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Ind. Eng. Chem. Res. 1999, 38, 2210-2215
KINETICS, CATALYSIS, AND REACTION ENGINEERING Selective Catalytic Reduction of NO by NH3 over a Bulk Sulfated CuO/γ-Al2O3 Catalyst Sang Mun Jeong, Soon Hwa Jung, Kyung Seun Yoo,† and Sang Done Kim* Department of Chemical Engineering and Energy & Environment Research Center, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
Selective reduction of nitric oxide by ammonia over sulfated CuO/γ-Al2O3 was carried out in a fixed-bed reactor. The optimum temperatures of the fresh and sulfated CuO/γ-Al2O3 in the selective catalytic reduction of NO are found to be 350 and 450 °C, respectively. NO reduction with the sulfated catalyst is somewhat higher than that of the fresh catalyst. The amount of N2O formation over the fresh and bulk sulfated catalysts is below 10% of NO in the feed stream. The intermediate in NO reduction by NH3 over the bulk sulfated catalyst is found to be ammonium sulfate from IR study. The activation energies of the fresh, surface, and bulk sulfated catalysts are found to be 29.4, 76.6, and 76.3 kJ/mol, respectively. Introduction Sulfur dioxide (SO2) and nitrogen oxides (NOX) are the major air pollutants, which have to be removed before emitting flue gas into the atmosphere. Various processes are under operation to remove SOX and NOX from flue gas.1 It has been reported that metal oxides impregnated in porous metal support provide one of the most effective ways to remove NOX by the selective catalytic reduction (SCR) method with ammonia.2 Vanadium oxide supported on TiO2 is known to be the most effective catalyst for NOX removal.3 However, this catalyst deactivates in the presence of SO2 due to fouling by ammonium bisulfate and formation of an inactive vanadium sulfate species.4,5 Numerous research works on simultaneous removal of SOX and NOX have been carried out to find costeffective catalysts/sorbents. In simultaneous removal of SOX and NOX, the SCR process with CuO/γ-Al2O3 has distinct advantages to remove SOX and NOX in flue gas because of the predominant reactivity with SO2 in the presence of oxygen and easy regeneration under the reductant gas atmosphere.6-9 Recently, studies7,8,10,11 on sulfation, regeneration, and enhancement of the sorption capacity of CuO/γ-Al2O3 as the sorbent to remove SO2 have been reported by the present authors. It has been reported that the SO2 removal capacity of CuO/γ-Al2O3 can be largely enhanced by the sulfation conditions and alkali-salt promoters.7,11 In the simultaneous SOX and NOX removal process, the amount of sulfates in CuO/γ-Al2O3 sorbent/catalyst will gradually increase with the reaction time. The SCR characteristics of NO over the fresh and surface sulfated CuO/γ-Al2O3 catalysts have been reported in previous studies.5,12 However, bulk sulfated * Corresponding author. E-mail:
[email protected]. Tel: 82-42-869-3913. Fax: 82-42-869-3910. † Present address: Department of Environmental Engineering, Kwangwoon University, Seoul 139-701, Korea.
