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Article

Pore-Rich Ni-Co Spinel Oxides for Treating Soot Oxidation in Engine Exhausts

by
Linsheng Xu
,
Kaixuan Chen
,
Yuanfeng Li
,
Yaxiao Ma
,
Baolong Cui
,
Jing Xiong
and
Yuechang Wei
*
State Key Laboratory of Heavy Oil Processing, Key Laboratory of Optical Detection Technology for Oil and Gas, China University of Petroleum, Beijing 102249, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(3), 267; https://doi.org/10.3390/catal15030267
Submission received: 17 February 2025 / Revised: 4 March 2025 / Accepted: 11 March 2025 / Published: 12 March 2025
(This article belongs to the Section Catalytic Materials)
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Versions Notes

Abstract

:
Noble metals are still in high demand for exhaust control catalysts in mobile sources. Designing highly efficient and less costly catalysts for soot purification from engine emissions is a challenge. Herein, the Ni-Co spinel oxide catalyst made of earth-abundant elements was synthesized by a precipitation method. Based on the test results of powder X-ray diffraction (XRD), N2 adsorption–desorption experiments, the temperature-programmed oxidation of soot (soot-TPO), the temperature-programmed oxidation of NO (NO-TPO), the temperature-programmed reduction in H2 (H2-TPR), and the advantages of Ni-Co synergistic catalysts relative to pure NiO and Co3O4 oxides were systematically investigated. The NiCo2O4 catalyst exhibits excellent catalytic performance and stability during soot oxidation compared with NiO and Co3O4 catalysts, i.e., its T10, T50, T90 and SCO2m are 316, 356, 388 °C and 99.95%, respectively. The mechanism of the Ni-Co synergy effect for boosting soot oxidation on the spinel oxide catalyst is proposed according to the experimental results of in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) and the theoretical knowledge of coordination chemistry of metal–NO. This study lays a good foundation for exhaust purification by non-noble metal catalysts for pollution control and sustainable environmental practices.

1. Introduction

Particulate matter (PM, containing mainly soot particles) from engine exhausts is one of the major sources of air pollution, which can cause acute health and environmental problems [1]. Catalytic after-treatment technology is an indispensable part of engine exhaust purification [2]. Continuously regenerating diesel particulate filter (CRDPF) technology, which combines a filter and a deep oxidation catalyst, is one of the most effective after-treatment technologies [3]. In the CRDPF reactor, platinum (Pt)/palladium (Pd) catalysts are located in the washcoat of the filter to accelerate the NO oxidation and soot purification. And the low efficiency and high cost of the currently used platinum (Pt)/palladium (Pd) catalysts are two of the major barriers to exhaust after-treatment technology [4,5]. The replacement of noble metal Pt/Pd catalysts by efficient non-noble metal catalysts is an important cost reduction measure [6]. Thus, it is urgent and challenging to design and construct a highly efficient catalyst made of earth-abundant elements to boost the soot purification of engine exhausts.
Currently, transition-metal oxide (TMO) catalysts have attracted a lot of attention [7,8]. Among all TMOs, the Co3O4 spinel oxide catalyst exhibits excellent deep oxidation reactivity due to its strong redox capacity [9,10]. It is well known that Co-based oxides are one of the most promising catalysts for the deep oxidation of pollutants due to the rapid redox cycles between Co2+ and Co3+ and the enhanced oxygen storage–release capacity [11,12]. The formation of binary oxides by combining Co3O4 oxides with other elements can modulate cationic properties and improve the redox properties of single oxides [13]. Bimetallic Ni-Co spinel oxides are expected to exhibit superior catalytic activity compared to pure NiO and Co3O4 oxides, which is attributed to the catalytic synergy between Ni and Co components [14]. Furthermore, NiCo2O4 is composed of Co and Ni elements, which are both earth-abundant elements. Thus, it promises to be a more active and certainly cost-effective catalyst compared with noble metal catalysts [15].
In this study, we present a precipitation method to synthesize the NiCo2O4 catalyst. The catalysts with pore-rich structures can accelerate gas-phase oxidation reactions. The NiCo2O4 catalyst shows excellent catalytic activity and stability for soot oxidation. Based on the results of the characterizations, it is revealed that the synergy of Co-Ni atoms can promote soot purification. This study provides a reliable solution for less costly Ni-Co spinel oxide catalysts with which to treat soot.

