Facile CC bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in CC bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C 2 H x adsorbates on a Co(0001) model catalyst and promote CC bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C 2 H x adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that CC bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.
Understanding the kinetics of oxygen removal from catalytically
active metal surfaces by hydrogen is important for several catalytic
reactions such as Fischer–Tropsch synthesis, methanation of
CO or CO2, and the reverse-water–gas-shift reaction.
Motivated by FTS, a Co(0001) single crystal model catalyst was used
to study the kinetics of oxygen removal through reaction with hydrogen.
Kinetic studies in the 10–7–10–4 mbar H2 pressure regime show that water formation is
first order in the surface hydrogen concentration while the order
in oxygen concentration changes from one at low oxygen coverage to
zero at high oxygen coverage. In situ XPS shows that the hydrogen
surface concentration saturates around 10–1 mbar
and on this basis the typical temperature of 450 K needed for water
formation in this pressure regime can be considered as typical for
high pressures as well. The absence of OH buildup during the reaction
points to O + H as the rate-limiting step, with a barrier of ∼120
kJ mol–1. Such a high barrier shows that slow removal
of adsorbed oxygen from the surface of reactive metal catalysts such
as cobalt may be rate-limiting for the overall reaction.
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