Nanoscale metal - organic ligand interactions for applications in environmental remediation and catalysis
Jha, Prashant Kumar
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Ever since the proposal of correct structure of CoCl3*6NH3 by Alfred Werner in 1893 the field of coordination chemistry has only grown and has become the center of chemical science owing to wide ranging applications in the fields of catalysis, medicine, biochemistry and organometallic chemistry. This dissertation deals with novel application of coordination chemistry from three different perspectives. The first explores the fate of a known contaminant (uranyl ion) in ground water with natural organic matter (NOM) using simple analogous aromatic and aliphatic molecules (catechol and oxalic acid). It was found that not only the immediate complex formed, long term interactions between the metal ion and the organic molecule need to be taken into account for any remediation process, as uranyl can act as a catalyst in polycondensation of polyphenols. The second part deals with modifying a natural polysaccharide to improve its metal complexation capacity using photochemical enhancement in an electrochemical pathway. Metal adsorption capacity of the novel polymer was tested with oxyanions (Cr2O72-) as well as oxycations (UO22+). It was found that a model based on monolayer adsorption occurring on an energetically uniform surface without interactive molecules best explained adsorption and kinetics of the reaction followed a pseudo second order model. In the final part strong complex formation between chitosan and metal ions was exploited to grow stable nanoparticles for catalysis applications. Protonated chitosan was deposited on stainless steel using an electrochemically-induced localized zone of high pH near the electrode. The hydrogel layer was found to be well adhered and conductive. The nature of conductivity is not known but was found to be sufficient for subsequent electro-reduction of silver salt to metallic nanoparticles. Nanoparticle size as well as distribution over the chitosan layer was optimized by controlling operating parameters including voltage, time of deposition and thickness of polymer film. The reduced nanoparticles were tested for their catalytic activity using the oxygen reduction reaction (ORR) and it was found that under oxidative potentials the silver nanoparticles were not just stable up to oxygen reduction potential under alkaline conditions (0.23V Vs Ag/AgCl) but they exceeded the limit by 200mV thus providing a very broad electrochemical window for the 4 electron pathway for oxygen reduction. The stability of silver is expected to have increased due to complex formation with amine as well as hydroxyl group in chitosan.