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Water oxidation catalysis by metal oxides derived from coordination complexes: from complexes to natural minerals
thesisposted on 15.05.2017, 07:15 by Singh, Archana
The only naturally occurring system that catalyses water oxidation is the Mn4CaO4 cluster of Photosystem II (PSII), and it is preserved in all oxygenic photosynthetic organisms. The unique role of the Mn-cluster in this system has stimulated particular interest in synthetic manganese catalysts. Previous work in the group demonstrated that efficient catalyst could be made by impregnating a tetra-nuclear manganese cluster [Mn4O4L6]+ where L = diarylphosphinate, into a Nafion. Peak turnover frequencies (TOF) of 100-270 molecules of O2/h/catalyst were observed at overpotentials of ~ 350 mV. In subsequent work, XAS spectroscopy and high resolution transmission electron microscopy (TEM) was used to show that manganese cluster dissociated in Nafion forming a nanoparticulate manganese oxide on application of a potential bias. The oxide is responsible for the sustained water oxidation catalysis. In this thesis, the ability of molecular complexes to be converted into active metal oxide water oxidation catalysts was further investigated in an examination of families of Mn and Ni complexes. Despite the same metal oxide material being generated from each family of complex the catalytic activity was found to vary substantially from precursor to precursor. In the first series of experiments, manganese complexes which differ in metal oxidation states, coordination environment and nuclearity were applied in water oxidation using the methodology developed for [Mn4O4L6]+ complex. Although XAS studies confirmed that the different complexes were converted to the same material in Nafion, a birnessite like phase, the catalytic activity strongly depended on the initial precursor. Of all complexes tested, the metal oxides derived from [Mn(Me3TACN)(OMe)3]+ (2) and [(Me3TACN)2MnIII2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N',N"-trimethyl-1,4,7- triazacyclononane) (3) showed the highest activity. At overpotential of 350 mV, 2 and 3 showed same TOF of ~ 45 (O2 evolved per hour per Mn center) which was over 10-fold higher than for the MnOx generated from [Mn(OH2)6]2+. TEM studies revealed that whereas [Mn(OH2)6]2+ resulted in both a higher manganese oxide loading and aggregated nanoparticles (50 nm diameter) the molecular complexes (2 and 3) generated highly dispersed MnOx nanoparticles (10-20 nm) with a high surface area which were more effective water oxidation catalysts. Electrochemical methods and EPR spectroscopy were used to follow the catalytic cycle and to further evaluate the origin of difference in the catalytic activity. EPR spectroscopy revealed that on incorporation in Nafion, the Mn complexes breakdown to produce Mn(II) species. The Mn(II) species generated in Nafion are then oxidized on application of bias to form an EPR silent Mn species; which was again confirmed to be a manganese oxide and structure similar to birnessite by XAS spectroscopy. The oxide was found to be photoreduced to Mn(II) species on illumination with visible light. Thus, catalysis involves cycling between Mn(II) and MnOx. The applicability of metal complexes as precursors for the electrodeposition of metal oxides was further examined through the study of nickel(II) complexes. Initially, NiOx films were deposited on FTO coated glass from [Ni(en)3]2+ , [Ni(NH3)6]2+ and [Ni(OH2)6]2+ dissolved in aqueous borate buffer at pH 9.2. Although electrochemical and XAS studies confirmed the formation of γ-NiOOH films from each precursor, the [Ni(en)3]2+ derived oxide films achieved catalytic activities that were about 1.5-fold higher than films derived from the other precursors. SEM images showed that [Ni(en)3]2+ complex leads to the deposition of very homogeneous, uniform NiOx films with dendritic structure whereas other [Ni(NH3)6]2+ and [Ni(OH2)6]2+ results in films that were non-uniform with numerous pin holes. Capacitance measurement confirmed that the surface area of the NiOx films deposited from [Ni(en)3]2+ is approximately 1.5 times higher than the films deposited from other precursors and related closely to the observed higher catalytic activity. Electrochemical deposition of NiOx films was also carried out using macrocyclic Ni(II) amine complexes dissolved in borate buffer (pH = 9.2) and alkaline medium (0.1 M NaOH). Although these complexes lead to the deposition of active NiOx films, the catalytic activity was close to that of observed for films derived from [Ni(OH2)6]2+. These electrochemically deposited films were shown to exhibit electrochromic properties, as confirmed by in-situ UV-Vis spectroscopy. The catalytic activity of the films was also tested with visible light illumination and a significant increase in the current density was observed. Nickel Oxide (NiO) prepared by a thermolysis method were used to deposit nickel oxide films using a facile and cost effective screen-printing method. The water oxidation catalytic activity of the microball films was compared to that of the films prepared from commercial NiO-nanoparticles and the microball films shown to have approximately 1.5 times higher activity. BET measurements have shown that the NiO microball film have a more porous structure and higher surface area than the films from nanoparticles. This difference could be responsible for the higher catalytic activity of the former. In summary, the present work contributes to the development of cheap and abundant metal (Mn and Ni) oxides water oxidation catalyst using cost effective, electrochemical and screen-printing methods. Different analytical techniques were used to characterize the films and water oxidation catalysis was confirmed by electrochemical methods and oxygen detection techniques. For electrochemically derived films, the films produced using metal complexes as pre-catalyst show improved catalytic efficiency than catalyst derived from corresponding metal salt. In addition, NiO microballs with well defined morphology were found to be catalytically more active than films prepared from commercially available nickel oxide nanoparticles.
Awards: Vice-Chancellor’s Commendation for Doctoral Thesis Excellence in 2013.