NO reduction with NH 3 over an activated carbon-supported copper oxide catalysts at low temperatures

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1 Applied Catalysis B: Environmental 26 (2000) NO reduction with NH 3 over an activated carbon-supported copper oxide catalysts at low temperatures Zhenping Zhu a, Zhenyu Liu a,, Shoujun Liu a, Hongxian Niu a, Tiandon Hu b, Tao Liu b, Yaning Xie b a State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyuan, , PR China b State Key Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, , PR China Received 18 October 1999; received in revised form 9 December 1999; accepted 9 December 1999 Abstract NO reduction with NH 3 over activated carbon-supported copper oxide (CuO/AC) catalyst was studied at low temperatures ( C). The attention was focused on the effects of preparation parameters, reaction temperature and SO 2 on the structure and activity of the catalyst. The catalysts show high activities for NO reduction with NH 3 in the presence of O 2 at temperatures above 180 C and are gradually deactivated at temperatures below 180 C. Cu 2 O exists in the catalyst and results in low initial activity, but it is easily oxidized into active CuO by O 2 during the NO NH 3 O 2 reaction. Calcination temperature and Cu loading of the catalyst strongly influence the activity and structure of the catalyst. The catalyst with 5 wt% Cu loading and calcined at 250 C shows the highest activity. The activities of the catalysts with higher Cu loadings and/or calcined at higher temperatures are relatively low, mainly derived from aggregation of copper species. The CuO/AC catalyst is greatly deactivated by SO 2 due to the formation of CuSO 4 which is inactive at low temperatures Elsevier Science B.V. All rights reserved. Keywords: CuO/AC catalyst; Nitric oxide; Reduction; Ammonia; EXAFS 1. Introduction NO x, mainly formed from combustion of fossil fuels, is very harmful for the ecosystem and humanity. A well-proven technique to remove NO x from the stationary sources is the selective catalytic reduction (SCR) of NO with NH 3 in the presence of oxygen [1,2]: 4NO + 4NH 3 + O 2 4N 2 + 6H 2 O For this reaction, some effective catalysts, such as V 2 O 5 WO 3 /TiO 2 [3], have been developed and suc- Corresponding author. Fax: address: zyl@public.ty.sx.cn (Z. Liu). cessfully commercialized due to their high activities and low sensitivity towards sulfur dioxide poisoning. However, these catalysts suffer from many disadvantages [4]. Due to high operating temperature (>350 C), the catalyst bed must be located at upstream of the desulfurizer and/or the particulate control device to avoid re-heating of the flue gas and thus is subject to deactivation from high concentration of sulfur dioxide and dust. Additionally, the retrofit of the SCR device into existing systems for flue gas cleaning is costly because space and access in many power plants are extremely limited. Therefore, it is needed to develop low-temperature SCR catalysts so that the catalyst bed can be located at the downstream of the desulfurizer and/or the particulate control /00/$ see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 26 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Table 1 The physical characteristics of the AC measured by N 2 adsorption at 77 K Activated carbon BET surface area (m 2 /g) Micropore area (m 2 /g) Total pore volume (ml/g) Micropore volume (ml/g) Un-oxidized Oxidized by nitric acid device, at which the temperature is in the range of C. Various catalysts, such as metal halide supported on carbon [5] and alumina [6], carbon-supported copper oxide [7,8], alumina-supported manganese oxides [9], titanium silicalites [10], amorphous chromia [11] and activated carbon [12] show high activity for NO reduction with NH 3 at low temperatures. Among these catalysts, activated carbon is mostly studied and has been commercialized [12]. However, in the temperature range of interest ( C) activated carbon shows low activity, which must be compensated by using larger volume of catalyst resulting in increased capital and operating cost. In contrast, carbon-supported copper oxide catalyst has a high activity for NO reduction with NH 3 in that temperature range [7,8]. In addition, the activated carbon-supported copper oxide (CuO/AC) catalyst, as a sorbent and a catalyst, shows high SO 2 removal efficiency in the same temperature range [13]. More importantly, Singoredjo et al. [8] showed that CuSO 4 supported on AC exhibits considerable activity for the SCR reaction at temperatures of C, although lower than that of the CuO/AC catalyst. If it is true, the CuO/AC catalyst may be used in the SCR of NO in the presence of SO 2, and used in simultaneous SO x and NO x removal at low temperatures (<250 C), as fully studied CuO/A1 2 O 3 catalyst at temperatures of above 350 C [14,15]. However, the previous reports [7,8] only dealt with some activity measurements in the absence of SO 2 for the CuO/AC catalyst. No information is available about the catalyst structures and SO 2 effect. Therefore, further investigations are needed to evaluate the possibility of the CuO/AC catalyst being used in simultaneous SO x and NO x removal at low temperatures (<250 C) and to understand the nature of the catalyst. For these purposes, the effects of preparation parameters, reaction temperature and SO 2 on the activity of the CuO/AC catalyst are studied in the present work. To understand the nature of the effects, the catalysts are characterized by X-ray absorption fine structure (XAFS) technique. 