REACTIVE SILICA .2. NATURE OF SURFACE SILICON HYDRIDES PRODUCED BY CHEMISORPTION OF HYDROGEN

The pyrolysis of the methoxy groups of methylated Aerosil silica leads to the production of surface Si-OH and SiH2 groups, as indicated by infrared absorptions at 3747 and near 2300 cm-1, respectively. Prolonged out-gassing causes the elimination of hydrogen from these groups and leads to the formation of surface Si radicals. Experiments with electron spin resonance confirm this mechanism. The radical is very reactive and permits the dissociative chemisorption of hydrogen at temperatures as low as 25°. A second surface silane, Si-H, is also formed, but is not stable above 550°. Adsorbed water does not interact extensively with the surface silanes. The latter are stable to oxygen up to 350°. The nature of the surface, the surface species, and their interactions, are discussed.