Hydrogen Atom Recombination on Glass Surfaces

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Abstract
The adsorption and recombination of H atoms at temperatures between 77 and 110 °K have been studied on various silicate glass surfaces consisting of fused silica, porous Vycor with surface adsorbed water, and porous Vycor with chemically altered surfaces achieved by replacement of the surface functional hydroxyl group by chlorine, fluorine, and methoxy substituents.The concentration of adsorbed H atoms was determined by a measurement of the e.s.r. signal and the kinetics of the disappearance of the hydrogen atoms were observed to be second order in mobile atoms, consistent with a diffusion controlled recombination process.The results require that there be at least two types of surface adsorbed atoms, namely, weakly physically adsorbed atoms that are mobile and immobile. Identification of these adsorbed atom types with a particular surface functional feature leading to the adsorption is proposed.The activation energy for the second order decay of hydrogen atoms at the surface was found to vary from 1.0 to 2.3 kcal/mol, depending upon the chemical nature of the surface. A correlation between this activation energy and an electronegativity function of the surface group is presented.

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