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.

Dans le contexte du réchauffement climatique et de l’usage abusif de combustibles fossiles, la recherche de sources d’énergie propres et durables est l’un des défis les plus importants de notre époque. Récemment, le stockage d’énergie solaire par la réduction de CO2 a fait l’objet d’un nouvel intérêt. Bien que la réduction de CO2 en carburants liquides ou gazeux soit une question à la fois fascinante et fondamentale, sa mise en œuvre dans les dispositifs technologiques reste très difficile à cause de la grande stabilité de CO2 et du caractère endergonique de sa transformation. On outre, les réactions impliquent multiples électrons et protons et ainsi demandent des catalyseurs efficaces et stables pour diminuer les barrières cinétiques importantes.Cette comprend deux parties. Après une introduction, la première partie décrit des études sur des catalyseurs homogènes en combinaison avec un photosensibilisateur, soit séparément soit connecté par liaison covalente. Grâce à la possibilité de les modifier par synthèse et à leur facile caractérisation, les photosystèmes moléculaires homogènes sont plus modulables et peuvent permettre un meilleur contrôle de la sélectivité des réactions et l’étude des mécanismes réactionnels.Cependant, les catalyseurs moléculaires ne peuvent être facilement transposés pour des applications à plus large échelle dans un contexte industriel. En effet, les catalyseurs homogènes sont moins stables et plus difficilement recyclables que les catalyseurs hétérogènes. Dans ce contexte, l’intégration de catalyseurs moléculaires au sein d’un support solide a l’avantage de maintenir leur activité catalytique tout en permettant une séparation et un recyclage plus faciles. La deuxième partie de cette thèse porte donc sur l’immobilisation de catalyseurs moléculaires dans les matériaux. Le but ultime de cette thèse est d’incorporer à la fois le catalyseur et le photosensibilisateur dans le support solide. / In the context of global warming and the necessary substitution of renewable energies (solar and wind energy) for fossil fuels, efficient energy-storage technologies need to be urgently developed. Recently, energy storage via the reduction of CO2 has seen renewed interest. Although reduction of CO2 into energy-dense liquid or gaseous fuels is a fascinating fundamental issue, its practical implementation in technological devices is highly challenging due to the high stability of CO2 and thus the endergonic nature of its transformation. Furthermore, the reactions involve multiple electrons and protons and thus require efficient catalysts to mediate these transformations.The objective of this thesis is to investigate different strategies for the storage of solar energy in chemical compounds, through visible-light-driven CO2 reduction. This thesis comprises of two main parts. After an introduction, the first part describes the investigation of homogeneous catalysts in combination with a photosensitizer, either separately or connected covalently. Due to the easily-tunable synthesis and facile characterization of molecular catalysts, homogeneous photosystems are more controllable and can give deep insight into product selectivity and mechanistic issues.With regards to future applicability, however, homogeneous catalysis often suffers from additional costs associated with solvents, product isolation and catalyst recovery, amongst other factors. The integration of molecular catalysts into solid platforms offers the possibility to maintain the advantageous properties of homogeneous catalysts while moving towards practical system designs afforded by heterogeneous catalysis. The second part of this thesis is therefore the immobilization of molecular catalysts within solid materials, namely MOFs and PMO. The ultimate goal of this thesis is to incorporate both catalyst and photosensitizer into the solid support.

