Differentiation between O2 and NO2 impact on PtOx formation in a diesel oxidation catalyst

Abstract

NO oxidation activity of platinum-based catalysts undergoes reversible changes during operation in oxidative atmosphere typical for Diesel engine exhaust gas due to platinum oxides (PtOx) formation and reduction. In recent years, these changes were studied mostly with the mixtures containing both O2 and NO2 so that it was difficult to quantify individual impacts of the oxidants on PtOx formation. In this paper, Pt/Al2O3 catalyst deactivation and PtOx formation with O2 and NO2 was examined separately by the means of transient isothermal experiments at 150, 175 and 200 °C with alternating deactivation and probe periods. The largest extent of deactivation was detected at 175 °C where the NO conversion in the isothermal experiment dropped from 85% (reduced catalyst) down to 10% (steady-state PtOx level) over 4 h. The experimental data were then utilized for parameter evaluation in global kinetic model of NO oxidation on a diesel oxidation catalyst extended by the reactions describing PtOx formation and reduction. Though the rate coefficient for PtOx formation with NO2 is higher than with O2, the overall PtOx formation rate and the steady-state PtOx level are higher with O2 at typical component concentrations (8% O2 or 250 ppm NO2). Similar activation energies were observed for both PtOx formation reactions. Validity of the model was verified also at lower O2 and NO2 concentrations (4% O2 or 125 ppm NO2). The developed model is thus capable of sufficiently precise prediction of catalyst deactivation at various O2 and NO2 concentrations relevant to diesel exhaust gas.