Efficient Pt_1Ni single-atom alloy catalyst for hydrogen-free catalytic fractionation of lignocellulose

Domain: biology
Published: 2026-03-23
Editorial outcome: minor_revision

Abstract
The production of monophenols from lignocellulose with cellulose intact without exogenous hydrogen via self-hydrogen supplied fractionation is promising, yet requires high metal loadings and yields saturated products. Herein, we report a single-atom alloyed Pt Ni catalyst that achieves 50.9 wt% yield of phenolic monomers from birch sawdust with about 50% selectivity to valuable propenyl side-chained products under mild conditions (140 °C, 1 atm N ), while preserving cellulose intact. Reaction pathway studies and density functional theory calculations based on a β-O-4 model compound reveal three coexisting pathways. The Pt Ni alloy preferentially promotes the dehydroxylation of C -OH and forms a key C =C intermediate due to the oxygen affinity of Ni sites, and ultimately, enhance the production of valuable propenyl products via the synergistic effect of Pt and Ni. This work provides a strategy for maximizing Pt utilization and producing unsaturated chemicals from biomass under hydrogen-free conditions, advancing sustainable biorefining.

Introduction
Given with the ongoning concerns about global warming due to the overuse of fossil energy, more and more attention is being paid to the green, sustainable and renewable biomass resources. Lignocellulosic biomass containting cellulose (30~50%), hemicellulose (20~35%), and lignin (15~35%), represents the most abundant renewable carbon resource on the earth.1-5 Efficient and rational fractionation, depolymerization, and upgrading of lignocellulose for the production of high-value chemicals, fuels, and functional materials are of great importance.6-9 Traditionally, fractionation of lignocellulose is a carbohydrate-oriented process, aiming to valorize (hemi)cellulose, whereas the valorization of lignin is ignored. While lignin, as one of the three major components of lignocellulose and the largest source of aromatics in nature (mainly composed of monolignols (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol10), its valorization with (hemi)cellulose is paramount for the full utilization of lignocellulose.

The lignin-first biorefining has emerged as a pivotal strategy. This overarching strategy facilitates the active stabilization of reactive lignin intermediates during biomass fractionation to prevent condensation reactions. Broadly, the lignin-first encompasses various stabilization methods including reductive catalytic fractionation (RCF),11,12 oxidative catalytic fractionation (OCF),13,14 protection-group approaches.15 Among these methods, RCF is widely used in biomass fractionation due to the high lignin phenolic monomers yield and good (hemi)cellulose retention.16,17 The product selectivity of RCF is highly tunable based on the catalytic system,18-24 typically favoring propyl-, propanol-, or propenyl-substituted phenols. Of these, the propenyl variants have garnered significant attention due to their unique double bonds, which enable functionalization into various high-value chemicals.25 Samec et al. achieved 49 wt% yield of 2,6-dimethoxy-4-(prop-1-enyl)-phenol from birch wood using ethanol/water as solvent over 5 wt%Pd/C catalyst.26 In addition, a choline chloride/ethylene glycol-based system was developed for hydrogen transfer reduction catalytic fractionation, yielding 36.2% propenyl phenols with 87.6% selectivity, but resulting in partial degradation of carbohydrates.27 Besides, catalysts with low loading and high dispersion also demonstrate outstanding performance in RCF. Song and coworkers reported the atomic dispersed ruthenium catalyst (Ru/ZnO/C) and palladium catalyst (Pd/ZnO/C) for acquiring propenyl-catechol (selectivity of 77%) and propenyl-phenols (selectivity of 62%), respectively.28,29 Despite of the progress mentioned above, these processes were carried out at high temperature in organic solvent and consumed exogenous hydrogen (H or alcohols).

Recently, our group developed an innovative self-hydrogen supplied catalytic fractionation (SCF) strategy by using structural hydrogens in hemicellulose as a hydrogen source. Over 2%Pt/NiAl O catalyst, the fractionation and depolymerization of lignocellulose to phenolic monomers (yield of 46.6 wt%) was realized in water with cellulose intact.30,31 Despite obvious advantages, such as operating without exogenous hydrogen, using only water as a solvent, and mild reaction conditions (140 °C), the SCF process still produces products with saturated side chains and requires a high Pt loading (2 wt%). Given that propenyl- or propanol-terminated products are more valuable, we questioned whether unsaturated side groups could be retained through the design of single-atom or single-atom alloy (SAA) catalysts. It well studied that SAA catalysts can offer unique active sites with high activity and improved sintering resistance,32-34 so a tailored SAA catalyst for SCF process to enable unsaturated product formation under mild conditions was designed.

Herein, we reported a single-atom alloy (SAA) catalyst comprised of isolated Pt atoms stabilized by Ni atoms on NiAl O spinel support for SCF of birch wood. The Pt Ni SAA catalyst was prepared by co-impregnation followed by H reduction, achieved nearly theoretical maximum yield of phenolic monomers (50.9 wt%), corresponding to turnover numbers (TON) of 270.5 mol mol-1 . Surprisingly, propenyl side-chained syringol (selectivity 51.8%) co-existed with the propyl side-chained syringol at 140 °C for 12 h, the former has more functionality and can be used for bioactive molecules, pharmaceuticals, and materials synthesis.35 The SCF over Pt Ni SAA catalyst resulted in a cellulose-rich pulp with high glucan retention (90%) due to the mild reaction conditions. This catalyst also demonstrated good stability in recyclability experiments. Model compound catalytic experiments and density functional theory (DFT) calculations reveal that three pathways including C -OH deH, C -OH deOH-I, and C -OH deOH-II co-existed in lignin depolymerization. Pt Ni SAA catalyst could reduce the energy barrier for dehydroxylation, rendering it to outperform pure Pt catalyst in C -OH deOH. Moreover, C -OH deOH is superior to the C -OH deH pathway, thereby promoting the selectivity towards useful propenyl-ended products. During the preparation of this manuscript, we noted that Liao’s work36 reported a 79.6% selectivity for alkene-functionalized side-chain products, which was achieved under N at 235 C with methanol serving as solvent and hydrogen source via a different reaction pathway. Both studies efficiently produced unsaturated side-chain products, offering new strategies for the valorization of lignocellulose. The following work would be concentrated on designing more efficient catalyst to achieve high yield of alkene-functionalized side-chain products via SCF.

Results
Characterizations of the structure of the single-atom alloyed Pt Ni catalyst

0.2Pt5Ni/NiAl O and 2Pt5Ni/NiAl O catalysts were prepared by co-impregnation followed by H reduction. As comparison, 2Pt/NiAl O and 5Ni/NiAl O catalysts were also prepared with the same method. The former (monometallic Pt loaded on spinel) was selected as a high-performance benchmark based on our previous work,30 and the latter (monometallic Ni loaded on spinel) was chosen to determine the intrinsic baseline activity of pure Ni catalyst. The phase structures of these catalysts were characterized by X-ray diffraction (Figure S1) and only the diffraction peaks (2θ = 19.1°, 31.4°, 37.0°, 45.0°, 59.7°, and 65.5°) corresponding to the (111), (220), (311), (400), (511), and (440) planes of NiAl O (PDF #10-0339) were observed. No diffraction peaks of Pt and Ni were detected in all of the catalysts, probably due to the low loading or small particle size. N sorption showed that the surface area changed less, all in the range of 61.6~93.0 m2·g-1. ICP-AES measurement gave the element composition of Pt (0.095, 0.97, and 1.7 wt%) and Ni (4.3, 4.7 wt%) in all samples and CO pulse chemisorption clearly revealed the good dispersion of Pt (58.5~95.5%). The physicochemical parameters of various samples were summarized in Table S1. The H -TPR profiles of various catalysts were shown in Figure S2. It shows that monometallic 5Ni/NiAl O catalyst has a single reduction peak at 416 °C, attributed to the reduction of NiO species.37 For 2Pt/NiAl O catalyst, the reduction peak at 193 °C can be assigned to the reduction of PtO and the peak at 401 °C is related to the co-reduction of PtO and NiO species in NiAl O .38 For PtNi/NiAl O catalysts with two different Pt loadings, the reduction peaks of NiO species moved to lower temperature because the doping of Pt makes it easier for NiO to be reduced. In addition, the reduction peaks of PtO were not detected for the 0.2Pt5Ni catalyst. This is likely due to the low loading of Pt (0.2 wt%), which falls below the detection limit of the TCD. Specifically, the H -TPR profiles reveal that Pt facilitates NiO reduction via hydrogen spillover, where dissociated hydrogen on Pt reduces adjacent NiO species. This confirms the structural intimacy of Pt and Ni, supporting potential alloy formation and enhancing overall catalyst reducibility.

To identify the position of Pt in 0.2Pt5Ni/NiAl O catalyst, aberration-correction high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) and energy-dispersive spectroscopy (EDS) elemental mapping were conducted. The EDS elemental mappings illustrate a high dispersion and close contact of Pt on Ni nanoparticles (Figure 1a, b). HAADF-STEM image displays the lattice spacing of ~0.181 nm in 0.2Pt5Ni catalyst (Figure 1c). This lattice spacing corresponds to that of Ni (200) plane, but larger than that of pure Ni (200) plane of 0.176 nm, indicating the occurrence of lattice expansion. Enlarged STEM image shows that number of atom-sized bright spots (highlighted by the red cycles) distributed on the surface of Ni NPs and these atoms are attributed to individual Pt atoms, Meanwhile, no Pt clusters or nanoparticles were observed (Figure 1d), which demonstrated the formation of Pt Ni single-atom alloy. In comparison, the HAADF-STEM image of 2Pt5Ni/NiAl O catalyst was also presented (Figure S3) and both Pt single atoms and Pt clusters are detected on Ni NPs, indicating an aggregation of Pt atoms due to the increase in loading. Meanwhile, the 2 wt% Pt/NiAl O was synthesized as a control sample, in which Pt clusters were uniformly distributed on the surface of NiAl O support with an average particle size of 1.4 nm (Figure S4).

To further explore the structural details of 0.2Pt5Ni catalyst, in situ DRIFTS spectra of CO adsorption was recorded. As comparison, 2Pt/NiAl O , 5Ni/NiAl O and 2Pt5Ni/NiAl O catalysts were also investigated (Figure 2a). CO adsorption on 2Pt/NiAl O catalyst generates dominant peak at 2066 cm-1, which is assigned to the linearly absorbed CO at Pt clusters.38,39 For monometallic 5Ni/NiAl O catalysts, almost no CO adsorption is detected. In the case of 0.2Pt5Ni catalyst, the observed peak at 2034 cm-1 is assigned to linearly absorbed CO at Pt single atom, the red shift of the linearly absorbed CO peak indicates an enhanced metallicity of Pt species in PtNi catalyst. This would come from the electron transfer from Ni to Pt, which increases the d-π electron feedback from Pt to CO 2π* antibonding orbital.40,41 Interestingly, a broad peak at 1835 cm-1 is also observed, which is assigned to bridged-bonded CO at contiguous Ni atoms. For 2Pt5Ni catalyst, in addition to the main peak at 2066 cm-1, an obvious shoulder centered at ~2034 cm-1 is observed, indicating that the catalyst contains a mixture of Pt single atom and Pt cluster, consistent with the STEM observations.

