Cu-Mn-Ce mixed oxides catalysts for soot oxidation and their mechanistic chemistry

Applied Surface Science 512 (2020) 145602

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Cu-Mn-Ce mixed oxides catalysts for soot oxidation and their mechanistic chemistry

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Shafqat Alia, Xiaodong Wua, , Zareen Zuhrab, Yue Maa, Yasir Abbasc, Baofang Jina, Rui Rana, Duan Wenga a

The Key Laboratory of Advanced Materials of the Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China c State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Mixed Metal Oxides Soot oxidation Oxygen vacancy Durability Mechanism

A series of flower-like CuxMnyCez mixed oxides catalysts for soot removal were prepared via a facile co-precipitation approach followed by calcination. The structural and surface properties of these catalysts were investigated by powder X-ray diffraction, N2 physisorption, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, oxygen temperature-programmed desorption and NO temperature-programmed oxidation. Superior soot oxidation performance was obtained over these ternary mixed oxides catalysts and the corresponding mechanism was explored. In particular, the Cu1Mn1Ce1 catalyst showed a remarkable improvement in soot oxidation activity in both NO + O2 and O2 atmospheres. The reusability and durability of these catalysts were assessed, and these materials showed high soot oxidation activity after the hydrothermal stability test.

1. Introduction Significant advances are being made in diesel engine technology that have improved fuel efficiency, range, dynamic performance, economics, reliability and durability [1]. However, diesel exhaust continues to be problematic because it contains particulate matter (often called soot) [2]. nitrogen oxides (NOx) and volatile organic compounds, all of which can have negative impacts on air quality, the general environment and human health [3,4]. Soot, in particular, is a primary air pollutant and can cause respiratory and cardiovascular diseases as well as skin cell alterations [5–7]. Therefore, increasingly strict regulatory limits on soot generation have been introduced that are forcing the development of advanced technologies for reducing soot particulate emission levels [8,9]. Among various methods developed to reduce soot particulate emissions from diesel engines, filtration [10] followed by catalytic oxidation is one of the most promising [11,12]. In such systems, the regeneration of the catalyst is achieved at significantly lower temperatures [13,14]. The majority of oxidation catalysts currently employed are based on noble metals (Pt, Pd and Rh) that have high activities for soot oxidation [15] but are expensive. Therefore, the development of noble-metal-free catalysts is of great interest [16,17].

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To date, various soot oxidation catalysts have been reported, including alkali metal titanates [18], alkali metal zirconates [19], alkali-modified aluminosilicates [20], transition metal oxides [21] and rare earth-based compounds [22]. Many other materials, such as perovskite-type oxides [23,24], spinel-type oxides [25] and hydrotalcite derived oxides [26], have also been studied, and have exhibited high activities during the removal of soot. In this regard, ceria (CeO2) has also been widely studied and used as a remarkable catalytic metal oxide owing to its distinctive physicochemical and redox features [27]. Ceria has special oxygen storage capacity (OSC), which can store and release highly active oxygen species and a defective structure able to accommodate many oxygen vacancies [28,29]. In order to improve its catalytic activity, many researchers have proposed addition of various transition metals resulting in catalytic systems extremely active even at low temperatures, due to fine metal dispersion and strong interaction with ceria [30]. Moreover, copper and manganese are considered as environmental friendly and economically advantageous materials [31]. Copper oxides have gained much attention in heterogeneous catalysis because of their good catalytic activity for oxidation reactions with active oxygen species, watergas shift reaction, methanol synthesis, and wet oxidation of phenol and fast electron transfer characteristics [32–34]. Significant efforts have

Corresponding author. E-mail address: [email protected] (X. Wu).

https://doi.org/10.1016/j.apsusc.2020.145602 Received 21 November 2019; Received in revised form 13 January 2020; Accepted 30 January 2020 Available online 03 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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20 h respectively.

