School of Chemistry (ETDs)
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Item Studies on the chemistry and biochemistry of gold(III) carboxamide pincer chelates(University of the Witwatersrand, Johannesburg, 2024-06) Razuwika, Rufaro; Nowakowska, Monika; Mathura, adhnaCancer, a group of diseases characterised by the uncontrollable growth of abnormal or mutated cells within an organ, is a global concern. Metallodrugs have emerged as promising solutions to this pandemic, leading to intense research on different metal complexes. In this study, gold(III) carboxamide pincer complexes were evaluated as potential chemotherapeutic agents. The novel NNN-type carboxamide pincer molecules (ligands) effectively stabilising the gold(III) metal centre. The strong σ-donor properties of both the anionic and pyridine N groups further enhanced this stability. Ligands 1a-1f exhibited atropisomerism, a common feature in drug discovery, and containing special heterocycles such as quinolones, indazole, benzophenone, and phenanthroline, which are particularly relevant in drug development. Atropisomerism, however, was lost upon metalation of the ligands. Three complexes, 2d, 2e, and 2f, were successfully synthesised and isolated. Complex 2d was subjected to biochemical property testing and in vitro analysis due to its superior stability and solubility compared to 2e (poor stability) and 2f (poor solubility in the buffer solution used in the study). Speciation studies, combined with computational studies, suggested that 2d exists as a neutral complex under physiological conditions. This inert complex demonstrated stability against the reducing agent glutathione, indicating resilience to reduction under physiological conditions. DNA spectroscopic titration studies revealed that 2d exhibited intensive interaction with ct-DNA, with binding constants Ka1 = 1.48 x109 M-1 and Ka2 = 6.59 x105 M-1. This interaction resulted in a notable increase in the DNA melting point by 4 °C and an enhancement in viscosity in a dose-responsive manner. The DNA titrations, melting point, and viscosity studies suggested a dual binding mode of 2d to ct-DNA, involving base binding with a nearly equal preference for A, T, G, and C bases, and groove binding. Complex 2d exhibited a high affinity towards the transport protein HSA (Ka values were 1.57 x104 M-1), suggesting that it can be transported in the body by means of the HSA-mediated pathway, enhancing its efficacy and stability. In comparison to its affinity towards DNA, there is a significant difference allowing for the successful transfer of 2d from HSA to DNA. The poor solubility of complex 2d in aqueous environments may have hindered its cellular uptake, but binding to HSA could mitigate this, ensuring minimal interference with its cytotoxicity towards different cancer cell lines. MTT studies demonstrated that 2d has comparable cytotoxicity towards the breast cancer cell line MCF-7 with an IC50 of 9 µM. The IC50 for HT-29 was, however, too high to measure accurately (>100 µM). In conclusion, complex 2d exhibits promising anticancer properties based on its DNA binding studies and cytotoxicity evaluations. This suggests that this class of compounds can be applied in cancer treatments, with potential modifications to compounds 2e and 2f to improve their solubility and stability.Item High entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-08) Mongwe, Agnes Monosi; Ozoemena, Kenneth Ikechukwu; Haruna, Aderemi B.The development of effective and stable rechargeable zinc-air batteries (RZABs) using noble-metal free bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has been a key challenge to its practical applications. This MSc research work strategically investigated some synthetic methods aimed at tuning the physicochemistry and electrochemical properties of two electrocatalysts (i.e., noble-metal free high entropy spinel oxide (HESOx) ((CoCuFeMnNi)3O4) and spinel Fe2CoO4 for rechargeable zinc-air batteries). For HESOx, a simple and reproducible Pechini method was used to synthesize a homogeneous nanosized electrocatalyst HESOx-550. The HESOx-550 was thereafter supported on onion-like carbon (OLC) in (1) an acidic environment to produce HESOx-550/OLCAT (where AT stands for acid-treated) and (2) a nonacidic environment to produce HESOx-550/OLC. The effects of the different synthesis environments on these three samples were thoroughly investigated using different analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA) and Nitrogen Gas Adsorption analysis. The Nitrogen Gas Adsorption