PHOTOREDUCTIVE SEQUESTRATION OF CO2 TO FORM C1 PRODUCTS AND FUEL
We have investigated the photochemical reduction of CO{sub 2} in aqueous solution to form C{sub 1} products using a variety of oxide semiconductors and near UV light. Conventional oxide photosemiconductors including TiO{sub 2} and SrTiO{sub 3}, as well as modified oxide semiconductors including Pt/TiO{sub 2} and SrTiO{sub 3}/Cr-Sb, TiO{sub 2}/Fe{sub 3}O{sub 4} or TiO{sub 2}/Fe(CN){sub 6}{sup 3-} were used. Analysis for formate, acetate, formaldehyde, methanol and methane were conducted by a variety of analytical methods. Despite extensive published data on the facile reduction of CO{sub 2} by these semiconductors, we found only trace quantities of formate and acetate (typically less than 1 ppm) and no unequivocal evidence for formation of the other products. The quantum efficiencies of TiO{sub 2} and SrTiO{sub 3} for forming formate and acetate from CO{sub 2} are estimated to be 0.1-0.2% or less. The spectral properties of the unmodified oxides (TiO{sub 2}, SrTiO{sub 3}) restricts their use of solar irradiation to about 10% of the available near UV and visible spectrum, thereby giving an overall quantum and spectral efficiency of less than 0.01%. Semiconductor oxides modified with, Cr and Sb, Fe{sub 3}O{sub 4} or Fe(CN){sub 6}{sup 3-} exhibited no enhanced efficiency. However, Pt/TiO{sub 2} does produce more formate and acetate than TiO{sub 2} alone by about a factor of two. Addition of 0.6 mM 2-propanol to an irradiated CO{sub 2}/TiO{sub 2} suspension led to formation of larger amounts (0.55 mM, 25 ppm) of formate, but no formaldehyde. The higher yield is probably because re-oxidation of formate by semiconductor holes was competitively blocked by propanol acting as a sacrificial electron donor. Reoxidation of formate by TiO{sub 2} under argon is 80 times faster than reduction. Thus, reoxidation could be an important process limiting the accumulation of products and will require new design strategies for reducing accumulation of products in the photocatalytic zone if the an efficient process is to be developed. New photosemiconductors are needed which have broad spectral absorbance extending to 600 nm. Preliminary estimates were made of the physical size of a solar CO{sub 2} photoreduction unit large enough to reduce the CO{sub 2} produced from a 1000 MW coal-fired electricity plant. Although a perfectly efficient (100%) system could be as small as 10 km{sup 2}, to begin to approach that size plant footprint will require a dramatic improvement in semiconductor photocatalysis efficiency over that which is currently available.
- Research Organization:
- SRI International (US)
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- FC26-02NT41224
- OSTI ID:
- 840114
- Resource Relation:
- Other Information: PBD: 24 Feb 2004
- Country of Publication:
- United States
- Language:
- English
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