X-ray spectroscopic fingerprints of reactive oxygen sites at the MoO3(0 1 0) surface

The identification of oxygen sites at metal oxide surfaces and the characterization of their properties is of great importance for an understanding of the catalytic activity of such materials and, thus, for a rational design of efficient and selective catalysts. In the case of the clean MoO3(0 1 0) surface we show that an unambiguous discrimination of the different reactive oxygen sites can be obtained by angle-resolved near-edge X-ray absorption fine structure (NEXAFS) combined with density functional theory (DFT) based spectrum analyses for different photon polarization directions. In particular, we are able to unequivocally discriminate the characteristic spectral signatures of singly coordinated molybdenyl oxygen covering the topmost molybdenum layers from those of other oxygen centers that have very similar local environment and only differ by their spatial orientation in the crystal. Theoretical predictions are also successfully used to identify and interpret characteristic features in the NEXAFS spectra that arise from specific vacancy sites present at oxygen deficient surfaces.