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Article

Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

by
Baofang Jin
1,
Yuxin Liu
1,
Yue Ma
1,
Zhenguo Li
2,
Kaixiang Li
2,
Shuang Liu
3,
Rui Ran
1 and
Xiaodong Wu
1,*
1
The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2
National Engineering Laboratory for Mobile Source Emission Control Technology, China Automotive Technology & Research Center, Tianjin 300300, China
3
School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(2), 135; https://doi.org/10.3390/catal14020135
Submission received: 20 January 2024 / Revised: 5 February 2024 / Accepted: 6 February 2024 / Published: 9 February 2024
(This article belongs to the Section Industrial Catalysis)
Browse Figures
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Abstract

:
A series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) were prepared via a citric acid sol–gel method for catalytic soot oxidation. The catalysts were characterized by powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy analysis (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The soot oxidation activity of the mullite was significantly promoted by the addition of silver and affected by the loading amount of the metal. Herein, the influences of silver loading on the metal size distribution and its interactions with the mullite were studied. Based on these characterizations, a possible soot oxidation reaction mechanism was proposed for silver-modified SmMn2O5.

Graphical Abstract

1. Introduction

Diesel particulate matters (PM) pose a serious threat to the atmospheric environment and human health [1,2,3]. A catalytic diesel particulate filter (CDPF) has been considered as an efficient way to reduce diesel PM emissions [4,5]. PM from diesel exhaust gases can be removed through two processes. The first process is the capture of PM, which can be achieved by the wall-flow structure of the CDPF to capture PM effectively in the exhaust gas. When PM is accumulated to a certain extent, the captured PM needs to be burned clean. Then, the CDPF can continue to capture and oxidize the PM in the exhaust gas, i.e., the so-called passive regeneration process. The core of this technology is to develop highly efficient catalysts, which can convert soot, one of the most difficult oxidized components in the PM, to CO2 at low temperatures. Pt-based materials have been widely used as commercial catalysts due to their high NO oxidation ability, which can promote soot oxidation through a NO2-assisted mechanism [6,7,8]. However, Pt as a scarce metal is expensive. Therefore, there is an urgent need to seek alternatives with lower cost.
It has been reported that many non-noble metal oxides including Ce-based oxides [9,10], transition metal oxides [11,12,13,14], and alkaline metal oxides [8,15] are effective for soot oxidation. Among them, Mn-based metal oxides are one of the most promising soot oxidation catalysts due to their abundance, non-toxicity, and high activity. Single manganese oxides with different valence states are prone to sintering and phase changes at high temperatures [16]. By contrast, Mn-based composite oxides not only have superior catalytic activity but also high thermal stability, especially Mn-based mullites. For example, Wang et al. [17] reported that Mn-mullite (Sm,Gd)Mn2O5 was able to oxidize NO in simulated diesel exhaust at temperatures as low as 75 °C, which is comparable with the commercial Pt/Al2O3 catalyst. Liu et al. [18]. synthesized SmMn2O5 by in situ dismutation of solid-state Mn3+ in bulk SmMnO3 perovskite for catalytic oxidation of VOC and found that it exhibits excellent catalytic activity and stability. Chen et al. [19]. compared three kinds of Mn-based mullites (SmMn2O5, YMn2O5, and PrMn2O5) for NO oxidation and found that SmMn2O5 exhibited the best NO oxidation ability due to its abundant surface-adsorbed oxygen species and good reducibility.
Although Ag also belongs to noble metals, it has attracted widespread attention for catalytic oxidation reactions due to its relatively low price and high reactivity. Research has found that introducing Ag into metal oxides such as CeO2 [20], MnO2 [21], MnCo2O4 [22], and LaMnO3 [23] is beneficial for activating oxygen species by accelerating dissociation of adsorbed O2 and migration of lattice oxygen in the bulk of oxides. Due to its distinct characteristics, Ag exhibits potential application prospects in the catalytic oxidation of soot. Cui et al. [24] developed Ag-supported CoCe porous nanosheet catalysts for soot oxidation. They reported that the introduction of Ag significantly reduced the ignition temperature of the catalyst under a loose contact mode without the assistance of other exhaust gases such as NOx, due to the good contact with soot and low energy barrier for O2 dissociation. Chen et al. [25] reported that Ag/Co3O4 presented competitive catalytic activity toward soot combustion with a T50 (the temperature at soot conversion of 50%) below 290 °C in 10% O2/N2. They attributed the high activity of Ag/Co3O4 catalyst to the enhanced metal–support interaction induced by the formation of uniform, dispersive, and suitable sized metallic Ag nanoparticles. In our previous work [26,27], we also discovered that SmMn2O5 had good catalytic performance in oxidation of soot and VOCs. Additionally, the influence of preparation methods of silver on SmMn2O5 was also investigated. It was found that in situ introduction of Ag by a sol–gel method exhibited better catalytic activity than the impregnated one. However, the effect of Ag loading on the catalytic performance of the mullite remains unknown.
Following our previous method [26,27], this work synthesized a series of Ag-modified manganese-mullite catalysts with different Ag contents. As expected, introducing Ag significantly boosted soot oxidation. More importantly, we found that there was an optimal Ag loading to achieve a balance between Ag loading and Ag particle size. This work presents new insights into the interactions between SmMn2O5 mullite and silver by tuning the loading amount of the metal and developing catalyst candidates for oxidation reactions.

