrates that trace CO appears only upon CO co‑feeding. Together, these results indicate that CO formation is strictly dependent on the presence of CO . Therefore, we have removed the ambiguous phrase “CO serves only as an oxygen source” from the revised manuscript. Additionally, to further clarify the formation pathway of CO, we have added DFT calculations that detail the *H coverage-induced CO → CO reaction mechanism, providing further explanation for the carbon origin of CO (please see the revised manuscript, page 13, first paragraph and renumbered Supplementary Fig. 37). These results and corresponding discussions have been integrated into the revised manuscript.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 13, first paragraph)\nThis indicates that consecutive dehydrogenation on two lattice oxygens leads to difficult desorption of *H, ultimately leading to deactivation of the catalyst due to *H coverage on the [Cu–O ] active sites. Further theoretical calculations (Supplementary Fig. 37) reveal that these two *H species can be transformed into H O via a low energy barrier of 0.40 eV, accompanied by a structural reconstruction from [Cu–O ] to [Cu–O ]. However, this process is still endergonic from a thermodynamic perspective, and the *H species preferentially remain adsorbed on the catalyst surface. Notably, in the presence of CO , CO can cooperatively interact with •C H to form a *COOH intermediate, which promotes the further transformation of the [Cu–O ] species generated during the dehydration process. This, in turn, drives the continuous consumption of the accumulated *H while simultaneously regenerating the [Cu–O ] sites.\n(2) (Supplementary Information, Page 39, Renumbered Supplementary Fig. 37)\nSupplementary Fig. 37. Reaction pathway for CO -assisted removal of surface *H species.\nComment 8: What temperature was used for the Ea calculation under illumination? Was the photothermal effect excluded?\nResponse: We sincerely thank you for raising this important question. In determining the apparent activation energy (Ea) under illumination, we deliberately selected a relatively low temperature range of 60–140°C, while ensuring that the reaction rate could still be measured accurately. It is important to emphasize that, to reduce significant light-induced heating arising from full-spectrum irradiation and to avoid discrepancies between the actual catalyst temperature and the set temperature, we employed ultraviolet light with a fixed light intensity as the irradiation source in the Ea measurements. We utilized the temperature control system of the PLS-LED 100 photothermal reactor to precisely control and adjust the temperature within the 60~140°C range. During the reaction, the temperature was monitored by a K-type thermocouple inserted into the center of the catalyst bed; thus, all temperatures used for Arrhenius fitting are the measured temperatures at the catalyst surface.\nBased on this experimental design and the associated validation results, we believe that the Ea values obtained under these optimized conditions reliably reflect the intrinsic kinetic characteristics of the photocatalytic reaction. To avoid any misunderstanding, we have included a detailed description of the specific experimental conditions and temperature control methodology for the Ea measurements in the revised manuscript (Please see the revised manuscript, page 14, “Flow-type performance test” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 14, “Flow-type performance test” section)\nFor each test, 10 mg of catalyst was uniformly coated onto a heat-resistant quartz fiber filter membrane (fixed coating area: 3.14 cm2), and the flow rate of the feed gas (20 vol% C H /20 vol% CO /Ar) was set as 10 mL min-1. The apparent activation energy measurements were carried out in the same reactor, in which a UV light source with fixed intensity was employed to mitigate photothermal effects, and a relatively low temperature window of 60~140 ℃ was selected to ensure that the reaction proceeded in the kinetic regime and to minimize interference from internal and external mass transfer and thermal effects.\nComment 9: Cu/TiO is widely reported for RWGS. Why was no CO detected in the H /CO control experiment?\nResponse: We sincerely thank you for raising this important question. Indeed, Cu-based catalysts are widely recognized as promising candidates for the RWGS reaction owing to their weak CO adsorption and facile CO desorption. However, the RWGS reaction is highly temperature-sensitive, which is the key reason why no CO was detected in our H /CO co-feeding control experiment carried out at a low temperature (108°C).\nInherently, the RWGS reaction is an endergonic reaction (ΔH° ≈ +41.2 kJ·mol-1). In traditional thermal catalysis systems, Cu-based catalysts typically require temperatures of ≥300°C to be significantly activated to initiate the reaction and generate CO at detectable levels (DOI: 10.1016/j.cej.2025.165414, 10.1021/jacs.3c01336, 10.1021/acscatal.0c01584). Although photothermal synergy strategies developed in recent years have effectively reduced the reaction's onset temperature threshold, existing studies indicate that even the current state-of-the-art Cu-based RWGS catalytic systems still maintain an onset temperature of no less than 150°C. For instance, the high-entropy alloy Cu catalyst developed by Xiong's group (DOI: 10.1002/adma.202409689) and the high-entropy oxide Cu-based catalyst (DOI: 10.1038/s41467-023-38889-5) reported by Ye's group both require temperatures above 220°C for CO generation to be detected; while the Cu single-atom doped In O photothermal catalyst subsequently developed by Xiong's group further reduced the reaction temperature, its RWGS activity onset threshold remains at 150°C (DOI: 10.1002/anie.202505244).\nBased on the above analysis, the absence of CO in the H /CO co-feed control is consistent with prior reports and is more likely attributable to differences in reaction conditions, particularly the low temperature used in our work, which is insufficient to trigger the RWGS reaction. Furthermore, given that CO is detectable under C H /CO co-feeding, we infer that, under the present reaction conditions, this catalyst likely lacks sufficiently active sites for H activation and is therefore unable to efficiently initiate a redox-type RWGS pathway. This interpretation has been explicitly incorporated into the revised manuscript for clarification (Please see the revised manuscript, page 10, second paragraph).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 10, second paragraph)\nAs shown in Fig. 4g, trace CO formation was detected only under C H /CO co-feeding, but was absent in the CO -free atmosphere. Control experiments with H /CO co-feeding also yielded no detectable CO, indicating that the catalyst is unable to effectively activate molecular H under the present conditions and thus cannot initiate a redox-mediated reverse water–gas shift (RWGS) pathway. These findings therefore rule out RWGS as the source of CO formation in the C H /CO co-feeding system.\nComment 10: In the NAP-XPS results, what changes are observed in Ti?\nResponse: We sincerely thank you for your attention to the evolution of Ti species in the NAP-XPS measurements. During the revision, we conducted a careful analysis of the Ti 2p signals. The results show that under both C H -only and C H /CO co-feeding conditions (Fig. R2), neither the binding energies nor the relative areas of the Ti 2p peaks exhibit any systematic trend with reaction time or gas composition. These observations indicate that the chemical environment of Ti remains essentially unchanged under the reaction conditions and that the redox dynamics relevant to the reaction mechanism are predominantly localized on the Cu centers rather than on the Ti sites. For this reason, we did not discuss the Ti signals in detail in the main text, in order to keep the mechanistic analysis focused on the evolution of the oxidation state and local structure of the core active sites (Cu species).\nFig. R2. In situ Ti 2p NAP-XPS spectra of Cu /TiO under (A) C H and (B) C H /CO atmosphere.\nComment 11: In the DFT section, the reported barrier reduction of 62.3% for C–H activation at the same O Cu br site in Path 1 (1.22 eV to 0.46 eV) is clear. Please explain how the percentages 94.1%, 96.6%, and 71.7% were obtained for Paths 2, 3, and 4. For example, Path 4 is listed as 2.83 eV to 0.83 eV, which gives about 70.7% if computed as (2.83-0.83)/2.83, not 71.7%.\nResponse: We sincerely thank you for raising this question, and we apologize for the confusion caused by not explicitly providing the calculation formula for these percentages in the original version. Actually, the percentage data is calculated using the following formula:\nE -E side TS IS *100% / E -E mian TS IS\nWhere 𝐸 is the energy of the corresponding initial state, 𝐸 is the transition-state energy of the competing side pathway, and 𝐸 is the transition-state energy of the desired main pathway on the same surface site. Using this main TS definition, the values were obtained as follows:\nfor Path 2, it was (1.79-0.03)/[1.79-(-0.08)]=94.1%;\nfor Path 4, it was (2.82-0.82)/(2.82-0.03)=71.7%;\nPath 3 showed an energy increase of [(0.2-(-0.08)]/[(-0.08)-(-0.37)]=96.6%.\nTaking into account your comment in Comment 18 regarding the potential misleading nature of percentage expressions, we have now removed all percentage values from Fig. 5c and the related text, and instead directly report the corresponding free-energy differences (ΔG) between the main and side pathways. In addition, incorporating suggestions from other reviewers, we have substantially revised the presentation of Fig. 5c (Please see the revised manuscript, page 11, Fig. 5c). We believe these revision presents the DFT results in a clearer manner for a broad readership. All relevant descriptions and figure captions in the revised manuscript have been updated accordingly.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 11, Fig. 5c)\nFig. 5. CO regeneration mechanism and DFT calculations. c Calculated energy diagrams and spin density distributions for C H photocatalytic conversion to C H and H on Cu /TiO . The Ti, O, Cu, C, and H atoms are in blue, red, orange, brown, and dark pink, respectively.\nComment 12: In the dark-state DRIFTS experiments with C H and CO , gas-phase C H shows bands in the same region as surface species. The observed signals may therefore originate from gas-phase C H rather than adsorbates. On this basis, the conclusion of preferential C H adsorption over CO is not supported. Please perform CO -DRIFTS and competitive C H /CO adsorption.\nResponse: Thank you very much for your careful review of the dark-state DRIFTS experimental results, which has greatly assisted us in further enhancing the rigor of our conclusion statements. We have carefully re-examined the DRIFTS data obtained under both C H -only and C H /CO co-feeding conditions. As you correctly pointed out, under the present experimental and analytical conditions, a contribution from gas-phase C H to the observed signals therefore cannot be excluded, which introduces uncertainty in peak assignment. This consideration highlights a limitation in our original discussion regarding adsorption competition based on this dataset. In light of this important concern, we have removed the inferential statements regarding “preferential adsorption of C H or distinct adsorption sites of C H and CO ” and Fig. S22 (the dark-state DRIFTS experiments with C H and CO ) from the revised manuscript.\nIn addition, regarding the suggestion to perform CO -only DRIFTS measurements, after careful consideration we concluded that in the vibrational range relevant to this work the adsorption features of CO do not significantly overlap with the C–H region. As a result, such experiments would provide limited insight into the competitive adsorption between C H and CO under realistic reaction conditions and would not substantially strengthen the core mechanistic arguments. We have therefore chosen to adopt a more cautious interpretation of the existing DRIFTS data rather than adding separate adsorption experiments, and we now explicitly restrict their role to qualitative support for the evolution of surface intermediates during reaction.