been recently made to develop efficient manganese oxides catalysts with unique morphologies and structures [35]. It has also been verified that manganese oxides are highly active in the oxidation of hydrocarbons and soot because of their high efficiency in the redox cycles and overflow ability of lattice oxygen [36,37]. Moreover, optimization of catalyst compositions and structural morphologies are useful solutions to improve the catalytic activity through enhancing the soot-catalyst (solid-solid) contact efficiency [38,39]. In recent years, the controlled preparation of nano- or micro-sized metal oxides with different morphologies has attracted significant research interests in the use of catalysis [40]. Our group and many other researchers have confirmed that the ceria based binary mixed oxides especially CuOx-CeO2 and MnOx-CeO2 exhibited excellent catalytic activity for soot oxidation [41–46]. However, there have been few reports on the development of ternary metal oxides catalysts with flower-like morphology for the oxidation of soot. In addition, the morphology of mixed metal oxide catalysts should be improved as it has been demonstrated to be crucial for the catalytic performance of the catalysts [47,48]. Therefore, it is necessary to develop the syntheses of multi-component metal oxide catalysts with a special morphology and for catalytic oxidation of soot at lower temperatures. In the work reported herein, these materials were synthesized using a co-precipitation approach, and the morphology and surface properties of the resulting materials were characterized. A mechanism for soot oxidation over these materials is also proposed.

2.2. Characterization Powder X-ray diffraction (XRD; Shimadzu XRD 6000) was employed to identify the structures of the catalysts, operating at 40 kV and 10 mA and using Cu Kα radiation (k = 1.54184 Å) and a Ni filter. The diffraction patterns were acquired at a scanning velocity of 4° min−1. N2 adsorption and desorption analyses were carried out using a Micromeritics ASAP 2020 instrument at −196 °C. The samples were degassed in vacuum and purged with N2 at 300 °C for 3 h prior to each trial and the specific surface areas were calculated using the BrunnerEmmet-Teller (BET) method. The morphologies of the materials were observed via scanning electron microscopy (SEM), employing a Quanta 200F instrument, and transmission electron microscopy (TEM) with a JEOL JEM-100CX. In preparation for analysis, samples were applied to a portion of adhesive conductive carbon tape attached to a copper disk and subsequently coated with Au. X-ray photoelectron spectroscopy (XPS) was performed using a Perkin–Elmer PHI-1600 ESCA spectrometer with a Mg Kα (hν = 1253.6 eV) X-ray source. Each binding energy (BE) was calibrated relative to the C1s peak of adventitious carbon (BE = 284.6 eV) as an internal standard. Temperature-programmed reduction with H2 (H2-TPR) was performed using a Micromeritics Auto Chem II 2920 apparatus. In each trial, a 50 mg sample was placed in a U-shaped quartz tube and pretreated in an Ar stream at 300 °C for 1 h. After the specimen was cooled to 30 °C, the gas was changed to a 10% H2/Ar flow (50 mL min−1). The signal from a thermal conductivity detector was recorded from 30 to 1000 °C at a linear heating rate of 10 °C min−1. Temperature-programmed desorption with O2 (O2-TPD) was performed with the same equipment. 100 mg sample of the catalyst was pretreated in pure helium at a flow rate of 10 mL min−1 at 500 °C for 1 h and then cooled to room temperature prior to the adsorption of oxygen for 2 h. The desorption profile was acquired at a heating rate of 10 °C min−1. Fourier transform infrared (FT-IR) spectra of catalysts were obtained using a Bruker vertex 70 FT-IR spectrometer, averaging 30 scans for each spectrum.

2. Experimental 2.1. Catalyst preparation The synthesis of mixed metal oxides is typically performed via coprecipitation [49], and so this technique was adopted for the preparation of Cu-Mn-Ce mixed oxides in this work. In preparation process, the nitrates of the three transition metals (Cu, Mn and Ce; Beijing Yili, China, AR) were dissolved in deionized water to give total metal cation concentrations in the range of 2.0–3.0 mol L−1. NaOH and Na2CO3 (Beijing Yili, China, AR) were also dissolved in deionized water to give total anion concentration of 4.0 mol L−1. These two solutions were subsequently mixed in a four-neck flask at room temperature and the combined solution was maintained within the pH range of 9.5–10 at 65–85 °C for 12 h. Finally, the resulting precipitate was collected by filtration, washed with hot deionized water and methanol until the wash solution had a pH of 7, then dried at 90 °C for 10 h. The CuMnCe powders were obtained by calcination in air at 600 °C for 6 h in a tube furnace. This process and the structures of the products are illustrated in Scheme 1. In comparison, CuO, MnOx and CeO2 were synthesized through the same precipitation and calcination of the corresponding metal nitrates. Artificially aged catalysts (H2O-Cu1Mn1Ce1 and SO2Cu1Mn1Ce1) were obtained by treatment in air containing 10% H2O at 600 °C for 5 h and containing 100 ppm SO2 and 10% H2O at 300 °C for

