Cordierite Catalyst for Selective Catalytic

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Ind. Eng. Chem. Res. 2008, 47, 4295–4301

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APPLIED CHEMISTRY Honeycomb CuO/Al2O3/Cordierite Catalyst for Selective Catalytic Reduction of NO by NH3sEffect of Al2O3 Coating Junhua Su,†,‡ Qingya Liu,§ Zhenyu Liu,*,†,§ and Zhanggen Huang† State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001 People’s Republic of China; Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100039 People’s Republic of China; and State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029 People’s Republic of China

Al2O3-coated cordierite has been widely used as a catalyst’s support. In this work, three Al2O3 precursors or alumina sols are studied to understand the effect of type, viscosity, and grain size of the sols and Cl- on properties and SCR activities of honeycomb CuO/Al2O3/cordierite catalysts. X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy and energy dispersive X-ray (SEM-EDX) techniques are used for analyses. Results show that the viscosity and average grain size of the sol have significant influences on the amount and stability of the Al2O3 coating. The sol synthesized from AlCl3 and aluminum is the best among the three sols in terms of less coating cycle, stability of the coating, and selective catalytic reduction (SCR) activity. Cl1- introduced in preparation must be washed out because it deactivates the SCR activity. 1. Introduction

2. Experimental Section

Selective catalytic reduction (SCR) of NO by NH3 is an effective approach to solve stationary NOx emission problems.1,2 Honeycomb V2O5/TiO2 has been used for this purpose for more than 20 years.3–5 However, porous anatase required in the formulation was reported to be difficult to prepare and physically weak,6 which, along with the high cost of V2O5, hampers the application of V2O5/TiO2. Cost-effective catalysts are, therefore, widely studied for advancement of the SCR technology. CuO supported on Al2O3 pellets (CuO/Al2O3) was reported to have high SCR activities in a temperature range of 300-400 °C.7–10 For industrial application, however, this catalyst must be made into a honeycomb shape to handle flue gases of large volume and to avoid plugging by dust. Our previous work showed that supporting CuO on an Al2O3-coated cordierite honeycomb yields a good SCR catalyst,11 but repeated Al2O3 coating is needed to attain a desired Al2O3 loading of 20 wt %. This complicates the preparation process and increases the cost. Al2O3-coated cordierite has been widely used as the support of three-way catalysts, where a high Al2O3 loading is obtained by Al2O3 slurry. However, the dispersivity and purity of the coating are reported to be not very good.12 To improve the properties of Al2O3 coating, sol-gel methods have been studied in recent years. Raw materials used for making Al2O3 sol and coating conditions may have significant influence on properties of the coating. However, few reports can be found in the literature in this regard. In this paper, properties of different alumina sols, such as the type of the raw material, viscosity, and average grain size of the sol, are studied and compared to understand their influences on the properties of Al2O3 coating and on the activity of SCR of NO.