2. Results and Discussion

2.1. Characterization of Catalysts

2.1.1. XRD Analysis

The crystalline phases of the catalysts were characterized by XRD, and the results are shown in Figure 1. In Figure 1a, the XRD pattern of the SMO sample shows main diffraction peaks at 2θ values of 28.7°, 30.5°, 33.8°, 35.5°, and 41.3° corresponding to the (121), (211), (130), (112), and (212) planes of a typical mullite-type mixed oxides SmMn2O5. The introduction of Ag does not change the crystal structure of the catalyst. Additionally, the peak at 2θ = 33.2° attributed to the perovskite-type mixed oxides (SmMnO3) can be detected, indicating the incomplete phase transformation from the perovskite to the mullite for all the samples. Although the diffraction peaks of Ag overlap partially with those of mullite, it can be observed by the slow scanning technique in Figure 1b that the diffraction peak at 2θ = 38.1° attributed to the metallic silver (111) plane increases significantly in intensity in the patterns of 3Ag/SMO and 6Ag/SMO catalysts, implying the aggregation and sintering of silver with increasing Ag loading.
Table 1 lists the crystal cell parameters and the average crystallite sizes of SmMn2O5 over the catalysts. Compared with pure mullite, the addition of silver leads to a shrinkage of the mullite crystal cell, which may be related to the transformation of Mn3+ (with the ionic radii of 0.065 nm [28]) to Mn4+ (0.053 nm [29]) according to the charge balance law arising from the replacement of the Sm3+ (0.108 nm [30]) sites by Ag+ (0.115 nm [31]) in the mullite. Another possible explanation is the electron transfer from Mn3+ ions in the mullite to silver at the metal–support interface. At a high loading of 6 wt.%, the aggregation and sintering of silver weakens its interaction with the mullite, resulting in an expansion of the mullite crystal cell again. Overall, the greatest crystal cell shrinkage occurs over 3Ag/SMO, implying the maximum lattice distortion in the mullite resulting from the addition of 3 wt.% silver. Additionally, the introduction of Ag accelerates the sintering of the mullite, with the average crystallite size increasing from 30 to ca. 40 nm. Correspondingly, the specific surface areas of the SMO, 1Ag/SMO, 3Ag/SMO, and 6Ag/SMO catalysts are 16.2, 13.7, 10.6, and 11.7 m2 g−1, respectively, indicating that the sintering of the mullite crystallites and loss of the textural feature of the Ag-containing catalysts assisted with the metal.

2.1.2. Raman Analysis

Raman spectroscopy is a powerful technique for characterizing the molecular structure of the materials. To further evaluate the effect of Ag addition on M-O bonds in the mullite, Raman spectroscopy was performed and the results are shown in Figure 2. Both the peaks at 612.1 and 678.5 cm−1 are assigned to Mn-O stretching vibrations, which shift towards lower frequencies and are strengthened in intensity after introduction of Ag. These peak shifts are closely related to the changes in lattice defects as reported by Wang’s work [32], suggesting the prolonged Mn-O bond and the shared lattice oxygen in the mullite with Ag species. The highest shifts appear in the Raman peaks of 3Ag/SMO due to the creation of more lattice defects, which agrees with the variations in crystal cell parameters obtained by XRD.

