3.7 mL of NH₄OH (28-30 wt.%), 70 mL of ethanol, and 12 mL of deionized water were combined and stirred at room temperature for 10 min to form a homogeneous mixture. Subsequently, 0.4 g of resorcinol and 0.5 mL of formaldehyde (37 wt.%) were introduced, and the mixture was stirred for an additional 40 min to ensure complete dissolution and pre-polymerization. Thereafter, 3 mL of tetrapropoxysilane (TPOS) was added dropwise to initiate the co-condensation reaction at room temperature. After 12 h of reaction, the resulting solid product was collected by centrifugation and washed alternately with deionized water and ethanol at least three times to remove residual reactants. The obtained resorcinol-formaldehyde/silica (RF/SiO₂) composite nanospheres were subsequently carbonized at 800 °C for 2 h under an N₂ atmosphere with a heating rate of 3 °C min^-1. The silica template was then etched by immersion in 10% hydrofluoric solution for 12 h, followed by thorough washing with deionized water and ethanol (at least three times each) and drying at 60 °C overnight to obtain MCS, as illustrated in Figure 1.

The water uptake capacity of MCS was determined to be 4.27 mL g^-1 prior to impregnation.

PtCo-S/MCS: 22 mg of H₂PtCl₆ and 6.7 mg of sodium thioglycolate (STG) were dissolved in 640 μL of deionized water and stirred at room temperature for 4-8 h. During this period, the Pt species were evidenced by a visible color change in the solution from yellow to orange-yellow, indicating successful Pt-thiolate complex formation. Subsequently, 33.6 mg of CoCl₂·6H₂O was added to the above solution and stirred for 15 min to obtain a homogeneous metal precursor solution. The precursor solution was then introduced into 150 mg of MCS via incipient wetness impregnation to ensure complete and uniform infiltration into the mesoporous carbon framework. After aging at room temperature for 6 h, the impregnated sample was transferred to a tube furnace and subjected to a two-stage thermal treatment under a H₂/Ar atmosphere: first at 900 °C for 2 h, then at 600 °C for 6 h, with a heating rate of 10 °C min^-1, to yield the PtCo-S/MCS catalyst, as illustrated in Figure 1.

PtFe-S/MCS: The synthesis procedure followed that of PtCo-S/MCS, with the exception that CoCl₂·6H₂O (33.6 mg) was replaced by FeCl₃ (24 mg). The thermal treatment was adjusted to 900 °C for 2 h, followed by 700 °C for 6 h under a H₂/Ar atmosphere at a heating rate of 10 °C min^-1, yielding the PtFe-S/MCS catalyst.

PtCu-S/MCS: The synthesis procedure followed that of PtCo-S/MCS, with the exception that CoCl₂·6H₂O (33.6 mg) was replaced by Cu(NO₃)₂·3H₂O (38.6 mg). The thermal treatment was adjusted to 800 °C for 2 h followed by 600 °C for 6 h under a H₂/Ar atmosphere at a heating rate of 10 °C min^-1, yielding the PtCu-S/MCS catalyst.

For comparison, a reference catalyst without sulfur anchoring was prepared following a similar procedure. Specifically, for PtCo/MCS: 22 mg of chloroplatinic acid (H₂PtCl₆) and 33.6 mg of CoCl₂·6H₂O were dissolved in 640 μL of deionized water and stirred at room temperature for 15 min to obtain a homogeneous metal precursor solution. The precursor solution was then introduced into 150 mg of MCS via incipient wetness impregnation. After aging at room temperature for 6 h, the impregnated sample was subjected to two-stage thermal treatment under an H₂/Ar atmosphere at 900 °C for 2 h, followed by 600 °C for 6 h, with a heating rate of 10 °C min^-1, yielding the PtCo/MCS catalyst, as illustrated in Figure 1.

Twenty-two mg of H₂PtCl₆ and 33.6 mg of CoCl₂·6H₂O were dissolved in 640 μL of deionized water and stirred at room temperature for 10 min. The precursor solution was then introduced into 150 mg of MCS via incipient wetness impregnation to ensure complete and uniform infiltration into the mesoporous carbon framework. After aging at room temperature for 6 h, the sample was transferred to a tubular furnace and calcined at 900 °C for 2 h under a H₂/Ar atmosphere to ensure complete alloying of Pt and Co. After cooling to room temperature, the obtained powder was placed downstream in a tubular furnace under an Ar atmosphere, while a porcelain boat loaded with thiourea was positioned upstream. The sample was subsequently heated at 350 °C for 2 h to introduce sulfur species, yielding the final PtCo/S/MCS catalyst.

