Supported noble metal catalysts for complete propane oxidation: structure-activity relationships and reaction mechanisms

The effect of the chemical state of Pt

Studies have shown that the chemical state of platinum species (such as Pt⁰, Pt²⁺, etc.) is one of the most important factors influencing the activity and stability of catalysts in propane catalytic oxidation^{[42,43,50,55]}. However, the true active species of Pt for propane oxidation remains a subject of extensive debate. Most studies propose that metallic Pt (Pt⁰) is the primary active center, while some emphasize the role of oxidized Pt (PtO_x), or suggest a synergistic effect between Pt⁰ and PtO_x. Li et al. proposed a cooperative mechanism in which C–H bond activation and cleavage primarily occur at Pt⁰ sites, while subsequent oxidation steps are facilitated by PtO_x^[41]. Thus, optimal catalytic performance is achieved when the Pt⁰/PtO_x ratio approaches 1:1. Similarly, Enterkin et al. deposited Pt onto SrTiO₃ perovskite supports using atomic layer deposition^[66]. The results show that the strong epitaxial interaction between SrTiO₃ and Pt enables Pt to exist in a stable metallic state, and under oxidative conditions, a Pt/PtO core-shell structure is formed, thereby significantly enhancing the catalytic oxidation performance of propane [Figure 2A]. Bal’zhinimaev et al. found that Pt catalyst prepared by ion-exchange method on silicate glass fibers (FG) [Pt(v)/FG] and Pt catalyst prepared by ion-exchange method on N-modified silicate glass fibers (FG) [Pt(v)/N-FG] catalysts containing both Pt²⁺ and Pt⁰ exhibit higher activity than the Pt catalyst prepared by incipient wetness impregnation on silicate glass fibers (FG) [Pt(s)/FG] catalyst with only Pt^0[67]. Shan et al. achieved a controllable surface transformation from PtNiCo alloy to PtNiCo-PtNiOCoO structure by adjusting the calcination conditions^[68]. The result revealed that the synergistic effect at the alloy-oxide interface stabilized Pt^δ+, significantly improving propane oxidation performance and thermal stability (with unchanged activity after aging at 800 °C). However, O’Brien et al. observed a significant decrease in the activity of Pt/Al₂O₃ catalysts under oxygen-rich conditions where the O₂/C₃H₈ ratio exceeds 5 for propane oxidation^[69]. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments reveal that under these conditions, metallic platinum nanoparticles are oxidized to PtO_x, leading to catalyst deactivation. This indicates that PtO_x formation does not promote the reaction but instead inhibits catalytic activity, thereby supporting the conclusion that Pt⁰ is the true active species. Liu et al. further demonstrated that under high-temperature reaction conditions, propane molecules exhibit reductive capabilities, progressively reducing PtO_x to Pt⁰ during the reaction process, thereby enhancing catalyst performance [Figure 2B and C]^[70]. You et al. constructed tri-coordinated Al (Al^III) sites in the Pt/Al₂O₃ catalyst using a facile solvothermal method^[39]. The presence of these Al^III sites promotes the dispersion and increases the surface electron density of Pt on the Pt/Al₂O₃ catalyst. As a result, the elevated proportion of metallic Pt species demonstrates exceptional propane adsorption and activation, leading to remarkable activity, long-term stability, and water resistance in propane oxidation. Peng et al. proposed that the excellent catalytic activity of the Pt/Ni_0.5Co₁AlO_x catalyst is primarily attributed to its abundant Pt⁰, Co³⁺, active oxygen species, and high specific surface area^[71]. Dun et al. found that the weak Pt-MnSO₄ interaction stabilized the Pt⁰-MnSO₄ active center, facilitating the initial C–H bond cleavage and rapid C–C bond cleavage of propane^[72]. A recent study by our group on the Pt/CeO₂ system^[56] revealed that platinum species exist as Pt^δ+ at the metal-support interface, while Pt⁰ dominates in regions distant from the interface. Moreover, metallic Pt⁰ exhibits higher reactivity than interfacial Pt^δ+. Although the precise active species of Pt in propane oxidation remains controversial, substantial evidence indicates that metallic Pt⁰ demonstrates outstanding catalytic performance. Consequently, developing structural control strategies to efficiently expose and stabilize Pt⁰ has become a central focus in designing high-performance Pt-based propane oxidation catalysts.

Figure 2. (A) Transmission electron microscopy images of Pt/SrTiO₃ after five ALD cycles and propane oxidation light-off curves over two temperature cycles (25-400 °C) for 1c, 3c, and 5c Pt/SrTiO₃, 4.7%Pt/γ-Al₂O₃, and platinum-free STO nanocuboids, with C₃H₈/O₂ = 1:16 and C₃H₈ WHSV = 4 h^-1. Reproduced with permission from Ref.^[66]; (B) Relationship between TOFs at 220 °C and Pt/Ptⁿ⁺ ratios for various catalysts. Reproduced with permission from Ref.^[70]; (C) Evolution of the chemical state of Pt species on a Pt/BN catalyst during propane oxidation. Reproduced with permission from Ref.^[70]. ALD: atomic layer deposition; STO: SrTiO₃; WHSV: weight hourly space velocity; TOFs: turnover frequencies; BN: boron nitride.

