Molecular fence Cu-based catalyst for CO_2 hydrogenation to CO with high activity and durability

Domain: chemistry
Published: 2026-03-06
Editorial outcome: major_revision

Abstract
The conversion of CO into high-value syngas (CO and H ) using Cu-based catalysts has garnered
2 2
considerable interest. However, these catalysts deactivate rapidly due to Ostwald-ripening and thermal
aggregation. Herein, we propose to encapsulate Cu-cluster into mMOR zeolite for engineering a
separation-site catalyst Cu@mMOR. In the reverse water-gas shift reaction (RWGS), mMOR zeolite of
Cu@mMOR directly captures and then activates CO , and simultaneously serves as a molecular fence to
2
prevent CO contact with Cu-clusters surrounding with small pore of mMOR (< 3.0 Å) channels. These
2
encapsulated Cu-clusters in mMOR possess considerable Cu vacancies, which significantly facilitate H
2
heterolytic dissociation, transferring H* species towNards activated CO
2
on mMOR and converting them
into CO. This separation-site strategy efficientl y increases catalytic activation while intelligently altering
2
the CO reaction pathway, preventing Cu from thermal agglomeration and Oswald ripening in the catalyst.
2
Consequently, Cu@mMOR attainTs a space-time yield of CO as high as 3290 mmol g -1 h-1, with catalytic
cat
stability extending up to 7A88 hours.

Introduction
The ongoing and excessive emissions of CO have led to global climate change, which is currently
2
receiving considerable attention. The conversion of CO into high-value chemicals is essential for
2
mitigating the greenhouse effect and maintaining a sustainable environment and energy supply1. Among
these, syngas (CO and H mixture) is reported to be an essential chemical used in the large-scale
2
production of alcohols, esters, and liquid fuels2. Recently, the insufficient provision stemming from
reliance solely on fossil resources has spurred growing interest in catalytic CO hydrogenation-a
2
promising strategy for converting greenhouse gases into valuable syngas3. Despite its apparent appeal,
this approach presents formidable obstacles.

The reverse water-gas shift reaction (RWG S) is a desirable and promising CCU technology that
catalytically converts CO into syngas under a H environment. Operating the RWGS at atmospheric
2 C 2
pressure and temperatures abovTe 500 °C enables direct conversion of CO into syngas with the ideal
2
H /CO ratio for Fischer-TAropsch synthesis, requiring only moderate energy input and avoiding extra
2
downstream separation4. For this reaction, Cu-based catalysts are promising non-noble metal candidates
for RWGS because of their high hydrogenation activity and high CO selectivity5. In general, high
temperature during the RWGS reaction usually tends to lead to the sintering of Cu, which can result in
catalyst deactivation via the aggregation of Cu NPs. Encapsulating the active metal inside thermally robust
zeolites is an effective way to curb high-temperature aggregation. For instance, Xiao et al. embedded Pt
NPs within Beta zeolite via a seed-mediated growth route, constructing a rigid “protective shell” around
the Pt that markedly suppressed thermal migration and aggregation6. Likewise, Liu et al. confined sub-
nanometer Pt clusters in the straight channels of MEL zeolite, firmly anchoring the naked Pt species and
achieving excellent stability in methyl-cyclohexane dehydrogenation7. These reports confirm that zeolite-
confinement can efficiently mitigate thermally induced particle aggregation. However, high-temperature
sintering can also proceed via an Ostwald-ripening path, where volatile metal species migrate from
interior pores to the outer surface and aggregate. For example, Julian et al. reported that confined Mo
zeolite still diffuses outward during high-temperature methane dehydrogenation, causing agglomeration
and coking8. Analogous to the above catalysts, Cu species are susceptible to forming volatile Cu-carbonyl
complexes under CO-rich conditions, leading to significant catalyst deactivation9. Therefore, the
simultaneous inhibition of Ostwald-ripening alongside the migration and aggregation of Cu species in the
RWGS reaction remains a crucial challenge for acNhieving high-performance catalysts that exhibit both
efficient activation and enduring stability.

Herein, we proposed to construct Cu@mMOR (copper-encapsulated modified mordenite zeolite) that
encapsulate Cu clusters within TmMOR (modified mordenite zeolite) crystals to realize catalytic site
separation for CO and H A activation in RWGS reaction. For our designed catalyst, mMOR was triggered
2 2
to crystallize around Cu clusters and encapsulate Cu species insides via its Na+-induced crystallized small
pore channels (< 3.0 Å) linking with Cu-O bonds. CO was efficiently captured and activated on outside
2
mMOR, which served as the molecular fence to isolated CO from Cu clusters depended on its small
2
channels. On the other hand, H was dissociated into H* on internal Cu clusters and spillover toward
2
activated CO via Cu-O interface of Cu@mMOR, and finally produced CO with high selectivity. The
2
catalytic site separation strategy significantly enhanced catalytic activation for both CO and H , and
2 2
rationally changed the CO reaction pathway, thereby effectively preventing Cu Oswald ripening and
2
thermal aggregation in catalysts. Therefore, a space-time yield of 3290 mmol g -1 h-1 was attained in
cat
Cu@mMOR at high-temperature conditions (600 °C), showcasing the long-term stability of catalyst
performance over a duration of 788 hours (49.1% CO -conversion-rate and 99.6% CO-selectivity).
2

Results
Design idea and synthesis strategy for Cu@mMOR
Fig. 1 showed our design idea and the corresponding synthesis strategy for mMOR-confined Cu catalyst
(denoted as Cu@mMOR) having separated catalytic sites for H and CO . Firstly, SiO nanospheres were
2 2 2
used to encapsulate Cu-based precursor via a reversed-phase micro-emulsion method (seen Fig. 1a-b).
Next, Cu-based precursor were calcined and reduced into Cu particles surrounding with SiO (named
2
Cu@SiO ,Fig. 1c). In this process, Na+ was used as a directing agent to induce the growth of small Si-O
2
cages (< 3.0 Å) in mMOR to isolate CO from Cu species. Next, the whole part of SiO of Cu@SiO was
2 2 2
gradually converted into mMOR zeolite under the iNnduction of N-methyl-pyrrolidine and followed by the
second calcination and H reduction (seen Fig. 1d-f and Supplementary Fig. 1). Finally, a portion of Na+
2
was selectively retained while the remainder was converted to NH + via low-concentration NH Cl
4 4
exchange, followed by calcinatioTn to ultimately form H⁺, resulting in the formation of Cu@mMOR.

Morphology of different Cu-based catalysts
It is evident from Fig. 2a-b that Cu@mMOR contained numerous Cu black dots (1.5 ± 0.3 nm) that were
homogenously dispersed throughout the nanosheets. These amorphous Cu nanodots were encapsulated in
mMOR, which was identified by the lattice fringes of (110) plane10 (Fig. 2c). According to the elemental
energy-dispersive X-ray spectroscopy (EDS)-mapping images depicted in Fig. 2d, the Cu element was
homogeneously distributed throughout the mMOR zeolite. However, the particle size of Cu in spherical
Cu@SiO (~20 nm, Fig. 2e) was about ~2.1 nm (Fig. 2f), which was obviously larger than that in
2
Cu@mMOR. It can be inferred that Cu was further reduced into smaller clusters in a confined space
during the crystal transformation from SiO to mMOR. In contrast, Cu-impregnated mMOR (denoted as
2
Cu/mMOR) contained nano-sized particles of approximately 4.4 nm on its outer surface of mMOR (Fig.
2g), and these nanoparticles displayed distinct lattice patterns corresponding to the metallic Cu (111)
crystal plane11 (Fig. 2h). It can be seen that the copper acetylacetonate impregnation method proved
challenging for incorporating Cu nanoparticles into mMOR crystals and effectively controlling their
particle size within mMOR.

X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses (Supplementary Fig. 2 and
Supplementary Table 1) revealed that Cu/mMOR and Cu@mMOR had high crystallinity of MOR type
zeolite, whereas Cu@SiO showed a diffuse X-ray peak of amorphous silica phase. This indicated that
2
the amorphous silica nanosphere was completely transformed into a highly crystalline MOR zeolite
structure in Cu@mMOR12. In addition, non e of the three catalysts (Cu@SiO , Cu/mMOR, and
2
Cu@mMOR) showed clear diffraction peaks of Cu crystals. However, the Cu element detected in XRF
(Supplementary Table 1) had aT very similar percentage composition in the range of 2.6-4.1 wt.%,
suggesting that Cu speciAes were highly dispersed in these catalysts. Additionally, the XPS survey
(Supplementary Fig. 3) showed the slight presence of Na element in Cu@mMOR but not in Cu/mMOR,
implying that the former remains some Na+ sites. Moreover, the Cu 2p XPS patterns of Cu@mMOR and
Cu@SiO exhibited much lower peak area than that of impregnated Cu/mMOR (Supplementary Fig. 4),
2
suggesting the extensive encapsulation of Cu nanoparticles rather than their exposure on the MOR
external surface13. Next, the pore texture (Supplementary Fig. 5 and Supplementary Table 2) of
Cu@mMOR showed a slight reduction in N adsorption uptake (427.1 m2 g-1) of type I isotherm compared
2
to that of pure mMOR (530.1 m2 g-1). This meant that most of the micropore structure of mMOR
maintained upon encapsulating Cu nanoparticles in mMOR. In contrast, Cu/mMOR exhibited a lower
surface area of 310.7 m2 g-1, indicating a partial loss of the mMOR porous structure due to the obstruction
caused by the impregnated Cu nanoparticles14. As anticipated, Cu@SiO had minimal porous structures
2
attributed to the low porosity of SiO . The textural structure analysis inferred that Cu@mMOR maintained
2
the mMOR porous property after encapsulating Cu species with homogeneous distribution, which would
favor the adsorption and transport of reactants and active species during the reaction. Further, positron
annihilation lifetime spectroscopy (PALS) was conducted to determine the pore size distribution of the
ultra-microporous region in MOR, mMOR, and their Cu-based catalysts15. The deconvoluted PALS
spectra display four distinct peaks corresponding to different positron annihilation lifetimes (τ -τ ;
1 4
Supplementary Fig. 6a). An increase in τ value indNicates larger pore sizes within the MOR framework.
Here, τ denotes the bulk lifetime, which is attrib uted to direct annihilation within the non-porous regions16.
1
The remaining lifetimes τ -τ correspond to positron annihilation within the 5-membered, 8-membered,
2 4
and 12-membered ring channels Tof MOR, respectively (Supplementary Fig. 6b-c). In comparison to the
support, the τ componenAts associated with 8-MRs exhibited distinctly opposite trends in Cu@mMOR
3
and Cu/mMOR. Specifically, the calculated pore size for Cu/mMOR increased to 3.6 Å. This observation
aligns with the pore-blocking effect identified via N adsorption isotherms (Supplementary Fig. 5),
2
ascribed to the obstruction of pore mouths by external Cu particles. Thus, it compels positrons to
annihilate within larger inter-particle voids on the surface17. In contrast, the encapsulation strategy yielded
a shrunken pore size of 2.8 Å for Cu@mMOR, directly confirming the effective occupation of the 8-MR
channels by Cu species. Furthermore, coincidence doppler broadening (CDB) analysis revealed a higher
electron density for Cu@mMOR in the low-momentum valence electron region (< 4×10-3 m c;
0
Supplementary Fig. 7), corroborating the tight confinement of copper clusters within the unique micro-
environment constructed by the 5-MRs and 8-MRs18. Notably, this shrunken pore mouths of Cu@mMOR
(2.8 Å) precisely matches the kinetic diameter of H (2.8 Å) while remains smaller than that of CO (3.3
2 2
Å)19. Conceptually, this double-ring encapsulation architecture could act as a "molecular fence" for
suppressing thermal aggregation and Ostwald ripening via the spatial separation of CO and H .
2 2

Structural characterization of different Cu-based catalysts
To better understand the local structure of Cu species in different micro-environments, X-ray absorption
spectroscopy (XAS) was performed in order to evaluate the Cu chemical state and coordination status in
various catalysts. The edge energy of the two catalysts (Cu@mMOR and Cu/mMOR) at the Cu K-edge
(Fig. 3a) was positioned between the Cu foil and CuN2 O standards, with no pre-edge band of Cu2+ detected
in the rising edge. This implied that Cu species were in the form of Cu0 and Cu+ rather than Cu2+ in the
two catalysts20. Compared to Cu/mMOR, Cu@mMOR exhibited a much higher Cu adsorption edge
energy, attributing to a higher proTportion of Cu+ species in Cu@mMOR21. To quantify the Cu+ species in
the catalyst, a mathematicAal relationship between the absorption edge energy position and the valence
state was established by fitting data from the corresponding standards22, 23. As shown in Supplementary
Fig. 8, the Cu+ content in Cu@mMOR was calculated to be ⁓38.1 at.%. In contrast, the Cu+ proportions
in Cu/mMOR and Cu@SiO were determined to be only 9.0 and 6.9 at.%, respectively. The significantly
2
higher Cu+ content in Cu@mMOR is indicative of enhanced interfacial charge transfer from Cu0 to
mMOR, a process facilitated by the abundant interfaces inherent to the encapsulation structure.

The local coordination environment of Cu species in various catalysts was identified by fitting the
Fourier transform extended X-ray absorption fine structure (FT-EXAFS) of the various catalysts, as
illustrated in Fig. 3b. As shown, Cu@mMOR exhibited a notable Cu-O peak at 1.46 Å, as well as minor
Cu-O-Cu and Cu-Cu peaks at 2.57 and 2.08 Å, respectively, indicating that most of its Cu species were
coordinated with O within Cu@mMOR24. Furthermore, the wavelet transform (WT) plot of Cu@mMOR
in Fig. 3c-1 showed an obvious Cu-O bond (k = 3.2-5.6 Å-1, R = 1.8-1.9 Å) and a weak Cu-Cu bond (k =
6.2-7.4 Å-1, R = 2.1-2.7 Å) when compared to Cu O (Fig. 3c-2) and Cu foil (Fig. 3c-3) counterparts25,
2
corroborating the results from the FT-EXAFS spectra. The shell fitting analysis presented in
Supplementary Table 3 depicts that most of the copper in Cu@mMOR coordinated with oxygen (O) and
formed Cu-O clusters, with only a small amount of Cu-Cu bonding remaining26. The XANES, FT-EXAFS,
and WT plot results proved that Cu-O clusters dominated the main position in the Cu@mMOR, with a
small portion of metallic Cu clusters in mMOR. Differently, Cu/mMOR and Cu@SiO
2
(Supplementary
Fig. 9) displayed significant similarities to Cu- foil in their EXAFS and WT plot features, reflecting the
predominant presence of metallic Cu0 with larger nanoparticle sizes in Cu/mMOR and Cu@SiO .
2
From the above results, the schematic rTepresentation of Cu@mMOR formation is illustrated in Fig. 3d. Initially,
the SiO in Cu@SiO begAan to crystallize into mMOR. Then, Cu grains confined in mMOR underwent
2 2
mMOR compression and tore into smaller Cu nanoclusters. During this process, confined space
continuously generated strong interactions between Cu and silanol, resulting in enrichment of the Cu-O
coordination within mMOR. Thus, Cu@mMOR contained abundant Cu-O clusters with atomic
distribution in the mMOR crystal. Conversely, this character was not observed for Cu/mMOR due to the
lack of the in situ mMOR crystallization process.

Moreover, the types and contents of defects in Cu clusters within Cu@mMOR were investigated via
UV-Raman spectroscopy, with the results shown in Fig. 3e. As seen, the confined Cu@mMOR exhibited
a sharp peak at 221 cm-1 and a broad peak around 400-463 cm-1, corresponding to the defective Cu-O of
Cu vacancies (the missing Cu atom in Cu-O-Cu motifs) and Si-O of MOR framework, respectively27, 28.
In comparison, Cu/mMOR displayed a negligible peak of defective Cu-O around Cu vacancies at 218 cm-
1 and a minor peak of Si-O of MOR at 397 cm-1. The intensities of these peaks indicated that Cu@mMOR
had a much higher concentration of Cu vacancies than Cu/mMOR, which was attributed to the Cu-O
defective coordination during the crystallization of mMOR from SiO . Additionally, Raman spectra
2
(Supplementary Fig. 10) were also measured for Cu@mMOR and Cu@SiO . As observed, the defective
2
Cu-O around Cu vacancies had a significantly higher peak intensity with a blue shift from 215 to 221 cm-
1 in Cu@mMOR compared to Cu@SiO . This once again demonstrated that the lattice compression of
2
crystal transformation induced the contraction of CNu-O bonds in Cu clusters, resulting in the generation
of additional Cu vacancies29.

It has been reported that Cu vacancies can significantly influence the redox properties of catalysts30, 31.
Furthermore, H temperature-pTrogrammed reduction-mass spectrometry (H -TPR-MS) analyses of
2 2
various catalysts were caArried out to investigate the redox properties of Cu. Initially, all catalysts
underwent complete oxidation to CuO prior to measurement. Fig. 3f showed the TPR-MS profiles of
Cu@mMOR and Cu/mMOR. As shown, hydrogen reduction profile of Cu@mMOR exhibited a single
hydrogen reduction peak at 262°C, whereas the corresponding peak for Cu/mMOR appeared at a higher
temperature (ca. 300°C). This significant shift to a lower temperature suggested that the copper species in
Cu@mMOR possessed smaller sizes and higher reducibility32. To quantitatively assess the hydrogen
spillover effect, the hydrogen spillover capacity was determined by calculating the additional H
2
consumption beyond that used for water formation. The results showed that each Cu atom in Cu@mMOR
consumed an extra 0.45 H atoms, while the Cu/mMOR exhibited almost no additional H consumption
2
(0.01 H/Cu). These results suggest that a significant hydrogen spillover phenomenon occurred in
Cu@mMOR, thereby demonstrating that the synergy between its confined microenvironment and copper
vacancies effectively promotes H activation and spillover.
2

Importantly, the electron transfer induced by metal vacancies can alter the surface acidic properties of
support33. In order to better understand this effect, NH -TPD (Supplementary Fig. 12) was performed to
3
explore the acidic concentration and acidity of mMOR in various catalysts. As displayed, pristine mMOR
had a total acid content of 580 μmol g-1, mainly consisting of strong (peak > 350 ºC) and weak (peak <
250 ºC) acids34. Compared to pristine mMOR, Cu@mMOR showed a notable reduction in total acid
content to 197 μmol g-1, with the retained portNion almost composed of strong acids. Conversely,
Cu/mMOR exhibited a slight reduction in all a cids, yielding a total value of 398 μmol g-1. Furthermore,
pyridine-IR spectra were utilized to gain insight into the effect of Cu loading mode on the acid distribution
of catalysts, as shown in SupplemTentary Fig. 13. Compared to Cu/mMOR, Cu@mMOR demonstrated a
much larger abatement inA content of Lewis acid sites (LAS), owing to the numerous coordination-
saturated Cu clusters in confined microenvironment35. Interestingly, the remaining Brønsted acid sites
(BAS) in Cu@mMOR almost belonged to strong acid sites36, aligning well with our NH -TPD curves
3
(Supplementary Fig. 12). As calculated, its strong BAS ratio in total BAS was up to 81.7%, far higher
than those of mMOR (62.3%) and Cu/mMOR (57.0%). This increase was mainly contributed to the
enriched Cu-acid interactions in mMOR confined spaces, which induced massive electron transfer from
Cu to the O of BAS (as evidenced by Cu XANES in Fig. 3a and O 1s XPS in Supplementary Fig. 14).
Meanwhile, this charge transfer could further enable Cu@mMOR to retain more Na+ sites (mentioned in
Supplementary Fig. 3), which are stabilized by stronger electrostatic interactions towards O of
Cu@mMOR37.

Reaction performance of Cu@mMOR and related catalysts
The effects of Cu diverse microenvironment within prepared Cu-based catalysts on their catalytic activity
for CO hydrogenation to syngas were evaluated, and the results are presented in Fig. 4. As seen in Fig.
2
4a, there was an upward tendency in the CO conversion of each catalyst when the reaction temperature
2
increased from 450 to 650 °C. By comparison, Cu@mMOR exhibited a superior RWGS activity and was
very close to the thermodynamic equilibrium (calculated using HSC6.0 software)38, which could reach
49.1% of CO conversion at 600 °C. In addition, it had the lowest CO apparent activation energies (34.09
2
± 6.88 kJ mol-1) among the three catalysts (SuppNlementary Fig. 15), signifying its superior catalytic
activity for CO conversion.
2

Additionally, a higher H ratio in the feed gas was beneficial in improving CO conversion for all three
2
catalysts (Fig. 4b). Notably, Cu@TmMOR and Cu@SiO demonstrated a similar variation trend in CO
2 2
conversion with H /CO raAtios to the thermodynamic equilibrium curve, with the former one approaching
2 2
close to the dashed line of reaction thermodynamic equilibrium, demonstrating much higher catalytic
activity of Cu@mMOR than that of Cu@SiO . On the contrary, the trend of Cu/mMOR shifted away from
2
the dashed line of reaction thermodynamic equilibrium with an increase in H /CO ratios. The discrepant
2 2
RWGS trends in encapsulated catalysts and exposed catalysts mainly presented different activation
mechanisms. For the encapsulated catalyst Cu@mMOR, the reaction likely followed an H-assisted
mechanism39. In this process, H facilitates CO activation through active species, which is a typical
2 2
feature in dual-site catalysts40. However, as an exposed catalyst, Cu/mMOR may require H and CO to
2 2
compete for the same site for activation (the Redox mechanism)41. Therefore, the excessive dosage of H
2
would pre-occupy Cu active sites and suppress the trend of H promoting CO conversion. Fig. 4c depicts
2 2
the variation trend in CO conversion over various catalysts as space velocity increased. As seen, it
2
showed a plateau and then a slight descend of CO conversion in Cu@mMOR with the increase of space
velocity. By contrast, Cu@SiO and Cu/mMOR displayed obvious descend trend, indicating Cu@mMOR
2
possessed an ultra-rapid pathway from CO into CO compared to Cu/mMOR and Cu@SiO 42.
2 2
Based on the above results, H /CO ratios of 2, space velocities of 270 L g -1 h-1, and a reaction
2 2 cat
temperature of 600 °C were chosen as the plausible conditions for the subsequent durability test. As
presented in Fig. 4d, Cu@mMOR exhibited a steady period of up to 788 hours with its corresponding
CO
2
conversion remaining similar at around 49.1%N, which consistently approached the thermodynamic
limit of CO conversion (50.2%). On the contra ry, Cu/mMOR showed much faster catalytic deactivation
2
with markedly reduced CO conversion. Moreover, it demonstrated a significant decline in catalytic
2
stability, with its CO conversionT plummeting from 30.8% to 13.5% after 158 h of reaction. Obviously,
2
Cu@mMOR with the uniqAue MOR-confined microenvironment could withstand the ordeal of a long-term
RWGS reaction. Through calculation, the highest space-time yield (STY) of CO over Cu@mMOR was
3290 mmol g -1 h-1 and the max stability duration reached up to 788 hours, displaying the highest STY
cat
and notable durability, surpassing most of the reported Cu-based catalysts under a severe condition of
high temperature and high flow43-52 (Fig. 4e and Supplementary Table 4).

