Chemical deposition of foreign metals on a Pd sheet and its application to continuous hydrogenation of 4-methylstyrene

Chemical deposition of foreign metals such as Pt, Au, Cu and Zn on a Pd sheet was carried out by using hydrogen atoms passing through the Pd sheet (thickness, 50 mu m) which served as a working electrode for water electrolysis to produce H atoms, a separator between the electrochemical and chemical reaction compartments, a selectively permeable membrane of H atoms and a chemical reaction field for hydrogenation. The amounts of Pt and Au were found to increase linearly with electrolysis time in the 6 M KOH side of a two-compartment cell, whereas Cu and Zn were scarcely deposited (less than 0.2 mg cm(-2)). This result can be explained in terms of the dissociative adsorbability of hydrogen on these metals. Its application to the hydrogenation of 4-methylstyrene was also investigated by using the same electrochemical cell. It became clear that the deposition of noble metals such as Pt and Au on a Pd sheet greatly improves the hydrogenation rate of 4-methylstyrene as compared with a bare Pd sheet, while no hydrogenation products were detected in the case of the Pd sheet on which Cu or Zn had been deposited. This is because, in contrast to Pd, Pt and Au, both Cu and Zn have only very weak catalytic activity for hydrogenation, and they rather suppress the hydrogen diffusion through the Pd sheet, as proved by measurements of H permeation. (C) 1998 Elsevier Science S.A. All rights reserved.