\nIt is important to emphasize that this adjustment only narrows the interpretation of the dark-state DRIFTS data in the context of adsorption competition and does not affect our main conclusions regarding the nature and function of the active sites. We would like to thank you again for your highly constructive comments, which have prompted us to make more careful and rigorous revisions to the relevant statements.\nComment 13: Figure S24 shows only the infrared spectrum of Cu /TiO . Please also include the spectrum of TiO to enable comparison of the OH region. Use the same enlarged view and scaling of the OH region in Fig. S24a and Fig. S24b to allow a direct comparison.\nResponse: We sincerely thank you for your valuable suggestions. Your comments on the original Fig. S24 have significantly improved the comparability of our spectroscopic data. As recommended, we have supplemented the infrared spectrum of TiO and standardized the enlarged view and scaling of the -OH region. The updated results are now presented in Fig. R3. Meanwhile, we further streamlined the organization of the spectra in the Supplementary Information. The original Fig. S24 was intended to highlight the change in -OH related signals before and after introducing the reaction atmosphere, and this key message is also captured in the renumbered Supplementary Fig. 27. Therefore, to avoid redundant presentation and to improve the conciseness and readability of the Supporting Information, we have decided to remove the original Fig. S24.\nWe appreciate the reviewer’s careful evaluation and constructive feedback, which have greatly helped us improve the manuscript. We would be happy to make further revisions if additional suggestions are provided.\nFig. R3. Infrared spectrum of (a) TiO and (b) Cu /TiO within the wavenumber range of 2800 cm-1 to 4000 cm-1. In situ DRIFTS spectra of Cu /TiO under dark conditions in the wavenumber range of 3200 cm-1 to 4000 cm-1 with continuous flow of (c) 20 vol% C H /Ar and (d) 20 vol% C H /20 vol% CO /Ar atmospheres.\nComment 14: The authors state in line 157 that “[Cu–O] sites effectively suppress side reactions, including alkene hydrogenation and over-dehydrogenation.” This conclusion is not supported by comparative data. There is no direct comparison with TiO or other controls that would demonstrate suppression rather than simply low intrinsic activity under the tested conditions. In addition, C H is detected in reactions 2, 3, and 4, which indicates some hydrogenation occurs. Figure S8 supports only that Cu/TiO is not efficient for C H and C H hydrogenation under the specific test conditions. Please provide additional evidence clarifying the role of Cu in the reaction and the role of CO .\nResponse: We sincerely thank you for pointing out the imprecision in the expression of our manuscript, and apologize for any misunderstanding caused by our insufficient distinction between \"suppression of side reactions\" and \"low intrinsic activity of side reactions\" in the manuscript.\nActually, our intention in switching the feed from C H to C H /H and C H /H was to probe whether over-dehydrogenation of ethane to C H or re-hydrogenation of C H to C H occurs on Cu /TiO , thereby assessing whether [Cu–O] sites can confine ethane dehydrogenation at the C H stage. As shown in Fig. S8, no significant products were detected under these control conditions over our Cu /TiO catalyst, indicating that the [Cu–O] sites effectively avoid deep dehydrogenation of ethane. Thus, compared to the commonly encountered challenge of uncontrollable deep dehydrogenation in this field, our catalyst demonstrates favorable suppression of this side reaction. To prevent potential misinterpretation, we have revised the original description that equated \"low intrinsic activity observed in control experiments\" with \"suppression of side reactions.\" The relevant statement has been modified to: \" Switching the feed gas from C H to C H /H or C H /H (over-dehydrogenation product) yielded no significant products, further indicating that, under the present conditions, [Cu–O] sites show low intrinsic activity toward side reactions, including alkene hydrogenation and over-dehydrogenation. (Please see the revised manuscript, page 5, last paragraph) \" Additionally, we have thoroughly reviewed and unified the terminology throughout the manuscript, removing or adjusting any expressions that might lead to misunderstanding, thereby improving the overall accuracy of our discussion.\nFurthermore, in response to your suggestion to further clarify the roles of Cu and CO in the reaction through additional evidence, we have conducted dedicated DFT calculations to provide mechanistic insights. Specifically, to elucidate the key role of Cu in promoting β‑H activation, we compared the reaction pathways for C H → C H on [Ti–O] and [Cu–O] sites (Please see the revised manuscript, page 11, Fig. 5c). The results show that the activation barrier for β‑H at the Ti site is as high as 1.60 eV, while at the Cu site it is only 0.11 eV. This pronounced difference directly confirms the efficient promoting effect of Cu on β‑H activation, which is consistent with the experimental observation of C H formation after quenching Cu sites with NH (Fig. 4b). Furthermore, regarding the role of CO in facilitating *H removal, the calculations indicate that CO can effectively drive *H conversion and restore the active‑site structure via the *COOH intermediate pathway (please see renumbered Supplementary Fig. 37). These supplementary calculations provide theoretical support for the catalytic promotion function of Cu and the *H‑scavenging mechanism of CO₂, and the corresponding analysis has been integrated into the revised manuscript (Please see the revised manuscript, page 12, last paragraph).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 5, last paragraph)\nIn striking contrast, Cu /TiO demonstrated dramatically enhanced near-stoichiometric production of C H and H with 97% C H selectivity, highlighting efficient and controlled selective dehydrogenation to C H at the localized [Cu–O] sites. Switching the feed gas from C H to C H /H or C H /H (over-dehydrogenation product) yielded no significant products, further indicating that, under the present conditions, [Cu–O] sites show low intrinsic activity toward side reactions, including alkene hydrogenation and over-dehydrogenation (Supplementary Fig. 9).\n(2) (Revised manuscript, page 11, Fig. 5c)\nFig. 5. CO regeneration mechanism and DFT calculations. c Calculated energy diagrams and spin density distributions for C H photocatalytic conversion to C H and H on Cu /TiO . The Ti, O, Cu, C, and H atoms are in blue, red, orange, brown, and dark pink, respectively.\n(3) (Supplementary Information, Page 39, Renumbered Supplementary Fig. 37)\nSupplementary Fig. 37. Reaction pathway for CO -assisted removal of surface *H species.\n(4) (Revised manuscript, page 12, last paragraph)\nThe •C H radical then undergoes two successive mild endergonic steps to transfer a *H to the Cu site via the HAT mechanism, involving adsorption at Cu site (0.29 eV) followed by β–H cleavage with a reaction energy of only 0.11 eV. In contrast, although the C–H activation ability of O site is also significantly enhanced after trapping photogenerated holes, with the corresponding barriers being comparable to that at O site, the resulting •C H radical exhibits an adsorption energy of 0.58 eV at the five-coordinated Ti site (Ti ), and the subsequent β–H bond cleavage has a reaction energy as high as 1.60 eV (Fig. 5c and Supplementary Fig. 31), both of which are significantly larger than those on the Cu site. This demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32). Finally, we calculated the generation of H at the [Cu–O] units, where *H–*H coupling energy barrier is 0.79 eV. Notably, •C H is also possible to continue to activate the β–H bond via hole-trapped O with an energy barrier of 0.07 eV (Supplementary Fig. 33), following the first C–H bond cleavage mechanism. The resulting *H is captured by the O adjacent to [Cu–O] units. Consequently, the H formation is significantly hindered at the O and O sites (Fig. 5c and Supplementary Fig. 34), with an energy barrier as high as 2.79 eV. We also considered the possibility of *H migrating from O or O to Cu sites for *H–*H coupling, yet these processes remain challenging. For migration from the O to the Cu site, *H could first relocate to the O site (three-fold coordinated lattice oxygen) with an energy barrier of 1.48 eV, then proceeds from O to the Cu site with a barrier of 3.12 eV (Supplementary Fig. 35), while direct migration from O to the Cu site requires overcoming a 2.92 eV barrier (Supplementary Fig. 36). This indicates that consecutive dehydrogenation on two lattice oxygens leads to difficult desorption of *H, ultimately leading to deactivation of the catalyst due to *H coverage on the [Cu–O ] active sites. Further theoretical calculations (Supplementary Fig. 37) reveal that these two *H species can be transformed into H O via a low energy barrier of 0.40 eV, accompanied by a structural reconstruction from [Cu–O ] to [Cu–O ]. However, this process is still endergonic from a thermodynamic perspective, and the *H species preferentially remain adsorbed on the catalyst surface. Notably, in the presence of CO , CO can cooperatively interact with •C H to form a *COOH intermediate, which promotes the further transformation of the [Cu–O ] species generated during the dehydration process. This, in turn, drives the continuous consumption of the accumulated *H while simultaneously regenerating the [Cu–O ] sites.\nComment 15: Figure S9A does not show a dark control at an equivalent catalyst surface temperature.\nResponse: We appreciate you raising this important comment, and sincerely apologize for the lack of clarity in the original manuscript. We fully agree that conducting a dark control experiment at an equivalent catalyst surface temperature is essential to confirm the photocatalytic nature of the reaction. We would like to clarify that, in our experiments, the dark control was indeed conducted at the same surface temperature as the photocatalytic reaction. To avoid any potential misunderstanding, we have now explicitly stated this important condition in the revised caption of Fig. S9 (now renumbered Supplementary Fig. 10).\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 12, caption of Renumbered Supplementary Fig. 10)\nSupplementary Fig. 10. a Photocatalytic C H and H yields over Cu /TiO under varied reaction conditions. b Photocatalytic C H and H yields as a function of the light intensity over Cu /TiO . c Photocatalytic yields over Cu /TiO under different wavelength regions. Note: Experiments in (a-c), except for the ice-bath cooled one, were measured under constant surface temperature (108 ℃) maintained by external heating. d UV-visible diffuse reflectance spectra and corresponding AQY values of 50 mg Cu /TiO after 1 h of irradiation with different monochromatic lights. Standard conditions (a-d): 1 atm 99.9 vol% C H , batch reactor.\nComment 16: How do C H Fe, K S O , (NH ) C O , and NH affect the surface properties, including the Cu valence and C H adsorption? How to rule out residual effects of these reagents on the surface as a cause of the reduced activity?\nResponse：We greatly appreciate your insightful comments and reminder, which are invaluable for strengthening the rigor of our mechanism analysis. We fully acknowledge the concern that quenchers may alter the surface properties of the catalyst or the adsorption behavior of reactant molecules, thereby introducing uncertainty into the mechanistic interpretation. This limitation is in fact common to conventional quenching experiments, which are usually based on the idealized assumption that sacrificial agents selectively remove the targeted reactive species without inducing other interfering effects. To assess the reliability of the mechanistic conclusions drawn from the quenching experiments in this work, we supplemented with additional control and cross-check experiments that were specifically aimed at minimizing potential interference arising from non-specific effects.