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

摘要

In-ZSM-5 catalysts for the delective catalytic reduction (SCR) of NO by methane have been prepared by different routes (aqueous ion exchange at different pH alues;solid-state ion exchange,sublimatiin,and transport reaction,with an InCl_3 indium source) and compared with respect to their catalytic behavior in the SCR (1000 ppm NO,1000 PPM METHANE,2% O_2 in He ,at 30,000 h~-1) to elucidate the originof 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,adn materials containing exclusively intrazeolite InO_xCl_y species on the basis of structural information availble from EXAFS and XPS studies.In addition,physical mixtrues of In(OH)_3 and NH_4-ZMS-5 were studied.THe results confirm that intrazeolite indium oxo species are active SCR sites.Their catalytic properties depend on details of hte coordination environment.The presence of extrazeolite InO_x species is hardly reflected in the SCR activity but induces unselective methane activation.The comp;ate failure of an In-silicalite-1 catalyst in NO reduction despite InO_xCl_y species exhibit low NO reduction activity,but are able to activate methane as well.with physical mixtures of In_2O_3 and H-ZSM-5(ex In(OH)_3/NH_4-ZSM-5) activities compasrable to those of In-ZSM-5 prepard by aqueous techniques were measured.These activities were obtained only when the constituents were well mixed in the catalyst pellets,while segregation in ceparate pellets strongly decreased the NO coversions.