DFT studies of the structure and vibrational spectra of isolated molybdena species supported on silica

Different cluster models of isolated Mo-VI molybdate species anchored on silica surface were investigated using density functional theory (DFT). Isolated molybdate centers were modeled as either a penta-coordinated mono-oxo species or a tetra-coordinated di-oxo species. Standard free energy changes for interconversion between mono-oxo and di-oxo species indicate that these two species can coexist in equilibrium on the surface of amorphous silica with di-oxo species being the favored species at high temperature and low partial pressures of water. Comparison of Raman-spectra from experiment and DFT suggests that di-oxo species might be the prominent species responsible for the peak at 988 cm(-1). This conclusion is strongly supported by the similarity in the EXAFS spectra obtained from experiment and theory. The thermodynamics for H-2 reduction of isolated molybdate species were determined and found to be in reasonable agreement with experimental observation. DFT calculations of the structure and properties of the reduced Moll centers and comparison of these with experimental results suggest that the reduced centers are present as mono-oxo species.