Dual-photon–driven hydrogen evolution in copper-based photocatalysts under near-infrared and visible light

Domain: physics
Published: 2025-12-09
Editorial outcome: minor_revision

Abstract
While the dynamic restructuring of Cu-metalated metal-organic frameworks for photocatalysis has recently been explored, its effect on electronic excitation remains under-investigated. For better mechanistic understanding, we study the light-induced activity of Cu-metalated UiO-66(COOH)2 metal-organic framework, known as UiO-66(COOH)2-Cu under varied irradiations, using gas-phase formic acid dehydrogenation at ambient conditions as a model reaction. A photocatalytic logic gate behavior is observed. The UiO-66(COOH)2-Cu remains photo-catalytically OFF under visible (>390-720 nm) or near-infrared (>700 nm) light alone, but shows high H2 production of 6.1 mmol·g⁻¹·h⁻¹ (ON state) when both are applied (≥390 nm). Operando Fourier transform infrared spectroscopy and X-ray absorption spectroscopy demonstrate that both visible and near-infrared irradiations are required for metalated Cu2+/1+ restructuring inside the framework to form photoactive Cu0/Cu+ binary center, and thereafter for the photocatalytic dehydration of formic acid. X-ray absorption spectroscopic analysis suggests a distinct initiation behavior of Cu+ and Cu0 species under visible and near-infrared irradiation, respectively. However, operando Fourier-transform infrared spectroscopy reveals a cascade mechanism requiring both irradiations to progress the catalytic reaction. This photocatalytic logic gate behavior also appears in bare Cu0/Cu2O system used as reference. These findings provide insights into dual-photon-driven photocatalysis and aid advanced hydrogen catalyst design.

Introduction
Plasmonic metal/metal-oxide hetero-structures offer a powerful strategy for enhancing photocatalysis and enabling value-added fuel production through efficient solar energy harvesting1,2. These hybrid systems leverage localized surface plasmon resonance (LSPR) in plasmonic metals or direct excitation of metal oxides to generate hot carriers, facilitating charge separation across heterojunctions3,4. Additionally, they extend the light absorption range beyond the intrinsic limitations of conventional metal oxides, such as TiO2 and ZnO, which are primarily active under ultraviolet (UV) light (~5% of the solar spectrum). By incorporating plasmonic metals, these composites effectively utilize visible and near-infrared (NIR) light, significantly improving photocatalytic efficiency and expanding their potential for sustainable energy applications5,6. Initial studies predominantly focused on coupling metal oxides to expensive noble metals, such as Au, Pt or Ag, enabling the composites to function as active photocatalysts through plasmonic excitation in the visible region, which constitutes approximately 43% of the solar spectrum7. However, to meet the economic-friendly standards, research has increasingly shifted toward Cu-based systems in recent years8–10. Notably, the LSPR of Cu can be tuned over a wider range from visible to NIR by varying its size, surface morphology, substrate pattern, etc11,12. As the solar energy contains ca. 52% NIR contribution, it makes inexpensive metallic Cu versatile for solar-energy-driven photocatalysis13. Furthermore, the corresponding metal oxide, Cu2O, has a low band gap (~2 eV), making it active under visible light, unlike TiO2 and ZnO. Thus, the combined advantages of metallic Cu and metal oxide Cu2O make Cu/Cu2O based composites promising candidates for sunlight-driven photocatalytic applications. Recently, several Cu/Cu2O composites were constructed with or without template, and showed higher photocatalytic activities attributed to greater charge separation through Cu/Cu2O heterojunctions14–17. Upon visible-light irradiation, photo-excited electrons transfer from Cu2O to metallic Cu, whereas holes remain in the valence band (VB) of Cu2O, resulting in a better charge separation for photocatalytic reactions14,16,17. Dai et al. demonstrated that if the SPR of metallic Cu was in the visible region, it would generate hot electrons upon visible irradiation to be subsequently involved in the catalytic reaction14. On the other hand, if the Cu SPR is NIR active, only metallic Cu produces hot electrons under NIR irradiation which are transferred directly to Cu2O12,15. Thus, in case of NIR based plasmonic Cu composites, irradiation with both visible and NIR sources results in a dual excitation from both Cu2O and Cu centers, respectively, thus demonstrating a cumulative effect on the overall photocatalytic activity12,15. Apart from charge separation, an energy transfer from LSPR of metallic Cu can also facilitate an electronic excitation at Cu2O for photocatalysis17. Likewise, reports have mostly focused so far on the charge separation mechanism at the interface of Cu/Cu2O composites, by single electron excitation at either Cu2O or metallic Cu, or both, using different wavelengths. Although acceptable catalytic activities have been achieved using these composites, the individual roles of visible and NIR light in the photocatalytic activity were not independently assessed. Furthermore, their poor stability under acidic conditions challenges their use in certain photocatalytic reactions. In this regard, metal-organic frameworks (MOFs), a well-known family of inorganic-organic hybrid materials with porous, tunable, and well-defined extended structures18–20, can be appealingly used as scaffolds to anchor the photo-active centers while maintaining structural stability and preventing leaching of active species, thus leading to enhanced photo-activity. The flexibility and tunability of MOFs, attributed to the potential modification of both metal nodes and organic linkers, allow for the precise adjustment of pore size, shape, and functionality to meet specific photocatalytic requirements. Particularly, Zr-based MOFs stand out as potential candidates in this regard due to their robust nature, structural diversity, and the ease of incorporation of advantageous guest species21.

In a previous study, some of us investigated the photocatalytic dehydrogenation of formic acid (FAc) under continuous vapor flow and λ≥390 nm irradiation over a Zr-based MOF, UiO-66(COOH)2, containing free carboxylate groups, post-metalated with Cu, known as UiO-66(COOH)2-Cu22. UiO-66(COOH)2-Cu shows higher activity, selectivity, and stability, compared to Cu/Cu2O and Cu‒Fe‒O spinal counterparts23,24, competing the non-functionalized UiO-66-Cu framework22. Operando Fourier-Transform Infrared (FTIR) analysis revealed the progressive formation of anhydride bridges in the UiO-66(COOH)2-Cu framework, which is possibly accompanied by an in-situ restructuring process involving the reduction of Cu2+/1+ to a Cu0/Cu2O composite. This restructuring was probably the key factor driving the high photocatalytic activity. It is important to mention that, operando FTIR was the only technique capable of monitoring the in-situ surface modifications related to the organic components of UiO-66(COOH)2-Cu, as evidenced by the indirect observation of anhydride formation. In contrast, bare Cu2O/Cu0 that is used as a reference material in this study suffers from low IR transmission of the surface under photocatalytic conditions, which hampers spectroscopic analysis and limits our mechanistic investigations23. However, the in-depth investigation of the mechanism under different irradiation wavelengths which could unravel the details of this restructuring behavior remains insufficiently explored.

Herein, we report a photocatalytic logic-gate behavior of UiO-66(COOH)2-Cu where the coordinated Cu sites within the MOF structure undergo in-situ restructuring to form active Cu species through anhydride network formation. The robust and well-organized framework not only acts as a stabilizing scaffold, effectively preserving the restructured copper and preventing sintering or agglomeration, but also enables indirect monitoring of the in-situ restructuring of Cu species through the evolution of surface anhydride bridges. Thus, we choose this MOF system to investigate the structural evolution of Cu oxidation states and coordination environments during photocatalytic FAc dehydrogenation, which serves as a model reaction to gain deeper insight into in-Cu restructuring at the molecular and electronic levels. We explore the performance of UiO-66(COOH)2-Cu under different irradiation wavelengths with complementary operando FTIR and X-ray absorption spectroscopy (XAS) techniques. More specifically, we evaluate the effect of light source(s) in two ranges, visible (λ>390–720 nm) and NIR (λ>810 nm, λ>800 nm, λ>750 nm, and λ>700 nm), independently. Their radiations are either combined or separated depending on the experiment. The intrinsic mechanistic pathway of the reaction is investigated by using separate wavelength bands of the entire Vis-NIR region, which reveals an enhanced effect of visible+NIR excitation (≥390 nm) resulting in a high H2 conversion rate of 6.1 mmol·g⁻¹·h⁻¹ under ambient conditions. However, the photocatalytic activity upon individual excitation either in visible or in NIR region is negligible. Unlike conventional Cu-based photocatalysts that are active under individual visible or NIR light, this system exhibits significant activity only under simultaneous dual irradiation, demonstrating logic-gate photocatalytic behavior. Operando studies under various irradiation conditions are then applied in order to propose a plausible mechanism behind this behavior. Double excitation proceeds in a cascade manner, resulting in the formation of dynamic Cu+/Cu0 redox cycle that underlies this logic-gate photocatalytic activity, which is evidenced by operando XAS. Similar behavior is also demonstrated with bare Cu/Cu2O reference material. These findings do not only reveal the nature of wavelength-dependent double-photon-process of Cu/Cu2O embedded in the UiO-66(COOH)2 framework, but also suggest strategies to engineer plasmonic Cu-based composites and frameworks that would optimize the use of wavelengths for enhanced photocatalytic activity.

Results
Synthesis and characterization
UiO-66(COOH)2 is synthesized using 1,2,4,5-benzenetetracarboxylic acid (BTCA) as a linker via a modulated-solvothermal method yielding crystals (ca. nano range) having free ‒COOH groups and exposed defect sites (more details in the Method section). Cationic Cu(II) was incorporated into the framework via post-synthetic metalation, through wet impregnation method. The concentration of the Cu(II) precursor was adjusted to reach a value of 6-8 wt% copper with respect to the final composite, i.e. a Cu/Zr ratio of ~0.2, as quantified by atomic absorption spectroscopy (AAS). The cationic copper species were expected to anchor to the linkers' free carboxylate functionalities, and the oxygen atoms of hydroxy capping groups at the Zr-defect sites25,26, as illustrated in Fig. 1A. The octahedral shape of UiO-66(COOH)2 and its homogeneous morphology were retained after the metalation process, as confirmed by the corresponding scanning electron microscopy (SEM) images (Fig. 1B). The powder X-ray diffraction (PXRD) patterns of as-synthesized as well as Cu-metalated samples match with the simulated profile of UiO-66(COOH)2 (Fig. 1C). The observed diffraction peak at ~12° in as-synthesized and Cu-metalated MOFs could be due to the presence of adsorbed water molecules in the materials27. No extra peaks that could be attributed to crystalline Cu species were observed in the post-metalation pattern, indicating the successful incorporation of cationic Cu into the functionalized framework. For further verification, energy-dispersive X-ray spectroscopy (EDX) mapping analysis of the UiO-66(COOH)2-Cu sample was performed. A homogeneous distribution of both Zr and Cu throughout the MOF crystals is observed, indicating uniform metalation (Supplementary Fig. S1). Similar data were also reported in our previous study22. UiO-66(COOH)2 demonstrated a type I N2 sorption isotherm, revealing its microporous nature, that was further validated by its pore size distribution profile highlighting the presence of small pores of approximately 1.7 nm (Supplementary Fig. S2). Upon post-synthetic Cu metalation, a decrease in its surface area from 239 to 32 m2/g was noticed, accompanied by a decrease in pore volume from 0.22 to 0.11 cm3/g due to Cu(II) species occupying the microporous framework void (Fig. 1D). Thermogravimetric analysis (TGA) was used to assess the thermal effect on the chemical composition of the synthesized MOF, which could withstand temperature up to 300 °C, beyond which significant linker degradation occurs (Supplementary Fig. S3). The defect number in the as-synthesized UiO-66(COOH)2 was estimated to be 1.8 by comparing to the theoretical and experimental weight losses of the framework (Supplementary Fig. S3, Table S1), validating the presence of structural vacancies within the parent MOF structure.

