Electrochemical study on the catalytic activity of Fe-N-C materials in the activation and fixation of CO2 in acetonitrile

The electrochemical reduction of carbon dioxide is a promising way to transform this gas into a useful resource. The Fe-N-doped carbons are emerging noble metal free electrocatalysts where the active site is represented by an iron metal centre coordinating 4 nitrogen atoms, in a structure similar to that observed in porphyrin or phthalocyanine molecular systems. However, while the study and use of Fe-N-C catalysts is widely documented in aqueous electrolytes even for the CO2 reduction reaction, their behaviour in organic solvent is poorly investigated. This is somehow astonishing since metal porphyrin/phtalocyanine molecular systems have been used and characterized since the sixties of the last century to catalyse, in homogeneous phase, the reactivity of small molecules including CO2 itself. This thesis work tries to fit into this context trying to fill this gap. In this thesis, Fe-N-C catalysts were tested in acetonitrile, a solvent with ca. 10 times higher CO2 solubility than water. Three types of Fe-N-doped catalysts, differing for the % of FeN4 sites, were tested for the direct reduction of CO2 and the fixation of CO2 using organic halides. It was found that the direct reduction of CO2 is not catalysed on F-N-doped carbons, but the cleavage of the C-Cl bond was catalysed and anticipated of 300 mV with respect to the glassy carbon electrode. However, even in this case the catalytic action seems to be due not to the presence of FeN4 sites but to the presence of nitrogen functional groups such as pyridine, pyrrole or N-graphite. This has allowed to hypothesize the use of Fe-N-C systems for dehalogenation reaction as well as for the in-situ formation of nucleophilic species available to react for example with CO2 to give carboxylation reaction. In particular, the dehalogenation reaction of benzyl chloride has been studied. The reaction occurs through a concerted mechanism, where the first electron transfer leads directly to the breaking of the carbon-halogen bond with the formation of a radical which is rapidly reduced to a carbanion. In the presence of CO2 the carbanion leads to the formation of phenylacetic acid, which however, given the applied potential, undergoes further reduction reaction to the corresponding carbonate.