Addition of H-2 in the lean exhausts significantly improves the NOx reduction activities at low temperatures by hydrocarbons over Ag/Al2O3 catalyst. This discovery "hydrogen effect" provides a novel approach to the design of SCR systems. We concluded that the catalytic activity is promoted by the reductive activation of O-2 with H-2 and the production of moderately agglomerated Ag-n(delta+) clusters on the catalyst surface. However, the other research groups concluded that the formation of Ag-n(delta+) cluster is not essential for the enhancement of the SCR activity and it is proposed that hydrogen itself participates directly in the reaction mechanism. Although the mechanism of the "hydrogen effect" is under debate, the oxidative activation of hydrocarbons is the key step to obtain high SCR activity at lower temperatures. (c) 2007 Elsevier Ltd. All rights reserved.