Density functional theory calculation on the promotion effect of H2 in the selective catalytic reduction of NOx over Ag–MFI zeolite

Density functional theory (DFT) calculation was used to study the hydrogen promotion effect for the selective catalytic reduction (HC-SCR) of NOx 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 Ag4 cluster is achieved by the AgH diffusion as follows: 2AgH + Ag-Z-AgimageHAg4H-ZimageAg4-Z + H2. With the presence of oxygen, the HAg4H cluster prefers the formation of HOO− adsorbate to the desorption of H2. O2 is physisorbed on the Ag4 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.