X-ray Photoelectron Spectroscopy (XPS) is a powerful
technique for characterization of heterogeneous catalysts,
which can provide information about elemental composition,
chemical state, coverage and thickness of adlayers on
surface. For application of XPS to fuel cell study, the
most important issue is to avoid contamination and
oxidation of the catalytic electrode surface by air during
emersion process from electrolyte. To avoid contamination,
we employ a closed system to transfer the catalytic
electrode from solution to UHV. All experiments can be
performed in the system which consists of two connected
chambers: one for the electrochemical treatment of the
surface to be analyzed and one for the subsequent XPS
measurement. The above photograph shows the entire UHV
apparatus. The main XPS chamber is equipped with an ESCA
M-Probe high-resolution, multi-channel hemispherical
electron analyzer (Surface Science Instruments) and
an ion gun for surface cleaning. The electrochemical
chamber can be backfilled with pure Ar during
electrochemical measurement. The electrochemical cell is
connected with a solution delivery system, which can store,
de-aerate, inject, and eject solutions by pressure of pure
Ar.
The experimental procedure is as follows: After cleaning in
the main XPS chamber, the sample electrode is transferred
to the electrochemical chamber. After the electrochemical
chamber is backfilled with pure Ar up to atmospheric
pressure, the electrochemical cell is inserted into the
electrochemical chamber, and the sample electrode is
immersed into electrolyte. After electrochemical
measurement, the electrode is emersed from the electrolyte
under potential control. The residual droplet of solution
on the electrode surface is sucked away via capillary
tubing. Then, the electrochemical chamber is evacuated with
2 sorption pumps and a cryo-pump, and the sample electrode
is finally transferred to the main XPS chamber for
measurement.
Previous XPS results of submonolayer Ru on Pt(111) indicate
that the presence of metallic ruthenium is a prerequisite
for effective Ru/Pt methanol electrooxidation catalysis.
Notably, exposure of the Ru/Pt surface to a
methanol-containing solution at a fixed electrode potential
simulates the effect of reduction in the positive value of
the electrode potential. That is, participation of the
metallic Ru phase is enhanced due to the exposure. This
most likely reflects the physical phenomena that occur when
Pt/Ru catalysts are "activated" in real-world fuel cell
devices. Current studies focus on the chemical state of Mo-modified Pt3Co catalysts and the shifts in binding energy of Pt, Ag, and Cu on Rh surfaces.