The effect of Pd ensemble structure on the O₂ dissociation and CO oxidation mechanisms on Au-Pd(100) surface alloys

The reactivity of various Pd ensembles on the Au-Pd(100) alloy catalyst toward CO oxidation was investigated by using density functional theory (DFT). This study was prompted by the search for efficient catalysts operating at low temperature for the CO oxidation reaction that is of primary environmental importance. To this aim, we considered Pd modified Au(100) surfaces including Pd monomers, Pd dimers, second neighboring Pd atoms, and Pd chains in a comparative study of the minimum energy reaction pathways. The effect of dispersion interactions was included in the calculations of the O₂ dissociation reaction pathway by using the DFT-D3 scheme. The addition of the dispersion interaction strongly improves the adsorption ability of O₂ on the Au-Pd surface but does not affect the activation energy barriers of the Transitions States (TSs). As for O₂ to dissociate, it is imperative that the TS has lower activation energy than the O₂ desorption energy. DFT-D3 is found to favor, in some cases, O₂ dissociation on configurations being identified from uncorrected DFT calculations as inactive. This is the case of the second neighboring Pd configuration for which uncorrected DFT predicts positive Gibbs free energy (ΔG) of the O₂ adsorption, therefore an endergonic reaction. With the addition of D3 correction, ΔG becomes negative that reveals a spontaneous O₂ adsorption. Among the investigated Au-Pd (100) ensembles, the Pd chain dissociates most easily O₂ and highly stabilizes the dissociated O atoms; however, it has an inferior reactivity toward CO oxidation and CO₂ formation. Indeed, CO strongly adsorbs on the palladium bridge sites and therefore poisoning the surface Pd chain. By contrast, the second neighboring Pd configuration that shows somewhat lower ability to dissociate O₂ turns out to be more reactive in the CO₂ formation step. These results evidence the complex effect of Pd ensembles on the CO oxidation reaction. Associative CO oxidation proceeds with high energy barriers on all the considered Pd ensembles and should be excluded, in agreement with experimental observations.