Al2O3-Coated Honeycomb Cordierite-Supported CuO for

Al2O3-Coated Honeycomb Cordierite-Supported CuO for Simultaneous SO2 and ..... Table 1 indicates that CHC-5Al, the substrate, has a BET surface area o...
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Ind. Eng. Chem. Res. 2004, 43, 4031-4037

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Al2O3-Coated Honeycomb Cordierite-Supported CuO for Simultaneous SO2 and NO Removal from Flue Gas: Effect of Na2O Additive Qingya Liu, Zhenyu Liu,* Zhenping Zhu, Guoyong Xie, and Yanli Wang State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P.R. China

Effect of Na2O additive in an Al2O3-coated cordierite honeycomb ceramic-supported CuO catalyst on simultaneous SO2 and NO removal activities was studied at 400 °C. Results show that SO2 removal activity increases with increasing sodium loading from 0 to 2.0 wt % but decreases with further increasing sodium loading to 3.0 wt %, while NO removal activity monotonically decreases with increasing sodium loading, from 99 to 65%, due to increased NH3 oxidation activity. Mechanism of the effect of Na2O additive on SO2 removal activity is studied using N2 adsorption, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray powder diffraction (XRD). Results show that Na2O additive (at sodium loadings of 1.0-2.0 wt %) inhibits the formation of Al-Si-O species and improves the distribution of CuO, which limits pore plugging and increases BET surface area of the catalyst. These behaviors account for the promoting effect of Na2O additive on SO2 removal activity. However, with increasing sodium loading from 2.0 to 3.0 wt %, more well-crystallized CuO are observed, which leads to a lower utilization rate of copper and thus SO2 removal activity. Chemical analysis and XRD results indicate that for the catalyst without Na2O additive, only CuSO4 is formed, whereas for the catalysts containing Na2O, the adsorbed SO2 exist in three forms: CuSO4, NaCu2(SO4)2OH‚ H2O, and Na2SO4. 1. Introduction SO2 and NOx, mainly from stationary sources, are very harmful to the ecosystem and humans, and must be removed before emission. Because of the simplicity and better economics, combined or simultaneous removal of SO2 and NOx is advantageous.1 CuO supported on pellet Al2O3 (CuO/Al2O3) has been widely studied for this purpose because of its good capacity for SO2 adsorption and high activity for selective catalytic reduction (SCR) of NO with NH3 in the presence of O2 in the temperature range of 300-400 °C.2-4 However, a large volume of catalyst pellets may cause high flow resistance and plugging by particulates in flue gas in industrial application.5 It is reported that monolithic catalysts not only can overcome these problems, but also present other advantages over conventional pellet/ granular catalysts: uniform flow distribution, ease of handling, attrition-free, etc.6,7 Therefore, developing honeycomb-type CuO/Al2O3 catalyst for simultaneous SO2 and NO removal presents technical merit. Although molding pellet Al2O3 into monolith may produce honeycomb CuO/Al2O3, the instability of γ-Al2O3 at temperatures higher than 700 °C makes it difficult to calcine to yield sufficient mechanical strength. For this reason, Al2O3 is usually coated on cordierite honeycomb ceramics, such as in preparation of the support of three-way catalyst (known as TWC).8,9 However, our earlier work10 found that loading of CuO directly on a support of TWC yielded a catalyst of a low SO2 conversion. To improve the catalytic activities, it was found * To whom correspondence should be addressed. Tel: +86351-413-4410. Fax: +86-351-405-0320. E-mail: zyliu@ sxicc.ac.cn.

