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

Semiconductor photocatalysts for overall water splitting into H-2 and O-2 require metal cocatalyst, such as Pt, to catalyze H-2 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 H-2 and O-2 due to the decrease of adsorption energies of H-2 and O-2 on Pt. The adsorption energy decrease of H-2 and O-2 followed the order of F/Pt < Cl/Pt < I/Pt < Br/Pt. H-2-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 H-2 and O-2 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 (H2O)-O-18 confirmed the over-all water splitting to H-2 and O-2. This study may help scientist to develop high-efficient photocatalyst for overall water splitting.