LaMnO₃‑Pt Whisker-Structured Cordierite Monoliths for Low-Temperature Soot Oxidation

Abstract

Diesel particulate filters (DPFs) require catalysts with both high activity and durability to achieve efficient low-temperature soot oxidation under practical exhaust conditions. In this work, a hierarchically structured LaMnO₃–Pt monolithic catalyst was constructed by growing aluminum borate whiskers on a cordierite substrate via a vapor-phase method to form a biomimetic tufted layer, followed by coating with LaMnO₃ perovskite and subsequent Pt loading. The whisker-induced structure significantly improves catalyst dispersion and soot–catalyst contact efficiency, while the synergistic interaction between LaMnO₃ and Pt effectively regulates surface oxygen species and their migration behavior. Temperature-programmed oxidation (TPO) tests demonstrate that the optimized C-LaMnO₃–Pt_0.9 wt % monolithic catalyst exhibits outstanding soot oxidation performance, with half-conversion temperature (T₅₀) reduced to 453 °C (under loose contact) and 403 °C (under tight contact), respectively, which are significantly lower than those of bare cordierite (T₅₀ = 610 °C). In addition, the catalyst maintains a CO₂ selectivity exceeding 99% during soot oxidation and shows no obvious activity degradation during repeated TPO cycles, indicating excellent thermal and structural stability. This hierarchical structural design provides a viable strategy for developing efficient and durable monolithic catalysts for soot oxidation in DPF applications.

1. Introduction

With the increasingly stringent emission regulations, the efficient removal of particulate matter (PM) from diesel exhaust remains one of the key challenges in aftertreatment technologies. Diesel particulate filters (DPFs) are regarded as the most mature control strategy owing to their ability to physically trap particulate matter. However, the accumulated soot must be periodically regenerated during operation to prevent filter blockage. , Conventional active regeneration relying on high temperatures not only results in high energy consumption but may also cause local overheating, thermal shock, and structural failure. Therefore, the development of catalyzed diesel particulate filters (cDPFs) capable of achieving sustained soot oxidation at lower temperatures has become an important direction in diesel aftertreatment technology. ,

In cDPF catalytic systems, He et al. enhanced the catalytic soot combustion performance of CeZrO catalysts by adding transition metal oxides (Mn, Fe, Co), among which the addition of Mn and Co transition metals exhibited the best catalytic performance. Ruan et al. introduced alkali metal ions (Cs⁺, K⁺) into zeolite supports via an ion-exchange method, followed by loading the noble metal Ag onto the supports using an impregnation method. The results showed that the Ag–K/ZSM-5–0.1 M catalyst exhibited the best catalytic activity, with a T _max value of 423.0 °C. Noble metal catalysts exhibit excellent low-temperature soot oxidation performance due to their strong oxygen activation ability. , Nevertheless, their high cost and limited thermal stability under high-temperature exhaust conditions have motivated continuous exploration of alternative catalysts based on transition metal oxides. Among them, perovskite-type mixed oxides have attracted considerable attention owing to their tunable structure, good thermal stability, and favorable redox properties. , Four LaBO₃ (B = Cr, Mn, Fe, Ni) perovskite catalysts loaded with 2% Au were prepared by solution combustion synthesis, among which Au-LaNiO₃ exhibited the best performance: a peak soot combustion temperature of 431 °C and a CO half-conversion temperature of 156 °C. Studies on Ag-doped La_0.6Ce_0.4CoO₃ showed that an Ag loading greater than 5% significantly enhanced the activity for diesel soot oxidation. Substituted lanthanum manganite perovskites were prepared by a coprecipitation/in situ method and supported on cordierite honeycomb ceramics. It was found that an Al₂O₃ washcoat precoated with La₂O₃ facilitates the in situ synthesis of perovskites, leading to improved surface area and adhesion. The addition of a small amount of Pt further enhanced the catalytic activity. For clearer comparison, the catalytic performance of the above catalysts is summarized in Table .

