Disproportionation and coupling reactions of alkyl iodides on a Au(111) surface

The chemistry of linear (C 2-C 4) alkyl iodides on a Au(111) surface has been studied by temperature programmed reaction (TPR) spectroscopy, surface work function change (ΔΦ) measurements, Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). Alkyl iodides adsorbed at 110 K remain molecularly intact on the surface. Thermal dissociation of the C-I bond occurs above 200 K to give surface bound alkyl groups which then undergo coupling and disproportionation reactions. Isotope labelling experiments suggest that the disproportionation reaction is initiated by rate-limiting β-hydride elimination to give the corresponding alkenes. The surface hydrogen atoms produced by these reactions undergo rapid reductive elimination with unreacted alkyl groups to evolve alkanes. The net result is a strict disproportionation reaction with no detectable molecular H 2 desorption from the surface. Photodissociation studies of alkyl bromides show that the reaction pathways and kinetics are unaffected by whether the coadsorbed species is Br or I. An interesting observation is that, unlike alkyl reactions on other metals where either the coupling reaction (Ag) or the decomposition reaction (Pt, Cu) is predominant, on gold, both of these reactions occur and give appreciable yields at all exposures. It appears to be coincidental that both alkyl coupling and disproportionation occur at ~265 K on Au(111). Factors influencing the relative rates of these processes are also discussed.