|M.Sc Student||Gluz Naama|
|Subject||Synthesis and Characterization of Water-Soluble Manganese|
Clusters as Molecular Electrocatalysts for
Homogeneous Water Oxidation
|Department||Department of Chemistry||Supervisor||Professor Galia Maayan|
|Full Thesis text|
As part of the ongoing efforts of finding alternative energy sources, it is desirable to develop an artificial photosynthesis system that would split water into oxygen and hydrogen gas, the later can be used as a cheap and “green” source for alternative fuel. This process requires the coupling of two half-reactions: water oxidation and proton reduction, the former being a four-electron oxidation and thus highly challenging.
The biological water splitting process is catalyzed in nature by the oxygen-evolving center (OEC) of the PSII, the metallo-oxo cluster Mn4O5Ca, producing O2, protons and electrons. Clusters based on high oxidation manganese ions, which are abundant, cheap and environmentally friendly, and often have rich redox chemistry, represent an outstanding opportunity for mimicking the OEC as water oxidation catalysts.
This work describes the design and characterization of manganese-based molecular electrocatalyst for homogenous water oxidation, inspired by the OEC.
The catalyst is composed of poly-nuclear high-oxidation-state manganese cluster, from the Mn12acetate family (Fig. 1), having the shared molecular formula of [Mn12O12(O2CR)16(H2O)4] (R= Me, Ph etc.). The cluster is composed of inorganic core and an outer organic shell. The organic group shell can be varied by carboxylate substitution reaction.
Mn12acetate, Mn12O12(O2CCH3)16(H2O)4, was chosen to serve as potential catalyst mainly because it contains high oxidation state manganese ions (8MnIII, 4MnIV) and displays multiple one-electron redox processes. Mn12 acetate is soluble in water but rapidly undergoes hydrolysis to form manganese oxides or hydroxides. In order to overcome this problem, the carboxylate substitution reaction method was applied. By using a ligand, which is both bulky, hydrophilic and has suitable pKa for the substitution (<4.75), a water soluble and water stable Mn12 acetate derivative was developed. Based on this work, four water-soluble and water stable clusters were produced and characterized:
1. Mn12O12(O2CC6H3(OH)2)16(H2O)4 (Mn12DH). The catalyst was previously developed and its catalytic performance was validated in this thesis.
2. Mn12O12(O2CC6H3(NH2)2)16(H2O)4 (Mn12DA).
3. Mn12O12(O2CC4H8N)16 (H2O)4 (Mn12Pro).
4. Mn12Asb - a low-oxidation-state manganese complex containing L-ascorbic acid as a ligand.
The water-soluble and water stable clusters were then examined as water oxidation electrocatalysts. Mn12Pro and Mn12Asb showed poor catalytic performances. The CV of Mn12DH showed significant catalytic activity, indicated by O2 reduction. The catalytic activity of Mn12DH was quantified by bulk electrolysis and direct oxygen measurements and gave TON of ~26.65 in 5 hours (TOF ~ 1.476x10-3 s-1) at low overpotential of η= 334mV, with a faradic efficiency of ~62%. A possible explanation for the electrocatalytic activity of Mn12DH may be related to the nature of the organic group shell - the phenol type hydroxyl groups of the 3,5-dihydroxybenzoate ligands. The surrounding ligands might act as additional proton acceptors in this reaction, shifting the reaction equilibrium toward the direct reaction side; thus, facilitating the oxygen evolution. The correlation between the pKa of the ligands and the catalytic activity of the Mn cluster was further studied by using the Mn12DA cluster.
This work demonstrates, for the first time, electrochemical homogeneous water oxidation catalysis by a manganese-based molecular cluster.