Coordination restraint of Rh-Cu diatomic catalyst and C-H bond oxygen insertion for methanol synthesis

Domain: chemistry
Published: 2026-02-28
Editorial outcome: minor_revision

Abstract
Direct oxidation of methane to methanol is valuable, but reactive oxygen species over oxidize methane and lower selectivity. Here we report a Rh-Cu heteronuclear dual atom catalyst on nitrogen doped graphite that separates sites and restrains oxygen intermediates, enabling methanol through oxygen insertion into C-H bonds. An encapsulated pyrolysis strategy yields an Rh-Cu-N motif with a Rh-Cu distance of 2.42 Å. Electronic coupling between Rh and Cu induces charge polarization and improves reactant activation. Cu captures active oxygen species, suppressing excessive oxygen insertion and deep oxidation. The catalyst achieves up to 81% methanol selectivity and about threefold higher activity than a single atom Rh catalyst. In situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations reveal stable oxygen bridged intermediates, Rh-O-O-Cu and Rh-O-Cu. Rh stabilizes methyl intermediates, while Cu restrains nearby oxygen reactivity and directs oxygen insertion into C-H bonds to form methanol.

Introduction
The one-step oxidation of CH to CH OH is widely regarded as the "Holy Grail" during catalytic reactions[1-3]. CH molecules have a symmetrical tetrahedral structure and a high C−H bond dissociation enthalpy of 440 kJ·mol-1, which makes the C−H bond difficult to polarize[4, 5]. The highly reactive methyl radical (·CH ) is prone to further C−H bond cleavage or side reactions with active oxygen, which reduces methanol selectivity and the economic viability of the process[6, 7]. This contradiction of "efficient activation and high selectivity being hard to reconcile" essentially stems from the insufficient ability of catalysts to precisely control the reaction pathway. Active sites need to have an appropriate electronic state to polarize and break the C−H bond while providing multiple stable sites for carbon-hydrogen intermediates and oxygen-containing groups to inhibit over oxidation[8-10]. Traditional catalytic systems fail to meet these dual requirements simultaneously.

In recent years, single-atom catalysts (SACs) have attracted widespread attention due to their high atomic utilization and unique electronic structure[11, 12]. However, the singularity of their active sites limits the multidimensional control of reaction pathways[13, 14]. When oxygen is used as the oxidant, the single metal site is unable to flexibly regulate the utilization of active oxygen species, making it challenging to balance the dual requirements of methane activation and intermediate stabilization[15, 16]. To overcome these limitations, dual-atom catalysts (DACs) have shown breakthrough advantages through the spatial proximity and electronic coupling effects of dual metal sites[17]. The introduction of two active sites not only changes the energy band structure but also adjusts the d-center and work function[18]. The synergistic effect of the dual sites can achieve charge distribution polarization and an increase in electronic states near the Fermi level, significantly enhancing the adsorption capacity for reactant methane and intermediates[19-21], thereby improving methane activation.

More importantly, the adjacent dual-metal centers can cooperatively promote the adsorption and activation of both O and CH [22], while providing differentiated stable sites for carbon-hydrogen intermediates and oxygen-containing groups, fundamentally inhibiting deep dehydrogenation and overoxidation side reactions, which greatly aligns with the reaction demands for methane-to-methanol conversion[23-25].

Compared with single-metal centers, the introduction of a second metal facilitate the capture of active oxygen to form coordination restraints, weakening the excessive binding of active oxygen to reactive carbon-hydrogen intermediates during methane activation, thus effectively preventing deep oxidation and dehydrogenation[26, 27]. However, the controllable synthesis of DACs still faces challenges in precise construction at the nanoscale: first, the stability of the dual-metal sites is highly dependent on the metal-support interaction, requiring precise control of the bonding mode to prevent the migration or aggregation of active centers[28]; second, due to differences in the chemical properties and coordination environments of heteronuclear dual-metal atoms, they are difficult to arrange directionally and tend to form inactive aggregates[29].

Recent work on methane-to-methanol catalysts has largely relied on aqueous- or peroxide-mediated pathways. For example, Sun et al.[30] showed that Cu–CHA zeolites under trace O /H O conditions operate via rapid Cu2+/Cu+ cycling with CuOOH-type intermediates (with water acting as the apparent oxygen source), while Wang et al.[31] demonstrated that Cu–Pd/anatase systems can generate H O in situ and use Pd–Cu synergy plus halide/CO additives to promote methane activation and tune selectivity. These studies highlight the central role of liquid-phase/peroxide chemistry and support confinement in contemporary Cu-based MTM systems; however, the mechanistic picture under dry, gas-phase O conditions remains unclear. To address these challenges, this study innovatively adopts an encapsulated pyrolysis method to design and construct a heteronuclear Rh-Cu dual-metal catalytic system (Rh-Cu DACs). This catalyst, using a high surface area nitrogen-doped two-dimensional graphene support, successfully achieves the precise construction of spatially proximal and functionally cooperative Rh-Cu dual-metal centers. Kinetic studies show that when O is used as the oxidant, the CH conversion rate reaches 232 μmol/(mol ·s), with a methanol selectivity of 81%, approximately 3 times higher than that of the single Rh atomic catalyst. The products shift from CO -dominated to CH OH, demonstrating excellent catalytic performance and application prospects. Theoretical calculations further reveal that the RhCu bimetallic centers catalyze methane oxidation through a synergistic effect: the Rh sites attack and activate the C-H bonds of methane, breaking its stable tetrahedral structure; the Cu sites, on the other hand, form a coordination constraint on the adsorbed oxygen species, preventing their deep insertion during methane oxidation, thereby forming the key intermediate *CH O, which ultimately combines with active hydrogen to generate the target product CH OH.