1. Summary of Soot Combustion Performance of Various Catalysts.

Samples	T m/°C	Reaction conditions
CZ6	551	Loose contact&NO+O2
Mn/CZ6	531	Loose contact&NO+O2
Fe/CZ6	546	Loose contact&NO+O2
Co/CZ6	531	Loose contact&NO+O2
Ag–K/ZSM-5–0.1M7	423	Loose contact&O2+Ar
Au-LaNiO3 12	431	Loose contact&On real diesel exhaust gases

In particular, LaMnO₃ perovskite exhibits good intrinsic activity for soot oxidation due to the reversible Mn³⁺/Mn⁴⁺ redox couple and high lattice oxygen mobility. To further regulate the surface oxygen chemistry and promote the participation of oxygen species in the low-to-medium temperature range, the introduction of small amounts of noble metals as promoters has been widely investigated. Noble metal incorporation enhances oxygen adsorption and activation and promotes the formation of surface active oxygen and lattice oxygen migration through metal–support interactions, thereby further improving low-temperature oxidation behavior while retaining the intrinsic advantages of the perovskite structure.

On the other hand, in monolithic cDPF systems, catalytic performance is influenced not only by chemical composition but also by the structural characteristics of the support and reaction interfaces. Conventional cordierite (2MgO·2Al₂O₃·5SiO₂) honeycomb ceramics, as the mainstream DPF substrate, possess relatively low specific surface area and limited filtration precision, which restrict the effective capture of nanoscale particulates and hinder uniform catalyst loading. , Consequently, improving the surface area through structural design while constructing catalyst coatings with both low-temperature activity and high-temperature stability remains a major challenge in monolithic cDPF systems. Recent studies have shown that introducing whisker layers with micro–nanoscale features onto cordierite substrates can effectively improve soot capture and catalytic reaction conditions. ,

Based on the synergistic regulation between perovskite and noble metals, combined with the macroscopic structural design of monolithic catalysts, this work proposes a hierarchically structured LaMnO₃–Pt composite monolithic catalyst. The core innovations of this study are as follows: First, biomimetic-structured aluminum borate whiskers are grown in situ on cordierite honeycomb ceramic supports via a vapor-phase method, forming a three-dimensional hierarchical interface with high specific surface area and robust structural stability. Subsequently, the LaMnO₃ perovskite active layer and a small amount of noble metal Pt are sequentially loaded using a sol–gel method and incipient wetness impregnation. By utilizing this “whisker-perovskite-noble metal” hierarchical structure, only trace Pt is required to significantly enhance low-temperature soot oxidation performance. Through systematic structural characterization, oxygen-related behavior analysis, and temperature-programmed soot oxidation tests, this work systematically investigates the synergistic mechanism of Pt promotion and hierarchical structural design on the low-temperature oxidation performance of monolithic catalysts.

2. Experimental Section

2.1. Preparation of Aluminum Borate Whiskers

Aluminum borate whiskers were grown in situ on the surface of cordierite honeycomb ceramics via a vapor-phase method. Specifically, an aluminum source (Al₂O₃), a boron source (B₂O₃), and a mineralizer (AlF₃) were mixed at mass fractions of 50, 3, and 12 wt %, respectively. The mixture was thoroughly blended and ball-milled to obtain a homogeneous precursor powder. Subsequently, the mixed powder and cordierite honeycomb substrates were placed together in a sealed corundum crucible and calcined at 1250 °C for 5 h in an air atmosphere. After the reaction, the samples were naturally cooled to room temperature inside the furnace. As a result, aluminum borate whisker/cordierite composite substrates with a high-surface-area, cilia-like structure were obtained.

2.2. Preparation and Loading of LaMnO₃-Pt Catalysts

The LaMnO₃–Pt monolithic catalysts were prepared via a two-step process. First, a LaMnO₃ perovskite layer was synthesized using a sol–gel method. Lanthanum nitrate and manganese nitrate were employed as metal precursors, while citric acid was used as the chelating agent, with a molar ratio of total metal ions to citric acid of 1:1.5. The precursors were mixed and continuously stirred at 80 °C to form a homogeneous LaMnO₃ precursor sol. The obtained sol was then loaded onto the aluminum borate whisker/cordierite substrates by an impregnation method. After drying, the samples were calcined in air at 500 °C for 4 h to form a stable LaMnO₃ perovskite active layer.

Subsequently, platinum was introduced by an incipient wetness impregnation method using an aqueous solution of chloroplatinic acid (H₂PtCl₆). After impregnation and drying, the samples were thermally treated at 350 °C for 4 h under a reducing atmosphere (H₂/Ar), yielding the final LaMnO₃–Pt/aluminum borate whisker/cordierite monolithic catalysts.