analysis results show that the HESOx-550/OLCAT has the largest surface area and more volume. The electron paramagnetic resonance (EPR) and O1s XPS data consistently proved that HESOx-550/OLCAT has improved oxygen vacancies which are essential in improving conductivity and offering abundant reaction sites. The HESOx-550/OLCAT shows the best bifunctional ORR and OER electrocatalytic performance with a bifunctionality index (ΔE) of 0.70 V in 1 M KOH. In addition, the RZAB air electrode with HESOx 550/OLCAT exhibits high areal capacity (60 mAh cm-2) and areal energy density (73.2 mWh cm-2) with a long-term cycle stability over 112 h in 6.0 M KOH and 0.2 M zinc acetate. The HESOx-550/OLCAT RZAB shows better electrochemical performance than 10wt.% Pt/C- IrO2 when cycled over 315 h under 27% depth of discharge condition. For Fe2CoO4, iron cobalt-based electrocatalysts on Vulcan carbon support were synthesized using a simple reduction method to produce two composites (FeCo-Fe2CoO4/CAnnealed), and (FeCo Fe2CoO4/CMicrowave). The physicochemical analytical methods such as XRD, XPS, Raman, TGA and Nitrogen Gas Adsorption analysis were used to investigate the samples. The electrochemical analysis showed that the FeCo-Fe2CoO4/CAnn had a very low “bifunctionality index” (ΔE) of 0.76 V and the FeCo-Fe2CoO4/CAnn air cathode RZAB demonstrated good stability for over 50 h under harsh DOD conditions (35.2%). The assembled RZABs have areal energy densities of 48.4 mWhcm-2 and 60.5 mWhcm-2 which are higher than the minimum recommended areal energy density of 35 mWhcm-2 (and better than most electrocatalysts reported in the literature). This study has significant contributions to the progress of practical applications of RZABs.Item A systematic study on the use of the sol-gel synthetic method for lithium manganese oxide-based cathode materials(University of the Witwatersrand, Johannesburg, 2024-09) Muntswu, Zwivhuya; Billing, Caren; Ferg, Ernst E.; Billing, David G.This dissertation investigated the synthesis of two lithium manganese oxide-based cathode materials (Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4) using the sol-gel method and probing the phase transitions during the synthesis. The sol-gel synthetic method involved dissolving stoichiometric amounts of lithium nitrate, manganese nitrate hydrate, and citric acid in distilled water forming an aqueous solution. The starting precursor materials were dried at 140 °C which formed a crystalline phase of -Aqua-S-citrato (2-)-manganese(II) with an orthorhombic crystal system and P222 space group. The thermal behaviour of the precursor was explored to understand the effects of calcination/annealing temperatures. Thermal analysis of precursors prepared using nitrate salts with a 1:1 total metal ion to citric acid ratio displayed thermal stability to temperatures higher than 380 °C with the formation of a final metal oxide after 70% mass loss due to the decomposition of the organic and nitrate materials. However, when increasing the concentration of the complexing agent, an increase in material decomposition due to an increase in organic material is seen. The precursor materials prepared with a lower complexing agent concentration result in materials that have thermal instability when exposed to high temperatures. Thermal analysis of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 prepared using acetate salts as starting materials shows material decomposition at high temperature of ~600 °C Calcining both undoped and Al-doped nitrate precursors at moderate temperatures (380 °C to 500 °C) resulted in the formation of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 with a pure cubic spinel structure and an Fd-3m space group, however, increasing the calcining temperature to 800 °C for the undoped nitrate-based precursor revealed an impurity phase formation relating to dilithium manganese oxide with a monoclinic crystal system. On the other hand, calcining acetate-based precursors at moderate temperatures (380 °C to 500°C) results in metal oxides with low crystallinity compared to metal oxides prepared with nitrate-based precursors. Calcining acetate-based precursors at 800 °C was more favourable since they form the desired metal oxides without any impurities which might imply structural phase stability at high temperatures. The local and average crystallographic structures (via PDF and XRD respectively) of various nitrate-based metal oxides were investigated, where a good agreement between collected data and a calculated structural model revealed the formation of a cubic spinel structure of space group Fd-3m. Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 metal oxides were achieved from calcining precursors at moderate temperatures of 380 °C and 450 °C. The PDF high r-value signal displays a good fit which confirms to the average structure data information where the r-value signal which correspond to the local structure refinements have a minor discrepancy when fitted with a cubic spinel of space group Fd-3m.Item Synthesis and electrochemical properties of high-entropy spinel oxides, cobalt atomic clusters and zinc oxide as electrode materials for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-07) Gaolatlhe, Lesego; Ozoemena, Kenneth IkechukwuThis thesis investigated cathode and anode electrode materials for application in rechargeable zinc-air battery (RZAB). Two types of cathode materials were strategically studied in RZAB applications: (a) cobalt carbon composites of (i) cobalt atomic clusters (Co AC@CBPDC) and (ii) cobalt nanoparticles (Co NP@CBPDC), and (b) high-entropy spinel oxide (HESOx, containing five transition metals – Cu, Mn, Fe, Ni, and Co). The activities of these materials toward oxygen reduction reaction (ORR and oxygen evolution reaction (OER) were investigated in both half- and full-cell configurations as a proof-of-concept in RZAB cells in alkaline electrolyte. Considering that conventional zinc plate has several short-comings as an anode for RZAB, a new material, polydopamine-derived carbon-coated zinc oxide (ZnO@PDA-DC), was also synthesised and applied in RZAB as a possible alternative anode to the popular zinc plate. First, Co AC@CBPDC and Co NP@CBPDC were prepared using the metal-organic framework (MOF) route through the microwave-assisted solvothermal method and acid treatment. From the XRD results, the spectra showed dominant {111} and {200} phases, characteristic of metallic cobalt with a face-centred cubic (fcc). There were trace amounts of CoO observed indicating the coexistence of Co/CoO. From TEM imaging, Co AC@CBPDC was highly defective with a visible porous carbon structure than its counterpart (Co NP@CBPDC) and showed dispersed atomic clusters. BET data showed that Co AC@CBPDC had a higher surface area (144.8 m2/g) than the Co NP@CBPDC (33.25 m2/g). The improved physicochemical merits of the Co AC@CBPDC allowed for better ORR and OER activities than the Co NP@CBPDC in terms of low halfway potential (E1/2), onset potential (Eonset), overpotential at 10 mA/cm2 (ƞ10), potential gap (∆E) between the overpotential of OER and the halfway potential, and a higher kinetic current density (jk). The enhanced electrochemistry of the Co AC@CBPDC was attributed to the defects created by the acid treatment. As proof of real-life applicability, the Co AC@CBPDC electrocatalyst delivered an excellent air cathode in a parallel plate RZAB cell with notable OCV (1.23 V), peak power density (49.9 mW/cm2), a real energy density (477 mAh/cm2), long-term stability for 210 h, enhanced voltage retention, Coulombic efficiency (ca. 100 %) and DOD (51.3%), comparable to literature. In addition, an all-solid-state RZAB based on the Co AC@CBPDC catalyst gave a higher and constant OCV (1.73 V) at varied bending angles (0 – 180 degrees) and excellent stability. Second, new HESOx materials were prepared via the Pechini method at two different annealing temperatures of 500 and 750 oC (abbreviated herein as HESOx-500 and HESOx-750). P-XRD results showed that these are inverse spinel oxides, with {311} as the dominant phase. HR-TEM images proved that they are single nanocrystalline materials. XRD and BET data showed that the HESOx-500 is smaller in size, more porous, and has a higher surface area than its counterpart (HESOx-750). HESOx-500 showed superior ORR performance with an onset potential of 0.93 V and a E1/2 of 0.88 mV. The OER performance also showed improved ƞ10 compared to IrO2 with an overpotential of 340 mV at a current density of 10 mA/cm2, and a 45 ± 5.0 mV/dec Tafel slope, above the performance of IrO2 (66 ± 6.1 V/dec). The ∆E of HESOx-500 was 0.69 V. The material was further tested as a cathode material in a RZAB cell. The optimised RZAB cell showed remarkable performance with a theoretical potential of 1.67 V and long-term stability of 375 h at 10 mA/cm2. The performance was attributed to the high-entropy compositional design with a high number of surface oxygen vacancies and different metal oxidation states. Finally, having dealt with the issue of bifunctionality in RZAB, a new ZnO@C anode material was also considered. The