Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1994-05-05
1996-01-09
Niebling, John
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
2041576, 2041579, C07C 100
Patent
active
054825997
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a photochemical method for converting carbon dioxide gas to organic substances such as methanol using visible light such as solar light.
DESCRIPTION OF THE RELATED ART
It has been advocated that the emission of carbon dioxide gas to the atmosphere be limited, and concurrently therewith, research on the reduction and conversion of CO.sub.2 molecules have begun to be developed rapidly. The current trend of such researches is towards catalytic reactions, organic photosynthetic reactions, electrochemical reactions, photoelectrochemical reactions, and the like for decomposing CO.sub.2 molecules or decomposing CO.sub.2 molecules while at the same time incorporating decomposed molecules into organic molecules. Known as one of such reactions is the Fischer-Tropsch reaction comprising decomposing carbon dioxide gas into carbon monoxide and oxygen, followed by subjecting the resulting carbon monoxide to a catalytic reaction with hydrogen to produce hydrocarbon having a higher molecular weight. However, this process requires a large quantity of hydrogen gas, so that the resulting hydrocarbon products will be too expensive to be put to practical use in an industrial application. Furthermore, it requires highly safe and hence expensive facilities in consideration of the possibility of explosion of hydrogen gas.
Besides the foregoing, studies have been conducted on catalytic reactions for synthesizing methanol or methane from CO.sub.2 and H.sub.2 O molecules, but none of them have been found advantageous in that they use costly H.sub.2 as a raw material.
Studies are still in the early stage on the photosynthetic reactions (relying on light and catalysts) which are considered the simplest, as represented by the following formula (1):
Fujishima et al deal with this type of photosynthetic reaction using various semiconductors as a catalyst in the "Nature" Vol. 277, February 1979, pp 637-638. Disclosed in this paper is the process of converting carbon dioxide to methanol using powders of TiO.sub.2, ZnO, CdS, GaP, Sic, WO.sub.3 or the like, for example as a semiconductor catalyst and irradiating the catalyst with ultraviolet radiation from a xenon lamp. However, the conversion efficiency has proved so poor as to be unsuitable for practical application. Moreover, the wavelength components taking part in the photosynthesis are ultraviolet light which is little contained in the solar radiation reaching the surface of the earth as it attenuates before reaching the surface. Consequently, the use of a source of ultraviolet light such as a xenon lamp is essentially required, making the process far from practical and economical.
In view of the foregoing, the Japanese Patent Application No. 2-144227 proposed a photosynthetic process comprising heating metallic oxide to activate it so as to produce free electrons and free holes locally on the surface thereof, and applying CO.sub.2 and H.sub.2 O gases to this metallic oxide while irradiating the metallic oxide with visible light energy to photosynthesize organic substances. More specifically, fine particles of ZnO (zinc oxide) are heated under vacuum to eliminate relatively many of O atoms bonded to Zn near the surfaces of the fine particles, and to create locally on the particle surfaces a number of regions where imperfect ZnO lattices with excessive Zn.sup.++ (positive ions) are generated and a number of regions where the elimination of O atoms scarcely occurs.
As seen in FIG. 7 illustrating the surface energy band structure of the ZnO particle, in the region (A) (the lefthand surface region of the particle 11) where numerous O atoms have been eliminated, positive ions (Zn.sup.++) become excessive at the surface layer where the energy band is bent downwardly, and free electrons will accumulate or build up at the downwardly bent portion of the band, resulting in generation of surface conductance. Also present on the surface are protons H.sup.+ which have been produced by decomposition of water. In FIG. 7, E.sub.v E.sub.f and E.sub.c
REFERENCES:
Aurian-Blajeni et al., Photoreduction of Carbon Dioxide and Water into Formaldehyde and Methanol on Semiconductor Material, Solar Energy, vol. 25 (1980), pp. 165-170.
Advantest Corporation
Niebling John
Wong Edna
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