Additive Effect of Silver on the Catalytic Activity of TiO2–ZrO2 for the Selective Reduction of NO with Propene, 2-Propanol, and Acetone

The catalytic performance of metal-impregnated TiO2–ZrO2 binary metal oxide for the selective reduction of NO was investigated. The addition of silver to TiO2–ZrO2 catalyst caused a drastic enhancement of NO reduction activity with propene. This silver additive effect was accounted for by the assumption that silver promotes the reaction of NO2 with propene to form a partially oxidized hydrocarbon, which is the rate-determining step. However, the catalytic activity of silver-supported TiO2–ZrO2 was depressed by the presence of water vapor because of its inhibiting effect on the formation of the oxygenated intermediate. Oxygen-containing compounds such as 2-propanol and acetone were also found to be good reductants on silver-supported TiO2–ZrO2 in the absence of water vapor. Moreover, the presence of water vapor significantly enhanced NO reduction activity of silver-supported TiO2–ZrO2 with these reductants. It was presumed that water vapor accelerates the selective reduction of NO by suppressing the undesirable oxidation of the reductants by O2.