Simultaneous catalytic removal of NOx and soot particulates over CuMgAl hydrotalcites derived mixed metal oxides

Simultaneous catalytic removal of NOx and soot particulates over CuMgAl hydrotalcites derived mixed metal oxides

Applied Clay Science 55 (2012) 125–130 Contents lists available at SciVerse ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/lo...

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Applied Clay Science 55 (2012) 125–130

Contents lists available at SciVerse ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Simultaneous catalytic removal of NOx and soot particulates over CuMgAl hydrotalcites derived mixed metal oxides Zhongpeng Wang a, b, Qian Li a, Liguo Wang a,⁎, Wenfeng Shangguan b a b

School of Resources and Environment, University of Jinan, 106 Jiwei Rd., Jinan 250022, PR China School of Mechanical and Power Engineering, Shanghai Jiao Tong University,800 Dongchuan Rd., Shanghai 200240, PR China

a r t i c l e

i n f o

Article history: Received 1 July 2011 Received in revised form 3 November 2011 Accepted 8 November 2011 Available online 10 December 2011 Keywords: Hydrotalcites Mixed oxides Copper NOx Soot Simultaneous removal

a b s t r a c t A series of CuMgAl hydrotalcites derived oxides were prepared by co-precipitation and calcination methods and tested for the simultaneous catalytic removal of NOx and soot. The obtained samples were characterized by XRD, N2 adsorption-desorption, H2-TPR and ICP-AES techniques. The crystal phases, porous structures and redox properties of the catalysts were strongly influenced by Cu substitution contents and calcination temperatures. The CuMgAl mixed oxides with mesoporous properties exhibit high activity for the simultaneous NOx-soot removal. Among the tested catalysts, 3.0Cu-800 sample shows the best performance with the ignition temperature of soot = 260 °C and the total amounts of N2 = 6.0 × 10 − 5 mol. Based on the experimental work, a primitive kinetics analysis was carried out from the non-steady (dynamic) TPR measurements. Linear Arrhenius plots of rates of CO2, N2 and N2O formation were observed around the onset of formation curves where the substantial amount of the soot still remains in the soot/catalyst mixture and the effective area of the soot/catalyst contact can be regarded as constant. Finally, a compensation effect was found for the formation of CO2, N2 and N2O over CuMgAl mixed oxides with CuO as the predominant phase. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen oxides (NOx) and soot particulates,which are the main pollutants in diesel engine emissions, can cause serious problems to global environment and human health. Thus, specific and severe limits to the exhaust-gas emissions have been introduced (Twigg, 2007; van Setten et al., 2001). As the legislation limitation goes more stringent, there is a growing interest in developing the process which enables the reduction of such emissions. Because of the trade-off effects for the reduction of NOx and soot present in engine design modifications, it is necessary to develop after-treatment technologies in order to meet the more and more stringent emission regulations. As a promising alternative, the simultaneous catalytic removal of NOx and soot particulates in a single trap was proposed by Yoshida et al. (Yoshida et al., 1989). Teraoka and coworkers conducted systemic research on the simultaneous catalytic removal of NOx and soot and found that both the perovskite-type oxides (Teraoka et al., 1995, 1996; 2001b) and AB2O4 spinel oxides (Shangguan et al., 1996, 1997, 1998) were active catalysts. Subsequently, many works have been carried out focusing on these two types of materials (Fino et al., 2006; Liu et al., 2008; Peng et al., 2007; Wei et al., 2011). In addition, Co,K supported catalysts

⁎ Corresponding author. Tel.: + 86 531 89736032; fax: + 86 531 89736032. E-mail address: [email protected] (L. Wang). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.11.003

