Browsing by Subject "Catalysis"

The chemical industry depends on heterogeneous thermocatalytic processes to satisfy the ever-increasing demand for fuels and fertilizers. High temperatures and high pressures are generally required to accelerate chemical transformations and operate practical rates. These harsh conditions, however, lead to huge energy consumption and other side effects, such as the lifetime of catalysts and parasitic formation of by-products. Light is used as an alternative energy input to drive chemical reactions on semiconducting photocatalysts, but the slow reaction rates and insufficient control of product selectivity hinder wide adaptation of photocatalysis. Plasmonic metal nanoparticles have been recently proposed as a new type of catalysts with photoactivities. As already been widely used in thermocatalytic reactions, the strong light absorption capability from excitation of localized surface plasmon resonance (LSPR) of plasmonic catalysts could combine light and thermal energy to work cooperatively in enhancing rates of chemical reactions. This dissertation summarizes our efforts aiming to design plasmonic catalysts with high efficiency and high product selectivity. The catalytic properties of synthesized rhodium (Rh) and ruthenium (Ru) catalysts are investigated in two model reactions, carbon dioxide (CO2) hydrogenation and ammonia (NH3) synthesis.

Chapter 2 describes the development of slow-injection polyol methods to synthesize monodispersed Rh nanocubes with tunable size and resonant energy. The wide size tunability of slow-injection methods allows for the red-shift of resonant wavelength of small Rh nanostructures, which are in the deep ultraviolet (UV) region, to more accessible and practical near-UV and visible regions by increasing the size of Rh nanocubes.

Chapter 3 focuses on the product selectivity of plasmonic Rh nanocubes in CO2 hydrogenation. Rh nanocubes supported on aluminum oxide (Al2O3) nanoparticles equally produce methane (CH4) and carbon monoxide (CO) in pure thermal conditions. Under illumination of UV and blue light, the rate of CH4 production is significantly enhanced, and almost exclusive CH4 production is observed. This photo-selectivity can be attributed to selective activation of specific reaction intermediate by photo-generated hot electrons among competing reaction pathways.

Chapter 4 describes the effects of catalyst support and morphology of plasmonic Rh nanostructures on the catalytic activities in plasmon-enhanced CO2 hydrogenation. Significant improvements of reaction rates are observed by switching to reducible titanium oxide (TiO2) support and shrinking the size of Rh nanostructures. The enhancement of reaction rates by light can be partially attributed to local heating of catalyst bed.

Chapter 5 focuses on the catalytic activities of Ru-based catalysts for NH3 synthesis under light illumination. Photo-enhanced NH3 production, which highly depends on the size, support, and promoter of catalysts, is observed.

Chapter 6 discusses conclusion and future directions of this project. Molecular level insights of plasmon-enhanced catalysis are highly desired for both fundamental research and practical applications.

Cationic gold(I) complexes have recently contributed to significant developments in homogenous catalysis. Such complexes have been praised as highly effective catalysts for the functionalization of C-C multiple bonds, leading to research on cationic gold-catalysts developing at an aggressive pace. Despite the progress being made surrounding gold(I)-catalysis, there are still many gaps in our fundamental understanding of the key intermediate complexes and their reactivity in these transformations, exemplified by the often evoked gold alpha-oxo carbene species. While there are existing computational studies suggesting the instability of gold alpha-oxo carbene species, there lacks any experimental evidence to support the stability and reactivity of alternate key intermediate species, such as gold alpha-oxo carbenoid species and gold N-alkenoxypyridinium/sulfonium complexes. Herein, we address the issues surrounding the formation of gold alpha-oxo carbene species in reported literature. We report the synthesis and reactivity of gold pyridinium alpha-oxo carbenoid complexes, gold sulfonium alpha-oxo carbenoid complexes, and gold alpha,alpha-dioxo carbenoid complexes. We then report the direct observation of a gold N-alkenoxysulfonium complex in a gold-catalyzed alkynyl sulfoxide rearrangement reaction and the synthesis of a series of gold-oxide compounds. Together, this research addresses the gaps in knowledge surrounding key intermediate species in gold(I)-catalyzed transformations.

