Propyne chemistry on Ni(111) and Cu(110); analogies with ethyne adsorption

Abstract

There now exists an accepted database for ethyne and ethene adsorbed on metals, but for larger hydrocarbons, even C₃ molecules, this is not the case and it becomes increasingly difficult to assign spectra arising from the adsorption of these molecules on surfaces. In this paper we report reflection–absorption IR spectroscopy (RAIRS) data for propyne adsorbed on Ni(111) and Cu(110) surfaces at low temperatures. The difficulty of unambiguous band assignments was overcome by studying molecules with targeted isotopic substitution, e.g. the partially deuteriated molecule CD₃CCH. Direct observation of the acetylinic hydrogen environment establishes that the molecule is significantly perturbed upon adsorption, with the gas phase CC triple bond suffering a reduction in bond order to ca. 1 upon adsorption. Propyne is shown to be molecularly adsorbed with the formation of a diσ/diπ-bonded species. We find a strong correlation between the adsorption complexes formed by propyne and those formed by ethyne on these surfaces and, therefore, conclude that the surface chemistry of these molecules is largely determined by the CC triple bond functionality. The diσ/diπ surface species formed at low temperature is shown to be stable up to room temperature on both surfaces. At higher temperatures, the propyne adsorbate is seen to trimerise to form trimethylbenzene on the Cu(110) surface.