1616
Znd. Eng. Chem. Res. 1995,34, 1616-1623
Effect of Copper Contents on Sulfur Poisoning of Copper Ion-Exchanged Mordenite for NO Reduction by N H 3 Sung-Won Ham, Hoon Choi, In-Sik Nam,*and Young Gul Kim Research Center for Catalytic Technology, Department of Chemical Engineering, Pohang University of Science & Technology (POSTECH)IResearch Institute of Industrial Science & Technology (RIST), Pohang 790-330,Korea
Deactivation of copper ion-exchanged mordenite (CuHM) catalysts with different copper contents by SO2 for NO reduction with N H 3 was examined in a fured bed flow reactor. A larger amount of deactivating agent deposits on the catalyst surface with the increase of copper contents under the same operating conditions. The deactivating agents deposited on the copper-exchanged mordenite catalysts were mainly ("€&)2S04 andor =so4 from the results of thermal analyses and temperaturesuch as thermogravimetric analysis (TGA), differential thermal analysis (DTA), programmed desorption (TPD). The formation of the ammonium salts largely depends on the reaction temperature and SO3 concentration generated from SO2 oxidation which is a reaction catalyzed primarily by copper ions on the catalyst surface. The catalytic activity and surface area of the deactivated catalysts are well-correlated with the sulfur content deposited on the catalyst surface, depending upon the reaction temperatures and its catalyst copper contents.
Introduction During the past few decades, much effort has been focused on the development of active catalysts for the removal of NO. Various types of catalysts, including noble metals, transition metal oxides, and mixed metal oxides, show high activity when used for the reduction of NO with NH3 (Meier and Gut, 1978; Nam et al., 1986a; Wong and Nobe, 1986). Recently, copper ionexchanged Y zeolite and H-mordenite have also been reported to be efficient catalysts for this reaction (Williamson and Lunsford, 1976; Mizumoto et al., 1979; Kiovsky et al., 1980; Nam et al., 1988; Medros et al., 1989). However, since sulfur compounds, mainly SOz, which can easily inactivate the catalyst are found in most flue gas streams containing NO,, any catalyst chosen for commercial use must be highly resistant to poisoning by the sulfur compounds. The deactivation of SCR (selective catalytic reduction) catalysts was primarily due to the blocking and filling of catalyst pores caused by the formation of sulfate compounds on catalyst support or active components from SO2 (Yoshida et al., 1984; Nam et al., 1986b). Several investigators also reported the pore filling and blocking phenomena by ammonium salts such as (NHd2S04 and NH4HSO4 formed by the reaction between SO2 and NH3 (Matsuda et al., 1982; Kusakabe et al., 1988, 1990). In the previous study (Ham et al., 19921, it has been shown that moderately copper-exchanged hydrogen mordenite (CuHM,copper 2.3 wt %) catalyst did not lose its activity a t reaction temperatures above 300 "C, despite the deposition of deactivating agents on the catalyst surface. Although the addition of copper ions to HM catalyst greatly improved the activity for NO removal at relatively lower reaction temperatures, it also enhanced the NH3 oxidation reaction producing NO andlor N2 at higher temperatures, which was attributed to the oxidation ability of copper ions on zeolite. Considering the formation of the deactivating agents such as (NH4)2S04 andor NH&IS04 on the catalyst surface, it could be anticipated that SO2 in the feed gas should be first oxidized into SO3. These results il-
* To whom all correspondence should be addressed.
lustrate that the copper ions on zeolite also affect the deactivation of the catalyst by SO2 as well as its catalytic activity for NO removal. In this study, the poisoning characteristics of HM and CuHM catalysts with different copper contents for NO removal by NH3 were examined to elucidate the role of copper on the sulfur poisoning of the catalysts. Fresh catalyst kinetics, which was confirmed to be first order with respect to NO concentration for this catalytic system (Ham et al., 19921, was employed to interpret the activity change of deactivated catalysts. The variation of the physicochemical properties for the deactivated catalysts by SO2 in the feed stream was also examined t o determine the deactivation parameters of the catalysts, such as catalyst s u l h content and surface area. In order to understand the mechanism of deactivation by SO2, the deactivating agents deposited on the catalyst surface were investigated by thermal analyses (DTA, TGA, and TPD) and X-ray photoelectron spectroscopy (XPS).In addition, the activity of the catalysts for SO2 oxidation to SO3 was also examined to understand the effect of SO3 concentration on the formation of deactivating agents.