To further understand the structural evolution of Cu species during the long-term reaction process, the
structure and morphology of fresh catalysts and spent catalysts after 120 h reaction were determined by
using XRD and TEM, as shown in Fig. 4f-h. Spent Cu/mMOR displayed obvious diffraction peaks at 43°
and 51° assigned to metallic Cu bulks53, which did not appear in the fresh samples. This suggested that
the local aggregation of Cu increased with reaction time. On the other hand, no distinct Cu diffraction
peaks were found in spent Cu@mMOR, implying a high dispersion of Cu species within mMOR.
Additionally, TEM images revealed that Cu species in fresh and spent Cu@mMOR had nearly the same
particle size of approximately 2 nm. In the case of Cu/mMOR, the Cu nanoparticles increased in size from
an initial range of 3-6 nm in the fresh sample to a range of 15-33 nm in the spent sample. Taking account
of these above characterizations, the sintering of Cu species was the main cause of the Cu/mMOR
deactivation.

RWGS reaction mechanism of Cu@mMOR
To validate the sites separated effect of Cu@mMONR for CO
2
and H
2
, N
2
O was employed to replace CO
2
for touching with Cu species in Cu@mMOR an d Cu/mMOR owing their similar molecular configuration
and diffusion properties54. At medium temperature, N O selectively oxidized the surface and subsurface
2
Cu0 and Cu+ species of the catalTysts55. The resulting oxidized Cu species were subsequently quantified
via H -temperature prograAmmed reduction (H -TPR). As depicted in Fig. 5a and Supplementary Fig. 16,
2 2
the Cu@mMOR sample exhibited only trace H reduction peak around 281 °C, whereas Cu/mMOR
2
displayed a pronounced reduction peak at 294 °C. Correspondingly, the H O signal generated during the
2
reduction of Cu@mMOR was significantly weaker than that of Cu/mMOR. These results confirm that
most Cu sites in Cu@mMOR were effectively shielded from N O oxidation, demonstrating the 'molecular
2
fence' function of the tight encapsulation. In contrast, the absence of such restriction in Cu/mMOR led to
substantial exposure of Cu species to the N O atmosphere. Finally, CO -DRIFTS was conducted at
2 2
elevated temperatures to further validate the molecular fence effect under realistic reaction conditions. As
demonstrated in Fig. 5b, Cu@mMOR exclusively showed absorbed CO and carbonate species upon CO
2 2
introduction without other reacted species, while the conventional Cu/mMOR catalyst immediately
exhibited characteristic CO vibrational bands. These results demonstrated that the molecular fence
structure of Cu@mMOR effectively prevented CO from accessing Cu clusters even under actual reaction
2
conditions, thereby maintaining spatial isolation of the active sites.

Next, CO -TPD was employed to analyze the influence of Na+ alkaline sites on mMOR for CO
2 2
adsorption and active sites. As displayed in Supplementary Fig. 17, Cu@mMOR demonstrated strong
CO chemisorption at 231 °C, whereas Cu/mMOR without Na+ displayed an upward-trending peak above
2
450 °C. This implied that Na+ may serve as the CO chemisorption site in Cu@mMOR56. Furthermore,
2
CO
2
-TPD, NH
3
-TPD, and CO
2
hydrogenation perfoNrmance of the Cu@mMOR with varying Na contents
were systematically studied to investigate the influence of Na+ on the CO chemisorption, acidic site
2
density, and CO hydrogenation activity. As shown in Supplementary Fig. 18a-b, CO chemisorption
2
capacity of Cu@mMOR exhibiTted a well-defined linear increase with rising Na+ content, while the
protonation degree (acidicA site density) decreased progressively with Na+ incorporation. Meanwhile, CO
2
hydrogenation rate (Supplementary Fig. 18c) displayed a characteristic volcano-shaped trend with
increasing Na+ content, which initially rose to a maximum and then declined. This volcano-type behavior
suggested that Na+ played an auxiliary role in activating CO . Specifically, Na+ enhanced the adsorption
2
of CO on adjacent Si-O--Al sites via electrostatic interactions, thereby increasing the surface coverage
2
and residence time of CO 56. This enhancement ultimately contributed to the superior CO hydrogenation
2 2
performance of Cu@mMOR.

Subsequently, the CO
2
diffusion process in Cu@mMOR was validated using ab initio molecular
dynamics simulations (AIMD). As seen in Supplementary Movie 1, CO was only prone to diffuse in the
2
channels of mMOR at a reaction temperature of 600 °C. It is noteworthy that the distance between CO
2
and Na+ site gradually transitioned from a fluctuating state to a stabilizing one at 3.4 Å (Supplementary
Fig. 20). This indicated that CO was rapidly adsorbed by the Na+ site during diffusion, occurring within
2
approximately 10 ps. Constrained ab initio molecular dynamics (cAIMD) was further employed to probe
the CO adsorption-activation mode at 600 °C (Supplementary Movie 3-7). As illustrated in
2
Supplementary Fig. 21, the formed carbonate involved the lowest energy barrier (0.7 eV) to bend O-C-O
bond. This demonstrated that Na+ markedly reduced the energy barriers for both CO adsorption and
2
activation. Therefore, it was proved that CO rapidly bonded with Na+ and then formed carbonate during
2
its diffusion through the channels of Cu@mMOR, Neffectively triggering the reaction57.

H adsorption behaviors of Cu@mMOR and Cu/mMOR were carried out through H -TPD. As seen in
2 2
Supplementary Fig. 22, Cu@mMOR had a higher desorption temperature than Cu/mMOR, suggesting
that the Cu-O clusters in Cu@TmMOR are more favorable for adsorbing H
2
. Next, H
2
dissociation
behaviors of various catalyAsts were evaluated by using H-D exchanged tests, and the obtained results were
shown in Fig. 5c. The results clearly revealed that Cu@mMOR exhibited varying HD production rates
compared to Cu/mMOR as the temperature increased (Eq. 5, SI). In contrast, the HD curve of Cu/mMOR
grew slowly until it significantly rose at 435 °C. Its formation rate of HD was only 78 μmol g -1 s-1 at
cat
500 °C with an H dissociation rate of 3.9%. By comparison, Cu@mMOR showed about 11.5 times the
2
H* formation rate than Cu/mMOR, attributing to abundant Cu vacancies in Cu@mMOR boosting H
2
dissociation capacity58. To further validate this hypothesis, Fig. 5d illustrated the free energy profiles of
the H dissociation process on various models (the detailed models see Supplementary Fig. 34-35). These
2
models include Cu@mMOR with Cu vacancies (denoted as Cu@mMOR-V ), Cu@mMOR without Cu
Cu
vacancies (denoted as Cu@mMOR-absent-V ), and Cu/mMOR. As shown, the energy barrier of H
Cu 2
dissociation of Cu@mMOR-V , Cu@mMOR-absent-V and Cu/mMOR were 0.37, 0.70, and 1.23 eV,
Cu Cu
respectively. It indicated that Cu vacancies promote accelerated H-H bond cleavage. Notably, in
Cu@mMOR systems, the dissociated H* species preferentially coordinate with both Si-O--Al groups and
Cu+ sites, leading to the formation of H+/H- pairs (Supplementary Fig. 34). Consequently, Cu@mMOR
utilized Cu-O clusters to adsorb and activate H , promoting its heterolytic dissociation through its Cu
2
vacancies. This phenomenon boosted CO reduction by providing a steady supply of highly reductive H*
2
species.

To unravel the chemical nature and dynamic behNavior of hydrogen spillover at the molecular level, the
evolution of hydrogen-containing species on Cu@mMOR were monitored using steady-state isotopic
transient kinetic analysis coupled with FTIR (SSITKA-FTIR) during H /D switching. As depicted in
2 2
Supplementary Fig. 36, under a HT atmosphere, distinct signals assigned to hydride species (Al-H and Si-
2
H) and characteristic peAaks for protic species (Si-OH-Al and Si-OH) were clearly observed on
Cu@mMOR59, 60. The coexistence of these H+ and H- species provided direct evidence for the heterolytic
cleavage of H on Cu@mMOR and identifies the specific nature of the resulting fragments. Upon
2
switching the feed to D , the intensities of these hydrogenated species gradually decayed, with a
2
concomitant growth of deuterated species peaks at the lower wavenumbers59, 60. This rapid H/D isotopic
exchange demonstrated that the generated H+/H- pairs exhibited high mobility and dynamic exchange
activity over the framework acid-base sites61, 62.

Next, in situ EPR was carried out to investigate the electronic mechanism driving H heterolytic
2
activation. As shown in Supplementary Fig. 37, Cu@mMOR displayed a strong signal associated with
unpaired electrons on the Cu clusters under H feeding63. When the atmosphere was switched to CO /H ,
2 2 2
this EPR signal decreased markedly. This indicated that the electrons were transferred to adsorbed
hydrogen to generate H- species, which subsequently migrate into the channels and participate in the
RWGS reaction. Furthermore, XPS analysis was conducted to elucidate the migration and consumption
dynamics of hydrogen species by monitoring valence shifts of Cu and Al in Cu@mMOR under H and
2
CO /H atmospheres. As depicted in Supplementary Fig. 38, H treatment decreased the binding energies
2 2 2
of both Cu and Al to indicate an electron-rich state, whereas the subsequent introduction of CO /H
2 2
triggered a synchronous positive shift in these values64, 65. These findings corroborated the spillover of H-
species from Cu surfaces onto the framework, wherNe they were consumed by the reaction with CO
2
within
the pores. Collectively, this unique H+/H- pai r migration mechanism facilitated an efficient hydrogen
supply. Furthermore, it modulated the hydrogenation pathways of formate intermediates via electrostatic
interactions to enhance both the sTustainability and selectivity of the RWGS reaction.

Furthermore, the spillovAer kinetics of the catalyst were evaluated using the kinetic isotope effect (KIE).
In this context, the CO formation rate under CO /H and CO /D atmosphere was respectively measured
2 2 2 2
and the corresponding KIE values with reaction time increasing were shown and calculated in
Supplementary Fig. 39. As shown, the KIE value (r /r ) of Cu@mMOR increased progressively from
H D
1.03 at 300 °C to 1.43 at 600 °C, revealing a pronounced temperature-dependent discrepancy. Generally,
the magnitude of KIE is governed by the zero-point energy (ZPE) and the tunneling effect66. The observed
rise in KIE with temperature indicated that the hydrogen spillover process on the Cu@mMOR catalyst
was accompanied by a significant quantum tunneling effect, contrasting with the temperature-diminishing
ZPE contribution67. This tunneling process facilitated the rapid spillover of active hydrogen species from
Cu sites into the mMOR channels, thereby promoting CO reduction.

Notably, the increasing KIE value suggested that more hydrogen-containing species were involved in
the H* spillover process of Cu@mMOR68. Additionally, the DRIFTS of Cu@mMOR revealed the
presentation of H(H O) *, which initially increased before reaching a steady state (Supplementary Fig.
2 n
40). These findings implied that H(H O) * may function as a transporter of H* during the spillover process
2 n
in Cu@mMOR69, 70. To deeply investigate the spillover progress of H(H O) *, an AIMD simulation was
2 n
carried out and displayed in Supplementary Movie 2. The video shows that H* on the Cu-O cluster was
prone to coordinate with H O and subsequently migrated within the mMOR channel. As calculated in
2
Supplementary Fig. 42, the energy barrier of H
2
O-aNssisted spillover was only 0.29 eV, lower than most of
the reported values (0.4 ~ 0.8 eV)71-73. In this sy stem, [H(H O)*] had a higher tendency to migrate through
2 n
the channels of Cu@mMOR owing to its delocalization. Thus, the transfer efficiency of H* in
Cu@mMOR was significantly imTproved, allowing adsorbed CO to be rapidly reduced. Furthermore, the
2
independent gradient modAel based on Hirshfeld partition (IGMH, Supplementary Fig. 43a) indicated that
within the Cu@mMOR channels, H- could form H(H O) - clusters with water molecules via strong
2 n
hydrogen bonds74. This interaction facilitated the dispersion of the negative charge on H-, thereby
enhancing thermodynamic stability. Simultaneously, the stability of H(H O) - was found to increase with
2 n
the number of water molecules (Supplementary Fig. 43b-c), implying that H- was thermodynamically
driven to approach the CO reaction sites where water was produced75. To further investigate the water
2
effects on hydrogen spillover, H /D and H +H O/D +D O switching SSITKA experiments were
2 2 2 2 2 2
performed on Cu@mMOR (Supplementary Fig. 44). At which, water-assisted hydrogen spillover
exhibited an extremely long residence time of 199.4 s under H +H O/D +D O switching conditions
2 2 2 2
(Supplementary Fig. 44b), significantly exceeding the 46.6 s observed during H /D switching
2 2
(Supplementary Fig. 44a). The markedly extended residence time indicates that water enables hydrogen
spillover to occur over a significantly much longer distance on Cu@mMOR via facile the O-H bond
cleavage and the pronounced quantum tunneling effects of H(H O) +/H(H O) - 76. Thereby, it established
2 n 2 n
a highly efficient cycle for the spatially separated RWGS reaction by maximizing the active hydrogen and
CO contact.
2

In order to elucidate the reaction species of RWGS over Cu@mMOR, DRIFTS tests under mixed
CO /H atmospheres (CO /H -DRIFTS) were respectively conducted to analyze the variation of RWGS
2 2 2 2
intermediates from CO
2
and H
2
, and the obtainNed spectra are illustrated in Fig. 5e. As observed,
Cu@mMOR displayed distinct peaks of CO * (carbonate species) and Al-H around 1310-1450 cm-1,
2
which demonstrated an upward trend with increasing reaction time. Meanwhile, the peaks of HCOO*
species (1603 cm-1), H O (3363 cTm-1), B-acids (3588 cm-1), and CO* (2152 cm-1) appeared and maintained
2
a dynamic equilibrium77-8A0. These intermediates confirmed that the RWGS reaction over Cu@mMOR
likely followed exclusively the formate pathway, and H O molecules actively participated in H+ transfer
2
during H* spillover. Furthermore, steady-state isotope transient kinetic analysis (SSITKA) experiments
were performed to the Cu@mMOR system to gain deeper kinetic insight. The procedure involved
equilibrating the catalyst under a CO /H atmosphere, followed by a rapid switch to an isotopic gas
2 2
mixture (13CO /H or CO /D ) supplemented with an inert Kr internal standard at an identical flow rate.
2 2 2 2
The dynamic evolution of surface species and effluent products were simultaneously monitored in real-
time via infrared spectroscopy and mass spectrometry, respectively. As shown in Supplementary Fig. 46-
52, the residence times τ of key reaction species over Cu@mMOR followed the order: 13CO * (3.4 s, CO
2 2
chemisorption) < H13COO* (11.4 s, CO hydrogenation) < H12COO* (46.4 s, formate dissociation) < Al-
2
H (57.5 s, hydrogen spillover). Among which, Al-H species exhibited the longest isotopic residence time,
indicating that hydrogen spillover was kinetically limited and thus identified as the reaction’s rate-
determining step (RDS)81-84.

Density functional theory (DFT) calculations were performed to investigate the CO hydrogenation
2
mechanism over the Cu@mMOR at the atomic level. As shown in Fig. 5g, H heterolytically dissociated
2
on the Cu cluster, generating active H+ and H- that migrate to the Si-ONa-Al site. Meanwhile, CO was
2
chemisorbed to carbonate at the same site. In this chemisorption configuration, CO * (carbonate species)
2
could be hydrogenated to COOH+ via the carboxyNl pathway or to HCOO- via the formate pathway. A
comparison of energy barriers exhibited that for ming COOH+ needed 0.98 eV, whereas producing HCOO-
required only 0.64 eV. Due to the lower barrier, CO * preferred to follow formate pathway, which was
2
consistent with the DRIFTS obTservation that detected only the formate intermediate (Fig. 5e). Next,
HCOO- may either becomAe protonated or directly dissociate into CO. Protonation is kinetically much
more favorable, with a very low barrier of only 0.43 eV, leading easily to HCOOH formation. In contrast,
direct dissociation pathways face much higher barriers (1.72 eV for HCOO⁻ and 2.04 eV for COOH⁺),
making them far less likely. These substantial barriers (particularly for COOH+) were attributed to the
necessity of rupturing an additional Al-H bond to facilitate electron transfer, strong antibonding orbital
repulsion, and the requirement to overcome hydrogen bonding interactions (Supplementary Fig. 53-54
and Supplementary Movies 11-14) 74, 85-88. Subsequently, the generated HCOOH could either dissociate
directly or undergo further hydrogenation. By comparing both energy barrier, direct dissociation of
HCOOH required merely 0.92 eV, whereas successive hydrogenation must overcome higher barriers of
2.04 eV and 0.93 eV. In addition, AIMD further exhibited that the dissociation energy barrier of HCOOH
inside the mMOR channel was 0.24 eV lower than that outside the channel (Supplementary Fig. 55).
Therefore, HCOOH rapidly decomposed to CO and H O within the Cu@mMOR channels, which
2
explained why Cu@mMOR achieved nearly 100 % CO selectivity. Notably, DFT calculations supported
that Cu@mMOR followed the formate pathway and identified H+/H- migration from Cu to the mMOR as
the highest-energy step, establishing it as the rate-determining step in line with our SSITKA analysis
(Supplementary Fig. 52).

By comparison, Cu/mMOR showed no apparent vibration peaks of hydrogenated intermediates in
CO /H -DRIFTS (Fig. 5f), signifying the absenceN of H* spillover for the reduction of CO
2 2
2
to CO. In
addition, its KIE curve displayed that the K IE value was almost similar in CO /H and CO /D
2 2 2 2
(Supplementary Fig. 39). In addition, the intensities of typical vibration peaks (1900 to 2100 cm-1) of
metal carbonyl compounds increTased with reaction time, indicating a gradual rise of Cu(CO) species in
n
Cu/mMOR89. These speciAes were typically linked to the Oswald-ripening phenomenon resulting from
excessive CO adsorption90. Therefore, unencapsulated Cu was prone to form highly mobile species in the
absence of an effective barrier to prevent CO adsorption (Fig. 6). It triggered Ostwald ripening, which
eventually led to the deactivation of Cu/mMOR.

In summary, confined Cu-O clusters in MOR zeolite (Cu@mMOR) were designed for a highly efficient
CO RWGS reaction through a separation strategy. We induced the growth of small cages (<3 Å) tightly
2
encapsulating Cu-O clusters in order to separate catalysis sites of Cu@mMOR for H and CO ,
2 2
respectively. At which, confined Cu-O clusters generated abundant Cu vacancies and robust Cu-O bonds
within the mMOR shell, thereby promoting the heterolytic dissociation of H by an 11.5-fold enhancement
2
compared to unencapsulated Cu/mMOR. Additionally, mMOR zeolite captured and activated CO with
2
fast kinetics (a rapid progress of ~10 ps), effectively forming a molecular fence to isolate CO from the
2
Cu-O clusters. In the continuous RWGS reaction, the generated H* species migrated from Cu-O clusters
to mMOR and contacted CO , realizing the reduction of CO to CO via the formate pathway. Interestingly,
2 2
H O was proved to serve as an effective H* carrier, which significantly promoted H* delocalization and
2
its reduction efficiency by a 20.5-fold increase. The established synergistic interactions endowed
Cu@mMOR with high catalytic activity in the RWGS reaction. During the long-term reaction,
Cu@mMOR prevented thermal aggregation owing to its strong Cu-O interactions. Moreover, the spatial
separation of CO prevented the Ostwald ripening Nof Cu-O clusters. Eventually, Cu@mMOR exhibited
2
remarkable RWGS activity of 3290 mmol CO g -1 h-1 under high space velocity conditions, along with
cat
788 hours of high-temperature stability, surpassing the majority of the latest reported Cu-based catalysts.
This work offers valuable insighTts for the development of catalysts that can overcome the challenges
associated with Ostwald riApening and agglomeration.

Methods
Catalyst preparation
Cu@SiO was prepared by a modified reported method from the literature91. Briefly, 3.37 g of Brij-58
2
was dissolved in 15 mL of cyclohexane in a 50 mL sample bottle at 50 °C under stirring. Then, 0.4 mL of
copper nitrate solution (2 M) was added to form a reverse microemulsion system. After 1 hour of stirring,
1 mL of ammonium hydroxide was added dropwise, followed by 2 g of tetraethyl orthosilicate after 30
minutes. The silica coating proceeded for 2 hours, after which 15 mL of isopropanol was added to
demulsify the microemulsion. The mixture was centrifuged to obtaiSn the precipitate (8000 rpm, with a
relative centrifugal force of 5800 g), which was dried in a vacuum oven at 70 °C for 12 hours. The dried
powder was then calcined at 500 °C in an air stream to remRove organic species. During calcination, the
temperature was increased slowly at a rate of 1 °C per minute to avoid the formation of quartz phases and
to prevent the rapid diffusion of decomposition gases, which could create excessive meso-porosity.
Eventually, the product Cu@SiO was reduced at 773 K for 3 hours under a 30 mL/min flow of 5% H in
2 E 2
N .
2
Cu@mMOR was prepared via Ia 1-methylpyrrolidine (NMP)-assisted crystallization method. In a
typical preparation, 500 mg oRf Cu@SiO
2
was mixed with sodium aluminate (0.513 g), sodium bromide
(1.28 g), aqueous sodium hydroxide (8.6 g, 30 wt.%), and deionized water (66.37 g) in a homogenizer at
a high speed of 20000 rpm to form a uniform solution. The purpose of this step was to enable the pre-
adsorption of Na+ on the Cu@SiO interface. Then, 23.3 g of silica sol and 6 g of the structure-directing
2
agent NMP were slowly added dropwise to this uniform solution. The purpose of adding NMP was to
accelerate the crystallization of the MOR framework. Next, the mixture was kept at 60 °C and stirred for
6 hours to form a uniform gel. Subsequently, the gel was transferred to a 150 mL Teflon-lined autoclave
for hydrothermal crystallization at 170 °C. After 4 days of heating, the solid product was separated by
centrifugation (8000 rpm, with a relative centrifugal force of 5800 g), washed with deionized water, and
then dried overnight at 120 °C. The sample was then calcined at 550 °C for 8 hours to remove the organic
components. To retain the Na as CO adsorption sites and avoid the damage of framework by Al leaching,
2
a “low-concentration, multi-step” exchange was adopted to precisely adjust the Na content. The calcined
sample was subjected to ion exchange with 0.1 M ammonium chloride solution three times for 2 hours
each, washed with deionized water, and dried overnight at 60 °C. Eventually, the dried sample was
followed by air calcination and H reduction at 500 °C to obtain Cu@mMOR.
2
Typically, the preparation of mMOR was similar to that of Cu@mMOR. The main difference was that
Cu@SiO was not added, and the amount of silica sol was adjusted according to the XRF result of
2
Cu@SiO .
2
As a comparison material, Cu/mMOR was prepared via a modified impregnation method. To increase
the presence of more Cu species on the catalyst surface, copper (II) acetylacetonate with a larger molecular
size was used as the Cu source. In a typical procedure, copper (II) acetylacetonate was first dissolved in
an appropriate amount of ethanol to prepare a uniform solutioSn. Subsequently, mMOR powder was
impregnated with this solution and ultrasonically dispersed in a sealed glass bottle for 30 minutes. The
excess solvent in the mixture was then removed by rotPary evaporation. Finally, the solid powder was
calcined at 500 °C for 4 hours and subsequently redNuced by H at 500 °C to obtain the Cu/mMOR sample.
2

Catalytic reaction test
The catalytic reaction tests for the rIeverse water-gas shift (RWGS) reaction were conducted in a mixed
bed reactor with an internal dRiameter of 14 mm and a length of 300 mm in downflow mode. To prevent
temperature fluctuations due to exothermic and endothermic reactions, a mixture of 20 mg of catalyst
(120 mesh) and 300 mg of quartz sand (100 mesh) was placed in the reactor and supported between quartz
wool. The catalyst was exposed to a CO /H flow. The reaction was carried out at 600 °C, 0.1 MPa,
2 2
CO /H = 1/2, and a GHSV of 270 L g -1 h-1, unless otherwise specified. The outlet light gases (CO, CH ,
2 2 cat 4
and CO ) were analyzed online via an Agilent 7820A gas chromatograph (GC) equipped with a TCD and
2
a PoraPak Q column (Agilent G3591-70010).
The conversion of CO (X ) and selectivity to CO (S ) were calculated as follows:
2 CO2 CO
X CO2 =(N CO2,in −N CO2,out )/N CO2,in 100% (1)
S CO = N CO,out /(N CO2,in −N CO2,out )100% (2)
At which, the N is the molar quantity of CO fed into the reactor, while N and N are the
CO2,in 2 CO,out CO2,out
molar quantities of CO and CO in the outlet gas, respectively.
2
All results were obtained with a carbon mole balance greater than 95%.