ZnO@PDA-DC (where PDA-DC means polydopamine-derived carbon) was used due to its ability to form Zn2+ pathways. Electrochemical potentiodynamic polarisation tests were performed to understand and compare the corrosion inhibition effects in an alkaline medium (6 M KOH). The ZnO@PDA-DC showed better corrosion inhibition properties than the zinc plate and other samples: low corrosion current (icorr = 0.107 uA/cm2) and corrosion potential (Ecorr = 1.077 V), and a mixed inhibition effect, indicating reduced hydrogen evolution reaction and zinc dissolution. Due to the excellent corrosion inhibition properties of the ZnO@PDA-DC, it was then evaluated in the RZAB cell. The shallow galvanostatic charge-discharge cycle stability at 2 mA/cm2 was able to maintain 150 h in a RZAB at a voltage gap of 0.76 V to 0.80 V. The results demonstrated that enhanced rechargeability is possible with ZnO@PDA-DC for RZAB.Item Synthesis of platinum-based electrocatalysts using nitrogen doped onion-like carbon and WS2 composites as the support for electrooxidation of ethanol in direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-10) Bila, Laercia Rose; Gqoba, Siziwe; Maubane-Nkadimeng, Manoko S.The study reports on the synthesis of onion-like carbons (OLCs)/tungsten disulfide (WS2) composites as catalyst support for direct alcohol fuel cells (DAFC). OLCs were synthesized using waste engine oil over a flame pyrolysis (FP) method. The pristine OLCs (p-OLCs) were functionalized and purified using nitric acid (HNO3). The functionalized OLCs (F-OLCs) were further doped with nitrogen using melamine to increase the electronic properties of the OLCs. WS2 was synthesized using the colloidal method and oleylamine was used as the capping agent. Pt/p-OLCs, Pt/F-OLCs, and Pt/N-OLCs were synthesized using a reflux method where ethylene glycol was the reducing agent. Finally, WS2/N-OLCs were synthesized using the colloidal method and then Pt was dispersed on WS2/N-OLCs to form Pt/WS2/N-OLCs. High-resolution transmission electron microscopy showed the presence of onion-like rings in the OLCs and the quasi-spherical morphology, while a flower-like morphology was observed for WS2. Powder X-ray diffraction revealed that the synthesized WS2 had traces of WO3 due to the oxidation of WS2 which introduces WO3 impurities. Energy Dispersive X-ray Spectroscopy revealed that the OLCs derived from waste engine oil present some impurities that were attributed to the motor wear as well as the fuel. When Pt was loaded onto the WS2/N OLCs composite, the WS2 lost its original nanoflower morphology, which was attributed to the presence ethylene glycol used as a reducing agent. X-ray photon spectroscopy confirmed the successful synthesis of the Pt electrocatalysts. Cyclic voltammetry was used to determine the oxidation of ethanol and the current density of the synthesized electrocatalysts. Interestingly, the Pt/p-OLCs electrocatalyst had a higher current density compared to Pt/F-OLCs and Pt/N-OLCs. This was attributed to metal impurities found in p-OLCs, which were reduced during the purification process. The Pt/WS2/N-OLCs electrocatalyst showed higher current density compared to Pt/WS2 but this was low compared to Pt/N-OLCs. The data reveals that the addition WS2 shows a co-catalyst behaviour, rather than a support.Item Biocatalytic oxidative conversion of valencene to nootkatone mediated by lipoxygenase and cytochrome P450(University of the Witwatersrand, Johannesburg, 2024-03) Raboya, Christopher; Ngwira, Kennedy; Brady, DeanNootkatone (NK) is an oxygen-containing sesquiterpene with a significant grapefruit aroma and plays an important role in the flavour and fragrance industry. The natural production of NK through extraction produces trace amounts and is therefore not a viable option to meet industrial needs. The chemical synthesis often utilises reagents harmful to the environment. The purpose of this research was to explore the use of crude lipoxygenase (LOX) enzyme extracted from soya beans, commercial mutants of CYP450 as well as laccase enzymes for the conversion of valencene (VL), the aroma components of citrus fruits to NK. For the LOX reactions, a conversion of 28.79% (mol/mol) was obtained when the reactions were performed with LOX only. The best conversion of 74.46 % was realised when FeSO4.7H2O and MnSO4 were added to the reaction. In the temperature studies, 70 ˚C was shown to be the optimal temperature for the conversion. In addition, we observed that vegetable oils provided sufficient unsaturated fatty acids to facilitate the conversion of