Adsorption and decomposition of acetylene on evaporated metal films

Abstract

The interaction of C₂H₂ with evaporated films of Fe, Ni, Pd, W, Al and Cu has been investigated in the temperature range 195 to 473 K. Adsorption at 195 K and under a gas pressure of 4 N m^–2 occurred rapidly on the first four metals but at an appreciably slower rate on the other two films. The reaction of the gas with Fe, Ni, W and Al films at temperatures 303 K was accompanied by a steady evolution of hydrogen as confirmed by careful analysis of the gas phase using a pre-calibrated Piranigauge. Such behaviour began at 393 K on Pd and at around 453 K on Cu. Extensive C₂H₂ uptake occurred on films of Fe, Ni, Pd and Cu in the temperature range 303–473 K subsequent to the completion of the surface layer and the process was accompanied by H₂ evolution. No self-hydrogenation of C₂H₂ was possible on any film below 473 K, but above this temperature there was slow H₂ uptake by Ni films which was followed by desorption of ethylene at a similar rate. The activation energy (E) of C₂H₂ adsorption on each metal increased rapidly as the extent of the gas uptake (θ) increased. The pre-exponential factor (A) for the reaction in each case was shown to decrease with increasing values of the energy of activation. The variation of log A with E was always linear suggesting the operation of the anti-compensation effect in C₂H₂ interaction with the metal films.