CO and NO adsorption and co-adsorption on the Pd(1 1 1) surface

Adsorption and co-adsorption of nitric oxide (NO) and carbon monoxide (CO) on a Pd(1 1 1) surface were studied using pseudopotential density-functional theory calculations. For individual NO and CO molecules we determined the energetically most stable adsorption sites and harmonic frequencies. The results show that at 0.25 monolayer (ML) coverage both the molecules prefer to reside on a hollow site in agreement with experimental and earlier theoretical results. At half of monolayer coverage pure NO occupies only hollow sites but CO molecules have two energetically degenerate structures namely fcc–hcp and bridge–bridge. The harmonic vibrational frequencies were calculated for the most stable structures and the changes of frequencies from 0.25 ML coverage to 0.5 ML were compared to the corresponding experimental shifts. Our results suggest a hollow site adsorption for NO and a hollow–bridge mixture for CO. CO/NO co-adsorption was studied at 0.5 ML coverage with a molecular ratio 1/1. We investigated 16 co-adsorption geometries and found five almost equally stable structures, where NO is mainly on a hollow site, whereas CO occupies either a hollow or a bridge site. For all the examined structures the interaction between co-adsorbed molecules is repulsive. The strength of the repulsion depends on a inter-molecule distance and sharing of the common Pd atom. We calculated harmonic wave numbers for three structures, and found that different adsorbates do not vibrate in resonance.