CuO/γ-Al2O3 is somewhat different from the fresh and the surface sulfated catalysts in the structure, chemical, and physical properties.7,8,11 Because NO reduction with the bulk sulfated catalyst are not available in the literature, NO removal characteristics by ammonia over the bulk sulfated catalyst have to be determined. Therefore, in the present study, the effects of reaction temperature and O2 and NH3 concentrations on SCR of NO by ammonia over the fresh and bulk sulfated CuO/ γ-Al2O3 catalysts have been determined in a fixed flow reactor. Also, FT-IR and temperature-programmed desorption (TPD) studies have been carried out to examine NH3 adsorption and desorption behavior on the catalysts. Experimental Section Catalyst Preparation. The catalyst was prepared by impregnating a copper precursor solution [Cu(NO3)2‚ 3H2O] into γ-Al2O3 of 0.65 mm mean diameter (Alcoa Co., Pittsburgh, PA). The alumina support was dried at 110 °C for 24 h and allowed to cool in a desiccator. After complete evaporation of moisture from the impregnated alumina in an oven, it was taken out and then calcined at 600 °C in a thermobalance reactor under air flow. The concentration of copper oxide in the calcined sorbent based on dry alumina was measured by atomic absorption spectroscopy. After calcination, uniformity of CuO distribution in CuO/γ-Al2O3 was verified by scanning electron microscopy and energydispersive X-ray spectroscopy (EDX). To prepare the sulfated catalyst, sulfation of CuO/γAl2O3 was carried out in a fixed-bed flow reactor under 1.5% SO2/air flow rate of 1 L/min for 3 h at 300 and 500 °C, which are the conditions to produce the surface and bulk sulfations, respectively. The sulfation type of CuO/γ-Al2O3 was confirmed by an X-ray diffractometer (XRD). Experimental Procedure. SCR of NO by ammonia over CuO/γ-Al2O3 catalyst was carried out in a quartz
10.1021/ie9807147 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2211
Figure 1. Schematic diagram of a quartz tubular fixed-bed reactor.
tubular reactor (0.02 m i.d.) as shown in Figure 1. The experimental equipment consists of three sections: reactor, gas feeding system, and gas analyzer. The CuO/ γ-Al2O3 catalyst (4 cm3) was packed in the reactor and heated to the desired reaction temperature by an IR heater with a proportional-integral-differential controller. When the system reached steady state at a desired temperature, the premixed simulated gas (400 ppm NO, 4% O2, and balanced N2) and 400 ppm ammonia ([NH3]/[NO] ) 1) were introduced into the upper part of the reactor with a space velocity of 30 000 h-1. The concentrations of NO, N2O, and NH3 were measured by gas analyzers (Chung Eng. Co., Korea; Hartmann & Braun Co., Germany; and Siemens Co., Germany) using a nondispersed Infrared (ND-IR) method. Also, the behaviors of adsorption and desorption of NH3 over the catalysts were measured by using a FT-IR spectrometer (Bomen, MB154 model) and mass quadrupoles (Balzers Quadstar 421). Results and Discussion Effect of Temperature on the Catalytic Activity of a Sulfated CuO/γ-Al2O3 Catalyst. When the CuO/ γ-Al2O3 catalyst is exposed in flue gas (SOX, NOX, etc.), a sulfation reaction takes place with SO2 and excess O2. The sulfation degree of alumina support can be classified into three types (surface, slightly deep, and bulk sulfations) as a function of reaction temperature and CuO loading.11 The effect of reaction temperature on catalytic activity and N2O formation with different sulfation types of CuO/γ-Al2O3 is shown in Figure 2 in which the S/Cu mole ratio was calculated by the following equation:
S/Cu )
(W - W0)/MSO3 (W0fCuO)/MCuO
(1)
where W is the weight of the catalyst at reaction time t, W0 is the initial weight of the catalyst, fCuO is the weight fraction of loaded CuO, and Mi is the molecular weight of species i.
Figure 2. Effect of the reaction temperature on (A) NO conversion and (B) N2O formation over 8 wt % CuO/γ-Al2O3 with a variation of S/Cu mole ratio: (b) fresh and (1) surface sulfated 4 wt % CuO/ γ-Al2O3 by Centi et al. (1992); (2) fresh and (9) sulfated (S/Cu ) 0.8) 5.7 wt % CuO/SiO2 by Kiel et al. (1992).