\nFirst, additional photoluminescence measurements were carried out to verify the effective quenching of photogenerated charges by the employed scavengers (please see renumbered Supplementary Fig. 16). Upon introduction of either hole or electron scavengers into the Cu /TiO system, the fluorescence intensity decreased markedly, confirming that the selected scavengers efficiently capture the targeted charge carriers. Second, in order to exclude possible interference of the quenchers with C H adsorption or the local surface environment, we performed cross validation experiments using alternative scavengers with distinct structures and chemical properties, that is, sodium oxalate (Na C O ) as a hole scavenger and potassium dichromate (K Cr O ) as an electron scavenger. As shown in the optimized Fig. 4b (Please see the revised manuscript, page 8, Fig. 4b), the product formation rates and distribution profiles obtained with these alternative scavengers are highly consistent with those observed in the original system. These results indicate that the observed quenching effects mainly arise from the selective removal of the targeted reactive species, rather than from indirect perturbations of the surface properties induced by the quencher.\nFurthermore, in the analysis of the results, we considered not only changes in overall catalytic activity but also the evolution of product distribution. For instance, in the electron quenching experiments, the H /C H ratio decreased significantly, indicating that photogenerated electrons play a key role in the kinetics of H formation. Such selective changes in product distribution are difficult to attribute solely to modifications in C H adsorption behavior. In addition, the experiments targeting Cu sites were designed by using NH to selectively coordinate with Cu and thereby deliberately switch off its catalytic function. This strategy is inherently intended to probe the role of a specific type of active site and does not rely on the assumption that all other processes remain completely unaffected. As a result, the corresponding data more directly reflect the functional role of Cu sites in the reaction.\nBased on the above supplementary data and analyses, we have revised and refined the mechanistic discussion in the manuscript in order to clarify how non-specific effects of the quenchers and NH on surface properties and adsorption behavior were minimized or excluded (Please see the revised manuscript, page 8,9).\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 18, Renumbered Supplementary Fig. 16)\nSupplementary Fig. 16. Photoluminescence spectra of Cu /TiO with and without hole or electron scavengers.\n(2) (Revised manuscript, page 8, Fig. 4b)\nFig. 4. b Reactive species quenching experiments. Note: Catalysts and sacrificial agent pre-ground at 10:1 ratio for 0.5h before use.\n(3) (Revised manuscript, page 8,9)\nActive species quenching experiments were conducted to further understand the mechanism of efficient C H activation under mild conditions (Fig. 4b). It should be noted that, to avoid interference arising from the quenchers themselves occupying catalyst surface sites or altering surface properties, two different quenchers were employed for each target species (h+ or e-) for cross-validation. The quenching efficiencies of all scavengers toward photogenerated electrons and holes were confirmed by photoluminescence spectroscopy (Supplementary Fig. 16). Specifically, when ferrocene (C H Fe) and sodium oxalate (Na C O ) were introduced as hole (h⁺) scavengers, the yields of C H and H decreased in an equimolar manner, indicating that h⁺ predominantly participates in the initial step, namely the cleavage of the first C–H bond of C H . In contrast, when potassium persulfate (K S O ) and potassium dichromate (K Cr O ) were introduced as electron (e⁻) scavengers, the C H /H ratio markedly deviated from the theoretical value of 1.0 and increased to 4.7 and 5.0, respectively, demonstrating that e⁻ plays a crucial role in promoting the kinetics of H formation. Meanwhile, quenching e⁻ led to a >90% decrease in the total product yield, which likely implies that the accumulation of *H species on the catalyst surface blocks active sites and ultimately causes catalyst deactivation. To further elucidate the role of Cu sites in the reaction, NH was introduced into the system to coordinate with Cu, forming Cu–NH complexes and thereby specifically blocking the Cu sites; this blocking effect was verified by NH -TPD measurements (Supplementary Fig. 17). The results show that the decrease in C H yield is more pronounced than that of H , and the additional decrease in C H (relative to H ) is stoichiometrically correlated with the formation of C H . This correspondence indicates that, once Cu sites are occupied, the reaction pathway leading to C H is effectively suppressed, and a fraction of the reaction intermediates is diverted toward a C–C coupling pathway to form C H . Accordingly, we propose that atomically dispersed Cu sites predominantly mediate β–H activation, thereby driving the dehydrogenation of the *C H intermediate to form C H . Once these Cu sites are blocked, the lack of accessible active sites for further dehydrogenation may lead to the accumulation and prolonged residence of *C H species on the catalyst surface, which in turn is more likely to promote C–C coupling pathways and ultimately yield C H .\nComment 17: The way the ethyl is coordinated to the surface as shown in the *C H +*H panel in Fig. 5c seems odd for an intermediate (unless it’s a transition state), since based on coordination chemistry principles one would expect coordination through the carbon – is there experimental evidence supporting H-coordination to the surface?\nResponse: We appreciate your insightful comment regarding the coordination mode of the ethyl species shown in the *C H +*H panel in Fig. 5C. Indeed, direct experimental identification of such a configuration is challenging due to the transient nature of surface intermediates. However, according to the theory of agostic interaction, the computed structure is chemically reasonable. In this configuration, the surface metal atom interacts with the σ (C–H) bond of the ethyl group, forming a three-center two-electron (3c–2e) M···H–C bond. Such agostic-like interactions are known to enable electron donation from the C–H σ bond to an empty orbital of the metal center, thereby stabilizing alkyl intermediates and facilitating β-C–H activation or elimination. This type of H-coordination has been widely observed in both homogeneous organometallic complexes and heterogeneous metal or oxide surfaces. (DOI: 10.1073/pnas.0610747104; 10.1002/anie.200200548) Moreover, the optimized geometry displays typical agostic characteristics, including a shortened M···H distance (1.67 Å) and a slightly elongated C–H bond (1.25 Å), consistent with literature precedents.\nComment 18: For Fig. 5C, what does each of the paths mean, and why do paths 1 and 2 become disconnected? I would avoid using percentages when comparing two different energies since it may be misleading and at times confusing to the reader (i.e. the 96.6% has a smaller gap than the 62.3%) but instead report the energy difference directly.\nResponse: Thank you for your valuable questions and suggestions, which have provided key guidance for us to optimize the presentation of the manuscript and improve the clarity of data visualization. In response, we have made a systematic revision to Fig. 5c and the related text in the revised manuscript. Specifically, all percentage-based comparisons have been replaced with explicit energy differences. We have also performed additional calculations and refined the figure layout/annotations to minimize potential confusion arising from discontinuous pathways. The updated Fig. 5c and the corresponding description have been incorporated into the manuscript (Please see the revised manuscript, page 11, Fig. 5c and page 12-13).\nWith regard to your inquiry about the specific meaning of each pathway in the original manuscript, we would like to provide the following clarification herein: Path 1 corresponds to the activation process of the C–H bond at the bridging oxygen-copper (O ) site under the ground state; Path 2 refers to the β–H activation process of ·C H at the five-coordinate Ti site; Path 3 represents the adsorption process of ·C H on the O site; Path 4 stands for the coupling process of two *H species adsorbed at the O site and O site respectively. Furthermore, regarding the issue of \"discontinuous pathways\", it is important to clarify that the relevant pathways in the original manuscript are independently designed comparative calculations for key elementary steps of the main reaction occurring at different potential competing sites; therefore, they are not intended to form a continuous reaction sequence. For example, Path 1 (the C–H bond activation process under the ground state) exhibits a significantly higher transition state energy and reaction energy (0.76 eV and 1.16 eV higher than that of the corresponding step in the main pathway), indicating that this route is both kinetically and thermodynamically highly unfavorable. Accordingly, we did not pursue subsequent steps along this high-energy pathway, which explains why it is shown as a truncated segment in the figure. To avoid confusion, we have presented Path 1 from the original manuscript separately in Supplementary Fig. 30.\nWe hope that the above revisions and clarifications adequately address your concerns and will help prevent similar confusion for other readers. We again sincerely appreciate your insightful review and constructive comments, which are crucial for improving the quality of this work. We would be pleased to further refine the manuscript if you have any additional suggestions on the revised content.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 11, Fig. 5c)\nFig. 5. CO regeneration mechanism and DFT calculations. c Calculated energy diagrams and spin density distributions for C H photocatalytic conversion to C H and H on Cu /TiO . The Ti, O, Cu, C, and H atoms are in blue, red, orange, brown, and dark pink, respectively.\n(2) (Revised manuscript, page 12-13)\nCritically, Fig. 5c demonstrates that upon hole capture by the O site, the C–H bond of C H undergoes facile cleavage via the hydrogen atom transfer (HAT) mechanism27, with a notably low barrier of 0.46 eV and a negative free energy change (ΔG) of -0.37 eV. This barrier and ΔG are 0.76 eV and 1.16 eV lower than those for ground-state C–H activation at the same O site, respectively (Supplementary Fig. 30), thus serving as a prerequisite for high PEDH activity under mild conditions. This process generates a free •C H radical and an adsorbed *H at the O site. The •C H radical then undergoes two successive mild endergonic steps to transfer a *H to the Cu site via the HAT mechanism, involving adsorption at Cu site (0.29 eV) followed by β–H cleavage with a reaction energy of only 0.11 eV. In contrast, although the C–H activation ability of O site is also significantly enhanced after trapping photogenerated holes, with the corresponding barriers being comparable to that at O site, the resulting •C H radical exhibits an adsorption energy of 0.58 eV at the five-coordinated Ti site (Ti ), and the subsequent β–H bond cleavage has a reaction energy as high as 1.60 eV (Fig. 5c and Supplementary Fig. 31), both of which are significantly larger than those on the Cu site. This demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32). Finally, we calculated the generation of H at the [Cu–O] units, where *H–*H coupling energy barrier is 0.79 eV. Notably, •C H is also possible to continue to activate the β–H bond via hole-trapped O with an energy barrier of 0.07 eV (Supplementary Fig. 33), following the first C–H bond cleavage mechanism. The resulting *H is captured by the O adjacent to [Cu–O] units. Consequently, the H formation is significantly hindered at the O and O sites (Fig. 5c and Supplementary Fig. 34), with an energy barrier as high as 2.79 eV.\nComment 19: For the computational methods, a +U correction is typically not applied to the oxygen but only the metal – if there’s any special reason for this it should be discussed; for the Ti, why was 4.2 eV chosen (is there a reference to it an experimental/computational benchmark?)