Asymmetric Pt_1C_3-Pt_1O_1C_3 catalytic pairs for efficient transfer hydrogenation of azobenzene

Domain: neuroscience
Published: 2026-02-02
Editorial outcome: major_revision

Abstract
Atomic catalytic pairs (CPs) have shown great promise in driving multi-step catalytic transformations, yet the influence of spatial arrangement and coordination asymmetry on homonuclear CPs remain poorly understood. Herein, we construct atomically dispersed homonuclear Pt-Pt CPs with asymmetric Pt1C3-Pt1O1C3 coordination anchored on reduced graphene oxide. By precisely tuning the spacing between the adjacent Pt1C3-Pt1O1C3 CPs to approximately 5.3 Å, the catalyst achieves an exceptional turnover frequency of 27,218 h-1 for transfer hydrogenation of azobenzene via ammonia-borane hydrolysis, surpassing benchmarking catalysts by more than an order of magnitude. The Pt1C3-Pt1O1C3 CPs separated by 5.3 Å can facilitate co-adsorption of sterically hindered intermediates and at the same time the asymmetric Pt1C3-Pt1O1C3 coordination enables facile hydrogen shuttling and barrier-suppressed hydrogenation. These synergistic effects enhance the overall azobenzene hydrogenation efficiency. Our findings uncover a fundamental spatial design principle for atomically precise homonuclear asymmetric CPs, offering new opportunities for sustainable and efficient fine chemicals synthesis.

Introduction
Catalysis lies at the heart of transformative innovations in sustainable chemical synthesis and energy conversion technologies. Single-atom catalysts (SACs) have emerged as a powerful platform due to their maximal atom utilization efficiency and highly tunable electronic properties1–4. However, the intrinsic single-site nature limits their efficacy in complex chemical reactions that require cooperative multi-site interactions5–10. To address this challenge, heterogeneous catalytic pairs (CPs) comprising two adjacent active atoms offer a promising solution, which can enable cooperative multiple substrate adsorption/activation, and thereby unlock unconventional reaction pathways11–14. For example, heteronuclear Ir-P CPs supported on carbon exhibit outstanding hydrogen oxidation activity, arising from the spontaneous H2 dissociation on Ir and the cooperative *H desorption by *OH generated on oxophilic P via spontaneous water dissociation15. Similarly, heteronuclear Rb-Ir CPs embedded in a poly(heptazine imide) framework achieve selective photooxidation of glycerol to hydroxypyruvic acid16. Despite these advances, homonuclear CPs remain largely unexplored, particularly regarding the impact of inter-CP spacing and coordination asymmetry on their catalytic performance. A fundamental understanding of these factors is critical to unlock the full potential of CPs, enabling enhanced catalytic efficiencies and novel reaction pathways.

In this work, we prepare atomically precise homonuclear Pt-Pt CPs with asymmetric Pt1C3-Pt1O1C3 coordination, anchored on reduced graphene oxide (termed as Pt1C3-Pt1O1C3/rGO). By finely tuning the inter-distance between the Pt1C3-Pt1O1C3 CPs to an optimal ~5.3 Å, we achieve a turnover frequency (TOF) of 27,218 h-1 for transfer hydrogenation of azobenzene, exceeding benchmarking catalysts by more than an order of magnitude. First-principles calculations reveal that the asymmetric Pt1C3-Pt1O1C3 CPs separated by a distance of 5.3 Å modulate the electronic structure of the Pt sites, facilitate the co-adsorption of sterically hindered intermediates, and enable an energetically favorable hydrogen shuttling and hydrogenation pathway. These synergistic effects collectively enhance the overall transfer hydrogenation efficiency.

Results
The Pt1C3-Pt1O1C3/rGO catalysts with tunable Pt loadings (0.5–7.5 wt.%) were synthesized by an interface confinement method, leveraging the diatomic structure of the Pt precursor and SiO2-induced physical confinement17. Specifically, the (ethylenediamine)iodoplatinum(II) dimer dinitrate precursor was adsorbed onto graphene oxide (GO), followed by amorphous SiO2 layer deposition via tetraethylorthosilicate hydrolysis and thermal treatment. NaOH etching then removed the SiO2 layer, yielding well-dispersed Pt-Pt CPs on rGO. X-ray diffraction (XRD) analysis (Supplementary Fig. 1) confirmed the reduction of GO to rGO, while atomic force microscopy (Supplementary Fig. 2) demonstrated that Pt1C3-Pt1O1C3/rGO retained a two-dimensional (2D) morphology with atomic thickness.

Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images (Fig. 1a–j) revealed atomic dispersion of Pt atoms (bright spots) on rGO surface at varying Pt loadings (0.5 wt.%, 1.0 wt.%, 2.5 wt.%, 5.0 wt.%, and 7.5 wt.%, as confirmed by inductively coupled plasma-optical emission spectroscopy), with no observable formation of Pt clusters or nanoparticles (NPs). Magnified HAADF-STEM images (Fig. 1k–o) clearly confirmed the presence of Pt dimers even at low Pt loadings (Fig. 1k), highlighted by solid orange boxes. As the Pt loading increased, the density of Pt dimers correspondingly rose, accompanied by a noticeable reduction in the inter-distance between Pt dimers (the inter-distance is defined as the shortest distance between two Pt atoms from adjacent Pt dimers, namely each Pt atom belongs to a different Pt dimer unit). Notably, the minimal inter-distance between Pt dimers significantly decreased from ~18.1 Å at 0.5 wt.% Pt loading to ~4.6 Å at 7.5 wt.% Pt loading (Fig. 1p–t).

To elucidate the coordination environment of Pt-Pt CPs, atomic-resolution electron energy-loss spectroscopy (EELS) and 4D-STEM were performed. EELS measurements of an individual Pt-Pt CP simultaneously detected the C-K, O-K, and Pt-M signals, indicating that Pt atoms are coordinated with both C and O (Fig. 1u–w). Complementary 4D-STEM charge-density mapping revealed two adjacent positively charged Pt centers, each coordinated by negatively charged atoms (Fig. 1x). Strikingly, the coordinating atom adjacent to the lower-left Pt displayed a markedly stronger negative charge density than other Pt-coordinating atoms, consistent with the Pt1C3-Pt1O1C3 pair owing to the higher electronegativity of O relative to C. In addition, HAADF-STEM images reconstructed from 4D-STEM datasets consistently resolved abundant Pt-Pt CPs (Supplementary Fig. 3). A small fraction of isolated single Pt atoms was also observed in the HAADF-STEM images, likely due to pair dissociation or projection overlap in the 2D images18. At high Pt loadings (≥8.5 wt.%), small crystalline Pt NPs began to appear (Supplementary Fig. 4)19. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirmed the homogeneous distribution of C, O, and Pt in the Pt1C3-Pt1O1C3/rGO catalysts (Fig. 1y).

To examine the electronic property of Pt, X-ray photoelectron spectroscopy (XPS) was performed20,21. All Pt1C3-Pt1O1C3/rGO samples exhibited similar Pt 4f7/2 and 4f5/2 peaks at ~73.0 eV and ~76.3 eV, indicating a stable Pt2+ oxidation state (Fig. 2a and Supplementary Fig. 5)22–24. In contrast, Pt NPs on rGO (PtNPs/rGO), synthesized via wet-chemical reduction of H2PtCl6 (Supplementary Figs. 6 and 7), displayed zero-valent Pt states with Pt 4f7/2 and 4f5/2 peaks at 71.4 eV and 74.7 eV, respectively. To further determine the Pt atomic coordination in Pt1C3-Pt1O1C3/rGO, X-ray absorption spectroscopy (XAS) measurements were conducted. The normalized Pt L3-edge X-ray absorption near-edge structure (XANES) spectra showed that the white-line of all Pt1C3-Pt1O1C3/rGO samples was positioned between that of Pt foil and PtO2 (Fig. 2b), indicating the oxidative state of Pt in Pt1C3-Pt1O1C3/rGO samples, in good alignment with the XPS results25. Fourier-transformed (FT) k3χ(k) extended X-ray absorption fine structure (EXAFS) spectra for Pt foil, PtO2, and Pt precursor revealed dominant peaks at 2.48 Å, 1.68 Å, and 2.45 Å, corresponding to Pt-Pt, Pt-O, and Pt-I first-shell scattering, respectively (Fig. 2c)26–28. In contrast, Pt1C3-Pt1O1C3/rGO samples showed only one prominent peak at around 1.56 Å, which could be attributed to the Pt-C/O first-shell scattering, suggesting that Pt atoms were atomically dispersed on the rGO support, matching well with the HAADF-STEM observations.

To determine the exact atomic structure of Pt1C3-Pt1O1C3/rGO, we proposed several possible models informed by the above results (Supplementary Fig. 8). The most plausible structure consists of two Pt atoms anchored on adjacent benzene rings in the rGO network (Fig. 2d–f). Specifically, one Pt atom coordinates with three carbon atoms (Pt1C3 moiety), while the other coordinates with three carbon atoms and one oxygen atom (Pt1O1C3 moiety), forming an asymmetric paired Pt1C3-Pt1O1C3 configuration. This assignment is corroborated by the charge-density distribution observed in 4D-STEM (Fig. 1x). The average Pt-C/O bond length (approximately 2.11 Å) and the coordination number (around 3.5) of this model agree well with the EXAFS fitting results (Supplementary Table 1). Notably, the average distance between adjacent Pt atoms in a Pt1C3-Pt1O1C3 pair is approximately 5.0 Å, aligning closely with the statistical data derived from HAADF-STEM imaging. All other proposed structural models failed to replicate the key experimental features, as evidenced by discrepancies in their XANES simulation spectra (Supplementary Fig. 9) and variations in bond lengths and coordination numbers (Supplementary Table 2).

In summary, converging evidence from precursor-directed synthesis, HAADF-STEM, atomic-resolution EELS, 4D-STEM charge-density mapping, electron localization function (ELF) calculation (Supplementary Fig. 10), and XAS, together with XANES simulations and EXAFS fittings, unequivocally establishes the asymmetric Pt1C3-Pt1O1C3 pair. For clarity, we also present a merged comparison of the atomic structure, 4D-STEM charge-density mapping, and ELF analysis (Supplementary Fig. 11), which supports the proposed Pt1C3-Pt1O1C3 configuration. Notably, charge-transfer analysis reveals a charge difference of only ~0.07 e- between the two Pt sites, below the threshold for a detectable XPS shift and consistent with prior reports15,29. This is corroborated by ELF calculation and 4D-STEM charge-density mapping results, both of which demonstrate two adjacent Pt centers in comparable charge environments, confirming the persistence of a uniform Pt2+ oxidation state in Pt1C3-Pt1O1C3/rGO.

