A transient kinetic study of the mechanism of the NO+H2 reaction over Pt/SiO2 catalysts -: 1.: Isotopic transient kinetics and temperature programmed analysis

The NO + H-2 reaction has been studied over a supported Pt catalyst using temperature programmed reaction (TPRxn), temperature programmed desorption (TPD), and steady state and non-steady state isotopic transient kinetic analysis (SSITKA and NSSITKA), N-2, N2O, and NH3 formation are noted during the TPRxn. N2O is the most prevalent species formed at lower temperatures while N-2 is most prevalent at higher temperatures. NH3 is formed in relatively low yields at intermediate temperatures, The SSITKA shows that N2O is the isotopically first product and that N-2 is isotopically second. The concentration of sites producing N2O and N-2 increases with temperature, and the increase in the effective activity of the sites is greater for those sites producing N-2 than those sites producing N2O, The chemisorption of NO is also found to be a reversible process under actual reaction conditions. Non-steady state transient kinetic experiments show that after removal of the gas phase NO, there is a conversion of some of the residual NO on the surface into N-2 precursors following a short treatment in a H-2 whereas N2O precursors on the surface are destroyed by this treatment. It is concluded that the formation of N2O requires the presence of gas-phase or very weakly adsorbed NO. (C) 1999 Academic Press.