Alkaline Earth Carbonate Engineered Pt Electronic States for High-Efficiency Propylene Oxidation at Low Temperatures

Abstract

Alkaline earth elements have emerged as crucial electronic modifiers for regulating active sites in catalytic systems, yet the influence of metal–support interactions (MSIs) between alkaline earth compounds and active metals remains insufficiently understood. This study systematically investigated Pt nanoparticles supported on alkaline earth carbonates (Pt/MCO₃, M = Mg, Ca, Ba) for low-temperature propylene combustion. The Pt/BaCO₃ catalyst exhibited outstanding performance, achieving complete propylene conversion at 192 °C, significantly lower than Pt/MgCO₃ (247 °C) and Pt/CaCO₃ (282 °C). The enhanced activity stemmed from distinct MSI effects among the supports, with Pt/BaCO₃ showing the poorest electron enrichment and lowest propylene adsorption energy. Through kinetic analyses, ¹⁸O₂ isotope labeling, and comprehensive characterization, the reaction was confirmed to follow the Mars–van Krevelen (MvK) mechanism. Pt/BaCO₃ achieves an optimal balance between propylene and oxygen adsorption, a critical factor underlying its superior activity.

1. Introduction

Volatile organic compounds (VOCs), as a class of carbon-based compounds readily volatilized at ambient temperature, not only serve as primary precursors for photochemical smog and ozone layer depletion but also exhibit significant correlations with the incidence of respiratory diseases, posing dual threats to both ecological systems and public health [1,2]. Among these, propylene, a representative unsaturated hydrocarbon VOC emitted by the petrochemical industry, presents a critical challenge for atmospheric pollution control in chemical industrial parks [3,4,5]. Among various VOC combustion catalysts, Pt-based catalysts have demonstrated prominent application potential in catalytic combustion due to their exceptional C-H bond activation and oxygen dissociation capabilities [6,7]. Notably, the catalytic combustion mechanism of propylene fundamentally differs from that of propane; its carbon–carbon double bond structure induces strong chemisorption on noble metal surfaces, while competitive adsorption with oxygen at active sites leads to complex catalytic kinetic behaviors [8,9]. Consequently, regulating the electronic states of active sites through metal–support interactions to balance the adsorption energies of propylene and oxygen has emerged as a pivotal scientific challenge for achieving efficient olefin catalytic combustion [10,11].

Alkali and alkaline earth metal elements (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺), with their distinctive electron transfer properties, offer an effective strategy for precisely modulating the electronic structure of metal active centers [12]. Among these, alkaline earth metal carbonates (e.g., CaCO₃) have demonstrated remarkable performance in catalytic systems such as CO₂ hydrogenation and transesterification reactions, owing to their moderate Lewis basicity, superior CO₂ adsorption capacity, and high-temperature stability [13,14,15]. Recent studies have further unveiled novel mechanisms underlying metal–carbonate interfacial effects; Zhu et al. reported H₂ pretreatment-induced dynamic reconstruction in Ni/BaCO₃ systems, where the surface migration of BaCO₃ formed a mesoporous overlayer encapsulating Ni nanoparticles, successfully constructing electron-enriched Ni-BaCO₃ interface. This unique strong metal–support interaction (SMSI) not only suppressed high-temperature sintering of metal particles (stabilizing particle size at ~5 nm) but also significantly enhanced CO₂ activation efficiency [16]. Nevertheless, critical knowledge gaps remain regarding structure-activity relationships in alkaline earth metal carbonate-supported Pt catalysts for low-temperature propylene catalytic combustion, particularly the regulatory mechanisms of carbonate carrier types on Pt electronic structures and the quantitative correlation between metal–support interaction strength and propylene–oxygen adsorption equilibrium.