Characterizations
Various characterization methods, including HRTEM, EDS mapping, XRD, XPS, XRF, N adsorption-
2
desorption isotherms, temperature-programmed desorption (TPD), XANES, EXAFS, isotope analysis,
the in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and positron
annihilation spectroscopy (PAS), were adopted to reveal the strucSture, physical and chemical properties
and reaction mechanism over Cu@mMOR and related catRalysts. Specifically, the positron annihilation
lifetime (PAL) spectra were recorded using a PAL spectrometer (DPLS4000, Anhui Nuclear and
Electronics Technology Co., Ltd., China) with a time resolution of approximately 180 ps and a source
intensity of 20 µC. The coincidence LDoppler-broadening measurements were performed using a
DCDB3000 spectrometer (Anhui Nuclear and Electronics Technology Co., Ltd., China). The detailed
processes are provided in the Supplementary Information.

Theory calculations
Various theoretical calculation methods—including transition states calculation, charge density difference
contour plots, work function, crystal orbital Hamilton population (COHP), independent gradient model
based on Hirshfeld partition (IGMH) and Ab initio molecular dynamics (AIMD)—were adopted to gain
a deep insight for reaction mechanism of Cu@mMOR. The details of theory calculations are provided in
the Supplementary Information.

Data availability
The data that support the findings of this study are available within the paper and its Supplementary
Information. The data generated in this study are pNrovided in the Source Data file. All data are available
from the corresponding authors upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the present study, the authors designed a strategy of catalytic function separation to achieve highly efficient and durable CO2 hydrogenation to CO over Cu-based catalysts. Specifically, Cu nanoparticles were confined within the micropores of MOR zeolite, making these Cu sites only accessible to H2 dissociation but not to CO2 hydrogenation. Na+ cations residing in the channel of MOR were supposed by the authors as active sites to catalyze CO2 hydrogenation with H* species transferred from the separated Cu nanoparticles via hydrogen spillover. Although the proposed strategy is attractive and a seemingly excellent performance has been obtained for CO2 hydrogenation on the designed Cu@mMOR catalyst, I am concerned that some of the conclusions lack compelling evidence, particularly regarding the role of Na⁺ ions as efficient active sites for CO₂ hydrogenation. I am afraid I cannot recommend the publication of this manuscript in Nature Communications at least with the current content. My main concerns are addressed as follows.\n1. The authors claimed that Cu nanoparticles confined within the micropores of MOR zeolite were not accessible to CO2 and CO molecules. This conclusion was reached mainly based on the weak H2-TPR signals for the N2O-treated Cu@mMOR sample (Fig. 5a). However, it is well known the N2O treatment at mild temperatures can only oxidize metallic Cu to Cu+, and Figure 3 shows that the Cu+ species had already prevailed in the Cu@mMOR sample before the N2O treatment, indicating the Cu species was not able to be further oxidized in the N2O treatment. Therefore, the weak signals observed in the H2-TPR experiment are unsurprising, and this evidence is invalid to prove the inaccessibility of CO2 to the Cu surface.\n2. Related to the above issue, it is indeed surprising to see that Cu species in the Cu@mMOR sample cannot be reduced fully to the metallic state even the temperature in the H2-TPR process increased to 873 K (Figure 3f). Could the authors provide an explanation on this phenomenon? It is also suggested to quantify the H2 consumption per Cu atom in the H2-TPR shown in Figure 3f.\n3. It is noticeable that the authors removed most of Na+ cations in the synthesized Cu@mMOR catalyst via ion exchange with NH4Cl, while they claimed that the residual Na+ cations served as the active sites for catalyzing CO2 hydrogenation. I am afraid the authors need to examine whether the residual Na+ cations are exchangeable with NH4+ and whether the amount of the Na+ cations can correlate with the hydrogenation rate of CO2.\n4. What is the rate-determining step in CO2 hydrogenation over the Cu@mMOR catalyst? The authors did not make an unambiguous assessment. In general, the dissociation of H2 is a fast step on Cu, and the cleavage of C-O bond in CO2 is crucial. The authors showed the possible occurrence of CO2 hydrogenation on the Na+ sites via DFT calculations, while they did not demonstrate the hydrogenation can occur faster on the Na+ sites than on the Cu surface, which would be the basis for the excellent catalytic performance of Cu@mMOR. Moreover, if the Na+ cations are the key active sites for CO2 hydrogenation, how will the authors explain the better catalytic activity of Cu@SiO2 compared with that of Cu/MOR?\n5. The authors stated that CO2 hydrogenation on the Na+ cations preferred the formate pathway (via the HCOO* intermediate) over the carboxyl pathway (via the COOH* intermediate). The DFT evidence provided by the authors is not convincible. They only showed that the formation of HCOO* from CO2 possesses a lower activation barrier than that for the formation of COOH*, while they did not examine the activation barriers for their decompositions into CO* and OH*. It seems that the decomposition of HCOO* is much more difficult because it requires the cleavages of the C-H and C-O bonds at the same time.\n6. What is the nature of H* species transferred from the Cu sites to the Na+ sites via the hydrogen spillover, a radical or a proton? Generally, the diffusion of protons in the zeolite is much easier, but CO2 hydrogenation cannot be completed only with protons.\n7. Regarding the catalytic performance, the durability test for Cu@mMOR was conducted under a condition close to reaction equilibrium (Figure 4d). In such a case, the amount of catalyst is overqualified, making it unsuitable to assess the true catalyst stability. Similarly, the strong dependence of CO2 hydrogenation rate on the space velocity (Figure 4c) indicates the rate measurement was not within the reaction-controlled regime.\n8. The authors are recommended to provide clear definitions for mMOR and Cu vacancy before using or discussing these concepts in the manuscript.", "reviewer_2": "This paper deals with the selective hydrogenation of CO2 to CO over copper clusters encapsulated in mMOR zeolite (denoted as Cu@mMOR) at 600 °C. The catalysts achieved performances markedly superior to that of recently reported in the literature, with a space-time yield of CO as high as 3290 mmol gcat-1 h-1, a CO selectivity of up to 99.6% and a catalytic stability extending up to 788 hours. To comprehend how the catalyst works the authors compared its properties with those of mMOR impregnated (Cu/mMOR) or SiO2 encapsulated (Cu@SiO2) copper nanoparticles respectively. The bulk and surface properties of the catalysts were analyzed by several techniques (X-ray absorption spectroscopy, Fourier transform extended X-ray absorption fine structure, X-ray Absorption Near Edge Structure, UV-Raman spectroscopy, H2-TPR, H2-TPD, NH3-TPD, CO2-TPD, H/D exchange. The hydrogenation of CO2 was conducted in a flow-bed reactor. The reaction paths were investigated by DRIFTS-tests and simulated by ab initio calculations.\nCu@mMOR was prepared from Cu@SiO2 precursor obtained by reversed-phase micro-emulsion method. Na+ ions induced the formation of small cages (  3 nm) in the zeolite structure and also ensured a better separation of metal and oxide phases. Copper species were in the form of Cu0 and Cu+ rather than Cu2+ with a higher proportion of Cu+ species in Cu@mMOR. Most of the copper in Cu@mMOR coordinated with oxygen and formed Cu-O clusters, with only a small amount of Cu-Cu bonding remaining. The interactions of copper with the O atoms of mMOR were the strongest and gave rise to a better dispersion of copper sites and their isolation from mMOR sites. In contrast, the presence of metallic Cu0 with larger nanoparticle sizes was predominant in Cu/mMOR and Cu@SiO2. Cu@mMOR had a much higher concentration of Cu vacancies than Cu/mMOR. Moreover, additional Cu vacancies were generated by the contraction of Cu-O bonds in Cu clusters induced by the lattice compression. Brönsted acidity of mMOR notably diminished in Cu@MOR as compared to Cu/mMOR, however with the retained portion almost composed of strong acids. Cu@mMOR exhibited great amounts of hydrogen spillover incorporated and strong interactions between CO2 and Na+.\nBased on the above information (characterization and testings) the reaction pathway for Cu@mMOR was deduced as follows: CO2 was first captured on the Na+ sites of mMOR, then reduced by spillover H* to HCOO*, and finally formed CO and H2O.\nComments:\nThis is an excellent paper describing an original method of selective hydrogenation of CO2 to CO, a hot topic in both academic and applied research. The catalyst prepared exhibited higher performances than that recently reported in the literature The conclusions on the structural properties of the catalysts are supported by the use of a wide range of appropriate physical and physicochemical tools of fresh and spent catalysts. The results may be reproduced. It fulfills the standards required in heterogeneous catalysis. However, it presents some weaknesses.\nFirst, the content of the manuscript is an illustration of the hydrogen spillover effect in the case of hydrogenation reactions over zeolite encapsulated metal catalysts. Also the work of authors dealing with this topic should be cited (e. g. Choi et al., Nat. Commun. 2014; ChemCatChem. 2015).\nSecond, the role of Na+ ions and the triggering of the reaction are unclear.\nIndeed, according to the authors, these ions would form an additional sublayer strenghening the separation between copper and the support shell, preventing any contact between the former and CO2 (Figure 1d). However, a strong interaction was evidenced between the sodium ions and CO2. This is a contradiction, unless some sodium ions exist at the zeolite surface. In such a case, CO2, as a Lewis acid, would be adsorbed on surface O atoms or OH hydroxyls of the zeolite, forming (hydrogeno)carbonate species. In the presence of surface Na+ ions it would interact with surface ONa species forming carbonate entities. Such entities were not evidenced by the authors.\nThis point should be clarified.\nA reasonable pathway would be the direct reaction of CO2 with H spiltover species forming the formate species evidenced by the authors. In this way it is worth noting the recent claim (Bettahar, Catal. Rev., 2022; Catalysts, 2024) that the structure of hydrogen spillover in the case of non-reducible oxide supports consists in surface Hδ-/OHδ+ (0<δ<1) pairs resulting from the adsorption of the H spiltover species on conjugated Mm+ Op- (p=1 or 2) pairs. The conjugated pairs pre-exist in basic (e.g. MgO) or amphoteric (e.g. alumina) oxide supports. For hydroxylated/acidic supports (silica, alumina, zeolites) they are formed by a dehydroxylation process during the catalyst reduction. The reaction between the hydridic fragment of Hδ-/OHδ+ pairs and CO2 would produce formate species which are further decomposed to CO and H2O by the protonic fragment:\n+CO2\nMm+Op- + 2 Hspilt <=> M(m- δ)+Hδ- O(p- δ)-Hδ+ => M(m- δ)+HCOO δ- O(p- δ)Hδ+ =>=> Mm+Op- + CO+H2O\nFinally the above considerations pose the question of the effect of the degree of protonation of the zeolite on the Cu@mMOR catalyst performances? Would this degree beneficial or detrimental to the catalyst activity? The answer to this question may help the comprehending of the structural and chemical role of the Na+ ions.\nIn conclusion, the submitted article is of high experimental and theoretical level. As a whole, the interpretations of the results are very good, except that related to the role of the additional layer of Na+ ions. References should be completed. Therefore it may be published in Nature Communications with a revision tacking into account the above remarks.", "reviewer_3": "Authors report a highly active and selective Cu catalysts for high-T RWGS. It is an interesting case study to push toward the equilibrium limit. The characterizations and DFT studies help to gain understanding of active sites and mechanism.\nHowever, the significance of current study is not clear.\n1. Synthesis of particles in a confined spaces has been reported previously to limit aggregation under reaction. These studies need to be cited and discussed to highlight the significant contribution from current study as compared to the previous.\n2. RWGS on Cu or Cu-based catalysts has been extensively studied. Under CO2 and H2, Cu alone is known to produce CO as the major product. Although the high selectivity is reported here, the reaction temperature is very high compared to the typical. It is not surprising that increased temperature promote CO2 conversion and decomposition of methanol to CO, thus increasing CO selectivity. What is the benefit of this high-T RWGS inconsideration of energy cost?\n3. Apparent activation energies reported for three systems are all below 1 eV. It is not straightforward that such low barrier requires high temperatures (> 450C) to overcome.\n4. Surface of MOR was simulated. Why? How representative to the cage structure of MOR?\n5. It was reported that formate is very stable on Cu. The barrier for formate decomposition to CO needs to be calculated. In addition, the barriers for competitive hydrogenations to *HCOOH and *H2COO should also be compared to ensure the claimed pathway. Without that, the current conclusion is not convincing.\n6. There is no structure in Supplementary Fig. 20-21."}, "author_response": "Author reply: Thank the reviewer for the thoughtful comment. We agree with your speculation regarding the N O-H -TPR experiment. However, in this work, our Cu@mMOR catalyst is proved to contain a considerable amount of Cu0, which was not clearly confirmed in our original manuscript. This ambiguity likely led the reviewer to misinterpret the weak H -TPR signals as insufficient evidence for the interaction between CO and Cu nanoparticles.\nTherefore, in the revised manuscript, (1) we have clearly identified the proportions of Cu0 and Cu+ in each catalyst based on the white-line peak intensities (newly added Supplementary Fig. 6). (2) To further demonstrate the “molecular fence” effect of Cu@mMOR, we have added Si solid-state NMR (newly added Supplementary Fig. 14) characterization for Cu@mMOR to prove the isolation of CO from Cu nanoparticles. (3) We supplied content in CO -DRIFTS spectra in Fig. 5b via CO reaction with Cu nanoparticles.\nThe revised sections and supplying contents (Three Parts) were highlighted in the revised manuscript and shown as following:\nPart I:\nAdded part of the 1st derivative patterns of white-line (Supplementary Fig. 6), revised manuscript, page 9: “Furthermore, the Cu+ proportion of various catalysts was linearly correlated with the white-line peak location of standard samplesR1. As estimated in the 1st derivative patterns of the white-line peak (Supplementary Fig. 6), Cu@mMOR contained 48.0 at. % Cu0 and 52.0 at. % Cu+, whereas the Cu+ proportion in Cu/mMOR and Cu@SiO were 25.2 at. % and 28.1 at. %, respectively.”\nPart II:\nAdded part of Si-NMR (Supplementary Fig. 14), revised manuscript, pages 15: “To validate the molecular fence effect of Cu@mMOR for CO and H , the chemical environment of interface between Cu and mMOR was firstly investigated via the 29Si solid-state NMR. As seen in Supplementary Fig. 12, Cu@mMOR possessed three- and four-membered rings in the interfaces, which were generated from the lattice strain and expected to act as molecular fences that block CO R2.”\nPart III:\nRevised content of CO -DRIFTS (Fig. 5b), revised manuscript, pages 16: “Finally, CO -DRIFTS was conducted at elevated temperatures to further validate the molecular fence effect under realistic reaction conditions. As demonstrated in Fig. 5b, Cu@mMOR exclusively showed absorbed CO and carbonate species upon CO introduction without other reacted species, while the conventional Cu/mMOR catalyst immediately exhibited characteristic CO vibrational bands. These results demonstrated that the molecular fence structure of Cu@mMOR effectively prevented CO from accessing Cu clusters even under actual reaction conditions, thereby maintaining spatial isolation of the active sites.”\nSupplementary Figure 6 The 1st derivative patterns of white-line for Cu foil, Cu O, Cu@mMOR, Cu/mMOR, and Cu@SiO .\nSupplementary Figure 14 The (a) 29Si-NMR spectra and (b) magnified view for Cu@mMOR, Cu/mMOR and Cu@SiO .\nFig. 5b CO -DRIFTS of Cu@mMOR and Cu/mMOR.\nRevised manuscript references:\nR1. Pankin, I. A. et al. Identifying Cu-oxo species in Cu-zeolites by XAS: A theoretical survey by DFT-assisted XANES simulation and EXAFS wavelet transform. Catal. Today 345, 125-135 (2020).\nR2. Cheng, C.-H. & Shantz, D. F. 29Si NMR studies of zeolite precursor solutions. J. Phys. Chem. B 110, 313-318 (2006).\n2. Related to the above issue, it is indeed surprising to see that Cu species in the Cu@mMOR sample cannot be reduced fully to the metallic state even the temperature in the H -TPR process increased to 873 K (Figure 3f). Could the authors provide an explanation on this phenomenon? It is also suggested to quantify the H consumption per Cu atom in the H -TPR shown in Figure 3f.\nAuthor reply: We are grateful to the reviewer for the insightful suggestion. At first, the reason of remaining Cu+ specie under 873 K was mainly owing to the strong Cu-O interaction between Cu and mMOR zeolite. According to the FT-EXAFS (Fig. 3b), these strong Cu-O interactions exhibit bond lengths shorter than those in Cu O, which makes it extremely difficult to fully reduce the Cu species to their metallic state Cu (Cu0). Additionally, Cu clusters are commonly found their resistance to reduction in zeolitesR3.\nSecondly, in response to the reviewers’ request for supplemental quantitative TPR measurements, we have thoroughly revised and expanded the relevant section, and added the TPR data for CuO standard and Cu@SiO sample. Both qualitative and quantitative results of TPR are added in the supplementary information. The relevant sections of the manuscript have been revised and highlighted accordingly as follows:\nAdded and revised manuscript, pages 11: “Furthermore, H -TPR analyses of various catalysts were carried out to investigate the redox properties of Cu. Initially, all catalysts underwent complete oxidation to CuO prior to measurement. As shown in Fig. 3f and Supplementary Fig. 9, the reduction peaks of Cu@mMOR were mainly concentrated in the 170-260 °C range, whereas those of Cu/mMOR displayed a broad distribution (190-440 °C). This difference was maybe attributed to Cu species having uniform particle size in Cu@mMOR, which derived from the formation of size-uniform Cu clusters under the compression of rapid crystallizationR4. In addition, its temperature of reduction peak was much lower than that in Cu/mMOR, indicating that oxidative Cu clusters in Cu@mMOR exhibited smaller particle sizes than those on Cu/mMORR5. Furthermore, H consumption for the reduction of Cu@mMOR was calculated to 1.56 H per Cu atom, which was consistent with the half Cu+/Cu0 ratio obtained from XANES (Fig. 2a and Supplementary Fig. 6). This mixed-valence structure features multiple asymmetric site pairs (e.g., Cu0-Cu+) that effectively polarize and destabilize H molecules, promoting their heterolytic cleavage and consequently boosting the catalyst's hydrogenation performance.”\nAnd the quantitative analysis of TPR was shown in Supplementary Fig. 9 of revised supplementary information, which was as follow:\nSupplementary Figure 9 | The quantitative TPR analysis for various catalysts. The TPR profiles for (a) Cu@mMOR, (b) Cu/mMOR, (c) Cu@SiO , and (d) CuO standard.\nRevised manuscript references:\nR3. Cui, W.-G., Li, Y.-T., Yu, L., Zhang, H. & Hu, T.-L. Zeolite-encapsulated ultrasmall Cu/ZnO nanoparticles for the hydrogenation of CO to methanol. x 2 ACS Appl. Mater. Interfaces 13, 18693-18703 (2021).\nR4. Sagar, G. V., Rao, P. V. R., Srikanth, C. S. & Chary, K. V. R. Dispersion and reactivity of copper catalysts supported on Al O -ZrO . J. Phys. Chem. B 110, 2 3 2 13881-13888 (2006).\nR5. Wang, B. et al. Nature of active sites on Cu-CeO catalysts activated by high- 2 temperature thermal aging. ACS Catal. 10, 12385-12392 (2020).\n3. It is noticeable that the authors removed most of Na+ cations in the synthesized Cu@mMOR catalyst via ion exchange with NH Cl, while they claimed that the residual Na+ cations served as the active sites for catalyzing CO hydrogenation. I am afraid the authors need to examine whether the residual Na+ cations are exchangeable with NH + and whether the amount of the Na+ cations can correlate with the hydrogenation rate of CO .\nAuthor reply: The residual presence of Na ions is an intentional design in our preparation process to enhance the adsorption and activation of CO by the molecular sieve. In response to your inquiry, our reply is as follows:\nWe adopted a “low-concentration and multi-step” exchange to precisely adjust the Na content in mMOR to prevent all Na ions from being exchanged out. The residue Na sites in Cu@mMOR showing strongly bound with mMOR (evidenced by O XPS and XANES) were scarcely exchangeable under our preparation conditions.\nAccordingly, we have added ion exchange procedure for Na⁺ in Experimental section of the revised Supplementary Information, page 3: “The sample was then calcined at 550 °C for 8 hours to remove the organic components. To retain the Na as CO adsorption sites, a “low-concentration and multi-step” exchange was adopted to precisely adjust the Na content. The calcined sample was subjected to ion exchange with 0.1 M ammonium chloride solution three times for 2 hours each, washed with deionized water, and dried overnight at 60 °C. Eventually, the dried sample was followed by air calcination and H reduction at 500 °C to obtain Cu@mMOR.”\nSecondly, to answer “whether the amount of the Na+ cations can correlate with the hydrogenation rate of CO ”, we precisely modulated the Na+ content in Cu@mMOR through ion exchange. Furthermore, CO -TPD, NH -TPD, and CO hydrogenation performance tests (supplementary Fig. 16) were added to elucidate the impact of Na+ on the RWGS reaction performance. The corrected contents were highlighted in the revised manuscript and shown as following:\nRevised Supplementary Information, pages 17: “As shown in Supplementary Fig. 16a-b, CO chemisorption capacity of Cu@mMOR exhibited a well-defined linear increase with rising Na+ content, while the protonation degree (acidic site density) decreased progressively with Na+ incorporation. Meanwhile, CO hydrogenation rate (Supplementary Fig. 16c) displayed a characteristic volcano-shaped trend with increasing Na+ content, which initially rose to a maximum and then declined. These results elucidated that increased protonation degree (decline in Na+ content) effectively accelerated the CO hydrogenation in Cu@mMOR. Notably, excessive protonation weakened acidic CO chemisorption, ultimately impairing CO hydrogenation performance.”\nSupplementary Figure 16 | The role of Na+ in Cu@mMOR. The correlation of Na+ content with (a) CO chemisorption amount, (b) acid sites amount, (c) CO hydrogenation rate, which were obtained from CO -TPD, NH -TPD, and CO hydrogenation performance tests, respectively.\n4. What is the rate-determining step in CO hydrogenation over the Cu@mMOR catalyst? The authors did not make an unambiguous assessment. In general, the dissociation of H is a fast step on Cu, and the cleavage of C-O bond in CO is crucial. The authors showed the possible occurrence of CO hydrogenation on the Na+ sites via DFT calculations, while they did not demonstrate the hydrogenation can occur faster on the Na+ sites than on the Cu surface, which would be the basis for the excellent catalytic performance of Cu@mMOR. Moreover, if the Na+ cations are the key active sites for CO hydrogenation, how will the authors explain the better catalytic activity of Cu@SiO compared with that of Cu/MOR?