Hydrothermal Aging Mechanism of CeO₂-Based Catalytic Materials and Its Structure–Activity Relationship Study on Particulate Matter Oxidation Performance

Abstract

With the increasing emphasis on environmental protection and sustainable development, improving air pollution control technology has become imperative. In this study, Ce-based catalysts are used as research objects to explore the effects of hydrothermal aging on their performance in oxidizing PM. Different Mn, Na, Pt and Zr-doped Ce-based catalysts were prepared based on the impregnation method and the PM oxidation performance of Ce-based catalysts before and after hydrothermal aging was investigated using thermogravimetric experiments, and the catalytic activity change pattern of fresh/hydrothermal aging Ce-based catalysts was analyzed by comparing the comprehensive combustion index S and combustion stability index Rw, revealing the PM oxidation process. The conclusion showed that the cerium-based catalyst significantly enhanced the oxidation efficiency of PM compared with PU. By comparing the performance of different metal-modified catalysts, it was found that the order of activity was: Pt > Na > Mn > Zr. With the metal doping increased, only the comprehensive combustion index S and combustion stability index Rw of Na/CeO₂ catalysts decreased. After hydrothermal aging treatment, the Zr/CeO₂ catalysts showed the best hydrothermal aging resistance, and the comprehensive combustion index S and combustion stability index Rw remained stable (<5%). Ce-based catalysts have the strongest to weakest hydrothermal aging resistance in the following order: Zr > Mn > Pt > Na. This study not only provides an important scientific reference for the application of Ce-based catalysts in the field of environmental purification but also contributes new ideas and methods to promote the green and sustainable development of air pollution control technology.

1. Introduction

With the acceleration of industrialization and rising energy consumption, air pollution has become increasingly severe. In particular, PM pollution poses a significant threat to both human health and the environment [1]. To address this challenge, the development of efficient and stable catalysts for promoting PM oxidation has emerged as a key research focus in the field of air pollution control. Cerium-based catalysts demonstrate considerable potential in PM oxidation due to their unique redox properties. However, a major challenge in their practical application is the decline in performance after long-term use, a phenomenon known as hydrothermal aging [2]. This study investigates the influence of hydrothermal aging on the PM oxidation performance of Ce-based catalysts, with the aim of providing a scientific foundation for their sustainable application.

Diesel engines are widely used in several fields due to their high economy, efficiency, and reliability [3]. However, the pollutants they emit, especially respirable PM, are produced at low temperatures and under anoxic conditions, posing a serious threat to human health [4]. DPF is currently an effective device to reduce PM emissions, and the regeneration of DPF is achieved by capturing and oxidizing combusted PM through a wall-flow ceramic filter body [5]. However, the regeneration process of DPF needs to increase the temperature to more than 550 °C, which is a contradiction between diesel engine fuel economy [6]. For this reason, the research and development of new PM capture and regeneration technologies to improve the reliability of DPF and reduce fuel consumption has become an urgent problem for after-treatment systems.

A promising aftertreatment solution for PM regeneration involves the use of a CDPF, which enables low-temperature PM oxidation and enhances filter regeneration [7]. By coating the surface of the wall-flow ceramic filter with a catalyst, the CDPF system eliminates the need for complex control systems and external heating sources, thereby reducing both energy consumption and operational costs compared to non-catalytic DPFs [8]. An effective CDPF catalyst must satisfy three essential requirements: (1) it should exhibit high catalytic activity, good NOx selectivity, and long-term durability [9]; (2) it should be capable of utilizing exhaust waste heat to improve thermal efficiency [10]; and (3) in the case of precious metal-based catalysts, cost control must be carefully considered to ensure economic feasibility [11].

