고상이온교환법으로 제조된 Co/ZSM-5 촉매상에서의 선택적 질소산화물 환원 반응기구 연구

S.S. Goryashenko, 박용기, 김대성, 이진우, 박상언

한국화학연구소 공업촉매실

The Mechanistic Study of the Selective Catalytic Reduction of NO with Propylene over Solid state-ion exchanged Co/ZSM-5.

S.S. Goryashenko, Y.K. Park, D.S. Kim, J.W. Lee, S.-E. Park

Industrial Catalysis Research Team, KRICT, P.O. Box 107, Taejon 305-606, Korea

INTRODUCTION

Selective Catalytic Reduction (SCR) of NO with propylene was studied over Co/ZSM-5. The formation and behaviour of the surface species was monitored using DRIFT in steady-state and transient conditions. The SCR of NO with propylene was shown to proceed through the formation of the NCO intermediates which are converted to the final products by the interaction with NO2. Among the surface NOx species, the ONO (1524cm-1) and NO2 (1605cm-1) were found to be the most sensitive to the presence of propylene in reaction mixture. The transient-response study showed that mainly ONO species are involved in the NCO conversion to the final products.

Co-exchanged ZSM-5 (Co/ZSM-5) remains one of the DeNOx candidate catalysts since it is active when CH4 is used as a reductant and is stable in the presence of water and SO2 [1]. Still the mechanism of SCR of NO with hydrocarbons remains unclear. When CH4 is used as a reductant the process most probably proceed through the formation of CN-intermediates and the rate-limiting step was shown to be the methane activation by the NOx surface species [2]. However, in the case of isobutane hydrocarbon activation is not rate-limiting step [3] and in the case of propylene the main intermediates were shown to be the NCO species [4]. In order to determine the rate-limiting step and the surface NOx species involved in SCR of NO over Co/ZSM-5 we studied the SCR of NO using C3H6 as a reductant and applied DRIFT in steady-state and transient conditions to monitor the formation and behaviour of the surface species formed on the catalyst.

EXPERIMENTAL

Co/ZSM-5 (55% exchange, SiO2/Al2O3=30) was prepared by the solid-state ion-exchange. Catalytic performance of the catalyst was studied in SCR of NO with C3H6 (1500 ppm of NO, 1200 ppm of C3H6, 3% of O2) and NO + O2 reactions (1500 ppm NO, 3% of O2) at space velocities of 30,000 h-1.

FT/IR data was obtained using Nicolet Magna 560 spectrometer equiped with liquid nitrogen cooled MCT detector and Spectra-Tech DRIFT cell. IR spectra were taken in gas mixtures containing 1500 ppm NO, 1300 ppm C3H6 and 3%O2 at space velocities of 45.000 h-1. The HNCO adsorption and the behaviour of the surface intermediates were studied using static in-situ IR cell. HNCO was prepared by the thermal decomposition of cyanuric acid at 250oC on alumina catalyst.

RESULTS and DISCUSSION

In our reaction conditions the SCR of NO with propylene over Co/ZSM-5 starts at 300oC, achieve maximum conversion of 48% at 570oC and sharply decrease at higher temperature. In NO SCR conditions besides hydrocarbon and NOx only NCO species were detected on the catalyst surface. The activation of propylene leading to the formation of the surface NCO species was found to proceed on Co/ZSM-5 at as low temperature as 200oC independent on the presence of O2 in reaction mixture. This observation provide evidence that NO2 is not necessary for the propylene activation and that this step hardly could be considered to be the rate limiting. The NCO surface species were assigned on the basis of the experiment with HNCO adsorption. The experiment proved the formation of the surface Si- and Al-bonded NCO (2300 and 2264 cm-1 respectively) accompanied with two types of Co-bonded NCO (2235 and 2180-2200 cm-1) which may be accounted for the presence of two types of Co cations with different coordination or location in the zeolite lattice. The maximum of the NCO bands intensity in steady-state reaction conditions at 550oC (Fig. 1) is in good correlation with the temperature for the maximum of the NO SCR activity. Beside this, the surface NCO species formed on Co/ZSM-5 appeared to be stable at 400oC in NO or O2 but in NO+O2 mixture they were rapidly converted to the final products (Fig. 2). These observations provide evidence that SCR of NO with propylene in excess oxygen over Co/ZSM-5 occurs through the NCO intermediates. The NCO intermediates also proved to block the complete oxidation of propylene due to the strong adsorption on the active sites of Co/ZSM-5. Hence, the high selectivity of NO SCR with C3H6 over Co/ZSM-5 is accounted for the highly selective interaction of the NCO intermediates with NO2 and the main role of NO2 is the NCO conversion to the final products.

Fig. 1. Temperature dependence of IR bands on Co/ZSM-5 in NO-SCR conditions.

Fig. 2. The NCO stability in the presence of NO+O2. (1) Prereacted Co/ZSM-5 in SCR condition, (2) treated in NO(13Torr)+O2(20Torr) at 300℃, and (3) 400℃.

In order to determine the rate limiting step and the main NO2-type surface species involved in the SCR of NO over Co/ZSM-5, the transient-response method was applied. The pulse of C3H6 to the NO+O2 mixture at 550oC caused rapid appearance of the NCO bands with simultaneous decrease of the ONO (1524 cm-1) and NO2 (1605 cm-1) bands (Fig. 3). The rate of the NCO formation was much higher than the rate of their disappearance indicating that the rate-limiting step of the SCR of NO over Co/ZSM-5 is the NCO transformation to the final products. Though both the NO2 (1605 cm-1) and the ONO (1524 cm-1) surface species appeared to be highly sensitive to the pulses of propylene, their behaviour was noticeably different especially when the hydrocarbon was switched off (Fig. 3). The amount of the surface NO2 species was restored rapidly even in the presence of NCO intermediates. The restoration of the ONO species occurred in good correlation with the consumption of the NCO intermediates, hence mainly the ONO species interact with the surface NCO intermediates.

Fig. 3 The responce of the surface species on Co/ZSM-5 on the pulse of propylene to NO+O2 at 550℃.

References

1. J.N. Armor, Catal. Today, 26 (1995) 147.

2. A.D. Cowan, R. Dumpelmann and N.W. Cant, J. Catal., 151 (1995) 356.

3. N.W. Cant and A.D. Cowan, Catal. Today, 35 (1997) 89.

4. S.S. Goryashenko, Y.K. Park and S.-E. Park, "Selective Catalytic          Reduction Behaviour of NO over CoZSM-5", will be submitted to the     Catal. Letters.