HETEROGENEITY OF V(2)O(5)/Al(2)O(3) AS STUDIED BY WATER VAPOUR ADSORPTION MICROCALORIMETRY

The adsorption of water vapour on 773 K-outgassed gamma-Al(2)O(3), V(2)O(5) and alumina-supported V(2)O(5) has been studied calorimetrically at 303 K. Measurements have been made incrementally in the region of sub-monolayer H(2)O coverage in order to map the hydrophilicity of the surface. For gamma-Al(2)O(3) the heat of adsorption is initially very high (155 kJ mol(-1)) and about 30% of the surface chemisorbs water with a heat of adsorption greater than 120 kJ mol(-1). The heat then falls rapidly with increasing coverage, reaching 55-60 kJ mol(-1) at the monolayer value. V(2)O(5), by contrast, is much less hydrophilic, with the heat of adsorption initially 90 kJ mol(-1) and a more shallow dependence of the heat on coverage to give a final value of 55 kJ mol(-1). This distinction in hydrophilicity between the two oxides has been exploited to investigate V(2)O(5), supported on gamma-Al(2)O(3) in the region of loading up to an amount capable of giving monolayer coverage of vanadia. Oxides containing 10, 15 and 20 wt.% V(2)O(5) have been compared. 10 wt.% loading, capable of similar to 40% coverage if spread as a monolayer, leads to a marked change in the hydrophilic profile of alumina. The high energy sites are eliminated preferentially. Increasing the loading to 15 and 20 wt.% produces further changes, but they am relatively much smaller, On the basis of the measured water vapour isotherms an estimate is made of the vanadia coverage. The conclusion is that up to 10 wt.% V(2)O(5) there is monolayering of the alumina by vanadia, but that further loading leads to much less effective spreading. At 20 wt.% loading there is still much alumina surface which is not covered by vanadia.