2. Results

2.1. Microstructure Properties

2.1.1. XRD

The crystal structure and composition can affect the physicochemical properties of catalysts, which are closely related to catalytic performance [16]. The X-ray diffraction (XRD) patterns can be used to investigate the crystal structure of catalysts. As shown in Figure 1, the diffraction peaks located at 19.0, 31.2, 36.9, 44.8, 59.3, and 65.2° can be indexed to the (111), (220), (311), (400), (511), and (440) crystal faces of Co3O4 spinel oxide structures (PDF#42-1467) [17]. And the diffraction peaks located at 37.3, 43.1, 62.8, 75.4, and 79.3° can be indexed to the (111), (200), (220), (311), and (222) crystal faces of NiO (PDF#47-1049) [14]. The Co3O4 and NiCo2O4 catalysts are consistent with the typical crystal structure of Co3O4 spinel oxides (PDF#42-1467) and without the appearance of NiO features (PDF#47-1049) on the surface of NiCo2O4 catalysts, which indicates that Ni enters completely into the lattice of Co3O4.

2.1.2. The N2 Adsorption–Desorption Experiments

N2 adsorption–desorption testing can be used to explore the pore structure of catalysts. The N2 adsorption–desorption isotherms of all catalysts (Figure 2a) exhibit obvious H3-type (P/P0 = 0.9–1.0) hysteresis loops and an average pore size of 8.3–12.2 nm, suggesting the formation of a macroporous structure [18]. As shown in Figure 2b, the NiO, Co3O4, and NiCo2O4 catalysts have a narrow pore size distribution of about 3–13 nm, which matches the size of the mesopores [19]. After the introduction of Ni into Co3O4, the curves of the NiCo2O4 catalyst were significantly enhanced by 23–80 nm, which indicates that the catalyst possesses a macroporous structure. As shown in Table 1, the specific surface area, pore volume, and average pore size of the NiCo2O4 catalyst is much higher than that of the NiO and Co3O4 catalysts. This may be attributed to the fact that Ni doping into the Co3O4 lattice makes the pore structure of the NiCo2O4 catalyst more developed relative to NiO and Co3O4 catalysts, therefore, enhances the specific surface area of the NiCo2O4 catalyst (21.7 m2 g−1). The increase in the specific surface area may promote the exposure of the active sites of the NiCo2O4 catalyst, which results in the enhancement of catalytic performance [20,21].

2.1.3. XPS

X-ray photoelectron spectroscopy (XPS) can be used to reveal the elemental components and valence distribution on the catalyst surface (Figure 3) [22]. And the deconvoluted XPS spectra for Ni, Co, and O are shown in Table 2. As shown in Figure 3a, the deconvolved results of Ni 2p2/3 spectra contain two characteristic peaks belonging to the Ni2+, Ni3+, and two satellite peaks, and the Ni3+ species ratios of NiO and NiCo2O4 catalysts are 64.0% and 75.4%, respectively (Table 2). Similarly, in Figure 3b, the Co 2p spectra can be deconvoluted into three components, which include Co2+, Co3+, and satellite peaks. It has been demonstrated that Co3+, as an active component, exhibits a high correlation with a deep oxidation reaction [23]. The ratio of Co3+ on the surface of the NiCo2O4 catalyst (79.8%) is higher than that of the Co3O4 catalyst (75.9%). The O 1s XPS spectra and detailed fitting results of the NiO, Co3O4, and NiCo2O4 catalysts are shown in Figure 3c and Table 2. The O 1s XPS are deconvoluted into three peaks corresponding to lattice oxygen (O2−), surface peroxide (O22−), and super-oxygen (O2−). The O22− and O2− species are considered two active oxygen species for deep oxidation reactions [24]. The (O22− + O2−) ratio in the NiCo2O4 catalyst is the highest (35.1%), suggesting that the binary Ni-Co components enhance the adsorption and activation of O2, which is responsible for boosting soot oxidation.