2.1. Preparation. Three Al2O3 sols were synthesized. Sol-1 was prepared from an aqueous solution of Al(NO3)3 and urea with a mass ratio of 1.9:1. Sol-2 was synthesized from a mixture containing Al powder and AlCl3 with a mole ratio of 5:1, which was refluxed at 90 °C for 24 h and then filtrated. Sol-3 was prepared from pseudoboehmite, which was added into a HNO3 solution at 90 °C ([H+]/[Al3+] ) 0.26) and held for 24 h. The alumina concentration of the three Al2O3 sols is 1.5 mol/L. The cordierite honeycomb used in this work is a commercial product with a cell density of 200 cells per square inch (cpsi) and a Brunauer-Emmett-Teller (BET) surface area of 0.7 m2/ g. Cordierite honeycombs with a size of Φ22 × 75 mm were treated with a 50 wt % oxalic acid solution for 6 h at 100 °C before being coated with Al2O3. The acid-treated cordierite honeycomb was then immersed into one of the alumina sols for 6 h, followed by drying at 110 °C and calcining at 500 °C. In some cases, the coating process was repeated a number of times (termed coating cycles) until the Al2O3 loading was at least 20 wt %. The Al2O3-coated honeycombs were impregnated with a 2 wt % Cu(NO3)2 solution and subjected to drying and calcining. The resulting catalysts all contain 1 wt % CuO and are named CuO/n-Al2O3(sol type), where “n” denotes Al2O3 loading in weight percent. For example, CuO/20-Al2O3(Sol-1) denotes a sample containing 1 wt % CuO and 20 wt % Al2O3 prepared from Sol-1. 2.2. Characterization. The grain size of all the sols were measured with Hitachi H-600 transmission electronic microscopy (TEM) with an accelerated voltage of 75 kV. The apparent viscosity of the sols was measured by a RS-75 rheometer using a Z40D rotor at a constant temperature of 30 ( 0.05 °C. X-ray diffraction (XRD) was performed on a Riguku D/Max 2500 system equipped with Cu KR (λ) 0.154 nm) radiation with an accelerating voltage of 40 kV and a current of 100 mA. XRD patterns were recorded from 5 to 80° at a scan rate of 5 °/min and a scan space of 0.02 °C.

* Corresponding author. E-mail: [email protected]. Fax: +86-3514053091. † Institute of Coal Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Beijing University of Chemical Technology.

10.1021/ie800105p CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

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BET surface area (ABET) was determined by nitrogen adsorption at -196 °C using an ASAP2000 volumetric adsorption analyzer. Total pore volume (V) was calculated from the amount of nitrogen adsorbed at a relative N2 pressure (P/P0) of 0.98. The average pore size was assumed to be 4V/ABET. All the samples were degassed at 150 °C for 12 h before the measurement. Scanning electron microscopy (SEM) analysis was carried out on JEC JEOL JSM-35C to observe surface morphology of the samples. Element concentration was measured by KEVEX SUPERDRY energy dispersive X-ray analysis (EDX). Stability of the Al2O3 coatings was determined by a CSF-1A supersonic equipment with a current of 200 mA. Constant-temperature reduction (CTR) and temperatureprogrammed reduction (TPR) experiments were performed on a fixed-bed reactor (Φ10 × 350 mm) loaded with 0.5 g of catalyst with sizes less than 20 mesh. The reactor effluent was analyzed via a capillary tube by a mass spectrometer (OmniStar200, Blazers) operated in a MID (multiple ion detection) mode. For CTR, the samples were in situ preheated in an Ar stream at 400 °C for 30 min before being exposed to a 5 vol % H2/Ar stream at 12 mL/min at the same temperature. For TPR, the samples were in situ preheated in an Ar stream at 100 °C for 30 min and then exposed to 5 vol % H2/Ar at 12 mL/min and heated up from 100 to 600 °C at a heating rate of 10 °C/ min. 2.3. Activity Measurement. Activity measurement for SCR of NO was carried out in a fixed-bed reactor (Φ22 × 450 mm). A monolithic catalyst sample (Φ20 × 11 mm) was fitted to the reactor and then heated to 400 °C in an Ar stream of 150 mL/ min. At steady state, a stream containing 1600 ppm SO2, 600 ppm NO, 600 ppm NH3, 5.0% O2, 2.5% H2O, and balance N2 was introduced into the reactor. In all the runs, the total flow rate was maintained at 450 mL/min, corresponding to a superficial space velocity of 7800 h-1. The concentrations of SO2, NO, and O2 at the outlet of the reactor are measured online by a flue gas analyzer (KM9106, Kane, U.K.), which is equipped with electrochemical sensors with an accuracy of (5 ppm for NO and SO2. Since no NO2 and only a trace amount of N2O are observed in the reactor effluent by mass spectroscopy, the catalysts’ SCR activity is expressed by NO conversion as defined below, where NOin and NOout denote concentrations of NO at the inlet and outlet of the reactor, respectively. NO conversion (%) ) NOin - NOout/NOin × 100%. 3. Results and Discussion 3.1. Effect of Alumina Sol on Al2O3 Loading and Characteristics of the Coating. 3.1.1. Al2O3 Loading of Different Sols. Figure 1 shows Al2O3 loading of the three alumina sols with the same Al concentration in continued coating cycles. As expected, Al2O3 loading increases with increasing coating cycles, but the amounts of Al2O3 coated in each cycle by each sol are very different. For Sol-1 and Sol-2, the accumulative Al2O3 loading increases almost linearly with the coating cycle, but the slope for Sol-1 is more than twice that for Sol-2. Three coating cycles are sufficient to achieve an Al2O3 loading of 20 wt % by Sol-1, while nine coating cycles are needed to achieve the same Al2O3 loading. The cumulative Al2O3 loading by Sol-3 shows a nonlinear relationship toward coating cycle, and four coating cycles are needed to achieve an Al2O3 loading of 20 wt %. In principle, the amount of Al2O3 coated onto the cordierite is determined by the properties of the sol and the interactions

