| Abstract |
In recent years, the reckless consumption of fossil fuels and the energy demands of people in general, has led in the formation of a new worldwide energy map, which inevitably leads to huge carbon dioxide (CO2) emissions. As one of the main greenhouse gases, it is a key driver of global climate change and other serious environmental impacts. Therefore, a step towards a sustainable development of human society is the strengthening of renewable sources, such as sunlight, wind, sea, etc. In fact, the solar system needs special attention due to the extensive and huge amount of energy that is delivered daily to the Earth. The conversion of solar energy into chemicals through the decomposition of H2O has been taking place in nature for millions of years through photosynthesis, which has inspired the development of numerous artificial photosynthetic systems. In particular, the production of fuels through photocatalytic systems has been the subject of intensive research due to its ability to mimic photosynthesis and convert abundant substances (water, CO2) into fuels (H2, CH4, etc.). Photocatalytic reduction of CO2 in hydrocarbons is considered as one of the most promising methods of conversion and use of CO2. Usually, photocatalytic CO2 reduction systems consist of a photosensitizer that absorbs sunlight, a catalyst that reduces CO2, and a sacrificial electron donor that provides electrons to the system and regenerates it. Metalloporphyrins, and more specifically iron porphyrins, have been widely used in both electro- and photocatalytic CO2 reduction. FeIII porphyrins are among the most efficient molecular catalysts in photocatalytic CO2 reduction systems, both in the absence and in the presence of a photosensitizer. By taking advantage of their π-coupled electronic structure, they can achieve fast charge transfer and while in the excited state, CO2 can be stabilized and coordinated in the metal center. Recent developments have shown that changing the second coordination sphere of porphyrin, using different types of meso-substituents, leads to improved catalytic performance through the formation of hydrogen bonds and proton transfer effects. It has also been shown that the charge of peripheral porphyrin substituents affects the catalytic activity of the system. In fact, the construction of a secondary coordination sphere stabilizes more effectively the intermediate CO2, which occurs at the rate determining step, during the reduction process. Based on these, in the present postgraduate thesis, the synthesis of suitably modified FeIII porphyrins and the study of their catalytic activity in terms of photocatalytic reduction of CO2, are presented. Various modifications were made to the structure of porphyrin such as - +N(Me) 3, -NMe2 groups located in the para substituted- phenyls, as well as fluorine (–F) groups at various positions of the phenyls. Moreover, porphyrins capable of forming hydrogen bonds were synthesized, having in the ortho positions of their phenyls, amines such as 4-Hexylaniline and histidine, bound to porphyrin via urea bonds, as well as porphyrins with carboxyl groups and corresponding methyl esters in the ortho position. The main objective of this master thesis is the study of the photocatalytic activity of these, iron porphyrins, regarding the reduction of CO2 and consequently the production of CO. The study focused on porphyrin 12, which affords fluoride and +NMe3 groups, placed in orthometa and para positions of the porphyrin phenyls, respectively. Various system parameters were tested and compared, such as the light source, the solvent, and the concentrations of the catalyst, photosensitizer, and sacrificial electron donor. Finding the optimal CO2 reduction conditions, a comparison was made between the other iron-porphyrins. As expected, porphyrin 12 was more efficient in producing CO than the other porphyrins that carried various combinations of fluorine, +NMe3 and NMe2 groups. In fact, experimental results have shown that this system can effectively reduce CO2 quickly, showing excellent turnover numbers (up to 5500 TONs). The main product of this system was CO and only a small amount of hydrogen was detected, showing the selectivity of 85% for CO2.
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