Decomposition of NO over Cu-ZSM-5 Zeolites

175

CatalysisToday,17 (1993) 175-180 ElsevierScience PublishersB.V., Amsterdam

Decomposition of NO over Cu-ZSM-5 A Transient Kinetic Study

Zeolites.

Z. Schay and L. Guczi Surface Science and Catalysis Laboratory,Institute of Isotopes of the Hungarian Academy of Sciences, P. 0. Box 77, Budapest, Hungary, H-1525

Abstract Transient kinetic method has been utilized to investigate the mechanism of NO dcomposition over Cu-ZSM-5 zeolite catalysts prepared by solid state ion exchange of the H+ form with CuClz at 800 K. At higher temperatures the activity decreased and the decomposition was found to be of first order with respect to the NO in the ranges of 0.4-2 ~01% NO in the range of 600-750 K. The apparent energy for activation was found to be 36kJ mol-l. The TPD of NO revealed three peaks one of them being at 690 K corresponds to the working temperature range of the catalyst. Isothermal transients during NO pulses indicated reversible poisoning of the catalyst by NO and oxygen. N3 overshoot at the leading edge and 03 peak at the tailing part of the pulse was also observed. The rate of reaction under the steady state condition was controlled by the desorption of 09 which in turn was hindered by NO. Keywords: NO decomposition,

copper zeolite, transient-kinetics

INTRODUCTION Reduction of NOx in the exhaust gas emissions from stationary sources is of growing importance. The selective catalytic reduction (SCR) of the NOx content of flue gases by NH is already industrialized in Japan and in Europe [12]. In the presence of 5-l?)% 0 a typical range for gas turbines and large stationary diesel engines, the S& cannot be applied because of the excess amount of oxygen leading to unselective oxidation of NH,. Decomposition of NO molecule seems to be an attractive alternative as NO is thermodynamically unstable below 900 K. Copper exchanged zeolites seem to be promising candidates as catalysts [3-61, but at present their activity is not high enough for practical applications. In the present work the kinetics of NO decomposition on Cu-ZSM-5 zeolite will be studied in order to obtain deeper insight into the mechanism of the reaction and to explore the possibilities for increasing the activity of the catalyst.

0920-5861/93/$6.00 0 1993ElsevierScience PublishersB.V. All rightsreserved.

176 EXPERIMENTAL The Cu-ZSM-5 zeolite was prepared by solid state ion exchange of H-ZSM-5 by CuCl at 800 K. The ion exchange capacity of the zeolite was 0.64 mmol gl anb half of the protonic sites have been replaced by CuCl+, resulting in about 2wt% Cu loading. The Cu-ZSM-5 zeolite was mixed with equal amounts of magadiit (a special form of SiOz), pressed and crushed. The 0.250.5mm size fraction was used in the experiments. 0.56 g catalyst containing 0.28 g zeolite was placed into a tubular quartz glass reactor of 13 mm i.d. equipped with a 5 mm diameter thermocouple well. Quartz glass layers of the same size fraction before and after the catalyst bed ensured a uniform gass flow through the catalyst. The zeolite was activated by heating to 940 K in flowing air for 2 hours. The reactor was operated as a flow through reactor with external circulation for the kinetic measurements to avoid any mass transfer limitations from the gas phase and to ensure CSTR conditions. By switching off the external circulation it was operated as a plug flow reactor for the TPD and transient studies to ensure a good time resolution. 2 ~01% NO/Ar and Ar were supplied by BOC England as N 4.8 purity gas and used without further purification. The whole catalytic system was helium leak tested to avoid any contamination of the gases. MKS mass flow controllers were used to mix the gases and to form the pulses of NO in Ar. A Balzers QMG 420 quadrupole mass spectrometer (QMS) in multiple ion detection mode has been applied for the analysis. A Balzers TPU 60 turbomolecular pump ensured the vacuum for the QMS. A Spectramass gas inlet system consisting of a heated stainless steel capillary diferentially pumped by a rotary pump followed by an orifice to the turbomolecular pump made the QMS signal proportional to the gas concentration in the effluent and ensured a high stability of the QMS calibration. The NO, N , 0, and NpO signals were referenced to the m/e=36 Ar signal (about 0.36% aYJundance in Ar) to measure all QMS signals in a reasonable narrow sensitivity range. 0.1-l mL air pulses in Ar and in 2 ~01% NO/Ar were used to calibrate the m/e=28 and m/e=32 QMS signals and to convert them into N, and 0, concentrations, respectively. The 2 ~01% NO/Ar mixture was used to calibrate the m/e=30 signal. In spite of all of our efforts we could not reach a good mass balance as about 20-30% of the oxygen were missing even in the kinetic measurements where in some cases stationary signals for 10 hours were observed. The origin of the loss is unknown for the moment therefore we report only conversions of NO in the followings. For the kinetic measurements the NO concentration, the gas flow rate and the temperature have been varied. In the TPD measurements 20 mL min-l Ar has been used as a carrier gas and 2 ~01% NO/Ar was adsorbed at room temperature. The heating rate was 10 K min-I. In the transient kinetic studies the catalyst was heated in Ar to the desired temperature. First short pulses of about 2 minutes containing 0.41 ~01% NO/Ar were introduced which were followed by longer pulses containing 2 ~01% NO/Ar.

