Semiconductor photocatalysis: visible light induced photoreduction of aromatic ketones and electron-deficient alkenes catalysed by quantised cadmium sulfide

Abstract

Colloidal CdS suspensions (CdS-0) prepared at 0 °C from methanolic Cd(ClO₄)₂ and Na₂S solutions consist of quantised CdS microcrystallites (2–5 nm) and their loose aggregates, which catalyse the effective photoreduction of aromatic ketones and electron-deficient alkenes with triethylamine as electron donor. Under visible light induced photolysis, the methanolic CdS-0 suspension becomes brown owing to the reduction of lattice Cd²⁺ to Cd⁰, leading to the effective formation of alcohols from ketones, and dihydro compounds from alkenes. With the reduction potential < – 1.56 V vs. standard calomel electrode (SCE), ketones were partly photoreduced to pinacols in methanol. Compared with highly pure bulk CdS (CdS-Ald), CdS-0 is more effective for photoreduction because of the size quantisation effect. The presence of an excess of sulfide ion (S^2–) in the CdS-0 system, however, suppresses the formation of lattice Cd⁰, inducing one-electron transfer photoreductions which result in the exclusive formation of pinacols and 1,2,3,4-tetra(methoxycarbonyl)butane from the respective ketones and dimethyl maleate. The relationship between the two-electron reductions and photogenerated lattice Cd⁰ is discussed in terms of the regulation of semiconductor photocatalysis.