Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO hydrotalcite-based mixed oxides

Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO hydrotalcite-based mixed oxides

JOURNAL OF RARE EARTHS, Vol. 32, No. 2, Feb. 2014, P. 176 Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO ...

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JOURNAL OF RARE EARTHS, Vol. 32, No. 2, Feb. 2014, P. 176

Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO hydrotalcite-based mixed oxides LI Qian (ᴢ ‫)׽‬, WANG Xiao (⥟ ᰧ), CHANG Wei (ᐌ ӳ), CHEN Hui (䰜 ᜻), ZHANG Zhaoliang (ᓴᰁ㡃)* (School of Chemistry and Chemical Engineering, Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan 250022, China) Received 16 September 2013; revised 4 December 2013

Abstract: A series of MnMgAlO samples with different amounts of Ce doping were facilely prepared using coprecipitation method and their catalytic soot combustion activity was evaluated by temperature programmed oxidation reaction (TPO). The methods of X-ray diffraction (XRD), Brumauer-Emmett-Teller (BET), H2-TPR, NO-TPO and in situ IR were used to characterize the physiochemical properties of these samples. Dopant Ce improved the soot combustion performance of MnMgAlO catalyst due to the enhanced redox ability. Introduction of NOx led to the further increase of catalytic soot oxidation activity on these samples. Over Ce-containing samples, the catalytic activity was slightly decreased as the amount of dopant Ce increased in O2. Differently, in NO+O2, a certain amount of dopant Ce was much more favorable and excess amount of Ce resulted in a sharp drop of the catalytic soot combustion activity. Both NO2 and nitrates were found to have great contributions to the effects of NOx on the soot combustion activity of Ce-doped catalysts. More NO2 was generated as dopant Ce increased. When appropriate amount of Ce was introduced, the as-formed NO2 was stored as bridging bidentate nitrate on Mn-Ce site, which was confirmed to have higher reactivity with soot than nitrite or monodentate nitrate on Mn and/or Ce sites. Overall, Mn0.5Mg2.5Ce0.1Al0.9O was considered as the most potential catalyst for soot combustion. Keywords: hydrotalcite; soot combustion; NO2; bridging bidentate nitrate; monodentate nitrate; rare earths

During the last few decades, diesel engines have attracted more and more attentions in vehicle markets in view of their high fuel efficiency, low operating costs and high durability[1]. However, the emitted exhausts contain a large amount of soot due to the incomplete combustion of diesel. Soot has been considered as one of the hazardous materials, causing considerable environmental pollutions such as haze weather and human health problems, like asthma. As soot elimination becomes more and more urgent[2], many methods have been proposed and the catalytic technique is one of the most effective after-treatment ways for eliminating the pollution of soot[3]. A number of catalysts have been employed for soot combustion, such as metal oxides[4–6], precious metals[7,8], zeolites[9], perovskite-related oxides[10–12] and spinel phases[13–16]. Among them, Mn-based compounds exhibit excellent catalytic performance, including the high soot combustion activity and low costs[17–19]. Meanwhile, it has been found that calcined hydrotalcite-like compounds (HTlcs) modified by transition metal ions exhibit excellent redox properties owing to their large surface areas, basic properties, high metal dispersions and high thermal

stability[20–22]. As a combination, a series of hydrotalcite-derived Mn-containing catalysts, namely MnxMg3 xAlO (x=0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0), were developed for the soot combustion process, as reported in our previous studies[23]. Moreover, many studies have been carried out on the promotion of Mn-based catalysts by other metals, such as Ce, Zr, Cu, Co, Ag, etc[24–27]. In view of the superior redox properties and oxygen storage capacity of Ce oxides, Ce-containing catalysts have been extensively applied in the removal of soot particles[4,6]. Thus, in this work, the Ce-doped MnMgAlO hydrotalcite-based catalysts for soot combustion were studied. Additionally, it is well known that NOx is simultaneously generated in the diesel engines when soot is formed. The influence of NOx on soot combustion has also been studied extensively by many researchers[28–30]. In general, NO2 or nitrate with stronger oxidation capacity than O2 was considered to contribute to the positive effects of NOx [31,32]. How do these two species affect the soot combustion process over Ce-doped MnMgAlO hydrotalcite-based catalysts in NOx atmosphere? To make this question clear, the soot oxidation under O2 and NO+ O2 atmosphere were carried out and compared.