(Pisarello et al., 2002), potassium modified iron oxide(Kureti et al., 2003), and Cu ion exchanged zeolites (Teraoka et al., 2001a; Xiao et al., 2001) also display activities for the simultaneous removal of nitric oxide and soot. Fino et al. (Fino et al., 2003) investigated the reaction mechanism for the soot/NO/O2 reaction over layered perovskites by FT-IR analysis and isothermal oxidation tests, and proposed that two different reaction mechanisms may occur simultaneously. The technology of simultaneous NOx-soot removal in a single trap is ambitious, though there are some technical problems to be solved especially under the real working conditions of diesel engine (Fornasiero et al., 2008). However, the more exigent research subject now is to develop new catalysts possessing low-temperature activity for both soot oxidation and NOx reduction. Hydrotalcite (HT) and Hydrotalcite-like compounds (HTlcs), also called layered double hydroxides (LDH), are widely applied in catalysis and adsorption. These materials can be chemically expressed by the formula [M(П)1-x M(Ш)x(OH)2] x+[(A n−)x/n·mH2O] x−, where M(П) represents any divalent metal cation, M(Ш) any trivalent metal cation, and A n− an anion; the value of x is equal to the molar ratio of M(П)/( M(П) + M(Ш)) and generally ranges between 0.2 and 0.4 (Vaccari, 1998; Xu et al., 2011). At higher temperatures hydrotalcites can be transformed into mixed metal oxides or spinels, which are excellent catalysts or catalyst support because of their large surface areas, basic properties, high metal dispersions and stability against sintering. Such mixed oxides also show redox properties and their oxidation performances are strongly related to the metal species, contents and calcined temperature (Chmielarz et al., 2003;

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Turco et al., 2004; Yu et al., 2006). In the last decade, the interests in application of HT derived mixed oxides as environmental catalysts have significantly increased. Transitional and noble metals containing catalysts show high catalytic performance. Cu-, Co- or Ni-containing oxides have been found to be active and selective catalysts of DeNOx with ammonia (Chmielarz et al., 2002), catalytic decomposition of NO and N2O (Kannan, 2006; Marquez et al., 2001), as well as methanol steam reforming (Turco et al., 2004). Noble metal and/or Transitional metal containing HT catalysts (Basile et al., 2004; Li et al., 2010) also exhibited excellent activity in NOx storage and reduction. The widely application of the prepared HT or the calcined HT samples is due to their high versatility, easily tailored properties and low cost, which make it possible to produce materials designed to fulfill specific requirements. In our pervious works (Wang et al., 2006; Zhang et al., 2010), hydrotalcites derived mixed oxides have been found to be active in catalytic soot oxidation. In particular, CoAl-HT, CuAl-HT and CuCoAlHT (Wang et al., 2007, 2009; Zhang et al., 2008) derived oxides displayed good activities for the simultaneous catalytic removal of NOx and soot due to the higher redox performance. In the present work, a series of CuxMg3-xAl HT derived oxides with different Cu contents and different calcinations temperatures were prepared for the simultaneous NOx-soot removal. Based on the experimental work, kinetics analysis was carried out from the non-steady (dynamic) TPR measurements.

diesel soot, with specific surface area of 120.6 m 2/g − 1, heating loss of less than 0.4% and ash content of less than 0.3%. The catalytic activity tests were carried out by the Temperature-programmed reaction (TPR) technique. The well mixed catalyst and soot (20:1 w/w) were palletized, crushed and sieved to 20–40 mesh. The tight mixture (0.33 g) was placed in an 8 mm U-shaped quartz reactor, and pretreated in a helium flow at 300 °C for 2 h in order to eliminate possible contaminants. After cooling down to 100 °C and replacing the helium flow with the reaction gas flow, TPR was started at a heating rate of 1.6 °C/min. The reaction gas consisted of 0.25 vol.% NO, 5 vol. % O2 with the balance being helium. The total flow rate was 20 cm 3/ min. The outlet gas was analyzed with intervals of about 15 min using a TCD gas chromatograph (Shimazu GC-8A) with columns of Porapak Q for separating CO2 and N2O and molecular sieve 5A for N2, O2, NO and CO. From TPR results, three parameters were derived in order to evaluate the catalytic performance. First is the ignition temperature of soot (Ti) estimated by extrapolating the steeply ascending portion of the CO2 formation curve to zero CO2 concentration. Second is the selectivity to CO2 formation defined as that the total amounts of CO2 divided by the sum amounts of the CO and CO2 formed during reactions, i.e., SCO2 = VCO2/(VCO + VCO2).Third is the total amounts of N2 (VN2) formed during the TPR run, which are obtained by integrating the N2 formation curves. The lower the Ti value, as well as the higher the SCO2 and the VN2 value, the more active the catalyst.

