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The use of CO2 as a chemical feedstock has been the focus of much research in recent years due to the promise for carbon neutral fuel storage in chemical bonds. Specifically, complexes of the type Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine and analogues thereof) have been studied for their ability to electrocatalytically reduce CO2 to CO. These catalysts are among the most active, selective, and robust homogeneous catalysts for CO2 reduction in the literature. Previous work has focused on mechanistic studies and determining the inductive effect of bipyridine functional groups on catalysis. The work presented in this dissertation focuses on the structural and biomimetic modification of these catalysts. In order to determine the optimal point of modification, the CO2 reduction capabilities of a series of Re(n,n'-dimethyl-bpy)(CO)3Cl (n = 3, 4, and 5) catalysts with a bpy modified at the 3, 4, and 5 positions with methyl substituents were assessed. A decreased catalytic current response in the n = 3 case can be explained by steric hindrance in the 3,3'- substituted catalyst disfavoring optimal charge transfer in the catalytic cycle. A series of rhenium catalysts were synthesized with bpy substituents amenable to common surface- and bio-conjugation techniques. Specifically, aminomethyl, groups were found to contribute to competent catalysts and were interrogated further. Interestingly, before the complex was coupled to amino acids, simple acylated amines (mimicking peptide bonds) on the 4- and 4,4'- positions of the bpy ligand were found to alter the mechanism by the rhenium catalyst operated electrochemically. Instead of a single metal site catalyzing the proton-dependent reduction of CO2 to CO and H2O, the bimetallic site, templated by hydrogen-bonding of the peptide bonds, reduces CO2 to CO and CO32- at potentials up to 240 mV more positive than previously studied catalysts of this type. The catalysts were then incorporated into amino acids and short peptides to investigate the advantageous effects of adding proton sources (tyrosine) and readily modifiable platform (peptides) on catalysis. In addition to proton sources, Lewis-acids can serve to increase the rates of catalysis; however insoluble carbonates prevent the reaction from being catalytic with respect to the Lewis acid. Macromolecules were used to bind the metal Lewis acids to prevent metal-carbonate formation over the course of the reaction. Furthermore, hydrogen-bonding could be utilized to template these heterobimetallic interactions and the preliminary work is presented.
Electrocatalytic reduction of carbon dioxide (CO2) is a profoundly challenging problem that is of interest, not only as a means of counteracting unsustainable emissions of CO2, but also as a method for the development of renewable fuels. Rhenium and manganese bipyridine tricarbonyl complexes are among the most active and robust catalysts for proton-coupled CO2 reduction to carbon monoxide (CO). X-ray Absorption Spectroscopy studies are reported to reveal the electronic ground state of the Re catalysts, which help explain origins for high selectivity for CO2 reduction over proton reduction. Stopped-flow mixing in tandem with rapid-scan IR spectroscopy is utilized to probe the direct reaction of the Re catalysts with CO2, observing, for the first time, the binding of CO2 to these catalysts. Manganese bipyridine catalysts are desirable, in comparison with their Re analogs, due to the earth-abundance of Mn and the ability for these catalysts to operate at lower overpotentials. One distinct difference between these Mn catalysts and their Re counterparts is a high tendency for dimerization after one-electron reduction, which contributes to the potential necessary to access their active state and to limiting their catalytic activity. Synthetic modification of the bipyridine ligand (by adding bulky mesityl groups) is used to completely eliminate dimerization for these Mn complexes, allowing the active catalyst to be generated at a 300 mV more positive potential than in typically Mn bipyridine complexes. CO2 reactivities in the presence of weak Brønsted acids, strong Brønsted acids, and Lewis acids have been explored in order to encourage this bulky Mn catalyst to reduce CO2 at low overpotentials. Mechanistic tools, including IR-spectroelectrochemistry, are described to gain insight into these unique catalytic processes. In order to further enhance stability and facilitate product separation, the use of metal-organic frameworks (MOFs) is explored as a means of anchoring molecular catalysts on a heterogeneous platform. A Mn bipyridine catalyst attached to a highly robust Zr(IV)-based MOF is used to enhance photochemical CO2 reduction. By utilizing an iron porphyrin catalyst, anchored into the linkers of a MOF thin film, we demonstrate, in a proof of principle, electrochemical CO2 reduction by this heterogenized molecular catalyst.