The effect of support

Different types of supports significantly influence the catalytic performance of Pt by regulating its electronic structure, dispersion, and redox properties. Avila et al. prepared supported Pt catalysts using CeO₂, TiO₂, and Al₂O₃ as supports via the impregnation method^[73]. They found that the turnover frequency (TOF) for propane catalytic oxidation followed the order Pt/TiO₂ > Pt/CeO₂ > Pt/Al₂O₃. It was concluded that using reducible oxides (such as CeO₂ and TiO₂) as supports enhances the interaction with Pt active sites, thereby improving the catalytic activity for propane oxidation. In addition, the acid-base properties of the support can also influence the chemical state of the supported Pt. Increasing the surface acidity can promote the presence of Pt species in the metallic state (Pt⁰), thereby significantly affecting the catalytic performance of the catalyst. Yazawa et al. synthesized a series of supported Pt catalysts, including Pt/MgO, Pt/La₂O₃, Pt/ZrO₂, Pt/Al₂O₃, Pt/SiO₂, Pt/SiO₂-Al₂O₃, and Pt/SO₄^2--ZrO₂^[74,75]. The results showed that the catalytic activity for propane oxidation is closely related to the acidity of the support surface, with the stronger the acidity, the higher the catalytic activity. Further research revealed that the acid-base properties of the supports directly regulate the chemical state of Pt. On acidic supports, part of the Pt exists in the metallic state (Pt⁰), whereas on basic supports, Pt mainly exists in an oxidized form^[75]. Huang et al. and Wang et al. directly compared the performance differences of Pt catalysts supported on redox oxide CeO₂ and acidic oxide Nb₂O₅ in propane oxidation [Figure 3A and B]^[30,76]. Their studies showed that the Pt/Nb₂O₅ catalyst surface contains a higher proportion of metallic Pt (Pt⁰) species, which facilitates the activation of C–H bonds and significantly enhances the catalytic oxidation performance of propane. In contrast, the Pt-CeO₂ interface in Pt/CeO₂ catalysts more effectively activates oxygen species and promotes CO oxidation [Figure 3C-E]. Furthermore, the type of support strongly influences the reaction pathway of propane oxidation. On Pt/CeO₂ catalysts, propane primarily forms propionate intermediates, which are then oxidized to formate and acetate species, whereas on Pt/Nb₂O₅ catalysts, propane tends to undergo dehydrogenation to propylene first, followed by oxidation to acrylate species, demonstrating a more efficient oxidation pathway. Xian et al. pointed out that, in addition to serving as supports, acidic carriers can also activate saturated alkanes, allowing propane and oxygen to avoid competing for Pt sites^[77]. Consequently, a synergistic catalytic effect arises between the Pt surface and the acidic sites, thereby enhancing the performance of propane oxidation. Wen et al. also reported that the physical mixture of acidic Mo₂O₃ with Pt/CeO₂ exhibits superior propane oxidation performance compared to catalysts possessing solely acidic (Pt/Mo₂O₃) or redox properties (Pt/CeO₂)^[78].

Figure 3. (A) Pt 4f XPS spectra and (B) catalytic activity of propane oxidation of Pt/CeO₂, Pt/SnO₂, Pt/ZrO₂, and Pt/Nb₂O₅ catalysts. Reproduced with permission from Ref.^[30]; XPS spectra of (C) Pt 4f and (D) O 1s of Pt/CeO₂ and Pt/Nb₂O₅ catalysts. Reproduced with permission from Ref.^[76]; (E) Catalytic activity of propane oxidation over Pt/CeO₂ and Pt/Nb₂O₅ catalysts. Reproduced with permission from Ref.^[76]. XPS: X-ray photoelectron spectroscopy.

The effect of the size of Pt nanoparticles

Extensive research indicates that as the size of noble metal particles shrinks from the nanoscale to the single-atom scale, their geometric structure and chemical state undergo significant changes, which profoundly influence their catalytic performance. As a structure-sensitive reaction, propane oxidation exhibits high sensitivity to the size of Pt particles, and the catalytic performance can be effectively enhanced by precisely tuning the particle size. Luo et al. prepared Pt/Al₂O₃ catalysts with average Pt particle sizes of 1.5, 2.5, 3.1, and 11.0 nm, respectively, and applied them to propane oxidation^[79]. The result showed that the catalyst with a 2.5 nm particle size exhibited optimal performance. Park et al. pointed out that on hydrogen-type Zeolite Socony Mobil-5 (H-ZSM-5, SiO₂/Al₂O₃ = 23), the specific reaction rate for propane oxidation decreases with increasing Pt particle size^[80]. For active oxide supports such as CeO₂, the regulation of electronic structure by Pt particle size is particularly important. Lykhach et al. combined density functional theory (DFT) calculations with synchrotron radiation experiments to quantitatively study the relationship between electron transfer at the Pt/CeO₂ interface and Pt particle size^[81]. The results revealed that when the Pt particle contains approximately 50 atoms, electron transfer from Pt to CeO₂ reaches a maximum, about one electron transferred per ten Pt atoms. For larger Pt particles, the CeO₂ support itself imposes stronger limitations on electron transfer, while for smaller, even atomic-scale Pt particles, the concentration of oxygen vacancies on the CeO₂ surface becomes the dominant factor, and the reduced CeO₂ has limited capacity to accept additional electrons.