Photocatalytic logic gate activity of UiO-66(COOH)2-Cu
The photocatalytic performance of UiO-66(COOH)2-Cu under flow operation conditions was systematically evaluated using a Sandwich cell reactor, with operando-FTIR spectroscopy enabling real-time investigation of both the photocatalyst surface and reaction gas phase products simultaneously (Supplementary Fig. S4). More details regarding the experimental conditions can be found in the Methods section. To gain deeper insight into the role of light in the reaction mechanism, we further examined the influence of irradiation wavelength on the photocatalytic activity by performing FAc dehydrogenation reaction over UiO-66(COOH)2-Cu under different irradiation wavelengths, as shown in Supplementary Fig. S6. No detectable activity was observed under visible light (390–720 nm) or combined UV-visible irradiation (240–720 nm) (Supplementary Fig. S6). Furthermore, when the reaction was further examined under NIR wavelength ranges, such as λ>810 nm, λ>800 nm, λ>750 nm, and λ>700 nm (Supplementary Fig. S6), no H2 production was observed. However, when the reaction was carried out under broad-spectrum irradiation encompassing both visible and NIR regions (≥390 nm), a significant enhancement in the activity was observed, resulting in 100% selectivity with a H2 production yield of 6.1 mmol·g⁻¹·h⁻¹. These results are very consistent with our previous work with similar Cu loading (7 wt.%) where ca. 5 mmol·g⁻¹·h⁻¹ was obtained under simulated-solar light irradiation22.

These observations revealed that neither visible, nor NIR could solely trigger the photocatalytic dehydrogenation of FAc over UiO-66(COOH)2-Cu. Importantly, their combination was necessary to promote the activity. These observations unveil a possible photocatalytic logic-gate behavior of UiO-66(COOH)2-Cu, where it is photocatalytic OFF under either visible or NIR irradiation, but ON when both are combined. Moreover, the Cu-free UiO-66(COOH)2 and UiO-66(COOH)2-Cu with different Cu loadings were also tested under similar conditions as reference materials (Supplementary Fig. S7). No activity was observed for the Cu-free sample, confirming the crucial role of Cu as active sites. At 3 wt.% Cu, the H2 production rate remained relatively low (0.55 mmol·g⁻¹·h⁻¹), but it increased to 4.3 mmol·g⁻¹·h⁻¹ at 6 wt.%. Comparable activity values were obtained for Cu loadings of 8 and 9 wt.%. Further raising the Cu content to 16 wt.% did not lead to a significant improvement, with the activity reaching only ca. 6.6 mmol·g⁻¹·h⁻¹. These results highlight the importance of Cu coordination on the free carboxylates and the formation of active sites which influence the activity of the system.

To assess the potential influence of the scaffold (i.e., MOF support) on the photocatalytic logic-gate behavior of UiO-66(COOH)2-Cu, control experiments were carried out using a pure Cu0/Cu2O nanocomposite. This approach helps determine whether this behavior is solely related to the active Cu centers or if the support material also plays a contributing role. The phase structure of the as-synthesized material was confirmed through PXRD (Supplementary Fig. S8) while SEM imaging (Supplementary Fig. S9) revealed a flower-like morphology composed of agglomerated nanosized crystals (17 nm). The pure structure of the Cu0/Cu2O was also verified by the Cu-XANES, where spectrum of the pristine material showed a pre-edge peak at ~8982 eV corresponding to Cu (I) dipole-allowed 1s→4p transition (Supplementary Fig. S10). The FT-EXAFS signal revealed two nearest coordination shells of Cu absorber at ~1.47 and ~2.78 Å attributed to Cu–O and Cu–Cu SS of Cu2O material. A little hump at ~2.22 Å could suggest the presence of small metallic Cu in the synthesized material (Supplementary Fig. S10). Photocatalytic H2 production under identical reaction conditions showed similar results whereby no activity was detected under visible light (390–720 nm) or NIR irradiation (>700 nm). However, under combined visible + NIR irradiation (≥390 nm), a significant H2 production rate of 8.2 mmol·g⁻¹·h⁻¹ was observed (Supplementary Fig. S11), demonstrating its photocatalytic logic-gate behavior. These findings suggest that the wavelength-dependent photocatalytic phenomena depend solely on the Cu active centers in both cases, whereby in the UiO-66(COOH)2-Cu, the coordinated Cu in the framework also favors the in-situ restructuring process. Thus, this phenomenon could be applicable to other Cu-based metal-semiconductor materials.

In a previous study, our group investigated the nature of the photocatalytic activity of UiO-66(COOH)2 using operando FTIR spectroscopy. Owing to the FTIR-active surface of the MOF, structural changes during the reaction could be effectively monitored, allowing the MOF to function as a probe. In contrast, bare Cu0/Cu2O suffers from low IR transmission of the surface under photocatalytic conditions, which hampers spectroscopic analysis and limits our mechanistic investigations22,23. Indeed, FTIR analysis showed the progressive formation of the anhydride bridge on the surface of the catalyst evidenced by characteristic IR bands in the 1700–1800 cm⁻¹ region22,28. This anhydride bridge originates from the condensation of two free carboxylate groups present on the MOF backbone, correlating with a significant enhancement in photocatalytic activity. Notably, the formation of this anhydride bridge facilitates the generation of photo-active Cu clusters (Cu0/Cu⁺) which serve as the primary catalytic sites for H2 evolution.

In this study, a real-time investigation of the UiO-66(COOH)2-Cu surface was conducted under, various irradiation conditions as shown in Fig. 2A, to elucidate the wavelength-dependent nature of the in-situ restructuring process. Under single visible light irradiation (>390–720 nm) for 30 min, no detectable anhydride formation was observed, as confirmed by both subtracted and direct FTIR spectra (Fig. 2B, Supplementary Fig. S12). In contrast, under NIR irradiation (>700 nm), a slight initial increase in anhydride intensity was observed, followed by a plateau over the remaining 30 minutes (Fig. 2B, Supplementary Fig. S12). However, neither visible nor NIR irradiation alone induced measurable hydrogen production, given the detection limits of our apparatus (Fig. 2D).

Notably, when combining visible-NIR irradiations (≥390 nm), a pronounced evolution of anhydride peaks was observed, probably due to the condensation of two adjacent carboxyl groups of the BTCA linker (Fig. 2C), accompanied by a significant H2 production rate of ca. 6 mmol·g⁻¹·h⁻¹ (Fig. 2D). These findings strongly suggest that the in-situ restructuring of Cu2+ sites via anhydride formation and consequently H2 production are highly wavelength-dependent. Then, the photocatalytic stability was evaluated under continuous-flow conditions by monitoring the catalyst’s activity over an extended period of 72 hours. These results align with our previous study22, in which the system exhibited excellent stability and maintained high activity over three consecutive cycles (24 hours/cycle) without any significant deactivation (Supplementary Fig. S13). Notably, the photocatalytic performance of UiO-66(COOH)2-Cu demonstrated in this study exceeds that of previously reported MOF-based photocatalysts for the dehydrogenation of formic acid, particularly with respect to product selectivity, operational stability, the incorporation of a cost-effective and earth-abundant metal (Cu), and the ability to function under simulated solar irradiation (Supplementary Table S2 for comparative data).

Two possible mechanistic hypotheses could then be proposed: (i) a double-photon mechanism, requiring simultaneous absorption of visible and NIR photons, or (ii) a cascade reaction, necessitating sequential irradiation with visible and NIR irradiation. However, the first hypothesis is unlikely, since this kind of phenomena is rather associated with the presence of intermediate excited states giving rise to quantum cascade phenomena. These excited states are currently engendered by UV-Vis photons. In the present case, we have photons of lower energy and stable intermediates, as witnessed by the slight increase in anhydride intensity under NIR irradiation, which strongly supports, on its own, the second hypothesis, indicating a stepwise restructuring pathway, that will be further investigated in the following section.

Ex-situ characterization of UiO-66(COOH)2-Cu before and after FAc dehydrogenation
To further investigate the structural and electronic changes of UiO-66(COOH)2-Cu before and after reaction, PXRD and high-resolution transmission electron microscopy (HRTEM) post-reaction analysis was conducted. PXRD analysis indicated that UiO-66(COOH)2-Cu retained its crystallinity and phase purity after photocatalytic reaction (Supplementary Fig. S14), highlighting the good structural stability of the material.

TEM analysis has provided significant insights into coordination changes occurring during the reaction, particularly highlighting the formation of Cu2O clusters in UiO-66(COOH)2-Cu. High-angle annular dark-field scanning TEM (HAADF-STEM) images (Supplementary Fig. S15A) display bright contrast regions corresponding to these copper clusters. Particularly the HRTEM image (supplementary Fig. S15B) shows monodispersed Cu2O nanoparticles, which can be indexed to the cubic Cu2O phase (space group Pn3̅m, lattice parameter a=0.426 nm; ICSD 172174), with particle sizes of 1–2 nm, consistent with our previous findings22. Notably, no Cu0 was detected due to their high dispersion. The lack of Cu0 detection might also be stemmed from their air sensitivity or possible amorphous (non-crystalline) nature. It should be noted that to mitigate beam-induced reduction of Cu2O to Cu0 during imaging, all measurements were performed using a low electron beam voltage. This precaution was essential to ensure that the detected Cu species represent the true post-reaction state of the sample, free from electron-beam artifacts.

Operando XAS
To get more insights into the in-situ formation of active species and any effect of the framework under various irradiation conditions, operando XAS was carried out using a reactor developed at SOLEIL-synchrotron29 under similar reaction conditions to the ones used in the operando-FTIR. XANES and EXAFS analyses at both the Zr and Cu K-edges were conducted to examine the structural and electronic modifications of these metals throughout different reaction stages. During activation under NIR irradiation (>720 nm), Zr K-edge XANES showed a flattening in the white line intensity (Supplementary Fig. S16). The same trend was observed by Valenzano et al, comparing the Zr K-edge XANES of an as-synthesized UiO-66 sample and the corresponding de-solvated materials; and was explained by the removal of solvent molecules27. This was confirmed by the Zr K-edge EXAFS signals for which the shoulder at ~2.1 Å in the first coordination shell (Zr–O single scattering (SS)) disappeared. Furthermore, a significant change was found in the second coordination shell, corresponding to the Zr–Zr SS, where the intensity of the peak at ~3.3 Å decreased while a peak at ~3 Å appeared upon NIR irradiation, confirming the removal of adsorbed water molecules inducing a change in the local order as explained by Valenzaeo et al.27. The corresponding online MS data shows the desorption of water during activation (Supplementary Fig. S17). Further irradiation with visible light (>390–720 nm) along with NIR caused additional adsorbed water removal, where the Zr – Zr coordination shell at ~ 3.3 Å almost disappears to the benefit of a peak at ~3 Å exhibiting a shoulder at ~3.1 Å. These results suggest that structural distortions in the Zr center are initially triggered by NIR irradiation, followed by visible-light exposure. The Zr K-edge XANES and EXAFS showed no significant changes after FAc adsorption on the catalyst surface (Supplementary Fig. S18) apart from effect of adsorbed FAc. Under the reaction’s conditions, Zr K-edge XANES and EXAFS analyses revealed no significant local or long-range structural changes under either NIR or visible irradiation alone (Supplementary Fig. S16). However, under combined visible and NIR irradiation, EXAFS revealed an intensity alteration of both Zr–O SS and Zr–Zr SS. These could be due to a change in the Debye-Weller factor, related to the disorder in the structure. The anhydride formation and associated in-situ Cu restructuring during the photocatalytic reaction could affect the adjacent Zr-cluster environment.