VL to NK with sunflower oil being the best. In exploring the potential of LOX to oxidise other organic molecules, caryophyllene was oxidised to novel caryophyllene oxide, and styrene was oxidised to benzoic acid, 1-phenylethane-1,2-diol, and 2-hydroxyl-2-phenylethyl benzoate. This is the first time that such oxidations are reported, and this underlines the potential of LOX in biotransformation and organic synthesis. For CYP450 reactions, the best conversion of 16.70 % was obtained using a variant sourced from Prozomix. Evaluation of parameters such as temperature, pH (7.0 to 7.5), using buffer solutions should be explored to optimise the activity of the enzyme. Laccase from Novoprime Base 268 showed no activity for the conversion of VL to NK despite making use of mediators. Therefore, we should explore changing the reaction conditions, varying the pH of the reaction, buffer strength or mediator and laccases from other sources such as Cerrena unicolor and Trametes versicolor, as well as a fresh batch of laccase from Trichoderma (Merck) should be assessed in the conversion of VL to NK.Item Preparation of nitrogen-doped multiwalled carbon nanotubes anchored 2D platinum dichalcogenides for application as hydrogen evolution reaction catalysts(University of the Witwatersrand, Johannesburg, 2024-09) Mxakaza, Lineo Florence; Moloto, Nosipho; Tetana, ZikhonaThe alkaline hydrogen evolution reaction (HER) (H2O + 2e − → H2 + 2OH−) is fast gaining traction as a sustainable hydrogen gas generation route but suffers from slow reaction kinetics because of the additional water dissociation step and large reaction overpotential. As such, the current state-of-the-art acidic medium Pt and Ru catalysts suffer from considerable loss of catalytic activity in an alkaline medium. We propose the development and use of platinum metal dichalcogenides for alkaline HER. Platinum dichalcogenides are 2D materials that offer the advantage of more exposed catalytic sites, show dramatic chalcogen-dependent electronic properties, and have a band gap (0.24 eV - 1.8 eV for PtS2 and PtSe2) thus extending the use of these materials to light-stimulated photo-electrochemical (PEC) HER. As such, PtS2 is reported to be a semiconductor, PtSe2 is semi-conductive/semi-metallic depending on the number of layers, and PtTe2 is metallic. The Pt-chalcogen covalent bond intensifies down the chalcogen group. Additionally, the interlayer interactions in Pt dichalcogenides are covalent, and just like the Pt-chalcogen bond, intensify as the chalcogen atom changes from sulphur to selenium to tellurium. This behaviour of Pt dichalcogenides results from the Pt bonding d orbitals and the chalcogen bonding p orbitals that are relatively close in energy than in other TMDs, and the difference in the energy becomes smaller and smaller down the chalcogen group. Herein, we report on the synthesis of PtSe2 and PtTe2 using the colloidal synthesis method for the first time and then applying them as electrocatalysts in alkaline HER. As mentioned, developing 2D materials results in band gap development, particularly in PtS2 and PtSe2. Following this, PtSe2 was explored as a photocathode in light-induced photo-electrochemical HER. Generally, semiconductors are poor electron transporters and one of the major requirements for an efficient PEC cathode is solar absorption, charge generation, and efficient charge separation. The charge separation properties of PtSe2 were improved by supporting this material on highly conductive, mechanically, and thermally stable nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs). In Chapter 3, we report on the effect of varying selenium precursors from elemental selenium, sodium selenite to selenourea on the colloidal synthesis of PtSe2 in a mixture of oleylamine and oleic acid at 320 ℃. All the reactions resulted in the formation of PtSe2 although PtSe2 prepared from selenourea is amorphous, evidenced by relatively broader XRD peaks and a smaller crystallite size. HER activity of the three PtSe2 catalysts was evaluated in 1 M KOH at a scan rate of 5 mV/s and PtSe2 prepared from selenium exhibited the earliest onset potential of 46 mV, overpotential of 162 mV, and a smaller Tafel slope of 112 mVdec-1. This material exhibits the smallest resistance to electron transport and a high electrochemical surface area. We then explored the effect of altering tellurium precursor from elemental tellurium to tellurium tetrachloride, and sodium tellurite. Unlike the PtSe2 synthesis, different platinum tellurite phases, PtTe2, PtTe, and the mixed phase