Preparation and reaction mechanistic characterization of sol–gel indium/alumina catalysts developed for NOx 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 NOx reduction with propene. Active In/alumina catalysts with a very high surface area (270 m 2 /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 NOx 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 In 2O3 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 m 2 /g) and In2O3 powders (27 m 2 /g), where the probability for molecular-scale intimate contact between indium and aluminium sites was very low (particle size 10‐250 m). 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.