Experimental Section Catalyst Preparation. The catalysts employed in the present study were hydrogen mordenite (HM) and copper ion-exchanged hydrogen mordenite (CuHM).HM was prepared by ion-exchanging the sodium mordenite (NaM) with aqueous NH&l solution at 90 "C, followed by drying at 100 "C for 12 h and then calcining in an air stream at 500 "C for 2 h. NaM was obtained from Norton Co. under the designation of 900Na. The copper contents of the two CuHM catalysts employed in this study were 2.3 and 4.2 wt % and were designated as CuHM31 and CuHM58, respectively. The number in the name of the catalysts indicates the exchange ratio of cupric ions as a percentage of the total exchangeable cations. The catalysts CuHM31 and CuHM58 contained 1.2 and 2.3 cupric ions per unit cell, respectively. CuHM catalysts were prepared by ionexchanging the HM with cupric nitrate solution. Reaction Apparatus and Procedure. Parametric and durability studies of HM and CuHM catalysts in
0888-588519512634-1616$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1617 the presence of SO2 were conducted with a tubular and downflow reador operating under isothermal and slightly above atmospheric pressure conditions. Details of the reaction apparatus and experimental procedure employed in this study were already described (Nam et al., 1990; Ham et al., 1992). The apparatus for the SO2 oxidation experiment was identical to that used for parametric and durability studies, except for the so3 absorption train. Since only a small percentage of SO2 is converted to sos, a conventional analysis of SO3 cannot be made by a GC and/or an IR detector. Therefore, so3 in the downstream of the reactor was absorbed into a solution of 80% isopropyl alcohol in deionized water. For the measurement of SO3concentration, an aliquot of the absorbing solution was titrated with 0.01 N barium perchlorate (Ba(C10&3HzO) using a thorin indicator. The precipitate formed by the reaction between barium ions and sulfate ions remains in a colloidal state a t the high concentration of nonaqueous solvent. The color of thorin adsorbed on the precipitates changes from yellow to pink in a n excess of Ba2+. Sulfur Contents and BET Measurements. The total sulfur contents of the deactivated catalysts were measured by the conventional oxidation method using a LECO, SC32 sulfur analyzer. BET surface area was examined by Micromeritics Accousorb 2100E using Nz a t 77 K. For the measurement of BET surface area, the catalyst was pretreated in vacuo a t 150 "C for 12 h. Thermal Analysis. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed in a Perkin-Elmer 1700 system. The heating rate was 10 "C/min in flowing Nz. Temperatureprogrammed desorption (TPD) of deactivated catalysts was carried out with the heating rate of 10 "C/min under the flow of He of 40 cm3/min. The evolving gases during the TPD experiments were detected with quadrupolemass spectroscopy (VG Instrument Ltd. MMPC-300D). XPS Analysis. X-ray photoelectron spectra were examined by Perkin-Elmer PHI 5400 XPS using Mg Ka as a radiation source. After each deactivation run, the catalyst pellets were crushed and then pressed into selfsupporting wafers for XPS analysis. The charging effects of the X P S spectra were calibrated by the carbon (1s)line at 284.6 eV associated with the hydrocarbon impurities on the catalyst surface.
Results Effect of Copper Contents on Catalytic Activity. Figure 1 shows the activity of HM and CuHM catalysts for the NO reduction by NH3 in the absence of SO2 a t reaction temperatures from 170 to 500 "C. The catalytic activity is expressed as a fractional NO conversion at the same reaction conditions. HM catalyst itself shows considerable activity for NO removal, and the activity is enhanced with the increase of reaction temperatures. The addition of copper ions t o HM greatly increases its NO removal activity, especially at relatively low reaction temperatures. The dramatic increase of catalytic activity for NO removal of CuHM compared with that of HM at lower reaction temperatures can be considered to be due to the formation of a copper-amine complex on copper ions offering new active sites for this reaction system (Williamson and Lunsford, 1976; Choi et al., 1991). However, CuHM catalyst reveals a maximum activity with respect to reaction temperatures, which is commonly observed for most NO reduction catalysts. In
5
.-
E
0.6
1 / E] S.V. : 100,OW h"
0.4
0
0
0.2 00
100
200
300
400
500
Temperature ( "C) Figure 1. Effect of copper content of CuHM catalysts on SCR actvity.