2.1.3. TEM Observation

TEM can be utilized to observe the morphology of the catalysts and determine the size distribution of silver, and the results are shown in Figure 3a,c,e,g. It can be observed in Figure 3a that SMO consists of aggregated nanoparticles. As the solid was obtained by calcination at 800 °C for 5 h, aggregation between nanoparticles occurred significantly, which leads to the reduction of the free surface with the secondary elimination of the grain boundary area and low specific surface area of the mixed oxides. To further confirm the lattice spacing of the catalyst, HRTEM was conducted. As shown in Figure 3b, the mullite is well crystallized. The measured lattice fringes are 0.279 and 0.287 nm, corresponding to the (220) and (002) planes of SmMn2O5, respectively. All the Ag-containing catalysts (Figure 3c,e,g) show similar morphology with larger mullite particle size. The lattice fringes of 0.235 nm corresponding to the Ag (111) facet can be clearly observed in these Ag-containing catalysts (Figure 3d,f,h). A total of 100–150 silver nanoparticles were measured for size distribution statistics of each sample and the results are shown in the inserted figures. The average Ag nanoparticle size is 2.53, 3.49 and 5.64 nm for 1Ag/SMO, 3Ag/SMO and 6Ag/SMO, respectively. Clearly, the average size of Ag nanoparticles increases with the metal loading, agreeing with the XRD results.

2.1.4. XPS Analysis

XPS can be used to detect the elementary composition and elemental chemical state of the catalysts, and the results are shown in Figure 4. The XPS peaks at 641.5 and 653.0 eV in Figure 4a, corresponding to the binding energies (BE) of Mn 2p3/2 and 2p1/2 levels, can be assigned to Mn3+, while the other two peaks located at 642.7 and 654.2 eV are attributed to Mn4+ [27,33]. The Mn4+/Mn3+ ratio was calculated by deconvoluting the Mn 2p XPS spectra and the results are listed in Table 2. Compared with pure mullite, the introduction of silver results in an increase in the Mn4+/Mn3+ ratio, which agrees with the XRD results about shrinkage of crystal cell. However, the Mn4+/Mn3+ ratio varies little for the Ag-modified catalysts. This is probably because of the formation of large Ag particles with increasing Ag loading, which weaken the interactions between the metal and the mullite support.
The O 1s spectra were fitted into two peaks in Figure 4b. The higher binding energy peak at 531.3 eV is assigned to chemisorbed oxygen (Oads), while the lower binding energy one at 529.3 eV is assigned to lattice oxygen (Olatt) [34]. As listed in Table 2, the introduction of Ag increases the Oads/Olatt ratio, arising from the generation of lattice defects accompanied with the significantly increased Mn4+/Mn3+ ratio. Among the prepared catalysts, 3Ag/SMO exhibits the highest Mn4+/Mn3+ and Oads/Olatt ratios, although the differences are not obvious. These implies the strongest interactions between the metal and the support over this catalyst.
The Ag 3d XPS spectra are shown in Figure 4c. Ag exists in the form of metallic Ag0, with the Ag 3d5/2 and Ag 3d3/2 peaks at 368.0 and 374.0 eV [35], respectively, which agrees well with the HRTEM and XRD results. It is interesting to note in Table 2 that the surface contents of Ag on the catalysts by XPS are higher than their nominal Ag contents, resulting from the enrichment of the metal on the mullite surface. Although the nominal Ag contents in Ag/SMO are in multiple relations, the surface contents of Ag on xAg/SMO are close to each other. As is well-known, XPS measurements are influenced by some factors, such as the probing depth (generally a few nanometers), the distribution of the aimed elements, and the sample homogeneity. In this work, the size of Ag nanoparticles in Ag/SMO increases with increasing Ag content according to the HRTEM results. Relatively large Ag nanoparticles and poor dispersion of Ag in 6Ag/SMO results in incomplete detection of surface Ag by XPS. Therefore, the obtained surface Ag content on Ag/SMO does not increase significantly along with their nominal loadings. Similar phenomena have been previously reported [27,36].