2. Experimental 2.1. Catalyst preparation The support, activated carbon (AC), was prepared from a commercial coal-derived semicoke (Datong Coal Gas Co., China) through steam activation at about 900 C. Before use, the AC was generally pre-oxidized with concentrated HNO 3 (3 ml/g AC) at 60 C for 1 h, followed by filtration, full washing with distilled water and drying at 120 C for 5 h. The physical characteristics of the AC with and without pre-oxidization by nitric acid, measured by N 2 adsorption at 77 K, are shown in Table 1. Some of the AC was also pre-oxidized with concentrated H 2 SO 4 at 200 C for 2 h. CuO/AC catalysts were prepared by pore volume impregnation of the AC with an aqueous solution of Cu(NO 3 ) 2 3H 2 O. The AC pre-oxidized by HNO 3 was used in the preparation of the CuO/AC catalysts unless specially mentioned. Cu loadings in the catalyst were determined by the used concentration of Cu(NO 3 ) 2 3H 2 O. After impregnation, the catalysts were dried overnight at 50 C and then at 120 C for 5 h, followed by calcination in Ar stream for 2 h at desired temperatures. To understand the effect of the AC pre-treatment, the CuO/AC catalysts were also prepared from the un-oxidized AC and the H 2 SO 4 pre-oxidized AC XAFS measurement To determine the chemical forms of the copper species in the CuO/AC catalysts, XAFS spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure

3 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) (XANES), was used, and measured on wiggler beam line 4W1B at the Beijing Synchrotron Radiation Facility (BSRF). A double crystal Si(111) was used to monochromatize X-rays from the 2.2 GeV electron storage ring with an average ring current of 80 ma. The Cu K-edge absorption spectra were recorded in the transmission mode in the range of photon energies from 8780 to 9800 ev at an interval of 0.5 ev in the XANES region and at an interval of 3 ev in EAXFS region. Before measurement, the samples were broken into fine particles less than 200 mesh and coated onto Scotch tape. EAXFS data analyses are performed following a standard procedure [16]. Fourier transformation was performed on the k 3 -weighted EXAFS oscillation in the range of 3 14 Å. The structural parameters of copper species in CuO/AC catalysts were calculated by r-window adding, Fourier-filtering from r-space into k-space, and curve-fitting in k-space. The curves were fitted with a single or double model based on the least-squares method and using crystalline Cu 2 O and metallic Cu as primary standards for the approximation of phase shift and amplitude functions. 3. Results and discussion 3.1. XAFS characterization of the catalysts To understand the form and structure of the copper species in the CuO/AC catalysts prepared under different conditions, Cu K-edge XAFS (XANES, EX- AFS) was used for local structure analysis of the Cu species. The XANES spectra of the catalysts with Cu loading of 5 wt% and calcined at 180, 250, 350 and 450 C are shown in Fig. 1, along with those of Cu, Cu 2 O and CuO. Both Cu and Cu 2 O exhibit a low energy peak in the region between ev, which has been assigned as a Cu ls 4p transition [17 19]. The X-ray absorption pre-edge feature of CuO also shows a peak at about 8987 ev, but its intensity is much weaker than those in the spectra of Cu and Cu 2 O. This is in agreement with the observations in the literature [17 19], although the assignment of the weak pre-edge peak remains controversial and could be either 1s 4s, 1s 4p, or 1s 4p simultaneous with a ligand-to-metal shake-down transition [19]. In addition, after the absorption maximum, the absorption intensities of Cu and Cu 2 O show a little decrease, while 2.3. Activity test The activity tests of the catalysts for the NO+NH 3 +O 2 reaction were carried out in a fixed-bed quartz reactor of 8 mm i.d. and 350 mm length. 0.5% NO in Ar, 0.5% NH 3 in Ar and 15% O 2 in Ar were used as the source of the flue gas components, and Ar was used as the balance gas. Gas flow rates were controlled by mass flow controllers for NO/Ar and NH 3 /Ar and by rotary flow controllers for O 2 /Ar and Ar. In all the test runs, the total gas flow rate was controlled at 300 ml/min. Prior to the reactor entrance, the feed gases were mixed in a chamber filled with glass wool. For the experiments of SO 2 influence, 0.16% SO 2 in Ar was fed to replace the same volume of Ar. To avoid reaction between SO 2 and NH 3 in the mixing chamber and tubing, NH 3 /Ar was fed directly into the reactor. The concentrations of NO, NO 2,SO 2 and O 2 at both the inlet and the outlet of the reactor were continuously analyzed by an on-line Flue Gas Analyser (KM9006 Quintox, Kane International), which is equipped with NO, NO 2, SO 2 and O 2 sensors for simultaneous analysis. Fig. 1. XANES profiles of Cu, Cu 2 O, CuO and some CuO/AC catalysts with Cu loading of 5 wt% and calcined at different temperatures.

