Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 319

10.2.3 Modified Electronic Structure of Pt Skin Layer
and Pt-Ru Alloy
According to the EQCM and EC-STM results, we cannot invoke the bifunctional
mechanism for CO tolerance of the present alloys, because of the absence of atoms
of the second metal in the surface layer. It should also be noted that CO ad cannot be
oxidized at the HOR potential E , 0.1 V even on a Pt-Ru alloy, as shown below. In
contrast, we found very interesting features in the CO adsorption on these alloys
distinct from those of pure Pt. The first is the type of CO ad . Using in situ FTIR spec-
troscopy at electrodes with u CO , 0.6, linearly adsorbed CO was found to be dominant
at Pt alloys, whereas a large fraction of CO adsorbed on pure Pt is in the bridged form,
by which the HOR active sites were blocked more dramatically [Watanabe et al., 2000;
Igarashi et al., 2001]. The same tendency was also confirmed at 30 - 70 8C based on
CV analysis [Uchida et al., 2006]. Another important point is the deceleration of
the CO adsorption rate on the alloys [Uchida et al., 2006]. These results strongly
suggest a modified electronic structure of these alloys.
Here, we demonstrate clear and direct evidence for the modified electronic struc-
tures of surface Pt atoms in Pt-Co and Pt-Ru by using EC-XPS [Wakisaka et al.,
2006]. The sample electrode was transferred between an XPS chamber and an electro-
chemical (EC) chamber without exposure to air (to minimize contamination of the
surface). All photoelectron spectra, including the valence level region) were taken
by using a monochromatic Al Ka (hn ¼ 1486.58 eV). The uncertainty of binding
energy measurement was less than +0.03 eV.
10.2.3.1 Electronic Structures of Pt in As-Prepared Alloys Figure 10.4a
shows Pt4f 7/2 core level (CL) spectra for pure Pt, Pt 58 Co 42 , and Pt 60 Ru 40 as-prepared
alloys (before electrochemical stabilization). The CLs for both Pt-Co and Pt-Ru alloys
clearly shift to higher binding energy with respect to pure Pt. The magnitude of the CL
shift for the Pt-Ru alloy was 0.39 eV, which is larger than the 0.19 eV for the Pt-Co
alloy. A positive CL shift of an atom has been generally interpreted as electron loss
from the atom [Briggs and Seah, 1990]. However, since the order of the work func-
tions is Pt . Ru . Co, the electron transfer must occur from Ru or Co to Pt. The posi-
tive Pt4f 7/2 CL shift in the XPS is rather explained by re-hybridization of the d-band as
well as the sp-band [Weinert and Watson, 1995] as follows. The change in the work
function leads to a reference level (E F ) shift in the XPS measurement. The negative
shift of E F results in a positive shift of the Pt4f 7/2 CL as well as the d-band center.
The observed values of the CL shifts for Pt 58 Co 42 and Pt 60 Ru 40 were in good
agreement with those calculated from the electronic parameters of the pure elements
given in the literature [Miedema et al., 1980].
10.2.3.2 Electronic Structures of Pt after Electrochemical Stabilization
Each test electrode was transferred to the EC chamber and subjected to electrochemical
stabilization. The EC chamber was then evacuated rapidly by two sorption pumps and
a cryopump to transfer the electrode to the XPS chamber again.