The formation of dissolved gaseous mercury (DGM) can be an important pathway for mercury removal from an aquatic environment. DGM evasional fluxes from an aquatic system can account for up to 95% of atmospheric Hg and its deposition pathways. While this makes DGM an important species of mercury to investigate, the difficulty of accurately analyzing DGM has prevented many from studying it. In this study, DGM was measured in two different types of estuarine environments and with two different methods, discrete and continuous analysis. The discrete technique works reasonably well and is reproducible, but it does not allow one to observe rapid changes in DGM concentration due to long analysis times (~2 hr per sample). When used in this study, the discrete sampling technique agreed well with the continuous technique for Offatts Bayou, Galveston, Texas, and Georgiana Slough in the California Bay-Delta region. The average DGM concentration during the March continuous study at Offatts Bayou was 25.3 + 8.8 pg L-1. This is significantly higher than the average DGM concentration from Georgiana Slough during late March 2006 (9.6 + 6.6 pg L-1). DGM seemed to correlate best with photosynthetically active radiation (PAR) data in every study, suggesting that the primary control of its formation is solar irradiation. Stronger positive correlations with PAR were seen when DGM data was shifted back one hour, indicating that mercury photoreactions take time to complete. DGM also correlated positively with wind speed in most instances. However, increased wind speed should enhance air to water transfer of elemental mercury, thus one would expect a negative correlation. DGM co-varied negatively with salinity during the continuous studies, suggesting that the DGM pool is reduced in surface waters by chloride mediated oxidation. Three predictive flux models were used in the study to assess the potential for DGM water to air transfer. For both the Georgiana Slough and Offatts Bayou studies, the predicted flux dropped to or below zero after sunset. This study does contribute to the understanding of DGM cycling in aquatic environments as there are few studies that have made continuous DGM measurements in estuarine environments.

The formation of dissolved gaseous mercury (DGM) can be an important pathway for mercury removal from an aquatic environment. DGM evasional fluxes from an aquatic system can account for up to 95% of atmospheric Hg and its deposition pathways. While this makes DGM an important species of mercury to investigate, the difficulty of accurately analyzing DGM has prevented many from studying it. In this study, DGM was measured in two different types of estuarine environments and with two different methods, discrete and continuous analysis. The discrete technique works reasonably well and is reproducible, but it does not allow one to observe rapid changes in DGM concentration due to long analysis times (~2 hr per sample). When used in this study, the discrete sampling technique agreed well with the continuous technique for Offatts Bayou, Galveston, Texas, and Georgiana Slough in the California Bay-Delta region. The average DGM concentration during the March continuous study at Offatts Bayou was 25.3 ± 8.8 pg L-1. This is significantly higher than the average DGM concentration from Georgiana Slough during late March 2006 (9.6 ± 6.6 pg L-1). DGM seemed to correlate best with photosynthetically active radiation (PAR) data in every study, suggesting that the primary control of its formation is solar irradiation. Stronger positive correlations with PAR were seen when DGM data was shifted back one hour, indicating that mercury photoreactions take time to complete. DGM also correlated positively with wind speed in most instances. However, increased wind speed should enhance air to water transfer of elemental mercury, thus one would expect a negative correlation. DGM co-varied negatively with salinity during the continuous studies, suggesting that the DGM pool is reduced in surface waters by chloride mediated oxidation. Three predictive flux models were used in the study to assess the potential for DGM water to air transfer. For both the Georgiana Slough and Offatts Bayou studies, the predicted flux dropped to or below zero after sunset. This study does contribute to the understanding of DGM cycling in aquatic environments as there are few studies that have made continuous DGM measurements in estuarine environments.

No description available.