Also, NO conversion is defined as
NO conversion (%) )
(
)
CNOi - CNO CNOi
× 100
(2)
where CNOi is initial NO concentration and CNO is the effluent NO concentration. As can be seen in Figure 2A, NO conversions exhibit the maximum values with variation of reaction temperature regardless of the sulfation types of the catalysts. The temperature at which NO conversion exhibits the maximum value with the bulk sulfated CuO/γ-Al2O3 catalyst is 450 °C, which is about 100 °C higher than that with the fresh catalyst. The optimum temperature shift to higher temperature with the bulk sulfated catalyst could be related to adsorption and oxidation of NH3 on the catalysts. The decrease of NO conversion above the optimum temperature may be due to an increase in the parallel reaction rate of NH3 oxidation to N2 (4NH3 + 3O2 f 2N2 + 6H2O), which causes a decrease in surface ammonia for the reaction with NO (4NO + 4NH3 + O2 f 4N2 + 6H2O).5 Maximum NO conversion with the bulk sulfated catalysts (S/Cu ) 1.3 and 3.2) is higher than that with the fresh catalyst. This finding is contrary to the results of Centi et al.5 with the surface sulfated catalyst in regards to the maximum NO conversion and the optimum temperature shift. This difference between the bulk and surface sulfated catalysts may result from the variation of crystal and chemical structure in the catalyst by sulfation conditions.7 This enhanced activity of the bulk sulfated catalyst compared to that of the fresh catalyst may be
2212 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
Figure 4. IR spectra of NH3 adsorbed on (a) fresh and (b) bulk sulfated CuO/γ-Al2O3 catalysts treated in the flow of 1200 ppm NH3/He (30 cm3/min) at 300 °C. Figure 3. Effect of the reaction temperature on conversion of NH3 oxidation and NO formation over the fresh and sulfated catalysts in the atmosphere of 400 ppm NH3/3.0 vol % O2/N2 balance: (O, b) fresh catalyst, (0, 9) bulk sulfated catalyst, (], [) NiO-CuO/ γ-Al2O3 (Blanco et al., 1986). Table 1. Basic Reactions in the Selective Catalytic Reduction of NO by NH3 4NO + 4NH3 + O2 f 4N2 + 6H2O 4NO + 4NH3 + 3O2 f 4N2O + 6H2O 6NO + 4NH3 f 5N2 + 6H2O 8NO + 2NH3 f 5N2O + 3H2O 4NH3 + 3O2 f 2N2 + 6H2O 4NH3 + 5O2 f 4NO + 6H2O
(1) (2) (3) (4) (5) (6)
due to increases of the superacidity and the Brφnsted acidity that are caused by a surface ion of SO42- on the catalyst.12-14 As can be seen, NO conversion with a bulk sulfated catalyst having a higher S/Cu ratio ()5.7) is about 20% lower compared to the catalyst having a lower S/Cu ratio because the surface area and pore volume of the catalyst decrease significantly with the degree of sulfation.11 The amount of N2O produced in the reduction of NO by NH3 over the fresh and sulfated catalysts is shown in Figure 2B. The formation of N2O in reaction 2 (4NO + 4NH3 + 3O2 f 4N2O + 6H2O) below 400 °C in Table 1 over sulfated CuO/γ-Al2O3 catalysts is somewhat lower than that over the fresh catalyst; however, it becomes higher at higher temperature, as found previously with the CuO/SiO2 catalyst.12 Nevertheless, N2O formation over the alumina-based catalysts is below 50 ppm, which is much lower than that of the silica-based catalyst in the given temperature range. Oxidation of NH3 on the Sulfated CuO/γ-Al2O3. It is known that oxidation of NH3 directly affects the optimum temperature for NO conversion.5,15 NH3 is mainly oxidized by reaction 5 (4NH3 + 3O2 f 2N2 + 6H2O) and in part by reaction 6 (4NH3 + 5O2 f 4NO + 6H2O) in Table 1. The effect of reaction temperature on NH3 oxidation over the fresh and the bulk sulfated CuO/γ-Al2O3 in the absence of NO is shown in Figure 3. As can be seen, oxidation of NH3 over the bulk sulfated catalyst (S/Cu ) 3.2) starts to proceed at higher temperature by about 100 °C compared to that over the fresh catalyst. The temperatures at which conversion of NH3 oxidation reaches 90% conversion are 350 °C for the fresh catalyst and 450 °C for the bulk sulfated catalyst because of the difference in the surface compound with or without
