Hydrogen production through microheterogeneous photocatalysis of hydrogen sulfide cleavage. The thiosulfate cycle

Abstract

Cleavage of hydrogen sulfide into hydrogen and sulfur occurs in alkaline aqueous CdS dispersions under visible light illumination (⩾400 nm). Small quantities of a noble metal catalyst (RuO 2) loaded onto ‘naked’ CdS particles markedly improve the yield of hydrogen formation. The effect of RuO 2 is ascribed to catalysis of electron transfer to proton. Simultaneous and efficient photogeneration of hydrogen and thiosulfate occurs in CdS dispersions containing both sulfite and bisulfide (or sulfide) ions. Electron transfer from the conduction band of CdS to that of TiO 2 particles occurs in alkaline suspensions containing these HS − ions and has been exploited to improve the performance of a system achieving decomposition of H 2S by visible light. Equally important is a recent finding that the performance of a system containing ‘naked’ CdS in combination with RuO 2-loaded TiO 2 particles is far better than that of CdS/RuO 2 alone. Additionally, conduction band electrons produced by bandgap excitation of TiO 2 particles efficiently reduce thiosulfate to sulfide and sulfite. The valence band process in alkaline TiO 2 dispersions is thought to involve oxidation of S 2 O 3 2− to tetrathionate, S 4O 6 2−, which quantitatively dismutates into sulfite and thiosulfate, the net reaction being 2h vb +( TiO 2 )+0.5S 2 O 3 2−+1.5 H 2 O→ SO 3 2−+3 H +. The photodriven disproportionation of thiosulfate into sulfide and sulfite is of great interest in systems that photochemically cleave hydrogen sulfide into hydrogen and sulfur 1.5 H 2 O+1.5S 2 O 3 2− ▪+2 SO 3 2−+ SO 2−+3 H +.