γ-Al2O3 Sorbents for SO2 in

Ind. Eng. Chem. Res. 2007, 46, 1975-1980

1975

Thermogravimetric Study of CuO/γ-Al2O3 Sorbents for SO2 in Simulated Flue Gas Qingchun Yu,† Shichao Zhang,*,‡ and Xindong Wang† Department of Physical Chemistry, UniVersity of Science and Technology-Beijing, Beijing 100083, People’s Republic of China, and Department of Applied Chemistry, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China

A series of CuO/γ-Al2O3 sorbents with different CuO loading prepared by impregnation were characterized using X-ray diffraction (XRD). Temperature-programmed reduction (TPR) was used to study the reducibility of the sorbent. The sulfation activity and sulfation cycles of sorbents were investigated by thermogravimetric techniques, and the change in the pore structure of the sorbent was also studied by Brunauer-EmmettTeller (BET) methods. It was determined that the monolayer coverage of CuO determined by X-ray quantitative analysis was 0.275 g CuO/(g γ-Al2O3). Below monolayer coverage, CuO was highly dispersed on the γ-Al2O3. Compared to unsupported CuO, microparticles of CuO were reduced easier by H2, but the highly dispersed CuO was difficult to reduce by H2. The optimal CuO loading was 0.12 g CuO/(g γ-Al2O3), far below its monolayer coverage. When CuO loading was equal or more than 0.07 g CuO/(g γ-Al2O3), the sulfated sorbent could be fully reduced by H2. γ-Al2O3 greatly participated in the sulfation, and the negligence of that was reasonable during the reaction between CuO and SO2. Higher temperature could reduce the sulfation time of the copper compound from CuO to CuSO4. The pore structure of the sulfated sorbent could recover to that of the fresh sorbent after regeneration, but long sulfation times could cause an increase in surface area and a decrease in regeneration of the sulfated sorbent. 1. Introduction Air pollution arising from the emission of SOx from the combustion of fossil fuels has been recognized as a problem. Various sorption processes are under operation to remove SOx from flue gas. Currently, the primary method of SO2 removal from flue gas is a scrubbing process with calcium-based sorbents, which results in a huge amount of waste. As a result, there is a strong incentive worldwide to develop efficient processes for dry regenerative sorbents. Recently, several impregnated metal oxides supported on alumina for the removal of SO2 from a SO2-N2-O2-H2O blend in a fixed bed were tested. It has been observed that CuO supported on γ-Al2O3 is an effective means to remove SO2, because sulfation and regeneration reactions readily occur,1-4 and it also is able to convert the captured SO2 to highly valued byproducts, such as sulfuric acid, elementary sulfur, ammonia sulfate, and concentrated SO2 gas, which can be used to compensate the process costs. Meanwhile, the copper oxide technology can reduce both SO2 and NOx in a single unit. Recently, much attention has been given to the sulfation of γ-Al2O3 supports. Lin5 reported that at e450 °C, sulfation on the γ-Al2O3 support became negligible. Some authors reported this sulfation step to proceed already at temperatures of 420600 K,6 whereas others claimed that sulfates were not formed at temperatures of 750 K, but the amount of SO2 adsorbed, even at those higher temperatures, is much lower than that observed for the copper-on-alumina system.8 Centi and co-workers9,10 reported that the direct participation of the γ-Al2O3 support, which caused the conversion of all CuO to CuSO4, was >1.0 in thermobalance * To whom correspondence should be addressed. Tel.: 86-1082338148. Fax: 86-10-82339319. E-mail: [email protected]. † University of Science and Technology-Beijing. ‡ Beijing University of Aeronautics and Astronautics.

