Modelling of catalytic monolith converters with low- and high-temperature NO x storage compounds and differentiated washcoat

Abstract

Two types of catalytic washcoats with different properties can be present simultaneously in a monolith channel, e.g. the flat foils and the corrugated ones in metallic monolith can be individually coated with specific type of the washcoat. The model of such multiphase, differentiated NO x storage and reduction (NSR) catalytic monolith converter has been developed. The important reactions as are the oxidation of carbon monoxide, hydrocarbons and hydrogen, the reduction of nitrogen oxides (NO x ), the water gas shift and the steam reforming reactions, the NO/NO 2 transformation, and the oxygen and NO x storage are considered. Unknown kinetic parameters of the NSR are evaluated from transient experiments with the samples of two different types of NSR catalysts. The first catalyst is of the Pt–γ-Al 2O 3–CeO 2 type with alkali earth metals (e.g. barium) as NO x storage components (active at lower temperature). The second one is of the PtRh–γ-Al 2O 3–CeO 2 type with both alkali earth metals and alkali metals (e.g. potassium) as NO x storage components (active at higher temperature). Simulations of periodic lean/rich operation of the low-temperature, the high-temperature and the differentiated (combined) NSR converters are performed. The results of the computations agree well with the experimental data. The dependences of integral NO x conversion on the lengths of the lean and rich phases and on the temperature of the inlet gas are discussed. The efficiencies of the low-temperature, the high-temperature and the differentiated NSR converters are compared at different operating conditions.