Supported Ag catalysts for the lean reduction of NO with C₃H₆

The activities of 2 and 6 wt% Ag/Al₂O₃ catalysts for lean NO reduction with C₃H₆ were compared. High conversions of NO to N₂ were obtained over 2 wt% Ag/Al₂O₃. In contrast, the NO conversions to N₂ were much lower over 6 wt% Ag/Al₂O₃, and this catalyst formed a substantial amount of N₂O. The difference in the behavior of the two catalysts was attributed to the much higher Ag dispersion for the 2 wt% than the 6 wt% sample, such that the oxidation states of Ag were different under reaction conditions. The 2 wt% Ag/Al₂O₃ was believed to contain silver in the +1 oxidation state under reaction conditions, while the 6 wt% Ag/Al₂O₃ catalyst contained Ag⁰ particles, the amount of which decreased at higher temperatures. The presence of Ag⁰ resulted in a high rate of C₃H₆ combustion at the expense of NO_x reduction. A synergistic effect was observed over a mixture of Al₂O₃ and 2 or 6 wt% Ag/Al₂O₃ and was attributed to the transfer of a very short-lived intermediate from Al₂O₃ to Ag/Al₂O₃ or vice versa.