2.3. Characterization

Microstructural analysis was carried out using a field-emission scanning electron microscope (SEM, APERO-LowVac, FEI, USA) equipped with an energy-dispersive X-ray spectroscopy (EDS) system. Phase structures were analyzed using an X-ray diffractometer (XRD, Bruker D8 Advance). Fourier-transform infrared (FT-IR) spectra were recorded using a Magna 560 spectrometer (Thermo Fisher Scientific, USA). Surface elemental composition, valence states, and chemical environments were analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). O₂-TPD and H₂-TPR measurements were conducted using a Micromeritics AutoChem II 2920 chemisorption analyzer. Thermogravimetric–differential scanning calorimetry (TG–DSC) measurements were performed using a STA 449 F5 Jupiter thermal analyzer (NETZSCH, Germany). O₂-TPO tests were carried out using a Thermo Antaris IGS Fourier-transform infrared gas analyzer to monitor outlet gas concentrations and obtain temperature-CO₂ concentration profiles.

2.4. Catalytic Performance Evaluation

The catalytic soot oxidation performance of the monolithic catalysts was systematically evaluated using a temperature-programmed oxidation (TPO) method in a fixed-bed reactor. Printex U is used in the burn test in place of actual carbon black. Printex U was ultrasonically dispersed in absolute ethanol to prepare a uniform soot suspension, which was then precisely deposited into the channels of the monolithic catalysts. The samples were dried at 75 °C to ensure loose contact between soot and catalyst under gravity-contact conditions. The mass ratio of catalyst to soot was fixed at 10:1. TPO tests were conducted under a simulated diesel exhaust atmosphere (10% O₂ + 500 ppm of NO, balanced with N₂), with a linear heating rate of 5 °C·min^–1 from room temperature to 650 °C and a gas hourly space velocity of 30,000 h^–1. The concentrations of CO and CO₂ in the outlet gas were continuously monitored using an FT-IR gas analyzer.

Based on the soot conversion–temperature curves, the ignition temperature (T₁₀), half-conversion temperature (T₅₀), and high-conversion temperature (T₉₀), corresponding to 10%, 50%, and 90% soot conversion, respectively, were selected to evaluate catalytic performance. CO₂ selectivity was calculated from the integrated areas of CO and CO₂ evolution curves. To assess catalytic stability under repeated oxidation conditions, the catalyst exhibiting the best overall performance (LaMnO₃–Pt_0.9 wt %, evaluated mainly by T₅₀) was subjected to four consecutive TPO cycles, and changes in T₁₀, T₅₀, and T₉₀ were analyzed to evaluate activity degradation.

2.5. Calculation of CO₂ Selectivity

During O₂-TPO testing, soot oxidation proceeds gradually with increasing temperature, mainly producing CO₂ along with minor amounts of CO as incomplete oxidation products. To comprehensively reflect the product distribution throughout the soot oxidation process, CO₂ selectivity (S _CO2 ) was calculated by integrating the gas concentration–temperature curves.

$$S_{{\mathrm{CO}}_2}=\frac{{\int }_{T_0}^T{C}_{{\mathrm{CO}}_2}\mathrm{d}T}{{\int }_{T_0}^T{C}_{{\mathrm{CO}}_2}\mathrm{d}T+{\int }_{T_0}^T{C}_{\mathrm{CO}}\mathrm{d}T}\times 100\%$$ 1

3. Results and Discussion

3.1. Samples Characterization

3.1.1. Morphology and Elemental Distribution Analysis

As shown in Figure a shows the cordierite/aluminum borate whiskers without catalyst. The scanning electron microscopy (SEM) images of the monolithic catalyst reveal that the LaMnO₃-based perovskite catalyst is uniformly deposited on the aluminum borate whiskers grown on the surface of the cordierite substrate, forming a relatively dense catalytic active layer. This hierarchical structure not only enhances the adhesion stability of the catalyst coating but also effectively increases the accessible surface area, thereby exposing more active sites and promoting the low-temperature catalytic oxidation of soot particles (PM) captured in diesel particulate filters (DPFs). Notably, despite the high dispersion and good coverage of the catalyst on the whisker surface, the original morphology of the aluminum borate whiskers remains clearly distinguishable. The preserved whisker structure maintains its physical filtration function, enabling efficient interception of soot particles, while simultaneously providing open channels for gas diffusion and soot–catalyst contact, which is beneficial for improving the overall reaction efficiency.

1.

