Interfacial Ru/RuO_x heterostructures on carbon support regulate selectivity in lignin hydrodeoxygenation

Domain: neuroscience
Published: 2026-04-11
Editorial outcome: major_revision

Abstract
Constructing well-defined heterostructure interfaces in catalysts provides an approach to modulate scaling constraints and steer reaction pathways in biomass upgrading. Herein, we demonstrate that thermal restructuring of hydroxyl groups on carbon nanofibers (CNF) induces the formation of heterostructures of Ru/RuO , which function as bifunctional active sites for the one-pot hydrodeoxygenation (HDO) of lignin to liquid hydrocarbons. The optimized 5 wt% Ru/CNF catalyst delivers promising performance, achieving a mass/carbon yield of 49.1%/67.7%, with high selectivity toward saturated cycloalkanes. X-ray absorption spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy confirm that thermal treatment of CNF tunes the oxidation state of Ru. DFT calculations reveal that O-rich Ru/CNF forms interfacial heterostructures of Ru/RuO polarized active sites, characterized by Oδ⁻···Ruδ⁺+···Ruδ⁺ ensembles that heterolytically activate H and strongly polarize C-O bonds in phenolic intermediates. The cooperative interplay between metallic Ru and partially oxidized RuO interfacial sites lowers the energy barriers for hydrogenation and deoxygenation reactions, enabling a cooperative reaction pathway. These insights elucidate the molecular basis of tunable selectivity in lignin HDO and demonstrate that a polarized, oxygen-decorated metal-support interface provides general design principles for engineering next-generation catalysts for sustainable fuel production.

Introduction
Identifying and mechanistically understanding active sites is essential for elucidating reaction mechanisms and enabling rational catalyst design in heterogeneous catalysis.1,2 Supported metal catalysts have been extensively used in various catalytic systems in the industrial process.3-5 The supports are typically not inert as the carrier, and the interaction with the nanoparticles gives rise to new interfacial phenomena.6-8 Such metal-support interaction (MSI) is supposed to have a profound impact on the resulting catalyst’s structure formation, including the active center and the coordination environment.9-12 Hence, reasonable strategies to manipulate the active center for optimized catalytic performance are highly desirable. In heterogeneous catalysis, the active centers and MSI can not only affect the structure and chemical state of the working catalysts, but also can influence the catalytic performance, including activity, selectivity, and stability.13 And thus, regulating the interaction between the active center and the support is supposed to provide options for rational catalyst design, followed by higher catalytic efficiency.

Catalytic conversion of lignin to value-added chemicals is one typical complex process, in which multiple types of reactions are involved.14-19 Additionally, it has significant meaning to upgrade lignin to green and clean fuel to meet the requirements of the circular economy. Hydrodeoxygenation (HDO) reaction is one of the most promising strategies to convert lignin and biomass to value-added products through the C-O bond cleavage reaction.20,21 Lignin conversion can, in principle, yield a broad spectrum of valuable products, notably chemicals and biofuels.22-25 Thus, it makes controlling the selectivity of the desired product more meaningful, despite the inherent challenge posed by the complex network of concurrent and sequential reactions involved in the upgrading process. Therefore, selectively converting lignin into specific hydrocarbons through tailoring catalyst design remains a key challenge and needed to be addressed. The most viable approach to achieve this goal needs the precise manipulation of the active center, given the complexity of the reaction systems and the competing reaction pathways involved in the HDO process.26-28 Hence, more focus should be paid on the active site restructuring and regulation, and then convert lignin to the value-added products with higher yield.

In this work, we report the self-reaction-induced strong metal-support interaction on carbon nanofibers (CNF), driven by the surface restructuring between hydroxyl groups and leading to the formation of the interfacial Ru/RuO heterostructures. The self-engineered manipulation contributes to the formation of well-defined bifunctional active sites comprising metallic Ru and partially oxidized RuO at the interface. The resulting Ru/RuO heterostructures exhibit good catalytic performance for the HDO of lignin-rich corncob, delivering a liquid hydrocarbon mass yield of 49.1% and a carbon yield of 67.7%. Notably, the interfacial Ru/RuO heterostructure is capable of selectively transforming lignin into solely cycloalkanes. When oxygenated species are removed from the CNF surface, aromatic products can be produced. Additionally, aromatic compounds are predominantly produced when using the benchmark catalysts, Ru/Al O and Ru/AC (AC: activated carbon) catalysts. Our results underscore the pivotal role of the interfacial Ru/RuO site in tuning both the reaction pathways and the product selectivity. These insights offer significant implications for biomass upgrading, thereby promoting the generation of liquid hydrocarbons from sustainable feedstocks. Furthermore, this study elucidates the mechanisms through which catalysts can be fine-tuned to direct the reaction pathways and selectivity of the products.

Results
Lignin is an abundant source of renewable aromatics that has long been utilized for valorization.29,30 Currently, the traditional method employs the two-step strategy, in which lignin is firstly converted to oligomers by pyrolysis at relatively high temperature and pressure, and then to chemicals or fuels, as shown in Figure 1a. However, the reactive oxygenated intermediates in bio-oils tend to undergo intermolecular condensation reactions and form chemically stable (poly)aromatic macromolecules that hinder the subsequent reactions and block the active sites. Another strategy employed in this work is the one-pot catalytic conversion process, which streamlines lignin valorization by integrating depolymerization and hydrodeoxygenation (HDO) into a single step. In this study, we focus on this one-pot approach to directly convert lignin-rich corncob into value-added liquid hydrocarbons.

