A Steady-State Isotopic Transient Kinetic Analysis of the NO/O2/H2 Reaction over Pt/SiO2 Catalysts

Abstract

The NO/O2/H2 reaction under strongly oxidising conditions has been studied over a 5% Pt/SiO2 catalyst using Steady-State Isotopic Transient Kinetic Analysis (SSITKA). The 14N-containing reactants and products were monitored following a 14NO/O2/H2→15NO/O2/H2 switch. N2O was found to be the isotopically first product and N2 the isotopically second. It was found that there was a constant desorption of NO from the catalyst surface under steady-state conditions. Recently introduced transformations (IDIMP, TRIMP, and the semilogarithmic plot of the α function versus time) of the product profiles, which concentrate on the isotopic distribution of the product molecules following the 14NO→15NO switch, are used to analyse the mechanism of N2 formation from this reaction. These show that N2 formation is predominantly through an “impact” route in which gaseous or physisorbed NO reacts with a reduced N species on the catalyst in a modified Eley–Rideal mechanism. A second, less active, mode of N2 formation is through the interaction of two equivalent species on the surface, each of which gives one N atom to the N2 molecule. The presence of O2 results in a change in the relative contribution of each type of N2 production.