Researchers have found that CeO₂ has a good ability to store and release oxygen and has a low oxygen vacancy generation energy, which can provide activated oxygen for the oxidation of PM, and thus can be used as a potential catalyst for CDPF applications [12]. Since PM oxidation is a gas–solid–solid reaction between O₂, PM, and catalyst, the contactivity between the reacting components and the catalyst activity are two key factors affecting the PM oxidation efficiency [13]. To further enhance the regeneration performance of CDPF, can be modified by doping different metallic. The conclusions of related studies showed that rare earth metal oxides such as Ce_0.5Zr_0.5O₂ [14], LaFeO₃ [15], and LaMn_0.8Pt_0.2O₃ [16] have the ability to catalyze PM oxidation reactions, and therefore, under the synergistic effect of the rare earth metal oxides, CeO₂ exhibits a better oxygen storage capacity and catalytic performance in the low-temperature range than the other conventional metal oxide catalysts [17]. Studies by Atribak et al. [18] and Gross et al. [19] showed that Zr and K doping could enhance the high-temperature stability and lattice oxygen mobility of Ce-based catalysts, respectively. The results of Shimokawa et al. [20] showed that the PM oxidation activity of the Ag/CeO₂ system was close to that of noble metal Pt catalysts under looser contact conditions. A systematic review by Shuang Liu et al. [21] indicated that Mn, K, Ag, and Zr are the most promising modifying elements for application. However, for the noble metals, Pt performs better than Ag, and Na elements have advantages in terms of wide temperature windows, high hydrothermal stability, and resistance to carbon deposition. Therefore, the optimal doping ratios of the above four metals are Ce_0.95Mn_0.05O_x, Ce_0.96Na_0.04O_x, Ce_0.95Pt_0.05O_x, Ce_0.76Zr_0.24O₂.

This study systematically investigated the effects of hydrothermal aging on the PM oxidation performance of Ce-based catalysts. The observed activity loss after aging was closely associated with structural changes and the reduction in active sites. Strategies for enhancing catalyst stability are proposed, which hold significant importance for advancing the sustainable development of air pollution control technologies. Through optimization of Ce-based catalysts, this research aims to extend service life, reduce resource consumption, and improve the efficiency of PM pollution treatment, thereby providing scientific support for environmental sustainability goals. In summary, while metal ion doping can effectively modulate the PM oxidation activity of ceria-based catalysts, its impact on hydrothermal stability varies significantly across different metal species. Previous studies have primarily focused on how metal modification affects the physicochemical properties of Ce-based catalysts, whereas systematic investigation remains limited regarding how hydrothermal aging influences PM oxidation performance—particularly concerning the mechanisms by which metal species (Mn, Na, Pt, Zr, etc.), doping concentration, and aging duration affect PM oxidation during hydrothermal treatment. To address this gap, the present study examines the influence of metal-modified Ce-based catalysts subjected to varying hydrothermal aging durations on the PM oxidation process using TGA. The research was conducted in two stages: first, a systematic evaluation of the differential effects of Mn, Na, Pt, and Zr modifications on PM oxidation reactivity; second, an investigation into how doping concentration and hydrothermal aging time affect PM oxidation performance, to elucidate the degradation patterns of catalytic performance following hydrothermal aging.

2. Results and Discussion

2.1. Metal Modification

In this study, the catalytic oxidation performance of metal-modified Ce-based catalysts/PU was investigated by the TG/DTG system, as shown in Figure 1. The experimental data revealed a distinct low-temperature shift in the TG curves after the introduction of Ce-based catalysts, compared to the oxidation behavior of PU. Generally, the TG profiles can be categorized into three stages: The first stage (200–300 °C) involves a gradual mass loss of PU, with the TG curve decreasing by approximately 8%, primarily due to the removal of adsorbed water and volatile organics from the catalyst surface. In the catalytic oxidation stage (300–700 °C), PU undergoes deep oxidation facilitated by the synergistic effect of the Ce³⁺/Ce⁴⁺ redox cycle and gas-phase oxygen [22], leading to a pronounced decrease in mass. During the complete oxidation stage (700–800 °C), the remaining particulate matter is fully oxidized, and the TG curve stabilizes, indicating the termination of the oxidation process. As reflected in the DTG curves, metal-modified Ce-based catalysts effectively reduced the peak temperature Tp, with the extent of reduction varying significantly depending on the metal dopant species. The Tp of the peak temperature of the PU was 582 °C. In comparison, the Tp of the peak temperature of Ce_0.95Mn_0.05O₂, Ce_0.96Na_0.04O₂, Ce_0.95Pt_0.05O₂, Ce_0.76Zr_0.24O₂, Ce_0.95Pt_0.05O₂, Ce_0.76Zr_0.24O₂ and CeO₂ with Tp of 420 °C, 380.5 °C, 341.5 °C, 479.5 °C and 371 °C, respectively. The highest Wmax occurred in the oxidation reaction of Ce_0.96Na_0.04O₂ with PU, which reached 141.09%/min.