\nAuthor reply: We thank the reviewer for the valuable suggestion.\n(1) The response to the question of “What is the rate-determining step in CO hydrogenation over the Cu@mMOR catalyst?”:\nWe apologize for not having clearly identified the corresponding rate-determining step in the original manuscript, which led to reviewer confusion. First, we reconstructed a model that more closely represents our catalyst structure. Next, based on the in situ DRIFTS experimental observations, we recalculated the DFT of reaction pathway (see added Fig. 5g). We then correlated these new results with the original KIE data (Supplementary Fig. 38). From this combined analysis, we conclude that the migration of an “H+/H- pair” from the Cu clusters to mMOR is the rate-determining step. The relevant details (KIE and reaction energy barrier) have been added in two paragraphs in the Mechanism section of revised manuscript (Two Parts), as follows:\nPart I:\nAdded and revised manuscript, pages 20: “As shown, the KIE value (r /r ) of Cu@mMOR increased progressively from 1.03 at 300 °C to 1.43 at 600 °C, revealing a pronounced temperature-dependent discrepancy. The significantly elevated KIE value of 1.43 (substantially greater than unity) strongly suggests that H* spillover served as the rate-determining step (RDS) in this reactionR6. This mechanistic distinction set it apart from the RWGS reaction observed in other catalytic systemsR7.”\nPart II:\nRevised manuscript, pages 22: “Notably, DFT calculations supported that Cu@mMOR followed the formate pathway and identified H+/H- migration from Cu to the mMOR as the highest-energy step, establishing it as the rate-determining step in line with our KIE analysis (Supplementary Fig. 38).”\nSupplementary Figure 38 | The kinetics of hydrogen spillover. The kinetic isotope effect patterns of Cu@mMOR and Cu/mMOR.\nFig. 5g Gibbs free energy profiles of the CO hydrogenation in Cu@mMOR.\nRevised manuscript references:\nR6. Zhu, J. et al. Boosting hydrogen evolution reaction activity of Ru anchored binary oxyhydroxide by F-doping in alkaline seawater. Nano Energy 121, 109249 (2024).\nR7. Kunkes, E. L., Studt, F., Abild-Pedersen, F., Schlögl, R. & Behrens, M. Hydrogenation of CO to methanol and CO on Cu/ZnO/Al O : Is there a common intermediate or not? J. Catal. 328, 43-48 (2015).\n(2) The response to the question of “The authors showed the possible occurrence of CO hydrogenation on the Na+ sites via DFT calculations, while they did not demonstrate the hydrogenation can occur faster on the Na+ sites than on the Cu surface, which would be the basis for the excellent catalytic performance of Cu@mMOR.”:\nOur Cu@mMOR catalyst has been confirmed to possess spatially separated CO - and H -activation sites, as demonstrated by N O-H -TPR and CO -DRIFTS analyses (Fig. 5a and Fig. 5b). In this manuscript, metallic Cu clusters dissociate H to generate H⁺/H⁻ species that spill over, whereas the Na⁺ sites within the mordenite framework adsorb CO and drive the subsequent hydrogenation step. These two functions were isolated by the mMOR zeolite channels. Therefore, when the two catalytic sites do not undergo hydrogenation simultaneously, their reaction rates cannot be compared.\n(3) The response to the question of “if the Na+ cations are the key active sites for CO hydrogenation, how will the authors explain the better catalytic activity of Cu@SiO compared with that of Cu/MOR?”:\nIn our system, factors other than Na can also influence the hydrogenation rate. For the Na-free catalysts of Cu@SiO and Cu/mMOR, they serve as references lacking mMOR and lacking encapsulated structure, respectively. Thus, they do not possess the significance for making direct comparisons. The superior CO hydrogenation performance exhibited by Cu@SiO may originate from its smaller catalyst particles, higher Cu content, and the more Cu-O interaction at the interface, etc.\n5. The authors stated that CO hydrogenation on the Na+ cations preferred the formate pathway (via the HCOO* intermediate) over the carboxyl pathway (via the COOH* intermediate). The DFT evidence provided by the authors is not convincible. They only showed that the formation of HCOO* from CO possesses a lower activation barrier than that for the formation of COOH*, while they did not examine the activation barriers for their decompositions into CO* and OH*. It seems that the decomposition of HCOO* is much more difficult because it requires the cleavages of the C-H and C-O bonds at the same time.\nAuthor reply: We thank the reviewer for pointing out the shortcomings in our theoretical calculations.\nIn response to the “while they did not examine the activation barriers for their decompositions into CO* and OH*.”, we have added the barriers for direct dissociation of HCOO* and COOH* (Fig. 5g), which required high barriers of 1.72 eV and 2.07 eV, respectively. Furthermore, we added the barriers for protonation of HCOO* and dehydration of HCOOH to CO, which possessed lower barriers than the direct dissociation of HCOO*.\nTo answer “It seems that the decomposition of HCOO* is much more difficult because it requires the cleavages of the C-H and C-O bonds at the same time.”: COOH+ must acquire two electrons from H- during its conversion to CO, making long-range electron transfer energetically unfavorable. In contrast, the decomposition of HCOOH required no additional electron transfer and could form a stable OC-H O transition state via the intramolecular hydrogen transferR8 (Supplementary Movie 8). As a result, the barrier of HCOOH dissociation was lower than that for COOH+.\nAccordingly, we have revised the DFT section in the manuscript and highlighted, as follow:\nRevised manuscript, pages 21-22: “Density functional theory (DFT) calculations were performed to investigate the CO hydrogenation mechanism over the Cu@mMOR at the atomic level. As shown in Fig. 5g, H heterolytically dissociated on the Cu cluster, generating active H+ and H- that migrate to the Si-ONa-Al site. Meanwhile, CO was chemisorbed to carbonate at the same site. In this chemisorption configuration, CO * (carbonate species) could be hydrogenated to COOH+ via the carboxyl pathway or to HCOO- via the formate pathway. A comparison of energy barriers exhibited that forming COOH needed 0.98 eV, whereas producing HCOO- required only 0.64 eV. Due to the lower barrier, CO * preferred to follow formate pathway, which was consistent with the DRIFTS observation that detected only the formate intermediate (Fig. 5e). Next, HCOO- faced a competition between direct dissociation and protonation. DFT calculations showed that the barrier of protonation was only 0.43 eV, far below the 1.72 eV needed for direct dissociation. This implied that HCOO- would rapidly protonate to yield HCOOH in Cu@mMOR.”\nFig. 5g Gibbs free energy profiles of the CO hydrogenation in Cu@mMOR.\nRevised manuscript references:\nR8. Fujita, E., Grills, D. C., Manbeck, G. F. & Polyansky, D. E. Understanding the role of inter- and intramolecular promoters in electro- and photochemical CO 2 reduction using mn, re, and ru catalysts. Acc. Chem. Res. 55, 616-628 (2022).\n6. What is the nature of H* species transferred from the Cu sites to the Na+ sites via the hydrogen spillover, a radical or a proton? Generally, the diffusion of protons in the zeolite is much easier, but CO hydrogenation cannot be completed only with protons.\nAuthor reply: We thank the reviewer for the very valuable question. We have added H -DRIFTS to probe the evolution of chemical groups during hydrogen spillover on Cu@mMOR, and introduced in situ EPR and ex situ XPS to monitor electron migration throughout the spillover process. Meanwhile, we have refined the model and made corresponding revisions to the descriptions regarding the TDOS and Cu-PDOS. Based on these added experiments, we infer that the nature of hydrogen spillover in Cu@mMOR is the migration of H+ and electrons within the mMOR framework and their recombination into H+/H- at Si-ONa-Al sites. Subsequently, H- attacks CO to trigger the RWGS reaction. Accordingly, we have revised the mechanism section in the manuscript and highlighted, as follow:\nRevised manuscript, pages 19: “Additionally, H -TPD (Supplementary Fig. 20) also exhibited that Cu@mMOR had a markedly higher amount (20.5 times) of spillover H* compared to Cu/mMOR. Furthermore, H was introduced into CO -saturated Cu@mMOR, and then in situ DRIFTS was employed to probe the evolution of surface species during the spillover process (Supplementary Fig. 34a). As the reaction progressed, the adsorption band of CO * gradually weakened, while the HCOO* band intensifiedR9-R12. Concurrently, sharp Brønsted-acid bands appeared and the Al-H signature grew steadilyR13. These results indicate that CO was reduced to HCOO* by H+/H- species, and H+ acted as the mobile carrier during hydrogen spilloverR14-R17.”\nRevised manuscript, pages 19-20: “Next, in situ EPR was carried out to determine whether the spillover process was accompanied by electron or radical migration. As shown in Supplementary Fig. 35, Cu@mMOR displayed a strong signal associated with unpaired electrons on the Cu clusters under H feeding. When the atmosphere was switched to CO /H , this signal decreased markedly, indicating that electron transfer occurred during hydrogen spillover. Furthermore, ex situ XPS offered direct evidence of the oxidation-state varies of Cu@mMOR (Supplementary Fig. 36). Under H feeding, the oxidation states for Cu and Al sites increased simultaneously, whereas they decreased in concert under CO /H reaction conditions. This indicated that those electrons were transferred from Cu clusters to Al sites. Meanwhile, the density of states (Supplementary Fig. 37) exhibited that Cu vacancies in Cu@mMOR shift the Cu electronic bands closer to the Fermi level, indicating Cu vacancies effectively accelerated the transfer of free electrons from the Cu clusters to mMORR18. In summary, H underwent heterolytic cleavage into H+ and H- species on the Cu clusters, followed by their independent migration through the mMOR framework via proton transfer and electron conduction. Ultimately, these species recombined into reactive H+/H- pairs at the CO adsorption sites, facilitating the rapid hydrogenation of CO . Evidently, hydrogen spillover bridged spatially isolated CO and H for subsequent reaction.”\nSupplementary Figure 34 | The variation of chemical groups in various catalysts during the hydrogen spillover and hydrogenation period. The H -DRIFTS profiles of (a) Cu@mMOR and (b) Cu/mMOR after adsorbing CO .\nSupplementary Figure 35 | The free electron dynamics in the RWGS progress of Cu@mMOR. The EPR spectra of Cu@mMOR under various conditions.\nSupplementary Figure 36 | The variation of elemental valences in Cu@mMOR during RWGS. The ex situ XPS (a) Cu 2p, (b) Cu 2p enlarged plot and (c) Al 2p spectra of Cu@mMOR under various conditions.\nSupplementary Figure 37 | The electron state of various catalysts. The (a) total density of states and (b) Cu projected density of states for the Cu@mMOR with Cu vacancy, the Cu@mMOR without Cu vacancy and the mMOR.\nRevised manuscript references:\nR9. Sun, Q. et al. Zeolite encaged Pd-Mn nanocatalysts for CO hydrogenation and formic acid dehydrogenation. Angew. Chem. Int. Ed. 59, 20183-20191 (2020).\nR10. Rabee, A. I. M. et al. Uncovering the synergy between gold and sodium on ZrO for boosting the reverse water gas shift reaction: in situ spectroscopic investigations. Appl. Catal. B 345, 123685 (2024).\nR11. Kondo, J. N., Iizuka, M., Domen, K. & Wakabayashi, F. IR study of H O adsorbed on H-ZSM-5. Langmuir 13, 747-750 (1997).\nR12. Osuga, R., Yokoi, T. & Kondo, J. N. IR observation of activated ether species on acidic OH groups on H-ZSM-5 zeolites. Mol. Catal. 477, 110535 (2019).\nR13. Manciu, F. S. et al. Spectroscopic and structural investigations of α-, β-, and γ- AlH phases. J. Raman Spectrosc. 42, 512-516 (2011).\nR14. Im, J., Shin, H., Jang, H., Kim, H. & Choi, M. Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures. Nat. Commun. 5, (2014).\nR15. Choi, M., Yook, S. & Kim, H. Hydrogen spillover in encapsulated metal catalysts: New opportunities for designing advanced hydroprocessing catalysts. ChemCatChem 7, 1048-1057 (2015).\nR16. Bettahar, M. M. The hydrogen spillover effect. A misunderstanding story. Catal. Rev. Sci. Eng. 64, 87-125 (2022).\nR17. Bettahar, M. M. The hydrogen spillover effect-a misunderstanding study II: Single oxide and zeolite supports. Catalysts 14, (2024).\nR18. Yan, G., Khan, S. A. & Vlachos, D. G. Charge transfer drives hydrogen adsorption, spillover, and hydroxylation at the Pt/γ-Al O interface. ACS Catal. 14, 13579-13590 (2024).\n7. Regarding the catalytic performance, the durability test for Cu@mMOR was conducted under a condition close to reaction equilibrium (Figure 4d). In such a case, the amount of catalyst is overqualified, making it unsuitable to assess the true catalyst stability. Similarly, the strong dependence of CO hydrogenation rate on the space velocity (Figure 4c) indicates the rate measurement was not within the reaction-controlled regime.\nAuthor reply: We thank the reviewer for the important comment.\n1. During the experimental design phase, we extensively referenced evaluation conditions from high-performance RWGS systems (see comparative analysis in Table S4). To ensure meaningful benchmarking, our reaction conditions were deliberately aligned with mainstream methodologies in the fieldR19-R28.\n2. Durability testing under equilibrium conditions is not intrinsically correlated with catalyst overloadingR29. In the kinetically controlled regime, an increase in space velocity reduces the contact time between the catalyst and reactants, consequently leading to a decrease in CO conversion. The trend of our CO conversion data with increasing space velocity (Fig. 4c) aligns with this behavior. In contrast, catalysts not operating within the reaction-controlled regime (e.g., those governed by external diffusion control) would exhibit increased CO conversion with rising space velocity. Clearly, this contradicts our experimental observations, confirming that our catalyst does not fall under the \"not within the reaction-controlled regime\" scenario as suggested by the reviewer.\nRevised manuscript references:\nR19. Zhang, X. et al. Highly dispersed copper over beta-Mo C as an efficient and stable catalyst for the reverse water gas shift (RWGS) reaction. ACS Catal. 7, 912-918 (2017).\nR20. Liu, H. X. et al. Partially sintered copper-ceria as excellent catalyst for the high-temperature reverse water gas shift reaction. Nat. Commun. 13, 867 (2022).\nR21. Jin, R., Easa, J. & O’Brien, C. P. Highly active CuO /SiO dot core/rod shell catalysts with enhanced stability for the reverse water gas shift reaction. ACS Appl. Mater. Interfaces 13, 38213-38220 (2021).\nR22. Yu, J. et al. Ultra-high thermal stability of sputtering reconstructed Cu-based catalysts. Nat. Commun. 12, 7209 (2021).\nR23. Bahmanpour, A. M. et al. Cu–Al spinel as a highly active and stable catalyst for the reverse water gas shift reaction. ACS Catal. 9, 6243-6251 (2019).\nR24. Pahija, E. et al. FeO nanoparticle doping on Cu/Al O catalysts for the reverse water gas shift. J. CO Util. 64, (2022).\nR25. Tarifa, P., González-Castaño, M., Cazaña, F., Monzón, A. & Arellano-García, H. Development of one-pot Cu/cellulose derived carbon catalysts for RWGS reaction. Fuel 319, 123707 (2022).\nR26. Xu, N. et al. Structure and activity comparison of Cu/Ce Zr O series catalysts y 1–y x in the reverse water–gas shift reaction. Energy & Fuels 38, 7158-7166 (2024).\nR27. Zhang, J., Wang, Y., Tian, J. & Yan, B. Cu/LaFeO as an efficient and stable catalyst for CO reduction: exploring synergistic effect between Cu and LaFeO . AlChE J. 68, e17640 (2022).\nR28. Li, Z.-X., Xu, K., Wang, W.-W., Fu, X.-P. & Jia, C.-j. Stabilized inverse Y O /Cu interfaces boost the performance of the reverse water gas shift reaction. Catal. Sci. Technol. 14, 3483-3492 (2024).\nR29. Zhang, M., Wang, M., Xu, B. & Ma, D. How to measure the reaction performance of heterogeneous catalytic reactions reliably. Joule 3, 2876-2883 (2019).\n8. The authors are recommended to provide clear definitions for mMOR and Cu vacancy before using or discussing these concepts in the manuscript.\nAuthor reply: We appreciate this constructive suggestion. The definitions for mMOR and Cu vacancy were added and highlighted as follows:\nRevised manuscript (the definition for mMOR), page 4: “Herein, we proposed to construct Cu@mMOR (copper-encapsulated modified mordenite zeolite) that encapsulate Cu clusters within mMOR (modified mordenite zeolite) crystals to realize catalytic site separation for CO and H activation in RWGS reaction.”\nRevised manuscript (the definition for Cu vacancy), page 10: “As seen, the confined Cu@mMOR exhibited a sharp peak at 221 cm-1 and a broad peak around 400-463 cm-1, corresponding to the defective Cu-O of Cu vacancies (the missing Cu atom in Cu-O-Cu motifs) and Si-O of MOR framework, respectively.”\nReviewer #2 (Remarks to the Author):\nComments:\n1. First, the content of the manuscript is an illustration of the hydrogen spillover effect in the case of hydrogenation reactions over zeolite encapsulated metal catalysts. Also the work of authors dealing with this topic should be cited (e. g. Choi et al., Nat. Commun. 2014; ChemCatChem. 2015).\nAuthor reply: We sincerely thank the reviewer for highlighting these important references, which we have now carefully cited in the revised manuscript. To further elucidate the mechanistic role of hydrogen spillover, we have conducted additional in situ H -DRIFTS experiments. The results have provided new insights into the hydrogen spillover process, which were summarized below:\nRevised manuscript, page 19: “Additionally, H -TPD (Supplementary Fig. 20) also exhibited that Cu@mMOR had a markedly higher amount (20.5 times) of spillover H* compared to Cu/mMOR. Furthermore, H was introduced into CO -saturated Cu@mMOR, and then in situ DRIFTS was employed to probe the evolution of surface species during the spillover process (Supplementary Fig. 34a). As the reaction progressed, the adsorption band of CO * gradually weakened, while the HCOO* band intensified. Concurrently, sharp Brønsted-acid bands appeared and the Al-H signature grew steadily. These results indicate that CO was reduced to HCOO* by H+/H- species, and H+ acted as the mobile carrier during hydrogen spilloverR14-R17.”\nRevised manuscript, page 19-20: “In summary, H underwent heterolytic cleavage into H+ and H- species on the Cu clusters, followed by their independent migration through the mMOR framework via proton transfer and electron conduction. Ultimately, these species recombined into reactive H+/H- pairs at the CO adsorption sites, facilitating the rapid hydrogenation of CO . Evidently, hydrogen spillover bridged spatially isolated CO and H for subsequent reaction.”\nSupplementary Figure 34 | The variation of chemical groups in various catalysts during the hydrogen spillover and hydrogenation period. The H -DRIFTS profiles of (a) Cu@mMOR and (b) Cu/mMOR after adsorbing CO .\nRevised manuscript references:\nR14. Im, J., Shin, H., Jang, H., Kim, H. & Choi, M. Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures. Nat. Commun. 5, (2014).\nR15. Choi, M., Yook, S. & Kim, H. Hydrogen spillover in encapsulated metal catalysts: New opportunities for designing advanced hydroprocessing catalysts. ChemCatChem 7, 1048-1057 (2015).\nR16. Bettahar, M. M. The hydrogen spillover effect. A misunderstanding story. Catal. Rev. Sci. Eng. 64, 87-125 (2022).\nR17. Bettahar, M. M. The hydrogen spillover effect-a misunderstanding study II: Single oxide and zeolite supports. Catalysts 14, (2024).\n2. Second, the role of Na+ ions and the triggering of the reaction are unclear.\nIndeed, according to the authors, these ions would form an additional sublayer strenghening the separation between copper and the support shell, preventing any contact between the former and CO (Figure 1d). However, a strong interaction was evidenced between the sodium ions and CO . This is a contradiction, unless some sodium ions exist at the zeolite surface. In such a case, CO , as a Lewis acid, would be adsorbed on surface O atoms or OH hydroxyls of the zeolite, forming (hydrogeno)carbonate species. In the presence of surface Na+ ions it would interact with surface ONa species forming carbonate entities. Such entities were not evidenced by the authors.\nAuthor reply: We thank the reviewer for the suggestion regarding the role of Na+ ions and CO adsorption-activation section.\nRegarding the reviewer’s concerns about the Na+ layer, it should be clarified that the Na+-rich layer only formed during Cu@mMOR crystallization, guiding SiO crystallization into small cages as the molecular fence around Cu NPs. After crystallization, most Na+ gradually migrated into mMOR channels, serving as CO adsorption sites. Furthermore, we have re-analyzed CO -DRIFTS and added cAIMD calculation to investigate Na+ trigger for the reaction. The revised manuscripts were shown and highlighted in Three Parts as follows:\nRevised manuscript (Part I), page 6: “In this process, Na+ was used as a directing agent to induce the growth of small Si-O cages (< 3.0 Å) in mMOR to isolate CO from Cu species. Next, the whole part of SiO of Cu@SiO was gradually converted into mMOR zeolite under the induction of N-methyl-pyrrolidine and followed by the second calcination and H reduction (seen Fig. 1d-f and Supplementary Fig. 1).”\nRevised manuscript (Part II), page 16: “As demonstrated in Fig. 5b, Cu@mMOR exclusively showed absorbed CO and carbonate species upon CO introduction without other reacted species, while the conventional Cu/mMOR catalyst immediately exhibited characteristic CO vibrational bands.”\nRevised manuscript (Part III), page 17: “Constrained ab initio molecular dynamics (cAIMD) was further employed to probe the CO adsorption-activation mode at 600 °C (Supplementary Movie 3-7). As illustrated in Supplementary Fig. 19, the formed carbonate involved the lowest energy barrier (0.7 eV) to bend O-C-O bond. This demonstrated that Na+ markedly reduced the energy barriers for both CO adsorption and activation. Therefore, it was proved that CO rapidly bonded with Na+ and then formed carbonate during its diffusion through the channels of Cu@mMOR, effectively triggering the reactionR30.”\nFig. 1 | Design idea and synthesis strategy for Cu@mMOR. a Model illustration of the inverse microemulsion step. b Model illustration of the inverse microemulsion after adding TEOS and NH ·H O. c Model illustration of Cu@SiO . d Model illustration of Cu@SiO with Na+-rich layer. e Model illustration of Cu@mMOR. f Atomic-scale illustration of Cu@mMOR.\nFig. 5b CO -DRIFTS of Cu@mMOR and Cu/mMOR.\nSupplementary Figure 19 | The CO adsorption configuration in Cu@mMOR.\nFree energy and O-C-O bond angle variations of CO in adsorption modes of (a) single-O and Na+, (b) single-C and Na+, (c) double-O and Na+, (d) single-C and O , (e) single-O and Na+ & single-C and O on Cu@mMOR.\nRevised manuscript references:\nR30. Mir, S. H., Yadav, V. K. & Singh, J. K. Efficient CO capture and activation on novel two-dimensional transition metal borides. ACS Appl. Mater. Interfaces 14, 29703-29710 (2022).\n3. This point should be clarified.