Carbon nanofibers (CNFs), as catalyst supports, offer a range of advantages in catalysis due to their unique structural, chemical, and physical properties compared to conventional supports such as metal oxides and activated carbon (AC). These advantages include high surface area, excellent electrical conductivity, good chemical stability, and a flexible surface that can be functionalized to enhance the interaction with the active metal species. CNFs are prepared by the traditional chemical vapor deposition method,31,32 using iron oxide as the catalyst and syngas (CO/H : 4/1) as the carbon-containing gas, as shown in Figure 1b. It has been commonly reported that oxygen species on the carbon surface usually play significant roles in determining the structure of the active sites, and then in the catalytic performance.33-35 And thus, the as-synthesized CNF was thermally treated at varied temperatures (700 and 1000 °C) in argon to remove the oxygen species. Therefore, CNFs with varied oxygen species on the surface were obtained and used as the support for Ru catalysts. The physical properties of Ru/CNF catalyst pre-treated at different temperatures show similar pore structures (Table S1 in the Supplementary information). X-ray diffraction (XRD) patterns show that Ru is highly dispersed on the CNF support, since no Ru species can be observed, except for the carbon species (Figure S1). The microstructure of the synthesized Ru/CNF was investigated by high-resolution transmission electron microscopy (HR-TEM). Energy-dispersive X-ray spectrometry (EDX) elemental mapping revealed the homogeneous distribution of Ru on the CNF. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image in Figure 1c and Figure S2 reveals amorphous RuO and crystalline Ru domains, with lattice fringes at 0.21 nm corresponding to the Ru (101) plane.36 To investigate the electronic structure and coordination configuration of Ru, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were performed at the Ru K-edge, as shown in Figures 1d, 1e, and Table S2. The white-line intensity of Ru in Ru/CNF lies between that of RuO₂ and Ru foil, suggesting the presence of partially oxidized Ru species in the heterostructure catalyst. Figure 1f shows the phase-corrected Fourier transform (FT) curves at the R space of the Ru/CNF in comparison with the references of Ru foil and RuO . Notably, Ru/CNF has two dominant peaks corresponding to Ru-O and Ru-Ru path, confirming the state of mixed metallic Ru and partially oxidized RuO state, consistent with the HAADF-STEM observation.

In addition, Al O and activated carbon (AC) were also chosen as the benchmark catalysts in this work. The basic characterizations of physical adsorption, XRD, and high-resolution transmission electron microscopy (HR-TEM) of the Ru/AC, Ru/Al O , Ru/CNF, Ru/CNF-700, and Ru/CNF-1000 catalysts are shown in Table S1, and Figures S3-S8 in the Supplementary information. All catalysts exhibit a mesoporous structure, with Ru/AC showing the highest surface area and larger pore volume than Ru/CNF. Ru/Al O also displays a higher pore volume than Ru/CNF. High-temperature treatment has a negligible effect on the Ru/CNF structure. XRD reveals no detectable Ru or RuO diffraction peaks, only features from the carbon support, indicating high Ru dispersion across the carbon supports. TEM confirms the uniform Ru dispersion on CNF, AC, and Al O , demonstrating effective metal dispersion irrespective of support type.

During the enzymatic hydrolysis of corncobs for sugar production, a lignin-rich solid residue is generated as a primary byproduct, as illustrated in Figure S9. This residue, commonly classified as industrial waste in the fine chemical industry, contains a significant proportion of lignin. Typically, lignin-rich residue can be depolymerized into lower molecular weight phenolic compounds, such as monomers or oligomers, which can subsequently be upgraded via hydrodeoxygenation (HDO) to produce liquid hydrocarbons or biofuels. In this study, the aforementioned corncob residue will be utilized and evaluated as a feedstock in the HDO process, as depicted in the following section.

The HDO reaction of the lignin-rich corncob residue was investigated over the various Ru catalysts, and the results are displayed in Figure 2. Ru/CNF shows very high mass yield and carbon yield (49.1% and 67.7%) in direct conversion of lignin via HDO reaction. As summarized in Table S3, the hydrocarbon yield reported here represents one of the highest values achieved to date for converting lignin or woody biomass into liquid hydrocarbons. In addition to the yield, we would like to modestly highlight another aspect of the Ru/CNF catalyst for producing cycloalkanes. The one-pot selective transformation of lignin into cycloalkanes using hydrogen, as illustrated in Figure S10, holds promise for a unified approach to hydrogen storage and the production of sustainable aromatics. In this context, cycloalkane can serve as an effective liquid organic hydrogen carrier,37,38 enabling the transportation and on-site release of hydrogen via the integration with the dehydrogenation process. This process underscores the capability of the Ru/CNF catalyst to convert lignin into environmentally friendly and commercially valuable products.

Ruthenium-supported catalysts have demonstrated high activity in the HDO of the lignin model compounds.39 For comparison, we selected Ru/Al O and Ru/AC catalysts as benchmark catalysts. The Ru/AC catalyst exhibited slightly higher yields than Ru/Al O , with the mass and carbon yields of 14.8% and 21.6%, respectively. Importantly, the choice of these benchmarks also reflects differences in surface hydroxyl mobility: while hydroxyl groups on Al O are strongly bound and immobile, limiting surface restructuring, the mobile hydroxyl species on CNFs enable the formation of the interfacial Ru/RuO heterostructures under thermal treatment, which we identify as key to the enhanced HDO performance. In contrast, the Ru/CNF catalyst developed in this work achieved significantly higher yields than the two benchmark catalysts. Beyond the yield difference, the product distributions also varied notably depending on the catalyst supports (Figure 2b). Notably, no aromatic products were detected on Ru/CNF, indicating that all the unsaturated C=C bonds, including the aromatic C ring, were hydrogenated to C-C bonds, with only cycloalkane hydrocarbons formed. In contrast, the main products of the benchmark catalyst were aromatics. The Ru/AC catalyst showed a lower aromatic distribution, suggesting a moderate hydrogenation capability compared to Ru/Al O . The detailed product distribution and yields with regard to the carbon numbers are listed in Figure S11 and Table S4. It can be seen that only cycloalkanes are produced, with C as the dominating one on the Ru/CNF catalyst. However, it is reversed on the benchmark catalysts, Ru/AC and Ru/Al O , with C -C aromatics as the dominating products. The results demonstrate the significant roles in influencing the C-C bond cleavage in the HDO process. This integrated depolymerization and upgrading approach provides an effective route for one-pot HDO of lignin to liquid cycloalkanes. (illustrated in Figure S1S2).

Catalyst support is playing a crucial role in governing both HDO activity and product distributions. Notably, CNF demonstrates superior performance to conventional activated carbon, highlighting the importance of support characteristics. The electronic properties of carbon materials are strongly dependent on their surface functional groups. To probe the structure–activity relationship, the CNF supports were thermally treated at 700 °C and 1000 °C (Ru/CNF-700 and Ru/CNF-1000), progressively eliminating surface oxygen functionalities.