contrast, the activity of HM catalyst does not decline over the reaction temperatures covered in this study. The decline of the activity for CuHM catalysts at higher reaction temperatures is primarily attributed t o the oxidizing ability of cupric ions converting NH3 to NO andfor Nz. It has been well-known that copperexchanged zeolites were very active for oxidation reaction and so have been extensively studied as an oxidation catalyst (Naccache and Ben Taarit, 1971; Flentge et al., 1975). Figure 1 also shows that the operating window for the maximum conversion of NO for CuHM31 is wider than that for CuHM58. The activity of CuHM31 catalyst begins to decline a t about 400 "C, while that of the CuHM58 catalyst begins to decline at about 350 "C. It indicates that the NH3 oxidation reaction becomes important at reaction temperatures of about 400 "C for CuHM31 and 350 "C for CuHM58. The decrease of the reaction temperature by the copper contents of the catalysts simply reveals that the copper contents on the catalyst surface play a critical role for the operating window of SCR technology over CuHM catalyst. On the other hand, HM catalyst, showing no decline in SCR activity, seems to have less oxidizing ability than CuHM catalysts over reaction temperatures studied in this work. Physicochemical Properties of the Deactivated Catalysts. Table 1shows a significant decrease in the surface area and an increase in the catalyst sulfur contents with the increase of SOz-exposingtime and its concentration in all catalysts examined in this study. It also indicates that the higher sulfur deposition on the catalyst is observed at 250 "C, compared with the higher reaction temperatures of 350 and 400 "C under the same on-stream time and SO2 feed concentrations. The similar trend can also be observed for HM and CuHM58 catalysts. These results suggest that the deactivating agent can be formed more easily at low reaction temperatures (-250 "C) than at high reaction temperatures (>350 "C). The deactivating agent appears to be a temperature-sensitive compound. Since the net conversion of NO over CuHM58 decreases with reaction temperatures above 350 "C due to the onset of NH3 oxidation, the reaction temperature of 350 "C is chosen as a high reaction temperature for the comparative
1618 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 Table 1. Physicochemical Properties of the CuHM Catalysts Deactivated by SO2 temperature catalyst ("C) HM fresh 250 250 250 400 400 400 CuHM31 fresh 250 250 250 400 400 400 CuHM58 fresh 250 250 250 250 350 350 350
running time (h) 0 0.5 3.0 15.0 0.5 5.0 15.0 0 0.1 12.0 50.0 0.2 12.0 48.0 0 2.0 10.0 18.0 24.0 2.0 10.0 19.0
SO2
S
concn (wm) 0 2 000 2 000 2 000 2 000 2 000 2 000 0 2 000 2 000 20000 2 000 2000 20000 0 2 000 2 000 2 000 2 000 2 000 2 000 2 000
content (wt%)
0 0.18 0.69 1.01 0.12 0.62 0.81 0 0.85 1.98 2.88 0.72 1.13 1.78 0 1.37 1.98 3.10 3.32 1.20 1.30 2.08
surface area (m2d 404 190 118 39 366 337 323 467 261 36 13 402 332 26 1 389 5 5 2 4 4 3 3
study for the effect of reaction temperature on the deactivation. For the dependence of the deactivation on the catalyst copper contents, it is observed that the amount of sulfur deposited on the catalyst surface increases in the order of CuHM58 > CuHM31 > HM, on the basis of the loading amounts of copper ions on catalysts under the same reaction conditions. It suggests that not only the reaction temperature but also the contents of cupric ion play an important role for the formation and deposition of the deactivating agent on the catalyst surface. As shown in Figure 2A,B, the change in surface area of deactivated HM and CuHM31 catalysts largely depends on the reaction temperature covered in this work. At the higher reaction temperature of 400 "C, the surface area of deactivated catalyst linearly decreases with the increase of sulfur contents, while it decreases exponentially with the increase of sulfur contents at 250 "C for both HM and CuHM31. Thus, a strong decay in the surface area is observed at 250 "C, while the decrease is rather moderate a t 400 "C at the same level of catalyst sulfur contents. This type of deactivation behavior has been frequently observed as proof of pore filling or blocking by a deactivating agent (Nam et al., 1986b). For CuHM58 catalyst as shown in Figure 2C, however, the surface areas of all