Experimental and mechanistic study of NOx adsorption over NOx trap catalysts

The storage capacity of NO(x) trap catalysts is affected by the gas composition. The capacity increases when the oxygen content goes from 0% to 3%. Above 3%, no effect is exhibited. The presence of NO(2) enhances the storage capacity. Storage is significant even if no oxygen is present in the gas phase and even in the absence of platinum. Two kinds of sites for NO, NO(2) and O(2) adsorption seem to operate. Platinum sites close to BaO crystallites (sites 1) are responsible for nitrate formation. The other platinum sites allow the catalyst to behave as an oxidation catalyst. A mechanism based on NO, NO(2) and O(2) adsorption and reaction over type 1 sites is able to qualitatively explain the experimental results. The rate of nitrate formation depends on the gas composition while the rate of nitrate decomposition increases with the amount of nitrate formed. This explains the effect of gas composition on the storage capacity obtained at steady-state, when formation and decomposition rates are the same. (C) 1999 Elsevier Science B.V. All rights reserved.