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 dehydroxylatcd samples chemisorption is rapid and the major product initially appears to be a surface SiOBF2 species. This species is also formed when BF3 interacts wtih totally dehydroxylatcd silicas which have been prepared by heating silica in vacuum at about 1250cC. Spectroscopic evidence indicates that on highly or totally dehydroxylatcd 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 docs 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 arc 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.