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Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions


Metallic cobalt nanoparticles are used as catalyst in the Fischer-Tropsch reaction. This chemical process is used on a commercial scale to convert synthesis gas, a mixture of carbon monoxide and hydrogen, into liquid transportation fuels and other long chain hydrocarbons. Synthesis gas is predominantly produced from fossil fuels such as coal and natural gas, but it can also be produced in a sustainable manner using biomass or green hydrogen and CO2. These alternative sources of synthesis gas, followed by Fischer-Tropsch synthesis to produce liquid fuels, is often mentioned in scenarios for the sustainable production of hydrocarbon-based products such as kerosene needed for air transport and as a renewable feedstock for the chemical industry. 
The Fischer-Tropsch synthesis process is highly complex and the mechanistic details about how the long chain products form on the surface of the catalyst have been debated for many years. Identification of the surface intermediates involved in the chain growth reaction is practically impossible when the reaction is on-going: the surface under reaction conditions is highly covered by a mixture of reactants, intermediates and products and the concentration of active intermediates is very small. The use of simplified model systems is therefore required to obtain molecular level insight into the intermediates involved in the formation of C-C bonds on the surface of the catalyst. 
In our work we use a single crystal surface of cobalt Co(0001) as a simplified model system. Ethene (C2H4) was chosen as a precursor to generate C2Hxadsorbates on the Co surface to study their reactivity. The high resolution photoelectron spectroscopy (HR-XPS) together with the fast data acquisition (fast-XPS) available at the SuperESCA beamline of Elettra allowed us to determine the concentration and chemical identity of the surface intermediates that form when the ethylene precursor reacts on the surface. During the reaction in industry the surface is covered to a significant extent by CO molecules, and we simulated this by using a high CO coverage in our experiments.The results provide detailed information about the mechanism by which carbon-carbon bonds form on a cobalt catalyst, an important elementary step in the formation of long chain hydrocarbon products in the Fischer-Tropsch synthesis process.
The experiment started by dosing C2Hon the Co(0001) surface at very low temperature so that the molecules adsorb intact. Next, the temperature was increased, leading to decomposition of C2H4into acetylene (C2H2) and two H atoms which remain adsorbed alongside acetylene. The temperature was then decreased and CO was introduced at a pressure of 1×10-7mbar. Figure 1 shows the 2D plot of the C1s intensity with selected high resolution C1s spectra obtained during subsequent heating in the presence of CO. We observed that the co-adsorbed CO causes C2Hto react around 250 K to form the ethylidyne (CCH3) intermediate, adsorbed on the surface with one carbon atom and the C-C bond perpendicular to the catalyst surface. This position turns out to be the intermediate needed for chain growth as the two ethylidynes dimerize around 300 K to form 2-butyne, a chain of four carbon atoms. XPS experiments performed at near-ambient pressure at the HIPPIE beamline of MAX IV confirm that these findings, obtained at low temperature and in ultrahigh vacuum conditions, also occur at higher reactant pressures. By comparing these results with experiments without CO we demonstrate that the CO molecules are not directly involved in the C-C bond forming reaction but stabilize CxHadsorbates as alkylidyne intermediates that readily react to form new C-C bonds. Our work sheds light onto the processes involved in the production of long chains in applied catalysis because the intermediates observed in our model system are likely to be present under realistic conditions. 
 

Figure 1.    (a) 2D plot of the C1s intensity and (b) high resolution C1s spectra during heating a C2H2ad/2Had-covered Co(0001) surface in CO. From the behavior of the C1s spectra while heating we determine the temperatures at which CxHy adsorbates react while the spectral deconvolution into different components reveals the identity of the intermediates formed at each stage. 
 

 

This research was conducted by the following research team:

C.J. (Kees-Jan) Weststrate1, Hans O.A. Fredriksson1, Devyani Sharma2, Daniel Garcia Rodriguez2, Michael A. Gleeson2, J.W. (Hans) Niemantsverdriet1,3

 

Syngaschem BV, Eindhoven, The Netherlands
Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven, The Netherlands
SynCat@Beijing, Huairou, China


Contact persons:

Kees-Jan Weststrate, email: c.j.weststrate@syngaschem.com

 

Reference

C.J. Weststrate, D. Sharma, D. Garcia Rodriguez, M.A. Gleeson, H.O.A. Fredriksson, J.W. Niemantsverdriet, “Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions” Nature Communications 11, 750 (2020), DOI: 10.1038/s41467-020-14613-5

 
Last Updated on Thursday, 20 August 2020 18:23