Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 275

Figure 9.5 Anion adsorption on Pt in several electrolytes: bisulfates and TFMSA as deter-
mined by radiotracers (RT), and OH and H as reflected in Dm, the amplitude of the XANES
peak. The effect of 6 M TFMSA suppressing OH formation can be seen. (Reproduced with per-
mission from Teliska et al. [2007].)
of oxides on the Pt surface. Figure 9.5 compares anion adsorption on Pt in several elec-
trolytes as determined by radiotracers showing the adsorption of bisulfates [Zelenay
and Wieckowski, 1991] and TFMSA, and OH and H as reflected in Dm, the amplitude
of the X-ray absorption near-edge structure spectroscopy (XANES) peak [Teliska
et al., 2007]. The effect of 6 M TFMSA suppressing OH ads formation is seen.
Enhanced ORR kinetics in this solution indicates that the suppression of OH is not
due to impurities that are often observed in concentrated TMFSA.
Earlier work on Pt oxidation was summarized in [Conway, 1995]. Recent EXAFS
studies, by Teliska and co-workers have revealed new information on Pt oxidation
[Teliska et al., 2007]. For Pt/C nanoparticles in a fuel cell electrode at 80 8C, the
peak corresponding to the Pt - O bond occurred concomitantly with the decrease in
the intensity of the peak corresponding to the Pt - Pt bond. This is indicative of the
restructuring of a Pt surface. The adsorption of O and OH on a Pt/C electrode com-
prising 1.5 - 3 nm Pt nanoparticles in 0.1 M HClO 4 or H 2 SO 4 electrolytes were studied
in situ with extended X-ray absorption fine structure (EXAFS) and XANES tech-
niques. Differences in the L3 XANES spectra at different potentials were used to sep-
arate the effects of O or OH adsorption in the XANES spectra. At low coverages, OH
adsorbs primarily in 1-fold coordinated atop sites. As the coverage increases, O binds
in the bridge/fcc sites, and at still higher coverages and further oxidation above 1.05 V
(vs. RHE), O adsorbs into a higher coordinated n-fold or subsurface site, which is
apparently the result of Pt - O site exchange and oxide formation [Sasaki et al., 2008].
Studies have shown that the Pt oxides are not hydrated [Birss et al., 1993;
Harrington, 1997; Jerkiewicz et al., 2004]. Electrochemical quartz crystal microba-
lance [Birss et al., 1993] and nanobalance [Jerkiewicz et al., 2004] experiments