sulfate species on the catalyst. NH3 over the bulk sulfated catalyst may not be oxidized by oxygen but adsorbs on the sulfate surface [Al2(SO4)3 and CuSO4] of the catalyst to produce ammonium sulfate below 300 °C. This ammonium sulfate is decomposed to produce SO2 above 600 K, and thereafter NH3 is oxidized to N2 and a small amount of NO.5,16 NO conversion starts to decrease at 350 °C with the fresh catalyst and 450 °C with the bulk sulfated catalysts (Figure 2), whereas NH3 oxidation exhibits their maximum values at those temperatures. Therefore, it can be claimed that the decrease of NO conversion in higher temperatures may be caused by oxidation of NH3 over the catalysts. The amount of NO formation from NH3 oxidation over the bulk sulfated catalyst is much smaller than that of the fresh catalyst. In the case of the fresh catalyst, effluent NO can be detected at 350 °C and increases up to about 25% of NH3 feed, whereas NO evolution cannot be detected up to 550 °C with the bulk sulfated catalyst. Adsorption and Desorption of NH3 on the Bulk Sulfated Catalyst. IR spectroscopy was employed to examine the adsorption behavior of NH3 on the catalysts. The catalysts were treated for 1 h in a 1200 ppm NH3/He atmosphere at 300 °C and thereafter purged by He flow to remove the physisorbed NH3 at 300 °C. The IR spectra of the catalysts at 300 °C are shown in Figure 4. As can be seen, IR spectra of NH3 adsorbed on the bulk sulfated catalyst (S/Cu ) 3.2) exhibit three bands (1090, 1360, and 1418 cm-1) and one shoulder (1131 cm-1) in the 1000-1500 cm-1 region. These peaks can be characterized by sulfate species produced during sulfation of the fresh catalyst. The sulfate species at the band of 1418 cm-1 is assigned to the surface sulfates on copper aluminate by the interaction of copper sulfate with alumina in the monolayer coverage.4 This sulfated species may be less stable and easily regenerated by the reducible gas.17 The broader band at 1360 cm-1 and the shoulder at 1131 cm-1 are assigned to the surface sulfates (SdO) with lower symmetry likened to Cu2+ or Al3+ ions. Also, the bulklike copper or aluminum sulfate species can be found at 1090 cm-1, which may be attributed to a S-bonded chemisorbed SO3 that interacts with the bulk Al site during the bulk sulfation of fresh CuO/γ-Al2O3.18 A multiplet in the NH stretching region (3000-3300 cm-1) and a broad band at 1638 cm-1 are observed to be due to NH3 adsorbed on the Lewis acid site.13,19 Also, a shoulder at 2867 cm-1 due to NH4+ may indicate the presence of Brφnsted acid sites at the surface of sulfated
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2213
Figure 5. TPD profiles of NH3 from (a) fresh and (b) bulk sulfated CuO/γ-Al2O3.
CuO/γ-Al2O3.20 Therefore, the chemical species on the bulk sulfated catalyst after NH3 adsorption can be assigned to ammonium sulfate as in the case of the surface sulfated catalyst5 that is formed by chemisorption of NH3 on sulfates [CuSO4 and Al2(SO4)3] in the catalyst. However, distinguished IR peaks cannot be observed in the case of the fresh catalyst because physisorbed NH3 was purged thoroughly. It was reported that ammonia desorbs preferentially from Brφnsted sites and readsorbs on Lewis sites freely.20 The above results may indicate that the sulfation of CuO/ γ-Al2O3 increases Lewis acid sites on the catalyst surface. To examine the desorption behavior of NH3 adsorbed on surfaces of the fresh and bulk sulfated (S/Cu ) 3.2) catalysts, a TPD technique was employed as shown in Figure 5. After the catalysts (0.1 g) were pretreated at 500 °C under a He flow for 1 h, those catalysts were kept in 1200 ppm NH3/He balance at room temperature for 2 h. The catalysts were flushed by He flow to remove the physisorbed NH3 at room temperature. Thereafter, the temperature was raised to 600 °C under a He flow (50 L/min) at a rate of 10 °C/min, and the concentration of NH3 in the gas phase was measured by mass quadrupoles (17 amu). NH3 adsorbed on the fresh catalyst (Figure 5a) is evolved at the temperature range from 100 °C to about 350 °C into