CHEMICAL-STATES OF BISMUTH AND SULFUR ADATOMS ON THE POLYCRYSTALLINE PT ELECTRODE SURFACE TOWARDS HCOOH OXIDATION COMBINED STUDIES OF CYCLIC VOLTAMMETRY, IN-SITU FTIRS AND XPS ON THE ORIGIN OF ELECTROCATALYTICACTIVITY OF ADATOMS

The modification of platinum electrode surfaces by bismuth and sulfur adatoms was studied using cyclic voltammetry, in situ FTIR spectroscop y and X-ray photoelectron spectroscopy (XPS). The apparent coverage of saturation adsorption of bismuth and sulfur on Pt electrode from solu tions containing 10(-3) M Bi3+ or S2- ions were measured at approximat ely 0.69 and 0.90, respectively. The in situ FTIR spectroscopic data d emonstrated that both sulfur and bismuth adatoms can prevent, by a sur face geometric arrangement, the formation of poison species which is i dentified as adsorbed CO species derived from the dissociative adsorpt ion of HCOOH on Pt electrode. However, a big difference in electrocata lytic activity of Pt/S-ad and Pt/Bi-ad electrodes for HCOOH oxidation has been determined. HCOOH cannot be oxidized on a Pt/S-ad electrode a t the saturation adsorption of sulfur. Nevertheless, when the S adatom s have been partially removed by oxidation at potentials above 1.0 V ( Pd/H), the oxidation of HCOOH on the Pt/S-ad electrode can take place and yield a larger current than a Pt electrode does in the positive go ing potential sweep. It has been found that the Pt/Bi-ad electrode at the saturation adsorption of bismuth maintains a high electrocatalytic activity towards HCOOH oxidation, which was determined both in the cy clic voltammetric studies and in the potential step experiments of a r elatively long time window. The difference in electrocatalytic propert ies of bismuth and sulfur adatoms in HCOOH oxidation was attributed to the different chemical states of these adatoms on the Pt electrode su rface. It has been revealed by combined studies of electrochemistry an d X-ray photoelectron spectroscopy that the ions of S2- can discharge on Pt surface during adsorption forming sulfur adatom under conditions with or without electrochemical polarization. In addition, the adsorb ed sulfur adatom is mainly in an atomic state, but charged partially w ith negative charge. The adsorbed sulfur (S-ad) can be oxidized to sul fate species at potentials above 1.20 V (Pd/H). However, the Bi3+ ions in solution cannot be reduced on Pt surface during adsorption. It was determined that 67% adsorbed bismuth was reduced to its atomic state at 0.0 V (Pd/H) and 33% bismuth remained in an oxidized state even at this relatively low potential. A transition oxidized state of adsorbed Pi has been observed from the XPS spectrum recorded near 1.0 V (Pd/H) , for which the higher binding energy of Bi-ad appeared near 160 (4f(7 /2)) and 165 eV (4f(5/2)). At potentials above 1.1 V (Pd/H), all adsor bed bismuth is in an oxidized state. The present study has placed emph asis on the importance of chemical states of electrode surface in elec trocatalysis and thrown new insight to understand the origin of electr ocatalytic effect of adatoms. (C) 1998 Elsevier Science B.V.