that pretreatment of the cordierite with an acid solution and addition of Na2O were important in catalyst preparation. For CuO/Al2O3 catalyst, the promoting effect of sodium additive on SO2 removal was attributed to the increased sulfation of Al2O3.11,12 However, this may not be the case for the cordierite-supported CuO/Al2O3 because the properties of Al2O3, which is in situ synthesized on the cordierite using a sol-gel method and subjected to repetitious calcinations, are still unclear. Furthermore, the introduction of cordierite support may complicate the effect of Na2O additive on SO2 removal. Hence, the aim of this work is to make a further contribution in these aspects. In this paper, favorable sodium loadings are obtained first, and then the mechanism of the effect of Na2O on SO2 removal activity is elucidated with the aid of N2 adsorption, scanning electron microscopy (SEM), energydispersive X-ray (EDX), X-ray powder diffraction (XRD), and chemical analysis. 2. Experimental Section 2.1. Preparation of Catalysts. The cordierite ceramic used in this work (marked as HC) is a commercial product with a cell density of 200 cells per square inch (cpsi) and a BET surface area of 0.7 m2/g. The preparation procedure includes three steps. Briefly, (1) HC was pretreated in oxalic acid; (2) the resulting sample (marked as CHC) was in situ coated with γ-Al2O3 5× to obtain the substrate CHC-5Al; (3) CuO or CuO-Na2O was introduced onto CHC-5Al by dip-impregnating with a cupric nitrate solution or a mixed solution of cupric nitrate and sodium nitrate to give a copper loading of about 6.0 wt % and a sodium loading of 0-3.0 wt %.

10.1021/ie049942t CCC: $27.50 © 2004 American Chemical Society Published on Web 06/15/2004

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The final catalysts are named according to the metal content. For example, Cu6Na1.5 refers to a catalyst with 6.0 wt % copper and 1.5 wt % sodium supported on CHC-5Al (the name of the substrate is omitted in this notation). It should be pointed out that the metal loadings in this notation are estimated (the actual values determined by ICP analysis are shown in Table 2). 2.2. Activity Test. The activity test was carried out in a fixed-bed reactor of 22-mm diameter. A monolithic catalyst sample (30 mm long and 20 mm diameter) was fitted in the reactor and then heated to 400 °C under an Ar flow. At steady state, a gas mixture containing 1960 ppm SO2, 500 ppm NO, 5.5% O2, 2.5% H2O, 500 ppm NH3, and balance Ar was introduced into the reactor. In all the runs, the total flow rate was controlled at 440 mL/min, which corresponded to a superficial space velocity of 2800 h-1 (based on the monolith volume). The concentrations of NO, SO2, and O2 in the inlet and outlet of the reactor were simultaneously measured on-line by a flue gas analyzer (KM9106). 2.3. Characterization Methods. BET surface area was determined by nitrogen adsorption at 77 K using an ASAP2000 volumetric adsorption analyzer (Micromeritics). The total pore volume (V) was calculated from the amount of nitrogen adsorbed at P/P0 ) 0.98. The average pore size was obtained by the formula of 4V/BET surface area. XRD was performed on a Riguku D/Max 2500 system. Diffraction patterns were recorded with Cu KR (λ ) 0.1542 nm) radiation and the X-ray tube was operated at 40 KV and 100 mA. Step scans were taken over a range of 2θ from 5 to 85° at a speed of 8°/min. SEM photographs were obtained on a JEOL JSM-35C scanning microscope. Prior to SEM analysis, the samples were embedded in epoxy resin, metallographically polished, and coated with a conductive layer of gold. The magnification is marked in the photograph, such as 200× and 2000×. Energy-dispersive X-ray (EDX) analysis was performed on a combined system of LEO 438VP scanning electron microscopy and a KEVEX SUPERDRY X-ray analyzer. Total copper and sodium contents of the catalysts were determined by dissolution of ground catalysts in acid solutions and then analysis on an inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer Optima 3300DV). Quantification of CuSO4 and Al2(SO4)3 in the catalysts consisted of dissolution of the samples in distilled water and analysis of the dissolved metal by ICP. 3. Results and Discussion 3.1. Effect of Sodium Loading on the Catalytic Activities. To all the catalysts, the initial SO2 conversion is 100%. With increasing time on stream, SO2 conversion decreases due to transformation of copper oxide to copper sulfates. To facilitate the discussion on SO2 removal, two terms are introduced. The term t80 is the time at which SO2 conversion decreases to 80%. The term SC80 represents the amount of SO2 adsorbed on the catalyst, in wt %, when the adsorption time reaches t80, or in short, sulfur capacity at t80. Figure 1a shows the effect of sodium loading on SO2 conversion. The behavior of Cu6 and Cu6Na0.5 is similar, with t80 of about 55 min, indicating that introduction of 0.5 wt % sodium additive into Cu6 has little effect on SO2 removal activity. However, when

