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A major contribution to global warming is CO2 emitted from the combustion of fossil fuels. Electrochemical processes can help to mitigate the elevated CO2 emissions through either the conversion of CO2 into value-added chemicals or the replacement of fossil fuels with clean fuels such as hydrogen produced from water oxidation. The present dissertation focuses on the mechanistic aspects of electrochemical processes. Electrochemical water oxidation is hindered by the low efficiency of oxygen evolution reaction (OER) at the anode whereas electrochemical reduction of CO2 (ERCO2) is hampered by high overpotentials and poor product selectivity. In this dissertation, we studied the catalytic activity of transition metal-based catalysts, including FeNi spinels, metal-oxide/copper, and d metal cyclam complexes, for both OER and ERCO2 using the density functional theory (DFT) computational approach.We report a combined effort of fabricating FeNi oxide catalysts and identifying the active component of the catalyst for OER. Our collaborators at the University of California, Santa Cruze fabricated a series of FeNi spinels-based materials including Ni(OH)Fe2O4(Cl), Ni(OH)Fe2O4, Fe(OH)Fe2O4(Cl), Fe(OH)Fe2O4, Ni(OH)O(Cl), Ni(OH)O and some show exceptional activity for OER. Combined experimental characterization and computational mechanistic study based on the computational hydrogen electrode (CHE) model revealed that Ni(OH)Fe2O4(Cl) is the active ensemble for exceptional OER performance. We also investigated CO2 reduction to C1 products at the metal-oxide/copper interfaces ((MO)4/Cu(100), M = Fe, Co and Ni) based on the CHE model. The effect of tuning metal-oxide/copper interfaces on product selectivity and limiting potential was clearly demonstrated. This study showed that the catalyst/electrode interface and solvent can be regulated to optimize product selectivity and lower the limiting potential for ERCO2. Applied potential affects the stability of species on the surface of the electrode. The proton-coupled electron transfer (PCET) equilibrium assumed in the CHE model does not capture the change in free energy under the influence of the applied potential. In contrast, the constant electrode potential (CEP) model captures changes in free energy due to applied potential, we applied the CEP model to ERCO2 and OER on (MO)4/Cu(100) and compared the results with those from the CHE model. The results demonstrate that the CHE and the CEP models predict different limiting potentials and product selectivity for ERCO2, but they predict similar limiting potentials for OER. The results demonstrate the importance of accounting for the applied potential effect in the study of more complex multi-step electrochemical processes. We also studied transition metal-based homogeneous catalysts for ERCO2. We examined the performance of transition metal(M) - cyclam(L) complexes as molecular catalysts for the reduction of CO2 to HCOO- and CO, focusing on the effect of changing the metal ions in cyclam on product selectivity (either HCOO- or CO), limiting potential and competitive hydrogen evolution reaction. Our results show that among the complexes, [LNi]2+ and [LPd]2+ can catalyze CO2 reduction to CO, and [LMo]2+ and [LW]3+ can reduce CO2 to HCOO-. Notably, [LMo]2+, [LW]3+, [LW]2+ and [LCo]2+ have a limiting potential less negative than -1.6 V and are based on earth-abundant elements, making them attractive for practical application. In summary, the dissertation demonstrates high-performance catalysts can be designed from earth-abundant transition metals for electrochemical processes that would alleviate the high CO2 level in the environment. On the other hand, completely reversing the increasing trend of CO2 level in the atmosphere requires a collective human effort.

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Characterization of a material’s surface, structural and physical properties is essential to understand its chemical reactivity. Control over these properties helps tailor a material to a particular application of interest. The research presented in this dissertation focuses on characterizing a synthetic method for carbon nanomaterials and the determination of structural properties of manganese oxides that contribute to its reactivity for environmental chemistry. In particular, one research effort was focused on the tuning of synthetic parameters towards the formation of carbon nanomaterials from gaseous methane and gaseous mixtures containing various mixtures of methane, argon and hydrogen. In a second research effort, photochemical and water oxidation chemistry were performed on the manganese oxide, birnessite, to aid in the remediation of arsenic from the environment and provide more options for alternative energy catalysts, respectively. With regard to the synthesis of novel carbonaceous materials, the irradiation of gaseous methane with ultrashort pulse laser irradiation showed the production of carbon nanospheres. Products were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), ultraviolet (UV) Raman spectroscopy, and infrared spectroscopy. Increasing the pressure of methane from 6.7 to 133.3 kPa showed an increase in the median diameter of the spheres from ~500 nm to 85 nm. Particles with non-spherical morphologies were observed by TEM at pressures of 101.3 kPa and higher. UV Raman spectroscopy revealed that the nanospheres were composed of sp2 and sp3 hybridized carbon atoms, based on the presence of the carbon D and T peaks. A 30% hydrogen content was determined from the red shift of the G peak and the presence of a high fluorescence background. Upon extending this work to mixtures of methane, argon, and hydrogen it was found that carbon nanomaterials with varying composition and morphology could be obtained. Upon mixing methane with other gases, the yield significantly dropped, causing flow conditions to be investigated as a method to increase product yield. Raman spectra of the product resulting from the irradiation of methane and argon indicated that increasing the argon content above 97% produced nanomaterial composed of hydrogenated amorphous carbon. In a second research effort, the effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (kobs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h−1 , respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process. Further research on the triclinic form of birnessite focused on its activation for water oxidation. Experiments were performed by converting triclinic birnessite to hexagonal birnessite in pH 3, 5, and 7 DI water with stirring for 18 hrs. Once the conversion was complete, the solid samples were characterized with TEM and x-ray photoelectron spectroscopy (XPS). The resulting hexagonal birnessites from experiment at pH 3, 5, and 7 possessed the same particle morphology and average surface oxidation states within 1% of each other. This observation supported the claim that upon transformation, Mn(III) within the sheet of triclinic birnessite migrated into the interlayer region of the resulting hexagonal birnessite. Furthermore, the migration of Mn(III) into the interlayer and formation of the hexagonal birnessite led to an increased chemical reactivity for water oxidation compared to the bulk. Electrochemical studies showed that the overpotential for water oxidation associated with the pH 3, 5, and 7 samples was 490, 510, and 570 mV, respectively. In another set of experiments, ceric ammonium nitrate was used to test birnessite for water oxidation reactivity. These experiments showed that the pH 3 birnessite produced the most O2 of all the samples, 8.5 mmol O2/mol Mn, which was ~6 times more than hexagonal birnessite which did not undergo post-synthesis exposure to low pH conditions. / Chemistry

