Mechanism of the H2 Effect on NH3-Selective Catalytic Reduction over Ag/Al2O3: Kinetic and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Studies

The mechanism of H-2-assisted NH3-selective catalytic reduction (SCR) over Ag/Al2O3 was systematically investigated by kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). H-2 was an essential reactant for the NH3-SCR reaction over Ag/Al2O3, and the overall chemical reaction equation for this reaction was as follows: 2NO + 2NH(3) + 2O(2) + 3H(2) = 2N(2) + 6H(2)O. Kinetic studies indicated that this reaction obeyed the Langmuir-Hinshelwood mechanism, such that both NO and NH3 adsorbed on the Ag/Al2O3 surface before their further reaction. In situ DRIFTS experiments showed that Ag species catalytically oxidized NO to yield nitrate species, which further migrated to the Al2O3 surface. Then, the bridging and bidentate nitrates reacted with the B-NH, species to produce N-2 and H2O. Afterward, the -OH species consumed by the adsorption of B-NH, species were restored by H-2 dissociation on the metallic Ag species. Notably, the rate-determining step in the H-2-NH3-SCR reaction was the reduction of oxidized Ag species, which further governed the formation of nitrate species. The reduction of NOx primarily occurred at the surface of Al2O3, while Ag species and H-2 contributed to the oxidation of NO to generate nitrates and to the regeneration of -OH species.