Foundation item: Project supported by National Natural Science Foundation of China (21107030, 21277060, 21077043) and the Foundation for Outstanding Young Scientist in Shandong Province (BS2011HZ002) * Corresponding author: ZHANG Zhaoliang (E-mail: [email protected]; Tel.: +86-531-89736032) DOI: 10.1016/S1002-0721(14)60048-X

LI Qian et al., Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO …

Thus, in this work, two parts were investigated, including the promotional effects of Ce on the soot combustion activity over MnMgAlO hydrotalcite-based catalysts, as well as the influence of NOx on the catalytic performance of these samples. Moreover, the methods of XRD, BET and H2-TPR were used to characterize the physiochemical properties of the samples before and after the addition of Ce. NO-TPO and in situ IR were also applied to investigate the effects of NO2 and nitrate on the soot combustion activity over these Ce-doped catalysts.

1 Experimental 1.1 Catalyst preparation The hydrotalcite-like compounds Mn0.5Mg2.5CexAl1–x (x=0, 0.1, 0.3, 0.5) were prepared using coprecipitation method by adding mixed salt solution and mixed basic solution dropwise into distilled water simultaneously at constant pH (10±0.5) under vigorous mechanical stirring. The mixed salt solution consists of Mn(CH3COO)2·4H2O, Mg(NO3)2·6H2O, Ce(NO3)3·6H2O and Al(NO3)3·9H2O with the designed molar ratio. The mixed basic solution contains NaOH and Na2CO3 with [OH–]=2.0 mol/L and [OH–]/[CO32–]=16. The resulting precipitates were kept in suspension at 65 ºC for 30 min under stirring, then ltered, thoroughly washed with distilled water and dried overnight at 120 ºC. The prepared hydrotalcites were calcined at 800 ºC for 4 h to get the desired catalysts Mn0.5Mg2.5CexAl1–xO, where x represents the theoretical atomic content of Ce in the samples. 1.2 Catalyst characterization X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/max-2500/PC diffractometer employing Cu KD radiation ( =0.15418 nm) operating at 50 kV and 200 mA. The Brunauer-Emmett-Teller (BET) surface area and pore structure were measured by N2 adsorption/desorption using a Micromeritics ASAP 2020M instrument. Before N2 physisorption, the sample was outgassed at 300 °C for 5 h. H2-TPR experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H2 consumed. 50 mg of the sample was pretreated in situ at 500 °C for 1 h in a flow of O2 and cooled to room temperature in the presence of O2. TPR was conducted at 10 °C/min up to 900 °C in a 30 mL/min flow of 5 vol.% H2 in N2. The in situ IR spectra were recorded on a Bruker Tensor 27 spectrometer over 400–4000 cm–1 after 32 scans at a resolution of 4 cm–1. Prior to the experiments, the background spectra were obtained in He flow at room temperature. The catalyst was pressed into a thin self-supporting wafer with a thickness of 7.5 mg/cm2 and then loaded into an in situ infrared transmission cell which is capable of oper-

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ating up to 500 °C and equipped with gas flow system. All experiments were performed in the gas flow of 100 mL/min with the heating rate of 5 °C/min. 1.3 Activity measurement For soot combustion, the catalytic activity of the samples was evaluated by TPO technique using Degussa Printex-U as the model soot, with a surface area of 100 m2/g (C, 92.2 wt.%; H, 0.6 wt.%; volatiles, 6 wt.%)[33]. Soot was mixed with the catalyst in a weight ratio of 1:9 in an agate mortar for 30 min to obtain a tight contact. The TPO reactions were conducted in a fixed bed microreactor consisting of a quartz tube (6 mm i.d.). 50 mg of the mixture was pretreated in a flow of He (50 mL/min) at 200 °C for 1 h to remove adsorbed species on surface. After cooling down to 100 °C, a gas flow with 5% O2 or 5% O2+1000 ppm NO balanced by He (100 mL/min) was introduced and then TPO was started at a heating rate of 5 °C/min until 700 °C. NOx (NO and NO2) and CO2 in the effluent were online analyzed by a chemiluminescence NOx analyser (42i-HL, Thermo Environmental) and a quadruple MS (OmniStar 200, Balzers) with a m/z of 14, respectively. In this work, the soot ignition temperature (denoted as Ti) and the temperature corresponding to maximal soot combustion rate (denoted as Tm) were used to evaluate the performance of catalysts.

