CHARACTERIZATION OF GAMMA-ALUMINA-SUPPORTED VANADIUM-OXIDE MONOLAYERS

The structure and properties of vanadium oxide deposited on gamma-alum ina at submonolayer coverage have been investigated. Textural analysis , electron microscopy, and low energy ion scattering (LEIS) show that, for loadings up to at least 3.4 V/nm(2), the vanadium oxide is presen t as a highly dispersed phase. LEIS experiments show that the developm ent of the vanadium oxide monolayer proceeds in three steps. In the fi rst step vanadium oxide is molecularly dispersed on gamma-alumina as i solated monomeric and dimeric species. Above approximately 1.5 V/nm(2) a second layer of vanadium oxide is formed on top of the dimeric spec ies, resulting in the formation of oligomeric species. At coverages ab ove 2.5 V/nm(2) these small oligomeric species grow into polymeric van adium oxide chains. TPR experiments show that reduction of these polym eric species proceeds at lower temperatures than reduction of the mono meric species, probably as a result of the weakening of the vanadium-o xygen bond. (C) 1995 Academic Press, Inc.