Abstract
Abstract The rational design toward developing energy-efficient and cost-effective processes to generate H2 remains as an outstanding challenge. To come up with this challenge, various transition metal complexes of macrocycles have been extensively reported as catalysts, with little attention given to porphyrin-based systems. In line with this, we reported electrochemical H2 evolution from organic acids and neutral aqueous solution using cheap and earth abundant transition metal complexes of porphyrin as catalysts. The thesis is divided in to 6 chapters. The 1st chapter is introduction part, chapters 2-5 author’s original works and the last chapter is conclusion of thesis. Encouraged by literature reports of H2 evolution based on cobalt(II) porphyrins in organic solvents or aqueous solution, we reported a water-soluble cobalt(II) tetrakis(p-sulfonatophenyl) porphyrin (CoTPPS) as a stable, active, and efficient electrocatalyst for H2 generation from neutral aqueous solution without any organic additives. The molecule features nearly quantitative Faradaic efficiency with a TOF of ~1.83 s-1 measured over 1 h and a TON of 1.9 x 104 moles of H2 per mole of catalyst with no loss in activity over 73 h at an applied potential of -1.29 V vs SHE in neutral phosphate buffer solution. In the 3rd chapter, H2 evolution activity of a series of cobalt(II) porphyrins with EW groups (COOMe, COOH, and SO3H), unsubstituted phenyl and ED groups (NH2, OH and OMe) at para position of meso-phenyl rings has been investigated in DMSO using acetic acid as a proton source. The study showed that nature of substituents significantly influences catalytic performance and overpotential. Faradaic efficiencies ranging from 44 to 99 %, TONs from 1.5 to 104 (~11 h electrolysis), TOFs from 0.23 to 9.1 h-1 and onset overpotentials ranging from 25 to 445 mV were obtained by tuning the substituents. Molecules with -SO3H, -COOH and -NH2 groups showed high activity and efficiency with more positive onset potentials as compared to a parent molecule, (CoTPP) and other molecules in this study. The key factor in enhancing activity at lower overpotential was supposed to be acidity of a functional group at para position of meso-phenyl ring during catalysis process. In chapter four, the effect of position and electronic nature of substituents were examined. Cobalt(II) porphyrins with pendant orth-amino, orth-nitro, and para-amino groups have been prepared to examine the role of substituent position and electronic nature in tuning overpotential and activity. Enhanced catalytic activity with H2 evolution rate constant of 1.12 x 105 M-1 s-1 with onset overpotential as low as 20 mV was obtained for cobalt(II) porphyrin with ortho-amino pendant group, confirming the substantial role of pendant amino group in intramoleular shuttling of proton to cobalt center thereby thermodynamically favouring proton-hydride interactions to evolve H2. In the 5th chapter, M-(p-NH2phenyl)4porphyrins (where M = Fe, Co, Ni, Cu and Zn) were synthesized and employed for H2 evolution study in organic solvents using weak and strong acids as well as in neutral aqueous solution to explore the roles of nature of central metal ion and proton source in H2 evolution activity and overpotential. In acetic acid (weak acid), the catalytic current appeared at a potential close to M1+/M0 redox couple, implying formation of M(II)-H as reactive intermediate through protonation of M(0) and the activity order of catalysts is Fe> Co> Ni> Cu>. However, in trifluoroacetic acid (strong acid) catalysis occur close to M2+/M1+ redox couple for Co and Cu while close to M1+/M0 redox event for Fe and Ni complexes with activity order of Co>Fe>Cu>Ni. In neutral aqueous solution, high activities were noticed for Co and Fe complexes than others. Based on our results, the redox potential of central metal ion and thermodynamic reduction potential of a proton source seem to play roles in tuning catalytic activity. In the last chapter, summary of author’s original work is presented. Thus, electrocatalytic H2 evolution studies using various metalloporphyrins has been reported in organic solvents and aqueous solution.