Results
Structural Characterizations
The XRD results indicate that after introducing Cu and Rh species into ZIF-8, Rh@ZnCu-MOF still retains a crystallographic structure similar to ZIF-8 (Figure S14)[32]. In the XRD pattern of RhCu DACs, two broad carbon characteristic peaks are observed at approximately 22.8° and 43.6° (Figure S15)[33], and no characteristic crystalline peaks of metals, metal oxides, or metal carbides are detected, suggesting the absence of metal particle formation[34]. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) results show that the metal Rh and Cu are atomically dispersed and evenly distributed on the carbon substrate (Figures 1a, 1b, 1c). Due to the different scattering abilities of Rh (atomic number Z = 45) and Cu (atomic number Z = 29) elements in HAADF-STEM imaging, they appear with different brightness. Using this property, we can visually observe the formation of RhCu paired sites. Randomly selected different bright spots were measured, and the average distance between them was found to be 2.46 Å (Figures 1b , 1c ), indicating the possible formation of metal-metal bonds. Furthermore, quantitative analysis based on heatmap-processed STEM images reveals that the targeted Rh–Cu dual-atom sites account for approximately 67% of the total observed metal sites (Figure S12 and S13). Energy dispersive X-ray spectroscopy (EDS) results (Figure S19) show that Rh, Cu, N, and C are uniformly distributed throughout the structure. Furthermore, the metallic Rh in the Rh SACs catalyst exists in an atomic-scale dispersed state (Figures S18, S20).

The H -TPR results of Rh SACs and RhCu DACs exhibit distinct reduction behaviors (Figure 2a). Specifically, the Rh SACs catalyst shows a higher reducibility, with its reduction peak appearing in a lower temperature range. In contrast, the reduction peak of the RhCu DACs catalyst shifts to a significantly higher temperature, which indicates that the interaction between rhodium and copper modifies the reducibility of the species.

X-ray photoelectron spectroscopy (XPS) confirmed the coexistence of C, N, O, Rh, and Cu elements in the RhCu dual-atom catalyst (RhCu DACs). The C 1s high-resolution spectrum (Figure S21) reveals three carbon species corresponding to C–C, C=N, and C–N, demonstrating the successful doping of nitrogen atoms into the carbon framework. The C–C component in RhCu DACs is lower than that in Rh SACs and Cu SACs, indicating a reduction in the sp2 hybridization degree of its carbon structure. The N 1s spectrum (Figure S22) can resolve four nitrogen species: pyridine nitrogen, metal-coordinated nitrogen, graphitic nitrogen, and oxidized nitrogen[35]. The metal–N coordination peak (399.07 eV) is significantly higher in RhCu DACs than in Rh SACs, indicating the formation of stable M–N (M = Rh, Cu) structures in DACs, while pyridine and graphitic nitrogen together form a nitrogen-doped network, providing electronic environment support for metal anchoring. The high-resolution Rh 3d and Cu 2p spectra (Figures 2b, 2c) show that in the reduced state, Rh and Cu mainly exist in high oxidation states[36, 37], while in the reaction state, Rh tends toward a low oxidation state, and Cu further oxidizes. This charge behavior suggests that Rh remains more stable during the reaction, while Cu may be involved in the transfer of oxygen species.

Further analysis of the local coordination environment was performed using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES results show that the K-edge absorption energy (E ) of reduced Rh lies between metallic Rh0 and oxidized Rh, indicating that it is partially oxidized with a positive charge (Figures 2d). In the reaction state, E is closer to Rh0, indicating that Rh tends to maintain a reduced state during the reaction. Similarly, Cu exists as Cu+ in the reduced state (E between Cu0 and Cu2+), while it further oxidizes under reaction conditions (Figures 2e). EXAFS fitting (Figures 2f, Figures 2g, Figure S25) reveals that the main peak of Rh is located at 1.96 Å, corresponding to Rh–N coordination, and the secondary peak at 2.45 Å corresponds to Rh–Cu coordination, indicating that Rh atoms are anchored on nitrogen-doped carbon and form atomically adjacent Rh–Cu bimetallic structures. The main peak of Cu at 1.92 Å is attributed to Cu–N, and a secondary peak also appears at 2.45 Å, matching the Rh–Cu bond, further supporting the stable presence of Rh–Cu dual-atom sites. The small bond length difference (ΔR = 0.04 Å) between Rh–N and Cu–N suggests that the Cu–N bond has stronger covalent characteristics, while the 2.45 Å Rh–Cu metal bond reflects significant electronic coupling between the two atoms.

Raman spectroscopy (Figure 2h，Figure S24) further reveals the structural ordering and defect characteristics of the carbon support. RhCu DACs show distinct peaks at 1335 cm-1 (D-band) and 1580 cm-1 (G-band). The intensity ratio (I /I = 1.01) is higher than that of Rh SACs and the NC support. This indicates that the synergistic doping of bimetallic atoms disrupts the ordering of the sp2 carbon framework. It induces structural distortion. It also leads to the generation of sp3 defects. This significantly enhances the defect density. Electron paramagnetic resonance (EPR) spectroscopy (Figure 2i) was used to analyze the metal–support interactions and defect structures. The nitrogen-doped carbon support (NC) shows a strong EPR signal at g = 2.003, corresponding to nitrogen vacancy (NV)-related paramagnetic defects. After introducing Rh SACs and RhCu DACs, the EPR signal gradually weakens, indicating that metal atoms preferentially coordinate with nitrogen vacancies to form the Rh–Nx–Cu structure, effectively passivating free electrons and reducing defect states. In conclusion, RhCu DACs achieve dual-atom synergy by constructing a stable Rh–N–Cu–N bridged structure, demonstrating advantages in charge state regulation and defect structure optimization, which provide a structural foundation for their high activity in CH selective oxidation reactions.

