Selective oxidation of glucose to gluconic acid by bimetallic catalysts

1 Introduction

Under the dual constraints of escalating land scarcity and limited biomass availability, the deepening implementation of sustainable development has further underscored the urgent need for efficient conversion of renewable resources (Popp et al., 2021; Svazas et al., 2019). Consequently, the valorization of biomass resources has become a key research direction in the fields of energy science and chemical engineering (Bai et al., 2022; Sun et al., 2024; Zhang et al., 2024). Solid biomass wastes, including agricultural wastes, forestry residues, and food-processing by-products, are rich in cellulose and cellulose-derived saccharides, representing abundant and renewable carbon resources with substantial potential for value-added conversion (Isikgor and Becer, 2015). As a representative biomass-derived platform molecule, glucose can be selectively converted into a variety of value-added chemicals, making it an important feedstock in green chemistry and sustainable industrial systems (Kong et al., 2026). Among these routes, the selective oxidation of glucose to gluconic acid has attracted considerable attention because of its relatively clean reaction pathway and the high practical value of the target product (Qamar et al., 2025).

In catalytic glucose oxidation systems, noble-metal catalysts generally exhibit high activity and selectivity. Among them, Pt has been widely used in the oxidation of saccharides and alcohols because its unique electronic structure facilitates oxygen-species activation and the conversion of organic molecules (Awad et al., 2026). Pt active sites can promote C-H bond activation and the adsorption of oxygen species, thereby accelerating the oxidation reaction (Lavrič et al., 2024). However, monometallic Pt catalysts still suffer from high cost, particle aggregation, and insufficient stability. Therefore, maintaining or even enhancing catalytic performance while reducing Pt loading is critical for the development of efficient Pt-based catalytic systems. Previous studies have shown that reducing the particle size of Pt can improve metal dispersion and utilization efficiency. However, nanoscale Pt particles are prone to migration, sintering, or aggregation during the reaction, which can lead to catalytic deactivation (Chen et al., 2023; Mahmud et al., 2022). Therefore, using suitable supports to stabilize and highly disperse Pt species at low loadings, while optimizing the electronic structure and intrinsic activity of Pt through metal-support interfacial modulation, is an important strategy for improving the performance of Pt-based catalysts (Wang et al., 2024).

In heterogeneous catalytic systems, supports not only affect the dispersion state and structural stability of metal particles but also regulate the electronic structure of active metals through metal-support interactions, thereby influencing reactant adsorption, oxygen-species activation, and the transformation of key intermediates (Li et al., 2022). Transition metal oxides (MOx), with tunable surface oxygen species, oxygen mobility, and electronic structures, are widely used to construct metal-oxide interfacial catalytic systems and regulate oxidation performance (Daelman et al., 2019). Different oxide supports exhibit pronounced differences in redox properties, defect structures, and surface structures, which can lead to significant variations in catalytic performance. Among them, NiO possesses good chemical and thermal stability and can provide active oxygen-related sites. Moreover, tunable Pt-NiO interactions can optimize the interfacial electronic structure and promote oxidation processes, making NiO a promising support for constructing Pt-based oxidation catalysts (Wang et al., 2024).

In addition to the type of support, regulation of the reaction environment during catalyst preparation can significantly affect the hydrolysis, deposition, nucleation, and interfacial structure formation of metal species. In solvothermal systems, the solution pH and alkaline environment are important factors governing the hydrolysis, deposition, and nucleation growth of metal precursors (Huo et al., 2023). Urea, a commonly used slow-release alkaline reagent, can gradually hydrolyze under heating to generate alkaline species and increase the OH⁻ concentration, thereby enabling mild and homogeneous pH regulation that favors the uniform nucleation and growth of metal species (Su et al., 2016). Meanwhile, urea can also serve as a nitrogen source during material construction, introducing nitrogen-containing structures under thermal or solvothermal conditions and further affecting the surface electronic structure and distribution of active sites in catalysts (Liu et al., 2022). Previous studies have shown that nitrogen doping can modulate the electronic properties and active-site structures of catalytic materials, thereby enhancing catalytic activity (Bai and Xu, 2025; Minotaki and López, 2024). However, in urea-assisted systems, alkaline regulation and the introduction of nitrogen-containing structures often occur simultaneously, making it difficult to distinguish their respective contributions to catalyst structure and performance. Therefore, introducing inorganic bases such as NaOH and Na₂CO₃ as control systems under comparable alkaline conditions can help exclude the effect of alkalinity alone and further clarify the structural regulation role of urea during catalyst construction.

