| Abstract |
The increasingly energy-demanding society requires the use of renewable energy sources, such as solar energy, as a viable solution to the problem of limited reserves of polluting fossil fuel, which will play an important role in sustainable development. The fact that H2 (the most environmentally clean fuel material) is produced in nature by photosynthesis splitting H2O, has triggered a large field of research activity attempting to synthesize appropriate models that can mimic these processes. Towards this direction the use of environmentally friendly water-soluble molecular models of natural chlorophyll (the main light-capturing compound of photosynthesis) is a challenge for both the synthetic chemistry of porphyrins and the study the photocatalytic activity.
This work deals with bio-inspired materials, based on synthetic analogues of chlorophyll, and their study on the conversion of solar energy. Thus we present a study of the production of H2 from H2O, irradiating in the visible light region a system composed of a water-soluble artificial photosensitizer (synthetic analogues of chlorophyll) and a catalyst of H2 production (a complex compound of cobalt). In this case the direct conversion of solar energy to chemical energy takes place. The process initially involves the absorption of light by the porphyrin, which is the chromophore compound / photosensitizer, causing excitation of its electrons, which are then involved in a series of redox reactions with the catalyst to result in the reduction of protons of H2O to H2. The catalysis is completed with a continuous supply of electrons from an amine which acts as a sacrificial electron donor and it is used in large excess in the reaction. Gas Chromatography is used to monitor the H2 production.
Specifically, the photocatalytic H2 production by combining a water soluble metal porphyrin [ZnTMPyP]4+(Cl-)4 (photosensitizer), a cobaloxime complex [Co(dmgH)2Cl(py)] (catalyst) and an amine [TEOA, triethanolamine] (sacrificial e-donor) was found to be quite effective with maximum efficiency to occur at neutral pH (TON of H2 production relative to porphyrin ~ 300). Production of H2 is observed even when selective stimulation of Q-bands of the porphyrin takes place, confirming that the porphyrin is the moiety that acts as the photosensitizer of the overall process. It was also observed that the production of H2 stops after prolonged irradiation of the reaction mixture, due to gradual decomposition of both the Co-catalyst and the porphyrin. Similar experiments with other water-soluble porphyrins showed that H2 is not produced even after many hours of photolysis, and even if the lamp used for the irradiation works at higher energy.
In the same direction, and in order to be used for photocatalytic H2 production, it was synthesized a star-shaped pentaporphyrin analogue of [ZnTMPyP]sup>4+/sup>Cl-)4 found in theoretical calculations to adopt a structure that resembles a propeller with the four porphyrins (blades) of the periphery to be twisted in the same direction relative to the central porphyrin. The synthetic approach involves a condensation reaction of a formyl- porphyrin, [5,10,15-tris(4-pyridyl)-20-(4-formylphenyl)] porphyrin with pyrrole in propionic acid. The water soluble and metallated with Zn derivative of this pentaporphyrin was tested for photocatalytic H2production under conditions analogous to the abovementioned, but unfortunately with no success even after increasing both the energy of the lamp and the irradiation time as well as the concentration of the catalyst.
Attempts were also made towards the synthesis of complex compounds wherein the photosensitizer (porphyrin) and the catalytic center of H2 production [Co(dmgH) 2Cl(py)] are joined in a supramolecular way. In this case, although one would expect the performance of the photocatalytic H2 production to be increased (as it would not depend on diffusion processes necessary to bring together all the components involved) surprisingly enough there was not produced any detectable amount of H2.
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Βιοεμπνεόμενα υλικά για τη φωτοκαταλυτική παραγωγή Η2
