The current energy and environmental scenario in the world demands acute attention on sustainable repurposing of waste CO2 to high value hydrocarbons that not only addresses the CO2 mitigation problem, but also provides pathways for a closed loop synthetic carbon cycle. Difference in the scales of global CO2 emissions (about 40 Gtpa, 2017) and the carbon capture and sequestration (CCS) facilities (estimated cumulative 40 Mtpa, 2018) provokes active research on this topic. Solar thermochemical (STC) and visible light photocatalysis are two of the most promising routes that have garnered attention for this purpose. While STC has the advantages of high CO2 conversion rates, it operates at high temperatures (more than 1000 °C) limiting its industrial implementation. Photocatalysis, on the contrary, is plagued by the poor quantum efficiency and conversion rates, although its exhibits the benefits of low temperature operation. Thus, any significant progress towards low temperature STC and visible light photocatalytic CO2 reduction is a giant leap towards a greener and sustainable energy solution. This dissertation is an effort towards improving both the STC and photocatalytic CO2 reduction. Reverse water gas shift - chemical looping (RWGS-CL) is a modified STC approach that has the potential for low temperature CO2 conversion. RWGS-CL process uses mixed metal oxides like perovskite oxides (ABO3) for the conversion to CO, a potential feedstock for subsequent hydrocarbon production. Generation of oxygen vacancy defects on these perovskite oxides is a key step of RWGS-CL and thus, oxygen vacancy formation energy has been found to be a key descriptor for this process. Using density functional theory based calculations, this intrinsic material property has been used towards rational design of better catalysts. Highest rate of CO2 conversion at the low temperatures of 450 °C was demonstrated by earth abundant perovskite oxide via RWGS-CL. This low temperature and stable CO2 conversion process enables thermal integration with subsequent Fischer Tropsch processes for the hydrogenation of CO to hydrocarbons. Parallel to the developments on materials discovery, another crucial parameter that deserves attention is the surface termination effects of the perovskite oxides. Hence, the site specificity of the bulk and surface oxygen vacancies have been probed in detail towards elucidating the CO2 conversion performance over these materials. In the view of recent progress on the growth of selective crystal facets and terminations, this study opens new avenues for enhanced CO2 conversion performance not only through bulk composition variation, but also via exposing desired crystal facets. Type-II semiconductor heterojunctions (staggered type) are promising candidates for efficient photocatalytic reactions, not only because of their capabilities of electronic density of states tuning, but also their ability to segregate the excited electrons and holes into different materials thereby restricting exciton recombination. Metal oxynitride heterojunctions have recently demonstrated promising activity on visible light water splitting. Elucidating the structure-function relationships for these materials can pave the way towards designing better CO2 conversion photocatalysts. This dissertation focuses on unravelling the roles of material composition, anion vacancy defects and lattice strain towards modulating the electronic density of states of lateral and vertical heterojunctions of (ZnO)X(AlN)1-X and (ZnO)X(GaN)1-X. The heterojunctions consist of periodic potential wells that allows for restricting interlayer charge transport. Increased ZnO concentration was explicitly shown to decrease the band gap due to N 2p and Zn-3d repulsion. Biaxial and vertical compressive strain effected increased band gap while tensile strain reduced the same. Oxygen vacancies was found to have different effect on the electronic state of the materials. When present in charged state (+2), it promotes mid gap state formation, while in neutral state it revealed increased electronic densities near the valence band and conduction band edges. These fundamental site specific material property tuning insights are essential for designing better photocatalysts for future.

CO2 global emissions exceed 30 Giga tonnes (Gt) per year, and the high atmospheric concentrations are detrimental to the environment. In spite of efforts to decrease emissions by sequestration (carbon capture and storage) and repurposing (use in fine chemicals synthesis and oil extraction), more than 98% of CO2 generated is released to the atmosphere. With emissions expected to increase, transforming CO2 to chemicals of high demand could be an alternative to decrease its atmospheric concentration. Transportation fuels represent 26% of the global energy consumption, making it an ideal end product that could match the scale of CO2 generation. The long-term goal of the study is to transform CO2 to liquid fuels closing a synthetic carbon cycle. Synthetic fuels, such as diesel and gasoline, can be produced from syngas (a combination of CO and H2) by Fischer Tropsch synthesis or methanol synthesis, respectively. Methanol can be turned into gasoline by MTO technologies. Technologies to make renewable hydrogen are already in existence, but CO is almost exclusively generated from methane. Due to the high stability of the CO2 molecule, its transformation is very energy intensive. Therefore, the current challenge is developing technologies for the conversion of CO2 to CO with a low energy requirement. The work in this dissertation describes the development of a recyclable, isothermal, low-temperature process for the conversion of CO2 to CO with high selectivity, called Reverse Water Gas Shift Chemical Looping (RWGS-CL). In this process, H2 is used to generate oxygen vacancies in a metal oxide bed. These vacancies then can be re-filled by one O atom from CO2, producing CO. Perovskites (ABO3) were used as the oxide material due to their high oxygen mobility and stability. They were synthesized by the Pechini sol-gel synthesis, and characterized with X-ray diffraction and surface area measurements. Mass spectrometry was used to evaluate the reducibility and re-oxidation abilities of the materials with temperature-programmed reduction and oxidation experiments. Cycles of RWGS-CL were performed in a packed bed reactor to study CO production rates. Different metal compositions on the A and B site of the oxide were tested. In all the studies, La and Sr were used on the A site because their combination is known to enhance oxygen vacancies formation and CO2 adsorption on the perovskites. The RWGS-CL was first demonstrated in a non-isothermal process at 500 °C for the H2-reduction and 850 °C for the CO2 conversion on a Co-based perovskite. This perovskite was too unstable for the H2 treatment. Addition of Fe to the perovskite enhanced its stability, and allowed for an isothermal and recyclable process at 550 °C with high selectivity towards CO. In an effort to decrease the operating temperature, Cu was incorporated to the structure. It was found that Cu addition inhibited CO formation and formed very unstable oxide materials. Preliminary studies show that application of this technology has the potential to significantly reduce CO2 emissions from captured flue gases (i.e. from power plants) or from concentrated CO2 (adsorbed from the atmosphere), while generating a high value chemical. This technology also has possible applications in space explorations, especially in environments like Mars atmosphere, which has high concentrations of atmospheric carbon dioxide.