Single-Atom High-Temperature Catalysis on a Rh1O5 Cluster for Production of Syngas from Methane

Single-atom catalysts are a relatively new type of catalyst active for numerous reactions but mainly for chemical transformations performed at low or intermediate temperatures. Here we report that singly dispersed Rh₁O₅ clusters on TiO₂ can catalyze the partial oxidation of methane (POM) at high temperatures with a selectivity of 97% for producing syngas (CO + H₂) and high activity with a long catalytic durability at 650 °C. The long durability results from the substitution of a Ti atom of the TiO₂ surface lattice by Rh₁, which forms a singly dispersed Rh₁ atom coordinating with five oxygen atoms (Rh₁O₅) and an undercoordinated environment but with nearly saturated bonding with oxygen atoms. Computational studies show the back-donation of electrons from the d_z² orbital of the singly dispersed Rh₁ atom to the unoccupied orbital of adsorbed CH_n (n > 1) results in the charge depletion of the Rh₁ atom and a strong binding of CH_n to Rh₁. This strong binding decreases the barrier for activating C-H, thus leading to high activity of Rh₁/TiO₂. A cationic Rh₁ single atom anchored on TiO₂ exhibits a weak binding to atomic carbon, in contrast to the strong binding of the metallic Rh surface to atomic carbon. The weak binding of atomic carbon to Rh₁ atoms and the spatial isolation of Rh₁ on TiO₂ prevent atomic carbon from coupling on Rh₁/TiO₂ to form carbon layers, making Rh₁/TiO₂ resistant to carbon deposition than supported metal catalysts for POM. The highly active, selective, and durable high-temperature single-atom catalysis performed at 650 °C demonstrates an avenue of application of single-atom catalysis to chemical transformations at high temperatures.