Preparation and reaction mechanistic characterization of sol–gel indium/alumina catalysts developed for NO _x reduction by propene in lean conditions

Abstract

The impregnation and sol–gel preparation methods were investigated to develop high activity catalysts and understand the significance of the indium–aluminium interaction on alumina‐supported indium catalysts in NO _x reduction with propene. Active In/alumina catalysts with a very high surface area (270 m²/g) and thermal stability were prepared in controlled conditions by sol–gel processing. When Al isopropoxide and In nitrate in ethyl glycol were used as precursors in aqua media, indium atoms were incorporated evenly distributed as a thermally stable form in the aluminium oxide lattice structure. In wet impregnation it was beneficial to use a certain excess of aqueous In solution (volumes of solution : pores = 2 : 1) to have the highest NO _x reduction activity. The catalyst containing dispersed Al on In oxide (58 wt% In, phase‐equilibrium preparation method) showed activity at lower temperatures than any other In–Al oxide catalyst or pure In₂O₃. The adsorption of different reaction intermediates on alumina and stable In₂O₃ sites were detected 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 indium and aluminium was evident even in the mechanical mixture of sol–gel prepared alumina (301 m²/g) and In₂O₃ powders (27 m²/g), where the probability for molecular‐scale intimate contact between indium and aluminium sites was very low (particle size 10–250 μm). Short‐lived gaseous intermediates and surface migration are the possible reasons for the high catalytic activities on the two physically separated active sites both necessary for the reaction sequence.