Electron transfer at alumina surfaces. Part 7.—Nature of the radical species formed from perylene on the surface of an activated alumina—cation or anion?
The preadsorption of Lewis acids or Lewis bases on the surface of an activated alumina leads to deactivation of the reducing or the oxidizing function of the catalyst with respect to the 1,3,5-trinitrobenzene (TNB) or the perylene couple. The incorporation of small amounts of fluoride ion into alumina decreases the electron-donor properties of the oxide, as shown by radical formation with TNB or tetracyanoethylene. With perylene, 9,10-dimethylanthracene or triphenylamine as the adsorbate, small additions of fluoride ion to the oxide have exactly the opposite effect, a dramatic increase in the radical-forming power of the catalyst being observed. The effect of adsorbed TNB or benzoic acid is to decrease the reducing power of alumina for iodine, but adsorbed perylene or triphenylamine produces a marked enhancement of this activity.
These results prove the existence of two different electron-transfer sites on the surface of an activated alumina: a site that can generate TNB ion radicals and a site that can produce a free-radical form of perylene. Since the radical derived from TNB is negatively charged, present findings demonstrate beyond reasonable doubt that the species formed when perylene is adsorbed on an active alumina surface is the cation radical.