In our recent study^[56], we systematically investigated the effect of Pt size on the efficiency of complete propane oxidation by synthesizing nanocubes of CeO₂ with well-defined (100) facets to support Pt nanoparticles of various sizes, ranging from 1.3 to 7 nm [Figure 4A-C]. We found that for Pt/CeO₂(100) catalysts, when the Pt particle size is in the range of 1.3-4.0 nm, a strong MSI between Pt and the CeO₂(100) support leads to partial oxidation of Pt to Pt^δ+. In addition, the catalytic activity is mainly influenced by the chemical state of Pt. As the Pt particle size exceeds 4.0 nm, the ability of the CeO₂(100) support to regulate electron transfer weakens, and the chemical state of Pt stabilizes, with the catalytic activity then depending more on the geometric structure of the Pt particles. Therefore, both the chemical state and geometric structure of Pt jointly determine the catalytic performance, resulting in a volcano-shaped relationship between propane oxidation activity and Pt particle size. Additionally, the TOF increases consistently as the particle size grows within this range. The study shows that the apparent performance for propane oxidation peaks when the Pt particle size is approximately 5 nm, while the intrinsic activity continues to rise with increasing Pt particle size. Gao et al. conducted a similar study, loading Pt single atoms, clusters, and nanoparticles separately onto ceria to systematically investigate the effect of Pt particle size on propane oxidation catalytic performance^[59]. However, their results showed that despite differences in Pt morphology and size, these catalysts exhibited similar trends in apparent activation energy (E_a) and TOF. The discrepancy between the two studies mainly arises from the different Pt particle size ranges investigated: our work focused on different uniform sizes within the 1-7 nm range, while the other study examined smaller sizes (from single atoms to about 2.5 nm Pt nanoparticles) and mixed-size species. This indicates that the effect of Pt particle size on catalytic performance in propane oxidation exhibits a distinct nonlinear behavior, which is fundamentally governed by multiple factors, including the strength of the MSI, electronic structure modulation, and the exposure of geometric structures. Although reducing the size of Pt nanoparticles to the single-atom level can maximize noble metal atom utilization and reduce material consumption, for propane oxidation, single-atom Pt lacks metal ensemble sites that are essential for the simultaneous activation of oxygen molecules and cleavage of C–H bonds. As a result, single-atom Pt exhibits relatively poor catalytic activity. Jeong et al. also confirmed this finding, showing that Pt with metal ensemble structures demonstrates superior catalytic performance in propane oxidation compared with fully dispersed single-atom Pt^[82].

Figure 4. (A) Transmission electron microscopy images and particle size distributions of Pt particles with different sizes. Reproduced with permission from Ref.^[56]; (B) Propane oxidation activity of Pt nanoparticles with different particle sizes. Reproduced with permission from Ref.^[56]; (C) Correlation between the reaction rate and TOF for complete propane oxidation at 210 °C and the Pt particle size. Reproduced with permission from Ref.^[56]. TOF: Turnover frequency.

The influence of promoters

To enhance the catalytic performance and stability of supported Pt catalysts, promoters are often introduced to modify the catalyst structure and surface properties. Yoshida et al. investigated the effects of various additives, including alkali metals, alkaline earth metals, and fifth-period elements, on the propane oxidation activity of Pt/Al₂O₃ catalysts^[29,75]. The results showed that additives such as niobium (Nb), tungsten (W), and molybdenum (Mo), which possess strong electron-withdrawing abilities and electronegativities greater than 2.7, effectively suppressed the oxidation of Pt species under oxygen-rich conditions, stabilizing Pt in its metallic state and thereby improving propane oxidation activity. Based on this understanding, many Pt/MO_x (MO_x = Al₂O₃, CeO₂, TiO₂, ZrO₂, etc.) catalytic systems have been developed by introducing strongly acidic transition metal oxides (such as NbO_x, MoO_x, WO_x, VO_x, etc.), significantly enhancing the catalytic performance in propane oxidation. Shao et al. reported that modifying 0.5% Pt/ZrO₂ with WO_x led to the incorporation of WO_x into the ZrO₂ lattice, forming a highly dispersed Zr_1-xW_xO_2+x/2 structure^[83]. This structure facilitates electron transfer from W to Pt, generating strong acid sites and Pt⁰ species that accelerate the reaction process [Figure 5A]. Similarly, Liao et al. synthesized a series of 1 wt% Pt and different W contents (x wt%) supported on hexagonal boron nitride (Pt-xW/BN) catalysts with varying tungsten contents via a secondary impregnation method^[84]. They found that the propane catalytic oxidation activity was highest when the tungsten content was 7 wt%, and the activity decreased when the tungsten content exceeded this value. The results of the characterization demonstrated that the Pt-WO_x interface acts as a novel active site, effectively promoting the heterolytic cleavage of C–H bonds in propane and exhibiting higher propane oxidation activity than the conventional Pt⁰-Pt^δ+ dual active sites. Zhao et al. also reported that NbO_x modification of Pt/Al₂O₃ promoted the formation of Pt⁰ species, significantly enhancing the catalytic performance of Pt/Al₂O₃^[40]. Moreover, the catalyst exhibited the highest activity when the Nb content was 10 wt% [Figure 5B]. Wen et al. found that introducing an ultralow loading of Mo (0.1 wt%) as a promoter in Pt/SiO₂ catalysts not only effectively reduced the Pt particle size but also donated electrons to Pt particles, significantly facilitating propane activation^[85].