Figure 1. Accumulative Al2O3 loading from different sols in continued coating cycles.

Figure 2. Al2O3 loading of each coating cycle from different sols. Table 1. Physical and Chemical Properties of Different Al2O3 Sols at the Same Al Concentration Al2O3 sol

grain size (nm)

apparent viscosity (Pa s)

Al concentration (mol/L)

Sol-1 Sol-2 Sol-3

10 40 20

0 0 6

1.5 1.5 1.5

of the sol with the surface of the substrate. The surface of the substrate is cordierite initially but turns into Al2O3 after several coating cycles. Figure 2 shows that Al2O3 loadings of the first coating cycle of the three sols are different. Sol-3 with a higher viscosity (Table 1) results in a small Al2O3 loading, while Sol-1 and Sol-2 with lower viscosities result in higher Al2O3 loadings. Apparently, a high viscosity is not helpful for a high Al2O3 loading. Figure 2 also shows that Al2O3 loadings of the first cycle for Sol-1 and Sol-2 are slightly more than those in the subsequent cycles, indicating the cordierite is able to absorb more of these sols than Al2O3 does. It is clear that, with increasing coating cycles, Al2O3 loading of each coating cycle by Sol-2 is low and constant while that by Sol-1 is high but decreasing. These behaviors may be attributed to difference in grain size of these sols, 10 nm for Sol-1 and 40 nm for Sol-2, as shown in Table 1. A sol with a smaller grain size may be more likely to enter most of the pores of the cordierite, resulting in a larger Al2O3 loading. However, this effect would decrease as the pores of the cordierite are filled up. In comparison, a sol with a larger grain size may be difficult

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Figure 3. XRD results of Al2O3-coated samples (o, R-cordierite; *, γ-Al2O3). Scrap-1 and Scrap-3 denote scraps that fell off from 20-Al2O3 (Sol-1) and 22-Al2O3 (Sol-3) during an ultrasonic treatment.

