Recombination coefficient of atomic oxygen on silica surfaces at high temperature: experiment and molecular dynamics simulations

We present, in this paper, the experimental set-up and method we have developed for the measurement of the atomic oxygen recombination coefficient ?O at high temperature (900 - 2000 K) using a direct method on silica surfaces (?-cristobalite and quartz). The atomic oxygen concentration evolution is described by the diffusion equation as the mean free path of atoms is very small in comparison with the reactor radius. The recombination coefficient values are calculated from the concentration profile obtained by VIS-spectroscopy and for different temperatures of the sample. We also have measured the gas temperature profile above the sample using molecular emission spectroscopy with very precise measurement close to the surface sample where recombination occurs. Finally, we can reach the activation energy of the atomic oxygen recombination reaction for each material in the studied temperature range. The experimental results are compared with semiclassical Molecular Dynamics calculations performed on the same catalytic system aiming at determining, at a molecular level, the basic features of the surface catalytic activity. A substantial agreement between the experimental and the theoretical recombination coefficients has been found in the surface temperature range explored in the experiments. In addition to that, the collision dynamics simulation has revealed interesting aspects concerning the energetic and the mechanism of the surface reaction, still under current discussion. In particular, results are reported and discussed on the relative importance of the Langmuir-Hinshelwood and Eley-Rideal reaction mechanisms and the fraction of the energy flux deposited to the silica substrate.