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To combat the escalating environmental challenges and alleviate the current energy crisis, CO2 conversion to fuels and chemical feedstocks provides a reliable approach to mitigate the devastating impact of greenhouse emissions on climate change. CO2 conversion/reduction could be carried out by several methods; however, the electrochemical CO2 reduction (CO2R) approach has coupled several advantages. For instance, CO2R occurs in near-ambient reaction conditions and could be driven through the employment of renewable energy resources (wind or solar) to generate electricity. However, this reaction has a large energy barrier which requires a catalyst to facilitate its pathway. In this context, various catalyst designs were developed and investigated during the last decades, such as heterogenous (metal and metal oxide) and homogenous (organic molecules) catalysts. A new class of materials – atomically dispersed metal nitrogen–doped carbon support (M–N–C)– has emerged recently and showed remarkable enhancement for CO2R compared to the state-of-the-art. In particular, Ni–N–C catalysts have demonstrated an improved selectivity toward CO production compared to precious metal catalysts. Researchers have postulated this superior performance to the high atomic utilization (theoretically 100%) of the metal sites under reaction conditions and the enhanced electronic properties. In addition, intermetallic carbides have been included as a promising class of catalysts for CO2R due to their unique physical and chemical characteristics. These catalysts could be synthesized using different precursors; among them, MOFs are currently one of the most promising platforms that generate several catalyst designs. It was demonstrated that MOF’s unique characteristics, such as high surface area and porosity, would be transitioned to the derived catalysts. In this thesis, two MOF architectures (ZIF-8 and MOF-74) were initially selected to be employed as precursors for deriving atomically dispersed Ni–N–C catalysts. Both MOF-derived catalysts were evaluated for CO2R using a customized electrochemical cell (E-cell) with a 3–electrode configuration. The derived Ni–N–C catalysts using ZIF-8 and MOF-74 have achieved enhanced CO selectivity with Faradaic efficiencies (FE) > 90% at less negative applied potentials, –0.68 and –0.76 V vs RHE, respectively. Further, various synthetic conditions were explored in these studies, such as the role of the Ni content and the pyrolysis temperature on the resulted catalyst structure, and the electrocatalytic performance during CO2 electrolysis. Subsequently, one of the MOF topologies – ZIF-8 – was further utilized to develop other designs of electrocatalysts by introducing different synthetic conditions. This has resulted in generating various moieties that are able to produce CO during CO2R. For example, one derived catalyst design consists of homogenously distributed atomically dispersed dual Ni–Zn–NX/C sites. Whereas the other design demonstrated a heterogenous structure of Ni3ZnC-based particles anchored on atomically dispersed dual Ni–Zn–NX/C sites. Both electrocatalyst designs were integrated into a gas diffusion electrode (GDE) and evaluated for CO2R using an MEA-based electrolyzer. Our findings revealed that the co-existence of Ni3ZnC particles and dual Ni–Zn–NX/C active sites in a heterogenous structure has boosted the electrocatalytic activity towards CO production, achieving near unity CO FE at 448 mA/cm2 at an overall cell voltage of 3.1 V. Aside from the electrocatalytic performance, the nature of active sites in the developed catalyst designs has been studied using in-situ and ex-situ X-ray absorption spectroscopy. Other analytical techniques such as transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) have also been used to identify the catalysts’ composition and morphology. / Thesis / Doctor of Philosophy (PhD) / This PhD thesis aims to develop and implement a sustainable technology that tackles increased CO2 emissions in the atmosphere and mitigates the greenhouse effect on climate change. The approach of this thesis focuses on developing efficient catalyst designs for CO2 electroreduction (CO2R) to CO as a beneficial chemical feedstock, and then pursues the practical implementation of these catalysts in an industrially relative reactor design in the form of a membrane electrode assembly (MEA)-type electrolyzer. This study selected atomically dispersed metal-doped nitrogen-carbon (M–N–C) and intermetallic carbide electrocatalysts as promising materials for CO2R. Among different precursors, metal-organic frameworks (MOFs) have been employed to synthesize the desired electrocatalysts due to their unique geometric structure and high surface area. On a fundamental level, our findings demonstrated that all MOF-derived catalysts have exhibited high selectivity towards CO during CO2 R. However, the conversion rates were governed by the nature of the active sites and the implemented electrochemical systems.