Cationic gold carbene complexes have attracted significant attention, being postulated as intermediates in a range of gold-catalyzed transformations. Regardless of the remarkable progress in the gold (I) catalysis, our fundamental understanding on the key intermediate species and the subsequent reactivity, and mechanistic insight is deficient. This is mainly due to the lack of proper model system with sufficient reactivity, as the majority of known gold carbene complexes are heteroatom stabilized or sterically hindered, and because of a dearth of direct intermediate observations in catalytic systems. Lewis acid mediated leaving group abstraction from a neutral gold precursor provides a convenient method for the generation of rare examples of reactive gold carbene species in high yield and purity, addressing the issue with isolation of such transient species as well as allowing in situ spectroscopic analysis. Subsequent trapping experiment with nucleophiles provides kinetic information about relevant catalytic transformations, and the -ionization strategy is further extended toward generation of transient  -cationic propyl gold species for studying gold to alkene carbene transfer reaction.

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.

Amines are ubiquitous in medicinal compounds and essential to human health. We have addressed the limitations of amine synthesis by significantly expanding the utility of 2-azaallyl anions and 2-azaallyl anion-like reagents in the production of imine-protected amines (Scheme 1).

Chapter Two

We have developed the first intermolecular, stereoselective reactions that directly generate 1,3-amino alcohols bearing three contiguous stereogenic centers through a coupling of 2-azaallyl anions with substituted epoxides. This method tolerates an expansive range of substrates, including variations on the substitution patterns of the epoxide and the azaallyl anion nucleophile. Both cis- and trans-1,2-disubstituted epoxides, electron-rich and electron-poor, readily react. Alkyl groups, including heteroatom containing ones, α-branching, and aromatic groups are tolerated within styrenyl epoxides. Terminal epoxides are also effective coupling partners, providing wide access to a range of primary, secondary, and tertiary alcohol products. Several azaallyl anions, including those with aryl, heteroaryl, vinyl, and alkynyl substituents, are excellent partners for transformations with trans-disubstituted epoxides. Importantly, these products can be further functionalized through deprotection and Mitsunobu reaction to access highly-substituted azetidines.

Chapter Three

Fluorine is known to enhance the pharmacology of compounds in several important ways, including improving pharmokinetics, lipophilicity, cell permeability, and metabolic rates. To improve access to these crucial compounds, we have developed a method for chiral α-CF3 amines through generation of an α-CF3-substituted azaallyl–Ag intermediate that is coupled with aryl iodides through a Pd-catalyzed process. We were able to expand this method to over twenty examples of electron-rich and electron-poor aryl iodides with various substitution patterns as well as one heteroaromatic compound. These products are immensely useful and we have explored their utility by developing a concise, 3-step synthesis of an HDAC6 inhibitor. Through spectroscopic studies, we were also able to elucidate the structure of the intermediate in this reaction, demonstrating that the likely species is an azaallyl–Ag intermediate and the formation of this intermediate is likely catalyzed by XPhos.

Chapter Four

Chiral α-amino boronates are found in medicinal compounds with great significance to human health, such as bortezomib and ixazomib, proteasome inhibitors used for treatment of cancer. We have begun to develop a method for hydroboration of 2-azadienes to afford α-amino boronates. Utilizing our 2-azadiene substrate, we can access these moieties regioselectively, enantioselectively, and in high yield. Ongoing work will expand substrate scope through broadening the substitution patterns of the azadiene.