SEM images of the monolithic catalysts: (a) C/A; (b) C-LaMnO₃; (c) C-LaMnO₃–Pt_0.6 wt %; (d) C-LaMnO₃–Pt_0.9 wt %; and (e-f) C-LaMnO₃–Pt_1.1 wt %.

Furthermore, the magnified SEM image shown in Figure f, corresponding to the region marked in Figure e, clearly displays highly dispersed Pt nanoparticles, confirming the successful loading of Pt onto the perovskite catalyst.

As shown in Figure , the EDS elemental mapping images and corresponding surface elemental composition of the C-LaMnO₃–Pt_0.9 wt % monolithic catalyst are presented. The EDS mapping analysis clearly shows that the substrate is mainly composed of Mg, Al, Si, and B elements, whose spatial distributions are consistent with the chemical compositions of cordierite (2MgO·2Al₂O₃·5SiO₂) and aluminum borate (Al₁₈B₄O₃₃), confirming that the support possesses a stable composite oxide framework.

2.

Elemental mapping images and EDS spectrum of C-LaMnO₃–Pt_0.9 wt % monolithic catalyst.

Meanwhile, the active components La, Mn, and Pt are uniformly distributed over the surfaces of both the cordierite substrate and the aluminum borate whiskers, indicating good penetration of the catalyst precursor and effective loading during the synthesis process. Although the relative contents of these active elements are low compared with the bulk substrate, their homogeneous dispersion facilitates intimate contact between soot particles and catalytic active sites, thereby promoting catalytic oxidation of soot at lower temperatures.

3.1.2. XRD Analysis

Figure a presents the XRD patterns of the monolithic catalytic diesel particulate filter (cDPF) samples together with the cordierite substrate, while Figure b shows the corresponding XRD patterns of the catalyst powders, aiming to provide a detailed analysis of the crystalline structures of the materials.

3.

XRD patterns of the catalysts: (a) monolithic catalysts; (b) powder catalysts.

By comparing the diffraction patterns of cordierite/aluminum borate whiskers, it can be observed that the monolithic catalyst retains the characteristic diffraction peaks of the cordierite substrate, while several new diffraction peaks appear after catalyst loading. In particular, the prominent peaks located at 2θ = 32.411° and 57.754° can be indexed to the (112) and (024) crystal planes of perovskite-type LaMnO₃, respectively. , These results clearly demonstrate that the LaMnO₃ perovskite active phase was successfully deposited onto the cordierite/aluminum borate whisker composite support via the sol–gel method, forming a catalytic coating with the desired crystalline structure.

It is worth noting that no characteristic diffraction peaks corresponding to metallic Pt or its oxides are detected in the XRD patterns. This phenomenon can be mainly attributed to the low Pt loading, which generally indicates that Pt species are highly dispersed on the surface of LaMnO₃ without forming large crystalline particles. Meanwhile, the absence of Pt-related diffraction peaks also suggests that the introduction of a small amount of Pt does not disrupt the original crystal lattice of the LaMnO₃ perovskite. Such structural stability is crucial for maintaining catalytic activity and durability under high-temperature exhaust conditions.

3.1.3. FT-IR Analysis

Figure shows the FT-IR spectra of the monolithic catalysts and the corresponding powder catalysts. All samples exhibit a pronounced absorption band in the range of 530–650 cm^–1, which can be attributed to the Mn–O–Mn stretching vibration, a characteristic vibrational feature of MnO₆ octahedra in the LaMnO₃ perovskite structure. , This result indicates that the fundamental perovskite framework of LaMnO₃ is well preserved after coating onto the cordierite substrate and after the introduction of noble metal Pt. In addition, absorption bands observed at approximately 1600 cm^–1 and 3300–3350 cm^–1 are associated with O–H vibrations of adsorbed water molecules, suggesting that a certain amount of surface hydroxyl groups or adsorbed water species remains on the catalyst surface. These surface species are beneficial for the adsorption and activation of oxygen, thereby providing active interfaces for the low-temperature oxidation of particulate matter in diesel exhaust.

4.

FT-IR patterns of the catalysts: (a) monolithic catalysts; (b) powder catalysts.

Overall, the incorporation of Pt leads only to slight shifts in the FT-IR band positions, indicating that the LaMnO₃–Pt catalyst maintains a stable perovskite structure during monolithic construction and noble metal loading.

