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Infra-red spectroscopy has been used to study the adsorption of boron trifluoride on silica. On highly dehydroxylated samples chemisorption is rapid and the major product initially appears to be a surface SiOBFz species. This species is also formed when BF3 interacts wtih totally dehydroxylated silicas which have been prepared by heating silica in vacuum at about 1250°C. Spectroscopic evidence indicates that on highly or totally dehydroxylated silica, BF3 preferably reacts with a new reactive site, probably a siloxane bridge site, which is formed as a result of the removal of the surface silanol groups during heating. Only after the reaction is complete on the " siloxane " sites does chemisorption take place with silancjl groups if these are present. During prolonged evacuation the surface SiOBFz groups react further with trace amounts of water to produce (Si0)2BF groups; the latter are also produced during the initial stages of chemisorption when the silanol concentration is high. When water or air is admitted after chemisorption of BF3, a spectrum characteristic of boric acid is observed.

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