experiments. They suggested that the mechanism of reaction could be described as a stage of catalytic oxidation of SO2 on Cu sites to SO3. The chemisorbed SO3 migrates from the Cu site to the Al site, and sulfate species linked to copper also formed, leaving a reduced copper oxide site that can be rapidly regenerated by O2. The metal loading affects the metal oxide crystallite and the bonding interaction between the metal oxide and the support. Strohmeier et al.11 studied the appearance of CuO in the X-ray difraction (XRD) patterns of sorbents, and they determined that after the support is saturated, agglomeration of the copper species would occur, forming microparticles large enough to be detected by XRD. The saturation value of CuO loading is 13% Cu. CuO microparticles are observed to be less active in the sulfation reaction, because of the formation of a first layer sulfate, which inhibits the sulfation of the subsurface layers from proceeding further. However, the effect of CuO loading on sulfation and regeneration of the sorbent has not been reported. In the present work, sorbents with different CuO loadings have been characterized by XRD. The reducibility, activity, and sulfation cycles of sorbents with different CuO loading have been investigated by temperature-programmed reduction (TPR) and thermogravimetric techniques. Also, the change in the pore structure of the sorbent has been discussed. 2. Experimental Section 2.1. Materials and Preparation of Sorbents. Cu(NO3)2‚ 3H2O and KCl were of high purity and commercially available. γ-Al2O3 (with a Brunauer-Emmett-Teller (BET) surface area of 277.8 m2/g) was obtained from SINOPEC. CuO/γ-Al2O3 sorbent was prepared by wet impregnation with a known mass of γ-Al2O3 and solution containing a calculated amount of Cu(NO3)2‚3H2O. While being kept under static conditions for 1 h at room temperature, the excess water was evaporated at 90 °C with stirring. The samples were dried in an oven at 120 °C for

10.1021/ie0612358 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

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Figure 1. Schematic diagram of the experimental setup.

24 h and were subsequently calcined at 450 °C in stagnant air for 5 h in a muffle furnace to convert Cu(NO3)2 to CuO. In this way, 11 samples, containing 0.03-0.55 g of CuO, were prepared and labeled according to their copper content (e.g., 0.03CuAl). 2.2. Experimental Setup. Sulfation, regeneration, and TPR tests were all conducted in a thermogravimetric setup similar to that depicted in Figure 1. The experimental setup consisted of three sections: a reactor, a feeding system, and an analytical balance. The output signals corresponding to the weight changes were automatic recorded in a computer. The gas mixture was measured and controlled by a flowmeter. A humidifier was placed in a constant-temperature water bath apparatus to control the saturation content of the water vapor. XRD patterns were recorded in the range of 10° e 2θ e 80°, using a model D/MAX-γ A diffractometer with Cu KR radiation (40 kV, 50-100 mA). The data of BET surface areas, pore volume, and pore size of samples were measured on the AUTOSORB-1C Instruments, using liquid N2 as the adsorbent. 2.3. Procedure. A sample pan loaded with 50 mg of sorbent was placed in the reaction zone and then heated to the desired temperature. Argon was introduced from the head of the setup, to protect the balance from erosion by SO2 in the flue gas. Humidified mixtures of N2 and O2 were fed into the reactor. When the weight change of the sample reached steady state, SO2 was introduced. The calculated typical gas composition was 2000 ppm SO2, 5% O2, and 3% H2O, with N2 as the balance. Regeneration of the sulfated CuO/γ-Al2O3 sorbent was performed in a 5 vol % H2/Ar mixture at the desired reduction temperatures. The reactor then was purged with argon and the temperature was adjusted to the desired sulfation temperatures. To avoid the mixing of H2 and O2, argon was introduced for at least 20 min before the next sulfation. When the weight change of sample recovered to steady state, SO2 was fed to the reactor again for the consecutive sulfation. The sulfation temperature and reduction temperature were labeled as Ts and Tr, respectively. To reduce a possible effect of external and internal transport phenomena, a weight of 50 mg in the 20-80 µm size range, with an average fractional porosity of 0.1 and a flow rate of 300 mL/min, was used. TPR tests of the sorbents were conducted with a 5 vol % H2/Ar mixture and a heating rate of 10 °C/min.

