Mechanism and reaction pathways of CO2 AND HCO3-photoreduction to hydrocarbons

Abstract

CO2/HCO3- photocatalytic reductions in water, using P25 Titanium dioxide, produce gas products at the order of the following rate: CO>H2>CH4, using bicarbonate as a substrate, reduction rates are ca. 5 times faster than using CO2 due to the adsorption constant of CO2 on the TiO2 surface is smaller than the bicarbonate. Langmuir-Hinshelwood kinetic model was used to describe the mechanism; therefore, adsorption constant (K) and the rate of maximum substrate coverage (k) at different charges of P25-surface were readily obtained. The K constant of TiO2-adduct formation favors no charge surface but k constant favors positive charge surface. Most of the products formed come from partitioning of the radical anion (TiO2-CO2.-) in the rate-limiting step. However, the bicarbonate absorption at OV promotes alternative pathways. This provides more CO and H2 production. Solvent isotope effect (H2O Vs. D2O) is ca. 1.5. This means that during the radical anion formation in the transition state, there is H being transferred from water in the radical anion intermediate step. In addition, the collapse of anatase and rutile phases from sonication increases CH4 production rate and selectivity due to improved charge separation that reduces alternative pathways.