abstract: Over the past years, an interest has arisen in resolving two major issues: increased carbon dioxide (CO2) emissions and depleting energy resources. A convenient solution would be a process that could simultaneously use CO2 while producing energy. The photocatalytic reduction of CO2 to fuels over the photocatalyst titanium dioxide (TiO2) is such a process. However, this process is presently inefficient and unsuitable for industrial applications. A step toward making this process more effective is to alter TiO2 based photocatalysts to improve their activity. The interactions of CO2 with oxygen-deficient and unmodified (210) surfaces of brookite TiO2 were studied using first-principle calculations on cluster systems. Charge and spin density analyses were implemented to determine if charge transfer to the CO2 molecule occurred and whether this charge transfer was comparable to that seen with the oxygen-deficient and unmodified anatase TiO2 (101) surfaces. Although the unmodified brookite (210) surface provided energetically similar CO2 interactions as compared to the unmodified anatase (101) surface, the unmodified brookite surface had negligible charge transfer to the CO2 molecule. This result suggests that unmodified brookite is not a suitable catalyst for the reduction of CO2. However, the results also suggest that modification of the brookite surface through the creation of oxygen vacancies may lead to enhancements in CO2 reduction. The computational results were supported with laboratory data for CO2 interaction with perfect brookite and oxygen-deficient brookite. The laboratory data, generated using diffuse reflectance Fourier transform infrared spectroscopy, confirms the presence of CO2- on only the oxygen-deficient brookite. Additional computational work was performed on I-doped anatase (101) and I-doped brookite (210) surface clusters. Adsorption energies and charge and spin density analyses were performed and the results compared. While charge and spin density analyses showed minute charge transfer to CO2, the calculated adsorption energies demonstrated an increased affinity for CO2adsorption onto the I-doped brookite surface. Gathering the results from all calculations, the computational work on oxygen-deficient, I-doped, and unmodified anatase and brookite surface structures suggest that brookite TiO2 is a potential photocatalysts for CO2 photoreduction. / Dissertation/Thesis / M.S. Chemical Engineering 2012

abstract: The mitigation and conversion of carbon dioxide (CO2) to more useful carbon chemicals is a research topic that is at the forefront of current engineering and sustainability applications. Direct photocatalytic reduction of CO2 with water (H2O) vapor to C1-C4 hydrocarbons has significant potential in setting substantial groundwork for meeting the increasing energy demands with minimal environmental impact. Previous studies indicate that titanium dioxide (TiO2) containing materials serve as the best photocatalyst for CO2 and H2O conversion to higher-value products. An understanding of the CO2-H2O reaction mechanism over TiO2 materials allows one to increase the yield of certain products such as carbon monoxide (CO) and methane (CH4). The basis of the work discussed in this thesis, investigates the interaction of small molecules (CO, CH4,H2O) over the least studied TiO2 polymorph - brookite. Using the Gaussian03 computational chemistry software package, density functional theory (DFT) calculations were performed to investigate the adsorption behavior of CO, H2O, and CH4 gases on perfect and oxygen-deficient brookite TiO2 (210) and anatase TiO2 (101) surfaces. The most geometrically and energetically favorable configurations of these molecules on the TiO2 surfaces were computed using the B3LYP/6-31+G(2df,p) functional/basis set. Calculations from this theoretical study indicate all three molecules adsorb more favorably onto the brookite TiO2 (210) surface. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was used to investigate the adsorption and desorption behavior of H2O and CH4 on Evonik P25 TiO2. Results from the experimental studies and theoretical work will serve as a significant basis for reaction prediction on brookite TiO2 surfaces. / Dissertation/Thesis / M.S. Chemical Engineering 2012