Reaction Kinetics
Figure 3a shows that as oxygen partial pressure (𝑃 ) increases, the methane catalytic reaction exhibits distinct kinetic regimes. In Region A (𝑃 < 0.12 kPa), the CH conversion rate increases linearly with 𝑃 , consistent with first-order reaction kinetics. During this stage, oxygen adsorption on the active sites adopts a bridging configuration (Rh-O*-O*-Cu or Rh-O*-Cu). When 𝑃 exceeds the optimal value (Region B), the catalyst surface active sites gradually become saturated with adsorbed oxygen (Rh-O * + Cu-O *), weakening the interaction between the metal sites and the C-H bond, leading to a decrease in the methane conversion. Figure 3b illustrates the influence of oxygen partial pressure and reaction regime on the product distribution. The primary product in Region A is methanol. Comparing rate constants, CH OH selectivity reaches 81.0%, superior to some literature reports (Table S6). Compared to the inferior methane conversion efficiency and CO as the main product observed on single-atom Rh catalysts (Figure S27), this indicates that single active sites struggle to control the oxidation reaction pathway. The introduction of Cu sites provides a coordination confinement effect on the oxygen species, restricting the attack by molecular oxygen or active oxygen monomers on the C-H bond, resulting in methanol as the partially oxidized product. Additionally, as shown in Figure S28, Cu single-atom catalysts exhibit significantly lower methanol selectivity and methane activity compared to RhCu DACs, further confirming that Cu alone does not effectively promote methanol formation. After oxygen adsorption saturation (Region B), the regulatory role of Cu diminishes, causing a rapid decline in methanol yield and a sharp increase in CO production. Ultimately, under surface oxygen oversaturation coverage, the reaction rate becomes uncorrelated with oxygen pressure and proportional only to methane partial pressure (Figure S26).

Figure 3c presents the activation energy barriers for the two reaction regimes: Ea = 18.9 kJ/mol (Region A) and Ea = 89.7 kJ/mol (Region B). Under unsaturated oxygen adsorption on the active site surface, the bonding and coordination of oxygen are primarily facilitated by the Cu site. This allows the Rh site to possess stronger electronic potential for activating the C-H bond of methane, thereby lowering the activation barrier. Conversely, when the metal sites become saturated, their activation capability decreases, leading to a significant increase in the reaction activation energy.

Figure 3d reveals the variation of RhCu DACs catalytic activity with temperature and 𝑃 . Across different reaction temperatures, a positive correlation regime exists between the production rate and 𝑃 . As the reaction temperature increases, the optimal inflection point for oxygen partial pressure also shifts to higher values. This is likely because high activity rapidly depletes surface oxygen, maintaining an unsaturated surface oxygen state.

Figure 3e displays the kinetics of oxygen exchange and its isotope (18O ) on the dual-atom catalyst. With fixed 16O partial pressure (0.01 mbar), the 18O partial pressure was varied, and the oxygen exchange rate was measured at different temperatures (270–360 °C) using gas chromatography-mass spectrometry (GC-MS). The results show that the oxygen exchange rate constant increases with rising temperature, indicating that high temperature promotes oxygen dissociation and recombination processes. In addition, we evaluated catalyst durability under the optimized feed composition by a continuous 200 h time-on-stream test (Figure S48a): the CH conversion remained essentially constant over the entire run, demonstrating good operational stability of the Rh–Cu DAC, and post-reaction EXAFS (Figure S48b and Table S9) shows no significant change in the local coordination environment of the Rh–Cu sites, consistent with preservation of the dual-atom structure.

Figure 3f presents the kinetics of the CH –D exchange reaction on both Rh-Cu dual-atom and Rh single-atom surfaces. With methane pressure fixed at 5 kPa, the exchange rate increases similarly on both Rh-Cu DACs and Rh SACs as D pressure rises. This demonstrates that C-H bond activation primarily occurs on the Rh site. By contrast, Cu primarily tunes oxygen reactivity via its local coordination environment—for example, confined Cu-OH sites, Cu-O-Cu (oxo) dimers, or Cu atoms paired with a second metal—which stabilizes oxygenated intermediates and steers subsequent O-insertion/oxidation steps toward partial oxidation (methanol) rather than over-oxidation. This mechanistic picture is consistent with previous experimental and theoretical reports[38-40].

Reaction Mechanism Study
As shown in Figure S29, the two transition metal atoms in RhCu DACs are connected by a metal bond, and the pyridine-type nitrogen atoms in a tridentate coordination form a stable Rh–Cu–N configuration. Previous theoretical studies have indicated that this structure is the most stable form of a metal dimer embedded in N-doped graphene[41]. The geometric optimization results show that the Rh–Cu bond length is 2.42 Å, which is in excellent agreement with the EXAFS experimental results, verifying the validity of the model construction. In the structures of Rh SACs and Cu SACs, metal atoms in each catalyst are coordinated with four N atoms, respectively. The comparative results show that the RhCu DACs has a lower formation energy, thus exhibiting higher thermodynamic stability (Table S4). The differential charge density (Figure S30) and Bader charge analysis (Table S5) reveal that the Rh–Cu dimer undergoes electron transfer to the coordinated nitrogen atoms, while the nitrogen atoms also gain electrons from the carbon backbone, demonstrating a typical "dual electron donation" effect.

The differential charge density (Figure S30 ) and Bader charge analysis (Table S5) reveal pronounced interfacial charge redistribution in the Rh–Cu system. Specifically, electrons are transferred from the Rh–Cu dimer to the coordinated pyridine-type nitrogen atoms, and these nitrogen atoms in turn draw electron density from the surrounding carbon backbone. This cascade-like electron migration establishes a continuous electron-flow pathway—a typical “dual electron donation” effect. Such electronic reorganization both stabilizes the Rh–Cu–N₆ coordination framework and renders the metal centers relatively electron-deficient, thereby tuning their adsorption affinity and activation propensity toward reactant molecules and intermediates.