\nA reasonable pathway would be the direct reaction of CO with H spillover species forming the formate species evidenced by the authors. In this way it is worth noting the recent claim (Bettahar, Catal. Rev., 2022; Catalysts, 2024) that the structure of hydrogen spillover in the case of non-reducible oxide supports consists in surface Hδ-/OHδ+ (0<δ<1) pairs resulting from the adsorption of the H spillover species on conjugated Mm+ Op- (p=1 or 2) pairs. The conjugated pairs pre-exist in basic (e.g. MgO) or amphoteric (e.g. alumina) oxide supports. For hydroxylated/acidic supports (silica, alumina, zeolites) they are formed by a dehydroxylation process during the catalyst reduction. The reaction between the hydridic fragment of Hδ-/OHδ+ pairs and CO would produce formate species which are further decomposed to CO and H O by the protonic fragment:\n+CO\nMm+Op- + 2 Hspilt <=> M(m- δ)+Hδ- O(p- δ)-Hδ+ => M(m- δ)+HCOO δ- O(p- δ)Hδ+ =>=> Mm+Op- + CO+H O\nAuthor reply: We appreciate the reviewer’s suggestion and references. In revised manuscript, H -DRIFTS (Supplementary Fig. 34a) was added to clarify the direct reaction of CO with H spillover species. Following the reviewer’s constructive suggestion, we have added the relevant analysis/discussion in the revised version (marked in yellow for easy identification):\nRevised manuscript, page 18-19: “Furthermore, H was introduced into CO -saturated Cu@mMOR, and then in situ DRIFTS was employed to probe the evolution of surface species during the spillover process (Supplementary Fig. 34a). As the reaction progressed, the adsorption band of CO * gradually weakened, while the HCOO* band intensified. Concurrently, sharp Brønsted-acid bands appeared and the Al-H signature grew steadily. These results indicated that CO was reduced to HCOO* by H+/H- species, and H+ acted as the mobile carrier during hydrogen spilloverR14-R17”\nSupplementary Figure 34 | The variation of chemical groups in various catalysts during the hydrogen spillover and hydrogenation period. The H -DRIFTS profiles of (a) Cu@mMOR and (b) Cu/mMOR after adsorbing CO .\nRevised manuscript references:\nR14. Im, J., Shin, H., Jang, H., Kim, H. & Choi, M. Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures. Nat. Commun. 5, (2014).\nR15. Choi, M., Yook, S. & Kim, H. Hydrogen spillover in encapsulated metal catalysts: New opportunities for designing advanced hydroprocessing catalysts. ChemCatChem 7, 1048-1057 (2015).\nR16. Bettahar, M. M. The hydrogen spillover effect. A misunderstanding story. Catal. Rev. Sci. Eng. 64, 87-125 (2022).\nR17. Bettahar, M. M. The hydrogen spillover effect-a misunderstanding study II: Single oxide and zeolite supports. Catalysts 14, (2024).\n4. Finally, the above considerations pose the question of the effect of the degree of protonation of the zeolite on the Cu@mMOR catalyst performances? Would this degree beneficial or detrimental to the catalyst activity? The answer to this question may help the comprehending of the structural and chemical role of the Na+ ions.\nAuthor reply: Following your suggestion, we precisely modulated the Na+ content in Cu@mMOR through ion exchange to indirectly control the protonation degree. Furthermore, CO -TPD, NH -TPD, and CO hydrogenation performance tests (Supplementary Fig. 16) were added to elucidate the impact of protonation degree on the RWGS reaction performance. To further investigate the influence of Na+ on CO chemisorption, cAIMD calculations were added (Supplementary Fig. 19). The revised manuscripts were shown and highlighted as follows:\nRevised manuscript, page 16-17: “As shown in Supplementary Fig. 16a-b, CO chemisorption capacity of Cu@mMOR exhibited a well-defined linear increase with rising Na+ content, while the protonation degree (acidic site density) decreased progressively with Na+ incorporation. Meanwhile, CO hydrogenation rate (Supplementary Fig. 16c) displayed a characteristic volcano-shaped trend with increasing Na+ content, which initially rose to a maximum and then declined. These results elucidated that increased protonation degree (decline in Na+ content) effectively accelerated the CO hydrogenation in Cu@mMOR. Notably, excessive protonation weakened acidic CO chemisorption, ultimately impairing CO hydrogenation performance.”\nRevised manuscript, page 17: “Constrained ab initio molecular dynamics (cAIMD) was further employed to probe the CO adsorption-activation mode at 600 °C (Supplementary Movie 3-7). As illustrated in Supplementary Fig. 19, the formed carbonate involved the lowest energy barrier (0.7 eV) to bend O-C-O bond. This demonstrated that Na+ markedly reduced the energy barriers for both CO adsorption and activation. Therefore, it was proved that CO rapidly bonded with Na+ and then formed carbonate during its diffusion through the channels of Cu@mMOR, effectively triggering the reactionR30.”\nSupplementary Figure 13 | The role of Na+ in Cu@MOR-x. The correlation of Na+ content with (a) CO chemisorption amount, (b) acid sites amount, (c) CO hydrogenation rate, which were obtained from CO -TPD, NH -TPD, and CO hydrogenation performance tests, respectively.\nSupplementary Figure 19 | The CO adsorption configuration in Cu@mMOR.\nFree energy and O-C-O bond angle variations of CO in adsorption modes of (a) single-O and Na+, (b) single-C and Na+, (c) double-O and Na+, (d) single-C and O , (e) single-O and Na+ & single-C and O on Cu@mMOR.\nRevised manuscript references:\nR30. Mir, S. H., Yadav, V. K. & Singh, J. K. Efficient CO capture and activation on novel two-dimensional transition metal borides. ACS Appl. Mater. Interfaces 14, 29703-29710 (2022).\nReviewer #3 (Remarks to the Author):\nAuthors report a highly active and selective Cu catalysts for high-T RWGS. It is an interesting case study to push toward the equilibrium limit. The characterizations and DFT studies help to gain understanding of active sites and mechanism. However, the significance of current study is not clear.\n1. Synthesis of particles in a confined spaces have been reported previously to limit aggregation under reaction. These studies need to be cited and discussed to highlight the significant contribution from current study as compared to the previous.\nAuthor reply: We would like to thank the reviewer for the attention to and interest in the distinctions between our work and conventional confined catalysts. We have added three new examples of metal in confined zeolite to replace the previous examples in the introduction section and have discussed them. Additionally, the description of our innovations compared the previous was constructed a high-RWGS-performance catalyst of Cu@mMOR with the simultaneous inhibition of Ostwald-ripening alongside the migration and aggregation via molecular fence. This had been included in the introduction section (final paragraph) of origin manuscript: “Herein, we proposed to construct Cu@mMOR that encapsulate Cu clusters within mMOR crystals to realize catalytic site separation for CO and H activation in RWGS reaction. For our designed catalyst, mMOR was triggered to crystallize around Cu clusters and encapsulate Cu species insides via its Na+-induced crystallized small pore channels (< 3.0 Å) linking with Cu-O bonds. CO was efficiently captured and activated on outside mMOR, which served as the molecular fence to isolated CO from Cu clusters depended on its small channels. On the other hand, H was dissociated into H* on internal Cu clusters and spillover toward activated CO via Cu-O interface of Cu@mMOR, and finally produced CO with high selectivity. The catalytic site separation strategy significantly enhanced catalytic activation for both CO and H , and rationally changed the CO reaction pathway, thereby effectively preventing Cu Oswald ripening and thermal aggregation in catalysts.”. The specific modifications are as follows:\nRevised manuscript, page 3: “For this reaction, Cu-based catalysts are promising non-noble metal candidates for RWGS because of their high hydrogenation activity and high CO selectivity. In general, high temperature during the RWGS reaction usually tends to lead to the sintering of Cu, which can result in catalyst deactivation via the aggregation of Cu NPs. Encapsulating the active metal inside thermally robust zeolites is an effective way to curb high-temperature aggregation. For instance, Xiao et al. embedded Pt NPs within Beta zeolite via a seed-mediated growth route, constructing a rigid “protective shell” around the Pt that markedly suppressed thermal migration and aggregationR31. Likewise, Liu et al. confined sub-nanometer Pt clusters in the straight channels of MEL zeolite, firmly anchoring the naked Pt species and achieving excellent stability in methyl-cyclohexane dehydrogenationR32. These reports confirm that zeolite-confinement can efficiently mitigate thermally induced particle aggregation. However, high-temperature sintering can also proceed via an Ostwald-ripening path, where volatile metal species migrate from interior pores to the outer surface and aggregate. For example, Julian et al. reported that confined Mo zeolite still diffuses outward during high-temperature methane dehydrogenation, causing agglomeration and cokingR33. Analogous to the above catalysts, Cu species are susceptible to forming volatile Cu-carbonyl complexes under CO-rich conditions, leading to significant catalyst deactivationR34. Therefore, the simultaneous inhibition of Ostwald-ripening alongside the migration and aggregation of Cu species in the RWGS reaction remains a crucial challenge for achieving high-performance catalysts that exhibit both efficient activation and enduring stability.”\nRevised manuscript references:\nR31. Zhang, J. et al. Sinter-resistant metal nanoparticle catalysts achieved by immobilization within zeolite crystals via seed-directed growth. Nat. Catal. 1, 540-546 (2018).\nR32. Wang, H. et al. Subnanometer Pt catalysts encapsulated in MEL zeolite mesocrystals for H production from methylcyclohexane dehydrogenation. J. Am. Chem. Soc. 147, 21789-21802 (2025).\nR33. Julian, I. et al. Supercritical solvothermal synthesis under reducing conditions to increase stability and durability of Mo/ZSM-5 catalysts in methane dehydroaromatization. Appl. Catal., B 263, 118360 (2020).\nR34. Hou, P., Yu, Q., Luo, F. & Liu, J.-C. Reactant-induced dynamic active sites on Cu catalysts during the water gas shift reaction. ACS Catal. 15, 352-360 (2025).\n2. RWGS on Cu or Cu-based catalysts has been extensively studied. Under CO and H , Cu alone is known to produce CO as the major product. Although the high selectivity is reported here, the reaction temperature is very high compared to the typical. It is not surprising that increased temperature promotes CO conversion and decomposition of methanol to CO, thus increasing CO selectivity. What is the benefit of this high-T RWGS inconsideration of energy cost?\nAuthor reply: We appreciate the reviewer’s concern regarding the energy demand of high-temperature RWGS.\nFirst, the ultimate purpose of RWGS is to supply synthesis gas to the mature, downstream Fischer-Tropsch (FT) process. Today’s large-scale syngas is produced predominantly by natural-gas steam reforming or coal gasification, both of which operate under very harsh conditions (≥850 °C and ≥3 MPa) R35, R36. In comparison, RWGS proceeds under much milder temperatures and pressures.\nSecond, RWGS is an endothermic reaction, and raising the temperature substantially boosts CO conversion. According to the techno-economic analysis, operating at around 600 °C provides the best compromise between conversion and energy inputR37.\nFinally, FT synthesis requires a specific H : CO ratio of 0.5 ~ 2R38, R39. Syngas generated at lower RWGS temperatures contains a relatively low CO fraction, which would force the addition of separation or recycle units to adjust the ratio, and these auxiliary units themselves consume significant additional energy.\nRevised manuscript, page 3: “The reverse water-gas shift reaction (RWGS) is a desirable and promising CCU technology that catalytically converts CO into syngas under a H environment. Operating the RWGS at atmospheric pressure and temperatures above 500 °C enables direct conversion of CO into syngas with the ideal H /CO ratio for Fischer-Tropsch synthesis, requiring only moderate energy input and avoiding extra downstream separationR37.”\nRevised manuscript references:\nR35. Lee, K. B., Beaver, M. G., Caram, H. S. & Sircar, S. Novel thermal-swing sorption-enhanced reaction process concept for hydrogen production by low-temperature steam methane reforming. Ind. Eng. Chem. Res. 46, 5003-5014 (2007).\nR36. Gao, Y. et al. Syngas production from biomass gasification: Influences of feedstock properties, reactor type, and reaction parameters. ACS Omega 8, 31620-31631 (2023).\nR37. Portillo, E., Gandara-Loe, J., Reina, T. R. & Pastor-Pérez, L. Is the RWGS a viable route for CO conversion to added value products? A techno-economic study to understand the optimal RWGS conditions. Sci. Total Environ. 857, 159394 (2023).\nR38. Leckel, D. Diesel production from Fischer-Tropsch: The past, the present, and new concepts. Energy & Fuels 23, 2342-2358 (2009).\nR39. Damma, D. & Smirniotis, P. G. Recent advances in the direct conversion of syngas to oxygenates. Catal. Sci. Technol. 11, 5412-5431 (2021).\n3. Apparent activation energies reported for three systems are all below 1 eV. It is not straightforward that such low barrier requires high temperatures (> 450C) to overcome.\nAuthor reply: We thank the reviewer for the comment regarding whether high temperatures are required for RWGS. The reasons that our system operates at high temperature despite its low apparent activation energy are as follows:\n1. Despite RWGS possesses low activation barriers, operation above 450 °C is better to forestall methanation and other side reactions that reduce CO selectivityR40.\n2. For Cu@mMOR, the coupled proton-electron transfer that forms the H+/H- pair is the rate-determining step. Elevated temperatures can enhance entropic effects to accelerate H O+ migration within the zeolite channels and thermally activate electron transfer to adjacent orbitalsR41-R43.\n3. At high temperatures, Cu@mMOR extensively dissociates H to produce high-energy H+ and electrons whose light mass enables quantum tunneling deep into the zeolite frameworkR44-R46, directly surmounting energy barriers and thus significantly accelerating CO reduction.\nRevised manuscript references:\nR40. Su, X., Yang, X., Zhao, B. & Huang, Y. Designing of highly selective and high-temperature endurable RWGS heterogeneous catalysts: Recent advances and the future directions. J. Energy Chem. 26, 854-867 (2017).\nR41. Wang, M. et al. Genesis and stability of hydronium ions in zeolite channels. J. Am. Chem. Soc. 141, 3444-3455 (2019).\nR42. Jin, Y., Yin, X., Yu, G., Sun, Q. & Wang, J. Investigation of electron transfer properties on Silicalite-1 zeolite for potential electrocatalytic applications. J. Am. Chem. Soc. 146, 35109-35116 (2024).\nR43. Shin, H., Choi, M. & Kim, H. A mechanistic model for hydrogen activation, spillover, and its chemical reaction in a zeolite-encapsulated Pt catalyst. Phys. Chem. Chem. Phys. 18, 7035-7041 (2016).\nR44. Waluyo, I., Ren, Y. & Trenary, M. Observation of tunneling in the hydrogenation of atomic nitrogen on the Ru(001) surface to form nh. J. Phys. Chem. Lett. 4, 3779-3786 (2013).\nR45. Jewell, A. D. et al. Quantum tunneling enabled self-assembly of hydrogen atoms on Cu(111). ACS Nano 6, 10115-10121 (2012).\nR46. Gao, L. G., Zhang, R. M., Xu, X. & Truhlar, D. G. Quantum effects on H 2 diffusion in zeolite RHO: Inverse kinetic isotope effect for sieving. J. Am. Chem. Soc. 141, 13635-13642 (2019).\n4. Surface of MOR was simulated. Why? How representative to the cage structure of MOR?\nAuthor reply: We thank the reviewer for raising concerns about our use of an interfacial slab model to simulate Cu@mMOR in the original manuscript. Owing to the significant disparity in electronic characteristics between surface-exposed and fully encapsulated structuresR47, we have reconstructed the model and calculated the corresponding DFT results. Specifically, the original surface Cu cluster model was replaced by a Cu cluster encapsulated within the MOR framework, with its geometry refined to better reflect the experimentally observed features. In addition, we replaced the previous MOR surface model with a MOR channel model to more accurately represent the RWGS reaction occurring inside the confined Cu@mMOR. We have made revisions in three places in the revised manuscript and added calculation relevant details in the revised supplementary information, as follows:\nPart I:\nAdded and revised manuscript, pages 18: “To further validate this hypothesis, Fig. 5d illustrated the free energy profiles of the H dissociation process on various models. These models include Cu@mMOR with Cu vacancies (denoted as Cu@mMOR-V ), Cu@mMOR without Cu vacancies (denoted as Cu@mMOR-absent-V ), and Cu/mMOR. As shown, the energy barrier of H dissociation of Cu@mMOR-V , Cu@mMOR-absent-V and Cu/mMOR were 0.37, 0.70, and 1.23 eV, respectively. It indicated that Cu vacancies could facilitate faster H-H bond cleavage. It was worth noting that dissociated H* in Cu@mMOR models readily coordinated with the Si-O--Al and the Cu+ site, forming H+/H- pairs (Supplementary Fig. 32).”\nFig. 5d Gibbs free energy profiles of the H dissociation for Cu@mMOR and Cu/mMOR.\nPart II:\nAdded and revised manuscript, pages 19: “Meanwhile, the density of states (Supplementary Fig. 37) exhibited that Cu vacancies in Cu@mMOR shift the Cu electronic bands closer to the Fermi level, indicating Cu vacancies effectively accelerated the transfer of free electrons from the Cu clusters to mMOR.”\nSupplementary Figure 33 | The electron state of various catalysts. The (a) total density of states and (b) Cu projected density of states for the Cu@mMOR with Cu vacancy, the Cu@mMOR without Cu vacancy and the mMOR.\nPart III:\nRevised manuscript, pages 21-22: “Density functional theory (DFT) calculations were performed to investigate the CO hydrogenation mechanism over the Cu@mMOR at the atomic level. As shown in Fig. 5g, H heterolytically dissociated on the Cu cluster, generating active H+ and H- that migrate to the Si-ONa-Al site. Meanwhile, CO was chemisorbed to carbonate at the same site. In this chemisorption configuration, CO * (carbonate species) could be hydrogenated to COOH+ via the carboxyl pathway or to HCOO- via the formate pathway. A comparison of energy barriers exhibited that forming COOH needed 0.98 eV, whereas producing HCOO- required only 0.64 eV. Due to the lower barrier, CO * preferred to follow formate pathway, which was consistent with the DRIFTS observation that detected only the formate intermediate (Fig. 5e). Next, HCOO- faced a competition between direct dissociation and protonation. DFT calculations showed that the barrier of protonation was only 0.43 eV, far below the 1.72 eV needed for direct dissociation. This implied that HCOO- would rapidly protonate to yield HCOOH in Cu@mMOR. The generated HCOOH could either dissociate directly or undergo further hydrogenation. By comparing both energy barrier, direct dissociation of HCOOH required merely 0.92 eV, whereas successive hydrogenation must overcome higher barriers of 2.04 eV and 0.93 eV. In addition, AIMD further exhibited that the dissociation energy barrier of HCOOH inside the mMOR channel was 0.24 eV lower than that outside the channel (Supplementary Fig. 42). Therefore, HCOOH rapidly decomposed to CO and H O within the Cu@mMOR channels, which explained why Cu@mMOR achieved nearly 100 % CO selectivity. Notably, DFT calculations supported that Cu@mMOR followed the formate pathway and identified H+/H- migration from Cu to the mMOR as the highest-energy step, establishing it as the rate-determining step in line with our KIE analysis (Supplementary Fig. 38).”\nFig. 5g Gibbs free energy profiles of the CO hydrogenation in Cu@mMOR.\nSupplementary Figure 42 | The mechanism of fast HCOOH dissociation in Cu@mMOR. The free energy variation profiles of HCOOH dissociation in/outside Cu@mMOR.\nRevised manuscript references:\nR47. Fan, Q.-Y. et al. Entropy in catalyst dynamics under confinement. Chem. Sci. 15, 18303-18309 (2024).\n5. It was reported that formate is very stable on Cu. The barrier for formate decomposition to CO needs to be calculated. In addition, the barriers for competitive hydrogenations to *HCOOH and *H COO should also be compared to ensure the claimed pathway. Without that, the current conclusion is not convincing.\nAuthor reply: We thank the reviewer for the discussion and suggestions.\nAs requested by the reviewer, we have added the formate dissociation, formate protonation, and further hydrogenation processes. The modifications of revised manuscript and corresponding DFT results are presented below:\nRevised manuscript, pages 22: “Next, HCOO- faced a competition between direct dissociation and protonation. DFT calculations showed that the barrier of protonation was only 0.43 eV, far below the 1.72 eV needed for direct dissociation. This implied that HCOO- would rapidly protonate to yield HCOOH in Cu@mMOR. The generated HCOOH could either dissociate directly or undergo further hydrogenation. By comparing both energy barrier, direct dissociation of HCOOH required merely 0.92 eV, whereas successive hydrogenation must overcome higher barriers of 2.04 eV and 0.93 eV. In addition, AIMD further exhibited that the dissociation energy barrier of HCOOH inside the mMOR channel was 0.24 eV lower than that outside the channel (Supplementary Fig. 42). Therefore, HCOOH rapidly decomposed to CO and H O within the Cu@mMOR channels, which explained why Cu@mMOR achieved nearly 100 % CO selectivity. Notably, DFT calculations supported that Cu@mMOR followed the formate pathway and identified H+/H- migration from Cu to the mMOR as the highest-energy step, establishing it as the rate-determining step in line with our KIE analysis (Supplementary Fig. 38).”\nFig. 5g Gibbs free energy profiles of the CO hydrogenation in Cu@mMOR.\nSupplementary Figure 42 | The mechanism of fast HCOOH dissociation in Cu@mMOR. The free energy variation profiles of HCOOH dissociation in/outside Cu@mMOR.\n6. There is no structure in Supplementary Fig. 20-21.\nAuthor reply: Thank you to the reviewer for pointing out this issue. We apologize for the numbering error in the original manuscript. The reaction-related calculations have been re-modeled and recomputed, and the new manuscript has been carefully proofread. The results for these new structures are now presented in Supplementary Fig. 21-33 and Supplementary Fig. 44-55 (revised supplementary information, pages 37-47, 59-70)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I appreciate the authors’ efforts to address the comments raised in my initial review. While some additional data and explanations have been provided, I remain unconvinced by several of the key responses due to insufficiently compelling evidence. Below I elaborate on my continuing concerns regarding the manuscript.\nRegarding the reply of Comment 1:\nThe authors claim to have quantified Cu⁰ and Cu⁺ species based on the white-line peak intensities and their derivatives. However, it is unclear how such quantification was rigorously performed, particularly regarding the assumptions involved and the potential errors in the deconvolution. The method requires further justification, including a clear discussion of quantification accuracy. Additionally, in Supplementary Fig. 14b, the signals attributed to three- and four-membered rings are very weak, raising doubts about their reliability and whether they can robustly support the proposed “molecular fence” effect.\nRegarding the reply of Comment 2:\nThe H₂-TPR peak fitting appears somewhat arbitrary. The authors do not clearly describe the principles used for peak assignment and separation. In particular, how were the peak position and shape corresponding to H-spillover determined? Without a more systematic analysis, the reliability of the deconvolution and the conclusions drawn from it remain questionable.\nRegarding the reply of Comment 3:\nI am afraid the authors’ explanation for the volcano-type dependence of CO₂ hydrogenation rate on Na⁺ content (Supplementary Fig. 16c) is unconvincing. Regardless of the proposed balance between CO₂ adsorption and surface acidity, the observed trend clearly suggests that Na⁺ is not the primary active site for CO₂ hydrogenation. If Na⁺ were indeed the catalytic site, one would expect a monotonic, positive correlation between Na⁺ content and reaction rate.