2.3. Activity measurements Temperature-programmed oxidation with soot (soot-TPO) was carried out using an infrared spectrometer (Thermo Nicolet iS10). In each trial, a 10 mg quantity of soot and 100 mg of the catalyst were mixed with a spatula for 10 min to provide loose contact. The mixture was then diluted with 300 mg silica to prevent excessive heat transfer during soot oxidation. The resulting mixture was placed in a quartz reactor and heated from 30 to 600 °C at a rate of 10 °C min−1 under an atmosphere containing 500 ppm NO and 10% O2. The temperatures at

Scheme 1. A schematic diagram showing the preparation of CuxMnyCez mixed oxides catalysts. 2

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in the copper content, implying that CuOx might be highly dispersed on the ceria surface [52]. In contrast, increases in the a value of the ceria lattice were observed following the addition of increasing quantities of manganese [53]. Both the incorporation of manganese ions into the ceria lattice and fine dispersion of copper oxides on the ceria surface can lead to charge transfer between the transition metals and cerium and produce more lattice defects including oxygen vacancies. Table 1 also summarizes the data obtained from N2 adsorption/ desorption analyses of the catalysts. All the CuxMnyyCez samples possess high surface areas in the range of 129–139 m2 g−1. The catalyst surface area reaches the maximum value when the Cu:Mn:Ce atomic ratio equals to 1:1:1, and it decreases to some extent when further enhancing the copper and manganese contents. The specific surface areas of the CuxMnyCez catalysts were also much larger than those of CuO, MnOx and CeO2, which suggested that soot would interact more effectively with the high-surface-area catalyst surface. It has been reported that metal particles in mixed oxides materials are highly dispersed within a metal oxide matrix [54]. This characteristic is important in terms of promoting heterogeneous catalysis over the CuMnCe catalysts. The morphologies of the CuMnCe samples were determined using TEM and SEM, and representative images are provided in Fig. 2 and S1, respectively. These catalysts retained their flower-like morphology as the copper, manganese and cerium proportions increased from 1.0 to 1.5 (Fig. 2a, b, d and f), whereas a further increase to 2.0 in each metal led to the formation of an irregular morphology (Fig. 2c, e and g). This observation indicates that the range of 1.0 to 1.5 is optimal. The spacing of the petal fins over Cu1Mn1Ce1 is about tens of nanometers (Fig. S1a), implying that the soot particles can enter into the petals. Moreover, HRTEM analysis of our best catalyst (Cu1Mn1Ce1) was performed. As shown in Fig. S2, the lattice fringes for (2 0 0) and (1 1 1) planes are 0.268 and 0.309 nm, respectively, which are smaller than the corresponding values of pure ceria (0.274 and 0.314 nm) [55]. It could be attributed to substitution of substitution of Cex+ with larger ionic radii by smaller ions including Mnx+ and Cux+ and correlated with the XRD results.

Fig. 1. XRD patterns obtained from (a) Cu1Mn1Ce1, (b) Cu1.5Mn1Ce1, (c) Cu2Mn1Ce1, (d) Cu1Mn1.5Ce1, (e) Cu1Mn2Ce1, (f) Cu1Mn1Ce1.5 and (g) Cu1Mn1Ce2 catalysts.

soot conversion levels of 10%, 50% and 90% were defined as the T10, T50 and T90 values, respectively. The amounts of CO and CO2 generated during this process were also determined using an on-line analyzer. NO temperature-programmed oxidation (NO-TPO) was also carried out using the same instrument with similar reaction conditions in the absence of soot.