The morphology and porous structure of the as-synthesized MCS were systematically characterized. As shown in Figure 2A, the transmission electron microscopy (TEM) image of the RF-SiO₂ precursor reveals smooth, solid spherical particles, confirming the successful co-condensation of the resorcinol-formaldehyde resin and silica components. Following carbonization at 800 °C and subsequent HF etching to remove the SiO₂ template, the resulting MCS retained its spherical morphology with a uniform size distribution, as evidenced by the SEM image in Figure 2B. The TEM image in Figure 2C further reveals a highly developed mesoporous interior structure within the carbon nanospheres, consistent with the removal of the silica framework and the formation of an interconnected pore network. N₂ adsorption-desorption measurements were performed to quantitatively assess the mesoporous characteristics of MCS. As shown in Figure 2D, the isotherm exhibits a typical Type IV profile with a distinct H₂-type hysteresis loop at relative pressures of 0.4-0.9, characteristic of mesoporous materials with ink-bottle-shaped pores^[30], yielding a BET specific surface area of 1,482.8 m² g^-1. The corresponding pore size distribution derived from the desorption branch using the Barrett-Joyner-Halenda method [Figure 2E] shows a narrow distribution centered at approximately 6.8 nm, further confirming the uniform mesoporous architecture of MCS. Such a high surface area and well-defined mesoporosity are expected to facilitate metal precursor infiltration during impregnation and provide abundant accessible active sites for catalytic reactions. The X-ray diffraction (XRD) pattern of MCS [Figure 2F] displays two broad diffraction peaks centered at approximately 24° and 43°^[31], indicative of the amorphous carbon nature of MCS with a low degree of graphitization.

Systematic characterizations were performed to elucidate the morphology and structure of MCS-supported Pt-based bimetallic catalysts. As shown in Figure 3A, the (111) diffraction peak of PtCo-S/MCS lies between those of metallic Pt (PDF#001-1194, 40.0°) and Co (PDF#001-1255, 44.4°), supporting the formation of a PtCo alloy phase^[32]. By contrast, the corresponding peak of sulfur-free PtCo/MCS is located closer to metallic Co, implying the formation of Co-rich alloy domains with less pronounced Pt incorporation. Similar peak shifts are also observed for PtFe-S/MCS and PtCu-S/MCS, supporting the formation of PtFe and PtCu alloy structures, respectively. No obvious diffraction peaks corresponding to isolated Co, Fe, Cu, or their oxide phases are detected, indicating that the secondary metals are mainly incorporated into Pt-based alloy domains. The broad diffraction profiles further suggest the nanosized nature of the alloy species, consistent with the confinement effect of the mesoporous carbon framework.

Scanning electron microscopy (SEM) images show that PtCo-S/MCS and PtCo/MCS retain the uniform spherical morphology of the MCS support after metal loading and high-temperature annealing [Figure 3B and C], confirming the structural robustness of the carbon nanospheres. Similar morphologies are observed for PtFe-S/MCS and PtCu-S/MCS [Figure 3D and E], indicating the versatility of the sulfur-anchored impregnation strategy across different Pt-based bimetallic catalysts. Furthermore, the incorporation of bimetallic alloy clusters into MCS reduces both pore size and BET specific surface area compared to pristine MCS [Figure 3F], which is attributed to the partial occupation of mesopores by the impregnated metal species. Nevertheless, all catalyst samples retain substantial surface areas and well-developed mesoporous structures, ensuring adequate exposure of the alloy active sites and efficient mass transport of reactants during catalytic reactions.

To further probe the internal structure of the catalysts, TEM characterization was performed. PtCo-S/MCS preserves the spherical mesoporous architecture of MCS, with ultrasmall metal clusters uniformly distributed throughout the carbon framework [Figure 4A and B]. The corresponding size distribution gives an average cluster diameter of 2.1 ± 0.5 nm, indicating that sulfur anchoring effectively suppresses metal migration and sintering during high-temperature annealing. The high resolution transmission electron microscopy (HRTEM) image [Figure 4C] reveals a lattice spacing of 0.218 nm, assignable to the PtCo (111) plane^[33,34], further supporting the alloy structure indicated by XRD. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings [Figure 4D] further show homogeneous Pt and Co distributions with clear spatial co-localization, together with dispersed S species across the carbon nanosphere, supporting the formation of sulfur-stabilized PtCo alloy clusters. In contrast, PtCo/MCS prepared without sulfur anchoring exhibits pronounced metal aggregation, with large nanoparticles scattered across and inside the carbon nanospheres [Figure 4E and F]. Although the lattice spacing of 0.215 nm can also be indexed to the PtCo (111) plane^[35], the larger particle size and less uniform elemental distribution indicate insufficient control over metal nucleation and growth in the absence of sulfur-mediated stabilization [Figure 4G]. The EDS mapping of PtCo/MCS [Figure 4H] shows a markedly inhomogeneous distribution of Pt and Co, with Co-rich domains associated with the larger particles, highlighting the compositional heterogeneity caused by uncontrolled sintering.

These results establish sulfur anchoring as a key structural regulator for confining PtCo alloy clusters within the mesoporous carbon framework. Rather than merely promoting alloy formation, the sulfur species primarily govern the dispersion, size confinement, and compositional uniformity of the bimetallic domains under reductive thermal treatment.