\nRegarding the reply of Comment 4:\nThe authors interpret the increasing KIE values with temperature as evidence that H⁺/H⁻ migration is the rate-determining step (RDS). This conclusion is ambiguous, as typical H/D KIE values tend to decrease with temperature. Moreover, if the migration of H⁺/H⁻ is indeed the RDS, one would expect the reaction rate to be largely independent of CO₂ partial pressure—yet no evidence is provided to support or refute this. Additionally, the argument that the hydrogenation rates on Cu and Na⁺ cannot be compared is not persuasive. While the Cu and Na⁺ sites are physically separated, kinetic and energetic comparisons remain scientifically valid. Since Cu is a well-known hydrogenation catalyst, the burden of proof lies with the authors to demonstrate that the encapsulated Cu sites are indeed inactive for CO₂ hydrogenation.\nRegarding the reply of Comment 5:\nThe DFT results indicate that direct dissociation of HCOO* has a lower energy barrier than COOH*. However, the authors do not provide clear physical reasoning for this difference. Given the mechanistic importance, I strongly suggest including transition state structures and performing detailed analysis to explain why HCOO* dissociates more readily. This would improve the robustness of the mechanistic proposal.\nRegarding the reply of Comment 6:\nZeolites are generally considered insulating materials. The mechanism by which electrons are proposed to migrate through the mordenite framework during hydrogen spillover remains unclear. Although EPR and XPS data were added, a more rigorous discussion on the feasibility of electron conduction in such systems is required. Without further clarification, the fundamental validity of the proposed H⁺/H⁻ transport mechanism remains questionable.", "reviewer_2": "The authors improved the manuscript for a better understanding of the results obtained. I agree that hydrogen spillover exists and diffuses over the support as H-/H+ pairs. I also agree with the overall reaction paths proposed. For that it merits to be published in Nat. Comm. Nevertheless, at the level of the discussion, the mechanism of diffusion of hydrogen spillover suggested is questionable.\n1. According to the authors the initial H-/H+ pairs formed on Cu first become two pairs of H+/e_ entities after the crossing of the interface, then diffuse independently as such on the support and finally recover their H-/H+ identity on the active sites. It is stated that H- is re-formed by recombination of H+ and 2 e-. This is hard to admit.\nIs it not more reasonable and simpler to consider that the acid-base nature of the H-/H+ pairs allows easy interactions with and migration on acid-base centers of the support? This was suggested in our previous report.\nHydrogen spillover for non reducible oxides would consist in hydrogenating H/OH pairs (more or less polarized)in vicinal position, formed by an interface dehydroxylation process of OH/OH pairs with water production (Bettahar, Catal. Rev., 2022). After desorption (or reaction) these hydrogenating sites become Mm+/Op-(p=1or2) hydrogen receiving sites. Hydrogenating and receiving sites are denoted as conjugated sites. It comes that the diffusion of hydrogen spillover necessarily proceeds by migration of H/H pairs from Mm+ O- site to Mm+ O- site (Bettahar, Catalysts, 2024).\nIn the presence of Na+ the hydrogenating sites are deprotonated and this favors the adsorption of CO2 (see suggested reaction paths in attached file)\nThis mechanism is compatible with the physical characterization of the catalysts, notably with the DRIFTS experiments (Figure 34). Indeed, after adsorbing CO2 the introduction of H2 led to concomitant formation of acidic OH groups, Al-H hydrides and adsorbed water which may correspond to the formation of the AlH/SiOH hydrogenating sites (water may arose from both dehydroxylation and WGSR processes).\n2. In these DRIFTS experiments the authors evidenced the existence of hydrogen spillover during the hydrogenation reaction. Why? Do they assume that hydrogen spillover is induced by the presence of the organic molecule (see Choi et al., Nanoresearch, 2022)? If it is such the statement is wrong because H spiltover species are incorporated in the support in the absence of any organic molecule, as the authors know. Why DRIFTS experiments were not conducted in H2 atmosphere solely?", "reviewer_3": "The responses address the reviewer's comments and the revised manuscript is recommended for acceptance."}, "author_response": "Author reply: Thanks for the reviewer’s suggestion. This question comprises two sub-questions, which are answered below.\n[I] In response to the reviewer’s concern, it is acknowledged that the intensity of the white-line peak is susceptible to interference from factors such as the coordination environment and symmetryR1, R2, thereby limiting its reliability for precise quantification of the Cu valence state. Therefore, the revised version of this manuscript employs edge position as the basis for quantitative calculation and correlation for the Cu atom’s valence stateR3, R4. Comprehensive justifications of the methodologies, error analyses, and revision details have been added to the corresponding sections of the manuscript. For further information, please refer to the description of revisions:\nRevised manuscript, pages 10: \"Compared to Cu/mMOR, Cu@mMOR exhibited a much higher Cu adsorption edge energy, attributing to a higher proportion of Cu+ species in Cu@mMOR. To quantify the Cu+ species in the catalyst, a mathematical relationship between the absorption edge energy position and the valence state was established by fitting data from its corresponding standardsR3, R4. As shown in Supplementary Fig. 8, the Cu+ content in Cu@mMOR was calculated to be ⁓38.1 at.%. In contrast, the Cu+ proportions in Cu/mMOR and Cu@SiO were determined to be 2 only 9.0 and 6.9 at.%, respectively. The significantly higher Cu+ content in Cu@mMOR is indicative of enhanced interfacial charge transfer from Cu0 to mMOR, a process facilitated by the abundant interfaces inherent to the encapsulation structure.”\nRevised supplementary information, pages 28: “Supplementary Fig. 8a compares the first-derivative spectra of the absorption edge (K-edge) for the catalysts with those of the standards (Cu foil, Cu O, and CuO). It is known that the energy position of this 2 derivative peak has a linear correlation with the average valence state of CuR3, R4. First, by fitting the derivative peak positions of the three standard reference samples with their corresponding valence states (0, +1, and +2), a calibration curve with a high linear relationship (R2 > 0.99) was established (Supplementary Fig. 8b). Based on this calibration curve, the average valence states of Cu in Cu@mMOR, Cu/mMOR, and Cu@SiO were calculated to be +0.381, +0.090, and +0.069, respectively. Given that 2 no characteristic Cu2+ signal was observed in the XANES spectra, the average valence state of the catalysts can be attributed entirely to contributions of Cu0 and Cu+. Consequently, it was calculated that the atomic percentage of Cu+ in Cu@mMOR was calculated to be as high as 38.1 at.%, a value far exceeding that of Cu/mMOR (9.0 at.%) and Cu@SiO (6.9 at.%), which confirms its unique Cu+-rich electronic structure.”\nSupplementary Figure 8 | The Cu+ proportion of various catalysts. (a) The 1st derivative patterns of k-edge for Cu-foil, Cu O, CuO, Cu@mMOR, Cu/mMOR, and 2 Cu@SiO . (b) Linear fitting of Cu-foil, Cu O, and CuO. 2 2\n[II] We thank the reviewers for their insightful comments. To further support the proposed “molecular fence” hypothesis, we conducted additional positron annihilation spectroscopy (PAS) and N O chemisorption/temperature-programmed reduction (TPR) 2 experiments, which provide more direct evidence to elucidate the mechanism. The PAS characterization confirms the presence of ultra-micropores closely surrounding the Cu nanoclusters. These pore structures effectively sieve CO from H , allowing only H 2 2 2 molecules to access the Cu active sites, thereby structurally validating the “molecular fence” mechanism. To strengthen this conclusion, we increased the contact temperature of N O with the catalyst from 90 °C to 160 °C to fully eliminate possible mass transfer 2 limitations on the experimental observations. It is noteworthy that even under these more stringent conditions, no signal of N O interacting with the catalyst was detected. 2 The combined results from structural characterization and reaction experiments rigorously confirm the exclusive “molecular fence” effect of the designed catalyst toward CO . Based on these additional experiments, we have incorporated the new PAS 2 data and the revised N O-TPR-MS analysis into the main text and the Supplementary 2 Information (SI), respectively. All modifications have been highlighted in yellow for your convenience.\nRevised manuscript, page 8-9:“…The textural structure analysis inferred that Cu@mMOR maintained the mMOR porous property after encapsulating Cu species with homogeneous distribution, which would favor the adsorption and transport of reactants and active species during the reaction. Further, positron annihilation lifetime spectroscopy (PALS) was conducted to determine the pore size distribution of the ultra-microporous region in MOR, mMOR, and their Cu-based catalystsR5. The deconvoluted PALS spectra display four distinct peaks corresponding to different positron annihilation lifetimes (τ -τ ; Supplementary Fig. 6a). An increase in τ value indicates 1 4 larger pore sizes within the MOR framework. Here, τ denotes the bulk lifetime, which 1 is attributed to direct annihilation within the non-porous regionsR6. The remaining lifetimes τ -τ correspond to positron annihilation within the 5-membered, 8-membered, 2 4 and 12-membered ring channels of MOR, respectively (Supplementary Fig. 6b-c). In comparison to the support, the τ components associated with 8-MRs exhibited 3 distinctly opposite trends in Cu@mMOR and Cu/mMOR. Specifically, the calculated pore size for Cu/mMOR increased to 3.6 Å. This observation aligns with the pore-blocking effect identified via N adsorption isotherms (Supplementary Fig. 5), ascribed 2 to the obstruction of pore mouths by external Cu particles. Thus, it compels positrons to annihilate within larger inter-particle voids on the surfaceR7. In contrast, the encapsulation strategy yielded a shrunken pore size of 2.8 Å for Cu@mMOR, directly confirming the effective occupation of the 8-MR channels by Cu species. Furthermore, coincidence doppler broadening (CDB) analysis revealed a higher electron density for Cu@mMOR in the low-momentum valence electron region (< 4 × 10-3 m c; 0 Supplementary Fig. 7), corroborating the tight confinement of copper clusters within the unique micro-environment constructed by the 5-MRs and 8-MRsR8. Notably, this shrunken pore mouths of Cu@mMOR (2.8 Å) precisely matches the kinetic diameter of H (2.8 Å) while remains smaller than that of CO (3.3 Å)R9. Conceptually, this 2 2 double-ring encapsulation architecture could act as a \"molecular fence\" for suppressing thermal aggregation and Ostwald ripening via the spatial separation of CO and H .” 2 2\nRevised manuscript, page 16-17:“To validate the sites separated effect of Cu@mMOR for CO and H , N O was employed to replace CO for touching with Cu 2 2 2 2 species in Cu@mMOR and Cu/mMOR owing their similar molecular configuration and diffusion properties. At medium temperature, N O selectively oxidized the surface and 2 subsurface Cu0 and Cu+ species of the catalystsR10. The resulting oxidized Cu species were subsequently quantified via H -temperature programmed reduction (H -TPR). As 2 2 depicted in Fig. 5a and Supplementary Fig. 16, the Cu@mMOR sample exhibited only trace H reduction peak around 281 °C, whereas Cu/mMOR displayed a pronounced 2 reduction peak at 294 °C. Correspondingly, the H O signal generated during the 2 reduction of Cu@mMOR was significantly weaker than that of Cu/mMOR. These results confirm that most Cu sites in Cu@mMOR were effectively shielded from N O 2 oxidation, demonstrating the 'molecular fence' function of the tight encapsulation. In contrast, the absence of such restriction in Cu/mMOR led to substantial exposure of Cu species to the N O atmosphere. Finally, CO -DRIFTS was conducted at elevated 2 temperatures to further validate the molecular fence effect under realistic reaction conditions…”\nRevised supplementary information, page 11:“Positron annihilation lifetime spectroscopy (PALS) measurements were carried out using a DPLS-4000 digital spectrometer with a time resolution of approximately 190 ps. The samples were pressed into pellets (diameter ~ 10 mm; thickness > 1 mm) to ensure complete positron absorption within the sample volume. A standard sandwich geometry was employed, wherein a 20 μC 22Na source encapsulated in 7.5 μm Kapton foil was placed between i two identical sample pellets. All spectra were collected at ambient temperature and pressure, with a total count exceeding 1.5×106 to ensure statistical reliability. The contribution from the source and background was subtracted based on the reference measurement of an yttria-stabilized zirconia standard. The lifetime spectra were deconvoluted into individual lifetime components and intensities using the LTv9 software package.\nCoincidence doppler broadening (CDB) spectroscopy was performed using the identical source-sample sandwich configuration described in the PALS section. The detection system consisted of two High-Purity Germanium (HPGe) detectors (ORTEC, Model GEM40P476) coupled to a multiparameter acquisition system (FAST ComTec, Model MPA4). The energy resolutions of the two detectors were 1.5 keV and 1.3 keV at 511 keV, respectively. For each measurement, a total of at least 1 × 107 counts were accumulated with a count rate exceeding 2000 cps. The one-dimensional CDB profiles were extracted from the two-dimensional coincidence spectra by applying an energy conservation window of 2m c2 ± 2.0 keV (i.e., |E + E − 2m c2| ≤ 2.0 keV) along the 0 1 2 0 diagonal.”\nRevised supplementary information, page 25-26:“For the investigation of free volume in materials such as polymers, silicones, mixed catalysts, and zeolitesR11, positrons were assumed to annihilate within free volume cavities. Given the strong dependence of positron lifetime on cavity size, the simplified quantum mechanical Tao-Eldrup model was employedR12, R13. This framework modeled ortho-positronium (o-Ps) as confined within a spherical cavity of radius R surrounded by an electron layer of thickness ∆R, corresponding to an infinite spherical potential well of radius R (where 0 R = R + ∆R). Solving the associated Schrödinger equation yielded the following relationship between o-Ps lifetime and cavity radius:\n1 b 1 R R R 2 1 s i n ( 2 R R R )     =  − +  + +   (25)\nHere τ denoted the o-Ps lifetime in nanoseconds (ns) and R the cavity radius in nanometers (nm). The parameter λ was fixed at 2 ns-1 R14. Through calibration against b molecular solids of known pore dimensions, the electron layer thickness ∆R was determined to be 0.1656 nmR15.\nHowever, the MOR zeolite framework comprised elliptical twelve-membered ring (12-MR) main channels interconnected by eight-membered ring (8-MR) channels. Since these elongated voids were better approximated as cylinders or cuboids, an infinite cylindrical geometry model was applied rather than the standard spherical approach. Adapting the quantum mechanical formalism to cylindrical coordinates yielded the following relationshipR16:\n1 2 (1 P 1 P 2 ) 1  = − − (26)\nL 1 L (27)\nP 1 = + sin( ) L+d  L+d\nP 2 =  a 0 (1 R  /( a 0 R 1 + x d J )) 20 x ( J x 20 ) ( d x x ) d x (28)\nHere τ denoted the o-Ps lifetime (ns), while R and L represented the radius and length of the cylindrical cavity, respectively. ∆R was the electron layer thickness, J the zeroth- 0 order Bessel function, and a the first zero of J . Given that L significantly exceeded R, 1 0 an infinite cylinder geometry was assumed, for which P = 1.\nSupplementary Figure 6 | Positron annihilation spectroscopy (PAS) of various catalysts. (a) The positron annihilation lifetime spectroscopy derived pore size distribution curves of Cu@mMOR, Cu/mMOR, MOR and mMOR. (b-d) Schematic representations of the electron clouds (van der Waals surfaces) and atomic structures (inset) of the 5-, 8-, and 12-membered-ring (MR) channels.\nSupplementary Figure 7 | Coincidence doppler broadening (CDB) analysis of various catalysts. The coincidence doppler broadening ratio curve of Cu@mMOR and Cu/mMOR.\nSupplementary Figure 16 | N O-H -TPR of various catalysts. Online mass 2 2 spectrometry profiles of (a) H consumption and (b) H O evolution during H -TPR of 2 2 2 Cu@mMOR and Cu/mMOR after N O treatment at 160 °C. 2\nReferences:\n[R1] Kosog, B., La Pierre, H. S., Denecke, M. A., Heinemann, F. W. & Meyer, K. Oxidation state delineation via U L -edge XANES in a series of isostructural III uranium coordination complexes. Inorg. Chem. 51, 7940-7944 (2012).\n[R2] Ressler, T. Bulk structural investigation of the reduction of MoO with propene 3 and the oxidation of MoO with oxygen. J. Catal. 210, 67-83 (2002). 2\n[R3] Caballero, A. et al. An in situ XAS study of Cu/ZrO catalysts under de-NO 2 x reaction conditions. J. Catal. 235, 295-301 (2005).\n[R4] Vitova, T., Hormes, J., Peithmann, K. & Woike, T. X-ray absorption spectroscopy study of valence and site occupation of copper in LiNbO : Cu. 3 Phys. Rev. B 77, 144103 (2008).\n[R5] Bosnar, S., Vrankic, M., Bosnar, D., Ren, N. & Saric, A. Positron annihilation lifetime spectroscopy (PALS) study of the as prepared and calcined MFI zeolites. J. Phys. Chem. Solids 110, 227-233 (2017).\n[R6] Huang, S. J. et al. Anylasis and comparison of several methods for calculation of positron bulk lifetime in perfect crystals. Acta Physica Sinica 63, 217804 (2014).\n[R7] Cízek, J. Characterization of lattice defects in metallic materials by positron annihilation spectroscopy: A review. Journal of Materials Science & Technology 34, 577-598 (2018).\n[R8] Taylor, C. N. et al. Catalyst deactivation probed by positron annihilation spectroscopy. ACS Catal. 11, 14967-14976 (2021).\n[R9] Mehio, N., Dai, S. & Jiang, D. E. Quantum mechanical basis for kinetic diameters of small gaseous molecules. J. Phys. Chem. A 118, 1150-1154 (2014).\n[R10] Jensen, J. R., Johannessen, T. & Livbjerg, H. An improved N O-method for 2 measuring Cu-dispersion. Applied Catalysis a-General 266, 117-122 (2004).\n[R11] Consolati, G., Mariani, M., Millini, R. & Quasso, F. Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy. Nuclear Instruments & Methods in Physics Research Section B- Beam Interactions with Materials and Atoms 267, 2550-2553 (2009).\n[R12] Tao, S. J. Positronium annihilation in molecular substances. The Journal of Chemical Physics 56, 5499-5510 (1972).\n[R13] Eldrup, M., Lightbody, D. & Sherwood, J. N. The temperature dependence of positron lifetimes in solid pivalic acid. Chem. Phys. 63, 51-58 (1981).\n[R14] Goworek, T. Positronium as a probe of small free volumes. J. Nucl. Radiochem. Sci. 1, 11-13 (2000).\n[R15] Nakanishi H., Jean Y. C. & Schrader D M. Positron and positronium chemistry. Studies in physical and theoretical chemistry. 57, 159 (1988).\n[R16] Olson, B. G., Prodpran, T., Jamieson, A. M. & Nazarenko, S. Positron annihilation in syndiotactic polystyrene containing α and β crystalline forms. Polymer 43, 6775-6784 (2002).\n2. The H -TPR peak fitting appears somewhat arbitrary. The authors do not clearly 2 describe the principles used for peak assignment and separation. In particular, how were the peak position and shape corresponding to H-spillover determined? Without a more systematic analysis, the reliability of the deconvolution and the conclusions drawn from it remain questionable.\nAuthor reply: Thank you for your valuable suggestions. In response to the questions regarding the assignment of reduction peaks and the identification of hydrogen spillover, the related catalysts were re-characterized via the combined H -TPR-MS technique. 2 During the tests, the consumption of hydrogen and the generation of water were monitored simultaneously. By calculating the additional hydrogen consumption caused by hydrogen spillover, the presence of hydrogen spillover in the Cu@mMOR catalyst during the reduction process was confirmed. The relevant experimental details, data processing methods, and discussion have been supplemented and added to the manuscript, and are highlighted as follows:\nRevised supplementary information, pages 6-7: “The reducibility of the as-calcined samples was determined by H -TPR-MS on a TP-5080D chemisorption 2 analyzer (Tianjin Xianquan) equipped with an online mass spectrometer (BSD Technology). 100 mg of as-calcined sample was pretreated at 450 °C for 1 hour in Ar flow to remove surface impurities, then cooled to room temperature. Subsequently, the sample was heated in a 10% H /Ar flow to 600 °C at a ramp rate of 10 °C/min. All gas 2 flow rates were set at 30 mL/min. The signals corresponding to H (m/z = 2) and H O 2 2 (m/z = 18) were recorded simultaneously by the online MS. The amounts of H 2 consumed and H O produced were quantified by calibrating the detector responses 2 against the reduction of a CuO standard. Hydrogen spillover was identified by the extra H consumption of the catalyst. The amount of spilled-over hydrogen was quantified 2 via the following equation:\nS n n H2,i = H2,i H2,CuO (2) S N H2,CuO Cu,icp\nIn this equation, n was the amount of hydrogen consumed by the catalysts. S H2, i H2, i and S corresponded to the consumed H peak area of catalysts and CuO standard, H2, CuO 2 respectively. The n was determined by the stoichiometric ratio for the complete H2, CuO reduction of CuO. The N represented the molar amount of Cu in the catalyst, as Cu,icp determined by ICP.\nn H O2 ,i = S S  H O ,i 2 H O ,C u O 2 n  H O2 N ,C u O C u ,ic p (3)\nIn this equation, n was the amount of water produced by the catalysts. S and H2O, i H2O, i S corresponded to the produced H O peak area of catalysts and CuO standard, H2O, CuO 2 respectively. The n was determined by the stoichiometric ratio for the complete H2O, CuO reduction of CuO. The N represented the molar amount of Cu in the catalyst, as Cu,icp determined by ICP.\nn H ,e 2 x tra = n H ,i 2 − 2 n H O2 ,i (4)\nIn this equation, n , n and n correspond to the extra hydrogen H2, extra H2, i H2O, i consumption, total hydrogen consumption and water production of catalyst.”\nRevised manuscript, pages 12-13: “It has been reported that Cu vacancies can significantly influence the redox properties of catalysts. Furthermore, H temperature- 2 programmed reduction-mass spectrometry (H -TPR-MS) analyses of various catalysts 2 were carried out to investigate the redox properties of Cu. Initially, all catalysts underwent complete oxidation to CuO prior to measurement. Fig. 3f showed the TPR-MS profiles of Cu@mMOR and Cu/mMOR. As shown, hydrogen reduction profile of Cu@mMOR exhibited a single hydrogen reduction peak at 262°C, whereas the corresponding peak for Cu/mMOR appeared at a higher temperature (ca. 300°C). This significant shift to a lower temperature suggested that the copper species in Cu@mMOR possessed smaller sizes and higher reducibilityR17. To quantitatively assess the hydrogen spillover effect, the hydrogen spillover capacity was determined by calculating the additional H consumption beyond that used for water formation. The 2 results showed that each Cu atom in Cu@mMOR consumed an extra 0.45 H atoms, while the Cu/mMOR exhibited almost no additional H consumption (0.01 H/Cu). 2 These results suggest that a significant hydrogen spillover phenomenon occurred in Cu@mMOR, thereby demonstrating that the synergy between its confined microenvironment and copper vacancies effectively promotes H activation and 2 spillover.”\nAnd the TPR-MS was shown in Fig. 3f of revised manuscripts, which was as follow:\nFig. 3f TPR-MS profile for Cu@mMOR and Cu/mMOR.\nReferences:\n[R17] Wang, B. et al. Nature of active sites on Cu-CeO catalysts activated by high- 2 temperature thermal aging. ACS Catal. 10, 12385-12392 (2020).\n3. I am afraid the authors’ explanation for the volcano-type dependence of CO 2 hydrogenation rate on Na+ content (Supplementary Fig. 16c) is unconvincing.