Design of Non-Noble Metal Catalysts for CO2 Reduction and Utilization

Abstract

Industrial development and anthropogenic emissions have significantly increased atmospheric carbon dioxide (CO2) levels, presenting a major environmental challenge. This rise in CO2 not only exacerbates global warming but also disrupts ecological balances, highlighting the need for effective strategies for its mitigation and utilization. Utilizing CO2 as a feedstock in chemical reactions, particularly through catalytic conversion, helps reduce atmospheric concentrations and moves towards carbon neutrality. Among the various CO2 conversion strategies, dry reforming of methane (DRM) is a promising pathway for converting not only CO2 but also the major greenhouse gas methane (CH4) into synthetic feedstocks (syngas), while hydrogenation of CO2 offers a route to produce methane or synthetic natural gas (SNG), converting these gases into valuable products such as synthetic fuels and chemicals. However, the real-world application of DRM has been hindered by durability issues with catalysts that do not maintain high catalytic activity continuously due to metal sintering and carbon deposition. In addition, the CO2 methanation reactions often suffer from lower methane yields due to the formation of hot spots resulting from the exothermic reaction. Consequently, these challenges highlight the critical requirement for further catalyst research to overcome the limitations of both DRM and CO2 methanation and enhance the catalytic performance of these CO2 conversion processes. In this study, we report on the development of a non-precious metal-based catalyst for CO2 and methane conversion that is facile to prepare, cost-effective, and exhibits stable catalytic performance. These properties enhance its suitability for large-scale industrial use as a practical alternative in sustainable chemical processes. Chapter 1 introduces the motivation and organization of the research focused on developing non-noble metal catalysts for dry reforming of methane and hydrogenation of CO2, and Chapter 2 provides a comprehensive overview of DRM and hydrogenation of CO2 reactions. Chapters 3 and 4 of this dissertation detail the innovative development of non-precious metal-based catalysts for methane dry reforming (DRM), with emphasis on the synthesis and improvement of these catalysts using the sol-gel method. Chapter 3 provides an introduction to the catalyst development process with sol-gel, starting with the screening of non-noble metals (Ni, Fe, Co and Cu) for DRM activity. Among the metals activity screening, nickel (Ni) was identified as the most promising due to its high catalytic performance. Subsequently, a re-precipitation method was used to develop a catalyst with Ni nanoclusters encapsulated in a silica matrix. This catalyst demonstrated high CO2 and CH4 conversions over long-term DRM reactions, surpassing the performance of previously reported catalysts. Furthermore, the effect of varying the number of re-precipitation times on DRM activity was investigated, with results indicating that a single re-precipitation produced the most effective results in terms of catalytic activity and stability. In Chapter 4, controlled silica synthesis was conducted using a silica precursor with amine groups, specifically aimed at regulating the acid/base properties of inert silica. The silica was synthesized according to the molar ratio of structuring agent tetraethyl orthosilicate (TEOS) and functionalized precursor aminopropyl triethoxysilane (APTES) and impregnated with nickel to confirm DRM activity. A detailed characterization of the silica samples and Ni-impregnated catalysts was conducted, and the pore structure, acid/base characteristics, and nickel phase were confirmed to differ depending on the ratio of the two precursors. In Chapter 5 details the one-step preparation of a 2D nickel silicate molecular sieve known as Ni-DML, derived from a borosilicate MWW precursor (B-MWW(P)), for hydrogenation of CO2 to methane. Through various analytical techniques, including IR spectroscopy, UV-diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and 29Si MAS NMR spectroscopy, the presence of framework nickel in Ni-DML is confirmed. Furthermore, the formation of metallic nickel clusters via the extraction of framework nickel during high-temperature treatment is observed using scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDS). The resulting fine nickel clusters located on the surface of Ni-DML are identified as active sites for the hydrogenation of CO2 to CH4. In this study, we successfully developed non-noble catalysts for the dry reforming of methane and the hydrogenation of CO2. We anticipate that this study will offer valuable insights into the design of catalysts for CO2 conversion reactions.