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Catalytic selective reduction of NO with propylene over Cu-Al2O3 catalysts: influence of catalyst preparation method
Applied Catalysis B: Environmental 23 (1999) 259–269
Catalytic selective reduction of NO with propylene over Cu-Al2 O3 catalysts: influence of catalyst preparation method Laiyuan Chen, Tatsuro Horiuchi ∗ , Toshihiko Osaki, Toshiaki Mori Ecoceramic Lab, Ceramics Technology Department, National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan Received 8 April 1999; received in revised form 7 July 1999; accepted 7 July 1999
Abstract NO reduction to N2 by C3 H6 was investigated and compared over Cu-Al2 O3 catalysts prepared by four different methods, namely, the conventional impregnation, co-precipitation, evaporation of a mixed aqueous solution, and xerogel methods. It was found that the catalyst preparation method as well as the Cu content exerts a significant influence on catalyst activity. For the catalysts prepared by the first three preparation methods, with the increase of Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased slightly, but the temperature for the maximum NO reduction also decreased. For the xerogel Cu-Al2 O3 , there was a significant decrease in NO reduction conversion with the increase of Cu content from 5 to 10 wt%. In the absence of water vapour, the Cu-Al2 O3 catalyst prepared by the impregnation method exhibited the highest activity toward NO reduction. The purity of alumina support was found to be a crucial factor to the activity of the Cu-Al2 O3 catalyst prepared by impregnation. In the presence of water vapour, a substantial decrease in NO conversion was observed for the Cu-Al2 O3 catalysts prepared by the first three methods, especially for the impregnated Cu-Al2 O3 catalyst. In contrast, the presence of water vapour showed only a minor influence on the xerogel 5 wt% Cu-Al2 O3 and it showed the highest activity for NO reduction in the presence of 20% water vapour. The xerogel 5 wt% Cu-Al2 O3 catalyst was also found to be less affected by a 5 wt% sulfate deposition than the Cu-Al2 O3 catalysts prepared by other methods. ©1999 Elsevier Science B.V. All rights reserved. Keywords: NO reduction with C3 H6 ; Copper/alumina; Catalyst preparation method
1. Introduction The control of NOx emission from automobiles remains a challenging problem for both academic research and industry. The conventional ‘three-way’ catalyst shows a low NO reduction conversion under lean burn conditions. Recently, the selective reduction of ∗ Corresponding author. Tel.:+81-52-911-2111; fax: +81-52-916-6993 E-mail address: [email protected] (T. Horiuchi)
NO with injected hydrocarbon is believed to be a possible way to solve this problem, and has been extensively studied [1–2]. Although a large number of catalyst systems has been investigated; so far, no suitable catalyst has been found for this purpose. The copper ion-exchanged ZSM-5 zeolite is known to be very effective for NO reduction by hydrocarbon [3]. However, its poor thermal and hydrothermal stability makes it impossible to be used under practical conditions. Alumina-based catalysts, which are of high hydrothermal stability, are possible candidates for NO removal
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 8 4 - 3
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[4]. Among them, the alumina-supported copper catalyst has been found to be as active as Cu-ZSM-5 and has a much higher hydrothermal stability [5–9]. It seems that Cu-Al2 O3 is a promising catalyst for NO removal. However, a few reports claimed that the Cu-Al2 O3 catalyst was inferior to Cu-ZSM-5 [10–11]. This discrepancy in the activity of Cu-Al2 O3 in different reports is possibly a result of the differences in the catalyst preparation method, copper content and alumina source. It was reported that increasing the copper loading from 0.36 to 1.65 wt% CuO resulted in a decrease of NO reduction conversion [8]. The Cu-Al2 O3 catalyst prepared by co-precipitation showed a higher activity than Cu-ZSM-5 only at low O2 concentration and below 623 K [9]. The Cu-Al2 O3 catalyst prepared by adsorption of Cu2+ from a basic solution showed a higher activity than the one prepared by the conventional impregnation method. This is because the basic solution preparation is favourable to the formation of isolated CuO-like species, which are believed to be the active sites for NO reduction [12]. Apparently, the preparation method is a critical factor to the activity of Cu-Al2 O3 catalyst for NO reduction by hydrocarbons, and hence a comparison of different catalyst preparation methods becomes necessary. In this study, Cu-Al2 O3 catalysts with different Cu contents were prepared by four different methods, namely, impregnation, co-precipitation, direct drying the mixed solution and xerogel methods. The structures of these catalysts have been characterized by XRD, UV-Vis absorption and TPR. NO reduction with propylene over these catalysts has been investigated and compared with those of Pt/Al2 O3 and Cu-ZSM-5. The influence of water and sulfate deposition on the catalytic performances of these catalysts has also been studied.
2. Experimental 2.1. Catalyst preparation Cu-Al2 O3 catalysts with a copper content from 2 to 15 wt% were prepared by four different methods, namely, the conventional impregnation [denoted as Cu-Al2 O3 (I)], co-precipitation [denoted as Cu-Al2 O3 (CO)], directly evaporating the mixed aque-
ous solution [denoted as Cu-Al2 O3 (S)] and xerogel [denoted as Cu-Al2 O3 (X)] methods. Details of the preparation of these catalysts are described as follows. The impregnated Cu-Al2 O3 (I) catalysts were prepared by impregnating a ␥-alumina powder with an appropriate amount of copper nitrate aqueous solution, stirred at room temperature for 6 h, then dried at 100◦ C overnight, calcined in static air at 800◦ C for 10 h. ␥-alumina was prepared by calcining an Al(OH)3 powder (Wako Pure Chemical Industries) in air at 800◦ C for 10 h. Two other kinds of reference alumina, JRC-ALO-2 and JRC-ALO-4 from the Catalysis Society of Japan, were also used for the purpose of comparison. The co-precipitated Cu-Al2 O3 (CO) catalysts were prepared from a mixed aqueous solution of copper nitrate and aluminum nitrate. The precipitate was obtained by adding aqueous ammonia (25%) slowly to the solution to a final pH of 7.0–7.5, then washed, dried at 100◦ C and calcined in air at 800◦ C for 10 h. Cu-Al2 O3 (S) catalysts were prepared by directly evaporating a mixed aqueous solution of copper nitrate and aluminum nitrate under stirring. After that, the gel mixture was dried at 100◦ C overnight and calcined in air at 800◦ C for 10 h. Xerogel Cu-Al2 O3 (X) catalysts were prepared according to Osaki et al. [13], but the drying process is different. Briefly, a boehmite solution was prepared by hydrolyzing aluminum tri-isopropoxide in a diluted nitric acid solution at 95◦ C. Then, an ethylene glycol solution of copper nitrate was added to the boehmite solution. A small amount of urea was added for gelation, if necessary. The gel was then transferred to a teflon-lined autoclave and heated at 95◦ C for 12 h, washed with ethanol, dried at room temperature for 2 days and at 100◦ C for 8 h, finally calcined in air at 800◦ C for 10 h. For comparison, a Cu-ZSM-5 catalyst was prepared by ion-exchange of a NH4 -form ZSM-5 zeolite (JRC-Z5-25H, a reference catalyst from the Catalysis Society of Japan) with a 0.5 M Cu(NO3 )3 solution at 80◦ C for 24 h. The copper content in the ion-exchanged Cu-ZSM-5 was 3.2 wt%. A 0.5 wt% Pt/Al2 O3 catalyst was prepared by the impregnation method. In order to investigate the effect of sulfur on the catalytic performance of the Cu-Al2 O3 catalyst prepared by different methods, 5 wt% SO2− 4 was deposited on the catalysts by impregnating the calcined Cu-Al2 O3 powder with a dilute H2 SO4 solution. Then
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Table 1 BET surface areas and pore volumes of 5 wt% Cu-Al2 O3 catalysts prepared by different methods Catalyst Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3
(I) (CO) (S) (X)
BET surface area (m2 /g)
Pore volume (ml/g)
Water adsorption (g/g.cat)
103.9 112.1 28.2 238.8
0.282 0.286 0.102 0.461
0.167 0.154 0.079 0.109
the catalysts were dried at 120◦ C, calcined at 550◦ C in air for 6 h before reaction tests were carried out. 2.2. Catalyst characterization BET surface area and pore volume were obtained from N2 adsorption isotherms measured at −196◦ C using a Bellsorp36 instrument. The BET surface areas and pore volumes of a few representative samples of differently prepared Cu-Al2 O3 catalysts are given in Table 1. XRD spectra were recorded using an MXP powder diffractometer with a Cu K␣ radiation and operated at 40 kV and 20 mA. UV-Vis absorption spectra were recorded using a U-3000 spectrophotometer in a similar way as reported previously for the solid sample [14]. TPR experiments were carried out using a H2 /Ar gas (5% H2 by volume) with a flow rate of 30 ml/min. The catalyst weight was 50 mg and the temperature ramp was 10◦ C per minute. Calibration was made by reduction of a known amount of pure CuO powder under the same experimental conditions. 2.3. NO reduction with propylene NO reduction with propylene in the presence of an excess amount of O2 was performed with a fixed-bed flow-type quartz reactor. Prior to the reaction, the catalyst (usually 0.2 g) was pretreated with flowing pure O2 at 500◦ C for 1 h, or treated in an oxygen flow containing 20% water at 600◦ C for 1 h if the catalyst was to be evaluated in the presence of water. The feed mixture consisted of 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , with or without 20% water, and N2 as a balance gas. The total gas flow rate was 145 ml/min, corresponding to a space velocity of about 15 000 h−1 . NO and NO2 concentrations were analyzed with a
HORIBA CLA-510SS NOx analyzer. Other products were analyzed using a SHIMADZU 8A GC equipped with a TCD. The concentrations of C3 H6 , CO2 and N2 O were analyzed by a Parapak Q column. A molecular sieve 5A column was used to monitor the formation of CO. Over all the catalysts tested, propylene was totally oxidized to CO2 and H2 O. NO was converted mainly to N2 with a small amount of NO2 and traces of N2 O and NH3 . The concentrations of NH3 and N2 O were lower than the detection limitation (less than 10 ppm) of our GC. No other N-containing compound was detected.