2. Experimental 3. Results and discussion 2.1. Catalyst preparation 3.1. Characterization of catalysts The synthesis of CuxMg3-xAl HT with (Cu 2+ + Mg 2+)/Al 3+ molar ratio fixed at 3.0 where x was set at 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 (designated as xCu-HT) followed the reference (Jiang et al., 2005) by coprecipitation of an aqueous solution of suitable metal nitrates with an aqueous solution of 2 M NaOH and 1 M Na2CO3. The two solutions were mixed under vigorous stirring at 25 °C with the pH maintained constant at 10.0 ± 0.5. The resulting slurry was aged in the mother liquor at 80 °C for 18 h. It was then filtered and washed with distilled water until the pH of the filtrate was around 7. The precipitate was then dried at 100 °C for 12 h to obtain CuMgAl HT. The hydrotalcite precursors were calcined at 800 °C in air for 5 h. In order to investigate the effect of calcination temperature, 3.0Cu-HT was also calcined at 600 and 700 °C for 5 h. The derived oxides catalysts are denoted as xCu-m, where m represents calcinations temperature. 2.2. Catalyst characterization Chemical analyses were carried out on a Thermo Elemental ICPAES spectrometer after dissolution of the solid sample in an HCl solution. XRD patterns were recorded on a Rigaku D/max-rc diffractometer employing Cu Kα radiation. N2 adsorption–desorption isotherms were measured on a Quantachrome NOVA1000 Sorptomatic apparatus. The specific surface area was calculated with the BET equation. The pore size distributions were obtained by the Barrett–Joyner– Halenda (BJH) methods using the desorption branch of the isotherms. Temperature-programmed reduction with H2 (H2-TPR) experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H2 consumed. A 50 mg 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. H2-TPR was conducted at 10 °C/ min up to 800 °C in a 50 ml/min flow of 5 vol.% H2 in N2. 2.3. Catalytic activity testing

3.1.1. Elemental and XRD analysis The chemical composition of the dried hydrotalcites, as well as the structural parameters obtained from the XRD results are shown in Table 1. The molar ratios of cations in solids corresponded approximately to those in the nitrate solutions used for coprecipitation. The XRD patterns of CuMgAl HT precursors are given in Fig. 1. All the precursors showed the typical diffraction patterns of hydrotalcitelike materials (JCPDS 22–0700, marked with H) having layered structure with intercalated carbonate anions (Cavani et al., 1991; Yu et al., 2006). Only pure hydrotalcite phase was detected in 0.0Cu-HT, 0.5CuHT and 1.0Cu-HT samples. A further increase in the Cu content results in the appearance of small impurity peaks corresponding to malachite phase(Cu2(OH)2CO3, JCPDS 10–0399, marked with M), being known to form in Cu/Al Hydrotalcite especially at high Cu/Al ratios (Lwin et al., 2001). Hence, there is a limitation on the amount of Cu incorporation in the Mg/Al HT framework. Our results (with 1.0Cu-HT) reveal that about 33% (molar ratio) of Mg ions can be isomorphously substituted by Cu in the Mg–Al brucite layer to obtain pure and homogeneously dispersed CuMgAl HT samples. The lower stability of the copper hydrotalcite phase, and the subsequent formation of Table 1 Chemical composition and structural parameters of CuMg/Al HT. HT Samples

0.0Cu-HT 0.5Cu-HT 1.0Cu-HT 1.5Cu-HT 2.0Cu-HT 2.5Cu-HT 3.0Cu-HT a

In this paper, a commercially available carbon black (Shanxi Luan Carbon Black Science & Technology Co. Ltd, China) was used as model