3. Results and discussion 3.1. Structural properties

3.2. XPS analyses Fig. 1a–h show the XRD patterns of the entire series of CuMnCe catalysts. Evidently, the thermal treatment at 600 °C led to the formation of ceria based mixed oxides. These patterns contain major peaks at 28.9°, 33.2°, 47.5°, 57.2°, 70.1° and 78.1° that correspond to the CeO2 phase [50,51]. In the case of the Cu1Mn1Ce1.5 and Cu1Mn1Ce2 catalysts, these CeO2 peaks are more intense due to the growth of ceria crystals. None of the patterns exhibit peaks corresponding to manganese oxides or copper oxides, indicating a high degree of dispersion of these metal oxides and/or the formation of CuMnCe. The lattice parameter (a) and crystallite size (d) were calculated for each material by full pattern fitting of the data, and the results are summarized in Table 1. The values of a for the CuMnCe catalysts decreased from that of pure CeO2 (0.5421 nm) owing to the substitution of Ce ions by Cu and Mn in the fluorite structure. Almost no variation in a was observed with increases

The surface compositions and elemental oxidation states of the catalysts were examined through XPS analysis. The deconvoluted Cu 2p, Mn 2p3/2, Ce 3d5/2 and O 1 s spectra are presented in Fig. S3 and the ion percentages are provided in Table 2. The actual Cu/Mn ratios on the surface of Cu1Mn1Ce1 (1.2), Cu1.5Mn1Ce1 (2.2) and Cu2.0Mn1Ce1 (2.8) were higher than the theoretical values (i.e. 1.0, 1.5 and 2.0), indicating that copper was highly dispersed on the catalyst surfaces. The differences between the actual Cu/Mn ratios on the surface of Cu1Mn1.5Ce1 (0.7) and Cu1Mn2Ce1 (0.5) and their theoretical values (0.66 and 0.5) are not considered significant, suggesting the possible incorporation of manganese into the ceria lattice [56]. These findings were also confirmed by the pronounced increase in the surface Cu/Mn ratio as the cerium content increased from 1.0 to 2.0. The active surface oxygen percentage (Osur.) values are provided in Table 2 and demonstrate that the Cu1Mn1Ce1 catalyst had a larger quantity of surface oxygen than the other samples. Thus, incorporating equal amounts of the metal oxides appears to facilitate the adsorption of oxygen species and to provide a higher oxygen vacancy density. Oxygen vacancies could represent the active centers during the soot oxidation reaction, as oxygen in defective oxides is readily released and transferred, which would be expected to enhance the catalytic activity [57]. It is worth noting that the proportion of low-valence metal ions increased with increases in the copper and manganese proportions, while the Ce4+/Ce3+ ratio also increased.

Table 1 Structural characteristics of the catalysts. Catalyst

SBET (m2 g−1)

Dp (nm)

d (nm)

a (nm)

Cu1Mn1Ce1 Cu1.5Mn1Ce1 Cu2Mn1Ce1 Cu1Mn1.5Ce1 Cu1Mn2Ce1 Cu1Mn1Ce1 Cu1Mn1Ce2 CuO MnOx CeO2

139 136 131 130 129 133 131 0.3 8 48

7.3 6.5 3.8 6.4 2.8 5.3 4.9 2.8 3.2 2.3

14.5 13.4 11.5 10.4 8.4 12.3 13.1 – – –

0.5401 0.5402 0.5402 0.5408 0.5411 0.5415 0.5418 – – 0.5421

3.3. H2-TPR The H2-TPR data were used to assess the redox properties of the 3

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Fig. 2. TEM images of (a) Cu1Mn1Ce1, (b) Cu1.5Mn1Ce1, (c) Cu2.0Mn1Ce1, (d) Cu1Mn1.5Ce1, (e) Cu1Mn2Ce1, (f) Cu1Mn1Ce1.5 and (g) Cu1Mn1Ce2 catalysts.

representative mixed oxides and monoxide samples (Fig. 3a). CeO2 exhibited two reduction peaks at 402 and 699 °C, assigned to the reduction of surface/subsurface oxygen and lattice oxygen, respectively [58]. Two overlapped reduction peaks were produced by MnOx at 320 and 403 °C, ascribed to the reduction of MnO2/Mn2O3 to Mn3O4 and that of Mn3O4 to MnO, respectively [36]. Pure copper oxide showed a single broad reduction peak at approximately 260 °C. Interestingly, the TPR profiles of the mixed oxides catalysts exhibited lower reduction temperatures than their single oxide counterparts. In addition, each of the mixed oxides catalysts displayed a sharp reduction peak below

