Optimizing Adsorption Energies of Reaction Products and Intermediates on Metal/Metal Oxide Catalysts to Achieve High Activity and Tunable Selectivity in Solid-Gas Phase Reactions

The solid-gas phase reactions, such as CO2 hydrogenation, the Fischer-Tropsh process, CO oxidation, and ammonia synthesis are one of the heterogenous catalytic reactions which have been emerged as a critical process in chemical and energy industries for a sustainable future. It needs a catalyst to change the chemical reaction pathway and enables the reaction to happen under milder condition. Metal catalysts supported on the metal oxide or unsupported metal crystallites have shown various catalytic behavior in different catalytic reactions. It can be divided into categories, catalysts with small sizes, such as single atoms, nanoclusters and nanoparticles, and also bulk catalysts. Bulk metal oxides or mixed oxides are widely used as heterogeneous catalysts in the current industry due to their ability for large-scale synthesis. However, the complex surface structures of metal oxides and mixed metal oxides, such as various oxidation states, oxygen vacancies, chemical nature of the active site (acid or base), are difficult to be characterized and developed by empirical methods. With the development of nanoscience and nanomaterials, nano-sized catalysts are well-studied. However, it lacks large-scale synthetic methods for practical use. To design catalysts for heterogeneous reactions, the Sabatier’s principle is used that the relationship of the catalytic activity and adsorption energies of reactants, products or intermediates is a volcano curve. To fit the volcano curve and predict the most optimal catalyst composition. The reaction mechanisms need to be studied and understood to determine the rate-limiting step. Based on that, better design of process and catalyst composition can be developed for efficiently producing a desirable product. In Chapter 2, the rate-limiting step for producing methanol in CO2 hydrogenation reaction under ambient pressure is to desorb methanol from the indium oxide surface. Therefore, we’ve designed a two-temperature process to use a photothermal effect to desorb methanol by quickly flipping the reaction temperature to a higher set temperature. In Chapter 3, the key to achieving high CO selectivity in CO2 hydrogenation reaction is to control the binding strength of reaction intermediate *CO. A one-step synthesis method, glycine-nitrate combustion was developed to synthesize a rhodium-based catalyst supported on high entropy oxide. The selectivity of this reaction can be tuned by changing the composition of elements in metal oxide support. In Chapter 4, the rate-limiting step of the ammonia decomposition reaction is desorbing N2. Following the same combustion synthesis method in Chapter 3, we used empirical experiments to determine the most optimal composition in bulk CoMo bicatalyst when Co/Mo molar ratio is at 6:1. And the same ammonia decomposition catalytic activity can be achieved compared to the noble ruthenium-based catalyst, just by increasing the mass of catalyst, which is accessible here. One variable to be tuned in the catalyst composition limits the enhancement of catalytic activity. However, multiple variables to be tuned at the same time is impractical to analyze data and conclude it by human-being. With the simple synthesis method that we’ve developed for synthesizing bulk catalysts. It’s promising and practical to provide training data for artificial intelligence to optimize the composition of earth-abundant catalysts to replace the noble metal catalyst in the future in the solid-gas phase reactions.