the gas phase, whereas it evolved up to 600 °C from the bulk sulfated catalyst (Figure 5b). It has been reported that ammonia desorption from Brφnsted acid sites occurs at lower temperatures compared to Lewis acid sites because NH3 adsorbs more weakly on Brφnsted acid sites than on Lewis acid sites.21 Therefore, sulfate in the catalyst may increase Lewis acid sites, which enhances the catalytic activity for NO conversion.13 Effect of Reactant Concentration on Catalytic Activity. The optimum amount of NH3 supply in the SCR process is important for NO conversion, unreacted NH3 slip, and the operating cost. NO conversion, N2O formation, and the unreacted NH3 slip as a function of NH3 concentration are shown in Figure 6. As can be seen in Figure 6A, NO conversions over the fresh and bulk sulfated catalysts (S/Cu ) 3.2) at 350 and 400 °C increase above 90% with increasing NH3/NO mole ratio up to 1.2 and remain constant with a further increase in the mole ratio. However, NO conversion over the bulk sulfated catalyst at 350 °C, which is not the optimum temperature for the catalytic activity, increases with increasing NH3/NO mole ratio. This result may be due to the increase of the driving force in the reaction caused
Figure 6. Effect of NH3/NO mole ratio on (A) NO conversion and N2O formation and (B) NH3 slip over the fresh and bulk sulfated CuO/γ-Al2O3.
by stronger adsorption of concentrated NH3. Centi et al.5 has reported that NO conversion over 4.8 wt % CuO/ γ-Al2O3 passes through a maximum value at NH3/NO ) 1. However, reduction of NO conversion is not observed with higher concentrations of NH3 in the present study. N2O formation slightly increases with increasing mole ratio of NH3/NO at a concentration of NO below 10% of the NO feed due to strong adsorption of NH3 on the catalyst. The unreacted NH3 slip in NO reduction over the different catalysts as a function of the NH3/NO mole ratio is shown in Figure 6B. As can be seen, the unreacted NH3 slip over the fresh catalyst at 350 °C increases with increasing NH3/NO mole ratio up to 1.2 and then remains constant with a further increase in NH3 concentration due to active oxidation of unreacted NH3 over the fresh catalyst (Figure 3). However, the NH3 slip over the sulfated catalysts increases with the NH3/NO mole ratio at 300-400 °C because this temperature range is ineffective for oxidation of NH3 (Figure 3). Also, the NH3 slip in NO reduction over the bulk sulfated catalyst exhibits a trend similar to that over the surface sulfated catalyst as reported previously.5 Therefore, it can be claimed that the amount of unreacted NH3 slip can be related with oxidation of NH3. The evolved NH3 in the lower NH3/NO mole ratios for all of the catalysts is higher than the theoretical value from reaction 1 (4NO + 4NH3 + O2 f 4N2 + 6H2O) in Table 1, which may result from the insufficient gas mixing and low reactivity with NO at that temperature range. The effect of O2 concentration on NO conversion with the fresh catalyst at 350 °C and the bulk sulfated catalyst at 450 °C is shown in Figure 7 because the optimum temperatures for NO reduction over the fresh
2214 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
Figure 7. Effect of O2 concentration on NO conversion over the fresh and bulk sulfated CuO/γ-Al2O3 at 350 and 450 °C, respectively.
Figure 8. Arrhenius plot of the intrinsic reaction rate constants for the fresh and bulk sulfated CuO/γ-Al2O3 with the literature data: (a) CuO/CuSO4/bauxite, (b) CuO/CuSO4/Al2O3, (c) sulfated CuO/SiO2.
and bulk sulfated catalysts are 350 and 450 °C, respectively. As can be seen, only a small amount of O2 (0.3%) provides the maximum NO conversion, as observed previously with the NiO-CuO/γ-Al2O3 catalyst.22 Also, NO conversion reaches about 80% in the O2-free condition regardless of catalyst types in the present study. This high NO conversion in the O2-free condition may come from reaction 3 (6NO + 4NH3 f 5N2 + 6H2O) in Table 1 and participation of lattice oxygen ions in CuO or CuSO4 crystal for the NO-NH3 reaction.