This study systematically investigates the influence of carrier properties on low-temperature propylene catalytic combustion performance by constructing a series of alkaline earth metal carbonate-supported Pt catalysts (Pt-MCO₃, M = Mg/Ca/Ba). Activity evaluations reveal that Pt-BaCO₃ exhibits optimal performance, with its T90 (complete propylene conversion temperature) reduced by 55 °C and 90 °C compared to Pt/MgCO₃ and Pt/CaCO₃, respectively. Combining kinetic analysis and ¹⁸O isotopic labeling experiments, we first confirm that the Pt-MCO₃ system follows a modified Mars–van Krevelen (MvK) reaction mechanism, where adsorbed propylene directly reacts with lattice oxygen at Pt-MCO₃ interfaces. Through in situ DRIFTS characterization and density functional theory (DFT) calculations, we elucidate that the weaker metal–support interaction in the Pt-BaCO₃ system results in a higher proportion of Pt⁴⁺ species. This electronic configuration effectively mitigates excessive site occupation induced by propylene adsorption, while simultaneously accelerating the regeneration kinetics of interfacial lattice oxygen. This research not only provides new insights for designing high-efficiency VOC purification catalysts but also deepens the theoretical understanding of electronic synergy mechanisms at metal–carbonate interfaces.

2. Result and Discussion

2.1. Synthesis and Characterization of Pt-MCO₃

The Pt-MgCO₃, CaCO₃, and BaCO₃ samples were synthesized through a wet-impregnation method, with Pt loading controlled at approximately 1 wt.% (ICP analysis confirmed actual loadings of 0.92 wt.%, 0.94 wt.%, and 0.98 wt.% for MgCO₃, CaCO₃, and BaCO₃ substrates, respectively). The XRD patterns of Pt/MCO₃ (M = Mg, Ca, Ba) catalysts and their corresponding supports reveal that both Pt-CaCO₃ and Pt-MgCO₃ catalysts exhibit characteristic diffraction patterns corresponding to the calcite phase with rhombohedral symmetry, while Pt-BaCO₃ displays distinct witherite phase reflections with orthorhombic symmetry (Figure 1a). This adoption of the BaCO₃ witherite phase (instead of the calcite phase) is necessitated by the limited synthetic accessibility of calcite-phase BaCO₃ [17,18]. Notably, no diffraction peaks associated with metallic Pt or PtO_X species are detected in any of the Pt-loaded samples, suggesting that Pt is highly dispersed [19]. The Raman spectra of Pt-BaCO₃, Pt-CaCO₃, and Pt-MgCO₃ exhibit characteristic carbonate vibrational modes, with all catalysts showing the ν₃(CO₃²⁻) asymmetric stretching vibration at 1088 cm⁻¹ (Figure 1b). Significantly, Pt-BaCO₃ displays an additional symmetric stretching band at 1062 cm⁻¹ [20]. In Pt-CaCO₃ and Pt-MgCO₃, the high symmetry of the trigonal crystal system preserves the degeneracy of CO₃²⁻ vibrational modes (yielding a single peak), whereas the diminished symmetry in orthorhombic Pt-BaCO₃ lifts this degeneracy, inducing ν₃ vibrational splitting. These spectroscopic observations correlate precisely with XRD structural determinations [21,22].

To elucidate the morphological and textural characteristics of Pt-MCO₃ (M = Mg, Ca, Ba) catalysts, a combination of N₂ physisorption analysis and scanning electron microscopy (SEM) is systematically employed. As shown in Figure 1c, all three materials demonstrated limited N₂ adsorption capacities across the relative pressure range (P/P₀ = 0.05–0.95). BET surface area calculations established the following specific surface area sequence: Pt-MgCO₃ (27.6 m²·g⁻¹) > Pt-BaCO₃ (3.5 m²·g⁻¹) > Pt-CaCO₃ (1.4 m²·g⁻¹). SEM characterization reveals distinct morphological features that correlate precisely with the crystallographic phases identified by XRD and Raman spectroscopy. Pt-BaCO₃ exhibits characteristic columnar structures (Figure S1), consistent with the orthorhombic symmetry of witherite, while both Pt-CaCO₃ and Pt-MgCO₃ display well-defined cubic particles (Figures S2 and S3), matching the expected morphology for trigonal calcite systems.

