A closer look inside TiO2 (P25) photocatalytic CO2/HCO3− reduction with water. Methane rate and selectivity enhancements

Abstract

Abstract The P25 CO2 and HCO3− photoreduction mechanism in water is described. Methane formation kinetics were measured at different pHs and [HCO3−]o conditions. Langmuir Hinshelwood K and k constants were readily obtained. Their values at different pHs are explained in terms of the TiO2 surface charge and the adduct and radical anion intermediates stabilities. DFT calculations show the monodentate HCO3-TiO2 adduct as the most stable interaction. HCO3-TiO2 with a nearby Ov (oxygen vacancy), H2O, OH and O were also simulated to mimic different TiO2 surface conditions. The results support the experimental findings. Solvent isotope effects also confirm the proposed mechanism. Non-constant with time product ratios reveal two paths for CH4, CO and H2 formation. [CO]/[CH4] t\u202f=\u202f0 extrapolation gives the kinetic ratio that corresponds to the cleavage of the adduct at the CB, and meanwhile, at t\u202f>\u202f0, a higher ratio value corresponds to the cleavage of the adduct at Ov. This cleavage fills the vacancy and produces additional CO and H2. To avoid the last CO contribution, P25 was sonicated to load the anatase phase on the rutile phase. This loading induces charge separation, where electrons and holes are redirected to the anatase conduction band and rutile valence band, respectively. As a result, the CH4 rate and its selectivity increase up to 4 times.