2.1.5. H2-TPR Analysis

The redox property of catalysts is crucial to the catalytic oxidation of soot. H2-TPR characterization was used to determine the redox properties of catalysts, and the results are shown in Figure 5. The overlapped peaks were fitted by Gaussian curves. It can be seen that SMO mainly exhibits four reduction peaks at 251.5, 319.5, 382.5, and 418.2 °C. The first two peaks are ascribed to the reduction of surface-adsorbed oxygen and the reduction of Mn4+ to Mn3+ in the mullite, respectively. The last two peaks are both ascribed to the reduction of Mn3+ to Mn2+, maybe due to the different coordination environments of Mn atoms in the mullite. The introduction of Ag significantly promotes the reduction of the mullite, with the reduction peaks shifting towards lower temperatures by more than one hundred degrees centigrade. For 1Ag/SMO, the peak at 182.1 °C is attributed to the reduction of surface-adsorbed oxygen. The peaks at 246.6 and 286.7 °C are attributed to the successive reduction of Mn4+ and Mn3+ in the mullite promoted by silver, while the small peak at 325.3 °C may be attributed to the reduction of Mn3+ in the mullite without interaction with silver [27]. Apparently, it can be seen that the relative peak area ratio of Mn4+ to (Mn4+ + Mn3+) reaches the maximum over 3Ag/SMO, indicating the strongest promotion effect of silver on the reducibility of the mullite. As listed in Table 2, the Mn4+/Mn ratio follows the order of SMO (0.43) < 1Ag/SMO (0.69) < 6Ag/SMO (0.70) < 3Ag/SMO (0.74), suggesting that introducing Ag increases the content of high-valent Mn4+ significantly. However, at a higher Ag loading, the interactions between the metal and the mullite is weakened in 6Ag/SMO due to the aggregation of segregated metal.

2.2. Catalytic Activity for Soot Oxidation

The catalytic activity of the catalysts for soot oxidation under loose contact between soot and the catalyst was evaluated. Figure 6a shows the soot conversion profiles of the catalysts in the presence of NO as a function of temperature, and Table 3 lists the corresponding values of T10, T50 and T90 for soot oxidation. It was found that compared with SMO, all the Ag-containing catalysts behaves much better in the soot conversion within the whole temperature range due to their superior redox properties. Among these modified catalysts, 3Ag/SMO exhibits some advantage especially at the soot catalytic ignition stage, achieving the lowest T10. Both the mullite and Ag-containing catalysts exhibit high CO2 selectivity (SCO2 > 99.5%) during soot oxidation, which can effectively avoid the generation of secondary pollutants such as CO. Additionally, the NO2 production over Ag-containing catalysts, taking 3%Ag/SMO as an example, during the TPO runs of NO and soot oxidation is shown in Figure 6b. The NO2 concentration during the NO-TPO is higher than that obtained during the soot-TPO tests at the temperatures above 220 °C, which is due to the consumption of NO2 by reacting with soot. The catalytic performance of this material was similar to those modified SmMn2O5 [37,38,39] and superior to those of many other previously reported compounds, such as Ag [40,41], Pt [42,43], Pd [44], Cu [45], and Ce-based catalysts [15,46,47], as briefly summarized in Table S1. The reusability of the catalysts was evaluated via a cycled soot-TPO test, taking 3%Ag/SMO as an example (Figure S1). Compared with the fresh catalyst, the used counterparts even exhibit somewhat improved activity in O2, maybe due to the redispersion of the metal on the mullite during the TPO runs.

2.3. The Proposed Reaction Mechanism

Based on the above results and previous studies, a possible soot oxidation reaction mechanism over Ag/SMO is proposed in Figure 7. During reactions in the presence of O2 and NO, NO interacts with active oxygen species provided by the activation of oxygen vacancies in the mullite and especially dissociation of gaseous oxygen on silver nanoparticles, resulting in the formation of NO2. As is well known, NO2 has stronger oxidizing ability than O2, which can directly oxidize soot. As evidenced by the difference in NO2 concentration between NO-TPO and soot-TPO results (Figure 6b), the consumption of NO2 indicates its involvement in the oxidation of soot, known as the NO2-assisted soot oxidation mechanism. After the loading of Ag in SMO, the reducibility and catalytic activity of the catalysts are improved remarkably. This suggests that the introduction of Ag plays critical roles for soot oxidation. On the one hand, Ag with low work function could stimulate the lattice oxygen in the mullite support with abundant oxygen vacancies via back spillover effect to produce active oxygen species [48,49], leading to a favorable environment for NO and soot oxidation. On the other hand, gaseous O2 can be adsorbed on the Ag sites and dissociated into atomic oxygen efficiently [48,49,50]. During these processes, the loading content of Ag is crucial to the amount and reactivity of active sites by affecting the particle size of Ag and the interactions between Ag and the mullite. The lower the Ag loading, the smaller the metal particle size, which facilitates the oxygen activation and interaction with the mullite. At a low content of Ag, it is not sufficient for 1Ag/SMO to produce sufficient active oxygen species and high valence Mn4+ in the mullite. At a much higher Ag loading, the aggregation and sintering of the metal weaken the interactions with the mullite, resulting in a decrease in redox ability of 6Ag/SMO catalyst. Therefore, a proper Ag loading such as 3 wt.% enables the catalyst with sufficient active sites with high reactivity and thereby the lowest soot ignition temperature T10 (Table 3).

