Efficient Photocatalytic H2O2 Formation by a Porphyrin-TiO2 Hybrid Catalyst in Pure Water

Abstract Efficient production of hydrogen peroxide (H2O2) has attracted much interest because of the growing importance of H2O2 as an energy source. In this study, we have developed photocatalytic H2O2 production in ultrapure water catalyzed by an organic–inorganic hybrid photocatalyst. The hybrid photocatalyst was prepared by immobilizing a diprotonated dodecaphenylporphyrin derivative (H4DPP2+-C) having carboxyl groups at the periphery to be anchored on a TiO2 surface. Photoirradiation to the hybrid photocatalyst (H4DPP2+-C/TiO2) dispersed in ultrapure water resulted in efficient H2O2 production by O2 reduction, as confirmed by iodometry for the supernatant. The concentration of aqueous H2O2 was determined to be 0.30 mM after 1 min of photoirradiation and the initial rate of H2O2 formation by H4DPP2+-C/TiO2 was determined to be 29 mmol g–1 h–1, which was higher than previously reported values in ultrapure water without any sacrificial reagents. In addition, the external quantum yield at 365 nm reached 27%, which is the highest value among semiconductor photocatalysts for H2O2 production. Based on the results from femtosecond transient reflectance spectroscopy, photoinduced electron transfer from the conduction band of photoexcited TiO2 to H4DPP2+-C on TiO2 was confirmed to afford the one-electron reduced species of H4DPP2+-C, which is further reduced to form a putative isophlorin intermediate as a two-electron reduced species to reduce O2 for H2O2 formation. Thus, H4DPP2+-C/TiO2 performs water oxidation at the TiO2 surface to gain electrons and two-electron reduction of O2 at the H4DPP2+-C moiety, respectively.