3. Results Fig. 1 shows the NO reduction conversions over the Cu-Al2 O3 catalysts prepared by different methods. It can be seen that there are some similar trends in NO reduction behaviours with the Cu content irrespective of the catalyst preparation method. That is, with the increase of the Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased, but the temperature required for the maximum NO reduction conversion shifted to the low temperature range. NO conversion at a high temperature decreased significantly with the increase of Cu loading. As a result, the operating temperature window for NO reduction became narrower. It is interesting to note that the catalyst preparation method exerts significant influence on the catalyst performance. Upon increasing the Cu content from 5 to 15 wt%, only a slight decrease in the maximum NO reduction conversion is observed for Cu-Al2 O3 (I), Cu-Al2 O3 (CO) and Cu-Al2 O3 (S) catalysts. However, there is a drastic decrease in conversion from 45 to 31% with the increase of the Cu content from 5 to 10 wt% for the Cu-Al2 O3 (X) catalyst. With the same Cu content, the activity of Cu-Al2 O3 catalyst toward NO reduction is also
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Fig. 1. NO reduction with C3 H6 over Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel. Catalyst weight: 0.2 g, gas flow rate: 145 ml/min. 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , N2 as a balance gas.
greatly dependent on the catalyst preparation method. For the Cu-Al2 O3 catalysts with 5 wt% Cu, the NO reduction activity decreases in the following order: Cu-Al2 O3 (I) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (X) > Cu-Al2 O3 (S). The impregnated catalyst shows the highest activity and the widest operating temperature window, followed by the co-precipitated catalyst. The Cu-Al2 O3 catalyst prepared from the evaporation of a mixed solution of Cu(NO3 )2 and Al(NO3 )3 shows a much lower activity than the impregnated and co-precipitated Cu-Al2 O3 catalysts.As it is found that the Cu-Al2 O3 catalyst prepared by the impregnation method exhibited the highest activity for NO reduction, further attention was paid to the influence
of the nature of alumina support on the catalytic performance of impregnated catalyst. It is shown that the purity of support alumina exerts significant influence on the catalyst activity. For the two reference ␥-alumina JRC-ALO-2- and JRC-ALO-4-supported catalysts, the activity of the Cu-Al2 O3 (JRC-ALO-4) is much higher than that of Cu-Al2 O3 (JRC-ALO-2) (Fig. 2). The BET surface areas of JRC-ALO-2 and JRC-ALO-4 are 285 and 177 m2 /g, respectively. Apparently, the observed difference in their activities is not due to the difference in BET surface area, although a higher BET surface area is considered to be advantageous to CuO dispersion. As these two supports have the same structure, the different activities
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Fig. 2. Influence of alumina source on NO conversion over impregnated 5 wt% Cu-Al2 O3 (I) catalysts. Alumina supports were (a) JRC-ALO-4, (b) ␥-Al2 O3 from calcination of Al(OH)3 at 800◦ C and (c)JRC-ALO-2. Reaction conditions were the same as those in Fig. 1.
Fig. 3. NO reduction by C3 H6 over the 0.5% Pt/Al2 O3 and Cu-ZSM-5 catalysts. Solid lines are the NO reduction conversions in the absence of water and dotted lines are those in the presence of 20% water. Reaction conditions were the same as those in Fig. 1 in the absence of water and those in Fig. 4 in the presence of water.
can possibly be attributed to the different purity of these alumina supports. It is known that the purity of JRC-ALO-4 is higher than JRC-ALO-2, which contains a significant amount of impurities like Fe, Si and Na, and 1.72% SO2− 4 . Apart from the influence of Fe, Si and Na species, it is supposed that a large amount of sulfur is also a poison to the catalyst, as a previous report showed that pre-sulfurized alumina had lower activity than pure alumina [15]. This conclusion is also in agreement with that of Iwamoto [16]. We also compared these Cu-Al2 O3 catalysts with the ion-exchanged Cu-ZSM-5 and Pt/Al2 O3 for NO reduction with C3 H6 under the same reaction conditions. The results of Cu-ZSM-5 and Pt/Al2 O3 are given in Fig. 3. By comparing Fig. 1 with Fig. 3, it is interesting to find that the Cu-Al2 O3 catalyst prepared by any of the aforementioned methods shows higher activity for NO reduction than the 0.3% Pt/Al2 O3 catalyst. Besides, the 0.3% Pt/Al2 O3 catalyst produced a sig-
nificant amount of N2 O (its selectivity is about 50%), which is a greenhouse effect gas. Over any Cu-Al2 O3 catalyst, NO was mainly converted to N2 with a small amount of NO2 , and traces of N2 O and NH3 . In comparison with the Cu-ZSM-5, all Cu-Al2 O3 catalysts showed higher activity at a relatively lower temperature than Cu-ZSM-5, whereas Cu-ZSM-5 exhibited high activity only at high temperature (450–500◦ C). This observation is in agreement with those of Hattori et al. [9]. Fig. 4 shows NO reduction conversion with C3 H6 in the presence of 20% water vapour over the differently prepared 5 wt% Cu-Al2 O3 catalysts, as a function of reaction temperature. By comparing of these results with those in the absence of water (Fig. 1), it is seen that the presence of water leads to a decrease in NO conversion and shifted the commencing temperature of NO reduction to the high temperature range. However, the presence of water showed only a
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much higher NO reduction activity than either the Pt/Al2 O3 or the Cu-ZSM-5. It is demonstrated that both the co-precipitated 5 wt% Cu-Al2 O3 and xerogel 5 wt% Cu-Al2 O3 are superior to the 0.5 wt% Pt/Al2 O3 and Cu-ZSM-5 catalysts. In the presence of water, the xerogel 5 wt% Cu-Al2 O3 catalyst exhibited the highest NO reduction conversion. The order of activities of these catalysts is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). The differently prepared Cu-Al2 O3 catalysts also showed different tolerance to sulfate deposition. Fig. 5 shows the NO reduction capabilities of sulfate-treated 5 wt% Cu-Al2 O3 catalysts. Comparing these results with those of the unsulfated catalysts in Fig. 1, it can be seen that the xerogel 5 wt% Cu-Al2 O3 shows a lesser influence by the deposition of SO2− 4 , whereas the activities of other catalysts decrease significantly. Therefore, it can be concluded that the xerogel 5 wt% Cu-Al2 O3 (X) is the best among the four differently prepared Cu-Al2 O3 catalysts for NO reduction with C3 H6 in the presence of water. Fig. 4. Influence of water on the catalytic activities of differently prepared 5 wt% Cu-Al2 O3 catalysts. Catalyst weight: 0.2 g, gas flow rate: 145 ml/min. 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , 20% water vapor, N2 as a balance gas.
minor influence on the xerogel 5 wt% Cu-Al2 O3 catalyst. Only a decrease of NO conversion at low temperature is observed. The maximum NO conversion decreases slightly from 45 to 41% in the presence of 20% water vapour. It is surprising to find that the activity of the 5% Cu-Al2 O3 (I), which is the highest in the absence of water, decreased substantially from 48.3 to 26.1% in the presence of water vapour. Comparison of the results in Fig. 4 with those of Pt/Al2 O3 and Cu-ZSM-5 (Fig. 3, dotted lines) reveals that the Cu-Al2 O3 catalysts prepared by impregnation and directly drying the solution are inferior to both Pt/Al2 O3 and Cu-ZSM-5, for NO reduction with C3 H6 in the presence of water. The Cu-Al2 O3 prepared by co-precipitation showed higher NO reduction conversion than Pt/Al2 O3 . Its maximum NO reduction conversion was comparable to that of Cu-ZSM-5, but the maximum NO reduction took place at a much lower temperature. It is of importance to note that the xerogel 5 wt% Cu-Al2 O3 catalyst exhibited a
Fig. 5. Influence of sulfate deposition on the catalytic activities of differently prepared 5 wt% Cu-Al2 O3 catalysts. Reaction conditions were the same as those in Fig. 1.