Based on the aforementioned considerations, this study fabricated a series of Pt/MOx catalysts via a solvothermal method, utilizing H₂PtCl₆ as the platinum precursor, various metal oxides as the secondary component, and ethylene glycol as the reaction medium. By modulating the support types and alkaline source conditions, the relationships among catalyst composition, interfacial structure, and glucose selective oxidation performance were systematically investigated. Moreover, the catalytic performance under mild conditions was evaluated using the selective oxidation of glucose to gluconic acid as a model reaction. This study aims to elucidate the origins of the performance enhancement of Pt-based catalysts from the dual perspectives of support effects and urea-assisted structural modulation, thereby providing an experimental foundation for the design of highly efficient noble metal catalysts and the green conversion of biomass platform molecules.

2 Experimental section

2.1 Materials

Nickel oxide (99%), cerium oxide (99.95%), aluminum oxide (99.8%), zirconium oxide (99%), zinc oxide (99%), copper oxide (98%), trimanganese tetraoxide (99.5%), urea (99.5%), sodium carbonate (98%), chloroplatinic acid (99.995%), ethylene glycol (98%), and nickel nitrate (99%) were purchased from Aladdin (China). Manganese (III) oxide (99%) and sodium hydroxide (NaOH) were obtained from Macklin (China). Anhydrous ethanol (98%) was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. All chemicals were used as received without further purification.

2.2 Material preparation

Pt/MOx series catalysts were prepared by a solvothermal method using H₂PtCl₆ as the Pt source, different metal oxides (ZnO, CuO, Mn₂O₃, Mn₃O₄, NiO, CeO₂, ZrO₂, Al₂O₃, etc.) as the second-metal sources, urea, NaOH, or Na₂CO₃ as the base source, and ethylene glycol (E.G.,) as the reaction medium.

During the experiment, H₂PtCl₆ (0.02 g), the corresponding metal oxide at a Pt/metal molar ratio of 1:3, and 0.3 g urea or an equivalent alkalinity amount of NaOH/Na₂CO₃ were added to 20 mL, EG. The mixture was transferred into a 100 mL polytetrafluoroethylene (PTFE)-lined stainless-steel autoclave and magnetically stirred at 900 rpm for 1 h at room temperature to ensure sufficient disperse on. For the control experiments, 0.4256 g Ni(NO₃)₂·6H₂O was additionally introduced into the above system as a metal salt precursor.

The sealed autoclave was then heated at 195 °C for 12 h in an oven. After cooling naturally to room temperature, the reaction product was collected and separated by filtration. The obtained solid was washed several times with deionized water and absolute ethanol to remove residual precursors and solvent, followed by vacuum drying at 60 °C to afford the Pt/MOx series catalysts. According to the type of metal oxide, base source, and whether a metal salt precursor was introduced, the samples were denoted as Pt/ZnO-U, Pt/CuO-U, Pt/Mn₂O₃-U, Pt/Mn₃O₄-U, Pt/NiO-U, Pt/CeO₂-U, Pt/ZrO₂-U, Pt/Al2O₃-U, Pt/NiO-OH, Pt/NiO-CO3, Pt/NiO-Ni-U, and Pt/NiO-Ni-OH.