\nResponse: We sincerely thank you for this valuable comment. In this work, on-site Hubbard U corrections were applied to both Ti 3d and O 2p orbitals to more properly describe the localized electronic states associated with photoexcited charge carriers. The U value of 4.2 eV for Ti 3d orbitals was adopted based on previous computational studies of TiO , which demonstrated that this value accurately reproduces the experimental band gap and electronic structure of TiO (DOI: https://doi.org/10.1103/PhysRevB.82.144119; 10.1016/j.susc.2007.08.025; 10.1039/c1cp21547g). In addition, a U correction was applied to the O 2p orbitals to cancel the self-interaction error and facilitate the localization of photogenerated holes on surface O sites. This approach has been successfully employed in several earlier DFT+U studies to model hole trapping and defect-related electronic states in TiO and related oxides. (DOI: 10.1038/s41929-018-0055-z; 10.1021/acscatal.2c01339). We have now added an explicit explanation of this choice in the “Theoretical calculations” section of the revised manuscript (Please see the revised manuscript, page 16-17, “Theoretical calculations” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 16-17, “Theoretical calculations” section)\nThe on-site Hubbard U term (PBE+U) was added for O orbitals and Ti orbitals with a value of 4.0 eV and 4.2 eV to facilitate the localization of trapped holes and electrons. In particular, the U correction on the O 2p orbitals helps to cancel self-interaction errors and enables a more realistic localization of photogenerated holes on surface O sites. In addition, to simplify the treatment of the photoexcited system by excluding the uncertain electron–hole interactions that depend on the specific trapping center, while simultaneously preserving the charge neutrality of the periodic slab and avoiding artificial background-charge corrections, an -OH group was introduced on the bottom surface as an electron acceptor63.\nComment 20: The authors report delta G values and provide the equation associated with it, but there was no mention of how the entropy was obtained (i.e. were any vibrational/phonon calculations done, and how are the associated contributions to the vibrational part of the entropy obtained?)\nResponse: We sincerely thank you for the important question regarding the entropy contribution. All reported Gibbs free energies were evaluated at 383 K. For adsorbed intermediates, vibrational frequencies were calculated at the optimized geometries and used to obtain ΔZPE and the vibrational contribution to ΔS. For gas-phase species, translational, rotational and vibrational entropy contributions at 383 K were taken from standard thermochemical data (NIST). These methodological details have now been explicitly clarified in the “Theoretical calculations” section of the revised manuscript (Please see the revised manuscript, page 17, “Theoretical calculations” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 17, “Theoretical calculations” section)\nThe Brillouin zone was sampled with a 3×2×1 Monkhorst-Pack k-point mesh. The free energy calculations are based on Eq.5:\nΔG=ΔE+ΔZPE-ΔTS [5]\nwhere E is the calculated total energy of the model, ZPE is the change of zero-point vibrational enthalpy, and TS is the entropy. For adsorbed intermediates, vibrational frequencies were calculated at the optimized geometries and used to obtain ΔZPE and the vibrational contribution to ΔS at 383 K. For gas-phase species, translational, rotational and vibrational entropy contributions at 383 K were taken from standard thermochemical data (NIST).\nComment 21: For the TS search, how the constrained optimization technique was used should be discussed.\nResponse: We sincerely thank you for this insightful comment. The transition states (TSs) were searched by a constrained optimization scheme. In this approach, we keep the C-H bond distance fixed at preselected values, and minimize the total energy with respect to all remaining orthogonal degrees of freedom. In particular this means that the molecules are free to rotate and translate subject to the above constraint, and in addition the TiO surface are allowed to relax. The TSs were verified when (i) all forces on the atoms vanish and (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. Previous work has demonstrated that the above DFT setup affords a good accuracy, especially for the calculation of reaction barriers in heterogeneous catalysis (DOI: https://doi.org/10.1103/PhysRevLett.80.3650; 10.1021/ja0207551). These methodological details on the TS search have now been explicitly added to the “Theoretical calculations” section of the revised manuscript (Please see the revised manuscript, page 17, “Theoretical calculations” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 17, “Theoretical calculations” section)\nThe transition states (TSs) were searched using a constrained optimization scheme. In this approach, we keep the C–H bond distance fixed at preselected values, and minimize the total energy with respect to all remaining orthogonal degrees of freedom. In practice, this allows the adsorbed molecules to rotate and translate under the imposed constraint, and the TiO2 surface is fully relaxed. A transition state is identified when the forces on all atoms vanish and the total energy exhibits a maximum along the chosen reaction coordinate while remaining a minimum with respect to all other degrees of freedom.\nResponse to Reviewer #3\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: Thank you for your thorough review and valuable comments.\nResponse to Reviewer #4\nGeneral Comment: This manuscript by Lingzhi Wang and coworkers reported a site-defined photocatalytic strategy for the selective light-driven dehydrogenation of ethane (C H ) to ethylene (C H ) using atomically dispersed Cu–O ensembles on TiO (Cu /TiO ). The study highlights that Cu atoms coordinated with bridging oxygen form well-defined [Cu–O] motifs, which serve as active centers for stepwise C–H activation and H evolution. Photogenerated holes localize at O sites to initiate the first C–H cleavage, while adjacent Cu centers mediate β–H elimination, resulting in efficient and hydrogen-conserving C H upgrading. This catalyst achieved an impressive C H yield of 21.1 mmol g-1 h-1, 97% C H selectivity, and a quantum efficiency of 6.1% at 365 nm, outperforming state-of-the-art photocatalytic ethane dehydrogenation systems. The co-feeding of CO was found to selectively scavenge surface *H species, thereby preventing catalyst deactivation and preserving long-term activity. The mechanism is well-supported by a combination of in situ EPR, DRIFTS, NAP-XPS, TRFL, fs-TAS, and DFT calculations, which collectively reveal that [Cu–O] ensembles orchestrate cooperative charge transfer and redox functionality, enabling controlled C–H activation and H evolution under mild conditions. This paper provides atomic-level insight into the design of redox-cooperative single-atom photocatalysts for alkane upgrading and contributes significantly to the development of sustainable, low-temperature C–H activation chemistry. This paper will be potentially acceptable for publication in Nature Communications. However, I do not recommend publication in the present form because some questions need to be answered.\nResponse: We sincerely appreciate your recognition of our study’s potential and your valuable insights. Your comments have provided crucial guidance for us to clarify key points and deepen our analysis, thereby significantly enhancing the overall quality of our work. All the concerns you raised have been point-by-point and thoroughly addressed in the revised manuscript. We believe that the revised version now fully meets the publication standards of Nature Communications, with point-by-point responses presented below.\nComment 1. The authors' characterization (XPS, XAS) clearly indicates that the as-prepared catalyst contains a substantial fraction of Cu+ (approx. 38% based on XPS deconvolution), coexisting with Cu2+. The manuscript's mechanistic discussion, however, focuses almost exclusively on the role of the [Cu2+−O ] motif as the primary active site for C-H activation and H evolution. The role of the significant initial population of [Cu+−O ] sites remains ambiguous. Are the Cu+ sites catalytically active for any step of the reaction, or are they merely spectators? Can these sites act as electron traps or play another role in the charge carrier dynamics that influence the overall efficiency? Is there any evidence for the interconversion between Cu+ and Cu2+ under reaction conditions, beyond the deactivation pathway identified?\nResponse: We sincerely appreciate your in-depth attention to the potential role of Cu+ species ([Cu+−O ] motif). Given that Comment 1 and Comment 10 are closely related, we present a consolidated response herein to more clearly address your questions regarding the roles of different Cu species in the reaction. By combining in situ spectroscopic characterization with first principles calculations, we can identify the [Cu–O ] unit as the primary active center, whereas the [Cu–O ] motif is essentially inactive. Quasi-in situ EPR (renumbered Supplementary Fig. 11) and in situ NAP-XPS (Figs. 5a and 5b) results consistently show that under feed conditions containing only C H the catalyst deactivation is accompanied by a pronounced decrease in the Cu2+ signal associated with [Cu–O ], whereas under co-feeding of C H and CO the catalytic activity remains stable and the Cu2+ content is effectively preserved, which provides strong experimental support for the core role of [Cu–O ] in the reaction.\nWith respect to the question of “Is there any evidence for the interconversion between Cu+ and Cu2+ under reaction conditions, beyond the deactivation pathway identified?”, quasi-in situ Raman spectroscopy under stepwise feeding of C H and CO provides relevant support. Under a C H atmosphere, the Eg vibrational mode of TiO gradually shifts to higher wavenumbers with increasing reaction time (renumbered Supplementary Fig. 23), which can be attributed to unit-cell expansion or enhanced lattice strain in TiO induced by the reduction of Cu2+ to Cu+. When the gas atmosphere is switched from C H to CO , this blue shift is reversed. This reversible behavior indicates that a certain degree of dynamic adjustment between Cu+ and Cu2+ occurs during the reaction and that this redox equilibrium is strongly influenced by the presence of co-fed CO .\nTo strengthen the above conclusion and following your helpful suggestion in the comment 10, we have incorporated additional DFT calculations in the revised manuscript for the β–H bond activation of the ·C H radical at both types of Cu sites (please see renumbered Supplementary Fig. 32). We focus on the β–H activation step because radical scavenger experiments (Fig. 4b) indicate that, in this system, Cu mainly participates in β–H bond activation processes. The calculations show that at a representative [Cu–O ] site, the reaction energy for cleavage of the β–H bond in the ·C H intermediate is as high as 1.06 eV, indicating a clearly endergonic process, whereas at the [Cu–O ] site the barrier for the same step is only 0.11 eV. This pronounced difference in activation energy clearly demonstrates that [Cu–O ] is the key active center for β–H bond activation, while the direct contribution of [Cu–O ] to this critical step is rather limited. All computational results have been incorporated into the revised manuscript (Please see the revised manuscript, page 12, second paragraph).\nMeanwhile, we fully understand your concern regarding whether Cu+ might play a beneficial role in charge separation and transport. However, due to limitations in current synthesis strategies and the lack of precise control over copper valence states, the presence of the [Cu–O ] motif is an inherent feature of the current catalytic system that is difficult to completely eliminate, and it is experimentally challenging to adjust the Cu2+/Cu+ ratio in an accurate and independent manner. As a result, we are not yet able to isolate and quantitatively evaluate the specific contribution of Cu+ as an electron trap or as a charge modulation center. This is indeed a limitation of the present study, which we have explicitly acknowledged in the revised manuscript, and we regard it as an important direction for future efforts to further optimize and extend this catalytic system. It should be emphasized that even if Cu+ were to contribute to improved charge separation or transport to some extent, its influence remains non-decisive, and it does not challenge the core conclusion that Cu2+ serves as the dominant center for β–H bond activation. All experimental and theoretical results consistently support this judgment. In the revised manuscript, we have further strengthened the mechanistic discussion to enhance its rigor and coherence.