A Theoretical Investigation on CO Oxidation by Single-Atom Catalysts M₁/γ-Al₂O₃ (M=Pd, Fe, Co, and Ni)

Single-atom catalysts have attracted much interest recently because of their excellent stability, high catalytic activity, and remarkable atom efficiency. Inspired by the recent experimental discovery of a highly efficient single-atom catalyst Pd₁/γ-Al₂O₃, we conducted a comprehensive DFT study on geometries, stabilities and CO oxidation catalytic activities of M₁/γ-Al₂O₃ (M=Pd, Fe, Co, and Ni) by using slab-model. One of the most important results here is that Ni₁/Al₂O₃ catalyst exhibits higher activity in CO oxidation than Pd₁/Al₂O₃. The CO oxidation occurs through the Mars van Krevelen mechanism, the rate-determining step of which is the generation of CO₂ from CO through abstraction of surface oxygen. The projected density of states (PDOS) of 2p orbitals of the surface O, the structure of CO-adsorbed surface, charge polarization of CO and charge transfer from CO to surface are important factors for these catalysts. Although the binding energies of Fe and Co with Al₂O₃ are very large, those of Pd and Ni are small, indicating that the neighboring O atom is not strongly bound to Pd and Ni, which leads to an enhancement of the reactivity of the O atom toward CO. The metal oxidation state is suggested to be one of the crucial factors for the observed catalytic activity.