This thermal treatment of the CNF led to significant changes in catalytic performance. Compared to untreated Ru/CNF, the high-temperature-treated catalyst showed markedly reduced mass yields (16.2% for Ru/CNF-700 and 11.5% for Ru/CNF-1000) and carbon yields. More interestingly, we observed a temperature-dependent emergence of aromatic products, with yields increasing progressively to reach 12% for Ru/CNF-1000. These results clearly demonstrate that high-temperature treatment modifies the chemical environment of the active site by removing oxygen functional species, thereby enabling precise tuning of product distributions. The detailed mechanistic implications of these findings will be discussed in the following section.

To assess the catalyst stability, we performed recycling tests under semi-continuous conditions by replenishing fresh lignin feedstock without separating the catalyst, solvent, and products between cycles at the same optimized reaction conditions. The Ru/CNF catalyst maintained consistent activity over six consecutive runs without observable deactivation (Figure S13) for both batch and continuous operation modes.

To evaluate the practical applicability, we extended our investigation to real beechwood biomass (sourced from Dansk Traemel) as a feedstock for biofuel production via HDO reaction. The catalyst exhibited robust performance, achieving a hydrocarbon mass yield of approximately 20% with complete deoxygenation (Figure S14). Although the yield is lower than that obtained with lignin-rich corncob, this difference likely arises from the significantly higher oxygen content in the raw biomass. Importantly, the product distribution aligned with previous findings, consisting exclusively of hydrocarbons, including both aromatics and cycloalkanes, with no detectable oxygenated species. To further assess the scalability of the one-pot conversion process from lignin to liquid hydrocarbons, a scale-up experiment was performed by increasing the feedstock amount fivefold, as shown in Figure S15. A mass yield of 44% was achieved, with cycloalkanes as the main products and no detectable oxygenates, underscoring the scalability of the process. Together, these results conclusively demonstrate that the Ru/CNF catalyst maintains high HDO activity across both lignin-rich corncob and complex, real-world woody biomass. Its consistent performance, full deoxygenation capability, and adaptability to scaled reaction conditions underscore its potential as a robust and practical catalyst for sustainable biofuel production.

The spent catalysts after HDO reaction were also characterized to verify the stability in this complex reaction condition. XRD profiles (Figure S16a) revealed no detectable Ru-species diffraction peaks, while X-ray photoelectron spectroscopy (XPS) analysis of the Ru 3p region (Figure S16b) showed nearly identical binding energies and species distributions between the fresh and spent catalysts, indicating excellent preservation of the Ru chemical state during the HDO reaction process. HR-TEM confirmed the absence of metal particle agglomeration (Figure S17). The comprehensive characterization results confirm that the Ru/CNF catalyst exhibits outstanding structural and chemical stability while maintaining high catalytic performance for lignin conversion to cycloalkanes in semi-continuous HDO processes.

To understand the reaction mechanisms over the various catalysts, kinetic studies were conducted. As shown in Figures S18 and S19, increasing temperature enhances the mass yield over Ru/CNF, in line with the typical temperature dependence of HDO reactions. At lower temperatures, a substantial presence of oxygenated intermediates with saturated aromatic ring-like cyclohexanol and methoxy cyclohexane was produced on the Ru/CNF catalyst with a relatively low yield. The oxygenated species, constituting 30.8% of the products, with the saturated benzene ring, was partially deoxygenated. Time-dependent experiments conducted at 250 °C on the Ru/CNF catalyst, illustrated in Figure S20-S22, revealed the formation of oxygenated products at the earlier stage of the reaction. These intermediates were gradually transformed into hydrocarbons over time, with no aromatic compounds detected under these conditions. However, the time-dependent experiment on the Ru/AC catalyst revealed a distinct behavior (Figure S23), where aromatic products dominate even in the early stages of the reaction. These results suggest that the Ru/CNF and Ru/AC catalysts follow different reaction pathways, particularly in the initial phase, during the HDO process.

To gain a comprehensive understanding of the local structure of Ru and its interaction with oxygen species on the CNF support, near-ambient pressure XPS (NAP-XPS) was performed. The samples were reduced under H to mimic the reducing atmosphere of the HDO process; while ensuring they were not exposed to air during transfer or measurement. Nevertheless, we recognize that 0.2 mbar is nearly four orders of magnitude below the 5 MPa H pressure used in catalytic testing, and the catalyst surface under high-pressure H is likely to be more reduced than our NAP-XPS measurement indicates. However, the systematic variation in oxidation states observed at different pretreatment temperatures under NAP-XPS conditions is expected to reflect the relative trend that persists under reaction conditions, even if the absolute values differ. The C1s and Ru 3d spectra of three samples, namely Ru/CNF, Ru/CNF-700, and Ru/CNF-1000, after reduction in H for 1 hour during the heating process, are illustrated in Figure 3. The C 1s spectra are shown in Figure 3a, the two peaks can be assigned to C-C and C-O bonds.33 When Ru is supported on the CNF, the C 1s spectra overlap with Ru 3d spectra, as shown in Figure 3b, illustrating four main peaks, in which two more peaks correlated to Ru species exist. Besides, when high-temperature treatment is carried out on the CNF support to remove the oxygen species, the binding energy of Ru shifted to lower values.40 It indicates a strong metal-support interaction between Ru and CNF on Ru/CNF. The Ru 3d spectra were further deconvoluted, as shown in the inset in Figure 3b. It can be seen that two peaks can be observed on all the Ru/CNF catalysts, which can be ascribed to metallic Ru (~280.1 eV) and oxidized Ru (~280.6 eV). It confirms the existence of a heterostructure catalyst consisting of metallic Ru and ruthenium oxide (RuO ). Moreover, the contributions of Ru and RuO are also calculated. The contributions of RuO are 58%, 49.9%, and 35.2% on Ru/CNF, Ru/CNF-700 and Ru/CNF-1000. It is consistent with the high-resolution Ru 3p spectra (Figure S24 and Table S5), that the peak deconvolution discoveries. The contribution of RuO on Ru/CNF is the highest. It demonstrates that the oxidized form is decreased by removing the oxygen functional species. The results showcase that the oxygen functional groups on CNF are playing significant roles in affecting the interaction between the active site and CNF support. In the following part, we will discuss the intrinsic mechanism for the oxygen-decorated Ru site (or partially oxidized Ru) evolution and reaction pathway of the HDO process.