Figure 1. Effect of sodium loading on SO2 removal activity. Experimental conditions: 1960 ppm SO2, 500 ppm NO, 5.5% O2, 2.5% H2O, 500 ppm NH3; total flow rate of 440 mL min-1; space velocity of 2800 h-1; reaction temperature of 400 °C.

sodium loading is at 1.0 wt % or higher, an obvious promoting effect of Na2O additive on SO2 removal activity is observed. The t80 increases from 55 to 175 min with increasing sodium loading from 0.5 to 1.5 wt %, and stays at 175 min for Cu6Na2. The t80 then decreases to 165 min for Cu6Na2.5 and 85 min for Cu6Na3. SC80 values of various catalysts are shown in Figure 1b. It can be seen that Cu6 and Cu6Na0.5 exhibit similar SC80 values of about 2.1 wt %, corresponding to the equal t80. The SC80 values of Cu6Na1, Cu6Na1.5, Cu6Na2, Cu6Na2.5, and Cu6Na3 are 5.0, 5.5, 6.8, 5.1, and 3.0 wt %, respectively. Clearly, SC80 increases with increasing sodium loading from 0.5 to 2.0 wt %, and then decreases with further increasing sodium loading. It should be pointed out that, because of a difference in catalyst loading in the reactor, the same t80 for Cu6Na1.5 and Cu6Na2 yields different SC80. Anyway, the t80 and SC80 data indicate that Na2O additive (1.0-3.0 wt % sodium) largely enhances SO2 removal activity. However, it is interesting to note that the effect of Na2O additive at sodium loadings of 0.5-2.0 wt % is different from that at 2.0-3.0 wt %, as studied in the following. Figure 2 shows NO conversion vs time on stream over the catalysts of different sodium loadings, obtained in the same experiments shown in Figure 1. For the catalysts with sodium loading of 0, 0.5, 1, 1.5, 2, and 2.5 wt %, the steady-state NO conversions are 99, 95, 94, 92, 85, and 80%, respectively. However, within the time period of the experiment (from 0 to t80), NO conversion of Cu6Na3 does not reach a steady state, with a maximum value of about 65%. These results indicate that sodium additive has a negative effect on NO conversion, as reported for V2O5/TiO2 catalyst,13 and SCR activity monotonically decreases with increasing

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Figure 2. Effect of sodium loading on NO removal activity obtained in the same experiment in Figure 1.

Figure 4. Relationship of BET surface area and SO2 removal activity. Table 1. Physical Properties of Various Catalysts

Figure 3. Effect of sodium loading on NH3 oxidation at 400 °C.

