Electron-transfer at alumina surfaces. Part 1.—Electron-acceptor properties of aluminas

Abstract

When catalytic aluminas are heated to a high temperature, sites are produced on the surface which are capable, at room temperature, of oxidizing polynuclear aromatic hydrocarbons to the corresponding ion-radical, provided oxygen is present. The oxidizing activity depends markedly on the conditions of activation. Fully dehydrated alumina (corundum) is inactive as is alumina dehydrated below 320°C. Maximum activity occurs in samples heated to about 900°C in which the crystalline phase is identified as γ-alumina. The sites responsible for electron-transfer on the alumina surface differ in important respects from the characteristic oxidizing centres on silica-alumina, although in some silica-alumina catalysts both types of site may be present. The site, which is characteristic of alumina itself, involves molecular oxygen. Hydrocarbons adsorbed on this type of site react rapidly with oxygen and cannot readily be recovered unchanged from the catalyst surface. A second type of active centre predominates in the silica-aluminas of low alumina content. On this type of site the adsorbed hydrocarbon is stable for long periods. The electron acceptor at the latter site may be the SiO₄ residue which occurs at the alumina-silica domain boundary. In the active oxidizing centre of the alumina type an oxygen molecule may occupy this role.