Thermal gravimetric (TG) tests were conducted on Pt-MCO₃ (M = Mg, Ca, Ba) catalysts to study their thermal stability under reaction temperature conditions (150–300 °C). As Figure S4 shows, all three catalysts exhibit excellent thermal stability within the reaction temperature range (150–300 °C), with no significant mass loss observed. Pt-BaCO₃ and Pt-CaCO₃ in particular demonstrate superior high-temperature structural stability, with almost no weight loss within this range. In contrast, Pt-MgCO₃ exhibits relatively lower thermal stability, beginning to decompose at temperatures between 300 and 500 °C. However, it remains structurally stable within the reaction temperature range (below 300 °C), with a weight loss rate of less than 5.6%.

The dispersion characteristics of Pt nanoparticles supported on various carbonate substrates (MCO₃, M = Mg, Ca, Ba) were quantitatively evaluated by CO pulse chemisorption measurements (Figure 1d). CO pulse chemisorption analysis indicates that the Pt-BaCO₃ catalyst exhibits the highest CO uptake, with a quantitatively derived average Pt particle size of 1.6 nm, confirming the predominance of highly exposed sub-nanoclusters or small nanoparticles. This phenomenon is presumably attributed to the enhanced surface alkalinity of BaCO₃ during wet impregnation, which facilitates superior Pt dispersion [23]. In contrast, Pt-CaCO₃ shows the lowest CO adsorption capacity, corresponding to a larger Pt grain size of 3.5 nm. Pt-MgCO₃ presents an intermediate adsorption behavior, with an estimated particle size of 2.6 nm. The slightly distinct Pt particle sizes observed across the three catalysts, likely originating from variations in morphology and specific surface area, remain confined within a narrow nanoscale range. More investigation of Pt-MCO₃ (M = Mg, Ca, Ba) samples is performed by HRTEM, HAADF-STEM, and EDS mapping. For Pt-BaCO₃ (Figure 2a–e), lattice-resolved TEM reveals highly dispersed Pt species, as evidenced by well-defined 0.21 nm fringes corresponding to the Pt (111) plane. In contrast, Pt-CaCO₃ (Figure 2f–j) exhibits distorted lattice fringes with a spacing of 0.24 nm [6,24]. The observed bending and local distortion indicate a significantly larger Pt grain size with pronounced lattice strain [25]. The observed bending and local distortion suggest the presence of larger Pt domains and indicate a higher degree of lattice strain at the Pt-CaCO₃ interface. All catalysts exhibit highly dispersed Pt species, as evidenced by the homogeneous Pt distribution in EDS mapping, with minimal variation in dispersion across different samples. The Pt remains uniformly anchored on the support surfaces, primarily existing as small nanoparticles or sub-nanometric clusters without significant differences among the catalysts.

2.2. The Catalytic Performance Tests

The catalytic performance of Pt catalysts supported on alkaline earth carbonates (MgCO₃, CaCO₃, BaCO₃) for low-temperature propylene combustion was systematically evaluated through temperature programmed oxidation experiments. As depicted in Figure 3a, distinct support effects manifested in the light-off profiles, as follows: Pt-BaCO₃ demonstrated exceptional low-temperature activity with catalytic ignition initiating at 140 °C, achieving complete conversion by 192 °C. Comparatively, Pt-MgCO₃ exhibited a higher activation threshold, requiring gradual temperature elevation to 247 °C for full conversion. The Pt-CaCO₃ system showed the most sluggish kinetics, initiating conversion at 180 °C but necessitating heating to 282 °C for complete oxidation. These results conclusively establish the following support-dependent activity hierarchy: Pt-BaCO₃ > Pt-MgCO₃ > Pt-CaCO₃, highlighting the critical role of support composition in modulating the metal–support interaction and redox properties.