The Cu K-edge XANES spectra of pristine UiO-66(COOH)2-Cu show weak quadrupole-allowed (1s→3d) pre-edge peak at ~8977.5 eV that is relevant of Cu(II) followed by a feature at ~8985.5 eV representing the dipole-allowed (1s→4p) transition, which is characteristic for Cu(II) (Fig. 3A)30,31. The EXAFS signal reveals peaks at ~1.57 and ~2.57 Å, which are attributed to Cu–O and Cu–Cu coordination in Cu(II), respectively (Fig. 3B)30,31. Moreover, a comparison of the Cu K-edge XANES and EXAFS spectra of Cu2O, as a reference, would suggest presence of minimal Cu (I), probably formed under post-activation process (solvent removal), along with major Cu(II) in UiO-66(COOH)2-Cu (supplementary Fig. S19). Nonetheless, during activation under NIR irradiation (>700 nm), the intensity of the white line decreases while the intensity of the shoulder at ~8985.5 eV slightly increases (Supplementary Fig. S20A). The Fourier Transform of the EXAFS signal exhibits a decrease in the intensity of the first and second coordination shell peaks (Supplementary Fig. S20B). Under visible irradiation (>390-720 nm) along with NIR, the observed trend continues to a higher extent (Supplementary Fig. S20C, D). Together with the observations made at the Zr K-edge, these results indicate the desorption of adsorbed water molecules. However, this does not affect the Cu oxidation state, apart from the local symmetry (Supplementary Fig. S21). Indeed, the Cu geometry remains similar after FAc adsorption on the MOF surface prior to the photocatalytic reaction (Supplementary Fig. S18, S21). Then we studied the reaction under different irradiation conditions. The obtained results are in line with the operando FTIR results. Under bare visible irradiation (Fig. 3C, D), the Cu K-edge XANES revealed the evolution of a very weak feature at ~8981.8 eV (relevant of Cu(I) 1s→4p transition) accompanied by a decrease in Cu–O and Cu–Cu distances, which suggest the reduction of a small amount of Cu(II) to Cu(I) (Fig. 3C inset and Fig. S22). This step was followed by bare NIR irradiation, during which a peak ~2.2 Å appeared in the Fourier Transform of the Cu K-edge EXAFS spectra attributed to Cu–Cu coordination sphere of metallic Cu (Fig. 3F). It should be noted that, when we applied a NIR irradiation as a first irradiation type (before exposing the sample to the visible-light), no significant changes in the Cu K-edge XANES and Cu K-edge EXAFS signals were observed (Supplementary Fig. S23). All these observations highlight the impact of individual irradiation on the in-situ restructuring phenomenon. Then, the reaction was carried out under simultaneous NIR and visible irradiation. We observed that the Cu K-edge XANES spectra displayed a gradual rise of pre-edge peak at ~8982 eV, attributed to Cu(I) species (1s→4p), which revealed more Cu(I) species formation during the reaction (Fig. 3G). Moreover, the gradual flattening of white line also suggested metallic Cu0 formation. Furthermore, Cu K-edge EXAFS spectra displayed a major peak evolution at ~2.20 Å corresponding to metallic Cu–Cu first coordination (Fig. 3H). The Cu–O coordination shell position was also shortened to ~1.43 Å corresponding to Cu(I) species. A comparison of the Cu K-edge XANES and EXAFS spectra of the restructured UiO-66(COOH)2-Cu with Cu2O and metallic Cu, as reference, clearly demonstrated the post-formation of Cu2O and metallic Cu (Supplementary Fig. S24). The absence of Cu2+ peak during visible+NIR irradiation indicate its transformation to Cu0/Cu+ species. Furthermore, the XAS analysis of post-catalytic UiO-66(COOH)2-Cu, in dark, showed that the Cu centers remained intact (Supplementary Fig. S25), revealing their reversible Cu restructuring during the photocatalytic reaction. However, this restructured Cu center would act as an active center for consecutive photocatalytic cycles. The stable photocatalytic dehydrogenation rate of UiO-66(COOH)2-Cu over three consecutive cycles (Supplementary Fig. S13) further supports the stability and reusability of the restructured MOF catalyst. As a summary, the restructuring process was initiated under visible light and promoted by NIR irradiation, however remained non-significant under individual irradiation. When both visible and NIR irradiations were applied simultaneously, the in-situ restructuring phenomenon from Cu2+ to Cu0/Cu+ is accelerated leading to high photocatalytic H2 production.

The operando XAS was also carried out on the Cu0/Cu2O nanocomposite under conditions similar to the ones used for UiO-66(COOH)2-Cu. The pure structure of the Cu0/Cu2O was verified by the Cu K-edge XANES. The pristine material showed a pre-edge peak at ~8982 eV corresponding to Cu (I) dipole-allowed 1s→4p transition (Supplementary Figs. S10A, S26A). The FT- EXAFS signal revealed two nearest coordination shells of Cu absorber at ~1.47 and ~2.78 Å attributed to Cu–O and Cu–Cu SS of Cu2O material (Supplementary Figs. S10B, S26B). A little hump at ~2.22 Å could suggest the presence of small metallic Cu in the synthesized material. When performing the reaction under combined visible + NIR irradiation (λ≥390 nm), Cu K-edge XANES spectra showed lower-energy shifting of 1s→4p pre-edge peak and flattening of white band, supporting the formation of metallic Cu species (Supplementary Fig. S26C). The Fourier transform of the Cu K-edge EXAFS spectra also showed the intensification of Cu–Cu SS at ~2.19 Å (from metallic Cu) and lowering of Cu – O coordination shell peak (from Cu2O), revealing the co-existence of greater metallic Cu and lesser Cu(I) which are considered as active catalytic species in this material (Supplementary Fig. S26D). These observations reveal a consistent photocatalytic mechanism for Cu0/Cu2O-based catalysts, both with and without a support template, and underscore the broader relevance of Cu-based photocatalysts.

Investigation of the reaction mechanism under NIR/visible periodic irradiations in cascade
Building on the previous findings, we further explore the photochemical nature of the in-situ restructuring process through experiments involving sequential cycles of visible and NIR irradiation. Each half-cycle of visible or NIR irradiation was maintained for 4 minutes, followed by a dark interval as illustrated in Fig. 4A, C, E. During the half-visible cycle each spectrum was subtracted from the previous one before visible irradiation in dark conditions. It is worth noting to mention that the initial spectrum (subtracted from the reference before starting the sequential experiment) exhibited no detectable changes in the anhydride bands, as illustrated in Fig. 4B. After starting the reaction, a slight decrease in the evolution of anhydride bands in the 1900–1870 cm-1 was shown (Fig. 4B). The cumulative addition of those spectra, after each cycle, revealed a noticeable decrease in the intensity of these bands under visible light (Fig. 4B, inset top left), suggesting partial hydration of anhydride in the presence of water vapor28, as illustrated in Fig. 4B, inset top right. Subsequently, we studied the evolution of the anhydride during the half-NIR cycles. Spectra were subtracted from the previous one before NIR irradiation in dark (Fig. 4C). Results showed an increase in the anhydride bands after each NIR irradiation cycle. This was also verified by the cumulative addition of those spectra where a clear increase in the anhydride evolution was observed (Fig. 4D, inset top left). These data suggest that the in-situ restructuring phenomenon and the anhydride formation are primarily assisted by NIR irradiation (Fig. 4D, inset top right), and is associated with a reverse reaction as well: hydration of the anhydride in the presence of water vapor. Moreover, it should be noted that the rate of the anhydride formation in 2.5 times greater than that of the hydration reaction. This was further confirmed when we analyzed the spectra of the UiO-66(COOH)2-Cu after six full visible-NIR cycles. A significant evolution of the anhydride was observed (Fig. 4F). It is worth to mention that as previously discussed, a single cycle of either visible or NIR irradiations with a duration of 30 min equivalent to around 6 half-cycles, induced very low anhydride formation after exposing the sample to visible light followed by NIR. However, successive alternating cycles of visible and NIR irradiation facilitated its evolution on the surface. This evidence strongly confirms the cascade nature of the reaction, wherein visible-light initiates the process, and NIR irradiation promotes its progression.

Methods
Materials
All chemical reagents and solvents used in this study were commercially sourced and employed without further purification. Formic acid (99%) and Ethanol (96%) were purchased from Carlo Erba. Zirconyl chloride octahydrate (ZrOCl2·8H2O, 98%), 1,2,4,5-benzenetetracarboxylic acid (96%), copper(II) nitrate (Cu(NO3)2·3H2O, 99%) were procured from Thermo Fisher Scientific. Acetic acid (≥99.8%), methanol (99.93%), and N, N’-dimethylformamide (DMF) (>99.8%) were purchased from Sigma-Aldrich. Deionized water and absolute ethanol were used during the reaction wherever required.

Preparation of UiO-66(COOH)2
Equimolar quantities of ZrOCl2·8H2O (59.4 mg, 0.148 mmol) and 1,2,4,5-benzenetetracarboxylic acid (47.1 mg, 0.148 mmol) were dissolved in 4 mL DMF in a 20 mL scintillation via vigorous sonication22. 4 mL of HCOOH was added thereafter as a modulator and the vial was then transferred to a pre-heated oven at 130 °C for 12 h. The resulting white powder was then washed thoroughly with DMF (3 times) and methanol (3 times) over 3 days. The obtained MOF was vacuum-dried at 120 °C overnight before proceeding with the metalation procedure. It should be noted that due to its hygroscopic nature, the MOF could adsorb moisture when exposed to open air.

Preparation of UiO-66(COOH)2-Cu
Copper metalation of UiO-66(COOH)2 frameworks was carried out post-synthetically22. Specifically, 150 mg of copper(II) nitrate trihydrate was dissolved in 20 mL of DMF in a 100 mL glass reagent bottle. Subsequently, 75 mg of the MOF powder was added to the solution, followed by sonication. The mixture was then sealed and stirred at 300 rpm at 100 °C for 21 h. Upon heating, the initial blue solution turned gradually to dark brown color, indicating successful metalation. The resulting Cu-functionalized MOF was isolated via centrifugation, thoroughly washed with DMF and methanol over three days until the supernatant turned transparent. The powder was finally vacuum-dried at 120 °C, overnight, to ensure complete solvent removal prior to characterization and testing. However, due to its hygroscopic nature, it could adsorb moisture from air.