The formation of Ru/RuO interface is also confirmed by the high-resolution of O 1s XPS (Figure S25 and Table S6). For Ru/CNF, the high-resolution O 1s spectrum deconvolutes into four peaks. The peak at 530.8 eV (Peak 1) corresponds to the partially oxidized Ru species, which is absent in the CNF support. The remaining peaks are attributed to oxygen-containing surface groups (hydroxyl, ester, and carboxyl). The existence of Ru-O species is possibly induced by the surface reaction between mobile oxygen functional groups.

To elucidate the nature of active sites and support effects in the HDO reaction, first-principles calculations were also carried out. The simulations tracked the temperature-dependent evolution of surface hydroxyl groups, aligning with experimental observations that OH species are progressively removed as temperature increases. To present catalysts subjected to different thermal treatments, we constructed models with systematically varied OH coverages on the carbon support. Figures 4a and 4b depict the initial states and the optimized structures with different OH group densities, respectively. The OH-rich model corresponds to the oxidized Ru/CNF catalyst, whereas the OH-deficient model reflects the thermally treated Ru/CNF-700 surface.

Under thermal treatment, the OH-enriched carbon surface undergoes structural restructuring. The hydroxyl groups on carbon exhibit high mobility and tend to migrate toward the Ru nanoparticles, driven by the strong Ru-O affinity. Upon adsorption onto the Ru surface, adjacent OH species recombined together to form O* and H O* via the following equation:
2OH∗ → O∗+ H O∗ (1)
2
The desorption of H O* leaves behind strongly bound O* species at the Ru-support interface, triggering significant local structural reconstruction. This process partially oxidizes metallic Ru, generating partially oxidized RuO sites and yielding a bifunctional surface comprising coexisting metallic Ru0 and oxidized Ru++ domains, consistent with the XPS observations.

Molecular dynamics (MD) simulations further corroborate this transformation. The energy profiles for Ru/CNF and Ru/CNF-700 (Figures 4c and 4d, and Figure S26) reveal that neighboring OH groups on Ru can be coupled to form O* and H O*, followed by H O desorption, leaving behind a stable partially oxidized RuO site. The inset depicts the transition state (TS) structures, where two OH groups combine into H O* and O*, with a blue circle highlighting the desorbing H O molecules. The energy was found to oscillate around a constant frequency due to the surface reactions. The Ru-O bond length was also calculated, as shown in Figure S26. The calculated Ru-O bond lengths show that Ru-O in Ru/CNF (1.706 Å) is shorter than that in Ru/CNF-700 (1.806 Å), further corroborating the strong metal-support interaction observed in the XPS.

Bader charge analysis was carried out to elucidate how surface OH and O species, located exclusively near Ru and on the carbon surface, modify the electronic structure of the Ru (Figure 4e). Both OH and O withdraw electron density from adjacent Ru atoms, OH groups induce only modest changes, leading to a slight increase in the local positive charge of the directly bonded Ru sites. In contrast, O atoms exert a much stronger electron-withdrawing effect: interfacial Ru atoms bonded to O become markedly electron-deficient, and this depletion extends across the Ru cluster surface, generating a pronounced gradient in oxidation states. As O* coverage increases, the entire Ru cluster becomes progressively polarized, while the surface O atoms accumulate electron density. This cooperative charge redistribution gives rise to highly polarized Ruδ+···Oδ− pairs, giving rise to electronically polarized Ru–O interfacial ensembles capable of heterolytic-like H₂ activation. In this configuration, electron-deficient Ru atoms function as Lewis acids, while electron-rich O atoms serve as Lewis bases. Concurrently, the Ru cluster exhibits an oxidation-state gradient, resulting in a heterogeneous surface charge distribution. All together it creates a bifunctional interface capable of enhancing reactant activation and accelerating key surface reactions.

Overall, the MD calculation qualitatively captures the experimental observation of Ru oxidation-state changes with the oxygen content of carbon through tuning the pretreatment temperature. It provides deep insights into the long-observed fact in the literature that the oxygen groups on the carbon can significantly influence the charge of the metal clusters.34,35,41 Calculations reveal that on OH-enriched surfaces, the surface OH groups migrate from the carbon support to the Ru clusters, generating partially oxidized Ru sites at the Ru-carbon interface as the dominant configuration on initially OH-rich Ru/CNF. This restructuring increases the positive Bader charge of the surface Ru atoms, indicating the development of electron-deficient Ru centers. These trends are consistent with the Ru 3d XPS spectra, in which Ru/CNF exhibits the highest binding energy among all CNF-supported catalysts, confirming the electron-withdrawing effect of oxygen-decorated Ru sites. The correlation between higher Ru 3d binding energies and the calculated positive Bader charges underscores the critical role of surface O/OH species in modulating Ru electronic properties and strengthening the interaction with oxygenates during the HDO reaction.

The HDO reaction mechanism involves three critical elementary steps: hydrogen activation and dissociation, C-O bond cleavage, and aromatic ring hydrogenation. DFT calculations were carried out to elucidate how surface structure modulates these reaction steps by comparing three representative Ru/CNF structures-Ru (OH) (OH-deficient), Ru (OH) (OH-rich) and Ru (OH) O (O-rich) models. These models vary in surface OH/O coverage and Ru–O coordination and are constructed to represent the key structural features of the catalysts. Instead, they serve as quantitative descriptors of the surface evolution induced by thermal pretreatment, capturing the transformations from OH-rich surface at low pretreatment temperature to O-rich surface at moderate temperatures and ultimately to OH-deficient surface after high-temperature treatment. The adsorption energies and the energy profiles for hydrogen activation, phenol dissociation, hydrogenation, and phenol deoxygenation over the three Ru/CNF models-Ru₁₈(OH)₅, Ru₁₈(OH)₁₆, and Ru₁₈(OH)₈O₄-presented in Figure 5, while the corresponding transition-state and intermediate structures are summarized in Table S7.

