Liquid metal catalysis for carbon dioxide splitting
Creators: name: Zuraqi, Karma
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials
classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)

description: For decades, the vast majority of chemical reactions were driven by solid heterogeneous catalysts. These solid catalysts, however, have been determined to suffer from rapid deactivation due to coking. The growing need to make industrial processes and manufacturing more efficient, quicker, and highly versatile, suggests that there is a need to probe and design innovative catalytic materials that meet industrial demands. Devising new catalysts with controlled activity, stability, and physicochemical properties is a continuously evolving science, that is progressively shifting away from conventional catalysts to curtail limitations
that are associated with their use.
Liquid metals have recently emerged as a new class of catalysts with the potential to replace solid catalysts. Liquid metals have been employed in a range of catalytic applications, predominantly in the hydrocarbon decomposition space, exhibiting immense capabilities in terms of activity, selectivity, and stability. Most importantly, these catalysts have been demonstrated to be highly resilient to coking. Liquid metals feature unique physicochemical properties, particularly their relatively low viscosity, and high thermal and electrical conductivity, that render them appealing mass and heat transfer media, and ultimately favorable catalytic materials. Additionally, liquid metals possess a unique feature that further differentiates them from other non-metallic solvents, their ability to dissolve a wide range of metals to obtain a library of liquid metal alloys with varying properties. Since the metal
composition and stoichiometry dictate the physical and chemical properties of the alloys, this feature promotes liquid metals as advanced materials that can enable the synthesis of catalysts with fine-tuned properties, tailored for a target application.
However, while liquid metals have been well-investigated in a wide array of applications, of which a non-exhaustive list includes: two-dimensional materials synthesis, electronics, heat transfer systems, and antennas, research on liquid metals as catalytic materials that facilitate chemical reactions remains limited. The field of liquid metal catalysis is still in its infancy,
an understanding of the surface chemistry of these catalysts is inadequate, and the mechanistic models that define and explain the factors driving the chemical reaction using liquid metals is largely lacking. As such, predicting the effectiveness of a liquid metal catalyst in successfully initiating a specific chemical reaction is very challenging.
This research investigates the potential application of liquid metals in what is arguably amongst the most fundamental conversion reactions, carbon dioxide (CO2) conversion, to address one of the most notable persisting global issues, CO2 mitigation. The need to cut CO2 emissions is uncontentious, but the decomposition of CO2 is an enormous undertaking
since CO2 is not only the most common greenhouse gas but is also the most stable state of carbon. The work presented in this thesis is primarily directed at using bulk liquid metals, specifically gallium and eutectic gallium indium alloy (EGaIn), to decompose CO2. A bubbling column reactor is designed to investigate the effectiveness of liquid metals in activating the stable CO2 molecule, enabling the direct conversion of CO2 to solid carbon. A temperature study is also performed to shed light on the kinetics of the reaction and investigate the selectivity of the process to carbon production. The stability of liquid metals
over prolonged periods of carbon generation and their resistance to coking is investigated. Moreover, a combination of experimental and computational approaches is deployed to understand the underlying mechanistic driving forces that promote the use of gallium-based liquid metals in reduction reactions. Due to the unique nature of liquid metals, characterization methods such as an in-situ spectroscopy study are developed in-house to analyze the formation of carbon over the liquid metal surface.
Moreover, since liquid metals are capable of dissolving a broad range of metals, metal solubility can be exploited to enhance their catalytic performance. In essence, this feature can enable the use of known metals that are well-established catalysts and facilitate their integration into liquid metal applications despite their high melting points. Therefore, targeted metal selection can be effectively exercised in designing liquid metal catalysts, depending on the application criteria. To enhance the performance of bulk gallium-based liquid metals in the decomposition of CO2, the use of iron as a metal additive is considered. Owing to
its high activity toward CO2 conversion reactions, iron is a highly investigated metal, most commonly in its oxide deficient forms magnetite and wustite. However, the high melting point of iron entails its utilization conventionally as a solid catalyst, hence, its integration into gallium-based liquid metals in this thesis signifies a significant shift in the employment of metallic iron in liquid phase reactions. The use of iron in homogeneous reactions overcomes the coking issues concomitant to using iron in CO2 conversion, and demonstrates the efficacy of different liquid metal alloys in driving the CO2 conversion reaction.
Current approaches of liquid metal catalysis are impeded by mass transfer limitations. These approaches now need to be re-evaluated and re-designed to ensure effective interaction between gas reactants and liquid metal catalysts. While increasing the surface area of solid catalysts by developing strategies that increase the pore volume is well investigated, improving the surface area of their liquid metal counterparts is not. One effective method of enhancing the contact between the gaseous reactants and liquid metals that could potentially be applied to liquid metal catalytic applications is by reducing the size of the liquid metal to the nanoscale. By reducing the droplet size, the surface area-to-volume ratio is increased and the contact area between the reactants and liquid metals is consequently improved. This thesis explores methods of liquid metal nanodroplets generation, with special emphasis on approaches independent of the use of ultrasound. To that end, this work investigates the production of sub 50 nm liquid metal nanodroplets using an emulsification-shearing method.