Figure 5. (A) Reaction mechanism and rate of propane oxidation over Pt/WO_x/t-ZrO₂ and Pt/WO_x/m-ZrO₂ catalysts. Reproduced with permission from Ref.^[83]; (B) Relationship between the propane conversion rate and reaction temperature of Pt-Nb/Al₂O₃ catalysts. Reproduced with permission from Ref.^[40]; (C) Light-off curves for propane oxidation over Pt/CeO₂-yPO_x catalysts. Reproduced with permission from Ref.^[57]; (D and E) Light-off curves (D) and reaction rates (E) for C₃H₈ oxidation over Pt/F-TiO₂ and Pt/TiO₂. Reproduced with permission from Ref.^[50].

In addition to doping with metal elements, introducing non-metal elements such as phosphorus (P), sulfur (S), or fluorine (F) to regulate the acidity of the support and the electronic structure of Pt is also an effective strategy to enhance the catalytic performance for C₃H₈ oxidation. Huang et al. modified the Pt/CeO₂ via phosphorus doping (Pt/CeO₂-yPO_x) and found that both the reaction rate and TOF increased progressively with increasing P content^[57]. When y = 0.3, the Pt/CeO₂-0.3PO_x catalyst exhibited the highest reaction rate and TOF, indicating optimal catalytic performance. Further investigations revealed that the surface phosphate (PO_x) species significantly enhanced the acidity and redox properties of the support, promoting the formation of Pt^δ+-(CeO₂-PO_x)^δ- dipolar active sites. These sites effectively facilitated C–H bond activation, thereby significantly improving the catalytic performance for propane oxidation [Figure 5C]. Yu et al. doped fluorine into the TiO₂ support, successfully tuning the electronic structure of the supported Pt to generate electron-rich Pt^δ+ active sites^[50]. These electron-rich Pt^δ+ sites not only promoted propane adsorption and oxygen activation, alleviated the competitive adsorption between them, but also enhanced C–H bond cleavage efficiency, significantly improving the propane oxidation activity of the catalyst at low temperatures [Figure 5D and E]. Zhou et al. also observed a similar phenomenon on P-modified Pt/Al₂O₃ catalysts, finding a significant synergistic effect between Pt active sites and acidic sites^[86]. This synergy effectively suppressed the competitive adsorption of oxygen and propane on the Pt sites, resulting in a notable decrease in the apparent E_a for propane oxidation, from 88.2 ± 4.7 to 59.2 ± 3.3 kJ·mol^-1, thereby significantly enhancing the catalytic reaction efficiency.

Sulfation of metal oxides is a commonly used and effective method for preparing solid superacids. Introducing sulfate groups onto the surface of a metal oxide support not only significantly enhances the surface acidity but also greatly improves their catalytic performance. Pt catalysts supported on such sulfated metal oxide supports exhibit excellent activity in the oxidation of propane, primarily due to the promotional effect of the strong acidity on the support surface. Specifically, the acidic sites on the support surface can efficiently adsorb and activate propane molecules, thereby lowering the E_a, increasing the reaction rate, and facilitating the efficient oxidative conversion of alkanes^[42,87]. Xu et al. conducted a systematic comparison of sulfated Zeolite Socony Mobil-5 (ZSM-5), Al₂O₃, ZrO₂, and CeO₂ supports, and the results demonstrated a significant synergistic catalytic effect between surface sulfate species and platinum species, which effectively promoted propane oxidation^[88]. Among the catalysts studied, the sulfated Pt/ZSM-5 catalyst exhibited the highest catalytic activity. Further analysis revealed that the 1 g 2Pt/ZSM-5 catalyst treated by 0.05 g (NH₄)₂SO₄ (5S-2Pt/ZSM-5) with Pt/S atomic ratio of 0.5 regulated the electron transfer and interaction strength between platinum and sulfate species, thereby optimizing their synergistic catalytic effect and significantly enhancing the catalytic performance for propane oxidation. Similarly, Shao et al. synthesized nanoflower-like Pt/ZrOSO₄ catalysts via a solvothermal method and found that the SO₄^2- and adsorbed SO₃ [(SO₃)_ad] species in the support promoted the formation of Pt⁰ and Pt⁺ species through Pt⁰-SO₄^2- and Pt⁺-(SO₃)_ad interactions, respectively^[87]. This synergistically enhanced the cleavage of C–C bonds and the heterolytic dissociation of C–H bonds in propane, thereby significantly improving catalytic oxidation performance. Gawthrope et al. loaded Pt onto pre-sulfated Al₂O₃ and found that propane combustion activity was positively correlated with the surface coverage of aluminum sulfate sites, with the best propane combustion performance observed for alumina impregnated with 0.1 M sulfuric acid^[89]. Wang et al. investigated the propane oxidation mechanism over Pt/SO₄^2-/CeO₂-ZrO₂ catalysts using in situ infrared spectroscopy, confirming that sulfation not only enhanced the catalyst’s ability to adsorb propane but also that the Pt-SO₄^2- interfacial active sites favored efficient C–H bond cleavage^[90]. Huang et al. introduced SO₂ directly into the reaction gas and found that while the addition of SO₂ only slightly increased propane adsorption, it significantly suppressed oxygen adsorption on Pt sites, thereby promoting C–C bond cleavage^[91].