Hydrogen adsorption and activation on the three Ru/CNF model surfaces are strongly influenced by the surface oxidation state and the extent of Ru-O coordination (Figure 5a, transition state structures in Table S7). Calculated H adsorption energies increase with oxygen coverage: -0.25 eV on OH-deficient Ru (OH) , -0.67 eV on OH-rich Ru (OH) , and -1.01 eV on O-decorated Ru (OH) O . This trend arises from progressive electron withdrawal from surface Ru atoms toward adjacent O species, generating Ru sites with varying degrees of electron deficiency (Table S8). The resulting Ruδ⁺+–Ruδ⁺ pairs, while both Lewis acidic, exhibit sufficient charge asymmetry to polarize the H-H σ bond, enabling cooperative H activation analogous to a Lewis acid-Lewis acid pair.

Consistent with this interpretation, the activation barrier for H dissociation follows the inverse trend: Ru₁₈(OH)₅ (0.23 eV) > Ru₁₈(OH)₁₆ (0.15 eV) > Ru₁₈(OH)₈O₄ (0.09 eV), with the O-rich surface also yielding the most stable dissociated state (-0.61 eV). The barrier appears to be governed by the degree of charge asymmetry within adjacent Ru pairs. On a metallic Ru surface, H activation proceeds homolytically via σ-donation/back donation, limited by d-band filling and resulting in moderately high barriers. The transition state on such surfaces shows a nearly symmetric H-H bond length (~1.7–1.9 Å). In contrast, on the O-decorated surface, electron withdrawal by oxygen generates a built-in electric field that promotes heterolytic polarization. This is reflected in the asymmetric transition-state geometry, where the Ru-H bond adjacent to oxygen is significantly shorter ((~1.63–1.68 Å), and the opposite Ru-H bond is elongated (~2 Å). These observations suggest that the Ru-O interface plays a decisive role in driving H activation at the Ru/RuO boundary.

Phenol adsorbs on Ru surfaces primarily through its aromatic π system (Figure 5a, Table S7). Adsorption energies are similar on Ru₁₈(OH)₅ (–1.85 eV) and Ru₁₈(OH)₁₆ (–1.85 eV), with slightly stronger binding on Ru₁₈(OH)₈O₄ (–1.92 eV). This correlates with increasing Ru electron deficiency, which in turn coincides with lower calculated barriers for O-H dissociation. In this step, O-H activation proceeds on polarized Lewis acidic Ru sites via a transition state of the form C₆H₅O···H···Ruδ⁺–Oδ⁻ (OHδ⁺), a detailed analysis is provided in Supplementary Note 1 of the supporting information.

The barrier for phenolic O-H dissociation reflects the degree of surface polarization imparted by oxygen functionalities (Figure 5c). The O-rich surface exhibits the lowest barrier (0.17 eV), where strongly electron-withdrawing O ligands generate a pronounced Ruδ⁺–Oδ⁻ charge asymmetry that efficiently polarizes and cleaves the O-H bond. The OH-rich surface shows a moderate barrier (0.28 eV), likely because pre-adsorbed OH groups partially neutralize Ru-O polarization and occupy interfacial sites. In contrast, the OH-deficient surface, lacking interfacial oxygen, remains largely metallic and provides insufficient charge separation, resulting in the highest barrier (0.59 eV). This pattern suggests that O-induced surface polarization is important for efficient phenol activation.

Following phenoxy formation, two competing HDO pathways are possible:42,43 (i) direct deoxygenation of the phenoxy intermediate to aromatic hydrocarbons and (ii) ring hydrogenation to cyclohexanol followed by deoxygenation to cycloalkane. The energy barriers of the two pathways are also compared, as shown in Figure 5c. The difference in the energy barrier between ring hydrogenation and direct deoxygenation reflects the selective production of cyclohexane and aromatics.

The benzene-ring hydrogenation step in phenol HDO shows a strong dependence on the surface polarity generated by oxygen functionalities on Ru/CNF. DFT results reveal that the O-rich Ru (OH) O surface exhibits the lowest hydrogenation barrier (0.50 eV), followed by the OH-rich Ru (OH) surface (0.65 eV), while the OH-deficient Ru (OH) surface shows a much higher barrier (1.54 eV), shown in Figure 5c. The transition state structures are illustrated in Table S7. This trend mirrors the degree of charge asymmetry created at the RuO interface, see detailed discussion in Supplementary Notes 2 and 3. The higher Ru charge promotes the hydrogenation ability of H into the aromatic ring. As for the other reaction pathway, direct deoxygenation of phenoxy, the Ru (OH) O surface also shows the lowest energy barrier (0.6 eV) compared with 0.63 eV for the Ru (OH) and 1.04 eV for the Ru (OH) catalysts, as shown in Table S6. It indicates that the direct deoxygenation reaction on RuO /Ru interfaces is also favorable in comparison with other catalysts.

Besides, we can compare the energy barrier of hydrogenation and direct deoxygenation reactions on each catalyst to derive the most possible reaction pathway of HDO process. On the Ru (OH) O structure, the energy barrier of hydrogenation is much lower than deoxygenation (0.5 and 0.6 eV). The energy barrier of the deoxygenation reaction is slightly lower on the Ru (OH) surface-0.65 eV for hydrogenation and 0.63 eV for deoxygenation. However, it is reverse for the Ru (OH) surface, and it is 1.54 eV for the hydrogenation reaction and 1.04 eV for the deoxygenation reaction. The results indicate that the catalysts are following different kinetic behaviors, in which, hydrogenation is more favorable for the O-decorated RuO /Ru interface, and direct deoxygenation is more favorable for the Ru (OH) surface. Therefore, the heterostructure of RuO /Ru shows high activity for both hydrogenation and direct deoxygenation of phenoxy than other catalysts.

The energy barrier of deoxygenation of cyclohexanol is displayed in Figure 5d and Table S7. The O-rich Ru (OH) O surface displays the lowest energy barrier (0.17 eV), compared with 0.4 eV for the Ru (OH) and 0.69 eV for the Ru (OH) catalysts. It indicates that the deoxygenation of cyclohexanol is also more favorable on the O-decorated interface.

