모더나이트형 제오라이트 촉매상에서 탄화수소에 의한

NO 제거반응에 대한 SO2의 영향

김 문 현, 남 인 식, 김 영 걸, 하창호*, 이정빈*, 최기선*

포항공과대학교 화학공학과·환경공학부/포항산업과학기술원 환경촉매연구팀, 한국전력연구원 수화력발전연구실*

Effect of SO2 on the Catalytic Activity of Mordenite-type Zeolite Catalysts

for NO Reduction by Hydrocarbons

Moon Hyeon KIM, In-Sik NAM, Young Gul KIM, Chang Ho HA*, Chung Bin LEE*, Ki Sun CHOI*

Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science & Technology/ Research Institute of Industrial Science & Technology, Korea Electric Power Research Institute*

1. INTRODUCTION

Most of flue gas from the emission sources also contains H2O and SO2 as well as NOx, which may cause the activity loss of SCR catalysts for NO reduction by hydrocarbons.  Many studies for the removal reaction mainly concerned on the water tolerance of the catalysts and the cause of the loss of their NO removal activity by H2O in the feed gas stream[1-3].  The role of SO2 in the activity deterioration of the catalyst was examined for the reduction of NO by hydrocarbons over Cu-ZSM-5[4].  The effect of both SO2 and H2O on the reduction of NO was also reported for Co-ZSM-5 and Co-ferrierite catalysts[5].

The water tolerance of mordenite-type zeolite catalysts and the cause of their deactivation by H2O vapor have been extensively studied for NO reduction by hydrocarbons such as C2H4 and C3H6[6,7].  In the present study, not only their sulfur tolerance for NO removal reaction but a deactivating agent existing on the catalyst surface was examined to elucidate the cause of the loss of their NO removal activity.  A role of Cu2+ site for the catalyst deactivation by SO2 was also investigated by the various catalyst characterizing methods such as TGA, TPSR, Raman and XAS.

2. EXPERIMENTAL

The mordenite-type zeolite catalysts, HM, CuHM and CuNZA were prepared by an ion exchanging method as previously described elsewhere[6,7].  The copper contents of CuHM and CuNZA catalysts are 3.5 and 2.0 wt.%, respectively.  The reaction mixture including 500 ppm of NO, 1,000 ppm of C2H4 or 2,000 ppm of C3H6, 4.2% of O2, 1,000 ppm SO2 and He (balance) was employed to investigate the sulfur tolerance for NO reduction in a fixed-bed flow reactor.  To observe the effect of both SO2 and H2O on the reduction of NO by hydrocarbons, 7.3% of H2O was also fed to the feed gas stream in addition to the reaction condition described as above.  The reactants and products were analyzed by an on-line gas chromatography.

To identify a deactivating agent leading the activity loss of NO removal reaction by SO2, the physicochemical properties of the catalyst including the sulfur contents and BET surface areas of the catalyst were obtained before and after reaction with SO2.  A Spex 20 spectrometer with 514.5 nm of Ar ion laser was used for the Raman study.  Cu K-edge absorptions for CuHM and CuNZA catalysts exposed to the feed stream containing SO2 as well as for the references such as Cu(OH)2 and CuSO4□5H2O were also measured at Beam Line 7C of Photon Factory in National Laboratory for High Energy Physics (KEK-PF), Japan.

3. RESULTS AND DISCUSSION

Effect of SO2 on NO Removal Activity of Mordenite-type Zeolite Catalysts

As shown in Figure 1(A), about 60% of NO conversion on HM catalyst at 360oC was maintained for the SO2-free condition even during 11 h of the continuous operation of the reactor.  However, its NO removal activity gradually decreased when 1,000 ppm of SO2 is introduced to the reaction system.  The feed of SO2 to the reaction system caused more rapid loss of NO removal activity of CuHM catalyst than that of copper-free catalyst, as depicted in Figure 1(B).  It indicates that the formation of a deactivating agent on the catalyst surface was notably promoted by Cu ions on the zeolite catalyst surface.

Figure 1. Sulfur tolerance of the synthetic mordenite catalysts : (A) HM and (B) CuHM.  (○) without SO2 ; (●) with SO2.  Reaction condition : NO 500 ppm, C2H4 1,000 ppm, O2 4.2%, SO2 1,000 ppm and T=360oC.