Cu2+: Mg2+: Al3+ atomic ratio In solution

In solid

0: 3.0:1.0 0.5:2.5:1.0 1.0:2.0:1.0 1.5:1.5:1.0 2.0:1.0:1.0 2.5:0.5:1.0 3.0:0:1.0

0:2.92:1.00 0.52:2.44:1.00 1.05:1.89:1.00 1.54:1.56:1.00 2.05:0.98:1.00 2.57:0.44:1.00 3.12:0:1.00

FWHMa (2θ)

tb (nm)

1.084 1.423 1.646 0.786 0.868 1.099 0.813

7.2 4.9 4.6 10.1 8.1 6.6 8.7

Full width at half maximum (FWHM) of (003) plane. Average crystallite size in the c direction calculated from (003) and (006) plane using Debye–Scherrer equation. b

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3.0Cu-800, i.e., Cu3Al mixed oxide, a well-defined terronite phase (CuO, JCPDS 45–0937, marked with T) together with a small amount of spinel phase (CuAl2O4, JCPDS 33–0448, marked with C) was detected. As for the tertiary oxides, with the increase of Cu content, the MgO phase tended to disappear, while the CuO phase became more evident. Besides, an obvious MgAl2O4 phase (JCPDS 33–0448, marked with M) also appeared for 1.5Cu-800 sample. 3.0Cu-HT derived mixed oxides with calcinations at 600, 700 and 800 °C exhibit similar CuO phase accompanied with traces of spinel phase. The crystallite sizes of CuO phase for 3.0Cu-600, 3.0Cu-700 and 3.0Cu-800 were 13.9, 20.8 and 36.5 nm, respectively, as estimated from the peak width using Debye-Scherrer equation. This suggests that the increase in calcinations temperature from 600 to 800 °C caused increase of crystallinities of CuO phase.

Fig. 1. XRD patterns of CuMg/Al HT samples. (H = hydrotalcite, M = malachite).

malachite can be explained by the Jahn–Teller distortion of copper in octahedral coordination (Cavani et al., 1991). The copper content also influenced the crystallinity of the CuMgAl HT precursors, which can be seen from both XRD patterns (in Fig. 1) and structural parameters (in Table 1). After Cu was incorporated, the crystalline perfection decreased, as represented by the intensity and width of the characteristic peaks such as the (110) and (113) reflection double peaks around 2θ values of 60°. With the increase of Cu content, the full width at half maximum (FWHM) of (003) plane (2θ ≈ 12°) increased until that of 1.0Cu-HT sample reached a maximum of 1.646, and then decreased largely. The average crystallite sizes in the c direction were calculated from (003) and (006) plane using Debye–Scherrer equation and summarized in Table 1. Obviously, the crystallite sizes of the HT samples varied in the range 4–10 nm. The fluctuation trend of crystallite sizes versus Cu content was opposite to that of the FWHM. The 1.0Cu-HT sample, with the largest Cu content in a single hydrotalcite phase, exhibits the smallest crystallite size at about 4.6 nm. Fig. 2 describes the XRD patterns of CuMgAl mixed oxides derived from HT precursors. As reported earlier (Kannan et al., 2004), the hydrotalcite phases were completely destroyed and new oxide derivatives were formed after calcinations at higher temperatures. It is obvious from Fig. 2(a) that the gradual replacement of Mg with Cu leads to the gradual changes in the oxides phase composition. For sample 0.0Cu-800, i.e., Mg3Al mixed oxides, three peaks with the 2θ angle centered at 37°, 42° and 62° were ascribed to the periclase phase (MgO, JCPDS 45–0946, marked with P). In contrast, for sample