2.2. Catalytic Performance

2.2.1. Soot-TPO

The catalytic performances of NiO, Co3O4, and NiCo2O4 catalysts for soot oxidation were investigated by temperature-programmed oxidation (TPO) reactions (Figure 4a). And the catalytic activity for soot oxidation was assessed by the T10, T50 and T90 values, defined as temperatures at 10%, 50%, and 90% for the conversion of soot (Table 3).
Under the conditions of no catalyst, it is very difficult to achieve soot oxidation, the T10, T50, and T90 values of which are 485, 599, and 651 °C, respectively, which is higher than the temperature of the engine exhaust (<400 °C). It is important to design a less costly catalyst to overcome the reliance on traditional noble metal catalysts, which can boost soot oxidation in engine exhausts. As shown in Figure 4a and Table 3, the NiCo2O4 catalyst exhibited better soot oxidation performance in comparison with that of the NiO and Co3O4 catalysts, i.e., the T90 value of soot oxidation was 388 °C, which was 19 and 108 °C lower than those of Co3O4 and NiO catalysts for soot oxidation, respectively.
As shown in Figure 4b, the selectivity of soot oxidation to CO2 products over NiO, Co3O4, and NiCo2O4 catalysts is 100% above 425 °C. But the NiCo2O4 catalyst shows optimal CO2 selectivity below 425 °C, which is consistently above 95%. It indicates that the NiCo2O4 catalyst can more immediately remove CO pollutants derived from engine exhausts relative to the NiO and Co3O4 catalysts. As shown in Table 3, this is also illustrated by the SCO2m value of the NiCo2O4 catalyst (99.95%) relative to the Co3O4 catalyst (99.80%). The catalytic stability of the NiCo2O4 catalyst was tested by consecutive TPO reactions. As shown in Figure 4c, the catalytic performance for soot oxidation rarely decreased in the five consecutive testing processes; the T10, T50, and T90 values varied within 5 °C during the five cycle tests of TPO. Therefore, the NiCo2O4 catalyst possesses excellent catalytic stability. Moreover, the XRD test was performed on the catalysts used after the stability tests. As shown in Figure 4d, the crystal phase structure remained unchanged after the five-cycle TPO tests, which indicates that the NiCo2O4 catalyst possesses excellent structural stability.

2.2.2. NO-TPO

It is well known that nitrogen oxides (NOx) in engine exhausts play an important role in catalytic soot oxidation via the NOx-assisted oxidation mechanism [25]. During soot oxidation, active oxygen species on the surface of catalysts can react with NO species, forming a NO2 intermediate with strong oxidizing properties, which promotes soot oxidation to CO2 [26]. Thus, the purification efficiency of soot can be improved by oxidizing NO to NO2 via the NO2-assisted mechanism. NO temperature-programmed oxidation (NO-TPO) was applied to evaluate the NO oxidation properties of catalysts. As shown in Figure 5, the NO2 peak temperatures for catalysts NiO, Co3O4, and NiCo2O4 were 426, 316, and 303 °C, respectively. The Co3O4 and NiCo2O4 catalysts exhibit superior NO oxidation properties (higher NO2 peak and lower peak temperature) compared to the NiO catalyst. But the NO2 peak temperature of the NiCo2O4 catalyst obtained after Ni substitution for Co was lower compared to catalyst Co3O4. It indicates that the Co element may be the key component to the excellent NO oxidizing ability of Co-Ni spinel oxide catalysts.

2.2.3. H2-TPR

The redox property is crucial for the catalytic activity during the deep oxidation reaction, which can be evaluated by the temperature-programmed reduction in H2 (H2-TPR) [27]. As shown in Figure 6, one broad peak appears for the NiO catalyst, which is attributed to the reduction of Ni2+ to Ni0 [28]. The Co3O4 and NiCo2O4 catalysts exhibit new peaks at lower temperatures, located at 308 and 287 °C, respectively, which reflect the reduction of Co3+ to Co2+ or Ni3+ to Ni2+ [29]. It is well known that the Co3+ and Ni3+ ions, as active components in spinel oxides, are essential for the deep oxidation of soot [30]. The temperature of the reduction peak for Co3+ to Co2+ or Ni3+ to Ni2+ can reflect the reactivity of the lattice oxygen coordinated to the metal ion. In Figure 6, the reduction peak of Co3+ or Ni3+ ions for the NiCo2O4 catalyst shifts to a lower temperature range and becomes larger relative to the Co3O4 catalyst. It confirms that doping Ni into the Co3O4 lattice can enhance the redox performance of the catalysts.