200 °C owing to the synergistic effects of the metal phases and facile oxygen transfer in these materials. The coexistence of these metal oxides could mutually facilitate their reduction [59]. The shoulder in the range of 110–135 °C can possibly be assigned to the reduction of highly dispersed copper and manganese oxides that interact strongly with the adjacent ceria. In the case of the Cu1Mn1Ce1 catalyst, another weak shoulder is seen at approximately 70 °C, assigned to the reduction of surface-active oxygen species, including O¯, O2¯ and O22¯. It is also evident that the Cu1Mn1Ce1 catalyst generated a primary reduction peak at the lowest temperature (Fig. 3b) and showed the highest degree 4

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Table 2 Proportions and chemical states of surface elements on the catalysts. Catalyst

Cu (%)

Mn (%)

Ce (%)

O(%)

Cu (%) Cu

Cu1Mn1Ce1 Cu1.5Mn1Ce1 Cu2Mn1Ce1 Cu1Mn1.5Ce1 Cu1Mn2Ce1 Cu1Mn1Ce1.5 Cu1Mn1Ce2

11.21 16.86 21.38 10.33 9.15 9.00 6.36

9.30 7.71 7.68 14.51 18.50 4.85 2.06

34.80 34.11 28.29 30.85 28.32 43.52 45.73

44.69 41.32 42.65 44.31 43.68 42.63 44.64

+

16.4 20.5 23.7 22.4 23.6 23.2 25.8

Ce4+/Ce3+ (%)

Mn (%) 2+

2+

Cu

Mn

83.6 79.5 76.3 77.6 76.4 76.8 74.3

15.3 20.9 18.7 17.6 21.7 18.5 18.7

3+

4+

Mn

Mn

38.1 38.4 38.3 37.7 36.4 37.9 38.3

46.6 44.9 43.0 44.7 41.9 43.6 43.0

27.6 28.4 27.8 27.4 29.3 38.4 41.8

O (%) Olatt

Osurf

36.6 38.3 52.4 42.4 52.8 53.2 59.9

63.4 61.7 47.6 57.6 47.2 46.8 40.1

Fig. 3. (a) H2-TPR and (b) O2-TPD profiles of the selected catalysts.

metals. The light-off temperature (T50) values followed the sequence: Cu1Mn1Ce1 (347 °C) < Cu1.5Mn1Ce1 (360 °C) < Cu1Mn1.5Ce1 (382 °C) < Cu1Mn1Ce1.5 (398 °C) < Cu2Mn1Ce1 (403 °C) ≅ Cu1Mn2Ce1 (404 °C) < Cu1Mn1Ce2 (416 °C). Compared to Pt/Al2O3, the optimized catalyst Cu1Mn1Ce1 reduced the T50 significantly by about 100 °C. The performance of this material was also superior to those of many other previously reported compounds, such as Pt, V, Mn, Co, Ni, Cu, Zn and Ce-based catalysts, as briefly summarized in Table S3. The soot oxidation activity of Cu1Mn1Ce1 in the absence of NOx was also investigated, and the T50 value was found to increase from 347 to 384 °C (Fig. 4d). This demonstrates the high reactivity of this material for soot oxidation in the presence of O2 due to its superior ability to activate oxygen. On the other hand, the catalytic performance in the presence of NO is related to the adsorption and activation abilities of active sites for both O2 and NO [64,65]. The metal cations with surface oxygen vacancies are the major active sites in facilitating the adsorption, activation and reaction of NO and O2 molecules and promoting NO oxidation [35].

of H2 consumption (Table S1), indicating that it had the best redox properties among the synthesized catalysts. 3.4. O2-TPD The oxygen species associated with the catalysts were examined using O2-TPD, and the results obtained from the selected samples are shown in Fig. 3b. The mixed oxides materials produced two main oxygen desorption peaks. The peak at approximately 525 °C (labeled β) is related to the release of chemisorbed oxygen, while that in the vicinity of 750 °C (γ) is attributed to the release of lattice oxygen from the mixed oxides. It has been generally accepted that surface oxygen species are more reactive due to their ready availability on the catalyst surface [60]. The desorption peak centered at approximately 180 °C was assigned to the removal of O¯, O2¯ and O22¯ adsorbed on oxygen vacancies (α) [61], which was only observed over the Cu1Mn1Ce1 catalyst and correlated well with the H2-TPR results. This, as well as the largest amount of total O2 desorption from this material (Table S1), indicated that the Cu1Mn1Ce1 catalyst had more readily available active oxygen than the other specimens.