3. Experimental

3.1. Chemicals and Materials

All analytical-grade chemicals were used without further purification. Samarium nitrate hexahydrate (Sm(NO3)3·2H2O, 99.99%), manganese nitrate solution (Mn(NO3)3, 50%), citric acid monohydrate (C6H10O8, 99.5%), and silver nitrate (AgNO3, 9.5%) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. High-purity nitrogen (N2, ≥99.999%), nitric oxide standard gas (NO, 1%) mixed with the base gas of highly pure nitrogen, and synthetic air were purchased from Beijing Praxis Practical Gases Co., Ltd., Beijing, China. Deionized water was obtained from a Millipore water purification system (Darmstadt, Germany).

3.2. Catalyst Preparation

Manganese-mullite SmMn2O5 was prepared by a facile citric acid sol–gel method. In a typical preparation, stoichiometric Sm(NO3)3, Mn(NO3)3, and citric acid were mixed in deionized water under stirring to form a homogeneous solution at room temperature. To ensure complete complexation of the metal ions, the molar ratio of citric acid and total metal nitrates in the solution was 1.1:1. The mixed solution was then heated to 90 °C and continuously stirred. After evaporation of water, a dark brown gel formed. The gel was dried at 110 °C overnight to form a spongy amorphous solid. The obtained solid was heated to 800 °C at a rate of 1 °C min−1 and kept for 5 h. The obtained mullite was denoted as SMO. Ag-modified SMO catalysts (xAg/SMO) with various Ag to SMO mass ratios (1:100, 3:100, and 6:100) were also prepared using the same method with additional additive of AgNO3 as one of the precursors and these solids were calcined at 800 °C for 5 h. The obtained catalysts were denoted as 1Ag/SMO, 3Ag/SMO, and 6Ag/SMO, respectively.

3.3. Characterizations

X-ray diffraction (XRD) was carried out on a Bruker D8 Advance (Billerica, MA, USA) operating at 40 kV and 10 mA and using Cu Kα radiation (λ = 1.54184 Å), and the XRD patterns were collected in a 2θ range of 10° to 60° at a scanning rate of 6°/min. In addition, slow scanning XRD analysis was performed in the range 30° to 40° at a scanning rate of 1°/min. The crystallite size of mullite was calculated by Scherrer equation.
N2 adsorption/desorption measurement was carried out at −196 °C using a JW-BK122 F (Beijing JWGB, Beijing, China) instrument. Prior to the analysis, the samples were degassed at 220 °C under vacuum for 1 h. The specific surface area of the samples was obtained from the Brunauer–Emmett–Teller (BET) theory in the relative pressure (p/p0) range of 0.05–0.30.
Raman spectra of the solids were recorded on a confocal micro-Raman spectrometer (IDSpec Aurora, Hong Kong, China) under ambient conditions. Prior to the analysis, the samples were fixed on a glass slide, and the slide was then placed onto the platform of the microscopy. After the laser beam was focused onto the sample surface, the Raman spectra were excited by a detective laser of 632.8 nm and recorded in the region of 500–800 cm−1.
The morphology of the samples was characterized by transmission electron microscopy (TEM) (JEOL-2100, JEOL, Tokyo, Japan) operated at an accelerating voltage of 200 kV. High-resolution transmission electron microscopy analysis (HRTEM) (JEOL-2100, JEOL, Tokyo, Japan) of the samples was further carried out with a point resolution of 0.19 nm.
X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250 Xi system (Thermo Fisher Scientific, Waltham, MA, USA) equipped with monochromatic Al Kα (1486.6 eV) X-ray source. The elemental composition of the catalyst was derived through a simple conversion formula according to the atomic ratios given by XPS without applying any standardization procedure. To obtain more information of Mn and O species, the obtained XPS spectra were deconvoluted employing the XPSPEAK41 software (version 4.1) after deducting the background signals using the Shirley algorithm. The C 1s line of adventitious hydrocarbon on air exposed samples with a binding energy of 284.6 eV was used as the reference to calibrate the XPS results.
H2 temperature-programmed reduction (H2-TPR) analysis was performed using a chemisorption analyzer (Micromeritics Auto Chem II 2920, Norcross, GA, USA) for the characterization of the redox properties of the catalysts. For each measurement, 0.05 g of catalyst was loaded into a quartz U-tube. Before the measurement, the catalyst was heated from room temperature to 300 °C at a rate of 10 °C·min−1 and kept in a stream of air (20 mL/min) for 1 h. After cooling to 50 °C, the gas was switched to 10 vol. % H2/Ar, and then the catalyst was heated to 900 °C at a rate of 10 °C·min−1 under this atmosphere (30 mL·min−1). During the tests, the H2 consumption was monitored as a TCD signal by the chemisorption analyzer.