sodium loading and a drastic decrease occurs between sodium loadings of 2.0-3.0 wt %. Our previous work on pellet CuO/Al2O3 catalyst14 showed that the inhibiting effect of sodium additive on NO removal is from increased NH3 oxidation to form NO. To understand the effect of sodium loading on the extent of NH3 oxidation for the honeycomb catalyst, an NH3 oxidation experiment was carried out in a stream containing 550 ppm NH3 and 5.4 vol % O2 at 400 °C. The outlet NOx concentration (monitored on-line by a flue gas analyzer) was used to estimate the extent of NH3 oxidation (NH3 to NO in a stoichiometric ratio of 1:1). Results in Figure 3 show that the outlet NO concentration increases linearly with increasing sodium loading, indicating an increase in NH3 oxidation. These suggest that NH3 oxidation is also a main reason for the monotonic decrease in SCR activity with increasing sodium loading for the honeycomb catalyst. On the basis of the above data, sodium loadings of 1.0-2.0 wt % may be proper compositions of the catalyst for simultaneous SO2 and NO removal. 3.2. Physical Properties of Various Catalysts. Generally, surface area of a catalyst is one of the main factors influencing catalytic activities. To study the effect of Na2O additive on SO2 removal, physical properties of the catalysts are analyzed. Table 1 indicates that CHC-5Al, the substrate, has a BET surface area of 87.6 m2/g, and loading of 6.0 wt % copper decreases it to 42.9 m2/g (see Cu6). Accordingly, the total pore volume decreases from 0.090 cm3/g for CHC-5Al to 0.043 cm3/g for Cu6. These data suggest that impregnation of CuO on the substrate results in pore plugging. However, introduction of sodium additive to Cu6 increases its BET surface area and pore volume to 52.5, 67.3, and 47.0

sample

BET surface area m2/g

average pore size nm

pore volume cm3/g

CHC-5Al Cu6 Cu6Na1 Cu6Na2 Cu6Na3

87.6 42.9 52.5 67.3 47.0

4.10 4.01 3.99 4.17 4.58

0.090 0.043 0.052 0.070 0.054

m2/g and 0.052, 0.070, and 0.054 cm3/g for Cu6Na1, Cu6Na2, and Cu6Na3, respectively. This suggests that sodium additive may prevent aggregation of some species (CuO particles or others) and in turn result in the lesser extent of pore plugging, which is similar to that demonstrated by NaCl on calcined limestone15 and ZnO on CuO/Al2O3 catalyst.16 However, it is worth noting that increasing sodium loading from 2.0 to 3.0 wt % causes decreases in surface area and pore volume and an increase in average pore size, suggesting the increased pore plugging resulted from formation of large or more particles in the impregnation. To visually understand the relationship between SO2 removal activity and physical properties of the catalyst, Figure 4 gives the curve of SC80 vs BET surface area. As can be seen, SC80 increases roughly linearly with increasing surface area, suggesting the importance of catalyst surface area to SO2 removal. 3.3. XRD and SEM Analyses. The catalysts with different Na2O loadings show different colors, black for Cu6 and green for Cu6Na2, suggesting improved dispersion of CuO by Na2O additive. To understand the distribution of CuO and changes of physical properties caused by Na2O additive, Cu6, Cu6Na2, and Cu6Na3 were characterized by XRD and SEM, along with the substrate CHC-5Al for comparison. Figure 5 shows the XRD patterns of these catalysts. Although CHC-5Al shows typical diffraction patterns of cordierite ceramics, the slightly increased intensities of peaks at 39, 45.5, and 66°, compared to the XRD pattern of CHC (not shown here), suggest that γ-Al2O3 has successfully synthesized onto the cordierite monolith. With introduction of 6.0 wt % copper onto CHC-5Al, a new strong peak at 35.6° appears and the intensity of the peak at 38.8° increases (see Cu6), indicating the existence of crystallized CuO (2θ ) 35.6 and 38.8°) in Cu6 catalyst. However, with introduction of 2.0 wt % sodium additive into Cu6 catalyst, the intensities of all peaks decrease (see Cu6Na2), suggesting that some changes take place. The visible peaks corresponding to CuO suggest that they still exist in the congregated form in Cu6Na2 catalyst. With increasing sodium loading to

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Figure 5. XRD patterns of (a) CHC-5Al, (b) Cu6, (c) Cu6Na2, and (d) Cu6Na3.

Figure 6. SEM photograph of CHC-5Al.

