Role of silver species in H2-NH3-SCR of NOx over Ag/Al2O3 catalysts: Operando spectroscopy and DFT calculations

Abstract

The role of silver species in the H2-assisted NH3 selective catalytic reduction of NOx (H2-NH3-SCR) over Ag/Al2O3 catalysts was investigated by operando spectroscopy (DRIFTS-MS and DR-UV–Vis) and Density Functional Theory (DFT) calculation. UV–Vis analysis showed that several kinds of silver species were present on the Ag/Al2O3, including dispersed silver cations, partially oxidized silver clusters, metallic silver clusters, and silver nanoparticles. Operando DRIFTS-MS experiments showed that the reduction of NOx was dependent on the formation rate of nitrates, which was closely related to the silver content, H2 concentration, and reaction temperature. Notably, operando DR-UV–Vis experiments unambiguously confirmed that the metallic silver clusters were the active sites for nitrates formation, and the reduction of silver nitrates was much slower than NO oxidation. DFT calculations further revealed that reducing silver nitrates required higher energy barriers than NO oxidation, while H2 dissociation could occur on metallic silver clusters with a moderate energy barrier. In conclusion, metallic silver clusters contributed to the formation of nitrates and H2 dissociation, and the reduction of silver nitrates was the rate-determining step in the overall H2-NH3-SCR reaction.