to enter the small pores of the cordierite and may coat mainly on the outer surface of the cordierite, resulting in a small and constant Al2O3 loading. These suggestions are supported by observation in the coating process, where more small air bubbles were releasing from the substrate during coating of Sol-1, compared to the coating of Sol-2. Behavior of Al2O3 loading of Sol-3 is very interesting. Although its grain size is small (20 nm), the Al2O3 loadings of the first two coating cycles are also small, indicating difficulties of the highly viscous sol in entering the pores of the cordierite, as evidenced by less and bigger air bubbles releasing from the substrate during the coating process. It is a surprise to see that Al2O3 loading of each coating cycle increases quickly with increasing coating cycles, indicating that interaction between the coating and the sol is stronger than that between the cordierite and the sol. This behavior may also be attributed to formation of reticulate structure by Sol-3, where the Al2O3 coated in the previous coating cycles acts as the crystal seeds.13 3.1.2. XRD Results of Al2O3-Coated Samples. Figure 3 shows XRD patterns of the cordierites with or without Al2O3 coating. Plate samples, instead of powders, were used for the analysis. The cordierite presents typical diffraction peaks of R-cordierite.14 After loading 20 wt % Al2O3 by Sol-1 or Sol-2, the sample 20-Al2O3(Sol-1) or 20-Al2O3(Sol-2) also shows peaks of R-cordierite but with decreased peak intensity. This observation, along with the absence of new peaks, indicates formation of a thin layer of amorphous materials on the cordierite surface. The sample coated with 22 wt % Al2O3 by Sol-3, 22-Al2O3(Sol3), again shows a behavior different from that of the other two samples, with weak γ-Al2O3 diffraction peaks and no R-cordierite peaks, indicating coverage of the cordierite by thick amorphous Al2O3. To further understand the types of Al2O3 formed by the three sols, the sols themselves are dried and calcined under the same conditions as that for preparation of Al2O3-coated samples. XRD patterns of these Al2O3 powders, P-Al2O3(Sol-1), P-Al2O3(Sol2), and P-Al2O3(Sol-3), are shown in Figure 4. P-Al2O3(Sol1) shows very weak diffraction peaks of γ-Al2O3 (2θ ) 66.6, 36.8, 45.8°), and P-Al2O3(Sol-3) shows slightly stronger diffraction peaks of γ-Al2O3, while P-Al2O3(Sol-2) shows strong diffraction peaks of R-Al2O3 (2θ ) 43.4, 57.6, 35.2°). Comparisons of these results with those in Figure 3 indicate that strong interactions occur between the cordierite and the

Figure 4. XRD results of powder Al2O3 formed directly from different sols (o, R-Al2O3; *, γ-Al2O3).

Al2O3 coating, especially for Al2O3 made from Sol-1 and Sol2, which lost their crystal characteristics when supported on the cordierite. These two sols may penetrate into the pores of the cordierite and form only very thin layers of amorphous Al2O3, which is invisible by XRD. The similarities between XRD patterns of the coated sample 22-Al2O3(Sol-3) and the uncoated powder P-Al2O3(Sol-3) indicate that the Al2O3 is mainly coated on the outer surface of the cordierite for Sol-3. As a consequence, the Al2O3 layer of 22-Al2O3(Sol-3) is thicker than those of 20-Al2O3(Sol-1) and 20-Al2O3(Sol-2), which made the cordierite invisible by XRD analysis. It is interesting to see that the diffraction peaks of the cordierite in 20-Al2O3(Sol-2) are much larger than those in 20-Al2O3(Sol-1), indicating that Al2O3 coated on the outer surface of 20-Al2O3(Sol-1) is more than that of 20-Al2O3(Sol2), although the grain size of Sol-1 is much smaller than that of Sol-2. This behavior may be attributed to more coating cycles used for preparing 20-Al2O3(Sol-2), 9 cycles, in comparison to that for preparing 20-Al2O3(Sol-1), 3 cycles. The repeated calcination during the repeated coating cycles may promote diffusion of Al2O3 into the bulk of the cordierite. 3.1.3. Surface Appearance and Stability of the Coatings. Figure 5 is SEM-EDX results of the cordierite and the Al2O3coated cordierites. It should be noted that EDX analyses were made at many locations of each sample, and the one shown in the figure represents the general surface composition of the sample. The cordierite has a scraggly surface with many large pores, with Si as the major element and Al and O as the minor elements. Sample 20-Al2O3(Sol-1) shows a smooth surface with many fine cracks and Al and O as the dominant elements, indicating the presence of a thin Al2O3 layer. Sample 22-Al2O3(Sol-3) also shows a smooth surface dominated by Al and O elements, but with only a few large cracks, indicating the presence of a thick Al2O3 layer. On the basis of XRD analyses in Figure 3, it is certain that the Al2O3 coatings formed by Sol-1 and Sol-3 are amorphous Al2O3. The SEM-EDX result of 20-Al2O3(Sol-2) is clearly different from those of 20-Al2O3(Sol-1) and 22-Al2O3(Sol-3). Its surface seems not smooth and contains many small scraps. Furthermore, its surface consists of a significant amount of Si in addition to Al and O. These suggest absence of a fully developed Al2O3 layer due possibly to diffusion of the Al2O3 into the cordierite bulk during repeated calcination, as suggested earlier.