3.0 wt %, the intensities of the peaks at 35.6 and 38.8° increase again (see Cu6Na3) and become stronger than those in Cu6 catalyst, suggesting that the particle size of CuO in Cu6Na3 is much larger than that in other catalysts and/or that the degree and the number of crystalline CuO phase are increased. This observation agrees with the report by El-Shobaky et al.17 which showed increased intensities of CuO diffraction lines with increasing amount of Na2O additive. It is worth pointing out that no diffraction patterns corresponding to Na2O (2θ ) 46.5, 32.3, and 27.3°) can be observed on Cu6Na2 and Cu6Na3 catalysts, indicating the weakly crystalline nature (amorphous) of Na2O additive. To visually understand the particle size of CuO, SEM photographs of the catalyst samples used for the XRD study are shown in Figures 6-8 and 11. Figure 6

exhibits the morphology of CHC-5Al. Cracks can be clearly observed, possibly due to the thick coating of Al2O3 or the dilute sol used.18 Introduction of 6.0 wt % copper into CHC-5Al results in the appearance of some cotton-like species with sizes ranging from 13 to 20 µm (see Figure 7). EDX analyses indicate that these species are composed of O, Al, Si, and Cu (least). It is surprising to see Si on the surface because Si is a component of the cordierite which is supposed to be under the Al2O3 coating. This suggests that Al2O3 coating is very thin or that Si may migrate to the surface in the Al2O3coating process. Comparing Figures 6 and 7, it can be deduced that loading of CuO promotes the formation of cotton-like Al-Si-O species. In addition, some wellcrystallized flat particles with sizes of about 3 µm are visible on the cotton-like species. EDX results indicate that these particles are composed of Cu and O, i.e., CuO. The congregation of CuO is not a surprise. Strohmeier et al.19 reported that the monolayer coverage of CuO on an Al2O3 support with a surface area of 100 m2/g resulted in a Cu loading of 4-5 wt %. On the basis of this relation, the copper loading for a monolayer coverage on CHC-5Al (BET surface area of 87.6 m2/g) would be about 4.0 wt %, which is much smaller than the CuO loading used in this work. Because only the outer surface of the CuO particle is easily available to react with SO2, the congregated CuO particles will lead to a low utilization rate of copper. Figure 8 shows that with introduction of 2.0 wt % sodium additive into Cu6 catalyst, the size of cottonlike species decreases to about 8 µm or below and many thread-like materials radiating from the cotton-like species are visible. The EDX result shown in Figure 9 indicates that the threads are composed of Si (most), Al, O, and Cu (least), and no Na is detected. This observation suggests that sodium additive may inhibit the formation of congregated Al-Si-O species. SEM studies on uncalcined Cu6 and Cu6Na2, shown in Figure 10, suggest that this inhibiting effect of Na2O additive occurs in the precipitation process, but cotton/ thread-like species are not formed in this process. The behavior of Na2O additive must result in less pore plugging and higher BET surface area of Cu6Na2 (see Section 3.2). These in turn may improve the distribution of CuO, as evidenced by XRD result. All of these contribute to the enhanced copper utilization rate and SO2 removal activity. Figure 11 shows the morphology of Cu6Na3, which is clearly different from that of Cu6Na2. The cottonlike species nearly disappear and two types of new particles appear. One is octahedron shaped with sizes of about 10 µm (see Figure 11b) and the other is a compressed sphere with sizes of about 5 µm (see Figure

Figure 7. SEM photographs of Cu6 with magnifications of (a) 200 and (b) 2000.

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Figure 8. SEM photographs of Cu6Na2 with magnifications of (a) 200 and (b) 2000.

Figure 9. EDX result of the threads in Figure 8.

Figure 10. SEM images of uncalcined Cu6 and Cu6Na2.

