Photothermal CO_2 methanation over (NiO/Ru^0)/TiO_2 catalysts via hydrogen spillover

Domain: chemistry
Published: 2026-02-27
Editorial outcome: major_revision

Abstract
Photothermal CO methanation presents a promising strategy for mitigating the energy crisis and
2
reducing CO emissions, however, the critical role of hydrogen migration dynamics in addressing reaction
2
kinetics and thermodynamics has not been thoroughly investigated. Here, we demonstrate the design a
(NiO/Ru0)/TiO photothermal catalyst with optimized interfacial architecture and enhanced hydrogen
2
mobility, which facilitates exceptionally selective conversion of CO -to-CH . Both experimental and
2 4
theoretical analyses reveal that H dissociates efficiently on Ru0, subsequently undergoing spillover to O
2
in NiO (O ). This process not only redistributes active sites but also influences the reaction kinetics,
NiO
thereby fundamentally altering the energy landscape associated with CO methanation. Consequently, the
2
(NiO/Ru0)/TiO catalyst achieves complete CO conversion and CH selectivity, with a CH production
2 2 4 4
rate of 2552.49 μmol h-1 (85.08 mmol g-1 h-1) under an irradiation of 25.5 suns without external heat or
pressure. This research underscores an innovative engineering approach that leverages hydrogen spillover
to enhance photothermal catalytic efficiency and selectivity, thereby providing a robust framework for the
advancement of sophisticated photothermal catalysts for selective CO hydrogenation.
2

Introduction
Carbon dioxide methanation represent a viable strategy for addressing the greenhouse effect and
S
achieving carbon neutrality.1-3 However, CO methanation is aS multi-step surface catalytic reaction
2
E
involving the synergistic action of multiple intermediates and active sites. That is to say, the process is
R
hindered by sluggish kinetics due to multiple proton-Pcoupled electron transfer steps, which require
substantial energy input to activate CO moleculesN, thereby limiting the overall conversion efficiency.4
2
I
Although the hydrogen spillover (H-spillover) e ffect has been proposed as a potential means to overcome
E
the kinetic barriers associated with hydroLgen activation in CO
2
methanation,5,6 the underlying dynamic
C
and thermodynamic regulation mechanisms by which H-spillover influences the reaction pathway remain
I
T
insufficiently understood. One reason is that the mechanism by which H-spillover promotes CO
2
R
methanation is a complexA issue involving multiple steps, scales, and factors. H-spillover may affect the
reaction pathway by altering the distribution of surface hydrogen. Additionally, the mechanism of
cooperative action between metal sites and carrier sites of H-spillover may be involved in the reaction,
and the mechanism of this cooperation needs to be verified systematically.

H-spillover refers to the process in which hydrogen, after undergoing dissociative adsorption and
accumulation on a metal catalyst surface, spill into adjacent typically oxides surfaces with relatively low
energy barriers.7-9 10 This phenomenon enables more precise control over catalytic reaction pathways for
more efficient chemical synthesis.11-13 The increased reactivity associated with H-spillover arises mainly
from its dual functionality: (1) providing a rich supply of active hydrogen species for hydrogenating
reaction via rapid H
2
dissociation, and (2) accelerating hydrogenation steps in the kinetics.5,14 In the
content of commentary reports regarding H-spillover, Pd/Cu nanomaterials utilize this phenomenon to
enhance the selective hydrogenation of alkynes,15 and Ni nanoparticles improve methane dry reforming
through effective H-spillover.16 In another case, hydrogen activated by Pd spills over to
phosphomolybdate enhances the efficiency of methane oxidation.17 H-spillover plays a significant role in
the hydrogenation of CO to CO.18 However, the impact of catalyst structure on H-spillover is not yet
2
fully comprehended.19,20 Consequently, it is imperative to design catalyst structure to modulate the
electronic structure, thereby advancing the exploration of the structure-activity relationship between H-
spillover and the performance of surface catalytic reactions.

The structural design of catalysts is pivotal in promoting H-spillover during CO methanation.
2
This
structural modulation is particularly critical for improving the kinetics of H dissociation and
S 2
thermodynamically regulating reaction pathways, both of which arSe essential for hydrogen activation and
E
migration, as well as for the overall efficiency of the CO methanation process.18,21 Prior studies have
2R
demonstrated that reducible catalysts exhibit more proPnounced H-spillover effects compared to non-
reducible catalysts, given the strong correlatioNn between H-spillover phenomenon and catalyst
I
reducibility .11,22,23 Although metal Ni remains the predominant choice for facilitating H-spillover due to
E
its cost-effectiveness, from an electronic Lstructure perspective, Ru presents a superior alternative.24 This
C
is attributed to lower energy barrier for H dissociation, higher surface hydrogen coverage, and enhanced
2
I
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hydrogen migration capabilities of Ru. Among transition metal catalysts, Ru is distinguished by its unique
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catalytic properties arisingA from its [Kr]4d75s1 electron configuration. The optimal positioning of its d-
band center enables an ideal binding strength for hydrogen intermediates (ΔG ≈0 eV), achieved through
H*
a balanced H interaction that avoids both under- and over-binding. These intrinsic electronic
characteristics, combined with exceptional capability for H dissociation and dynamic surface properties
2
of Ru, establish it as an excellent candidate for the engineering of H-spillover within catalytic systems.25-
27 In addition to hydrogen activation, the adsorption and activation of CO are equally critical for CO
2 2
methanation. NiO demonstrates superior capabilities in CO conversion, attributable to Ni2+ ([Ar]3d8)
2
possessing a higher d-band center (-3.5 eV relative to the Fermi level). The surface of NiO is rich in
oxygen atoms and Lewis acid (Ni2+), which facilitate chemical adsorption on basic site (O2-) or interact
with Ni2+ to form multiple adsorption states, thereby providing abundant pathways for CO activation.
2
This work primary innovation lies in designing a catalyst that optimizes hydrogen dissociation sites
beyond those found in Ni-TiO or Ru-TiO systems (Table S1), while simultaneously introducing more
2 2
effective CO adsorption sites via NiO at activation centers.1 The energy alignment of the Ni2+ d-band
2
between metallic Ni and higher oxides optimizes the binding environment, preventing both excessive
adsorption and insufficient activation. Serving as a redox-active mediator, NiO plays a vital role in
selectively breaking C=O bonds during the CO conversion reaction.28-30 Furthermore, the presence of
2
high efficient hydrogen dissociation sites is essential for the effective implementation of H-spillover
effects.31 The synergistic combination of Ru and NiO, functioning respectively as sites for hydrogen
evolution and terminals for CO alkylation, can substantially enhance the photon-to-electron conversion
2
efficiency of catalytic system by effectively spatially separating and utilizing photogenerated carriers.32
S
The development of a composite material comprising Ru and NiOS may signify a notable progression in
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the field of catalytic design for CO hydrogenation. This approach potentially harnesses the synergistic
2 R
advantages offered by both constituent leveraging compoPnents.