2.4. Data Evaluation. The chemical reactions that occur during the sulfation and reduction of sorbents are as follows:

1 CuO + SO2 + O2 ) CuSO4 2

(1)

3 Al2O3 + 3SO2 + O2 ) Al2(SO4)3 2

(2)

CuSO4 + 2H2 ) Cu + SO2v + 2H2Ov

(3)

1 Cu + O2 ) CuO 2

(4)

CuO + H2 ) Cu + H2O

(5)

Weight gain (∆m), plotted as a function of time, is obtained from the thermogravimetry results. ∆m is calculated at each time value (∆m ) mt - m0, where m0 is the mass of the sorbent at the start of sulfation, and mt is the mass of the sorbent at time t). Correspondingly, the weight loss ∆m is negative. Conversion (X), which is defined as X ) ∆m/∆mmax, is calculated for each time value and plotted as a function of time t. ∆mmax is the theoretical weight gain obtained according to eq 1, considering the composition and weight of the sorbent. 3. Results and Discussion 3.1. XRD Characterization. The state and dispersion of copper oxide supported on γ-Al2O3 is dependent on both the metal loading and the calcination temperature.11 At a moderate temperature (350-500 °C), we use XRD to study the influence of CuO loading. Figure 2 shows the XRD patterns of the CuO/γ-Al2O3 sorbents with different CuO loadings. As can be seen from XRD patterns 1 and 3 in Figure 2, there are no characteristic peaks of the γ-Al2O3 support and sorbents with low CuO loading (i.e., 0-0.1 CuAl). In comparison, XRD pattern 2 in Figure 2 shows the XRD pattern of a physically combined mixture containing the same amount of CuO and γ-Al2O3 as that of the sorbent from XRD pattern 3. The characteristic peaks of crystalline CuO are evident. These results indicate that the disappearance of the CuO peaks in the low-CuO-loading sorbents is due to the high dispersion of the supported species, rather than the detection sensitivity of the apparatus. However, as shown in XRD patterns 4-6 in Figure 2, when the CuO loading exceeds a critical value

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Figure 4. Temperature-programmed reduction (TPR) profiles of unsupported CuO, 0.1CuAl, and 0.5CuAl. Figure 2. X-ray diffraction (XRD) patterns of CuO/γ-Al2O3 samples with different CuO loadings: (1) γ-Al2O3, (2) 0.10 CuAl (physically mixed), (3) 0.10 CuAl, (4) 0.35 CuAl, (5) 0.40 CuAl, and (6) 0.50 CuAl.

Figure 5. Effect of CuO loading on the sulfation of sorbents.

Figure 3. XRD quantitative analysis results.

(i.e., the monolayer coverage), the characteristic peaks of crystalline CuO can clearly be detected and the peak intensities increase as the CuO loading increases. XRD quantitative analysis12,13 is used to determine the dispersion capacity of the copper oxide on the γ-Al2O3. KCl is used as a reference material, to eliminate any matrix effect. Figure 3 shows the ratios of the diffraction peak intensities of CuO versus the total mass of CuO in the sorbents. The monolayer coverage of copper oxide is determined to be 0.275CuAl, corresponding to 13.4% Cu (the γ-Al2O3 surface area is 196 m2/g), which is in accordance with the saturation value of CuO loading11 but slightly greater than the value reported by Freedman.14 Strohmeier11 noted that it the difference was related to the calcination temperature. Gao et al.15 studied the monolayer dispersion of oxide additives on SnO2, and they reported that monolayer coverage measured by XRD was consistent with that measured by X-ray photoelectron spectroscopy (XPS). 3.2. Temperature-Programmed Reduction. With the objective of understanding of the reducibility of the prepared sorbents with different CuO loading, the results of TPR experiments performed on unsupported CuO, 0.1CuAl, and 0.5CuAl are shown in Figure 4. Differential thermogravimetry (DTG) peaks are obtained using a differentiation method. In the case of unsupported CuO, weight loss begins at ∼310 °C, then a quick decrease in weight follows, with a DTG peak at ∼350 °C. This result shows that unsupported CuO is easily reduced by H2. At low metal loadings, the formation of a “surface spinel” (which “resembles” CuAl2O4) predominates,11,14 and most of the Cu2+ ions are in a distorted octahedral geometry. These Cu2+ ions