Preparation of Cu nanocomposite
Cu(NO3)2·3H2O (181.2 mg, 25 mmol) was dissolved in 30 mL ethanol in a Teflon flask. 1.5 mL acetic acid was added gradually and sonicated for 5 min. The resulting solution was then transferred to a Teflon-lined stainless-steel autoclave (80% filling) and heated at 180 °C for 2 h. The resulting brick-red solid was then recovered from the reaction mixture via centrifugation, subjected to multiple washes with deionized water and ethanol, and then dried at 60 °C for 12 h.

Characterization
Powder XRD patterns of the synthesized samples were recorded using a Bruker D8 Advance diffractometer equipped with Cu Kα radiation (λ=1.5418 Å), operating at 40 kV and 40 mA. For scanning electron microscopy (SEM) analysis, powder samples were dispersed on carbon tape affixed to an aluminum stub and subsequently coated with 10 nm gold layer via sputtering to enhance conductivity. SEM images were acquired using a Mira3 Tescan microscope, enabling detailed visualization of the morphology and surface characteristics of the samples. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 F1 Libra TGA apparatus. For each MOF, approximately 10 mg of powder was accurately weighed and placed in an alumina crucible, then heated from 30 to 1000 °C at a rate of 10 °C/min under air. The 1st derivative thermogravimetry (dTG) curve was plotted along with the thermogram, to better identify the weight losses and calculate the missing linkers defects in each MOF structure. Nitrogen sorption measurements were conducted for each MOF using a Micromeritics 3Flex analyzer at 77 K to evaluate surface area and pore volume. Prior to analysis, the samples were degassed under nitrogen at 120 °C overnight to remove adsorbed species and ensure accurate measurements. The actual Cu(II) content in the metalated MOFs was quantified using Atomic Absorption Spectrophotometry (AAS). Approximately 2 mg of each sample was precisely weighed and sonicated in 400 µL of aqua regia to ensure complete framework decomposition. The resulting solution was diluted with deionized water, and the Cu concentration was measured using a Thermo Fisher Scientific Atomic Absorption Spectrophotometer in flame mode. Diffuse reflectance spectra were recorded using a Cary 4000 UV-Vis spectrophotometer equipped with a HARRICK Praying Mantis diffuse reflectance accessory. Measurements were conducted with an average acquisition time of 0.2 s and a scan rate of 300 nm/min. Solid state photoluminescence spectra were acquired from a FS1000 spectrometer (Edinburgh, UK) equipped with an N-J03 as sample holder with a front-face geometry, which also used a Xenon lamp for sample excitation, placed between the sample holder and emission monochromator. The TR decay was recorded using the same spectrometer equipped with a femtosecond laser as excitation source (Satasuma Display AMPLITUDE) and a High-Speed hybrid photodetector.

Estimation of missing linker defects (from TGA)
The MOF’s thermogram was normalized to have its final weight (WLfinal) equivalent to 100%. The first derivative curve (dTG) was plotted in parallel to identify the temperature beyond which no more trapped solvent and modulator molecules could be lost (Tlink). The MOF structure, having originally a general formula of Zr6O4(OH)4(BTCA)6 (BTCA: 1,2,4,5-benzenetetracarboxylic acid), will only be composed of dehydroxylated zirconium nodes connected to a number of linkers (NLexp), and NLexp <NLtheoretical in such defective structures. Under an oxygenated atmosphere, this UiO-66(COOH)2 structure is thought to be decomposed giving 6 ZrO2 which mass corresponds to the final plateau in the thermogram. The following series of formulas was then applied:

WLPth = (MWMOF / MW6ZrO2) × WLfinal (1)

WLPth : theoretical weight loss
MWMOF : MOF’s molecular weight
MW6ZrO2 : Molecular weight of the remaining zirconium oxide
WLfinal : Final weight, equivalent to 100%
If the UiO-66(COOH)2 structure was ideal, the number of linkers (NLth) per MOF formula would be equal to 6, and the weight loss per linker (denoted as WLlink) can be calculated by:

WLlink = (WLPth - WLfinal) / NLth (2)

In this work, UiO-66(COOH)2, synthesized using formic acid as a modulator, is defective, and the actual and missing number of linkers (NLexp and NLmis respectively) can be calculated according to the following formulas, taking WLPexp to be the normalized weight that corresponds to Tlink.

NLexp = (WLPexp - WLfinal) / WLlink (3)
NLmis = NLth - NLexp (4)

all parameters used in the aforementioned calculations are included in Supplementary Table S1.

Photocatalytic tests
A Sandwich IR reactor-cell was employed to assess the performance towards the FAc dehydrogenation under different irradiation conditions at ambient temperature. The catalyst, prepared as a self-supported pellet by weighing ca. 20 ± 1 mg of the MOF or a ratio of 1:1 (SiO2:Cu/Cu2O) composite of a thickness of ~65 μm were activated first at 25 °C under visible light irradiation (>390–720 nm) (unless described otherwise in the text) under Ar flow (25 cc·min−1). The photocatalytic reaction was conducted in vapor phase under a flow of 2400 ppm FAc in Ar with a total flow rate of 25 cc·min−1. The adsorption of FAc on the catalyst surface was performed in the dark prior to irradiation steps. Finally, different irradiation conditions were applied to the cell. The output gases from the IR reactor-cell were analyzed simultaneously by an FTIR spectrometer (Thermo-Nicolet NEXUS 670 FTIR) equipped with a mercury cadmium telluride (MCT) detector (spectral resolution of 4 cm−1 and accumulating 64 scans) and by mass spectrometry (Quadrupole Pfeiffer Omnistar GSD301). The amount of CO2 production was calculated from the IR band area under 2395–2282 cm-1. The amount of H2 production was revealed from the mass spectrometer at m/z=2. Whereas, the catalyst (pellet) was monitored by FTIR throughout the whole catalytic processes.

To study the effect of single wavelength on the photocatalytic activity, a Xenon lamp (LC8 Hamamatsu, irradiance of 60 mW·cm⁻2) was used for visible irradiation (wavelength range 390–720 nm) and a Xenon lamp (LC8 Hamamatsu, irradiance of 70 mW·cm⁻2) for visible-near infrared irradiation equipped with a high-pass filter (≥390 nm). For different NIR wavelengths, a MAX-303 compact Xenon light source (irradiance of 170 mW·cm⁻2) equipped with various high-pass filters (λ>810 nm, λ>800 nm, λ>750 nm, and λ>700 nm) was utilized. Depending on the irradiation wavelength being tested, the appropriate lamp was activated. To investigate the effect of combined wavelengths on the catalytic activity, a double optical fiber was connected to the two lamps. The optical fiber had a single inlet that was inserted into the reactor cell, as shown in Fig. S5A (configuration 1). It is important to show that the irradiated area of the pellet is ~1.6 cm2, where the peripheral 20% of the total pellet surface (2 cm2) was not irradiated due to the holder shadow effect. The global temperature of the catalyst was measured directly using a Raytek infrared camera after removing the KBr pellet from the reactor cell.

Quantification of the concentration of the reactants and products in FAc dehydrogenation reaction
The concentration of FAc in the gas phase was calculated using the surface area of the IR band at 1109−1101 cm−1 and the MS signals at m/z=45 and 46. The amounts of H2 were determined by its MS signal at m/z=2 after correction for water contributions.
The conversion of FAc, expressed as a percentage or (mmol/g. of photocatalyst/ unit time) or (mmol/ g. of photocatalyst/ irradiated surface area) and the selectivity (expressed as a percentage) were determined using equations 5–8 at steady state, based on the calibration curve for various reaction products. The formic acid conversion and conversion rate were calculated as follows:

FAc conversion (%) = ([FA]0 (ppm) - [FA]t (ppm)) / [FA]0 (ppm) × 100 (5)

where, [FA]0 is the initial concentration of FAc sent to the cell and [FA]t was the concentration of FAc at a certain time (t) during the reaction.

FAc conversion rate (mmol·g⁻¹·h⁻¹) = (Total flow (L·h⁻¹) × [FA]0 (ppm) × 10⁻⁶ × conversion% × 1000) / (g.mol⁻¹ × 1 L·g⁻¹ × mcat (g)) (6)

FAc conversion rate (mmol·cm⁻²·h⁻¹) = (FAc conversion rate mmol·g⁻¹·h⁻¹ × mcat (g)) / S irradiated cm2 (7)

Selectivity (%) = (Concentration of target product (ppm) / Concentration of FAc converted (ppm)) × 100 (8)

Calculation of apparent quantum yield of H2 production
The apparent quantum yield for H2 production was determined according to the following equation:

Φ H2 = (2 × molecules of H2 produced (molecules/s)) / (Incident photons (molecules/s)) (9)

With f = S(irradiated pellet) / S(reactor used for actinometry experiment) = 1.6 cm2 / 18.1 cm2 = 0.0884

The amount of H2 produced was quantified by mass spectrometry (MS), and the incident photon flux was determined using potassium ferrioxalate (K3Fe(C2O4)3) as a chemical actinometer. A 15 mL (V1) aqueous solution containing 0.15 M K3Fe(C2O4)3 and 0.05 M H2SO4 was prepared and introduced into the photocatalytic reactor. Prior to irradiation, a 0.180 mL aliquot (V2) was withdrawn and mixed with 2 mL of a phenanthroline buffered solution (0.015 M phenanthroline in 0.5 M H2SO4), followed by the addition of distilled water to reach a total volume of 25 mL (V3). The mixture was kept in the dark for 90 minutes to allow complete formation of Fe–phenanthroline complexes, establishing the baseline. Subsequently, the reactor was irradiated for varying durations (30–180 s) using the 150 W Xe lamp (≥390 nm) applied in the photocatalytic experiments. The absorbance at λmax = 500 nm was then used to determine the number of Fe2+ ions (nFe2+) generated during irradiation according to the following equation:

nFe2+ = (V1 × V3 × (A - A0)) / (V2 × ε × 1000) (10)

where V1 is the volume of the irradiated actinometer solution (mL), V2 is the aliquot volume taken for analysis (mL), and V3 is the final dilution volume (mL). A represents the measured absorbance at 500 nm, while A0 corresponds to the absorbance of a non-irradiated (dark) sample. ε is the molar extinction coefficient of the Fe2+-phenanthroline complex (11,100 L·mol⁻¹·cm⁻¹).

The number of incident photons per second (I0) was then determined according to the following equation:

Incident Photons (s⁻¹) = (6.022 × 10^23 × nFe2+) / (Φt × t) (11)

where Φt denotes the quantum yield for Fe2+ formation (average value of 1.1), and t represents the irradiation time (s). The calculated photon flux incident on the K3Fe(C2O4)3 solution was 4.10176 × 10^16 s‒1 under the experimental conditions.

Based on these calculations the Φ H2 = 12.8 ± 0.37%. The measurements were repeated three times to ensure reproducibility.