To further study the electronic and geometric structure of PtNi catalysts, the Pt L -edge X-ray absorption near-edge (XANES) spectra were conducted with Pt foil and PtO as reference. As shown in Figure 2b, the 0.2Pt5Ni and 2Pt5Ni catalysts have lower intensity of “white line” than that of 2Pt/NiAl O catalyst, indicating a more reduced state of Pt species of PtNi catalyst, in accordance with CO-DRIFTS results. The Fourier transforms of the extended X-ray absorption fine spectrum (EXAFS) in the R space was performed to investigate the detailed coordination structure of Pt (Figure 2c). The 0.2Pt5Ni catalyst exhibits a unique peak located at ~2.1 Å, which can be assigned to the Pt-Ni coordination.41,42 The EXAFS data-fitting results (Table S2 and Figure S5) show that the coordination number (CN) of Pt-Ni path is 6.6, while Pt-Pt coordination is absent in 0.2Pt5Ni catalyst. This verifies that predominant Pt atoms are atomically dispersed and surrounded by Ni to form single-atom alloy. For 2Pt5Ni and 2Pt/NiAl O catalysts, the Pt-Pt coordination number are 2.5 and 3.4, respectively, indicating the Pt atoms aggregate gradually upon increasing Pt loading. The k2-weighted wavelet transforms (WT) for the Pt L -edge XAFS signals were further employed to study coordination environment of Pt species. As shown in Figure 2d~f, the WT contour plots of 0.2Pt5Ni catalyst display only one Pt-Ni coordination intensity maximum at 2.1 Å, 7.2 Å-1, without other scattering paths compared with the WT plots of Pt foil and PtO , demonstrating the formation of SAA. In brief, the results of AC-HAADF-STEM, CO-DRIFTS as well as XAFS experiments confirm the successful preparation of Pt Ni SAA catalyst.

Performance of PtNi/NiAl O in the self-hydrogen supplied catalytic fractionation (SCF)

Birch wood, a typical hardwood substrate containing 43.6 wt% of cellulose, 18.4 wt% of hemicellulose and 20.1 wt% of lignin, was initially used for SCF. The reaction was performed at 140 C and 1 atm N in water with the substrate to catalyst ratio at 0.5/0.2. Firstly, we evaluated the performance of pristine NiAl O support and found that almost no product was detected, indicating that the pure carrier is basically inactive (Table 1). The 5Ni/NiAl O catalyst exhibited no measurable activity, which is consistent with the finding that its surface is primarily composed of NiO species, confirming the inactivity of NiO in this reaction. In contrast to monometallic catalysts, all PtNi catalysts showed significantly enhanced catalytic activity (Table 1). As shown in Table 1, 0.2Pt5Ni catalyst achieved the yield of phenolic monomers (38.6 wt%) with 51.8% selectivity to 4-propenyl guaiacol/syringol ( Pe-G/S) at 140 °C for 12 h (Figure S6), corresponding to TON as 216.6 mol mol-1 (The TON was calculated based on the total number of moles of Pt in 0.2Pt5Ni catalyst). When the reaction time was prolonged to 24 h at 140 °C, the phenolic monomers yield further increased to 50.9 wt%, approaching the theoretical maximum yield of phenolic monomers,14 with the TON reached 270.5 mol mol-1 , larger than most results of reported catalytic systems (Figure S7), indicating the superiority of single atom alloyed Pt Ni catalyst. In contrast, the 0.2Pt/NiAl O catalyst (Table 1, entry 7) was inert, indicating that Pt-Ni interactions are crucial for the SCF process (Tables S3 and S4). This conclusion was further supported by a control experiment using a mechanical mixture of the monometallic catalysts (Table 1, entry 8). Collectively, the characterization and control experiments indicated that the high activity of the Pt Ni catalyst originates from the single-atom alloy structure, which effectively stabilizes metallic Ni species. Interestingly, among all monomers obtained over Pt Ni catalyst, 4-propenyl syringol (Pe-S, 19.9 wt%) was always co-existed with 4-propyl syringol (Pr-S, 23.5 wt%) as major products, with 4-propenyl guaiacol (Pe-G, 0.9 wt%), 4-propyl guaiacol (Pr-G, 3.2 wt%) and 4-ethyl syringol (Er-S, 3.4 wt%) as the minors (corresponding birch lignin monomers concentrations are shown in (Table S5). The structural features and molecular weight distribution of the obtained lignin oil were characterized by 2D HSQC NMR and GPC, respectively (Figures S8 and S9). In addition, the glucan in the pulp showed the good retention (90 %) based on the 43.6 wt% of cellulose in the birch and the pulp also showed the 70 % of the crystallinity index (CrI) (Figure S10). 2Pt and 2Pt5Ni catalyst also obtained good phenolic monomers yield of 36.2 wt% and 48.5 wt%, respectively. But almost no 4-propenyl guaiacol(syringol) were detected in the obtained products, and the TONs were calculated as 34.9 and 30.5 mol mol-1 , respectively, which were far lower than that from 0.2Pt5Ni catalyst, the latter indeed realize the maximization of Pt utilization efficiency.

Furthermore, we conducted SCF of birch wood over different PtNi catalysts based on same Pt usage (by adjusting catalyst dosage, Table S6), and found only 2.2 wt% phenolic monomers yield were achieved over 2Pt5Ni catalyst, but 0.2Pt5Ni catalyst still remain high performance (38.6 wt% phenolic monomers yield), further confirming the high efficiency of 0.2Pt5Ni alloy catalyst.

To better elucidate the unique role of the NiAl O support in the reaction, we synthesized a 0.2Pt5Ni/Al O catalyst using Al O as the support for comparison. The results showed that NiAl O support exhibited superior performance in the selective depolymerization of lignin. This is driven by abundant oxygen vacancies, promoted hydrogen spillover, and robust metal-support interactions (Tables S7 and S8, Figure S11-S13). Notably, the superior hydrothermal stability of the NiAl O support relative to Al O support plays an indispensable role in this process. In addition, various oxygen affinity transition metals (e.g., Nb, Mo, W) were also screened for SCF of birch wood (Table S9). Results indicate that the 0.2Pt5W and 0.2Pt5Mo catalysts yield only 14.7% and 8.9% phenolic monomers, respectively. Notably, no phenolic monomers were detected on 0.2Pt5Nb catalyst. The reason for the above results may be that Nb, Mo, and W are difficult to reduce and cannot form alloy structures with metal Pt, further demonstrating the crucial role of Ni in self-hydrogen supplied fractionation.

Above results showed that over 0.2Pt5Ni catalyst, 4-propenyl syringol (Pe-S) and 4-propyl syringol (Pr-S) are all co-existed as main products, which was different from traditional RCF and SCF strategy over 2%Pt/NiAl O catalyst. In the latter processes, 4-propenyl syringol was less detected although it was more useful due to the existence of the active C=C bond in C tail chain. In order to investigate the reaction pathway, the reaction time profile of SCF was conducted and the results were presented in Figure 3. It is shown that in the first 16 h, the yields of Pr-S and Pe-S both increased with the prolonging of time, indicating probably two reaction pathway co-existed, different from that over 2%Pt/NiAl O as we reported before (Figure S14).30 With the further extension of the reaction time to 24 h, the yield of Pe-S decreased and the yield of Pr-S further increased. A plausible explanation is that as the reaction proceeds, most of the lignin is removed (delignification up to 94%) and no new monomers are produced, hence the Pe-S is gradually hydrogenated to Pr-S in the presence of hydrogen. The hydrogen source origins from hemicellulose as we described recently.41

Furthermore, the reusability of the catalyst was investigated. The total phenolic monomers yield of 30.5 wt% was achieved with 47.5% selectivity of 4-propenyl guaiacol/syringol (Pe-G/S) at 140 °C for 12 h in the third run (Table S10), a slight decrease yield compared to the first run, indicating the catalyst has good stability in SCF process. The above results imply 0.2Pt5Ni catalyst has excellent activity for SCF of birch wood, and can be further extended to other biomass (pine wood and wheat straw), the total phenolic monomers yield of 28.5 wt% with 26% selectivity of 4-propenyl guaiacol/syringol (Pe-G/S) and 10.8 wt% with 40.7% selectivity of 4-propenyl guaiacol/syringol (Pe-G/S) for pine wood and wheat straw, respectively (Table S11). In addition, the obtained monophenols (lignin oil) could further convert to 4-alkylphenols by demethoxylation in the absence of exogenous hydrogen over RuNi/NiAl O catalyst.43 Under optimized conditions, 20.5 wt % yield of 4-alkylphenols could be acquired from birch lignin oil (Figure S15).

Intrinsic catalytic activity and reaction pathways

To further investigate the possible reaction pathways in the SCF process, typical β-O-4 lignin model compound, syringyl-glycerol-β-guaiacyl ether (SG, 1a), was used to evaluated the reaction pathways over 0.2Pt5Ni/NiAl O and 2Pt/NiAl O catalysts. To be consistent with the reaction conditions of birch wood, we then added xylan as hydrogen source in the transformation of lignin model compounds. As shown in Table 2, when 1a was used as the reaction substrate over the 0.2Pt5Ni catalyst for 1 hour, the dimeric intermediates, 1-(4-hydroxy-3,5-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)-1-propanone (2a), 4-(3-hydroxy-2-(2-methoxyphenoxy)propyl)-2,6-dimethoxyphenol (3a), and 4-(3-Hydroxy-2-(2-methoxy-phenoxy)-propenyl)-2,6-dimethoxy-phenol (4a) were detected with yields of 2 wt%, 0.8 wt%, and 11 wt%, respectively, indicating there are three potential reaction pathways and the main reaction pathway is the dehydroxylation of C -OH and formation of C =C double bond. Importantly, the unsaturated C=C products, derived from the cleavage of 4a, sinapyl alcohol (6a) and its dehydroxylation product Pe-S (11a), were detected with yields of 6.1 wt% and 19.5 wt%, respectively. This is consistent with the results of the SCF reaction of birch wood, which primarily yields 4-propenyl products. Additionally, the phenolic monomers with side-chain saturated and oxygen-containing (7a, 8a, 9a, 10a, and 12a) were also detected with yields of 8 wt%, 0.4 wt%, 1.2 wt%, 0.7 wt%, and 1.3 wt%, respectively. With the reaction time extended to 4 hours, the unsaturated C=C products gradually decreased due to hydrogenation and lead to over-reaction, with the yields of 8a and 10a increased. In addition, we conducted control experiments on model compound SG using different types of hydrogen sources, such as xylan, isopropanol, and H . The consistent reaction pathway was observed, demonstrating that the formation of propenyl-substituted phenolic products is primarily determined by intrinsic catalytic properties rather than hydrogen sources (Table S12).