\nRegardless of the proposed balance between CO adsorption and surface acidity, the 2 observed trend clearly suggests that Na+ is not the primary active site for CO 2 hydrogenation. If Na+ were indeed the catalytic site, one would expect a monotonic, positive correlation between Na+ content and reaction rate.\nAuthor reply: We appreciate the reviewer’s insightful suggestion. Based on a comprehensive literature review and our experimental data, we concur with your assessment that Na⁺ is not the primary active site for CO hydrogenation. Instead, it 2 functions more accurately as a catalytic promoter within the system. Specifically, the Na+ sites facilitate the adsorption and temporary capture of CO molecules, thereby 2 prolonging their residence time within the pore channels. This extends the duration of contact between CO and the intrinsic active sites, which in turn enhances the overall 2 CO hydrogenation efficiency to a certain extent. Accordingly, we have revised the 2 relevant discussions concerning the role of Na⁺ in the manuscript. These changes have been highlighted for ease of review. The specific revisions are detailed below:\nRevised manuscript, page 18: “Meanwhile, CO hydrogenation rate (Supplementary 2 Fig. 18c) displayed a characteristic volcano-shaped trend with increasing Na+ content, which initially rose to a maximum and then declined. This volcano-type behavior suggested that Na+ played an auxiliary role in activating CO . Specifically, Na+ 2 enhanced the adsorption of CO on adjacent Si-O--Al sites via electrostatic interactions, 2 thereby increasing the surface coverage and residence time of CO R18. This 2 enhancement ultimately contributed to the superior CO hydrogenation performance of 2 Cu@mMOR. ”\nReferences:\n[R18] Delgado, J. A. & Gómez, J. M. Estimation of adsorption parameters from temperature-programed-desorption thermograms: Application to the adsorption of carbon dioxide onto Na- and H-mordenite. Langmuir 21, 9555-9561 (2005).\n4. The authors interpret the increasing KIE values with temperature as evidence that H+/H- migration is the rate-determining step (RDS). This conclusion is ambiguous, as typical H/D KIE values tend to decrease with temperature. Moreover, if the migration of H+/H- is indeed the RDS, one would expect the reaction rate to be largely independent of CO partial pressure—yet no evidence is provided to support or refute this. 2 Additionally, the argument that the hydrogenation rates on Cu and Na+ cannot be compared is not persuasive. While the Cu and Na+ sites are physically separated, kinetic and energetic comparisons remain scientifically valid. Since Cu is a well-known hydrogenation catalyst, the burden of proof lies with the authors to demonstrate that the encapsulated Cu sites are indeed inactive for CO hydrogenation. 2\nAuthor reply: We appreciate the reviewer’s constructive questions. We have addressed these concerns in the following three points.\n[I]. First, regarding the reviewer’s comment that “the increasing KIE values with temperature as evidence that H+/H- migration is the rate-determining step (RDS). This conclusion is ambiguous, as typical H/D KIE values tend to decrease with temperature”, we fully agree with this assessment. According to relevant reviews, the kinetic isotope effect (KIE) is generally considered to be governed by both the difference in zero-point energy (ZPE) and the tunneling effectR19. Generally, as the temperature increases, atomic motion intensifies, and the contribution of the ZPE difference to the KIE gradually diminishes, resulting in a decrease in the apparent KIE valuesR20. However, the quantum tunneling effect can be significantly enhanced in specific environments, such as structures rich in weak-interaction donors/acceptors or hydrogen-bond networks where hydrogen atoms resideR21-R24. This enhancement can consequently lead to an increase in KIER25, R26.\nIn the Cu@mMOR system, we observed that CO conversion increased with 2 temperature. This process also generated more dissociated hydrogen species and water molecules. Since these water molecules are confined within the zeolite channels, we believe they interact weakly with spillover hydrogen species. This interaction creates favorable conditions for hydrogen tunneling. Accordingly, we have clarified the KIE mechanism in the revised manuscript and highlighted the relevant sections.\nRevised manuscript, page 21-22: “Furthermore, the spillover kinetics of the catalyst were evaluated using the kinetic isotope effect (KIE). In this context, the CO formation rate under CO /H and CO /D atmosphere was respectively measured and the 2 2 2 2 corresponding KIE values with reaction time increasing were shown and calculated in Supplementary Fig. 38. As shown, the KIE value (r /r ) of Cu@mMOR increased H D progressively from 1.03 at 300 °C to 1.43 at 600 °C, revealing a pronounced temperature-dependent discrepancy. Generally, the magnitude of KIE is governed by the zero-point energy (ZPE) and the tunneling effectR19. The observed rise in KIE with temperature indicated that the hydrogen spillover process on the Cu@mMOR catalyst was accompanied by a significant quantum tunneling effect, contrasting with the temperature-diminishing ZPE contributionR26. This tunneling process facilitated the rapid spillover of active hydrogen species from Cu sites into the mMOR channels, thereby promoting CO reduction.” 2\n[II]. In the present Cu@mMOR-catalyzed RWGS system, the reaction is prone to follow a dual-sites reaction kinetics mechanismR27, R28. Consequently, the dependence of the reaction rate on CO concentration cannot be directly used to determine the rate- 2 determining step (RDS). To thoroughly elucidate the kinetic characteristics of the Cu@mMOR system, we have conducted additional Steady-State Isotope Transient Kinetic Analysis (SSITKA) experiments under varying CO and H partial pressures in 2 2 the revised manuscript. These experiments were designed to unambiguously identify the RDS. The results indicate that the active hydrogen species exhibit the longest H/D isotope residence time on the catalyst surface. This finding further confirms that the migration of H+/H- is the rate-determining step for the RWGS reaction over Cu@mMOR. Following the reviewer's advice, we have incorporated these analyses and discussions into the main text and the experimental section of the supplementary information. All changes have been highlighted in yellow, as detailed below:\nRevised supplementary information, page 11-13: “Steady-state isotopic transient kinetic analysis (SSITKA) measurements were performed in a transmission in situ IR cell with a dead volume of less than 400 μL. The cell was coupled to a mass spectrometer (Pfeiffer Vacuum) and an FTIR spectrometer (Thermo Fisher Scientific iS50) equipped with an MCT detector. For sample preparation, the catalyst (5 mg) was mixed with KBr (5 mg), finely ground, and pressed into a self-supporting pellet. The pellet was then loaded into the in situ cell and sandwiched between two CaF windows. 2 Prior to the reaction, the catalyst was reduced at 600 °C under a flow of 10%-H /90%- 2 Ar for 30 min. Subsequently, the system was purged with Ar for 30 min at the same temperature, and a background spectrum was collected. The reaction was then conducted at 600 °C and 0.1 MPa using a gas mixture of 12CO /H /Ar (flow rate: 40 2 2 mL/min, 12CO /H /Ar = 1/2/37). After maintaining steady-state reaction conditions for 2 2 30 min, the feed was abruptly switched via a four-way valve (VICI) to an isotopic gas mixture of 13CO /H /Ar/Kr (flow rate: 40 mL/min, 13CO /H /Ar/Kr = 1/2/36.9/0.1) or 2 2 2 2 12CO /D /Ar/Kr (flow rate: 40 mL/min, 12CO /D /Ar/Kr = 1/2/36.9/0.1) with the same 2 2 2 2 overall composition and flow rate. An inert tracer (Kr) was included in the feed to determine the gas-phase holdup of the system. The transient responses of the effluent gases were continuously monitored by the MS, while time-resolved in situ FTIR spectra were simultaneously recorded to track the dynamic evolution of surface reaction intermediates. The surface residence times (τ) were calculated based on the integral of the normalized isotopic decay curves. The residence time for the growing isotopic component (wash-in) is calculated as followsR29, R30:\nt2 t1 [ Z K (r t ) Z g ro w th ( t ) ] d t  =  − (9)\nThe residence time of the decaying isotopic component is calculated as follows:\nt2 t1 [ Z K (r t ) (1 Z d e c a y ( t ) ) ] d t  =  − − (10)\nHere, τ denotes the residence time of the isotopic species. The terms t and t 1 2 represent the time points corresponding to the steady states before and after the isotopic switch, respectively, while Z(t) refers to the normalized mass spectrometry signal profile.\nFor the SSITKA measurements investigating hydrogen spillover, the system was first equilibrated under a H /Ar flow (total flow rate: 40 mL/min, H /Ar = 2/38). 2 2 Subsequently, the feed was switched to a D /Ar/Kr mixture (total flow rate: 40 mL/min, 2 D /Ar/Kr = 2/37.9/0.1) via the four-way valve, while the MS signals of the effluent 2 gases and in situ FTIR spectra were simultaneously recorded.\nThe procedure for water-assisted hydrogen spillover SSITKA was similar to that described above, with the exception that the feed gases were humidified prior to entering the in situ FTIR cell. Specifically, the H /Ar stream (total flow rate: 40 mL/min, 2 H /Ar = 2/38) was directed through a temperature-controlled bubbler containing H O. 2 2 Upon reaching steady state, the feed was switched via the four-way valve to a D /Ar/Kr 2 mixture (total flow rate: 40 mL/min, D /Ar/Kr = 2/37.9/0.1) that had been passed 2 through a D O bubbler. The in situ FTIR spectra and MS signals of the effluent were 2 recorded continuously throughout the process. To maintain identical vapor pressures, the temperatures of the H O and D O bubblers were set to 10.0 °C and 11.6 °C, 2 2 respectively, as determined by the Antoine equation:\nB log P= A− (11) 10 T +C\nwhere P represents the vapor pressure, T is the temperature. A, B, and C are empirical parameters obtained from the NIST database (A (5.40221), B (1838.675), C (- H2O H2O H2O 31.737), A (5.15107), B (1700.073), C (-44.013))R31, R32.”\nRevised manuscript, page 23-24: “… These intermediates confirmed that the RWGS reaction over Cu@mMOR likely followed exclusively the formate pathway, and H O molecules actively participated in H+ transfer during H* spillover. Furthermore, 2 steady-state isotope transient kinetic analysis (SSITKA) experiments were performed to the Cu@mMOR system to gain deeper kinetic insight. The procedure involved equilibrating the catalyst under a CO /H atmosphere, followed by a rapid switch to an 2 2 isotopic gas mixture (13CO /H or CO /D ) supplemented with an inert Kr internal 2 2 2 2 standard at an identical flow rate. The dynamic evolution of surface species and effluent products were simultaneously monitored in real-time via infrared spectroscopy and mass spectrometry, respectively. As shown in Supplementary Fig. 46-52, the residence times τ of key reaction species over Cu@mMOR followed the order: 13CO * (3.4 s, CO 2 2 chemisorption) < H13COO* (11.4 s, CO hydrogenation) < H12COO* (46.4 s, formate 2 dissociation) < Al-H (57.5 s, hydrogen spillover). Among which, Al-H species exhibited the longest isotopic residence time, indicating that hydrogen spillover was kinetically limited and thus identified as the reaction’s rate-determining step (RDS)R29, R30, R33, R34.”\nSupplementary Figure 46 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different CO partial pressures. Normalized transient MS responses of Kr, 13CO , and 2 2 13CO signals at 600 °C upon switching from 12CO /H to 13CO /H . The flow rate ratios 2 2 2 2 of CO /H correspond to (a) 1/2, (b) 1.4/2, (c) 1.8/2, and (d) 2.2/2. 2 2\nSupplementary Figure 47 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different CO partial pressures. Normalized transient in situ FTIR responses of 2 13CO * and H13COO* signals at 600 °C upon switching from 12CO /H to 13CO /H . The 3 2 2 2 2 flow rate ratios of CO /H correspond to (a) 1/2, (b) 1.4/2, (c) 1.8/2, and (d) 2.2/2. 2 2\nSupplementary Figure 48 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different CO partial pressures. Normalized transient in situ FTIR responses of 2 reaction intermediates signals at 600 °C upon switching from 12CO /H to 13CO /H . 2 2 2 2 The flow rate ratios of CO /H correspond to (a) 1/2, (b) 1.4/2, (c) 1.8/2, and (d) 2.2/2. 2 2\nSupplementary Figure 49 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different H partial pressures. Normalized transient MS responses of Kr, 13CO , and 2 2 13CO signals at 600 °C upon switching from 12CO /H to 13CO /H . The flow rate ratios 2 2 2 2 of CO /H correspond to (a) 1/2, (b) 1/2.4, (c) 1/2.8, and (d) 1/3.2. 2 2\nSupplementary Figure 50 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different H partial pressures. Normalized transient in situ FTIR responses of 13CO * 2 3 and H13COO* signals at 600 °C upon switching from 12CO /H to 13CO /H . The flow 2 2 2 2 rate ratios of CO /H correspond to (a) 1/2, (b) 1/2.4, (c) 1/2.8, and (d) 1/3.2. 2 2\nSupplementary Figure 51 | SSITKA of CO hydrogenation over Cu@mMOR at 2 different H partial pressures. Normalized transient in situ FTIR responses of reaction 2 intermediates signals at 600 °C upon switching from 12CO /H to 13CO /H . The flow 2 2 2 2 rate ratios of CO /H correspond to (a) 1/2, (b) 1/2.4, (c) 1/2.8, and (d) 1/3.2. 2 2\nSupplementary Figure 52 | SSITKA of RWGS over Cu@mMOR upon switching from CO /H /Ar to CO /D /Ar/Kr. (a) Normalized transient MS responses of Kr, H , 2 2 2 2 2 D , H O, and D O signals. (b) Normalized transient IR profiles of the C-H bond of 2 2 2 HCOO* and Al-H species. (c) Time-resolved in situ FTIR contour map showing the dynamic isotopic exchange of surface intermediates and hydroxyl groups. (d) in situ FTIR spectra before (purple) and after (green) the switch to D . 2\n[III]. We appreciate the reviewer’s suggestion to supplement the kinetic and energetic analyses of CO hydrogenation over encapsulated Cu clusters. To theoretically address 2 the concerns regarding the activity of Cu sites, a Cu@mMOR model based on the cleaved MOR surface was constructed (i.e., the Cu clusters situated on the MOR cleavage plane) and performed detailed DFT calculations in the attachment. This model was employed to compare the kinetic and energetic differences between the encapsulated Cu clusters and the active sites of mMOR. Our calculations indicate that although the Cu clusters in Cu@mMOR can theoretically catalyze CO hydrogenation 2 via the carboxyl pathway, their strong interaction with C atoms favors the formation of copper carbonyl species, which subsequently induces Ostwald ripening and accelerates catalyst deactivation. As requested, we have included the relevant analysis and discussion in the Supplementary Information. The newly added content is highlighted in yellow and presented below:\nAttachment，page 1: “To investigate the direct hydrogenation mechanism of CO 2 on mMOR-supported Cu clusters at the atomic scale, the Gibbs free energy profile for the relevant reaction processes was constructed using DFT calculations. As shown in Fig. A1, the adsorption energy of CO on Cu clusters is only -0.06 eV, which is 2 significantly weaker than that on the Na+ sites in mMOR (-0.44 eV). This indicates that in the Cu@mMOR system, the Cu clusters possess a weak capability for CO capture, 2 making the CO molecules thermodynamically more inclined to be adsorbed by the Na+ 2 sites. In contrast, the adsorption of H atoms on Cu clusters leads to a system energy decrease of 0.50 eV, confirming a stronger affinity of the Cu surface for H species. Regarding the hydrogenation pathways, CO and H on Cu clusters can react via either 2 the formate or the carboxyl pathway. Calculations reveal that the hydrogenation barrier for the former is as high as 2.35 eV, whereas that for the latter is only 0.74 eV. Furthermore, the dissociation barrier of COOH on Cu clusters (1.85 eV) is also lower than that of HCOO (2.08 eV). Therefore, if the RWGS reaction were to occur on Cu clusters, it would kinetically prioritize the carboxyl pathway. Notably, the desorption barrier of CO on Cu clusters exhibits significant configurational dependence: the 2 desorption barrier for the C-bound configuration is as high as 0.99 eV, far exceeding the 0.20 eV for the O-bound configuration. This reveals a strong interaction between the Cu clusters and the C atoms of carbon-containing species (e.g., COOH and CO) during the reaction. According to the transition-state/adsorption-strength scaling relationsR35, R36, this strong Cu-C interaction effectively lowers the transition state barrier, thereby establishing the thermodynamic and kinetic dominance of the carboxyl pathway.\nHowever, this excessively strong CO adsorption also implies that Cu species are 2 highly susceptible to Ostwald ripening under high-temperature reaction conditions, which serves as a critical consideration in our catalyst design. Most critically, DRIFTS spectroscopy (Fig. 5d in the manuscript) has confirmed that Cu@mMOR follows the formate pathway, which stands in stark contrast to the DFT-derived conclusion that the carboxyl pathway is preferred on Cu clusters. This fundamental mechanistic contradiction provides compelling evidence that the active sites for CO hydrogenation 2 in Cu@mMOR are not the encapsulated Cu clusters, but rather the mMOR framework sites.”\nFig. A1 Gibbs free energy profile for the RWGS reactions on Cu clusters.\nReferences:\n[R19] Hammes-Schiffer, S. Explaining kinetic isotope effects in proton-coupled electron transfer reactions. Acc. Chem. Res. 58, 1335-1344 (2025).\n[R20] Chen, Benjamin W. J. Equilibrium and kinetic isotope effects in heterogeneous catalysis: A density functional theory perspective. Catal. Commun. 177, 106654 (2023).\n[R21] Kyakuno, H., Matsuda, K., Ishizeki, K., Yamamoto, T. & Maniwa, Y. Proton transport through ice nanoribbons. J. Phys. Chem. C 128, 11289-11297 (2024).\n[R22] Ceriotti, M. et al. Nuclear quantum effects in water and aqueous systems: Experiment, theory, and current challenges. Chem. Rev. 116, 7529-7550 (2016).\n[R23] Salna, B., Benabbas, A. & Champion, P. M. Proton-coupled electron transfer and the \"linear approximation\" for coupling to the donor-acceptor distance fluctuations. J. Phys. Chem. A 121, 2199-2207 (2017).\n[R24] Pudney, C. R., Johannissen, L. O., Sutcliffe, M. J., Hay, S. & Scrutton, N. S. Direct analysis of donor acceptor distance and relationship to isotope effects and the force constant for barrier compression in enzymatic H-tunneling reactions. J. Am. Chem. Soc. 132, 11329-11335 (2010).\n[R25] Wu, Q. Y. et al. Catalytic hydrogenation dominated by concerted hydrogen tunneling at room temperature. ACS Cent. Sci. 11, 2180-2187 (2025).\n[R26] An, B. et al. Molecular iridium complexes in metal-organic frameworks catalyze CO hydrogenation via concerted proton and hydride transfer. J. Am. 2 Chem. Soc. 139, 17747-17750 (2017).\n[R27] Hu, M. C., Yao, Z. H., Hui, K. N. & Hui, K. S. Novel mechanistic view of catalytic ozonation of gaseous toluene by dual-site kinetic modelling. Chem. Eng. J. 308, 710-718 (2017).\n[R28] Pino, L., Italiano, C., Lagana, M., Vita, A. & Recupero, V. Kinetic study of the methane dry (CO ) reforming reaction over the Ce La Ni O catalyst. 2 0.70 0.20 0.10 2-δ Catal. Sci. Technol. 10, 2652-2662 (2020).\n[R29] Otroshchenko, T., Kondratenko, V. A., Zanina, A., Zhang, Q. Y. & Kondratenko, E. V. Progress through temporal analysis of products and steady-state isotopic transient kinetic analysis to elucidate oxidation, CO hydrogenation and lower 2 olefins production reactions. ChemCatChem 16, e202301447 (2024).\n[R30] Janssens, P., Poissonnier, J., Chakkingal, A., Bos, R. & Thybaut, J. W. Recent advances in the use of steady-state isotopic transient kinetic analysis data in (micro)kinetic modeling for catalyst and process design. Catal. Commun. 179, 106688 (2023).\n[R31] Bridgeman, O. C. & Aldrich, E. W. Vapor pressure tables for water. J. Heat Transfer 86, 279-286 (1964).\n[R32] Liu, C. T. & Lindsay, W. T. Vapor Pressure of D O from 106 to 300°C. J. Chem. 2 Eng. Data 15, 510-513 (1970).\n[R33] Wang, K. et al. DRIFTS-SSITKA-MS investigations on the mechanism of plasmon preferentially enhanced CO2 hydrogenation over Au/γ-Al O . Appl. 2 3 Catal., B 328, 122531 (2023).\n[R34] Wang, X. Y. et al. The nature of interfacial catalysis over Pt/NiAl O for 2 4 hydrogen production from methanol reforming reaction. J. Am. Chem. Soc. 145, 905-918 (2023).\n[R35] Cheng, J. et al. Brønsted-evans-polanyi relation of multistep reactions and volcano curve in heterogeneous catalysis. J. Phys. Chem. C 112, 1308-1311 (2008).\n[R36] Honkala, K. et al. Ammonia synthesis from first-principles calculations. Science 307, 555-558 (2005).\n5. The DFT results indicate that direct dissociation of HCOO* has a lower energy barrier than COOH*. However, the authors do not provide clear physical reasoning for this difference. Given the mechanistic importance, I strongly suggest including transition state structures and performing detailed analysis to explain why HCOO* dissociates more readily. This would improve the robustness of the mechanistic proposal.\nAuthor reply: We appreciate the reviewer’s suggestion. We have supplemented detailed transition state structures in the revised supplementary information.\nFurthermore, we added crystal orbital Hamilton population (COHP) and independent gradient model based on Hirshfeld partition (IGMH) analyses to reveal the physical origin of this differenceR37-R41. Our analysis indicated that the higher dissociation energy barrier for COOH* primarily stemmed from three factors: (1) the necessity of breaking an additional Al-H bond to facilitate electron transfer; (2) the significant antibonding character of the transition state C≡O bond near the Fermi level, as revealed by COHP; and (3) the strong hydrogen bonding interactions between the initial state hydroxyl group and the framework oxygen, as confirmed by IGMH, which increased the difficulty of dissociation. These revisions were integrated into the main text. Additionally, detailed explanations, figures, and animations were included in the revised supplementary information. All changes were highlighted in yellow, as shown below:\nRevised manuscript, page 24-25:“… Due to the lower barrier, CO * preferred to 2 follow formate pathway, which was consistent with the DRIFTS observation that detected only the formate intermediate (Fig. 5e). Next, HCOO- faced a competition between direct dissociation and protonation. Next, HCOO- may either become protonated or directly dissociate into CO. Protonation is kinetically much more favorable, with a very low barrier of only 0.43 eV, leading easily to HCOOH formation. In contrast, direct dissociation pathways face much higher barriers (1.72 eV for HCOO⁻ and 2.04 eV for COOH⁺), making them far less likely. These substantial barriers (particularly for COOH+) were attributed to the necessity of rupturing an additional Al-H bond to facilitate electron transfer, strong antibonding orbital repulsion, and the requirement to overcome hydrogen bonding interactions (Supplementary Fig. 53-54 and Supplementary Movies 11-14) R37-R41. Subsequently, the generated HCOOH could either dissociate directly or undergo further hydrogenation.…”\nRevised supplementary information, page 14“: … The orbital-transformation (OT) scheme was activated during the self-consistent field iterations to accelerate convergence, and the electronic energy convergence criterion was set to 1 × 10-6. The independent gradient model based on Hirshfeld partition (IGMH) and crystal orbital Hamilton population (COHP) analyses were performed using the Multiwfn code based on the molden files generated by CP2KR40, R41.”\nSupplementary Figure 53 | Direct dissociation processes of intermediates in Cu@mMOR via different pathways. Direct dissociation process via (a) the formate pathway and (b) the carboxyl pathway. The O, Na, C, H, Al, and Si atoms are depicted in red, purple, black, light-gray, pink, and yellow, respectively.