The conversion of ethanol to hydrogen or syngas can be achieved by reacting ethanol with water via steam reforming, CH3CH2OH + (1-x)H2O = (4-x)H2 + (2-x)CO + xCO2 (R.1) CH3CH2OH + H2O = 4H2 + 2CO (R.2) CO + H2O = H2 + CO2 (R.3) Ideally, the ethanol steam reforming reaction can achieve a hydrogen yield of 6 moles of hydrogen per mole of ethanol when the value of x in (R.1) equals 2. High theoretical H2 yield makes ethanol steam reforming a very attractive route for H2 production. Thermodynamic equilibrium studies have shown that ethanol steam reforming produces mixtures of H2, CO, CO2, and CH4 below 950 K, while above 950 K the ethanol steam reforming reaction (R.1) adequately describes the product composition In this study a series of 10wt% Ni loaded Mg-Al mixed oxide supported catalysts were evaluated for the production of hydrogen via the steam reforming of ethanol. Mg-Al mixed oxide supported nickel catalysts were found to give superior activity, steam reforming product selectivity (H2 and COx), and improved catalyst stability than the pure oxide supported nickel catalyst at both temperatures investigated. Activity, product selectivity, and catalyst stability were dependent upon the Al and Mg content of the support. At 923 K, the Mg-Al mixed oxide supported nickel catalysts were the best performing catalysts exhibiting the highest steam reforming product yield and were highly stable, showing no signs of deactivation after 20 h of operation. The improved performance of the Mg-Al mixed oxide supported catalysts was related to the incorporation of the pure oxides, MgO and Al2O3, into MgAl2O4. The formation of MgAl2O4 reduced nickel incorporation with the support material since MgAl2O4 does not react with Ni; therefore, nickel was retained in its active form. In addition, incorporation of Mg and Al in to MgAl2O4, a slight basic material, modified the acid-base properties resulting in a catalyst that exhibited moderate acidic and basic site strength and density compared to the pure oxide supported catalysts. Moderation of the acid-base properties improved the activity, selectivity, and stability of the catalysts by reducing activity for by-product reactions producing ethylene and acetaldehyde. At lower reaction temperatures, below 823 K, Mg-Al mixed oxide supported nickel catalysts experienced substantial deactivation resulting in reduced ethanol conversion but interestingly, the H2 and CO2 yields increased, exceeding equilibrium expectations with time on stream while CH4 yield decreased far below equilibrium expectations, suggesting a direct ethanol steam reforming reaction pathway. Over stabilized Mg-Al mixed oxide supported nickel catalysts, direct ethanol steam reforming was activated by a reduction in the catalyst’s activity for the production and desorption of CH4 from the surface. The effect of pressure on the direct ethanol steam reforming reaction pathway over stabilized Mg-Al mixed oxide supported nickel catalysts was investigated at 673 and 823 K. At 823 K, increasing the total pressure resulted in a product distribution that closely matched the thermodynamic expectations. However, at 673 K, the product distribution deviated from thermodynamic expectations, giving substantially greater yields for the steam reforming products, H2, CO, and CO2, while CH4 yield was consistently less than equilibrium expectations. The identification of an alternative direct ethanol steam reforming reaction pathway at relatively low temperatures (below 823 K) that could be operated at elevated pressures will result in an energy efficient process for the production of hydrogen from bio-ethanol.