DFT simulation results reveal significant differences in O adsorption behaviors among RhCu DACs, Rh SACs, and Cu SACs (Figure S31). Fourier-transform infrared (FTIR) spectroscopy results (Figure 4a) show that under low oxygen pressure (0.01–0.10 kPa), the CH signal weakens as 𝑃 increases, and C–O vibration peaks at 1088 cm-1 (*CH O) and 1042 cm-1 (*CH OH) appear, confirming the correlation between *OCH and CH OH[42-47]. Under higher oxygen pressure (0.2–0.4 kPa), the *CH O vibration weakens, and the characteristic vibration peaks of CO (2377 and 2333 cm-1) become stronger, reflecting the oxygen coverage effect and the change in the reaction path (Figure S46a). Importantly, no *OCH vibrations are observed in Rh SACs and Cu SACs (Figure S46b, c), further confirming that *OCH is a key intermediate in the RhCu DAC-catalyzed CH oxidation to CH OH.

Specifically, both Rh SACs and Cu SACs stably bind O molecules through end-on adsorption modes (corresponding to Rh SACs-O -b and Cu SACs-O -a configurations, respectively). These configurations exhibit lower adsorption energies, representing their most stable adsorption states (Figure 4b). Orbital analysis indicates a distinct coupling effect between the d orbitals of the metal centers and the 2p orbitals of O (Figure S32). In contrast, RhCu DACs possess two types of O adsorption configurations. In the lateral configuration (RhCu-O -a), the Rh-O and Cu-O bond lengths are 1.97 Å and 2.06 Å, respectively, while the O-O bond length increases to 1.36 Å, which is significantly longer than that of free O molecules (1.21 Å). In the vertical configuration (RhCu-O -b), the Rh-O and Cu-O bond lengths are 1.93 Å and 2.50 Å, respectively, with an O-O bond length of 1.31 Å, indicating a lower degree of O activation. Among these, the RhCu-O -a configuration has the lowest adsorption energy (Figure 4b), making it the optimal O adsorption configuration for RhCu DACs. Electronic structure analysis (Figure S33) shows that RhCu-O -a transfers 0.63 electrons to O , with a higher electron transfer amount compared to Rh SACs-O₂-b and Cu SACs-O₂-a configurations. This makes O molecules electron-rich and the metal sites electron-deficient, significantly enhancing the metal-oxygen polarization effect.

The simulation results of reaction pathways for each catalyst are presented in Figures S35-S45. For RhCu DACs, under the optimal RhCu-O -a adsorption state, CH undergoes two consecutive adsorption and activation reactions to generate 2 molecules of CH OH. In the initial state (IS), CH reacts with *O to form *OCH and *OH (IM1). After overcoming an energy barrier of 0.76 eV (TS-2), it is converted to CH OH (IM2) with the release of bridge oxygen (Rh-O-Cu). The bridge oxygen then reactivates CH (IM4→IM5), ultimately generating a second molecule of CH OH, which desorbs (FS). The Gibbs free energy change (ΔG) of the overall reaction is < 0, indicating an exothermic process. In contrast, the pathway leading to CO formation requires overcoming a higher energy barrier (0.93 eV, TS-2’), making it a non-preferred pathway. This confirms that the RhCu-O₂-a configuration is more favorable for the selective oxidation of CH to CH OH (Figures 4c, 4d). In contrast, for Rh SACs and Cu SACs, under their optimal O adsorption configurations, the dominant pathway of CH oxidation is towards CO formation. Both catalysts generate *CH and *OOH intermediates, leading to the continuous dehydrogenation of *CH and its final conversion to CO .

The projected density of states (PDOS) analysis (Figure 5a) further confirms the electronic coupling characteristics: Rh 4d and Cu 3d orbitals significantly overlap, and both hybridize with the N 2p orbitals, indicating the formation of M–N bonds, consistent with the XPS results. Notably, the d-band center of Rh in RhCu DAC is located at -0.82 eV, which is significantly downshifted compared to -0.58 eV in Rh SACs. This phenomenon is attributed to the introduction of Cu, which breaks the high symmetry of Rh-N4 and forms a lower symmetry Rh-N3 structure. The downward shift of the d-band center directly weakens the adsorption strength of Rh towards oxygen species, laying the electronic structural foundation for the aforementioned "coordination constraint effect".

To further understand the reaction mechanism of CH selective oxidation to CH OH catalyzed by RhCu DACs, the following analysis was conducted. The adsorption energy calculation results (Figure 5b) show that, compared with Rh SACs and Cu SACs, RhCu DACs have lower adsorption energy for CH and higher adsorption energy for the product CH OH. The diatomic sites exhibit stronger activation ability for CH and, at the same time, are more conducive to the timely desorption of the product CH OH. PDOS analysis under O₂ adsorption (Figure 5c) shows that in the RhCu–O –a configuration, both the 4d orbitals of Rh and the 3d orbitals of Cu hybridize with the 2p orbitals of O, reflecting the synergistic adsorption effect of the bimetallic sites on O . After O adsorption, the d-band center of Rh shifts significantly downward (from –0.82 eV to –1.86 eV), while the d-band center of Cu shifts only slightly downward (from –2.28 eV to –2.73 eV). The significant downward shift of the Rh d-band center directly leads to a weakened adsorption effect on oxygen species (O , O), which is consistent with the weakly oxygen-adsorbed state (Region A) observed in the previous kinetic study and its characteristics (low activation energy barrier, high methanol selectivity).