Hydrogenation of azobenzene
The transfer hydrogenation of azobenzene (C6H5N=NC6H5) to hydrazobenzene (C6H5NH-HNC6H5) via in-situ ammonia borane (NH3BH3) hydrolysis was conducted to evaluate the catalytic performance of Pt1C3-Pt1O1C3/rGO in complex transformations (Fig. 3a). The overall stoichiometric reaction involves one NH3BH3 molecule, three C6H5N=NC6H5 molecules, and three H2O molecules, yielding three C6H5NH-HNC6H5 products30. First of all, we optimized the reaction conditions, including solvent, temperature, and reaction time, and revealed methanol as the optimal solvent at a reaction temperature of 80°C (Supplementary Figs. 12 and 13). As shown in Fig. 3b, at a constant total Pt mass, increasing the Pt loading of Pt1C3-Pt1O1C3/rGO from 0.5 wt.% to 5.0 wt.% leads to a pronounced enhancement in the yield of C6H5NH-HNC6H5, rising from 26.6% to as high as 99.8%. However, at a Pt loading of 7.5 wt.%, the yield decreased to 33.6%. It is found that the TOF value strongly depends on the inter-distance between the adjacent Pt1C3-Pt1O1C3 pairs, peaking at ~5.3 Å with a maximum TOF of 27.2×103 h–1 (Fig. 3c). On the other hand, both rGO and conventional PtNPs/rGO (5.0 wt.%) catalysts displayed poor performance in hydrogenation of azobenzene (Fig. 3d). Quantitative comparison of TOF and yield with previously reported catalysts further highlights the superior activity of the Pt1C3-Pt1O1C3/rGO (Supplementary Table S3). Besides excellent catalytic activity, the Pt1C3-Pt1O1C3/rGO catalyst also showed outstanding catalytic stability, which could maintain high catalytic activity throughout 10 hydrogenation cycles with negligible activity decay (Fig. 3e). The Pt1C3-Pt1O1C3/rGO morphology was well preserved after 10 cycles of hydrogenation of azobenzene (Supplementary Figs. 14 and 15), highlighting the structural stability of the catalyst. Besides azobenzene, a broad range of azobenzene derivatives could be hydrogenated via in situ NH3BH3 hydrolysis, producing the corresponding products with high efficiency (Fig. 3f). The reaction exhibited excellent tolerance for various substituents, including halogen and alkyl groups at different positions, yielding hydrazobenzene derivatives in remarkable yields (Supplementary Figs. 16–25). This broad applicability highlights the versatility of the Pt1C3-Pt1O1C3/rGO catalyst for catalyzing the hydrogenation reactions.

Theoretical studies
To provide a deep understanding of the transfer hydrogenation of azobenzene taking place on Pt1C3-Pt1O1C3, we performed ab initio density functional theory calculations. The Pt1C3-Pt1O1C3 motif was embedded into four defective graphene supercells to simulate different Pt1C3-Pt1O1C3/rGO catalysts with varying Pt1C3-Pt1O1C3 densities. These models, labeled as Models 1-4 (Supplementary Fig. 26), were assigned to the Pt1C3-Pt1O1C3/rGO catalysts with Pt loading of 1.0 wt.%, 2.5 wt.%, 5.0 wt.%, and 7.5 wt.%, respectively, based on the close agreement between the theoretical inter Pt1C3-Pt1O1C3 pair spacings and the experimental values obtained from HAADF-STEM (Fig. 1q–t).

We first computed the projected density of states and the d-band center for each Pt in Pt1C3-Pt1O1C3 across different structures (Fig. 4a and Supplementary Fig. 27). At an inter-spacing between adjacent Pt1C3-Pt1O1C3 pairs of 9.38 Å, both Pt sites show similar d-band centers at around −1.11 eV. As the inter-pair distance decreases from 7.24 Å to 5.45 Å, the d-band center of Pt1C3 increases to −0.04 eV, while that of Pt1O1C3 remains at −1.37 eV. At an inter-pair distance of 5.23 Å, this trend reverses, with Pt1O1C3 showing a slightly higher d-band center (−0.33 eV) than Pt1C3 (−0.62 eV). Notably, Model 3 shows a highest d-band center of −0.04 eV at the Pt1C3 site, indicative of an optimal electronic structure for reactant adsorption31–33, as confirmed by adsorption energy calculations (Fig. 4b and Supplementary Fig. 28). Specifically, the Pt site with a Pt1C3 configuration for Model 3 displays an optimal adsorption energy of −1.53 eV for NH3BH3 and −0.43 eV for C6H5N=NC6H5. H2O adsorption is moderate at both sites, indicating that water does not severely compete with the main reactants for Pt sites (Supplementary Table S4).

Further analysis reveals that local structural distortion is the primary cause of varying electronic properties among different models. As shown in Supplementary Fig. 26, the Pt atom in Pt1C3 for Model 3 exhibits the greatest protrusion, driven by the balance between Pt and internal energy. Bader charge and charge differential analyses reveal that 0.3 e- can be transferred from NH3BH3 to Pt1C3 (Supplementary Fig. 29), while 0.1 e- can be transferred from C6H5N=NC6H5 to Pt1C3 site (Supplementary Fig. 30).

These findings demonstrate that the inter-distance between adjacent Pt1C3-Pt1O1C3 pairs and the asymmetric Pt1C3-Pt1O1C3 configuration can synergistically modulate the electronic properties of Pt sites. The Pt1C3-Pt1O1C3/rGO catalyst with Model 3 configuration not only offers spatially optimized sites for co-adsorption of sterically hindered C6H5N=NC6H5 and NH3BH3 (Fig. 4c and Supplementary Fig. 31), but also ensures optimal adsorption strength for these reactants. The azobenzene hydrogenation pathway on Pt1C3-Pt1O1C3/rGO catalyst with Model 3 configuration was further studied by density functional theory (DFT), which involved the following three key steps (Supplementary Fig. 32): 1. Transfer hydrogenation of C6H5N=NC6H5 by NH3BH3 hydrolysis, producing one C6H5NHHNC6H5 and leaving NH3BH3* at the Pt1C3 site (from co-adsorption to intermediate V). 2. Transfer hydrogenation with NH3BH3* and H2O to form NH3BOH* and a second C6H5NHHNC6H5 (from intermediate VI to XII). 3. Transfer hydrogenation with H2O as the proton donor to form NH3B(OH)3* and a third C6H5NHHNC6H5 (from intermediate XIII to final desorption). The overall pathway and associated energy barrier are clearly depicted in Fig. 4d, as well as in Supplementary Figs. 33 and 34. Additionally, an alternative pathway was also considered, in which the azo group in azobenzene bonded directly to the Pt1C3 site (Supplementary Fig. 35). However, this pathway was energetically unfavorable due to the strong co-adsorption strength and a high energy barrier of 1.08 eV for step III.

The reaction coordinate simulations identified the first H abstraction as the rate-determining step with the highest energy barrier of 1.73 eV (step I). Climbing nudged elastic band (cNEB) simulations further suggested that the first H abstraction process could be broken into two steps by introducing a new H* intermediate at a neighboring C atom (Supplementary Fig. 36)34. By adopting this two-step H abstraction mechanism, initialized with co-adsorption of NH3BH3 and C6H5N=NC6H5, the first H atom from BH3 group of NH3BH3 transferred to a neighboring C atom to form a new H* intermediate with an energy barrier of 0.83 eV. Subsequently, the H* intermediate at the C site was transferred to the Pt atom at the Pt1O1C3 site with an energy barrier of 0.90 eV. The reaction barrier heights were predicted to be 1.98 eV and 0.93 eV for TS3 and TS4, respectively. Importantly, NH3BH3 hydrolysis was also evaluated on Pt1C3-Pt1O1C3/rGO with different Pt loadings (Supplementary Table S5), and Model 3 was found to catalyze NH3BH3 hydrolysis and generate active H* intermediates with barriers comparable to those for the first hydrogenation step of azobenzene.

Comparatively, we have conducted theoretical calculations on other Pt-based catalysts for transfer hydrogenation of azobenzene, including Pt1C3-Pt1C3/rGO, Pt1O1C3-Pt1O1C3/rGO, Pt/rGO, and PtNPs (Supplementary Fig. 37). In Pt1C3-Pt1C3/rGO, where all Pt sites adopt the symmetric Pt1C3 configuration, and the dissociated H atoms bind strongly to the Pt sites, with an adsorption energy of −8.26 eV (Supplementary Fig. 38). This strong binding inhibits H-atom transfer, making Pt1C3-Pt1C3/rGO ineffective for hydrogenating C6H5N=NC6H5. Pt1O1C3-Pt1O1C3/rGO exhibits universally weak adsorption for azobenzene, NH3BH3, and H2O (Supplementary Table S6). Pt/rGO with Pt1O1C3 configuration exhibits weak binding to NH3BH3, with an adsorption energy of −0.15 eV (Supplementary Fig. 39). Pt NPs are suboptimal due to the excessive adsorption strength of NH3BH3 (−2.16 eV) and C6H5N=NC6H5 (−2.20 eV) (Supplementary Fig. 40), leading to catalyst poisoning.

We further established a volcano relationship by correlating ΔEads (azobenzene) with the reaction energy of the first N=N hydrogenation for various models (Fig. 4e). In this plot, Pt (111) resides on the over-binding branch, whereas Pt1C3-Pt1O1C3/rGO (Models 1, 2, and 4), Pt1O1C3-Pt1O1C3/rGO, Pt1C3/rGO, and Pt1O1C3/rGO fall on the weak-binding branch. The first hydrogenation of azobenzene becomes energetically disfavored for Pt1C3-Pt1C3/rGO. By contrast, the Pt1C3-Pt1O1C3/rGO (Model 3) sits at the apex of the volcano plot, exhibiting a balanced adsorption strength coupled with the lowest reaction energy for the first hydrogenation step. These calculation results highlight the crucial asymmetric paired Pt1C3-Pt1O1C3 configuration with an optimized inter-pair spacing, which can effectively promote the transfer hydrogenation of azobenzene.

Methods
Materials
(Ethylenediamine)iodoplatinum(II) dimer dinitrate (96%, Sigma-Aldrich), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.9%, Adamas), graphene oxide (GO, 0.5–5 µm, XFNANO), tetraethylorthosilicate (TEOS, SiO2 > 28.4%, Sigma-Aldrich), ammonia solution (25–28%, Sigma-Aldrich), sodium hydroxide (NaOH, 98%, Aladdin), sodium borohydride (NaBH4, 98%, Energy chemical), azobenzene (97%, Macklin), borane-ammonia complex (NH3BH3, 97%, Macklin), and other reagents were purchased from commercial suppliers and used without further purification.

