Adsorption, desorption and surface decomposition of formaldehyde and acetaldehyde on metal films nickel, palladium and aluminium

Abstract

The interaction of formaldehyde and acetaldehyde with clean and oxidized films of Ni, Pd and Al has been investigated in the temperature range 195 or 223 to 510 K. Extensive adsorption of formaldehyde (HCHO) occurred on the films in the temperature range 195–273 K, and a substantial fraction of the adsorbed aldehyde could be removed from the films on heating to 510 K. Dissociative adsorption of HCHO, which took place to a much less extent than molecular adsorption, produced CO and H₂ gases. Acetaldehyde (CH₃CHO) adsorption on the films at 223 K was less extensive as compared with the adsorption of HCHO on the same metals under similar experimental conditions. A slow desorption of CH₃CHO, together with CO, H₂ and CH₄ gases, was shown to occur in the temperature range 273–300 K. Further slow CH₃CHO uptake, which was accompanied by the evolution of CO, H₂ and CH₄ gases took place on Ni and Pd films above 300 K. Adsorption of each aldehyde on the oxidized films was both molecular and dissociative, and the nature of the decomposition products was similar to those given for the clean films. Adsorption of each aldehyde, under similar conditions of pressure and temperature, was less extensive on oxidized surfaces than on clean films. Above 423 K, some CO₂ gas was also detected subsequent to the dissociative adsorption of each aldehyde on oxidized films. The energies of activation (E) for the adsorption of each aldehyde on the films were approximately the same. The variation of the rates of adsorption and consequently the activities of the films were ascribed to the variation of the values of the pre-exponential factor (A) in the rate equation. The values of E and A for the adsorption of CH₃CHO were greater than those obtained for HCHO adsorption on the same metals. The energies of activation for the desorption of each aldehyde, which was adsorbed at temperatures 273 K, were within the range of the energies of physical adsorption on various surfaces.