4 28 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Fig. 2. The Cu K-edge EXAFS DSF of some copper compounds. that of CuO decreases more sharply. Compared with the XANES features of Cu, Cu 2 O, and CuO, those of the four catalyst samples are less structured, especially at the post-edge, which indicate that the copper particle on the AC surface are smaller in size than those in metallic Cu, Cu 2 O crystal, or CuO crystal. For the 180 and 250 C samples, no pre-edge peak is observed, but it can be estimated that the copper species in them exist mainly as Cu 2+ because their sharply decreased absorption features are similar to that of CuO. Similar to the XANES features of Cu and Cu 2 O, the spectra of the 350 and 450 C samples show a pre-edge peak at about 8984 ev, which suggests that Cu + and/or Cu 0 exist in the two samples. Fig. 2 shows the radial structure functions (RSF) of some pure compounds, Cu, Cu 2 O, CuO, CuSO 4 5H 2 O and H 2 O-free CuSO 4, obtained by k 3 -weighted Fourier transformation. Metallic Cu shows a strong peak at about 2.10 Å corresponding to the Cu Cu distance. Cu 2 O shows two strong peaks at about 1.48 and 2.60 Å corresponding to the Cu O and Cu Cu distances, respectively. CuO shows three distinct peaks at about 1.56 Å (Cu O), 2.57 Å (Cu Cu) and 2.99 Å (Cu Cu). These features of Cu, Cu 2 O, and CuO are in agreement with the observations by Fukumi et al [17]. Both CuSO 4 5H 2 O and H 2 O-free CuSO 4 show a strong peak at 1.56 Å (Cu O), which is similar to that of CuO, and hence it is difficult to differentiate them from CuO when the latter is well dispersed on the support surface. Note that in the RSF spectra (also in the following ones of catalysts) all data are presented without any correction for phase shift, and hence the distance indicated along the abscissa does not represent the true distance between copper and the neighboring atoms. Fig. 3 illustrates the RSF spectra of the catalysts calcined at 180, 250, 350 and 450 C. The RDF spectrum of the un-oxidized sample (without calcination) is also shown as a reference. The un-oxidized sample and the one calcined at 180 C show a strong peak at 1.56 Å, corresponding to the Cu O distance in CuO. With increasing calcination temperature this peak becomes weaker and shifts to 1.6 Å, meanwhile a new peak appears at about 1.4 Å and becomes stronger with temperature. Additionally, a strong peak appears at about 2.1 Å for the 350 and 450 C samples. These observations indicate that with increasing calcination temperature part of CuO is reduced by the activated carbon into Cu 2 O and subsequently into metallic Cu. In the 180 C sample copper mainly exists as CuO, in the 250 C sample as CuO (may be dominant) and Cu 2 O, and in the 350 and 450 C samples as CuO, Cu 2 O and metallic Cu. It can be estimated that metallic Cu is the dominant species in the 450 C sample. Interestingly, in the high-r region (>3 Å) of RSF, the Fig. 3. The Cu K-edge EXAFS DSF of the CuO/AC catalysts with 5 wt% Cu loading and calcined at different temperatures.

