The selective catalytic reduction of NOx with C3H6 (C3H6-SCR of NOx) was carried out in the presence of water vapor over various trace noble metal (Pt, Au and Pd) co-impregnated with Ag/Al2O3 catalysts. The Ag-Pd (0.01%)/Al2O3 shows a higher NOx, conversion than Ag/Al2O3 does, especially at temperatures ranging from 300 to 500degreesC. However, the additions of An and Pt to Ag/Al2O3 only resulted in a decrease of activity for the C3H6-SCR of NOx. In situ DRIFTS spectra suggests that the presence of trace Pd (0.01%) catalyzed the formation of an enolic species which was converted from C3H6, whereas the presence of Pt and Au did not show this effect. The surface enolic species is very active towards NO2 and NO3-, resulting in the formation of an NCO species which is the key reaction intermediate in the selective catalytic reduction of NOx (C) 2004 Elsevier B.V. All rights reserved.