Crystal orbital Hamilton population (COHP) analysis (Figure S34) further confirms that the interaction strength of the Rh–O bond (–ICOHP = –2.67 eV) is significantly weaker than that of the Cu–O bond (–ICOHP = –0.37 eV), indicating that the Rh–O bond has weaker covalency. Bader charge analysis (Figure 5d) shows that Rh loses 0.92 e⁻, Cu loses 0.71 e⁻, and the charge transfer is mainly concentrated on one of the O atoms (O1 gains 0.38 e⁻). Rh has stronger electronegativity, and the Rh–O bond has an obvious charge polarization effect, which is consistent with the COHP analysis results. In terms of methane activation and product selectivity control, Cu, relying on its stronger electron transfer ability, effectively exerts a "coordination constraint" effect on oxygen species, stabilizes adsorbed oxygen, and promotes the cleavage of the O-O bond; while Rh, due to the weak adsorption state of oxygen species after the downward shift of its d-band center, can more efficiently activate the C–H bond of methane, triggering its first step of dissociation. This is consistent with the conclusion of the previous CH –D exchange experiment, that is, the activation of the C–H bond is mainly undertaken by the Rh site.

On Rh SACs, the key intermediate CH –OOH has insufficient stability because Rh transfers limited electrons to it. The highly active OOH group is prone to decompose to produce H O and react with CH triggering deep dehydrogenation to generate the by-product CO (Figure 5e). In the RhCu DACs system (Figure 5f), the key intermediates are *OCH and *OH. Differential charge and Bader charge analysis show that Rh transfers 0.36 e⁻ to *OCH , and Cu transfers 0.54 e⁻ to *OH, indicating a stronger interaction between *OH and Cu. In the subsequent reaction, *OCH and *OH directly combine through H atoms to form CH OH and desorb, and *OCH₃ preferentially dissociates from the Rh site (rather than *OH dissociating from Cu to form H₂O). Therefore, the relative stability of the intermediate *OCH on the Rh site is the decisive factor for inhibiting the deep oxidation pathway and achieving high selective generation of CH OH.

In summary, the significant advantage of RhCu DACs over single-atom catalysts stems from the unique electronic coupling effect between the bimetallic sites and the charge polarization effect caused by it. This synergistic effect achieves precise functional division: the Cu site implements "coordination constraint" on oxygen species through strong electron transfer, stabilizes adsorbed oxygen, and regulates its activity; the Rh site maintains a weak adsorption state for oxygen species due to the significant downward shift of its d-band center, thus enabling efficient activation of the C-H bond of CH . The synergy of the two sites not only optimizes the adsorption and activation of reactant molecules but, more importantly, stabilizes the key intermediate *OCH and drives its efficient conversion to the target product CH OH. This mechanism effectively inhibits the excessive insertion of active oxygen and the deep combination of reaction intermediates with active oxygen, fundamentally avoiding the occurrence of over-oxidation (generating CO ).

Methods
Synthesis of RhCu DACs
This study successfully constructed a Rh–Cu dual-atom synergistic catalytic system using a host-guest confinement strategy (Figure 1d). The preparation of the precursor and Rh3+ confinement encapsulation process begins with the self-assembly of Zn2+, Cu2+, and 2-methylimidazole as ligands to synthesize ZnCu-MOF (200 mg) as the precursor. Using the heteronuclear metal nodes and the multilevel pore structure of the MOF (with a pore size of approximately 0.55 nm, Figure S15), the MOF was ultrasonically dispersed in 30 mL of n-hexane. Next, 1000 μL of RhCl ·xH O solution (10 mg·mL-1) was added dropwise to the dispersion and sonicated for 30 minutes at room temperature, followed by stirring for 2 hours. This process encapsulates Rh3+species, smaller than 0.5 nm, within the MOF cavities. The encapsulated sample was then centrifuged, vacuum-dried overnight at 333 K to obtain Rh@ZnCu-MOF. The next step involves high-temperature pyrolysis and structural reconstruction. The Rh@ZnCu-MOF was placed in a nitrogen atmosphere and pyrolyzed at 1173 K for 2 hours. During this process, Zn volatilized preferentially, inducing the formation of a gradient pore structure in the skeleton and driving the MOF to restructure into nitrogen-doped graphene. The remaining Cu2+ ions strongly coordinate with the pyridine-type nitrogen atoms in the carbon substrate, forming stable Cu-N anchoring sites. The Rh3+ species pre-constrained in the cavities of the MOF migrate toward the adjacent Cu-N sites under the stress induced by the contraction of the carbon layer. Ultimately, the synergistic interaction between Rh and Cu atoms forms a stable RhCu-N heteronuclear bimetallic active configuration on the carbon substrate. To validate the dual-metal synergistic effect, Rh single-atom catalysts (Rh SACs) were prepared using ZIF-8 without Cu2+ as the precursor, following the same confinement encapsulation and pyrolysis steps. Meanwhile, Cu single-atom catalysts (Cu SACs) were obtained by directly pyrolyzing ZnCu-MOF without encapsulating Rh3+.

Characterization
The phase composition and crystal structure of the catalysts were analyzed using a powder X-ray diffractometer (XD-6 model, Cu Kα). The specific surface area and pore size distribution were measured using a physical adsorption instrument (Micromeritics ASAP 2020 PLUS HD88) under liquid nitrogen, with calculations based on the BET equation. The microscopic structure and elemental distribution were characterized by transmission electron microscopy (FEI Talos F200X, 200 kV) combined with HAADF-STEM and EDS, and high-resolution structural images were obtained using a Hitachi HD3000C microscope (200 kV). X-ray photoelectron spectroscopy (XPS) was performed on a SPECS analyzer (MCD-10 analyzer, Al Kα, hv = 1486.6 eV). In situ X-ray absorption spectroscopy (XAS) was conducted at the Beijing Synchrotron Radiation Facility 8-ID and 8-BM beamlines, encompassing XANES and EXAFS information to analyze the metal's electronic state and local structure. In situ Fourier-transform infrared (FTIR) spectroscopy was measured using a PerkinElmer FTIR spectrometer (MCT detector, resolution 0.2 cm-1), combined with CO adsorption studies to examine catalytic active sites. Raman spectroscopy was conducted on a confocal Raman system (Xplora Plus, HORIBA Scientific, excitation source: 532 nm) and calibrated with a silicon wafer for Raman shifts. Hydrogen temperature-programmed reduction (H -TPR) experiments were carried out on a Micromeritics Auto Chem II 2920 (USA) to investigate the reduction behavior and hydrogen adsorption capacity of the catalyst. Electron paramagnetic resonance (EPR) spectroscopy was performed using an EMX plus 10/12 (Bruker, Germany) instrument to detect defect information, such as nitrogen vacancies.