2.3 Characterization

The surface morphology and overall structure of the synthesized materials were characterized by scanning electron microscopy (SEM, TESCAN MIRA LMS). Powder samples were dispersed in ethanol and ultrasonicated for 5 min before analysis, and morphological information was acquired in secondary electron imaging mode. The microstructure and nanoscale morphology of the materials were examined using a transmission electron microscope (TEM, JEM-2100 F) operated at an accelerating voltage of 200 kV. For TEM sample preparation, the samples were dispersed in ethanol, ultrasonicated, drop-cast onto copper grids, and air-dried before observation. The crystal structures of the catalysts were analyzed by X-ray diffraction (XRD, Rigaku Ultima IV) using Cu Kα radiation (λ = 0.154 nm) at an operating voltage of 40 kV and a current of 40 mA. Diffraction patterns were collected over a 2θ range of 5°–90° at a scanning rate of 8°min⁻¹. The surface elemental compositions and chemical states of the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using monochromatic Al Kα radiation (1,486.6 eV) as the excitation source, with each sample scanned 200 times to improve measurement accuracy. The surface chemical bonding structures were characterized by Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Scientific Nicolet iS20) over the range of 400–4,000 cm⁻¹. Structural defects and other structural characteristics were investigated by Raman spectroscopy (Horiba LabRAM HR Evolution) using an excitation wavelength of 532 nm. The metal contents of the catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5,110). The specific surface areas and pore structures of the samples were measured using an automated surface area and porosity analyzer (Micromeritics ASAP 2460) through N₂ adsorption–desorption measurements at 77 K. Surface-active oxygen species were investigated by oxygen temperature-programmed desorption (O₂-TPD, TP-5080), while the thermal stability of the samples was evaluated by thermogravimetric analysis (TGA, Netzsch STA449F3) under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹.

Conversion mol % = 1 − moles sugar in product initial moles subtrate × 100 % (1)

Yield mol % = 1 − moles sugar acid in product initial moles subtrate × 100 % (2)

2.4 Product analysis

After the reaction, the reaction mixture was filtered through a polyethersulfone (PES) membrane with a pore size of 0.2 μm. The obtained filtrate was analyzed by high-performance liquid chromatography (HPLC, Waters e2489) equipped with an Aminex HPX-87H column, and separation was performed at 50 °C. The detection system was equipped with a refractive index (RI) detector and an ultraviolet (UV) detector. The mobile phase was 5 mM H₂SO₄ solution at a flow rate of 0.5 mL·min⁻¹, and the injection volume was 20 μL. After the reaction, the recovered catalyst was washed successively with ethanol and deionized water four times to remove adsorbed reactants and by-products on the surface. The washed catalyst was then dried under vacuum at 60 °C for 12 h, and the dried sample was used for subsequent characterization or reuse. The glucose conversion and gluconic acid yield were calculated according to Equations 1, 2, respectively.

3 Result and discussion

3.1 Support screening and performance differences

The glucose oxidation performance of Pt catalysts supported on different metal oxides under identical reaction conditions (100 °C and 1 MPa O₂) is summarized in entries 1–9 of Table 1. Overall, the catalytic performance varied markedly with the oxide support. Among the catalysts examined, Pt/NiO-U exhibited the best performance, achieving complete glucose conversion and a substantially higher target-product yield than the other metal oxide-supported catalysts. Comparison of the catalysts prepared using different alkaline additives (entries 3 and 10–14 in Table 1) showed that Pt/NiO-U, synthesized using urea as a synthesis-regulating agent, markedly outperformed Pt/NiO-OH and Pt/NiO-CO₃, which were prepared using NaOH and Na₂CO₃, respectively. Although all three additives provided an alkaline environment during solvothermal synthesis, Pt/NiO-U afforded a higher product yield and superior selectivity, indicating that its enhanced catalytic performance cannot be attributed solely to differences in synthesis alkalinity. Combined with the structural characterization results, these findings suggest that urea not only acted as a slowly decomposing base to regulate the pH during solvothermal synthesis but also introduced nitrogen-containing species that may have modified the surface electronic structure of the catalyst. Moreover, the introduction of an additional Ni salt precursor in the Pt/NiO-Ni-U and Pt/NiO-Ni-OH systems did not result in a further significant improvement in catalytic performance. This result indicates that simply introducing an additional Ni source is insufficient to enhance catalytic activity and may instead adversely affect performance by promoting particle aggregation or the formation of heterogeneous interfacial structures.

