Browsing Theses and Dissertations by Subject "Water Oxidation"
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Characterization of Carbon Nanomaterial Formation and Manganese Oxide ReactivityCharacterization 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.
THE SYNTHESIS AND MODIFICATION OF 2D MATERIALS FOR APPLICATION IN WATER OXIDATION CATALYSISThe unifying goal of this work is the design of a heterogeneous catalyst that can facilitate the energy intensive oxygen evolution reaction (OER) in water splitting, considered one of the ‘holy grails’ in catalytic science. In order for this process to be industrially feasible, an efficient catalyst composed of first row transition metal based materials must be used. To accomplish this, existing systems must be studied in order to determine which properties are important and subsequent creation and modification of new systems based on lessons learned must be employed. Birnessite, a 2D layered manganese dioxide, comprises the majority of the effort. In the studies leading to this work, this material was primarily studied by mineralogists with the majority focusing on structural characterization. However, the material’s moderate activity toward performing the OER has revived interest. In this work, we look to determine important species, the role dopants play in activity, and the function of the interlayer and surface chemistry. From these findings, an enhanced, earth abundant OER catalyst will be designed. We determine that Mn3+ in the system plays and important role in producing a catalytic species with large oxygen production capabilities. By increasing the amount of Mn3+ in the system via a simple comproportionation reaction by exposing the Mn4+ to Mn2+ ion, we increase the total turnover of birnessite 50-fold. Additionally, the addition of dopants to the system , both within and between the sheets, has a positive effect on the activity of birnessite. In particular, incorporation of cobalt into the lattice of birnessite brings the activity level on par to that of precious metal oxide catalysts due to the cobalt offering a deeper electron acceptor than in birnessite alone. In conjunction with these studies, the role of the interlayer species and catalyst confinement has demonstrated the ability to greatly enhance a catalyst’s ability to perform the OER by ordering and orienting the water around the active confined catalyst. Combining confinement effects with the cobalt-doped birnessite sheets resulted in further enhancement in the material’s OER capabilities. This system mimics that of an enzyme where the cobalt-doped birnessite sheets facilitate greater electron-hole transfer to the interlayer active site, where the confinement effects enhance electron transfer kinetics and water organization for O-O bond formation. Additionally, metal chalcogenide OER catalysts were explored with mattagamite phase cobalt pertelluride. Through the work, we determine the formation of a Te-Co-O heterostructure as the catalytically active phase, where the metallic nature of the cobalt pertelluride facilitates charge mobility between the electrode and catalyst’s cobalt oxide surface functioning as the active OER species.