Electrochemical experiments
Electrochemical and photoelectrochemical measurements were performed using an Autolab PGSTAT302N potentiostat (biologic SP200) in a standard three-compartment cell equipped with a standard three-electrode configuration: 1 cm2 FTO substrate with the catalyst film as the working electrode, while a platinum wire served as the counter electrode and a saturated calomel electrode (Hg/Hg2Cl2/sat. KCl) was used as the reference electrode. The electrolyte consisted of 0.5 mol·dm⁻3 Na2SO4 aqueous solution, adjusted to pH 6.4 ± 0.2 and saturated with argon prior to use. Catalyst films were prepared by dispersing 2 mg of the photocatalyst in 0.5 mL ethanol, followed by 30 min sonication. A 65 µL aliquot of the suspension was drop-cast onto the FTO substrate (1 cm2) and dried overnight in air. For photoelectrochemical measurements, a Xenon lamp (LC8 Hamamatsu, irradiance 60 mW·cm⁻2; 0.6 Sun AM1.5) provided visible irradiation in the 390–720 nm range. Photocurrent vs. time was recorded under chopped illumination at an applied potential of 1.12 V vs. RHE without iR correction. Electrochemical impedance spectroscopy was performed in the frequency range of 0.1 Hz to 200 kHz at −0.38 V. Prior to all measurements, the electrolyte was purged with argon for 1 h.

The potential was given with respect to RHE using the following equation:

ERHE = ESCE + 0.241 V + 0.059 V × pH (12)

The resistance measurements are tabulated in supplementary Table S3.

Operando XAS
The operando quick-XAS experiments at Cu (8979 eV) and Zr (17998 eV) K-edges were performed at the ROCK beamline (SOLEIL synchrotron)44. The monochromatic beam was obtained using Si(111) quick-EXAFS channel-cut monochromators aligned at 12.3° and 6.1° and an oscillation amplitude of 3.9° and 0.6° for Cu and Zr K-edges, respectively. The monochromators were calibrated by setting the first inflexion point of a Cu and a Zr metallic foil to 8979 eV and 17998 eV, respectively. The harmonic rejection at Cu and Zr K-edges was ensured using respectively B4C and Pd coated mirrors, that were installed before and after the monochromator at a grazing incidence of 2.8 mrad with respect to the pink and monochromatic beam. The oscillation frequency of the monochromators was set to 2 Hz, allowing to record one XAS spectrum in 250 ms. Measurements were performed in transmission mode using three ionization chambers (Ohyo Koken) filled with N2 and a mixture of N2/Ar (50/50) for Cu and Zr K-edge, respectively. The two first ionization chambers were used to measure the spectra of the sample, and a copper and zirconium foil were placed on the third to be used as reference for further energy calibration and alignment. The sample was prepared and the photocatalytic reaction was performed following the same procedure as for operando FTIR, except for the cell which was a reactor developed at ROCK beamline and suitable for operando quick-XAS under light irradiation. The gases outcoming from the cell were analyzed using a Pfeiffer mass spectrometer.

Data extraction, energy alignment, and normalization were performed using the Python-based graphical interfaces extract_gui and normal_gui, developed at the ROCK beamline45. The FASTOSH software is utilized for the analysis of the XAS data46.