are difficult to reduce to Cu0, because of their strong interaction with the support. For 0.1CuAl, a gentle weight loss, noted by a DTG peak at ∼570 °C, can be observed in Figure 4. Above monolayer coverage, CuO microparticles form on the γ-Al2O3 support.8 The TPR profile of 0.5CuAl exhibits a quick weight loss, with a DTG peak at ∼260 °C, and a slow weight loss, with a DTG peak at ∼550 °C. The appearance of two peaks in a TPR profile denotes the single-stage reduction (Cu2+ to Cu0) of two different types of Cu2+ species, or a two-stage reduction (Cu2+ to Cu+ to Cu0) of a single Cu2+ phase. Bond16 reported that there is no evidence for Cu+ species as an intermediate in the reduction of dispersed Cu2+ species on γ-Al2O3. Therefore, the first peak is related to the reduction of CuO microparticles, which are reduced slightly easier to Cu0 than unsupported CuO, because of their small size and greater reactivity;16 the latter peak is related to the reduction of Cu2+ species in a distorted octahedral geometry and is similar to the peak of 0.1CuAl. It can be concluded that the interaction between the copper oxide and the support exerts an influence on the change in the reduction temperature of copper oxide. 3.3. Activity Tests. Activity tests of sorbents have been performed by examining the CuO loading on the sulfation of sorbents, as shown in Figure 5. The specific rate of sulfation passes through a maximum for a defined surface coverage of copper oxide on alumina (i.e., 0.12 g CuO/(g γ-Al2O3), which is far below the monolayer coverage). In the preparation of sorbents for the flue gas desulfurization, monolayer coverage dispersion is highly desirable, to maximize the amount of active species that can react with SO2 and, hence, to improve their sorption capacity for SO2. However, that is not the case for the sulfation of CuO/γ-Al2O3 sorbents. To clarify this phenomenon, the specific state of CuO supported on γ-Al2O3 should be

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Figure 6. Effect of CuO loading on the first and second sulfation processes of sorbents: (1) first 0.03CuAl, (2) second 0.03CuAl, (3) first 0.05CuAl, (4) second 0.05CuAl, (5) first 0.07CuAl, (6) second 0.07CuAl, (7) first 0.1CuAl, and (8) second 0.1CuAl.

analyzed. Over the past two decades, several surface models concerning the monolayer have been proposed, and these can be divided into five categories: (1) Closed-packed model: the metal oxide might cover the surface of the support with the formation of a monolayer on the surface.13 (2) Incorporation model: metal ions are incorporated into the vacant sites on the surface of the support.17 (3) Surface symmetry model: metal ions will be arranged according to the symmetry of the cations on the support surface.18 (4) Modified monolayer structure: monolayer structures include isolated species and/or monolayer islands.19 (5) Solid-solid wetting model: metal ions migrate over the surface of the support and envelop them in a thin overlayer.20 According to the modified monolayer structure, the state of CuO possibly changes from isolated species to monolayer islands with increased CuO loading. Further study on the state of CuO supported on γ-Al2O3 shows that CuO can be classified into several types:21 (1) isolated Cu2+, (2) weakly magnetic associates, (3) small two- or three-dimensional clusters, and (4) large three-dimensional clusters. The former three types exist below monolayer coverage and cannot be detected via XRD. When isolated Cu2+ ions are sulfated, the CuSO4 that is formed has no influence on each other. The rate of weight gain will increase with increased CuO loading. As the CuO loading exceeds the amount of isolated Cu2+ species, CuO begins to link with each other, and weakly magnetic associates and small two- or three-dimensional clusters will form. When these sorbents are sulfated, the formed CuSO4 will cover adjacent CuO, because of size expansion of the copper compound from CuO to CuSO4,22 and cause the adjacent CuO to be difficult to be sulfated. The rate of weight gain decreases as the CuO loading increases and makes the optimal CuO loading be realized far below its monolayer coverage. 3.4. Sulfation Cycles. 3.4.1. Effect of CuO Loading. Experimental results of the first and second sulfation of CuO/ γ-Al2O3 sorbents with different CuO loadings are given in Figure 6. The values of Ts and Tr are 350 and 400 °C, respectively. The weight gain of these four sorbents increase with sulfation time. Because the CuO loading in these sorbents are below optimal CuO loading, the effect of CuO loading on the rate of sulfation becomes more significant as the CuO loading increases (as shown in plots 1, 3, 5, and 7 in Figure 6). Habashi et al.23 reported that the reduction of crystalline CuSO4 and Al2(SO4)3 starts to occur at temperatures above 280 and