Performance Evaluation
This study focuses on the catalytic performance of single-atom and dual-atom catalysts, evaluated using a self-built fixed-bed tubular reactor system for gas-solid reactions (Figure S8). The experimental setup uses a quartz tube reactor with an outer diameter of 10 mm and a wall thickness of 1 mm. The reaction gas mixture consists of fuel gas (CH /Ar), synthetic air (O /N ), and high-purity N as a diluent. To ensure accurate gas flow control, a soap film flow meter is used to calibrate the feed gas flow, and the calibrated values are applied during the experiment to maintain precision. A total of 100 mg of catalyst sample is uniformly mixed with quartz sand at a 1:1 mass ratio and loaded into the isothermal section of the reactor, with quartz wool isolating both ends. Prior to the reaction, the catalyst undergoes pretreatment under a high-purity N flow of 100 sccm: the temperature is ramped to 200 °C at a rate of 5 °C/min and maintained for 1 hour to remove surface-adsorbed impurities. Subsequently, the catalyst is reduced in a 5 vol% H /Ar mixed gas flow (50 mL·min⁻¹) at 200 °C for 1 hours to generate the active metal sites. After the reduction, the temperature is increased to the target reaction value at the same rate and held for 1 hour to fully activate the catalyst. After the reduction, the temperature is increased to the target value at the same rate and held for 1 hour to activate the catalyst. Upon completion of activation, the reactant gases, mixed in controlled proportions, are introduced into the system with a total flow rate of 100 sccm. The effluent gases are collected after condensation in a cold trap and quantitatively analyzed by gas chromatography (GC). By optimizing the catalyst particle size (100–200 mesh) and total gas flow rate (100 sccm), both internal and external diffusion limitations are eliminated, ensuring that the reaction is controlled by intrinsic kinetics (Figure S9). Product analysis is performed using an Agilent 8890 GC system equipped with a hydrogen flame ionization detector (FID) and a thermal conductivity detector (TCD), with high-purity N as the carrier gas. TCD is connected to an HP-MOLESIEVE column to detect CH , CO , and Ar; FID is connected to an HP-PLOT/Q capillary column for the quantitative analysis of CH OH (methanol) products. All gaseous products are analyzed in online mode.

Calculational method
All density functional theory (DFT) calculations in this study were performed using the Vienna Ab-initio Simulation Package (VASAP) version 5.4.4[48]. The exchange-correlation energy was described using the Perdew-Burke-Ernzerhof (PBE) function within the generalized gradient approximation (GGA)[49], and the electron-ion interactions were described using projector augmented-wave (PAW) pseudopotentials[50, 51]. A plane-wave energy cutoff of 450 eV was used in the calculations. The geometry optimization was carried out with strict convergence criteria, including a maximum force of 0.05 eV/Å, maximum pressure of 0.1 GPa, maximum displacement of 1×10-3 Å, single-atom energy change no greater than 1×10-5 eV, and a self-consistent field (SCF) convergence threshold of 1×10-5 eV[52]. Transition states were searched using the climbing image elastic band method (CI-NEB)[53], and the transition state structures were validated through imaginary frequency calculations. For the construction of the computational models, the heteronuclear bimetallic system embedded in N-doped graphene (NC) was based on a p(6×6) single-layer graphene structure, while the single-metal system embedded in the NC substrate used a p(5×5) single-layer structure. A vacuum layer of 15 Å was applied in the vertical direction of the two-dimensional monolayer to eliminate interactions between periodic images. K-point sampling was performed using the Monkhorst-Pack grid, with geometry optimization carried out on a 3×3×1 grid, Gibbs free energy calculations on a 4×4×1 grid, and projected density of states (PDOS) analysis on a 6×6×1 grid[54]. The electronic structure calculations were analyzed for charge transfer using the Bader charge analysis method, and post-processing was performed using VASPKIT[55].