3.1.4. Redox Property Analysis (H₂-TPR and O₂-TPD)

As shown in Figure a, the H₂ temperature-programmed reduction (H₂-TPR) profiles reveal distinct reduction behaviors of the catalysts. For pristine LaMnO₃, a weak reduction peak appears at approximately 207 °C, which is generally attributed to the reduction of high-valence Mn⁴⁺ to Mn³⁺, while the main reduction peak located at 461 °C corresponds to the reduction of Mn³⁺ to Mn²⁺. −

5.

(a) H₂-TPR patterns of the catalysts; (b) O₂-TPD patterns of the catalysts.

In contrast, the reduction peaks of the Pt-modified samples shift markedly toward lower temperatures. LaMnO₃–Pt_0.6 wt %, LaMnO₃–Pt_0.9 wt %, and LaMnO₃–Pt_1.1 wt % all exhibit a pronounced low-temperature reduction peak at approximately 133–134 °C, which is commonly associated with the reduction of PtO _x species and the synergistic reduction of surface-active oxygen at the Pt–Mn interfacial sites, indicating that the introduction of Pt significantly enhances the low-temperature reducibility of the catalyst. Meanwhile, the Pt-modified samples retain reduction peaks related to lattice oxygen in the intermediate-high temperature region (530–620 °C). However, these peaks exhibit a reduced intensity and a slight shift toward higher temperatures compared with pristine LaMnO₃. This behavior suggests that the reduction process is more concentrated on surface and interfacial oxygen species at lower temperatures upon Pt incorporation. These results indicate that noble metal Pt mainly promotes surface-related reduction processes associated with Mn⁴⁺ species.

As shown in Figure b, O₂ temperature-programmed desorption (O₂-TPD) experiments were conducted on LaMnO₃ and LaMnO₃–Pt catalysts with different Pt loadings. It is generally accepted that desorption peaks below 500 °C are associated with the release of surface reactive oxygen species, such as superoxide (O₂ ^–), whereas peaks above 500 °C correspond to the desorption of surface lattice oxygen. , Compared with pristine LaMnO₃, which exhibits a low-temperature desorption peak at approximately 100 °C, the introduction of Pt and the increase in Pt loading lead to a gradual shift of the low-temperature desorption peak toward higher temperatures, reaching up to 142 °C, accompanied by enhanced peak intensity and higher desorption temperatures. This behavior indicates a higher concentration of oxygen defects and a stronger adsorption capability for oxygen species (e.g., O₂ ^– and O^–). In the 350–450 °C range, all LaMnO₃–Pt samples display pronounced oxygen desorption peaks at approximately 380–412 °C, whereas the corresponding peak of pristine LaMnO₃ is relatively weak. Moreover, the intensity of the medium-temperature desorption peak increases with increasing Pt content, suggesting that Pt loading facilitates the generation and migration of mobile oxygen species. At temperatures above 600 °C, pristine LaMnO₃ exhibits a distinct high-temperature desorption peak at approximately 746 °C, which is attributed to the release of lattice oxygen (O^2–). In contrast, the high-temperature desorption peaks of the Pt-modified samples are significantly weakened, indicating that Pt incorporation suppresses the retention of lattice oxygen at high temperatures and promotes its participation in oxidation reactions at lower temperatures, thereby enhancing the overall oxygen availability.

3.1.5. XPS Analysis

As shown in Figure , XPS was used to analyze the surface chemical states of elements over the C-LaMnO₃–Pt_0.9 wt % catalyst. The survey spectrum proves the coexistence of La, Mn, O, and Pt, and the relevant quantitative data are presented in Table . All binding energies were calibrated with the C 1s peak at 284.8 eV. This characterization helps explore elemental states and their synergistic effects during soot oxidation.

6.

X-ray photoelectron spectroscopy (XPS) results of the C-LaMnO₃–Pt_0.9 wt % catalyst.

2. XPS Quantitative Analysis (for C-LaMnO₃–Pt_0.9 wt % Catalyst).

Element	Peak	Binding Energy (eV)	Area CPS. (eV)	fwhm (eV)
C 1s	C–C/CC	284.8	16789.78	1.48
	C–O/CO	286.3	1743.13	1.56
	OCO	288.9	1863.76	1.98
O 1s	Metal-O (Olat)	529.6	48034.20	2.32
	O–H (OH–)	531.6	12783.79	2.01
	OCO/Oads	533.4	4372.54	2.20
La 3d	La 3d5/2 (La3+)	835.0	6800.53	3.32
	La 3d3/2 (La3+)	851.8	5110.78	3.32
Mn 2p	Mn 2p3/2 (Mn3+)	641.5	3409.69	2.82
	Mn 2p1/2 (Mn3+)	653.2	1609.22	2.82
Pt 4f	Pt 4f7/2 (Pt0)	71.2	749.18	1.92
	Pt 4f5/2 (Pt0)	75.6	441.79	1.92