Metal-organic frameworks (MOFs) are a class of hybrid materials that offer a promising platform for a range of catalytic reactions. Due to their complex structure, MOFs offer unique opportunities to serve as novel catalysts, or as host to improve the properties of previously studied species. However, while other catalytic approaches have been studied for many decades, the recency of their discovery means that significant work is still needed to develop MOFs as a viable option for large scale application. Herein, we aimed to advance the field of MOFs as both novel catalysts, and as host platforms for other catalytic species. To this end, we studied synthetic pathways to produce favorable MOF properties such as higher porosity and active site concentration through introduction of defects and macromorphological control, as well as utilization of molecular catalysts imbedded in the MOF structure for multicomponent, light driven reactivity. Chapter 1 introduces the history MOFs and the pursuit of the stable structures commonly associated with MOF chemistry. The synthesis process for zirconium-based MOFs will be discussed, with specific attention given to the modulated synthesis process which can harnessed to change MOF properties and improve catalysis. Two specific reactions will be introduced which serve as a basis for study in this work. First, the hydrolysis of organophosphate nerve agents by MOFs acting as novel catalysts will be introduced. The mechanism of reaction, as well as previous work in this field will be discussed. Finally, water oxidation as part of artificial photosynthesis through incorporated molecular catalysts will be introduced. Chapter 2 presents a modulator screening study on a zirconium-based MOF, UiO-66. One of the most commonly studied MOFs, UiO-66 provides an excellent platform for synthetic modulation. Particle size and defect level were measured of 26 synthetic variations and synthetic conditions were found to isolate changes in defect level and particle size, which typically change coincident with each other. Hydrolysis of the organophosphate compound dimethyl 4-nitrophenylphosphate (DMNP) was used to study the impact of particle size and defect level on reactivity. The reaction was found to be surface limited, even at high levels of missing linker defects. In Chapter 3, the macromorphology of three zirconium-based MOFs were tuned through synthesis modification. MOF powders and xerogels were prepared and characterized to highlight the desirable properties obtained through the gelation process. The materials were compared in the hydrolysis of DMNP and significant enhancement was observed for UiO-66 and NU-1000 xerogels. This was largely attributed to the introduction of mesoporosity and nanocrystalline particle sizes, which significantly increase the number of reactive sites easily accessible for catalysis. In Chapter 4 the authors examine MOFs as a host for molecular catalysts for use in photoelectrochemical water oxidation. A ruthenium-based catalyst [Ru(tpy)(dcbpy)]2+ was incorporated into UiO-67 through a mixed linker synthesis and grown on a WO3 substrate (Ru-UiO-67/WO3). Previous work from our group demonstrated Ru-UiO-67 retained the catalytic activity as the molecular species, while improving the recyclability of the material. In this work, addition of WO3 as a light harvester allowed for the reaction to be driven at a photoelectrochemical underpotential, a first for MOF-based water oxidation. Finally, Chapter 5 offers a perspective of the field of MOF-based artificial photosynthesis. Particular attention is given to issues of diffusion, selectivity, stability, and moving towards integration of multiple components rather than the study of half-reactions. / Doctor of Philosophy / Catalysts are a key component of chemistry that has a major impact on everyday life. From biological examples to industrial settings, catalysts are used to facilitate chemical conversions to new products and compounds. Because of the high demand, development of new catalysts with improved reactivity is a significant scientific challenge. A new class of materials known as metal-organic frameworks (MOFs) have been recently shown to acts as new catalysts or improve the properties of existing catalysts. Herein, we discuss the use of MOFs as catalysts for both development of new catalysts and improving known species. MOF-based catalysts have been used in a range of reactions from destruction of toxic chemical weapons to the production of renewable energy through artificial photosynthesis. This work is intended to highlight the potential for MOF-based catalysts and the next steps to further realize their potential.

