CO oxidation catalyzed by single gold atoms supported on aluminum oxide clusters

The single gold atom doped aluminum oxide clusters AuAl₃O₃⁺, AuAl₃O₄⁺, and AuAl₃O₅⁺ have been prepared and mass-selected to react with CO, O₂, and mixtures of CO and O₂ in an ion trap reactor under thermal collision conditions. The reactions have been characterized by mass spectrometry with isotopic substitution (¹⁶O₂ → ¹⁸O₂) and density functional theory calculations. The AuAl₃O₅⁺ cluster can oxidize two CO molecules consecutively to form AuAl₃O₄⁺ and then AuAl₃O₃⁺, the latter of which can react with one O₂ molecule to regenerate AuAl₃O₅⁺. The AuAl₃¹⁶O₃⁺ ions interact with a mixture of C¹⁶O and ¹⁸O₂ to produce the fully substituted ¹⁸O species AuAl₃¹⁸O_3-5⁺, which firmly identifies a catalytic cycle for CO oxidation by O₂. The oxidation catalysis is driven by electron cycling primarily through making and breaking a gold-aluminum chemical bond. To the best of our knowledge, this is the first identification of catalytic CO oxidation by O₂ mediated with gas-phase cluster catalysts with single-noble-metal atoms, which serves as an important step to understand single-atom catalysis at strictly a molecular level.