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 electrocatalytic activity of adatoms
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The modification of platinum electrode surfaces by bismuth and sulfur adatoms was studied using cyclic voltammetry, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). The apparent coverage of saturation adsorption of bismuth and sulfur on Pt electrode from solutions containing 10(-3) M Bi3+ or S2- ions were measured at approximately 0.69 and 0.90, respectively. The in situ FTIR spectroscopic data demonstrated that both sulfur and bismuth adatoms can prevent, by a surface geometric arrangement, the formation of poison species which is identified as adsorbed CO species derived from the dissociative adsorption of HCOOH on Pt electrode. However, a big difference in electrocatalytic 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 at the saturation adsorption of sulfur. Nevertheless, when the S adatoms 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 going 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 cyclic voltammetric studies and in the potential step experiments of a relatively long time window. The difference in electrocatalytic properties of bismuth and sulfur adatoms in HCOOH oxidation was attributed to the different chemical states of these adatoms on the Pt electrode surface. It has been revealed by combined studies of electrochemistry and 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 adsorbed sulfur adatom is mainly in an atomic state, but charged partially with negative charge. The adsorbed sulfur (S-ad) can be oxidized to sulfate 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 adsorbed bismuth is in an oxidized state. The present study has placed emphasis on the importance of chemical states of electrode surface in electrocatalysis and thrown new insight to understand the origin of electrocatalytic effect of adatoms. (C) 1998 Elsevier Science B.V.