Rational design and synthesis of bismuth-doped metal borate and defect-rich metal oxide catalysts for water splitting and chemical conversions
Creators: name: Thomas, Belvin
Publisher: name: No Publisher Supplied
Year: 2025

description: As the world grapples with the harmful effects of climate change due to the excessive use of fossil fuels that emit greenhouse gases, sustainable and carbon-neutral energy sources are urgently needed. Hydrogen produced from water is considered the potential fuel that can replace our current dependence on fossil fuels. The hydrogen production can be achieved by water electrolysis or photocatalytic water splitting. The water electrolysis involves two half-reactions - the hydrogen evolution reaction (HER), which produces hydrogen, and the oxygen evolution reaction (OER), which produces oxygen. Among these two reactions, the OER is kinetically sluggish and is detrimental to the overall efficiency of water electrolysis to produce hydrogen. In the case of photocatalytic water splitting, excellent photocatalysts that can work in the visible region of sunlight with high efficiency are still lacking. Thus, catalysts that are low cost and abundant and have high catalytic activity are required for producing hydrogen from water. This thesis explores doping and introducing defect sites as strategies to improve the catalytic activity of existing catalysts for water splitting. Furthermore, sustainable synthesis of catalysts for chemical conversions using ionic liquids as reaction media is also studied.In Chapter 1, a broad overview of climate change and how it is fueled by our fossil fuel consumption are discussed. Since the hydrogen economy is one of the potential ways forward to mitigate climate change, the production of hydrogen via water electrolysis and photocatalytic water splitting are reviewed in detail. As excellent catalysts are required to produce hydrogen from water, various strategies to design and develop such catalysts are also presented. Finally, brief overviews of metal borates, transition metal oxides, titanium dioxide, and zeolitic materials, which are parts of the studies in this thesis, are also included. In Chapter 2, a series of Bi-doped cobalt borates are synthesized, and their electrocatalytic activities towards OER in alkaline media are tested. Here, we demonstrate that the incorporation of oxophilic p-block metals such as Bi into cobalt borate can enhance its electrocatalytic activity towards OER. Furthermore, we show that pyrolyzing the catalyst under various temperatures enhances its catalytic activity because it increases the synergistic interactions between Bi with Co and B atoms. The best electrocatalytic performance is obtained when the Co-to-Bi ratio in the catalyst is 9:1, and the pyrolysis temperature is set at 450 oC.In Chapter 3, bismuth’s ability to enhance the electrocatalytic activities of first-row transition metal oxides towards acidic OER is demonstrated. First, a series of Bi-doped cobalt oxides are synthesized in situ on fluorine-doped tin oxide (FTO) glass slides and are then tested as electrocatalysts for acidic OER. In this study, the incorporation of Bi into cobalt oxides is found to not only improve their electrocatalytic activity towards acidic OER but also not compromise the durability of the catalysts. The best catalytic performance is obtained when the catalyst is synthesized with a Co:Bi ratio of 9:1. The experiments and density functional theory-based calculations indicate that the Bi incorporated into cobalt oxide can act as catalytic sites during acidic OER. The study is extended further to understand the versatility of Bi in improving the catalytic activity of other transition metal oxides by making a series of Bi-doped iron oxides and nickel oxides on FTO. The results demonstrate that Bi can also improve the catalytic activities of these metal oxides towards acidic OER.Chapter 4 presents the synthesis of defect-rich TiO2 or reduced TiO2 (which is also called TiO2-x) materials using polyaniline-derived mesoporous carbon (PDMC) as a hard template and the investigation of their photocatalytic activities towards hydrogen production from water. Here, specifically, titanyl sulfate, which is an industrially relevant titania precursor, is used to make the defect-rich TiO2 materials, which makes the work scalable and commercially viable in the end. The level of defect sites in the materials can be varied by varying the TiO2 loading in the PDMC template. These defect-rich TiO2 materials exhibited unusual charge-discharge phenomena when exposed to slow electron beams. Preliminary studies also show that they are active towards photocatalytic hydrogen production from water.In Chapter 5, the synthesis of aluminophosphates (AlPOs) using double salt ionic liquids (DSILs) prepared from imidazole-based ionic liquids (ILs) with fast resistive conventional and microwave heating is demonstrated. Notably, by using DSILs as both solvent and structure-directing agents, highly crystalline AlPOs are synthesized without the use of corrosive mineralizers such as hydrofluoric acid (HF). Additionally, the composition of DSILs is found to dictate the morphology of the obtained AlPO crystals. The fast resistive conventional heating, which mimics microwave-based heating, also enables rapid crystallization of AlPOs.In summary, this dissertation explores (i) the enhancement in the catalytic activities of metal borates and first-row transition metal oxides by doping them with the p-block metal bismuth for electrocatalytic OER, (ii) the improvement in the photocatalytic activities of TiO2 by introducing defect sites in them, and (iii) the synthesis of catalytically active AlPOs using DSILs through a facile synthetic route.