the catalysts obtained after reaction in the presence of SO2 are below 13 m2/g,regardless of the deposited sulfur contents and reaction temperatures employed in this study. Note that the BET surface area of fresh CuHM58 catalyst is 389 m2/g. Since mordenite has a two-dimensionalpore structure with nonintersecting parallel channels, the internal surface area of the catalyst may be easily blocked by the adsorption of reactants as well as by the deposition of deactivating agents. To confirm the speculation that the adsorbed reactants can block the pores, the changes in surface area of each catalyst after the adsorption of NH3 and the activity test in the absence of SO2 were measured as shown in Table 2. Note that NH3 adsorption was carried out a t room temperature for 1h with anhydrous NH3. The surface area of CuHM58 remarkably decreases to 9 mz/g only with the adsorption of NH3 molecules. Although the CuHM31 and HM catalysts
also lose their original surface areas to some extent, the degree of decrease in surface area is much less than that of CuHM58. It reveals that adsorbed NH3 also serves to decrease the surface area by blocking the pores of the catalyst. However, since the reaction temperature used for the deactivation test is higher than the temperature employed for the pretreatment of the catalyst for the measurement of the BET surface area, the effect of adsorbed NH3 on the decrease of surface area may not be significant for the actual reaction system. The changes in surface area after the activity test in the absence of ,302 were also shown in Table 2. HM and CuHM31 catalysts do not show an appreciable decrease of surface area after only the activity test without SO2 in the feed stream, while the surface area of CuHM58 catalyst decreases to less than half of its initial value. There will be plenty of NH3 molecules adsorbed on the catalyst surface even after the activity test. In the case of CuHM31 and HM, however, it does not block the pore from accessing the reactants into the internal surface of the catalyst. It may indicate that the blocking effect of adsorbed NH3 is pronounced when the copper contents of the mordenite catalyst exceed a limit. It should be noted that CuHM31 and CuHM58 contain 1.2 and 2.3 cupric ions in a unit cell of zeolite, respectively. With these results, the decrease of surface area of deactivated CuHM58 exhibiting no general trend with its sulfur contents is partly due t o the superposition of adsorbed NH3 molecules, especially in the form of a copper-amine complex and deactivating agents that block the pore from accessing even small molecules such as NZwhen the conventional BET method is employed t o measure the surface area.
Discussion Deactivating Agents Deposited on the Catalysts. It is important t o identify the final form of the deactivating agent on the catalyst surface to examine the mechanism of catalyst deactivation by SOZ. XPS analysis was conducted to investigate the oxidation state of sulfur deposited on the catalyst surface after the deactivation test as shown in Figure 3. The number following the catalyst denotes the reaction temperature ("C) at which the deactivation test was conducted. For example, HM-250 represents the deactivation test conducted at 250 "C for HM catalyst. The binding energy (eV) for S 2p of all catalysts lies at 169.0-169.4 eV. It is typical for S6+, indicating that sulfur is present in the form of sulfate or sulfite (Mullenberg, 1978). Thermal analyses such as TGA, DTA, and TPD were also carried out to identify the deactivating agents. Figures 4 and 5 show the TGA, DTA, and TPD spectra of the deactivated catalysts obtained from deactivation tests a t two distinctive reaction temperatures, i.e., 250 "C (Figure 4) and 350-400 "C (Figure 51, respectively. For CuHM58-250 and CuHM31-250, apparent weight losses are observed at two temperature ranges, which is believed to be due to dehydration (-100 "C) and decomposition of sulfur compounds (250-5.00 "C) as shown in Figure 4B,C. For these two catalysts, two maxima of the endothermic peak in DTA appeared at around 180 and 430 "C, respectively. The first peak showing a maxima at around 180 "C may be due to dehydration, while the second peak around 430 "C is due to the decomposition of sulfur compounds, which is confirmed by the SO2 evolution at 450 "C from the TPD spectra.
