Promotion effect of hydrogen on surface steps in SCR of NO by propane over alumina-based silver catalyst as examined by transient FT-IR

Abstract

The mechanistic cause of enhancement of C₃H₈-SCR activity by addition of H₂ over Ag/Al₂O₃ was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C₃H₈ + O₂, nitrates were mainly formed on Ag/Al₂O₃. An addition of H₂ into a C₃H₈-SCR atmosphere increased the concentration of surface acetate significantly, but decreased the concentration of surface nitrates. Formation and consumption rates of acetate and nitrates were estimated with transient in-situ IR measurement. By the addition of H₂, both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C₃H₈ + O₂ and acetate in a flow of NO + O₂ were also increased by the addition of H₂. From a comparison between the evolutions of adsorbed species (nitrate and acetate) and gaseous species (NO and C₃H₈), it was clarified that the NO reduction activity is controlled by partial oxidation of C₃H₈ to mainly surface acetate. The addition of H₂ results in remarkable promotion of partial oxidation of C₃H₈ to mainly surface acetate, which is the rate-determining step of C₃H₈-SCR in the absence of H₂.