Transition metal oxide layered materials as catalyst and precatalyst for green energy applications: Oxygen evolution reaction and Fischer-Tropsch synthesis

Abstract

Fossil fuels constitute 86% of global energy consumption. Even though fossil fuels have satisfied our energy needs for decades, they are non-renewable source of energy, and burning of fossil fuels is detrimental for the environment. Mining and extraction release toxic and heavy metals in the environment. The burning of fossil fuels release greenhouse carbon dioxide, SO2, NOX and volatile organic compounds into the atmosphere. Hence, the development of non-fossil fuel based alternative sources of energy is a logical solution to address these concerns. This thesis work primarily focused on design, development and understanding the chemistry of two-dimensional (2D) layered materials, particularly transition metal oxides, birnessite and lithium cobalt oxide as catalytic materials for the conversion of renewable energy into fuels and. In order to accomplish this, we principally studied the energy intensive oxygen evolution reaction (OER) in water splitting, and Fischer-Tropsch synthesis (FTS) to generate synthetic fuels. Birnessite is a 2D layered manganese dioxide material with intercalated Lewis cations and water molecules. Birnessites have been extensively investigated for their catalytic activity towards oxygen evolution reaction. In this work, we studied the influence of interstitial hydration structure on the catalytic efficiency of birnessite towards OER. The results of this study facilitated the development of upgraded, low-cost and, earth abundant catalyst for the OER. We demonstrated that the layered materials constructed from the same batch of nanosheets, but with different interlayer hydration structure exhibited significant differences in catalytic activity for chemical and electrochemical water oxidation. The dominant factor in these differences was the enhancement of relevant water fluctuations due to geometric frustration leading to enhanced electron transfer rate in the oxidation step of water splitting. Furthermore, lithium cobalt oxide (LiCoO2) and Co-doped birnessite were explored for their competence as precatalysts for Fischer-Tropsch synthesis (FTS). FTS is a commercial technology that allows converting synthesis gas, a mixture of CO and H2, into fuels and chemicals. This process is fundamentally important in the reduction of fossil fuel dependency for the energy needs. It has a great potential for generating synthetic fuels from renewable sources, such as biomass, after its gasification into synthesis gas. The synthetic fuels produced via this technology have a lower local environmental impact as compared to the conventional fuel, since it is practically free of sulfur and nitrogen impurities and yields lower exhaust emissions of hydrocarbons. The present study focused on the use of cobalt-based catalysts for the production of small to medium chain hydrocarbons (paraffins and olefins). In particular, the correlation between product selectivity and varying catalyst properties and reaction parameters was studied. In-situ studies revealed that LiCoO2 was reduced to metastable Co(hcp) and Co(fcc) nanoparticles during the activation process, providing a high surface area medium for the adsorption and hydrogenation of CO. The catalyst exhibited a high %CO conversion with small to medium chain hydrocarbon products (C2-C7). Co-doped birnessited was reduced to Co(hcp) and MnO(ccp) phases during the activation step of the FTS reaction. MnO provided an excellent medium for the dispersion and stabilization of the cobalt nanoparticles to catalyze CO-hydrogenation. Lower olefins and paraffins (C2-C4) were selectively synthesized in conjunction with low CO2 production and methane selectivity. These studies suggested that transition metal oxide based layered heterogeneous catalysts are capable of producing chemicals and fuels directly from H2-rich synthesis gas. This gas-to-chemicals process can greatly reduce CO2 emissions, thereby contributing to the mitigation of climate change and the energy needs of the future generations.