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1619
I \
l \
0
0.0 0.2 0.4 0.6
0.8 1.0
1
1.0
0.0
2.0
3.0
0.0
I
1.0
2.0
3.0
4.0
Sulfur content (wt.%) Figure 2. Effect of sulfur content on surface area for (A)HM, (B) CuHM31, and (C) CuHh458. Table 2. Change in Surface Area after NHs Adsorption and an Activity Test in the Absence of SO2 surface area (m2/gp catalyst HM CuHM31 CuHM58
fresh 404 467 389
after N H 3 adsorption 261 242 9
after an activity test 392 457 162
Measured by BET method with Nz and pretreated in vacuo at 150 "C for 12 h.
A review of the decompositiontemperature of possible sulfur compounds on the catalyst surface shows that the weight loss around 250-500 "C would represent the decomposition of ammonium sulfate (230 "C). Therefore, an ammonium sulfate-impregnated catalyst containing 7.3 w t % of sulfur was tested, and it shows a similar weight loss trend for the deactivated CuHM58250 and CuHM31-250 catalysts from this study as shown in Figure 4D. As discussed in the previous work (Ham et al., 19921, however, the deactivating agent causing weight loss around 250-500 "C might be partly ammonium bisulfate (NH4HS04) since the molar ratio of NH4+ to Sod2- ranged from 1.14 to 1.84 for deactivated CuHM31 catalyst by ion chromatography measurement. It revealed that 90% of the sulfur was existing in the form of S042-. In contrast, the TGA result for HM-250 catalyst shows no appreciable weight loss at around 250-500 "C, indicating less formation of ammonium salts on this catalyst than on CuHM catalysts. Note that the sulfur contents of HM catalysts are much lower than those of CuHM catalysts deactivated a t a similar condition as shown in Table 1. It presents two small weight losses at around 150-240 "C and above 600 "C instead. The first peak showing a small weight loss at around 150240 "C may be attributed to the dehydration. The second peak is due to the decomposition of sulfur compounds which is consistent with SO2 evolution at 670 "C as shown in the TPD curve of Figure 4A. The late detection of SO2 for TPD of HM-250 catalyst may simply be due to the amount of sulfur compounds deposited on the catalyst compared with other CuHM catalysts. For the TPD experiment, other species of gases such as NH3, Ha, and H20 are also detected over the tem-
175
172
169
166
BINDING ENERGY (eV) Figure 3. S 2p XPS spectra for deactivated catalysts: (A) CuHM58-350, (B) CuHM31-400, (C) HM-400, (D) CuHM58-250, (E) CuHM31-250, and (F)HM-250.
perature ranges above 150 "C by a mass spectrometer, although they appear not to be appreciable quantities compared to the quantities of SOz. It may indicate that the NH3 released by the decomposition of ammonium salts can readsorb on the catalyst surface and/or consecutively react with 0 2 on the zeolite surface, producing N2 and HzO (Knoblauch et al., 1981). TGA curves of each catalyst obtained from the deactivation experiments conducted a t higher temperatures show nearly the same shape as the corresponding curves at lower temperatures as shown in Figure 5. DTA curves also present similar shapes, except for CuHM31400, which has its maximum endothermic peak corre-
1620 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
C
1
TGA 6o
1
I
1
\
DTA
a
5 > I
/
-
t
t
v)
z
150
Y
DTA
TPD
TPD
m'
60
1I TGA
60
240
330
420
510
600
690
780
A
-60
150
240
330
420
510
600
690
780
TCMDFIYATIIIYE/ o r \
TEMPERATURE ('C)
Figure 4. TGA, DTA, and TPD spectra of the catalysts deactivated by SO2 a t lower temperatures: (A) HM-250, (B)C-58250, (C) CuHM31-250, and (D) (NH&SOa-impregnated CuHM31.