11c). EDX results show that both of them are CuO particles, without Al or Si. The increased well-crystallized CuO phase was also observed on CuO/Al2O3 catalyst at sodium loading of 5 wt % or more,20 which was attributed to the formation of a sodium aluminate layer that hindered the thermal diffusion of Cu2+ into the Al2O3 matrix.17 In any case, the more and larger CuO particles of higher crystallinity in Cu6Na3, com-

pared to those in Cu6Na2, agrees with the increased intensities of peaks corresponding to CuO in XRD pattern, and may cause more pore plugging and thus a decreased BET surface area and pore volume, as indicated in Table 1. These behaviors in turn result in a lower utilization rate of copper and SO2 removal activity. 3.4. Characterization of Sulfates Formed on the Catalysts. To further understand the reactive property of Al2O3 coating synthesized in this work and the promoting effect of sodium additive on SO2 removal activity, the forms of sulfate formed on the catalysts during SO2 removal were determined by XRD and chemical analysis. Figure 12 shows the XRD pattern of sulfated Cu6Na2, along with that of a fresh one for comparison. It can be seen that three new peaks at 13.4, 25.1, and 31.9° appear in the XRD profile of the sulfated catalyst and the intensities of peaks at 21.6, 26.3, 34.2, and 54.3° increase. Peak-search indicates that (1) no diffractions corresponding to Al2(SO4)3 (2θ ) 25.4, 15.7, and 20.4°) can be observed on the sulfated catalyst, which provides evidence for the limited formation of Al2(SO4)3; (2) there are two kinds of sulfates, CuSO4 (2θ ) 21.2, 25.3, and 34.3°), usually observed in sulfated CuO/Al2O3 catalyst, and NaCu2(SO4)2OH‚H2O (2θ ) 13.4, 26.3, 31.9, 35.5, 38.8, and 54.3°), never reported in the literature to our knowledge. It is surprising that no Na2SO4 is detected because it was believed to be a product of the reaction between Na2O and SO2+H2O+O2. To further identify the chemical forms of the sulfates, chemical analyses were performed, and the results are shown in Table 2. For sulfated Cu6 catalyst, the watersoluble copper and water-soluble alumina are 2.52 and 0 wt %, respectively. Because the atomic weight of Cu (63.5) is very close to the molecular weight of SO2 (64), the amount of SO2 associated with the Cu is also 2.52 wt %. This value is very close to the corresponding SC80 shown in Figure 1b, suggesting that most of the sulfates formed on Cu6 catalyst are CuSO4 and little Al2(SO4)3 is formed. This is rather surprising because the formation of Al2(SO4)3 has been reported in the literature for CuO/Al2O34 and sodium-doped Al2O321 catalysts. This difference suggests that Al2O3 synthesized and supported on the cordierite ceramic in this work is different from pellet Al2O3 generally used, possibly due to interactions between Al2O3 and SiO2, as evidenced by the formation of cotton-like Al-Si-O species. Table 2 also shows that the amount of water-soluble copper increases with increasing sodium loading, from 2.52 wt % for sulfated Cu6 to 4.02, 4.98, and 6.14 wt % for sulfated Cu6Na1, Cu6Na1.5, and Cu6Na2, respectively. These results clearly indicate the enhanced copper utilization rate by sodium additive. It is worth

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Figure 11. SEM photographs of Cu6Na3.

not more than 0.05 wt %, although it increases with increasing sodium loading due to the promoting effect of Na2O additive,21 it cannot account for the differences between SC80 and the amount of SO2 linked to copper. This leaves sodium as the only candidate to account for SO2 adsorption, besides in the form of NaCu2(SO4)2OH‚ H2O. Table 2 shows that the water-soluble sodium in the sulfated Cu6Na2 (S) is 1.24 wt %. Supposing all the water-soluble sodium is from Na2SO4, with little formation of NaCu2(SO4)2OH‚H2O, then the overall amount of SO2 adsorbed on Cu6Na2 (S) would yield a SC80 of 7.86 wt %, which is significantly higher than the SC80 shown in Figure 1b (6.83 wt %). This indicates that only a fraction of sodium forms amorphous Na2SO4 upon adsorption of SO2, which is not detectible by XRD. 4. Conclusions Figure 12. XRD patterns of Cu6Na2 catalyst before and after sulfation.