The conversion of ethanol to hydrogen or syngas can be achieved by reacting ethanol with water via steam reforming, CH3CH2OH + (1-x)H2O = (4-x)H2 + (2-x)CO + xCO2 (R.1) CH3CH2OH + H2O = 4H2 + 2CO (R.2) CO + H2O = H2 + CO2 (R.3) Ideally, the ethanol steam reforming reaction can achieve a hydrogen yield of 6 moles of hydrogen per mole of ethanol when the value of x in (R.1) equals 2. High theoretical H2 yield makes ethanol steam reforming a very attractive route for H2 production. Thermodynamic equilibrium studies have shown that ethanol steam reforming produces mixtures of H2, CO, CO2, and CH4 below 950 K, while above 950 K the ethanol steam reforming reaction (R.1) adequately describes the product composition In this study a series of 10wt% Ni loaded Mg-Al mixed oxide supported catalysts were evaluated for the production of hydrogen via the steam reforming of ethanol. Mg-Al mixed oxide supported nickel catalysts were found to give superior activity, steam reforming product selectivity (H2 and COx), and improved catalyst stability than the pure oxide supported nickel catalyst at both temperatures investigated. Activity, product selectivity, and catalyst stability were dependent upon the Al and Mg content of the support. At 923 K, the Mg-Al mixed oxide supported nickel catalysts were the best performing catalysts exhibiting the highest steam reforming product yield and were highly stable, showing no signs of deactivation after 20 h of operation. The improved performance of the Mg-Al mixed oxide supported catalysts was related to the incorporation of the pure oxides, MgO and Al2O3, into MgAl2O4. The formation of MgAl2O4 reduced nickel incorporation with the support material since MgAl2O4 does not react with Ni; therefore, nickel was retained in its active form. In addition, incorporation of Mg and Al in to MgAl2O4, a slight basic material, modified the acid-base properties resulting in a catalyst that exhibited moderate acidic and basic site strength and density compared to the pure oxide supported catalysts. Moderation of the acid-base properties improved the activity, selectivity, and stability of the catalysts by reducing activity for by-product reactions producing ethylene and acetaldehyde. At lower reaction temperatures, below 823 K, Mg-Al mixed oxide supported nickel catalysts experienced substantial deactivation resulting in reduced ethanol conversion but interestingly, the H2 and CO2 yields increased, exceeding equilibrium expectations with time on stream while CH4 yield decreased far below equilibrium expectations, suggesting a direct ethanol steam reforming reaction pathway. Over stabilized Mg-Al mixed oxide supported nickel catalysts, direct ethanol steam reforming was activated by a reduction in the catalyst’s activity for the production and desorption of CH4 from the surface. The effect of pressure on the direct ethanol steam reforming reaction pathway over stabilized Mg-Al mixed oxide supported nickel catalysts was investigated at 673 and 823 K. At 823 K, increasing the total pressure resulted in a product distribution that closely matched the thermodynamic expectations. However, at 673 K, the product distribution deviated from thermodynamic expectations, giving substantially greater yields for the steam reforming products, H2, CO, and CO2, while CH4 yield was consistently less than equilibrium expectations. The identification of an alternative direct ethanol steam reforming reaction pathway at relatively low temperatures (below 823 K) that could be operated at elevated pressures will result in an energy efficient process for the production of hydrogen from bio-ethanol.