Figure 5. TGA, DTA, and TPD spectra of the catalysts deactivated by SO2 at higher temperatures: (A) HM-400, (B)CuHM58350, and (C) CuHM31-400.
sponding to the decomposition of sulfur compounds at 380 "C. According to the thermal decomposition mechanism of ammonium sulfate by DTA, TGA, and X-ray diffraction, ammonium sulfate can be decomposed into triammonium hydrogen sulfate or ammonium bisulfate, subsequently producing ammonium polysulfate or sulfamic acid and finally several gases (Kiyoura et al., 1970). Since the deactivation test was conducted at a temperature higher than the decomposition temperature of ammonium sulfate and ammonium bisulfate, the presence of these salts may be a cause for the weight loss peaks of TGA a t the slightly lower decomposition temperature. TPD spectra of the catalysts deactivated a t higher temperatures show the two steps in SO2 evolution, indicating the presence of two kinds of sulfur compounds. The first small peak of all catalysts appears at around 100 "C, and the maximum of the peak is at around 200 "C. The original forms of the sulfur com-
pounds evolving SO2 at this temperature cannot be precisely identified. However, since the deactivation temperature is higher than the decomposition temperature of ammonium salts, which are believed to be the primary deactivating agents on C u m catalysts, it may be the residue of decomposition of these salts at high reaction temperatures. Further study is required t o closely examine the SO2 evolution at the lower temperature of TPD. F'rom thermal analyses of deactivated catalysts, (MI&so4 andor NH4HS04 are found to be the primary deactivating agents deposited on copper-exchanged mordenite, CuHM31 and CuHM58, while less salts are formed on HM catalyst. For the formation of the ammonium salts such as ammonium sulfate and ammonium bisulfate on the catalyst surface, SO2 should be first oxidized to SO3 and then SO3 should react with NH3 and HzO. Moreover, the formation of ammonium salts can be regarded as equilibrium processes, the
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1621 Table 3. Comparison of the Activity of CuHM Catalysts for SO2 Oxidation catalyst Hh4
CuHM58
reaction temp ("C) 250 300 350 400 250 300 350 400
so3 concn
conversionu
(PPd 2.3 4.1 10.8 15.2 12.5 26.8 48.2 86.0
(%)
0.12 0.21 0.54 0.76 0.63 1.34 2.41 4.30
Based on the SO2 feed concentration of 2 000 ppm. Space velocity: 50 000 h-l.
higher concentrations of NH3 and SO3 favoring the formation of these ammonium salts at a given temperature. Therefore, whether these ammonium salts form may depend on the oxidizing ability of the catalyst converting SO2 into SO3, which is a reaction mainly catalyzed by copper ions on the catalyst surface. The bell-shaped activity with reaction temperatures due to NH3 oxidation as shown in Figure 1 may be indirect evidence for a higher oxidizing ability for converting SO3 to SO3 of copper-exchanged catalysts compared with that of HM catalyst. On the other hand, the reason for less formation of ammonium salts on the HM catalyst is the lack of oxidizing ability, revealing no maximum conversion of NO at reaction temperatures up to 500 "C as shown in Figure 1. Morever, the cause of the formation of larger amounts of sulfur compounds with the increase of cupric ion contents under the same operating conditions as shown in Table 1 can also be elucidated by the difference in the oxidation activities of the catalysts. SO2 Oxidation into SO3 over CuHM Catalysts, The oxidation of SO2 to SO3 over SCR catalysts has been another important topic along with their activity for NO removal since SO3 produced by the oxidation of SO2 was known to react with ammonia and water to form ammonium salts, the primary deactivating agents for this reaction system. These ammonium salts can reduce the catalytic performance and cause corrosion and plugging of the reactor and heat-exchanger in the downstream of the SCR reactor. In order to distinguish the contribution of copper ions on the catalyst surface to the oxidation of S02, CuHM58 and HM catalysts were examined as shown in Table 3. The SO2 oxidation experiment was conducted a t a space velocity of 50 000 h-l with 2000 ppm of SO2 feed concentration. It is observed that SO2 oxidation activity is enhanced with the presence of copper on the catalysts and the increase of reaction temperatures. Although the formation of so3 by SO2 oxidation is thermodynamically favored over the entire range of temperatures examined in this study, the activity of SO2 oxidation over HM and CuHM58 catalysts in very low. SO2 conversions for both catalysts are less than a few percent. HM catalyst, especially, has very low activity for SO2 oxidation with less than 1% conversion. The conversion over CuHM58 catalyst was 4 to 6 times higher than that over HM catalyst at each reaction temperature examined in this work. It may indicate that copper ions on the catalyst surface play a decisive role for SO2 oxidation as well as for NO removal by NH3. Although the activity of CuHM58 catalyst for SO2 oxidation is not so high, it is appreciably higher than that of HM catalyst at each reaction temperature.