In contrast, no dimer intermediate 4a and the corresponding monomers 6a and 11a were produced over the 2Pt catalyst within 1 hour of reaction, while a large amount of monomer 7a (17.3%) was produced, indicating a distinct reaction pathway compared to the 0.2Pt5Ni catalyst. The same pattern was also observed in experiments using hydroxyphenyl-glycerol-β-guaiacyl ether (HG) and guaiacyl-glycerol-β-guaiacyl ether (GG) as the substrate (Tables S13 and S14), suggesting that three reaction pathways co-exist over the 0.2Pt5Ni catalyst, while the 2Pt catalyst has a relatively single reaction pathway (C -OH dehydrogenation) and is unable to produce the unsaturated dimer intermediates 4a, 4b, and 4c. This might be due to the stronger hydrogen generation and hydrogen migration capabilities of the 2Pt catalyst compared to the 0.2Pt5Ni catalyst (Figure S16).

DFT calculations

To gain a deeper understanding of the reaction mechanism and pathways, the DFT calculations were performed to investigate the mechanism of SG hydrogenation on the exposed (100) facet of Pt and Pt Ni. Figure S17 shows the SG could preferentially chemisorb on the Pt Ni(100) and Pt(100) surfaces via the flat-lying configuration, at which both benzene rings in SG were bonded to the surfaces. The higher binding strength between benzene rings and Pt(100) indicates the stronger carbophilicity of platinum. The conversion of chemisorbed SG can potentially start with the dehydrogenation of (C )OH, the hydrogenolysis of C -OH, the ether C-O bond cleavage, or the dehydrogenation of C -H. Among these steps, the lowest free energy barrier step is the hydrogenolysis of C -OH over Pt Ni(100), while it is the dehydrogenation of (C )OH over Pt(100) (Figure S18). This indicates that the hydrogenolysis of C -OH would preferentially occur over Pt Ni(100), while the dehydrogenation of (C )OH would be more favorable over Pt(100). This could be attributed to the higher oxophilicity of nickel compared to platinum.44 The Crystal Orbital Hamilton Populations (COHP) analysis (Figure S19) confirms that the strong adsorption of oxygen atoms by Ni sites on the PtNi (100) surface weakens the C -OH bond compared to Pt(100), resulting in a 0.04 Å elongation and thereby facilitating C -OH bond cleavage. It indicates that Pt Ni (100) might be more favorable to produce the intermediates with unsaturated C=C bond, such as 4a, while Pt (100) is more favorable to produce the intermediates with carbonyl group, such as 2a, at the initial stage of the SG hydrogenation. This aligns with the observed differences in product distribution (Table 2) and the reaction pathways proposed based on experimental observations.

To distinguish how these initial steps govern subsequent reactivity, we conducted a further mechanistic study of the reaction pathways, commencing with the hydrogenolysis of C -OH and the dehydrogenation of C -OH over the two catalysts. The corresponding pathways were labelled as C -OH deOH towards the production of 8a, 6a, and 11a, and C -OH deH towards the production of 7a, respectively. In the C -OH deH pathway (Figure S20), each elementary step exhibits a higher energy barrier on Pt Ni (100) than Pt (100). Specifically, the water-mediated dehydrogenation of the C -H for the production of 2a encounters a free energy barrier of 1.11 eV (TS2-1) on Pt Ni(100), while the subsequent ether C-O bond cleavage requires an even higher barrier of 1.51 eV (TS3-1). In contrast, the higher carbophilicity of Pt sites lead to the reduced energy barriers for the dehydrogenation of C -H (TS2-1’, 0.77 eV) and the ether C-O bond breaking (TS3-1’, 0.80 eV) over Pt(100). These results imply higher selectivity towards 7a over Pt(100) compared to Pt Ni(100), which aligns well with the product distribution listed in Table 2. The C -OH deOH branches into two pathways after the dehydroxylation: (I) hydrogenation of C -H to produce 3a first and (II) dehydrogenation of C -H to produce 4a first. In the C -OH deOH-I pathway (Figure S21), the least stable TS was found in the hydrogenolysis of C -OH (TS1-2’) on Pt(100), which is less stable than the rate-determining TS of the ester C-O bond breaking (TS3-2, 0.73 eV) on Pt Ni(100). This would limit the 8a production on Pt(100). Figure S22 shows that the energy barriers of most steps on Pt Ni(100) are lower than those on Pt(100) in the C -OH deOH-II pathway. The TS of the hydrogenolysis of C -OH on Pt(100) (TS1-2’) is still the most unstable TS, which is less stable than the rate-determining TS for the ester C-O bond breaking on Pt Ni(100) (TS3-3). Moreover, the barrier of the ester C-O bond breaking on Pt Ni(100) (G = 1.43 eV) is lower that on Pt(100) (G = 1.46 eV). These suggest that Pt Ni(100) would surpass Pt(100) in the C -OH deOH-I and C -OH deOH-II pathways, while Pt(100) would be superior to Pt Ni(100) in the C -OH deH pathway. Hence, the oxophilicity of nickel could facilitate the production of propenyl side-chained compounds on Pt Ni(100).

In Figure 4, we energetically compared the three pathways to understand the selectivity of SG hydrogenation on the Pt Ni(100) surface. Notably, the rate-determining step in each pathway is the ether C-O bond cleavage. It needs to overcome the free energy barrier of 1.51 eV in C -OH deH pathway, corresponding to the step from IM2-1 to IM3-1. Similarly, the ether C-O bond breaking step of IM2-2 to IM3-2is the most difficult step in the C -OH deOH-I pathway, requiring an energy input of 1.48 eV. In the C -OH deOH-II pathway, the ether C-O bond breaking of IM2-3 to IM3-3 requires the lowest energy input of 1.43 eV among the three pathways. The lower energy barrier of the rate-determining C-O ether bond break indicates that the C -OH deOH-II would be superior to the C -OH deOH-I and C -OH deH pathways, indicating a higher yield of 6a and 11a for SG hydrogenation. It is also in good agreement with our experimental observation. Noteworthily, the 6a produced in C -OH deOH-II pathway at the metal Pt Ni site is an enol ether intermediate, which is a crucial intermediate in lignin depolymerization. The C -O bond of this intermediate could be further cleaved to produce propenyl-ended products.

Based on the results of model compound catalytic experiments and DFT calculations, we provide a possible mechanism of lignin depolymerization in this work (Figure 5). Three pathways (C -OH deH, C -OH deOH-I, and C -OH deOH-II) co-existed in lignin depolymerization. (i) C -OH deH: dehydrogenation of the C -OH in β-O-4 units to ketone intermediates. Then, the C -O bond of the intermediate will cleave to monomers, followed by hydrogenolysis and hydrogenation process to obtain 4-propyl-syringol (guaiacol). (ii) C -OH deOH-I: hydrogenolysis of C -OH in β-O-4 units, then cleavage of C -O bond, further to obtain 4-propyl-syringol (guaiacol). (iii) C -OH deOH-II: hydrogenolysis of C -OH and dehydrogenation of C -H to enol ether intermediate, then cleavage of C -O bond, further to obtain 4- propenyl-syringol (guaiacol). In addition, acid formed in biomass depolymerization may also be involved in the reaction, which also promotes the C -OH deOH-II pathway and therefore produces more propenyl-ended products.

Methods
Catalyst preparation

NiAl O spinel was synthesized by a coprecipitation method at a Ni/Al mole ratio of 1:2.38, 45 A typical procedure is as follows: 40 mmol nickel nitrate hexahydrate and 80 mmol aluminum nitrate nonahydrate were dissolved in 200 ml of ultrapure water under vigorous stirring at room temperature. Aqueous ammonia (25-28 wt%) was added dropwise to the above solution until the pH reached 8 to obtained precipitates. Then, the formed precipitates were filtered and washed with deionized water until the pH of the filtrate reached 7. Finally, the filter cake was dried at 100 C for 24 h, followed by calcination at 800 C for 8 h under Ar atmosphere. xPtyNi/NiAl O (where x, y represents the loading weight of Pt and Ni, respectively) was prepared by the co-incipient wetness impregnation method with appropriate amounts of aqueous solution of chloroplatinic acid (H PtCl ) and NiCl ·6H O, respectively. The obtained samples were dried at 60 C for 12 h and then reduced in a 10% H /Ar flow at 400 C for 3 h. To make the Tables/Figures look more concise, we use xPtyNi represent xPtyNi/NiAl O catalysts.

Catalyst characterizations

The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2550VB/PC diffractometer by using Cu Kα radiation (λ=0.15406 nm). Nitrogen adsorption/desorption isotherms of the catalysts were measured on a Micromeritics ASAP 2020M sorption analyser at 77K. The Brunauer-Emmett-Teller (BET) method was used to calculate the surface area. Aberration-corrected-HAADF-STEM images and EDS mappings were collected at 300 kV on a ThermoFisher Scientific Spectra 300 scanning transmission electron microscope equipped with a Super-X EDS detector system. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to determine the Pt/Ni loading content by using an Agilent 725ES.

H temperature-programmed reduction (H -TPR) experiments were conducted on VDSorb-91i automatic chemisorption system. Typically, 0.1 g of catalyst was injected into the quartz sample tube. Before each run, the catalyst sample was in situ pretreated with Ar at 150 C for 1 h and cooled down to ambient temperature. Then, the samples were heated to 700 oC with a 10 °C min−1 ramp in 10% H /Ar flow.

CO pluse chemisorption tests were operated on VDSorb-91i automatic chemisorption system. Typically, 0.1 g of catalyst was injected into the quartz sample tube. Before each run, the catalyst sample was in situ reduced in flowing 10% H /Ar at 400 C for 1 h and cooled down to ambient temperature with He. The CO pluse chemisorption results were calculated on the basis of the peak areas of the CO pulse injection (220 μL per pulse, 50% CO/He), which were monitored by a TCD detector (45 °C, 80 mA).

In situ diffuse reflectance infrared Fourier transform (DRIFTS) of CO adsorption was collected with a Nicolet iS50 FTIR spectrometer equipped with an MCT/A detector. Firstly, putting around 20 mg of catalyst into the chamber, the catalyst was in situ reduced in flowing 10% H /Ar at 400 C for 1 h and decreased to 30 C, followed by collecting the background spectrum. 1% CO/Ar was then introduced into the chamber for 30 min. Next, the chamber was purged with Ar for another 30 min and the adsorption spectra of CO was recorded. All spectra were collected after subtracting the background with a resolution of 4 cm-1 and an accumulation of 32 scans.