The different metal-modified Ce-based catalysts exhibit, in addition to obvious performance differences, which is due to the different mechanisms of synergistic effects between different metals and CeO₂, leading to the differentiation of oxidation performance. The metals used for modification can either accelerate the generation of reactive oxygen species, e.g., Mn, Zr, or the metal itself participates in the PM oxidation reaction as a redox-active site, e.g., Pt, or change the physical properties of CeO₂ to increase the mobility of the catalyst, thus enhancing the contact between PM and the catalyst, e.g., Na. The experimental results show that due to the excellent valence conversion between Ce³⁺ and Ce⁴⁺, CeO₂ has good storage and discharge performance, which makes it have good oxygen storage and discharge performance and PM catalytic oxidation activity [23]. Based on the CeO₂ catalyst reaction mechanism established by Bueno-Lopez [24], the oxygen required for the PM oxidation reaction is provided by the lattice oxygen of CeO₂.

In order to investigate the effects of different metal-modified ceria catalysts on the oxidation characteristics of PM, S, Rw, and Tp of the mixtures of PU with four metal-modified ceria catalysts are compared in Figure 2. It is shown that the introduction of ceria catalysts promotes the oxidative reaction of PM, which is manifested by the enhancement of S and Rw as well as the decrease in Tp. Under the condition of the same PU/catalyst mixing ratio, the S and Rw of different metal-modified ceria-based catalysts exhibited the same trend, showing a decreasing order: Na> Pt >Mn> Zr. In contrast, the Tp showed the following decreasing order: Zr > Mn > Na > Pt.

2.2. Metal Doping Concentration

As shown in Figure 3, the effects of different metal doping ratios (Mn: 0.05–0.1; Na: 0.04–0.08; Pt: 0.05–0.1; Zr: 0.24–0.4) on the thermal oxidation behaviors of Ce-based catalysts/PU were investigated by the TG-DTG analysis system. Figure 3a presents the TG-DTG curves of samples with different Mn doping ratios. It can be observed that as the Mn doping content increases from 0.05 to 0.1, the corresponding Tp shifts toward lower temperatures, indicating a negative correlation between the Mn doping ratio and the oxidation performance of PM. Figure 3b displays the TG-DTG curves for different Na doping ratios. With an increase in Na doping from 0.04 to 0.08, the maximum rate of mass change (Wmax) increases, while Tp remains largely unchanged, suggesting a positive correlation between the Na doping level and PM oxidation performance. Figure 3c illustrates the TG-DTG curves for different Pt doping ratios. As the Pt doping amount rises, the Tp shifts to a lower temperature region, implying that the Pt doping ratio is positively correlated with PM oxidation performance. Figure 3d shows the TG-DTG curves at different Zr doping ratios. When the Zr doping increases from 0.24 to 0.4, the Wmax value increases, whereas Tp changes only slightly, demonstrating a negative correlation between the Zr doping ratio and the oxidation performance of PM. The results show that the characteristic oxidation temperature Tp of the Mn- and Pt-doped systems exhibits an obvious low-temperature shift trend, whereas the Na- and Zr-modified catalysts exhibit different behavioral characteristics: the maximum reaction rate Wmax is increased by Na doping, and Wmax is increased by Zr doping, and the magnitude of the change in the Tp temperature is small in both cases. The doping ratios of the metals Mn and Zr were negatively correlated with the oxidizing properties of PM, while the doping ratios of the metals Na and Pt were positively correlated with the oxidizing properties of PM [25]. The doping ratios of Mn and Zr metals are negatively correlated with PM oxidizability, while the doping ratios of Na and Pt metals are positively correlated with PM oxidation performance. Figure 4 systematically compares the characteristic oxidation parameters (S, Rw, Tp) of CeO₂-based catalysts with gradient metal doping concentrations. Experimental data reveal significant trends in both S and Rw parameters with increasing dopant content across all modified catalyst systems. The optimal S, Rw, and Tp achieved by each type of metal-doped CeO₂ catalyst at its respective optimal doping ratio are summarized in

Table 1. Highest S, Rw, and lowest Tp values for Ce-based catalysts with optimal metal doping ratios.

Catalyst	S/%2min−2 °C−3	Rw/%min−1 °C−2	Tp/°C
Ce0.9Mn0.1O2	46.65 × 107	39,972.41	389.5
Ce0.95Na0.04O2	93.72 × 107	84,128.23	381.5
Ce0.9Pt0.1O2 Ce0.6Zr0.4O2	64.50 × 107 18.69 × 107	61,471.91 18,783.95	335.5 476.8

.

