Selective reduction of nitric oxide with methane on gallium and indium containing H-ZSM-5 catalysts: Formation of active sites by solid-state ion exchange

H-ZSM-5 catalysts containing Ga and In were prepared by the ion-exchange method and by physically mixing H-ZSM-5 and Ga2O3 or In2O3, and they were used in the NO reduction with methane to determine the active species of these metals, particularly of In, for this reaction. Ga-loaded H-ZSM-5 prepared by the ion-exchange method gave a higher catalytic activity than the physical mixture of Ga2O3 and H-ZSM-5, of which the catalytic activity was comparable to that of H-ZSM-5. On the contrary, both ion-exchange and physical mixing methods yielded equally high catalytic activities of In-loaded H-ZSM-5. The reduction of NO on these catalysts proceeds consecutively by way of oxidation of NO to NO2 and reduction of NO2 with CH4 to N-2 With increasing heat treatment temperature, the catalytic activity of the mixture of In2O3 and H-ZSM-5 for the reduction of NO2 increased, while that for NO oxidation decreased. An IR study of OH groups on this catalyst indicated that the solid-state exchange reaction occurred between In2O3 and H-ZSM-5 leading to the increase in the number of indium cation sites and consequently the catalytic activity for NO2 reduction. (C) 1996 Academic Press, Inc.