MODELING OF THE HYDROXYL GROUP POPULATION USING AN ENERGETIC ANALYSIS OF THE TEMPERATURE-PROGRAMMED DESORPTION OF PYRIDINE FROM SILICA-GEL

The temperature-programmed desorption (TPD) of pyridine from silica gel has been analysed with the variable heating, constant coverage procedure proposed by Richards and Rees (R. E. Richards and L. V. C. Rees, Zeolites, 1986, 6, 17). This results in curves of activation energies of desorption as a function of coverage. A weak heterogeneity of the silica gel surface is observed in contrast with other pyridine sorption studies. Two adsorption sites, the free and bridged hydroxyl groups, contribute to the surface heterogeneity. The relative amounts of the two sites on the surface are deduced from the calculated activation energy vs. coverage profiles. An attempt is made to extrapolate the absolute concentrations of free and bridged hydroxyls from the relative site coverages.