3.1.2. N2 adsorption–desorption characterization The N2 adsorption-desorption isotherms of CuMgAl mixed oxides exhibit similar Type-IV isotherms with a H3 hysteresis loop at a high relative pressure as our previous study (Wang et al., 2006). Adsorption isotherms of this type are representative of mesoporous materials with no or few micropores and strong interactions between adsorbent and adsorbate molecules. As displayed by the pore size distribution curves (not shown here due to the illustrations restriction), most of the pores fall in meso size range (2 nm b rp b 50 nm). However, calcination of HT precursors even to 800 °C preserves some amount of micropores, though their proportion in the total pore volume is not very high. The BET specific surface area (SBET), Total Pore Volume (VP) and Average Pore Diameter (DP) of the mixed oxides determined from isotherms are listed in Table 2. Obviously, the composition and calcination temperature severely affect the three textural parameters. The surface areas of all catalysts range from 18 to 115 m 2/g − 1. A decreasing trend is observed in the surface areas with increase of Cu content or calcination temperature. The interparticle pore diameter falls in the range of 8–25 nm, and thus gas molecule diffusion in the pores should not be the rate-determining step for adsorption and desorption (Yu et al., 2006). 3.1.3. H2-TPR results H2-TPR was used to examine the redox properties of catalysts. Fig. 3 presents H2-TPR profiles for mixed oxides catalysts, and the H2 consumption amounts in the temperature range 100–450 °C are summarized in Table 2. It is very clear that the reduction behaviors of these catalysts are strongly related to the Cu content and calcination temperature. For 0.0Cu-800, i.e., free of copper, no reduction peaks were detected until 800 °C. For Cu-containing oxides, the broad peaks in the temperature range of 200–400 °C were related to the reduction of Cu 2+ cations in the CuO phase and CuAl2O4 spinel phase (Jiang et al., 2005; Rives and Kannan, 2000; Turco et al., 2004). With the gradual replacement of Cu for Mg, the reduction peaks shifted toward low temperatures and multiple weak peaks Table 2 Characteristics of the porous structure of CuMg/Al mixed oxides. Sample

SBETa

VP b

DP c

H2 uptaked

0.0Cu-800 0.5Cu-800 1.0Cu-800 1.5Cu-800 2.0Cu-800 2.5Cu-800 3.0Cu-800 3.0Cu-700 3.0Cu-600

115 103 82 24 28 21 18 38 57

0.549 0.065 0.043 0.058 0.064 0.045 0.052 0.082 0.118

19.1 25.3 20.8 9.8 9.2 8.8 11.5 8.7 8.2

0 13.8 39.3 57.1 83.6 93.3 101.5 95.8 91.6

a b

Fig. 2. XRD patterns of CuMg/Al mixed oxides derived from HT. (P = perclase, S = spinel, T = terronite).

c d

BET specific surface area, m2·g− 1. Total pore volume, cm3·g− 1. Average pore diameter, nm. H2 uptake in the range 100–450 °C, mL·g− 1.

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Fig. 3. H2-TPR patterns of CuMg/Al mixed oxides.

were transformed into a single intense peak, implying the rapid reduction of copper ions and the enhancement of reducibility of the catalyst. H2 consumption amounts were increased with the increase of Cu content, which means a higher amount of copper cations in the hydrotalcite causes an increase in the reducible copper particles. This may be helpful in the catalytic oxidation reactions. The increase in calcination temperature from 600 to 800 °C also improved the reducibility, as can be seen in Fig. 3 and Table 2. In comparison with 3.0Cu-800 sample, the higher reduction temperatures for 3.0Cu-600, and 3.0Cu-700 may be related to undecomposed carbonate which was coordinated tightly with Cu ions according to thermogravimetric analysis. This results in difficult reduction of Cu 2+ and higher reduction temperatures.