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Fig. 6. XRD spectra of Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel.
4. Discussion Cu-Al2 O3 catalysts prepared by different methods exhibited quite different activities for NO reduction with C3 H6 . The difference is possibly caused by the presence of different Cu species in these catalysts. Fig. 6 shows the XRD spectra of Cu-Al2 O3 catalysts prepared by the aforementioned four different methods. No CuO peak is detected for all the catalysts prepared by impregnation, co-precipitation and direct drying of the mixed aqueous solution methods, although the Cu content is as high as 15 wt%. For all the Cu-Al2 O3 catalysts except the xerogel Cu-Al2 O3 with a Cu content over 5 wt%, only a spinel struc-
ture is detected. The XRD peak intensities increase with increasing Cu content in the catalyst indicating an enhanced formation of the spinel phase, possibly CuAl2 O4 . For the xerogel 5 wt% Cu-Al2 O3 , only a ␥-Al2 O3 phase was detected. However, with a further increase in Cu content, CuO peaks are observed at 2θ of 35.6 and 38.8◦ . To gain further information on the state of Cu species in these catalysts, the UV-Vis spectroscopic studies are carried out and the spectra are shown in Fig. 7. Generally, three bands were observed for these Cu-Al2 O3 catalysts in the range of 210–350 nm, 400–500 nm and 600–900 nm. The band at 210–350 nm is attributed to the Cu2+ –O2− charge
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Fig. 7. UV-Vis absorption spectra of Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel.
transfer [10]. The band at 400–500 nm is assigned to the three-dimensional Cu+ clusters in the CuO matrix in the literature [10]. However, as the catalysts were used and measured in the oxidized form, the existence of Cu+ species was less likely. In addition, bulk CuAl2 O4 also shows strong absorption in this region. We thus prefer to assign the band at 400–500 nm to the bulk-like CuAl2 O4 species. The band at 600–900 nm is assigned to d–d transitions of Cu2+ situated in an octahedral environment with an Oh symmetry [10]. The significant difference in the UV-Vis spectra between the Cu-ZSM-5 and Cu-Al2 O3 catalysts indicates that the Cu species in Cu-Al2 O3 are quite different from those in Cu-ZSM-5. For the impregnated Cu-Al2 O3 catalysts with a low Cu content, bands at 210 and
225 nm with a tail to 400 nm, and a weak band at 600–900 nm are observed. With the increase of the Cu content, the Cu2+ –O2− charge transfer band shifted to a higher wavelength range and the intensity of the band at 600–900 nm slightly increased. In addition, a new band at 400–500 nm was observed if the Cu content was above 10 wt%. The appearance of this band is an indication of the existence of new Cu species, possibly the formation of bulk-like spinel CuAl2 O4 species [17]. This suggestion is in agreement with the previous observation that the intensity of XRD peaks increased with the increase of Cu loading. For the Cu-Al2 O3 catalysts prepared by other methods, a strong band at 600–900 nm is always observed. This implies that a significant amount of octahedral Cu2+
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species is present even in the Cu-Al2 O3 catalyst with a 5 wt% of Cu. Upon increasing the Cu content, the minimum of the low wavelength side of this band shifts to the high wavelength region. This is an indication of a less tetragonal distortion of the Cu species in the octahedral environment with the increase of Cu content in the catalysts [10,17]. For the Cu-Al2 O3 catalysts prepared from drying the solution, the UV-Vis spectra are different from those of other catalysts. It shows a broad band from 220 to 500 nm in addition to the sharp band at 215 nm. The observed tailing in the region 400–500 nm and a relatively weak absorption band around 200–300 nm for the Cu-Al2 O3 (S) catalysts imply that bulk-like CuAl2 O4 or CuO species are possibly present in this type of catalysts. The Cu-Al2 O3 system has been extensively investigated owing to its potential application as a catalyst in various types of reactions. It is known that Cu loading and calcination temperature are the most crucial factors for the catalyst structure [17,18]. At low Cu loading and calcination temperature below 500◦ C, formation of a surface spinel which resembles CuAl2 O4 , predominates. At high Cu loading and low calcination temperature, bulk-like CuO forms. A higher calcination temperature like 900◦ C leads to the formation of bulk CuAl2 O4 . In the present study, since the calcination temperature is fixed at 800◦ C, it is more probable that the surface spinel CuAl2 O4 or bulk CuAl2 O4 species would predominate in our Cu-Al2 O3 catalysts. XRD and UV-Vis results reveal that isolated Cu2+ , tetrahedrally distorted octahedral Cu2+ and surface CuAl2 O4 species exist in all the Cu-Al2 O3 catalysts used in this study. The difference possibly is the different amount of these Cu species in each catalyst. The absence of CuO in most of these catalysts even with a Cu loading as high as 15 wt% could be attributed to the high calcination temperature. Another observation is that the UV-Vis spectra of impregnated Cu-Al2 O3 catalysts are quite different from those of other catalysts. That is, the octahedral Cu2+ absorption band at 600–900 nm is rather weak and the 210–350 nm bands are strong. This possibly implies that the concentration of octahedral Cu2+ species in these catalysts is lower compared to other catalysts, and the isolated Cu2+ species would predominate. Upon increasing the Cu content to 10 wt%, bulk CuAl2 O4 species could be formed. For the Cu-Al2 O3 catalysts prepared by the other three methods, while some isolated Cu2+
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Fig. 8. TPR spectra of 5 wt% Cu-Al2 O3 catalysts prepared by different methods. (a) impregnation , (b) co-precipitation, (c) drying mixed solution, and (d) xerogel.
species existed, a significant amount of distorted octahedral Cu2+ species, in the form of surface CuAl2 O4 species, are detected. In order to further elucidate the nature of Cu species in these catalysts, a Temperature-Programmed Reduction (TPR) experiment was performed. The TPR profiles are shown in Fig. 8, and the analysis results are given in Table 2. A sharp reduction peak appeared at around 200◦ C for the 5 wt% Cu-Al2 O3 (CO), Cu-Al2 O3 (I) and Cu-Al2 O3 (S) catalysts. However, for the xerogel Cu-Al2 O3 , the major reduction peak appeared at a higher temperature. At 222◦ C, only a rather weak peak is observed. According to literature reports [7,12], the reduction peak at around 200◦ C can be attributed to the reduction of isolated Cu2+ species to Cu+ and partial reduction of surface CuAl2 O4 to CuAlO2 . For the Cu-Al2 O3 prepared from the drying solution and xerogel methods, a peak at 270◦ C appeared. This peak is assigned to the reduction of amorphous bulk-like CuO, which could not be detected by XRD [19]. This is in agreement with the previous suggestion from the UV-Vis results.