5 Fig. 4. The Cu K-edge EXAFS DSF of the CuO/AC catalysts with different Cu loadings and calcined at 250 C. Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Cu as primary standards for the approximation of structural parameters. In most cases, good fitting was obtained. Two typical fitted curves from Cu O and Cu Cu shells for the sample with 5 wt% Cu loading and calcined at 350 C are shown in Fig. 5A,B, respectively. Table 2 presents the calculated interatomic distance (R), the number of neighbours (N) and the Debye Waller factor ( σ 2 ) of the CuO/AC catalysts. These data are in agreement with the qualitative descriptions shown above. It should be pointed out that during the calculations of Cu O shell a single shell fitting can result in good fitting for the catalysts with high Cu loadings (10 and 20 wt%), but a double-shells fitting is needed for the catalysts with lower Cu loadings (<10 wt%) due to the co-existence of CuO and Cu 2 O in the catalysts. For example, although the cat- samples calcined at temperatures below 250 C show very weak peaks, which is attributed to the small particle size of the copper species in the catalysts. On the other hand, the samples calcined at temperatures above 350 C show stronger peaks suggesting a grown particle of the copper species in these catalysts. In conclusion, the copper species loaded on the AC surface is obviously reduced (by carbon) and aggregated at calcination temperatures of above 350 C. The RSF spectra of the samples with Cu loadings of 5, 10, and 20 wt% and calcined at 250 C, are shown in Fig. 4. The sample with Cu loading of 5 wt% shows double peaks at 1.4 and 1.6 Å and very low intensity in the high-r region, indicating that part of CuO is reduced by carbon into Cu 2 O during calcination and that both CuO and Cu 2 O are well dispersed on the AC surface. Unlike this, the samples with Cu loading of 10 and 20 wt% show three peaks at 1.56, 2.6 and 3.0 Å similar to crystalline CuO (see Fig. 2), suggesting that in the cases of higher Cu loadings the CuO species are hardly reduced but significantly aggregated, and the aggregation gets heavy with increasing copper loading as indicated by the increased intensities of the peaks at 2.6 and 3.0 Å and in the high-r region. The structural parameters of the copper species in CuO/AC catalysts were calculated through adding r-window, Fourier-filtering and curve-fitting in k-space, and using crystalline Cu 2 O and metallic Fig. 5. Fourier-filtered EXAFS k 3 χ (k) vs. k(solid) and the fitted curves (dashed) for the (A) Cu O shell and (B) Cu Cu shell of the CuO/AC catalyst with copper loading of 5 wt% and calcined at 350 C.

6 30 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Table 2 The structural parameters of the copper species in the CuO/AC catalysts Sample Calcination temperature ( C) Cu loading (wt%) Cu O shell Cu Cu shell a R (Å) N σ 2 R (Å) N σ 2 Cu Cu 2 O CuO CuO/AC Un-calcined After SCR reaction b After pre-oxidation c a The Cu Cu shell is one in metallic Cu. b The catalyst was used for NO NH 3 O 2 reaction at 180 C for 2 h. c The catalyst was pre-oxidized by 10 vol% O 2 /Ar at 180 C for 30 min. alyst with 5 wt% Cu loading and calcined at 180 C shows a single peak (see Fig. 3) associating with the Cu O distance in CuO, a good fitting was obtained only by a double-shells fitting with R-values of 1.83 and 1.98 Å, which suggests the existence of Cu 2 O along with CuO. the range of C suggests SCR reaction over the CuO/AC catalyst occurs mostly at carbon sites in this temperature range. In other words, the copper species may be inactive at such low temperatures. This suggestion is supported by the observation that in the range of C the activity of the CuO/AC catalyst 3.2. Effect of reaction temperature and preparation parameter on the activity Fig. 6 shows NO conversion over the un-oxidized AC and AC-supported copper oxide catalyst (5 wt% Cu, calcined at 250 C) at reaction temperatures from 30 to 250 C. In the case of the AC, NO conversion monotonously decreases from 80 to 20% with increasing temperature in the whole temperature range. In contrast, the dependence of the CuO/AC activity on temperature is in a double way. In the range of C, the activity of the CuO/AC catalyst, similar to that of the AC, decreases significantly with increasing temperature, but, in the range of C, it increases greatly with temperature. This suggests that the SCR reaction over the CuO/AC catalyst follows different mechanisms in different temperature ranges. The similar behavior of the CuO/AC and the AC in Fig. 6. NO conversion vs. temperature over the AC and CuO/AC catalyst. Reaction conditions: 500 ppm NO; 560 ppm NH 3 : 3.3 vol% O 2 : Ar balance; WHSV, 1000 h 1.