TABLE 1. Catalytic performance of catalysts for glucose oxidation under identical conditions (100 °C, 1 MPa O₂).

The catalytic performance of Pt/NiO-U under different reaction conditions is shown in Figure 1. In the temperature optimization experiments (Figure 1a), increasing the reaction temperature from 60 °C to 80 °C increased the glucose conversion from 75.2% to 100% and the target-product yield from 56.7% to 85.2%. When the temperature was further increased to 100 °C, the yield increased to 92.5%. These results demonstrate that increasing the temperature accelerates the reaction kinetics, although the incremental improvement becomes less pronounced at higher temperatures, suggesting that the system approaches a kinetically favorable reaction regime. The effect of reaction time is shown in Figure 1b. When the reaction time was extended from 0.5 to 1 h, glucose conversion rapidly reached 100%, accompanied by a substantial increase in product yield. Further extending the reaction time to 3 h increased the yield to 95.1%. Thus, a longer reaction time promoted more complete product formation, whereas the additional improvement gradually diminished at prolonged reaction times. Oxygen pressure also exerted a pronounced effect on catalytic performance (Figure 1c). The catalyst exhibited relatively low activity in the absence of O₂. When the O₂ pressure was increased to 1.5 MPa, glucose conversion reached 100%, and the product yield attained a maximum of 87.6%. A further increase in O₂ pressure to 2.0 MPa resulted in a slight decrease in yield, suggesting that an appropriate O₂ pressure facilitates oxidant activation and the availability of reactive oxygen species, whereas excessive pressure may promote overoxidation or undesired side reactions. The effect of catalyst loading is presented in Figure 1d. Increasing the catalyst loading rapidly increased glucose conversion to 100%, while the product yield reached a maximum of 85.2% at a catalyst loading of 20 mg. This decrease in yield with increased active sites, despite complete conversion, is a classic kinetic signature of a consecutive or parallel over-oxidation reaction. With more active sites available, the desired product has a higher probability of being re-adsorbed and further oxidized to undesired byproducts before it can desorb into the bulk solution (Deng et al., 2021). Increasing the loading further to 30 mg decreased the yield, possibly because an excessive number of available active sites promoted nonselective or consecutive oxidation reactions, thereby lowering product selectivity. Furthermore, Table 2 summarizes representative Pt-containing heterogeneous catalysts recently reported for glucose oxidation under aqueous conditions without the addition of soluble alkali. The comparative results indicate that the Pt/NiO-U catalyst designed in this study affords complete glucose conversion and a gluconic acid yield of 92.5%, thereby achieving a desirable balance between conversion and target-product yield among the surveyed catalytic systems.

TABLE 2. Selective oxidation of glucose to sugar acids with Pt-containing heterogeneous catalysts in water without added soluble alkali.

3.2 Characterizations results

SEM observations first revealed the microscale morphology of the Pt/NiO-U catalyst. As shown in Figure 2a-b, the sample exhibited a hierarchical architecture composed of irregular agglomerates, within which rod- or plate-like structural units were interstacked. These units possessed relatively rough surfaces, together with abundant interparticle voids and pore-like features. Such an open hierarchical architecture may provide favorable pathways for reactant transport and diffusion (Dhas et al., 2022). TEM analysis further revealed the microstructure of the catalyst. As shown in Figure 2c, nearly spherical Pt nanoparticles were uniformly dispersed on the NiO surface, with no apparent formation of large agglomerates. Particle-size analysis showed that the Pt nanoparticles were predominantly distributed within the range of 2–4 nm, with an average diameter of approximately 2.89 nm. The relatively small particle size and uniform distribution may be associated with the gradual generation of OH⁻ through urea hydrolysis during the solvothermal process, which creates a mild and homogeneous alkaline environment. These conditions may regulate the deposition and nucleation rates of the Pt precursor, thereby suppressing excessive particle growth and aggregation (Li et al., 2025). The crystal structure of the catalyst was further examined by HRTEM. As shown in Figure 1d-e, well-resolved lattice fringes were observed, with interplanar spacings of 0.224 and 0.214 nm assigned to the (111) plane of metallic Pt (Wang et al., 2020) and the (200) plane of NiO, respectively. These results demonstrate the coexistence of metallic Pt and crystalline NiO and, together with the TEM observations, confirm that Pt nanoparticles were successfully deposited on the NiO surface with intimate interfacial contact. The HAADF-STEM-EDS elemental maps in Figure 2f show that Ni and O exhibited closely matched spatial distributions throughout the primary particles, confirming that NiO constituted the main structural framework. Pt was uniformly distributed across the NiO domains, further demonstrating its high degree of dispersion. A distinct N signal was also detected, suggesting that urea-derived nitrogen-containing species may have been introduced and partially retained during synthesis, although their specific chemical states require further verification by XPS. According to the ICP-OES results presented in Supplementary Table S1, the Pt and Ni contents of the Pt/NiO-U catalyst were 17.3 and 66.0 wt%, respectively.

