Photosynthesis of Methanol from CO2 and H2O on ZnO Surface.

Abstract

Photosynthesis of methanol from water and carbon dioxide.CO2+2H2Ohv→cat.CH3OH+3/2O2-63kcal/molis examined on ZnO fine powder and on ZnO(1010) single crystals. The ZnO samples are illumi-nated by a Xe lamp with a filter to cut UV lights with higher energies than 3.2eV (the forbidden band gap of ZnO crystal) as well as by the sun light. The reactive CO2 is suggested to be specific molecules which are weakly adsorbed by interacting with OH groups on ZnO. The activation of ZnO powder at 400°C under vacuum accumulated Si, S and P impurity atoms near and on the surface. The surface impurity levels may play an important role to generate free electrons and holes under the visible light illumination. Surface species were analyzed by FTIR spectroscopy and by a mass spectrometer. Surface conductance and photoexcitation of electrons and holes were observed with using the ZnO single crystal under vacuum or ambient CO2 and H2O. The activated ZnO powder is found to be capable of synthesizing CH3OH by visible light illumination especially under 32 atm. pressure of CO2 gas at 5°C, where the CO2 clathrate hydrate phase is formed around the ZnO surface. The best conversion rate to CH3OH is 2.1% with respect to the amount of reactant H2O.