The recycling of atmospheric molecules for use as fuels and chemicals is a goal which can only be achieved through a deeper understanding of catalytic processes, particularly electrocatalysis whereby redox transformations can be interfaced with solar or nuclear energy input. Carbon dioxide is a prototypical small molecule in many regards since it is chemically inert. In addition, because of the likely role of carbon dioxide in global temperature cycles, it will be imperative in the future to regulate the output from industrial processes. The purpose of this book is to present a unified discussion of the carbon dioxide chemistry which is necessary for the understanding and design of electrochemically-driven processes for the reduction of carbon dioxide and to provide an impetus for the further development of electrocatalytic carbon dioxide chemistry.
Efficient conversion of carbon dioxide, a major greenhouse gas, into liquid fuels and commodity chemicals will have a huge impact on the energy economy and environmental sustainability. Thermodynamic stability and kinetic inertness of the CO2 molecule pose a challenge for its conversion into useful products. Development of an efficient and selective catalyst for CO2 reduction is thus important. Rhenium and manganese-based complexes have been studied for their ability to catalyze the electrochemical reduction of CO2. Rhenium and manganese-based catalysts containing phenanthroline and substituted phenanthroline have been studied for their activity towards the reduction of CO2 to CO. Manganese carbonyl complexes containing the diimine ligand such as diphenylphenanthroline, dimethyldiphenylphenanthroline, and tetramethylphenanthroline catalyze the reduction of CO2 even in the absence of external proton source. Presence of external proton source such as water is a must for most manganese-based CO2 redcution (sic) electrocatalysts to complete the catalytic cycle. In chapter four, a new series of manganese-based catalysts containing phenylenediammine ligands is discussed. The generation of spectacular CO gas bubbles from the electrochemical reduction of CO2 was observed in the bulk electrolysis experiments. Effects of electron-donating and electron-withdrawing substituents in the diamine ligands were investigated. The role of acid on the catalytic rates was also studied. Chapter five presents the novel design, synthesis, and electrocatalytic studies of rhenium and manganese complexes containing [selecting]-donation and the steric feature resulting from the various substituents in the P and N ligands. Finally, few dinuclear and trinuclear manganese complexes were tested for their ability to catalyze the reduction of CO2. Di- and trinuclear systems investigated here did not display cooperative binding and the resulting increased catalytic current is due to the presence of multiple metal centers as active sites.
As global anthropogenic carbon dioxide (CO2) emissions continue to rise, there is a need not only to reduce production of CO2, but also an opportunity to use it as a substrate for value-added products. One viable solution is to reduce CO2 in the two proton, two electron coupled process to produce carbon monoxide (CO), which can in turn be utilized to recreate hydrocarbon fuels. One of the most active and selective molecular electrocatalysts for the reduction of CO2 to CO is Re(2,2′-bipyridine)(CO)3Cl (Re-bpy) and derivatives thereof. The best method to study electrocatalysts is cyclic voltammetry (CV), which affords both kinetic and thermodynamic information about catalysis. CV is the main technique used to characterize substituent, labile ligand, and Brønsted acid effects on Re-bpy based catalysts, which show increased activity with electron donating 4,4′-substituents and moderate Brønsted acids such as 2,2,2-trifluoroethanol and phenol. The Re-bpy catalyst motif is also extended to Group 6 Mo and W metals, which are not as active as their Group 7 counterparts due to high overpotentials and product poisoning of the catalyst. To build a fundamental understanding of how molecular catalysts interact with surfaces, Re-bpy derivatives were bound to Au substrates and studied by sum frequency generation spectroscopy (SFG). While cyano substituents deactivated the molecular catalyst, they adsorbed onto Au surfaces, allowing for determination of molecular orientation on the surface as well as characterization of surface-molecule vibratinal communication. Thiol groups were subsequently employed on the bpy ligand for both Re and Mn catalysts to create a covalent attachment to Au surfaces. These groups did not deactivate the molecular catalysts and reproducibly create monolayers on Au surfaces. Further studies are needed in order to fully understand the implications of surface bound Re-bpy based catalysis as well as apply the design principles learned from Re-bpy systems to future molecular electrocatalysts.
