XAFS Study of Tungsten L1- and L3-Edges:  Structural Analysis of WO3 Species Loaded on TiO2 as a Catalyst for Photo-oxidation of NH3

We analyzed the X-ray absorption fine structure (XAFS) spectra of W L1- and L3-edges in order to determine the structure of various W species on TiO2. Two 2p → 5d transitions were observed in the second derivative of the W L3-edge X-ray absorption near-edge structure (XANES) spectra of all samples containing W (WO3, Na2WO4, Cr2WO6, Ba2NiWO6, (NH4)10W12O41·5H2O, Sc2W3O12, H3PW12O40·13H2O, and TiO2-loaded WO3 samples). These two transitions can be assigned to 5d orbitals split by the ligand field. The split in the 5d orbital has a linear relationship with the area of pre-edge peak of the W L1-edge XANES spectrum. This linear relationship is supported by density functional theory (DFT) calculations of the octahedral and tetrahedral W models. On the basis of this linear relationship, we estimated the structure of various W species on TiO2. The obtained structure is in accordance with the curve fitting analysis of W L3-edge extended X-ray absorption fine structure (EXAFS). Additionally, we found that the octahedral WO3 species loaded on TiO2 sample exhibits the high activity for the photo-oxidation of NH3 (photo-SCO).