The influence of metal oxide additives on the molecular structures of surface tungsten oxide species on alumina. II. In situ conditions

• Published in: Journal of molecular catalysis. A, Chemical Chemical, 1998-05, Vol.132 (1), p.59-71
• Main Authors: Ostromecki, Marlene M., Burcham, Loyd J., Wachs, Israel E.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; In situ Raman spectroscopy; Secondary metal oxide additives; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Tungsten oxide
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/S1381-1169(97)00227-6

The dehydrated Raman spectra of unpromoted WO 3/Al 2O 3 indicate that the tungsten oxide surface species have the mono-oxo structure and are present as tetrahedrally coordinated monomers at low loadings and as a mixture of tetrahedrally and octahedrally coordinated surface polymers at high loadings. The effect of introducing secondary metal oxide additives to WO 3/Al 2O 3 catalysts, where tungsten oxide is present as a two-dimensional surface metal oxide overlayer, was investigated with in situ Raman spectroscopy under dehydrated conditions. The secondary metal oxide additives P 2O 5, SnO 2, Fe 2O 3, NiO, ZnO, CoO, CeO 2 and MgO coordinate directly to the oxide support without significantly interacting with the surface tungsten oxide phase. The secondary metal oxide additives La 2O 3, CaO, K 2O and Na 2O directly interact with surface tungsten oxide species to form both a surface mixed metal oxide complex and, for La 2O 3 and CaO, a crystalline tungstate compound. The noninteracting additives tend to be acidic (P 2O 5 and SnO 2) or mildly basic (Fe 2O 3, NiO, ZnO, CoO, CeO 2 and MgO) and the interacting additives tend to be more basic (La 2O 3, CaO, K 2O and Na 2O).