Relations between structure and catalytic activity of Ce-In-ZSM-5 catalysts for the selective reduction of NO by methane

In-ZSM-5 catalysts for the selective catalytic reduction (SCR) of NO by methane have been prepared by different routes (aqueous ion exchange at different pH values; solid-state ion exchange, sublimation, and transport reaction, with an InCl3 indium source) and compared with respect to their catalytic behavior in the SCR (1000 ppm NO, 1000 ppm methane, 2% O2 in He, at 30,000 h−1) to elucidate the origin of differing NO conversion-temperature characteristics observed with the In-ZSM-5 catalyst system. The samples were categorized into those containing intrazeolite indium oxo species exclusively or coexisting with extrazeolite indium oxo aggregates, and materials containing exclusively intrazeolite InOxCly species on the basis of structural information available from EXAFS and XPS studies. In addition, physical mixtures of In(OH)3 and NH4-ZSM-5 were studied. The results confirm that intrazeolite indium oxo species are active SCR sites. Their catalytic properties depend on details of the coordination environment. The presence of extrazeolite InOx species is hardly reflected in the SCR activity but induces unselective methane activation. The complete failure of an In-silicalite-1 catalyst in NO reduction despite considerable methane activation capability indicates a crucial role of Brønsted acidity for the SCR over In-ZSM-5 catalysts. Intrazeolite InOxCly species exhibit low NO reduction activity, but are able to activate methane as well. With physical mixtures of In2O3 and H-ZSM-5 (ex In(OH)3/NH4-ZSM-5) activities comparable to those of In-ZSM-5 prepared by aqueous techniques were measured. These activities were obtained only when the constituents were well mixed in the catalyst pellets, while segregation in separate pellets strongly decreased the NO conversions.