In summary, both metallic and non-metallic promoters typically show an optimal loading window. At insufficient loadings, the promotional effect is not fully manifested, whereas excessive amounts can deteriorate catalytic performance by blocking active sites, inducing overly strong or weak interactions, and/or altering the support structure, resulting in decreased catalytic performance. Moreover, when multiple promoters are introduced simultaneously, synergistic effects may emerge from cooperative tuning of acidity and redox properties and/or strengthened MSIs. Conversely, antagonistic effects may occur due to competitive adsorption or phase separation.

The effect of MSIs

Classical MSIs are typically achieved by reducing metal/oxide catalysts under a high-temperature (> 450 °C) hydrogen atmosphere. During this process, metal nanoparticles are partially encapsulated by a defective layer derived from the support oxide, forming structures rich in interfacial sites that significantly affect the catalytic performance. For example, in the Pt@TiO_x/TiO₂ catalyst constructed by Hao et al., Pt is encapsulated by an amorphous TiO_x layer under reductive conditions^[92]. The TiO_x layer not only stabilizes metallic Pt⁰ through electronic interactions, preventing its transformation into inactive PtO_x under high-temperature oxidative conditions, but also significantly enhances the activity and stability for propane oxidation [Figure 6A]. The strength of the MSI in catalysts typically depends on the physicochemical properties of the support. For other redox-type oxide supports, such as CeO₂, the MSI can also be directly regulated by adjusting the number of metal-oxygen bonds through other strategies. Tan et al. successfully modulated the interfacial strength of Pt₁-Ce_0.9Zr_0.1O₂ single-atom catalysts by varying the calcination temperature (350-750 °C)^[93]. Higher temperatures facilitated the incorporation of Pt atoms into the support lattice, creating stronger interactions and simultaneously increasing the concentration of Ce³⁺ species and oxygen vacancies. This enhanced O₂ activation and promoted the rapid conversion of reaction intermediates (such as carbonates/carboxylates), thereby boosting catalytic performance [Figure 6B]. Zhang et al. reported a Pt single-atom catalyst (Pt_SA/CeZrO₂) in which isolated Pt atoms are anchored on an ordered macroporous (OM) Ce_0.8Zr_0.2O₂ support^[94]. The improved high-temperature activity and durability were attributed to Zr-stabilized, dynamically low-coordinated Pt sites that weaken Pt–O bond occupation and increase free d-electron availability, thereby sustaining peroxide-related reactivity and promoting propane C–H activation. Consequently, the catalyst maintained 92% conversion at 450 °C after 50 h aging at 800 °C with 10 vol.% H₂O, and was further demonstrated in a 3.4-L commercial cordierite monolith for scalable converter applications. Liu et al. used a photo-assisted deposition method to load Pt nanoparticles onto CeO₂ nanorods with abundant oxygen defects, significantly enhancing the strong interaction between Pt and CeO₂, thereby improving the catalytic performance of the catalyst^[95].

Figure 6. (A) Schematic illustration of the effect of SMSI between Pt and TiO_x on the chemical state of Pt. Reproduced with permission from Ref.^[92]; (B) Effect of different Pt-CeO₂ interaction strengths and coordination environments on propane combustion performance. Reproduced with permission from Ref.^[93]; (C and D) Temperature-dependent C₃H₈ oxidation reaction of Pt/CeO₂ (C), and Pt/9Nb-CeO₂ (D). Reproduced with permission from Ref.^[55]; (E) The schematic of the evolution of Pt on Pt/CeO₂ and Pt/9Nb-CeO₂. Reproduced with permission from Ref.^[55]; (F) Correlation between the TOF for propane oxidation at 220 °C and the Pt-MgAl₂O₄ interaction strength. Reproduced with permission from Ref.^[97]. SMSI: Strong metal-support interaction; TOF: turnover frequency; MSI: metal-support interaction.

However, such strong interactions often induce the formation of partially oxidized Pt species (e.g., Pt^δ+ or Pt²⁺) at the metal-support interface, which can inhibit the adsorption and activation of propane, ultimately deteriorating the efficiency of propane oxidation. In contrast, inert supports exhibit weaker interactions with noble metals, favoring the preservation of metallic Pt⁰, though they provide limited stabilization against particle agglomeration. Although redox-type supports can provide active oxygen species, their more significant impact lies in regulating the valence state of Pt, thereby influencing the overall catalytic performance. It should be noted that enhancing the MSI mainly benefits reactions where the active sites are located at the interface, such as CO oxidation, whereas its effect is relatively limited for propane oxidation, where metallic Pt⁰ serves as the primary active center. Therefore, the key to enhancing propane oxidation performance lies in regulating the MSI to effectively prevent metal particle aggregation while maintaining an adequate amount of metallic Pt⁰ active sites. In our recent study^[58], we found that on octahedral CeO₂ (CeO₂-o), where the MSI is relatively weak, Pt remained predominantly in the metallic state, resulting in superior propane oxidation activity. In contrast, on rod-shaped CeO₂ (CeO₂-r), stronger interfacial interactions induced partial oxidation of Pt, which hindered the decomposition and removal of intermediate species, thereby lowering the catalytic efficiency.