Figure 7. Effect of prolonged time on the sulfation cycles of 0.07CuAl sorbent.

600 °C, respectively. For the 0.03CuAl and 0.05CuAl sorbents, the weight gain of the first sulfation is greater than that of the second sulfation, which shows that Al2(SO4)3 produced in the first sulfation cannot be reduced by H2. For the 0.07CuAl and 0.1CuAl, there is no discrepancy of weight gain between the first and second sulfation processes. These results indicate that γ-Al2O3 does not participate in the sulfation reaction at higher CuO loadings (i.e., above 0.07CuAl). According to Centi et al.,10 a discrepancy should exist, despite the CuO loading. However, our experimental results show that it is true only for sorbents with low CuO loadings, such as 0.03CuAl and 0.05CuAl. Wachs24 reported that supported metal oxide catalysts possess surface metal oxide phases that are formed by the reaction of the deposited metal oxides with the surface hydroxyls of the high-surface-area oxide supports. Consequently, the reactive surface hydroxyls are reduced as the CuO loading increases, which causes the amount of SO2 adsorbed by the reactive surface hydroxyls to decrease until it disappears. Because the CuO loading is equal to or more than 0.07CuAl, reactive surface hydroxyls of the γ-Al2O3 support are probably depleted, and the discrepancy involving the weight gain between the two sulfation processes disappears. 3.4.2. Effect of Sulfation Time. To confirm whether or not γ-Al2O3 participates in sulfation, we prolonged the sulfation time of the 0.07CuAl sorbent under the same conditions. The experimental results are shown in Figure 7. The weight gain discrepancy between the two sulfation processes shows that the γ-Al2O3 support actively participates in the sulfation. In the case of γ-Al2O3 support, SO2 can be chemisorbed at Lewis acid sites, basic surface hydroxyls, and O2- sites.7,25 Even if the surfacereactive hydroxyls are depleted, other adsorption sites can adsorb SO2 continuously and convert sulfites to sulfates upon oxidation. Long sulfation times limit the regeneration of sulfated sorbent and results in degradation of sulfation performance. 3.4.3. Effect of Temperature. Figure 8 shows the first and second sulfation processes of the 0.1CuAl sorbent with sulfation temperatures of 350 and 450 °C and reduction temperatures of 400 and 450 °C, respectively. It can be seen that, at Ts ) 350 °C, the first and second sulfation processes almost coincide. When Ts is increased to 450 °C, the sulfation rate obviously increases and the discrepancy between these two sulfation processes appears at ∼2000 s. This is because the higher sulfation rate of CuO shortens the sulfation time needed for CuO, and then the formation of Al2(SO4)3, which cannot be reduced, follows. Consequently, the discrepancy disappears. These results suggest that the sulfation of CuO occurs first, and after the completion of that activity, the bulk sulfation of γ-Al2O3 begins. Conversion approaching a value of X ) 1 at

