Interaction of hydrogen chloride with evaporated metal films. Part 2.—Adsorption and incorporation on films of Pd, Ag and Pb, deposited on iron substrates

Abstract

An attempt was made to deposit films of Pd, Ag and Pb on top of iron films and then to investigate the interaction of HCl with the resulting “two-metal” films of Pd/Fe, Ag/Fe and Pb/Fe in the temperature range –80 to about 200°C. The area of the Fe film increased by subsequent deposition of Pd and decreased on deposition of Ag or Pb. Adsorption of HCl at –80°C on these films occurred as on films of Pd, Ag and Pb respectively; no H₂ was evolved in any case in contrast to the behaviour of an Fe film. At temperatures 30°C, the Pb/Fe film preserved much of the properties of the Fe film. A solid solution or an alloy of Fe in Pb is assumed to be formed due to the diffusion of Fe atoms into the Pb phase; the ease with which the electron and the defect can migrate through Pb is thus disturbed by this process. The formation of such structures was probably less pronounced with an Ag/Fe film due to the smaller difference in the atomic diameter between Ag and Fe as compared with the difference between Pb and Fe and, also, because of the higher melting point of Ag than Pb; the Ag/Fe film was, therefore, slightly more active than Ag. With Pd/Fe film, where both metals have almost the same atomic diameter and melting point, there was no tendency to form a solid solution or an alloy and, therefore, the film behaved toward HCl at temperatures 30°C exactly in the same way as Pd film.

Extensive oxygen adsorption and incorporation occurred on Pd/Fe film, very much like that on Fe. The Fe atoms may have a considerable mobility for outward diffusion as a consequence of either a strong electric field on the oxidized surface or due to the lowering of the surface tension of the surface by oxygen.