CO as a promoting spectator species of C_xH_y conversions relevant for Fischer-Tropsch chain growth on cobalt:evidence from temperature-programmed reaction and reflection absorption infrared spectroscopy

Cobalt-catalyzed low temperature Fischer-Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that CxHy surface species react differently in the presence of COad, a notion of general importance for catalytic reactions where both CO and CxHy species are present. In the context of Fischer-Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CHad with a (Cn) alkylidyne adsorbate to produce the (Cn+1) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of CxHy surface intermediates is beneficial for efficient chain growth.