| AVS 66th International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Thursday Sessions |
| Session HC+SS+TL-ThA |
| Session: | Reaction Pathways and Addressing Challenges for Energy Production in the 21st Century & Heterogeneous Catalysis Graduate Student Award Presentation |
| Presenter: | Kees-Jan Weststrate, Syngaschem BV, Netherlands |
| Authors: | K.-J. Weststrate, Syngaschem BV, Netherlands D. Sharma, DIFFER, Eindhoven University, The Netherlands D. Garcia Rodriguez, DIFFER, Eindhoven University, The Netherlands M.A. Gleeson, DIFFER, Eindhoven University, The Netherlands H.O.A. Fredriksson, Syngaschem BV, Netherlands H.J.W. Niemantsverdriet, Syngaschem BV, Netherlands |
| Correspondent: | Click to Email |
Supported cobalt catalysts find their most widespread application in low temperature Fischer-Tropsch synthesis (FTS), a process in which C-C bond forming reactions produce long chain saturated hydrocarbon chains from synthesis gas, a mixture of CO and H2. The versatile FTS process may very well continue to play a role in future energy scenarios: synthesis gas can be derived from any carbon-containing source, e.g. biomass or even CO2 may be used. These renewable carbon sources offer a sustainable alternative to replace petroleum as the principal feedstock of chemicals and liquid transportation fuels.
The FTS reaction mechanism can be ranked among the most complex in the chemical industry. CO and H2 are converted into long chain hydrocarbons in a sequence of bond-breaking and bond-making steps that are catalyzed by metals such as cobalt, ruthenium and iron (the latter is active in the carbide form). As the steady state concentration of chain growth intermediates is below the detection limits of in-situ spectroscopies simplified model studies are needed to elucidate the mechanism by which long hydrocarbon chains grow on the cobalt catalyst surface. Since chains grow on a surface that is packed with CO, it is of crucial importance to consider how CO spectators influence the reactivity of hydrocarbon adsorbates. We use a Co(0001) single crystal surface as a model system to study how C2Hx adsorbates react on a cobalt surface, both in ultrahigh vacuum (~10-10 -10-7 mbar) as well as at near-ambient pressure (~0.1 mbar). By using the high resolution available of x-ray photoemission spectroscopy at the SuperESCA beamline of ELETTRA (Trieste, Italy), and the unique opportunity to combine these qualities with measurements at near-ambient pressure at the HIPPIE beamline of MAX IV (Lund, Sweden), we were able to elucidate the reaction mechanism by which carbon-carbon bonds form on a cobalt surface. We find that CO’s presence is of essential importance: It promotes hydrogenation of acetylene, HC≡CH [the most stable C2Hxad without CO] to ethylidyne, ≡C-CH3, a facile reaction that occurs around 250 K. Ethylidyne dimerization around 310 K produces 2-butyne (H3C-C≡C-CH3), a strongly bound alkyne adsorbate that hydrogenates to 2-butene (g) above 400 K. Extrapolated to FTS, the findings speak in favour of the alkylidyne chain growth mechanism: long chain alkylidynes (≡C-R), stabilized by the presence of CO spectators, react with a methylidyne (≡CHad) monomer to produce a 1-alkyne (R-C-CH) adsorbate. Partial hydrogenation of the 1-alkyne product is promoted by COad and produces the alkylidyne species needed for the next CH insertion step.