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Table 2 TPR results of 5 wt% Cu-Al2 O3 catalysts prepared by different methods Catalyst Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 a
(I) (CO) (S) (X)
Temperature of TPR peak
H2 /Cu below 400◦ C
209◦ C, 230◦ C (shoulder) 220◦ C 213◦ C, 270◦ C 222◦ C, 270◦ C, 318◦ C, 392◦ C
0.57 0.78 0.65 0.91a
Below 500 ◦ C
For the xerogel 5 wt% Cu-Al2 O3 catalyst, a sharp reduction peak appeared at 318◦ C. This peak did not appear for other catalysts indicating that a new type of Cu species probably exists in this catalyst. As the temperature of this peak is close to that of bulk CuO, we tentatively assign this peak to the reduction of the CuO species which strongly interacted with the alumina support, or highly dispersed CuO species in the alumina matrix. For the 5 wt% Cu-Al2 O3 (CO), Cu-Al2 O3 (I) and Cu-Al2 O3 (S) catalysts, a significant amount of Cu species could not be reduced at temperature below 500◦ C (Table 2). These un-reduced Cu species are possibly the supported Cu+ species from the partial reduction of isolated Cu2+ [20], or CuAlO2 species from surface CuAl2 O4 [21–24], whose reduction needs a higher temperature. For the Cu-Al2 O3 catalyst calcined at a high temperature (800◦ C), Hattori et al. [9] attributed the high activity of the catalyst to the surface aluminate phase. The correlation between copper species concentration profile analyzed by XANES and catalytic activity indicated that highly dispersed CuO-like aggregates are the active centres where NOx molecules are activated [12,25]. The reaction results in Fig. 1 show that the NO conversions over the impregnated, co-precipitated and xerogel 5 wt% Cu-Al2 O3 catalysts are not of much difference. Therefore, it is difficult to draw a conclusion on what are the active sites in our catalysts. It seems that the isolated Cu2+ species, surface CuAl2 O4 and highly dispersed CuO are all active toward NO reduction by propylene. However, it is certain that a Cu-Al2 O3 catalyst containing a significant amount of bulk CuAl2 O4 or bulk CuO would show low activity. The non-isolated copper ions (copper oxides) were found to be more active for the C3 H8 oxidation by O2 than for the reaction of C3 H8 with NO [26]. In the case of NO reaction with C3 H6 , a similar preference is plausible. Thus the appearance of bulk CuO
phase interprets the low NO reduction conversion of xerogel Cu-Al2 O3 (X) with a Cu content over 5 wt%. The low activity of the Cu-Al2 O3 prepared from drying the mixed solution can be attributed to the extremely low surface area and pore volume of this type of catalysts (Table 1), as it is shown that diffusion limitation is a key factor that affects the reaction rate [27]. In the presence of water, the activity order of the Cu-Al2 O3 catalysts with a 5 wt% Cu is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). This order is different from the one obtained in the absence of water. It is of importance to note that the presence of water showed only a minor influence on the xerogel 5 wt% Cu-Al2 O3 catalyst at a low temperature. However, the impregnated 5 wt% Cu-Al2 O3 (I) catalysts, which showed the highest activity in the absence of water, exhibited a very low NO reduction conversion in the presence of water. The simple interpretation for the decrease of NO reduction conversion in the presence of water is that water competes with NO and/or C3 H6 for the same adsorption sites. NO conversion decreases because water inhibits the adsorption of NO and/or C3 H6 . Kim et al. [28] have reported that the large hydrophobicity of zeolite plays a critical role in the activity loss of SCR by water. The higher water tolerance of Ga2 O3 /Al2 O3 is explained by the lower affinity of water for Al2 O3 than for ZSM-5 [29]. For the xerogel 5 wt% Cu-Al2 O3 catalyst, the major active centers are possibly these highly dispersed CuO species. It is reasonable to assume that water adsorption on these CuO species is difficult than it does on other kinds of Cu species. Furthermore, the xerogel Cu-Al2 O3 showed a lower water adsorption capacity compared to other catalysts (Table 1). The low affinity of water of this catalyst is possibly the reason why NO reduction is less affected by the presence of water.
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5. Conclusions The activity of the Cu-Al2 O3 catalyst for NO reduction by C3 H6 is largely dependent on the method of catalyst preparation and Cu content. For the Cu-Al2 O3 catalysts prepared by impregnation, co-precipitation and evaporation of a mixed solution, the maximum NO reduction conversion decreased slightly and the commencing temperature of NO reduction shifted to a low temperature with the increase of Cu content from 5 to 15 wt%. For the xerogel Cu-Al2 O3 , there was a significant decrease in NO reduction with the increase of Cu content from 5 to 10 wt%. For the impregnated Cu-Al2 O3 catalyst, the purity of alumina support is a crucial factor in determining catalyst activity. In the absence of water, the activity order of the catalysts with the same Cu content is: Cu-Al2 O3 (I) > Cu-Al2 O3 (CO)>Cu-Al2 O3 (X)>CuAl2 O3 (S). Although the impregnated Cu-Al2 O3 catalyst exhibited the highest activity in the absence of water, there was a substantial decrease in its NO reduction activity by the presence of water. In contrast, the xerogel 5 wt% Cu-Al2 O3 (X) was less affected by the presence of water and showed the highest activity. In the presence of water, the activity order of these catalysts is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). The xerogel 5 wt% Cu-Al2 O3 catalyst is also found to be more tolerant to sulfate deposition.
Acknowledgements L.Y. Chen is grateful to Japan International Science and Technology Exchange Center (JISTEC) for an STA fellowship. We also thank Dr. S. Velu for some helpful discussions and corrections.
References [1] A. Fritz, V. Pitchon, Appl. Catal. B. 13 (1997) 1. [2] V.I. Pârvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. [3] M. Iwamoto, H. Hamada, Catal. Today 10 (1991) 57.
269
[4] H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito, Appl. Catal. 75 (1991) L1. [5] M. Ozawa, H. Toda, O. Kato, S. Suzuki, Appl. Catal. B 8 (1996) 123. [6] M. Ozawa, S. Suzuki, H. Toda, J. Am. Ceram. Soc. 80 (1997) 1957. [7] J.A. Anderson, C. Márques-Alvarez, M.J. Lopez-Muñoz, I. Rodr´ıguez-Ramos, A. Gurrero-Ruiz, Appl. Catal. B 14 (1997) 189. [8] F. Radtke, R.A. Koeppel, E.G. Minardi, A. Baiker, J. Catal. 167 (1997) 127. [9] K. Shimizu, H. Maeshima, A. Satsuma, T. Hattori, Appl. Catal. B 18 (1998) 163. [10] H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B 6 (1998) 359. [11] Z. Chajar, VC.L. Chanu, M. Primet, H. Praliaud, Catal. Lett. 52 (1998) 97. [12] M. Fernández-Garc´ıa, I. Rodriguez-Ramos, P. Ferreira-Aparicio, A. Guerrero-Ruiz, J. Catal. 178 (1998) 253. [13] T. Osaki, T. Horiuchi, T. Sugiyama, S. Suzuki, T. Mori, Catal. Lett. 52 (1998) 171. [14] L. Chen, S. Jaenicke, G.K. Chuah, H.G. Ang, J. Electron. Spectrosc. Rel. Phen. 82 (1996) 203. [15] R. Burch, E. Halpin, J.A. Sullivan, App. Catal. B 17 (1998) 115. [16] N. Okazaki, Y. Shiina, H. Itoh, A. Tada, M. Iwamoto, Catal. Lett. 49 (1997) 169. [17] R.M. Friedman, J.J. Freeman, J. Catal. 55 (1978) 10. [18] B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal. 94 (1985) 514. [19] W.P. Dow, Y.P. Wang, T.J. Huang, J. Catal. 160 (1996) 155. [20] C. Torre-Abreu, M.F. Ribeiro, C. Henriques, G. Delahay, Appl. Catal. B 14 (1997) 261. [21] J.Y. Yan, G.D. Lei, W.M.H. Sachtler, H.H. Kung, J. Catal. 161 (1996) 43. [22] S. Sato, M. Iijima, T. Nakayama, T. Sodesawa, F. Nozaki, J. Catal. 169 (1997) 447. [23] F. Severino, J.L. Brito, J. Laine, J.L.G. Fierro, A.L. Agudo, J. Catal. 177 (1998) 82. [24] A. Alejandre, F. Medina, A. Fortuny, P. Salagre, J.E. Sueiras, Appl. Catal. B 16 (1998) 53. [25] C. Márquez-Alvarez, I. Rodr´ıguez-Ramos, A. Gurrero-Ruiz, G.L. Haller, M. Fernández-Garc´ıa, J. Amer. Chem. Soc. 119 (1997) 2905. [26] Z. Chajar, M. Primet, H. Praliaud, J. Catal. 180 (1998) 279. [27] K. Katagi, A. Shichi, S. Komai, A. Satsuma, T. Hattori, Preprints of 82th Catalysis Conference, Catalysis Society of Japan, September, 1998, p.135. [28] M.H. Kim, I.S. Nam, Y.G. Kim, in: H. Chon, S.K. Ihm, Y.S. Uh (Eds.), Progress in Zeolite and Microporous Materials, Stud. Surf. Sci. Catal. 105(1997)1493 [29] K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B 16 (1998) 319.
