Structures and thermodynamic stability of cobalt molybdenum oxide (CoMoO₄-II)

This contribution reports density functional theory (DFT) calculations on structural and electronic properties of bulk and surfaces of cobalt molybdenum oxide CoMoO₄-II; i.e., a material that enjoys a wide array of chemical catalytic and optical applications. Estimated lattice constants and atomic charges for bulk CoMoO₄-II reproduce limited analogous experimental measurements. Bader's charges confirm the ionic nature for metal-O bonds in bulk and surfaces of CoMoO₄-II. Plotted partial density of states reveal a narrow band gap of 1.8 eV for bulk CoMoO₄-II. We found that cleaving bulk of CoMoO₄-II along the low-Miller indices afford twelve distinct surfaces. Upward displacement of oxygen atom becomes evident when contrasting bulk positioning of atoms with relaxed surfaces. The two mixed Mo/O- and Co/O–terminated surfaces dominate the thermodynamic stability diagram at 1 atm and 300 – 1400 K, and across a wide range of oxygen chemical potential. The presence of surface oxygen atoms in these stable surfaces is expected to facilitate the occurrence of oxygen reduction reactions as experimentally demonstrated. Likewise, the adjacent surface cations (Mo⁴⁺/Co²⁺) and anions (O²⁻) serve as Lewis-acid pairs; i.e., very potent active sites in prominent catalysis reactions.