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript (NCOMMS-25-64080) reports a heteronuclear Rh–Cu dual atom catalyst synthesised via an “encapsulation—high-temperature carbonisation” strategy and proposes a synergistic mechanism in which “Cu captures and constrains oxygen while Rh activates the C–H bond.” The catalyst achieves high methanol selectivity in the reaction of CH4 oxidation to CH3OH. This study is well-structured, with experimental results and DFT calculations complementing each other. Overall, I recommend the publication of this paper after minor revision. My specific comments are as follows:\n1.The calculation and characterisation parameters should be standardised and clearly presented. For instance, Section 2.4 of the main text states that the plane-wave cutoff energy is 450 eV, whereas Supplementary Information S1.4 lists it as 400 eV. Please ensure consistency across all related parameters and specify the exact settings used for all DFT calculations. Note also that Supplementary Information S24 contains Chinese text, which should be corrected.\n2.The manuscript emphasises the high selectivity of the catalyst, but carbon balance data are not provided. Without a rigorous carbon balance analysis, the reported selectivity values may be overestimated. Please supply carbon balance data, such as a product distribution table.\n3.Catalyst stability is critical for potential industrial applications. Please provide time-on-stream data for long-term reactions or recycling tests to demonstrate the stability of the catalytic performance. In addition, please include post-reaction EXAFS or other characterisation results to confirm that the dual-atom sites remain intact without aggregation or loss.\n4.In some key discussions, additional references would strengthen the arguments. For example, when stating that “dual-atom/heteronuclear sites can modulate adsorption behaviour and reaction selectivity,” relevant studies on dual-atom catalysts should be cited. Similarly, when assigning infrared bands, it would be appropriate to reference previous literature or computational data for support.\n5.Although the catalyst is described as dual-atom structured, some isolated single-metal sites can still be observed in the electron microscopy images. Please clarify the proportion of dual-atom sites.\n6.In Figure 2, the in situ XPS spectra of Rh 3d and Cu 2p indicate that under reaction conditions, Rh tends to a lower oxidation state while Cu becomes more oxidised. Is this charge redistribution related to the reaction mechanism? Please analyse the process using DFT calculations and discuss the possible reasons for these valence changes.\n7.Please explain the calculation method used to determine the ID/IG ratio in the Raman spectra. Why does the dual-atom catalyst exhibit a higher ID/IG value than the other samples, while its EPR signal is the lowest?\n8.In the kinetic analysis (Section 3.2), Regions A and B are associated with different rate-controlling steps. Please provide a detailed discussion about this conclusion, specifically in the methane oxidation pathway.", "reviewer_2": "In this manuscript, the authors utilized an encapsulated pyrolysis strategy to synthesize Rh–Cu heteronuclear dual-atom catalysts over nitrogen-doped graphite carbon supports. A methanol selectivity of 81% could be achieved for methane partial oxidation by molecular oxygen when the partial pressure of dioxygen was optimized. However, the proposed mechanism of the manuscript is not convincing. The synthesis method is not novel, and the origin of the construction of spatially proximal and functionally cooperative Rh-Cu dual-metal remained unclear. Moreover, the performance of catalysts for direct methane to methanol needs to be further confirmed. At this stage, I could not recommend it be published in Nature Communications. The detailed questions are as follows:\n1. It is confusing that Rh and Cu mainly exist in high oxidation states in the reduced state, while Rh tends toward a low oxidation state, and Cu further oxidizes in the reaction state. As discussed in the mechanism section, molecular oxygen would chemisorb at Rh and Cu during methane oxidation followed by O-O bond breaking. Thus, both metallic centers should be oxidized. The authors should clarify the issue.\n2.The coordination numbers of Rh and Cu and the construction of spatially proximal and functionally cooperative Rh-Cu dual-metal are crucial for the reaction. The key strategy for controllable synthesis of targeted coordination number of Rh-Cu dual-metal should be analyzed.\n3. Except for Rh-Cu dual metal sites, Rh particles, Rh-Rh, Cu-Cu dual-metal sites also exist. More evidence should be provided to confirm the catalytic performance from Rh-Cu dual-metal sites.\n4. The reaction mechanism just discussed about methane oxidation towards methanol. The deep oxidation of methanol should be further discussed. Following the presented methane activation process in the manuscript, the radical-like mechanism indicates that methanol would be easier to be oxidized, indicating deep oxidation.\n5. The Hessian should be calculated to confirm the reliability of the TS. In addition, the electronic state of Rh and Cu should be performed to confirm the reliability of the model.\n6.Direct methane to methanol generally faces the conversion-selectivity trade-off (ACS Catal. 8(2018), 6894). The high selectivity could be easily obtained by the lower conversion that is generally lower than 20%, even though the methane conversion is not prior to methanol deep oxidation (Angew. Chem. Int. Ed. 56(2017), 16464). The conversion data should be provided.", "reviewer_3": "The authors report an encapsulated pyrolysis strategy and successfully synthesized Rh–Cu heteronuclear dual-atom catalysts (RhCu DACs) on nitrogen-doped graphite carbon supports for the methane-to-methanol reaction. The Rh-Cu-N6 structural catalyst features an optimal bond length of 2.42 Å and exhibits a charge polarization effect at the bimetallic sites, enabling efficient activation of reactant molecules. This bimetallic active site effectively inhibits the complete oxidation of methane, achieving 81% methanol selectivity. In-situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations were employed to investigate the molecular adsorption and methanol desorption mechanisms at the Rh-Cu bimetallic centers. While this work presents a highly interesting mechanism for selective methanol synthesis from methane and O2 under dual-atom catalysis, the following critical issues must be addressed prior to acceptance.\n1. The authors synthesized Rh-Cu dual-atom catalysts (RhCu DACs) via a host-guest confinement strategy, involving pyrolysis at 1173 K for 2 h. Such extreme thermal treatment inevitably induces significant compositional changes. Crucially, the elemental composition and mass fractions of Rh and Cu in the final catalyst are not provided. The author uses mol/(molcat·s) as the unit when measuring the CH4 turnover rate and C product formation rate. The molar amount of the catalyst cannot be determined when its elemental composition is unknown. Although the author explicitly mentions catalyst mass in Section 2.3 Assessment, the expression of catalyst molar quantity is still used. The calculation process is needed. To better compare with other studies using CH4 and O2 (eg. J. Am. Chem. Soc. 2024, 146, 23, 16039–16051; Chem, 2021, 7(6): 1557-68; Nat Catal 5, 45–54 (2022)), the author should also use the common units mol/(gcat·h) or mol/(molmetal·h).