A molecular beam study of the oxidative dehydrogenation of alcohols on Cu(110)

Abstract

The adsorption and reaction of ethanol, propan-1-ol, and propan-2-ol have been studied on clean and oxygen-covered copper(110) by using a thermal molecular beam. The preadsorption of oxygen greatly enhances the sticking probability of all the alcohols, although a saturation coverage ( p (2 × 1) LEED pattern) poisons the oxidative dehydrogenation reaction. The reaction mechanism has been worked out in detail with the help of isotopic labeling experiments and is common to all alcohols examined. There is a change of stoichiometry observed with temperature, at low temperatures (at ambient or just above) 2 R 1 R 2CHOH + O (a) → 2 R 1 R 2CO + H 2O + H 2, changing to R 1 R 2CHOH + O (a) → R 1 R 2CO + H 2O at higher temperature, the exact temperature depending on the alcohol involved. The rate of reaction follows the stability of the intermediate alkoxy species except for 2-propoxy which shows a slower rate than expected. This effect and the poisoning observed for saturation coverages of preadsorbed oxygen are due to the stabilization of the alkoxy species by coadsorbed surface hydroxyl groups or oxygen atoms. Overall the reaction rate is limited by the decomposition of the alkoxy species.