| Abstract |
It is well known that our environment is changing day by day. The human activity has caused enormous environmental problems, especially the last decades. Pollution of air, water, global warming are some examples of the problems that have been created and the impact on humans are catastrophic (healthy issues, etc.). Energy is the most important tool of modern communities in order to facilitate their daily life. Energy is incident to fossil fuels that cause most of the environmental problems. This fact along with their depletion due to their continuous use has forced the scientific community to find clean and abundant sources of energy. Although hydrogen has some drawbacks such as its storage, it could be a solution and especially its production using water and sunlight. The splitting of water produce hydrogen and oxygen. Nature is able to do this, using sunlight as energy source, a natural process called photosynthesis. Scientists try to mimic the basic mechanisms of photosynthesis, a field of research called artificial photosynthesis, and apply them to catalysis in order to produce hydrogen from water and sunlight. Photocatalysis is one of the most promising ways to produce clean hydrogen. The photocatalytic hydrogen production from aqueous protons constitutes the reductive side of water splitting. Each photocatalytic system, contains three essential components. A photosensitizer that absorbs light, a catalyst that receives the excited electron from the photosensitizer, a sacrificial electron donor that feeds the photosensitizer with electrons and many times an electron relay that facilitates the electron transfer.
In the present study, we report novel photocatalytic systems for hydrogen evolution. The first systems consist of tin porphyrins as photosensitizers and cobaloximes as catalysts. Porphyrins are excellent chromophores due to their high stability, high absorption in the visible region and their long life time in their excited states. Cobaloximes are molecular catalysts that are easy to prepare and their cost is low (noble metal free). In the second part we present systems consisting of triethanolamine (TEOA) as sacrificial electron donor, TiO2 nanoparticles, porphyrins [ZnTMPyP4+]Cl4 (P1), ZnPCNCOOH (P2) and ZnP(SP)CNCOOH (P3) as photosensitizers and cobaloximes CoN-Methyl-imidazole (C1), CoCNCOOH (C2) and Co(SP)CNCOOH (C3) as catalysts. In the third one, we tested two novel nickel and two novel cobalt complexes as catalysts for hydrogen evolution and make a comparison among them as far as their catalytic activity concerns. We tried to optimize the conditions, using different photosensitizers (fluorescein, [Ir(ppy) 2 (bpy)](PF6), [ZnTMPyP4+]Cl4, [Ru(bpy) 3]Cl2 and different sacrificial electron donors (TEOA, TEA and ascorbic acid). We obtained results for all the complexes using Ir-PS, as SED TEA and mixed aqueous-organic media.
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Synthesis and physicochemical characterization of new catalysts and photosensitizers
