Poisoning of Hydrogen Recombination on Silica Due to Water Adsorption

The role of water adsorption in poisoning hydrogen recombination on silica is studied using a two-layer Langmuir isotherm model. In the model, hydrogen atoms adsorb directly on the silica surface, while water molecules physisorb above the surface, blocking gas atoms from reaching or reacting on the surface. Model parameters such as heat of adsorption and activation energy are informed using experimental values and data from atomic-scale simulations. Using this model for a gas-phase radical concentration of 1016 cm–3 (representative of a room-temperature gas at 1 Torr), the addition of water vapor is predicted to lower the room-temperature recombination coefficient of hydrogen (γH) from 3 × 10–3 to 2 × 10–4, consistent with experimental data. In general, surface reaction rates are reduced significantly for temperatures below 400 K. An analytical model is used to estimate the effect of reactive walls on radical concentration in a silica tube used in hydrogen plasma experiments. In the model, radical concentration is held constant at one end of the tube, and radical species diffuse down its length and to the walls of the tube, where they are destroyed by recombination reactions. The poisoning effect of water on surface reactions greatly enhances radical concentration in the tube, increasing the fraction of unreacted radicals 30 cm from the inlet by nearly 2 orders of magnitude. Both the reduction of the room-temperature recombination coefficient and the increased concentration of atomic hydrogen in the silica tube are in quantitative agreement with experimental findings.