3.6. Effect of NO on soot oxidation and the reaction mechanism

3.5. Catalytic activity

NO can be catalytically oxidized to NO2, and this plays a crucial role in soot oxidation in the presence of NOx [66]. The role of NOx in the soot oxidation process was assessed by examining variations in the conversion of NO to NO2 during NO oxidation and soot oxidation over Cu1Mn1Ce1 as a model catalyst. The resulting data are presented in Fig. 5. The NO2 concentration during soot-TPO trials was found to decrease over the temperature range of 200–450 °C compared to values obtained during the NO-TPO tests, which was consistent with the temperature range for soot oxidation (right-side inset in Fig. 5) [67]. These results demonstrate that the consumption of NO2 occurs during soot oxidation, and so the presence of this compound plays an important role in promoting the combustion of soot, as demonstrated in

The catalytic reaction of soot oxidation takes place at a three-phase interface among a solid catalyst, a solid reactant (soot) and gaseous reactants (O2, NO) [62]. The activities of the catalysts for soot oxidation were evaluated in the presence of NO and O2 under loose catalyst–soot contact conditions, with the results presented in Fig. 4a–c. As shown in Table S2, the activities of the CuxMnyCez catalysts were much higher than those of the single oxides (CuO, MnOx and CeO2) and of the reference catalyst 1%Pt/Al2O3. These results indicate that the chemically mixed oxides produced an important synergistic effect [63]. The activity of the catalysts was also greatly affected by the proportion of each 5

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Fig. 4. The effects of (a) Cu, (b) Mn and (c) Ce on the soot oxidation activities of the catalysts in the presence of 500 ppm NO/10% O2/N2 and (d) in the presence of O2.

the top half of Fig. 5. The XPS data indicated a greater concentration of surface chemisorbed oxygen on the Cu1Mn1Ce1 than on the other mixed oxides catalysts, which was also confirmed by the H2-TPR and O2-TPD results. This finding is related to the redox couples of different metal species and the flower-like shape which favors to develop more contact between soot and catalyst. Additionally, it should be noted that the activation of oxygen and its reaction with soot affect soot oxidation in NO2 atmosphere [68]. As shown in Fig. 4d, the CuMnCe catalyst provides a significant advantage with regard to these processes.

3.7. Recyclability The stability of the mixed oxides catalysts was assessed by performing four consecutive cycles using the Cu1Mn1Ce1 catalyst under the same conditions, with the results shown in Fig. 6a. There was no significant deactivation after three cycles, although a small increase in the T50 value (approximately 5–9 °C) was observed during soot conversion. It should be noted that the test method adopted in this work meant that the catalyst mass decreased (from 100 mg catalyst + 300 mg silica to 360, 352 and 338 mg of the catalyst and silica mixture) in each consecutive trial, which would lead to continuous decreases in the catalyst/ soot mass ratio and thus increases in the light-off temperature. The SEM (Fig. 6b) and XRD (Fig. 6c) data obtained from the used catalyst also confirmed the excellent stability of the morphology and crystallinity of this material, although the peaks related to silica were found at about 20° and below 10° in the diffraction pattern of the used catalyst. These observations indicate the potential reusability of CuMnCe catalysts in soot combustion.

Fig. 5. NO2 production over a representative Cu1Mn1Ce1 catalyst during NOand soot-TPO tests. The insert plots the conversion of soot to CO2 during the soot-TPO test.