Synthesis of Pt1C3-Pt1O1C3/rGO catalysts
One hundred milligrams of GO was dispersed in 50 mL of deionized water through sonication, followed by the addition of 200 mL of ethanol. A specific amount of Pt2 precursor was dispersed in 10 mL of ethanol and subsequently added to the GO ethanol solution. The mixture was stirred at room temperature for 12 h. The TEOS (2.75 mL in 25 mL ethanol) was then added dropwise to the mixture and stirred for an additional 13 h. The resulting product was centrifuged, washed thoroughly with water and ethanol, and dried under vacuum. The dried powder was calcined at 400 °C under N2 atmosphere for 2 h. Finally, silicon species in the sample were removed by etching with 3 M NaOH solution at room temperature for 3 h, yielding the final Pt1C3-Pt1O1C3/rGO catalyst. The Pt loading in the Pt1C3-Pt1O1C3/rGO catalyst was controlled by varying the amount of Pt2 precursor used in the synthesis. The Pt loadings in catalysts were confirmed by inductively coupled plasma-optical emission spectroscopy.

Synthesis of PtNPs/rGO catalysts
PtNPs/rGO catalysts were synthesized using a procedure similar to that of Pt1C3-Pt1O1C3/rGO, with H2PtCl6·6H2O as the Pt precursor and NaBH4 as the reducing agent. Specifically, an amount of H2PtCl6·6H2O was dispersed in 10 mL of ethanol and subsequently added to the ethanol solution containing GO. The mixture was stirred at room temperature for 13 h, followed by the addition of NaBH4 as the reducing agent. The subsequent steps, including washing, drying, calcination, and etching with 3 M NaOH to remove silicon species, were identical to those used for Pt1C3-Pt1O1C3/rGO synthesis. The final product was denoted as PtNPs/rGO.

Reaction conditions for azobenzene and derivative hydrogenation
In an oven-dried 10 mL Schlenk tube equipped with a magnetic stir bar, azobenzene (0.2 mmol) and ammonia borane (0.4 mmol) were sequentially added. The tube was sealed with a rubber septum, evacuated, and refilled with N2 gas. Then, 0.01 mol.% Pt1C3-Pt1O1C3/rGO catalyst was introduced. For example, in the case of Pt1C3-Pt1O1C3/rGO (5 wt.%), 8 mg of catalyst was ultrasonically dispersed in 50 mL of deionized water, and 0.5 mL of this dispersion, together with 1 mL of solvent (typically methanol), was transferred into the reaction tube. A nitrogen balloon was attached as shown in Supplementary Fig. 41. The mixture was stirred at 80 °C for 22 min, then cooled to room temperature. The catalyst was separated by centrifugation, washed twice with ethyl acetate (2 × 5 mL), and the combined filtrate was dried over anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure at 40 °C using a rotary evaporator. The residue was dissolved in deuterated chloroform (CDCl3), and 1,1,2,2-tetrachloroethane (TCE) was added as an internal standard for nuclear magnetic resonance (NMR) analysis. Optimization studies of solvent, temperature, and time identified methanol at 80 °C as the optimal condition (Supplementary Figs. 12 and 13). Reactions involving substituted azobenzene derivatives were performed under identical conditions except that the reaction time was extended to 120 min. The crude products were purified by flash column chromatography on silica gel using hexane/ethyl acetate as the eluent to yield the corresponding hydrogenated products.

Catalytic performance evaluation
Catalytic performance was quantified using azobenzene hydrogenation over the Pt1C3-Pt1O1C3/rGO (5 wt.%) catalyst as a representative case. Product yields were determined from NMR data according to Eq. (1):
Yield (%) = n(obtained products) / n(initially added substrates) = 99.8%

The conversion and selectivity were calculated according to Eqs. (2) and (3), respectively:
Conversion (%) = 1 - n(remained substrate) / n(initial substrate) * 100% = 100%
Selectivity (%) = n(obtained product) / (n(initial substrate) - n(remained substrate)) * 100% = 99.8%

The turnover frequency (TOF) was calculated at the 22-min time point from NMR spectra according to Eq. (4):
TOF = moles of reactant converted / (mole of total active sites * reaction time) = 27218 h-1

Characterization
Wide-angle X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus Powder X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA) at room temperature. Aberration-corrected high-annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings were collected on an aberration-corrected Thermo Fisher Spectra 300 system equipped with a cold field emission gun and an ASCOR probe corrector at 300 kV. The metal loadings in all the samples were measured by inductively coupled plasma-optical emission spectroscopy (Perkin Elmer Avio 500, UK). Atomic force microscopy (AFM) topography measurements were performed with an XE-100 AFM (Park Systems) in noncontact mode. The X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) measurements of Pt L3-edge were carried out at the beamline of the Singapore synchrotron light source. Pt foil was used for the energy calibration, and all samples were measured under transmission mode at room temperature. Si(111) double crystal monochromator was used to filter the X-ray beam. EXAFS oscillations χ(k) were extracted and analyzed using the Demeter software package. Energy-loss spectroscopy (EELS) spectrum imaging was conducted using a Gatan Quantum ER spectrometer with an exposure time of 2 ms per pixel. Four-dimensional (4D) STEM datasets were acquired on a JEOL ARM200F microscope equipped with an ASCOR aberration corrector and a Merlin Medipix3 1R pixelated detector (256 × 256 pixels, 6-bit depth), with a convergence semi-angle of approximately 21 mrad. Charge-density mapping was carried out via Gauss’s law by computing the divergence of the inverted electric-field distribution reconstructed from the 4D-STEM datasets using the open-source LiberTEM software35. To minimize long-range background features, a Gaussian filter with a kernel size of two pixels was applied36.