2 Results 2.1 Characterizations The Mn0.5Mg2.5CexAl1–x precursors just after drying at 120 ºC were characterized by XRD technique, as shown in Fig. 1. The major phase corresponding to hydrotalcite-like compound was obtained for the precursor of all the four samples, with the typical diffraction peaks at 11.3º, 23.1º, 34.5º, 38.9º, 46.4º, 60.4º and 61.8º, which can be assigned to the (003), (006), (009), (015), (018),

Fig. 1 XRD patterns of the precursors (1) Mn0.5Mg2.5Al; (2) Mn0.5Mg2.5Ce0.1Al0.9; (3) Mn0.5Mg2.5Ce0.3Al0.7; (4) Mn0.5Mg2.5Ce0.5Al0.5

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(110) and (113) crystal planes in the layered structure with a rhombohedral symmetry (3R)[34,35]. With the addition of Ce, new peaks at 28.8º and 57.1º appeared, which can be attributed to Ce(OH)3. As the content of Ce increases, the peaks attributed to hydrotalcite-like compound became weaker, accompanied by the stronger peaks corresponding to Ce(OH)3. This suggests that with increasing the amount of dopant cerium in the samples, the content of hydrotalcite is decreased correspondingly, leading to the decrease of the peak intensity for the hydrotalcite-like structure. After calcination of the hydrotalcite-like compounds Mn0.5Mg2.5CexAl1–x at 800 ºC, the Mn0.5Mg2.5CexAl1–xO mixed oxides were obtained. The XRD patterns are displayed in Fig. 2. It can be found that MgO, Mg2MnO4, MgAl 2 O 4 and MnAl 2 O 4 are the major phases in Mn0.5Mg2.5AlO sample, which was also reported in our previous study[23]. When the cerium was added, the hightemperature calcination process resulted in the formation of CeO2 phase, characterized by a set of peaks at 28.6º, 33.0º, 47.5º, 56.4º, 59.1º, 69.4º, 76.9º and 79.0º. As the dopant Ce increases, the peaks ascribed to the CeO2 phase become stronger, accompanied by the weaker peaks attributed to MgO, Mg2MnO4, MgAl2O4 and MnAl2O4 phases. This suggests that the introduction of cerium may affect the crystallinity of the Mn-related and Mg-related phases. The possible reason is that strong interaction may exist between the dopant Ce and Mn or

Fig. 2 XRD patterns of catalysts (1) Mn0.5Mg2.5AlO; (2) Mn0.5Mg2.5Ce0.1Al0.9O; (3) Mn0.5Mg2.5Ce0.3Al0.7O; (4) Mn0.5Mg2.5Ce0.5Al0.5O

Mg, leading to the decrease in crystallinity of the Mn-related and Mg-related phases. Additionally, the specific surface area and the pore volume of Mn0.5Mg2.5CexAl1–xO oxides were also measured, as listed in Table 1. It can be found that for all the samples, as high as 90 m2/g of surface area can be obtained even after calcination at 800 ºC, which can serve as one advantage of the hydrotalcite-like precursors. However, it can also be observed that the addition of cerium leads to the decrease of the specific surface area and the pore volume of the Mn0.5Mg2.5AlO sample. Combined with the above XRD results, it can be attributed to the destroyed framework of the hydrotalcite-like Mn0.5Mg2.5Al compounds after the introduction of Ce. In order to study the redox properties of the Mn0.5Mg2.5CexAl1–xO catalysts, H2-TPR experiments were performed, as displayed in Fig. 3. It can be found that as the dopant Ce increases, more H2 is consumed, the calculated results of which are listed in Table 1. This suggests that more oxidative species are present in the catalysts. Moreover, it can be found that after the curve fitting resolution, two peaks at 458 and 515 ºC can be obtained for Mn0.5Mg2.5AlO, which can be ascribed to the reduction of Mn4+ in the Mg2MnO4-like phase and the further reduction of Mn3+ to Mn2+ [36,37]. After the introduction of Ce, two peaks at lower temperatures (360 and 444 ºC) can be obtained. This enhancement of redox properties by dopant Ce can be attributed to the strong interaction of Ce and Mn species, facilitating the reduction of Mn-related species in the Mg2MnO4 phase. Additionally, it is noted that for all Ce-doped samples, the low-temperature reduction peaks were located at almost the same position, indicating that no big difference of the oxidative ability was present for these Ce-doped catalysts. Moreover, it can be observed that for Mn0.5Mg2.5Ce0.3 Al0.7O and Mn0.5Mg2.5Ce0.5Al0.5O with larger amounts of dopant Ce, two new peaks were observed, with one weak peak at ~500 ºC and the other one at ~800 ºC. Based on the H2-TPR results for pure CeO2 using the same preparation method as that of other samples in this work, these two peaks can be attributed to the reduction of the surface and bulk CeO2, respectively. Additionally, a new and weak peak centered at ~200 ºC was also observed for Ce-doped samples. This is possibly due to the reduction of adsorbed oxygen species stimulated by oxygen va-