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Figure 8. Effect of temperature on the sulfation cycles of 0.1CuAl sorbent. Figure 10. N2 adsorption-desorption isotherm plots of 0.12CuAl sorbents: (a) sulfation time ) 5000 s and (b) sulfation time ) 5000 s. Table 1. Pore Structure Parameters of 0.12CuAl Sorbent before and after Sulfation and Also after Regeneration

Figure 9. Pore-size distribution of 0.12CuAl sorbent before and after sulfation and also after regeneration: (b) fresh sorbent, (9) sulfated sorbent (sulfation time ) 5000 s), (2) regenerated sorbent (sulfation time ) 5000 s, reduction time ) 3000 s), and (1) sulfated sorbent (sulfation time ) 12 000 s).

2000 s can confirm the aforementioned discussion. Note that the sulfation of the γ-Al2O3 support, which begins after the completion of sulfation of CuO, is not absolute, because of the existence of other adsorption sites; however, the negligence of that is reasonable. This simplification is helpful in the theoretical calculations of the reaction between SO2 and the CuO/γ-Al2O3 sorbent. 3.5. Changes in the Pore Structure of the Sorbent. Pore structure parameters of the 0.12CuAl sorbent before and after the sulfation and also after the regeneration, tested using a N2 isothermal adsorption method, are shown in Table 1. The pore size distribution of these sorbents is shown in Figure 9. Table 1 shows that the surface area, pore volume, and pore size of sulfated sorbent with a sulfation time of 5000 s decreases because of the sulfation of CuO,22 compared to that of fresh sorbent. After regeneration, the surface area, pore volume, and pore size of the regenerated sorbent with a sulfation time of 5000 s approach the values of the fresh sorbent. An abnormal observation is made in that the surface area and pore volume of sorbent with a sulfation time of 12 000 s are higher than those of a sorbent with a sulfation time of 5000 s and even higher than that of pure γ-Al2O3 (BET surface area ) 277.8 m2/g). To analyze the effect of sulfation time on the change of pore structure of sorbent, the N2 adsorption-desorption isotherm plots of sorbents with different sulfation time are shown in Figure 10. Hysteresis loops of this type may be given by cylindrically shaped capillaries that are open at both ends.26 At low relative pressure, the isotherms exhibit high adsorption, indicating that the powder contains micropores. However, at

sorbent

sulfation time (s)

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (Å)

fresh sulfated regenerated sulfated

5000 5000 12000

1.381 × 102 1.250 × 102 1.403 × 102 3.608 × 102

5.037 × 10-1 4.146 × 10-1 5.287 × 10-1 1.166 × 100

1.459 × 102 1.326 × 102 1.424 × 102 1.293 × 102

middle and high relative pressures, the curves exhibit hysteresis loops, which indicate the presence of mesopores.27 As shown in Figure 10, there is an increase both in micropores and mesopores of sorbent with a sulfation time of 12 000 s, which means long sulfation times exert different influence on the change in the pore structure of the sulfated sorbent. In the literature, there exists two opinions about the change of surface area before and after sulfation: one suggests that, after sulfation, the surface area decreases because of the pore plugging that is caused by the formation of Al2(SO4)328 or CuSO4;22 the other proposes that the increase in surface area might be ascribed to a certain amount of opening-up of previously blocked pores that occurs during sulfation.3 Both cases are observed in the present study. During a continuous increasing temperature treatment, the surface area of γ-Al2O3 increases to >400 °C and then decreases,29,30 which means the amount of surface OH is related to temperature. Therefore, one remote possibility for the sulfated sorbent with a short sulfation time (5000 s) is that micropores and mesopores formed at 350 °C adsorb water molecules again when the sample is taken out of the experimental setup at room temperature, and some water molecules remain in the BET experiment. For a sulfated sorbent with a long sulfation time (12 000 s), the surface adsorption sites are occupied by sulfates, which reduce the adsorption of water molecules. The micropore and mesopore structure can be retained, even in the BET experiment. The increase in the surface area that is caused by the removal of the surface OH group is so great that it can disguise the reduction in surface area that is caused by the formation of Al2(SO4)3 and CuSO4; however, quantitative analysis of the amount of surface OH requires further investigation. 4. Conclusion Monolayer coverage of CuO supported on γ-Al2O3, as determined by X-ray quantitative analysis, was 0.275CuAl. Below monolayer coverage, CuO was highly dispersed on γ-Al2O3. The experimental results show that thermogravimetric

