Infra-red studies of reactions on oxide surfaces. Part 1.—Boron trifluoride on silica

Abstract

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 SiOBF₂ species. This species is also formed when BF₃ interacts with 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 silanol groups if these are present. During prolonged evacuation the surface SiOBF₂ groups react further with trace amounts of water to produce (SiO)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.