This research utilizes an engineered approach to Ndesign a photothermal catalyst through the meticulous
I
assembly of NiO and Ru0 on TiO
2
, denoted as N R-TiO
2
. TiO
2
is chosen as the engineered support owing
E
to its remarkable stability and tunable suLpport for performance modification. Notably, the compatibility
C
of the matching lattice constants between Ru and TiO facilitates the stabilization and confinement of Ru
2
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within the TiO
2
lattice.33 The adjacent Ru0 and NiO facilitates an effective channel for H-spillover effect.
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The hydrogen that is rapidAly dissociated at the Ru0 atoms subsequently spills over to O
NiO
via the Ru0–
NiO channels, supplying hydrogen for the formation of the hydrocarbon intermediates on NiO. Notably,
the spillover of hydrogen from Ru to O on NiO results in the occupation of O sites by hydrogen,
NiO NiO
which alters the adsorption mode of *COOH from the traditional metal-oxygen (Ni-O ) bonding
NiO
configuration to a dual metal (Ni-Ni) coordination mode, thereby reducing energy barriers associated with
C-O bond breaking. The unique structure of the Ru0–NiO is capable of modulating the reaction pathway
for CO methanation on NR-TiO , leading to significant enhancements in both conversion rates and
2 2
selectivity. The photothermal effect induced by Ru0 and NiO accelerates the transfer of photogenerated
carriers and elevates the surface temperature of the catalyst, which synergistically enhances the kinetics
of the intermediate hydrogenation process. Notably, H-spillover significantly enhances both
thermodynamics and kinetic aspects of CO methanation. This leads to a remarkable conversion rate of
2
approximately 100% for CO and a selectivity of ~100% for CH , resulting in a production rate of μmol
2 4
h-1 (85.08 mmol g-1 h-1 under an irradiation of 25.5 suns, achieved without the application of external heat
or pressure. This study serves as a vital reference for elucidating the role of engineering in facilitating H-
spillover, thereby enhancing the efficiency of CO methanation (Table S2).
2

Results
Photothermal CO methanation performance
2
The selectivity and efficiency of CO methanation are influenced by the characteristics of active sites,
2
the efficiency of electron transfer, and the selection of hydrogenation by-products. The effective multiple
S
proton-coupled electron transfer plays a crucial role in determiningS the reaction rates, energy barriers, and
E
mechanism associated with CO hydrogenation reactions. H-spillover is proposed as a promising strategy
2 R
to overcome kinetic limitations in CO methanation.34 TPhe phenomenon of H-spillover necessitates two
2
fundamental conditions: the ability of active metalN sites to adsorb H and dissociate them into H atoms,
2
I
and the presence of a receptor that provides a h ydrogen transfer channel and a driving force for hydrogen
E
migration. Informed by a comprehenLsive understanding of catalyst design strategies, we have
C
meticulously developed a photothermal catalyst comprising NiO, Ru0 and TiO , which possesses the H-
2
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spillover effect through an immersion-calcination process (Fig 1a). The photothermal catalytic
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performance was assessedA in a sealed system utilizing a glass reactor with a capacity of 358.8 mL, which
was thoroughly vacuumed prior to the reaction. To identify the optimal catalyst for subsequent
mechanistic investigations, we analyzed the effects of varying loading amount of NiO and Ru0 on the
photothermal catalytic performance. Our findings indicate that the catalytic performance improves
progressively with the addition of Ru0 and NiO (Fig S1), with the most significant enhancement observed
at a loading of 40 wt.% NiO and 2 wt.% Ru0 on TiO , yielding performance metrics that are 37.7 times
2
and 2.5 times greater than those of synthetic parallel comparison catalysts (Table S3, 40 wt.% NiO-1 wt.%
Ru0-TiO and 20 wt.% NiO-2 wt.% Ru0-TiO ). The quantity of co-catalyst facilitating the reaction is
2 2
associated with the efficiency and effectiveness of H-spillover. However, this relationship is not invariably
linear with respect to the extent of H-spillover. 35 36 An excessive amount of co-catalyst may even exert
detrimental effects. H-spillover is a multifaceted, multi-step process involving: the dissociation of H
2
molecules on the co-catalyst, migration of H atoms to the interface, occurrence of H-spillover, subsequent
surface migration, and interaction with reactants. The overall rate of H-spillover is governed by the rate-
determining step (RDS). While increasing the co-catalyst quantity primarily augments the number of
active sites involved in the initial dissociation step, the third step typically serves as RDS.11 Consequently,
an excessive addition of co-catalyst does not proportionally enhance the overall H-spillover process. The
optimal loading amounts of NiO and Ru0, as determined by inductively coupled plasma optical emission
spectroscopy (ICP-OES) analysis, were found to be 39.96% and 1.26%, respectively. To further
investigate the influence of active sites on the catalytic performance and reaction pathways, 40 wt.% NiO-
TiO (designated as N-TiO ), 2 wt.% Ru0-TiO (denoted as R-TiO ) and 40 wt.% NiO-2 wt.%-Ru0-TiO
2 2 2 2 , S 2
(denoted as NR-TiO ) are selected for additional studies.
2
In the process of photothermal catalysis, the surface temperature of the catalyst is significantly
influenced by the intensity of light irradiation, which in turn affects the catalytic performance. As
illustrated in Fig 1b, S2 and S3, the correlation bNetween increased surface temperature and enhanced
I
catalytic performance due to elevated light inten sity exhibits a similar trend across three different catalysts.
E
Notably, NR-TiO
2
demonstrates higher reLaction temperatures under photoexcitation compared to N-TiO
2
C
and R-TiO . Specially, it has been observed that the temperature of NR-TiO can reach to 220 ℃ at the
2 2
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light intensity of 2.55 W/cm2, thereby highlighting the photothermal effect associated with NR-TiO .
2
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The
data points for NR-TiO
2
Aat 25.5 suns were recorded upon achieving complete conversion. CH
4
was
identified as the principal product in photothermal CO hydrogenation, with its concentration increasing
2
over time (Fig S2). The CH yields from N-TiO , R-TiO , and NR-TiO increase with rising light intensity,
4 2 2 2
with NR-TiO achieving the highest rate of CH production. The NR-TiO catalyst demonstrates
2 4 2
selectivity above 98%, achieving a peak value of 99.14% at 25.5 suns, surpassing both N-TiO and Ru-
2
TiO across all light intensities examined (Fig S3). At a light intensity of 2.55 W/cm2, the CH evolution
2 4
rate on NR-TiO reaches an impressive 2552.49 μmol h-1, which is 5.5 and 4.2 times greater than that of
2
N-TiO and R-TiO , respectively. This significant enhancement in performance cannot be solely attributed
2 2
to the mere combination of NiO and Ru0 components, but rather to specific interfacial interactions
occurring at the Ru0–NiO, which will be discussed in detailed. Simultaneously, in contrast to the
temperature variations of TiO under varying light intensities (Table S4), the observed temperature
2
elevation in N-TiO , R-TiO and NR-TiO indicates that the incorporation of NiO and Ru can significantly
2 2 2
improve the photothermal conversion efficiency of the NR-TiO catalyst. The photothermal conversion
2
efficiency is enhanced, along with specific interfacial interactions at the Ru0–NiO, which contribute to
improved photothermal effect of NR-TiO .37 And this improved photothermal effect is further identified
2
by the performance experiments with different cutoff wavelengths driving the selective CO
2
hydrogenation reaction (Fig S4). Cycling tests were performed on the NR-TiO photothermal catalyst at
2
complete conversion as well as within the 20-30% conversion range, along with characterization before
and after the reaction confirm the exceptional performance and structural stability of NR-TiO (Figs S5-
2
S6).