The electrocatalytic reduction of carbon dioxide (CO2) to carbon monoxide (CO) is explored for both rhenium and manganese complexes. Electrochemistry, X-ray crystallography, Infrared spectroelectrochemistry, and stopped-flow kinetics are employed in order to identify catalysts and probe their mechanism and selectivity. Two catalysts in particular, Re(bipy-tBu)(CO3(L) and Mn(bipy-tBu)(CO3L (where bipy-tBu = 4,4'-di-tert-butyl-2,2'-bipyridine and L = Cl−, Br− or (MeCN)(OTf)−), were studied extensively and displayed high activity, Faradaic efficiency, and selectivity for the reduction of CO2 to CO. The Re-Cl catalyst exhibits a turnover frequency of>200 s−1, one of the fastest reported rates for a catalyst with appreciable turnover number. The Mn catalysts, when Brönsted acid sources are added to the electrochemical solution, exhibit current densities rivaling those of the Re-Cl catalyst. Amazingly, these catalysts showed high selectivity for CO2 in both dry solvents and those with significant amounts of Brönsted acid added. Stopped-flow UV-Vis kinetics experiments showed that the reaction of the active form of the catalysts, [M(bipy-tBu)(CO)3]−1, is ca 50 times faster with CO2 than they do with protons. Stopped-flow IR kinetics experiments comparing the reactions [Re(bipy-tBu)(CO)3]−1 with CO2 and [Re(bipy)(CO)3]−1 with CO2 shows, that at equal CO2 concentrations, the bipy-tBu analog reacts ten times faster than the bipy analog. CO2 also appears to react with both complexes via a concerted, two-electron oxidative addition of CO2 to the metal center. The heterogenization of these catalysts was also explored with limited success. Intercalation, covalent bonds to gold, and covalent bonds to p-Si were all demonstrated, but none displayed activity towards the reduction of CO2. Future experiments are suggested to solve this issue.
Porphyrins play a vital role in many biological functions including oxygen transport, electron transfer and catalyzing the incorporation of oxygen into other molecules. This current survey discusses the use of modern physical techniques to probe porphyrin structure and function. The authors review the data available through a particular technique and show what can be learned therefrom about the (electronic) structure and function of biologically important porphyrins. The techniques include magnetic circular dichroism, X-ray absorption fine structure (EXAFS) and Mössbauer spectroscopies. All contributors are well known in their respective fields, enjoying world-wide reputation.
A guide to the effective catalysts and latest advances in CO2 conversion in chemicals and fuels Carbon dioxide hydrogenation is one of the most promising and economic techniques to utilize CO2 emissions to produce value-added chemicals. With contributions from an international team of experts on the topic, CO2 Hydrogenation Catalysis offers a comprehensive review of the most recent developments in the catalytic hydrogenation of carbon dioxide to formic acid/formate, methanol, methane, and C2+ products. The book explores the electroreduction of carbon dioxide and contains an overview on hydrogen production from formic acid and methanol. With a practical review of the advances and challenges in future CO2 hydrogenation research, the book provides an important guide for researchers in academia and industry working in the field of catalysis, organometallic chemistry, green and sustainable chemistry, as well as energy conversion and storage. This important book: Offers a unique review of effective catalysts and the latest advances in CO2 conversion Explores how to utilize CO2 emissions to produce value-added chemicals and fuels such as methanol, olefins, gasoline, aromatics Includes the latest research in homogeneous and heterogeneous catalysis as well as electrocatalysis Highlights advances and challenges for future investigation Written for chemists, catalytic chemists, electrochemists, chemists in industry, and chemical engineers, CO2 Hydrogenation Catalysis offers a comprehensive resource to understanding how CO2 emissions can create value-added chemicals.
This book provides a detailed description of metal-complex functionalized carbon allotrope forms, including classic (such as graphite), rare (such as M- or T-carbon), and nanoforms (such as carbon nanotubes, nanodiamonds, etc.). Filling a void in the nanotechnology literature, the book presents chapters generalizing the synthesis, structure, properties, and applications of all known carbon allotropes. Metal-complex composites of carbons are described, along with several examples of their preparation and characterization, soluble metal-complex carbon composites, cost-benefit data, metal complexes as precursors of carbon allotropes, and applications. A lab manual on the synthesis and characterization of carbon allotropes and their metal-complex composites is included. Provides a complete description of all carbon allotropes, both classic and rare, as well as carbon nanostructures and their metal-complex composites; Contains a laboratory manual of experiments on the synthesis and characterization of metal-complex carbon composites; Discusses applications in diverse fields, such as catalysis on supporting materials, water treatment, sensors, drug delivery, and devices.
Nanodroplets, the basis of complex and advanced nanostructures such as quantum rings, quantum dots and quantum dot clusters for future electronic and optoelectronic materials and devices, have attracted the interdisciplinary interest of chemists, physicists and engineers. This book combines experimental and theoretical analyses of nanosized droplets which reveal many attractive properties. Coverage includes nanodroplet synthesis, structure, unique behaviors and their nanofabrication, including chapters on focused ion beam, atomic force microscopy, molecular beam epitaxy and the "vapor-liquid- solid" route. Particular emphasis is given to the behavior of metallic nanodroplets, water nanodroplets and nanodroplets in polymer and metamaterial nanocomposites. The contributions of leading scientists and their research groups will provide readers with deeper insight into the chemical and physical mechanisms, properties, and potential applications of various nanodroplets.