A DFT Study on the Coordination Nature in Manganese Oxide Cluster

Silica supportec manganese-tungsten is among the best catalysts for oxidative coupling of methane. Experiment study suggests that manganese exists in the form of oxide clusters over the catalyst surface and play a key role for the activation of molecular oxygen. The calculation method is based on density functional theory, and an the calculation is carried out at the generalized gradient approximation level. Analysis of the total bonding energy, Mn-O bond lengths and the atom charges shows that the most stable structures are sir coordinated octahedral structure and four coordinated tetrahedral or planar structure. The energy levels of these model give two bands, contributed from the oxygen 2s orbitals, manganese 3d, 4s and oxygen 2p orbitals respectively. Compositions of the molecular orbitals reveal that the valence orbitals of manganese can combine in different ways to make maximal overlap with the 2p orbitals of oxygen, and so the unsaturately coordinated manganese oxides can combine and activate oxygen easily.