curves for CO2, N2 and N2O indicate a closer temperature dependency and closer apparent activation energy. Fig. 5 shows soot oxidation curves over CuMgAl mixed oxides, and the carbon mass balance for all the catalysts was nearly 100 ± 5%. The activity parameters are listed in Table 3. The blank experiment was performed mixing the soot with SiO2, and the ignition temperature was 530 °C while the soot was totally burnt at 790 °C. Compared with the non-catalyzed soot combustion (Blank curve in Fig. 5), the CO2 formation curves over the CuMgAl mixed oxides catalysts shift to lower temperature range and the catalyzed soot oxidation starts at 260–300 °C. There is a decreasing trend for Ti values with the increase of Cu content, meaning that the catalytic activities for soot combustion were increased. This may be partly related to the enhancement of the reducibility of the catalysts as mentioned in H2TPR results, because the effect of NOx on soot oxidation cannot be excluded. Similar case is also found with the increase of calcinations temperature from 600 to 800 °C. Regarding to the selectivity to CO2 formation, the non-catalytic oxidation of soot is very low with SCO2 = 60.1%, while all catalysts exhibits excellent selectivity with SCO2 = 100%, except for Cu-free sample (SCO2 = 92.3%). CO2 is the sole product for Cu-containing catalysts, which may be due to the enhancement of redox properties after Cu incorporation and/or the promotion effects of NOx species on soot oxidation (Wang et al., 2006). From the viewpoint of Ti and SCO2 values listed in Table 3, CuO phase may be the active phase for soot oxidation in comparison with the reference CuO. As for the NO conversion, different amounts of N2 with a small quantity of N2O were detected during the TPR runs over CuMgAl

3.2. Simultaneous NOx-soot removal reactions Fig. 4 shows the TPR results over 3.0Cu-800 catalyst in the NO–O2– He atmosphere. Similar to the previous reports (Shangguan et al., 1996, 1998; Wang et al., 2007), the formations of CO2, N2 and N2O were observed within the same temperature range of 200–500 °C evidencing the occurrence of the simultaneous NOx-soot removal reactions: the oxidation of soot by either NOx or O2, and the reduction of NOx by soot into N2 and N2O. The similar shapes of the formation

Fig. 4. TPR results of the simultaneous NOx-soot removal over 3.0Cu-800. Gas composition: NO 0.25 vol.% , O2 5 vol.% ,balance He; Flow rate: 20 cm3/min.

Fig. 5. Catalytic performances for soot oxidation over CuMg/Al catalysts during NOxsoot reactions. Same experimental conditions as Fig.4.

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Table 3 The catalytic performance and kinetics parameters CuMg/Al mixed oxides for the simultaneous NOx-soot removal. Catalysts

Catalytic performance Ti

Blank CuOa 0.0Cu-800 0.5Cu-800 1.0Cu-800 1.5Cu-800 2.0Cu-800 2.5Cu-800 3.0Cu-800 3.0Cu-700 3.0Cu-600 a

VN2

SCO2 −5

(°C)

(10

530 325 451 301 311 288 271 264 260 268 277

– 2.08 3.28 4.91 4.60 3.35 4.90 4.10 6.00 3.67 4.90

mol)

Ea (kJ⋅mol− 1)

ln ξA

CO2

N2

N2O

CO2

N2

N2 O

158.4 70. 6 72. 2 61.1 54.3 75.4 60.9 62.3 69.2 56.1 55.2

72.0 67.6 87.1 91.5 66.8 70.3 59.1 55.9 81.6 69.3

63.8 56.7 48.3 105.9 58.1 55.8 58.4 69.1 65.3 62.9

28.8 7.8 6.5 7.4 5.5 9.9 7.5 7.7 9.6 6.3 6.1

3.55 2.23 9.6 10.5 4.92 6.27 3.34 3.04 9.42 5.46

1.55 − 1.63 − 0.11 12.7 1.85 1.44 1.72 4.89 3.58 3.33

(%) 60.1 100 92.3 100 100 100 100 100 100 100 100

Reference CuO was prepared by decomposition of copper nitrate at 800 °C for 5 h. Fig. 6. Arrhenius plots of CO2, N2 and N2O formation over 3.0Cu-800.