\nRevised supplementary information, page 72:“As illustrated in Supplementary Fig. 53a, the dissociation of HCOO- entailed the cleavage of C-H and C-O bonds alongside the formation of C≡O and O-H bonds. In stark contrast, the dissociation mechanism for COOH+ (Supplementary Fig. 53b) was significantly more intricate, necessitating the concurrent rupture of Al-H, C-O and C-O(mMOR) bonds, as well as the generation of C≡O and O(mMOR)-H bonds. This structural disparity suggests that the dissociation of COOH+ mandates the breaking of an extra Al-H bond to facilitate electron transfer, which inherently results in the substantially higher energy barrier observed for this pathway.”\nSupplementary Figure 54 | Electronic orbital analysis of species involved in direct dissociation pathways of Cu@mMOR. Crystal orbital Hamilton population (COHP) curves of the initial and transition states for the direct dissociation along (a-b) the carboxyl pathway and (c-d) the formate pathway.\nRevised supplementary information, page 73-74“ : To probe the orbital interactions, COHP analyses were performed for the initial and transition states of both pathways (Supplementary Fig. 43). Occupied states (energy < 0 eV) with positive and negative -COHP values correspond to bonding and antibonding states, respectivelyR37. In the initial states, COOH+ bonds manifested distinct antibonding features near the Fermi level, in contrast to the subtle bonding character observed for HCOO-, which underscores the relative stability of the latter. Crucially, in the transition state, the C≡O bond along the COOH+ pathway exhibited substantial antibonding character from -2 to 0 eV, with an intensity surpassing both the HCOO- pathway and its own initial state. This indicates that C≡O formation via the COOH+ route faces strong electronic resistance within the Cu@mMOR frameworkR42, R43.\nTo probe non-bonded interactions during dissociation, the independent gradient model based on Hirshfeld partition (IGMH) analyses for the initial and transition states were performed (Supplementary Movies 11-14). Here, blue/red iso-surfaces correspond to attraction/repulsion, and green represents vdW forcesR38. As shown in Supplementary Movie 11, COOH+ was surrounded by broad green iso-surfaces, with a distinct blue region localized near the hydroxyl group. These features reveal that COOH+ is stabilized not only by framework vdW forces and Na+ electrostatics but also by strong hydrogen bonding with Cu@mMOR. Conversely, in the transition state (Supplementary Movie 12), this hydrogen bond dissipated, preserving only vdW and electrostatic contributions.”\nReferences:\n[R37] Dronskowski, R. & Blochl, P. E. Crystal orbital hamilton populations (COHP) - energy-resolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 97, 8617-8624 (1993).\n[R38] Lu, T. & Chen, Q. X. Independent gradient model based on Hirshfeld partition: A new method for visual study of interactions in chemical systems. J. Comput. Chem. 43, 539-555 (2022).\n[R39] Lu, T. Visualization analysis of covalent and noncovalent interactions in real space. Angewandte Chemie-International Edition 64, e202504895 (2025).\n[R40] Lu, T. & Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J. Comput. Chem. 33, 580-592 (2012).\n[R41] Lu, T. A comprehensive electron wavefunction analysis toolbox for chemists, Multiwfn. The Journal of Chemical Physics 161, 082503 (2024).\n[R42] Cai, Y. et al. Antiover-reduction of Ni/In O nanocatalysts by atomic layer 2 3 deposition of Al O films for durable CO hydrogenation to methanol. ACS 2 3 2 Catal. 14, 8463-8479 (2024).\n[R43] Deringer, V. L., Tchougréeff, A. L. & Dronskowski, R. Crystal orbital hamilton population (COHP) analysis as projected from plane-wave basis sets. The Journal of Physical Chemistry A 115, 5461-5466 (2011).\n6. Zeolites are generally considered insulating materials. The mechanism by which electrons are proposed to migrate through the mordenite framework during hydrogen spillover remains unclear. Although EPR and XPS data were added, a more rigorous discussion on the feasibility of electron conduction in such systems is required. Without further clarification, the fundamental validity of the proposed H⁺/H⁻ transport mechanism remains questionable.\nAuthor reply: We sincerely thank the reviewer for this insightful comment. We acknowledge that the poor electrical conductivity of zeolites was not fully accounted for in our original manuscript. Consequently, the previous hypothesis proposing the “separate transport of protons and electrons over the zeolite framework” was physically unsound, and we have now rectified this interpretation. To rigorously elucidate the hydrogen spillover mechanism within the Cu@mMOR system, we have replaced the original H -DRIFTS data with steady-state isotopic transient kinetic analysis (SSITKA) 2 using H /D switching. By coupling SSITKA with in situ FTIR spectroscopy, we have 2 2 confirmed that hydrogen spillover in Cu@mMOR is primarily mediated by the dynamic coordination of H+/H- pairs within the zeolite channels. Accordingly, the manuscript and supplementary information have been updated with revised text and new experimental data. We have also corrected the interpretations of the previous in situ EPR and XPS results to align with this mechanism. All modifications are highlighted in yellow, as detailed below:\nRevised manuscript, page 20-21:“To unravel the chemical nature and dynamic behavior of hydrogen spillover at the molecular level, the evolution of hydrogen-containing species on Cu@mMOR were monitored using steady-state isotopic transient kinetic analysis coupled with FTIR (SSITKA-FTIR) during H /D switching. As 2 2 depicted in Supplementary Fig. 36, under a H atmosphere, distinct signals assigned to 2 hydride species (Al-H and Si-H) and characteristic peaks for protic species (Si-OH-Al and Si-OH) were clearly observed on Cu@mMORR44, R45. The coexistence of these H+ and H- species provided direct evidence for the heterolytic cleavage of H on 2 Cu@mMOR and identifies the specific nature of the resulting fragments. Upon switching the feed to D , the intensities of these hydrogenated species gradually 2 decayed, with a concomitant growth of deuterated species peaks at the lower wavenumbersR44, R45. This rapid H/D isotopic exchange demonstrated that the generated H+/H- pairs exhibited high mobility and dynamic exchange activity over the framework acid-base sitesR49, R50.\nNext, in situ EPR was carried out to investigate the electronic mechanism driving H 2 heterolytic activation. As shown in Supplementary Fig. 37, Cu@mMOR displayed a strong signal associated with unpaired electrons on the Cu clusters under H feeding R46. 2 When the atmosphere was switched to CO /H , this EPR signal decreased markedly. 2 2 This indicated that the electrons were transferred to adsorbed hydrogen to generate H- species, which subsequently migrate into the channels and participate in the RWGS reaction. Furthermore, XPS analysis was conducted to elucidate the migration and consumption dynamics of hydrogen species by monitoring valence shifts of Cu and Al in Cu@mMOR under H and CO /H atmospheres. As depicted in Supplementary Fig. 2 2 2 38, H treatment decreased the binding energies of both Cu and Al to indicate an 2 electron-rich state, whereas the subsequent introduction of CO /H triggered a 2 2 synchronous positive shift in these valuesR47, R48. These findings corroborated the spillover of H- species from Cu surfaces onto the framework, where they were consumed by the reaction with CO within the pores. Collectively, this unique H+/H- 2 pair migration mechanism facilitated an efficient hydrogen supply. Furthermore, it modulated the hydrogenation pathways of formate intermediates via electrostatic interactions to enhance both the sustainability and selectivity of the RWGS reaction.”\nRevised supplementary information, page 12-13:“For the SSITKA measurements investigating hydrogen spillover, the system was first equilibrated under a H /Ar flow 2 (total flow rate: 40 mL/min, H /Ar = 2/38). Subsequently, the feed was switched to a 2 D /Ar/Kr mixture (total flow rate: 40 mL/min, 2 D /Ar/Kr = 2/37.8/0.2) via the four-way 2 valve, while the MS signals of the effluent gases and in situ FTIR spectra were simultaneously recorded.”\nSupplementary Figure 36 | Steady-state isotopic transient kinetic profiles of Cu@mMOR under hydrogen spillover. The in situ FTIR spectra of Cu@mMOR during H /D switch. 2 2\nReferences:\n[R44] Liu, R. et al. Increasing the number of aluminum atoms in T3 sites of a mordenite zeolite by low-pressure SiCl treatment to catalyze dimethyl ether 4 carbonylation. Angew. Chem. Int. Ed. 61, e202116990 (2022).\n[R45] Chen, G., Fang, L., Li, T. & Xiang, Y. Ultralow-loading Pt/Zn hybrid cluster in zeolite HZSM-5 for efficient dehydroaromatization. J. Am. Chem. Soc. 144, 11831-11839 (2022).\n[R46] Yang, X. et al. Understanding the synergistic catalysis of balanced Cu0-Cu+ sites and oxygen vacancies in Cu/ZrO catalysts for the efficient hydrogenation of 2 furfural. Chem. Eng. J. 501, 157796 (2024).\n[R47] Halder, A. et al. CO methanation on Cu-cluster decorated zirconia supports 2 with different morphology: A combined experimental in situ GIXANES/GISAXS, ex situ xps and theoretical dft study. ACS Catal. 11, 6210-6224 (2021).\n[R48] Liu, C. et al. In situ reconstruction of Cu-N coordinated MOFs to generate dispersive Cu/Cu O nanoclusters for selective electroreduction of CO to C H . 2 2 2 4 ACS Catal. 12, 15230-15240 (2022).\nReviewer #2 (Remarks to the Author):\nComments:\n1. According to the authors the initial H-/H+ pairs formed on Cu first become two pairs of H+/e- entities after the crossing of the interface, then diffuse independently as such on the support and finally recover their H-/H+ identity on the active sites. It is stated that H- is re-formed by recombination of H+ and 2 e-. This is hard to admit.\nIs it not more reasonable and simpler to consider that the acid-base nature of the H-/H+ pairs allow easy interactions with and migration on acid-base centers of the support? This was suggested in our previous report.\nHydrogen spillover for non-reducible oxides would consist in hydrogenating H/OH pairs (more or less polarized) in vicinal position, formed by an interface dehydroxylation process of OH/OH pairs with water production (Bettahar, Catal. Rev., 2022). After desorption (or reaction) these hydrogenating sites become Mm+/Op- (p=1or2) hydrogen receiving sites. Hydrogenating and receiving sites are denoted as conjugated sites. It comes that the diffusion of hydrogen spillover necessarily proceeds by migration of H/H pairs from Mm+ O- site to Mm+ O- site (Bettahar, Catalysts, 2024).\nIn the presence of Na+ the hydrogenating sites are deprotonated and this favors the adsorption of CO (see suggested reaction paths in attached file) 2\nThis mechanism is compatible with the physical characterization of the catalysts, notably with the DRIFTS experiments (Figure 34). Indeed, after adsorbing CO the 2 introduction of H led to concomitant formation of acidic OH groups, Al-H hydrides 2 and adsorbed water which may correspond to the formation of the AlH/SiOH hydrogenating sites (water may arose from both dehydroxylation and WGSR processes).\nAuthor reply: We appreciate the reviewer’s insightful suggestion and fully acknowledge the limitations of the 'proton/electron separation' mechanism originally proposed. After a thorough review of the relevant literature—including the reports cited by the reviewer—we have adopted the suggested direct H+/H- migration mechanism based on acid-base pairsR26, R49-R54. To elucidate the hydrogen spillover pathway on Cu@mMOR within this new mechanistic framework, we replaced the original H - 2 DRIFTS analysis with H /D switching SSITKA experiments. Through in situ IR 2 2 spectroscopy, we observed the formation of both hydride species (Al-H, Si-H) and protons (Brønsted acid sites, Si-OH), providing direct evidence for the H+/H- migration mechanism. Furthermore, our original KIE experiments (Supplementary Fig. 38), CO /H -DRIFTS (Fig. 5e), and AIMD results (Supplementary Movie 2) all indicated 2 2 that water significantly promotes hydrogen spillover. Drawing upon literature regarding the stabilization mechanisms of hydrated ions (e.g., H(H O) + and H(H O) -)R55-R60, we 2 n 2 n propose that water molecules within Cu@mMOR can function as mobile acid-base pairs to assist this process. Therefore, we posit that during the RWGS reaction, hydrogen spillover is facilitated not only by the intrinsic acid-base sites on the zeolite framework but is also reinforced by in situ formed water molecules acting as mobile acid-base pairs. To validate this hypothesis, we performed additional H +H O/D +D O 2 2 2 2 switching SSITKA experiments and corresponding DFT calculations. Both sets of results support the concept of water-assisted acid-base spillover. Finally, the corresponding textual revisions and new data have been updated in the revised manuscript and Supplementary Information (SI). All changes are highlighted in yellow, as shown below:\nRevised manuscript, page 20-21:“To unravel the chemical nature and dynamic behavior of hydrogen spillover at the molecular level, the evolution of hydrogen-containing species on Cu@mMOR were monitored using steady-state isotopic transient kinetic analysis coupled with FTIR (SSITKA-FTIR) during H /D switching. As 2 2 depicted in Supplementary Fig. 36, under a H atmosphere, distinct signals assigned to 2 hydride species (Al-H and Si-H) and characteristic peaks for protic species (Si-OH-Al and Si-OH) were clearly observed on Cu@mMORR44, R45. The coexistence of these H+ and H- species provided direct evidence for the heterolytic cleavage of H on 2 Cu@mMOR and identifies the specific nature of the resulting fragments. Upon switching the feed to D , the intensities of these hydrogenated species gradually 2 decayed, with a concomitant growth of deuterated species peaks at the lower wavenumbersR44, R45. This rapid H/D isotopic exchange demonstrated that the generated H+/H- pairs exhibited high mobility and dynamic exchange activity over the framework acid-base sitesR49, R50.\nNext, in situ EPR was carried out to investigate the electronic mechanism driving H 2 heterolytic activation. As shown in Supplementary Fig. 37, Cu@mMOR displayed a strong signal associated with unpaired electrons on the Cu clusters under H feedingR46. 2 When the atmosphere was switched to CO /H , this EPR signal decreased markedly. 2 2 This indicated that the electrons were transferred to adsorbed hydrogen to generate H- species, which subsequently migrate into the channels and participate in the RWGS reaction. Furthermore, XPS analysis was conducted to elucidate the migration and consumption dynamics of hydrogen species by monitoring valence shifts of Cu and Al in Cu@mMOR under H and CO /H atmospheres. As depicted in Supplementary Fig. 2 2 2 38, H treatment decreased the binding energies of both Cu and Al to indicate an 2 electron-rich state, whereas the subsequent introduction of CO /H triggered a 2 2 synchronous positive shift in these valuesR47, R48. These findings corroborated the spillover of H- species from Cu surfaces onto the framework, where they were consumed by the reaction with CO within the pores. Collectively, this unique H+/H- 2 pair migration mechanism facilitated an efficient hydrogen supply. Furthermore, it modulated the hydrogenation pathways of formate intermediates via electrostatic interactions to enhance both the sustainability and selectivity of the RWGS reaction.”\nRevised manuscript, page 22-23:“… As calculated in Supplementary Fig. 42, the energy barrier of H O-assisted spillover was only 0.29 eV, lower than most of the 2 reported values (0.4 ~ 0.8 eV). Thus, the transfer efficiency of H+ in Cu@mMOR was significantly improved, allowing adsorbed CO to be rapidly reduced. Furthermore, the 2 independent gradient model based on Hirshfeld partition (IGMH, Supplementary Fig. 43a) indicated that within the Cu@mMOR channels, H- could form H(H O) - clusters 2 n with water molecules via strong hydrogen bondsR38. This interaction facilitated the dispersion of the negative charge on H-, thereby enhancing thermodynamic stability. Simultaneously, the stability of H(H O) - was found to increase with the number of 2 n water molecules (Supplementary Fig. 43b-c), implying that H- was thermodynamically driven to approach the CO reaction sites where water was producedR59. To further 2 investigate the water effects on hydrogen spillover, H /D and H +H O/D +D O 2 2 2 2 2 2 switching SSITKA experiments were performed on Cu@mMOR (Supplementary Fig. 44). At which, water-assisted hydrogen spillover exhibited an extremely long residence time of 199.4 s under H +H O/D +D O switching conditions (Supplementary Fig. 44b), 2 2 2 2 significantly exceeding the 46.6 s observed during H /D switching (Supplementary Fig. 2 2 44a). The markedly extended residence time indicates that water enables hydrogen spillover to occur over a significantly much longer distance on Cu@mMOR via facile the O-H bond cleavage and the pronounced quantum tunneling effects of H(H O) +/H(H O) - R22. Thereby, it established a highly efficient cycle for the spatially 2 n 2 n separated RWGS reaction by maximizing the active hydrogen and CO contact.” 2\nSupplementary Figure 43 | Structure of hydrated hydride ions within the Cu@mMOR channels. (a) The IGMH analysis of H(H O) - inside Cu@mMOR. The 2 4 electrostatic potential, Hδ- Mulliken charges, and formation energies of (b) H(H O) -, 2 3 (c) H(H O) -, and (d) H(H O) -.\n2 4 2 6\nSupplementary Figure 44 | Steady-state isotopic transient kinetic profiles of Cu@mMOR under hydrogen spillover. The online MS signals of Cu@mMOR during (a) H /D and (b) H +H O/D +D O switch. 2 2 2 2 2 2\nReferences:\n[R22] Ceriotti, M. et al. Nuclear quantum effects in water and aqueous systems: Experiment, theory, and current challenges. Chem. Rev. 116, 7529-7550 (2016).\n[R26] An, B. et al. Molecular iridium complexes in metal-organic frameworks catalyze CO hydrogenation via concerted proton and hydride transfer. J. Am. 2 Chem. Soc. 139, 17747-17750 (2017).\n[R38] Lu, T. & Chen, Q. X. Independent gradient model based on Hirshfeld partition: A new method for visual study of interactions in chemical systems. J. Comput. Chem. 43, 539-555 (2022).\n[R44] Liu, R. et al. Increasing the number of aluminum atoms in T3 sites of a mordenite zeolite by low-pressure SiCl treatment to catalyze dimethyl ether 4 carbonylation. Angew. Chem. Int. Ed. 61, e202116990 (2022).\n[R45] Chen, G., Fang, L., Li, T. & Xiang, Y. Ultralow-loading Pt/Zn hybrid cluster in zeolite HZSM-5 for efficient dehydroaromatization. J. Am. Chem. Soc. 144, 11831-11839 (2022).\n[R49] Bettahar, M. M. The hydrogen spillover effect. A misunderstanding story. Catalysis Reviews-Science and Engineering 64, 87-125 (2022).\n[R50] Bettahar, M. M. The hydrogen spillover effect-a misunderstanding study II: Single oxide and zeolite supports. Catalysts 14, 458-477 (2024).\n[R51] Gronert, S. & Keeffe, J. R. Identity hydride-ion transfer from C-H donors to C acceptor sites. Enthalpies of hydride addition and enthalpies of activation. Comparison with C···H···C proton transfer. An ab initio study. J. Am. Chem. Soc. 127, 2324-2333 (2005).\n[R52] Huang, F. et al. Features and mechanism of H- anion emission from 12CaO·7Al O surface. J. Phys. Chem. B 109, 12032-12037 (2005).\n2 3\n[R53] Copéret, C., Estes, D. P., Larmier, K. & Searles, K. Isolated surface hydrides: Formation, structure, and reactivity. Chem. Rev. 116, 8463-8505 (2016).\n[R54] Yi, F. J. et al. Origin of weak lewis acids on silanol nests in dealuminated zeolite beta. J. Catal. 380, 204-214 (2019).\n[R55] Hack, J. H. et al. Structural characterization of protonated water clusters confined in HZSM-5 zeolites. J. Am. Chem. Soc. 143, 10203-10213 (2021).\n[R56] Armbruster, M., Haberland, H. & Schindler, H.-G. Negatively charged water clusters, or the first observation of free hydrated electrons. Phys. Rev. Lett. 47, 323-326 (1981).\n[R57] Mayberry, D. D., Linehan, J. C. & Appel, A. M. Designing catalytic systems using binary solvent mixtures: Impact of mole fraction of water on hydride transfer. Inorg. Chem. 60, 17132-17140 (2021).\n[R58] Mei, K. N., Borrelli, W. R., Sandoval, J. L. G. & Schwartz, B. J. How to probe hydrated dielectrons experimentally: Ab initio simulations of the absorption spectra of aqueous dielectrons, electron pairs, and hydride. J. Phys. Chem. Lett. 15, 9557-9565 (2024).\n[R59] Lee, H. M., Anick, D. J. & Kim, K. S. Structures of tri-, tetra-, and hexahydrated hydride anion clusters. Int. J. Quantum Chem. 109, 1820-1826 (2009).\n[R60] Prakash, M., Gopalsamy, K. & Subramanian, V. Studies on the structure, stability, and spectral signatures of hydride ion-water clusters. J. Chem. Phys. 135, 214308 (2011).\n2. In these DRIFTS experiments the authors evidenced the existence of hydrogen spillover during the hydrogenation reaction. Why? Do they assume that hydrogen spillover is induced by the presence of the organic molecule (see Choi et al., Nanoresearch, 2022)? If it is such the statement is wrong because H spiltover species are incorporated in the support in the absence of any organic molecule, as the authors know. Why DRIFTS experiments were not conducted in H atmosphere solely?\nRevised manuscript, page 20:“To unravel the chemical nature and dynamic behavior of hydrogen spillover at the molecular level, the evolution of hydrogen-containing species on Cu@mMOR were monitored using steady-state isotopic transient kinetic analysis coupled with FTIR (SSITKA-FTIR) during H /D switching. As 2 2 depicted in Supplementary Fig. 36, under a H atmosphere, distinct signals assigned to 2 hydride species (Al-H and Si-H) and characteristic peaks for protic species (Si-OH-Al and Si-OH) were clearly observed on Cu@mMORR44, R45. The coexistence of these H+ and H- species provided direct evidence for the heterolytic cleavage of H on 2 Cu@mMOR and identifies the specific nature of the resulting fragments. Upon switching the feed to D , the intensities of these hydrogenated species gradually 2 decayed, with a concomitant growth of deuterated species peaks at the lower wavenumbersR44, R45. This rapid H/D isotopic exchange demonstrated that the generated H+/H- pairs exhibited high mobility and dynamic exchange activity over the framework acid-base sitesR49, R50.”\nSupplementary Figure 36 | Steady-state isotopic transient kinetic profiles of Cu@mMOR under hydrogen spillover. The in situ FTIR spectra of Cu@mMOR during H /D switch. 2 2\nRevised manuscript references:\n[R26] An, B. et al. Molecular iridium complexes in metal-organic frameworks catalyze CO hydrogenation via concerted proton and hydride transfer. J. Am. 2 Chem. Soc. 139, 17747-17750 (2017).\n[R44] Liu, R. et al. Increasing the number of aluminum atoms in T3 sites of a mordenite zeolite by low-pressure SiCl treatment to catalyze dimethyl ether 4 carbonylation. Angew. Chem. Int. Ed. 61, e202116990 (2022).\n[R45] Chen, G., Fang, L., Li, T. & Xiang, Y. Ultralow-loading Pt/Zn hybrid cluster in zeolite HZSM-5 for efficient dehydroaromatization. J. Am. Chem. Soc. 144, 11831-11839 (2022).\n[R49] Bettahar, M. M. The hydrogen spillover effect. A misunderstanding story. Catalysis Reviews-Science and Engineering 64, 87-125 (2022).\n[R50] Bettahar, M. M. The hydrogen spillover effect-a misunderstanding study II: Single oxide and zeolite supports. Catalysts 14, 458-477 (2024).\n+Na+ + CO +H+ 2 AlOH/SiOH+2 H  AlH/SiOH  AlH/SiONa  AlH/SiOCOONa  Al/HCOO/SiONa   Al/SiO sp -H O H+ -(CO, H O, Na+) 2 2 AlOH/SiOH, AlH/SiOH and Al/SiO are hydroxyl precursors, hydrogenating sites and receiving sites respectively."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have made a significant improvement in this reversed version, and I would like to recommend its publication in Nature Communications.", "reviewer_2": "In my first report I concluded that the manuscript may be published as such. My remarks aimed to improve its scientific content. The authors took them into account in the revised manuscript and congratulate them for that. Therefore I confirm that the paper can be accepted for publication."}, "author_response": ""}]

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