Regarding the excessively strong interaction between Pt and CeO₂-r^[55], we found that this leads to the redispersion of metallic Pt ensembles and the formation of oxidized Pt single-atom species under propane oxidation reaction conditions, driven by the promotion of the reaction product H₂O. This phenomenon ultimately causes the deactivation of the Pt/CeO₂ catalyst during the complete oxidation of propane [Figure 6C]. To address this issue, we introduced NbO_x as a decorator to effectively block the strong interaction sites on CeO₂ that anchor Pt, the Pt-CeO₂ interaction is weakened, preventing the redispersion of Pt nanoclusters during the reaction and maintaining Pt in metallic ensembles form before and after reaction [Figure 6D], thus enhancing complete propane oxidation performance and preventing catalyst deactivation [Figure 6E]. Another study^[48,96] also revealed a strong MSI between Pt and rutile TiO₂, which leads to the epitaxial growth of PtO₂ species on the (101) facet of rutile TiO₂, forming atomic-layer structures. This phenomenon limits the exposure of metallic Pt and results in lower propane oxidation performance. After introducing a SiO_x coating layer (i.e., Pt@SiO_x/TiO₂), the interfacial interaction between Pt and TiO₂ is effectively weakened, suppressing the epitaxial growth of PtO₂. Consequently, Pt species are dispersed as metallic nanoparticles across the entire TiO₂ support surface. Simultaneously, the engineered Pt-SiO_x interface creates new active sites that facilitate the intermediate C–C bond cleavage, synergistically enhancing the catalytic activity along with the active metallic Pt species for propane oxidation. Remarkably, T₉₀ is achieved at just 210 °C with a low Pt loading of only 0.2 wt%.

For irreducible oxide supports, however, which cannot be easily regulated in terms of MSI due to their stable O^2- ions, the formation of metal-metal bonds is uncommon. We adopted a method to modify the irreducible MgAl₂O₄ (MAO) support via H₂O₂ treatment, reducing the density of Brønsted acid sites on MAO responsible for anchoring Pt, thereby weakening the MSI^[97]. This allowed Pt to be more readily reduced by propane into active metallic Pt⁰, enhancing the activation of C–H bonds in propane and facilitating the rapid consumption of intermediate species. As a result, catalytic activity significantly improved, exhibiting an impressive increase in intrinsic activity of over 32-fold compared to traditional Pt/MAO [Figure 6F].

In summary, by regulating macroscopic factors such as Pt particle size, morphology, MSIs, and the rational introduction of promoters, it is possible to precisely tune microscopic characteristics, including the electronic state, coordination environment, geometric structure, and dispersion of Pt. The synergistic effect of these factors significantly optimizes the surface properties of supported Pt catalysts, thereby effectively enhancing their catalytic activity and structural stability, and improving the overall performance and lifespan of the catalysts.

The effect of chemical state of Pd

Studies have shown that, unlike Pt, high-valent Pd is generally regarded as the key active species in propane oxidation^[101-103]. Hu et al. prepared Pd catalysts supported on CeO₂ nanomaterials with different morphologies and applied them to propane oxidation^[104]. They found that when Pd was loaded on octahedral CeO₂ exposing mainly the (111) crystal facets, Pd predominantly existed in the form of PdO_x nanoparticles, exhibiting superior catalytic activity compared with those supported on rod-like and cubic CeO₂ [Figure 7A and B]. Subsequently, Liu et al. regulated the oxygen vacancy concentration of CeO₂ by varying its calcination temperature, thereby tuning the chemical state of Pd^[61]. The results indicated that high-temperature calcined CeO₂ possessed fewer oxygen vacancies, leading to a weaker Pd-CeO₂ interaction and promoting the aggregation of Pd in the form of PdO. These PdO species effectively facilitated the decomposition of acetate intermediates, thus enhancing propane oxidation activity. In contrast, low-temperature calcined CeO₂ contained more oxygen vacancies, resulting in a stronger Pd-CeO₂ interaction, where Pd mainly existed in a solid-solution state, corresponding to lower catalytic activity. Furthermore, Li et al. revealed that in the Pd/Al₂O₃ and Pd/Ce_0.33Zr_0.67O₂ system, unencapsulated Pd species formed a tight interface with the alumina support^[105]. Due to the higher oxidation state of Pd, abundant surface PdO species were generated at the Pd-Al₂O₃ interface. These PdO species were identified as the main active sites for propane oxidation, leading to significantly enhanced catalytic performance. In addition to the support effect, the synthetic method also plays a crucial role in determining the dispersion and chemical state of Pd species. Peng et al. prepared a single-crystalline zeolite silicalite-1 (S-1) with intra-mesopores encapsulating palladium (Pd) NPs (Pd@IM-S-1) catalyst via an in situ dry-gel conversion method^[106]. Compared with conventionally prepared Pd@SiO₂ and Pd/S-1 catalysts, Pd@IM-S-1 exhibited superior propane oxidation activity, thermal stability, cycling durability, and water resistance. Further characterization revealed that during propane oxidation, metallic Pd particles were partially oxidized to form PdO_x species, and the Pd-PdO interface provided abundant active oxygen species, which facilitated C–H bond cleavage in propane molecules [Figure 7C and D].