Catalytic selective reduction of NO with propylene over Cu-Al2 O3 catalysts: influence of catalyst preparation method Laiyuan Chen, Tatsuro Horiuchi ∗ , Toshihiko Osaki, Toshiaki Mori Ecoceramic Lab, Ceramics Technology Department, National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan Received 8 April 1999; received in revised form 7 July 1999; accepted 7 July 1999
Abstract NO reduction to N2 by C3 H6 was investigated and compared over Cu-Al2 O3 catalysts prepared by four different methods, namely, the conventional impregnation, co-precipitation, evaporation of a mixed aqueous solution, and xerogel methods. It was found that the catalyst preparation method as well as the Cu content exerts a significant influence on catalyst activity. For the catalysts prepared by the first three preparation methods, with the increase of Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased slightly, but the temperature for the maximum NO reduction also decreased. For the xerogel Cu-Al2 O3 , there was a significant decrease in NO reduction conversion with the increase of Cu content from 5 to 10 wt%. In the absence of water vapour, the Cu-Al2 O3 catalyst prepared by the impregnation method exhibited the highest activity toward NO reduction. The purity of alumina support was found to be a crucial factor to the activity of the Cu-Al2 O3 catalyst prepared by impregnation. In the presence of water vapour, a substantial decrease in NO conversion was observed for the Cu-Al2 O3 catalysts prepared by the first three methods, especially for the impregnated Cu-Al2 O3 catalyst. In contrast, the presence of water vapour showed only a minor influence on the xerogel 5 wt% Cu-Al2 O3 and it showed the highest activity for NO reduction in the presence of 20% water vapour. The xerogel 5 wt% Cu-Al2 O3 catalyst was also found to be less affected by a 5 wt% sulfate deposition than the Cu-Al2 O3 catalysts prepared by other methods. ©1999 Elsevier Science B.V. All rights reserved. Keywords: NO reduction with C3 H6 ; Copper/alumina; Catalyst preparation method
1. Introduction The control of NOx emission from automobiles remains a challenging problem for both academic research and industry. The conventional ‘three-way’ catalyst shows a low NO reduction conversion under lean burn conditions. Recently, the selective reduction of ∗ Corresponding author. Tel.:+81-52-911-2111; fax: +81-52-916-6993 E-mail address: [email protected] (T. Horiuchi)
NO with injected hydrocarbon is believed to be a possible way to solve this problem, and has been extensively studied [1–2]. Although a large number of catalyst systems has been investigated; so far, no suitable catalyst has been found for this purpose. The copper ion-exchanged ZSM-5 zeolite is known to be very effective for NO reduction by hydrocarbon [3]. However, its poor thermal and hydrothermal stability makes it impossible to be used under practical conditions. Alumina-based catalysts, which are of high hydrothermal stability, are possible candidates for NO removal
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 8 4 - 3
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[4]. Among them, the alumina-supported copper catalyst has been found to be as active as Cu-ZSM-5 and has a much higher hydrothermal stability [5–9]. It seems that Cu-Al2 O3 is a promising catalyst for NO removal. However, a few reports claimed that the Cu-Al2 O3 catalyst was inferior to Cu-ZSM-5 [10–11]. This discrepancy in the activity of Cu-Al2 O3 in different reports is possibly a result of the differences in the catalyst preparation method, copper content and alumina source. It was reported that increasing the copper loading from 0.36 to 1.65 wt% CuO resulted in a decrease of NO reduction conversion [8]. The Cu-Al2 O3 catalyst prepared by co-precipitation showed a higher activity than Cu-ZSM-5 only at low O2 concentration and below 623 K [9]. The Cu-Al2 O3 catalyst prepared by adsorption of Cu2+ from a basic solution showed a higher activity than the one prepared by the conventional impregnation method. This is because the basic solution preparation is favourable to the formation of isolated CuO-like species, which are believed to be the active sites for NO reduction [12]. Apparently, the preparation method is a critical factor to the activity of Cu-Al2 O3 catalyst for NO reduction by hydrocarbons, and hence a comparison of different catalyst preparation methods becomes necessary. In this study, Cu-Al2 O3 catalysts with different Cu contents were prepared by four different methods, namely, impregnation, co-precipitation, direct drying the mixed solution and xerogel methods. The structures of these catalysts have been characterized by XRD, UV-Vis absorption and TPR. NO reduction with propylene over these catalysts has been investigated and compared with those of Pt/Al2 O3 and Cu-ZSM-5. The influence of water and sulfate deposition on the catalytic performances of these catalysts has also been studied.
2. Experimental 2.1. Catalyst preparation Cu-Al2 O3 catalysts with a copper content from 2 to 15 wt% were prepared by four different methods, namely, the conventional impregnation [denoted as Cu-Al2 O3 (I)], co-precipitation [denoted as Cu-Al2 O3 (CO)], directly evaporating the mixed aque-
ous solution [denoted as Cu-Al2 O3 (S)] and xerogel [denoted as Cu-Al2 O3 (X)] methods. Details of the preparation of these catalysts are described as follows. The impregnated Cu-Al2 O3 (I) catalysts were prepared by impregnating a ␥-alumina powder with an appropriate amount of copper nitrate aqueous solution, stirred at room temperature for 6 h, then dried at 100◦ C overnight, calcined in static air at 800◦ C for 10 h. ␥-alumina was prepared by calcining an Al(OH)3 powder (Wako Pure Chemical Industries) in air at 800◦ C for 10 h. Two other kinds of reference alumina, JRC-ALO-2 and JRC-ALO-4 from the Catalysis Society of Japan, were also used for the purpose of comparison. The co-precipitated Cu-Al2 O3 (CO) catalysts were prepared from a mixed aqueous solution of copper nitrate and aluminum nitrate. The precipitate was obtained by adding aqueous ammonia (25%) slowly to the solution to a final pH of 7.0–7.5, then washed, dried at 100◦ C and calcined in air at 800◦ C for 10 h. Cu-Al2 O3 (S) catalysts were prepared by directly evaporating a mixed aqueous solution of copper nitrate and aluminum nitrate under stirring. After that, the gel mixture was dried at 100◦ C overnight and calcined in air at 800◦ C for 10 h. Xerogel Cu-Al2 O3 (X) catalysts were prepared according to Osaki et al. [13], but the drying process is different. Briefly, a boehmite solution was prepared by hydrolyzing aluminum tri-isopropoxide in a diluted nitric acid solution at 95◦ C. Then, an ethylene glycol solution of copper nitrate was added to the boehmite solution. A small amount of urea was added for gelation, if necessary. The gel was then transferred to a teflon-lined autoclave and heated at 95◦ C for 12 h, washed with ethanol, dried at room temperature for 2 days and at 100◦ C for 8 h, finally calcined in air at 800◦ C for 10 h. For comparison, a Cu-ZSM-5 catalyst was prepared by ion-exchange of a NH4 -form ZSM-5 zeolite (JRC-Z5-25H, a reference catalyst from the Catalysis Society of Japan) with a 0.5 M Cu(NO3 )3 solution at 80◦ C for 24 h. The copper content in the ion-exchanged Cu-ZSM-5 was 3.2 wt%. A 0.5 wt% Pt/Al2 O3 catalyst was prepared by the impregnation method. In order to investigate the effect of sulfur on the catalytic performance of the Cu-Al2 O3 catalyst prepared by different methods, 5 wt% SO2− 4 was deposited on the catalysts by impregnating the calcined Cu-Al2 O3 powder with a dilute H2 SO4 solution. Then
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Table 1 BET surface areas and pore volumes of 5 wt% Cu-Al2 O3 catalysts prepared by different methods Catalyst Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3
(I) (CO) (S) (X)
BET surface area (m2 /g)
Pore volume (ml/g)
Water adsorption (g/g.cat)
103.9 112.1 28.2 238.8
0.282 0.286 0.102 0.461
0.167 0.154 0.079 0.109
the catalysts were dried at 120◦ C, calcined at 550◦ C in air for 6 h before reaction tests were carried out. 2.2. Catalyst characterization BET surface area and pore volume were obtained from N2 adsorption isotherms measured at −196◦ C using a Bellsorp36 instrument. The BET surface areas and pore volumes of a few representative samples of differently prepared Cu-Al2 O3 catalysts are given in Table 1. XRD spectra were recorded using an MXP powder diffractometer with a Cu K␣ radiation and operated at 40 kV and 20 mA. UV-Vis absorption spectra were recorded using a U-3000 spectrophotometer in a similar way as reported previously for the solid sample [14]. TPR experiments were carried out using a H2 /Ar gas (5% H2 by volume) with a flow rate of 30 ml/min. The catalyst weight was 50 mg and the temperature ramp was 10◦ C per minute. Calibration was made by reduction of a known amount of pure CuO powder under the same experimental conditions. 2.3. NO reduction with propylene NO reduction with propylene in the presence of an excess amount of O2 was performed with a fixed-bed flow-type quartz reactor. Prior to the reaction, the catalyst (usually 0.2 g) was pretreated with flowing pure O2 at 500◦ C for 1 h, or treated in an oxygen flow containing 20% water at 600◦ C for 1 h if the catalyst was to be evaluated in the presence of water. The feed mixture consisted of 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , with or without 20% water, and N2 as a balance gas. The total gas flow rate was 145 ml/min, corresponding to a space velocity of about 15 000 h−1 . NO and NO2 concentrations were analyzed with a
HORIBA CLA-510SS NOx analyzer. Other products were analyzed using a SHIMADZU 8A GC equipped with a TCD. The concentrations of C3 H6 , CO2 and N2 O were analyzed by a Parapak Q column. A molecular sieve 5A column was used to monitor the formation of CO. Over all the catalysts tested, propylene was totally oxidized to CO2 and H2 O. NO was converted mainly to N2 with a small amount of NO2 and traces of N2 O and NH3 . The concentrations of NH3 and N2 O were lower than the detection limitation (less than 10 ppm) of our GC. No other N-containing compound was detected.
