Partial Oxidation of Methane with Molecular Oxygen to Produce Hydrogen Catalyzed by Rh₃O₂⁺ Clusters

Catalytic partial oxidation of methane (POM) in the presence of molecular oxygen represents a highly promising strategy for the conversion of methane to hydrogen under mild conditions; however, it remains quite challenging due to the kinetic inertness of methane and catalyst instability under an oxidative atmosphere. Herein, benefiting from dual-ion-trap mass spectrometric experiments in conjunction with quantum chemical calculations, we successfully identify that the Rh₃O₂⁺ cluster cation can catalyze the POM reaction with O₂ to produce 2H₂ + CO₂ under thermal collision conditions. The ligand effect of the oxygen atoms to enhance the activity and stability of a cluster catalyst has been clarified. The coordinated oxygen atoms can tune the contribution of the active Rh 4d-orbital in the frontier orbitals of Rh₃O_0-2⁺ clusters, enabling enhancement of cluster reactivity toward C-H activation. The stability of reaction intermediate Rh₃O₃⁺ under an oxygen atmosphere is also ensured by the bridging oxygen atoms that can regulate the highest occupied orbitals of the active [Rh₃] site, dominated by the Rh 4d_z² orbital that is disadvantageous for reaction with O₂. This work not only provides an insightful guide in the design of excellent ligand-protected metal cluster catalysts in direct conversion of CH₄ with O₂ under mild conditions but also reveals the molecular-level mechanism of efficient hydrogen production in the POM process.