Photoactivation of Solids and its Effect on Adsorption

Abstract

CONSIDERABLE attention has been given recently to chemical processes involving an activating influence of a crystal excited by irradiation¹. The mechanism of such photosensitized reactions, although unknown in detail, is generally believed to be a more or less complete transfer of the energy absorbed by the crystal to the reacting components, physically or chemically. Accordingly, the essential difference between photosensitized processes and real photochemical ones is the distance between the place of absorption and the place of reaction. But there must also be another, more general, effect of irradiation on the activity of crystals. Due to the change in the electronic state of the particles in the lattice by the absorption of light (change in charge and degree of polarization, formation of space charges, etc.), the forces between the particles are changed and, consequently, there is also a change in potential of the surface field that determines the chemical activity of the crystal. As in a real crystallographic transition, which causes a difference in the reactivity of the modifications under comparable experimental conditions², so irradiation of an absorbing crystal causes a change of the surface activity that might have a detectable influence on sorption or catalytic processes or on real chemical reactions. Naturally, in many cases, such effects would be expected to be rather small.