Step-enhanced selectivity of NO reduction on platinum-group metals.

The conversion of NO to N2 is a key issue encountered in the control of emission from vehicles. The selectivity of NO reduction on two platinum group metals, Ir and Pt, including the close-packed flat surface and the monatomic steps are extensively studied within the first-principles density functional theory framework. A stepped Ir surface is found to possess high selectivity for NO reduction, which is attributed to both the electronic and geometric structures of the Ir steps. The other surfaces considered fail to combine both attributes, activity and selectivity. In particular, a stepped Pt surface has a poor N2 selectivity with a large tendency for N2O production. The results explain the observed metal dependency and the structure sensitivity of the NO reduction under excess O2 conditions.