To systematically compare the propylene catalytic combustion performance of three catalysts, catalytic reaction rates under different temperature conditions were also systematically tested. As shown in Figure 3b, after eliminating internal/external diffusion effects, the Pt-BaCO₃ catalyst demonstrated a propylene combustion rate of 370 mmol·g⁻¹·s⁻¹ at 200 °C, representing 10-fold enhancements over Pt-CaCO₃ and Pt-MgCO₃, respectively, further confirming its catalytic superiority. While all three catalysts showed significant activity improvement with increasing temperature, Pt-BaCO₃ maintained a 10 times rate advantage. Activation energy measurements revealed that the Pt-BaCO₃ system possessed the lowest energy barrier (41.7 kJ·mol⁻¹), substantially lower than those of Pt-CaCO₃ (65.1 kJ·mol⁻¹) and Pt-MgCO₃ (53.2 kJ·mol⁻¹). (Figure 3c) These experimental findings conclusively demonstrate that Pt-BaCO₃ exhibits optimal propylene catalytic combustion performance among carbonate-supported catalysts.

Complementary GHSV experiments (20,000–160,000 mL·g⁻¹·h⁻¹) on the optimal Pt-BaCO₃ catalyst (Figure 3d) revealed robust operational stability. While incremental flow rate increases caused moderate activity attenuation, the system exhibited remarkable process tolerance above 80,000 mL·g⁻¹·h⁻¹, where conversion rates stabilized within <5% variation despite the quadrupled space velocity. This threshold behavior suggests practical applicability in high-throughput industrial settings. Meanwhile, catalyst durability was also rigorously assessed through extended time-on-stream (50 h) and thermal cycling (3 cycles) experiments under harsh reaction conditions. Continuous operation at 190 °C (Figure 3e) revealed remarkable stability with propylene conversion maintaining stable conversion rates between 85–95% throughout the 50 h duration, without detectable deactivation. Thermal stress testing through repeated heating–cooling cycles (150–200 °C) demonstrated exceptional structural robustness, as evidenced by the superimposable conversion profiles in Figure 3f. Each cycle consistently achieved >90% conversion at 190 °C and complete oxidation at 200 °C, with less than 2% variation in T₅₀ values between cycles. This excellent stability indicates that under operating conditions, common deactivation mechanisms, such as active phase sintering or carbonate carrier decomposition, are effectively alleviated in Pt-BaCO₃.

2.3. The Reaction Path in Pt/MCO₃

To elucidate the divergent catalytic activities, we performed systematic kinetic analyses of the above catalysts. As shown in Figure 4a under fixed propylene partial pressure (1 kPa), the oxygen reaction orders for propane combustion rates were determined as 0.98 (Pt-BaCO₃), 1.08 (Pt-MgCO₃), and 1.61 (Pt-CaCO₃), respectively. The lower oxygen reaction order of Pt-BaCO₃ and Pt-MgCO₃ compared to Pt-CaCO₃ suggested that the influence of O₂ partial pressure to the combustion rate in the former two catalysts was weaker than Pt-CaCO₃ (Figure 4a). Conversely, the propylene reaction orders exhibited negative values (−0.36 for Pt-BaCO₃, −0.42 for Pt-MgCO₃, −0.56 for Pt-CaCO₃), indicating stronger propylene adsorption on Pt-CaCO₃ relative to the other catalysts (Figure 4b) [26,27]. The observed positive oxygen reaction orders coupled with negative substrate dependencies collectively support a Mars–van Krevelen (M-K) redox mechanism, where adsorbed hydrocarbons react with lattice oxygen species during the catalytic combustion process.

