A Transient Kinetic Study of the Mechanism of the NO+H2 Reaction over Pt/SiO2 Catalysts: 1. Isotopic Transient Kinetics and Temperature Programmed Analysis

Abstract

The NO+H2 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). N2, N2O, and NH3 formation are noted during the TPRxn. N2O is the most prevalent species formed at lower temperatures while N2 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 N2 is isotopically second. The concentration of sites producing N2O and N2 increases with temperature, and the increase in the effective activity of the sites is greater for those sites producing N2 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 N2 precursors following a short treatment in a H2 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.