Additive effect of palladium on the catalytic activity of In/TiO2-ZrO2 for the selective reduction of nitrogen monoxide in the presence of water vapor

The additive effect of noble metals (Pt, Pd, Rh, and Ir) on the catalytic activity of In/TiO2-ZrO2 for the selective reduction of NO with propene in the presence of water vapor was investigated. Although NO reduction activity of In/TiO2-ZrO2 was inhibited drastically by coexisting water vapor, the addition of palladium was capable of depressing the retarding effect. The presence of both indium and palladium was necessary for the enhancement of NO reduction activity, and this was accounted for by a synergistic effect between palladium and indium. Optimum indium and palladium loadings were at 2-5 wt% and 0.005 wt%, respectively. It is suggested that the NO reduction on Pd/WTiO2-ZrO2 by propene proceeds via the following consecutive steps: the formation of oxygenated intermediates by the reaction of propene with oxygen on palladium, a spillover of the intermediates from palladium to indium and reaction between the intermediates and NO2 to form N-2 on indium.