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Figure 5. SEM-EDX results of different Al2O3-coated samples.

To further understand the properties of the Al2O3 coatings, all the Al2O3-coated cordierite samples were subjected to an ultrasonic treatment for 30 min. The scraps fell off, the samples were analyzed by XRD, and their amounts were

determined. This experiment was repeated 3 times for each sample to yield an average data point. The amounts of scraps obtained are 4.3 wt % for 20-Al2O3(Sol-1), 0.5 wt % for 20Al2O3(Sol-2), and 8.1 wt % for 22-Al2O3(Sol-3). Obviously,

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Figure 6. SEM photographs of Al2O3-coated samples after an ultrasonic treatment. Table 2. Surface Area and Pore Structure of Different Samples samples cordierite 20-Al2O3(Sol-1) 20-Al2O3(Sol-2) 22-Al2O3(Sol-3) CuO/20-Al2O3(Sol-1) CuO/20-Al2O3(Sol-2) CuO/22-Al2O3(Sol-3) CuO/20-Al2O3(Sol-1)-wash

total pore average pore BET surface area (m2/g) volume (cm3/g) size (nm) 164.0 90.4 64.2 77.5 61.2 60.1 68.1 56.6

0.095 0.069 0.065 0.067 0.064 0.062 0.059 0.054

2.32 3.07 4.25 3.77 3.74 4.12 3.50 3.78

the Al2O3 coating by Sol-2 is the most stable, while that by Sol-3 is the least stable. The XRD patterns in Figure 3 shows that the scraps from 20-Al2O3(Sol-1) (termed Scrap-1) and 22-Al2O3(Sol-3) (termed Scrap-3) are amorphous Al2O3 with small amounts of γ-Al2O3. Since little scraps were collected during the ultrasonic treatment of 20-Al2O3(Sol-2), XRD analysis was not performed on Scrap-2. Figure 6 shows SEM photographs of the Al2O3-coated samples after the ultrasonic treatment. Compared to the corresponding photographs in Figure 5, it can be found that the ultrasonic treatment has little effect on the appearance of 20Al2O3(Sol-1) and 20-Al2O3(Sol-2) but a significant effect on the appearance of 22-Al2O3(Sol-3). These differences are consistent with the findings that low viscosities of Sol-1 and Sol-2 allow penetration of Al2O3 into the pores of the cordierite, and repeated calcination operations promote diffusion of Al2O3 into the cordierite bulk. 3.1.4. Pore Structure of the Al2O3-Coated Samples. Table 2 shows pore-structure parameters of the cordierites with or without Al2O3 coating. The BET surface area and pore volume of the cordierite are 164 m2/g and 0.095 cm3/g, respectively. In comparison, the Al2O3-coated samples show lower BET surface area and lower pore volume, indicating filling and/or blocking of the cordierite’s pores by Al2O3. The surface area and pore volume of 20-Al2O3(Sol-1) are the largest, 90.4 m2/g and 0.069 cm3/g, respectively, while those of 20-Al2O3(Sol-2) are the least,

Figure 7. SCR activities of the catalysts (1600 ppm SO2, 600 ppm NO, 600 ppm NH3, 5.0% O2, 2.5% H2O; 400 °C; GHSV of 7800 h-1).

