Stacking of elementary non-porous particles as XPS intensity model for supported heterogeneous catalysts: experimental assessment of validity

Mo 3/b d//Al2/b p/ XPS intensity ratios (/b R//sub Mo3d /) have been determined for a series of Al/sub 2/O/sub 3/ supported molybdenum oxides, characterized by a roughly constant Mo coverage and surface areas (/b S//sub BET/) ranging from 10 to 300 m/sup 2/ g/sup -1/. These experimental /b R//sub Mo3d / values are compared with theoretical values calculated according to three XPS intensity models. Of these three models, the stacking model the author has proposed earlier is shown to take the effect of surface area the best into account and to predict correctly the experimental values for the entire /b S//sub BET/ range. In contrast, the two other models are shown experimentally to be valid only at low or high surface area. Grain size turns out to be an important factor in XPS, in relation to diffusion-controlled impregnation which leads to concentration profiles.