Surface equilibration in adsorption microcalorimetry of bases on H-USY

It is commonly assumed that adsorbed basic probe molecules freely equilibrate with surface-acid sites in microcalorimetric experiments to determine the strength and distribution of the acid sites. The validity of this assumption was tested by comparing the differential heat of adsorption on H-USY as a function of surface coverage of CD3NH2, NH3 and CD3CN, which have widely different proton affinities, and by monitoring with FTIR the distribution of the adsorbed molecules between Bronsted-and Lewis-acid sites during adsorption and desorption. The results showed that full equilibration was achieved with the weakest base, CD3CN, but not with the stronger bases, CD3NH2 or NH3. (C) 1998 Elsevier Science B.V.