3. Results Fig. 1 shows the NO reduction conversions over the Cu-Al2 O3 catalysts prepared by different methods. It can be seen that there are some similar trends in NO reduction behaviours with the Cu content irrespective of the catalyst preparation method. That is, with the increase of the Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased, but the temperature required for the maximum NO reduction conversion shifted to the low temperature range. NO conversion at a high temperature decreased significantly with the increase of Cu loading. As a result, the operating temperature window for NO reduction became narrower. It is interesting to note that the catalyst preparation method exerts significant influence on the catalyst performance. Upon increasing the Cu content from 5 to 15 wt%, only a slight decrease in the maximum NO reduction conversion is observed for Cu-Al2 O3 (I), Cu-Al2 O3 (CO) and Cu-Al2 O3 (S) catalysts. However, there is a drastic decrease in conversion from 45 to 31% with the increase of the Cu content from 5 to 10 wt% for the Cu-Al2 O3 (X) catalyst. With the same Cu content, the activity of Cu-Al2 O3 catalyst toward NO reduction is also
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Fig. 1. NO reduction with C3 H6 over Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel. Catalyst weight: 0.2 g, gas flow rate: 145 ml/min. 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , N2 as a balance gas.
greatly dependent on the catalyst preparation method. For the Cu-Al2 O3 catalysts with 5 wt% Cu, the NO reduction activity decreases in the following order: Cu-Al2 O3 (I) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (X) > Cu-Al2 O3 (S). The impregnated catalyst shows the highest activity and the widest operating temperature window, followed by the co-precipitated catalyst. The Cu-Al2 O3 catalyst prepared from the evaporation of a mixed solution of Cu(NO3 )2 and Al(NO3 )3 shows a much lower activity than the impregnated and co-precipitated Cu-Al2 O3 catalysts.As it is found that the Cu-Al2 O3 catalyst prepared by the impregnation method exhibited the highest activity for NO reduction, further attention was paid to the influence
of the nature of alumina support on the catalytic performance of impregnated catalyst. It is shown that the purity of support alumina exerts significant influence on the catalyst activity. For the two reference ␥-alumina JRC-ALO-2- and JRC-ALO-4-supported catalysts, the activity of the Cu-Al2 O3 (JRC-ALO-4) is much higher than that of Cu-Al2 O3 (JRC-ALO-2) (Fig. 2). The BET surface areas of JRC-ALO-2 and JRC-ALO-4 are 285 and 177 m2 /g, respectively. Apparently, the observed difference in their activities is not due to the difference in BET surface area, although a higher BET surface area is considered to be advantageous to CuO dispersion. As these two supports have the same structure, the different activities
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Fig. 2. Influence of alumina source on NO conversion over impregnated 5 wt% Cu-Al2 O3 (I) catalysts. Alumina supports were (a) JRC-ALO-4, (b) ␥-Al2 O3 from calcination of Al(OH)3 at 800◦ C and (c)JRC-ALO-2. Reaction conditions were the same as those in Fig. 1.
Fig. 3. NO reduction by C3 H6 over the 0.5% Pt/Al2 O3 and Cu-ZSM-5 catalysts. Solid lines are the NO reduction conversions in the absence of water and dotted lines are those in the presence of 20% water. Reaction conditions were the same as those in Fig. 1 in the absence of water and those in Fig. 4 in the presence of water.
can possibly be attributed to the different purity of these alumina supports. It is known that the purity of JRC-ALO-4 is higher than JRC-ALO-2, which contains a significant amount of impurities like Fe, Si and Na, and 1.72% SO2− 4 . Apart from the influence of Fe, Si and Na species, it is supposed that a large amount of sulfur is also a poison to the catalyst, as a previous report showed that pre-sulfurized alumina had lower activity than pure alumina [15]. This conclusion is also in agreement with that of Iwamoto [16]. We also compared these Cu-Al2 O3 catalysts with the ion-exchanged Cu-ZSM-5 and Pt/Al2 O3 for NO reduction with C3 H6 under the same reaction conditions. The results of Cu-ZSM-5 and Pt/Al2 O3 are given in Fig. 3. By comparing Fig. 1 with Fig. 3, it is interesting to find that the Cu-Al2 O3 catalyst prepared by any of the aforementioned methods shows higher activity for NO reduction than the 0.3% Pt/Al2 O3 catalyst. Besides, the 0.3% Pt/Al2 O3 catalyst produced a sig-
nificant amount of N2 O (its selectivity is about 50%), which is a greenhouse effect gas. Over any Cu-Al2 O3 catalyst, NO was mainly converted to N2 with a small amount of NO2 , and traces of N2 O and NH3 . In comparison with the Cu-ZSM-5, all Cu-Al2 O3 catalysts showed higher activity at a relatively lower temperature than Cu-ZSM-5, whereas Cu-ZSM-5 exhibited high activity only at high temperature (450–500◦ C). This observation is in agreement with those of Hattori et al. [9]. Fig. 4 shows NO reduction conversion with C3 H6 in the presence of 20% water vapour over the differently prepared 5 wt% Cu-Al2 O3 catalysts, as a function of reaction temperature. By comparing of these results with those in the absence of water (Fig. 1), it is seen that the presence of water leads to a decrease in NO conversion and shifted the commencing temperature of NO reduction to the high temperature range. However, the presence of water showed only a
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much higher NO reduction activity than either the Pt/Al2 O3 or the Cu-ZSM-5. It is demonstrated that both the co-precipitated 5 wt% Cu-Al2 O3 and xerogel 5 wt% Cu-Al2 O3 are superior to the 0.5 wt% Pt/Al2 O3 and Cu-ZSM-5 catalysts. In the presence of water, the xerogel 5 wt% Cu-Al2 O3 catalyst exhibited the highest NO reduction conversion. The order of activities of these catalysts is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). The differently prepared Cu-Al2 O3 catalysts also showed different tolerance to sulfate deposition. Fig. 5 shows the NO reduction capabilities of sulfate-treated 5 wt% Cu-Al2 O3 catalysts. Comparing these results with those of the unsulfated catalysts in Fig. 1, it can be seen that the xerogel 5 wt% Cu-Al2 O3 shows a lesser influence by the deposition of SO2− 4 , whereas the activities of other catalysts decrease significantly. Therefore, it can be concluded that the xerogel 5 wt% Cu-Al2 O3 (X) is the best among the four differently prepared Cu-Al2 O3 catalysts for NO reduction with C3 H6 in the presence of water. Fig. 4. Influence of water on the catalytic activities of differently prepared 5 wt% Cu-Al2 O3 catalysts. Catalyst weight: 0.2 g, gas flow rate: 145 ml/min. 773 ppm NO, 579 ppm C3 H6 , 2.5% O2 , 20% water vapor, N2 as a balance gas.
minor influence on the xerogel 5 wt% Cu-Al2 O3 catalyst. Only a decrease of NO conversion at low temperature is observed. The maximum NO conversion decreases slightly from 45 to 41% in the presence of 20% water vapour. It is surprising to find that the activity of the 5% Cu-Al2 O3 (I), which is the highest in the absence of water, decreased substantially from 48.3 to 26.1% in the presence of water vapour. Comparison of the results in Fig. 4 with those of Pt/Al2 O3 and Cu-ZSM-5 (Fig. 3, dotted lines) reveals that the Cu-Al2 O3 catalysts prepared by impregnation and directly drying the solution are inferior to both Pt/Al2 O3 and Cu-ZSM-5, for NO reduction with C3 H6 in the presence of water. The Cu-Al2 O3 prepared by co-precipitation showed higher NO reduction conversion than Pt/Al2 O3 . Its maximum NO reduction conversion was comparable to that of Cu-ZSM-5, but the maximum NO reduction took place at a much lower temperature. It is of importance to note that the xerogel 5 wt% Cu-Al2 O3 catalyst exhibited a
Fig. 5. Influence of sulfate deposition on the catalytic activities of differently prepared 5 wt% Cu-Al2 O3 catalysts. Reaction conditions were the same as those in Fig. 1.
