Reactions of halomethanes with gamma-alumina surfaces .1. An infrared spectroscopic study

The reactions of several atmospherically relevant halomethane compounds (CF3Cl, CF2Cl2, CFCl3, and CCl4) with heat-treated gamma-alumina powders 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 formation 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 spectra were recorded as a function of temperature. Several absorption features in the 1300-1850 cm(-1) region of the spectra were observed that were common to all of the compounds studied. These features, which appear at temperatures as low as 120 K, are attributed to adsorbed carbonate, bicarbonate, and/or formate species (COn, n = 2,3), indicating that halomethanes dissociatively chemisorb on heat-treated gamma-alumina 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 coordinatively 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 well as residual OH groups also participate in the surface chemistry.