AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM2
Structurally Ordered Magnesium Vanadate Model Catalysts for Oxidative Dehydrogenation

Tuesday, October 3, 2000, 8:40 am, Room 208

Session: Reactions on Oxides and Environmental Chemistry
Presenter: A.G. Sault, Sandia National Laboratories
Authors: A.G. Sault, Sandia National Laboratories
J.E. Mudd, Sandia National Laboratories
J.A. Ruffner, Sandia National Laboratories
J.E. Miller, Sandia National Laboratories
Correspondent: Click to Email
A fundamental understanding of active sites in mixed metal oxide oxidative dehydrogenation (ODH) catalysts continues to be elusive. In an effort to simplify the complexities inherent in these materials we are growing single phase, oriented mixed metal oxide thin films. Using RF sputter deposition, we have grown 10-2000 Å films of Mg@sub 3@(VO@sub 4@)@sub 2@, a known ODH catalyst. Bulk and surface analysis of films grown on silicon wafers show the desired stoichiometry, but the films are amorphous. Deposition on a 500 Å gold layer grown on oxidized silicon results in films strongly oriented toward the (021) plane of Mg@sub 3@(VO@sub 4@)@sub 2@. This orientation is due to the tendency of gold to grow with (111) planes exposed, which provides an ideal epitaxial substrate for Mg@sub 3@(VO@sub 4@)@sub 2@ (021). This plane consists of close packed oxygen layers, with Mg and V ions in octahedral and tetrahedral sites, respectively. By varying oxygen pressure we can deposit films with stoichiometries ranging from fully oxidized Mg@sub 3@(VO@sub 4@)@sub 2@ to partially reduced Mg@sub 3@V@sub 2@O@sub 6@. We will detail the effects of heat treatments in reactive environments (e.g., oxygen/propane mixtures) on the structure and composition of these films, and report on the catalytic activity of these films for ODH. In general, heating the films above 623 K in vacuum or 100 Torr propane results in at least partial reduction of V(V) to V(III) and segregation of V to the surface. Prolonged reduction at higher temperatures results in complete reduction to Mg@sub 3@V@sub 2@O@sub 6@. Treatment in oxygen reverses these changes. This reversible redox chemistry is consistent with the known mechanism for ODH, which involves participation of lattice oxygen. Catalytic measurements of propane ODH show very low conversions due to the low surface areas of the films. Measurements with more facile ODH reactions (such a 1-butene to butadiene) are underway and will be reported.