SOLAR CONVERSION AND ENERGY STORAGE BY THE CHLOROPHYLL A DIHYDRATE PHOTOCATALYTIC DECOMPOSITION OF WATER AND REDUCTION OF CARBON-DIOXIDE

In this work the photoconversion and storage of visible light energy based on the half-cell reactions of water photolysis by crystalline chlorophyll a dihydrate is described. The role of water on the photochemical activity of chlorophyll a is examined by a study of a photogalvanic cell consisting of a Pt/Chl a photocathode and a Chl a-free anode. The production of H(\u27+) and OH(\u27-) ions in the water oxidation and reduction half reactions is established by acid-base titration. The observed photogalvanic current is decomposed into two contributions attributable to the markedly different rates of hydrogen evolution from elementary photochemical reactions initiating at the Chl a-H(,2)O and Pt-Chl a interfaces. The dependence of the photocurrent on sample equilibration with water is examined relative to the enhancement of the diffusion coefficient for electron-hole pair migration in (Chl a(.)2H(,2)O)(,n) on impregnation of the chlorophyll film by excess water. It is shown that (Chl a(.)2H(,2)O)(,n) catalyzes water photolysis on illumination by red light and that this capability is greatly enhanced by platinization of the chlorophyll. It is determined that Pt in the absence of Chl a is not photochemically reactive with water in the visible wavelength region. However, the thermochemical reaction of platinum with water in the absence of oxygen yields molecular hydrogen in the relatively low temperature range 130-210(DEGREES)C. No oxygen was detected from the Pt-H(,2)O thermal reaction, suggesting that the oxygen is consumed in the oxidation of Pt. The products of the Chl a-H(,2)O and Pt-H(,2)O reactions were determined by mass spectrometry, ion cyclotron resonance mass spectrometry, pyrolytic analysis, and gas chromatography. An investigation is made of the platinum surface by studying the catalytic action of Pt on the decomposition of hydrogen peroxide. It was found that (Chl a(.)2H(,2)O)(,n), unlike Pt, photocatalyzes hydrogen peroxide decomposition with visible light. The photosynthesis of organic molecules from carbon dioxide and water by chlorophyll a dihydrate is described