Data availability
All data generated in this study are provided in the source data file. Source data file is provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors investigate photocatalytic logic gate behavior in a Cu-metalated UiO-66(COOH)₂ MOF system. To gain mechanistic insights, the study examines the light-induced catalytic activity of the material under various irradiation conditions, using gas-phase formic acid (FAc) dehydrogenation at ambient conditions as a model reaction. The authors report that no detectable activity occurs under visible (>390–720 nm) or near-infrared (>700 nm) light alone, but a significant enhancement is observed when both irradiation sources are applied (≥390 nm). The material has been appropriately synthesized and characterized, and the photocatalytic activity has been systematically studied. The results are novel and scientifically interesting, and the manuscript could be suitable for publication in Nature Communications after addressing the following concerns, which would help improve the overall quality and robustness of the work:.\n1. The manuscript states that the conversion of Cu²⁺ to Cu⁺ occurs exclusively under visible light irradiation during the dynamic restructuring of Cu-metalated UiO-66(COOH)₂. The authors should provide a clearer explanation or hypothesis for this selective behavior, supported by experimental or literature evidence..\n2. For more accurate mechanistic interpretation, the authors are advised to monitor the surface temperature of the catalyst under full-spectrum irradiation. In addition, thermal control experiments should be performed to rule out the influence of temperature on the observed photocatalytic activity.\n3. The current characterization is insufficient to conclusively verify the formation of Cu₂O in the restructured catalyst. Additional structural and chemical analyses, such as XRD, XPS, and TEM, are recommended to strengthen this claim.\n4. The contribution of the UiO-66(COOH)₂ framework to the photocatalytic performance remains unclear. The authors are encouraged to explore and discuss its specific role within the restructured catalyst.\n5. In Figure S1, the authors estimate the defect number of the as-synthesized UiO-66 to be 1.8, based on weight loss comparisons. It is recommended that the authors provide the detailed calculation or derivation process to enhance transparency and reproducibility.\n6. To contextualize the photocatalytic performance, a comparison with similar Cu-based or MOF-based photocatalysts from the literature should be included. Presenting this comparison in a tabulated format would offer clearer insights into the advantages and novelty of the current system.\n7. To confirm the structural integrity and reusability of the catalyst, appropriate post-reaction characterizations (e.g., XRD, BET, TEM) should be performed and discussed.", "reviewer_2": "The manuscript by El-Roz and co-workers describes the synthesis, characterization of a UiO-66(COOH)2-Cu framework and its use in the photocatalytic formic acid dehydrogenation reaction. Interestingly, the UiO-66(COOH)2-Cu displayed a rarely photocatalytic logic gate behavior. The mechanisms were clearly uncovered by operando FT-IR, operando XAS and a series of control experiments. The presented manuscript is innovative and well organized, which can give a good reference for the design of novel photocatalysts. Hence, this work is worthy of publishing in Nature Communications after considering the following issues:\n1) The type of N2 sorption isotherms, hypothesis loop, and pore size distribution are suggested.\n2) In order to confirm the successful incorporation of Cu(II) into the framework of UiO-66(COOH)2, energy-dispersive X-ray (EDX) mapping analyses should be further conducted.\n3) The photocatalytic dehydrogenation of DCOOH and 13C-labeled H13COOH should be conducted to gain a deep understanding of the reaction mechanism.\n4) The structural stability and catalytic behavior of UiO-66(COOH)2-Cu in the photocatalytic cycles should be discussed.\n5) Line 181: To confirm the pure structure of Cu0/Cu2O nanocomposite, powder XRD is not enough, XPS spectroscopy is also needed.\n6) The Cu0/Cu2O nanocomposite showed similar photocatalytic activity to that observed in UiO-66(COOH)2-Cu, thus what is the role of MOF in the photocatalytic reaction. In addition, to further confirm the Cu species (Cu0/Cu⁺) acting as the catalytic active site, other control experiments should also be conducted by using UiO-66(COOH)2 as catalyst or reducing the Cu content in the UiO-66(COOH)2-Cu.", "reviewer_3": "In this manuscript, the authors report the fundamental studies on the catalytic mechanism of Cu-metalated MOFs that has been demonstrated as photocatalyst for formic acid dehydrogenation. It is interesting that this system shows photocatalytic activity neither under visit light illumination nor under NIR illumination but under the combined visible-NIR illumination. The thorough spectroscopic studies including in situ FTIR and X-ray absorption spectroscopy were performed to understand the mechanism and cascade logic-gate mechanism was proposed. These findings are interesting and fundamental studies are convincing to support their conclusions. This reviewer would recommend publication in Nature Communication after the authors address my minor concerns below.\nThe in situ XAS studies are impressive but the reversibility of the system under light conditions is not discussed. This can be easily measured using in situ XAS after turning off the light."}, "author_response": "Response to the Reviewers’ comments (Manuscript ID: NCOMMS-25-41463)\nWe thank all the reviewers for their valuable comments. In view of their feedback, new experiments\nwere performed, and the manuscript has been revised accordingly. Our point-by-point responses are\ngiven below. Modifications made to the text are highlighted in yellow in the revised version of the\nmanuscript.\nSummary on the new experiments performed: To address the comments of the reviewers on this work,\nvarious additional experiments were performed, as follows,\n Photocatalytic tests (including isotopic tests) using Operando FT-IR spectroscopy\n Thermal catalytic tests using Operando FT-IR spectroscopy\n Photoluminescence and time-resolve spectroscopy\n Photo-electrochemical experiments\n Scanning electron microscopy and EDX mapping\n Powder X-ray diffraction spectroscopy\n Transmission electron spectroscopy\nReviewer #1 (Remarks to the Author):\nOverall comments:\nIn this manuscript, the authors investigate photocatalytic logic gate behaviour in a Cu-metalated UiO-\n66(COOH) MOF system. To gain mechanistic insights, the study examines the light-induced catalytic\n2\nactivity of the material under various irradiation conditions, using gas-phase formic acid (FAc)\ndehydrogenation at ambient conditions as a model reaction. The authors report that no detectable\nactivity occurs under visible (>390–720 nm) or near-infrared (>700 nm) light alone, but a significant\nenhancement is observed when both irradiation sources are applied (≥390 nm). The material has been\nappropriately synthesized and characterized, and the photocatalytic activity has been systematically\nstudied. The results are novel and scientifically interesting, and the manuscript could be suitable for\npublication in Nature Communications after addressing the following concerns, which would help\nimprove the overall quality and robustness of the work.\nResponse: We appreciate the reviewer’s time in evaluating the manuscript. We are pleased that the\nreviewer has appreciated our work, novelty, and recommended our work for possible publication in\nNature Communications. We have addressed all the reviewer’s concerns in the revised manuscript.\nPlease find below our point-by-point responses.\n1. The manuscript states that the conversion of Cu²⁺ to Cu⁺ occurs exclusively under visible light\nirradiation during the dynamic restructuring of Cu-metalated UiO-66(COOH)₂. The authors should\nprovide a clearer explanation or hypothesis for this selective behaviour, supported by experimental or\nliterature evidence.\nResponse: We thank the reviewer for this insightful comment. As noted in the original manuscript, the\nformation of Cu+ under visible-light irradiation was already demonstrated through X-ray absorption\nspectroscopy (XAS), which showed clear features consistent with the photoinduced reduction of Cu2+\nto Cu+ (Figure 3C). We recognized the importance of directly demonstrating its selective formation only\nunder visible-light irradiation. So, we performed time resolved photoluminescence analysis to further\nunderstand this selective transformation. Results confirm a charge transfer from the LUCO of the UiO-\n66(COOH) to the Cu2+ (Figure 6). The new findings have been added to the revised manuscript as\n2\nfollows.\nOn page 25 line 10: “To gain deeper insight into the charge carrier dynamics of UiO-66(COOH) -Cu,\n2\nwe first examined the photo-electrochemical properties, both in its pristine form and after Cu\nmetalation. First the transient photocurrent response curve (Figure 6A) were employed to evaluate the\nseparation efficiency and transfer characteristics of the photo-generated charges. The UiO-66(COOH)\n2\nexhibits a lower photocurrent response compared to Cu metalated version. In contrast, UiO-\n66(COOH) -Cu exhibits a significantly increased photocurrent density, showing around 1.5-times\n2\nenhancement compared to pristine sample under similar irradiation condition. This indicates an\nimprovement in charge separation efficiency upon Cu incorporation which agrees with its superior\nactivity compared to the pristine sample showing a possible charge transfer. EIS studies by using\nNyquist plots were then carried out to determine the charge transfer resistance as illustrated in Figure\n6B. The Nyquist plots also clearly show that the charge transfer resistance is significantly reduced after\nadding Cu.\nFurthermore, the steady-state and time-resolved photoluminescence measurements (PL and TR-PL,\nrespectively) were performed to evaluate the charge carrier dynamics. The emission maps of UiO-\n66(COOH) and UiO-66(COOH) -Cu at steady state upon excitation from 340 nm to 650 nm are\n2 2\nillustrated on Figure 6C, D. A prominent emission band at 430 nm is observed with the UiO-\n66(COOH) . This band is attributed to the ligand-cluster charge transfer (LCCT) from the BTCA linker\n2\nto the Zr-oxo-clusters, generating a charge separated state (electrons and holes).35-36 Upon Cu\nincorporation, a notable decrease in luminescence intensity was observed compared to the UiO-\n66(COOH) pristine sample, indicating that Cu incorporation effectively suppressed electron–hole\n2\nrecombination (Figure 6E).37 This emission band was then monitored under 343 nm laser excitation\n(Figure 6F). A longer lifetime decay in the UiO-66(COOH) -Cu sample was observed compared to the\n2\npristine from 2.1 ns to 5.2 ns (Error! Reference source not found. S3), suggesting enhanced charge\ncarrier separation attributed to possible charge transfer from the UiO-66(COOH) framework to the\nReference source not found.G. Upon visible light irradiation and in presence of FAc, an electron is\nphoto-excited from the highest occupied crystal orbital (HOCO) of UiO-66(COOH) to its lowest\n2\nunoccupied crystal orbital (LUCO), generating a hole in the HOCO. The excited electron in the LUCO\nis subsequently transferred to a coordinated Cu²⁺ center, reducing it to Cu⁺ (Figure 6G, intermediate\n1b), while the photo-generated hole oxidizes HCOO‒ to HCOO• at the HOCO of UiO-66(COOH)₂\nframework. In the presence of FAc and both NIR and visible light, the neighboring carboxylates,\ncoordinated to Cu (I), rearrange (Figure 6G, intermediate 1b to 2c) which promotes the condensation\nof neighboring Cu (I) centers through oxide formation.”\nFigure 1. Investigation of the charge carrier dynamics in the UiO-66(COOH) -Cu (A), (B)\n2 2\npotential of 0 V upon irradiation with visible light and their Nyquist plots respectively. (C), (D)\n3D plots of the PL emission maps of UiO-66(COOH) and UiO-66(COOH) -Cu upon\n2 2\nexcitation from 320 to 650 nm respectively. (E) The PL spectra of the samples upon excitation\nat 340 nm (inset: their corresponding normalized intensities), (F) their corresponding decay\nlife time at 430 nm upon 343 nm excitation and (G) the proposed charge carrier mechanism.\n2. For more accurate mechanistic interpretation, the authors are advised to monitor the surface\ntemperature of the catalyst under full-spectrum irradiation. In addition, thermal control experiments\nshould be performed to rule out the influence of temperature on the observed photocatalytic activity.\nResponse: We appreciate the reviewer’s insightful comment regarding the thermal control experiments\nand surface temperature of the catalyst under full-spectrum irradiation. In addition, we have performed\nan additional thermal controlled experiment. The new data and related data and discussion were added to the\nrevised manuscript as follow:\nOn page 23 line 20: “To investigate the thermal catalytic activity of the material, thermal controlled\nexperiments were performed in dark under similar conditions, showing no activity till 200 °C\n(Supplementary Fig. S28A). This revealed that the catalytic activity under ambient condition was\nattributed majorly to light effect on the material. The surface IR at steady state of each temperature\nrevealed anhydride band formation (1802, 1851 cm-1) at 250 °C, depicting the importance of anhydride\nformation for the catalytic activity (Supplementary Fig. S28B). Figure S28C shows some formation of\nCO (due to dehydration of FAc) which increases with reaction temperature. This could be due to slight\nvariation in the mechanism under thermal condition. A similar thermal controlled catalytic experiment\nafter forming the active MOF catalyst (of Cu0/Cu+ binary system) was also performed. That\ndemonstrated catalytic activity at relatively lower temperature (at 200 °C) compared to bare MOF\ncatalyst under thermal condition (Supplementary Fig. S28C), suggesting the photothermal effect along\nwith its photocatalytic nature.