3.4. Catalytic Activity Measurements

The catalytic oxidation of soot was tested in a home-made fixed-bed quartz reactor using a temperature-programmed oxidation (TPO) program at a heating rate of 5 °C/min from room temperature to 600 °C. Printex-U (diameter 25 nm, surface area 100 m2/g, Degussa AG, Dusseldorf, Germany) was used as the model soot. Prior to the measurements, 10 mg soot and 100 mg catalyst were mixed using a spatula for 3 min to simulate a “loose contact” condition. To prevent reaction runaway, 0.3 g silica was then added to the mixture of soot and catalyst. The above mixtures were always sandwiched by two layers of quartz wool before put into the reactor. The inlet gas mixture containing 500 ppm NO and 10% O2 balanced with N2 was introduced into the reactor. The total flow rate of the feed mixture was 500 mL min−1 and the gas hourly space velocity (GHSV) was 100,000 h−1. Before heating the reaction tube, the gas concentration needed to achieve stable. The effluent gas stream from the reactor was analyzed online by an infrared spectrometer (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA). The temperatures at the soot conversion of 10%, 50%, and 90% were defined as the T10, T50 and T90, respectively. Temperature-programmed oxidation of NO over the catalysts was also carried out on the same apparatus and steps without adding soot. The selectivity to CO2 during soot oxidation (SCO2) was expressed as another important factor by the equation SCO2 = CCO2/(CCO + CCO2). CCO and CCO2 were defined as the total CO and CO2 released in the outlet gas during the soot-TPO test, respectively, which were obtained by integrating the outlet COx concentrations over time.

4. Conclusions

In this work, a series of Ag-modified mullite catalysts (xAg/SMO, x = 1, 3 and 6) with varied metal loadings were synthesized by the citric acid sol–gel method. Compared with pure mullite, the modified catalysts, especially 3Ag/SMO, exhibit significantly improved activity for soot oxidation in the presence of NO. Introduction of Ag increases the high-valence Mn4+ content and active adsorbed oxygen species in the mullite, leading to enhanced reducibility. Excessive loading of Ag, however, results in the formation of large Ag nanoparticles and weakened interactions of the metal with the mullite. Therefore, an appropriate Ag loading for SmMn2O5, which balances the amount and reactivity of Ag active species, is crucial to the catalytic oxidation of diesel soot. We hope this work can provide some theoretical foundations for the development of efficient soot oxidation catalysts operating at exhaust temperatures.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14020135/s1, Table S1: Comparative studies of catalytic activity for soot oxidation among various materials and our designed catalysts [15,37,38,39,40,41,42,43,44,45,46,47]; Figure S1: Cycled soot oxidation profiles of 3Ag/SMO. Reaction conditions: 10% O2/N2, GHSV = 100,000 h−1, heating rate = 5 °C/min [51,52,53,54]; Figure S2: C 1s spectra for the catalysts [40].

Author Contributions

Conceptualization, B.J. and X.W.; data curation, B.J., Y.L. and X.W.; formal analysis, B.J., Y.M. and Z.L.; funding acquisition, X.W. and B.J.; investigation, B.J. and S.L.; methodology, X.W.; project administration, R.R.; resources, Z.L., K.L. and R.R.; supervision, X.W.; visualization, Z.L. and K.L.; validation, Y.L.; writing—original draft, B.J.; writing—review and editing, B.J., X.W., Y.L., Y.M., Z.L. and K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by projects of the National Key Research and Development Program of China (No. 2023YFC3707202), the National Natural Science Foundation of China (Grant No. 21906091), and Mobile Source Emission Control Technology (NELMS2020A08).