¹⁸O₂ isotopic labeling experiments unambiguously demonstrate the prevalence of the Mars–van Krevelen (M-K) mechanism in propylene catalytic combustion. Time-resolved isotopic tracking (Figure 4c) reveals distinct oxygen participation kinetics on Pt-BaCO₃; pre-adsorbed surface O¹⁶ species are completely consumed within 30 min under He flow, while subsequent C₃H₆/O¹⁸₂/He exposure initially generates CO⁴⁴₂ (from lattice O¹⁶) with progressively increasing intensity. The absence of CO⁴⁶₂ and CO⁴⁸₂ signals provides definitive evidence for the M-K pathway [4]. Isotopic kinetic analysis (Figure 4d) quantifies the rate-determining role of lattice oxygen regeneration, showing a twofold enhancement in reaction rates with O¹⁶₂ versus O¹⁸₂ at elevated temperatures, confirming oxygen diffusion as the rate-limiting step. This mechanistic understanding rationalizes the observed activity trend; strong propylene adsorption on Pt-CaCO₃ induces competitive site blocking, simultaneously suppressing oxygen adsorption and compromising lattice oxygen replenishment, ultimately diminishing catalytic efficiency [28].

To validate the competitive adsorption mechanism, systematic O₂-TPD and C₃H₆-TPD analyses were performed. As illustrated in Figure 5a, the O₂-TPD profile of Pt-BaCO₃ exhibited a broad multi-shouldered desorption peak spanning 427–640 °C with a dominant feature at ~427 °C, indicating abundant surface oxygen species. In contrast, Pt-MgCO₃ demonstrated a single sharp desorption peak shifted to higher temperatures (~440 °C), suggesting reduced reactivity of its surface lattice oxygen. Notably, Pt-CaCO₃ showed negligible oxygen desorption, reflecting its poor oxygen activation capability [29,30]. Complementary C₃H₆-TPD results (Figure 5b) revealed minimal propylene desorption from Pt-BaCO₃ and Pt-MgCO₃, implying weak propylene adsorption. However, Pt-CaCO₃ displayed a prominent C₃H₆ desorption peak centered at ~411 °C. These TPD findings align with kinetic analyses, confirming that the propylene combustion pathway involves adsorbed C₃H₆ reacting with surface lattice oxygen. The superior performance of Pt-BaCO₃ correlates with its optimal combination of low C₃H₆ surface coverage and enhanced oxygen activation capacity [31].

2.4. Understanding the Structure–Performance Relationship in Pt/MCO₃

Given the poor lattice oxygen reactivity of carbonate-supported catalysts, the active lattice oxygen in Pt-BaCO₃ and Pt-CaCO₃ likely originates from PtO₂ or PtO species. To verify this hypothesis, a comprehensive XPS analysis was conducted to probe the electronic states of Pt. As shown in Figure 6a, all three catalysts exhibited characteristic Pt⁴⁺ and Pt²⁺ signatures, with binding energies at 72.5 eV and 74.8 eV corresponding to the d_7/2 orbitals of Pt²⁺ and Pt⁴⁺, respectively, confirming the predominantly oxidized state of Pt on carbonate supports. Notably, distinct Pt²⁺/Pt⁴⁺ ratios were observed across different carriers, following the sequence Pt-CaCO₃ (0.60) > Pt-BaCO₃ (0.39) > Pt-MgCO₃ (0.31), revealing the highest proportion of Pt²⁺ species in Pt-CaCO₃. These findings suggest that Pt²⁺ species may facilitate propylene adsorption, while Pt⁴⁺ ions likely correspond to reactive surface lattice oxygen species critical for catalytic oxidation.

While the Pt²⁺/Pt⁴⁺ ratio correlates with variations in propylene adsorption energy and lattice oxygen reactivity within Pt-MCO₃ catalytic systems, the subtle differences in these ratios fail to account for the distinct properties between Pt-BaCO₃ and Pt-MgCO₃. Subsequently, H₂-TPR analyses were performed to investigate the reducibility of PtO₂ and PtO species. As shown in Figure 6b, only Pt-BaCO₃ displays a distinct hydrogen consumption peak around 100 °C, whereas Pt-CaCO₃ and Pt-MgCO₃ show negligible reduction signals in this range, indicating divergent metal–support interactions that render PtO₂/PtO species on CaCO₃ or MgCO₃ more reducible compared with those on BaCO₃. This electronic modulation likely originates from interfacial charge transfer mechanisms.