The crystalline phases, surface functional groups, carbonaceous structures, and textural properties of the catalysts are presented in Figure 3. The crystalline structures of the samples were first examined by X-ray diffraction, as shown in Figure 3a. All samples exhibited distinct diffraction peaks at 2θ values of 37.2°, 43.3°, 62.9°, 75.4°, and 79.4°, which were indexed to the (111) (200) (220) (311), and (222) planes of NiO (PDF No. 89–7,131), respectively. These results indicate that the NiO framework retained good crystalline-phase stability under the different synthesis conditions. No discernible diffraction peaks attributable to crystalline Pt were detected in any of the samples, suggesting that the Pt species were highly dispersed and/or present as crystallites below the detection limit of XRD, rather than as large Pt crystallites (Zhong et al., 2024). Moreover, no additional crystalline phases or appreciable shifts in the NiO diffraction peaks were observed. Within the detection limit of XRD, these findings indicate that no detectable crystalline Pt-Ni alloy or other new phase was formed under the applied synthesis conditions and that the catalysts predominantly retained the structural characteristics of supported Pt/NiO catalysts.

FTIR spectroscopy was further employed to investigate the surface functional groups of the catalysts. As shown in Figure 3b, the broad absorption band centered at 3,431 cm⁻¹ was assigned to the O-H stretching vibrations of surface hydroxyl groups and/or adsorbed water molecules (Niu et al., 2024; Wang et al., 2017). The band at 1,627 cm⁻¹ was attributed to the H-O-H bending vibration, confirming the presence of adsorbed water on the catalyst surface (Liu et al., 2025). For the samples synthesized using urea, distinct absorption bands were observed at approximately 1,372 and 1,060 cm⁻¹. The band at 1,372 cm⁻¹ may be associated with nitrogen-containing surface species, such as C-N functionalities, although contributions from residual surface anions cannot be excluded. The band at 1,060 cm⁻¹ can be assigned to the stretching vibrations of C-O or C-N bonds. These results suggest that urea participated in modifying the surface chemical environment during catalyst synthesis and may have introduced nitrogen-containing functionalities, thereby potentially influencing the electronic properties of the catalyst. Nevertheless, because the vibrational assignments in these regions may overlap, the chemical identities and states of these surface species require further confirmation by XPS.

Raman spectroscopy was employed to investigate the local structural and defect characteristics of the catalysts. As shown in Figure 3c, Pt/NiO-Ni-U exhibited two distinct bands centered at approximately 1,356 and 1,521 cm⁻¹, which were tentatively assigned to the D and G bands of disordered carbon, respectively. The D band is associated with defect-induced vibrations in disordered sp²-carbon domains, whereas the G band arises primarily from the in-plane stretching vibrations of sp²-bonded carbon atoms. These features suggest the presence of carbonaceous species containing disordered or defective sp² domains on the catalyst surface (Wang et al., 2020). Nevertheless, because NiO may also exhibit two-magnon Raman scattering near 1,500 cm⁻¹, the assignment of the 1,521 cm⁻¹ band exclusively to the carbon G band should be further verified by spectral deconvolution and complementary characterization. In addition, all samples displayed a band at approximately 546 cm⁻¹, which was attributed to a defect-activated one-phonon Ni-O vibration of NiO. The persistence of this band across the samples, together with the XRD results, confirms that the NiO framework was retained under the different synthesis conditions.