Theoretical calculations
The spin-polarized density functional theory (DFT) calculations were performed utilizing the Vienna Ab Initio Simulation Package (VASP, version 5.4.4)37,38. The generalized gradient approximation in the Perdew-Burke-Ernzerhof format and the plane wave basis kinetic energy cutoff of 500 eV were used in all calculations39,40. The structural relaxations were carried out until the Hellmann-Feynman force on each atom was less than 0.01 eV/Å, and the energy convergence criterion was set to 10-4 eV. The gamma point sampling in the Brillouin zone was employed in all calculations. Along the direction perpendicular to the 2D graphene substrate sheet, a vacuum of 15 Å is included in the supercell to avoid the interaction of neighboring images. The reaction barrier was determined by the climbing-image nudged elastic band method34.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors proposed the asymmetric Pt1C3-Pt1O1C3 catalytic pairs for efficient transfer hydrogenation of azobenzene.\nHowever, some results and discussion are inconsistent, and several questions need to be further addressed. The current\nversion of the manuscript does not meet the high standard of nature communications. Below are the detailed comments:\n1. Structural identification of the Pt₁C₃–Pt₁O₁C₃ pair site remains inconclusive. The authors claim to have constructed\nasymmetric Pt catalytic pairs (Pt₁C₃–Pt₁O₁C₃), yet the provided HRTEM and XAFS data only confirm the presence of\natomically dispersed single Pt atoms. These techniques alone cannot directly demonstrate the existence of a homonuclear\nPt–Pt pair site composed specifically of one Pt₁C₃ and one Pt₁O₁C₃ center. It is recommended that the authors consider\nusing advanced techniques such as atomic-resolution EELS or other electron spectroscopy methods to better resolve the\nspatial and electronic structure of the proposed pair site.\n2. The DFT results suggest a notable difference in the electronic environments of the Pt₁C₃ and Pt₁O₁C₃ centers. However,\nthe XPS data shows a uniform Pt²⁺ oxidation state for all Pt species, which is inconsistent with the proposed electronic\nasymmetry.\n3. The catalytic tests are not described with sufficient details. For instance, only provided the 0.01 mol% catalyst loading in\nreactor. The authors should clearly describe the experimental setup, including the exact amount of catalyst used (in mg or\nµmol), and the basis for calculating yield and TOF. The formula used for TOF and yield calculations should be explicitly\nprovided, along with representative data on reaction conversion and selectivity.\n4. The catalytic performance (TOF and yield) should be compared quantitatively with those of other reported catalysts for\nazobenzene hydrogenation. A fair and objective benchmark comparison is necessary to substantiate the claim of superior\nactivity.\n5. DFT adsorption energy calculations appear inconsistent and potentially invalid. The adsorption configurations used in\nDFT simulations are inconsistent across models: Model 2 involves N atom adsorption, while Models 1, 3, and 4 show\nadsorption through C atoms. Comparing adsorption energies from fundamentally different configurations is not meaningful.\nAdditionally, Supplementary Fig. 26 suggests that the distances between the azobenzene molecule atom and the Pt active\nsites are too long to indicate chemisorption. Moreover, the significant structural distortion of the Pt₁C₃–Pt₁O₁C₃ site upon\nadsorption introduces large uncertainties in adsorption energy. The authors should reassess the modeling approach and\nensure structural stability and meaningful interaction between the adsorbate and the active site.\n6. The proposed reaction pathway lacks clarity and chemical plausibility. The DFT-derived reaction mechanism does not\nclearly delineate the elementary steps of azobenzene hydrogenation. It remains unclear how the N=N bond interacts with\nthe active site if it is not properly adsorbed. In particular, the origin and migration pathway of hydrogen atoms to the\nazobenzene substrate is insufficiently described. Without a clear depiction of N=N activation and hydrogen transfer, the\nproposed reaction pathway is difficult to accept.", "reviewer_2": "In this manuscript, the authors reported that atomically dispersed homonuclear Pt1-Pt1 catalytic pairs (CPs) on reduced\ngraphene oxide. By precisely tuning the spacing between the adjacent Pt1-Pt1 CPs to approximately 5.3 Å, the catalyst\nachieves an exceptional turnover frequency of 27,218 h-1 for transfer hydrogenation of azobenzene via ammonia-borane\nhydrolysis, surpassing benchmarking catalysts by more than an order of magnitude. They suggest that Pt1-Pt1 CPs\nseparated by 5.3 Å can facilitate co-adsorption of sterically hindered intermediates and at the same time an enable a low-\nbarrier hydrogen shuttling pathway, thus enhancing the overall azobenzene hydrogenation efficiency.\nWhile one major concern of this work is about the claim of the asymmetric Pt1C3-Pt1O1C3 coordination, which is just based\non XANE/XAFS simulation. What is the content of oxygen and Pt-to-O atomic ratios in these samples?\nThe other key concern is about the theoretical part. The rate determining step has an energy barrier as high as 1.73 eV (step\nI). This could question about the feasibility of reaction at 80  C. Therefore, the reviewer can’t recommend it for acceptance at\nthis time.\nBesides the above two key concerns, there are also other comments as follows for the consideration.\n1. It seems that the Pt1O1C3 configuration is just speculator, both azobenzene and NH3BH3 adsorb on the Pt1C3 sites. In\nfact, the adsorption of NH3BH3 on the Pt1C3 is much stronger than azobenzene? Under reaction conditions, NH3BH3\nadsorbs on Pt1C3, while azobenzene will adsorb on Pt1O1C3 sites due to the competitive adsorption? How about water\nadsorption on these two Pt sites?\n2. The reaction of ammonia-borane hydrolysis should be also carried out on these sets of Pt catalysts with different loadings.\nThe authors might like to correlate the activity in ammonia-borane hydrolysis and the activity in transfer hydrogenation of\nazobenzene\n3. If it forms the asymmetric Pt1C3-Pt1O1C3 coordination structure, the samples should have two different oxidation states\nfor Pt, is there any evidence from XPS?\n4. Figure 4 is not well presented. It can be hardly see the details in Figure 4d and e.", "reviewer_3": "This manuscript reports enhanced azobenzene hydrogenation efficiency with optimized Pt loading (5 wt.%) on graphene\noxide. The central claim revolves around the existence of asymmetric Pt1C3-Pt1O1C3 catalytic pairs and their systematic\ninfluence on catalytic properties as a function of inter-pair distance. Although the topic is relevant and potentially interesting,\nI have significant concerns regarding the clarity and validity of the provided evidence. Below, I detail the main issues:\nAmbiguity in Confirming Pt-Pt Pairs (Fig. 1):\nAlthough the authors claim the existence of Pt-Pt pairs with an interatomic distance of approximately 5 Å, the presented\nevidence appears inconclusive. Specifically, Figures 1f–j reveal many single Pt atoms along with potential Pt-Pt pairs. In\nFigures 1k–o, Pt-Pt pairs indicated by orange boxes are densely packed, raising doubts about accurate identification and\ndistance measurements, especially at higher Pt densities (e.g., Fig. 1n–o). This selective consideration of Pt-Pt pairs likely\nintroduces bias, undermining the reliability of conclusions drawn about the significance of Pt-Pt pairs relative to single Pt\natoms or clusters.\nAtomic Configuration and Stability (Fig. 2):\nThe authors propose that the Pt1C3-Pt1O1C3 structure (Fig. 2f) is the most probable configuration based on comparisons of\nexperimental data (XANES, EXAFS) to simulations. However, justification for selecting this specific configuration is weak.\nSupplementary Figure 9 (XANES spectra) does not convincingly differentiate the proposed configuration from others, as\nmultiple configurations appear similarly plausible. Additionally, Supplementary Tables 1–2 compare\nprimarily Pt-Pt pair configurations, neglecting potentially relevant configurations such as isolated single Pt atoms. Moreover,\nthe authors didn't assess relative energies of these configurations systematically, casting further doubt on their stability and\nplausibility.\nValence State Consistency:\nThe manuscript insufficiently addresses the valence state of Pt atoms within the proposed Pt pairs. The overlaid plots in Fig.\n2b obscure clear interpretation, and the similarity to PtO2 raises additional questions. Given that the authors report\nsignificantly different d-band centers for the paired Pt atoms, one would naturally expect variations in Pt valence states. A\ncareful and explicit valence state analysis is necessary to support the proposed configuration convincingly.\nSimplified Inter-Pair Distance Models (Fig. 4):\nThe theoretical models (Models 1–4) use simplified assumptions about inter-pair spacing. While models with greater inter-\npair distances (>7 Å) seem acceptable, shorter inter-pair distances (around 5 Å) complicate the identification of active\ncatalytic sites. The authors neglect possible roles of alternative configurations (e.g., PtC3-PtC3 or PtOC3-PtOC3 pairs),\nthereby weakening their central argument regarding the specificity and catalytic relevance of Pt1C3-Pt1O1C3 pairs.\nInsufficient Explanation of Catalytic Activity Trends:\nThe rationale provided for maximal catalytic activity at approximately 5.3 Å inter-pair spacing, based on d-band center\nanalysis, lacks sufficient theoretical rigor. Without establishing a clear volcano relationship or explicitly considering the\nkinetics of azobenzene adsorption and hydrogenation, the interpretation presented does not fully support the observed\nmaximal turnover frequency.\nGiven these critical issues, the manuscript in its current form does not convincingly support its conclusions. Major revisions\naddressing these fundamental points would be required to reconsider publication."}, "author_response": "1 Point-by-point response letter\n1 3 1 1 3\n5 hydrogenation of azobenzene. However, some results and discussion are inconsistent, and several\n6 questions need to be further addressed. The current version of the manuscript does not meet the\n7 high standard of nature communications. Below are the detailed comments:\n8 1. Structural identification of the Pt C -Pt O C pair site remains inconclusive. The authors claim\n1 3 1 1 3\n9 to have constructed asymmetric Pt catalytic pairs (Pt C -Pt O C ), yet the provided HRTEM and\n1 3 1 1 3\n11 alone cannot directly demonstrate the existence of a homonuclear Pt-Pt pair site composed\n12 specifically of one Pt C and one Pt O C center. It is recommended that the authors consider using\n1 3 1 1 3\n13 advanced techniques such as atomic-resolution EELS or other electron spectroscopy methods to\n14 better resolve the spatial and electronic structure of the proposed pair site.\n16 we performed electron energy-loss spectroscopy (EELS) and four-dimensional scanning\n17 transmission electron microscopy (4D-STEM) at the atomic scale to further substantiate the\n18 assignment of asymmetric Pt C -Pt O C pairs.