Water tolerance of mordenite-type zeolite catalysts for selective reduction of nitric oxide by hydrocarbons

Abstract

The water tolerance of the catalysts such as HM, CuHM and CuNZA prepared by the conventional ion-exchanged method was examined in a fixed-bed flow reactor. The copper-containing catalyst shows milder deactivation than the copper-free catalyst, and the mordenite type natural zeolite catalyst (CuNZA) reveals more stable activity maintenance than the synthetic mordenite catalysts (HM and CuHM) in the presence of water vapor. The sintering of copper ions and the formation of cluster species do not occur on the deteriorated catalysts by XANES and EXAFS studies. Copper on the catalyst surface exists in the form of Cu2+ regardless of the existence of H2O in the feed gas stream. The deteriorating trends of the catalysts are well elucidated by the hydrophobicity and the adsorption strength of water vapor on the catalyst surface. The water coordinated on the copper ions in the zeolites produces strong absorption bands at the region of 12,500 cm(-1). The amounts of NO adsorption on HM and CuHM catalysts are dramatically decreased due to the competitive adsorption of NO and H2O. However, its amount for CuNZA catalyst is not altered at all. It reveals that the inhibition of NO adsorption sites by water vapor is one of the crucial reasons of the activity loss for NO reduction by H2O. The contribution of the side reactions to the decrease of NO conversion by H2O, however, may not be eliminated for this reaction.