Table 1 Specific surface area (SSA), pore volume (PV), amount of H2 consumed from H2-TPR and soot combustion characteristic temperatures (Ti, Tm) of the Mn0.5Mg2.5CexAl1–xO catalysts (x=0, 0.1, 0.3, 0.5) Sample

SSA/

PV/

Amount of H2 consumed/

Ti, oxygen/

Tm,oxygen/

Ti,NOx/

Tm, NOx/

(m2/g)

(cm3/g)

(mol/g)

ºC

ºC

ºC

ºC

Mn0.5Mg2.5AlO

142

0.153

15.10

405

544

355

500

Mn0.5Mg2.5Ce0.1Al0.9O

111

0.159

19.03

375

495

310

448

Mn0.5Mg2.5Ce0.3Al0.7O

100

0.124

19.98

375

498

310

448

Mn0.5Mg2.5Ce0.5Al0.5O

92

0.056

24.53

375

503

335

476

LI Qian et al., Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAlO …

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2.3 Influence of NOx on the catalytic soot combustion activity

The soot combustion activity of Mn0.5Mg2.5CexAl1–xO catalysts in the atmosphere of 5 vol.% O2+He was evaluated by TPO method, as shown in Fig. 4 and the results of which are listed in Table 1. Over Mn0.5Mg2.5AlO, soot oxidation starts at ~405 ºC, with the Tm at 544 ºC. When a small amount of Ce was doped (Mn0.5Mg2.5Ce0.1 Al0.9O), the characteristic temperatures of soot combustion are decreased to a large degree, with the Ti and Tm decreased to 375 and 495 ºC, respectively. Combining with H2-TPR results, the promotional effects of Ce can be attributed to the enhanced redox ability of Mn0.5Mg2.5AlO by dopant Ce. As the dopant Ce increases (Mn0.5Mg2.5Ce0.3Al0.7O and Mn0.5Mg2.5Ce0.5Al0.5O), a slight increase of Tm of catalytic soot combustion was observed, possibly due to the decrease of the specific surface area of catalysts, as listed in Table 1.

In order to investigate the influence of NOx on the soot oxidation behaviors, the experiments in the atmosphere of 1000 ppm NO+5 vol.% O2 balanced by He were carried out, as displayed in Fig. 5 and the results are listed in Table 1. It can be observed that the introduction of NOx leads to the easier oxidation of soot over all the catalysts, as reflected by the much lower characteristic temperatures. For Ce-free catalyst, soot oxidation starts at ~355 ºC, with the Tm at 500, ~40 ºC lower than that on the same catalyst in O2 atmosphere. Additionally, Ce-doped samples show better catalytic performance than Ce-free sample. This is consistent with the notion that NOx can facilitate the soot combustion process compared with O2, as most researchers reported[7,28]. Different from the soot combustion behaviors in O2, much higher catalytic soot combustion activity was obtained in NOx on Mn0.5Mg2.5Ce0.1Al0.9O and Mn0.5Mg2.5Ce0.3Al0.7O than that on Mn0.5Mg2.5Ce0.5Al0.5O. On Mn0.5Mg2.5Ce0.1Al0.9O and Mn0.5Mg2.5Ce0.3Al0.7O, soot can be ignited at the temperature as low as 310 ºC and the maximum oxidation rate can be reached at 448 ºC, much lower than that on the Ce-free catalyst or that on the same catalyst in O2 atmosphere. When more Ce was added (Mn0.5Mg2.5Ce0.5Al0.5O), an increase of Ti or Tm by ~25 ºC can be observed compared to that on the samples with lower contents of dopant Ce. That is to say, for Mn0.5Mg2.5CexAl1–xO, appropriate amount of dopant Ce is more favorable for soot combustion in the presence of NOx. In view of the positive effects of NOx on soot combustion over Mn0.5Mg2.5CexAl1–xO catalysts, experiments were carried out to make clear the functional species of N-related species, such as NO2, nitrites or nitrates. Firstly, NO2 profiles over Mn0.5Mg2.5CexAl1–xO catalysts were provided in the atmosphere of 1000 ppm NO+5 vol.% O2