Photocatalytic reactions mediated by semiconductors such as ZnS, TiO2, ZnO, etc. can harvest solar energy into chemical bonds, a process with important prebiotic and environmental chemistry applications. The recycling of CO2 into organic molecules (e.g., formate, methane, and methanol) facilitated by irradiated semiconductors such as colloidal ZnS nanoparticles has been demonstrated. ZnS can also drive prebiotic reactions from the reductive tricarboxylic acid (rTCA) cycle such as the reduction of fumarate to succinate. However, the mechanism of photoreduction by ZnS of the previous reaction has not been understood. Thus, this thesis reports the mechanisms for heterogeneous photocatalytic reductions on ZnS for two model reactions in water with sulfide hole scavenger. First the reduction of CO2 is carried out under variable wavelength of irradiation and proposed to proceed thorough five steps resulting in the exclusive formation of formate. Second the reduction of the double bond of fumaric acid to succinic acid is reported in detail and compared to the previous conversion of CO2 to formic acid. Both reactions are carried out under variable wavelength of irradiation and proposed to proceed thorough one electron transfer at a time. In addition, a new method to measure the bandgap of colloidal ZnS suspended in water is established. Furthermore, the time scales of electron transfer and oxidizing hole loss during irradiation of ZnS for both reactions are reported and interpreted in terms of the Butler-Volmer equation. The sunlight promoted production of succinate introduced above, provides a connection of this prebiotic chemistry work to explore if central metabolites of the rTCA cycle can catalyze the synthesis of clay minerals. Clay minerals are strong adsorbents that can retain water and polar organic molecules, which facilitate the polymerization of biomolecules and conversion of fatty acid micelles into vesicles under prebiotic conditions relevant to the early Earth. While typical clay formation requires high temperatures and pressures, this process is hypothesized herein to be accelerated by central metabolites. A series of synthesis are designed to last only 20 hours to study the crystallization of sauconite, an Al- and Zn-rich model clay, at low temperature and ambient pressure in the presence of succinate as a catalyst. Succinate promotes the formation of the trioctahedral 2:1 layer silicate at ≥ 75 °C, 6.5 ≤ pH ≤ 14, [succinate] ≥ 0.01 M. Cryogenic and conventional transmission electron microscopies, X-ray diffraction, diffuse reflectance Fourier transformed infrared spectroscopy, and measurements of total surface area and cation exchange capacity are used to study the time evolution during the synthesis of sauconite. While the studies with ZnS presented above advanced the fundamental understanding of photocatalysis with single semiconductors, the environmental applications of this material appear limited. A common limitation to photocatalysis with single semiconductors is the rapid recombination of photogenerated electron-hole pairs, which reduces significantly the efficiency of the process that in the case of ZnS also suffers from photocorrosion in the presence of air. In order to overcome the fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, an effective strategy is developed in this work by combining two semiconductors into a nanocomposite. This nanocomposite is solvothermally synthesized creating octahedral cuprous oxide covered with titanium dioxide nanoparticles (Cu2O/TiO2). The nanocomposite exhibits unique surface modifications that provide a heterojunction with a direct Z-scheme for optimal CO2 reduction. The band structure of the nanocomposite is characterized by diffused reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy. The photoreduction of CO2(g) to CO(g) on the nanocomposite is investigated in the presence water vapor as the hole scavenger that generates the quantifiable hydroxyl radical (). The quantum efficiency of CO production under irradiation at λ ≥ 305 nm with the nanocomposite is 2-times larger than for pure Cu2O. The detection of and XPS analysis contrasting the stability of Cu2O/TiO2 vs Cu2O during irradiation prove that TiO2 prevents the photocorrosion of Cu2O. Overall, the studies of photocatalytic reductions on single component semiconductors reveal new knowledge needed for developing future photocatalytic application for fuel production, wastewater treatment, reducing air pollution, and driving important prebiotic chemistry reactions. Furthermore, the design of a photocatalyst operating under a Z-scheme mechanism provides a new proof of concept for the design of systems that mimic photosynthesis. Finally, this work also demonstrates how molecules obtained by mineral mediated photochemistry can catalyze clay formation; highlighting the important role that photochemistry may have played for the origin of life on the early Earth and other rocky planets.

This research is motivated by two significant challenges facing the planet: reducing the emission of CO2 to the atmosphere and production of sustainable fuels by harnessing solar energy. The main objective of this work is the study of promising photocatalysts for CO2 reduction. DFT modeling of CO2, subnanometer Ag&Pt clusters, and anatase TiO2 (101) surface is employed to gain fundamental understanding of the catalytic process, followed by validation using a guided experimental endeavor. The binding mechanism of CO2 on the surface is investigated in detail to gain insights into the catalytic activity and to assist with characterizing the photocatalyst. For CO2 photoreduction, the cluster induced sub-bandgap and the preferred adsorbate in the first and key step of the CO2 photoreduction are explored. It is found that TiO2-supported Pt octamers offer key advantages for CO2 photoreduction: 1. by providing additional stable adsorption sites for favored CO2 species in the first step, and 2. by aiding in CO2- anion formation. Electronic structure analysis suggests these factors arise primarily from the hybridization of the bonding molecular orbitals of CO2 with d orbitals of the Pt atoms. Also, structural fluxionality is quantified to investigate geometry dependent (3D-2D) CO2 adsorption. Geometric information, electronic information, and C-O bond breaking tendency of adsorbed CO2 species are proposed to connect to experimental observables (IR frequency). The CO2 adsorption sites on supported Pt clusters are also identified using IR as the indicator. A cluster-induced CO2 dissociation to CO pathway is also discovered. Finally, experimental work including dendrimer-encapsulated technique, TPD, and UV-Vis is performed to validate the computational results, the availability of adsorption sites and CO2 binding strength on supported Pt clusters.