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 34, Renumbered Supplementary Fig. 32)\nSupplementary Fig. 32. Comparison of calculated energy diagrams for β–H activation at the Cu sites in [Cu-O ] and [Cu-O ] motifs.\n(2) (Revised manuscript, page 12, second paragraph)\nThis demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H br–Cu 2 5 activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32).\nComment 2. The authors report a catalyst surface temperature of ~108 °C under illumination. While the control experiments (wavelength dependence, dark controls, and a significantly lower activation energy under light) provide strong evidence for a predominantly photocatalytic mechanism, the potential contribution of a photothermal effect at this temperature warrants a more detailed discussion. A localized thermal effect could still play a role in overcoming certain reaction barriers. The authors should provide a more quantitative analysis or discussion to deconvolve the quantum effects from the light-induced thermal effects to strengthen their mechanistic claims further.\nResponse: Thank you for your insightful comments and valuable suggestions, which have further improved the quality of our manuscript. We fully understand the core concern regarding decoupling and quantifying the contributions of quantum effects and light-induced thermal effects, a key and challenging issue in the field of photocatalysis that urgently requires clarification.\nGiven that phonon vibrations, or thermal energy, are inherent properties of materials under illumination, photothermal effects are indeed unavoidable in photocatalytic processes. Their mechanisms of action are notably complex. On one hand, thermal effects can influence catalytic performance through pathways unrelated to quantum efficiency, such as promoting reactant diffusion and optimizing molecular adsorption on catalyst surfaces. On the other hand, increased temperature may also act through processes related to quantum efficiency, including enhancing charge carrier migration and separation efficiency or accelerating carrier recombination (DOI: 10.1016/j.cej.2022.137063, 10.1021/acscatal.4c03685). Therefore, completely decoupling and quantitatively distinguishing their respective contributions at the microscopic scale remains a significant challenge in the field. Admittedly, progress has been made in quantifying thermal and non-thermal effects in plasmonic catalysis. For instance, He's group designed precise control experiments to selectively \"switch off\" pathways such as charge transfer, thereby isolating the contribution of individual mechanisms (DOI: 10.1038/s41467-025-57569-0). Zhang's group measured the actual reaction temperature in photothermal catalysis based on Le Chatelier's principle and calculated the \"pure hot electron contribution\" by subtracting the thermal effect rate from the total reaction rate under illumination (DOI: 10.1002/anie.202304452). Nevertheless, most of these studies have not systematically considered the temperature-dependent influence of photothermal effects on charge carrier dynamics, which may, to some extent, underestimate the complex role of photothermal effects.\nGiven the current state of research, there is no universal approach to quantify the contribution of light-induced thermal effects. Therefore, we have attempted to qualitatively evaluate the influence of the photothermal effect at the macroscopic level. Specifically, we additionally designed a control experiment in which an external ice bath was introduced to suppress the temperature rise of the system under the condition of maintaining the same light intensity, thereby distinguishing the photothermal contribution. The results of this experiment have been included in the revised manuscript as renumbered Supplementary Fig. 10. The data show that when an ice bath was used to weaken the photothermal effect, the catalytic performance decreased significantly from 8.4 mmol g-1 h-1 to 3.8 mmol g-1 h-1， while the product selectivity remained unaffected. Therefore, based on this new data and our existing evidence (wavelength dependence, dark controls, significantly reduced activation energy), we conclude that the process is best described as a photon-driven, phonon-assisted mechanism. In this mechanism, photogenerated charge carriers serve as the primary driving force and dictate the reaction pathway, while light-induced thermal effects (phonon assistance) play an auxiliary role by potentially optimizing processes such as diffusion and adsorption or moderately regulating carrier behavior.\nWe have added the above analysis in the revised manuscript to further clarify the relationship between these two effects (Please see the revised manuscript, page 7, first paragraph). Once again, we appreciate your suggestions, which have helped us refine the mechanistic arguments.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 12, Renumbered Supplementary Fig. 10)\nSupplementary Fig. 10. a Photocatalytic C H and H yields over Cu /TiO under varied reaction conditions. b Photocatalytic C H and H yields as a function of the light intensity over Cu /TiO . c Photocatalytic yields over Cu /TiO under different wavelength regions. Note: Experiments in (a-c), except for the ice-bath cooled one, were measured under constant surface temperature (~108 ℃) maintained by external heating. d UV-visible diffuse reflectance spectra and corresponding AQY values of 50 mg Cu /TiO after 1 h of irradiation with different monochromatic lights. Standard conditions (a-d): 1 atm 99.9 vol% C H , batch reactor.\n(2) (Revised manuscript, page 7, first paragraph)\nHowever, when an ice bath was introduced under constant light intensity to attenuate the photothermal effect, the photocatalytic yield decreased from 8.4 to 3.8 mmol g-1 h-1 while the product selectivity remained unchanged, clearly demonstrating that the light-induced thermal effect (that is, phonon) plays an important synergistic role in the reaction. Moreover, the linear correlation of C H production with light intensity and wavelength-dependent PEDH performance further collectively establish the photon-driven and phonon-assisted mechanism of PEDH over Cu /TiO (Supplementary Figs. 10b and 10c).\nComment 3. The strategy of using CO to regenerate the catalyst by scavenging poisonous *H species is a highlight of this work. However, this process produces CO as a byproduct. The 20-hour stability test is promising, but for potential practical applications, the long-term consequences should be considered. Can the accumulation of CO in a continuous or recycle reactor lead to catalyst poisoning (e.g., via carbonyl formation on Cu sites) or promote side reactions like the reverse water-gas shift reaction with the co-produced H ?\nResponse: Thank you for recognizing the value of the CO enabled regeneration strategy in this study and for highlighting potential risks associated with the accumulation of CO byproducts, catalyst poisoning, and the RWGS reaction. In response to your comments, we have provided the following clarifications based on our experimental data.\nFirst, it should be noted that the CO yield detected in both continuous-flow and batch reactor tests remained below 0.1 mmol g-1 h-1, which is considered a trace amount. Throughout the 20-hour stability test, no significant decline in catalytic activity was observed. Furthermore, in situ DRIFTS under C H and CO co-feeding conditions did not detect any carbonyl species on the catalyst surface. These results collectively suggest a low risk of CO-induced poisoning of the Cu active sites under the current reaction conditions.\nRegarding the possibility of the RWGS side reaction, control experiments involving H and CO co-feeding, as shown in Fig. 4g, clearly demonstrate that this reaction does not occur in our system. The core reason for this phenomenon is that the Cu /TiO catalyst lacks active sites for H activation and the reaction system operates at a low temperature, which prevents the initiation of the RWGS reaction through the redox mechanism. As for the detected trace CO, its specific formation mechanism involves the following process. During C H dehydrogenation, ethane undergoes a continuous dehydrogenation side reaction on lattice oxygen. This process increases the energy barrier for H formation, leading to partial *H species failing to desorb in a timely manner, then induces the subsequent conversion of CO to CO.\nTo more clearly illustrate the formation pathway of CO and address concerns about the RWGS side reaction, we have supplemented detailed discussions and explanations of the CO formation mechanism in the revised manuscript (Please see the revised manuscript, page 10, second paragraph and page 12, first paragraph).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 10, second paragraph)\nAs shown in Fig. 4g, trace CO formation was detected only under C H /CO co-feeding, but was absent in the CO -free atmosphere. Control experiments with H /CO co-feeding also yielded no detectable CO, indicating that the catalyst is unable to effectively activate molecular H under the present conditions and thus cannot initiate a redox-mediated reverse water–gas shift (RWGS) pathway. These findings therefore rule out RWGS as the source of CO formation in the C H /CO co-feeding system.\n(2) (Revised manuscript, page 12, first paragraph)\nUnder illumination in C H (Supplementary Fig. 27a), the immediate decay of C H adsorption peaks, accompanied by the simultaneous appearance of *CH CH species and gaseous C H , provided experimental evidence for the stepwise dehydrogenation of C H to C H . Concomitantly, the observed generation of –OH indicated *H accumulation on the catalyst surface (Supplementary Fig. 27b). In contrast, CO co-feeding led to the formation of *COOH intermediates (Supplementary Fig. 27c) and effective suppression of –OH generation (Supplementary Fig. 27d). These results indicate that CO promotes the removal of surface-adsorbed *H species, thereby regenerating the catalytic active sites and concurrently producing CO. This mechanistic insight further corroborates the intrinsic correlation between the CO formation rate and the ethane conversion behavior.\nComment 4. In the abstract, the phrase \"adjacent Cu centers mediate -H elimination\" should be specified as \"β-H elimination\" for clarity, consistent with the terminology used later in the manuscript.\nResponse: We sincerely thank you for your careful observation and correction. Accordingly, we have revised the expression in the abstract to “β-H elimination,” ensuring consistency with the terminology used throughout the manuscript. In addition, we have carefully re-examined the entire manuscript to identify and correct any similar inconsistencies.\nComment 5. The observation that C H is formed when Cu sites are blocked by NH is interesting. The authors state that this is because ∙C H radicals prefer coupling. A brief mechanistic explanation would be beneficial, clarifying why this occurs (e.g., due to increased surface concentration and lifetime of the radicals in the absence of a site for the second dehydrogenation step).\nResponse: We sincerely appreciate your helpful suggestion, which is indeed important for understanding the reaction pathway. Following your comment, we have added a brief mechanistic explanation for the formation of C₄H₁₀ in the revised manuscript. The revised content is as follows (or see the revised manuscript, page 9, first paragraph):\nTo further elucidate the role of Cu sites in the reaction, NH was introduced into the system to coordinate with Cu, forming Cu–NH complexes and thereby specifically blocking the Cu sites; this blocking effect was verified by NH -TPD measurements. The results show that the decrease in C H yield is more pronounced than that of H , and the additional decrease in C H (relative to H ) is stoichiometrically correlated with the formation of C H . This correspondence indicates that, once Cu sites are occupied, the reaction pathway leading to C H is effectively suppressed, and a fraction of the reaction intermediates is diverted toward a C–C coupling pathway to form C H . Accordingly, we propose that atomically dispersed Cu sites predominantly mediate β–H activation, thereby driving the dehydrogenation of the *C H intermediate to form C H . Once these Cu sites are blocked, the lack of accessible active sites for further dehydrogenation may lead to the accumulation and prolonged residence of *C H species on the catalyst surface, which in turn is more likely to promote C–C coupling pathways and ultimately yield C H .