Studies on the Conversion of Atmospheric Carbon Dioxide to Methanol

This thesis considers the fundamentals and applications of converting atmospheric CO₂ to methanol (MeOH), covering catalysts to support the selective synthesis of MeOH and approaches to interface intermittent renewable power with the production of methanol. For thermochemical hydrogenation of CO₂ to methanol, copper-based catalysts are the most established formulation, comprising metallic copper and a metal oxide support. Here, zinc oxide (ZnO), zirconia (ZrOₓ), and ceria (CeOₓ) were studied for use in copper-based catalysts, demonstrating the role of the supports in providing sites for the adsorption of CO₂, whereas Cu enables the dissociative adsorption of H₂ for the hydrogenation of CO₂ to MeOH. An optimum support should also suppress the inherent tendency for copper to catalyse the non-selective production of CO – a function performed best by the ZrOₓ, found here to be amorphous in catalysts produced by coprecipitation. Spectroscopic studies of the catalysts showed that Cu-ZnO and Cu-ZrOₓ catalyse methanol synthesis via a formate intermediate, with subsequent kinetic analysis then aiming to determine the active sites at which formate is converted to methanol. By comparing the copper-based catalysts against pure copper, the analysis revealed that the rate of CO₂ conversion is controlled by the sites on metallic copper at atmospheric pressure, whilst the contribution of sites from the support grows in prominence with rising pressure. Characterisations considered the structural-functional relationships governing catalyst activity for Cu-ZnO and Cu-ZrOₓ, as well as a ternary combination of copper-zinc oxide-zirconia (Cu-ZnZrOₓ). Studying crystallisation of the catalysts by in-situ X-ray diffraction showed strong evidence for amorphous zirconia incorporating both zinc and copper in a solid solution. Additional electron microscopy and in-situ X-ray photoelectron spectroscopy indicated that copper breaks from the solid solution under reducing conditions – effecting an evolution in morphology towards well-dispersed copper nanoparticles across the amorphous support. Overall, the considered catalysts each exhibited intimate mixing of both copper and support components throughout their structure, with the close contact of Cu and ZnO proving particularly effective in raising the overall conversion rate of CO₂, albeit with lower selectivity towards MeOH than the amorphous ZrOₓ or ZnZrOₓ supports. In seeking to explain the improved selectivity from these amorphous supports, the characterisation showed evidence for both hydroxyl groups and oxygen vacancies, each of which can stabilise formate on the zirconia-based supports and prevent non-selective decomposition to CO. Of the considered supports, CeOₓ showed the highest propensity to support CO formation from CO₂ at low pressure, leading to unfavourable MeOH yield over Cu-CeOₓ vs Cu-ZnO, Cu-ZrOₓ, or Cu-ZnZrOₓ. However, the activity of Cu-CeOₓ for CO formation proved utile when applied for plasma-driven splitting of CO₂ through dielectric barrier discharge. In supporting dielectric barrier discharge for non-thermal plasma, Cu-CeOₓ appeared to form oxygen vacancies able to scavenge oxygen after cleavage of the CO₂ molecule, whereas the Cu-ZnO and Cu-ZrOₓ catalysed the oxidation of CO back to CO₂, leading to poor overall efficiencies of plasma-driven CO₂ splitting. Additional experiments successfully applying non-stoichiometric strontium ferrite for plasma-driven CO₂ splitting further point to the role of oxygen vacancies in plasma, while the comparatively high conversion and efficiency achieved here over Al₂O₃ are noteworthy given the use of alumina as an inert support for copper catalysts, also suggesting a direction for future work on plasma-driven methanol synthesis. Finally, the work emphasises the need for dynamic production of chemicals to interface successively with renewable power, taking the example of a wind farm erected directly as part of a plantfor converting atmospheric CO₂ to MeOH, using stored reserves to enable continuous production. Through a consideration of worldwide candidate locations, the modelling shows an overarching trend of MeOH production becoming more cost-effective as the reliance on storage is reduced by curtailing production according to available power. Moreover, a consideration of future improvements to process units – electrolysers, direct air capture, catalysts, and wind farms – showed renewable MeOH reaching costs to supplant MeOH from fossil fuels, also delivering a substantial net-drawdown of atmospheric CO₂ into the methanol product.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Fulham, George

Advisor dc:contributor.advisor

• Marek, Ewa

• carbon capture and utilisation
• catalysis
• renewable power

Licence

• https://www.repository.cam.ac.uk/bitstreams/fee2ca1a-b842-46c7-ae08-475bd7b9f8a0/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0009-0005-1063-8141

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/398184