PtTe: PtTe2 were produced from Te, PtCl4, and sodium tellurite, respectively. Of the three, PtTe2 exhibited the highest alkaline HER activity with an onset potential of 29 mV, an overpotential of 107 mV, and a Tafel slope of 79 mVdec-1. In the same chapter, we compared the catalytic activity of PtSe2 (prepared from Se) and PtTe2 (prepared from Te) catalysts. We determined that PtTe2 has a high surface roughness and electrochemical surface, leading to relatively higher activity than PtSe2. However, PtTe2 is metallic and therefore does not have a band gap, which implies that it cannot be employed in light-stimulated catalysis reactions. In Chapter 4, we explored the use of PtSe2 as a light-stimulated PEC alkaline HER catalyst. We used in situ colloidal synthesis to grow PtSe2 on the walls of N-MWCNTs to improve the overall electron transport properties of PtSe2. PtSe2 anchored on N-MWCNTs was also studied in the dark and under illumination using 1 sun (100 mW/cm2) to determine the influence of light on the HER catalytic activity of the hybrid materials. This study demonstrates that the light-stimulated HER activity of PtSe2 improves when minimal amounts of N-MWCNTs are incorporated in the PtSe2 sample matrix. This then leads to employing these materials as photocathodes in PEC HER.Item Microwave-assisted synthesis of palladium-based ferroalloy electrocatalysts for application in alkaline direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-11) Ramashala, Kanyane Nonhlanhla Eugenia; Billing, Caren; Modibedi, R. Mmalewane; Ozoemena, Kenneth IkechukwuThis research work describes the study of Pd-based ferro-electrocatalysts for application towards direct ethanol fuel cells (DEFCs), direct ethylene glycol fuel cells (DEGFCs), direct glycerol fuel cells (DGFCs) and oxygen reduction reaction (ORR) operated in a basic environment. The initial part of the research was to explore the Pd-based monometallic and bimetallic (Pd/C and PdFe/C) by utilising varied methods such as the conventional sodium borohydride (NaBH4) and microwave-assisted technique (MW) towards the oxidation of glycerol (gly), intending to choose the best method viable for these catalysts. This study revealed that MW techniques tuned the physicochemical properties of Pd/C and PdFe/C by augmenting their crystallinity and defect. These led to improved electrocatalytic activities towards glycerol oxidation reaction (GOR) over NaBH4 technique. MW process as a powerful tool was further used in the entire study to synthesise bimetallic and trimetallic electrocatalysts in ethanol (EtOH), ethylene glycol (EG) and glycerol (Gly) oxidation reaction in an alkaline environment. The synthesised bimetallic catalysts studied in this research work were (PdFe/C, PdCo/C, and PdMn/C) at varied ratios of Pd: M (Pd2M/C (2:1) and PdM/C (1:1)). Amongst them all, Pd2Fe/C and PdFe/C were observed to be the most favourable catalysts towards all the alcohols, with the excellent specific activity of about, for EtOH (11.59 and 4.15 mA cm-2), EG (9.82 and 5.51 mA cm-2) and Gly (8.94 and 4.73 mA cm-2), respectively. The satisfactory performance exhibited by the PdFe/C electrocatalyst prompted the exploration of the second 3d transition metal (PdFeMn/C and PdFeCo/C), intending to investigate the synergistic behaviour between the non-noble metals and Pd. The XRD confirmed that these electrocatalysts are in a crystalline nature with a decrease in d spacing (from 0.2247 nm, PdFe/C to 0.2236 nm (PdFeMn/C)) after the insertion of Mn into PdFe/C. This was supported by the TEM images obtained for the PdFeMn/C catalyst with a particle size of sub 10 nm. The comparison studies towards EtOH, EG and Gly were investigated for all the electrocatalysts and there was a remarkable observation, which is dissimilar from the theoretical studies (DFT). Density Functional Theory (DFT) revealed that PdFeCo performed better in terms of Gibbs free energy, binding energy, and energy band gap than PdFeMn; however, the experimental studies favoring the performance of PdFeMn. The PdFeMn/C delivered the best electrochemical activities, including a superior electrochemical active surface area (ECSA), larger current densities and mass activity response, and less susceptibility to poisoning and high conductivity as compared to PdFe/C and PdFeCo/C electrocatalysts. Furthermore, the PdFeMn/C electrocatalyst exhibited remarkable electrochemical properties during the ORR (basic medium). Ultimately, the best two anode electrocatalysts (PdFe/C & PdFeMn/C) were explored and tested for the proof-of-concept in