The effect of H 2 addition on the selective catalytic reduction of NO x with propene (C 3 H 6 -SCR of NO x ) was investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The de-NO x activity of the Ag/Al 2 O 3 catalyst at low temperature was markedly enhanced by the addition of a small amount of H 2 ; the enhancement was associated with the improved propene activation at low temperatures. The addition of H 2 greatly promoted the nitrate decomposition at low temperatures, which was believed to alleviate nitrate poisoning and to initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H 2 addition will further reduce the freed Ag + species into Ag n δ+ and will eventually lead to the improved de-NO x activity, because Ag n δ+ is more active for the propene activation than Ag + at low temperatures. However, excess H 2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NO x . Additionally, a combination of this H 2 promotion effect and electrical heating produced a favorable de-NO x system with a quick response, a wide temperature-window and a high de-NO x activity.