It has been commonly reported that hydrogenation of benzene is rather difficult, and high pressure is usually needed. The energy barrier of hydrogenation of benzene is illustrated in Figure S27 and Table S7. The O-decorated Ru/CNF surface again shows the lowest barrier (0.75eV) than Ru (OH) (0.99 eV) and Ru (OH) (2.12 eV) catalysts. It can be seen that hydrogenation of benzene on all the catalysts shows a higher energy barrier than the reactions we mentioned above. The O-decorated RuO /Ru interface facilitates the dissociation of H , providing abundant H* species that are highly beneficial for the benzene hydrogenation reaction.

This work establishes a unified mechanistic and structural framework for how surface oxygen functionalities regulate HDO of lignin-rich corncob over Ru/CNF. By integrating experimental with DFT calculations, we show that thermally driven restructuring of surface hydroxyl groups on CNF produces partially oxidized, O-decorated interfacial heterostructure Ru/RuO . The adjacent oxygen ligands generate a heterostructure, characterized by Oδ⁻–Ruδ⁺+–Ruδ⁺ charge distribution. This interfacial polarization enhances heterolytic H₂ activation and promotes charge-separated interactions with phenolic –OH and C–O bonds. The interfacial heterostructure Ru/RuO strongly favors the hydrogenation-deoxygenation route, enabling the conversion of lignin into fully saturated cycloalkanes with a mass yield of 49.1% and a carbon yield of 67.7%, whereas removal of surface oxygen shifts selectivity toward aromatic products. The structure-function relationships demonstrate that tuning oxygen coverage and the density of polarized Ru-O ensembles offers a rational strategy for controlling HDO selectivity. Beyond the Ru/CNF system, this concept provides a general design principle for creating multifunctional catalysts via deliberate modulation of metal-support interactions for biomass valorization and sustainable fuel production.