To gain atomic-level insights into the electronic interactions between Pt and alkaline earth carbonate supports, density functional theory (DFT) calculations were performed for PtO₂ clusters anchored on the (104) facets of CaCO₃, MgCO₃, and BaCO₃. As illustrated by the charge density difference plots and Mulliken population analysis (Figures S5–S7), significant electron donation from the carbonate supports to Pt, with the magnitude of electron accumulation on Pt following the order MgCO₃ (+0.52 e) > CaCO₃ (+0.42 e) > BaCO₃ (+0.39 e), demonstrating the support-dependent electronic modulation of Pt nanoparticles. The diminished electron donation capacity of Ba²⁺, arising from its low electronegativity and highly ionic nature, accounts for its inefficient charge transfer to Pt in comparison to Ca²⁺ and Mg²⁺. This electronically inert environment at the Pt-BaCO₃ interface gives rise to weak electrostatic interactions, thereby favoring the stabilization of Pt species in a highly dispersed and oxidized state [32,33]. Consequently, BaCO₃ demonstrates a superior ability to maintain ultrasmall Pt structures, which can be rationalized by its intrinsically weaker metal–support interaction with Pt [34,35].

Although the Pt valence states are initially determined under static conditions via XPS analysis, the inherently reductive nature of Pt species suggests that the oxidative propylene reaction environment may lead to dynamic valence changes (Figure 5b). To probe this possibility, in situ CO desorption infrared spectroscopy is employed, where characteristic CO binding configurations are used to distinguish Pt oxidation states based on their distinct chemisorption patterns (Figure 7). Typically, Pt^δ+ species possess fewer d-electrons than Pt⁰, which limits effective d–π* back-donation, resulting in weaker CO adsorption and activation. Accordingly, high-frequency bands above 2090 cm⁻¹ are assigned to CO adsorbed on Pt⁴⁺ sites, while the lower-frequency region (2088–2068 cm⁻¹) corresponds to CO bound to Pt⁰. Notably, Pt–BaCO₃ shows a much stronger signal at 2108 cm⁻¹, corresponding to CO adsorbed on Pt⁴⁺ sites, indicating a higher proportion of Pt⁴⁺, consistent with the XPS results. In addition, Pt-CaCO₃ displays a distinct band at 2016 cm⁻¹, attributable to bridged CO, suggesting Pt aggregation and the formation of larger particles, in contrast to the more uniform dispersion of Pt species observed in Pt-BaCO₃ and Pt-MgCO₃ [36]. This observation aligns with the particle size trend obtained from CO pulse chemisorption. Moreover, compared to the other two catalysts, the CO adsorption band on Pt-BaCO₃ is shifted to a higher wavenumber, reflecting weaker Pt-CO bonding and lower electron density around Pt, further supporting the presence of weaker metal–support interactions (MSIs) in the Pt-BaCO₃ system.

To probe the evolution of surface intermediates under reaction conditions, Pt-BaCO₃ is exposed to a C₃H₆/O₂ mixture in a DRIFTS cell (Figure 8). In situ IR spectroscopy reveals the formation of carbonyl-containing intermediates on the catalyst surface. As temperature increases, a distinct band at 1745 cm⁻¹ emerges, assigned to the C=O stretch of oxidation products (e.g., acrolein or ketones). Bands at 1619 and 1591 cm⁻¹ are attributed to C₂ enolates, while signals at 1577, 1560, 1545, 1408, and 1200–1400 cm⁻¹ correspond to surface-stabilized C₁ (carbonates/formates) and C₂ (carboxylates) species, indicating progressive oxygenate formation. A minor increase in the 2800–3000 cm⁻¹ region suggests σ-bonded alcohols. These results demonstrate that Pt-BaCO₃ facilitates selective propylene oxidation to unsaturated aldehydes/ketones (e.g., acrolein, acetone), which further convert to C₂/C₁ species prior to final oxidation to CO₂ and H₂O [37,38].

