In the dehydration of catalytic aluminas, sites are formed on the surface which are capable of reducing tetracyanoethylene (TCNE) to the corresponding anion radical. Two different surface sites are responsible for the reducing properties. One, formed during dehydration at low temperatures (<350°C), may be associated with the presence on the surface of unsolvated hydroxyl ions. At higher temperatures (>500°C), a different site is produced which is believed to be a defect centre involving oxide ions. It is associated on the surface with an electron-acceptor site which is responsible for the oxidizing properties of alumina. The high-temperature aluminas therefore possess well-marked redox properties. Confirmation that the active sites on aluminas dehydrated at lower temperatures differ from those on the high-temperature aluminas is obtained from a study both of the surface products formed in the two cases and of the products obtained on desorption with water. Additional evidence is obtained from blocking experiments with n-butylamine. Aluminas that have been heated above 1100°C are inactive either as reducing or oxidizing agents.

Typical silica-alumina cracking catalysts (alumina content <25 %), which have previously been shown to possess strong oxidizing properties, have negligible reducing power when activated at any temperature. Silica-aluminas of higher alumina content (40–70 % Al₂O₃) possess appreciable reducing activity but this may be associated with the presence, in the conglomerate, of pure alumina phase.