REACTIONS OF HALOMETHANES WITH GAMMA-ALUMINA SURFACES .1. AN INFRAREDSPECTROSCOPIC STUDY

The reactions of several atmospherically relevant halomethane compound s (CF3Cl, CF2Cl2, CFCl3, and CCl4) with heat-treated gamma-alumina pow ders have been investigated using ill situ Fourier transform infrared absorption spectroscopy in an attempt to assess the impact of alumina exhaust particles from solid-propellant rocket motors on stratospheric chemistry. Infrared spectra revealed that heating the powders to 1000 K in vacuum promotes dehydroxylation of the alumina surface and forma tion of a new surface site which is proposed to consist of clusters of Al3+ and O2- ions. The heat-treated powders were dosed at 100 K with halomethanes and then gradually heated to promote reaction; infrared s pectra were recorded as a function of temperature. Several absorption features in the 1300-1850 cm(-1) region of the spectra were observed t hat were common to all of the compounds studied. These features, which appear at temperatures as low as 120 K, are attributed to adsorbed ca rbonate, bicarbonate, and/or formate species (COn, n = 2,3), indicatin g that halomethanes dissociatively chemisorb on heat-treated gamma-alu mina at temperatures below that of the lower midlatitude stratosphere. These COn species are apparently stable to temperatures in the range 250-400 K, depending on the degree of chlorination of the halomethane compound. Pyridine preexposure experiments revealed that coordinativel y unsaturated Al3+ sites that are formed during the heat treatment are essential to the low-temperature dissociative chemisorption process. The AlxOy ion-cluster sites formed as a result of heat treatment as we ll as residual OH groups also participate in the surface chemistry.