The generality of the sulfur-anchored strategy was further verified with PtFe-S/MCS and PtCu-S/MCS. PtFe-S/MCS preserves the spherical mesoporous architecture of MCS after high-temperature annealing, with ultrasmall metal clusters uniformly confined within the carbon framework [Figure 5A and B]. The narrow size distribution centered at 2.1 ± 0.6 nm indicates efficient suppression of metal sintering. HRTEM reveals a lattice spacing of 0.219 nm for the PtFe-S/MCS clusters, assignable to the PtFe (111) plane [Figure 5C]. HAADF-STEM and EDS mapping further reveal the homogeneous distribution and co-localization of Pt and Fe, together with dispersed S species throughout the carbon nanosphere, supporting the formation of sulfur-stabilized PtFe alloy clusters [Figure 5D]. PtCu-S/MCS also retains the mesoporous spherical morphology, but the metal species grow into larger nanoparticles with an average size of 5.6 ± 1.9 nm [Figure 5E and F], which can be attributed to the intrinsically higher aggregation tendency of Cu species during thermal treatment. The resolved lattice spacing of 0.221 nm can be indexed to the PtCu (111) plane, confirming alloy formation with a contracted Pt lattice [Figure 5G]. Elemental mapping shows broadly distributed Pt, Cu, C, and S signals across the nanosphere, indicating that sulfur anchoring remains effective at dispersing PtCu species [Figure 5H].

The elemental contents of the catalysts were further determined by Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES) and EDS analyses, and the results are summarized in Table 1. The Pt contents of PtCo-S/MCS, PtCu-S/MCS, PtFe-S/MCS, and PtCo/MCS were 5.1, 4.9, 5.0, and 4.8 wt.%, respectively, while the corresponding contents of Co, Cu, and Fe were close to the nominal values. Sulfur was detected only in the sulfur-containing catalysts, with contents ranging from 0.8 to 1.3 wt.%.

To further investigate the chemical states of Pt, Co, and S, X-ray photoelectron spectroscopy (XPS) measurements were performed. The Pt 4f spectra of all samples can be deconvoluted into metallic Pt⁰ and partially oxidized Pt²⁺ species^[36] [Figure 6A-C], the dominant Pt⁰ contribution indicates the effective reduction of Pt precursors during H₂/Ar annealing, whereas the residual Pt²⁺ component is likely associated with electron-deficient Pt species at the alloy/support interface or Pt species interacting with sulfur-containing groups. The Co 2p, Fe 2p, and Cu 2p spectra reveal mixed surface valence states of the secondary metals [Figure 6D-F]. For PtCo-S/MCS, the Co 2p region consists of Co²⁺/Co³⁺ components accompanied by satellite features^[37]. PtFe-S/MCS exhibits Fe⁰, Fe²⁺, and Fe³⁺ contributions, while PtCu-S/MCS shows reduced Cu species together with Cu²⁺ related components and satellite peaks^[38-40]. These partially oxidized surface species can be attributed to the high fraction of exposed metal atoms and the high surface energy of ultrasmall nanoparticles, which render their surfaces more susceptible to mild oxidation upon air exposure. Next, the S 2p spectra provide direct evidence for sulfur incorporation and metal-sulfur coordination [Figure 6G-I]. The low binding energy can be assigned to metal-sulfur bonds, including Co-S, Fe-S, and Cu-S, whereas the C-S-C contribution indicates the incorporation of sulfur species into the carbon framework^[41]. The coexistence of M-S and C-S-C species suggests that sulfur serves as an interfacial anchoring motif, coupling the metal domains with the carbon support and thereby stabilizing highly dispersed bimetallic clusters or nanoparticles during high-temperature annealing, which differs from the classical strong metal-support interaction generally observed for reducible oxide-supported metal catalysts.

Based on the above structural characterizations, the role of sulfur anchoring can be further understood. During the impregnation process, sulfur species introduced by STG coordinate with Pt precursors and subsequently serve as anchoring sites during high-temperature annealing. The resulting metal-S interactions suppress the migration and aggregation of metal species, leading to the formation of highly dispersed PtCo alloy clusters within the MCS framework. Compared with the sulfur-free PtCo/MCS catalyst, the smaller and more uniformly distributed PtCo alloy clusters provide a larger number of accessible active sites, which is beneficial for the transfer hydrogenation of FFA to FOL.

To further investigate the role of sulfur, a post-sulfurized catalyst (PtCo-S/MCS) was prepared by introducing sulfur species onto preformed PtCo/MCS using thiourea. As shown in Supplementary Figure 1, sulfur was successfully introduced into the catalyst, as confirmed by elemental mapping. However, PtCo-S/MCS exhibited a much larger average particle size (23.2 ± 6.6 nm) than PtCo-S/MCS (2.1 ± 0.5 nm) [Supplementary Figure 2]. These results suggest that sulfur introduced after catalyst formation cannot reproduce the dispersion-promoting effect observed in PtCo-S/MCS.