Experimental and kinetic study of NO oxidation on model Pt catalysts

Modeling and experimental studies on model Pt/Al2O3 and Pt/BaO/Al2O3 catalysts are performed to elucidate the kinetics of NO oxidation, which is a key step during the lean phase of NO, trap operation. Experiments show that a steady-state is never truly achieved during NO oxidation: a continuous decrease in the reaction rate with time is observed on both the catalysts. This decrease is distinct from and beyond the prompt inhibition of the NO oxidation reaction observed with NO2 in the feed or product. NO oxidation carried out after catalyst pretreatments with H-2, O-2, and NO2 indicates that NO2 is responsible for the deactivation while NO2 storage plays a negligible role. Experiments with NO2 as the feed elucidate its role in the production of NO, either by storage or by decomposition, for a wide range of temperatures. The highly oxidizing nature of NO2 suggests that the Pt surface could be covered with oxygen, either as chemisorbed O or as Pt oxides, which results in slow poisoning of the catalyst. Microkinetic analysis of the NO oxidation reaction shows O-2 adsorption as the rate-determining step and predominant surface species to be adsorbed NO and O. Based on the microkinetic studies, a global kinetic model is proposed which includes the inhibiting effect of NO2 on the NO oxidation reaction. The importance of including coverage of NO in the global model at low temperatures is shown, which is neglected in the current literature global models. The model predicts the experimental observations for a wide range of temperatures within acceptable error limits. However, prediction of the transient data requires modeling of NO2 storage, decomposition and the complex NO2 inhibition chemistry in addition to other surface reactions. (C) 2009 Elsevier Inc. All rights reserved.