Theoretical Study of NO Dissociation on Stepped Rh(221) and RhCu(221) Surfaces

Periodic density functional calculations are reported for NO dissociation on the Rh(221) and RhCu(221) surfaces. The analysis of the reaction pathways indicates that the presence of steps results not only in new adsorption sites for NO adsorption but also on the appearance of a completely new reaction pathway with activation energies which are smaller than those found previously for the regular Rh(111) surfaces. At low NO surface coverage and a small Cu density, the presence of Cu on Rh(221) does not change the reactivity of rhodium stepped surfaces toward NO dissociation, since the effective activation energy barrier is higher (1.32 eV for RhCu(221) versus 1.17 eV for Rh(221)). The essential reason is the increased metastability of the step sites. However, for stronger NO coverage or a higher Cu density, the stepped RhCu(221) surface is expected to exhibit an enhanced activity:  either by increasing the occupancy of these metastable sites along the RhCu steps or by removing the possibility of highly stable pure Rh bridge step sites.