A transient kinetic study of the mechanism of the NO/C3H6/O2 reaction over Pt-SiO2 catalysts -: Part 2:: Steady-state isotopic transient kinetic analysis

Steady-state isotopic transient kinetic analysis (SSITKA) of the NO/C3H6/O-2 reaction, using (NO)-N-14-->(NO)-N-15 switches, has been carried out over a 1% Pt-SiO2 catalyst at four different temperatures. The results indicate that below the temperature of the maximum NOx conversion there are no NO adsorption/desorption processes taking place under these steady-state conditions. Moreover, less than 15% of the catalyst surface is covered with N-2 or N2O precursors. N2O is the "isotopically first" product while Nz is the second. The surface lifetime of N2O precursors is relatively short while the surface lifetime of N-2 precursors is significantly longer. The production of N-2 and N2O are both increased by raising the temperature. In the case of N2O, this seems to be solely due to an increase in the concentration of active sites on the catalyst while in the case of N-2 the increase in the number of active sites is accompanied by an increase in their intrinsic activity. At temperatures above the maximum in the NOx conversion there is a discernible amount of NO desorption taking place under steady-state conditions, This is rationalised by the formation of NO2 ((ads)) species on the now-oxidised Pt surface. The SSITKA results are found to support the previously proposed reaction mechanism for the NO/C3H6/O-2 reaction over Pt-based catalysts. Finally, a general form for the calculation of concentrations of adsorbed species from any SSITK analysis is presented. (C) 1999 Academic Press.