mixed oxides. Obviously, N2O, which is one type of greenhouse gas, is an undesired product with a maximum conversion of 4–7% over all the catalysts. The VN2 values for CuMgAl mixed oxides are also larger than that for reference CuO, revealing the superiority of hydrotalcites derived mixed oxides. In comparison with the previous investigation in O2 (Yu et al., 2004), the presence of NO in the reaction gas considerably decreased soot combustion temperatures over CuMgAl mixed oxides. The promotion effect is contributable to NO2 as a strong oxidizing agent and nitrate and/or nitrite species formed in soot–NO– O2 reactions (Wang et al., 2006). Among the tested catalysts, 3.0Cu-800 exhibits the best activity with Ti =260 °C and VN2 = 6.0 × 10 − 5 mol. It should be noted that the catalytic performance is almost independent on the surface area as listed in Tables 2 and 3, because the simultaneous NOx-soot removal reaction supposedly takes place at the so-called “triple contact point” where the solid catalyst, the solid soot and the gaseous reactants (O2/NO) meet together (Shangguan et al., 1996, 1997; Teraoka et al., 1996).

ln ξA for the formation of CO2, N2 and N2O obtained from the slope and the intercept, respectively, are listed in Table 3. The Ea for CO2 formation is decreased from 158.4 kJ⋅mol − 1 for the non-catalytic oxidation to about 50–70 kJ⋅mol − 1 for the catalytic oxidation, which is corresponding to the decrease of the ignition temperature. However, there is not a linear relation between Ea and Ti values, which may be because there are many impact factors for soot oxidation in the O2/NO mixture. As listed in Table 3, the close Ea for the formation of CO2 and N2 corresponds to the parallelism among the formation curves in Fig. 4 over the CuMgAl mixed oxides. As shown in Fig. 7, a linear correlation or the compensation effect was observed between the Ea and ln ξA for the formation of CO2, N2 and N2O over CuMgAl mixed oxides with CuO as the predominant phase. This implies that the reaction mechanism and reaction orders with respect to reactants are the same over these catalysts. More detailed investigations on kinetics and mechanism are in progress at our lab.

3.3. Kinetics analysis of NOx-soot removal reaction 4. Conclusion It is quite difficult to carry out the kinetics study for the reaction involving solid reactants, such as soot, mainly because it is difficult to realize the steady-state reaction conditions. The kinetics analysis was followed the reference (Shangguan et al., 1996, 1997), which has demonstrated the possibility of the kinetics analysis of nonsteady TPR results on the simultaneous removal of soot and NO. TPR results on 3.0Cu-800 (Fig. 4) were converted into the formation rates of CO2, N2 and N2O, and logarithm of rates were plotted against the reciprocal of temperature in Fig. 6. The linear Arrhenius relation was observed below about 290 °C (T − 1/K − 1 = 1.78 × 10 − 3) and reaction rates tended to level off above that temperature. A sum of the soot consumed up to 290 °C was 4.4% of the initially charged soot. These results indicate that the kinetic analysis of the TPR result is possible around the onset of formation curves where the substantial amount of the soot still remains in the soot/catalyst mixture and the effective area of the soot/catalyst contact can be regarded as constant. The reaction rate can be written by the conventional power-law expression as follows: n1 n2

n3

r ¼ Aexpð−Ea =RT ÞC C C NO C O2

The CuMgAl hydrotalcites derived mixed oxides are active for the simultaneous removal of NOx and soot. The Cu incorporation into hydrotalcites strongly influenced the crystal phases, porous structures and redox properties of the catalysts. The enhancement of redox properties after Cu incorporation and/or the promotion effects

ð1Þ

or   n1 n2 n3 lnr ¼ −Ea =RT þ lnxA x ¼ C C C NO C O2

ð2Þ

where A is the pre-exponential factor, Ea the apparent activation energy, Ci the concentration and ni the reaction order. Thus, the Ea and

Fig. 7. Compensation effects with respect to CO2, N2 and N2O formation in the simultaneous NOx-soot removal over CuMg/Al catalysts. (1: 1.0Cu-800; 2: 1.5Cu-800; 3: 2.0Cu-800; 4: 2.5Cu-800; 5: 3.0Cu-800; 6: 3.0Cu-700; 7: 3.0Cu-600).

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