Tin-porphyrin sensitized TiO2 for the production of H2 under visible light

Abstract

Hydrogen was successfully produced under visible light irradiation in a tin porphyrin (SnP)-sensitized TiO₂ system in the wide pH range (pH 3–11) although SnP hardly adsorbs on TiO₂. The number of H₂ produced in the SnP/TiO₂ system after 9 h irradiation corresponds to the turnover number of 410. The apparent photonic efficiency for H₂ evolution was estimated to be 35% with the monochromatic radiation of 550 ± 10 nm. The photochemical production of hydrogen is mediated through the formation of the π-radical anion (SnP˙⁻) that subsequently transfers electron to TiO₂. The photogenerated SnP˙⁻ was monitored by transient absorption spectroscopy and its lifetime is long enough to survive the slow diffusion from the solution bulk to the TiO₂ surface, which makes the adsorption of SnP on TiO₂ not required for hydrogen production. This is clearly contrasted with the common ruthenium complex-sensitized TiO₂ system where the adsorption of the sensitizer complex is essentially required and the hydrogen production is limited to the acidic condition where the adsorption of the sensitizers on TiO₂ is allowed. The photocatalytic activity of SnP was mainly attributed to the Q-band (500–650 nm), not to the Soret band (420–430 nm) of which absorption intensity is much higher.