Microcalorimetric study of the acidic character of modified metal oxide surfaces. Influence of the loading amount on alumina, magnesia, and silica

The modification of the acid/base properties of a series of oxide supports (alumina, magnesia, and silica) modified by increasing loadings of additive ions (Li+, Ni2+, and SO42-) from 1% to 50% of the support surface coverage is reported using adsorption microcalorimetry of NH3 and SO2 as acidic and basic probe molecules. The curves of differential heat of adsorption versus the coverage have shown that the acidic properties of alumina were weakly modified when 1% of the support surface was covered by the guest ion. The acid/base properties of silica and magnesia were much more easily enhanced by the additives. A high guest oxide loading (50% of surface coverage) led to very strong modifications of the surface acid-base behavior depending on the support and the additive. Volumetric measurements allowed us to quantify the number of acid/base sites, to express them in terms of ion specific effect, and also to search for correlations, in the case of high loadings, with Tanabe's and Seiyama's hypotheses on the acidity of mixed oxides.