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

The mechanistic cause of enhancement of C3H8-SCR activity by addition of H-2 over Ag/Al2O3 was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C3H8 + O-2, nitrates were mainly formed on Ag/Al2O3. An addition of H-2 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 H-2, both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C3H8 + O-2 and acetate in a flow of NO + O-2 were also increased by the addition of H-2. 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 H-2 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 H-2.