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Fig. 6. XRD spectra of Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel.
4. Discussion Cu-Al2 O3 catalysts prepared by different methods exhibited quite different activities for NO reduction with C3 H6 . The difference is possibly caused by the presence of different Cu species in these catalysts. Fig. 6 shows the XRD spectra of Cu-Al2 O3 catalysts prepared by the aforementioned four different methods. No CuO peak is detected for all the catalysts prepared by impregnation, co-precipitation and direct drying of the mixed aqueous solution methods, although the Cu content is as high as 15 wt%. For all the Cu-Al2 O3 catalysts except the xerogel Cu-Al2 O3 with a Cu content over 5 wt%, only a spinel struc-
ture is detected. The XRD peak intensities increase with increasing Cu content in the catalyst indicating an enhanced formation of the spinel phase, possibly CuAl2 O4 . For the xerogel 5 wt% Cu-Al2 O3 , only a ␥-Al2 O3 phase was detected. However, with a further increase in Cu content, CuO peaks are observed at 2θ of 35.6 and 38.8◦ . To gain further information on the state of Cu species in these catalysts, the UV-Vis spectroscopic studies are carried out and the spectra are shown in Fig. 7. Generally, three bands were observed for these Cu-Al2 O3 catalysts in the range of 210–350 nm, 400–500 nm and 600–900 nm. The band at 210–350 nm is attributed to the Cu2+ –O2− charge
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Fig. 7. UV-Vis absorption spectra of Cu-Al2 O3 catalysts prepared by different methods. (A) impregnation, (B) co-precipitation, (C) drying mixed solution, and (D) xerogel.
transfer [10]. The band at 400–500 nm is assigned to the three-dimensional Cu+ clusters in the CuO matrix in the literature [10]. However, as the catalysts were used and measured in the oxidized form, the existence of Cu+ species was less likely. In addition, bulk CuAl2 O4 also shows strong absorption in this region. We thus prefer to assign the band at 400–500 nm to the bulk-like CuAl2 O4 species. The band at 600–900 nm is assigned to d–d transitions of Cu2+ situated in an octahedral environment with an Oh symmetry [10]. The significant difference in the UV-Vis spectra between the Cu-ZSM-5 and Cu-Al2 O3 catalysts indicates that the Cu species in Cu-Al2 O3 are quite different from those in Cu-ZSM-5. For the impregnated Cu-Al2 O3 catalysts with a low Cu content, bands at 210 and
225 nm with a tail to 400 nm, and a weak band at 600–900 nm are observed. With the increase of the Cu content, the Cu2+ –O2− charge transfer band shifted to a higher wavelength range and the intensity of the band at 600–900 nm slightly increased. In addition, a new band at 400–500 nm was observed if the Cu content was above 10 wt%. The appearance of this band is an indication of the existence of new Cu species, possibly the formation of bulk-like spinel CuAl2 O4 species [17]. This suggestion is in agreement with the previous observation that the intensity of XRD peaks increased with the increase of Cu loading. For the Cu-Al2 O3 catalysts prepared by other methods, a strong band at 600–900 nm is always observed. This implies that a significant amount of octahedral Cu2+
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species is present even in the Cu-Al2 O3 catalyst with a 5 wt% of Cu. Upon increasing the Cu content, the minimum of the low wavelength side of this band shifts to the high wavelength region. This is an indication of a less tetragonal distortion of the Cu species in the octahedral environment with the increase of Cu content in the catalysts [10,17]. For the Cu-Al2 O3 catalysts prepared from drying the solution, the UV-Vis spectra are different from those of other catalysts. It shows a broad band from 220 to 500 nm in addition to the sharp band at 215 nm. The observed tailing in the region 400–500 nm and a relatively weak absorption band around 200–300 nm for the Cu-Al2 O3 (S) catalysts imply that bulk-like CuAl2 O4 or CuO species are possibly present in this type of catalysts. The Cu-Al2 O3 system has been extensively investigated owing to its potential application as a catalyst in various types of reactions. It is known that Cu loading and calcination temperature are the most crucial factors for the catalyst structure [17,18]. At low Cu loading and calcination temperature below 500◦ C, formation of a surface spinel which resembles CuAl2 O4 , predominates. At high Cu loading and low calcination temperature, bulk-like CuO forms. A higher calcination temperature like 900◦ C leads to the formation of bulk CuAl2 O4 . In the present study, since the calcination temperature is fixed at 800◦ C, it is more probable that the surface spinel CuAl2 O4 or bulk CuAl2 O4 species would predominate in our Cu-Al2 O3 catalysts. XRD and UV-Vis results reveal that isolated Cu2+ , tetrahedrally distorted octahedral Cu2+ and surface CuAl2 O4 species exist in all the Cu-Al2 O3 catalysts used in this study. The difference possibly is the different amount of these Cu species in each catalyst. The absence of CuO in most of these catalysts even with a Cu loading as high as 15 wt% could be attributed to the high calcination temperature. Another observation is that the UV-Vis spectra of impregnated Cu-Al2 O3 catalysts are quite different from those of other catalysts. That is, the octahedral Cu2+ absorption band at 600–900 nm is rather weak and the 210–350 nm bands are strong. This possibly implies that the concentration of octahedral Cu2+ species in these catalysts is lower compared to other catalysts, and the isolated Cu2+ species would predominate. Upon increasing the Cu content to 10 wt%, bulk CuAl2 O4 species could be formed. For the Cu-Al2 O3 catalysts prepared by the other three methods, while some isolated Cu2+
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Fig. 8. TPR spectra of 5 wt% Cu-Al2 O3 catalysts prepared by different methods. (a) impregnation , (b) co-precipitation, (c) drying mixed solution, and (d) xerogel.
species existed, a significant amount of distorted octahedral Cu2+ species, in the form of surface CuAl2 O4 species, are detected. In order to further elucidate the nature of Cu species in these catalysts, a Temperature-Programmed Reduction (TPR) experiment was performed. The TPR profiles are shown in Fig. 8, and the analysis results are given in Table 2. A sharp reduction peak appeared at around 200◦ C for the 5 wt% Cu-Al2 O3 (CO), Cu-Al2 O3 (I) and Cu-Al2 O3 (S) catalysts. However, for the xerogel Cu-Al2 O3 , the major reduction peak appeared at a higher temperature. At 222◦ C, only a rather weak peak is observed. According to literature reports [7,12], the reduction peak at around 200◦ C can be attributed to the reduction of isolated Cu2+ species to Cu+ and partial reduction of surface CuAl2 O4 to CuAlO2 . For the Cu-Al2 O3 prepared from the drying solution and xerogel methods, a peak at 270◦ C appeared. This peak is assigned to the reduction of amorphous bulk-like CuO, which could not be detected by XRD [19]. This is in agreement with the previous suggestion from the UV-Vis results.
