Photodecomposition of Chloromethanes Adsorbed on Silica Surfaces

Irradiation of CCl₄, CFCl₃, and CF₂Cl₂ in the presence of C₂H₆ in vessels containing silica sand or fused quartz tubing results in the formation of chlorine-containing products. The formation of these compounds occurs at wavelengths extending up to approximately 400 nm, that is, at wavelengths well beyond the absorption threshold of the chloromethanes in the gas phase. It is suggested that CCl₄ adsorbed on silica surfaces photodissociates to yield CCl₃ and CCl₂ species. The poor material balance obtained in these experiments indicates that several of the chlorine-containing fragments are strongly adsorbed on the surface. At a CCl₄ pressure of 13 Pa (0.1 torr), photolysis with 366 nm light in the presence of sand results in the decomposition of one molecule for every 10⁴ photons striking the surface. Under otherwise identical conditions, the photon-induced breakdown of CFCl₃ and CF₂Cl₂ is respectively only 10 percent or 3 percent as efficient.