the two-electrode configuration with the micro-3D printed cell. The PdFeMn/C delivered improved µ-ethylene glycol fuel cell, µ-glycerol fuel cell, and µ-ethanol fuel cell activities with respective to high voltage and power density of 33.27 mW cm-2, 11.00 mW cm-2 and 45,80 mW cm-2 respectively, operated at 100 mV / s. These electrocatalysts have demonstrated promising results in advancing ADAFCs.Item Application of oxidative enzymes in membrane systems for the bioremediation of triazines in wastewater(University of the Witwatersrand, Johannesburg, 2024-10) Lesaoana, Mahadi; Richards, Heidi L.; Brady, DeanThe prevalence of herbicidal pollutants present in various environmental matrices have become a global concern. The discharge and accumulation of s-triazine agrochemicals in effluents remains a major challenge, threatening the quality of freshwater resources. These are newly identified recalcitrant contaminants of concern (CECs) with complex structures, and inadvertent exposure poses deleterious ecological risks and human health-related adverse effects. Unfortunately, they have shown resistance to conventional treatment strategies, hence their persistence in wastewater treatment plant (WWTP) effluents and water bodies. Therefore, there is an urgent need for the exploration of alternative technologies for the effective eradication of such contaminants from water samples. The bioconversion of such micropollutants using oxidative enzymes like laccase is a promising research avenue, providing a sustainable, economically and ecologically benign strategy. The current research examined the potential of a hybrid biocatalytic membrane system to degrade common s-triazine agrochemical herbicides in aqueous solutions. Specifically, the use of Novoprime base 268 laccase coupled with hollow fibre polyethersulfone (PES) membranes was investigated for the bioremediation of atrazine (ATZ), ametryn (AMT), simazine (SMZ) , prometon (PMT) and terbuthylazine (TERB) in wastewater. In batch-mode reactions, major operating parameters (i.e. pH and temperature profiles, enzyme dosage and contact time) were varied for the laccase-assisted catalysis of s-triazine compounds. Optimised conditions provided highest removal efficiencies (> 88.9%) at pH 5.0, combined with a temperature of 25°C and 1.0 mg L-1 solution concentration after 24h reaction time. Through the addition of redox mediators viz. 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS), violuric acid (VA), vanillin (VA), syringaldehyde (SRA) and acetosyringone (ASR) recalcitrant triazine degradation was enhanced by 10 to 20 % at 1.50 mm. Subsequently, the performance of a standalone continuous flow-mode membrane system was evaluated firstly, using a bed adsorption column only operated under various conditions. The efficiencies were compared to batch-mode enzymatic experiments. The adsorption of triazines by PES was only weakly influenced by pH, and the optimum removal was attained at pH 5.0 (5.0 mg L-1), 2.35 g bed mass (14.0 cm height) and 24h column operation time. The overall removal percentages were 72.6%, 75.2%, 71.4%, 67.4%, and 68.2% for ATZ, AMT, SMZ, PMT and TERB, respectively. Although the results indicated satisfactory performances by both systems, their performance is limited when used as separate units (continuous membrane vs laccase reactor). A biocatalytic membrane system was achieved by integrating laccase into the dynamic packed-bed membrane column. Relevant process control design parameters of the fixed-bed biocatalytic column were carefully evaluated and recorded an optimum of 93.2 % removal efficiency as observed at a feed flow rate 2.0 mL min-1, at a bed height of 14.0 cm using an atrazine influent concentration of 5.0 mg L-1. Equilibrium dynamics of the breakthrough modelling were best fitted by Thomas model. Results attained demonstrated selectivity for triazines in matrix-matched real river water samples with remarkable recyclability after six successive operational cycles. This reflects the potential workability of the integrated system for extended enzymatic reactions evaluated under robust experimental conditions. As a benchmarking exercise, cost-analysis studies showed comparable projected scalability of our configuration at 1200 m3/d capacity at an estimated total cost of R7.036 mil.Item Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation(University of the Witwatersrand, Johannesburg, 2023-06) Mpala, Josephine Tshepiso; Nthunya, Lebea; Richards, Heidi; Etale, AnitaA global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling.