Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum

The hydrogenation reaction of maleic acid, catalyzed by platinized platinum , was investigated in the presence and in the absence of gaseous hydrogen, and with/without an external control of platinum potential, in different ac id supporting electrolytes. The intrinsic catalytic activity of platinum, u sed as a catalyst and as an electrode, was measured at different potentials . A promotion of the catalytic activity of platinum, induced by an external control of its potential, in the presence of gaseous hydrogen, was found o nly in a perchloric acid supporting electrolyte. No promotion effect was ob tained in the presence of strongly adsorbed anions (HSO4-, Cl-). Moreover s trongly adsorbed anions induce a decrease in the catalytic activity of plat inum, whereas they do not affect the electrocatalytic activity of platinum catalyst-electrode. The observed anion effects on the hydrogenation of male ic acid were compared with those obtained in the electrocatalytic hydrogen oxidation (h.o.r.) and evolution (h.e.r.) reactions studied on a rotating d isk platinum electrode. In electrocatalytic oxidation of hydrogen the curre nt densities depend on the nature of the supporting electrolyte and decreas e in the order, j(HClO4) > j(H2SO4) > j(HCl). However, the hydrogen evoluti on reaction, studied in underpotential and in low overpotential regions, do es not depend on the nature of the supporting electrolyte. The ensemble of results is discussed in connection with the nature of active hydrogen speci es and/or active platinum sites involved in these reactions. (C) 2000 Elsev ier Science Ltd. All rights reserved.