Fischer-Tropsch catalysis: osmium and platinum promoted cobalt supported in/on hollow carbon spheres
Date
2024
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Abstract
The Fischer-Tropsch synthesis (FTS) is a process is used to convert traditional energy carriers such as coal, biomass, and natural gas into hydrocarbon products. The process involves a catalyst assistance in the conversion of syngas to hydrocarbons, which is usually composed of cobalt or iron supported on high surface area materials such as carbon-based materials, metal oxides, silica or alumina. In this thesis, the studies are focused on the fundamental processes involved in FT catalysts by using a hollow carbon sphere support. The study uses hollow carbon sphere (HCS) nanoreactors as the catalyst support and evaluates the effect of the nanoreactor's on the FT process, using Os and Pt nanoparticles as the reduction promoter for a Co catalyst encapsulated inside the HCSs. The synthesis and utilization of Os and Pt nanoparticles loaded on/in Co@HCS FT catalysts to give promoter-Co@HCS (e.g., OsCo@HCS & PtCo@HCS) and promoter/(Co@HCS) (e.g., Os/(Co@HCS) & Pt/(Co@HCS)) catalysts are reported.
The use of Os as a promoter for FTS has received very little attention. In previous studies, Os has been shown to be a poor FT catalyst, but its use as a catalyst promoter has not been studied. The hollow morphology of the HCSs nanoreactor was used to investigate the effect of Os and Pt promoter nanoparticle location relative to the Co3O4 (Co 10 wt %) nanoparticles (dCo = 3.5 – 12.5) and on the effect of Os and Pt on the reduction behaviour and activity of the Co FT catalyst. Electron microscopy, in situ PXRD, TGA, PDF, TPR and BET studies revealed that the prepared catalysts were successfully synthesized with Co nanoparticles well dispersed. The Co nanoparticles had a high degree of stability as catalysts because they were encased in a carbon nanoreactor shell with a large surface area which showed good stability against sintering. The use of PXRD, PDF and TPR studies provided information on the Co phases and reduction pathways of the Co3O4 metal catalyst and the spillover effect from the Os and Pt promoters. Co3O4, CoO and Co (fcc) phases were observed for the promoter-Co@HCS and promoter/(Co@HCS) catalysts. More interesting was the observation of the Co hcp phase in the promoter-Co@HCS catalysts, indicating the importance of the promoter-Co interaction. The Co (hcp/fcc) phase ratio increased with the increase in promoter percentage loading and intimacy
The synthesis of Co nanoparticles with varying thickness of a carbon shell was conducted with Os promotion on the outside of the shell to investigate the secondary hydrogen spillover effect. 4 The information obtained from the PXRD and TPR data revealed that the intimacy of the Os promoter with the Co catalyst in a thin and medium carbon shell (Os/(Co@HCS16) and (Os/(Co@HCS28)) promoted the reduction of CoOx to give metallic Co hcp and fcc phase due to secondary spillover effect. Whereas, when the Os promoter and Co were separated by a thicker carbon shell (Os/(Co@HCS51)) no metallic Co phase was observed. The study highlights the importance of Os-Co interaction and the ease of H2 diffusion through the carbon shell in determining the reduction of Co oxides. More interesting was the observation of the Co hcp phase in the Os/(Co@HCS16)) catalyst, indicating the importance of the promoter-Co intimate interaction.
Nitrogen doping of HCS support was investigated in this thesis. The XPS, BET, and TPR studies showed that N-HCSs can offer a special unique material that is suitable for use as a catalyst support. Nitrogen doping increased the number of catalyst anchoring sites on the support, improved the thermal stability of the material, and assisted in immobilizing the catalyst nanoparticles during the reaction. These properties can improve the efficiency and stability of the catalyst, making nitrogen-doped HCS an attractive material for use as a support in catalytic reactions.
In order to explain the synergism during the FT reaction, hydrogen spillover was invoked once more. The primary spillover process (Co and promoter inside a HCS) produced a catalyst that gave a higher Fischer-Tropsch activity (e.g. OsCo@HCS, 38.5 – 46.4 x10-6 molCO/gCo.s) and outperformed unpromoted catalyst (e.g. Co@HCS, 27.8 x10-6 molCO/gCo.s) and catalysts where the promoter and Co were separated (e.g. Os/(Co@HCS), 25.1 – 36.4 x10-6 molCO/gCo.s) by a mesoporous carbon shell (secondary spillover effect) in regards to the FTS activity and C5+ production. In short, secondary hydrogen spillover effect on FTS performance was studied as a function of shell thickness. A shorter distance between the Os promoter and Co catalyst improved CO conversion and enhanced the reduction of CoO to Co0 . Thicker shell (Co@HCS_51nm) reduced reactant flow and led to lower CO conversion and greater selectivity to C5+ hydrocarbon production. The Os promoter and nitrogen doping increased the FTS catalytic performance. Nitrogen doping improved the hydrocarbon selectivity of the Co FTS reaction towards the production of long-chain hydrocarbons. In the spent catalysts, nitrogen doped catalysts revealed less particle sintering.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2023
Keywords
Fischer-Tropsch synthesis (FTS), Osmium, Hollow carbon spheres