\n1 3 1 1 3\n20 (532 eV), and Pt-M (M : 2122 eV; M : 2202 eV) signals, indicating that Pt atoms are coordinated\n5 4\n21 with both C and O (Fig. 1u-w). Because spatial elemental mapping using EELS remains\n22 challenging due to beam-induced damage during extended exposure, we additionally employed\n23 4D-STEM to probe the atomic species and asymmetric structure. Atomic-scale charge density\n24 mapping revealed two adjacent positively charged Pt centers, each coordinated by negatively\n25 charged atoms (Fig. 1x). Strikingly, the coordinating atom adjacent to the lower-left Pt displayed\n26 a markedly stronger negative charge density than that adjacent to the other Pt, consistent with the\n27 proposed Pt C -Pt O C pair motif (Fig. 2f) owing to the higher electronegativity of O relative to\n1 3 1 1 3\n29 electron localization function (ELF) calculation result using VASP (Supplementary Fig. 10),\n30 providing compelling and mutually consistent evidence for the Pt C -Pt O C pair structure. For\n1 3 1 1 3\n31 clarity, we also present a merged comparison of the atomic structure, charge density mapping, and\n1 3 1 1 3\n34 4D-STEM charge-density mapping at atomic resolution, ELF calculations, and XAS, together with\n1 3 1 1 3\n36 pair.\n39 identification of the Pt C -Pt O C sites.\n1 3 1 1 3\n40\n1 1\n43 image reconstructed from the 4D-STEM dataset, resolving a Pt -Pt pair on rGO. Right: Projected\n1 1\n44 charge-density distribution derived from the inverted divergence of the center of mass (dCoM),\n45 highlighting the spatial positions of C, O, and Pt atoms.\n46\n1 3 1 1 3\n49 accumulation/localization. The asymmetric electron localization around the two adjacent Pt\n50 centers highlights the distinct coordination environments, with stronger electron localization at the\n1 3 1 1 3\n52\n54 mapping, and ELF analysis, collectively supporting the proposed Pt C -Pt O C configuration.\n1 3 1 1 3\n56 transfer, with red and blue regions denoting positive and negative charge accumulation,\n57 respectively. In the ELF map, blue indicates electron depletion/delocalization, while red reflects\n58 electron accumulation/localization.\n59 2. The DFT results suggest a notable difference in the electronic environments of the Pt C and\n1 3\n1 1 3\n61 which is inconsistent with the proposed electronic asymmetry.\n63 indeed reveal an electronic asymmetry between the Pt C and Pt O C centers, this difference does\n1 3 1 1 3\n64 not translate into a measurable change in oxidation state. To substantiate this, we have performed\n65 the charge transfer analysis, which indicates a charge difference of only ~0.07 e- between the two\n67 consistent with prior reports (Angew. Chem. Int. Ed. 64, e202421277 (2025); Nat. Catal. 6, 916-\n68 926 (2023)), which similarly show that the Co-N P and Co-N catalysts contain exclusive Co2+\n4 1 4\n69 species, and that the Ir atoms in Ir /NG and Ir -P /NPG remain in the same oxidation state.\n1 1 1\n1 3 1 1 3\n72 density mapping from 4D-STEM reveals two adjacent Pt centers with similar surrounding charge\n73 distributions (Fig. 1x). Therefore, the experimentally observed uniform Pt2+ state is fully consistent\n74 with our theoretical results.\n76 charge-density mapping from 4D-STEM, and the corresponding references and descriptions into\n77 the revised manuscript to explicitly demonstrate that Pt species are stabilized in a uniform Pt2+\n78 oxidation state.\n79 3. The catalytic tests are not described with sufficient details. For instance, only provided the 0.01\n80 mol% catalyst loading in reactor. The authors should clearly describe the experimental setup,\n81 including the exact amount of catalyst used (in mg or µmol), and the basis for calculating yield\n82 and TOF. The formula used for TOF and yield calculations should be explicitly provided, along\n83 with representative data on reaction conversion and selectivity.\n85 in an oven-dried 10 mL Schlenk tube equipped with a magnetic stir bar, azobenzene (0.2 mmol)\n86 and ammonia borane (0.4 mmol) were sequentially added. The tube was sealed with a rubber\n87 septum, evacuated, and refilled with N gas. Then, 0.01 mol.% Pt C -Pt O C /rGO catalyst was\n2 1 3 1 1 3\n88 introduced. For example, in the case of Pt C -Pt O C /rGO (5 wt.%), 8 mg of catalyst was\n1 3 1 1 3\n89 ultrasonically dispersed in 50 mL of deionized water, and 0.5 mL of this dispersion, together with\n90 1 mL of solvent (typically methanol), was transferred into the reaction tube. A nitrogen balloon\n91 was attached as shown in Supplementary Fig. 40.\n93 was separated by centrifugation, washed twice with ethyl acetate (2 × 5 mL), and the combined\n94 filtrate was dried over anhydrous Na SO . After filtration, the solvent was removed under reduced\n2 4\n95 pressure at 40 °C using a rotary evaporator. The residue was dissolved in deuterated chloroform\n96 (CDCl ), and 1,1,2,2-tetrachloroethane (TCE) was added as an internal standard for NMR analysis.\n3\n98 condition (Supplementary Figs. 12 and 13). Reactions involving substituted azobenzene\n99 derivatives were performed under identical conditions except that the reaction time was extended\n100 to 120 min. The crude products were purified by flash column chromatography on silica gel using\n101 hexane/ethyl acetate as the eluent to yield the corresponding hydrogenated products.\n1 3\n1 1 3\n𝑛 (𝑜𝑏𝑡𝑎𝑖𝑛𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠)\n𝑛 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙𝑙𝑦 𝑎𝑑𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒𝑠)\n𝑚𝑇𝐶𝐸×2×2.6\n106 = 𝑀𝑇𝐶𝐸×4\n𝑛 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙𝑙𝑦 𝑎𝑑𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒𝑠)\n25.75 𝑚𝑔×2×2.60\n107 = 167.85 𝑔/𝑚𝑜𝑙×4 = 99.8%\n0.2 𝑚𝑚𝑜𝑙\n𝑛 (𝑟𝑒𝑚𝑎𝑖𝑛𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)\n𝑛 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒\n110 = (1-0)% = 100%\n𝑛 (𝑜𝑏𝑡𝑎𝑖𝑛𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡)\n𝑛 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)−𝑛 (𝑟𝑒𝑚𝑎𝑖𝑛𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)\n𝑚𝑇𝐶𝐸×2×2.6\n112 = 𝑀𝑇𝐶𝐸×4\n𝑛 (𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)−𝑛 (𝑟𝑒𝑚𝑎𝑖𝑛𝑒𝑑 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒)\n25.75 𝑚𝑔×2×2.60\n113 = 167.85 𝑔/𝑚𝑜𝑙×4 =99.8%\n0.2 𝑚𝑚𝑜𝑙−0\n𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑\n𝑚𝑜𝑙𝑒 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑎𝑐𝑡𝑖𝑣𝑒 𝑠𝑖𝑡𝑒𝑠×𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒\namount of azobenzene(mol) × conversion(%) × selectivity(%)\n116 =\namount of catalyst(mol) × reaction time(h)\n117 = 0.2 𝑚𝑚𝑜𝑙×100%×99.8% =27218 h-1\n0.0002 𝑚𝑚𝑜𝑙×0.367 ℎ\n119 include full experimental details of the catalytic tests.\n120\n122 azobenzene hydrogenation.\n123 4. The catalytic performance (TOF and yield) should be compared quantitatively with those of\n124 other reported catalysts for azobenzene hydrogenation. A fair and objective benchmark comparison\n125 is necessary to substantiate the claim of superior activity.\n127 objective benchmark, we quantitatively compared the TOF and yield of our Pt C -Pt O C /rGO\n1 3 1 1 3\n128 catalyst with representative ones previously reported for azobenzene hydrogenation\n129 (Supplementary Table S3).\nx 2 3\n131 corresponding to a TOF of only 323 h-1, nearly two orders of magnitude lower than that of our\n132 catalyst (27218 h-1) (Angew. Chem. Int. Ed. 56, 3226-3231 (2017)). Dppe-NiCl (0.5 mol.%)\n2\n133 achieved 98% yield at 120 oC in methanol, but the TOF remained less than 200 h-1 (Org. Lett. 25,\n134 4198-4202 (2023)). In the presence of 20 mol.% 4-cyanopyridine as a co-catalyst, a maximum\n135 yield of 99% was obtained only with excess B (pin) (1.5 equiv.) in pentane, while the intrinsic\n2 2\n136 activity (TOF ≈ 1 h-1) was extremely limited (Angew. Chem. Int. Ed. 55, 5985-5989 (2016)). Under\n137 photocatalytic conditions, Au-NPs/CeO enabled only partial conversion (40%) and 31.2% yield\n2\n138 after 6 h, with a TOF of 2.6 h-1 (Chem. Commun. 48, 3509-3511(2012)).\n140 including a new comparison table (Supplementary Table S3) that summarizes representative\n141 catalysts and explicitly highlights the superior TOF and yield of Pt C -Pt O C /rGO relative to\n1 3 1 1 3\n142 previously reported catalysts.\n1 3 1 1 3\n144 other reported catalysts for azobenzene hydrogenation.\nCatalyst (mol.%) TOF (h-1) Yield (%) Reference\nPt C -Pt O C /rGO (0.01) 27218 99.8 This work\n1 3 1 1 3\nAngew. Chem. Int. Ed.\nALD FeS /Al O (0.9) 323 97\nx 2 3 56, 3226-3231 (2017)\nOrg. Lett. 25, 4198-4202\nDppe-NiCl (0.5) 196 98\n2 (2023)\nAngew. Chem. Int. Ed.\n4-cyanopyridine (20) 1.23 99\n55, 5985-5989 (2016)\nChem. Commun. 48,\nAu-NPs/CeO (0.02) 2.6 31.2\n2 3509-3511(2012)\n145 5. DFT adsorption energy calculations appear inconsistent and potentially invalid. The adsorption\n146 configurations used in DFT simulations are inconsistent across models: Model 2 involves N atom\n147 adsorption, while Models 1, 3, and 4 show adsorption through C atoms. Comparing adsorption\n148 energies from fundamentally different configurations is not meaningful. Additionally,\n151 of the Pt C -Pt O C site upon adsorption introduces large uncertainties in adsorption energy. The\n1 3 1 1 3\n152 authors should reassess the modeling approach and ensure structural stability and meaningful\n153 interaction between the adsorbate and the active site.\n156 reported in the manuscript by adopting the same adsorption configuration, namely adsorption\n157 through the C atom of the phenyl ring of azobenzene. In particular, we have re-examined the\n158 adsorption configuration of azobenzene on Model 2. In this revision, the previous Model 2 has\n159 been replaced with a configuration in which azobenzene adsorbs through the C atom of the phenyl\n160 group (Supplementary Fig. 28). In this updated model, the distance between the azobenzene\n161 molecule and the Pt active site is matchable with the calculated adsorption energy we present.\n163 recalculated the adsorption energies using two different formulations, as listed below:\nads\nads\nads\n1 167 1. In Equation 2, the reference reactant system was constructed as a combined model, where the\n168 initial geometry was built by positioning the azo molecule at least 4 Å away from the catalyst\n169 surface to avoid any interaction between the two components. The corresponding results obtained\n170 from both formulations are summarized in Table R1. It can be seen that the ΔE values calculated\nads\n171 using Equation 1 and Equation 2 are very similar, which gives us confidence that the adsorption\n172 energies reported in the manuscript are both reliable and meaningful.\n174 adsorbs via the C atom of the phenyl ring (Supplementary Fig. 28).\nads\nModel ΔE\nads\n(Equation 1, eV) ΔE\nads\n(Equation 2, eV)\nModel 1 (6x6) -0.03 -0.09\nModel 2 (5x5) -0.057 -0.06\nModel 3 (4x4) -0.43 -0.42\nModel 4 (4x5) -0.03 0.004\n176 6. The proposed reaction pathway lacks clarity and chemical plausibility. The DFT-derived\n177 reaction mechanism does not clearly delineate the elementary steps of azobenzene hydrogenation.