Among the studied metal dopants, only Pt doping significantly enhanced the PM oxidation performance of the ceria-based catalyst compared to the unmodified CeO₂ catalyst. The improvement in catalytic performance from other metal dopants was negligible. However, the different metal systems exhibited significant performance differences as shown in

Table 2. Trends of oxidation activity indexes of a series of metal-modified ceria-based catalysts.

Catalyst	S Trend/%	Rw Trend/%	Tp Trend/°C
Ce0.95Mn0.05O2→Ce0.9Mn0.1O2	160.2	130.54	−30.14
Ce0.95Na0.04O2→Ce0.95Na0.08O2	−3.75	−6.54	−2.16
Ce0.95Pt0.05O2→Ce0.9Pt0.1O2 Ce0.76Zr0.24O2→Ce0.6Zr0.4O2	100.25 9.23	90.25 10.12	−7.12 3.11

. Specifically, the Pt-, Na-, and Mn-modified systems obtained different degrees of performance enhancement, respectively. In contrast, the Zr-modified catalysts showed a reduced improvement in activity, which was comparable to that of pure CeO₂. An in-depth analysis of the effect of doping concentration reveals that the Mn- and Pt-modified systems exhibit a significant concentration dependence, with a notable increase in S and Rw, and a decrease in Tp, indicating that their PM oxidation performance is enhanced [26]. In contrast, the Na- and Zr-modified catalysts exhibited a low rate of change in the parameters within the tested concentration range, demonstrating a concentration-independent feature. Comparing Pt, Na, and Zr, the doping ratio of Mn had the most significant effect on the PM oxidation performance. Overall, the ranking was Pt > Na > Mn > Zr.

2.3. Hydrothermal Aging

The TG-DTG curves of CeO₂-based catalysts/PU with different metal doping (Mn, Na, Pt, and Zr) before and after hydrothermal aging (0 h, 12 h, and 24 h) are shown in Figure 5. Figure 5a presents the TG-DTG curves of fresh Ce-based catalysts and those subjected to 12 h and 24 h of hydrothermal aging with varying Mn doping ratios. It can be observed that the Tp shifts toward higher temperatures for all Ce-based catalysts after hydrothermal aging. This shift is likely attributed to structural alterations in the catalyst lattice induced by hydrothermal aging, which increase the activation energy for oxygen vacancy formation. As a result, the energy barriers for generating reactive oxygen species and initiating surface oxidation reactions are raised, leading to an elevated PM ignition temperature and a decreased reaction rate. Figure 5b displays the TG-DTG curves of fresh Ce-based catalysts and those hydrothermally aged for 12 h and 24 h with different Na doping ratios. A consistent shift of Tp toward higher temperatures is observed after aging, likely due to lattice changes in the Ce-based catalyst that raise the energy barriers for forming oxygen vacancies and reactive oxygen species, ultimately reducing the PM oxidation activity. In Figure 5c, TG-DTG curves are shown for Pt-doped Ce-based catalysts with different doping levels, including fresh, 12 h aged, and 24 h aged samples. Again, Tp shifts to higher temperatures following hydrothermal aging. This behavior may stem from lattice modifications in the Ce-based catalysts that increase the formation energy of oxygen vacancies. Consequently, the energy barrier for generating reactive oxygen species rises, increasing the activation energy of surface catalytic oxidation following the Mars–Van Krevelen mechanism, which in turn elevates the PM ignition temperature and lowers the reaction rate. Figure 5d illustrates the TG-DTG curves of Ce-based catalysts doped with different Zr ratios, including fresh, 12 h, and 24 h hydrothermally aged samples. A clear shift of Tp toward higher temperature ranges is evident post-aging, likely resulting from hydrothermal aging-induced lattice changes that raise the activation energy for oxygen vacancy formation. This enhances the energy barriers for reactive oxygen species generation and surface oxidation, leading to an increase in PM ignition temperature and a reduction in reaction rates. Figure 5e shows TG-DTG curves for fresh and hydrothermally aged (12 h and 24 h) Ce-based catalysts. The peak temperature Tp shifts toward higher regions after aging, possibly due to hydrothermal modification of the catalyst crystal lattice, which increases the energy barriers for forming oxygen vacancies and reactive oxygen species, thereby diminishing PM oxidation activity. From Figure 6a–e, it can be seen that the peak temperature Tp of all Ce-based catalysts shifted to the high-temperature range after hydrothermal aging. This is probably due to the change in the crystal lattice of Ce-based catalysts after hydrothermal aging, the increase in the oxygen vacancy generation energy of the catalysts, which leads to the increase in the reaction energy barrier for the formation of reactive oxygen species, and the increase in the activation energy of the surface-catalyzed oxidation reaction based on the Mars–Van-Krevelen mechanism [27], increasing the ignition temperature of the PM and the decrease in the reaction rate.

