D.L. Trimm et al. / Journal of Molecular Catalysis A: Chemical 307 (2009) 13–20
19
hydrogen atoms derived by the dissociation of hydrogen or acety-
lene. If the concentration of hydrogen atoms at the surface is lower,
then propagation of surface species should be favoured relative to
termination to gaseous products. If acetylene dissociation is dis-
favoured then less surface carbon will ensue and activity should
be higher as a result. The truncation of production of oligomers
greater than C10 is most likely a reflection of the condensation of
highly unsaturated products to carbonaceous material on the sup-
port given that the gain in catalyst weight during reaction is higher
when carbon monoxide is present.
Scheme 2.
sequential, acetylene to ethynyl ( CCH) to methylidyne and carbon
[34], as also indicated by calculations for a Rh(1 0 0) surface [49].
Formation of ethynyl can be represented in the following way with
the surface species depicted in di /di form as is the case for
the adsorption of acetylene on a Ni(1 1 1) surface [26] (Scheme 2).
The dissociation of adsorbed acetylene to form the ethynyl
species requires transfer of H to an adjacent vacant site (*) and
hence would be hindered by co-adsorbed CO. In this way carbon
monoxide could hinder acetylene dissociation and hence the pro-
duction of deactivating species. The lesser amount of CH4 formed
under steady state conditions, 0.8% with CO present versus 1.5% in
Acknowledgements
One of us (DLT) is most grateful to the Australian Research Coun-
cil for the award of a Federation Fellowship and to the CSIRO and the
University of New South Wales for the provision of the associated
research funding that supported this project.
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