The influence of metal oxide additives on the molecular structures of surface tungsten oxide species on alumina: I. Ambient conditions

The molecular structures of WO3/Al2O3 catalysts, with and without the presence of secondary metal oxide additives (P, Sn, Fe, Ni, Zn, Ce, Co, La, Ca, Mg, K and Na), were determined with Raman spectroscopy under ambient conditions where the surface is hydrated. The hydrated surface tungsten oxide species found on the alumina support (WO4-2, HW6O21-5, and U-metatungstate) were related to the net pH at the point of zero charge (pzc) of the thin aqueous layer. The isolated tungstate species was dominant at high pH values and the polytungstate species were dominant at lower pH values. The net pH at pzc was a function of tungsten oxide loading (decreasing from 8.9 to 4.4 for 1 to 25% WO3/Al2O3, respectively) and the specific secondary metal oxide additive (Na > K > Mg > Ca > La > Co > Ce > Zn > Ni > Fe > Sn > P). Below monolayer coverages, the secondary metal oxide additives preferentially interacted with the alumina support to form surface metal oxide species rather than with the tungsten oxide species to form mixed tungsten oxide crystalline compounds. Above monolayer coverages, both surface metal oxide species and mixed tungsten oxide crystalline compounds were formed. The more basic secondary metal oxide additives (Ca, La) formed mixed tungsten oxide crystalline compounds. In the absence of the formation of mixed tungsten oxide crystalline compounds, the alumina support was able to accommodate approximately two monolayer-equivalents of the surface metal oxides during calcination because the surface tungsten oxide species and the secondary surface metal oxide species occupied different sites on the alumina support. The surface tungsten oxide species preferentially interacted with the surface hydroxyls of the alumina support and the secondary surface metal oxide species preferentially interacted with coordinately unsaturated Al+3 sites. (C) 1998 Elsevier Science B.V. All rights reserved.