Photoreductive Sequestration of CO(sub 2) to Form C(sub 1) Products and Fuel. Quarterly Progress Report No. 4 for the Months of January to March and April to June 2003.
Analytical methods for determining formic, acetic and oxalic acids, formaldehyde, and methanol have been evaluated and/or optimized for measuring products from photoreduction of CO(sub 2) in illuminated, aqueous suspensions of photocatalysts. An electrophoresis anion separation method (CIA) can detect aqueous formate and oxalate ions at 22 and 17 (micro)M (1 ppm), respectively. Recalibration of the Nash formaldehyde determination shows that as little as 10 (micro)M (0.3 ppm) can be detected spectrally. Several experiments using suspensions of Pt/TiO(sub 2), SrTiO(sub 3), and SrTiO(sub 3) with Cr and Sb were illuminated in CO(sub 2) saturated solutions. No acids were detected in most experiments using CIA; however, ion chromatography (IC) was able to detect formate and acetate at low (micro)M (sub ppm) concentrations in several experiments using Pt/TiO(sub 2) and SrTiO(sub 3) in sunlight and with xenon uv light. Analysis for methanol by gas chromatography showed that not more than 2 ppm methanol could have formed and probably less. Adding 0.6 mM 2-propanol to an irradiated CO(sub 2)/TiO(sub 2) suspension led to formation of 550 (micro)M formate, but no formaldehyde, probably because re-oxidation of formate by semiconductor holes was competitively blocked. Loss of C(sub 1) products at higher concentrations by re-oxidation may be an important process, limiting the accumulation of products. 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. A perfectly efficient system could be as small as 2 to 3 km(sup 2).
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