For more than a decade, since 2009, biodiesel production has led to excessive production of its by-product, glycerol, consequently decreasing its market value and creating waste issues for the biodiesel industry. Valorization of glycerol is a promising strategy to enhance the sustainability of the biodiesel industry. Electrochemical oxidation of glycerol stands out among the other methods (i.e., hydrogenolysis, dehydration, and catalytic oxidation) due to its simplicity, eco-friendliness, and cost-effectiveness. Glycerol electrooxidation reaction has a wide range of products including C3 (i.e., glyceric acid), C2 (i.e., glycolic acid) and C1 (i.e., formic acid) products, and a complex reaction pathway. Moreover, some of the products spontaneously convert to each other or form decomposition products in the strong alkaline medium, making the product analysis challenging. Therefore, this thesis started by establishing a foundation for the quantitative technical analysis method of glycerol electrooxidation products. Proton Nuclear Magnetic Resonance (H-NMR) was developed as an alternative to High-Performance Liquid Chromatography (HPLC) by providing the capability to assess the products in their medium and detect the chemical conversions in alkaline medium. Additionally, H-NMR is a highly sensitive technique with a low detection limit of 0.01mM for the GOR product, and its accuracy was confirmed with less than 8% error by using a sample product mixture with known concentrations. Most importantly, the proposed chemical pathways were determined by using H-NMR, assisting in a deeper understanding of the glycerol electrooxidation mechanism in an alkaline medium. Subsequently, this thesis explores an efficient catalyst for the glycerol electrooxidation reaction since the state-of-the-art catalysts developed for the glycerol electrooxidation reaction mostly include noble metals hindering their commercialization due to their high price and low stability resulting from susceptibility to CO poisoning. Specifically, catalysts that can hinder the C-C cleavage provide more economic advantages since C3 products have higher market prices than the C2 and C1 products. Thus, the primary aim of this thesis is to develop a cost-effective catalyst with high selectivity towards C3 products, by demonstrating high activity, and stability to make the glycerol electrooxidation reaction economically feasible. This thesis uses the catalyst development strategy of alloying the noble metal with the transition metal, where Pt and Cu were chosen, respectively. PtxCu100-x/C bimetallic alloy catalysts were prepared using the chemical reduction method. The effect of Pt:Cu ratio on the electrochemical performance was studied by using a three-electrode cell in an alkaline medium, revealing the Pt31Cu69/C as the best-performing catalyst with the highest Pt-mass normalized current density (5.9 mA μgPt-1), highest geometrical current density (75.3 mA cm-2), and low onset potential (~0.38V vs RHE) among Pt/C and other PtxCu100-x/C catalysts. Conducting parametric studies on the electrolyte concentrations and applied potential, the C3 selectivity of Pt31Cu69/C catalyst demonstrated the highest selectivity to GLY (75 %) and C3 (86 %) after 10 h chronoamperometry at optimal conditions. Subsequently, the impact of the metal oxide effect on the Pt-based catalyst, PtCu/C-CeO2, and Pt/C-CeO2 were prepared using the ball milling method. The CeO2 impact on the selectivity towards C3 products was investigated. PtCu/C-CeO2 showed the highest selectivity and concentration for C3 products compared to Pt/C-CeO2, Pt/C and PtCu/C. Additionally, temperature impact on the performance towards glycerol electrooxidation reaction was studied, revealing temperature increase further increases the selectivity and concentration for C3 products (90%, and 18.4 mM). All the catalysts prepared in this thesis were characterized physically using analytical techniques of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma optical emission spectroscopy (ICP-OES). / Thesis / Doctor of Philosophy (PhD) / Fossil fuels need to be replaced by clean alternatives due to sustainability concerns. Consequently, as a renewable energy source, biodiesel production has been booming since 2009, producing large amounts of glycerol as a side-product. This overproduction devalorizes glycerol, creating a disposal problem for the biodiesel industry. Electrochemical valorization is a sustainable approach to upgrade glycerol into useful compounds such as glyceric acid and tartronic acid that have medical, food and cosmetic applications. However, glycerol electrooxidation reaction (GOR) has a complex reaction pathway with a wide range of products that can be converted to each other spontaneously in an alkaline medium, making it challenging to fully understand the GOR mechanism, thereby negatively impacts GOR catalyst development. This thesis develops the proton nuclear magnetic resonance method for quantitative analysis of GOR products, highlights the chemical reaction pathways in alkaline electrolyte, and develops PtCu/C and PtCu/C-CeO2 catalysts as selective GOR catalysts towards the valuable C3 products.