\nComment 6. The photocatalytic process is modeled under excited-state conditions, but the manuscript does not fully clarify how the \"excited\" surface states are constructed in DFT. Are the results from constrained hole localization sufficient to capture the photogenerated carrier distribution? A more detailed explanation of how the excited-state pathway is modeled would be better.\nResponse: We sincerely thank you for your valuable suggestions and kind reminders. In this work, the excited-state surface was modeled by introducing a localized hole to mimic the photoexcited condition, following previous DFT-based studies on photocatalytic TiO systems. To simulate a photogenerated hole in the consideration of simplifying the complicated photo-excited system to exclude the uncertain electron–hole interaction which varies with the trapping center while ensuring the electroneutrality of the periodic system (eliminating the energy correction resulting from the presence of background counter charge), we introduced an OH group as an electron acceptor on the bottom surface of the slab instead of removing electrons from the periodic system. Specifically, the on-site Hubbard U correction was applied to the O 2p orbitals to cancel the self-interaction error and facilitate hole localization the surface O atom. Similar approaches were used in previous work (DOI: 10.1038/s41929-018-0055-z).\nThe lattice oxygen that captures a photogenerated hole exhibits distinct characteristics compared to that in the ground state, which can be readily identified by the following features: (1) The ground-state lattice oxygen has no distinct spin-polarizing features and excited-state lattice oxygen exhibits distinct spin behavior. (2) Geometry structures: the hole localization is accompanied by the distinct elongations of Ti–O bonds by the outward movement of the lattice Ti4+ ions. (3) Electronic structure analysis: the localization of the hole can be further confirmed by the Bader charge difference (~0.31 |e|; positively charged with +0.31 |e| compared to the ground state lattice O) (please see Fig. R1). We have supplemented the “Theoretical calculations” section with additional details on the excited-state modeling and the transition-state calculations (Please see the revised manuscript, page 16-17, “Theoretical calculations” section).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 16-17, Theoretical calculations” section)\nThe on-site Hubbard U term (PBE+U) was added for O orbitals and Ti orbitals with a value of 4.0 eV and 4.2 eV to facilitate the localization of trapped holes and electrons. In particular, the U correction on the O 2p orbitals helps to cancel self-interaction errors and enables a more realistic localization of photogenerated holes on surface O sites. In addition, to simplify the treatment of the photoexcited system by excluding the uncertain electron–hole interactions that depend on the specific trapping center, while simultaneously preserving the charge neutrality of the periodic slab and avoiding artificial background-charge corrections, an -OH group was introduced on the bottom surface as an electron acceptor63. To validate the accuracy of the DFT+U method64, complementary HSE06 hybrid functional calculations were systematically performed for all reaction steps, with corresponding energy values exclusively derived from the HSE0627. Structure optimizations were performed using the Quasi-Newton Broyden minimization scheme until the atomic forces on each ion were less than 0.05 eV/Å.\nThe transition states (TSs) were searched using a constrained optimization scheme65. In this approach, we keep the C–H bond distance fixed at preselected values, and minimize the total energy with respect to all remaining orthogonal degrees of freedom. In practice, this allows the adsorbed molecules to rotate and translate under the imposed constraint, and the TiO surface is fully relaxed. A transition state is identified when the forces on all atoms vanish and the total energy exhibits a maximum along the chosen reaction coordinate while remaining a minimum with respect to all other degrees of freedom.\nFig. R1. Spin density plots of excited-state oxygen and ground-state oxygen.\nComment 7. The reported barrier for C–H activation at O under photoexcited conditions (0.46 eV) is impressively low compared to thermal activation (0.46 vs 1.2 eV). However, the paper would benefit from additional comparison across different sites (O vs O vs terrace Ti sites) under the same computational framework, to confirm the relative preference quantitatively.\nResponse: We greatly appreciate your insightful comment. To clarify the site-dependent reactivity and selectivity, we have additionally computed C–H activation and β–H activation capabilities at different sites. We note that on the anatase TiO (101) surface, all exposed Ti atoms are fivefold-coordinated (Ti ) and adjacent to bridging oxygens (O ). Therefore, the O configuration essentially represents the terrace Ti site on this surface. The C–H activation structure at the O site is thus equivalent to that at the terrace Ti site, and both correspond to the same local coordination environment of Ti . For the O site, the thermal (ground-state) barrier is 1.72 eV, while under photoexcited conditions it decreases markedly to 0.51 eV, indicating a significant hole-induced facilitation effect (please see Fig. R2). For comparison, at the O site the corresponding barriers are 1.20 eV (thermal) and 0.46 eV (photoexcited), as reported in the main text. Although the numerical values appear similar, previous analysis shows that the photogenerated hole preferentially localizes at the O site rather than O , due to the lower hole-trapping energy. Therefore, in realistic photoexcited systems, Cu modification promotes more efficient hole localization and C–H bond activation at the O site.\nIn addition, for the subsequent β–H activation step, quenching experiments at Cu sites confirm that they serve as the primary β–H activation centers, and DFT calculations further show that the β–H activation reaction energy at these sites is only 0.11 eV, whereas that at Ti sites is as high as 1.60 eV (Please see the revised manuscript, page 11, Fig. 5c and renumbered Supplementary Fig. 31). Taken together, the markedly lower barriers for both the initial C–H activation and the subsequent β–H elimination at the [Cu–O] unit, combined with the enhanced hole localization at O sites, provide strong quantitative evidence that Cu modification enhances both the activity and the selectivity of ethane dehydrogenation.\nThese additional results and the corresponding discussion have now been incorporated into the revised manuscript (Please see the revised manuscript, page 12, second paragraph). Specifically, the reaction pathway on the O site is presented in parallel with the ethane dehydrogenation pathway on O , aiming to further illustrates and reinforces the selective regulatory role of the [Cu–O] motif.\nFig. R2. Activation energy barriers for C–H bond activation at different sites in C H\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 11, Fig. 5c)\nFig. 5. CO regeneration mechanism and DFT calculations. c Calculated energy diagrams and spin density distributions for C H photocatalytic conversion to C H and H on Cu /TiO . The Ti, O, Cu, C, and H atoms are in blue, red, orange, brown, and dark pink, respectively.\n(2) (Supplementary Information, Page 33, Renumbered Supplementary Fig. 31)\nSupplementary Fig. 31. Calculated energy diagrams and spin density distributions for C H photocatalytic conversion to C H and H on [Ti-O] site of Cu /TiO in the excited and ground-state. The Ti, O, Cu, C, and H atoms are in blue, red, orange, brown, and dark pink, respectively.\n(3) (Revised manuscript, page 12, second paragraph)\nCritically, Fig. 5c demonstrates that upon hole capture by the O site, the C–H bond of C H undergoes facile cleavage via the hydrogen atom transfer (HAT) mechanism27, with a notably low barrier of 0.46 eV and a negative free energy change (ΔG) of -0.37 eV. This barrier and ΔG are 0.76 eV and 1.16 eV lower than those for ground-state C–H activation at the same O site, respectively (Supplementary Fig. 30), thus serving as a prerequisite for high PEDH activity under mild conditions. This process generates a free •C H radical and an adsorbed *H at the O site. The •C H radical then undergoes two successive mild endergonic steps to transfer a *H to the Cu site via the HAT mechanism, involving adsorption at Cu site (0.29 eV) followed by β–H cleavage with a reaction energy of only 0.11 eV. In contrast, although the C–H activation ability of O site is also significantly enhanced after trapping photogenerated holes, with the corresponding barriers being comparable to that at O site, the resulting •C H radical exhibits an adsorption energy of 0.58 eV at the five-coordinated Ti site (Ti ), and the subsequent β–H bond cleavage has a reaction energy as high as 1.60 eV (Fig. 5c and Supplementary Fig. 31), both of which are significantly larger than those on the Cu site. This demonstrates that the introduction of Cu not only markedly promotes the efficient localization of photogenerated holes from the bulk to surface O sites, but also facilitates the •C H adsorption and β–H activation capabilities. This synergistic advantage in both hole management and β–H activation underpins the high ethane activation capability and excellent ethylene selectivity of the Cu /TiO catalyst. In addition, the high reaction energy for β–H cleavage at the Cu+ site in [Cu–O ] (1.06 eV) further confirms its limited activity, highlighting [Cu–O ] as the dominant active center (Supplementary Fig. 32).\nComment 8. The calculated barriers for *H migration (2.9–3.1 eV) are high, supporting the conclusion of *H accumulation. However, the authors could further discuss whether such high-energy events are still feasible under photon-driven conditions (where local hot carriers may contribute). The neglect of non-adiabatic effects may lead to an underestimation of possible *H dynamics.\nResponse: We greatly appreciate your insightful comment regarding the potential role of non-adiabatic effects and hot-carrier–driven dynamics in facilitating *H migration. Indeed, under photon-driven conditions, energetic carriers may transiently modify reaction pathways. We fully acknowledge this possibility. However, previous studies show that hot-carrier relax extremely rapidly through electron–electron and electron–phonon scattering, so that only a very small fraction (<2%) can cross the interface before thermalization. (DOI: 10.1038/s41377-021-00609-3) Because hot-electron injection competes directly with ultrafast self-scattering, the available energy cannot be efficiently delivered to drive processes requiring high barriers. In addition, given that the vast majority of hot carriers lose their excess energy within tens to hundreds of femtoseconds (DOI: 10.1021/nn305080c; https://doi.org/10.1103/PhysRevB.57.12812; https://doi.org/10.1103/PhysRevB.67.113315). The efficiency of hot-carriers–assisted *H migration is likely very low, and our conclusion regarding *H accumulation remains robust.\nComment 9. The proposed *H scavenging pathway via CO → *COOH → CO is strongly supported by DRIFTS and isotope labeling, but the DFT section only briefly covers CO reactivity. It would strengthen the work if the energy profile for *COOH formation and decomposition were presented explicitly, in parallel with the C H dehydrogenation pathway.