Carbon-carbon bond formation on cobalt

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 used a single crystal surface of cobalt Co(0001) as a simplified model system. Ethene (C₂H₄) was chosen as a precursor to generate C₂H_x adsorbates on the Co surface to study their reactivity. The high resolution photoelectron spectroscopy (HR-XPS) together with the fast data acquisition (fast-XPS) 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 C₂H₄ on the Co(0001) surface at very low temperature so that the molecules adsorb intact. Next, the temperature was increased, leading to decomposition of C₂H₄ into 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. We observed that the co-adsorbed CO causes C₂H₂ to 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 C_xH_y adsorbates 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.