7 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) is lower than that of the AC at the same temperatures, which is possibly due to part of carbon surface being covered by the copper species. In addition, it has been suggested that on carbon surface the SCR reaction follows a mechanism controlled by the adsorption of reactates, NH 3, NO and O 2, especially NH 3 [20]. Such a mechanism results in the relationship between activity and temperature as shown in the range of C in Fig. 6. On the other hand, in the range of above 90 C, the dependence of CuO/AC activity on temperature suggests that the mechanism is distinct from that in lower temperatures. SCR reaction over the CuO/AC catalyst may proceed mainly at copper sites and thinly at carbon sites. The reaction at copper sites may be controlled by surface reaction steps. In this way, the reaction rate will increase with increasing temperature as shown by the CuO/AC catalyst in the range of above 90 C in Fig. 6. More importantly, the CuO/AC catalyst shows much higher activity than the AC in the interesting range of C. To improve the activity of the CuO/AC catalyst, especially at temperatures around 150 C, the support AC was pre-oxidized by nitric or sulfuric acids before the impregnation of copper nitrate. The effects of pre-oxidation on the catalytic activity are shown in Fig. 7. Nitric acid pre-oxidation significantly enhances catalytic activity, with NO conversion from 27, 39, and Fig. 7. The effect of AC pre-oxidization on the CuO/AC catalytic activity. (a) un-oxidized AC, (b) HNO 3 -pre-oxidized AC, (c) H 2 SO 4 -pre-oxidized AC. Reaction conditions: 500 ppm NO; 560 ppm NH 3 ; 3.3 vol% O 2 ; WHSV, h 1 ; catalyst, 5 wt% Cu loading and calcined at 250 C. 72% to 50, 78, and 93% at 90, 120, and 150 C, respectively. At temperatures above 180 C, NO is completely converted. Unlike the HNO 3 pre-oxidation, H 2 SO 4 pre-oxidation shows a negative effect on the activity; NO conversion is much lower than that of the un-oxidized sample in the whole used temperature range. The increased catalytic activity by HNO 3 preoxidation possibly results from the increase of oxygen-containing functional groups on the AC surface, which can be formed by HNO 3 oxidation [21] and, may lead to improved dispersion of copper species on the AC surface through an interaction between them and the metal ions during the impregnation of copper nitrate [22]. Note that the change in the surface area of the AC caused by HNO 3 pre-oxidation does not contribute to increased activity because HNO 3 pre-oxidation significantly reduces the BET surface area as shown in Table 1. For the case of H 2 SO 4 pre-oxidation, the catalytic activity was expected to increase because H 2 SO 4 oxidation can also increase oxygen-containing functional groups on the AC surface [23,24], but the result is opposite to it. The reason may be that some H 2 SO 4 molecules remain in the pore of AC and/or that some sulfonic or sulfinic groups are formed through reactions of H 2 SO 4 and carbon under the employed pre-treatment conditions. During the impregnation of copper nitrate these sulfur-containing groups (or molecules) may react with Cu 2+ ions to form copper sulfate, which remains during the catalyst calcination (at 250 Cin Ar stream) and hence reduces the catalytic activity. This conjecture is supported by two facts: (a) after the 180 C reaction shown in Fig. 7, the catalyst prepared from the H 2 SO 4 -pre-oxidized AC was heated in Ar stream of 300 m1/min from 180 to 400 C, during which a considerable amount of SO 2 is released; (b) as shown below, CuSO 4 shows rather low SCR activity at the used temperature range. Based on the above results, CuO/AC catalysts prepared from the HNO 3 -pre-oxidized AC are further studied below. Experiments showed that the CuO/AC catalyst deactivates gradually with reaction time at temperatures below 180 C, but the deactivation disappears at higher temperature (Fig. 8). As suggested by the reaction of pre-adsorbed NO and gas phase NH 3 over the same catalyst [25], the deactivation of the catalyst at low temperatures is due to the formation of some special

