Friday, November 12, 2010: 10:15 AM
151 D/E Room (Salt Palace Convention Center)
It is well known that monolayers of vanadia supported on other oxides can be uniquely active and selective for a number of reactions including the selective catalytic reduction of NO and the partial oxidation of methanol to formaldehyde. A number of explanations have been offered in the literature to explain the unique properties of vanadia monolayers and the influence of the support, but these have been hampered by the inability to control both the chemical interaction of the vanadia with the substrate and the structure of the vanadia layer. We have addressed this issue by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to grow ordered vanadia monolayers and multilayers on rutile (110), anatase (001) and (101), WO3(100) and LaCoO3(100). By control of the atomic oxygen flux, monolayers and multilayers with the same surface structure can be obtained. For the monolayers, oxygen adsorption oxidizes all of the vanadium to 5+ while for the multilayers the bulk stoichiometry is VO2 but oxygen adsorption oxidizes the surface to V2O5. Comparing the reactivity of monolayers and multilayers of vanadia on rutile (110) and anatase (001) reveals the importance of direct chemical interactions with the support versus the importance of maintaining non-bulk V2O5 structures. For both the rutile and anatase support, we find that as long as the films are epitaxial that the direct chemical interaction with the support is not required to observe oxidative dehydrogenation of 1-propanol to propionaldehyde in thermal desorption; vanadia on both supports desorbed 1-propionaldehyde in a peak at ~400 K independent of the thickness of the vanadia layer. Differences between anatase and rutile as well as monolayers and multilayers were observed however. In particular, the fraction of adsorbed alcohol that reacted on the vanadia monolayer was much higher than any other vanadia surface, a finding attributed to the propensity of the rutile (110) surface to form defects that are highly active for alcohol deprotonation, the first step in the reaction. Thus the titania supports can influence the reactivity by both stabilizing reactive, non-bulk structures, and by facilitating the first step in reactions of alcohols. The reactivity data will be compared with results for vanadia on LaCoO3(100) which has the same surface unit cell as anatase (001), and thus in the monolayer regime the influence of chemical interactions on the reactivity of the vanadia are characterized with the structure held constant.