How surface reparation prevents catalytic oxidation of carbon monoxide on atomic gold at defective magnesium oxide surfaces

Abstract

In this contribution, we study using first principles the co-adsorption and catalytic behaviors of CO and O₂ on a single gold atom deposited at defective magnesium oxide surfaces. Using cluster models and point charge embedding within a density functional theory framework, we simulate the CO oxidation reaction for Au₁ on differently charged oxygen vacancies of MgO(001) to rationalize its experimentally observed lack of catalytic activity. Our results show that: (1) co-adsorption is weakly supported at F⁰ and F²⁺ defects but not at F¹⁺ sites, (2) electron redistribution from the F⁰ vacancy via the Au₁ cluster to the adsorbed molecular oxygen weakens the O₂ bond, as required for a sustainable catalytic cycle, (3) a metastable carbonate intermediate can form on defects of the F⁰ type, (4) only a small activation barrier exists for the highly favorable dissociation of CO₂ from F⁰, and (5) the moderate adsorption energy of the gold atom on the F⁰ defect cannot prevent insertion of molecular oxygen inside the defect. Due to the lack of protection of the color centers, the surface becomes invariably repaired by the surrounding oxygen and the catalytic cycle is irreversibly broken in the first oxidation step.