Platinum oxide formation and reduction during NO oxidation on a diesel oxidation catalyst—Macrokinetic simulation

Abstract

The activity of a diesel oxidation catalyst (Pt-γ-Al2O3) is strongly influenced by the reaction conditions due to platinum oxidation, which drastically decreases the NO oxidation activity. Reactivation is possible with normal exhaust gas since platinum oxide can be reduced by NO in an atmosphere with high oxygen content at temperatures below 200°C. A macrokinetic model is presented which takes into account the deactivation due to platinum oxidation. A standard model is extended by an additional balance of the platinum fraction and by reactions for platinum oxide formation (Pt+0.5 O2⇌PtO) and reduction (PtO+NO→Pt+0.5 NO2). It is assumed (1) that platinum oxide is not inactive but has a lower activity, (2) that small amounts of NO2 have no impact on the oxidation in the presence of high oxygen concentrations and (3) that formation of PtO2 is negligible and that PtO is the main platinum oxide species formed. The NO oxidation rate is calculated for highly active sites (platinum) and sites with lower activity (platinum oxide). The actual reaction rate is calculated as a function of the platinum oxide fraction. With this extended macrokinetic model a good prediction of NO conversion is achieved. For model calibration and validation, isothermal experiments with either constant temperature or linear temperature ramps are employed. The model can be transferred to catalysts with lower platinum loading or aged catalysts, if only the NO oxidation on platinum and platinum oxide is reparameterised.