Previous research has unequivocally demonstrated that, in ceSrtain reactions, the photothermal effect
serves as the primary driving force,38 whereas in other reactions, non-thermal effects assume a more
prominent role.39-41 Therefore, it is imperative to differentPiate between the thermal and non-thermal effects
of NR-TiO in the context of photothermal CO meNthanation. In this reaction, temperature, light intensity,
2 2
I
and wavelength do not independently influence the process; rather, the CH
4
yield results from the interplay
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of these factors. To elucidate the respectiLve contributions of thermal and non-thermal effects, a series of
C
experiments was performed (Fig S7-10). The findings reveal that the photothermal effect predominates
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under low light intensities, whereas at elevated light intensities, the non-thermal effect gains prominence
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due to the increased photoAn flux generating additional hot electrons that facilitate the reaction. Notably,
the non-thermal effect is contingent upon both light intensity and temperature, while the thermal effect
exhibits greater complexity, being modulated by light intensity, temperature, and catalyst thickness. The
rates of CH evolution were measured under conditions both with and without light irradiation. It should
4
be noted that due to the significant temperature increase of the photothermal catalyst NR-TiO under
2
illumination, to achieve comparable experimental conditions, it is necessary to measure the surface
temperature of the catalyst under illumination. Here, we employed a thermocouple for temperature
measurement, and then maintained the reaction system at the corresponding temperature conditions
through electrical heating under non-illuminated conditions. The catalytic performance of NR-TiO when
2
exposed to illumination is markedly superior to the thermal catalytic process conducted without
photoexcitation (Fig 1c), suggesting that temperature serves a supplementary role in photocatalysis
throughout the reaction process. The activation energies for the reactions were determined using the
Arrhenius equation, taking into account the photothermal and thermal properties of CO methanation for
2
N-TiO , R-TiO and NR-TiO at their respective temperature (Fig 1d-e). Under full spectrum light
2 2, 2
irradiation (300W Xe lamp), the activation energies for N-TiO , R-TiO and NR-TiO are found to be
2 2, 2
99.29, 65.85, and 53.96 kJ mol-1, respectively, which are lower than those observed without light
irradiation (heated to the corresponding temperature) (104.26, 72.91, and 65.59 kJ mol-1). This reduction
in apparent activation energy suggests that specific interactions related to H-spillover at the Ru0–NiO may
effectively lower the activation energy required for CO methanation.
2
The photothermal catalyst NR-TiO has been theoretically demonstrated to possess the potential for H-
2 S
spillover effect, which is corroborated by experimental data indSicating a marked enhancement in its
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photothermal performance. Theoretically, the elevated d-band center of NiO enhances the activation of
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CO through π*-orbital electron injection, thereby weakePning C=O bonds. And Ru can being an optimal
2
equilibrium for H+ adsorption and desorption, pNromoting efficient H dissociation. Thus, it can be
2
I
reasonably deduced that the catalytic perform ance of NR-TiO
2
is intricately linked to the synergistic
E
interplay between NiO and Ru0, which faLcilitates rapid H
2
adsorption and dissociation. This is to say that
C
these two components engender a bidirectional charge-transfer platform: Ru swiftly generates active
I
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hydrogen species that subsequently spill over to the NiO surfaces, where CO
2
species were pre-activated,
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which is thoroughly evideAnced in the section dedicated to mechanistic analysis. Consequently, NR-TiO
2
demonstrates significant advantages in both photothermal catalytic performance and CH selectivity when
4
compared to currently reported advanced CO methanation catalysts (Table S5), further substantiating the
2
effectiveness of our proposed engineering approach to enhance H-spillover effect.

Characterization of photothermal catalysts
In order to investigate the correlation between photothermal catalytic performance and the structural
characteristics of catalysts, a comprehensive analysis was conducted focusing on crystal structure surface
morphology, and electronic coordination information of the catalyst. High Resolution Transmission
Electron Microscope (HR-TEM) was employed to ascertain the composition and morphology of the
photothermal catalyst. The lattice fringe of TiO is distinctly observed in NR-TiO , indicating its high
2 2
crystallinity (Fig 2a), which is further confirmed by X-ray diffraction (XRD) (Fig 2d).42 Specifically, a
lattice fringe of 0.214 nm corresponds to the Ru0(002) crystal plane (JCPDS PDF# 70-0274), while a
spacing of 0.209 nm is associated with the NiO(200) plane (JCPDS PDF# 89-7130), thereby confirming
S
the successful synthesis of the NR-TiO composite. The NiOS(200) surface displays an alternating
2
E
arrangement of Ni and O atoms in atomic rows, whereas the Ru(002) surface is characterized by an
R
exposed layer of Ru atoms. This structural arrangement Ppromotes the formation of short-range chemical
bonds between the surface Ru atoms and the O atoNms from NiO, consequently establishing atomic-scale
I
H-spillover channels. Furthermore, HR-TEM i maging across various regions corroborates the existence
E
of contact among the NiO, Ru0, and TLiO
2
phases (Figs 2a and S11). Based on the experimental
C
observations and the findings indicating that the surface energy of NiO(100) is lower than that of
I
T
compared to NiO(110),43 a model structure for the NR-TiO has been constructed for Density Functional
2
R
Theory (DFT) calculationAs (Fig S12). As illustrated in Fig 2b, there is a notable accumulation of charge
at Ru-NiO, Ru-TiO , and NiO-TiO , indicating a strong coupling interaction among the three contacted
2 2
surfaces.

Additionally, the elemental compositions of NR-TiO within a 2μm range were examined utilizing an
2
Energy Dispersive Spectrometer (Fig 2c). The results from the scanning images indicate the simultaneous
presence of Ru, Ni, Ti, and O, thereby affirming the successful synthesis of the photothermal composite
catalyst. The spatial overlap of Ru and Ni observed in the scanning positions is attributed to the interfacial
contact between Ru0 and NiO. Further analyses indicate that the synergistic interaction between the CO
2
activation capability of NiO and the optimized H adsorption-desorption equilibrium of Ru facilitates H-
spillover, which addresses the sluggish kinetics of H activation and alleviates the over-stabilization of
2
CO . XRD was employed to characterize the phase composition of the photothermal catalyst. The XRD
2
patterns for N-TiO , R-TiO , and NR-TiO predominantly exhibit diffraction peaks corresponding to TiO
2 2 2 2
(JCPDS PDF#71-1166) (Fig 2d). The relatively weak peaks observed for N-TiO and NR-TiO at 37.1º,
2 2
43.1º, and 62.6º are attributed to the (111), (200), and (220) planes of NiO (JCPDS PDF# 89-7130).
To elucidate the surface chemical composition of NR-TiO , X-ray photoelectron spectroscopy (XPS)
2
was conducted, calibrated at 284.8 eV (C 1s). The observed peaks at 854.3 and 872.0 eV correspond to
the spin splitting of Ni2+, accompanied by satellite peaks (Fig 2e). The binding energy position of Ru0 is
identified at 280.31 eV, while peaks associated with Ti4+ are detected at 457.9 eV and 463.57 eV (Fig
S13). The XPS findings confirm the presence of Ni and Ru on NR-TiO in the forms of NiO and Ru0,
2
respectively, which is consistent with the results obtained from XRD, HR-TEM, and energy dispersive
spectroscopy (EDS) mapping analysis. In comparison to N-TiOS, the characteristic Ni 2p peak in NR-
2
TiO exhibits a shift towards higher binding energy by 0.2 eV, while the binding energy of Ru0 shifts from
2 R
280.31 eV in R-TiO
2
to 280.16 eV in NR-TiO
2
. These Presults indicate that electrons from NiO can be
transferred to Ru0 due to the interfacial contact estNablished between NiO and Ru0. The light absorption
I
properties of the catalysts play a crucial role i n influencing their catalytic activity.44 To investigate the
E
impact of NiO and Ru0 on the light absoLrption characteristic of the catalyst, UV-Vis diffuse reflectance
C
spectroscopy (UV-Vis DRS) was employed. The results indicate that NR-TiO exhibits superior light
2
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T
absorption properties (Fig S14), suggesting that the coexistence of NiO and Ru0 enhances the capture of
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photon-energy during the eAnergy conversion process, thereby improving the performance of photothermal
catalysts.

