Density Functional Theory Investigation of Carbon- and Porphyrin-based Nanostructures

The present doctoral thesis examines the properties of carbon-based, porphyrin-based and hybrid carbon-porphyrin nanostructures as promising candidate materials for catalysis (including photocatalysis) applications. I use density functional theory simulations, together with experimental insights from collaborators, to both explain known behaviour and suggest ways in which these materials can be modified for improved catalytic efficiency. Since the catalytic activity of graphitic materials is concentrated on the edges, I investigate their properties in several ways. First, I attempt to understand the properties of folded edges of graphitic nanostructures and quantify their thermodynamic stability, and explain how the application of an electric field leads to their opening. Edge folding can reduce catalytic activity by allowing bond saturation at the edge, but at the same time they provide a way to achieve highly porous carbon-based materials, which could be very useful for catalytic applications. My calculations rationalise the experimental observations about these folded edges. Additionally, I investigate catalysts based on carbon- and iron-based nanostructures, in collaboration with experimentalists. I present models for N-doped graphitic/ferrihydrite nanocatalysts for CO2 reduction, and for Fe-N active sites in graphite-based catalysts. In contrast with carbon nanostructures, porphyrin nanostructures exhibit a welldefined band gap which makes them more useful in photocatalytic applications. In this thesis I explore possible routes to engineer the electronic properties of two types of porphyrin-based materials. The first type consists of fully-organic porphyrin nanostructures with various dimensionalities, and we show how the length of the linkers between porphyrin can be used to engineer their electronic band structures. The second type consists of two-dimensional (2D) porphyrin-based metal-organic frameworks, where we explored different strategies to optimise the photocatalytic behaviour, by changing metal centres, partially reducing the porphyrins or changing the bridges between the porphyrin units. Finally, I consider mixed graphitic/porphyrinic structures, based on the idea that such composites could combine the advantages of both types of structures, leading to superior photocatalytic behaviour. I discuss the adsorption of porphyrins on the surfaces or edges of graphene nanoribbon, and how the interaction affects the electronic properties of the combined structures. Overall, the thesis shows how computer simulation approaches can be used to understand, and also to design and optimise the electronic properties of carbon and porphyrin-based nanostructures to be applied in catalysis and photocatalysis