Interaction of alcohols with evaporated metal films. Part 1.—Adsorption and decomposition of methanol on metal films of nickel, palladium and aluminium

Abstract

The interaction of methanol with clean and oxidized films of Ni, Pd and Al has been investigated in the temperature range 223–450 K. Dissociative adsorption of methanol on the clean films occurred even at 223 K. A substantial fraction of the adsorption on oxidized Al film at 223 K was reversible. Further extensive adsorption of the alcohol took place on all surfaces, excluding oxidized Al, in the temperature range 290–450 K. Oxidation of Ni and Al films occurred through the reaction of the resulting surface methoxide and hydrogen adatoms in which gaseous methane was produced. The surface methoxide on Pd underwent a series of reactions by which CH₂O, CO and H₂ were formed. Carbon dioxide and water vapour were also detected in the gas phase subsequent to the methanol adsorption on oxidized Ni and Pd films at temperatures < 290 K. The rate of methanol adsorption on any film directly depended on the methanol pressure. The activation energy (E_a) of methanol adsorption as well as the pre-exponential factor (A) in the rate equation remained independent of the variation in the extent of adsorption. A linear relationship was found between the values of log A and those of E_a for the various surfaces, with the exception of Al, suggesting the operation of a compensation effect in the methanol adsorption and decomposition on such surfaces. Adsorption of monodeuterated methanol on the films over a similar temperature range was also carried out with the aim of examining the reactivity of the oxygen–hydrogen bond and to obtain information regarding the mode of alcohol adsorption and its subsequent decomposition on the surface.