Car exhaust catalysis from first principles:: Selective NO reduction under excess O2 conditions on Ir

Combining energetic data from density functional theory with thermodynamic calculations, we have studied in detail selective NO reduction under excess O-2 conditions on Ir. We show that excess O-2 can readily poison the Ir catalyst for NO reduction and the poisoning starts from a low O coverage on the surface. The adsorbed O switches the reaction selectivity from reduction (N-2 production) to oxidation (NO2 production). As the O coverage is built up, Ir metal can eventually be oxidized to IrO2, which is predicted to be thermodynamically possible under reaction conditions. To prevent O poisoning the surface, the presence of reductants is thus essential. We demonstrate that NO reduction is sensitive to the choice of reductant, and that alkenes are the most effective, mainly because they are able to produce surface C atoms that can selectively remove O atoms from Ir steps. On the basis of our analyses of the electronic structures, the mechanism of O-poisoning is elucidated and the reactant sensitivity in NO reduction is also discussed in terms of the bonding competition effect. We found that for different adsorbates, such as NO, O, and N, their bondings with surface d-states are remarkably similar. This gives rise to an indirect repulsion between adsorbates whenever they may bond with the same metal atoms. This energy cost can be qualitatively correlated with the valency of the adsorbate, and this is the key to understand the O-poisoning effect and the structure sensitivity in NO reduction.