Data Availability Statement

Data are contained within the article and supplementary materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Normal and (b) slow-scanning XRD patterns of the catalysts.
Figure 1. (a) Normal and (b) slow-scanning XRD patterns of the catalysts.
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Figure 2. Raman spectra of the catalysts.
Figure 2. Raman spectra of the catalysts.
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Figure 3. TEM and HRTEM images of (a,b) SMO, (c,d) 1Ag/SMO, (e,f) 3Ag/SMO, and (g,h) 6Ag/SMO.
Figure 3. TEM and HRTEM images of (a,b) SMO, (c,d) 1Ag/SMO, (e,f) 3Ag/SMO, and (g,h) 6Ag/SMO.
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Figure 4. (a) Mn 2p, (b) O 1s and (c) Ag 3d XPS spectra of the catalysts.
Figure 4. (a) Mn 2p, (b) O 1s and (c) Ag 3d XPS spectra of the catalysts.
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Figure 5. H2-TPR profiles of the catalysts.
Figure 5. H2-TPR profiles of the catalysts.
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Figure 6. (a) Soot conversion over the catalysts during soot-TPO and (b) NO2 production over a representative 3Ag/SMO during NO- and soot-TPO tests. Reactant gas: 500 ppm NO/10% O2/N2.
Figure 6. (a) Soot conversion over the catalysts during soot-TPO and (b) NO2 production over a representative 3Ag/SMO during NO- and soot-TPO tests. Reactant gas: 500 ppm NO/10% O2/N2.
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Figure 7. Schematic illustration of soot oxidation over Ag/SMO catalysts. The rectangle and O* represent oxygen vacancies and active oxygen species, respectively.
Figure 7. Schematic illustration of soot oxidation over Ag/SMO catalysts. The rectangle and O* represent oxygen vacancies and active oxygen species, respectively.
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Table 1. Structural and textural properties of the catalysts.
Table 1. Structural and textural properties of the catalysts.
CatalystCell Parameter adSmMn2O5 b (nm)SBET c (m2/g)
a (nm)b (nm)c (nm)a × b × c (nm3)
SMO0.75020.85030.57040.363830.416.2
1Ag/SMO0.74770.85480.56650.362039.513.7
3Ag/SMO0.74290.85250.56620.358540.410.6
6Ag/SMO0.74550.85300.56860.361540.311.7
a The lattice constants estimated from consecutive peaks using the Bragg formula. b Crystallite size of the mullite calculated by Debye–Scherrer equation. c Surface area derived from the BET equation.
Table 2. Surface elemental contents of the catalysts.
Table 2. Surface elemental contents of the catalysts.
CatalystSurface Content (wt.%) aMn4+/Mn3+ aOads/Olatt aMn4+/(Mn4+ + Mn3+) b
SmMnOAg
SMO40.232.427.4---0.530.480.43
1Ag/SMO40.428.524.86.30.700.630.69
3Ag/SMO38.228.126.17.60.730.650.74
6Ag/SMO38.727.325.88.20.710.620.70
a Calculated from XPS data. b Estimated from H2-TPR results.
Table 3. Catalytic features of the catalysts for soot oxidation.
Table 3. Catalytic features of the catalysts for soot oxidation.
CatalystT10 (°C)T50 (°C)T90 (°C)SCO2 (%)
SMO32340345699.4
1% Ag/SMO30037242099.7
3% Ag/SMO27836742099.8
6% Ag/SMO29037242199.9
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Jin, B.; Liu, Y.; Ma, Y.; Li, Z.; Li, K.; Liu, S.; Ran, R.; Wu, X. Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions. Catalysts 2024, 14, 135. https://doi.org/10.3390/catal14020135

AMA Style

Jin B, Liu Y, Ma Y, Li Z, Li K, Liu S, Ran R, Wu X. Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions. Catalysts. 2024; 14(2):135. https://doi.org/10.3390/catal14020135

Chicago/Turabian Style

Jin, Baofang, Yuxin Liu, Yue Ma, Zhenguo Li, Kaixiang Li, Shuang Liu, Rui Ran, and Xiaodong Wu. 2024. "Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions" Catalysts 14, no. 2: 135. https://doi.org/10.3390/catal14020135

APA Style

Jin, B., Liu, Y., Ma, Y., Li, Z., Li, K., Liu, S., Ran, R., & Wu, X. (2024). Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions. Catalysts, 14(2), 135. https://doi.org/10.3390/catal14020135

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