(1) SO2 removal activity increases with increasing sodium loading from 0 to 2.0 wt % and then decreases with further increasing sodium loading to 3.0 wt %, while SCR activity for NO removal monotonically decreases. Sodium loadings of 1.0-2.0 wt % in the catalyst may be favorable for simultaneous SO2 and NO removal. Under the conditions used, SC80 of 5.0 wt % and a NO conversion of about 85% can be reached. (2) The inhibiting effect of Na2O additive on NO removal is due to increased NH3 oxidation to form NO. The extent of NH3 oxidation increases linearly with increasing sodium loading. (3) Loading of CuO promotes the formation of cotton-like Al-Si-O species, which causes pore plugging and thus a decrease in surface area.

noting that both CuSO4 and NaCu2(SO4)2OH‚H2O, determined by XRD, can yield water-soluble copper with S/Cu atomic ratio of 1. This seems to suggest that the amount of SO2 adsorbed on the catalyst can be calculated from water-soluble copper. However, all the values of water-soluble copper, except 2.52 wt % for Cu6, are smaller than SC80 of the corresponding catalysts (shown in Figure 1b). This suggests that not all of the SO2 adsorbed is associated with copper in the presence of sodium additive, and some of the adsorbed SO2 may be linked to other species. Because the content of watersoluble alumina, in the form of Al2(SO4)3, is very small, Table 2. Results of Chemical Analyses of Various Catalysts

a

samplea

total copper (%)

total sodium (%)

water-dissolved copper (%)

water-dissolved sodium (%)

water-dissolved alumina (%)

Cu6(S) Cu6Na1 (S) Cu6Na1.5 (S) Cu6Na2 (S) Cu6Na2 (F)

6.69 6.34 6.49 6.40 6.40

1.00 1.51 2.04 2.04

2.52 4.02 4.98 6.14 0

0.88 1.00 1.24 0.70

0 0.01 0.03 0.05 0.54

Note that (F) represents the fresh catalyst; (S) represents the sulfated catalyst.

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However, sodium additive, at sodium loadings of 2.0 wt % or below, inhibits the formation of cotton-like species and thus increases the BET surface area of the catalyst and enlarges the pore size. Sodium additive also improves the distribution of CuO. All of these account for the promoting effect of Na2O additive on SO2 removal. (4) Addition of 3.0 wt % sodium causes completely different behavior, i.e., a marked increase of wellcrystallized CuO phase. This behavior results in a decreased BET surface area and pore volume, and a lower utilization rate of copper and thus SO2 removal activity. (5) For the catalyst without sodium additive, all SO2 adsorbed is linked to copper, i.e., CuSO4. However, for the catalysts containing sodium additive, the SO2 adsorbed exists in three forms: CuSO4, NaCu2(SO4)2OH‚H2O, and Na2SO4. Al2O3 coating synthesized in this work almost does not take part in SO2 adsorption to form Al2(SO4)3. Acknowledgment We gratefully acknowledge financial support from the Natural science Foundation of China (20276078, 90210034), the National High Technology Research and Development Program (The 863 Program, 2002AA529110), Chinese Academy of Sciences, and the Shanxi Natural Science Foundation. Literature Cited (1) Centi, G.; Riva, A.; Passarini, N.; Brambilla, G.; Hodnett, B. K.; Delmon, B.; Ruwet, M. Simultaneous removal of SO2/NOx from flue gas. Sorbent/catalyst design and performances. Chem. Eng. Sci. 1990, 45, 2679. (2) Macken, C.; Hodnett, B. K.; Paparatto, G. Testing of the CuO/Al2O3 Catalyst-Sorbent in Extended Operation for the Simultaneous Removal of NOx and SO2 from Flue Gases. Ind. Eng. Chem. Res. 2000, 39, 3868. (3) Centi, G.; Perathoner, S. Role of the Size and Texture Properties of Copper-on-Alumina Pellets during the Simultaneous Removal of SO2 and NOx from Flue Gas. Ind. Eng. Chem. Res. 1997, 36, 2945. (4) Xie, G.; Liu, Z.; Zhu, Z.; Liu, Q.; Ma, J. Reductive regeneration of sulfated CuO/Al2O3 catalyst-sorbent in ammonia. Appl. Catal. B 2003, 45, 213. (5) Jeong, S. M.; Kim, S. D. Removal of NOx and SO2 by CuO/ γ-Al2O3 Sorbent/Catalyst in a Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 2000, 39, 1911.