The electronic structure and coordination characteristic of NR-TiO were examined using X-ray
2
absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses.
As illustrated in Fig 2f, the Ni K-edge XANES spectra of NR-TiO exhibit similarities to the NiO standard,
2
however, the absorption edge is observed to shift to higher energies in comparison to the NiO standard.
This shift indicates that the average oxidation state of Ni in NR-TiO is elevated relative to the Ni2+ state
2
found in the NiO standard.45,46 The R-space diagrams for NR-TiO reveal that the Ni-O bond length in the
2
first shell layer is 2.03 Å, with a coordination number (N) of 1.0, while the second shell layer exhibits a
bond length of 2.98 Å and a coordination number of 6.0 (Table S6 and Fig S15a-b). Notably, the Ni-O
bond length in the second shell layer of NR-TiO is shorter than that of the NiO standard, suggesting that
2
robust interfacial interactions significantly decrease the average distance between adjacent Ni and O
atoms. This reduction in bond length is likely to influence the adsorption of *COOH intermediates,
thereby facilitating the C-O bond breaking process on Ni.47 Furthermore, as depicted in Fig 2g, the relative
intensity and line shape of the Ti K edge peaks in NR-TiO align more closely with the TiO standard,
2 2
indicating that the TiO in NR-TiO exists in the anatase form, characterized by Ti-O bonds.48,49 Compared
2 2
with the TiO standard, the increased intensity of characteristic peaks in NR-TiO is due to the enhanced
2 2
electron cloud of Ti4+ species caused by interfacial interactions, as corroborated by XPS results (Fig S15c-
d). These findings substantiate that the engineered Ru0–NiO can significantly influence the resulting
interfacial electronic structures and bond lengths, thereby creating favorable conditions for H-spillover to
modulate the reaction pathway for photothermal CO methanation.
2
Separation and transfer of photogenerated carriers
Based on the analysis of CO methanation performance, it is evident that a significant breakthrough has
2
been achieved due to the synergistic effect of thermal effects under photoexcitation conditions and the
efficient rapid migration of photogenerated carriers. Consequently, the migration mechanism of these
photogenerated carriers is a crucial factor in enhancing the performance of photothermal CO methanation.
2
A comprehensive analysis of the separation and transfer dynamics of these carriers was conducted
utilizing photophysical techniques, in-situ characterizations and theoretical evaluations. The directional
behavior of photogenerated carriers transfer was elucidated througSh in-situ XPS and in-situ Kelvin probe
E
force microscopy (KPFM). Initially, in-situ XPS measurements were performed after stabilizing the
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sample in darkness and subsequently exposing it to light fPor a duration of 10 minutes. A notable alternation
in binding energy of Ni2+ within NR-TiO is obserNved, increasing from 855.9 eV to 856.3 eV (Fig 3a),
2
I
which suggests that Ni is undergoing a ch emical state transition characterized by electron loss.
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Concurrently, the binding energies of RLu0 and Ti4+ are found to decrease by 0.23 eV and 0.1 eV,
C
respectively, under light irradiation, while the binding energy of Ti-O diminishes from 530.12 eV to
I
T
530.03 eV, indicating that these three atomic species are situated in an electronically aggregated chemical
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state50 (Fig 3b-d). Consequently, it is reasonable to deduce that photogenerated electrons migrate from
A
NiO to TiO and Ru0 upon photoexcitation, thereby establishing NiO as a significant electron donor in the
2
context of photothermal catalysis. To visually assess the migration and accumulation of photoelectrons,
in-situ KPFM was employed to examine the variation in surface photo potential across the photocatalysts.
Under illumination, NR-TiO , N-TiO and R-TiO all demonstrated distinct surface photo potential (SPV)
2 2 2
signals, indicative of their light-driven charge transport characteristics. Notably, NR-TiO exhibits the
2
most pronounced reduction in SPV signal (11.03 mV) under illumination (Fig 3e) when compared to N-
TiO and R-TiO (Fig S16a, b), which can be attributed to the surface charge accumulation resulting from
2 2
the transfer of electrons from the electron donor NiO to Ru0 and TiO . These findings suggest that the
2
incorporation of Ru0 and NiO enhances the separation and migration of photogenerated carriers on
catalyst surfaces, thereby providing the requisite kinetic conditions for the H-spillover effect through the
modulation of the interfacial electronic structure.

To elucidate the mechanisms and dynamics underlying the separation of photogenerated carriers, a
series of techniques were employed, including photoluminescence spectroscopy (PL), time-resolved
photoluminescence spectroscopy (TRPL), transient photocurrent measurements, and electrochemical
impedance spectroscopy (EIS). The PL spectrum of N-TiO reveals a peak at 436 nm, which is attributed
2
to the recombination of photogenerated carriers (Fig 3f). In comparison to R-TiO and N-TiO , NR-TiO
2 2 2
exhibits the lowest intensity of the photoluminescence signal, indicating that the incorporation of NiO and
Ru0 effectively inhibits the recombination of photogenerated carriers. This enhancement leads to an
increased number of photogenerated carriers and facilitates the kinetics of interfacial charge migration
during the photothermal CO methanation reaction. The lifetimeSs of the photogenerated carriers were
2
assessed through TRPL analysis, revealing that NR-TiO possess the shortest average lifetime, thereby
2 R
confirming the efficiency of the composite catalyst iPn promoting the separation and migration of
photogenerated carriers. Additionally, transient phNotocurrent and EIS measurements were performed to
I
further investigate the efficiency of charge se paration and transport within the catalysts.51 The results
E
indicate that NR-TiO
2
achieves the higheLst photocurrent density and the smallest arc radius, suggesting
C
superior charge separation efficiency and expedited interfacial charge transport compared to N-TiO and
2
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R-TiO (Fig S17). Consequently, it can be logically inferred that the constructed NR-TiO facilitates
2 2
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efficient separation and mAigration of photogenerated carriers, thereby enhancing the kinetic properties of
the carriers involved in the reaction and establishing favorable conditions for H-spillover modulation of
the reaction pathway, ultimately promoting highly selective CO hydrogenation.
2