\n180 insufficiently described. Without a clear depiction of N=N activation and hydrogen transfer, the\n181 proposed reaction pathway is difficult to accept.\n183 the concern regarding the clarity and chemical plausibility of the proposed reaction pathway, we\n184 have carefully refined our description and simplified the mechanism into three key stages\n185 (Supplementary Fig. 32): Step one: Hydrogenation of C H -N=N-C H by NH BH as a proton\n6 5 6 5 3 3\n186 donor, producing one C H -NH-NH-C H and leaving NH BH * at the PtC site. Step two:\n6 5 6 5 3 1 3\n3 1 2 3 1 6 5\n6 5 2 3 3\n189 a third C H -NH-NH-C H .\n6 5 6 5\n191 (1) C H -N=N-C H + NH BH +*→ *C H -N=N-C H + NH BH * (adsorption)\n6 5 6 5 3 3 6 5 6 5 3 3\n192 (2) *C H -N=N-C H + NH BH *→ NH BH * + C H -NH-NH-C H (step one)\n6 5 6 5 3 3 3 1 6 5 6 5\n193 (3) *C H -N=N-C H + NH BH * + H O → NH BOH * + C H -NH-NH-C H (step two)\n6 5 6 5 3 1 2 3 1 6 5 6 5\n194 (4) *C H -N=N-C H + NH BOH * + 2H O → NH B(OH) + C H -NH-NH-C H +* (step three)\n6 5 6 5 3 1 2 3 3 6 5 6 5\n196 of azobenzene hydrogenation (Fig. 4d, Supplementary Figs. 32 and 33).\n3 3 6 5 6 5\n198 catalyst to form adsorption states of NH BH * and C H NNC H * (N=N bond length: 1.277 Å). (I)\n3 3 6 5 6 5\n3 3 3 1 1 3\n200 site, forming an H* and NH BH * intermediate, which requires 1.73 eV. (II) The H* then\n3 2\n201 hydrogenates neighboring N=N bond in C H NNC H * at the Pt C site, forming C H NHNC H *\n6 5 6 5 1 3 6 5 6 5\n202 (the bond length between N and N: 1.334 Å, indicating effective activation). (III) Another H atom\n203 dissociates from NH BH * and adsorbs to the Pt O C site, forming H* and NH BH *, with an\n3 2 1 1 3 3 1\n204 energy cost of 0.20 eV. (IV) The generated H* reacts with C H NHNC H * to form\n6 5 6 5\n6 5 2 6 5\n6 5 2 6 5 1 3 3 3\n207 acts as the primary proton donor, and the N=N bond in C H NNC H is effectively activated to\n6 5 6 5\n208 form C H NH NC H .\n6 5 2 6 5\n6 5 6 5 2\n210 molecule attacks NH BH *, forming NH BH OH* and H* over the Pt C and Pt O C sites,\n3 1 3 1 1 3 1 1 3\n211 respectively. (VIII-IX) The second C H NNC H molecule adsorbs onto the proximate Pt C site\n6 5 6 5 1 3\n212 and interacts with the H* to form C H NHNC H *. (X) NH BH OH* dissociates into H* and\n6 5 6 5 3 1\n3 1 1 1 3 1 3\n6 5 6 5 6 5 6 5\n215 cost of 0.56 eV. This demonstrates that N=N in C H NNC H in stage two can also be effectively\n6 5 6 5\n216 activated to yield C H NH NC H .\n6 5 2 6 5\n6 5 6 5\n218 transfer, hydrogenation, and desorption processes. (XIII) An H O molecule attacks NH BH O*,\n2 3 1\n219 forming NH BO H *. (XIV) NH BO H * dissociates into NH BO H * and H* over the Pt C and\n3 2 3 3 2 3 3 2 2 1 3\n1 1 3 6 5 6 5\n221 onto the Pt C site and interacts with the H* to form C H NHNC H *. (XVII-XVIII) Another H O\n1 3 6 5 6 5 2\n222 molecule attacks NH BO H *, forming NH BO H *, which then dissociates into NH B(OH) * and\n3 2 2 3 3 4 3 3\n1 3 1 1 3\n6 5 6 5 6 5 6 5\n226 azobenzene hydrogenation (Fig. 4d, Supplementary Figs. 32 and 33).\n227\n229\n1 3\n1 1 3\n1 1\n234 pairs (CPs) on reduced graphene oxide. By precisely tuning the spacing between the adjacent Pt -\n1\n1\n236 h-1 for transfer hydrogenation of azobenzene via ammonia-borane hydrolysis, surpassing\n237 benchmarking catalysts by more than an order of magnitude. They suggest that Pt -Pt CPs\n1 1\n238 separated by 5.3 Å can facilitate co-adsorption of sterically hindered intermediates and at the same\n239 time an enable a low-barrier hydrogen shuttling pathway, thus enhancing the overall azobenzene\n240 hydrogenation efficiency.\n1 3 1 1 3\n242 coordination, which is just based on XANE/XAFS simulation. What is the content of oxygen and\n245 of Pt C -Pt O C /rGO is resolved by combining HAADF-STEM, XANES, and EXAFS analyses\n1 3 1 1 3\n246 with spectral simulations. Only asymmetric Pt C -Pt O C coordination reproduces the\n1 3 1 1 3\n247 experimental features. Alternative models deviate significantly in simulated spectra and structural\n248 parameters.\n1 3 1 1 3\n250 atomic-scale electron energy loss spectroscopy (EELS) and four-dimensional scanning\n251 transmission electron microscopy (4D-STEM) to further substantiate the assignment of\n252 asymmetric Pt C -Pt O C pairs. EELS measurements of individual Pt dimers simultaneously\n1 3 1 1 3\n253 detected the C-K (284 eV), O-K (532 eV), and Pt-M (M : 2122 eV; M : 2202 eV) signals, indicating\n5 4\n254 that Pt atoms are coordinated with both C and O (Fig. 1u-w). Atomic-scale charge density mapping\n255 from the 4D-STEM revealed two adjacent positively charged Pt centers, each surrounded by\n256 negatively charged atoms (Fig. 1x). Strikingly, the coordinating atom adjacent to the lower-left Pt\n257 displayed a markedly stronger negative charge density than that adjacent to the other Pt, consistent\n258 with the proposed Pt C -Pt O C pair motif (Fig. 2f) owing to the higher electronegativity of O\n1 3 1 1 3\n259 relative to C. This experimentally observed asymmetric electronic environment matches well with\n260 the electron localization function (ELF) calculation result using VASP (Supplementary Fig. 10),\n261 providing compelling and mutually consistent evidence for the Pt C -Pt O C pair structure. For\n1 3 1 1 3\n262 clarity, we also present a merged comparison of the atomic structure, charge density mapping, and\n1 3 1 1 3\n266 simulations and EXAFS fittings, unequivocally establishes the asymmetric Pt C -Pt O C pair.\n1 3 1 1 3\n1 3 1 1 3\n268 loadings (0.5, 1.0, 2.5, 5.0, and 7.5 wt.%), oxygen (wt.%) was quantified by combustion pyrolysis\n269 in H /He (“O mode,” CO captured and detected by TCD) after rigorous drying (120 oC, high\n2\n270 vacuum) to remove physiosorbed water. Pt (wt.%) was determined by inductively coupled plasma\n271 optical emission spectroscopy (ICP-OES) following complete acid digestion. The results show that\n272 the oxygen contents of Pt C -Pt O C /rGO at the respective loadings are 16.0 wt.%, 15.9 wt.%,\n1 3 1 1 3\n273 15.7 wt.%, 15.3 wt.%, and 14.9 wt.%. Based on these values, the calculated Pt-to-O atomic ratios\n274 are 0.003, 0.005, 0.01, 0.03, and 0.04, respectively. These stoichiometries confirm that sufficient\n276 graphene oxide supports, consistent with early reports (Nat. Commun. 15, 164 (2024); Nat\n1 3 1 1 3\n279 catalyst. Specifically, we added new atomic-resolution EELS, 4D-STEM, and ELF data (Fig. 1u-\n280 x and Supplementary Figs. 10 and 11). These revisions strengthen the experimental evidence\n281 supporting the asymmetric Pt C -Pt O C pair assignment.\n1 3 1 1 3\n282\n1 1\n285 image reconstructed from the 4D-STEM dataset, resolving a Pt -Pt pair on rGO. Right: Projected\n1 1\n286 charge-density distribution derived from the inverted divergence of the center of mass (dCoM),\n287 highlighting the spatial positions of C, O, and Pt atoms.\n288\n1 3 1 1 3\n290 by VASP. Blue indicates electron depletion/delocalization, while red reflects electron\n291 accumulation/localization. The asymmetric electron localization around the two adjacent Pt\n292 centers highlights the distinct coordination environments, with stronger electron localization at the\n1 3 1 1 3\n294\n296 mapping, and ELF analysis, collectively supporting the proposed Pt C -Pt O C configuration.\n1 3 1 1 3\n298 transfer, with red and blue regions denoting positive and negative charge accumulation,\n299 respectively. In the ELF map, blue indicates electron depletion/delocalization, while red reflects\n300 electron accumulation/localization.\n302 as high as 1.73 eV (step I). This could question about the feasibility of reaction at 80 oC. Therefore,\n303 the reviewer can’t recommend it for acceptance at this time.\n305 that the DFT simulations presented in the manuscript are expected to give the overestimated\n306 barriers, mainly due to the intrinsic limitations of the computational setup (vacuum slab, no explicit\n307 solvation/entropy). It is therefore common that the absolute barrier heights predicted by DFT\n308 appear higher than those under experimental conditions.\n310 9639-9647 (2021); Angew. Chem. Int. Ed. 59, 13379-13384 (2020)), where similar energy barriers\n311 (1.5-2.0 eV) were obtained from DFT, while the corresponding reactions were experimentally\n312 feasible in the 300-400 K range. In addition, following the reviewer’s suggestion, we estimated\n313 the reaction feasibility for a barrier of 1.73 eV using the Arrhenius equation:\n314 k = Ae-Ea/(k T)\nB\n315 k depicts reaction rate constant, A is pre-exponential factor, E stands for reaction barrier height,\na\n316 k is the Boltzmann constant (1.380649×10−23 J/kelvin), and T is the absolute temperature (in\nB\n317 kelvins). The calculated results, as summarized below, are strongly dependent on the choice of the\n319 versus temperature (Fig. R1). It depicts that when A factor is set as 1e16 s-1 at T = 300 K and E =\na\n320 1.73 eV, the rate constant is around 10-12 s-1, indicating a feasible reaction process.\n321\n324 1. It seems that the Pt O C configuration is just speculator, both azobenzene and NH BH adsorb\n1 1 3 3 3\n325 on the Pt C sites. In fact, the adsorption of NH BH on the Pt C is much stronger than azobenzene?\n1 3 3 3 1 3\n3 3 1 3 1 1 3\n327 sites due to the competitive adsorption? How about water adsorption on these two Pt sites?\n329 roles of the asymmetric Pt sites, we systematically compared the adsorption energies of\n330 azobenzene, NH BH , and H O on Pt C and Pt O C . The results are summarized below\n3 3 2 1 3 1 1 3\n331 (Supplementary Table S4).\n3 3 1 3\n333 strongly overall. However, this does not preclude azobenzene from adsorbing and participating in\n334 the reaction. Specifically, under realistic reaction conditions, not all Pt C sites are simultaneously\n1 3\n335 occupied by NH BH . Moreover, considering the steric effect and adsorption properties,\n3 3\n336 azobenzene tends to adsorb at a neighboring Pt C site rather than at Pt O C when NH BH is\n1 3 1 1 3 3 3\n337 present. This spatial separation naturally allows both reactants to be accommodated simultaneously.\n339 compete with the main reactants for active sites. Therefore, we propose the most plausible and\n340 energetically favorable reaction pathway in this work.\n1 1 3\n342 indispensable function as a hydrogen relay center (Supplementary Fig. 35). By transiently\n343 stabilizing H* intermediates and mediating their transfer to azobenzene, Pt O C effectively\n1 1 3\n344 complements the strongly binding Pt C site and enables the low-barrier hydrogen-shuttling\n1 3\n345 pathway. Therefore, Pt O C is not speculative but mechanistically essential.\n1 1 3\n3 3 2 1 3 1 1 3\n348 corresponding discussion, to elucidate the distinct roles of the asymmetric Pt sites in the revised\n349 manuscript.\n3 3 2 1 3\n351 and Pt O C .\n1 1 3\nAdsorbate ΔE\nads\non Pt\n3\n(eV) ΔE\nads\non Pt\n3\n(eV)\nAzobenzene -0.32 -0.43\nBH NH -0.63 -1.53\n3 3\nH O -0.18 -0.60\n2\n352 2. The reaction of ammonia-borane hydrolysis should be also carried out on these sets of Pt\n353 catalysts with different loadings. The authors might like to correlate the activity in ammonia-\n354 borane hydrolysis and the activity in transfer hydrogenation of azobenzene.