Electron transfer at alumina surfaces. Part 6.—Redox properties of fluorided aluminas

Abstract

An e.s.r. study of adsorbed perylene and adsorbed tetracyanoethylene has shown that the incorporation into alumina of up to 6% by weight of fluorine enhances the oxidizing activity and simultaneously lowers the reducing activity of the catalyst surface, on subsequent activation in the temperature range 180–800°C. The increase in oxidizing activity with increase in fluoride content is paralleled by the development of and increase in Brönsted acidity. This and other evidence indicate that fluoridation of alumina produces a surface with redox activity qualitatively intermediate between that of alumina and a typical silica–alumina cracking catalyst. Replacement by fluoride of hydroxyl ions in the surface of alumina samples activated at low temperatures (< 400°C) and of oxide ions in samples heated at higher activation temperatures accounts for the decrease in the reducing activity on fluoridation. A third type of site has been postulated to account for the reducing properties of some fluorided aluminas.