(6) Irandoust, S.; Andersson, B. Monolithic catalysts for nonautomobile applications. Catal. Rev.-Sci. Eng. 1988, 30, 341. (7) Williams, J. L. Monolith structures, materials, properties and uses. Catal. Today 2001, 69, 3. (8) Heck, R. M.; Farrauto, R. J. Automobile exhaust catalysts. Appl. Catal. A 2001, 221, 443. (9) Kasˇpar, J.; Fornasiero, P.; Hickey, N. Automotive catalytic converters: current status and some perspectives. Catal. Today 2003, 77, 419. (10) Liu, Q.; Liu, Z.; Xie, G. A honeycomb catalyst for simultaneous NO and SO2 removal from flue gas. Presented at the 3rd Asia-Pacific Congress on Catalysis, Oct 12-15th 2003, Dalian, China; Paper 244. (11) Yoo, K. S.; Kim, S. D.; Park, S. B. Sulfation of Al2O3 in Flue Gas Desulfurization by CuO/γ-Al2O3 Sorbent. Ind. Eng. Chem. Res. 1994, 33, 1786. (12) Jeong, S. M.; Kim, S. D. Enhancement of the SO2 Sorption Capacity of CuO/γ-Al2O3 Sorbent by an Alkali-Salt Promoter. Ind. Eng. Chem. Res. 1997, 36, 5425. (13) Khodayari, R.; Ingemar Odenbrand, C. U. Regeneration of commercial SCR catalysts by washing and sulphation: effect of sulphate groups on the activity. Appl. Catal. B. 2001, 33, 277. (14) Xie, G.; Liu, Z.; Liu, Q.; Ge, J. Effect of sodium on simultaneous removal of SO2 and NO from flue gas over CuO/ Al2O3 catalyst-sorbent. Presented at the 3rd Asia-Pacific Congress on Catalysis, Oct 12-15th 2003, Dalian, China; Paper 242. (15) Shearer, J. A.; Johnson, I.; Turner, C. B. Effect of sodium chloride on limestone calcinations and sulfation in fluidized bed combustion. Environ. Sci. Technol. 1979, 13, 1113. (16) El-Shobaky, G. A.; Fagal, G. A.; Mokhtar, M. Effect of ZnO on surface and catalytic properties of CuO/Al2O3 system. Appl. Catal. A 1997, 155, 167. (17) El-Shobaky, G. A.; El-Nabarawy, Th.; Fagal, G. A.; Amin, N. H. Mutual solid-solid interaction between CuO and sodium oxide-doped alumina. Thermochim. Acta 1989, 141, 195. (18) Agrafiotis, C.; Tsetsekou, A. Deposition of meso-porous γ-alumina coatings on ceramic honeycombs by sol-gel methods. J. Eur. Ceram. Soc. 2002, 22, 423. (19) Strohmeier, B. R.; Leyden, D. E.; Field, R. S.; Hercules, D. M. Surface spectroscopic characterization of Cu/Al2O3 catalysts. J. Catal. 1985, 94, 514. (20) Selim, M. M.; Youssef, N. A. Thermal stability of CuOAl2O3 system doped with sodium. Thermochim. Acta 1987, 118, 57. (21) Mitchell, M. B.; Sheinker, V. N.; White, M. G. Adsorption and Reaction of Sulfur Dioxide on Alumina and Sodium-Impregnated Alumina. J. Phys. Chem. 1996, 100, 7550.

Received for review January 17, 2004 Revised manuscript received April 30, 2004 Accepted May 12, 2004 IE049942T