ABSTRACT
The reduction of carbon dioxide to useful compounds such as fuels and chemical intermediates is inherently difficult, as it involves several steps of both electron and proton transfers. The primary steps in the reactions of carbon dioxide or the bicarbonate or carbonate ions in solution were identified mainly by pulse radi olysis, but also by flash photolysis, electron spin resonance, and electrochemical experiments. The radiolysis of water results in short-lived radicals, ions, and atoms, which may react with certain solutes to produce secondary radicals. If these secondary radicals have intense UV-or visible absorption bands, the rates of their formation and decay can be determined by kinetic spectrophotometry. Their interaction with other solutes may result in increases in the decay rates, from which the second-order rate constants of such interaction can be derived.^
which strongly absorbs in the ultraviolet region, with = 235 nm and € = 3000 M" ^ cm" The rate of reaction of the hydrated electron with carbon dioxide is close to the diffusion-controlled limit, with k2 = 7.7 x 10^ M~ radical anion is protonated only in strongly acidic solutions,
•COOH = + -CO2 pK ^ = 1.4
This
(2)
With a redox potential of —2.0V vs. NHE, the radical anion is strongly reducing.The decay of the radical followed second-order kinetics. At pH 7, the second-order rate constant for the decay
•CO; + -COö (3)
was 5 X 10® M ^ s" The carbonate radical, -C03", was obtained in flash photolysis and pulse radiolysis, by oxidation of carbonate and bicarbonate ions by hydroxyl radicals. These reactions were found to occur with second-order rate constants of 4.2 x 10® and 1.5 x 10”^ M~^ s“ ^ respectively,^
•OH + COi" OH-+ -CO;
•OH + HCO; H^O + -CO; (4) (5)
The carbonate radical anion was also formed by photoionization of the carbonate ion,'
Its broad absorption band in the visible region, with = 600 nm and e = 1.9 X 10^ c m i s convenient for the determination of rate constants of its interactions with other species. The rate constant for the recombination of the •CO3-radical has a negligible apparent activation energy, which seems to indicate a composite reaction sequence. The pK ^ for the conjugate acid.