Catalysts are essential in producing chemicals and materials that support the global economy, but the standard catalytic process consumes vast resources to meet this demand. It has now been shown that significant portions of the required energy for these reactions can be eliminated by replacing or supplementing expensive heat and pressure with free or inexpensive light. Localized surface plasmon resonances enable the coupling of light into catalytic reactions to improve reaction rates and control reaction specificity through a process called plasmon enhanced catalysis. As an effect, plasmonic enhanced catalysis can lead to reaction rate enhancements of over 500 percent when compared to their thermal catalytic counterparts, alter the catalyst selectivity, and lead to increased catalyst lifetimes. This enables the production of chemicals with decreased demand to thermal energy and pressure. In turn, this decreases the capital cost for reactors and equipment, permitting a new optimization in the scale of chemical production facilities and energy demands. Currently, as an example, the catalytic production of ammonia requires 1-2 percent of global energy annually and is limited to large scale performance by highly industrialized nations.77 The use of plasmonic enhanced catalysis can drive down both capital and operating costs, while decreasing the environmental impact and expanding access to these vital chemical reactions. Plasmonic enhanced catalysis is a growing and thriving field due to its potential to alter current operations and downstream processing for various industries. Further growth of the field into far reaching applications is dependent on the answer to a seemingly basic question: What are the mechanisms driving the observed reaction rate improvements under light at the chemical active site? We and others have demonstrated that excited plasmonic nanoparticles generate heat and enable electron-transfer to improve catalytic processes. This dissertation details our recent work to expand our understanding of relevant plasmonic enhancement mechanisms. In more detail, we will discuss our works on untangling the nature and mechanisms of plasmonic enhanced catalysis by specifically focusing on the catalytic active site. This work is formulated by six chapters discussing various plasmonic catalytic reactions and associated systems, which share a common set of goals to advance the understanding of the mechanisms and implications of plasmonic catalysts while encouraging the use of well formulated catalytic systems for high-impact applications. Plasmonic catalysts have the potential to reshape the production of specialty and commodity chemicals by supporting new reaction pathways and allowing for decreased operating costs while increasing reaction efficiency. The above goals are obtained through a focus on the individual roles and interactions of thermal and electron-transfer effects in plasmonic catalysts. Upon light illumination, a plasmonic catalyst rapidly undergoes photothermal heating and can transfer high-energy electrons to local absorbed molecules. These two processes amplify reaction rates individually, and on a plasmonic catalyst are likely to be codependent. In this work, a variety of methods will be used to analyze these two effects to increase the understanding of the relationship between thermal and electron transfer processes. This dissertation contains introduction material (chapter 1), five chapters concerning our investigations to characterize active-site behavior in plasmonic enhanced catalysis, and three appendixes with further supporting information and detailed experimentation. The primary objectives of the second and third chapter focus on improving the characterization of thermal processes at the active site occurring under plasmonic excitation and how these methods can be used to better understand the non-thermal electron transfer process. In the second chapter, we use both on- and off-resonant plasmonic excitation to demonstrate the presence of nanoscale thermal gradients within a few nanometers of the surface of the plasmonic catalyst particle. These gradients underly a significant increase in the active site temperature over the bulk catalyst temperature. The subsequent chapter uses the temperature dependent reaction kinetics to accurately characterize this temperature difference between the active site and the catalyst bulk temperature. This chapter also outlines a highly relevant possible application for plasmonic catalysis and includes a mechanistic study into the cause of non-thermal electron transport in this system. The third chapter also demonstrates that plasmonic enhancement of chemical reactions can increase catalyst lifetimes. With the lessons learned from the previous chapters, chapter four uses active-site engineering to synthesize four catalysts with similar plasmonic metal nanoparticles, but with different active sites, to better understand their electron transfer processes. This work compares multiple methods to describe the relative contributions from thermal and non-thermal mechanisms in the plasmonically enhanced catalytic reaction, with a specific focus on the gold catalyzed oxidation of carbon monoxide in a hydrogen rich system. Here, we suggest a modified pathway of non-thermal electron transfer by observing the apparent electron transfer from the plasmonic gold to oxygen absorbed on the support or at the metal-support interface. Chapters five and six focus on alternative methods to understand plasmonic catalysis. Chapter five uses a plasmonic rhodium catalyst for the release of hydrogen from a liquid organic hydrogen carrier. This reaction, normally requiring high temperatures, is shown to be improved significantly with light incident on the plasmonic catalyst. By using a liquid phase reaction with increased thermal conductivity relative to the gas-phase, rapid stirring, and thermally regulated experimentation, we suggest the presence of an electron transfer to drive the reaction directly. Chapter six explores an alternative method to modify the LSPR energy in a plasmonic catalyst without altering the catalyst itself, which is performed by magnetic field modulation. The plasmonic gold nanoparticles analyzed in this work are non-magnetic, yet the oscillating electrons which form the LSPR’s can interact with a magnetic field. This project utilizes a magneto-spectrometer to characterize the plasmon absorption of gold nanorods and nanospheres in a magnetic field of varied strength. Results demonstrate the existence of an interaction between the LSPR energy and the magnetic field, which may be due to Lorentz forces acting on the LSPR. The modification of the LSPR energy, without altering the catalyst permanently, may result in new experiments probing the importance of the LSPR energy on hot electron transfer and enable further control of reaction specificity in plasmonically enhanced catalytic reactions. This dissertation outlines several notable recent findings in plasmonic catalysis with a focus on high-impact applications. Plasmonic catalysts open new avenues in catalysis, enabling the production of a wide range of chemicals with a reduced energy input and cost. These observations provide new insights for expanded applications of plasmon enhanced catalysis to decrease the energetic requirements of catalysis on a small to industrial scale. Future work in this field will be to determine suitable reactions for scaling to industrial proportions. We hope the mechanistic work provided in this document provides strong evidence for the rapid utilization of these processes in many global industries.

