NOx storage/reduction over lean-burn automotive catalysts

Abstract

This paper shows the behavior of a Pt/Ba/γ–Al2O3 automotive catalyst in a fixed bed reactor during cyclic operation at lean and rich gas phase conditions at short (seconds) and long (hours) cycling times at different temperatures. Reactor exit gas phase concentrations have been measured and catalyst properties have been determined before and after selective cycling experiments. The experimental results indicate that: (i) Upon 9h lean and 15h rich cycling, the NO oxidation efficiency of the catalyst decreases with time while incomplete regeneration is seen, even after 15h rich exposure with H2. The cyclic steady state is reached after 3lean/rich cycles, at which only 60% of the available barium is involved in the NOx storage/reduction. (ii) The BET surface area, pore volume, and Pt dispersion decrease by approximately 40%, which may be a result of masking of Pt sites or blocking of pores of the barium clusters as BaCO3 becomes Ba(NO3)2. Experiments with catalyst pellet sizes of 180 and 280μm along with XPS measurements show that blocking of catalyst pellet pores is not taking place. (iii) When applying lean/rich cycling in the order of seconds, it appears that catalyst history and lean/rich timing affect the number of cycles required to arrive at a closed N balance. XRD results after lean exposure confirm the formation of barium nitrate in the bulk of the barium cluster.