64.2 m2/g and 0.065 cm3/g, respectively. This phenomenon is difficult to understand because it cannot be correlated directly with the samples’ SEM photographs and the grain size of the sols. 3.2. SCR Activities of the Catalysts. Loading 1 wt % CuO on Al2O3-coated cordierites yields SCR catalysts. Their pore properties are also shown in Table 2. As can be seen, the surface area and pore volume of the catalysts are all similar but smaller than those of the supports, Al2O3-coated cordierites. SCR activities of the catalysts are shown in Figure 7, along with that of CuO/cordierite (the cordierite loaded with 1 wt % CuO) for comparison. Obviously, NO conversions of all the catalysts are much higher than that of CuO/cordierite, indicating that Al2O3 coatings are a crucial component of the SCR catalyst. CuO/22-Al2O3(Sol-3) shows the highest steady-state NO conversion, about 82%. CuO/20-Al2O3(Sol-2) shows a NO conversion of 75%, and CuO/20-Al2O3(Sol-1) shows the lowest NO conversion, about 70%.

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Figure 8. SCR activities of catalysts with washed supports (1600 ppm SO2, 600 ppm NO, 600 ppm NH3, 5.0% O2, 2.5% H2O; 400 °C; GHSV of 7800 h-1).

It was reported that Cl- in the catalyst is unfavorable to the SCR reaction.15,16 Since Sol-1 is prepared from AlCl3, it is very possible that CuO/20-Al2O3(Sol-1) contains Cl-, which poisons its SCR activity. To avoid this possible effect of Cl-, all the supports (Al2O3-coated cordierites) were washed thoroughly with deionized water until no Cl- was found in the used water (tested by AgNO3). Results shows that the residual Cl content is 0.03 wt % for the washed 20-Al2O3(Sol-1) and zero for the other two washed supports. The washed supports were then loaded with 1 wt % CuO and subjected to the SCR reaction. Figure 8 shows that CuO/20-Al2O3(Sol-2)-wash and CuO/22Al2O3(Sol-3)-wash yield NO conversions similar to those of the unwashed catalysts (Figure 7), but CuO/20-Al2O3(Sol-1)-wash yields a much higher NO conversion than CuO/20-Al2O3(Sol-1) (Figure 7). To further verify the presence of Cl- in CuO/20-Al2O3(Sol1), H2-TPR was performed on all three catalysts along with a 1% CuCl2-loaded sample (termed CuCl2/20-Al2O3(Sol-1)). Figure 9 shows releases of Cl2 and HCl during the H2-TPR. Clearly, CuO/20-Al2O3(Sol-1) shows Cl2 release similar to CuCl2/20-Al2O3(Sol-1) but little HCl release. In comparison, CuO/20-Al2O3(Sol-2) and CuO/22-Al2O3(Sol-3) show neither Cl2 release nor HCl release. These phenomena suggest that the Cl- introduced in the catalyst during preparation of the support 20-Al2O3(Sol-1) finally associates with copper, which deactivates the SCR activity. Obviously, SCR activity of the catalysts follows the order of CuO/20-Al2O3(Sol-1)-wash > CuO/22-Al2O3(Sol-3)-wash > CuO/20-Al2O3(Sol-2)-wash. Since the surface properties of these catalysts are very close, the difference in SCR activity may be attributed to the difference in the form of Al2O3, which then affects distribution of CuO. The catalysts, therefore, were subjected to a constant-temperature reduction (CTR) at 400 °C in H2. Figure 10 shows that H2 consumption of the support 20Al2O3(Sol-1)-wash is very little, while those of the catalysts are significant, following an order of CuO/20-Al2O3(Sol-1)-wash > CuO/22-Al2O3(Sol-3)-wash > CuO/20-Al2O3(Sol-2)-wash. This order is the same as that of SCR activity, which confirms that the form of Al2O3 affects dispersion of CuO and, thus, SCR activity. 4. Conclusions The type of alumina sols has a significant influence on the activity of honeycomb CuO/Al2O3/cordierite catalysts for SCR of NO at 400 °C. The amount and the stability of the Al2O3

Figure 9. MS results of various samples during H2-TPR.

