AVS 56th International Symposium & Exhibition
    In Situ Microscopy and Spectroscopy: Interfacial and Nanoscale Science Topical Conference Friday Sessions
       Session IS+AS-FrM

Paper IS+AS-FrM1
In Situ Atomic-Scale Imaging of an Oxide Supported Catalyst during a Redox Change: WOx / a-Fe2O3 (0001)

Friday, November 13, 2009, 8:20 am, Room C4

Session: In-Situ Microscopy and Spectroscopy: Dynamic Nanoscale Processes
Presenter: Z. Feng, Northwestern University
Authors: Z. Feng, Northwestern University
J.W. Elam, Argonne National Lab
C.-Y. Kim, Canadian Light Source Inc.
Z. Zhang, Argonne National Lab
M.J. Bedzyk, Northwestern University
Correspondent: Click to Email
Ultrathin metal-oxide layers deposited onto oxide surfaces have wide applications in catalysis and chemical sensing. Supported tungsten oxides are among these. If the atomic-scale geometrical and electronic surface structure of WOx could be predicted, this would impact our understanding of numerous chemical processes. As a model catalytic system, atomic layer deposition (ALD) grown WOx on hematite (0001) was used for finding the positions of W with respect to the support lattice and its sensitivity to the reduction-oxidation (redox) cycle. AFM is used to study the surface morphology changes. X-ray fluorescence and in situ X-ray standing waves (XSW) are used in combination to determine the geometric structure changes during the redox reaction. The XSW results for 1/3 ML W show that W cations on the surface occupy different positions in the as-deposited, oxidized and reduced states. The ALD as-depostied W shows strong correlation vertically but no correlation laterally. Oxidization causes the W ordered and they occupy some special adsorption sites. However, in the reduced state, W cations change their adsorption sites. Atomic density maps created from in situ XSW measurements give us direct informaiton for the W cations surface site location. Finally, X-ray photoelectron spectroscopy (XPS) is used to correlate the W oxidation state(s) with the above redox induced structural changes. A model is proposed to explain the reversible geometrical/electronic structure changes during this redox reaction.