H-spillover mechanism favoring photothermal CO methanation
2
In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was employed to
investigate the alterations in surface chemical bonding on N-TiO and R-TiO during the photothermal
2 2
CO methanation (Figs 4a-b). This analysis elucidates the fundamental mechanisms underlying CO
2 2
methanation on N-TiO and R-TiO . Specifically, for N-TiO , the spectral peaks observed at 2349 cm-1
2 2 2
and 1837 cm-1 are indicative of CO adsorption and the bridged adsorption of *CO —. This observation is
2 2
further supported by vibrational frequency calculations (Fig S18, S19). All other computational settings,
including the exchange-correlation functional, basis set, k-point sampling, and Hubbard U parameters,
S
were kept consistent with those used in the structural optimizatioSns. And *CO adsorption species were
2
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detected in CO temperature-programmed desorption (CO -TPD) analyses (Fig S20). The peak at 1620
2 2R
cm-1 is associated with the carboxy intermediate (*COOH),52 while the peaks at 1098, 1473 and 1136 cm-
1 correspond to the intermediates *CHO, *CH O Nand *CH O, respectively (Fig 4a).52,53 The infrared
2 3
I
vibrational peaks of the final product (CH
4
) are observed at 1305 cm-1 and within the range of 3021-3105
E
cm-1, indicating the progressive reductionL of CO
2
to CH
4
.54 Based on these observations, it is proposed
C
that the CO methanation pathway on N-TiO follows a carboxylate mechanism (*CO →*CO —→
2 2 2 2
I
T
*COOH→*CH O→*CH ). In contrast, the activation of CO on R-TiO occurs through the direct
x 4 2 2
R
dissociation of the C-O boAnd, as evidenced by the *CO vibrational peak at 2019 cm-1 and the absence of
the *COOH peak (Fig 4b).55 The subsequent *CO hydrogenation pathway of *CO to CH on R-TiO is
4 2
consistent with that observed on N-TiO , with no shifts in the peak positions for *CHO, *CH O, *CH O,
2 2 3
and CH . Thus, it can be concluded that CO methanation on R-TiO follows a direct dissociation pathway
4 2 2
(*CO →*CO→*CH O→*CH ). So far, these findings clearly indicate that the mechanisms of
2 x 4
photothermal catalytic CO methanation on N-TiO and R-TiO proceed through two distinct pathways.56
2 2 2
Meanwhile, in the CO methanation performance analysis (Fig 1b, c), NR-TiO exhibited a significant
2 2
improvement compared to both N-TiO and R-TiO , which can be reasonably inferred that there may be
2 2
critical changes in the reaction mechanism. Therefore, it is necessary to conduct a systematic analysis
focusing on the photothermal catalytic CO methanation on NR-TiO .
2 2
DFT calculations were performed to further clarify the mechanism responsible for the remarkable CO
2
methanation performance of the catalyst. Utilizing HR-TEM images, structural models of the NiO(100)
and Ru0(001) surfaces were developed (Fig S21). Initially, the reaction site for CO methanation was
2
investigated. As illustrated in Figs S22-25, the CO molecule is weakly adsorbed on the Ru0(001) surface
2
through a physisorption mode, exhibiting an adsorption energy of -0.31 eV (Fig S22). Subsequently, the
generating *CO (where * denotes the reaction site) preferentially undergoes direct C-O bond breaking to
2
yield *CO, characterized by a relatively low energy barrier of 0.74 eV and an exothermicity of 0.66 eV
(in contrast to the hydrogenation pathway leading to *COOH formation, which has a barrier of 1.07 eV
and an endothermicity of 0.26 eV towards, Fig S23). This observation is consistent with the favorable
direct dissociation pathway identified in the in-situ DRIFTS analysis (Fig 4b). The *CO subsequently
reacts with hydrogen to produce *CHO, with a barrier of 1.10 eV Sand an endothermicity of 1.04 eV. The
E
elevated temperatures will facilitate the reaction crossing the high reaction barrier, thereby accelerating
R
the reaction process. This barrier is significantly higherP than that for the subsequent hydrogenation of
*CHO to form *CH O (x=2 and 3) intermediates anNd the CH product (Fig S24). These findings indicate
x 4
I
that the Ru0(001) surface alone demonstrates lim ited CO
2
methanation activity. Differently, the NiO(100)
E
surface exhibits markedly different catLalytic behavior (Fig S25). Under elevated temperatures, H
2
C
adsorption on NiO becomes even less favorable. Although the CO adsorption energy on the bare NiO(100)
2
I
T
surface (-0.31 eV) is comparable to that on Ru0(001), the overall barrier for the kinetically favorable
R
pathway reaches 1.13 eV Afor *COOH dissociation. Notably, the dissociation barrier for *COOH can be
significantly reduced from 1.60 eV to 0.78 eV upon hydrogen coverage on NiO(100), thereby providing
compelling evidence that surface hydrogenation, rather than adsorption strength, governs the kinetic
feasibility of C-O bond breaking. This underscores the pivotal role of hydrogen in CO methanation,
2
wherein the Ru0-NiO facilitates H-spillover to synergistically optimize reaction energetics(Fig 4c). Under
these conditions, the CO adsorption process is enhanced to -0.58 eV (indicating a chemical adsorption
2
mode), and the resulting *CO is readily hydrogenated to *COOH with a reduced barrier of 0.46 eV (Fig
2
S25). In the subsequent step, *COOH undergoes C-O bond breaking with an exceptionally low barrier of
0.04 eV and is exothermic by -0.29 eV, yielding *CO. This result can be attributed to the alignment of the
NiO d-band with the π* antibonding orbitals of CO , which promotes electron transfer from the d-orbitals
2
of NiO to the antibonding states of CO . Such an interaction results in the weakening of the C=O bond
2
through orbital hybridization. The subsequent reduction of *CO to CH proceeds through intermediates
4
with energy barriers all below 0.28 eV and exhibits exothermicities exceeding 0.46 eV. The observed
energy differentials indicate that the NiO(100) surface, rather than Ru0(001), serves as the primary
reaction site for CO methanation in the presence of a sufficient hydrogen source.
2
While NiO(100) presents kinetic advantages for the CO methanation, it is significantly hindered by its
2
limitations in H dissociation. As illustrated in Fig 4d, the H dissociation on NiO(100) encounters a
2 2
barrier of 0.72 eV and is characterized as endothermic by 0.65 eV, with one H atom adsorbing onto a Ni
atom and the other onto an O atom. The elevated thermodynamic energy associated with this process
renders H dissociation a critical kinetic bottleneck, which elucidates the susceptibility of Ni-based
2 S
catalysts to deactivation through mechanisms such as carbon poSisoning or sintering.57 In contrast, H
2
E
dissociation on the Ru0(001) surface occurs without an energy barrier and is highly exothermic,
R
underscoring its superior capacity to activate H . This dPisparity in H dissociation capabilities indicates
2 2
that transfer of hydrogen from Ru0 to NiO, facilitatNed by the H-spillover effect, may serve as an effective
I
strategy to enhance CO
2
methanation on NiO(00 1). The strategic design of NiO and Ru0 is likely to induce
E
synergistic enhancements in both the kineLtics and thermodynamics of photothermal CO
2
methanation.