Figure 10. Constant-temperature reduction of the catalysts in H2 at 400 °C.

coated onto the cordierite in each cycle are affected by viscosity and grain size of the sols. The sols prepared from Al and AlCl3 (Sol-1) and from Al (NO3)3 and urea (Sol-2) show very low viscosity and are able to fill some of the cordierite’s pores, which results in the formation of relatively thin but firm Al2O3 coatings. The sol prepared from pseudoboehmite and HNO3 (Sol-3) has a high viscosity, which inhibits its filling in the cordierite’s pores and results in a relatively loose and thick A2O3 coating. The form of Al2O3 coating prepared from these sols is different, which affects CuO distribution and, thus, the SCR activity. Water washing is crucial for a good SCR catalyst when Sol-1

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is used to coat Al2O3. Sol-1 is the best among the three sols in terms of less coating cycle, stability of the coating, and SCR activity. Acknowledgment The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (20736001, 20606002) and The 863 Program (2007AA05Z310). Literature Cited (1) Liu, Q. Y.; Liu, Z. Y.; Li, C. Y. Adsorption and activation of NH3 during selective catalytic reduction of NO by NH3. Chin. J. Catal. 2006, 27, 636. (2) Djerad, S.; Crocoll, M.; Kureti, S.; Tifouti, L.; Weisweiler, W. Effect of oxygen concentration on the NOx reduction with ammonia over V2O5WO3/TiO2 catalyst. Catal. Today 2006, 113, 208. (3) Richter, M.; Bentrup, U.; Eckelt, R.; Schneider, M.; Pohl, M.-M.; Fricke, R. The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al2O3. Appl. Catal., B 2004, 51, 261. (4) Pereira, C. J.; Phumlee, K. M. Grace Camet metal monolith catalytic emission control technologies. Catal. Today 1992, 13, 23. (5) Byrne, J. W.; Chen, J. M.; Speronello, B. K. Selective catalytic reduction of NOx using zeolitic catalysts for high temperature applications. Catal. Today 1992, 13, 33. (6) Beeckman, J.; Hegedus, L. Design of monolith catalysts for power plant nitrogen oxide (No.+-.) emission control. Ind. Eng. Chem. Res. 1991, 30, 969.

(7) Jeong, S. M.; Kim, S. D. Removal of NOx and SO2 by CuO/Al2O3 sorbent/catalyst in a fluidized-bed reactor. Ind. Eng. Chem. Res. 2000, 39, 1911. (8) Centi, G.; Passarini, N.; Perathoner, S.; Riva, A. Combined DeSOx/ DeNOx reactions on a copper on alumina sorbent-catalyst. 1. Mechanism of SO2 oxidation-adsorption. Ind. Eng. Chem. Res. 1992, 31, 1947. (9) Chi, Y.; Chuang, S. S. C. The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3. Catal. Today 2000, 62, 303. (10) Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined SO2/NOx Removal From Flue Gas. EnViron. Prog. 1985, 4, 223. (11) Liu, Q. Y.; Liu, Z. Y. A honeycomb catalyst for simultaneous NO and SO2 removal from flue gas: Preparation and evaluation. Catal. Today 2004, 93-95, 833. (12) He, Z. F.; Shao, Q.; Chen, N. Y.; Da, Z. J. China Patent CN1436599, 2003. (13) Shelleman, R. A.; Messing, G. L.; Kumagai, M. Alpha alumina transformation in seeded boehmite gels. J. Non-Cryst. Solids 1985, 82, 277. (14) Gonza′lez-VelascoT, J. R.; Ferret, R.; Lo′pez-Fonseca, R.; Gutie′rrezOrtiz, M. A. Influence of particle size distribution of precursor oxides on the synthesis of cordierite by solid-state reaction. Powder Technol. 2005, 153, 34. (15) Choung, J. W.; Nam, I. S. Characteristics of copper ion exchanged mordenite catalyst deactivated by HCl for the reduction of NOx with NH3. Appl. Catal., B 2006, 64, 42. (16) Lisi, L.; Lasorella, G.; Malloggi, S.; Russo, G. Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts. Appl. Catal., B 2004, 50, 251.

ReceiVed for reView January 20, 2008 ReVised manuscript receiVed April 5, 2008 Accepted April 16, 2008 IE800105P