Light-induced hydrogen production using waste compounds as sacrifical electron donors

Abstract

Systems for the conversion and chemical storage of solar energy are usually based on photo-induced electron transfer reactions from an excited sensitizer to an electron acceptor (i.e. an electron relay compound). We have investigated the photo- and electrochemical properties of two novel electron relay compounds: l,l″,r″′-trimethyl[4,2′; 4′,4″; 6′,4″′]quaterpyridinium trichloride (1) and 1,1′,1″,1″′-tetramethyl[4,2′; 4′,4′’; 6′,4″] quaterpyridinium tetrachloride (2). When solutions containing Ru(bpy)3Cl2 or Zn-porphyrin (as photosensitizer), compound1 or2 and EDTA (as sacrificial electron donor) are irradiated by visible light (λ > 400 nm), only the formation of reduced relay compound 1 occurs. Addition of platinum catalyst to such solutions with compound 1 leads to the formation of hydrogen. Quantum yields are in the range of 5%, calculated for absorbed light. Using other sacrificial electron donors such as alcohols or glucose (or waste compounds like 4-chlorophenol) did not result in hydrogen evolution. However,1 and2 are reduced by these sacrificial electron donors in the absence of an additional photosensitizer, when near UV irradiation light (λ > 280 nm) is used. Quantum yields for hydrogen production with compound1 are about 2%, calculated for absorbed light out of GC-measurements from H2 in the gas phase above the irradiated solution. The photo- and electrochemical properties of compounds1 and2 are discussed.