Nano-structures were investigated for the advancement of energy conversion technology because of their enhanced catalytic, thermal, and physiochemical interfacial properties and increased solar absorption. Hydrogen is a widely investigated and proven fuel and energy carrier for promising "green" technologies such as fuel cells. Difficulties involving storage, transport, and availability remain challenges that inhibit the widespread use of hydrogen fuel. For these reasons, in-situ hydrogen production has been at the forefront of research in the renewable and sustainable energy field. A common approach for hydrogen generation is the reforming of alcoholic and hydrocarbon fuels from fossil and renewable sources to a hydrogen-rich gas mixture.

Unfortunately, an intrinsic byproduct of any fuel reforming reaction is toxic and highly reactive CO, which has to be removed before the hydrogen gas can be used in fuel cells or delicate chemical processes. In this work, Au/alpha-Fe2O3 catalyst was synthesized using a modified co-precipitation method to generate an inverse catalyst model. The effects of introducing CO2 and H2O during preferential oxidation (PROX) of CO were investigated. For realistic conditions of (bio-)fuel reforming, 24% CO2 and 10% water the highest document conversion, 99.85% was achieved. The mechanism for PROX is not known definitively, however, current literature believes the gold particle size is the key. In contrast, we emphasize the tremendous role of the support particle size. A particle size study was performed to have in depth analysis of the catalysts morphology during synthesis. With this study we were also able to modify how the catalyst was made to further reduce the particle size of the support material leading to ~99.9% conversion. We also showed that the resulting PROX output gas could power a PEM fuel cell with only a 4% drop in power without poisoning the membrane electrode assembly.

The second major aim of this study is to develop an energy-efficient technology that fuses photothermal catalysis and plasmonic phenomena. Although current literature has claimed that the coupling of these technologies is impossible, here we demonstrate the fabrication of reaction cells for plasmon-induced photo-catalytic hydrogen production. The localized nature of the plasmon resonance allows the entire system to remain at ambient temperatures while a high-temperature methanol reformation reaction occurs at the plasmonic sites. Employing a nanostructured plasmonic substrate, we have successfully achieved sufficient thermal excitement (via localized surface plasmon resonance (LSPR)) to facilitate a heterogeneous chemical reaction. The experimental tests demonstrate that hydrogen gas can indeed be generated in a cold reactor, which has never been done before. Additionally, the proposed method has the highest solar absorption out of several variations and significantly reduces the cost, while increasing the efficiency of solar fuels.