Interaction of alcohols with evaporated metal films. Part 2.—Adsorption and decomposition of ethanol on metal films of nickel, palladium and aluminium

Abstract

The interaction of ethanol with clean and oxidized films of Ni, Pd and Al has been investigated in the temperature range 223–450 K. Dissociative adsorption of ethanol on the clean surfaces took place even at 223 K. Reversible molecular adsorption of ethanol occurred on oxidized Al films. Adsorption of ethanol on Ni and Al films gave rise to metal oxidation through the reaction of surface ethoxides by which gaseous ethane and hydrogen were formed. Dehydrogenation of ethanol occurred mainly on Pd films and the formation of acetaldehyde, together with carbon monoxide and some methane, was observed. Dehydration of ethanol proceeded on oxidized Ni film resulting in the liberation of ethylene gas. The production of CO₂ gas subsequent to ethanol adsorption on oxidized Ni and Pd films occurred, probably through the formation of acetate radicals on the surface. Parallel experiments using monodeuterated ethanol were carried out on all surfaces with the aim of understanding the modes of alcohol adsorption and decomposition on metal surfaces. The rate of ethanol adsorption on each surface depended directly on the ethanol pressure to the first power and the activation energy (E_a) of adsorption was not a function of the extent of adsorption. A linear relationship was found between E_a and the logarithm of the pre-exponential factor of the rate equation, suggesting the operation of a compensation effect in the alcohol adsorption and decomposition on the films. Ethanol was found to have a greater tendency for adsorption and decomposition on the metals than methanol. A sequence was suggested for the activity of the various surfaces in alcohol adsorption and decomposition.