In the O 1s spectrum, the peaks located at 529.6, 531.6, and 533.4 eV are assigned to lattice oxygen (O_latt), surface active oxygen (O_sur), and adsorbed oxygen species (O_ads), respectively. During the soot oxidation process, soot particles preferentially react with highly reactive adsorbed oxygen species (O_ads), while lattice oxygen from the bulk can migrate to the surface at elevated temperatures, forming a dynamically replenished oxygen supply that sustains catalytic activity.

The C 1s spectrum exhibits a characteristic peak at 284.8 eV, corresponding to the original soot carbon. In addition, peaks at higher binding energies associated with C–O/CO and OCO species are clearly observed, providing direct evidence that soot undergoes stepwise oxidation and is ultimately converted into CO₂.

In the La 3d spectrum, the La 2p_3/2 peak centered at 851.8 eV and La 2p_5/2 peak centered at 835.0 eV indicate that lanthanum exists predominantly in the +3 oxidation state, consistent with the chemical environment of La³⁺ in the perovskite lattice, suggesting that the support structure remains stable during the reaction. The Mn 2p_3/2 peak centered at 641.5 eV and Mn 2p_1/2 peak centered at 653.2 eV indicate that manganese mainly exists in the +3 oxidation state, further confirming the structural integrity of the LaMnO₃ perovskite phase. ,

Furthermore, the Pt 4f_5/2 peak located at 75.6 eV and Pt 4f_7/2 peak located at 71.2 eV suggest that platinum is present predominantly in the form of Pt⁰. The coexistence of Pt⁰ species and the LaMnO₃ perovskite framework establishes a highly active redox synergistic system, which enhances lattice oxygen migration and reactivity, thereby promoting efficient and complete oxidation of soot to CO₂.

3.1.6. O₂-TPO Analysis

As shown in Figure a, the catalytic oxidation behavior of soot particles over different monolithic catalysts was systematically evaluated using the O₂ temperature-programmed oxidation (O₂-TPO) method. The bare cordierite substrate exhibits almost no noticeable low-temperature catalytic activity, with a soot ignition temperature (T₁₀) and a half-conversion temperature (T₅₀) as high as 510 and 610 °C, respectively, indicating that soot oxidation mainly relies on thermal oxidation at high temperatures in the absence of active components.

7.

O₂-TPO performance of different monolithic catalysts. (a) O₂-TPO soot conversion curves; (b) comparison of soot oxidation characteristic temperatures of various catalysts.

After loading LaMnO₃ perovskite onto the surface of the cordierite substrate, the soot oxidation curves shift markedly toward lower temperatures, with T₁₀ and T₅₀ reduced to 366 and 528 °C, respectively. This result demonstrates that LaMnO₃ perovskite can effectively promote the participation of oxygen species in the reaction, thereby lowering the onset temperature required for soot oxidation. However, the conversion rate in the low-temperature region remains relatively limited, suggesting that the intrinsic activity of the perovskite alone is insufficient to fully exploit the low-temperature oxidation potential.

With the introduction of Pt, the soot oxidation performance of the monolithic catalysts is further significantly enhanced. As shown in Figure a, the conversion curves of samples with different Pt loadings are all clearly shifted toward lower temperatures and exhibit faster conversion rates in the low-temperature region (<400 °C). Among them, the C-LaMnO₃–Pt_0.9 wt % sample shows the best overall catalytic performance, with T₁₀ and T₅₀ reduced to 240 and 453 °C, respectively. Compared with bare cordierite, the T₁₀ and T₅₀ values are decreased by approximately 270 and 157 °C, respectively; relative to the Pt-free LaMnO₃ catalyst, the corresponding reductions are 126 and 75 °C.

Figure b further provides a direct comparison of the characteristic temperatures at different conversion stages for all samples. It can be clearly observed that the Pt-modified catalysts exhibit significantly lower reaction temperatures at the T₁₀, T₅₀, and T₉₀ stages than the unmodified samples, with the 0.9 wt % Pt-loaded catalyst showing the most pronounced advantage across all characteristic temperatures. Based on the overall TPO curve profiles and the variations in characteristic temperatures, the catalytic activity of the monolithic catalysts follows the order: C-LaMnO₃–Pt_0.9 wt % > C-LaMnO₃–Pt_1.1 wt % > C-LaMnO₃–Pt_0.6 wt % > C-LaMnO₃ > Cordierite.

Soot oxidation is a heterogeneous catalytic reaction. The contact mode between the catalyst and soot particles is one of the key factors affecting catalytic activity: loose contact more closely resembles the actual operating conditions of diesel engine exhaust, but the catalyst activity is generally lower; tight contact, although an idealized mode, enhances the interaction between soot and the catalyst, facilitates sufficient access to active sites, and promotes soot oxidation at lower temperatures. To quantitatively elucidate this effect, we also tested the soot oxidation activity of various samples under tight contact mode (Figure ). Taking LaMnO₃–Pt_0.9 wt % as an example, its T₅₀ under loose contact is 453 °C, whereas under tight contact, it decreases to 403 °C, indicating that tight contact significantly improves catalytic performance. Moreover, Pt doping further enhances catalytic activity: under tight contact conditions, the T₅₀ of the Pt-doped sample is approximately 80 °C lower than that of pristine LaMnO₃ (482 °C). This is attributed to the introduction of Pt, which increases the concentration of oxygen vacancies, improves oxygen mobility, and thereby promotes the contact between soot particles and oxygen.

8.

Catalytic performance of the catalyst under tight and loose contact conditions.

3.1.7. Catalytic Cycling Stability Analysis

As shown in Figure a, four consecutive O₂ temperature-programmed oxidation (O₂-TPO) cycles were performed on the optimal C-LaMnO₃–Pt_0.9 wt % monolithic catalyst to evaluate its catalytic stability under repeated oxidation conditions. It can be observed that the overall shapes of the soot conversion curves remain highly similar over different cycles, and no obvious shifts are detected in the medium-temperature rapid-conversion region, indicating that the catalyst maintains stable catalytic behavior during successive reactions.

9.

Cyclic stability test of C-LaMnO₃–Pt_0.9 wt % monolithic catalyst. (a) O₂-TPO soot oxidation profiles over four cycles; (b) characteristic temperatures for soot oxidation in each cycle.

A quantitative analysis of the characteristic temperatures is further presented in Figure b. Both the soot ignition temperature (T₁₀) and the half-conversion temperature (T₅₀) exhibit only slight variations after multiple cycles. In particular, the T₅₀ value gradually increases from 452 °C in the first cycle (Run 1) to 465 °C in the fourth cycle (Run 4), with an overall change of approximately 13 °C, without showing a continuous upward trend. Meanwhile, the variations in T₁₀ across different cycles are also minimal, suggesting that the low-temperature activity of the catalyst is well preserved.

Based on the consistent TPO curve profiles and the minor changes in characteristic temperatures, it can be concluded that the C-LaMnO₃–Pt_0.9 wt % monolithic catalyst does not undergo noticeable deactivation under repeated oxidation conditions. These results demonstrate that the synergistic interaction between the LaMnO₃ perovskite structure and the Pt promoter on the monolithic support exhibits good thermal stability, allowing the active interfaces to be effectively maintained during repeated high-temperature oxidation processes.

3.1.8. TG-DTG Analysis

To evaluate the catalytic performance toward soot oxidation, an experimental approach simulating the contact conditions in practical diesel particulate filters was employed. Carbon black (Printex U) was thoroughly mixed with the catalyst powders in an agate mortar, followed by temperature-programmed oxidation measurements using thermogravimetric (TG) analysis. The characteristic temperatures reflecting catalytic activity were precisely determined from the simultaneously recorded TG-DTG curves.

As shown in Figure , the thermogravimetric profiles exhibit two distinct weight-loss stages. The initial weight loss occurring at approximately 150 °C is attributed to the evaporation of physically adsorbed water on the sample surface, whereas the major weight-loss stage in the temperature range of 400–600 °C corresponds to the catalytic oxidation of carbon black, during which carbon is converted into CO and CO₂. Among the investigated samples, the LaMnO₃–Pt_0.9 wt % catalyst exhibits the best catalytic performance, with a DTG peak temperature (T _m) as low as 508 °C, which is significantly lower than that of pristine LaMnO₃ (T _m = 562 °C) and lower than those of most comparative samples. This result indicates that the catalyst effectively reduces the activation energy barrier for soot oxidation and displays superior catalytic oxidation activity. By comparing the TG-DTG results with the O₂ temperature-programmed oxidation (O₂-TPO) performance of the corresponding monolithic catalysts, a good consistency in the catalytic activity trends can be observed, further confirming the reliability of the activity evaluation and the effectiveness of Pt modification.

