376
J. Phys. Chem. 1980, 84, 376-381
(16) R. Sips, J. Chem. f%ys., 18 (9),1024 (1950);(a) W. A. Steele, "The Solid-Gas Interface", Vol. 1, E. A. Flood, Ed., Marcel Dekker, New York, 1967,Chapter 10. (17) J. M. Honig and P. C. Rosenbloorn, Can. J. Chem., 33, 193 (1955). (18) J. M. Honig, J. Phys. Chem., 57, 349 (1953). (19) G. F. Cerofolini, Surf. Sci., 51, 333 (1975). (20) P. Brauer, Wiss. b. Friedrich-Schiller-Univ., Jena, Math.-Natunviss. Reihe, 26, 719 (1977). (21) H. Zeise, Z.Phys. Chem., Stoechiom. Verwandschaftsl.,136,385 (1928).
(22) W. Fritz, Ph.D. Dissertation, Universitat I 1.2 V, all adsorbed organic species are practically absent from the platinum electrode.2 Therefore, after anodic activation a t E = 1.65 V for 1 min, we abruptly switch E to E , = 0.45 V a t T = 0. A t E,, methanol is oxidized on the electrode for a period of T s when the adsorption of methanol also takes place, and subsequently a rapid cathodic potential sweep of 8 V/s is applied to the electrode from E = E , to E = 0.05 V. When the sweep rate is rapid enough for A adsorbed a t E , to stay on the electrode during the potential sweep and also for the additional A not to adsorb on it, the site occupied by A at E , is not able to adsorb hydrogen. Consequently, the amount of
Binding Energy (eV)
Flgure 2. The XPS spectrum of Pt 4f electrons of 100 atom % Pt. The half-band width was taken as 1.5 eV in the deconvolutlon.
TABLE I: Binding Energy of Platinum 4f Electrons (eV) 4f5jz 74.5 76.1 74.5 76.0 76.8 78.4
44
assignment
( A ) 100 atom % Pt 71.2 PtO 72.8 Pt(OH)Z (B) 1 6 atom % Pt-Sn Oxide 71.2 PtO 72.8 Pt(OH)Z 73.6 PtO 75.1 PtO,/Pt(OH),
adsorbed hydrogen observed by the potential sweep of the present potential sequence (Figure 1)gives the number of sites which are still vacant when the electrode is polarized at E , for r s. The current for a sweep rate of 8 VIS was recorded as the iR drop with a Wave Memory (E-5001 of NF) through a differential amplifier. 7 was varied from 1 to 600 s. All electrochemical measurements were done potentiostatically at room temperature. We employed 2.0, reported for Sn02on glass,lg as a roughness factor for the Pt-Sn oxide. X-Ray Photoelectron Spectroscopy ( X P S ) . The XPS spectrometer was an ESCA 3 (Vacuum Generator). Catalysts were inserted into the spectrometer after being washed with ethanol. Spectra were taken with Mg Ka X rays at pressures