Figure 7. (A and B) Reaction rate in propane oxidation (A) and Pd 3d XPS spectra (B) of Pd/CeO₂ catalysts. Reproduced with permission from Ref.^[104]; (C and D) Schematic illustration of the synthesis (C) and catalytic performance in propane oxidation over Pd@IM-S-1 and Pd@S-1 (D). Reproduced with permission from Ref.^[106]. XPS: X-ray photoelectron spectroscopy.

The effect of the size of Pd nanoparticles

Besides the chemical state of Pd, the particle size of Pd also plays a crucial role in determining the catalytic performance of supported Pd-based catalysts for propane oxidation. Khudorozhkov et al. deposited Pd particles with sizes ranging from 3 to 6 nm on Al₂O₃ by adjusting the acidity of the Pd precursor solution^[107]. The results demonstrated that the TOF for propane oxidation increased monotonically with the Pd particle size and exhibited a clear linear correlation with the Pd²⁺/Pd⁰ ratio.

Similarly, Zhou et al. prepared a series of 0.5 wt% Pd/La-Al₂O₃ (PLA) catalysts using the impregnation method and found that the TOF for propane oxidation was jointly influenced by the redox properties of the catalyst and the Pd particle size^[108]. Larger Pd particles generally showed higher catalytic activity for propane oxidation, among which the aged catalyst supported on La-Al₂O₃ calcined at 500 °C exhibited the highest TOF value (0.125 s^-1). In addition, Bailey et al. prepared Pd particles of different sizes on Al₂O₃ by varying the Pd loading^[44]. The study revealed that the overall catalytic activity was primarily determined by the number of active Pd sites. As the Pd loading increased, the Pd particle size also grew; however, the total number of active sites increased correspondingly, leading to a significant enhancement in catalytic performance.

The influence of promoters

To further enhance the activity and stability of supported Pd-based catalysts, researchers commonly employ support doping and modification strategies. Li et al. reported that introducing La into Pd/Al₂O₃ catalysts leads to a strong interaction between Pd and La, which alters the oxidation state of Pd^[109]. The incorporation of La creates basic sites on the support surface, promoting the formation of more oxidized Pd species and significantly enhancing the catalytic activity of Pd/Al₂O₃ in propane oxidation. Yan et al. indicated that doping Co into Pd/Al₂O₃ catalysts can also markedly improve propane oxidation activity, primarily due to the enhanced redox properties of the catalyst and its increased ability to activate gaseous oxygen^[110]. In addition, acidic elements can be introduced to modify the support, thereby tuning the surface properties of the catalyst. Taylor et al. found that both Pd⁰ and Pd²⁺ species coexist in Pd/TiO₂ catalysts^[111]. Upon the introduction of W, a Pd-WO_x interface forms between PdO_x and TiO₂, causing Pd species to predominantly exist in the Pd²⁺ state, which significantly promotes propane catalytic oxidation. Liang et al. prepared Pd-based catalysts supported on Al₂O₃-TiO₂ composite oxides by combining mechanical ball milling with wet impregnation, followed by surface etching with phosphoric acid at various concentrations^[45]. The results showed that when the Al/P molar ratio was 3:1, the phosphoric acid-modified Pd/Al₂O₃-TiO₂ catalyst exhibited the best performance in propane catalytic oxidation.

The effect of chemical state of Ru

Unlike the conventional view that the primary active sites for propane catalytic oxidation are metallic Pt, Ru-based catalysts exhibit a significantly different reaction mechanism. Because Ru has a relatively weak ability to activate molecular oxygen, its catalytic activity does not mainly rely on isolated metal centers, but is more likely derived from RuO_x or the interface regions between Ru and the support^[46]. Therefore, constructing highly dispersed RuO_x and highly active interfacial sites is crucial for enhancing catalytic performance. Choosing a support that can form a strong MSI with Ru and possesses excellent redox properties (such as CeO₂) not only helps stabilize Ru species and inhibit sintering but also provides active oxygen species, thereby significantly improving the activity and stability of Ru-based catalysts in propane oxidation reactions. Okal et al. systematically studied the effect of CeO₂, Al₂O₃, and ZnAl₂O₄ supports on the propane oxidation performance of Ru-based catalysts^[112]. The results showed that CeO₂ can supply reactive oxygen species involved in the reaction, resulting in excellent propane oxidation activity. Xia et al. further pointed out that Ru-O-M interfaces or highly dispersed RuO_x are the main active sites, with the activity order as follows: Ru-O-Ce interface > Ru-O-Co interface ≈ Co₃O₄ > highly dispersed RuO_x >> CeO₂^[113]. Hu et al. demonstrated that the Ru-CeO₂ interface can provide additional propane adsorption and activation sites, thereby broadening the reaction pathway for propane oxidation [Figure 8A]^[114]. Building on this, Wang et al. found that the morphology of the CeO₂ support significantly affects the number of surface oxygen vacancies near the Ru-CeO₂ interface by modulating the interaction between Ru and different CeO₂ crystal planes^[65]. Among them, the Ru/CeO₂-R catalyst exhibited a higher concentration of surface oxygen vacancies, thereby showing stronger adsorption and activation abilities for both oxygen and propane [Figure 8B]. Wu et al. indicated that a dense and uniformly distributed Ru-O-Ce interface is a key active site for propane oxidation, which may be related to asymmetric bridging adsorption and activation^[115]. However, the presence of chlorine can inhibit the formation of highly dispersed RuO_x and Ru-O-Ce interfaces, and accelerate CeO₂ sintering and interface degradation at high temperatures. To further increase the number of Ru-O-Ce interfacial sites and RuO_x species, Yan et al. prepared a Ru/CeO₂-CS catalyst with more RuO_x and oxygen vacancies via a colloidal chemistry method^[64]. Compared with Ru/CeO₂-IM (IM: Impregnation Method) catalysts prepared by conventional impregnation, Ru/CeO₂-CS possesses more Ru-O-Ce interfacial sites, oxygen vacancies, and Lewis acid sites, which help maintain Ru in a low-valence state and significantly enhance propane adsorption, dissociation, and activation. Wang et al. prepared a structurally specific Ru/Ce@Co catalyst by dispersing thin layers of CeO₂ on Co₃O₄^[62]. Compared with Ru/CeO₂ and Ru/Co₃O₄ catalysts, the Ru/Ce@Co catalyst contains more Ce³⁺ species, which helps maintain highly active and stable RuO_x species [Figure 8C]. Sun et al. successfully constructed highly active and stable Ru-CeO₂ catalysts for low-temperature propane oxidation by optimizing the preparation method to control the distribution of small-sized RuO_x in CeO₂ [Figure 8D]^[63].