As shown in Figure 6, the key oxidation indices (S, Rw, Tp) of Ce-based catalysts were systematically compared before and after hydrothermal aging to evaluate their oxidation stability. Figure 6a compares the S, Rw, and Tp indices of fresh and hydrothermally aged cerium-based catalysts with different Mn doping ratios. It can be observed that Mn-modified cerium-based catalysts exhibit a decrease in S and Rw, and an increase in Tp. Figure 6b presents a comparison of the S, Rw, and Tp indices for fresh and hydrothermally aged cerium-based catalysts with different Na doping ratios. The results show that Na-modified cerium-based catalysts demonstrate a reduction in S and Rw, and a rise in Tp. Figure 6c illustrates the S, Rw, and Tp indices of fresh and hydrothermally aged cerium-based catalysts with varying Pt doping ratios. Pt-modified cerium-based catalysts are found to display decreased S and Rw, along with an increased Tp. Figure 6d provides a comparison of the S, Rw, and Tp indices for fresh and hydrothermally aged cerium-based catalysts with different Zr doping ratios. It can be seen that the changes in S, Rw, and Tp are relatively minor after hydrothermal aging for Zr-modified cerium-based catalysts. Figure 6e shows the comparison of S, Rw, and Tp indices between fresh and hydrothermally aged cerium-based catalysts. The results indicate that the cerium-based catalysts experience a decrease in S and Rw, and an increase in Tp. The results indicate that for Mn-, Pt-, and Na-modified Ce-based catalysts, the values of S and Rw decrease, while Tp increases after aging. In contrast, the Zr–CeO₂ system maintains good structural stability under the same hydrothermal aging conditions. Experimental data further reveal the optimal doping ratios and corresponding performance metrics for various metal-modified Ce-based catalysts after aging: Ce_0.9Mn_0.1O₂ exhibits an S of 22.50 × 10⁷%²min⁻² °C⁻³, Rw of 23,235.77%min⁻¹ °C⁻¹, and Tp of 475 °C; Ce_0.96Na_0.04O₂ shows an S of 19.95 × 10⁷%²min⁻² °C⁻³, Rw of 14,545.37%min⁻¹ °C⁻¹, and Tp of 508 °C; Ce_0.95Pt_0.05O₂ displays an S of 27.31 × 10⁷%²min⁻² °C⁻³, Rw of 22,595.23%min⁻¹ °C⁻¹, and Tp of 459 °C; Ce_0.6Zr_0.4O₂ demonstrates an S of 15.58 × 10⁷%²min⁻² °C⁻³, Rw of 14,545.47%min⁻¹ °C⁻¹, and Tp of 507 °C; and similarly, Ce_0.6Zr_0.4O₂ is also recorded with an S of 15.60 × 10⁷%²min⁻² °C⁻³, Rw of 16,158.75%min⁻¹ °C⁻¹, and Tp of 501 °C.

A comparison of fresh and hydrothermally aged samples in

Table 3. Trends of S, Rw and Tp indices of hydrothermally aged Ce-based catalysts compared to fresh samples.

Catalyst	Aging Temperature and Time	S Trend/%	Rw Trend/%	Tp Trend/°C
Ce0.95Mn0.05O2	750 °C, 12 h	−53.35	−55.15	45.5
Ce0.95Mn0.05O2	750 °C, 24 h	−27.30	−37.21	51
Ce0.9Mn0.1O2 Ce0.9Mn0.1O2 Ce0.96Na0.04O2	750 °C, 12 h 750 °C, 24 h 750 °C, 12 h	31.20 26.15 64.37	17.83 13.68 65.85	62 85.5 19
Ce0.96Na0.04O2	750 °C, 24 h	78.26	83.03	124
Ce0.92Na0.08O2 Ce0.92Na0.08O2	750 °C, 12 h 750 °C, 24 h	54.21 52.16	57.93 56.55	0 22.5
Ce0.95Pt0.05O2 Ce0.95Pt0.05O2 Ce0.9Pt0.1O2 Ce0.9Pt0.1O2 Ce0.76Zr0.24O2 Ce0.76Zr0.24O2 Ce0.6Zr0.4O2 Ce0.6Zr0.4O2 CeO2 CeO2	750 °C, 12 h 750 °C, 24 h 750 °C, 12 h 750 °C, 24 h 750 °C, 12 h 750 °C, 24 h 750 °C, 12 h 750 °C, 24 h 750 °C, 12 h 750 °C, 24 h	8.48 16.24 48.35 40.26 3.93 5.51 7.52 16.26 8.56 20.65	15.55 31.95 57.82 55.30 −5.95 1.24 5.55 14.28 5.06 18.25	30 116 105 1 0 −4 17 23 42 67

