A recent paper in ACS Catalysis by a team of researchers from University College London and the Research Complex at Harwell (RCaH) marks the 13,000th to be published as a result of innovative research at Diamond Light Source, the UK's national synchrotron. Its insights into the cobalt nanoparticles used during the synthesis of liquid fuels will help the world transition towards cleaner energy sources.
In the mid-1920s, Franz Fischer and Hans Tropsch developed a process to convert syngas (a mix of carbon monoxide and hydrogen) into fine chemicals and liquid hydrocarbons such as diesel and aviation fuel. Although syngas can be derived from coal or natural gas, it can also be made from any carbon-based feedstocks, including biomass, municipal solid waste, and carbon dioxide captured from industrial processes. Fischer−Tropsch synthesis (FTS) requires the use of a catalyst, and commercial production generally utilises cobalt nanoparticles (CoNPs) on a solid titania (TiO2) support. Co/TiO2 catalysts strike a balance in terms of reducibility, dispersion, and stability, they are not immune to deactivation. However, CoNPs are known to undergo changes that affect FTS performance, and understanding CoNP evolution is a topic of great interest.
Professor Andy Beale, from University College London, said:
Fischer-Tropsch synthesis is pretty agnostic about the source of the syngas. Essentially you can take any hydrocarbon source and break it down, separate out the carbon monoxide and the hydrogen and then recombine them. This technology is interesting now because we can take bio-derived hydrocarbons (i.e. methane) and turn them into syngas, or generate syngas by reducing carbon dioxide, and use it to create liquid fuels. And I think it's likely that this sort of technology will play a major role in aviation fuel, for example, because it's very difficult to imagine how you're going to fly long haul with batteries or hydrogen as a fuel.
Previous research has shown that the redox state of the CoNPs is an important factor affecting FTS performance, with metallic cobalt being the active state of this catalyst. The size of the CoNPs also affects FTS performance, and a limitation of previous studies performed using powdered forms of the catalyst is that the results are an average across a range of CoNP sizes, making it difficult to identify specific size effects. In order to understand the size-dependent evolution of CoNPs during FTS, the team investigated the redox state of individual CoNPs (6−24 nm) in situ during CO/syngas treatment, using X-ray photoemission electron microscopy (X-PEEM) on Diamond's I06 beamline.
Professor Beale explained:
The amazing thing about the XPEEM facility at Diamond is that it is so sensitive you can collect a very good spectrum off a single nanoparticle. We engineered our nanoparticles so that they were sufficiently spaced that there was just one in the field of view. We were able to collect a spectrum for not just the nanoparticle itself but the titanium and the oxygen. It's rare that you are able to record a spectrum from all of the components in the sample in a single imaging experiment. But when we focused the beam on a nanoparticle, we picked up a signal from the titanium and the oxygen surrounding it, which are common components in the nanoparticle and the support. And that allowed us to look at the evolution of the oxygen signal in particular under the controlled atmospheres that we're using, and then we can put the changes into context - this is what we know is happening with the titanium, this is what's happening with the cobalt. We look at all of those three components together to understand the redox state of those three elements. And the other neat thing is that we're even able to resolve that image down and are able to characterise the edge and the centre of the nanoparticle, and to resolve differences in their oxidation state.
The team had previously conducted research at I06 (also published in ACS Catalysis), which demonstrated that CoNPs reduced more easily in the presence of oxygen vacancies on the surface of the titania support. In these new experiments, they showed that smaller NPs are more dynamic than larger ones. They were also able to confirm that the combination of NP size and the presence of oxygen vacancies does very much affect the stability of these NPs in terms of their redox state in the presence of syngas.
These insights suggest potential avenues for improving the performance of the catalyst, such as partially reducing the titania support before adding CoNPs and that particles 12-15nm in size are particularly stable in their metallic form.
To find out more about the I06 beamline or discuss potential applications, please contact Principal Beamline Scientist Larissa Ishibe-Veiga: larissa.ishibe-veiga@diamond.ac.uk.
Qiu C et al. Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM. ACS Catalysis 13 (2023): 15956-15966. DOI:10.1021/acscatal.3c03214.
Qiu C et al. Resolving the effect of oxygen vacancies on Co nanostructures using soft XAS/X-PEEM. ACS catalysis 12.15 (2022): 9125-9134. DOI:10.1021/acscatal.2c00611.
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