THE INFLUENCE OF METAL-OXIDE ADDITIVES ON THE MOLECULAR-STRUCTURES OFSURFACE TUNGSTEN-OXIDE SPECIES ON ALUMINA - II - IN-SITU CONDITIONS

The dehydrated Raman spectra of unpromoted WO3/Al2O3 indicate that the tungsten oxide surface species have the mono-ore structure and are pr esent as tetrahedrally coordinated monomers at low loadings and as a m ixture of tetrahedrally and octahedrally coordinated surface polymers at high loadings. The effect of introducing secondary metal oxide addi tives to WO3/Al2O3 catalysts, where tungsten oxide is present as a two -dimensional surface metal oxide overlayer, was investigated with in s itu Raman spectroscopy under dehydrated conditions. The secondary meta l oxide additives P2O5, SnO2, Fe2O3, NiO, ZnO, CoO, CeO2 and MgO coord inate directly to the oxide support without significantly interacting with the surface tungsten oxide phase. The secondary metal oxide addit ives La2O3, CaO, K2O and Na2O directly interact with surface tungsten oxide species to form both a surface mixed metal oxide complex and, fo r La2O3 and CaO, a crystalline tungstate compound. The noninteracting additives tend to be acidic (P2O5 and SnO2) or mildly basic (Fe2O3, Ni O, ZnO, CoO, CeO2 and MgO) and the interacting additives tend to be mo re basic (La2O3, CaO, K2O and Na2O). (C) 1998 Elsevier Science B.V. Al l rights reserved.