Regensburg 2013 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 8: Surface Chemical Reactions and Heterogeneous Catalysis I
O 8.4: Vortrag
Montag, 11. März 2013, 11:15–11:30, H33
O K-edge NEXAFS for Silica-Supported Molybdena at Different Loadings: theoretical cluster studies — •Lili Sun1, Klaus Hermann1, Kazuhiko Amakawa1, Michael Hävecker2, Anette Trunschke1, and Robert Schlögl1 — 1Inorganic Chemistry Department, Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin (Germany). — 2Department of Solar Energy Research, Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Str. 15, 12489 Berlin. Germany.
Oxygen core excitations in different molybdena-silica clusters are evaluated using density-functional theory to explain the X-ray absorption fine structure (NEXAFS) near the O K-edge measured for molybdena supported by SBA-15 silica. Different oxygen species in the molybdena-silica clusters with molybdenum in different coordination can be clearly distinguished in the theoretical spectrum. The experimental NEXAFS spectra at low molybdenum loadings (< 6.6%) exhibit a double-peak structure in the O 1s to Mo 4d / O 2p excitation range of 529-536eV. This is reproduced by the theoretical results for tetrahedral dioxo MoO4 units, singly or doubly connected with the silica substrate. The experimental spectrum shows a transition from a double- to a broad asymmetric single-peak structure at high molybdenum loadings (about 13.3%). This can be modeled by decreasing the distance between the two molybdenyl oxygen atoms in the dioxo MoO4 unit leading to direct O-O coupling. Thus, the spectral transition may be explained by a simple structural effect resulting from the increased density (squeezing) of MoO4 units at the SBA-15 silica surface.