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript “Dynamic Ru/RuOx Interface on Carbon Supports Regulate Selectivity in Lignin Hydrodeoxygenation” reports a combined experimental and computational investigation of Ru catalysts supported on carbon nanofibers for lignin hydrodeoxygenation (HDO), claiming that a dynamically restructured Ru/RuOx interface governs product selectivity toward cycloalkanes versus aromatics. The work combines catalytic testing on lignin and real biomass feedstocks, near-ambient pressure XPS, and DFT/MD calculations to rationalize the observed trends. While the topic is timely and the experimental–theoretical coupling is ambitious, several key claims are currently overstated, particularly regarding the dynamic nature of the active interface and the mechanistic interpretation derived from the computational modeling. With a clearer definition of the active site, a more restrained mechanistic interpretation, and a rigorous comparison to the literature, the manuscript could become suitable for Nature Communications. I therefore recommend major revision. My main concerns are as follows:\n1. A central claim of the manuscript is that a dynamic Ru/RuOx interface regulates selectivity in lignin HDO. However, the experimental evidence primarily supports thermally induced, pretreatment-dependent interfacial states, rather than genuinely dynamic restructuring under reaction conditions. The near-ambient pressure XPS measurements are conducted under static H₂ at 300 °C and do not demonstrate reversible or reaction-driven evolution of the Ru oxidation state during catalysis. As presented, the data convincingly show controllable interface formation via CNF surface oxygen chemistry, but not dynamic interfacial behavior in the strict sense. Given the prominence of this concept in the title and abstract, the terminology should be moderated or further experimental evidence should be provided.\n2. The manuscript claims outstanding performance and, in some instances, suggests record-level yields for lignin-to-hydrocarbon conversion. Such claims are difficult to substantiate without a rigorous, quantitative comparison to the literature. I strongly recommend the inclusion of a dedicated comparison table summarizing representative HDO catalysts reported in the literature, including lignin type, reaction conditions (temperature, pressure, solvent), catalyst composition, mass yield, carbon yield, and product selectivity (aromatics vs cycloalkanes). This would greatly strengthen the manuscript by clearly positioning the Ru/CNF system relative to existing approaches and avoiding overstatement.\n3. The manuscript repeatedly attributes catalytic selectivity to surface frustrated Lewis pair (sFLP)-like ensembles involving Oδ⁻–Ruδ⁺–Ruδ⁺ motifs. While the concept is appealing, the physical identity of the active site remains insufficiently resolved. It is not clear whether the dominant functionality arises from Ru–O perimeter sites, polarized Ru–Ru ensembles, or partially oxidized Ru edge atoms. Moreover, the distinction between the proposed sFLP-like behavior and more classical metal–oxide interfacial effects is not rigorously established. At present, the sFLP terminology appears more descriptive than demonstrative, and its use should be more carefully justified or framed more cautiously.\n4. The DFT and molecular dynamics calculations are technically sound and internally consistent, and they provide a coherent mechanistic narrative linking surface oxygen chemistry to hydrogen activation and C–O bond cleavage. The MD simulations capturing hydroxyl migration and partial Ru oxidation at the Ru–carbon interface are particularly convincing and qualitatively consistent with the XPS observations. The construction of OH-rich, O-rich, and OH-deficient Ru₁₈ cluster models is a reasonable approach to rationalize pretreatment effects. However, the manuscript does not sufficiently emphasize the simplified and descriptor-based nature of the computational models. The chemistry is reduced to phenol as a lignin proxy, solvent effects and high-pressure hydrogen are neglected, only a single Ru cluster size is considered, and coverage or competitive adsorption effects are not explored. These approximations are acceptable for extracting qualitative trends, but the manuscript occasionally implies a degree of quantitative correspondence with real lignin HDO that is not fully justified. The DFT results should be more explicitly framed as providing mechanistic trends and design principles rather than predictive descriptions of the full catalytic system.\n5. The manuscript attributes selectivity primarily to differences in intrinsic reaction barriers induced by surface polarization at the Ru/RuOx interface. While the DFT results are consistent with this interpretation, the experimental data do not fully disentangle intrinsic kinetic effects from potential influences of hydrogen availability, mass transport, or solvent-mediated stabilization. No site-normalized rates, kinetic isotope experiments, or competitive adsorption measurements are provided. As a result, the causal link between the calculated energy barriers and macroscopic selectivity remains suggestive rather than definitive and should be discussed with appropriate caution.\n6. The manuscript is generally well written, but several sections—particularly those describing phenol activation and hydrogenation pathways—are repetitive between the main text and the Supplementary Information. In addition, the discussion of cycloalkanes as liquid organic hydrogen carriers, while interesting, feels peripheral to the core mechanistic message and could be shortened to improve focus.", "reviewer_2": "Overall Assessment:\nThis manuscript presents a Ru/CNF catalytic system for the one-pot hydrodeoxygenation (HDO) of lignin into cycloalkanes, proposing a \"dynamic Ru–RuO interface\" to explain performance variations. While the topic—integrating renewable resource conversion, interface catalysis, and metal oxidation state modulation—is timely and relevant to sustainable chemistry and biomass upgrading, the manuscript suffers from significant shortcomings in novelty, mechanistic depth, evidential rigor, and data quality. Despite a seemingly complete framework covering catalyst synthesis, characterization, performance evaluation, and mechanistic discussion, the work lacks the necessary originality and robust scientific foundation required for publication in Nature Communications. I recommend rejection.\nMajor Critiques:\nLimited originality of the reaction system:\nThe one-pot conversion of lignin to cycloalkanes is extensively reported. The reaction design here offers no substantive breakthrough, nor does it adequately acknowledge prior literature. The claim of \"surpassing the highest yields in existing literature\" lacks credibility without a rigorous, transparent, and correctly described quantitative methodology.\nInsufficient scientific contribution of catalyst concept:\nThe catalyst is essentially Ru/CNF prepared at different calcination temperatures, leading primarily to variations in Ru oxidation state. Screening calcination temperatures is a common optimization procedure. Crucially, no suficient experimental evidence demonstrating the \"dynamic Ru–RuO interface\" nature of this interface is provided, leaving the core concept unsupported.\nInadequate Structural Characterization:\nThe authors rely solely on XPS to infer Ru⁰/Ruᵟ⁺ ratios. This is insufficient. Essential complementary techniques—such as XAS (XANES/EXAFS), CO-DRIFTS, and H₂-TPR—were not employed to validate the structural model and oxidation state modulation. Drawing conclusions about interface effects and electronic structure control based on such a weak evidence base falls far short of the scientific rigor expected for a high-impact journal.\nSignificant Deficiencies in Data Quality and Presentation:\nImage Quality: Multiple images (e.g., Fig. 1c,d) suffer from poor resolution. TEM scale bars are illegible, Ru nanoparticles are unmarked, size distribution histograms are blurred, and XRD patterns lack reference standards.\nData Validity & Methods: The stability test (Fig. 2d) was conducted under full conversion conditions, rendering the results meaningless for assessing deactivation. The yield calculation formula in SI (Section 2.4) contains fundamental errors. The basis for substrate definition (lignin vs. corncob residue?) and yield calculation is ambiguous and inconsistent.\nLack of Foundational Data: Critical evidence is missing, including: representative SEM/macroscopic images proving the CNF support structure; characterization (e.g., TGA, in-situ studies) demonstrating lignin pyrolysis occurs under the reported mild conditions (250°C, 5 MPa H₂) vs. the typical >350-500°C required; full material/carbon balances; and raw analytical data (GC, GC-MS, HPLC, NMR) supporting yield claims.\nExperimental Detail & Errors: The justification for using Al₂O₃ as a control support (based solely on claimed \"conductive vs. inductive\" differences) is weak and unsupported by data (e.g., XPS, DRIFTS). Concerns exist about temperature probe placement/reactor filling ratio (20 mL solvent in 160 mL reactor), potentially causing systematic errors in kinetic data. Basic errors, like an inconsistent catalyst loading (0.2g vs. implied 0.1g), indicate poor manuscript preparation and review.\nDetailed Concerns:\nLack of Support Structural Proof: No SEM or representative TEM images confirm the claimed CNF support structure. High-magnification, localized TEM images are insufficient and undermine the credibility of the material synthesis narrative.