reveals distinct deactivation patterns among the metal-modified CeO₂ catalysts. A substantial loss in oxidative activity was observed for the Mn-, Pt-, and Na-CeO₂ formulations after aging. The Zr-CeO₂ catalyst, however, demonstrated remarkable resilience, with all measured parameters deviating by less than 10% from their initial values, indicating outstanding hydrothermal stability. Post-aging performance evaluation identified Ce_0.9Pt_0.1O₂ as the most effective catalyst for PM oxidation, a result of its proficient formation and mobilization of reactive oxygen species. This suggests that while Zr doping primarily enhances structural stability, Pt doping, even at low concentrations, optimally preserves catalytic function after aging. The resistance to hydrothermal aging conferred by Mn, Pt, and Na doping was found to be substantially inferior to that of Zr.

2.4. Extended Discussion on Hydrothermal Stability Trends

The observed hydrothermal stability trend (Zr > Mn > Pt > Na) aligns with prior studies on doped ceria systems and can be understood through ion radius, redox behavior, and the presence of dopants within the lattice.

• Zr⁴⁺ Doping and Solid Solution Stability: The exceptional hydrothermal stability of Zr-doped ceria has been well established in previous studies [28]. This property originates primarily from the formation of a highly stable Ce_1−xZr_xO₂ solid solution. The ionic radius of Zr⁴⁺ is close to that of Ce⁴⁺, facilitating its effective incorporation into the fluorite lattice with minimal strain. More significantly, this substitution induces a distorted yet reinforced lattice structure. By elevating the energy barriers for both oxygen vacancy migration and cation diffusion, this modified lattice effectively suppresses the sintering and grain growth of CeO₂ under harsh hydrothermal conditions. Additionally, the strong Zr–O bonding enhances the overall structural integrity, preventing collapse of the mesoporous framework and thereby maintaining the specific surface area and active sites crucial for PM oxidation.
• Mn²⁺/³⁺/⁴⁺ Doping and Redox-Induced Instability: The moderate deactivation of Mn-doped ceria aligns with its reported sintering susceptibility [29]. While Mn ions can enter the ceria lattice, a significant ionic radius mismatch introduces substantial lattice strain and point defects. Under hydrothermal conditions, the redox cycling between Mn³⁺ and Mn⁴⁺, while beneficial for low-temperature activity, may accelerate MnOx species segregation and promote crystallization of less active crystalline phases, leading to partial deactivation [30]. However, the residual Mn species maintain a connection with the CeO₂ lattice, allowing partial redox activity to persist after aging.
• Pt as a Surface Species and Its Sintering Behavior: The inferior hydrothermal stability of Pt–CeO₂ illustrates a classical case of noble metal sintering [31]. In contrast to Zr⁴⁺, platinum species do not incorporate into the ceria lattice but remain as metallic nanoparticles or surface oxide clusters. Under high-temperature steam, these Pt particles gain mobility and undergo coalescence, resulting in a pronounced increase in particle size. Consequently, the number of active Pt sites and Pt–CeO₂ interfacial sites—which are widely recognized as critical centers for oxygen activation and spillover in oxidation reactions—is markedly reduced. This sintering phenomenon constitutes the primary reason for the observed activity loss.
• Na⁺ Doping and Structural Degradation: The severe deactivation of Na-doped ceria can be attributed to both structural and chemical factors. The large ionic radius of Na⁺ makes it difficult to enter the ceria lattice, causing it to reside primarily at the surface [32]. This blocks active sites and, more critically, acts as a fluxing agent. Under hydrothermal conditions, mobile Na⁺ species react with silica/alumina impurities (originating from PM or the catalyst support itself) and ceria to form low-melting-point silicates, aluminates, or cerates. This leads to pore blockage, surface vitrification, and permanent loss of porosity and specific surface area—a more severe form of deactivation than sintering.

