Hybrid-DFT Modeling of Lattice and Surface Vacancies in MnO

We have investigated the properties of defects in MnO bulk and at (100) surfaces, as used in catalytic applications, using hybrid-level density functional theory (i.e., inclusion of exact exchange within the exchange-correlation evaluation) in a hybrid QM/MM embedded-cluster approach. Initially, we calculate the formation energy for bulk Mn and O vacancies, comparing charged-defect compensation with charge carriers at the Fermi Level (ϵf) and through Schottky defect formation. Oxygen vacancies were also investigated at the (100) surface, where the vacancy formation energy is very similar to the bulk. Defect levels associated with the most stable vacancies are calculated using the ΔSCF method: all are positioned mid band gap, with surface environments failing to alter strongly the overall nature of the defect relative to bulk. Chemical activity of the (100) MnO surface was considered through the adsorption of a probe CO2 molecule, which is considered the initial step in the transformation of CO2 into hydrocarbons. CO2 adsorption was strongest over a neutral oxygen vacancy, where the associated trapped electrons of the defect transfer to the adsorbate and thus activate it. However, we have shown with our embedded-cluster approach that the neutral oxygen vacancy is not necessarily the dominant species, which has implications when interpreting results for future catalytic applications