kr ) 1.01 × 104 exp(-29.4 × 103/RT) for the fresh catalyst (6)
Reaction Kinetics of the Sulfated Catalyst. The reaction kinetic study is limited in the temperature range to neglect NH3 oxidation as a side reaction and simply considered the SCR of NO by NH3 according to reaction 1 (4NO + 4NH3 + O2 f 4N2 + 6H2O) in Table 1. In NO reduction, the reaction rates have been reported to be first order for NO and zero order for NH3.12,18 Therefore, the intrinsic reduction rate of NO can be expressed as
r ) krCNO
(3)
The intrinsic reaction rate constant, kr, can be calculated with an assumption of the gas-phase flows in plug flow as
kr ) 2.99 × 107 exp(-76.6 × 103/RT) for the surface sulfated catalyst (7) kr ) 7.11 × 107 exp(-76.3 × 103/RT) for the bulk sulfated catalyst (8) The activation energy of the fresh catalyst is lower than that of the sulfated catalysts because of the difference in pore diffusion in the catalyst that governs the reaction rate in NO reduction.23 Sulfation of CuO/γAl2O3 brings about a decrease in the surface area and average pore diameter, which may inhibit pore diffusion of the reactant gas in the catalyst.7,8,11 Although activation energies of the sulfated CuO/γ-Al2O3 in the present study are somewhat higher than those of Fukuzawa and Ishihara,24 they are comparable with those of Kiel et al.,12 who reported that the obtained activation energy is good enough for simultaneous removal of SOX/NOX. Therefore, it can be claimed that the sulfated catalysts in the present study have enough catalytic activity to remove NOX for practical purposes. Conclusions
kr ) -
Ug (1 - )H
ln(1 - X)
(4)
Also,
kr ) k0 exp(-Ea/RT)
(5)
From Arrhenius plots (kr vs reaction temperature) for the various catalysts (Figure 8), Ea and k0 of the fresh, surface (S/Cu ) 1.3), and bulk (S/Cu ) 3.2) sulfated catalysts are derived. The intrinsic rate constants of the catalysts can be expressed as
The characteristics of SCR of NO by NH3 over the sulfated CuO/γ-Al2O3 catalyst have been determined in a fixed-bed reactor. The optimum temperature for NO reduction over the sulfated CuO/γ-Al2O3 is 450 °C, which is 100 °C higher than that over the fresh catalyst. Also, NO conversion over the sulfated catalyst is somewhat higher than that over the fresh catalyst. In NO reduction by NH3 over the bulk sulfated catalyst, the intermediate product is found to be ammonium sulfate by IR analysis. The amount of N2O formation in NO reduction over the fresh and the bulk sulfated catalysts is below 10% of NO in the feed. The activation energies of the fresh, surface, and bulk sulfated catalysts are found to be 29.4, 76.6, and 76.3 kJ/mol, respectively.
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2215
Acknowledgment The authors acknowledge a grant-in-aid from Ministry of Science and Technology, Korea, and the alumina supporter from Alcoa Co. Nomenclature CNO ) concentration of NO in gas phase, mol mg-3 Ea ) activation energy, J mol-1 fCuO ) weight fraction of loaded CuO k0 ) frequency factor of the intrinsic rate constant, mg3 ms-3 s-1 kr ) intrinsic reaction rate constant, mg3 ms-3 s-1 H ) catalytic bed height, m Mi ) molecular weight of species i, kg mol-1 r ) intrinsic reaction rate, mol ms-3 s-1 R ) gas constant, J mol-1 K-1 T ) reaction temperature, K Ug ) superficial gas velocity, m s-1 W ) weight of CuO/γ-Al2O3 after sulfation, kg W0 ) initial weight of CuO/γ-Al2O3, kg X ) conversion of NO Greek Symbol ) void fraction of the catalyst bed Subscripts s ) solid bed g ) gas phase
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Received for review November 12, 1998 Revised manuscript received March 2, 1999 Accepted March 7, 1999 IE9807147
