Spectrokinetic study of the mechanism of NOx reduction with propylene over ZrO2 in excess oxygen

Abstract

It has been demonstrated by quantitative spectrokinetic measurements that, on the surface of zirconia stabilized as a tetragonal phase, the rate-limiting step of the selective catalytic reduction of nitrogen oxides (SCR of NO_x) with propylene is the interaction of surface nitrates with C₃H₆ yielding organic nitro compounds. It is hypothesized that propylene reacts not with the nitrates themselves but with the activated complex NO₂^ads whose structure is intermediate between the structures of the monodentate NO₃⁻ and NO₂ species. Deep C₃H₆ oxidation exerts an adverse effect on the rate of the SCR of NO_x with propylene, and the interaction between O₂ and NO, which yields NO₂ and NO₃⁻ stimulates further nitrogen reduction to N₂. The effect of the reaction between oxygen and O₂N−C_nH_m on the NO_x reduction rate is variable and is determined by the C₃H₆/NO_x ratio. A generalized scheme of the SCR of NO_x with propylene on the surface of ZrO₂ partially stabilized as a tetragonal phase has been developed by comparing experimental data of this study and data available from the literature.