REACTIONS OF HALOMETHANES WITH GAMMA-ALUMINA SURFACES .2. X-RAY PHOTOELECTRON AND TEMPERATURE-PROGRAMMED REACTION SPECTROSCOPIC STUDIES

The reactions of CF3Cl, CF2Cl2, CFCl3, and CCl4 with gamma-alumina pow ders have been studied using temperature-programmed reaction (TPR) and X-ray photoelectron (XPS) spectroscopies. In the TPR experiments, pow der samples that had been dehydroxylated by healing to 1000 K in vacuu m were dosed at 100 K with halomethanes and then gradually heated. Des orbing species were monitored as a function of substrate temperature u sing a line-of-sight quadrupole mass spectrometer. Hydrogen chloride a nd halomethyl fragments, which are indicative of halomethane dissociat ive chemisorption, were observed to desorb at temperatures below 150 K . Carbon dioxide began to desorb between 240 and 320 K; the onset temp erature of CO2 desorption correlated inversely with the degree of chlo rination of the compound. The CO2 most likely arises from COn (carbona te and/or formate) species which are formed via the low-temperature di ssociative chemisorption of the halomethanes. Repeated heating and dos ing of the alumina sample resulted in a drop in CO2 production, implyi ng that surface active sites become depleted. In situ XPS analysis of heat-treated powders that had been dosed at 150 K with chlorofluoromet hanes revealed the presence of both organic and inorganic forms of flu orine. Powders dosed at 200 K had much lower organic halogen coverages . Halogen uptake probabilities, which are estimated to be similar to 1 0(-5) from the data, increased as the degree of chlorination of the ha lomethane increased. These results indicate that chlorofluoromethane c ompounds will probably decompose on alumina solid-propellant rocket mo tor exhaust particles in the stratosphere, forming adsorbed AI-X (X = F, CI) and COn species and releasing gas phase HCl and CFxCly fragment s. However, the impact of these processes on global stratospheric halo methane and ozone concentrations is likely to be minimal.