Combining the results of kinetic studies, isotopic labeling, XPS characterization, and in situ FT-IR spectroscopy reveals that the catalytic combustion of propylene over carbonate-supported catalysts follows a classical M-K mechanism. Among these systems, the BaCO₃ support exhibits the weakest interaction with PtO₂, as evidenced by its limited electron transfer capacity and the higher oxidation state of Pt species (as illustrated in Figure 6a). These characteristics synergistically lead to (i) the weakest propylene adsorption on active sites and (ii) highly reactive lattice oxygen species. The concerted effects of these two factors collectively contribute to the superior catalytic performance observed on Pt-BaCO₃.

3. Experimental

3.1. Preparation of Catalysts

The Pt-MCO₃ catalysts (M = Mg, Ca, Ba) with 1 wt.% Pt loading were synthesized via an impregnation method [39,40]. Specifically, 1.0 g of MCO₃ support was dispersed in 40 mL deionized water to form a slurry. Subsequently, 0.067 g of Pt(NO₃)₂ solution (containing 14.94% Pt) was added dropwise to the suspension under continuous stirring to achieve the designated platinum loading. The mixture was continuously stirred at room temperature for 6 h to ensure homogeneous dispersion of the active component, followed by 12 h of drying under ambient conditions. The dried precursors were then calcined in static air at 400 °C for 3 h with a heating rate of 2 °C/min [41].

3.2. Catalyst Performance Evaluation

The catalytic activity of the catalysts was evaluated in a fixed-bed quartz reactor (8 mm in diameter and 40 cm in length). To mitigate thermal effects and improve heat dissipation during the reaction, 30 mg of catalyst powder (40–60 mesh) was mixed with 100 mg of quartz sand (40–60 mesh) [28]. The reactant gas flow rate was controlled using a mass flow controller at 10 mL/min. The reactant mixture consisted of 1 vol% C₃H₆, 20 vol% O₂, 79 vol% N₂, with a gas hourly space velocity (GHSV) maintained at 20,000 mL·g⁻¹·h⁻¹. Product stream composition was continuously monitored by online gas chromatography (GC-9750, FULI Analytical Instruments, Wenling, China) equipped with a TDX-01 packed column and dual thermal conductivity detectors (TCD).

The C₃H₆ conversion was calculated using the following relationship:

$$X_{C_3{H}_6}={\displaystyle \frac{{\left[C_3{H}_6\right]}_{\mathit{inlet}}-{\left[C_3{H}_6\right]}_{\mathit{outlet}}}{{\left[C_3{H}_6\right]}_{\mathit{inlet}}}} \times 100\%$$

where [C₃H₆]_inlet and [C₃H₆]_outlet represent the inlet and outlet concentrations of propylene, respectively [37].