Fig. 4 TPO profiles of soot combustion over Mn0.5Mg2.5Cex Al1–xO catalysts (x=0, 0.1, 0.3, 0.5) in the atmosphere of 5 vol.% O2 balanced by He

Fig. 5 TPO profiles of soot combustion over Mn0.5Mg2.5Cex Al1–xO catalysts (x=0, 0.1, 0.3, 0.5) in the atmosphere of 1000 ppm NO+5 vol.% O2 balanced by He

Fig. 3 H2-TPR profiles of the Mn0.5Mg2.5CexAl1–xO catalysts (x=0, 0.1, 0.3, 0.5)

cancy, resulting from the strong interaction between dopant Ce and Mn in the catalysts. 2.2 Effects of Ce doping on the catalytic soot combustion activity

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balanced by He, as shown in Fig. 6. It can be observed that as the amount of dopant Ce increases, more NO2 can be formed above 300 ºC. This can be attributed to the increase of oxidative species in the catalysts by dopant Ce and this can be explained by H2-TPR results, in which more H2 are consumed as the dopant Ce increases. It is well known that NO2 possesses higher oxidation ability than O2. This can serve as the evidence that the soot combustion in NOx atmosphere proceeds at lower temperatures than that in O2. However, this cannot explain why the Mn0.5Mg2.5Ce0.5Al0.5O with higher content of Ce shows the much poorer soot oxidation activity compared with Mn0.5Mg2.5Ce0.1Al0.9O and Mn0.5Mg2.5Ce0.3Al0.7O. Thus, it is deduced that other N-related species should contribute to the excellent catalytic performance of the samples with lower contents of dopant Ce. Thus, in situ IR experiments were performed to study the effects of other N-related species over the Mn0.5Mg2.5CexAl1–xO catalysts, such as nitrite or nitrate species, the results of which are presented in Fig. 7. The IR bands and the corresponding stored NOx species over the catalysts are summarized in Table 2. It can be observed that on Mn0.5Mg2.5AlO, NOx are adsorbed as monodentate nitrate species on Mn sites characterized at 1284 cm–1 in the temperature range of 250–500 ºC [38–41]. With the introduction of proper amounts of Ce

(Mn0.5Mg2.5Ce0.1Al0.9O, Mn0.5Mg2.5Ce0.3Al0.7O), the band at 1284 cm–1 disappeared in the temperature range of 200–350 ºC, corresponding to the soot ignition temperature window on this catalyst in NOx atmosphere. Simultaneously, a new band appeared at 1305 or 1300 cm–1, which can be ascribed to the bridging bidentate nitrate on M n - C e s i t e s [ 4 2 ] . W h e n mo r e C e w a s a d d ed (Mn0.5Mg2.5Ce0.5Al0.5O), a peak at 1275 cm–1 came into

Fig. 6 NO2 profiles over Mn0.5Mg2.5CexAl1–xO catalysts (x=0, 0.1, 0.3, 0.5) in the atmosphere of 1000 ppm NO+5 vol.% O2 balanced by He

Fig. 7 In situ IR spectra of NOx adsorption over different catalysts (1) Mn0.5Mg2.5AlO; (2) Mn0.5Mg2.5Ce0.1Al0.9O; (3) Mn0.5Mg2.5Ce0.3Al0.7O; (4) Mn0.5Mg2.5Ce0.5Al0.5O

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181

Table 2 A summary of IR bands and the corresponding stored NOx species over the Mn0.5Mg2.5CexAl1–xO catalysts (x=0, 0.1, 0.3, 0.5) Sample