8 32 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Fig. 8. NO conversion vs. reaction time over the CuO/AC catalyst at different temperatures. Reaction conditions: 500 ppm NO; 560 ppm NH 3 ; 3.3 vol% O 2 ; WHSV, h 1 ; catalyst, 5 wt% Cu loading and calcined at 250 C. NO species (possibly nitrates) on the catalyst surface, which are inactive at low temperature and hence gradually cover the active sites during the SCR reaction. Fig. 8 also shows that the activity of the catalyst is initially low, but quickly increases with reaction time and reaches a steady-state value in about 25 min. A similar phenomenon was also observed for other prepared catalysts, especially for those calcined at temperatures of 350 and 450 C. The low initial activity of the catalysts may be derived from the existence of Cu 2 O and/or Cu in the catalysts as suggested by EX- AFS results (Fig. 3 and Table 2). During NO NH 3 O 2 reaction Cu 2 O and/or Cu are oxidized by O 2 (and NO) into active CuO and hence result in the increased activity. Fig. 9a,b show the Cu K-edge EXAFS DSF spectra of the catalysts before and after the NO NH 3 O 2 reaction, respectively. The relevant structural parameters of the copper species are presented in Table 2. Unlike the catalyst before reaction, the catalyst after reaction shows a single peak at 1.56 Å, indicating the existence of CuO only. To further verify it, the prepared catalyst was pre-oxidized by 10 vol% O 2 /Ar at 180 C for 30 min before the NO NH 3 O 2 reaction. As expected, the resulting catalyst contains only CuO as shown in Fig. 9c and Table 2, and shows high activity in whole reaction process. These results indicate that the active phase of the CuO/AC catalyst for the Fig. 9. The Cu K-edge EXAFS DSF spectra of the CuO/AC catalysts with 5 wt% Cu loading and calcined at 250 C. (a) original, (b) after two hours reaction of No NH 3 O 2 at 180 C, (c) after pre-oxidization by 10 vol% O 2 /Ar at 180 C for 30 min. NO NH 3 O 2 reaction is CuO other than Cu 2 O and that during the reaction Cu 2 O is easily oxidized to CuO by O 2 (and/or NO). The effect of calcination temperature on catalytic activity is shown in Fig. 10. The catalyst calcined at 250 C shows the highest activity, with complete NO Fig. 10. Effect of calcination temperature on the CuO/AC catalytic activity. Reaction conditions: 500 ppm NO; 560 ppm NH 3 ; 3.3 vol% O 2 ; WHSV, h 1 ; reaction temperature, 180 C; catalyst, 5 wt% Cu loading.

9 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) Fig. 11. Effect of Cu loading on the CuO/AC catalytic activity. Reaction conditions: 500 ppm NO; 560 ppm NH 3 ; 3.3 vol% O 2 ; WHSV, h 1 ; reaction temperature, 180 C; catalyst, calcined at 250 C. conversion under the employed reaction conditions, while the catalysts obtained at 180, 350, and 450 C show lower activities. The calcination temperature of 180 C may be too low to allow complete decomposition of the impregnated copper nitrate and hence results in low activity. At 350 and 450 C, on the other hand, the copper species is reduced and aggregated as suggested by EXAFS (see Figs. 1 and 3, and Table 2), which also results in declined activity. Fig. 11 shows the dependence of catalytic activity on Cu loading. With increasing Cu loading, catalytic activity increases initially, reaches 100% NO conversion at 5 wt% Cu loading, and then decreases. It is easily understood that the lower activities of the catalysts at lower Cu loadings are due to the low coverage of copper on the AC surface. For catalysts with high Cu loadings, the aggregation of copper species occurs as suggested by EXAFS (see Fig. 4) and thus results in decreased activities Effect of SO 2 The effect of SO 2 on the activity of the CuO/AC catalyst is studied in a transient experiment. The results are illustrated in Fig. 12. At the first stage of the experiment (section I), the NO NH 3 O 2 reaction was carried out at 180 C over the pre-oxided catalyst (5 wt% Cu, 250 C calcination, 180 C pre-oxidization). NO Fig. 12. Effect of SO 2 on the CuO/AC catalytic activity. (I) NO NH 3 O 2 reaction at 180 C; (II) adding 400 ppm SO 2 ; (III) removing SO 2 from feed gas; (IV) Ar purge and then TPD from 180 to 400 C; (V) cooling temperature from 400 to 180 C and then re-performing the NO NH 3 O 2 reaction. The initial reaction conditions: 500 ppm NO, 560 ppm NH 3, 3.3% O 2, WHSV of h 1. Catalyst: 5 wt% Cu loading and calcined at 250 C. conversion is about 95% under the employed conditions. Upon addition of SO 2 into the feed (section II), NO conversion rapidly drops to 17% in about 20 min, at the same time the exit SO 2 concentration gradually increases and reaches a stationary value in about 60 min. When SO 2 is cut off from the feed stream (section III), NO conversion does not recover. In the following step (section IV), all of the reactive gases, such as NO, NH 3 and O 2, are removed from the feed stream, only Ar remains to purge the physically adsorbed SO 2 on the catalyst surface. After the purge (30 min), temperature-programmed desorption (TPD) is performed from 180 to 400 C at a heating rate of 5 C/min in an Ar stream of 300 ml/min. During TPD, a large amount of SO 2 is desorbed. Desorption starts at about 280 C and shows a peak at 350 C. After TPD, the catalyst is coolled to the original reaction temperature of 180 C, and the feed of NO, NH 3 and O 2,is resumed (section 5) with the same flow rate as that in section I. During this process, the catalytic activity recovers to a NO conversion of 89%, which is roughly close to the original level (95%, section 1). The slight loss of activity may possibly be due to the incomplete desorption of SO 2 and/or the formation and aggrega-