\nResponse: We greatly appreciate your valuable suggestions and insightful comments. Following your recommendation, calculations for the CO →*COOH→CO pathway have been added, which are presented in the Supplementary Information due to length considerations. As outlined in manuscript, when the β–H species of C H desorbs at the O site, the resulting *H cannot readily desorb as H because H–H coupling is associated with a high activation barrier, leading to deactivation of the [Cu–O ] active centers due to *H surface accumulation. Further DFT calculations reveal that while the direct reaction between this *H species and CO to form CO and H O is thermodynamically hindered due to its high reaction energy (△G=3.5 eV), the *H can instead combine with another *H at a neighboring O site to form and desorb as H O, with a modest barrier of only 0.40 eV and is accompanied by a structural transformation from [Cu–O ] to [Cu–O ]. Under CO co-feeding conditions, CO cooperates with ·C H generated at other active sites, enabling the structural regeneration of the catalyst from [Cu-O ] to [Cu-O ] via the *COOH pathway. As shown in renumbered Supplementary Fig. 37, CO first adsorbs at the [Cu–O ] site, and the adsorbed CO then abstracts a β–H from ·C H to form an adsorbed *COOH intermediate, with an activation barrier of 1.32 eV and a reaction energy of 0.29 eV. In contrast, direct oxygen-vacancy refilling by CO via a Mars–van Krevelen (MvK) mechanism is less favorable both thermodynamically and kinetically, with an activation barrier and reaction energy as high as 1.96 eV and 1.85 eV, respectively (Fig. R3). The resulting *COOH subsequently decomposes into adsorbed CO with a barrier of only 0.04 eV. After CO desorption, the [Cu–O ] coordination environment is regenerated. Following the reaction pathway illustrated in renumbered Supplementary Fig. 37, the regenerated Cu sites can further capture β–H, which then couples with *H species at O sites to produce H . It is worth emphasizing that *H removal via H O formation remains an endergonic process, implying that under pure C H the system tends to retain *H on the catalyst surface rather than efficiently removing it through the H O pathway. The introduction of CO promotes the continuous conversion of [Cu–O ] back to [Cu–O ], thereby facilitating *H conversion toward H O from kinetic perspectives. This provides a rational explanation for the absence of observable *H accumulation on the catalyst surface under CO co-feeding. These results have been incorporated into the revised manuscript (Please see the revised manuscript, page 13, first paragraph), and we once again thank the reviewer for the thorough and constructive suggestions.\nIt has been modified in manuscript：\n(1) (Supplementary Information, Page 39, Renumbered Supplementary Fig. 37)\nSupplementary Fig. 37. Reaction pathway for CO -assisted removal of surface *H species.\n(2) (Revised manuscript, page 13, first paragraph)\nThis indicates that consecutive dehydrogenation on two lattice oxygens leads to difficult desorption of *H, ultimately leading to deactivation of the catalyst due to *H coverage on the [Cu–O ] active sites. Further theoretical calculations (Supplementary Fig. 37) reveal that these two *H species can be transformed into H O via a low energy barrier of 0.40 eV, accompanied by a structural reconstruction from [Cu–O ] to [Cu–O ]. However, this process is still endergonic from a thermodynamic perspective, and the *H species preferentially remain adsorbed on the catalyst surface. Notably, in the presence of CO , CO can cooperatively interact with •C H to form a *COOH intermediate, which promotes the further transformation of the [Cu–O ] species generated during the dehydration process. This, in turn, drives the continuous consumption of the accumulated *H while simultaneously regenerating the [Cu–O ] sites.\nFig. R3. Reaction energy pathway for CO → CO on the [Cu–O ] site and corresponding DFT-optimized initial state (IS), transition state (TS), and final state (FS) structures.\nComment 10. Following the first question, the experimental results show that the catalyst contains a significant amount of Cu+ (~38%). The current DFT model in Figure 5C focuses on what appears to be a Cu2+ site. Given the prevalence of Cu+, its potential role in the catalytic cycle cannot be ignored. Did the authors perform calculations on a model containing a Cu+ site (e.g., a [Cu+−O ] unit)? A comparative calculation of the C-H activation barrier on a modeled Cu+ site would be highly insightful. If the barrier is significantly higher, it would strengthen the claim that Cu2+ is the primary active state. If the barrier is comparable, it would suggest a more complex picture.\nResponse: We would like to thank you again for this valuable suggestion. The comparative DFT calculations for the [Cu+–O ] model that you requested have been performed and are now included in our response to Comment 1. The results consistently support [Cu2+–O ] as the primary active site.\nResponse to Reviewer #5\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: Thank you for your thorough review and valuable comments.\nResponse to Reviewer #1\nGeneral Comment: I have checked the revision of the manuscript. The authors have well addressed the issues raised by the reviewer and the revised manuscript can be accepted for publication.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #2\nGeneral Comment: I appreciate that many of the comments have been addressed in the revised manuscript, but several issues raised previously are still not fully resolved.\nResponse: We sincerely appreciate your comments and careful review of the revised manuscript. We have re-examined the issues that were not fully resolved and implemented further revisions accordingly. Below, we provide point-by-point responses to each of your concerns. All corresponding changes have been incorporated into the revised manuscript and are highlighted in yellow for your convenience.\nComment 1: How to role out the effect of C H Fe, K S O , (NH ) C O , NH , Na C O and K Cr O on the Cu valence and C H adsorption? As the author mentioned that the activity deactivation correlates with a decrease in the Cu2+.\nResponse: We sincerely thank you for raising this important question again. We would like to clarify that our mechanistic interpretation primarily relies on the consistency of functional responses. When quenchers with the same quenching function but different molecular structures are used as probes, the reaction behavior, particularly product distribution and selectivity, shows consistent and well-defined trends (Please see the revised manuscript, page 8, Fig. 4b). This cross validated consistency indicates that the observed effects mainly arise from selective interception of the targeted active species rather than from quencher specific surface perturbations. For example, to assess the role of photogenerated holes, we employed both C H Fe and Na C O as hole scavengers and obtained mutually consistent results. We acknowledge your concern that these quenchers may affect the valence state of Cu or the adsorption behavior of C H , which might in turn complicate the interpretation of the relationship between catalytic performance and photogenerated charge carriers. It should be noted, however, that even if additional characterizations were conducted, it would be difficult to distinguish whether any observed surface changes stem from non-specific interference or are a natural consequence of the functional quenching process. Therefore, we consider the evidence framework based on cross validation and selective reaction responses to be the more rigorous approach.\nImportantly, the conclusions derived from the quenching experiments are further supported by both theoretical calculations and in situ evidence. As detailed in Fig. 5c and Supplementary Figs. 30 and 31, the presence of photogenerated holes lowers the activation barrier for C–H bond activation in C H from 1.22 eV in the ground state to 0.46 eV. In addition, relative to Ti sites, Cu sites facilitate both the adsorption of •C H radical and the subsequent β–H activation step, with the corresponding energy requirements decreasing from 0.58 eV and 1.60 eV on Ti sites to 0.29 eV and 0.11 eV on Cu sites, respectively. Consistent with this mechanism, •C H radicals were directly detected in our in situ SVUV-PIMS experiments, providing direct experimental validation of the proposed radical-mediated pathway. Taken together, these cross validated functional probe experiments, corroborated by theory and in situ spectroscopy, provide a coherent and robust basis for our mechanistic conclusions.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 8, Fig.4b)\nFig. 4. b Reactive species quenching experiments. Note: Catalysts and sacrificial agent pre-ground at 10:1 ratio for 0.5h before use.\nComment 2: For the 20 vol% C H /Ar stability test, the activity shows a pronounced decline over time. To clarify the origin of this deactivation, please provide the Raman spectrum of the spent catalyst. Is there coke after the long-term reaction under 20 vol% C H /Ar? Many studies attribute EDH deactivation to coke formation or the consumption of lattice oxygen.\nResponse: We sincerely thank you for this important suggestion, which has helped us improve the manuscript. Following your comment, we have performed Raman spectroscopy on the spent catalyst after long-term testing under continuous-flow 20 vol% C H /Ar. The spectrum is now included in Fig. 3h (Please see the revised manuscript, page 6, Fig. 3) alongside those of catalysts used under PEDH and TEDH conditions (20 vol% C H /20 vol% CO /Ar). As shown in Fig. 3h, the spent catalyst from the 20 vol% C H /Ar test exhibits no detectable D band (~1350 cm-1) or G band (~1580 cm-1) signals, indicating the absence of significant coke formation. This result aligns with our earlier Raman data from batch-reactor tests in pure C H atmosphere (Supplementary Fig. 8), ruling out coke deposition as the primary cause of deactivation. Moreover, in situ NAP-XPS analysis (Fig. 5a) shows no appreciable increase in oxygen-vacancy concentration under C H atmosphere. Collectively, these findings suggest that the activity loss in this case stems mainly from *H surface coverage and the reduction of Cu2+ species, rather than from coke formation or lattice-oxygen consumption. The related discussion of these newly added Raman results has been included in the revised manuscript (Please see the revised manuscript, page 7).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 6, Fig.3)\nFig. 3. Catalytic performance. a Photocatalytic C H and H yields and C H selectivity over TiO and M/TiO (M=Cu, Au, Ag, Pd, Pt) under 1 atm 99.9 vol% C H . b Photocatalytic C H and H yields as a function of the catalyst dosage over Cu /TiO under 1 atm 99.9 vol% C H . c C H yield comparison with reported PEDH systems. d Time-dependent photocatalytic C H yield and C H conversion (%) over Cu /TiO in 20 vol% C H /Ar and 20 vol% C H /20 vol% CO /Ar. e Flow reactor product yields of Cu /TiO over a 20-h light-driven test under 20 vol% C H /20 vol% CO /Ar. f MS signals during C H /CO -TPSR test over Cu /TiO . g Product yield over a 10-h dark test at 550°C under 20 vol% C H /20 vol% CO /Ar. h Raman spectra of the spent catalysts post light/dark reactions. Standard conditions (a-e, g): 10 mg catalyst, 777 mW cm-2 xenon lamp, 4-h test, batch reactor. Error bars represent standard deviations obtained from three independent measurements.\n(2) (Revised manuscript, page 7)\nStability evaluation of Cu /TiO for TEDH at 550°C revealed low initial activity (ca. 1.2 mmol g-1 h-1) with rapid deactivation to 20% of the initial activity within 10 h (Fig. 3g). Notably, compared with the aforementioned PEDH processes under continuous-flow conditions, this TEDH process exhibits higher H yields than C H yields alongside substantial carbon deposition (Fig. 3h), signifying the predominance of deep dehydrogenation side reactions. In contrast, the light-driven selectivity of PEDH enables targeted C–H activation while suppressing deep dehydrogenation and over-oxidation, highlighting the intrinsic advantage of photocatalysis for controlled C H -to-C H conversion.\nComment 3: Please specify the software package used to compute the delta G from the vibrational frequencies of the adsorbed intermediates; I believe VASP doesn't have that routine and an external package has to be used to calculate the delta G from the vibrational frequencies obtained in VASP.