The inhibition of hydrogen and oxygen recombination reaction by halogen atoms on over-all water splitting over Pt-TiO2 photocatalyst

Abstract

Semiconductor photocatalysts for overall water splitting into H2 and O2 require metal cocatalyst, such as Pt, to catalyze H2 evolution efficiently. However, these metal cocatalysts can also catalyze hydrogen and oxygen recombination to form water. In this paper, we found that the pre-adsorbed halogen atom catalyst could inhibit the reverse reaction of water formation from H2 and O2 due to the decrease of adsorption energies of H2 and O2 on Pt. The adsorption energy decrease of H2 and O2 followed the order of F/Pt < Cl/Pt < I/Pt < Br/Pt. H2-TPD results exhibited similar dependence. This inhibition was achieved via the occupation of halogen atom on the Pt surface sites, and thereby the adsorption and activation of hydrogen and oxygen molecules were decreased. The occupation difference of halogen atoms are determined by radius of halogen ions, which further leads the different activity for H2 and O2 recombination. By inhibition of water formation reverse reaction, the over-all water splitting over various TiO2 photocatalysts has been achieved. Isotope experiments with D2O and H218O confirmed the over-all water splitting to H2 and O2. This study may help scientist to develop high-efficient photocatalyst for overall water splitting.