Kinetic Studies of Oxygen Chemisorption on the Rutile Single-crystal Surface by Means of Electrical Conductivity

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The reaction of oxygen with the surface of a rutile single crystal was studied in the temperature range from 212 to 362°C and in the pressure range from 10−1 to 102 Torr. The process of the surface oxidation was examined by monitoring the electrical conductance change due to oxygen chemisorption. Two types of chemisorption kinetics were found for the rutile which had been reduced to different extents in vacuo before the introduction of oxygen. For the low-reduced rutile, the kinetics obeyed the parabolic rate law; this was interpreted by the diffusion-controlled reaction theory. On the other hand, it was found for the high-reduced rutile that the kinetics obeyed the Elovich equation; this could be explained by the boundary layer theory. The apparent activation energy for the former was 14.2 kcal/mol. For the latter, it was 4 kcal/mol and increased with an increase in the chemisorbed surface-charge density. In addition, the rate of oxygen chemisorption in the latter case was proportional to the pressure at a constant density of the chemisorbed surface charge.
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