XAS measurements were conducted at the Canadian Light Source for the Pt L -edge on the Hard X-ray Micro-Analysis Beamline (HXMA). beamline. Pt foil was used for energy calibration and both fluorescence and transmission yields were recorded. fluorescence spectra were reported for samples with low Pt loading. Data analysis was performed using a standardized IFEFFIT package (including Athena and Artemis software).46

Catalytic reactions

In a typical SCF reaction, 0.5 g birch wood, 0.2 g xPtyNi/NiAl O catalyst and 10 mL H O were put into the 50 mL stainless-steel autoclave reactor. The reactor was sealed and purged with N at room temperature, then heated to different temperatures and kept for a certain time at a magnetic stirring speed of 700 rpm. At the end of the reaction, it was cooled down to room temperature in ice water. The soluble fraction and residue were subsequently separated by centrifugation.

Lignin-derived Products Analyses

After SCF reaction, the liquid phase was extracted by ethyl acetate and analyzed by GC-MS (Agilent 7890A) and quantitatively analyzed by GC (Agilent 7890B) with a flame ionization detector, both quipped with HP-5 capillary columns. Tridecane was used as an internal standard for the quantification of the liquid products. Nitrogen was employed as a carrier gas, and the following operating conditions were adopted: injection temperature 250 °C, column temperature program: 50 °C (hold time 3 min), 10 °C min-1 to 250 °C (hold time 5 min), detection temperature of 280 °C. Response factors for the different products were determined by calibration with commercial standards or through calculations based on the effective carbon number (ECN) theory.47 All experiments were performed at least three times. The phenolic monomers yields were calculated based on the Klason lignin weight, calculating formula as shown below:

Mass (total monomers)
Phenolic monomers (wt%)= ×100% (1)
Mass (Klason lignin)

Mole (total monomers)
Mole yields of monomers (%)= ×100% (2)
Mole (all monomers in lignin)

The mass of lignin in biomass was measured by Klason method and the mole of monomers in lignin was measured by NBO method.15

Computational details

All the calculations were performed in the framework of periodic density functional theory (DFT) using the Perdew-Burke-Ernzerhof (PBE) functional48 implemented in Vienna ab initio simulation package (VASP 6.2.0).49-51 The electron-ion interactions were modelled utilizing the projector augmented wave (PAW) pseudopotential and the plane wave basis-set was expanded to a converged cut-off energy of 450 eV. The energy calculation was performed using Methfessel-Paxton smearing method with the width of 0.05 eV. The van der Waals interaction was considered by the empirical DFT-D3(BJ) method.52,53 The adsorption energy (E ) was calculated by the following equation:

E = E - E - E (3)
ads X* surf X

where E , E , and E are the total energies of the adsorbate on the surface, bare surface, and gaseous adsorbate molecule. The asterisk represents the adsorbate on the surface.

The Gibbs free energy of adsorption (G ) was calculated by the E with the thermodynamic correction under the reaction condition of 413.15 K as follows:

 p 
G =E +ZPE+(U)−TS +RT +RTln   +RTln(x ) (4)
ads ads  p  i

where ∆ZPE, Δ(ΔU), and ∆S⊖ respectively represent the variation of zero-point energy, internal energy variation, and entropy of the surface adsorbate for the adsorption process under the reaction condition. The thermodynamic corrections were calculated based on the partition functions from Boltzmann distribution. The detailed formulae could be found in our previous work.54-57 P and p are the saturated vapor pressure of adsorbate at 413.15 K and the standard vapor pressure, respectively. x is the mole fraction of dissolved adsorbate in the liquid water. The negative G denotes that the adsorption process is thermodynamically favorable. The more negative G , the stronger the adsorption.

The rectangle p(6×6) supercells of the three-layer exposed Pt(100) and Pt Ni(100) surfaces were 1 used to model the exposed facets of 0.2Pt/NiAl O and 0.2Pt5Ni/NiAl O , respectively. For the γ-Al O and NiAl O supports, 1 × 1 surface supercells of the (111) facets were employed to represent the corresponding oxide surfaces. In all slab models, a vacuum region of 15 Å was introduced along the surface normal. Monkhorst-Pack k-point meshes of 2 × 2 × 1 were used for the Pt(100) and Pt Ni(100) slabs, while a 4 × 4 × 1 meshes was adopted for both γ-Al O (111) and NiAl O (111). The geometry optimization was performed using the conjugate gradient algorithm until the maximal force of all the relaxed atoms lower than 0.05 eV/Å. The transition state was searched using the constrained optimization scheme.58 and then verified by vibrational analysis, ensuring only one imaginary frequency. During the geometry optimization, the bottom layer metal atoms were fixed, while the upper two-layer surface atoms and adsorbates were allowed to relax.