10 34 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) tion of Cu 2 O and Cu during the TPD process at temperatures above 350 C as suggested above. These experimental results clearly indicate that SO 2 significantly deactivates the CuO/AC catalyst for the NO NH 3 O 2 reaction and that the deactivation may result from the change of catalyst form and/or structure other than from the gas phase reaction of SO 2. The release of SO 2 during TPD and the recovered activity after TPD suggest that CuO may be converted into CuSO 4 during the SCR reaction in the presence of SO 2. To identify the possibly formed CuSO 4, EX- AFS and XRD measurements were carried out, but no information was obtained because CuO and the newly formed copper species are well dispersed on the AC surface and because a distinction between CuSO 4 and CuO only by the Cu O distance in EXAFS is difficult as shown in Fig. 2. However, the formation of CuSO 4 on the CuO/Al 2 O 3 catalyst during the SCR reaction in the presence of SO 2 has been well established [1,14,15]. A similar process was also reported for other metal oxide catalysts, such as MnO x /Al 2 O 3 [26]. During the SCR reaction in the presence of SO 2, SO 2 is also oxidized by O 2 into SO 3, which reacts with metal oxide to form the corresponding sulfate salt [14,15,26]. It should be emphasized that based on the above experimental results, the formed CuSO 4 is inactive for the NO NH 3 O 2 reaction at 180 C. This is in disagreement with Singoredjo et al. [8], who showed that the activated carbon-supported copper sulfate (CuSO 4 /AC) catalyst had considerable activities in the temperature range of C. The reason may result from the calcination temperature (300 C) employed by Singoredjo et al. [8], at which part of the supported CuSO 4 is possibly converted into CuO (or metallic Cu, and subsequently into CuO during the NO NH 3 O 2 reaction) as suggested by the TPD result here (see Section IV in Fig. 12). The decomposition (or reduction by carbon) of the formed CuSO 4 starts at 280 C. The high activity of the CuSO 4 /AC catalyst observed by Singoredjo et al. may actually result from the newly formed CuO other than from CuSO 4. To clarify this matter, through impregnation of the AC with an aqueous solution of CuSO 4 5H 2 O, two CuSO 4 /AC catalysts, with Cu loading of 5 wt%, were prepared by calcination in an Ar stream at 250 and 300 C for 2 h. SO 2 release was observed during the 300 C calcination but not during the 250 C calcination. This suggests that the supported CuSO 4 can decompose (or be reduced by carbon) at 300 Cbut not at 250 C, which is in good agreement with the TPD results shown in Fig. 12 (Section IV). Activity tests showed that the CuSO 4 /AC catalyst calcined at 250 C shows low activity, with only 31% NO conversion under the same reaction condition as in Fig. 12 (section I). However, the CuSO 4 /AC catalyst calcined at 300 C exhibited higher activity with NO conversion of 72%. These results clearly show that CuSO 4 supported on the AC surface is inactive for the NO NH 3 O 2 reaction at low temperatures and that the high activity of the CuSO 4 /AC catalyst observed by Singoredjo et al. [8] is actually derived from CuO formed during catalyst calcination at 300 C. The deactivation effect of SO 2 on the CuO/AC catalyst for the SCR reaction is unfavourable for the simultaneous removal of SO x and NO x. However, this problem can be solved by using proper reactors, a moving bed reactor similar to that for combined SO x and NO x removal using activated coke [12], for example. The CuO/AC catalyst can be charged from top of the reactor and moves continuously down to the bottom. The flue gas can be fed from the bottom, and NH 3 at the middle part of the catalyst bed. In this process, SO 2 is removed firstly at the lower part of the catalyst bed, the SO 2 -free flue gas then moves to the upper part of the bed for NO x removal. The deactivated catalyst can be regenerated in a separate unit through the conversion of CuSO 4 to CuO [13]. 4. Conclusions The effects of reaction temperature, preparation parameters and SO 2 on the activity of the CuO/AC catalyst for NO reduction with NH 3 are studied, and the catalysts are characterized by XAFS. The CuO/AC catalyst shows high activity for NO reduction with NH 3 in the temperature range of C. The CuO/AC catalyst deactivates gradually with time at temperatures below 180 C but is stable at temperatures above 180 C. The initial activity of the catalyst is low due to the existence of Cu 2 O in the catalyst, but Cu 2 O can be easily oxidized into active CuO during the NO NH 3 O 2 reaction, which results in increased activity.