The N₂ adsorption-desorption isotherm and corresponding pore-size distribution of Pt/NiO-U are shown in Figure 3d. The sample exhibited a pronounced adsorption-desorption hysteresis loop, indicating capillary condensation within mesopores. Its pore-size distribution extended over a broad range of 1.72–148.55 nm, spanning the conventional micropore (<2 nm), mesopore (2–50 nm), and macropore (>50 nm) regimes. This broad distribution suggests a three-level hierarchical porous architecture comprising micro-, meso-, and macropores, with microporosity predominating. Such a multiscale pore network may provide interconnected mass-transport pathways, shorten the diffusion distance of reactants, and improve the accessibility of active sites. BET analysis revealed a specific surface area of 2.84 m² g⁻¹ and a total pore volume of 0.0146 cm³ g⁻¹. The BJH desorption average pore diameter was 13.08 nm, further confirming the predominantly mesoporous nature of the catalyst.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface elemental composition and chemical states of the catalysts, and the results are presented in Figure 4. The high-resolution C 1s spectrum in Figure 4a was deconvoluted into three principal components. The peak at 284.8 eV was assigned to C-C/C=C bonds in graphitic carbon, while the component at 286.0 eV was mainly attributed to C-N bonds, although a contribution from C-O species cannot be completely excluded. Together with the N 1s results, this component supports the incorporation of nitrogen into the carbon framework (Chen et al., 2021). The peak at approximately 288.5–289.0 eV was assigned to oxygen-containing functional groups such as O-C=O. The O 1s spectrum in Figure 4b was fitted with three components corresponding to lattice oxygen at 529.5–530.0 eV, surface hydroxyl groups and/or defect-associated oxygen species at 531.0–532.0 eV, and adsorbed oxygen species or molecular water above 532.5 eV.