The LOBSTER code was implemented to perform the Crystal Orbital Hamilton Population (COHP) analysis59,60 which is capable of understanding the chemical bonding of BM molecules chemisorbed on the Pt(100) and Pt Ni(100) surfaces.61 The integration of the contribution of the energy bands of the projected crystal orbital Hamilton population up to the Fermi level (IpCOHP) indicates the bonding strength between the functional groups and the surface. To be analogous to the Crystal Orbital Overlap Population (COOP) analysis, -COHP was plotted instead of COHP. Thus, the more negative the -IpCOHP, the stronger the chemical bonding between the adsorbate and the surface.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work reports that a single-atom alloy Pt1Ni catalyst can directly convert birch sawdust into propenyl side-chained phenols with a selectivity of 50%. The study is significant and has substantial potential impact; however, several aspects require revision prior to publication, particularly the mechanistic investigation.\nMajor comments that need to be addressed include:\n1. From the activity data in Table 1, comparison of entries 3 and 4 suggests that the presence of Ni species has only a minor effect on overall reaction yield. However, when comparing entries 1 and 7, introduction of Ni increases the yield from 0 to 38.6%. These observations indicate that Ni primarily enhances catalytic activity rather than selectively steering formation of propenyl side-chained phenols. The manuscript should present a clear, quantitative statement of this conclusion, and support it with additional experimental evidence.\n2. The current DFT modeling, using pristine Pt(100) to represent the “2Pt” single-atom catalyst and Pt1Ni(100) to represent the Pt1Ni material, is not an appropriate structural model for the catalysts used experimentally. The “2Pt” single-atom catalyst in this work is anchored on an oxide support (Pt–O environments) and therefore cannot be represented accurately by bulk Pt(100). Likewise, Ni in the synthesized catalyst is more likely present as NiO (or oxidized Ni species) rather than metallic Ni layers. Consequently, the present calculations risk drawing misleading mechanistic conclusions.\nOther comments:\n3. Reductive catalytic fractionation (RCF) is not equivalent to the lignin-first biorefining strategy. Please revise the introduction to clearly distinguish these two concepts and provide a more accurate and coherent summary of their relationship.\n4. The second paragraph of the introduction should be more concise and precise. If the key advancement of this work is the high selectivity toward propenyl side-chained phenols, the discussion should focus primarily on this point and avoid unrelated or secondary descriptions.\n5. In the first paragraph of the Results and Discussion section, a more detailed introduction of the catalyst systems is needed. Rather than only listing labels such as “2Pt/NiAl2O4” or “5Ni/NiAl2O4,” please briefly describe their compositions, structural features, and why they were selected for comparison.\n6. The current explanation linking PtOx reduction peaks in the H2-TPR profile to Pt dispersion is not scientifically accurate. The reduction behavior of PtOx does not directly reflect Pt dispersion. Additional clarification and a more appropriate interpretation of the TPR results are required.\n7. The manuscript should explicitly state what conclusions can be drawn from the H2-TPR experiments.", "reviewer_2": "The submitted manuscript reports a single-atom alloy Pt₁Ni catalyst that enables reductive catalytic fractionation (RCF) of birch sawdust, achieving a phenolic monomer yield of 50.9 wt.% with ~50% selectivity toward valuable propenyl-substituted products under relatively mild conditions (140 °C, 1 atm N₂), while largely preserving cellulose. Although the reported monomer yield is somewhat higher than that in several previous RCF studies, the catalyst design and mechanistic insights do not appear to offer sufficient novelty or depth relative to the existing literature on single-atom and bimetallic RCF catalysts. The advances claimed are therefore incremental rather than transformative. Consequently, I recommend rejection of the manuscript in its present form. The following comments are provided to help improve the work for future submission.\n1. Both 0.2Pt5Ni/NiAl₂O₄ and 2Pt5Ni/NiAl₂O₄ exhibit hydrogen-free lignin fractionation at 140 °C, which is significantly lower than the temperatures required for the catalysts shown in Fig. S6, whereas bare NiAl₂O₄ is inactive under identical conditions. This raises an important mechanistic question regarding the specific role of the NiAl₂O₄ support in enabling this unusually low-temperature activity. The authors should more clearly elucidate whether the support contributes through metal–support interactions, hydrogen spillover, oxygen vacancies, or promotion of lignin depolymerization pathways.\n2. The manuscript argues that Ni is not directly the active site, yet its oxophilicity is essential for promoting Cα–OH hydrogenolysis and the formation of propenyl side chains. If oxophilicity is indeed the dominant factor, the authors should rationalize why other more strongly oxophilic transition metals (e.g., V, Nb, Mo, W) were not considered or screened. A comparative discussion or preliminary screening would significantly strengthen the mechanistic argument.\n3. The reported turnover number (TON) of 270.5 for 0.2Pt5Ni/NiAl₂O₄ is only moderate compared to recently reported Ru-based single-atom catalysts for lignin RCF. Moreover, the long reaction time (12 h) required to achieve the high monomer yield suggests a relatively low intrinsic reaction rate. The authors should demonstrate that the improved monomer yield originates from genuinely enhanced site activity rather than simply from extended reaction time. Time-resolved kinetics, initial rate comparisons, or space–time yield (STY) analysis would be necessary to support this claim.\n4. Additional control experiments are required to verify the role of xylan as the hydrogen source. For example, reactions conducted under H₂ or using external hydrogen-donating solvents should be included to determine whether the formation of propenyl-substituted phenolic products is dictated primarily by the hydrogen source rather than by the intrinsic catalytic properties. Furthermore, the selectivity toward propenyl side-chained products should be quantitatively compared with literature values to convincingly demonstrate any advantage of single-atom alloy catalysts over conventional single-atom or nanoparticle catalysts for preserving unsaturated side chains.\n5. To enhance the reliability of the results, more experimental details are needed. For example, monomer yield quantitative method, GC column temperature program, representative GC chromatograms with peak assignments，GPC results for the lignin-oil.", "reviewer_3": "Zhou et al. report the development of a single-atom alloyed Pt₁Ni catalyst for lignocellulose fractionation that enables efficient production of phenolic monomers under hydrogen-free conditions while preserving cellulose integrity. This work addresses a critical challenge in biomass valorization by achieving high monomer yields and enhanced selectivity toward unsaturated side-chain products without the need for external hydrogen or excessive metal loading, and it proposes a mechanistically well-supported catalytic strategy. Overall, the manuscript is strong and appears suitable for publication. However, several concerns should be addressed, as outlined below. Addressing these points will further strengthen the manuscript and ensure that it represents a meaningful contribution to sustainable biomass conversion and catalytic lignin valorization.\n1. The authors attribute the enhanced Cα–OH activation by the oxophilicity of Ni to the higher selectivity towards valuable propenyl products. It was evidenced by the mechanistic studies over Pt1Ni(100) and Pt(100) based on DFT calculations. However, the specific role of the isolated Pt1 site in the Pt1Ni(100) model was not clearly explained. It should be clarified.\n2. Self-hydrogen supplied fractionation rather than exogeneous hydrogen gas was used as the hydrogen resources in this work. Compared with direct use of hydrogen gas, whether would self-hydrogen supplied fractionation affect the reaction mechanism or the resultant selectivity?\n3. Please clarify how the transition states were verified in the DFT calculations.\n4. In the current version, the colors of the corresponding atoms in the figures of reaction mechanisms, such as Figure 4, were not labelled. It is difficult for readers to clearly identify the active sites and visualize the adsorption geometries, especially for the Pt1 in the Pt1Ni(100) surface.\n5. The manuscript contains some typos. For example, “Tranditionally” should be corrected to “Traditionally” in page3, and “efficientcy” should be corrected to “efficiency” in page3 and page12. The authors are encouraged to carefully proofread the text and revise these issues for clarity and consistency."}, "author_response": "Response to referees’ comments and questions\nReviewer #1 (Remarks to the Author):\nThis work reports that a single-atom alloy Pt1Ni catalyst can directly convert birch sawdust into propenyl\nside-chained phenols with a selectivity of 50%. The study is significant and has substantial potential impact;\nhowever, several aspects require revision prior to publication, particularly the mechanistic investigation.\nMajor comments that need to be addressed include:\n1. From the activity data in Table 1, comparison of entries 3 and 4 suggests that the presence of Ni species\nhas only a minor effect on overall reaction yield. However, when comparing entries 1 and 7, introduction of\nNi increases the yield from 0 to 38.6%. These observations indicate that Ni primarily enhances catalytic\nactivity rather than selectively steering formation of propenyl side-chained phenols. The manuscript should\npresent a clear, quantitative statement of this conclusion, and support it with additional experimental\nevidence.\nReply: We thank the reviewer for raising this comment. Metal Ni plays a unique role in the single-atom\nalloy Pt Ni catalyst. The addition of Ni forms a highly active Pt -Ni single-atom alloy structure, which\n1 1\nsuccessfully depolymerizes lignin to yield a 50.9 wt% yield of phenolic monomers with about 50%\nselectivity to propenyl side-chained products. Furthermore, compared to 0.2Pt catalyst, the oxygen affinity\nof Ni sites facilitates the formation of propenyl side-chained phenols on Pt -Ni catalyst. To visualize this\n1\neffect, we compared the depolymerization activity of 0.2Pt5Ni and the 0.2Pt catalysts toward lignin model\ncompound, syringyl-glycerol-β-guaiacyl ether (SG, as shown in Table R1) by using xylan as the hydrogen\nsource. The results indicate that intermediate 4 is the primary product on the 0.2Pt5Ni catalyst, yielding\n28.5% of the olefinic phenolic monomer (6+11). In contrast, on the 0.2Pt catalyst, intermediates 2 and 4\npredominated, with a 13.7% yield of olefinic phenolic monomers originating from 6. This indicates that\n0.2Pt5Ni and 0.2Pt catalysts exhibit distinct reaction pathways, with the former producing more propenyl\nside-chained phenols via intermediate 4. In addition, the 0.2Pt5Ni catalyst demonstrates superior hydrogen\nproduction capacity compared to the 0.2Pt catalyst (Table R2), which could accelerate lignin\ndepolymerization.\nThese results further demonstrate the crucial role of Ni in enhancing catalytic activity and formation of\npropenyl side-chained phenols. The relevant description has been incorporated into the revised manuscript\nand supporting information (Tables S3 and S4).\nTable R1. Reaction pathway of lignin model compound SG over two different catalysts.\nYield (wt%)\nTime Conv.\nEntry Catalysts\n(h) (%)\n2 3 4 5 6 7 8 9 10 11 12\n1 0.2Pt5Ni 2 100 0.4 0.1 7.6 37.1 12.0 4.5 0.1 0.7 1.6 16.5 2.7\n2 0.2Pt 2 98.2 21.3 - 14.8 26.0 13.7 5.6 0.2 0.4 - 0.3 0.9\nReaction Conditions: Xylan (0.05 g), Substrate (0.1 g), catalyst (0.05 g), H O (10 mL), N 1 atm, 140 C.\n2 2\nTable R2. Hydrogen production of xylan over different catalysts.