Preparation and reaction mechanistic characterization of sol-gel indium/alumina catalysts developed for NOx reduction by propene in lean conditions

The impregnation and sol-gel preparation methods were investigated to devel op high activity catalysts and understand the significance of the indium-al uminium interaction on alumina-supported indium catalysts in NOx reduction with propene. Active In/alumina catalysts with a very high surface area (27 0 m(2)/g) and thermal stability were prepared in controlled conditions by s ol-gel processing. When Al isopropoxide and In nitrate in ethyl glycol were used as precursors in aqua media, indium atoms were incorporated evenly di stributed as a thermally stable form in the aluminium oxide lattice structu re. In wet impregnation it was beneficial to use a certain excess of aqueou s In solution (volumes of solution : pores = 2 : 1) to have the highest NOx reduction activity. The catalyst containing dispersed Al on In oxide (58 w t% In, phase-equilibrium preparation method) showed activity at lower tempe ratures than any other In-Al oxide catalyst or pure In2O3. The adsorption o f different reaction intermediates on alumina and stable In2O3 sites were d etected by FTIR studies. In/alumina catalysts have active sites to oxidize NO to NO2, partially oxidize HC, form the actual reductant which contains N -H or N-C bonding and react with NO to dinitrogen. The cooperation with ind ium and aluminium was evident even in the mechanical mixture of sol-gel pre pared alumina (301 m(2)/g) and In2O3 powders (27 m(2)/g), where the probabi lity for molecular-scale intimate contact between indium and aluminium site s was very low (particle size 10-250 mu m). Short-lived gaseous intermediat es and surface migration are the possible reasons for the high catalytic ac tivities on the two physically separated active sites both necessary for th e reaction sequence.