The difference in SO2 oxidation activity, Le., the SO3 concentration, seems to be a critical criterion for whether ammonium salts such as ammonium sulfate and ammonium bisulfate can easily form and deposit on the catalyst surface. Note that CuHM31 catalyst maintains its oxidation activity of SO2 to SO3 between HM and CuHM58 catalysts. Effect of Copper Contents on the SCR Activity in the Presence of SO2. Since the objective of this study was t o idestify the effect of copper contents on the deactivation behavior of mordenite catalysts by S02, the correlation of the variations of the catalytic activity for NO removal with respect to copper contents in the presence of SO2 has been examined. In the previous study (Ham et al., 19921, the reaction rate of the NONH3 system over CuHM was well-correlated by irreversible first-order kinetics with respect to NO concentration. Therefore, the first-order kinetics was also used for the investigation of the deactivation behavior of CuHM by S02. Here, the catalytic activity was defined as the ratio of the reaction rate for deactivated catalysts to that for a fresh catalyst based on the firstorder reaction kinetics: a = klk,. Figure 6A-C shows the activity change of HM, CuHM31, and CuHM58 catalysts for the reduction of NO with the sulfur contents deposited on the catalyst surface. The effect of sulfur compounds deposited on the catalysts due to the presence of SO2 in the feed gas stream on SCR activity significantly depends on both the reaction temperatures and the copper contents of the catalyst. For HM catalyst, the catalytic activity varies with its sulfur contents, depending on reaction temperatures, i.e., an exponential decay at 250 "C and a linear decay a t 400 "C as shown in Figure 6A. It has already been investigated that the surface area of deactivated HM catalyst exponentially decreases with sulfur contents at lower temperatures of 250 "C, while it linearly decreases at higher temperatures of 400 "C as shown in Figure 2A. Judging from these experimental results between catalytic activity and surface area with their catalyst sulfur content a t two distinctive reaction temperatures, it is considered that the decline of the catalytic activity for deactivated HM catalyst occurs probably due to the decrease of surface area. The dependence of SCR activity of CuHM31 on its sulfur contents with respect to reaction temperatures also shows a trend similar to that of HM catalyst as shown in Figure 6B. The catalytic activity reveals an exponential decrease with the sulfur contents of the catalyst at 250 "C, while no deactivation is observed at 400 "C, despite the deposition of sulfur up to 1.78 wt % on the catalyst surface. As discussed in the previous study (Ham et al., 19921, it is probably due to the depositing location of the deactivating agents, which are mainly ammonium sulfate and/or ammonium bisulfate in the pores of the catalyst structure. At low temperatures of 250 "C, these ammonium salts can form near the entrance of catalyst pores, causing severe pore blocking. However, at higher temperatures of 400 "C above the decomposition temperatures of ammonium salts, they can form at the deep inside pores by capillary condensation, causing pore filling. It is believed to be one of main reasons why the catalytic activities vary with respect to reaction temperatures. It may also be a reason why the catalyst surface areas vary with reaction temperature at the same level of sulfur contents as shown in Figure 2B. At 400 "C, more than one-
1622 Ind. Eng. Chem. Res., Vol. 34, No. 5,1995
1 0.0
0.2 0.4 0.6 0.8 1.0
0.0
1.0
2.0
3.0
0.0
1.0
2.0
3.0
4.0
Sulfur content (wt.%) Figure 6. Effect of sulfur content on NO removal activity for (A) HM,(B)Cu-HM31, and (C)CuHM58.