\nCatalysts H Content/μmol·g-1\n2\n0.2Pt5Ni 502\n0.2Pt 261\n5Ni 103\n2Pt 1113\n2Pt5Ni 1232\nReaction Conditions: Xylan (0.2 g), Catalyst (0.1 g), H O (10 mL), N 1 atm,\n2 2\n140 C, 2 h.\n2. The current DFT modeling, using pristine Pt(100) to represent the “2Pt” single-atom catalyst and\nPt1Ni(100) to represent the Pt1Ni material, is not an appropriate structural model for the catalysts used\nexperimentally. The “2Pt” single-atom catalyst in this work is anchored on an oxide support (Pt–O\nenvironments) and therefore cannot be represented accurately by bulk Pt(100). Likewise, Ni in the\nsynthesized catalyst is more likely present as NiO (or oxidized Ni species) rather than metallic Ni layers.\nConsequently, the present calculations risk drawing misleading mechanistic conclusions.\nReply: We appreciate this important comment and agree that the correspondence between computational slab\nmodels and experimentally synthesized supported catalysts should be stated more explicitly to avoid\nconfusion. In the revised manuscript, we have clarified what the DFT models represent. The reviewer’s\nconcern partly arises from an ambiguity in the description of the “2P” catalyst. In our experimental\ncharacterization, 2Pt/NiAl O is not a single-atom Pt catalyst. Instead, it contains Pt clusters (average size\n2 4\n~1.4 nm from HAADF-STEM shows in Figure S4) and shows a dominant linear CO-Pt band at ~2066 cm-1\nin Figure 2a, characteristic of Pt clusters. In addition, Pt-Pt coordination is observed by EXAFS fitting (Pt-\nPt CN reported in Table S2). Therefore, Pt(100) was used as a simplified metallic Pt surface model to\ndescribe the intrinsic reactivity trend of Pt clusters, rather than a Pt-O single-atom site. We agree that Ni\nspecies on oxide supports can be oxidized prior to reduction. In our catalyst preparation, all samples were\nreduced in 10% H /Ar at 400°C for 3 h (see SI). Consistently, H -TPR shows that monometallic 5Ni/NiAl O\n2 2 2 4\nexhibits a reduction peak attributable to NiO species, whereas Pt doping shifts the NiO reduction peaks to\nlower temperature, indicating that Pt promotes NiO reduction and facilitates the formation of metallic Ni\nspecies (Figure S2). More importantly, several in-situ and ex-situ characterizations support that the active\nPt₁Ni catalyst exposes a reduced Pt-Ni alloy surface with contiguous metallic Ni ensembles, rather than an\noxidized NiO surface. In-situ CO-DRIFTS for 0.2Pt5Ni shows a band at 2034 cm-1 (linear CO on isolated\nPt atoms) together with a broad band at 1835 cm-1 (bridged CO on contiguous Ni atoms), which is a\ncharacteristic signature of metallic Ni ensembles; in contrast, 5Ni/NiAl O exhibits almost no CO adsorption\n2 4\n(Figure 2a). Pt L -edge XANES/EXAFS further indicate a more reduced Pt state in PtNi catalysts, and\ncontribution, consistent with a SAA environment rather than Pt clusters or Pt-O anchored single sites (Figure\n2b; Figure 2c; Table S2; Figure S5). Experiments show that 5Ni/NiAl O is inactive and its surface is\n2 4\nprimarily composed of NiO species, whereas Pt Ni displays high activity, supporting that the Pt Ni SAA\n1 1\neffectively stabilizes metallic Ni under reaction-relevant conditions (Table 1). Accordingly, Pt Ni(100) in\nis evidenced experimentally, and is used to compare intrinsic mechanistic trends between a metallic Pt\nsurface and a Pt-Ni SAA surface. We have added explicit statements in the revised manuscript to clearly\ndistinguish these two surface models and to prevent potential misunderstanding.\nOther comments:\n3. Reductive catalytic fractionation (RCF) is not equivalent to the lignin-first biorefining strategy. Please\nrevise the introduction to clearly distinguish these two concepts and provide a more accurate and coherent\nsummary of their relationship.\nReply: We thank the reviewer for this suggestion. We agree that treating RCF as synonymous with the\nbroader lignin-first strategy is inaccurate. We recognize that lignin-first is an overarching strategy defined\nby the active stabilization of lignin during fractionation (including RCF, oxidative catalytic fractionation\n(OCF), and protection-group approaches), whereas RCF is a specific methodology relying on reductive\ncatalysis.\nIn the revised manuscript, we have rewritten the relevant paragraph in the introduction to explicitly clarify\nthis hierarchical relationship. The rewritten content is as follows:\n“The lignin-first biorefining has emerged as a pivotal strategy. This overarching strategy facilitates the\nactive stabilization of reactive lignin intermediates during biomass fractionation to prevent condensation\nreactions. Broadly, the lignin-first encompasses various stabilization methods including reductive catalytic\nfractionation (RCF),1,2 oxidative catalytic fractionation (OCF),3,4 protection-group approaches.5 Among\nthese methods, RCF is widely used in biomass fractionation due to the high lignin phenolic monomers yield\nand good (hemi)cellulose retention.6,7”\n1. Van den Bosch, S., et al. Reductive lignocellulose fractionation into soluble lignin-derived phenolic\nmonomers and dimers and processable carbohydrate pulps. Energy Environ. Sci. 8, 1748-1763\n(2015).\n2. Li, N., et al. Selective lignin arylation for biomass fractionation and benign bisphenols. Nature 630,\n381-386 (2024).\n3. Luo, H., et al. Oxidative Catalytic Fractionation of Lignocellulosic Biomass under Non-alkaline\nConditions. J. Am. Chem. Soc. 143, 15462-15470 (2021).\n4. Zhu, Y., et al. Oxidative Catalytic Fractionation of Lignocellulose to High-Yield Aromatic Aldehyde\nMonomers and Pure Cellulose. ACS Catal. 13, 7929-7941 (2023).\n5. Shuai, L., et al. Formaldehyde stabilization facilitates lignin monomer production during biomass\ndepolymerization. Science 354, 329-333 (2016).\n6. Abu-Omar, M.M., et al. Guidelines for performing lignin-first biorefining. Energy Environ. Sci. 14,\n262-292 (2021).\n7. Renders, T., Van den Bosch, S., Koelewijn, S.-F., Schutyser, W. & Sels, B. Lignin-first biomass\nfractionation: the advent of active stabilisation strategies. Energy Environ. Sci. 10, 1551-1557\n(2017).\n4. The second paragraph of the introduction should be more concise and precise. If the key advancement of\nthis work is the high selectivity toward propenyl side-chained phenols, the discussion should focus primarily\non this point and avoid unrelated or secondary descriptions.\nReply: We thank the reviewer for this advice. We have restructured the language in the second paragraph to\nfocus on the discussion of propenyl side-chained phenolic monomers, streamlining secondary points. The\nrevised paragraph is as follows:\n“The product selectivity of RCF is highly tunable based on the catalytic system,8-14 typically favoring\npropyl-, propanol-, or propenyl-substituted phenols. Of these, the propenyl variants have garnered significant\nattention due to their unique double bonds, which enable functionalization into various high-value\nchemicals.15 Samec et al. achieved 49 wt% yield of 2,6-dimethoxy-4-(prop-1-enyl)-phenol from birch wood\nusing ethanol/water as solvent over 5 wt%Pd/C catalyst.16 In addition, a choline chloride/ethylene glycol-\nbased system was developed for hydrogen transfer reduction catalytic fractionation, yielding 36.2% propenyl\nphenols with 87.6% selectivity, but resulting in partial degradation of carbohydrates.17 Besides, catalysts\nwith low loading and high dispersion also demonstrate outstanding performance in RCF. Song and coworkers\nreported the atomic dispersed ruthenium catalyst (Ru/ZnO/C) and palladium catalyst (Pd/ZnO/C) for\nacquiring propenyl-catechol (selectivity of 77%) and propenyl-phenols (selectivity of 62%), respectively.18,19”\n8. Zhai, Y., et al. Depolymerization of lignin via a non-precious Ni–Fe alloy catalyst supported on\nactivated carbon. Green Chem. 19, 1895-1903 (2017).\n9. Park, J., et al. Highly efficient reductive catalytic fractionation of lignocellulosic biomass over\nextremely low-loaded Pd catalysts. ACS Catal. 10, 12487-12506 (2020).\n10. Liu, Z., et al. Rational highly dispersed ruthenium for reductive catalytic fractionation of\nlignocellulose. Nat. Commun. 13, 4716 (2022).\n11. Ge, J., et al. Highly efficient metal-acid synergetic catalytic fractionation of lignocellulose under\nmild conditions over lignin-coordinated N-anchoring Co single-atom catalyst. Chem. Eng. J. 462,\n142109 (2023).\n12. Brienza, F., et al. Toward a Hydrogen-Free Reductive Catalytic Fractionation of Wheat Straw\nBiomass. ChemSusChem 16, e202300103 (2023).\n13. Chen, J., et al. Efficient fractionation and catalytic valorization of raw biomass in ϵ-caprolactone\nand water. ChemSusChem 16, e202202162 (2023).\n14. Zhang, K., et al. Catalytic Hydrogenolysis of Lignin into Propenyl-monophenol over Ru Single\nAtoms Supported on CeO2 with Rich Oxygen Vacancies. ACS Catal. 14, 16115-16126 (2024).\n15. Trullemans, L., et al. Renewable and safer bisphenol A substitutes enabled by selective zeolite\nalkylation. Nat. Sustain. 6, 1693-1704 (2023).\n16. Galkin, M.V. & Samec, J.S. Selective route to 2-propenyl aryls directly from wood by a tandem\norganosolv and palladium-catalysed transfer hydrogenolysis. ChemSusChem 7, 2154-2158 (2014).\n17. Li, Y., et al. Hydrogen-Transfer Reductive Catalytic Fractionation of Lignocellulose: High\nMonomeric Yield with Switchable Selectivity. Angew. Chem. Int. Ed. 62, e202307116 (2023).\n18. Wang, S., Zhang, K., Li, H., Xiao, L.P. & Song, G. Selective hydrogenolysis of catechyl lignin into\npropenylcatechol over an atomically dispersed ruthenium catalyst. Nat. Commun. 12, 416 (2021).\n19. Wang, S., Li, X., Fu, C., Li, H. & Song, G. Atomically Dispersed Palladium Driving Reductive\nCatalytic Fractionation of Lignocellulose into Alkene-Functionalized Phenols. ACS Catal. 14, 3565-\n3574 (2024).\n5. In the first paragraph of the Results and Discussion section, a more detailed introduction of the catalyst\nsystems is needed. Rather than only listing labels such as “2Pt/NiAl2O4” or “5Ni/NiAl2O4,” please briefly\ndescribe their compositions, structural features, and why they were selected for comparison.\nReply: We thank the reviewer for raising this comment. As comparison, 2Pt/NiAl O and 5Ni/NiAl O\n2 4 2 4\ncatalysts were also prepared with the same method. The former (monometallic Pt loaded on spinel) was\nselected as a high-performance benchmark based on our previous work,20 and the latter (monometallic Ni\nloaded on spinel) was chosen to determine the intrinsic baseline activity of pure Ni catalyst.\nThe above content has been incorporated into the revised manuscript.\n20. Zhou, H., Liu, X., Guo, Y. & Wang, Y. Self-hydrogen supplied catalytic fractionation of raw biomass\ninto lignin-derived phenolic monomers and cellulose-rich pulps. JACS Au 3, 1911-1917 (2023).\n6. The current explanation linking PtOx reduction peaks in the H2-TPR profile to Pt dispersion is not\nscientifically accurate. The reduction behavior of PtOx does not directly reflect Pt dispersion. Additional\nclarification and a more appropriate interpretation of the TPR results are required.\nReply: We thank the reviewer for raising this comment. We have removed the correlation between PtO\nx\nreduction behavior and Pt dispersion. The reduction peaks of PtO were not detected for the 0.2Pt5Ni catalyst.\nx\nThis is likely due to the low loading of Pt (0.2 wt%), which falls below the detection limit of the TCD.\nThe above content has been added into the revised manuscript.\n7. The manuscript should explicitly state what conclusions can be drawn from the H2-TPR experiments.\nReply: We thank the reviewer for raising this comment. We have incorporated the H -TPR conclusions into\n2\nthe revised manuscript as follows:\n“Specifically, the H -TPR profiles reveal that Pt facilitates NiO reduction via hydrogen spillover, where\n2\ndissociated hydrogen on Pt reduces adjacent NiO species. This confirms the structural intimacy of Pt and Ni,\nsupporting potential alloy formation and enhancing overall catalyst reducibility.”\nReviewer #2 (Remarks to the Author):\nThe submitted manuscript reports a single-atom alloy Pt₁Ni catalyst that enables reductive catalytic\nfractionation (RCF) of birch sawdust, achieving a phenolic monomer yield of 50.9 wt.