\nResponse: We sincerely thank you for your careful review and valuable comments. The vibrational frequencies of the adsorbed intermediates were obtained from VASP calculations. The Gibbs free energy (delta G) corrections were evaluated using the VASPKIT post-processing tool based on these frequencies. We have clarified this point in the revised manuscript (Please see the revised manuscript, page 17).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 17)\nwhere E is the calculated total energy of the model, ZPE is the change of zero-point vibrational enthalpy, and TS is the entropy. For adsorbed intermediates, vibrational frequencies were calculated at the optimized geometries and used to obtain ΔZPE and the vibrational contribution to ΔS at 383 K. The ΔG corrections were then evaluated using the VASPKIT post-processing tool based on these vibrational frequencies66. For gas-phase species, translational, rotational and vibrational entropy contributions at 383 K were taken from standard thermochemical data (NIST).\nResponse to Reviewer #3\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #4\nGeneral Comment: The authors have answered all the comments well. It is now ready for publication in its current form.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #5\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have checked the revision of the manuscript. The authors have well addressed the issues raised by the reviewer and the revised manuscript can be accepted for publication.", "reviewer_2": "I appreciate that many of the comments have been addressed in the revised manuscript, but several issues raised previously are still not fully resolved\n1. How to role out the effect of C10H10Fe, K2S2O8, (NH4)2C2O4, NH3, Na2C2O4 and K2Cr2O7 on the Cu valence and C2H6 adsorption? As the author mentioned that the activity deactivation correlates with a decrease in the Cu2+.\n2. For the 20 vol% C₂H₆/Ar stability test, the activity shows a pronounced decline over time. To clarify the origin of this deactivation, please provide the Raman spectrum of the spent catalyst. Is there coke after the long-term reaction under 20 vol% C₂H₆/Ar? Many studies attribute EDH deactivation to coke formation or the consumption of lattice oxygen.\n3. Please specify the software package used to compute the delta G from the vibrational frequencies of the adsorbed intermediates; I believe VASP doesn't have that routine and an external package has to be used to calculate the delta G from the vibrational frequencies obtained in VASP.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The authors have answered all the comments well. It is now ready for publication in its current form.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We sincerely appreciate your comments and careful review of the revised manuscript. We have re-examined the issues that were not fully resolved and implemented further revisions accordingly. Below, we provide point-by-point responses to each of your concerns. All corresponding changes have been incorporated into the revised manuscript and are highlighted in yellow for your convenience.\nResponse to Reviewer #1\nGeneral Comment: I have checked the revision of the manuscript. The authors have well addressed the issues raised by the reviewer and the revised manuscript can be accepted for publication.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #2\nGeneral Comment: I appreciate that many of the comments have been addressed in the revised manuscript, but several issues raised previously are still not fully resolved.\nResponse: We sincerely appreciate your comments and careful review of the revised manuscript. We have re-examined the issues that were not fully resolved and implemented further revisions accordingly. Below, we provide point-by-point responses to each of your concerns. All corresponding changes have been incorporated into the revised manuscript and are highlighted in yellow for your convenience.\nComment 1: How to role out the effect of C H Fe, K S O , (NH ) C O , NH , Na C O and K Cr O on the Cu valence and C H adsorption? As the author mentioned that the activity deactivation correlates with a decrease in the Cu2+.\nResponse: We sincerely thank you for raising this important question again. We would like to clarify that our mechanistic interpretation primarily relies on the consistency of functional responses. When quenchers with the same quenching function but different molecular structures are used as probes, the reaction behavior, particularly product distribution and selectivity, shows consistent and well-defined trends (Please see the revised manuscript, page 8, Fig. 4b). This cross validated consistency indicates that the observed effects mainly arise from selective interception of the targeted active species rather than from quencher specific surface perturbations. For example, to assess the role of photogenerated holes, we employed both C H Fe and Na C O as hole scavengers and obtained mutually consistent results. We acknowledge your concern that these quenchers may affect the valence state of Cu or the adsorption behavior of C H , which might in turn complicate the interpretation of the relationship between catalytic performance and photogenerated charge carriers. It should be noted, however, that even if additional characterizations were conducted, it would be difficult to distinguish whether any observed surface changes stem from non-specific interference or are a natural consequence of the functional quenching process. Therefore, we consider the evidence framework based on cross validation and selective reaction responses to be the more rigorous approach.\nImportantly, the conclusions derived from the quenching experiments are further supported by both theoretical calculations and in situ evidence. As detailed in Fig. 5c and Supplementary Figs. 30 and 31, the presence of photogenerated holes lowers the activation barrier for C–H bond activation in C H from 1.22 eV in the ground state to 0.46 eV. In addition, relative to Ti sites, Cu sites facilitate both the adsorption of •C H radical and the subsequent β–H activation step, with the corresponding energy requirements decreasing from 0.58 eV and 1.60 eV on Ti sites to 0.29 eV and 0.11 eV on Cu sites, respectively. Consistent with this mechanism, •C H radicals were directly detected in our in situ SVUV-PIMS experiments, providing direct experimental validation of the proposed radical-mediated pathway. Taken together, these cross validated functional probe experiments, corroborated by theory and in situ spectroscopy, provide a coherent and robust basis for our mechanistic conclusions.\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 8, Fig.4b)\nFig. 4. b Reactive species quenching experiments. Note: Catalysts and sacrificial agent pre-ground at 10:1 ratio for 0.5h before use.\nComment 2: For the 20 vol% C H /Ar stability test, the activity shows a pronounced decline over time. To clarify the origin of this deactivation, please provide the Raman spectrum of the spent catalyst. Is there coke after the long-term reaction under 20 vol% C H /Ar? Many studies attribute EDH deactivation to coke formation or the consumption of lattice oxygen.\nResponse: We sincerely thank you for this important suggestion, which has helped us improve the manuscript. Following your comment, we have performed Raman spectroscopy on the spent catalyst after long-term testing under continuous-flow 20 vol% C H /Ar. The spectrum is now included in Fig. 3h (Please see the revised manuscript, page 6, Fig. 3) alongside those of catalysts used under PEDH and TEDH conditions (20 vol% C H /20 vol% CO /Ar). As shown in Fig. 3h, the spent catalyst from the 20 vol% C H /Ar test exhibits no detectable D band (~1350 cm-1) or G band (~1580 cm-1) signals, indicating the absence of significant coke formation. This result aligns with our earlier Raman data from batch-reactor tests in pure C H atmosphere (Supplementary Fig. 8), ruling out coke deposition as the primary cause of deactivation. Moreover, in situ NAP-XPS analysis (Fig. 5a) shows no appreciable increase in oxygen-vacancy concentration under C H atmosphere. Collectively, these findings suggest that the activity loss in this case stems mainly from *H surface coverage and the reduction of Cu2+ species, rather than from coke formation or lattice-oxygen consumption. The related discussion of these newly added Raman results has been included in the revised manuscript (Please see the revised manuscript, page 7).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 6, Fig.3)\nFig. 3. Catalytic performance. a Photocatalytic C H and H yields and C H selectivity over TiO and M/TiO (M=Cu, Au, Ag, Pd, Pt) under 1 atm 99.9 vol% C H . b Photocatalytic C H and H yields as a function of the catalyst dosage over Cu /TiO under 1 atm 99.9 vol% C H . c C H yield comparison with reported PEDH systems. d Time-dependent photocatalytic C H yield and C H conversion (%) over Cu /TiO in 20 vol% C H /Ar and 20 vol% C H /20 vol% CO /Ar. e Flow reactor product yields of Cu /TiO over a 20-h light-driven test under 20 vol% C H /20 vol% CO /Ar. f MS signals during C H /CO -TPSR test over Cu /TiO . g Product yield over a 10-h dark test at 550°C under 20 vol% C H /20 vol% CO /Ar. h Raman spectra of the spent catalysts post light/dark reactions. Standard conditions (a-e, g): 10 mg catalyst, 777 mW cm-2 xenon lamp, 4-h test, batch reactor. Error bars represent standard deviations obtained from three independent measurements.\n(2) (Revised manuscript, page 7)\nStability evaluation of Cu /TiO for TEDH at 550°C revealed low initial activity (ca. 1.2 mmol g-1 h-1) with rapid deactivation to 20% of the initial activity within 10 h (Fig. 3g). Notably, compared with the aforementioned PEDH processes under continuous-flow conditions, this TEDH process exhibits higher H yields than C H yields alongside substantial carbon deposition (Fig. 3h), signifying the predominance of deep dehydrogenation side reactions. In contrast, the light-driven selectivity of PEDH enables targeted C–H activation while suppressing deep dehydrogenation and over-oxidation, highlighting the intrinsic advantage of photocatalysis for controlled C H -to-C H conversion.\nComment 3: Please specify the software package used to compute the delta G from the vibrational frequencies of the adsorbed intermediates; I believe VASP doesn't have that routine and an external package has to be used to calculate the delta G from the vibrational frequencies obtained in VASP.\nResponse: We sincerely thank you for your careful review and valuable comments. The vibrational frequencies of the adsorbed intermediates were obtained from VASP calculations. The Gibbs free energy (delta G) corrections were evaluated using the VASPKIT post-processing tool based on these frequencies. We have clarified this point in the revised manuscript (Please see the revised manuscript, page 17).\nIt has been modified in manuscript：\n(1) (Revised manuscript, page 17)\nwhere E is the calculated total energy of the model, ZPE is the change of zero-point vibrational enthalpy, and TS is the entropy. For adsorbed intermediates, vibrational frequencies were calculated at the optimized geometries and used to obtain ΔZPE and the vibrational contribution to ΔS at 383 K. The ΔG corrections were then evaluated using the VASPKIT post-processing tool based on these vibrational frequencies66. For gas-phase species, translational, rotational and vibrational entropy contributions at 383 K were taken from standard thermochemical data (NIST).\nResponse to Reviewer #3\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #4\nGeneral Comment: The authors have answered all the comments well. It is now ready for publication in its current form.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it.\nResponse to Reviewer #5\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We would like to express once more our profound appreciation for the considerable time and effort you have devoted to the review of our manuscript, and for your gracious acceptance of it."}, {"round": 3, "reviewer_comments": {"reviewer_2": "This work can be accepted, all my concerns have been addressed or improved.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter