Hydrogen Production Coupled to Hydrocarbon Oxygenation from Photocatalytic Water Splitting

Abstract

Splitting of water into hydrogen and oxygen, especially with visible light, is a reaction of great challenge. H2 generation from water has attracted much attention because of its potential use as a source of clean and renewable energy. In natural photosynthesis, splitting of water to oxygen (2H2OQO2+ 4H + 4e ) is driven by sunlight and carried out by the oxygen-evolving complex (OEC) in photosystem II (PSII). Besides water splitting, H2 production from alternative sources such as abundant biomass has also attracted a great interest. A number of metalloenzymes such as cytochrome P450s and methane monooxygenases catalyze the oxidation of various types of hydrocarbons. High-valent metal-oxo species, derived from molecular oxygen or peroxide, have been suggested as active intermediates for the oxidation of hydrocarbons. Recent years have seen significant progress in designing and modeling metalloproteins for highly selective and efficient oxidation of organic substrates. Inspired by natural photosynthesis, oxidation of hydrocarbons with light and water has also been reported to reduce the use of hazardous chemicals and waste production. Water splitting into H2 and O2 involves multielectron processes for water oxidation (4e ) and proton reduction (2e ). There has been significant progress in developing photocatalysts for overall water splitting (2H2OQ2H2 +O2) in heterogeneous systems. However, in homogeneous systems, studies of water splitting have been generally carried out separately in two half reactions: the water oxidation process using sacrificial oxidants such as Ce, and the water reduction reaction using sacrificial reductants such as triethylamine. However, for overall water splitting, sacrificial reagents should be avoided. The combination of water oxidation and proton reduction reactions in a homogeneous system remains a great challenge. Using water as oxygen source, we and others have reported photocatalytic oxidation of organic substrates in homogeneous systems containing metal catalysts such as Ru and Mn for hydrocarbon oxidation, [Ru(bpy)3] 2+ as photosensitizer (Ru PS; see Figure S1 in the Supporting Information), and [Co(NH3)5Cl] 2+ as electron acceptor. For example, an olefin can be oxidized to an epoxide with water as oxygen source, releasing two protons and two electrons [Eq. (1)]. As shown in Scheme 1, the electrons derived from water were eventually transferred to [Co(NH3)5Cl] 2+ in the