A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NOx by propene over Ag/USY and Ag/Al2O3 catalysts. Ag/USY was almost inactive in thermal SCR while it showed obvious activities in NTP-assisted SCR at 100 degrees C-200 degrees C. Although the NOx conversion over Ag/Al2O3 was also enhanced at 300 degrees C-400 degrees C by the assistance of NTP, it was ineffective below 250 degrees C. The intermediates over Ag/USY and Ag/Al2O3 were investigated using in situ DRIFTS method. It was found that key intermediates in HC-SCR, such as NCO, CN, oxygenates and some N-containing organic species were enriched after the assistance of NTP. The differences in the behaviors of above intermediates were not found between these two kinds of catalysts. However, some evidences suggested that different properties of the absorbed NO, species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al2O3. TPD profiles of Ag/Al2O3 showed that nitrates formed over the catalyst were quite stable at low temperatures, which might occupy the active sites and were unfavorable to SCR reactions. The nitrates over Ag/USY were unstable, among which the unidentate nitrate species is probably contributed to the SCR reactions at low temperatures. (C) 2007 Elsevier B.V. All rights reserved.