Abstract:
The interaction of HCl with evaporated metal films of iron, nickel, palladium, silver and lead has been investigated in the temperature range from –78°C to about 250°C. At –78°C, dissociative adsorption of HCl occurred on iron film, dissociative and molecular on nickel, and mainly molecular on palladium. In addition to dissociative chemisorption on lead, some incorporation also occurred at –78°C. At the latter temperature, the adsorption on silver proceeded to a less extent than on any of the other four metals. The presorbed hydrogen on iron was displaced by the subsequent HCl chemisorption at –78°C due probably to the formation of a stronger metal-chloride bond than that of metal-hydrogen. The formation of bulk chloride on all the five metals began at [gt-or-equal] 30°C with constant energies of activation. The rate of incorporation at a given temperature and HCl pressure decreased as the thickness θ of the chloride layer increased. This was attributed to the decrease of the frequency factor A in the rate expression with increasing θ the concentration of the reacting sites on the surface decreased exponentially as θ increased. From the plots of log A against θ it was possible to derive certain conclusions concerning the activity of the metal films for reaction with HCl. Lead was the most, and silver the least, active metals. The desorption of HCl from nickel at >200°C was accompanied by hydrogen adsorption; the reduction of the chloride layer by the adsorbed hydrogen seems to be possible at these temperatures. Oxidized films were, in general, more reactive toward HCl than the clean metal due to the higher A values on the former. The adsorbed oxygen probably provides additional sites for chlorine chemisorption as well as for hydrogen.