The decrease of the initial NO conversion was more than 60% when SO2 is fed to CuNZA-C3H6 reaction system, as shown in Figure 2.  It reveals that the catalyst is very sensitive to the catalyst deactivation by SO2, even though it exhibited a better NO removal activity than any other catalytic systems examined in this study even with H2O vapor in the feed gas stream without SO2[7].  The deteriorating trend for the oxidation reaction of hydrocarbons such as C2H4 and C3H6 in the presence of SO2 was similar to that for NO conversion, regardless of the types of catalyst.

As shown in Figure 3, the catalytic activity of CuHM catalyst for NO reduction immediately decreased up to about 20% within 10 min of the reactor on-stream time when SO2 and H2O are simultaneously fed to the reaction stream.  It is quite similar to the activity loss of the catalyst for NO reduction by C2H4 with Figure 2. Sulfur tolerance of CuNZA      Figure 3. Effect of H2O on NO removal

catalyst : (○) without SO2 ; (●) with    activity of CuHM (●) and CuNZA (■)

SO2.  Reaction condition : NO 500       catalysts.  Reaction condition : NO 500

ppm, C2H4 1,000 ppm, O2 4.2%, SO2     ppm, C2H4 1,000 (●) or C3H6 2,000 (■)

ppm and T=400oC.                      ppm, O2 4.2%, H2O 7.3% and T=360 (●)                                       or 400oC(■).

H2O[7].  It may indicate that H2O included in the feed gas stream played a dominant role for the loss of NO removal activity of the catalyst, while the deactivation by SO2 was apparently negligible.  NO conversion of CuNZA catalyst decreased up to about 30% within 2 h by both SO2 and H2O, which is similar to the loss of its NO removal activity with SO2 alone as revealed in Figure 2.  Therefore, the apparent loss of the catalytic activity for the removal reaction was mainly due to the presence of SO2 in the feed gas stream.

Characterization of Deactivated Catalysts by SO2

CuHM catalyst exhibited higher sulfur deposition on the catalyst surface than HM catalyst.  It well agrees not only with the decreasing trend of their BET surface areas but with milder deactivation of HM catalyst than CuHM catalyst.  It was also observed by Ham et al.[8] for the reduction of NO by NH3 over CuHM catalyst.

TPSR pattern of Cu-exchanged catalysts deactivated by SO2 showed an unique desorption pattern of SO2 compared to that of HM catalyst, which exhibits three types of desorption peaks at 450, 600 and 680oC.  It indicates that Cu ions are strongly involved for the formation of a sulfur species as a deactivating agent.  S 2p line in the binding energy ranges from 168.4 to 169.4 eV was observed for the catalysts deactivated by SO2.  About 975 cm-1 of the Raman shift band was also evident not only for the catalysts with SO2 but for the fresh catalysts treated with a 0.2N H2SO4 solution.  It indicates that the peak at 975 cm-1 is mainly due to the sulfate formed on their surface.

The Fourier transformed Cu K-edge k3-weighted EXAFS spectra for CuHM and CuNZA catalysts deteriorated by SO2 can be found in Figure 4.  Only a single major peak at 1.53 Å by the bond of Cu-O appeared for all the samples employed in this study.  It should be noted that the sulfate species on the catalyst surface as identified by XPS and Raman is not existing in the form of the cupric sulfate pentahydrate.  It might partially surround the copper ions, as previously suggested by Choi et al.[9] and Hamada et al.[10].  It is expected that the electrostatic interaction between Cu ions and sulfate species probably influences the local structure of Cu ions on the zeolite catalysts.

Distance (Å)	Distance (Å)

Figure 4. Cu K-edge EXAFS spectra for CuHM(A) and CuNZA(B) catalysts deteriorated by SO2.  (······) fresh catalyst ; ( □··□ ) after reaction without SO2 ; (□ □ □ □ □) after reaction with SO2 ; (□□ □□ □□) Cu(OH)2 ; ( □□·□□ ) CuSO4□5H2O.

4. CONCLUSIONS

Cu -exchanged zeolite catalysts such as CuHM and CuNZA showed poorer activity maintenance for NO removal reaction than HM catalyst, regardless of the types of reductant employed in the present study.  A sulfur species as a deactivating agent on the surface of the deactivated catalysts by SO2 is existing in the form of sulfate (SO42-) which causes the loss of the catalytic activity of the catalysts, as confirmed by XPS and Raman.  It may electrostatically interact with Cu2+ on the surface of CuHM and CuNZA catalysts as suggested by XANES and EXAFS spectra.

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