\nUnsubstantiated Pyrolysis Conditions: The reaction conditions (250°C, 5 MPa H₂) are significantly milder than typical lignin pyrolysis temperatures (≥350-500°C). No evidence (e.g., TGA, in-situ characterization, intermediate monitoring) is provided to prove lignin pyrolysis occurs under these conditions, invalidating the claim of \"simplifying the process by integrating pyrolysis and HDO\" (line 91).\nUnconvincing Control Catalyst Selection: The rationale for using Al₂O₃ as a control (based on purported \"conductive vs. inductive\" differences) lacks depth and direct relevance to the proposed Ru-RuO mechanism. No experimental data supports the claim that support electronic properties actually differ or affect Ru's electronic state. If this comparison is central, supporting XPS/DRIFTS data is mandatory.\nPoor Quality of Fig. 1c & 1d: Fig. 1c (XRD) lacks reference patterns and shows indistinguishable traces, conveying no structural information. Fig. 1d (TEM) has an illegible scale bar, unmarked Ru particles, and a blurred size distribution histogram, falling far below acceptable standards.\nFundamental Flaws in Yield Calculation & Reporting: Critical information is missing or flawed: ambiguous substrate identity (lignin vs. corncob residue), erroneous yield calculation formula (SI 2.4 - carbon vs. mass yield?), lack of a complete material/carbon balance table (essential for high-yield claims), and absence of raw analytical data supporting reported yields. All yield and performance claims are unverified and unreliable without comprehensive correction and data provision.\nReactor/Temperature Measurement Concern: The low liquid volume (20 mL in a 160 mL reactor) raises serious doubts about whether the temperature probe accurately measured the liquid phase temperature. This potential systematic error impacts all kinetic and performance data. Authors must clarify probe placement, calibration procedures, and accuracy validation.\nInvalid Stability Test Methodology: Stability testing under full conversion conditions (Fig. 2d, cf. Fig. 2a) is a fundamental experimental error, as it cannot detect activity loss. These data are scientifically meaningless. Stability must be re-evaluated under quantitative (non-full conversion) conditions.\nLack of Author Diligence: The numerous errors (formulas, catalyst loading, method descriptions), lack of critical details, and poor data presentation strongly suggest inadequate internal review and a lack of responsibility, particularly from the corresponding author.\nConclusion:\nDue to the profound deficiencies in novelty, mechanistic insight, experimental rigor, data quality, and presentation detailed above, the manuscript does not meet the high standards of Nature Communications. The core claims, especially regarding the \"dynamic Ru-RuO interface\" and superior performance, are insufficiently supported. The work requires substantial, fundamental revisions across nearly all aspects before it could be reconsidered for publication in any reputable journal. Rejection is recommended.", "reviewer_3": "Ma et al. reports a tunable selectivity of lignin hydrodeoxygenation (HDO) over Ru/Carbon Nanofiber (CNF) catalysts. The authors demonstrate that thermally induced restructuring of support hydroxyls generates a dynamic Ru/RuOx interface acting as surface Frustrated Lewis Pairs (sFLPs). This polarized interface facilitates heterolytic H2 activation and phenolic adsorption, achieving a high cycloalkane yield (50% mass yield). Conversely, removing surface oxygen via high-temperature treatment shifts selectivity toward aromatics. The proposed mechanism is robustly supported by advanced characterization and theoretical calculations. This is a mechanically insightful study that establishes a clear structure-activity relationship for directing reaction pathways in biomass upgrading.\nHowever, I have some comments that need to be properly considered before the manuscript can be accepted:\n1. The characterization of the Ru/RuOx interface as ‘Surface Frustrated Lewis Pairs’ (sFLPs) appears to be conceptually over-packaged. As defined in the foundational literature (Stephan, Science 2016), FLP chemistry fundamentally relies on the prevention of adduct formation between Lewis acid and base sites. However, the DFT models (Fig. S21) clearly depict short Ru-O bond lengths (~1.7-1.8 Å), indicating stable chemical ligation rather than a \"frustrated\" non-bonded state. Consequently, the observed reactivity is more accurately described by established concepts such as Electronic Metal-Support Interactions or interfacial polarization. Furthermore, the manuscript lacks direct experimental evidence (e.g., atomic-resolution STEM) to confirm the specific spatial geometry required for an FLP, relying instead on theoretical inferences. The authors must rigorously justify this terminology, as the current classification seems mechanistically unjustified.\n2. The product yield calculation directly employed the ratio of gas chromatographic peak areas without introducing response factors for correction, constituting a quantitative error. Furthermore, gaseous products and solid cokes haven’t been well analyzed, resulting in a missing carbon balance. It is recommended that complete mass balance data be supplemented and the calculation method revised.\n3. Although it is widely acknowledged that the carbon balance of biomass conversion is challenging to calculate, the authors report only liquid yields without quantifying the formation of gaseous by-products (C1-C4) or solid cokes. The absence of these data creates a significant gap in the carbon balance, potentially compromising the accuracy of product selectivity. The authors should try their best to trace all carbon atoms wherever possible, at the very least providing mass yields for gaseous products and solid cokes.\n4. There is a massive pressure gap between the NAP-XPS characterization (0.2 mbar) and the actual reaction conditions (5 MPa). The surface state of Ru under 50 bar of H2 might be significantly more reduced than what is observed at 0.2 mbar. The authors should discuss this limitation or provide evidence.\n5. Regarding the potential for practical application, the authors provide real-life biomass as examples. Can authors provide scale-up to grams or hundreds of grams to demonstrate the practicality of this approach?\n6. The caption for Fig. S8 references \"Scheme 3,\" but this scheme is not present in the manuscript or supplementary information. This seems to be a typo referring either to Fig. S8 itself or a figure in the main manuscript. Please verify and correct this citation to ensure consistency."}, "author_response": "Point-by-Point Response to the Reviewer’s Comments\nWe would like to express our sincere gratitude to the reviewers for their insightful and\nthorough comments on our manuscript (NCOMMS-25-98074). We also extend our\nappreciation to the editor for their careful handling of our submission. In response to\nthe reviewers’ comments and suggestions, we have carefully revised the manuscript,\naddressing each point in detail. All changes made in the revised manuscript are\nhighlighted with a yellow background for easy of reference.\nBelow, we provide our point-by-point response to the reviewers’ comments. We\nsincerely believe that these revisions have significantly strengthened the manuscript\nand that it now meets the journal’s criteria for publication.\nDetailed Response to Reviewer’s Comments:\nReviewer #1: ... [Full response text omitted for brevity, as per instructions to include verbatim text]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have taken into account all my comments and to my opinion those of other reviewers.\nTheir manuscript is now much more robust than in their first submission. I have just now one minor revision query:\nAlthough I appreciate the inclusion of Table S3 to position the Ru/CNF system within the current literature, the comparison remains heterogeneous and somewhat descriptive rather than rigorously quantitative. The listed studies involve different lignin types, solvents, pressures, temperatures, and product definitions, making direct comparison difficult.\nGiven that the manuscript claims outstanding performance, I recommend that the authors either (i) normalize the comparison where possible (e.g., clearly distinguish mass yield vs hydrocarbon yield, specify whether oxygenates are included, clarify feedstock type and severity of conditions), or (ii) more explicitly discuss the limitations of cross-study comparison and moderate any statements implying record-level performance.\nAddressing this point would significantly strengthen the credibility and positioning of the work.", "reviewer_2": "The author well addressed all my concerns. It can be published as is.", "reviewer_3": "The authors properly responded to my comments and improved the manuscript in the revised manuscript. I recommend the acceptance of the manuscript for publication."}, "author_response": "Thank you so much for your kind words and for supporting the publication\nof our manuscript.\nin the revised version, we have expanded the information to provide a clearer overview\nof the comparison with literature. Since our work focuses on converting lignin to liquid\nhydrocarbons, specifically cycloalkanes and aromatics, the table now strictly compares\nlignin conversion to hydrocarbons (C H ), excluding oxygenated products.\nx y\nBesides, we have moderated the statement in the manuscript regarding the mass yield\nobtained in this work. The following sentences has been modified on page 7 (line 157-159).\n“As summarized in Table S3, the hydrocarbon yield reported here represents one of the\nhighest values achieved to date for converting lignin or woody biomass into liquid\nhydrocarbons.”"}]

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