2.3. Intermediates and Mechanisms

In Situ DRIFTS of NO Oxidation

Previous studies have confirmed that NO2, the oxidation product of NO, can significantly improve the purification efficiency of soot particles [31]. To provide insight into the intermediate species and reaction mechanisms of NO oxidation over the Co3O4 and NiCo2O4 catalysts, the in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) under reaction atmosphere (0.1 vol% NO and 5 vol% O2 balanced with N2) were obtained. As shown in Figure 7a,b, a series of NOx intermediate species was observed on the surface of the NiCo2O4 catalyst after introducing NO and O2 into the reactor at 50 °C. These species include nitrates (831 cm−1), monodentate nitrites (1162 cm−1), free nitrates (1368 cm−1), and adsorbed NO2 species (*NO2) (1631 cm−1) [32,33,34,35]. The nitrates and free nitrates are clearly present in the range of 50–300 °C, and the characteristic peaks of the two become progressively weaker as the temperature rises above 300 °C. In contrast, the characteristic peak of monodentate nitrates becomes gradually stronger at temperatures above 300 °C. Moreover, the peak at 1631 cm−1 becomes progressively weaker, suggesting that the adsorbed NO2 species (*NO2) are gradually desorbed from the catalyst surface with the temperature increases. These changes indicate that there is a gradual transformation of nitrates and free nitrates to monodentate nitrate species with increasing temperature, which subsequently decompose into NO2. The gaseous NO2 produced by surface nitrate decomposition can boost catalyzing soot oxidation in the NO2-assisted mechanism (Figure 8) [36]. As shown in Figure 7c,d, the in situ NO oxidation DRIFTS and corresponding contour projection results of the Co3O4 catalyst exhibit a clear characteristic of bidentate nitrates (1589 cm−1) relative to the NiCo2O4 catalyst [37]. Bidentate nitrates are more difficult to desorb from the catalyst surface and generate NO2, which may cause a deficiency in the NO oxidation performance of the Co3O4 catalyst.

3. Materials and Methods

3.1. Synthesis of NiO, Co3O4, and NiCo2O4 Catalysts

In a typical preparation process of precursors, a certain amount of nickel and cobalt in the form of nickel nitrate hexahydrate and cobalt nitrate hexahydrate were dissolved in 3 mL nitric acid and 30 mL de-ionized water with a molar ratio of 1:2. The mixture was then heated under magnetic stirring to obtain a homogeneous solution. Then, this homogeneous solution was transferred into an oven and dried for 12 h, followed by calcination in the air at 500 °C for another 6 h (heating rate: 2 °C min−1). After being ground into powder, NiCo2O4 spinel oxides were then collected and kept in a dry place prior to use. All the chemicals were of analytical reagent grade purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China), without further purifications. Nickel nitrate hexahydrate and cobalt nitrate hexahydrate were calcined in static air at an elevated rate of 1 °C min−1 to 500 °C for 4 h to obtain NiO and Co3O4, respectively.

3.2. Catalysts Characterization

Powder X-ray diffraction (XRD) patterns were obtained by a diffractometer (Bruker D8 advance) using Cu-Kα radiation to obtain the phase structure of all as-prepared catalysts (Bruker, Bremen, Germany). Nitrogen adsorption−desorption experiments were operated by a Micromeritics TriStar-II3020 (Micromeritics, Shanghai, China). The temperature-programmed reduction in hydrogen (H2-TPR) was carried out on an HUASI DAS-7200 instrument (HUASI, Cangzhou, China). The temperature-programmed oxidation of NO (NO-TPO) was carried out on a fixed-bed tubular quartz reactor, and the products could be detected by online FT-IR (Bruker, Germany). In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) were carried on a Bruker FT-IR spectrometer (TENSOR II) equipped with a liquid nitrogen-cooling mercury–cadmium–telluride (MCT) detector (Bruker, Germany).

