RECOMBINATION OF O,N, AND H-ATOMS ON SILICA - KINETICS AND MECHANISM

The probability of recombination, gamma, was measured for recombination of O, N, and H atoms on pure silica between 700 and 1250 K (HT), 300 and 700 K (MT), and at 194 K (LT). The rate of recombination is first order with respect to the atom concentration from LT to HT. The Arrhenius plots, gamma-vs 1/T, are very complex. All observations are explained by assuming a surface with a small fraction of active sites that irreversibly bind chemisorbed atoms. Everything happens as if the active sites are surrounded by collection zones within which all atoms striking the surface are adsorbed reversibly with an assumed sticking probability of unity. These atoms then diffuse on the surface. Some of them reach the active sites where they can recombine with the chemisorbed atoms. At LT, all atoms striking the surface reach the active sites. As a result of desorption at MT, the collection zones shrink with increasing temperature. At HT, only atoms striking active sites directly from the gas phase lead to recombination. All data were fitted to an analytical solution of the diffusion-reaction equation obtained for a model where the active sites are distributed uniformly. A new result of this work is the identification of the active sites for recombination of H on silica at HT as surface hydroxyl groups.