\nMoreover, the global surface temperature of the catalyst was measured using infra-red thermal imaging\n(Supplementary Fig. S28D and MP4 video). The global temperature of UiO-66(COOH) -Cu shows\n2\nslow rise from RT to ~ 34.4 °C (steady state) in ~ 50 s under visible irradiation, and a rapid temperature\nincrease occurred to ~ 54 °C under NIR irradiation. Whereas, under both visible and NIR, the\ntemperature was increased significantly to ~ 62 °C. On contrast, UiO-66(COOH) , as a reference\n2\nmaterial, showed temperature rise to ~25 °C, ~26 °C, and ~29 °C in presence of visible, NIR, and\nvisible + NIR irradiation. This small temperature rise could be accounted as sample heating solely due\nto light irradiation. Therefore, the significant temperature enhancement in Cu metalated MOF confirms\nits photothermal effect. Nevertheless, this temperature rise is insufficient to reach the catalytic activity\n(Supplementary Fig. S28A), indicating that the MOF’s activity originates from a photocatalytic\nmechanism complemented by photothermal contributions. It should be noted that the local heating\ncould be much higher than the global temperature of the sample. However, this would not be much\nrelevant towards the overall photocatalytic activity of MOF.”\nFigure S28. (A) Rate of FAc conversion at steady state with increasing reaction temperature under\ndark, (B) the corresponding surface IR spectra at their steady state showing evolution of anhydride\nbands at 250 °C, (C) bar diagram of photo-reactivity and photo-selectivity comparison at different\ntemperature under various conditions (purple: in dark, gold: ≥ 390 nm light irradiation, green: at dark,\nfor restructured UiO66-(COOH) -Cu from photocatalytic reaction under similar condition (after active\n2\nconditions.\n3. The current characterization is insufficient to conclusively verify the formation of Cu O in the\n2\nrestructured catalyst. Additional structural and chemical analyses, such as XRD, XPS, and TEM, are\nrecommended to strengthen this claim.\nResponse: We appreciate the reviewer’s insightful comment regarding the need for more conclusive\nevidence to confirm the formation of Cu O in the restructured MOF catalyst. In the original manuscript,\n2\nthe presence of Cu O was supported by X-ray absorption spectroscopy (XAS) after simultaneous\n2\nirradiation of visible and NIR irradiation whereby the spectral features of both Cu+ and Cu0 were\nidentified. For more clarification, we conducted the HAADF-STEM analysis after the photocatalytic\nreaction, which confirmed the presence of Cu O (Figure S14). For better understanding, we plotted the\n2\nand metallic Cu, as reference.\nRegarding the PXRD, we were unable to visualize the diffraction pattern of Cu O in the post-catalytic\nTEM.\nThese new data have been included in the revised version of the manuscript as follows:\nOn page 17 line 7: “A comparison of the XANES and EXAFS spectra of the restructured UiO-\n66(COOH) -Cu with Cu O and metallic Cu, as reference, clearly demonstrated the formation of Cu O\n2 2 2\nand metallic Cu (Supplementary Fig. S23).”\n(A) (B)\nFigure S23. The operando XAS spectra of (a) UiO-66(COOH) -Cu at steady state during photocatalytic\n2 2\nXANES spectra comparison, (B) their corresponding Fourier Transform of EXAFS data.\nOn page 14 line 3: “TEM analysis has provided important insights into coordination changes occurring\nduring the reaction, particularly highlighting the formation of Cu O clusters in UiO-66(COOH) -Cu.\n2 2\nHigh-angle annular dark-field scanning TEM (HAADF-STEM) images (supplementary Fig. S14A)\ndisplay bright contrast regions corresponding to these copper clusters. Particularly the HRTEM image\n(supplementary Fig. S14B) showed monodispersed Cu O nanoparticles, which can be indexed to the\n2\ncubic Cu O phase (space group Pn3̅m, lattice parameter a = 0.426 nm; ICSD 172174), with particle\n2\nsizes of 1–2 nm, consistent with our previous findings.22 Notably, no Cu0 attributed to their high\ndispersion. The lack of Cu0 detection may also stem from their air sensitivity or possible amorphous\n(non-crystalline) nature. It should be noted that to mitigate beam-induced reduction of Cu O to Cu0\n2\nduring imaging, all measurements were performed using a low electron beam voltage. This precaution\nwas essential to ensure that the detected Cu species represent the true post-reaction state of the sample,\nfree from electron-beam artifacts.”\nFigure S14. (A) HAADF-STEM image of UiO-66(COOH) -Cu after reaction, showing morphological\n2\nchanges and formation of Cu-containing domains. Inset highlights lattice fringes corresponding to Cu O\n2\nwith an interplanar spacing of 0.25 nm, indexed to the (110) plane. (B) HRTEM image further\nconfirming the presence of crystalline Cu O nanodomains within the UiO-66 framework.\n2\n4. The contribution of the UiO-66(COOH) framework to the photocatalytic performance remains\n2\nunclear. The authors are encouraged to explore and discuss its specific role within the restructured\ncatalyst.\nResponse: We thank the reviewer for this constructive comment. As previously highlighted in the\nmanuscript, the use of the UiO-66(COOH) framework plays a key role in enhancing the dispersion of\nbridges, after restructuring, help stabilizing the Cu sites during the reaction, effectively preventing\nsintering. In addition, the new data (TR-PL and transient photocurrent) revealed enhanced charge\ntransfer upon Cu incorporation into UiO66-(COOH) structure as reported in our response on comment\n2\n1. This information is now highlighted and clarified in the revised version of the manuscript as\npreviously mentioned in comment 1.\n5. In Figure S1, the authors estimate the defect number of the as-synthesized UiO-66 to be 1.8, based\non weight loss comparisons. It is recommended that the authors provide the detailed calculation or\nderivation process to enhance transparency and reproducibility.\nResponse: We thank the reviewer for this insightful suggestion. The defect number estimation for UiO-\n66(COOH) is now detailed in the SI as follows:\n2\nreported procedure was followed (Chem. Mater. 28, 3749-3761 (2016)). In brief, the MOF’s\nthermogram was normalized to have its final weight (WL ) equivalent to 100%. The first derivative\nfinal\ncurve (dTG) was plotted in parallel to identify the temperature beyond which no more trapped solvent\nand modulator molecules can be lost (T ). The MOF structure, having originally a general formula\nlink\nof Zr O (OH) (BTCA) (BTCA:1,2,4,5-benzenetetracarboxylic acid), will only be composed of\n6 4 4 6\ndehydroxylated zirconium nodes connected to a number of linkers (NL ), and NL < NL in\nexp exp theoretical\nsuch defective structures. Under an oxygenated atmosphere, this UiO-66(COOH) structure is thought\n2\nto decompose giving 6 ZrO which mass corresponds to the final plateau in the thermogram. The\n2\nfollowing series of formulas was then applied:\n𝑀𝑊\n𝑀𝑂𝐹\n𝑊𝐿𝑃 = ×𝑊𝐿\n𝑡ℎ 𝑀𝑊 𝑓𝑖𝑛𝑎𝑙\n6𝑍𝑟𝑂2\nWLP : theoretical weight loss.\nth\nMW : MOF’s molecular weight.\nMOF\nMW : Molecular weight of the remaining zirconium oxide.\n6ZrO2\nWL : Final weight, equivalent to 100%.\nfinal\nIf the UiO-66(COOH) structure was ideal, the number of linkers (NL ) per MOF formula would be\n2 th\nequal to 6, and the weight loss per linker (denoted as WL ) can be calculated by:\nlink\n𝑊𝐿𝑃 −𝑊𝐿\n𝑡ℎ 𝑓𝑖𝑛𝑎𝑙\n𝑊𝐿 =\n𝑙𝑖𝑛𝑘 𝑁𝐿\n𝑡ℎ\nIn this work, UiO-66(COOH) , synthesized using formic acid as a modulator, is defective, and the actual\n2\nand missing number of linkers (NL and NL respectively) can be calculated according to the\nexp mis\nfollowing formulas, taking WLP to be the normalized weight that corresponds to T .\nexp link\n𝑊𝐿𝑃 −𝑊𝐿\n𝑒𝑥𝑝 𝑓𝑖𝑛𝑎𝑙\n𝑁𝐿 =\n𝑒𝑥𝑝 𝑊𝐿\n𝑙𝑖𝑛𝑘\n𝑁𝐿 = 𝑁𝐿 −𝑁𝐿\n𝑚𝑖𝑠 𝑡ℎ 𝑒𝑥𝑝\nThe following table includes all parameters used in the aforementioned calculations:\nSupplementary Table 1: Parameters used for defect number estimation in UiO-66(COOH) .\nWLP 296.5%\nth\nWLP 237.3%\nexp\nWL 100%\nfinal\nT 294°C\nlink\nNL 4.2\nexp\nNL 6\nth\nNL 1.8\nmis\n6. To contextualize the photocatalytic performance, a comparison with similar Cu-based or MOF-based\nphotocatalysts from the literature should be included. Presenting this comparison in a tabulated format\nwould offer clearer insights into the advantages and novelty of the current system.\nResponse: We thank the reviewer for the constructive suggestion to include a comparison of our MOF-\nbased photocatalyst with similar systems reported in the literature. While the primary focus of this\nmanuscript is to provide a detailed understanding of the wavelength-dependent mechanism underlying\nthe photocatalytic dehydrogenation of FAc, we have nonetheless included a comparative table in the\nSupporting Information (Table S2) in response to the reviewer’s comment. This table summarizes key\nperformance metrics and characteristics of relevant MOF-based photocatalysts reported in the literature.\nThis comparison underscores several key advantages of our photocatalytic system. Operating under\ncontinuous-flow gas-phase conditions, the catalyst exhibits excellent stability without any significant\ndeactivation over an extended period of experiment (72 hours). Furthermore, using copper as the active\nmetal offers a cost-effective and earth-abundant alternative to noble or rare metals typically employed\nin other MOF-based photocatalysts, such as Au or InS . Moreover, unlike systems that require UV light\n2\nactivation, our photocatalyst functions efficiently under simulated solar light, improving both the\nsustainability and practical applicability of the system.\nThese findings were added to the modified version of the manuscript as follows:\nOn page 12 line14: “Notably, the photocatalytic performance demonstrated in this study exceeds that\nof previously reported MOF-based photocatalysts for the dehydrogenation of formic acid, particularly\nwith respect to product selectivity, operational stability, the incorporation of a cost-effective and earth-\nabundant metal (Cu), and the ability to function under simulated solar irradiation (Supplementary\nTable S2 for comparative data).”\nTable S2. Conversion rates of FAc dehydrogenation to H₂ using homogeneous and\nheterogeneous photocatalysts under various reaction conditions.\nPhotocatalyst Light Reaction H Apparent Dehydrogenation Stability test Remarks Ref.\n2\nsource/Temperature phase production quantum selectivity %\nrate in effeciency\nmmol.g- (Φ)\n1.h-1\n(in mmol.\ng-1.h-1cm-\n2)\nCdS@ZIF-8 Xe lamp (300 W> Liquid 0.9 - 88 - Toxicity, no [4]\n420 nm), RT stability\nreported.\nCo-sal-NH- Xe lamp (300 W> Liquid 18.7 Φ = 99.9 No deactivation Rare metal [5]\n2 420nm\nMIL- 420 nm), RT 3.8 after 3 cycles used, corrision\n68@InS (24h/cycle) of InS and Co\n2 3 2 3\nleaching was\ndetected\nAu@Pd/MOF Xe lamp (500 W, Liquid 5.4 - 100 Slight decrease Use of [6]\n(UiO- 320 mW/cm2, > of activity after expnesive noble\n66(Zr Ti ) 420 nm), RT 3 cycles metals and no\n85 15\n(1h/cycle) isotopic\nexperiments\nwere performed\nto confirm the\norigin of the\nevolved\nproducts\nMIP-177-LT Xe lamp (300 W), Liquid 0.35 Φ = >99 No deactivation Activity limited [7]\n238nm\nRT 22 after 4 cycles to UV irradtion\n(2h/cycle)\nUiO-66- Xe lamp (150W, 71 Vapor 6.1 (3.8) Φ >99.9 No deactivation High stability This\nvisible light\n(COOH)-Cu mW/cm2, > 390 under =10.6 after under continous work\n2\nnm), RT continuous (24h/cycle) reaction\nflow conditions\n7. To confirm the structural integrity and reusability of the catalyst, appropriate post-reaction\ncharacterizations (e.g., XRD, BET, TEM) should be performed and discussed.\nResponse: We thank the reviewer for this insightful comment. We have performed stability test for\nphotocatalytic performance of our MOF. The steady dehydrogenation rate was observed over three\nconsecutive cycles (24 h. each.)\nFigure S12. Photocatalytic dehydrogenation rate of UiO-66(COOH) -Cu from FAc over three\n2\nconsecutive cycles (24 hours each) showing no significant deactivation.\nIn response to post-reaction characterizations, we have conducted TEM analysis (explained in response\n3) and powder XRD after the photocatalytic reaction. The crystallinity and phase purity remained intact\nafter catalysis, as revealed from the PXRD. These experiments support the reusability and structural\nintegrity of MOF after photocatalytic reaction.\nFigure S13: Powder XRD of UiO-66(COOH) -Cu before and after photocatalytic reaction.\nof material remaining after the reaction was insufficient for measurement (approximately 20 mg of\ncatalyst was used in the photocatalytic experiment).\nThe following texts have been included in the manuscript:\nOn page 12 line 10: “Furthermore, the photocatalytic stability was evaluated under continuous-flow\nconditions by monitoring the catalyst’s activity over an extended period of 72 hours, during which no\nsigns of deactivation were detected. These results align with our previous study,22 in which the system\nexhibited excellent stability and maintained high activity over three consecutive cycles (24 hours each)\nwithout any significant deactivation (Supplementary Fig. S12).”\nOn page 13 line 18: “PXRD analysis indicated that UiO-66(COOH) -Cu retained its crystallinity and\n2\nphase purity after photocatalytic reaction (Supplementary Fig. S13), suggesting the structural stability\nof the material.”\nReviewer #2 (Remarks to the Author):\nOverall comments:\nThe manuscript by El-Roz and co-workers describes the synthesis, characterization of a UiO-\n66(COOH) -Cu framework and its use in the photocatalytic formic acid dehydrogenation reaction.\n2\nmechanisms were clearly uncovered by operando FT-IR, operando XAS and a series of control\nexperiments. The presented manuscript is innovative and well organized, which can give a good\nreference for the design of novel photocatalysts. Hence, this work is worthy of publishing in Nature\nCommunications after considering the following issues:\nResponse: We value the reviewer’s time for evaluating our manuscript. We are pleased to hear our\nwork to be innovative and potentially sound for publication. Here are our responses of the reviewer’s\nconcerns that have also been added to the revised manuscript.\n1) The type of N sorption isotherms, hypothesis loop, and pore size distribution are suggested.\nN sorption isotherms are now added to the manuscript and the pore size distribution is now included\n2\nin the Supporting Information file. The following paragraph was modified in the Results section to\nhighlight these additional details.