Figure 8. (A) Catalytic activity for the total oxidation of propane on Ru/Al₂O₃ and Ru/CeO₂ catalysts. Reproduced with permission from Ref.^[114]; (B) Correlation between TOF (at 155 °C) and oxygen vacancy concentration of catalysts for propane oxidation. Reproduced with permission from Ref.^[65]; (C) Schematic illustration of the chemical states of Ru active sites on catalysts Ru/Ce@Co and Ru/CeO₂. Reproduced with permission from Ref.^[62]; (D) Illustration of the preparation procedures for the Ru-CeO₂-x and Ru/CeO₂-x catalysts. Reproduced with permission from Ref.^[63]. TOF: Turnover frequency; TPD: temperature-programmed desorption; XPS: X-ray photoelectron spectroscopy.

The effect of the size of Ru nanoparticles

The particle size of Ru also has a significant impact on the propane catalytic oxidation performance of supported Ru-based catalysts. Okal et al. found that treating Ru/γ-Al₂O₃ catalysts under different atmospheres led to variations in particle size and oxidation state: samples treated with H₂ at low temperature (250 °C) formed non-stoichiometric RuO_x structures encapsulating metallic Ru with smaller particle sizes, exhibiting the highest propane oxidation activity, in contrast, O₂ treatment at 600 °C caused severe sintering of RuO₂ nanoparticles (9-25 nm), resulting in the lowest catalytic activity^[116]. The treatment with O₂ at 250 °C produced well-crystallized RuO₂ nanoparticles (approximately 4 nm) with moderate activity. Subsequently, they synthesized Ru nanoparticles supported on γ-Al₂O₃ via a microwave-polyol method and found that catalysts with an average particle size of 1.6 nm exhibited excellent activity, whereas those with an average particle size of 6 nm showed significantly lower activity, further indicating that high dispersion and small particle size of Ru nanoparticles are key factors for their superior catalytic performance^[117]. Camposeco et al. prepared Ru/TiO₂ catalysts with different Ru loadings using a urea deposition method and found that hydrogen pre-treatment effectively reduced the Ru particle size, thereby significantly enhancing propane oxidation activity^[54]. In addition, Zhang et al. compared single-atom (Ru/CeO₂-SA), diatomic (Ru₂/CeO₂), and tetra-atomic cluster (Ru₄/CeO₂) catalysts, revealing the significant effect of ultra-low Ru loading on propane oxidation performance^[118]. The study showed that single-atom Ru catalysts excelled at suppressing propane adsorption, regulating oxygen activation, and promoting the formation of acrylic acid intermediates, thereby markedly improving catalytic efficiency.

The influence of promoters

To further enhance the propane oxidation performance of Ru/CeO₂ catalysts, the introduction of promoters to precisely regulate the concentration of oxygen vacancies in CeO₂ and the interfacial interaction between Ru and CeO₂ has been proven to be an effective strategy. This is because metal doping can not only modify the crystal structure and electronic properties of CeO₂, but also adjust its surface redox ability and oxygen mobility, thereby optimizing the dispersion of Ru species and promoting the formation of active RuO_x, ultimately enhancing the overall catalytic performance in propane oxidation and related reactions. Deng et al. demonstrated that rare-earth element doping not only helps regulate the interaction between Ru and CeO₂ but also effectively promotes the formation of oxygen vacancies^[119]. Among these, the Pr-doped CeO₂ catalyst exhibited the most favorable catalytic performance in propane oxidation. However, when Ba and Nb were introduced into Ru/CeO₂, the results showed that, unlike Pt- and Pd-based catalysts, the addition of basic or acidic promoters did not improve the propane oxidation performance of the Ru/CeO₂ catalyst; instead, the catalytic activity decreased due to the reduced formation of Ru-O-Ce interfacial structures^[120].

In summary, the rational design of supports and the regulation of Ru structure and dispersion, which promote the formation of metal-support interfaces, enhance the activation of reactive oxygen species, and optimize reaction pathways, constitute an effective strategy for improving both the catalytic activity and long-term stability of Ru-based catalysts in complete propane oxidation.