Thesis advisor: Dunwei Wang / Photosynthesis converts solar energy and stores it in chemical forms. It is one of the most important processes in nature. Artificial photosynthesis, similar to nature, can provide us reaction products that can potentially be used as fuel. This process promises a solution to challenges caused by the intermitted nature of solar energy. Theoretical studies show that photosynthesis can be efficient and inexpensive. To achieve this goal, we need materials with suitable properties of light absorption charge separation, chemical stability, and compatibility with catalysts. For large-scale purpose, the materials should also be made of earth abundant elements. However, no material has been found to meet all requirements. As a result, existing photosynthesis is either too inefficient or too costly, creating a critical challenge in solar energy research. In this dissertation, we use inorganic semiconductors as model systems to present our strategies to combat this challenge through novel material designs of material morphologies, synthesis and chemical reaction pathways. Guided by an insight that a collection of disired properties may be obtained by combining multiple material components (such as nanostructured semiconductor, effective catalysts, designed chemical reactions) through heterojunctions, we have produced some advanced systems aimed at solving fundamental challenges common in inorganic semiconductors. Most of the results will be presented within this dissertation of highly specific reaction routes for carbon dioxide photofixation as well as solar water splitting. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The focus of this thesis was on the synthesis, characterization and application of ceria nanocubes. This thesis is divided into two main sections; the first section investigates the electronic properties of ceria nanocubes, and the second explores their catalytic applications towards alcohol oxidation reactions. The first project of this thesis consisted of the X-ray characterization of hydrothermally synthesized ceria nanocubes of various sizes. For the first time, the electronic properties of such nanocubes were systematically studied using high resolution XPS and XANES. It was found that the concentration of Ce3+ present within the nanocubes was independent of the particle size, as well as the Ce precursor used during synthesis. Throughout the analysis of the Ce 3d and 4d XPS spectra, it was observed that the surface of the ceria nanocube samples was undergoing photoreduction/damage over time. This damage was attributed to the samples’ exposure to high intensity X-ray radiation. This was confirmed through examination of the Ce M4,5- and N4,5-edge XANES spectra. From these results, it was clear that the concentration of Ce3+ on the surface of the ceria nanocubes was independent of particle size. This fact may become important when investigating their potential catalytic activity. The second project of this thesis concentrated on the analysis of the catalytic activity of a variety of CeO2, Au and Au/CeO2 catalysts towards the oxidation of benzyl alcohol. The low temperature oxidation reactions were studied using 1H NMR spectroscopy. It was observed that Au NPs, Au/bulk CeO2, and Au/CeO2 nanocubes in water and K2CO3 were active catalysts for this oxidation reaction at 60°C in both air and O2 (g) atmospheres. Surprisingly, however, the Au/bulk CeO2 and Au/CeO2 nanocube catalysts showed very similar activities. It was also found that ceria nanocubes alone, and Au25(SR)18/bulk CeO2 showed no activity for this reaction under similar conditions. It was determined that below a substrate to catalyst ratio of ~ 1500:1, the Au/CeO2 catalysts, which showed the highest activities, were mass-transport limited with respect to the O2 in the system. The turnover frequencies of the supported catalysts were approximately double those of the unsupported NPs. Furthermore, these reactions have indicated that activating Au25(SR)18/CeO2 for catalysis is a non-trivial task, and more work needs to be done to understand the activation of such clusters.

No description available.

Methylmercury (MeHg) is a potent vertebrate neurotoxin and a contaminant of global concern. Increased anthropogenic emissions of mercury (Hg) to the atmosphere have led to increased bioaccumulation of MeHg in top predatory organisms such as fish, the consumption of which is the main exposure pathway of this toxin to humans and other animals. Forest canopies significantly increase the deposition of Hg in general to watersheds, but sources and fates of MeHg deposition in particular remain poorly understood. In this study, wet and dry loadings of MeHg to a watershed were quantified, and the retention and (photo)reduction of MeHg on foliage were measured using unique stable isotope experiments. We found that traditional methods of quantifying net deposition underestimate incoming sources of MeHg, and that retention of MeHg on forest canopies results in delayed transport of a significant portion of newly deposited MeHg from terrestrial catchments into adjacent lakes. / Ecology

abstract: Titanium oxide (TiO2), an abundant material with high photocatalytic activity and chemical stability is an important candidate for photocatalytic applications. The photocatalytic activity of the TiO2 varies with its phase. In the current project, phase and morphology changes in TiO2 nanotubes were studied using ex-situ and in-situ transmission electron microscopy (TEM). X-ray diffraction and scanning electron microscopy studies were also performed to understand the phase and morphology of the nanotubes. As prepared TiO2 nanotubes supported on Ti metal substrate were amorphous, during the heat treatment in the ex-situ furnace nanotubes transform to anatase at 450 oC and transformed to rutile when heated to 800 oC. TiO2 nanotubes that were heat treated in an in-situ environmental TEM, transformed to anatase at 400 oC and remain anatase even up to 800 oC. In both ex-situ an in-situ case, the morphology of the nanotubes drastically changed from a continuous tubular structure to aggregates of individual nanoparticles. The difference between the ex-situ an in-situ treatments and their effect on the phase transformation is discussed. Metal doping is one of the effective ways to improve the photocatalytic performance. Several approaches were performed to get metal loading on to the TiO2 nanotubes. Mono-dispersed platinum nanoparticles were deposited on the TiO2 nanopowder and nanotubes using photoreduction method. Photo reduction for Ag and Pt bimetallic nanoparticles were also performed on the TiO2 powders. / Dissertation/Thesis / M.S. Materials Science and Engineering 2014