Reduction of CO_2 to Methanol in Photoelectrochemical Cell: CM-n-TiO_2 /Cu

摘要

Reduction of CO_2 to methanol was carried out in a two compartment photoelectrochemical cell using carbon modified titanium oxide (CM-n-TiO_2) as a photoanode and Cu metal gauze as a dark cathode in an aqueous electrolyte of 5.0 M NaOH. Both compartments were connected by glass wool soaked in the same electrolyte. The CO_2 reduction at the cathode in aqueous medium was coupled with the energetically challenging H_2O oxidation reaction to O_2 at CM-n-TiO_2 photoanode. The reduction of CO_2 to methanol was found to depend on the way CO_2 solution was made. This is because most of the dissolved CO_2 in aqueous medium should be intact in the molecular form trapped by water molecules via hydrogen bonding and adsorbed on energetically compatible Cu metal surface. No measurable amounts of reduction of CO_2 to methanol were observed when it was mostly converted to CO_3~(2-) and HCO_3~- ions after dissolution. The highest amount of methanol, 2.5 × 10~(-8) moles cm~(-2) s~(-1) which is equivalent to 28.8 grams m~(-2) hr~(-1) was observed in first 5 min of reaction time. The amounts of methanol decreased after 5 min indicating back reaction at the photoanode. An average photoconversion efficiency of solar energy to methanol fuel energy was found to be ～11 % within 20 min. The reduction of CO_2 to methanol was found to increase the yield by two fold in presence of initially added 0.5 mM methanol that acted as an autocatalyst.