11 Z. Zhu et al. / Applied Catalysis B: Environmental 26 (2000) The pre-treatment of the AC by HNO 3 significantly enhances the activity of the CuO/AC catalyst, on the contrary, the pre-treatment by H 2 S0 4 shows a negative effect. The catalyst calcined at 250 C shows the highest activity. The calcination temperatures higher than 250 C result in lower activities because the formation and the aggregation of some reductive copper species, such as Cu 2 O and metallic Cu. The catalyst with Cu loading of 5 wt% shows maximum NO conversion. Cu loadings higher than 5 wt% result in growth of a CuO cluster and hence low activity. The CuO/AC catalyst is greatly deactivated by SO 2 through the formation of CuSO 4, which is inactive at low temperatures. The reported SCR activity of CuSO 4 for NO reduction at low temperatures may actually result from the formation of CuO during catalyst calcination. The CuO/AC catalyst can be used for simultaneous removal of SO x and NO x in a properly designed moving bed. Acknowledgements The authors gratefully acknowledge the experimental support of Beijing Synchrotron Radiation Laboratory and the financial support from the Natural Science Foundation China ( , ) and the Shanxi Natural Science Foundation. References [1] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [2] S. Morikawa, H. Yoshida, K. Takahashi, S. Kurita, Chem. Lett., 1981, 251. [3] M.D. Amiridis, I.E. Wachs, G. Deo, J.M. Jehng, D.S. Kim, J. Catal. 161 (1996) 247. [4] Y. Li, P.J. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [5] A. Nishijima, Y. Kiyozumi, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, N. Todo, Bull. Chem. Soc. Jpn. 52 (1979) [6] N. Todo, A. Nishijima, A. Ueno, M. Kurita, H. Hagiwara, T. Sato, Y. Kiyozumi, Chem. Lett. 1976, 897. [7] F. Nozaki, K. Yamazaki, T. Inomata, Chem. Lett., 1977, 521. [8] L. Singoredjo, M. Slagt, J. Van Wees, F. Kapteijn, J.A. Moulijn, Catal. Today 7 (1990) 157. [9] L. Singoredjo, R. Korver, F. Kapteijn, J.A. Moulijn, Appl. Catal. B 1 (1992) 297. [10] D. Roberge, A. Raj, S. Kaliaguine, D.T. On, S. Iwamoto, T. Inui, Appl. Catal. B 10 (1996) L237. [11] H.E. Curry-Hyde, H. Musch, A. Baiker, Appl. Catal. 65 (1990) 211. [12] K. Tsuji, I. Shiraishi, Fuel 76 (1997) 549 and 555. [13] S.J. Liu, Z.Y. Liu, Z.P. Zhu, H.X. Niu, 16th North Am. Catal. Soc. Meet., Boston, MA, 1999, P [14] G. Centi, A. Riva, N. Passarini, G. Brambilla, B.K. Hodnett, B. Delmon, M. Ruwet, Chem. Eng. Sci. 45 (1990) [15] J.T. Yeh, R.J. Demski, J.P. Strakey, J.I. Joubert, Environ. Prog. 4 (1985) 223. [16] B.K. Teo, EAXFS: Basic Principles and Data Analysis, Springer, Berlin, Chap. 6, pp [17] K. Fukumi, A. Chayahara, K. Kadano, H. Kageyama, T. Akai, N. Kitamura, M. Makihara, K. Fujii, J. Hayakama, J. Non-Crystal. Solids 238 (1998) [18] T.A. Smith, J.E. Penner-Hahn, K.O. Hodgson, M.A. Berding, S. Doniach, Springer Proc. Phys. 2 (1984) 58. [19] L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson, E.I. Solomon, J. Am. Chem. Soc. 109 (1987) [20] Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, Fuel, 1999, in press. [21] T. Grzybek, H. Papp, Appl. Catal. B. 1 (1992) 271. [22] T. Grzybek, Fuel 69 (1990) 604. [23] S.N. Ahmed, R. Baldwin, F. Derbyshire, B. McEnaney, J. Stencel, Fuel 72 (1993) 287. [24] B.J. Ku, J.K. Lee, D. Park, H.K. Rhee, Ind. Eng. Chem. Res. 33 (1994) [25] Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, to be published. [26] W.S. Kijlstra, Appl. Catal. B. 16 (1998) 327.