A Direct Relation between Adsorbate Interactions, Configurations, and Reactivity: CO Oxidation on Rh(100) and Rh(111)

Formation of carbon dioxide from preadsorbed O and CO has been modeled by Density Functional Theory (DFT) calculations on Rh(100) and Rh(111) for total surface coverages of 0.22, 0.50, 0.66, and 0.75 ML. The most stable coadsorption configurations are investigated for each case prior to the modeling of the reaction. It is seen that the activation barriers on Rh(100) continuously decrease (from 1.03 to 0.40 eV) with increasing coverage, due to surface configurations always leading to less strongly bound adsorbates. On Rh(111), however, the activation barriers remain relatively constant (changing from 1.28 to 1.00 eV) because adsorbate configurations are relatively independent of coverage, except the defect-like/transient structure (having a barrier of 0.81 eV) calculated for 0.66 ML. The results demonstrate the importance of adsorbate configurations for the rate of a surface reaction.