In-situ electrochemical generation of superoxide ion for the conversion of CO2 and chlorinated compounds in ionic liquid and nanohybrid-ionic liquid media / Ahmed Halilu

Ahmed , Halilu (2021) In-situ electrochemical generation of superoxide ion for the conversion of CO2 and chlorinated compounds in ionic liquid and nanohybrid-ionic liquid media / Ahmed Halilu. PhD thesis, Universiti Malaya.

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Abstract

Pollution remediation is one of the 21st-century global environmental concerns that need to be sustainable. As such, the conversion of pollutants such as CO2 or chlorinated organics to useful products is a judicious strategy to achieve sustainability of their remediation processes as it provides an opportunity to generate economic profit. Conversion of pollutants can be achieved using superoxide ions (O2●−) as demonstrated in this research in the case of CO2 and selected chlorinated organics. The conversion of pollutants using O2●− relies heavily on media with low nucleophilic susceptibility. This study successfully generated stable O2●− at –±1.0 V vs Ag/AgCl in ammonium [Amm+], pyrrolidinium [Pyrr+], piperidinium [Pip+] and morpholinium [Mor+] based ILs that satisfy the low nucleophilic criterion. The study further pioneered the development of IL-nanofluid media using fully characterized novel pseudocapacitive Fe/Ru-SiMWCNT nanohybrid. The nanofluid has low nucleophilic susceptibility and suitable as media for stable O2●− generation at – 0.445 V vs Ag/AgCl. Moreover, the interfacial and mass transfer parameters therein were determined using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) analysis. Long-term stability of the O2●− was determined in ILs and the nanofluid media using UV/Vis spectrometry. The kinetic parameter indicated that O2●− stays longer in some ILs; [MOEMPyrr+][PFTP−] (3.82 % decay, 0.34×10−5 s−1), [MOEMMor+][TFSI−] (4.81 % decay, 1.71×10−5 s−1), [BTMAmm+][TFSI−] (8.38 % decay, 0.82×10−5 s−1). Second, O2●− decayed by only 2.24 % in Fe/Ru-SiMWCNT-[MOEMPyrr+][PFTP−] nanofluid with a rate constant of 0.423×10−5 s−1 after at least 1 day. These long-term stability results validated the media for at least 60 min in-situ O2●− mediated electrolysis. The CV technique was used in the second part of the analysis to investigate selective CO2 conversion to peroxy-dicarbonate anion (C2O62–) by O2●− at – 0.54 V vs Ag/AgCl. The C2O62– was identified by O-O (proxy-bridge) symmetric stretching harmonic at 853 cm-1 using DFT-IR and ATR-IR spectroscopy. The energy efficiency of C2O62– production in the nanofluid was 97.6 %, higher than that in [MOEMPyrr+][PFTP−] (50 %). This study postulated that the pseudocapacitance of Fe3O4 and RuO2 phase of the nanofluid implicated a low overpotential required for CO2 conversion by O2●− unlike ordinary ILs. Moreover, the CO2 conversion mechanism followed direct nucleophilic addition of O2●− to CO2 since the charge transfer resistance of the CO2 saturated media is extremely high (6.86×1018 kΩ). In the third part of the study, in-situ C2O62– further carboxylated diethanolamine to produce methyl (2-hydroxyethyl) (methyl) carbamate. In the fourth part of this research, dechlorination of CH2Cl2 and C6H5Cl by O2●− was investigated in [BMPip+][TFSI−]. The dechlorination process produced non-dioxin product distribution, achieving 100 % dechlorination after 2 h of electrolysis. In all these accomplishments, quantum chemical calculations were conducted to support the results. The final part of the research entailed analysis of product distribution from the electrolysis of either CO2 or CH2Cl2 and C6H5Cl using GCMS, 1D (1H and 13C), 2D (COSY, HSQC and HMBC) nuclear magnetic resonance analysis. Overall, this research contributed by conceptualizing the development of stable nanofluid media for reactive oxygen species, energy storage and conversion applications.

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