10.

Thermogravimetric (TG-DTG) analysis of soot oxidation over different catalysts: (a) LaMnO₃; (b) LaMnO₃–Pt_0.6 wt %; (c) LaMnO₃–Pt_0.9 wt %; (d) LaMnO₃–Pt_1.1 wt %.

3.1.9. CO₂ Selectivity Analysis

CO₂ selectivity is an important indicator of the oxidation ability of oxidation-type catalysts, where a higher value corresponds to a more complete oxidation process. As shown in Figure , the integrated temperature–carbon oxide concentration curves illustrate the evolution of CO and CO₂ during the soot oxidation process over C-LaMnO₃–Pt monolithic catalysts with different Pt loadings. The corresponding quantitative CO₂ selectivity data are summarized in Table .

11.

Integrated CO and CO₂ evolution curves over different catalysts during soot oxidation. (a) C-LaMnO₃; (b) C-LaMnO₃–Pt_0.6 wt %; (c) C-LaMnO₃–Pt_0.9 wt %; (d) C-LaMnO₃–Pt_1.1 wt %.

3. CO₂ Selectivity Data of Different Catalysts.

Sample	${\int }_{{\tau }_0}^{\tau }{C}_{{\mathrm{CO}}_2}\mathrm{d}T$ (ml)	${\int }_{{\tau }_0}^{\tau }{C}_{\mathrm{CO}}\mathrm{d}T$ (ml)	S CO2 (%)
C-LaMnO3	107.47	0.885	99.18
C-LaMnO3–Pt0.6 wt %	66.289	0.1085	99.84
C-LaMnO3–Pt0.9 wt %	73.4	0.0959	99.87
C-LaMnO3–Pt1.1 wt %	84.45	0.516	99.39

It can be observed that all monolithic catalysts generate only trace amounts of CO during soot oxidation, indicating that they possess excellent CO₂ selectivity. The C-LaMnO₃ monolithic catalyst exhibits a CO₂ selectivity of 99.18%, already reflecting a relatively high oxidation capability. Notably, all Pt-modified C-LaMnO₃–Pt monolithic catalysts show higher CO₂ selectivity than the Pt-free catalyst. Among them, the C-LaMnO₃–Pt_0.9 wt % monolithic catalyst achieves the highest CO₂ selectivity of 99.87%.

The enhanced CO₂ selectivity upon Pt incorporation can be attributed to the increased concentration of surface-adsorbed oxygen species, which promotes a more complete catalytic oxidation of soot particles and favors the formation of CO₂ over CO.

4. Conclusions

In this study, a monolithic LaMnO₃–Pt catalytic system was constructed using cordierite honeycomb ceramics as the support. The in situ-grown aluminum borate whiskers formed a three-dimensional hierarchical interface, which improved catalyst dispersion and soot-catalyst contact efficiency. The LaMnO₃ catalyst modified with a small amount of Pt exhibited more readily reducible active sites in the low-temperature region, along with increased surface active oxygen and mobile oxygen species.

Soot oxidation is a heterogeneous catalytic reaction, and the contact mode has a significant impact on catalytic activity. Taking LaMnO₃–Pt_0.9 wt % as an example: under tight contact conditions, the T₅₀ of the Pt-doped sample (403 °C) was approximately 80 °C lower than that of pristine LaMnO₃ (482 °C), which is attributed to the enhanced soot-catalyst interaction under tight contact, as well as the increased oxygen vacancy concentration and improved oxygen mobility due to Pt incorporation. Under loose contact conditions, the T₅₀ was 453 °C, which is 157 °C lower than that of the bare cordierite substrate (610 °C), with a CO₂ selectivity exceeding 99%. Cyclic TPO tests showed that the T₅₀ increased by only about 13 °C after four cycles, indicating good catalytic stability under repeated high-temperature oxidation conditions. These results demonstrate the application potential of the developed monolithic catalyst in diesel particulate filters.

The authors declare no competing financial interest.

Published as part of ACS Omega special issue “Microporous Organic Frameworks”.