To further substantiate this perspective, DFT calculations were conducted to evaluate for H-spillover
at the Ru0–NiO. The simulated NR-TiO model and the corresponding DFT results are depicted in Figs
2 S
S12 and 5a. The adsorption energies of hydrogen on the N-TiO Sand R-TiO surfaces, both prior to and
2 2
E
following interaction, are illustrated in Fig S26. It is observed that H exhibited a stronger adsorption
R
affinity for the Ru0(001) surface compared to the NiOP(001) surface, with a notably more negative
adsorption energy of -2.88 eV. Consequently, an eNnergy barrier of at least 0.95 eV must be surmounted
I
for hydrogen to transfer from Ru0(001) to NiO (100). However, upon the establishment of the Ru0–NiO,
the adsorption energy of H on Ru0(001L) significantly decreases, while adsorption on NiO becomes
C
thermodynamically more favorable. Specifically, the hydrogen adsorption energy on Ru0 in NR-TiO rises
2
I
T
to -2.19 eV, whereas on O , it becomes more negative at -2.33 eV. In contrast, the hydrogen adsorption
NiO
R
energy on Ni is -1.51 eV, wAhich is 0.82 eV higher than that of O
NiO
. These findings result in a diminished
thermodynamic energy difference of merely -0.14 eV for hydrogen transfer from Ru0(001) to NiO(100),
with the migration target identified as the O on the NiO(100) surface, thus render the process
NiO
exothermic. The energy barrier for H-spillover is moderate at 0.78 eV (Fig 5a), further corroborating the
kinetic feasibility of this process, as prior research has indicated that H-spillover becomes
thermodynamically unfavorable at energies exceeding 1.15 eV.11 These results collectively affirm both
the kinetic and thermodynamic viability of H-spillover across the Ru0–NiO. To gain a deep understanding
of the H-spillover effect, H temperature-programmed reduction (H -TPR) tests were performed on N-
2 2
TiO , R-TiO , and NR-TiO , respectively. As shown in Fig 5b, two reduction peaks are observed for N-
2 2 2
TiO at 342.2 ℃ and 463.1 ℃, corresponding to the transitions Ni2+→Ni+ (α) and Ni+→Ni0 (β),
2
respectively.58 The dissociated H atom is generally regarded as possessing a stronger reducing capability
than molecular hydrogen. For NR-TiO , the reduction peaks of NiO shift to lower temperatures, indicating
2
that the energy barrier for breaking Ni-O bonds is reduced due to the presence of Ru0, thereby enhancing
the reduction process of Ni through the action of dissociated H atoms. This downward shift in the
reduction peaks of NR-TiO provides direct evidence for H-spillover from Ru0 to NiO. Additionally, the
2
H-spillover in NR-TiO was further corroborated using the WO color reaction, a well-established method
2 3
for indicating the activation of H at the catalyst (Fig S27-28).59 In this process, WO exhibits challenges
2 3
in undergoing direct reduction by H at low temperatures. Nevertheless, the incorporation of noble metals
2
facilitates the migration of hydrogen atoms from the metal to the WO lattice via a spillover mechanism,
3
resulting in the formation of hydrogen tungsten bronze H WO . This transformation is accompanied by a
x 3 S
notable color change from yellow to blue. Concurrently, XPS reveSal that the appearance of two additional
E
peaks associated with W5+ in H WO .21 Based on the aforementioned analyses, the structural modeling of
x 3 R
the H-spillover process at the Ru0–NiO is illustrated as FPig 5c. The hydrogen exhibits a rapid dissociation
from the Ru0 atom, subsequently spilling over to thNe O atom, thereby serving as a hydrogen source for
NiO
I
the subsequent formation of the *CH
x
O
y
key in termediates on NiO.
E
The H-spillover from Ru0(001) to LNiO(100) significantly alters the adsorption behavior of
C
intermediates, as demonstrated by structural and spectroscopic analyses (Figs 5d, 5e, S18, and S19).
I
T
Theoretically, when hydrogen occupies the O due to the H-spillover effect, it induces a notable shift in
NiO
R
the *COOH adsorption sitAe from the conventional metal-oxygen (Ni-O
NiO
) configuration to a bimetallic
(Ni-Ni) coordination mode. This reconfiguration leads to a substantial alteration in the vibrational
frequency, changing from 1634 cm-1 (O -COOH) to 1759 cm-1 (Ni-COOH). To further validate the
NiO
impact of H-spillover effect on the changes in adsorption sites, in-situ DRIFT was conducted. The
vibrational peak of *COOH was observed to shift from 1620 cm-1 (O -COOH) to 1743 cm-1 (Ni-COOH),
NiO
aligning closely with the theoretical calculations, with the minor discrepancy within an acceptable range
for theoretical and practical monitoring. This observed shift in the vibrational frequency of *COOH from
in-situ DRIFT spectral changes provide direct evidence for the reconfiguration process. The alternation
of *COOH adsorption sites facilitate the C-O bond breaking, significantly decreasing the dissociation
barrier of *COOH from 0.78 eV to 0.04 eV, thereby underscoring the role of H-spillover in promoting C-
O bond breaking through the reconfiguration of intermediate coordination (Fig 4c). Notably, the
occupation of O atom by hydrogen further reduces the barrier of this reaction process to 0.78 eV,
accompanied by an enthalpy change of -0.73 eV. The consistent frequency shift consistent from the
transition of O -COOH adsorption to Ni-COOH as a consequence of H-spillover highlights the
NiO
enhancement of catalytic activity of NR-TiO for CO methanation.
2 2
The analysis presented above systematically delineates the reaction mechanism underlying the H-
spillover effect that facilitates CO methanation, which can be encapsulated in two fundamental processes:
2
(1) the H-spillover from Ru0 to O circumvents the energy barrier associated with hydrogen dissociation
NiO
on the NiO surface, thereby supplying an adequate hydrogen source for the CO methanation reaction, (2)
2
the accumulation of H on the NiO surface effectively modulates the bidentate adsorption sites of the
intermediate *COOH, resulting in a fundamental alteration inS the kinetics of C-O bond breaking.
E
Consequently, a mechanism has been proposed to elucidate how the H-spillover effect influences the
R
adsorption configuration of key intermediates *COOPH, thereby regulating reaction pathways and
enhancing the efficiency of CO methanation (FigNs 5f and S29). Initially, H dissociates rapidly on the
2 2
I
Ru0(001) surface and subsequently spills over o nto the NiO(100)surface, providing a substantial hydrogen
E
source for CO
2
methanation on NiO(100)L. The H-spillover effect results in the occupation of O
NiO
in NiO
C
by H, which alters the bidentate adsorption of *COOH from Ni-O sites to Ni-Ni sites, significantly
NiO
I
T
lowering the energy required for C-O bond breaking. The CO methanation pathway comprises a series
2
R
of hydrogenation steps, wAherein the H-spillover from Ru0 to O
NiO
dynamically influences the reaction
energetics. This directional H-spillover leads to specific reactions in energy barriers for individual steps,
while the H-spillover at the Ru0–NiO enhanced proton-transfer kinetics, as demonstrated by in situ DRIFT
and DFT calculations. Collectively, the H-spillover effect on sequential hydrogenation processes over
NR-TiO results in a 37.7-fold increase in CH formation efficiency compared to the H-spillover-free N-
2 4
TiO catalyst. Furthermore, the H-spillover effect facilitates the breaking of the C-O bond, yielding the
2
*CO intermediate, which is subsequently hydrogenated to produce CH . Through the H-spillover effect
4
at the Ru0–NiO, CO
2
is effectively converted into CH
4
, achieving atomic-scale regulation of CO
2
methanation within the NR-TiO system.
2