Temperature range/ºC

IR bands/cm–1

Corresponding stored NOx species

nitrite, monodentate nitrate

Mn0.5Mg2.5AlO

250–500

1140, 1284, 1462, 1625

Mn0.5Mg2.5Ce0.1Al0.9O

200–350

1140, 1305, 1462, 1546, 1625

Mn0.5Mg2.5Ce0.3Al0.7O

250–350

1140, 1300, 1475, 1535, 1625

Mn0.5Mg2.5Ce0.5Al0.5O

250

1140, 1275, 1300, 1462, 1528, 1625

being at 250 ºC, which can also be attributed to monodentate nitrate species located on Ce sites. Moreover, it can be observed that on all these catalysts, higher temperature is favorable for the formation of the peak at ~1285 cm–1 which can be ascribed to the monodentate nitrate species on Mn-sites, indicating that NOx can be strongly adsorbed on Mn species. Additionally, it can be concluded that as the dopant Ce increased, the bridging bidentate nitrates on Mn-Ce sites located at 1305 or 1300 cm–1 are more and more unstable, reflected by the narrower temperature range from 200–350 ºC on Mn0.5Mg2.5Ce0.1Al0.9O to 250 ºC on Mn0.5Mg2.5Ce0.5Al0.5O. Based on the results above, it can be concluded that soot combustion behavior over Ce-doped MnMgAlO catalysts in NOx differs from that in O2 atmosphere. The difference of the soot combustion activity in NOx over Mn0.5Mg2.5CexAl1–xO catalysts with varied Ce doping amounts can be attributed to the comprehensive effects of NO2 and nitrates species. Dopant Ce increases the amount of NO2 because the oxidizing ability of the Ce-doped catalysts can be enhanced, as reflected by H2-TPR results. Moreover, when NOx were stored, the introduction of Ce can facilitate the formation of a new-style bridging bidentate nitrate species, suggesting that Mn sites adjacent to Ce sites are present possibly due to the strong interaction of Mn with Ce species. However, an excess of dopant Ce leads to the large amounts of CeO2 on the catalysts. Surface Mn-sites are covered by these CeO2, and thus result in the formation of monodentate nitrate again stored on Ce sites. Additionally, the interaction of Mn with Ce may be weakened, decreasing the stability of bridging bidentate nitrate species on Mn-Ce sites. That is to say, compared with monodentate nitrate species on Mn sites and/or Ce sites, this bridging bidentate nitrate species on Mn-Ce sites are suggested to have stronger reactivity with soot. The possible reason is that the added Ce may strongly interact with Mn species, inducing the formation of Ce-O-Mn structure. Therefore, the mobility of oxygen bonded to Mn can be enhanced due to the larger ionic radius of Ce than that of Mn, thus facilitating the adsorption of NOx to

Structure of nitrate species

nitrite, bridging bidentate nitrate

nitrite, monodentate nitrate, bridging bidentate nitrate

form bridging bidentate nitrate species on MnCe sites with lower stability than monodentate nitrate species on Mn sites. Thus, the reaction of soot and nitrate species became easier, resulting in the lower soot combustion temperatures on these catalysts when a certain amount of Ce was introduced.

3 Conclusions The soot combustion activity on MnMgAlO catalyst was enhanced after the doping of Ce, with the temperature corresponding to the maximal soot combustion rate lowered by ~50 ºC in O2. This promotional effect could be attributed to the enhanced redox ability of Mn0.5Mg2.5AlO by dopant Ce, derived from H2-TPR results. When NOx was introduced, higher soot combustion activity was obtained over all the catalysts. Based on in situ IR results, as well as the NO2 profiles derived from NOx storage results, it was observed that dopant Ce facilitated the formation of NO2, as well as the bridging bidentate nitrate on Mn-Ce sites which possessed higher reactivity with soot than nitrite or monodentate nitrate on Mn sites and/or Ce sites. However, thermal stability of this bridging bidentate nitrate was decreased on the samples with an excess of dopant Ce. Overall, Mn0.5Mg2.5Ce0.1Al0.9O was proposed as the most potential soot combustion catalysts in this work. Acknowledgement: Authors are grateful to the Doctor Foundation of University of Jinan (XBS1047) for financial support.

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