\n356 hydrolysis of ammonia-borane (NH BH ) as well as the first hydrogenation step of azobenzene on\n3 3\n1 3 1 3\n358 of NH BH hydrolysis are summarized in Supplementary Table S5, while the corresponding\n3 3\n359 azobenzene hydrogenation energetics are presented in Fig. 4e.\n3 3\n361 energetics (ΔE = -0.77 eV for the first step and 0.41 eV for the second step), indicating superior\n3 3 3 3\n363 and generate active H intermediates, as its energy barriers are comparable to those calculated for\n364 the hydrogenation step of azobenzene (Fig. 4d).\n366 transfer barrier and the most favorable geometry for co-adsorption of NH BH and azobenzene\n3 3\n367 (Fig. 4 a-d). NH BH preferentially binds on Pt C , and azobenzene is directed to adsorb on an\n3 3 1 3\n368 adjacent Pt C site, thereby establishing the spatial configuration required for the reaction.\n1 3\nads\n370 reaction energy of the first N=N hydrogenation for various models (Fig. 4e), including Pt(111),\n1 3 1 3 1 3 1 3 1 3 1 3 1 3\n1 3 1 3 1 3\n373 1), Pt C -Pt OC /rGO (Model 2), Pt C -Pt OC /rGO (Model 4), Pt OC -Pt OC /rGO, Pt C /rGO,\n1 3 1 3 1 3 1 3 1 3 1 3 1 3\n374 and Pt OC /rGO occupy the weak-binding branch. The first hydrogenation of azobenzene becomes\n1 3\n375 thermodynamically unfavored for Pt C -Pt C /rGO. By contrast, the Pt C -Pt OC /rGO (Model 3)\n1 3 1 3 1 3 1 3\n376 sits at the volcano apex, combining balanced adsorption with the lowest reaction energy for the\n377 first hydrogenation step.\n3 3\n379 volcano plot by correlating ΔE (azobenzene) with the reaction energy of the first N=N\nads\n380 hydrogenation step (Supplementary Table S5, Fig. 4e).\nads 3 3 3\n382 hydrolysis steps on Pt C -Pt OC /rGO with different pair densities.\n1 3 1 3\nModel ΔE (BH *, eV) ΔE (1st hydrolysis, eV) ΔE (2nd hydrolysis, eV)\nads 3\nModel 1 -0.42 -0.77 0.41\nModel 2 -0.98 0.64 1.40\nModel 3 -1.53 1.73 1.09\nModel 4 -0.96 0.85 1.24\n383\n385 hydrogenation step on various Pt-based catalysts.\n386 3. If it forms the asymmetric Pt C -Pt O C coordination structure, the samples should have two\n1 3 1 1 3\n387 different oxidation states for Pt, is there any evidence from XPS?\n389 indeed reveal an electronic asymmetry between the Pt C and Pt O C centers, this difference does\n1 3 1 1 3\n390 not translate into a measurable change in oxidation state. To substantiate this, we have performed\n391 charge transfer analysis, which indicates a charge difference of only ~0.07 e- between the two Pt\n392 sites, a value far below the threshold required to induce a detectable XPS shift. This finding is\n393 consistent with early reports (Angew. Chem. Int. Ed. 64, e202421277 (2025); Nat. Catal. 6, 916-926 (2023)),\n394 which show that the Co-N P and Co-N catalysts contain exclusively Co2+ species,\n4 1 4\n395 and that Ir atoms in Ir /NG and Ir -P /NPG remain in the same oxidation state. Furthermore, ELF\n1 1 1\n396 calculations using VASP show comparable electron localization patterns around Pt in Pt C and\n1 3\n1 1 3\n398 mapping from 4D-STEM reveals two adjacent Pt centers with similar surrounding charge\n399 distributions (Fig. 1x). Therefore, the experimentally observed uniform Pt2+ state is fully consistent\n400 with our theoretical results.\n402 charge-density mapping from 4D-STEM, and the corresponding references and descriptions into\n403 the revised manuscript to explicitly demonstrate that the Pt species are stabilized in a uniform Pt2+\n404 oxidation state.\n405 4. Figure 4 is not well presented. It can be hardly see the details in Figure 4d and e.\n408 moved them into Supplementary Fig. 33. Similarly, the schematic details of Fig. 4e have also been\n409 included in Supplementary Fig. 35, allowing clearer visualization of the reaction pathways and\n410 structural features.\n413 (5 wt.%) on graphene oxide. The central claim revolves around the existence of asymmetric Pt C -\n1 3\n1 1 3\n415 pair distance. Although the topic is relevant and potentially interesting, I have significant concerns\n416 regarding the clarity and validity of the provided evidence. Below, I detail the main issues:\n419 approximately 5 Å, the presented evidence appears inconclusive. Specifically, Figures 1f-j reveal\n420 many single Pt atoms along with potential Pt-Pt pairs. In Figures 1k-o, Pt-Pt pairs indicated by\n421 orange boxes are densely packed, raising doubts about accurate identification and distance\n422 measurements, especially at higher Pt densities (e.g., Fig. 1n-o). This selective consideration of\n424 significance of Pt-Pt pairs relative to single Pt atoms or clusters.\n426 dispersed Pt-Pt pairs with an interatomic distance of ~5 Å existing in the Pt C -Pt O C /rGO\n1 3 1 1 3\n427 catalyst is supported by multiple evidences:\n428 (1) Precursor strategy. The catalyst was synthesized using a di-platinum molecular precursor,\n429 (ethylenediamine)iodoplatinum(II) dimer dinitrate, which intrinsically contains a pre-organized\n432 7, 754-767 (2023)).\n433 (2) Structural assignment validated by spectroscopy and simulation. We constructed several\n434 atomic models based on HAADF-STEM, XANES, and EXAFS results. Only the asymmetric\n1 3 1 1 3\n436 and EXAFS coordination parameters (Fig. 2d-f, Supplementary Table 1). All alternative models\n437 fail to match the experimental features (Supplementary Figs. 8 and 9, Table 2).\n438 (3) Direct visualization at low Pt loading. At an ultralow Pt loading (0.5 wt.%), discrete Pt dimers\n439 with ~5 Å spacing can be clearly observed in HAADF-STEM images (Fig. 1k). This demonstrates\n440 that the paired Pt structure is present independently.\n441 (4) Preservation across Pt loadings. The XANES white-line features remain nearly unchanged\n442 when Pt loading increasing from 0.5 wt.% to 7.5 wt.% (Fig. 2b), and DFT-optimized Pt C -\n1 3\n1 1 3\n444 dimer structure. This indicates that the paired Pt geometry is stable across a large Pt loading range.\n445 (5) Explanation of “single atom” appearances. In Fig. 1f-j, the scale bar is 5 nm, whereas the\n447 feature, mimicking a single atom. This explains why many features in Figs. 1f-j appear as isolated\n448 atoms but are in fact Pt pairs (Fig. 1k-o, Supplementary Fig. 3).\n449 (6) Additional EELS and 4D-STEM verification. To further substantiate the assignment of\n450 asymmetric Pt C -Pt O C pairs, we performed atomic-resolution electron energy loss\n1 3 1 1 3\n451 spectroscopy (EELS) and four-dimensional scanning transmission electron microscopy (4D-\n454 (532 eV), and Pt-M (M : 2122 eV; M : 2202 eV) signals, indicating that Pt atoms are coordinated\n5 4\n455 with both C and O (Fig. 1u-w). Atomic-scale charge density mapping from 4D-STEM revealed\n456 two adjacent positively charged Pt centers, each surrounded by negatively charged atoms (Fig. 1x).\n458 negative charge density than that adjacent to the other Pt, consistent with the proposed Pt C -\n1 3\n1 1 3\n460 experimentally observed asymmetric electronic environment matches well with the electron\n461 localization function (ELF) calculations using VASP (Supplementary Fig. 10), providing\n462 compelling and mutually consistent evidence for the Pt C -Pt O C pair structure. For clarity, we\n1 3 1 1 3\n463 also present a merged comparison of the atomic structure, charge density mapping, and ELF\n464 analysis (Supplementary Fig. 11), which supports the proposed Pt C -Pt O C configuration. A\n1 3 1 1 3\n465 minor fraction of true single atoms does exist, likely arising from pair dissociation or projection\n466 overlap, as reported previously (Nat. Rev. Chem. 7, 754-767 (2023)).\n467 (7) Statistical robustness at higher Pt densities. At moderate Pt loadings (0.5-2.5 wt.%, Fig. 1k-\n468 m), the Pt-Pt pairs are well separated and reliably measured. At higher Pt loadings (Figs. 1n-o), we\n469 considered Pt pairs meeting the strict ~5 Å separation criterion. Moreover, to avoid selection bias,\n470 raw images were divided into multiple sub-regions, each with >50 atom pairs measured, and the\n471 full dataset was analyzed. This approach ensures, to the greatest extent possible, that the statistical\n472 conclusions are not affected by selective sampling.\n474 spectroscopic validation, atomic-resolution imaging at low Pt loadings, structural robustness\n475 across different Pt loadings, magnification-dependent statistical analysis, EELS characterization,\n476 and 4D-STEM charge-density mapping collectively and conclusively confirms that the Pt C -\n1 3\n1 1 3\n478 predominant in our catalysts.\n479\n1 1\n482\n1 1\n485 image reconstructed from the 4D-STEM dataset, resolving a Pt -Pt pair on rGO. Right: Projected\n1 1\n486 charge-density distribution derived from the inverted divergence of the center of mass (dCoM),\n487 highlighting the spatial positions of C, O, and Pt atoms.\n488\n490 mapping, and ELF analysis, collectively supporting the proposed Pt C -Pt O C configuration.\n1 3 1 1 3\n492 transfer, with red and blue regions denoting positive and negative charge accumulation,\n493 respectively. In the ELF map, blue indicates electron depletion/delocalization, while red reflects\n494 electron accumulation/localization.\n1 3 1 1 3\n497 based on comparisons of experimental data (XANES, EXAFS) to simulations. However,\n498 justification for selecting this specific configuration is weak. Supplementary Figure 9 (XANES\n499 spectra) does not convincingly differentiate the proposed configuration from others, as\n500 multiple configurations appear similarly plausible. Additionally, Supplementary Tables 1-2 compare\n501 primarily Pt-Pt pair configurations, neglecting potentially relevant configurations such as isolated\n502 single Pt atoms. Moreover, the authors didn't assess relative energies of these configurations\n503 systematically, casting further doubt on their stability and plausibility.\n505 justification of the proposed Pt C -Pt O C configuration requires considering not only spectral\n1 3 1 1 3\n506 fitting but also alternative structural models and energetic stability.\n1 3 1 1 3\n508 combination of spectral features, including the white-line energy position, rising-edge shape,\n509 constructive and destructive interference patterns, and the relative intensity ratio between the main\n510 edge and post-edge shoulder, in excellent agreement with experiment. Furthermore, the\n511 corresponding EXAFS analysis yields coordination numbers and Pt-C/O bond lengths consistent\n512 with the experimental fitting data (Fig. 2e; Supplementary Tables 1-2), whereas all alternative\n513 structural models fail to simultaneously reproduce these combined spectroscopic signatures. In\n514 addition, we have incorporated EELS and 4D-STEM evidence that independently corroborate the\n515 atomic-scale arrangement of the asymmetric pair (Fig. 1 u-w).