RESULTS

AND DISCUSSION

Results of the kinetic measurements are summarized in Fig.1. It is obvious that the conversion vs. flow rate cannot be approached by simple

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hyperbolic curves, even at low conversions there is strong deviation. The temperature dependence of the conversion between 600 and 750 K and at 15 mL min-1 flow rate can be described by an Arrhenius equation which gives E = 36 kJ mol-1. Above 800 K the conversion starts decreasing which indicates a drastic change in the decomposition mechanism. The conversion was found to be practically independent of the NO concentration between 0.41-2 ~01% NO in the 600 and 750 K temperature range, the reaction being first order with respect to NO. The TPD of the NO molecule shows two low temperature and one high temperature adsorption states (see Fig. 2.). The peak at 450 K is associated with N 0 formation. In the peak at 680 K Nz, N,O and 02 can also be detected. Peak temperatures for NO, N2 NzO and 02 are the same and are slightly below the temperature of the maximal conversion. The amount of the NO desorbed is 0.3 mmol g-1 which corresponds to half of the Cu participating in the total exchange capacity and involved in the adsoprtion process. Results of the isothermal transient measurements at three characteristic temperatures during the first short pulses of 0.41 ~01% NO are presented in Fig. 3. For the clarity of the figure only the m/e=30, 28 and 32 signals for NO, N2 and 02, respectively, are presented. There were no changes in the N20, NO and Hz0 signals during the & pulses. At each temperature the NO in the first pulse decomposed into Nz and 0, to a much larger extent than in the subsequent pulses. Especially at the leading edge of the first pulse large amount of Nz was formed. There was a temperature dependent delay in the appearance of the O2 signal. It can be explained

178

by the easy recombination and subsequent desorption of nitrogen, whereas oxygen remains adsorbed on the Cu. The oxygen evolved at 775 K was only slightly behind the Nz signal whereas at 676 K its maximum was shifted into the tailing part of the NO peak. The tailing of the N2 signal corresponds to the elution of N2 from the gas phase, whiIe the appearance of NO and --NO 02 molecules is typical for desorpN2 tion processes. In An effort to estimate the activation energies for desorption of NO and 0, has failed but it is evident that oxygen is more strongly tibmin bonded to the catalyst than NO. I I During the second puIse of NO 323 473 673 873 much less N2 and 02 were formed. After a long purge in Ar the shape temperature,K of the first pulse could be reproduced indicating a reversible poiFigure 2. Temperature prosoning of the catalyst with NO grammed desorption of NO and/or the product(s). into Ar. After the initial short pulses of NO long pulses _. of 2 ~01% _ NO/Ar were given. Here the catalyst reached a nearly steady state during the pulse. The higher NO concentration made the adsorption-desorption processes become evident. The important characteristics at all temperatures is the overshoots of the N2 at the leading edge of the NO pulse and the delay in the 02 formation. At the end of the pulse there is a large peak in the 0 signal, while both NO and N2 are already decreasing. The rise and fall of 2h t e NO signal is typical for signal without any adsorption-desorption process. The increase in the 0 additional N signal during the tailing part of the RJ0 peak indicates a hindrance of b desorption by NO. There was no separation in the N2 and 0 peaks during 0.‘4 mL pulses of air, indicating that the Nz overshoot is not cause d by leakage of air into the gas manifold. The transient experiments show that the Cu-ZSM-5 has a reasonable initial activity for the NO decomposition but NO itself acts as an inhibitor, too, by hindering the desorption of 02. It seems that the steady state reaction rate is controlled by the desorption rate of 0 . N is only weekly adsorbed. To maintain the high initial activity of the Cu-&MS zeolite it seems to be essential to increase the desorption rate of 02 in the presence of NO.

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180 ACKNOWLEDGEMENTS The research has been sponsored by the Hungarian Commission for Technological Development under the contract OMFB-IKI 7-03-0860. Dr. G. Borbely in the Central Research Institute for Chemistry, Budapest, prepared the Cu-ZSM-5 zeolite.

REFERENCES 1 2 3 2 6

Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S.; Kagawa, S.: J. Chem. Sot., Chem. Commun. 1986, 1273 Iwamoto, M.: Catalytic Decomposition of Nitrogen Monoxide in Future Opportunities in Catalytic and Separation Technology, (Eds.: by Misono, M.; Morooka, Y.; Kimura, S.), Elsevier, Amsterdam, 1990 Iwamoto, M.; Yahiro, H.; Tanada, K.; Mizuno, N.; Mine, Y.; Kagawa, S.: J. Phys. Chem. 95 (1991) 3727 Sato, S.; Yu-u, Y.; Mizuno, N.; Iwamoto, M.: Appl. Cat. 70 (1991) Ll Bosch, H.; Janssen, F, Catalysis Today 2 (1988) 369 Proceedings of the 20th Swedish Symposium on Catalysis, Lund, 13-14 October, 1987 edited by Odenbrand, C.U.I.; Catalysis Today 4 (1989) 139265