In summary, the hierarchy of stability is jointly determined by the ability of the dopant to integrate and stabilize the ceria crystal structure (Zr), its tendency to induce lattice strain (Mn), its presence as sinterable surface particles (Pt), or its capacity to trigger destructive chemical transformations (Na).

3. Introduction to the Experiment

3.1. Experimental Methods

The oxidation characteristics of PM were evaluated by monitoring the mass change in samples during heating via TGA. In this work, a METTLER-2(SF) thermogravimetric analyzer was employed; the key instrument parameters are summarized in

Table 4. Specifications of TGA.

Property	Numerical Value
Temperature range	1100 °C
Equilibrium accuracy	0.2 g
Sample Mass Thermogravimetric Drift Rising Rate	Max. 1 g <1 mg/h 0.02∼250 K/min

. The measurement accuracies for temperature and mass were ±0.5 °C and ±0.03 µg, respectively. Prior to TGA, all samples were subjected to a standardized pretreatment, which included maintaining a constant temperature of 110 °C under an air purge of 200 mL/min for 30 min. The mass of PU—used as a model PM compound—and the catalyst in each test sample were 4 mg and 40 mg, respectively. The experiments were conducted in a gas atmosphere of 21% O₂ and 79% N₂, with the temperature ramped from 100 °C to 800 °C at a heating rate of 10 °C/min. The principal properties of the PU material are provided in

Table 5. Physical properties of PU.

Property	Numerical Value
Carbon Black Grade	Printex-U
Particle Size	25~35/nm
Specific Surface Area Oil Content Ash Content	85~95/(m2/g) 440~460/(g/100 g) 0.02/%

. Explicitly state that PU was used as a model compound for PM.

3.2. Data Analysis Methods

The TG analysis was performed by continuously monitoring the mass change in the sample as a function of temperature. The DTG curve, representing the rate of mass loss, was obtained by differentiating the TG curve mathematically. From these curves, key oxidation parameters were determined, including the ignition temperature (T_s), end temperature (T_e), and peak temperature (T_p), as well as the maximum (W_max) and average (W_mean) mass loss rates. For further methodological details, see Reference [33].

The composite combustion index S is a standardized coefficient for the ignition and exhaustion characteristics of PM oxidation [34], and a higher value of S means better combustion performance the relevant formula is as follows:

$$S={\displaystyle \frac{W_{max}-W_{mean}}{T_s^2-T_e}}$$

(1)

The combustion stability index Rw reflects the stability of the PM oxidation process [35], the higher the value of Rw, the better the combustion stability, the relevant formula is as follows:

$$R_w=8.5875\times {10}^7{\displaystyle \frac{W_{max}}{T^s\times T_p}}$$

(2)

4. Conclusions

This study systematically evaluated the hydrothermal aging behavior and PM oxidation performance of CeO₂-based catalysts modified with Mn, Pt, Na, and Zr. The main findings are summarized as follows:

(1)

Metal-doped CeO₂ catalysts significantly improve PM oxidation activity, reducing peak temperature (Tp) and enhancing combustion indices (S and Rw). The activity order is Na > Pt > Mn > Zr, while Tp increases in the order: Zr > Mn > Na > Pt.

(2)

The oxidation performance was influenced by metal doping concentration. Mn and Pt doping promoted low-temperature activity, while Na and Zr mainly increased the maximum reaction rate (Wmax). Na doping positively correlated with catalytic performance, whereas Mn, Pt, and Zr exhibited negative correlations.

(3)

After hydrothermal aging treatment, the S, Rw, and Tp of the Mn-, Pt-, and Na-modified catalysts decreased drastically, but the changes in the Zr-modified catalysts were smaller, the Ce_0.9Pt_0.1O₂ catalyst showed relatively superior PM oxidation performance. In contrast, the Zr-modified catalyst exhibits excellent hydrothermal aging resistance and shows good structural stability, the hydrothermal aging resistance of Mn-, Pt-, and Na-doped Ce-based catalysts is relatively poor, so the ranking is Zr > Mn > Pt > Na.

These results highlight the potential of Zr-modified CeO₂ as a highly stable catalyst for practical applications such as diesel particulate filters, contributing to longer service life, reduced maintenance costs, and more sustainable emission control technologies.