3.3. Evaluation of Catalytic Activity

The soot oxidation catalytic activity and selectivity were evaluated via temperature-programmed oxidation (TPO) reaction with Printex-U-modeled soot particles (~25 nm in diameter). The TPO reaction ranged from 150 to 700 °C at a ramping rate of 2 °C min−1. The catalyst (100 mg) and soot particles (10 mg) were mixed with a spoon for 10 min until loose contact was formed, which could simulate realistic conditions for the catalytic purification of soot. The gaseous reactants flowed at a rate of 50 mL min−1 and consisted of O2 (5 Vol %) and NO (0.1 Vol %) balanced with N2. The gaseous products after the oxidation reaction occurred were analyzed by an online gas chromatograph (GC 9890B, Shanghai, China) equipped with the FID detector. Catalytic activity was assessed through the T10, T50, and T90 values, defined as temperatures at 10%, 50%, and 90% conversion soot, respectively. The total CO2 selectivity (SCO2) of soot oxidation was calculated using the following equation: S C O 2 = C C O 2 C C O 2 + C C O , from which CO2 and CO are produced by soot oxidation. SCO2m was defined as SCO2 with the maximum value of CO2 concentration.

4. Conclusions

The preparation of less costly Ni-Co spinel oxide catalysts can represent a significant advance in the field of environmental catalysis, especially in the context of soot oxidation in exhaust systems. The enhanced specific surface area and pore structure are conducive to the improvement of the NOx diffusion mass transfer rate, thus increasing the soot purification rate. The NiCo2O4 catalyst was obtained by doping Ni into the Co3O4 lattice, which increased the active oxygen concentration on the catalyst surface. The NiCo2O4 catalyst exhibits super catalytic performance for soot oxidation, i.e., its values of T10, T50, T90, and SCO2m are 316, 356, and 388 °C and 99.95%, respectively. The catalytic efficiency increases significantly at lower temperatures, which is the key to pollution control technology. This study not only facilitates the development of non-noble metal catalysts but also lays the foundation for further exploration into the design and application of cost-effective and environment-friendly catalysts. The significance of our work extends beyond the realm of particulate matter reduction in exhausts, offering potential solutions to broader environmental challenges. Future research will focus on extending these findings to practical applications and exploring the long-term stability and effectiveness of these catalysts under operation conditions.