half of the catalyst surface area remains active since the deactivating agents could exist only deep inside pores, creating pore-filling phenomena. It should be noted that pore filling hardly blocks the entire surface area of a catalyst. The activity change of deactivated CuHM58 catalyst shows a deactivation trend similar to that of CuHM31, depending on the reaction temperatures and its sulfur contents (Figure 6C). The catalytic activity exponentially decreases with the increase of sulfur contents on the catalyst surface at 250 "C, while no deactivation is observed a t 350 "C, despite the deposition of sulfur up to 2.08 w t % on the catalyst surface. Considering that the primary deactivating agents deposited on CuHM58 are also ammonium salts as observed for CuHM31, it is believed that these ammonium salts deposit at the different locations inside the pore, depending on reaction temperatures. For CuHM58 catalyst, however, the activity change with sulfur contents cannot be fully understood by the difference in the location of deactivating agents deposited on the catalyst as observed for CuHM31 since the surface area of CuHM58 catalyst significantly decreases to below 13 m2/g, regardless of sulfur contents and reaction temperatures. As already mentioned, the unusual decrease of surface area of CuHM58 is mainly due to the pore blocking by ammonium salts deposited near the entrance of the pores a t 250 "C, while it is due t o the blocking effect of the copper-amine complex and the pore filling by ammonium salts at 350 "C. Therefore, CuHM58 catalyst still maintains its initial catalytic activity at higher temperatures of 350 "C by the participation of the copper-amine complex as a supplier of NH3 for SCR reaction, although it serves to significantly decrease the surface area of CuHM58. On the other hand, a t 250 "C, the catalytic activity exponentially decreases with the catalyst sulfur content due t o the pore blocking by ammonium salts the access to the reactants into the surface area inside pores. Note that the formation of the copper-amine complex can occur at any temperatures with the catalysts containing copper for this reaction system. It is simply more evident for the higher reaction temperature and CuHM58 catalyst due t o its higher copper contents. More speculation for the fact that CuHM catalysts do not lose their activity a t higher reaction temperatures of 350 and 400 "C, despite the sulfur deposition on the
catalyst surface by SO2 in the feed, is that the intrinsic rate constant at high reaction temperatures is compensating for the activity loss due to the deactivation by SO2 for copper-exchangedmordenite catalysts, CuHM31 and CuHM58. In contrast, the activity of HM catalyst linearly decreases with its sulfur content even at high reaction temperatures of 400 "C. For a longer period of operation, it will eventually reveal the decrease of SCR activity.
Conclusions The deactivation behavior of CuHM and HM catalysts for selective catalytic reduction of NO by NH3 was examined in the presence of S02. The deactivation is strongly dependent on both reaction temperature and copper ion contents of the catalyst. The copper ions on zeolite greatly enhance the catalytic activity for NO reduction at relatively lower reaction temperatures. However, it serves not only to decrease the reactor operating temperatures window but also to increase the amount of sulfur compounds deposited under the same operating conditions, mainly due to the oxidation capability of copper ions on the catalyst surface. The surface area of deactivated HM and CuHM31 catalysts are well-correlated with the catalyst sulfur contents, while that of deactivated CuHM58 with higher copper contents greatly decreased, regardless of sulfur contents and reaction temperatures studied in this work. This unusual behavior of the change in surface area of CuHM58 was probably due to the blockage of pores by the copper-NH3 complex as well as by the deactivating agent. The deactivating agents are considered to be located mainly near the entrance of the catalyst pores, causing pore blocking at 250 "C, while it deposits on the deep inside of pores, causing pore filling at 350 and 400 "C. The deactivating agents on the surface of copperexchanged mordenite catalyst were considered to be primarily ammonium salts, such as (NH4)$304 and/or NH4HSO4. However, less ammonium salts form on HM catalyst than on CuHM catalysts, probably due to the lack of oxidation ability converting SO2 into SO3which is a reaction catalyzed by copper ions on the catalyst surface.
Ind. Eng. Chem. Res., Vol. 34, No. 5 , 1995 1623
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m.
Received for review May 2 , 1994 Revised manuscript received September 20, 1994 Accepted February 7,1995@
IE940284U @
Abstract published in Advance ACS Abstracts, March 15,
1995.
