Author(s)

V. Palanichamy, M. Frites, S. U. M. Khan

Abstract

Photocatalytic conversions of CO₂ to methanol were carried out in aqueous medium on visible light active bismuth vanadium oxide (BiVO₄) and on copper bismuth vanadium oxide (Cu₂BiVO₆) photocatalysts under illumination of light intensity of 1 sun (0.1 W cm⁻²). The percent solar to methanol conversion efficiency (% STME) was found to be 2.78% on of BiVO4 compared to 2.50% on Cu₂BiVO₆ photocatalyst under same illumination condition for 80 min reaction time. Such small difference in the photoconversion efficiencies can be attributed to similar band gap energies, absorption coefficient of light and particle sizes for both samples. At more than 80 min reaction time, the amount of methanol formation reduced drastically due to rapid uncontrollable back reaction on the same photocatalyst surface. Much higher solar to methanol photoconversion efficiency (% STME) of 5.83% was observed when CO₂ was reduced to methanol on Cu cathode and water was oxidized to O₂ on visible light active carbon modified titanium oxide (CM-n-TiO₂) photoanode in a photoelectrochemical cell (PEC) under minimal external bias of 0.3 V for the reaction time of 10 min. Methanol formation was found to be dependent on the way the CO₂ solution was prepared under pressure in deionized water and followed by addition of NaOH solution so that the most of the dissolved CO₂ remained intact in its molecular form. A mechanism of the photoelectrochemical reduction of adsorbed CO₂ (aq) to methanol was given in terms of multi electron and proton transfer reactions under illumination followed by removal of a water molecule from the intermediate.

Keywords

carbon dioxide, methanol, photocatalysts, photoelectrode, PEC