3.3. Characterization of Catalysts

X-ray diffraction (XRD) patterns were acquired using a Bruker AXS D8 Focus diffractometer equipped (Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA. The morphologies of the samples were characterized using scanning electron microscopy (SEM, Nova Nano SEM 450, Waltham, MA, USA) and transmission electron microscopy (TEM, Nion UltraSTEM 100, Karlsruhe, Germany). For TEM analyses, the dry carbon samples were first dispersed in ethanol through ultrasonication to form homogeneous suspensions. Approximately 5 μL aliquots of these suspensions were then drop-cast onto TEM grids and allowed to dry under ambient conditions prior to imaging. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo ESCALAB 250 spectrometer (Waltham, MA, USA) with a monochromatized Al Kα X-ray source (1486.6 eV). The analyzer pass energy was set to 20 eV for high-resolution scans. All binding energies were calibrated against the C1s peak of adventitious carbon (284.6 eV). Oxygen temperature programmed desorption (O₂-TPD) was performed on a Micromeritics AutoChem 2920 II analyzer (Shanghai, China) coupled with a Hiden HPR 20 mass spectrometer (Warrington, UK). Approximately 50 mg of sample (40–60 mesh) was loaded into a quartz reactor and pretreated under He flow (30 mL/min) at 250 °C for 30 min. After cooling to room temperature, the sample was exposed to a 3 vol% O₂/He mixture (30 mL/min) at 200 °C for 30 min (heating rate: 10 °C/min), followed by cooling to room temperature under He purge (20 mL/min). The desorption profile was recorded by heating the reactor to 600 °C at 10 °C min⁻¹ under He flow (20 mL/min), with m/z = 32 (O₂) monitored by mass spectrometry. H₂ temperature programmed reduction (H₂-TPR) experiments were carried out using conventional flow system equipment with a thermal conductivity detector (TCD). A 30 mg sample was loaded into the quartz tube reactor. The reduction gas consisted of 5% H₂/N₂ (40 mL/min). TPR was run from 30 °C to 450 °C at the rate of 5 °C/min.

3.4. Isotope-Labeling Experiments

The isotopic labeling experiments for the C₃H₆ + ¹⁸O₂ continuous reactions were monitored by real-time mass spectrometry. Prior to testing, all samples underwent pretreatment at 300 °C for 60 min under 5 vol% ¹⁶O₂/He flows, followed by He purging to eliminate residual ¹⁶O₂. Upon cooling the reaction beds to the target temperatures, the carrier gases were immediately switched to the reaction mixtures (20 vol% C₃H₆ + 5048 ppm ¹⁸O₂ balanced with He) while heating to 280 °C at 2 °C/min for 1 h, with the effluents directly analyzed by mass spectrometry. Reaction products were continuously quantified using online mass spectrometers (Thermo Fisher Delta V, Waltham, MA, USA) by tracking the characteristic signals of C¹⁶O₂ (m/z = 44), C¹⁶O¹⁸O (m/z = 46), and C¹⁸O₂ (m/z = 48).

3.5. Computational Method

All density functional theory (DFT) calculations were performed using CASTEP with the projector augmented wave method. The exchange–correlation effects were treated with the Perdew–Burke–Ernzerhof functional. After rigorous convergence tests, we employed plane-wave cutoff energies of 450 eV and 1 × 1 × 1 k-point meshes for Brillouin zone sampling. Vacuum layers of 15 Å were applied along the z-axes of the slab models to eliminate periodic interactions, with convergence criteria set at 10−5 eV for energies and 0.02 eV Å−1 for maximum stresses [42,43].

4. Conclusions

In summary, this study reveals a pronounced structure–composition correlation in alkaline earth metal carbonate-supported Pt catalysts for propylene combustion, with BaCO₃ identified as the optimal support. Notably, the propylene combustion rate over Pt/BaCO₃ surpasses those of Pt/CaCO₃ and Pt/MgCO₃ by an order of magnitude. Through kinetic studies and ¹⁸O₂ isotope labeling experiments, we demonstrate that while all Pt-MCO₃ systems follow the Mars–van Krevelen (M-K) mechanism, there are distinct differences emerge in propylene adsorption and activation pathways. The weakened metal–support interaction in Pt/BaCO₃ results in minimal electron transfer from the BaCO₃ support to PtO₂, rendering the Pt⁴⁺ centers in Pt/BaCO₃ the most electrophilic yet least capable of propylene chemisorption. Intriguingly, this electronic configuration modification effectively prevents poisoning of Pt active sites, while stabilizing Pt species in higher oxidation states, thereby dramatically enhancing the catalytic combustion performance. These findings provide fundamental insights into the structure–performance relationships of Pt/MCO₃ catalysts, establishing a framework for the rational design of advanced metal carbonate-supported catalytic systems.