The Pt 4f spectrum in Figure 4c exhibited the characteristic spin–orbit doublets of Pt species. The peaks at approximately 71.6 and 74.6 eV were assigned to the Pt 4f₇/₂ and Pt 4f_5/2 components of metallic Pt⁰, respectively, whereas the peaks at approximately 72.3 and 75.5 eV were attributed to partially oxidized Pt²⁺ species (Mosallanezhad et al., 2022). A Ni 3p contribution was also observed at approximately 68.1 eV and was considered during the fitting of the Pt 4f region because of the partial overlap between the two signals. Compared with the sample prepared without urea (Pt/NiO-OH), Pt/NiO-U exhibited a slight positive shift in the Pt 4f binding energy, consistent with a decrease in the electron density around Pt. Meanwhile, the Ni 2p peaks of Pt/NiO-U shifted toward lower binding energies relative to those of Pt/NiO-OH, indicating an increase in the electron density around Ni. Importantly, these opposite shifts are not contradictory; rather, they are consistent with a coordinated redistribution of interfacial charge associated with the urea-derived nitrogen-containing species. A plausible interpretation is that the nitrogen functionalities identified in the N 1s spectrum-including pyridinic, pyrrolic, and graphitic N species-interact preferentially with the NiO support, potentially through N-Ni coordination or partial incorporation into the near-surface NiO structure. Such interactions may increase the electron density of adjacent Ni sites, as suggested by the negative shift in the Ni 2p binding energy. To equilibrate the Fermi levels across the intimately contacted Pt-NiO interface, charge transfer from the Pt nanoparticles to the nitrogen-modified NiO support may occur, resulting in the observed electron depletion around Pt and the corresponding positive shift in the Pt 4f binding energy. This bidirectional electronic modulation is consistent with an electronic metal-support interaction (EMSI). The resulting polarized interface, comprising relatively electron-rich NiO domains and electron-deficient Pt sites, may influence molecular-oxygen activation and the adsorption or transformation of oxygen-containing intermediates. Electron-deficient Pt sites may modify the adsorption strengths and reaction pathways of oxygenated species, whereas electron-enriched NiO sites may facilitate the mobility and reactivity of surface oxygen species. Therefore, the enhanced catalytic performance of Pt/NiO-U is more appropriately attributed to the synergistic electronic coupling between nitrogen-modified NiO and interfacial Pt sites than to a simple unidirectional electron-withdrawing effect. Nevertheless, direct identification of N–Ni coordination and quantitative determination of the charge-transfer pathway would require further spectroscopic or theoretical investigation. This shift suggests that nitrogen incorporation modified the electronic environment of Pt and may have strengthened the metal-support interaction (Wang et al., 2020). In the Ni 2p spectrum shown in Figure 4d, the components at approximately 856.0 and 873.9 eV were assigned to the Ni 2p_3/2 and Ni 2p_1/2 signals of Ni²⁺, respectively. The accompanying satellite features at 860.9 and 879.6 eV are characteristic of NiO (Yan et al., 2012). Weak lower-binding-energy components at approximately 853.2 and 870.3 eV were detected in some samples and were tentatively assigned to metallic Ni⁰. Following Pt deposition, the Ni 2p peaks shifted slightly toward lower binding energies, indicating an increase in the electron density around Ni and providing evidence for interfacial electronic interactions between Pt and Ni-containing sites (Zhong et al., 2024). Deconvolution of the N 1s spectrum yielded three components centered at 398.8, 399.9, and 401.3 eV, which were assigned to pyridinic N, pyrrolic N, and graphitic N, respectively (Li et al., 2023).

Figure 5a presents the thermogravimetric (TG) profile of the Pt/NiO-U catalyst. The sample exhibited only a small overall mass loss during heating, indicating good thermal stability under the applied measurement conditions. The slight mass loss below 150 °C was primarily attributed to the desorption of physically adsorbed water and weakly bound surface hydroxyl species. No appreciable mass loss was observed between 150 °C and 400 °C, suggesting the absence of substantial amounts of thermally labile residual precursors. Above 400 °C, the sample mass remained nearly constant, further demonstrating the high thermal stability of the NiO framework (Niu et al., 2024; Wang et al., 2017).

Figure 5b compares the O₂ temperature-programmed desorption (O₂-TPD) profiles of the different catalysts. According to their desorption temperatures, the oxygen species were classified as weakly bound surface-adsorbed oxygen species, such as O₂ ⁻ and O⁻, in the low-temperature region below 300 °C; more strongly bound surface lattice oxygen species in the intermediate-temperature region of 300 °C–600 °C; and bulk or subsurface lattice oxygen species above 600 °C. The principal desorption peaks of all samples were concentrated in the intermediate-temperature region, indicating that surface lattice oxygen constituted the predominant desorbable oxygen species. Among the catalysts, Pt/NiO-U exhibited a stronger and more concentrated intermediate-temperature desorption peak, together with a discernible low-temperature signal. These features suggest that Pt/NiO-U possessed a greater abundance of accessible surface oxygen species, including readily activated adsorbed oxygen and relatively labile surface lattice oxygen. Such oxygen species may facilitate interfacial redox cycling and oxygen transfer, thereby contributing to the enhanced catalytic oxidation performance. In comparison, the other catalysts displayed weaker and broader desorption features, suggesting a lower abundance of desorbable surface oxygen species and a more heterogeneous distribution of oxygen-binding strengths.