% with ~50%\nselectivity toward valuable propenyl-substituted products under relatively mild conditions (140 °C, 1 atm\nN₂), while largely preserving cellulose. Although the reported monomer yield is somewhat higher than that\nin several previous RCF studies, the catalyst design and mechanistic insights do not appear to offer sufficient\nnovelty or depth relative to the existing literature on single-atom and bimetallic RCF catalysts. The advances\nclaimed are therefore incremental rather than transformative. Consequently, I recommend rejection of the\nmanuscript in its present form. The following comments are provided to help improve the work for future\nsubmission.\n1. Both 0.2Pt5Ni/NiAl₂O₄ and 2Pt5Ni/NiAl₂O₄ exhibit hydrogen-free lignin fractionation at 140 °C, which\nis significantly lower than the temperatures required for the catalysts shown in Fig. S6, whereas bare NiAl₂O₄\nis inactive under identical conditions. This raises an important mechanistic question regarding the specific\nrole of the NiAl₂O₄ support in enabling this unusually low-temperature activity. The authors should more\nclearly elucidate whether the support contributes through metal–support interactions, hydrogen spillover,\noxygen vacancies, or promotion of lignin depolymerization pathways.\nReply: We thank the reviewer for raising this comment. To better elucidate the unique role of the NiAl O\n2 4\nsupport in the reaction, we synthesized a 0.2Pt5Ni/Al O catalyst using Al O as the support for comparison.\n2 3 2 3\nIn the self-hydrogen supplied fractionation of birch wood (Table R3), only 7.3 % yield of phenolic monomers\nwas achieved over 0.2Pt5Ni/Al O catalyst, significantly lower than 38.6 % obtained over 0.2Pt5Ni/NiAl O\n2 3 2 4\ncatalyst. In the reaction of model compound SG (Table R4), the intermediate 4 is the primary product on\nthe 0.2Pt5Ni/NiAl O catalyst, yielding 28.5% of the olefinic phenolic monomer (6+11). In contrast, on the\n2 4\n0.2Pt5Ni/Al O catalyst, intermediates 2 and 4 predominated, with a 18% yield of ketone phenolic monomers\n2 3\n(7+9). This indicates that different reaction pathways are exhibited on NiAl O and Al O supports.\n2 4 2 3\nTable R3. SCF of birch wood over different catalysts.\nPhenolic monomers yield (wt%) Sel. to\nTotal\nCatalysts Pe-G/S\nPe-G Pr-G Er-S Pr-S Pe-S (wt%)\n(%)\n0.2Pt5Ni/NiAl O 0.5 1.2 2.4 15.0 19.5 51.8 38.6\n2 4\n0.2Pt5Ni/Al O - - - 3.4 3.9 53.4 7.3\n2 3\nReaction conditions: birch wood (0.5 g), catalyst (0.2 g), H O (10 mL), N 1 atm, 140 C, 12 h.\n2 2\nTable R4. Reaction pathway of lignin model compound SG over two different catalysts.\nYield (wt%)\nCatalysts Conv. (%)\n2 3 4 5 6 7 8 9 10 11 12\n0.2Pt5Ni/NiAl O 100 0.4 0.1 7.6 37.1 12.0 4.5 0.1 0.7 1.6 16.5 2.7\n2 4\n0.2Pt5Ni/Al O 100 9.2 - 13.1 32.5 5.7 15.2 0.4 2.8 0.3 5.2 3.7\n2 3\nReaction Conditions: Xylan (0.05 g), Substrate (0.1 g), catalyst (0.05 g), H O (10 mL), N 1 atm, 140 C, 2\n2 2\nh.\nFurthermore, we conducted H -TPR, electron paramagnetic resonance (EPR), and hydrogen spillover\n2\nexperiments over 0.2Pt5Ni/NiAl O and 0.2Pt5Ni/Al O (Figures R1-R3) to investigate the influence of the\n2 4 2 3\nsupport on the properties of the catalysts. The 0.2Pt5Ni/NiAl O catalyst exhibits a higher number of oxygen\n2 4\nvacancies (Figure R1) and stronger hydrogen spillover capacity compared to the 0.2Pt5Ni/Al O catalyst\n2 3\n(Figure R2). In addition, in the H -TPR profiles (Figure R3), the co-reduction peak of PtO and NiO species\n2 x x\non 0.2Pt5Ni/NiAl O catalyst appears at a higher temperature (371°C), indicating stronger metal-support\n2 4\ninteractions on 0.2Pt5Ni/NiAl O catalyst than that of 0.2Pt5Ni/Al O catalyst (360°C).\n2 4 2 3\nIn summary, the 0.2Pt5Ni/NiAl O catalyst exhibits superior performance in the selective\n2 4\ndepolymerization of lignin to propenyl side-chained phenolic monomers at 140°C. This is driven by\nabundant oxygen vacancies, promoted hydrogen spillover, and robust metal-support interactions. Notably,\nthe superior hydrothermal stability of the NiAl O support relative to Al O support plays an indispensable\n2 4 2 3\nrole in this process.\nThe relevant description has been incorporated into the revised manuscript and supporting information\n(Tables S7 and S8, Figures S11-S13).\nFigure R1. EPR profiles of 0.2Pt5Ni/NiAl O and 0.2Pt5Ni/Al O catalysts.\n2 4 2 3\nFigure R2. Hydrogen spillover experiments over 0.2Pt5Ni/NiAl O and 0.2Pt5Ni/Al O catalysts.\n2 4 2 3\nFigure R3. H -TPR profiles of 0.2Pt5Ni/NiAl O and 0.2Pt5Ni/Al O catalysts.\n2 2 4 2 3\n2. The manuscript argues that Ni is not directly the active site, yet its oxophilicity is essential for promoting\nCα–OH hydrogenolysis and the formation of propenyl side chains. If oxophilicity is indeed the dominant\nfactor, the authors should rationalize why other more strongly oxophilic transition metals (e.g., V, Nb, Mo,\nW) were not considered or screened. A comparative discussion or preliminary screening would significantly\nstrengthen the mechanistic argument.\nReply: We thank the reviewer for this advice. We supplemented the activity of various oxygen affinity\ntransition metals (e.g., Nb, Mo, W) in self-hydrogen supplied fractionation of birch wood as shown in Table\nR5. Results indicate that the 0.2Pt5Ni catalyst exhibits the highest activity with a 38.6% phenolic monomers\nyield, whereas the 0.2Pt5W and 0.2Pt5Mo catalysts yield only 14.7% and 8.9% phenolic monomers,\nrespectively. Notably, no phenolic monomers were detected on 0.2Pt5Nb catalyst. Instead, significant\nfurfural products were observed in the post-reaction solution, indicating that the strong acidity from Nb\nconverted hemicellulose into furfural in the lignin depolymerization process. The reason for the above results\nmay be that Nb, Mo, and W are difficult to be reduced and cannot form alloy structures with metal Pt,\nfurther demonstrating the crucial role of Ni in self-hydrogen supplied fractionation.\nThe relevant description has been incorporated into the revised manuscript and supporting information\n(Table S9).\nTable R5. SCF of birch wood over four different catalysts.\nPhenolic monomers yield (wt%) Sel. to\nTotal\nEntry Sample Pe-G/S\nPe-G Pr-G Er-S Pr-S Pe-S (wt%)\n(%)\n1 0.2Pt5Ni 0.5 1.2 2.4 15.0 19.5 51.8 38.6\n2 0.2Pt5Nb - - - - - - -\n3 0.2Pt5W - - 0.2 5.6 8.9 60.5 14.7\n4 0.2Pt5Mo - - 0.4 4.1 4.4 49.4 8.9\nReaction conditions: substrate (0.5 g), catalyst (0.2 g), H O (10 mL), N 1 atm, 140 C, 12 h.\n2 2\n3. The reported turnover number (TON) of 270.5 for 0.2Pt5Ni/NiAl₂O₄ is only moderate compared to\nrecently reported Ru-based single-atom catalysts for lignin RCF. Moreover, the long reaction time (12 h)\nrequired to achieve the high monomer yield suggests a relatively low intrinsic reaction rate. The authors\nshould demonstrate that the improved monomer yield originates from genuinely enhanced site activity rather\nthan simply from extended reaction time. Time-resolved kinetics, initial rate comparisons, or space–time\nyield (STY) analysis would be necessary to support this claim.\nReply: We thank the reviewer for raising this comment. To address the concern regarding “reaction time vs.\nintrinsic rate”, we calculated the space-time yield (STY). The results are shown in the Table R6. Compared\nto recently reported Ru-based RCF systems for generating propenyl side-chained phenols, this work exhibits\ncompetitive STY (20.3 g monomers per gram catalyst per hour) and operates at a low reaction temperature\n(140°C) without requiring organic solvents.\nTable R6. Comparison of the space–time yield (STY) obtained in this work with recently reported Ru-\nbased RCF systems under N atmosphere.\nCatalyst Feedstock Solvent Atmosphere Ref.\n(°C) (h) Yield (wt%) (g/g/h)\nThis\n0.2Pt5Ni birch H O 140 12 N 38.6 20.3\n2 2\nwork\nRu /C birch methanol 235 4 N 35.8 22.9 1\n0.8 2\n110%ChCl-\n5Ru/C birch 190 2 N 36.2 3.8 2\n1. Huang, H., Zhang, X., Ma, L. & Liao, Y. Reductive catalytic fractionation of lignocellulose toward\npropyl‐ or propenyl‐substituted monomers and mechanistic understanding. Angew. Chem. Int. Ed.\n64, e202502545 (2025).\n2. Li, Y., et al. Hydrogen-Transfer Reductive Catalytic Fractionation of Lignocellulose: High\nMonomeric Yield with Switchable Selectivity. Angew. Chem. Int. Ed. 62, e202307116 (2023).\n4. Additional control experiments are required to verify the role of xylan as the hydrogen source. For\nexample, reactions conducted under H₂ or using external hydrogen-donating solvents should be included to\ndetermine whether the formation of propenyl-substituted phenolic products is dictated primarily by the\nhydrogen source rather than by the intrinsic catalytic properties. Furthermore, the selectivity toward propenyl\nside-chained products should be quantitatively compared with literature values to convincingly demonstrate\nany advantage of single-atom alloy catalysts over conventional single-atom or nanoparticle catalysts for\npreserving unsaturated side chains.\nReply: We thank the reviewer for raising this comment. We conducted control experiments on model\ncompound SG using different types of hydrogen sources, such as xylan, isopropanol, and H . As shown in\nsuggesting a consistent reaction pathway. Notably, using xylan as the hydrogen source yields 28.5% (6+11)\nolefinic phenolic monomers, whereas isopropanol and H produce lower yields of 23.6% (6+11) and 17.7%\n2\n(6+11), respectively. This demonstrates that the formation of propenyl-substituted phenolic products is\nprimarily determined by intrinsic catalytic properties rather than hydrogen sources. Furthermore, the mass\nactivity (mass of propenylphenols per gram of metal) was evaluated to benchmark against state-of-the-art\nRu-based catalysts. Our single-atom alloy catalyst (0.2Pt5Ni/NiAl O ) achieved a yield of 126.3 g per gram\n2 4\nof Pt, significantly outperforming the recently reported 5Ru/C (6.6 g/g) 2 and Ru /C (73 g/g) 1 systems.\n0.8\nThis result highlights the distinct advantage of the single-atom alloy strategy in preserving unsaturated\nfunctional group.\nThe relevant description has been incorporated into the revised manuscript and supporting information\n(Table S12).\nTable R7. Reaction pathway of lignin model compound SG under different hydrogen sources.\nYield (wt%)\nConv.\nHydrogen Sources\n(%)\n2 3 4 5 6 7 8 9 10 11 12\nXylan (0.05 g) 100 0.4 0.1 7.6 37.1 12.0 4.5 0.1 0.7 1.6 16.5 2.7\nIsopropanol (0.02 g) 100 - - 1.1 44.3 6.8 1.7 0.1 0.1 2.2 16.8 14.4\n10%H /Ar 100 - - 0.5 44.7 0.7 0.6 1.4 1.8 1.4 17.0 18.2\n2 2\n10%H /Ar.\n2\n1. Huang, H., Zhang, X., Ma, L. & Liao, Y. Reductive catalytic fractionation of lignocellulose toward\npropyl‐ or propenyl‐substituted monomers and mechanistic understanding. Angew. Chem. Int. Ed.\n64, e202502545 (2025).\n2. Li, Y., et al. Hydrogen-Transfer Reductive Catalytic Fractionation of Lignocellulose: High\nMonomeric Yield with Switchable Selectivity. Angew. Chem. Int. Ed. 62, e202307116 (2023).\n5. To enhance the reliability of the results, more experimental details are needed. For example, monomer\nyield quantitative method, GC column temperature program, representative GC chromatograms with peak\nassignments，GPC results for the lignin-oil.\nReply: We thank the reviewer for raising this comment.\nThe quantification of phenolic monomers was carried out using GC-MS (Agilent 7890A) and\nquantitatively analyzed by GC (Agilent 7890B) with a flame ionization detector, both quipped with HP-5\ncapillary columns. Tridecane was used as an internal standard for the quantification of the liquid products.\nNitrogen was employed as a carrier gas, and the following operating conditions were adopted: injection\ntemperature 250 °C, column temperature program: 50 °C (hold time 3 min), 10 °C min-1 to 250 °C (hold\ntime 5 min), detection temperature of 280 °C. Response factors for the different products were determined\nby calibration with commercial standards or through calculations based on the effective carbon number\n(ECN) theory.3 All experiments were performed at least three times. The phenolic monomers yields were\ncalculated based on the Klason lignin weight, calculating formula as shown below:\nMass (total monomers)\nPhenolic monomers (wt%)= ×100%\nMass (Klason lignin)\nMole (total monomers)\nMole yields of monomers (%)= ×100%\nMole (all monomers in lignin)\nThe mass of lignin in biomass was measured by Klason method and the mole of monomers in lignin was\nmeasured by NBO method.4\nGC-MS spectrum and GPC profiles of the lignin-derived products from SCF of birch over\n0.2Pt5Ni/NiAl O catalyst were shown in Figure R4 and Figure R5. The weight-average (M ) and number-\n2 4 w\naverage (M ) molecular weights of lignin decreased significantly after the reaction. Specifically, M dropped\nn w\nfrom 4484 to 498 g/mol, while M declined from 2990 to 259 g/mol.