Molecular beam studies of the dehydrogenation of alcohols on oxidised copper {110}

Abstract

The dehydrogenation of a number of alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) have been studied on oxygen-covered copper (110) under UHV conditions using a thermal molecular beam system. This paper will illustrate how this technique has given new information about these complex reactions and their rates, showing a strong dependence on the surface temperature, oxygen precoverage, as well as the alcohol involved. This is due to the beam technique being able to give absolute values of the sticking probability of the reactant, while simultaneously measuring product evolution. All alcohols give common products of the appropriate aldehyde (or ketone) water and hydrogen, however, the reaction shows a change in stoichiometry from 2R1R2CHOH + O(a) → 2R1R2CO + H2 + H2O at low temperatures to R1R2CHOH + O(a) → R1R2CO + H2O at higher temperature temperature depending on the alcohol involved). The preadsorbed oxygen strongly enhanced the absolute sticking probability of all the alcohols studied, but also poisons the dehydrogenation reaction at higher coverages. The oxygen coverage variation studies clearly show that the alcohol adsorption involves a precursor state. Isotopic labelling experiments have also been carried out which help to explain clearly many of the effects observed, especially pointing to the importance of the stability of the intermediate alkoxy species.