Modeling of two-step CO oxidation light-off on Pt/γ-Al2O3 in the presence of C3H6 and NOx

Abstract

Abatement of pollutants in automotive exhaust converters can be negatively affected by interactions of reaction intermediates with the catalyst surface. This paper focuses on a specific inhibition of CO oxidation on Pt/γ-Al2O3 catalyst by C3H6 oxidation intermediates, leading to a so-called two-step CO light-off behavior. Two reaction pathways contributing to a temporary decrease of CO conversion during a temperature-programmed co-oxidation of CO and C3H6 are identified: (i) accumulation of C3H6 oxidation intermediates on the catalyst surface that block active sites and progressively inhibit both CO oxidation and total C3H6 oxidation, and (ii) CO by-product formation during propylene oxidation. The latter pathway seems to play a minor role while the inhibition effect dominates. The phenomenon is most pronounced at lower (nearly stoichiometric) oxygen concentrations and in the presence of NO. This indicates that the initial formation of C3H6 oxidation intermediates is less sensitive to O2 and NO concentrations, while the total oxidation steps depend more strongly on the actual exhaust gas composition. A novel global reaction kinetic model is proposed that captures all the experimentally observed phenomena including transient accumulation of the reaction intermediates on the catalyst surface and the resulting impact on CO and C3H6 conversion. The developed model covers a wide range of operating conditions, from three-way catalyst (stoichiometric mixture) to diesel oxidation catalyst (lean mixture).