3.3 Recycle and reuse

The recycling stability of the Pt/NiO-U catalyst is presented in Figure 6. Glucose conversion remained at 100% throughout five consecutive reaction cycles, demonstrating that the catalyst retained its apparent activity upon repeated use. Nevertheless, the gluconic acid yield gradually decreased from 84.2% in the first cycle to 79.3% in the fifth cycle, while the reported gluconic acid selectivity remained above 90%. Moreover, the metal content and metal-leaching behavior of the recycled Pt/NiO-U catalyst were characterized via ICP-OES. The Pt content of the fresh catalyst was 17.3 wt%, and the metal leaching was below 0.08% after each catalytic cycle. These results indicate that Pt/NiO-U maintained high conversion and product selectivity, although a modest decline in gluconic acid production was observed during recycling.

The ICP-OES results presented in Supplementary Table S1 showed that the leaching percentages of Pt and Ni after five consecutive cycles were only 0.082% and 0.046%, respectively. Such negligible metal loss demonstrates the strong resistance of the catalyst to the dissolution of its active components during the reaction. The post-reaction TEM image in Figure 7a revealed no appreciable morphological changes in the examined regions, and the Pt nanoparticles remained well dispersed on the NiO surface without pronounced aggregation. The lattice spacing of approximately 0.224 nm observed in the HRTEM image was assigned to the Pt (111) plane, confirming that the crystalline structure of metallic Pt was retained after recycling. The XPS spectra in Figure 7b-d showed that the characteristic C-C, C-N, and O-C=O components remained detectable in the C 1s region of the recycled catalyst, indicating the retention of nitrogen- and oxygen-containing surface functionalities. Quantitative XPS analysis (Table 3) showed that the surface Pt/Ni molar ratio decreased modestly from 0.91 to 0.83 after recycling, while the O-H/O_total and O_latt/O_total ratios changed only from 51.0% to 49.0%–50.7% and 49.3%, respectively. These minor variations indicate that the surface elemental composition and oxygen-species distribution were largely preserved during repeated use. Moreover, the Ni 2p spectrum remained dominated by Ni²⁺ species, with only a minor contribution from Ni⁰, demonstrating that the recycled catalyst retained a predominantly NiO-based surface chemical state.

TABLE 3. Surface atomic composition and oxygen species distribution of different Pt/NiO-based catalysts, including fresh and cycled samples, determined by XPS.

4 Conclusion

In this work, a series of Pt/MOx composite catalysts were prepared via a urea-assisted solvothermal route and evaluated for the selective oxidation of glucose to gluconic acid. Support screening identified Pt/NiO-U as the most active catalyst among the investigated metal oxide-supported systems, achieving complete glucose conversion and a gluconic acid yield of 92.5% at 100 °C under 1 MPa O₂. Structural characterization revealed that ultrasmall Pt nanoparticles, with an average diameter of approximately 2.89 nm, were highly dispersed on the NiO surface. N₂ adsorption-desorption analysis further demonstrated a broad pore-size distribution of 1.72–148.55 nm. The resulting micro-meso-macroporous hierarchical architecture was dominated by mesopores, with a BJH average pore diameter of 13.08 nm. This three-level pore network may provide multiscale transport pathways, shorten reactant diffusion distances, and improve the accessibility of Pt active sites. Notably, urea played a multifunctional role during catalyst synthesis: in addition to regulating the synthesis environment, it introduced nitrogen-containing surface species and modified the electronic structure of the Pt–NiO interface and the distribution of surface oxygen species. The combination of highly dispersed Pt nanoparticles, hierarchical three-level porosity, and urea-mediated interfacial regulation constitutes the principal structural innovation of the catalyst. Reaction-condition optimization further demonstrated that Pt/NiO-U achieved 100% glucose conversion and a gluconic acid yield of approximately 85.2% under the milder conditions of 80 °C, 1 h, and 1 MPa O₂. Recycling experiments and ICP-OES analysis confirmed its favorable stability, with the leaching percentages of both Pt and Ni remaining below 0.1% after five consecutive cycles. This work establishes an effective strategy integrating oxide-support effects, hierarchical pore engineering, and urea-assisted interfacial regulation for the rational design of efficient Pt-based catalysts for selective glucose oxidation.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2026.1943690/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Junfei Liang, North University of China, China

Reviewed by: Yaohui Zhang, North University of China, China

Guangyan Zhong, Apple United States, United States