\nOn page 6 line 21: “UiO-66(COOH) demonstrated a type I N sorption isotherm, revealing its\n2 2\nmicroporous nature, that was further validated by its pore size distribution profile highlighting the\npresence of small pores of approximately 1.7 nm (Supplementary Fig. S2). Upon post-synthetic Cu\nmetalation, a decrease in surface area from 239 to 32 m2/g was noticed, accompanied by a decrease in\npore volume from 0.22 to 0.11 cm3/g due to Cu (II) species occupying the microporous framework’s\nvoid (Figure 1D).”\n0.007\ng) 0.006\n3/\nm\nc 0.005\ne\n(\nm\nu 0.004\nol\nV\ne 0.003\nor\nP\nal 0.002\nt\ne\nm 0.001\ne\ncr\nI n 0.000\n−0.001\n0 20 40 60 80 100 120\nPore Width (Å)\nFigure S2: Pore size distribution of UiO-66(COOH)\n2\n2) In order to confirm the successful incorporation of Cu (II) into the framework of UiO-66(COOH) ,\n2\nenergy-dispersive X-ray (EDX) mapping analyses should be further conducted.\nResponse: We thank the reviewer for the fruitful suggestion regarding the need to confirm the\nsuccessful incorporation of Cu (II) into the UiO-66(COOH) framework using EDX mapping analysis.\nvisualization of the elemental distribution of Cu2+. The newly added SEM-EDX results show a\nhomogeneous distribution of both Zr and Cu throughout the UiO-66(COOH)₂-Cu crystals, confirming\nuniform metalation and supporting the successful incorporation of Cu into the framework\n(Supplementary Fig. S1). These results were highlighted in the modified manuscript as follows:\nOn page 6 line 17: “To further verify the incorporation of the Cu to the framework, the EDX mapping\nanalysis on the UiO-66(COOH)₂-Cu sample was performed. This confirmed the successful\nincorporation of copper into the MOF structure. The elemental maps display a homogeneous\ndistribution of both Zr and Cu throughout the MOF crystals, indicating uniform metalation\n(Supplementary Fig. S1). Similar data were also reported in our previous work.22”\nFigure S1: SEM-EDX of UiO-66(COOH)₂-Cu showing homogeneous distribution of each elements\nthroughout the sample.\n3) The photocatalytic dehydrogenation of DCOOH and 13C-labeled H13COOH should be conducted to\ngain a deep understanding of the reaction mechanism.\nResponse: We appreciate the reviewer’s suggestion to perform isotopic-label experiments to deepen\nthe mechanistic understanding of the photocatalytic dehydrogenation of FAc. In response, we conducted\ncontrol reactions under identical photocatalytic conditions using H13COOH and DCOOH. These\nexperiments allowed us to verify that HCOOH is the origin of the evolved gaseous products, CO and\n2\nexperiments were conducted and yielded comparable outcomes, further validating the mechanism\nproposed. The new data have been added to the in the revised version of the manuscript as follows:\nOn page 28 line 4: “To gain deeper insight into the photocatalytic reaction mechanism and to determine\nthe origin of the reaction products, isotopic labeling experiments were conducted using H13COOH and\nDCOOH under the same reaction conditions as those employed for H12COOH. When H12COOH was\nused as the reactant, operando gas-phase FTIR confirmed the formation of 12CO (Supplementary Fig.\nS29B). In contrast, when H13COOH was used, operando FTIR showed the exclusive formation of 13CO ,\n2\nclearly demonstrating that the carbon in the evolved CO originates from the formic acid molecule\n2\n(Supplementary Fig. S29A). The FTIR spectra of the surface show no modification of the anhydride\nbands’ position confirming their formation from structural carboxylate functions (Supplementary Figs.\n29C, D). Further, experiments with DCOOH shows the formation of 12CO in gas-phase and more\n2\nsignificantly, a combined production of H , H-D and D with mass spectrometry signals at m/z = 2, 3\n2 2\nand 4, respectively (Supplementary Fig. S29B). These results confirm the involvement of deuterium-\nlabeled protons in the reaction pathway and the origin of H production. The observed isotopic\n2\ndistributions of CO and H products strongly support the dehydrogenation mechanism and confirm\n2 2\nthat both CO and H originate from the formic acid feedstock. These findings are consistent with our\n2 2\nprevious work, in which similar isotopic-labeling experiments were performed and led to comparable\nresults, further reinforcing the validity of the proposed mechanism.”\nFigure S29. (A) Gas phase FTIR spectra of UiO-66(COOH) -Cu at steady state during photocatalytic\ncorresponding MS intensity comparison at m/z = 2 (for H ), 3 (for H-D), and 4 (for D ), (C) the spectra\n2 2\nwith 13CHOOH and when exchange with 12CHOOH and (D) the spectra with DCOOH and when\nexchange with 12CHOOH.\n4) The structural stability and catalytic behaviour of UiO-66(COOH) -Cu in the photocatalytic cycles\n2\nshould be discussed.\nResponse: We thank the reviewer for this important comment regarding the structural stability and\ncatalytic performance of UiO-66(COOH)₂-Cu during photocatalysis. In response to this comment, we\nassessed the long-term stability of the catalyst under continuous-flow condition over a period of 72 h.\n(24 h. /cycle). No significant deactivation was observed throughout the catalytic cycles, confirming its\nexcellent durability under continuous flow reaction conditions. (Please refer to Response #Reviewer 1,\nComment 7).\n5) Line 181: To confirm the pure structure of Cu0/Cu O nanocomposite, powder XRD is not enough,\nResponse: We thank the reviewer for this helpful suggestion regarding the need for XPS analysis to\nconfirm the composition of the Cu0/Cu O nanocomposite. Due to time constraints during the revision\n2\nprocess, we were unfortunately unable to access XPS facilities as is not available in our laboratories.\nHowever, the nature of the synthesized Cu0/Cu O composite was verified by the XAS analysis which\n2\nprovides reliable information on both oxidation state and local coordination environment. The XAS\nspectrum of the composite closely matched that of a Cu O reference, supporting the presence of Cu⁺\n2\nspecies. Moreover, Fourier transform of EXAFS of synthesized Cu0/Cu O showed a very small hump\n2\nat ~ 2.22 Å, suggesting the presence of very small amount of metallic Cu along with Cu O. The\n2\nformation of little metallic Cu during Cu O synthesis is logical due to sintering process and use of\n2\nexcess FAc (oxidizing agent). That’s the reason, we described the synthesized material as Cu0/Cu O\n2\ninstead of pure Cu O to obviate any further ambiguity. Also, the Cu K-edge XANES spectrum might\n2\nnot clearly resolve the weak 1s → 4p transition associated with Cu0, as it is likely overshadowed under\nintense 1s → 4p feature of Cu+. These findings, together with the powder XRD results, confirm the\nformation of a Cu0/Cu O nanocomposite. The manuscript has been revised as follows:\n2\nspectrum of the pristine material showed a pre-edge peak at ~ 8982 eV corresponding to Cu (I) dipole-\nallowed 1s → 4p transition (Supplementary Fig. S9A). The FT- EXAFS signal revealed two nearest\ncoordination shells of Cu absorber at ~ 1.47 Å and ~ 2.78 Å attributed to Cu – O and Cu – Cu single\nscattering of Cu O material (Supplementary Fig. S9B). A little hump at ~2.22 Å could suggest the\n2\npresence of small metallic Cu in the synthesized material.”\nFigure S9: The operando XAS spectra of (a) pristine Cu0/Cu O, compared with (b) metallic Cu, as\n2\nreference. (A) Cu-K edge XANES spectra comparison, (B) their corresponding Fourier Transform of\nEXAFS data.\n6) The Cu0/Cu O nanocomposite showed similar photocatalytic activity to that observed in UiO-\n2\n66(COOH) -Cu, thus what is the role of MOF in the photocatalytic reaction. In addition, to further\n2\nconfirm the Cu species (Cu0/Cu⁺) acting as the catalytic active site, other control experiments should\nalso be conducted by using UiO-66(COOH) as catalyst or reducing the Cu content in the UiO-\n2\n66(COOH) -Cu.\npreviously discussed in the manuscript, one of the main advantages of using a MOF e.g. UiO-\n66(COOH)₂ is that it overcomes bare Cu O semiconductor materials in terms of aging under ambient\n2\nconditions. For example, a fresh UiO-66(COOH)₂-Cu sample retains nearly the same photocatalytic\nactivity for formic acid dehydrogenation even after two years of storage under ambient conditions,\nwhereas a freshly synthesized Cu O sample loses most of its activity within just two weeks due to Cu\n2\noxidation.Furthermore, under our controlled reaction conditions UiO-66(COOH) -Cu with its unique\n2\nanhydride-type bridging carboxylates showed that it help stabilize the Cu sites during the reaction,\neffectively minimizing sintering and preserving active site integrity.\nIt is worth highlighting that the amount of Cu in both UiO-66(COOH) -Cu and Cu O:SiO samples are\n2 2 2\nnot comparable. For the bare Cu O:SiO sample, a 20 mg pellet containing 50% Cu O (10 mg) exhibits\n2 2 2\nan activity of 7.6 mmol·g⁻¹·h⁻¹ when normalized to the Cu content. In contrast, UiO-66(COOH) –Cu,\n2\nwith a Cu loading of ~7 wt%, shows an activity of 6.1 mmol·g⁻¹·h⁻¹ when normalized to the total pellet\nmass. However, if we consider normalizing to the amount of Cu in the UiO-66(COOH) -Cu, a much\n2\nhigher activity is recorded. Nevertheless, we agree that this structural role alone may not fully explain\nthe observed photocatalytic behavior. Notably, a similar point was also raised by Reviewer 1. In\nresponse, we carried out additional investigations to probe the influence of the UiO-66(COOH)\n2\nframework on charge carrier dynamics. Specifically, we performed electrochemical measurements,\nsteady-state photoluminescence (PL), and time-resolved photoluminescence (TR-PL) analyses. The\ndata confirms that in addition to the role of UiO-66(COOH) as support, it plays a crucial role in the\n2\nrestructuring of Cu2+ by promoting a charge transfer from the UiO-66(COOH) to Cu2+ and initiating\n2\nthe in-situ restructuring process which in turn enhances the lifetime of charge separated (e‒ ‒ h+) states.\nMore details can be found in our response to comment 4 of Reviewer #1. In addition, we also performed\nnew experiments on UiO-66(COOH) -Cu with different Cu loadings and the results were added and\n2\ndiscussed in the revised manuscript as follows:\nOn page 9 line 23: “Moreover, the Cu free UiO-66(COOH) 2 was also tested under similar\nconditions as a reference material along with UiO-66(COOH) -Cu with different Cu loadings\n2\nwere tested as shown in (Supplementary Fig. S6). No activity was observed for the Cu-free\nsample, confirming the crucial role of Cu as the active sites. At 3 wt.% Cu, the H₂ production\nrate remained relatively low (0.55 mmol·g⁻¹·h⁻¹), but it increased to 4.3 mmol·g⁻¹·h⁻¹ at 6 wt.%.\nComparable activity values were obtained for Cu loadings of 8 and 9 wt.%. Further raising\nthe Cu content to 16 wt.% did not lead to a significant improvement, with the activity reaching\nonly about 6.6 mmol·g⁻¹·h⁻¹.These results highlight the importance of Cu coordination on the\nfree carboxylates and the formation of active sites which impact the activity of the system.”\nFigure S6. H production rate of UiO-66(COOH) -Cu with different Cu wt% loadings at room\n2 2\ntemperature.\nComment the results with the copper loading\nReviewer #3 (Remarks to the Author):\nOverall comments:\nIn this manuscript, the authors report the fundamental studies on the catalytic mechanism of Cu-\nmetalated MOFs that has been demonstrated as photocatalyst for formic acid dehydrogenation. It is\ninteresting that this system shows photocatalytic activity neither under visit light illumination nor under\nNIR illumination but under the combined visible-NIR illumination. The thorough spectroscopic studies\nincluding in situ FTIR and X-ray absorption spectroscopy were performed to understand the mechanism\nand cascade logic-gate mechanism was proposed. These findings are interesting and fundamental\nstudies are convincing to support their conclusions. This reviewer would recommend publication in\nNature Communication after the authors address my minor concerns below.\nResponse: We thank reviewer’s effort to evaluate our manuscript. We are delightful that the reviewer\nrecognized the novelty and quality of our work. The minor concerns have been addressed to the revised\nversion.\nThe in situ XAS studies are impressive but the reversibility of the system under light conditions is not\ndiscussed. This can be easily measured using in situ XAS after turning off the light.\nResponse: We thank for the insightful suggestions. The XAS measurements in dark were always\nconducted after each analysis. We have included the XAS data during catalytic reaction after turning\noff the light in the revised manuscript. The similar Cu-K edge XANES and corresponding EXAFS\nspectra during photocatalytic reaction and after light off revealed that the active catalyst (Cu0/Cu+)\nremained preserved in the MOF structure after photocatalytic experiment. This restructured active Cu\ncentres are also responsible of the photocatalytic activity during consecutive cycles. The MOF showed\nstable photocatalytic dehydrogenation activity under continuous-flow condition over three consecutive\ncycles (24h / cycle). (Please refer to #Reviewer 1, response 3) Thus, the restructured Cu remains intact\nafter the catalytic reaction, and will act as active centre during further catalytic experiment.\nFollowing texts have been added to the revised manuscript:\nOn page 17 line 11: “Further, the XAS analysis of post-catalytic MOF showed the active Cu centre\nremained intact (Supplementary Fig. S24), revealing the irreversible Cu restructuring during\nphotocatalytic reaction. However, this restructured Cu centre would act as active centre for consecutive\nphotocatalytic cycles. The stable photocatalytic dehydrogenation rate of UiO-66(COOH) -Cu over\n2\nthree consecutive cycles (Supplementary Fig. S12) further support the stability and reusability of the\nrestructured MOF catalyst.”\nFigure S24. The operando XAS spectra of UiO-66(COOH) -Cu (a) during photocatalytic H production\n2 2\n(≥ 390 nm) from FAc at steady state and (b) after 30 min. of light OFF. (A) Cu-K edge XANES spectra,\n(B) their corresponding Fourier Transform of EXAFS data."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed my comments, resulting in a notable improvement in the quality of the work. I recommend the revised manuscript for publication in the journal.", "reviewer_2": "The authors have addressed the comments well, and the paper is now suitable for acceptance.", "reviewer_3": "The authors have addressed my concerns."}, "author_response": ""}]

Decision letter