First Principles Modeling of photo(electro-)catalytic conversion of carbon dioxide and methane

Abstract

Excessive accumulation of greenhouse gases such as methane (CH₄) and carbon dioxide (CO₂) in the atmosphere is a major contributor to global environmental challenges. An effective approach to mitigate this issue is to convert these gases into value-added products. However, their high thermodynamic stability presents a significant challenge to their efficient conversion. Conventional thermocatalytic conversion methods are highly energy-intensive and less environmentally sustainable. In contrast, photocatalytic and electrocatalytic pathways utilizing clean and renewable energy sources offer promising alternatives as they operate under relatively mild reaction conditions. However, industrial implementation of such technologies necessitates the development of highly efficient catalytic systems along with several other factors. A molecular-level insight into the interactions between CO₂/CH₄ and catalyst surfaces can facilitate the rational design of catalysts with enhanced performance. Work in my thesis employs density functional theory (DFT) based methods to investigate the interaction of CO₂ and CH₄ with catalysts. The study is divided into two problems: (i) CO₂ conversion to methanol on sulfur vacancy-rich MoS₂ monolayers (2H, 1T and 1T’ phases): It was found that S vacancies act as reaction sites, where CO₂ is reduced to methanol and formic acid via the formate pathway. Our calculated results are in good agreement with the experimental findings. (ii) Adsorption of CH₄ on small iron oxide moiety supported on anatase TiO₂ surfaces: In this work we studied interaction of methane with a monomer of Fe2O3 supported on (101), unreconstructed (001), and (1×4) reconstructed (001) surfaces of anatase TiO₂. Due to the presence of localized Fe-3d orbitals, we have looked into the effect of the value of Hubbard U(Fe-3d) on the electronic and magnetic properties of Fe2O3/anatase. It was found that irrespective of the value of U(Fe-3d) and type of surface, Fe2O3 prefers to bind in antiferromagnetic alignment of the two Fe atoms, both possessing the same oxidation state (+2). CH₄ adsorption studies on Fe2O3/anatase surface also reveal behavior independent of U(Fe-3d) value. However, presence of Fe-3d states at valence band maxima and conduction band minima of Fe2O3/anatase indicates that the course of reaction might depend on the value of U(Fe-3d), which needs further investigation of the reaction mechanism.