\nn\nThe relevant description has been incorporated into the revised manuscript and supporting information\n(Figure S6 and Figure S9).\nFigure R4. GC-MS spectrum of the lignin-derived monomers from birch over the 0.2Pt5Ni/NiAl O catalyst.\n2 4\nReaction conditions: birch wood (0.5 g), 0.2Pt5Ni catalyst (0.2 g), H O (10 mL), 140C, N 1 atm, 12 h.\n2 2\nFigure R5. GPC profiles of the lignin-derived products from SCF of birch over 0.2Pt5Ni/NiAl O catalyst.\n2 4\n3. Scanlon, J.T. & Willis, D.E. Calculation of Flame Ionization Detector Relative Response Factors\nUsing the Effective Carbon Number Concept. J. Chromatogr. Sci. 23, 333-340 (1985).\n4. Shuai, L., et al. Formaldehyde stabilization facilitates lignin monomer production during biomass\ndepolymerization. Science 354, 329-333 (2016).\nReviewer #3 (Remarks to the Author):\nZhou et al. report the development of a single-atom alloyed Pt₁Ni catalyst for lignocellulose fractionation\nthat enables efficient production of phenolic monomers under hydrogen-free conditions while preserving\ncellulose integrity. This work addresses a critical challenge in biomass valorization by achieving high\nmonomer yields and enhanced selectivity toward unsaturated side-chain products without the need for\nexternal hydrogen or excessive metal loading, and it proposes a mechanistically well-supported catalytic\nstrategy. Overall, the manuscript is strong and appears suitable for publication. However, several concerns\nshould be addressed, as outlined below. Addressing these points will further strengthen the manuscript and\nensure that it represents a meaningful contribution to sustainable biomass conversion and catalytic lignin\nvalorization.\n1. The authors attribute the enhanced Cα–OH activation by the oxophilicity of Ni to the higher selectivity\ntowards valuable propenyl products. It was evidenced by the mechanistic studies over Pt1Ni(100) and Pt(100)\nbased on DFT calculations. However, the specific role of the isolated Pt1 site in the Pt1Ni(100) model was\nnot clearly explained. It should be clarified.\nReply: We thank the reviewer for raising this comment. We agree that the specific role of the isolated Pt\n1\nsite should be clarified more explicitly. Here we emphasize the role supported by experimental evidence: Pt\nstabilizes Ni in a metallic state rather than oxidized Ni species, thereby sustaining the active surface required\nfor selective C –OH activation. Experimentally, 0.2Pt5Ni forms a Pt Ni single-atom alloy on Ni\nα 1\nnanoparticles: atom-sized bright spots assigned to individual Pt atoms are observed on Ni NPs with no Pt\nclusters, while 2Pt5Ni shows mixed Pt single atoms and clusters. The in-situ CO-DRIFTS spectra further\nreveal a CO band at 2034 cm-1 (linear CO on isolated Pt atoms) together with a broad band at 1835 cm-1\nassigned to bridged CO on contiguous Ni atoms, indicating the presence of metallic Ni ensembles adjacent\nto isolated Pt sites. In addition, Pt L -edge EXAFS fitting for 0.2Pt5Ni shows predominant Pt–Ni\n3\ncoordination (CN ≈ 6.6) and no Pt–Pt contribution, consistent with a single-atom alloy environment rather\nthan Pt clusters. Crucially, H -TPR demonstrates that Pt doping makes NiO easier to reduce, shifting NiO\n2\nreduction peaks to lower temperature relative to monometallic 5Ni, and 5Ni/NiAl O is catalytically inactive\n2 4\nwith a surface primarily composed of NiO species. These observations collectively support that the Pt₁Ni\nsingle-atom alloy structure promotes reduction and stabilizes metallic Ni species, which is essential for the\nobserved high activity and selectivity in SCF. Therefore, in the Pt₁Ni(100) model, the “Pt site” represents\n1\nthe single atom Pt that alloy-stabilizes a metallic Ni surface. This stabilization maintains active metallic Ni\nensembles that interact strongly with oxygen-containing functionalities, thereby favoring dehydroxylation\npathways leading to valuable propenyl products.\n2. Self-hydrogen supplied fractionation rather than exogeneous hydrogen gas was used as the hydrogen\nresources in this work. Compared with direct use of hydrogen gas, whether would self-hydrogen supplied\nfractionation affect the reaction mechanism or the resultant selectivity?\nReply: We thank the reviewer for raising this comment. We conducted control experiments on model\ncompound SG using different types of hydrogen sources, such as xylan, isopropanol, and H . As shown in\nsuggesting a consistent reaction pathway. Notably, using xylan as the hydrogen source yields 28.5% (6+11)\nolefinic phenolic monomers, whereas isopropanol and H produce lower yields of 23.6% (6+11) and 17.7%\n2\n(6+11), respectively. This demonstrates that the formation of propenyl-substituted phenolic products is\nprimarily determined by intrinsic catalytic properties rather than hydrogen sources.\nThe relevant description has been incorporated into the revised manuscript and supporting information\n(Table S12).\nTable R7. Reaction pathway of lignin model compound SG at different hydrogen sources.\nYield (wt%)\nConv.\nHydrogen Sources\n(%)\n2 3 4 5 6 7 8 9 10 11 12\nXylan (0.05 g) 100 0.4 0.1 7.6 37.1 12.0 4.5 0.1 0.7 1.6 16.5 2.7\nIsopropanol (0.02 g) 100 - - 1.1 44.3 6.8 1.7 0.1 0.1 2.2 16.8 14.4\n10%H /Ar 100 - - 0.5 44.7 0.7 0.6 1.4 1.8 1.4 17.0 18.2\n2 2\n10%H /Ar.\n2\n3. Please clarify how the transition states were verified in the DFT calculations.\nReply: Thanks for the comments, we have added explicit details in the revised SI (Computational details\nsection) on transition-state (TS) verification. Briefly: Each TS structure was converged using constrained\noptimization scheme,1 and then verified by vibrational analysis, ensuring only one imaginary frequency.\nThese details have added to the supporting information.\n1. Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. CO oxidation on Pt (111): an ab initio density\nfunctional theory study. Phys Rev Lett 80, 3650 (1998).\n4. In the current version, the colors of the corresponding atoms in the figures of reaction mechanisms, such\nas Figure 4, were not labelled. It is difficult for readers to clearly identify the active sites and visualize the\nadsorption geometries, especially for the Pt1 in the Pt1Ni(100) surface.\nReply: Thanks for the comments, we have revised Figure 4 in below (Figure R8), by Adding a clear color\nlegend for Pt, Ni, C, O, and H. Highlighting the isolated Pt atom (with a red circle out). We have added\n1\nthese figures to the revised manuscript.\nFigure R8. The comparative diagram of the reaction pathways. (a) energy profiles for C -OH deH, C -OH\nα α\ndeOH-I, and C -OH deOH-II on Pt Ni(100). (b) corresponding the molecular structure diagram. The Pt\nα 1\natoms in Pt Ni(100) have been circled in red in the last figure.\n1\n5. The manuscript contains some typos. For example, “Tranditionally” should be corrected to “Traditionally”\nin page3, and “efficientcy” should be corrected to “efficiency” in page3 and page12. The authors are\nencouraged to carefully proofread the text and revise these issues for clarity and consistency.\nReply: We thank the reviewer for pointing out this issue. We have corrected the specified typos and\nconducted a careful proofread of the full manuscript and SI to improve clarity and consistency."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Many thanks to the authors. Now the current version is more appropriate for publication.", "reviewer_2": "The manuscript provides detailed explanations for the proposed issues and supplements relevant experiments. There are still some questions that require to provide further clarification before publication.\n1) The manuscript suggests that Al2O3 and NiAl2O4 have different reaction pathways, which is driven by abundant oxygen vacancies, promoted hydrogen spillover, and robust metal-support interactions. The authors should further elaborate on the fundamental reasons underlying these observed differences.\n2) Aerobic metals Mo and W cannot form alloy structures with Pt, but still exhibit SCF activity and a certain selectivity for propenyl products compared to 0.2% Pt. Please provide a more detailed explanation to clarify this.", "reviewer_3": "My questions have been well addressed. I recommend the acceptance of the revised manuscrupt as is."}, "author_response": "Response to reviewer comments\nReviewer #2 (Remarks to the Author):\nThe manuscript provides detailed explanations for the proposed issues and supplements relevant\nexperiments. There are still some questions that require to provide further clarification before\npublication.\n1) The manuscript suggests that Al O and NiAl O have different reaction pathways, which is\n2 3 2 4\ndriven by abundant oxygen vacancies, promoted hydrogen spillover, and robust metal-support\ninteractions. The authors should further elaborate on the fundamental reasons underlying these\nobserved differences.\nReply: We thank the reviewer for the insightful comment and have performed additional DFT\ncalculations to elucidate the fundamental origins of the divergent reaction pathways observed on γ-\nAl O and NiAl O supports. As shown in Figure R1a, the reducible NiAl O exhibits a substantially\n2 3 2 4 2 4\nlower average oxygen vacancy formation energy (1.50 eV) compared to the non-reducible γ-Al O\n2 3\n(2.90 eV), indicating that NiAl O sustains a higher concentration of oxygen vacancies under\n2 4\nreaction conditions. These abundant oxygen vacancies are expected to enhance metal–support\ninteractions. Furthermore, the critical elementary step for spillover-mediated support participation\ninvolves hydrogen transfer from an OH species to a neighboring lattice oxygen. Figure R1b presents\nthe computed Gibbs free energy barriers for this step. Notably, NiAl O displays a moderate barrier\n2 4\n(Gₐ = 1.06 eV) for this OH-to-O hydrogen transfer, whereas the same process on γ-Al O is\n2 3\nkinetically prohibitive (Gₐ = 2.31 eV). Consequently, even if OH species form at the metal–support\ninterface, γ-Al O cannot effectively relay hydrogen migration across the support, limiting spillover\n2 3\nand confining reactivity largely to the metal and its immediate vicinity. In contrast, the combination\nof higher reducibility and a much lower hydrogen-transfer barrier on NiAl O facilitates rapid\n2 4\ninterfacial spillover, thereby promoting lattice-assisted hydrogenation pathways and shifting the\ndominant reaction mechanism.\nFigure R1. (a) Oxygen vacancy formation energies (E ) at different surface O sites on pristine\nOvf\nNiAl O (111) and γ-Al O (111). (b) The Gibbs free energy barriers (G ) for hydrogen transfer from\n2 4 2 3 a\nan adsorbed hydroxyl (OH) to a neighboring lattice oxygen on NiAl O (111) and γ-Al O (111),\n2 4 2 3\ntogether with the optimized initial state (IS), transition state (TS), and final state (FS) geometries.\nColor code: H, white; O, red; Al, purple; Ni, blue.\n2) Aerobic metals Mo and W cannot form alloy structures with Pt, but still exhibit SCF activity and\na certain selectivity for propenyl products compared to 0.2% Pt. Please provide a more detailed\nexplanation to clarify this.\nReply: We thank the reviewer for this insightful comment. Although Pt does not form alloys with\nMo or W, the enhanced catalytic performance can be attributed to the interfacial synergy between\nPt and the oxophilic MOₓ (M = Mo, W) species. Specifically, the oxophilic interface facilitates\nselective C–O bond cleavage, while hydrogen spillover from Pt promotes the reduction of interfacial\noxygen species. Although the stronger oxophilicity of Mo and W makes the subsequent desorption\nof H O and completion of the catalytic cycle more challenging, this interfacial mechanism still\n2\nenables a more efficient and selective pathway toward propenyl products compared to 0.2% Pt alone."}]

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