Density functional theory (DFT) calculation was used to study the hydrogen promotion effect for the selective catalytic reduction (HC-SCR) of NO over Ag-MFI zeolite The nature of the bond between an Ag atom and MFI is ionic One of the roles of hydrogen addition is to neutralize the cationic Ag atom by forming an AgH molecule We propose that the formation of a Ag-4 cluster is achieved by the AgH diffusion as follows 2AgH+Ag-Z-Ag -> HAg4H-Z -> Ag-4-Z+H-2 With the presence of oxygen, the HAg4H cluster prefers the formation of HOO- adsorbate to the desorption of H-2 O-2 is physisorbed on the Ag-4 cluster. Thus, the HAg4H cluster is important for the activation of oxygen Another role of hydrogen addition is to reproduce the HAg4H cluster from the Ag4 cluster which is generated by the SCR reaction from HOOAg4H species Since time dependent DFT (TD-DFT) calculation shows that the UV absorption bands of the HAg4H cluster are weak, UV-vis measurements are not adequate for the study of the reaction mechanisms of HC-SCR (C) 2010 Elsevier B V All rights reserved