Methods
Experimental Section
Materials. RuCl ·3H O (99.95%, Heowns), Na CO (98%, Tianjin Jiangtian Chemistry), P25 TiO (AR,
3 2 2 3 2
Degussa), and NiCl
3
·6H 2AO (AR, Tianjin Sanchang Chemistry), all of which were employed without
additional purification.
Preparation of catalyst.300 mg P25 was added into 30 mL DI water, followed by the addition of 488 mg
NiCl and 480 μL of an aqueous RuCl (25 mg mL-1) solution. The pH of mixture was subsequently
2 3
adjusted to 8 using a Na CO solution (1.2M). The resulting solution was then placed in a water bath at
2 3
80 ℃ for a duration of 12 h. The precipitate was washed for three times and dried at 120 ℃ for 12 h. The
dried products were ground and subjected to calcination in a tube furnace, following a temperature ramp
of 10 ℃ min-1 until reaching 300 ℃, where they were calcined for 1 h in an Ar atmosphere, followed by
an additional hour in a H /Ar mixture. This resultant sample was designated as NR-TiO . The N-TiO and
2 2 2
R-TiO samples were synthesized without the inclusion of RuCl and NiCl , respectively. Variations in
2 3 2
the ratios of Ni and Ru were achieved by modifying the quantities of NiCl and RuCl (either doubling or
2 3
halving their amounts). The samples were according labeled as 20 wt.% NiO-2 wt.% Ru0-TiO (with
2
halved NiCl ), 80 wt.% NiO-2 wt.% Ru0-TiO (with doubled NiCl ), 40 wt.% NiO-1 wt.% Ru0-TiO (with
2 2 2 2
halved RuCl ) and 40 wt.% NiO-4 wt.% Ru0-TiO (with doubled RuCl ).
3 2 3
Photothermal CO methanation test
2
Photothermal CO methanation was conducted with a sealed system utilizing a glass reactor with capacity
2
of 358.8 mL, which was thoroughly evacuated prior the initiation of the reaction. During the experimental
procedures, 30 mg catalyst was meticulously ground and uniformly dispersed onto quartz paper, while 50
mg catalyst was employed in cycling experiments. Following the vacuum process, 1.6 mmol high purity
absolute CO and 6.4 mmol H were introduced into the system aSnd allowed to adsorb in the dark for a
2 2
duration of 30 min. A full spectrum 300W Xe lamp served as the light source, complemented by a
compound eye lens and a Fourier lens to mitigate tempPerature discrepancies.60 The incorporation of a
homogenizer modified the beam path, ensuring anN even distribution of light across the catalyst surface,
thereby enhancing the precision of the tempera ture measurements and preventing the underestimation of
temperature that could occur if the beamL was not uniformly distributed at the measurement point. To
evaluate the respective contributions of thermal and non-thermal effects in the reaction, a reactor with
adjustable external heating temperature was utilized. By systematically varying both the light intensity
and the reaction temperatuAre, the non-thermal effects of NR-TiO
2
in the photothermal CO₂ methanation
were extracted. A GC-2014 gas chromatograph was employed to analyzed the concentrations of CH , CO,
4
CO , C H , C H and H on an hourly basis. Organic compounds were separated using a Sunpak-A (H-
2 2 4 2 6 2
2099, 50-80) column, while CO and CO were separated on a Porapak-QS (H-2100, 80-100) column after
2
passing through a methane conversion furnace. H was separated using a 5A molecular sieve column. The
2
detection of organic compounds and CO was performed using a hydrogen flame ionization detector (FID),
while a thermal conductivity detector (TCD) was utilized for H . The conversion of CO was calculated
2 2
using Equation (1):
Eq. (1) CO conversion=
CO2 initial-CO2 eventual
2
CO2 initial
The selectivity of CH was determined using Equation (2):
4
Eq. (2) CH selectivity=
CH4 yield
4
CO yield+CH4 yield+C2H
4
yield+C2H
6
yield
Characterizations
X-ray diffraction (XRD) analysis, utilizing a Cu Kα radiation source and conducted with a D8-Focus
instrument from Bruker (Germany), was employed to elucidate the crystal structures of the samples. In-
situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed using an
Invenio-S instrument. The concentration of metal ions was determined through inductively coupled
plasma optical emission spectrometry (ICP-OES) with an iCAP7000 series instrument from Thermo. The
chemical compositions of the samples were analyzed via X-ray photoelectron spectroscopy (XPS) using
a Thermo Scientific Escalab 250XI apparatus. The morphological characteristics of the samples was
examined using transmission electron microscopy (TEM) with Sa JEM-F200 instrument from Japan.
Energy dispersive spectroscopy (EDS) analysis focused on the Ni K, Ru L, Ti K, and O K peaks. The
atomic coordination environment was analyze using XP-ray Absorption Fine Structure (XAFS) with a
Table XAFS-500-A system. In-situ XPS measuremeNnts was conducted with by the Thermo Escalab 250Xi
(Thermo Fisher, USA). The light absorption spectra were derived from the ultraviolet-visible diffuse
reflectance spectra (UV-vis DRS), measuLred with UV-3600 spectrophotometer from Shimadzu (Japan).
Photoluminescence (PL) properties were investigated using a fluorescence spectrophotometer (Fluorolog-
3, HORIBA Scientific, USA) with an excitation wavelength of 320 nm, and the time-resolved
photoluminescence (TRPLA) lifetime was calculated as detailed in Text. S1. Furthermore, the samples were
deposited on an indium tin oxide (ITO) conducting glass for photoelectrochemical measurements, with
further details provided in Text S2. CO temperature-programmed desorption (CO -TPD) and H
2 2 2
temperature-programmed reduction (H -TPR) were conducted using an AutoChem II 2920 chemisorption
2
instrument.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors report a photothermal catalyst composed of NiO and Ru nanoparticles supported on TiO₂ P25. This catalyst has been applied to the photo-methanation reaction, achieving notable CH₄ productivity under 15 Suns light irradiation. Photothermal catalysis has been highlighted as a highly promising approach for the conversion of CO₂ into valuable products, based on the synergy between photocatalytic and thermocatalytic mechanisms. Irradiation with visible light excites electrons in the catalytic material, generating electron–hole pairs that can transfer energy to activate reactant molecules. Simultaneously, the temperature increases the kinetic energy of these molecules, promoting their adsorption and facilitating the breaking of chemical bonds. This dual action helps reduce activation energy, enhance the reaction rate, and improve selectivity toward desired products, as demonstrated in this work.\nHowever, the proposed mechanism is not entirely convincing, and several fundamental issues should be addressed prior to publication.\nThe authors propose hydrogen spillover from the Ru surface to NiO, which acts as an \"effective channel\" for H. They suggest that the interface between Ru and Ni–O atoms is crucial for efficient photothermal methanation. While I fully agree that Ru is an efficient site for H-spillover, and NiO has previously been proposed as a good candidate for CO₂ activation, some inconsistencies raise concerns. For instance, in the proposed mechanism, the formation of a continuous interface between Ru and NiO is essential. However, there is a significant mismatch between the amounts of Ru and NiO loaded. While the optimal catalyst contains approximately 1 wt% Ru, the Ni content reaches nearly 40 wt%. This implies that, despite HRTEM images showing some NiO nanoparticles in contact with Ru, the majority of NiO is likely isolated from Ru. Therefore, only a small fraction of the NiO would participate in H· transfer from Ru.\nMoreover, the synergistic effect of Ru and NiO in the present case do not surpass the production of other systems reported in the literature containing one or the other element, as the authors show in Table S3 (i.e. https://doi.org:10.1002/aenm.201601657; https://doi.org:10.1038/s41560-021-00867-w; https://doi.org:10.1002/adfm.202008244).\nAlternatively, Figure 1C shows a non-negligible contribution from light to the overall mechanism, which remains approximately constant regardless of reaction temperature. In addition, photocurrent measurements, XPS peak shifts, PL quenching, and EIS results confirm efficient charge separation. It is well known that NiO is a p-type semiconductor, while TiO₂ is an n-type semiconductor. In this system, they form a heterojunction. Upon visible light irradiation, NiO can promote photo-induced charge transfer to TiO₂. Moreover, small Ru metal nanoparticles exhibit broad absorption in the visible and near-infrared range, potentially acting as antennas to harvest photons, in addition to facilitating H-spillover. Similar systems have been previously reported by Chaudret and Garcia (https://doi.org/10.1002/chem.201803022).\nOverall, a photo-induced charge separation mechanism between NiO/TiO₂, enhanced by the photosensitizing effect of Ru, could be occurring instead of—or in addition to—the thermal mechanism proposed. Confirmation or refutation of this hypothesis should be supported by experimental and theoretical evidence, especially considering the low proportion of interfacial Ru/Ni–O contact sites.", "reviewer_2": "The authors investigate the role of hydrogen migration dynamics in modulating the kinetics and thermodynamics of photothermal CO2 methanation. To support their experimental findings the authors employ density functional theory (DFT) calculations to investigate reaction pathways and hydrogen spillover at the Ru0–NiO interface. The theoretical simulations provide a mechanistically sound explanation of the observed catalytic behavior, and their findings align with experimental results.\nNotably, the identification of a spillover-induced shift in *COOH adsorption configuration from Ni–O to Ni–Ni is an important insight.\nThe comparison of energy barriers across Ru(001), NiO(100), and their interface strengthens the central claim regarding enhancement through hydrogen spillover.\nThe authors utilize the PBE functional in combination with a Hubbard U correction, with effective U values of 6.3 eV (O 2p), 4.2 eV (Ti 3d), and 4.6 eV (Ni 3d), and they appropriately include van der Waals interactions. The choice of PBE+U is well justified, especially given the large unit cells in combination with AIMD simulations, where a trade-off between accuracy and computational cost is unavoidable.\nOverall, I consider the presented work as novel, original, highly significant and to be of good fit for Nature Communication.\nOverall, I recommend the manuscript to be published in Nature Communications if my comments have been addressed.\nSince my primary expertise is in theoretical simulations, my comments will focus solely on this aspect of the manuscript.