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Table 2 TPR results of 5 wt% Cu-Al2 O3 catalysts prepared by different methods Catalyst Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 Cu-Al2 O3 a
(I) (CO) (S) (X)
Temperature of TPR peak
H2 /Cu below 400◦ C
209◦ C, 230◦ C (shoulder) 220◦ C 213◦ C, 270◦ C 222◦ C, 270◦ C, 318◦ C, 392◦ C
0.57 0.78 0.65 0.91a
Below 500 ◦ C
For the xerogel 5 wt% Cu-Al2 O3 catalyst, a sharp reduction peak appeared at 318◦ C. This peak did not appear for other catalysts indicating that a new type of Cu species probably exists in this catalyst. As the temperature of this peak is close to that of bulk CuO, we tentatively assign this peak to the reduction of the CuO species which strongly interacted with the alumina support, or highly dispersed CuO species in the alumina matrix. For the 5 wt% Cu-Al2 O3 (CO), Cu-Al2 O3 (I) and Cu-Al2 O3 (S) catalysts, a significant amount of Cu species could not be reduced at temperature below 500◦ C (Table 2). These un-reduced Cu species are possibly the supported Cu+ species from the partial reduction of isolated Cu2+ [20], or CuAlO2 species from surface CuAl2 O4 [21–24], whose reduction needs a higher temperature. For the Cu-Al2 O3 catalyst calcined at a high temperature (800◦ C), Hattori et al. [9] attributed the high activity of the catalyst to the surface aluminate phase. The correlation between copper species concentration profile analyzed by XANES and catalytic activity indicated that highly dispersed CuO-like aggregates are the active centres where NOx molecules are activated [12,25]. The reaction results in Fig. 1 show that the NO conversions over the impregnated, co-precipitated and xerogel 5 wt% Cu-Al2 O3 catalysts are not of much difference. Therefore, it is difficult to draw a conclusion on what are the active sites in our catalysts. It seems that the isolated Cu2+ species, surface CuAl2 O4 and highly dispersed CuO are all active toward NO reduction by propylene. However, it is certain that a Cu-Al2 O3 catalyst containing a significant amount of bulk CuAl2 O4 or bulk CuO would show low activity. The non-isolated copper ions (copper oxides) were found to be more active for the C3 H8 oxidation by O2 than for the reaction of C3 H8 with NO [26]. In the case of NO reaction with C3 H6 , a similar preference is plausible. Thus the appearance of bulk CuO
phase interprets the low NO reduction conversion of xerogel Cu-Al2 O3 (X) with a Cu content over 5 wt%. The low activity of the Cu-Al2 O3 prepared from drying the mixed solution can be attributed to the extremely low surface area and pore volume of this type of catalysts (Table 1), as it is shown that diffusion limitation is a key factor that affects the reaction rate [27]. In the presence of water, the activity order of the Cu-Al2 O3 catalysts with a 5 wt% Cu is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). This order is different from the one obtained in the absence of water. It is of importance to note that the presence of water showed only a minor influence on the xerogel 5 wt% Cu-Al2 O3 catalyst at a low temperature. However, the impregnated 5 wt% Cu-Al2 O3 (I) catalysts, which showed the highest activity in the absence of water, exhibited a very low NO reduction conversion in the presence of water. The simple interpretation for the decrease of NO reduction conversion in the presence of water is that water competes with NO and/or C3 H6 for the same adsorption sites. NO conversion decreases because water inhibits the adsorption of NO and/or C3 H6 . Kim et al. [28] have reported that the large hydrophobicity of zeolite plays a critical role in the activity loss of SCR by water. The higher water tolerance of Ga2 O3 /Al2 O3 is explained by the lower affinity of water for Al2 O3 than for ZSM-5 [29]. For the xerogel 5 wt% Cu-Al2 O3 catalyst, the major active centers are possibly these highly dispersed CuO species. It is reasonable to assume that water adsorption on these CuO species is difficult than it does on other kinds of Cu species. Furthermore, the xerogel Cu-Al2 O3 showed a lower water adsorption capacity compared to other catalysts (Table 1). The low affinity of water of this catalyst is possibly the reason why NO reduction is less affected by the presence of water.
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5. Conclusions The activity of the Cu-Al2 O3 catalyst for NO reduction by C3 H6 is largely dependent on the method of catalyst preparation and Cu content. For the Cu-Al2 O3 catalysts prepared by impregnation, co-precipitation and evaporation of a mixed solution, the maximum NO reduction conversion decreased slightly and the commencing temperature of NO reduction shifted to a low temperature with the increase of Cu content from 5 to 15 wt%. For the xerogel Cu-Al2 O3 , there was a significant decrease in NO reduction with the increase of Cu content from 5 to 10 wt%. For the impregnated Cu-Al2 O3 catalyst, the purity of alumina support is a crucial factor in determining catalyst activity. In the absence of water, the activity order of the catalysts with the same Cu content is: Cu-Al2 O3 (I) > Cu-Al2 O3 (CO)>Cu-Al2 O3 (X)>CuAl2 O3 (S). Although the impregnated Cu-Al2 O3 catalyst exhibited the highest activity in the absence of water, there was a substantial decrease in its NO reduction activity by the presence of water. In contrast, the xerogel 5 wt% Cu-Al2 O3 (X) was less affected by the presence of water and showed the highest activity. In the presence of water, the activity order of these catalysts is: Cu-Al2 O3 (X) > Cu-Al2 O3 (CO) > Cu-Al2 O3 (I) > Cu-Al2 O3 (S). The xerogel 5 wt% Cu-Al2 O3 catalyst is also found to be more tolerant to sulfate deposition.
Acknowledgements L.Y. Chen is grateful to Japan International Science and Technology Exchange Center (JISTEC) for an STA fellowship. We also thank Dr. S. Velu for some helpful discussions and corrections.
References [1] A. Fritz, V. Pitchon, Appl. Catal. B. 13 (1997) 1. [2] V.I. Pârvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. [3] M. Iwamoto, H. Hamada, Catal. Today 10 (1991) 57.
269
[4] H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito, Appl. Catal. 75 (1991) L1. [5] M. Ozawa, H. Toda, O. Kato, S. Suzuki, Appl. Catal. B 8 (1996) 123. [6] M. Ozawa, S. Suzuki, H. Toda, J. Am. Ceram. Soc. 80 (1997) 1957. [7] J.A. Anderson, C. Márques-Alvarez, M.J. Lopez-Muñoz, I. Rodr´ıguez-Ramos, A. Gurrero-Ruiz, Appl. Catal. B 14 (1997) 189. [8] F. Radtke, R.A. Koeppel, E.G. Minardi, A. Baiker, J. Catal. 167 (1997) 127. [9] K. Shimizu, H. Maeshima, A. Satsuma, T. Hattori, Appl. Catal. B 18 (1998) 163. [10] H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B 6 (1998) 359. [11] Z. Chajar, VC.L. Chanu, M. Primet, H. Praliaud, Catal. Lett. 52 (1998) 97. [12] M. Fernández-Garc´ıa, I. Rodriguez-Ramos, P. Ferreira-Aparicio, A. Guerrero-Ruiz, J. Catal. 178 (1998) 253. [13] T. Osaki, T. Horiuchi, T. Sugiyama, S. Suzuki, T. Mori, Catal. Lett. 52 (1998) 171. [14] L. Chen, S. Jaenicke, G.K. Chuah, H.G. Ang, J. Electron. Spectrosc. Rel. Phen. 82 (1996) 203. [15] R. Burch, E. Halpin, J.A. Sullivan, App. Catal. B 17 (1998) 115. [16] N. Okazaki, Y. Shiina, H. Itoh, A. Tada, M. Iwamoto, Catal. Lett. 49 (1997) 169. [17] R.M. Friedman, J.J. Freeman, J. Catal. 55 (1978) 10. [18] B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal. 94 (1985) 514. [19] W.P. Dow, Y.P. Wang, T.J. Huang, J. Catal. 160 (1996) 155. [20] C. Torre-Abreu, M.F. Ribeiro, C. Henriques, G. Delahay, Appl. Catal. B 14 (1997) 261. [21] J.Y. Yan, G.D. Lei, W.M.H. Sachtler, H.H. Kung, J. Catal. 161 (1996) 43. [22] S. Sato, M. Iijima, T. Nakayama, T. Sodesawa, F. Nozaki, J. Catal. 169 (1997) 447. [23] F. Severino, J.L. Brito, J. Laine, J.L.G. Fierro, A.L. Agudo, J. Catal. 177 (1998) 82. [24] A. Alejandre, F. Medina, A. Fortuny, P. Salagre, J.E. Sueiras, Appl. Catal. B 16 (1998) 53. [25] C. Márquez-Alvarez, I. Rodr´ıguez-Ramos, A. Gurrero-Ruiz, G.L. Haller, M. Fernández-Garc´ıa, J. Amer. Chem. Soc. 119 (1997) 2905. [26] Z. Chajar, M. Primet, H. Praliaud, J. Catal. 180 (1998) 279. [27] K. Katagi, A. Shichi, S. Komai, A. Satsuma, T. Hattori, Preprints of 82th Catalysis Conference, Catalysis Society of Japan, September, 1998, p.135. [28] M.H. Kim, I.S. Nam, Y.G. Kim, in: H. Chon, S.K. Ihm, Y.S. Uh (Eds.), Progress in Zeolite and Microporous Materials, Stud. Surf. Sci. Catal. 105(1997)1493 [29] K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B 16 (1998) 319.