6

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Fig. 6. (a) Soot-TPO curves of Cu1Mn1Ce1 over three repeated reuse cycles, and (b) an SEM image of the used material and (c) XRD patterns of the fresh and used catalyst.

in Fig. 7b. The bands at 2067 and 1630 cm−1 represent the stretching of CueO and MneO bonds, respectively, while those at 1158 and 501 cm−1 are related to CeeO bonds [71] and the band at 3414 cm−1 indicates the presence of OH groups. From these data, the hydrothermal aging evidently did not change the surface groups on the catalyst, although the band at 3400 cm−1 was broadened owing to the contribution of water. Sulfur poisoning is another critical issue associated with the application of soot oxidation catalysts [72] and so the oxidative adsorption of SO2 on the surface of the catalyst was observed via FT-IR analysis, as in Fig. 7b. Following this test, a new band appeared at 2500 cm−1 that is attributed to the S = O bonds of SO2 molecules on the surface of the sulfated catalyst. The sulfur treatment also resulted in a significant loss of activity of the Cu1Mn1Ce1 catalyst and a T50 shift towards higher temperatures of approximately 30 °C (see Fig. 7a).

3.8. Hydrothermal stability and sulfur poisoning effects It is known that stability and moisture resistance have a substantial influence on practical applications of catalysts [69]. It is generally believed that a combination of high temperature and a wet atmosphere will severely degrade the performance of certain catalysts, such as Ptsupported zeolites [70]. However, in the present work, hydrothermal ageing at 600 °C did not reduce the performance of the CuMnCe catalysts during subsequent NOx-assisted soot oxidation, as shown in Fig. 7a. The T50 of the hydrothermally-aged catalyst was close to that of the fresh material, with an increase of only 4 °C, demonstrating the superior hydrothermal stability of the catalyst. The FT-IR transmission spectrum of the fresh catalyst was used to assess the metal-to-oxygen bonds present in the catalyst, and four characteristic bands are apparent

Fig. 7. (a) Soot conversion curves and (b) FT-IR spectra generated from the fresh Cu1Mn1Ce1 and the same material after hydrothermal aging and sulfur poisoning. 7

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However, this loss of activity was less than has been previously reported for catalysts based on Pt, Mn, Ce and Al2O3 (see Table S4).

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4. Conclusions Despite significant progress in the management of air pollution worldwide, pollutants continue to represent a hazard to health and the environment, and soot remains a major pollutant. The present study demonstrates CuMnCe mixed oxides as an alternative to commercially available catalysts based on precious metals for the purpose of soot remediation. The catalysts synthesized in this work, in particular Cu1Mn1Ce1, showed remarkable soot oxidation performance in both NO + O2 and O2 environments and were also superior to single oxides and Pt/Al2O3. Even after hydrothermal aging at 600 °C for 5 h, the aged Cu1Mn1Ce1 remained as a superior catalyst. This optimized catalyst has good reusability and sustained a high level of activity throughout four reuse cycles. It also shows acceptable sulfur tolerance. More importantly, the synergistic effects of different metals (Cu, Mn and Ce) in the mixed oxides, which derive from both the charge transfer and the unique flower-like planar sheet morphology, reduce the redox potentials of the catalysts and produce a large quantity of active surface oxygen. Owing to these features, the synthesized compounds have potential applications as environmentally-friendly, efficient, and cost-effective catalysts for soot oxidation. Therefore, this work could lead to the development of highly active structured mixed oxides catalysts based on Cu-Mn-Ce. 5. Credit Author Statement All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Applied Surface Science. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge financial support from the China Science and Technology Exchange Center (grant no. 2016YFE0126600), the National Key R&D Program of China (grant no. 2017YFC0211102) and the National Engineering Laboratory for Mobile Source Emission Control Technology (grant no. NELMS2017A13). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2020.145602. References [1] D. Pierce, A. Haynes, J. Hughes, R. Graves, P. Maziasz, G. Muralidharan, A. Shyam, B. Wang, R. England, C. Daniel, High temperature materials for heavy duty diesel engines: Historical and future trends, Prog. Mater. Sci. 103 (2019) 109–179. [2] A. Liati, D. Schreiber, P. Dimopoulos Eggenschwiler, Y. Arroyo Rojas Dasilva, Metal particle emissions in the exhaust stream of diesel engines: an electron microscope study, Environ. Sci. Technol. 47 (2013) 14495–14501. [3] T.J. Wallington, E.W. Kaiser, J.T. Farrell, Automotive fuels and internal combustion engines: a chemical perspective, Chem. Soc. Rev. 35 (2006) 335–347. [4] A. Liati, B.T. Brem, L. Durdina, M. Voegtli, Y.A.R. Dasilva, P.D. Eggenschwiler, J. Wang, Electron microscopic study of soot particulate matter emissions from

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