DFT+ U Study on Catalysis by Co3O4: Influence of U Value and a Surface-Bulk Bi- U Strategy

The DFT+U method provides an effective approach to correctly reproduce the material bulk properties of strongly correlated transition metal oxides (TMO) such as magnetic ground states, electronic structure, and redox reaction energies. Therefore, the catalysis processes over these strongly correlated TMO surfaces are frequently simulated utilizing the DFT+U method with the U value fit by these reported bulk properties. However, it is generally acknowledged that the calculated results utilizing the DFT+U method depend on the applied effective U value. Due to the significantly different coordination environments, the fit U value based on the bulk properties might not be proper for the simulation of the catalysis process. Moreover, the possibly caused calculation errors still remain elusive. To clarify it, we systematically investigate the influence of the Hubbard U value on the molecule adsorption at different sites, CO oxidation, and first C-H activation of methane over Co3O4(110). It is found that with the change of U value, the activities of surface Co cations and surface O anions exhibit reverse variation tendencies. This results in the fact that the transition state (TS) occupying both Co and O sites energetically exhibits lower dependences on the U value, especially for those TSs such as the first C-H bond activation of CH4 and CO oxidation by lattice oxygen. However, the strong dependency of the energies of intermediates occupying only either Co or O sites on the U value makes it still essential to identify the reliable U value to achieve the correct energy profile of the reaction pathway. To uniformly reproduce surface chemical properties of both Co and O sites, we put forward a surface-bulk bi-U strategy, utilizing the RPA results of adsorption energy as the benchmark for the surface U fitting combined with the bulk U fitting from formation enthalpy of oxides. The combination of surface U = 1.35 eV and bulk U = 3.4 eV is found to simultaneously describe the accurate reaction performance at both surface Co sites and O sites of Co3O4(110). These understandings may provide a benchmark to identify the influence of U value on the understanding of catalytic reactions and a possible strategy to accurately reproduce surface properties of catalyst. © 2018 American Chemical Society.