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techniques are useful in regard to studying the reducibility, sulfation, and regeneration of the CuO/γ-Al2O3 sorbent. For the reduction of sorbent with higher CuO loading, there are two weight loss processes, corresponding to the reduction of CuO microparticles and dispersed CuO phase, respectively. The former is reduced easier by H2, compared to unsupported CuO, but the latter is difficult. With increased CuO loading, the activity of sorbent increases and then decreases at 0.12CuAl. Long sulfation times can limit the regeneration of the sulfated sorbent and result in degradation in the sulfation performance. This will inevitably reduce the lifetime of the sorbent and discount the merits of copper oxide technology. Higher temperatures can reduce the sulfation time of the copper compound from CuO to CuSO4. When the CuO loading is equal to or more than 0.07CuAl, regeneration of sulfated sorbent can be fully realized. The reaction between CuO and SO2 begins first; after the completion of that process, sulfation of γ-Al2O3 begins at a slow rate. The negligence of sulfation of the γ-Al2O3 support is reasonable and can simplify some theoretical calculations of reaction between SO2 and the CuO/γ-Al2O3 sorbent. The surface area, pore volume, and pore diameter of the sorbent decrease when it is sulfated and recover as that of fresh sorbent after regeneration. Sorbents with a long sulfation time cause an increase in surface area and pore volume, compared to sorbents with a short sulfation time. Acknowledgment We acknowledge the financial support from the National Natural Science Foundation of China (Project No. 90210003) and Natural Science Foundation of Beijing City (Project No. 2012010). Literature Cited (1) McCrea, D. H.; Forney, A. J.; Myers, J. G. Recovery of sulfur from flue gases using a copper oxide absorbent. J. Air. Pollut. Contr. Assoc. 1970, 20, 819. (2) Vogel, R. F.; Mitchell, B. R.; Massoth, F. E. Reactivity of SO2 with supported metal oxide-alumina sorbents. Eng. Sci. Technol. 1974, 8, 432. (3) Yates, J. G.; Best, R. J. Kinetics of the reaction between sulfur dioxide, oxygen, and cupric oxide in a tubular, packed bed reactor. Ind. Eng. Chem Process Des. DeV. 1976, 15, 239. (4) Deberry, D. W.; Sladek, K. J. Rates of reaction of SO2 with metal oxides. Can. J. Chem. Eng. 1971, 49, 781. (5) Lin, Y. S.; Deng, S. G. Removal of trace sulfur dioxide from gas stream by regenerative sorption processes. Sep. Purif. Technol. 1998, 13, 65. (6) Pieplu, A.; Saur, O.; Lavalley, J. C.; Pijolat, M.; Legendre, O. A kinetic model for alumina sulfation. J. Catal. 1996, 159, 394. (7) Chang, C. C.; Infrared studies of SO2 on γ-Alumina. J. Catal. 1978, 53, 374. (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) Centi, G.; Passarini, N.; Perathoner, S.; Riva, A. Combined DeSOx/ DeNOx reactions on a copper on alumina sorbent-catalyst. 2. Kinetics of DeSOx reaction. Ind. Eng. Chem. Res. 1992, 31, 1956.

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ReceiVed for reView September 20, 2006 ReVised manuscript receiVed January 16, 2007 Accepted January 30, 2007 IE0612358