Gegenstand der vorliegenden Dissertation ist die Untersuchung von Ca-dotierten PrMnO3 (PCMO) als Katalysator für die (photo)elektrochemische Wasseroxidation. Im Fokus der Untersuchungen stehen die folgenden elementaren Schritte des Gesamtprozesses: i) Die optische Absorption in PCMO wird zunächst als Funktion der Ca-Dotierung und der Temperatur untersucht mit dem Ziel, den Einfluß von Korrelationseffekten auf die optischen Eigenschaften zu verstehen. Die präsentierten Ergebnisse zeigen, dass die Bildung kleiner Polaronen im PCMO als Folge starker Korrelationswechselwirkungen in breites Absorptionsmaximum im Nah-Infrarot bis sichtbarem Energiebereich verursacht, welches im Rahmen eines Photonen-assistierten Polaronenhüpfprozesses und einer Anregung zwischen Jahn-Teller-aufgespaltenen Zuständen diskutiert wird. Weiterhin legt die Dotierungsabhängigkeit der Spektren nahe, dass O 2p und Mn 3d Hybridzustände die Fermienergie-nahe elektronische Struktur bestimmen, wobei der relative Anteil von O 2p mit der Ca-Dotierung variiert. ii) Der aktive Zustand von PCMO in Kontakt mit Wasser bzw. Wasserdampf wird mit Hilfe von Zyklovoltammetrie und in situ ‚environmental‘ Transmissionselektronenmikroskopie (ETEM) für verschiedene Dotierlevels untersucht. Die Ergebnisse beider Methoden ergeben, dass die katalysierte Wasseroxidation gemäß $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4 \text{H}^+$ mit einem Korrosionsprozess in Form einer Pr/Ca Verarmung und Amorphisierung der PCMO-Elektrode konkurriert. Die höchste katalytische Aktivität sowie Korrosionsstabilität werden im mittleren Dotierungsbereich gefunden. Auf Basis der in situ ETEM Ergebnisse wird außerdem gezeigt, dass durch Zufügen von Monosilan zu Wasserdampf-basierten Elektrolyten im ETEM eine Elektronenstrahl-induzierte Wasseroxidation an aktiven PCMO Oberflächen über die Sekundärreaktion $\text{SiH}_4+2\text{O}_2\rightarrow\text{SiO}_2+2\text{H}_2\text{O}$ nachgewiesen werden kann. Elektronenenergieverlustspektroskopie von PCMO vor und nach der Reaktion in Wasserdampf ergeben, dass der aktive Zustand von PCMO die Bildung und Ausheilung von Sauerstoffleerstellen im Rahmen einer Interkalation des bei der Wasseroxidation freiwerdenden Sauerstoffs beinhaltet. Die Rolle des Elektronenstrahls als Triebkraft für die Wasseroxidation im ETEM wird mithilfe von Elektronenholographie und elektrischen Experimenten sowie theoretischer Modellierung basierend auf Sekundärelektronenemissionen als ein positives Elektronenstrahl-induziertes elektrisches Potential identifiziert.

<p dir="ltr">Insights from research into the natural photosynthetic processes are applied to inform the rational design of inorganic catalysts. The study of these synthetic systems – artificial photosynthesis – will lead towards the development of a device able to absorb light, convert and store the energy in the form of chemical bonds. The water-splitting reaction, a bottleneck of the photosynthetic process, is a key barrier to overcome in this endeavor. Thus, the focused study of water-oxidation catalysts able to facilitate this difficult reaction is performed, in order to develop a green-energy solution in the form of an artificial photosynthesis system.</p>

The theme of this thesis is the development of mononuclear Ru-based complexes that are capable of catalyzing the water oxidation (or O2-evolving) reaction, e.g. 2 H2O → O2 + 4 H+ + 4 e−. Several families of mononuclear Ru water oxidation catalysts were designed and prepared. They feature with anionic ancillary ligands that contain carboxylate or phenolate donors. The properties of the catalysts were investigated in various aspects including coordination geometry, electrochemical behavior, and ligand exchange. All catalysts showed outstanding catalytic activity towards water oxidation in the presence of cerium(IV) ammonium nitrate as a sacrificial oxidant. High-valent Ru intermediates involved in the reactions were characterized both experimentally and theoretically. The kinetics of catalytic water oxidation was examined based on one catalyst and a prevailing catalytic pathway was proposed. The catalytic cycle involved a sequence of oxidation steps from RuII−OH2 to RuV=O species and O−O bond formation via water-nucleophilic-attack to the RuV=O intermediate. By comparing properties and catalytic performance of Ru catalysts herein with that of previously reported examples, the effect of anionic ancillary ligands was clearly elucidated in the context of catalytic water oxidation. Aiming to further application in an envisaged artificial photosynthesis device, visible light-driven water oxidation was conducted and achieved primarily in a homogeneous three-component system containing catalyst, photosensitizer, and sacrificial electron acceptor. Moreover, one model Ru catalyst was successfully immobilized on ordinary glass carbon surface through a facile and widely applicable method. / <p>QC 20121112</p>