Aluminum oxide materials feature good thermal and mechanical stability and they are difficult to participate directly in chemical reactions under mild conditions. Recently, scientists found that trace amounts of rhodium can promote direct participation of lattice oxygen of aluminum oxide to oxidize reactant molecules. However, the fundamental mechanism of noble metal catalysis is elusive.

A research group at Institute of Chemistry of Chinese Academy of Sciences led by Prof. HE Shenggui demonstrated that a single rhodium atom can promote the participation of five oxygen atoms to oxidize carbon monoxide molecules from a nine-atom rhodium–aluminum oxide cluster RhAl₂O₆⁺. This work was published on Nature Communications.

The rhodium-aluminum oxide clusters were generated by laser ablation of a mixed-metal disk compressed with rhodium and aluminum powders. The generated RhAl₂O_m⁺ clusters were mass-selected, confined, and then interacted with CO in an ion trap reactor under mild conditions. On the interaction of RhAl₂O₆⁺ with CO, the consecutive transfer of five oxygen atoms to CO has been unexpectedly identified.

It was reported that a single noble metal atom doped heteronuclear oxide cluster can transfer at most two oxygen atoms to oxidize CO molecules. Calculation using density functional theory demonstrated that the rhodium atom functions as the trapping site to anchor and oxidize CO molecules by the oxygen atoms connected with rhodium.

Furthermore, rhodium atom acts as the primarily oxidative center to accumulate the large amounts of electrons that stored originally in the removed oxygen atoms, then the polarity of rhodium is ultimately transformed from positive to negative.

This gas-phase study sheds new insights into the nature of noble metal catalysis that takes place in the related condensed phase.