\n2. Stability testing and time-on-stream performance evaluation are critically required to assess catalyst durability and active sites. The author should provide comprehensive post-reaction characterization to verify the structural integrity of active species, including: AC HAADF-STEM to confirm atomic dispersion retention of Rh/Cu; XPS analysis to track oxidation state changes and coordination environment evolution; ICP-OES quantification of Rh/Cu loadings in spent catalysts to evaluate metal leaching; Leached metal concentrations in solution to identify potential active component loss. These data are essential to substantiate claims the stability under reaction conditions.\n3. The credibility of XPS data requires validation through the inclusion of raw data points. Crucially, the absence of quantified Rh/Cu mass fractions raises concerns that low metal loadings could compromise XPS signal quality due to insufficient photoelectron counts. Additionally, an inconsistency exists between catalyst pretreatment protocols: Section 2.3 Assessment explicitly states that catalyst activation occurred under N2 at 200 °C without reduction, yet XPS analysis in Section S1.3 Characterizations describes characterization under reduction (Figure 2b&c). If H2 reduction was deliberately performed for XPS samples, the rationale and detailed experimental procedure (e.g., reduction temperature, duration, gas flow) should be explicitly documented. Finally, the labels 'Red' and 'Rex' in Figures 2d&e lack definition beyond the vague caption 'metal foils, reactive state, and oxygen-covered state'. Precise descriptions of all sample states should be provided in the figure legends.\n4. Although the authors clearly characterized the presence of RhCu dual-atom sites on the catalyst surface, rhodium species may undergo agglomeration or oxidation during reaction if unstable. The authors must provide time-dependent profiles of methane conversion and product yield to clarify whether active sites undergo structural changes under operating conditions. Notably, Figures S18 and S19 (at 50 nm scale) reveal apparent clusters. To definitively attribute activity to atomically dispersed sites, the authors should either compare catalytic performance with Rh/Cu nanoparticle (NP) controls, or conduct acid leaching experiments to remove unstable surface species while preserving stable framework-embedded Rh-Cu dual atoms, followed by re-evaluation of activity. This validation is critical to exclude contributions from metallic clusters or nanoparticles to the reported performance.\n5. The possibility of Cu acting as an active site requires urgent verification. Although the authors demonstrated Rh-mediated methane activation on Rh SACs in Figure S25, this only confirms the capability of Rh–O bonds to activate methane, but does not exclude the potential role of Cu–O sites. While the CH4–D2 exchange experiment in Figure 3f attempts to identify the metal site responsible for C–H activation, the reaction likely occurs on Metal–O oxygen atoms rather than through direct methane-metal contact. Consequently, this experiment is insufficient to conclusively identify active sites. Critically, in-situ FTIR data (Figures 4a & S43) show no *OCH3 intermediate on Rh-only sites, whereas its emergence on RhCu DACs implies that Cu may be essential for *OCH3 formation. A pivotal question arises: Could Cu–O configurations generate Cu–*OCH3 intermediates and drive subsequent reaction pathways? As referenced in Table S6, Cu atoms are established active sites for CH4 + O2 reactions (Chem, 2021, 7(6): 1557-68). Analogous phenomena were reported on Cu–Pd/Anatase catalysts (Energy Environ. Sci., 2024,17, 9122-9133), where Pd efficiently activates methane but Cu determines methanol selectivity, with the synergy between Cu–Pd further promoting methanol formation. Therefore, this work must urgently supplement characterization and computational studies to clarify the role of Cu species.\n6. Full-range FTIR spectra should be provided in the Supporting Information. Notably, the absorption peak at 3015 cm-1 is assigned to the ν3 asymmetric stretching of CH4, but the expected rotational fine structure (a series of sharp split peaks) is absent, likely due to insufficient spectral resolution. We recommend using higher resolution for such critical data. The *OCH3 signal at ~2822 cm-1—a key intermediate fingerprint observable on both Rh and Cu sites (ACS Nano 2025, 19, 32981; JACS 2023, 145, 8609)—requires explicit assignment to determine whether this signal is present on the catalyst.\n7. The authors state that the Rh–N bond has stronger covalent characteristics than Cu–N (1.92 Å) due to a shorter bond length (ΔR=0.03 Å). However, the provided bond lengths contradict this claim, as Cu–N is actually shorter. Generally speaking, shorter bond lengths correspond to stronger covalent bonds. Please verify and correct the bond length values (1.95 Å in line 203, but 1.96 Å in Figure 2f) and ΔR calculation. If the data is accurate, provide additional evidence to support the assertion of stronger covalency in Rh–N.\n8. While the reported Rh–Cu bond length of 2.45 Å from EXAFS analysis is compelling, the fitting methodology requires clarification. Was phase correction applied during Fourier transform processing? And provide quantitative fitting metrics including: R-factor, S02, fitting ranges in R-space (ΔR) and k-space (k-range, Δk).\n9. The citation of the aforementioned references contributes to an enhancement in performance benchmarking, mechanistic discussions, and theoretical validation.\nMinor issues:\n10. Text in Figure 2c is blurred.\n11. Square symbols appear in the legend of Figure S24.\n12. Figure 4a at line 289 is mislabeled; it should be Figure 5a.\n13. Period missing at line 376.\n14. Supporting Information lacks page numbering."}, "author_response": "We sincerely thank the reviewer for the thoughtful and constructive comments regarding our manuscript, particularly those concerning the experimental details, data consistency, in-situ characterization, catalytic mechanism, and interpretation of electronic states. We have carefully studied each of your suggestions and made targeted revisions throughout the manuscript and Supplementary Information. Specifically, we standardized all computational and experimental parameters to ensure consistency; provided complete carbon balance and product distribution data; added long-term stability testing and post-reaction EXAFS and ICP analyses to confirm structural integrity; included quantitative STEM statistics to clarify the proportion of dual-atom sites; expanded the DFT discussion to explain the observed valence evolution of Rh and Cu; and supplemented key discussions and spectral assignments with relevant literature references. These revisions have made the manuscript more rigorous, transparent, and coherent. We are deeply grateful for your insightful feedback, which has greatly improved the quality and clarity of our work, and we sincerely welcome any further guidance you may have."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all the concerns proposed by the reviewer; hence, I recommend publishing this manuscript as it is.", "reviewer_3": "The authors have adequately addressed my concerns, and I am satisfied with the revised manuscript. The work is timely and provides useful insights for the direct, selective oxidation of methane with oxygen to methanol. I support publication in Nature Communications."}, "author_response": ""}]

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