\n517 based catalysts for transfer hydrogenation of azobenzene, including Pt C -Pt C /rGO, Pt O C -\n1 3 1 3 1 1 3\n1 1 3 1 1 3 1 3\n519 adopt the symmetric Pt C configuration, and the dissociated H atoms bind strongly to the Pt sites,\n1 3\n520 with an adsorption energy of -8.26 eV (Supplementary Fig. 37). Pt O C -Pt O C /rGO exhibits\n1 1 3 1 1 3\n521 universally weak adsorption for azobenzene, NH BH , H O, and H (Supplementary Table S6). We\n3 3 2\n522 have additionally evaluated Pt /rGO. The corresponding EXAFS fittings of isolated Pt C and\n1 1 3\n1 1 3\n524 measured one (~3.5). These results demonstrate that purely isolated single-atom models cannot\n525 account for the observed spectral characteristics. In addition, Pt /rGO with Pt O C configuration\n1 1 1 3\n526 exhibits weak binding to NH BH , with an adsorption energy of -0.15 eV (Supplementary Fig. 38).\n3 3\n1 3 1 1 3\n528 small fraction of isolated Pt atoms is indeed present (as noted in Figs. 1k-o and literature reports\n529 on pair dissociation), their contribution is minor. Pt NPs are suboptimal due to the excessive\n530 adsorption strength of NH BH (-2.16 eV) and C H N=NC H (-2.20 eV) (Supplementary Fig. 39),\n3 3 6 5 6 5\n531 leading to catalyst poisoning.\n533 relaxations. The results confirm that the proposed Pt C -Pt O C dimer and other plausible\n1 3 1 1 3\n534 configurations are all thermodynamically stable (see Methods and Supplementary Fig. 9).\n1 3 1 1 3\n536 3e and Supplementary Figs. 14-15) provides further experimental evidence for its high structural\n537 stability. Taken together, both theoretical energetics and experimental observations strongly\n538 support the plausibility and robustness of the proposed configuration.\n1 3 1 1 3\n540 integrating both spectroscopic and theoretical evidence (Fig. 1 u-w, Fig. 4e, Supplementary Figs.\n541 36-39, Supplementary Table S6) in the revised manuscript.\n2\n545 additional questions. Given that the authors report significantly different d-band centers for the\n546 paired Pt atoms, one would naturally expect variations in Pt valence states. A careful and explicit\n547 valence state analysis is necessary to support the proposed configuration convincingly.\n549 remain nearly unchanged when Pt loading increasing from 0.5 wt.% to 7.5 wt.%.(Fig. 2b). Both\n1 3 1 1 3\n551 stabilized in a +2 valence state. Although the absorption edge position in Fig. 2b appears close to\n552 that of PtO , the white-line intensity and overall spectral features are distinctly different.\n2\n554 of d-band centers reflect the modulations of the local electronic structure of Pt C and Pt O C\n1 3 1 1 3\n555 sites, rather than changes in formal oxidation state. To substantiate this, we performed charge\n556 transfer analysis, which indicated a charge difference of only ~0.07 e- between the two Pt sites, a\n557 value far below the threshold required to induce a detectable XPS shift. This finding is consistent\n558 with early reports (Angew. Chem. Int. Ed. 64, e202421277 (2025); Nat. Catal. 6, 916-926 (2023)),\n559 which similarly show that the Co-N P and Co-N catalysts contain exclusively Co2+ species, and\n4 1 4\n560 that the Ir atoms in Ir /NG and Ir -P /NPG remain in the same oxidation state. Furthermore, ELF\n1 1 1\n561 calculations using VASP show comparable electron localization patterns around Pt in Pt C and\n1 3\n1 1 3\n563 mapping from 4D-STEM reveals two adjacent Pt centers with similar surrounding charge\n564 distributions (Fig. 1x). Therefore, the experimentally observed uniform Pt2+ state is fully consistent\n565 with our theoretical results.\n1 3 1 1 3\n568 only as fine orbital-level adjustments, which are essential for catalytic activity but do not\n569 correspond to distinct oxidation states.\n571 manuscript by integrating complementary XPS, XANES, and 4D-STEM analyses with theoretical\n572 charge transfer and ELF calculations.\n575 models with greater inter-pair distances (>7 Å) seem acceptable, shorter inter-pair distances\n576 (around 5 Å) complicate the identification of active catalytic sites. The authors neglect possible\n577 roles of alternative configurations (e.g., PtC -PtC or PtOC -PtOC pairs), thereby weakening their\n3 3 3 3\n578 central argument regarding the specificity and catalytic relevance of Pt C -Pt O C pairs.\n1 3 1 1 3\n580 calculations on other Pt-based catalysts for transfer hydrogenation of azobenzene, including Pt C -\n1 3\n1 3 1 1 3 1 1 3 1 1 3\n1 3 1 3\n583 bind strongly to the Pt sites, with an adsorption energy of -8.26 eV (Supplementary Fig. 37). This\n584 strong binding inhibits H-atom transfer, making Pt C -Pt C /rGO ineffective for hydrogenating\n1 3 1 3\n6 5 6 5 1 1 3 1 1 3\n3 3 2\n587 reaction would proceed preferentially via an Eley-Rideal (ER) pathway, in which azobenzene\n588 directly approaches H from the gas phase, rather than H*. However, reactions dominated by the\n1 1 1 3\n591 exhibits weak binding to NH BH , with an adsorption energy of -0.15 eV (Supplementary Fig. 38).\n3 3\n3 3\n6 5 6 5\n595 calculations for alternative Pt-based configurations, namely Pt C -Pt C /rGO, Pt O C -\n1 3 1 3 1 1 3\n1 1 3 1\n597 address the reviewer’s concern.\nads 3 3 2 1 1 3\n1 1 3\nAdsorbate ΔE (eV)\nads\nAzobenzene -0.098\nNH BH -0.57\n3 3\nH O -0.42\n2\n600\n603 based on d-band center analysis, lacks sufficient theoretical rigor. Without establishing a clear\n604 volcano relationship or explicitly considering the kinetics of azobenzene adsorption and\n605 hydrogenation, the interpretation presented does not fully support the observed maximal turnover\n606 frequency.\n608 conclusions. Major revisions addressing these fundamental points would be required to reconsider\n609 publication.\n611 the density of states and the d-band center for each Pt in Pt C -Pt O C with different Pt density\n1 3 1 1 3\n612 (Fig. 4a, Supplementary Fig. 27). Notably, Model 3 shows the highest d-band center of -0.04 eV\n613 at the Pt C site, indicative of an optimal electronic structure for reactant adsorption, as confirmed\n1 3\n614 by adsorption energy calculations (Fig. 4b, Supplementary Fig. 28).\n616 ΔE (azobenzene) with the reaction energy of the first N=N hydrogenation step (Fig. 4e) for\nads\n617 various models, including Pt(111), Pt C -Pt OC /rGO (Models 1-4), Pt C -Pt C /rGO, Pt OC -\n1 3 1 3 1 3 1 3 1 3\n1 3 1 3 1 3\n619 hydrogenation of azobenzene; horizontal axis: ΔE (azobenzene)), Pt(111) lies on the over-\nads\n620 binding branch, while motifs, including Pt C -Pt OC /rGO (Model 1), Pt C -Pt OC /rGO (Model\n1 3 1 3 1 3 1 3\n621 2), Pt C -Pt OC /rGO (Model 4), Pt OC -Pt OC /rGO, Pt C /rGO, and Pt OC /rGO, occupy the\n1 3 1 3 1 3 1 3 1 3 1 3\n622 weak-binding branch. In addition, the first hydrogenation of azobenzene becomes\n623 thermodynamically unfavored for Pt C -Pt C /rGO. By contrast, the Pt C -Pt OC /rGO (Model 3)\n1 3 1 3 1 3 1 3\n624 sits at the volcano apex, combining balanced adsorption with the lowest reaction energy for the\n625 first hydrogenation step. These results provide a coherent structure-activity rationale,\n626 demonstrating that the ~5.3 Å inter-pair spacing uniquely balances adsorption and reactivity,\n627 thereby explaining the experimentally observed maximum turnover frequency.\nads\n629 of the first N=N hydrogenation step, and incorporated the corresponding analysis and discussion\n630 into the revised manuscript (Fig. 4e).\n631\n633 hydrogenation step on various Pt-based catalysts."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed some of the reviewer’s previous concerns; however, one key issue remains unresolved and\nmust be well addressed before this manuscript can be considered for publication in Nature Communications.\nIn response, the authors describe that “(I) A hydrogen atom dissociates from BH in NH BH * and transfers to the Pt atom at\n3 3 3\nthe Pt O C site, forming an H* and NH BH * intermediate, which requires 1.73 eV. (II) The H* then hydrogenates\n1 1 3 3 2\nneighboring N=N bond in C H NNC H * at the Pt C site, forming C H NHNC H * (the bond length between N and N:\n6 5 6 5 1 3 6 5 6 5\n1.334 Å, indicating effective activation). (III) Another H atom dissociates from NH BH * and adsorbs to the Pt O C site,\n3 2 1 1 3\nforming H* and NH BH *, with an energy cost of 0.20 eV. (IV) The generated H* reacts with C H NHNC H * to form\n3 1 6 5 6 5\nC H NH NC H * (HN-NH bond length: 1.356 Å, indicating complete hydrogenation).”\n6 5 2 6 5\nHowever, it is still unclear how steps (II) and (III) take place mechanistically: the N=N bond of azobenzene is not adsorbed or\nactivated at any metal site, so it is insufficiently justified how an H* species located at the Pt O C site can effectively attack\n1 1 3\nand hydrogenate a non-adsorbed N=N bond. To support the proposed mechanism, the authors should additionally compute\nand report the transition states and the associated energy barriers for these hydrogenation steps.", "reviewer_2": "In this revised manuscript, the authors have thoroughly addressed the reviewers' comments with substantial new\nexperimental evidence, most notably the atomic-resolution EELS data. These new results provide compelling support for the\nproposed symmetric Pt1C3-Pt1O1C3 catalytic pair structures. Given the material's remarkable catalytic performance and the\nprofound structural insights now provided, I recommend the manuscript for acceptance without further revision."}, "author_response": "Response: We sincerely thank the reviewer for the constructive comment and suggestion.\nFollowing which, we have computed the transition states (TS) and the associated energy barriers\nfor these hydrogenation steps using the climbing-image nudged elastic band (cNEB) method (J.\nChem. Phys. 2000, 113, 9901-9904). The optimized TS geometries are provided in Supplementary\nFig. 34. The first hydrogenation step, from intermediate I to intermediate II (labels consistent with\nFig. 4d), has an energy barrier of 0.20 eV (TS1). In TS1, the N atom of the N=N bond is only 1.65\nÅ away from the H* species at the Pt O C site, enabling efficient H transfer. The second\n1 1 3\nhydrogenation step, from intermediate III to intermediate IV, exhibits an ultralow barrier of 0.063\neV (TS2), where the N atom in the HN-N moiety is 2.19 Å from the H* species. Therefore, although\nthe N=N bond itself does not directly coordinate to Pt sites, azobenzene as a whole remains\nadsorbed on the Pt C -Pt O C pair, and the N=N bond in azobenzene is positioned within the\n1 3 1 1 3\nreactive vicinity of surface H* at the Pt C -Pt O C pair. As a result, sequential hydrogenation can\n1 3 1 1 3\nproceed with low energy barriers without requiring direct metal-N coordination. Notably, similar\nbehavior has been widely observed in heterogeneous hydrogenation reactions, where non-\ncoordinating functional groups within an adsorbed molecule undergo reaction without directly\nbinding to metal atoms (e.g., Nat. Commun., 2025, doi.org/10.1038/s41467-025-66809-2).\nAction: Transition states and energy barriers for the two hydrogenation steps have been added into\nthe revised manuscript (Supplementary Fig. 34), together with the corresponding discussion.\nSupplementary Fig. 34. Transition states and the associated energy barriers for the two\nhydrogenation steps from intermediate I to intermediate IV.\n\nResponse: We sincerely thank the reviewer for the positive evaluation and recommendation of\nacceptance."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed the issue I raised. The manuscript is now acceptable for publication in Nature\nCommunications."}, "author_response": ""}]

Decision letter