Nanostructured Catalysts for Conversion of CO2 into Hydrocarbons

Abstract

Increasing greenhouse gas emissions since the onset of industrialisation now pose a significant threat to modern society and the habitability our planet for future generations. It has been determined that we must achieve zero net emissions by 2050 with a 45% decrease in emissions by 2030 (relative to 2010 levels) in order to limit warming to 1.5 °C by the end of the century. A wide variety of solutions are being explored to limit the impact of modern emissions and transition to a more sustainable energy infrastructure. One such example is the capture, storage and conversion of CO2 into valuable products and fuels for emissions mitigation and energy storage.

Iron nanoparticles supported on carbon nanotubes known as Fe@CNT have proven themselves to be active for the catalytic hydrogenation of CO2 into hydrocarbons. This is due to their ability to catalyse both the reverse water-gas shift and Fischer-Tropsch processes, resulting in a coupled process that generates chemical fuels directly from a feed gas of hydrogen and CO2. Furthermore, the integration of the iron particles into the carbon nanotube support structure during synthesis results in a nanotube-particle bridged structure that enhances catalyst activity due to improved hydrogen spillover. However, the distribution of the resulting products is notoriously difficult to control, often requiring the addition of promoter metals to enhance catalyst activity and selectivity towards desirable products.

These promoters are typically doped onto the surface of the catalyst using a wet impregnation technique, and are said to enhance reactivity by increasing the catalyst’s Lewis basicity. Herein, however, an alternative method of increasing the basicity of the catalyst is explored by doping nitrogen heteroatoms directly carbon nanotube support structure during synthesis resulting in a novel catalyst referred to as Fe@NCNT. This thesis explores the synthesis, characterisation and reactivity of Fe@NCNT to determine the potential for nitrogen doping to enhance the activity of carbon-supported iron nanoparticles in CO2 hydrogenation. The influence of reaction conditions and the addition of synergistic promoter metals are also explored.

Nitrogen doping in Fe@NCNT serves to enhance the basicity of the catalyst, resulting in notably increased CO2 conversion and decreased CO selectivity relative to nitrogen-free Fe@CNT. However, methane production also increases as a consequence of nitrogen doping, and a trade-off is observed between CO2 conversion and high α values in the FT product distribution. This unexpected observation is largely attributed to the influence of local C—N dipoles in the catalyst surface upon the adsorption properties of the dipole-containing CO2 and CO reactant molecules and the significantly less polar hydrocarbon products. This behaviour is subsequently exploited further to develop a primarily iron-based, FT-driven methanation catalyst using a significantly lower ruthenium loading than similar catalysts in literature.

Date of Award	19 Feb 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Davide Mattia (Supervisor) & Matthew Jones (Supervisor)