\n(1) My main concern is that it is not clear to the reader how the authors obtained the U values. Were they calculated (e.g., via first-principles approaches) or simply taken from the literature? More critically, how do these specific U values influence the simulation results? Even if the values were derived in a somewhat first-principles manner, this still could affect the results as these methods are known to be error prone. More details on how the U values have been obtained and a brief discussion with maybe a test calculation on if the U values affect their results would improve the quality of the manuscript. This can be down additionally to the computational details already mentioned in the SI.\n(2) In Figure S12, some values related to the calculated vibrational frequencies appear to be missing (or maybe this is just in my copy of the file). Also, it would improve reproducibility if the authors could give more details on how they calculated these frequencies. I am guessing they also employed DFT with the same settings for all other calculations? If so how do U values alter this? This could be explained with either a clear justification for the U values used (see point 1) or just some simple test simulations.\n(3) The authors calculate binding energies and electronic activation barriers along the reaction pathway, which provide useful qualitative insights. However, it would strengthen the mechanistic interpretation if the manuscript included at least a brief discussion on the limitations of analyzing binding energies and electronic barriers alone, rather than full free energy profiles that account for entropic and thermal effects.\nI do not consider this a critical issue given the high complexity of such calculations and the fact that the experimental findings support the conclusions. Nonetheless, including a short discussion acknowledging these limitations would improve the overall quality and completeness of the manuscript.", "reviewer_3": "In this work, Nie and collaborators present a photothermal catalyst, (NiO/Ru⁰)/TiO₂, designed to enhance CO₂ methanation by improving hydrogen migration dynamics. The catalyst achieves highly selective and complete conversion of CO₂ to CH₄ through efficient H₂ dissociation on Ru⁰ and subsequent hydrogen spillover to NiO. This alters reaction kinetics and improves catalytic performance. Although the results of this work may be of interest to the photo-thermal catalysis community, from my point of view, this study is merely incremental and far from providing a ground-breaking advancement in the field of photothermal catalysis. In fact, Santamaria and colleagues recently reported a similar system to the one presented here, that is, a RuNi/TiO2 catalyst for the photo-thermal methanation of CO2 (see \"Continuous photothermal gas-phase CO2 hydrogenation over highly dispersed Ru-Ni on TiO2.\" Catalysis Today (2025): 115440). In addition to this, the methodology for the photo-thermo catalytic tests is not fully explained (for instance, authors did not provide any details on reaction time) and the relative contribution of light and heat on the reaction mechanism is poorly described.\nFor all these reasons, I cannot recommend the publication of this work in Nature Communications. Detailed comments to support this decision can be found below:\na) Authors wrote: “To assess the contributions of photons and thermions to the performance…”. Do the authors indeed refer to thermions (electrons emitted from a heated material) or phonons (vibrations) instead? Please provide an explanation for this statement.\nb) What is the reaction time for experiments shown in Figure 1? Are those productivities calculated at a single point corresponding to 100 % conversion? Authors should provide data points at shorter reaction times to study the evolution of the product distribution with time. This is particularly important in closed batch reactors where consecutive reactions may happen.\nc) Is there any effect of the composition of the catalyst on the product distribution? In other words, the selectivity towards CH4 is always 100 % irrespective of the catalyst?\nd) The influence of the irradiation wavelength is poorly described. As per these experiments, UV light would have a minor effect, whereas IR would largely contribute to the catalytic activity. However, authors should clarify if these experiments were performed at constant irradiance, otherwise, the comparison is not fair. Upon introducing cut-off filters, the total number of photons is reduced, which inevitably reduces the catalytic activity. To draw correct conclusions, authors need to make sure that the number of photons under different irradiation wavelengths remains constant.\ne) The relative contribution of thermal and non-thermal effects on the catalytic activity has not been discussed. Is the system dominated by thermal or non-thermal effects? Which is the larger contribution? Is the role of thermal and non-thermal effects influenced by reaction conditions (light intensity, irradiation wavelength, temperature). This is a work on photo-thermal catalysis and these questions need to be discussed.\nf) The methodology to assess catalyst stability should be improved. Authors performed a stability test consisting of 5 consecutive catalytic cycles, each lasting 3 hours, until the system reaches 100 % conversion. To have a better insight into the stability of the catalyst, I suggest authors to avoid testing the catalyst until full conversion is achieved. Stability tests should be performed at low conversions (below 50 %, ideally 20-30 %) to ensure that the system is at kinetic conditions. In the kinetic regime, the reaction rate is primarily determined by the intrinsic activity of the catalyst and is not significantly affected by external factors (for instance mass transport limitations). When stability tests are performed under kinetic conditions, any decrease in reaction rate over time can be attributed to catalyst deactivation. Therefore, in this case it would be more insightful to perform a larger amount of cycles (10 or more) at shorter reaction times.\ng) Authors claim that “Additionally, the H-spillover in NR-TiO2 was further corroborated using the WO3 color reaction, a well-established method for indicating the activation of H2 at the catalyst”. A reference should be included to demonstrate that this is indeed a well-established method.\nh) In Figure S20, the images corresponding to W-TiO2 are missing.\ni) In Figure 3, a), b), c) and e) letters are missing."}, "author_response": "We are grateful for the reviewer’s comments and suggestions. We have addressed the issues raised by the reviewer carefully and revised accordingly, further explanation of which can be found in the response to Specific Comment R1-3 below. [Detailed rebuttal text follows...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have now provided new experiments and explanantions, answering succesfully most of my questions and the concenrs arised from the previous version. In view of these new evidences I would like to recommend for publication in the present form.", "reviewer_2": "I appreciate that the authors have carefully and thoroughly addressed all of my comments. The manuscript has improved significantly and can be accepted for publication in its current form.", "reviewer_3": "I acknowledge the work and effort the authors have put into improving the manuscript. That said, I still believe that this study does not represent a significant advance in the field of photothermal catalysis that would justify its publication in Nature Communications. First, I reiterate my previous comment that the catalyst does not present any novelty. Beyond what was reported by Santamaria et al., there are additional works in this direction (see doi.org/10.1002/adfm.202414931). Of course, the use of Ru- or Ni-based catalysts on TiO₂ has been widely reported (see https://onlinelibrary.wiley.com/doi/full/10.1002/ange.202318166 , doi.org/10.1002/cctc.202200182, https://doi.org/10.1016/j.pnsc.2018.02.004 , https://doi.org/10.1016/j.apcatb.2023.123600 , https://onlinelibrary.wiley.com/doi/full/10.1002/ange.202300129 ). And this is not an exhaustive list, there are many more examples. TiO₂-based catalysts are, by far, among the most widely studied systems in photo- and photothermal catalysis. Regarding the H₂ spillover effect, this has also been reported previously in Ru/TiO₂ photocatalysts (see for example https://doi.org/10.1016/j.jcis.2023.06.206 ). With all these comments I do not intend to discredit the present work, I simply aim to justify my view regarding its incremental nature.\nSecond, the authors have included various experiments to address my comments on the reaction mechanism, for example the calculation of the OLE. I appreciate these experiments because they help in understanding the system, but once again, the same strategy used by Prof. Liu’s group in their Nature Catalysis work has been followed here. There is no new strategy, it is simply following the path that others have already established. With full respect for the authors’ work, these types of approaches are not the ones that merit publication in Nature.\nAside from all this, and although it should not be the sole or main reason, the catalytic activity is far from representing a milestone in the field. Nor has a new reaction system been proposed, or new characterization strategies to disentangle the complex reaction mechanisms governing photo-thermal systems.\nWith all this in mind, I confirm that this study does not represent a breakthrough in the field of photothermal catalysis, though I once again acknowledge the effort and work of the authors."}, "author_response": "We appreciate the reviewer very much for the comments and suggestions, which help us improve the quality of our work. We have thoroughly revised the manuscript accordingly. [Detailed rebuttal text follows...]"}, {"round": 3, "reviewer_comments": {"reviewer_3": "After evaluating the second revised version, I still find that the manuscript does not meet the novelty and significance standards required for publication in Nature Communications. The key concerns relate to fundamental aspects of originality that, in my view, cannot be resolved through additional revisions. For this reason, I cannot recommend publication, and I believe that further review rounds would not be productive. I would prefer not to review additional versions of this manuscript. Thanks for your understanding."}, "author_response": ""}]

Decision letter