Author Contributions

L.X. and Y.W. conceived and synthesized the catalyst, completed most of the experiments and characterizations, and wrote the paper. K.C. and Y.L. helped with the catalytic performance and analyses. Y.M. and B.C. helped analyze the experiment data. K.C. and J.X. edited the manuscript. All the authors discussed the results in the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Key Research and Development Program of China (2022YFB3504100, 2022YFB3506200), the National Natural Science Foundation of China (22376217), the Key Laboratory of Jiangxi Province for Persistent Pollutants Prevention Control and Resource Reuse (2023SSY02061), and Financial Support by Open Research Fund of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces in Zhejiang Normal University (KLMEACM202303).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00267 g001
Figure 1. XRD test results of NiO, Co3O4, and NiCo2O4 catalysts.
Figure 1. XRD test results of NiO, Co3O4, and NiCo2O4 catalysts.
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Catalysts 15 00267 g002
Figure 2. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of NiO, Co3O4 and NiCo2O4 catalysts.
Figure 2. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of NiO, Co3O4 and NiCo2O4 catalysts.
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Catalysts 15 00267 g003
Figure 3. XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) O 1s.
Figure 3. XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) O 1s.
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Catalysts 15 00267 g004
Figure 4. (a) Temperature-conversion curves of catalytic soot oxidation; (b) selectivity of CO2 products; (c) stability of NiCo2O4 catalyst after five-cycle tests for soot oxidation; and (d) comparison of XRD results for fresh and used NiCo2O4 catalysts after the five-cycle TPO tests.
Figure 4. (a) Temperature-conversion curves of catalytic soot oxidation; (b) selectivity of CO2 products; (c) stability of NiCo2O4 catalyst after five-cycle tests for soot oxidation; and (d) comparison of XRD results for fresh and used NiCo2O4 catalysts after the five-cycle TPO tests.
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Catalysts 15 00267 g005
Figure 5. NO-TPO results of NiO, Co3O4, and NiCo2O4 catalysts.
Figure 5. NO-TPO results of NiO, Co3O4, and NiCo2O4 catalysts.
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Catalysts 15 00267 g006
Figure 6. H2-TPR results of NiO, Co3O4, and NiCo2O4 catalysts.
Figure 6. H2-TPR results of NiO, Co3O4, and NiCo2O4 catalysts.
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Catalysts 15 00267 g007
Figure 7. (a) In situ NO oxidation DRIFTS and corresponding contour projection results of (a,b) NiCo2O4 and (c,d) Co3O4 catalysts.
Figure 7. (a) In situ NO oxidation DRIFTS and corresponding contour projection results of (a,b) NiCo2O4 and (c,d) Co3O4 catalysts.
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Catalysts 15 00267 g008
Figure 8. The NO2-assisted mechanism of catalytic soot oxidation.
Figure 8. The NO2-assisted mechanism of catalytic soot oxidation.
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Table 1. Specific surface areas, average pore volumes, and average mesopore sizes of NiO, Co3O4, and NiCo2O4 catalysts.
Table 1. Specific surface areas, average pore volumes, and average mesopore sizes of NiO, Co3O4, and NiCo2O4 catalysts.
CatalystSpecific Surface Areas
(m2/g)		Average Pore Volumes
(cm3/kg)		Average Mesopore Sizes
(nm)
NiO 14.974.08.3
Co3O410.565.88.5
NiCo2O421.7122.312.2
Table 2. Curve-fitting results (%) of Ni (2p), Co (2p), and O (1s) for NiO, Co3O4, and NiCo2O4 catalysts.
Table 2. Curve-fitting results (%) of Ni (2p), Co (2p), and O (1s) for NiO, Co3O4, and NiCo2O4 catalysts.
CatalystNi 2pCo 2pO 1s
Ni3+Ni2+Co3+Co2+O2−O22−O2−O22− + O2−
NiO64.036.0--81.112.96.018.9
Co3O4--75.924.158.934.07.141.1
NiCo2O475.424.679.820.254.936.68.535.1
Table 3. Catalytic activities of catalysts for soot oxidation.
Table 3. Catalytic activities of catalysts for soot oxidation.
CatalystT10 (°C)T50 (°C)T90 (°C)SCO2m (%) *
Without catalysts48559965122.83
NiO40346449699.69
Co3O431636940799.80
NiCo2O431635638899.95
* SCO2m is defined as SCO2 with the maximum value of CO2 concentration.
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Xu, L.; Chen, K.; Li, Y.; Ma, Y.; Cui, B.; Xiong, J.; Wei, Y. Pore-Rich Ni-Co Spinel Oxides for Treating Soot Oxidation in Engine Exhausts. Catalysts 2025, 15, 267. https://doi.org/10.3390/catal15030267

AMA Style

Xu L, Chen K, Li Y, Ma Y, Cui B, Xiong J, Wei Y. Pore-Rich Ni-Co Spinel Oxides for Treating Soot Oxidation in Engine Exhausts. Catalysts. 2025; 15(3):267. https://doi.org/10.3390/catal15030267

Chicago/Turabian Style

Xu, Linsheng, Kaixuan Chen, Yuanfeng Li, Yaxiao Ma, Baolong Cui, Jing Xiong, and Yuechang Wei. 2025. "Pore-Rich Ni-Co Spinel Oxides for Treating Soot Oxidation in Engine Exhausts" Catalysts 15, no. 3: 267. https://doi.org/10.3390/catal15030267

APA Style

Xu, L., Chen, K., Li, Y., Ma, Y., Cui, B., Xiong, J., & Wei, Y. (2025). Pore-Rich Ni-Co Spinel Oxides for Treating Soot Oxidation in Engine Exhausts. Catalysts, 15(3), 267. https://doi.org/10.3390/catal15030267

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