Electron spin resonance study of the interaction of perylene with alumina

Abstract

ESR spectra obtained from the interaction of perylene with alumina become less well resolved with time. The resolution decays with a rate that is dependent on the concentration of adsorbed radical and to a lesser extent on the activation temperature of the catalyst. Radical concentrations < 10 16 g −1 of catalyst and activation temperatures > 650°C give low rates of decay. Decreasing the temperature at which adsorption of the perylene occurs reduces the decay rate markedly. Under suitable conditions a well-resolved nine-line spectrum can be observed for the perylene cation-radical generated on the surface of pure alumina at ambient temperatures. Molecular oxygen plays a decisive role in this electron-transfer process. Subsequent chemical reaction between the perylene cation-radical and oxygen leads to the formation of new radical species and largely accounts for the decrease in resolution of the ESR spectrum with time. On some sites on the alumina surface, however, the perylene radical is stable for long periods. Present findings have particular importance for current theories of cationradical formation on the alumina surface.