Adsorption, decomposition and surface reactions of methyl chloride on metal films of iron, nickel, palladium, lead, gold and copper

Abstract

Adsorption and decomposition of methyl chloride on metal films of Fe, Ni, Pd, Pb, Au and Cu have been investigated in the temperature range 193–570 K. Both molecular and dissociative adsorption of CH₃Cl occurred on all films at 193 K. Above 300 K the dissociative chemisorption of CH₃Cl on all the films was accompanied by the evolution of CH₄, C₂H₆ and H₂ together with some C₂H₄ on Fe, Ni, Pb and Au. The gaseous products subsequent to CH₃Cl adsorption on the oxidized films of Fe, Ni, Pd, Pb and Cu above 350 K involved CH₄, C₂H₆, H₂ and CO gases. The structure of the surface phase was represented as C_nH_mCl and the minimum values of the ratio m/n on Fe, Ni, Pd, Au and Cu ranged from 2.69 to 2.93, but remained as 2.1 on Pb, indicating the existence of CH₃ and CH₂ radicals on the surface. Further support for the surface species was derived from the results of CH₃Cl adsorption on Fe and Ni films which had been saturated with deuterium. No HCl, CH₂O, CH₃OH, H₂O, CO₂ or other gases have been detected at any stage, and the adsorption of chlorine atoms, resulting from CH₃Cl decomposition, on the film surface was accompanied by some incorporation in the bulk of the metal. The kinetic data revealed the direct dependence of the reaction rate on CH₃Cl pressure and the operation of a compensation effect throughout the interaction of CH₃Cl with various surfaces. On the basis of such a compensation and the linearity of the relationship existing between the activation energies (E_a) and the pre-exponential factors of the rate equation (log A), it was possible to arrange the clean and oxidized films in the order of decreasing activity towards CH₃Cl adsorption and decomposition. An attempt was also made to estimate the enthalpy changes associated with the dissociative adsorption of CH₃Cl on various surfaces, as well as for the subsequent reactions of the resulting species on the basis of the standard enthalpy involved in the respective processes.