Steam deactivation of CoZSM-5 NO x reduction catalysts Prastawa Budi, Russell F. Howe * Department of Physical Chemistry, The University of New South Wales, Sydney 2052, Australia Abstract The effects of high-temperature steam treatment on the activity and selectivity of CoZSM-5 catalysts for the selective catalytic reduction of NO with methane have been examined. Loss of activity and selectivity on steam treatment is due to framework dealumination of the zeolite, causing an irreversible change in the coordination state of the Co 2 ions. Pre- exchange of the zeolite with La 3 cations inhibits the dealumination while stabilizing the catalysts. Keywords: NO x reduction; CoZSM-5; Steam; Catalyst stability; Dealumination 1. Introduction The selective catalytic reduction (SCR) of NO with hydrocarbons in the presence of excess oxygen has recently attracted wide interest in light of growing environmental concerns over NO x emissions from diesel and lean-burning gasoline engines [1]. The use of hydrocarbons in the SCR of NO as an alter- native to ammonia over CuZSM-5 zeolite catalysts was first proposed by Iwamoto et al. [2–4]. More recently, the SCR of NO x using methane as the redu- cing reagent over CoZSM-5 was demonstrated by Li and Armor [5]. A key issue in the practical implementation of metal zeolite catalysts for NO x reduction is catalyst durabil- ity. In particular, steam which is unavoidably present in combustion gases will irreversibly deactivate CuZSM-5 catalysts above a certain temperature [6–8]. We have recently shown that this deactivation is due to dealumination of the zeolite rather than to structural collapse; the resulting loss of ion-exchange capacity causes an irreversible change in the coordi- nation state of the Cu 2 and some migration of Cu to the external surface of the zeolite [9]. Steam deactiva- tion of CuZSM-5 catalysts can be inhibited by co- exchange of lanthanum ions into the zeolite, which prevents or at least reduces the extent of dealumina- tion [10]. The effect of H 2 O on the activity of SCR of NO by methane over CoZSM-5 was investigated by Armor et al. [11]. They suggested that the water effect is a kinetic phenomenon rather than due to structural damage of the catalyst, and concluded that CoZSM-5 was more stable in wet NO streams at high tempera- tures than CuZSM-5. We have reported briefly [9] that CoZSM-5 catalysts are not durable under long steam aging. In this paper, we examine the stability of CoZSM-5 catalysts to steam treatment in more detail, and show that co-exchange of lanthanum can also stabilize these catalysts. 2. Experimental All catalysts were prepared from HZSM-5 provided by PQ Corporation SiO 2 =Al 2 O 3  35. The zeolite was converted to either the ammonium or sodium Catalysis Today 38 (1997) 175–179 *Corresponding author. e-mail: r.howe@unsw.edu.au 0920-5861/97/$32.00 # 1997 Elsevier Science B.V. All rights reserved. PII S0920-5861(97)00063-1