Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 373

kinetics on millisecond time scales commensurate with rates of surface poisoning in
fuel cell electrocatalysis.

12.2 EXPERIMENTAL

All experiments were carried out at ambient temperature (21+2 8C). Potentials were
measured against a commercial Ag/AgCl electrode (BAS), and are reported versus
this reference scale. To date, we have investigated the following samples: a smooth
polycrystalline Pt disk, a Pt single crystal 6 mm in diameter cut along (111) and
(100) orientations, and a smooth polycrystalline Au disk with immobilized Pt nano-
particles (approximately 6 nm). The single crystal was prepared by annealing in a
hydrogen/air flame, and cooling in an Ar/H 2 atmosphere and in ultrapure water
[Clavilier, 1999]. The spontaneous deposition of Ru on Pt(111) was carried out
from solutions of 1 mM of RuCl 3 in 0.1 M of HClO 4 in a preparative electrochemical
cell [Crown and Wieckowski, 2001; Chrzanowski and Wieckowski, 1997; Crown et al.,
2001, 2002]. After Ru deposition, the electrode was removed from the cell, rinsed with
Millipore water, and “stabilized” by three voltammetric cycles between 0.01 and 0.60
V in 0.1 M H 2 SO 4 electrolyte (single spontaneous deposition [Crown et al., 2002]).
Cyclic voltammetric and chronoamperometric measurements were carried out using
a PAR 263A potentiostat through Corrware software. The chemicals used were
sulfuric, perchloric, formic, and acetic acids (GFS Chemicals), methanol (Fisher),
RuCl 3 . xH 2 O (Aldrich), CO (Matheson, research purity), and Millipore water.
The third-generation SFG apparatus is shown in Fig. 12.3. The heart of the system
is a compact femtosecond laser (Quantronix Integra-C 2.0) having a diode-pumped
fiber oscillator and a chirped-pulse Ti : sapphire amplifier. The output pulses at 800 nm
are 120 fs in duration with 2 mJ energy at a repetition rate of 1 kHz. Three-fourths
of the output energy is used to pump an optical parametric amplifier (OPA, Light
Conversion TOPAS-C 800 fs DFG) equipped with both AgGaS 2 and GaSe difference-
frequency output crystals. The OPA generates the broadband infrared (BBIR) pulses,
which are tunable from 2.5 to 16 mm. The remainder of the laser output is sent through
a Fabry - Perot air-spaced etalon (TecOptics) to create narrowband picosecond visible
pulses (NBVIS) at 800.0 nm. These NBVIS pulses are asymmetric in time, having a
steep (about 120 fs) rising edge and a gradual (a few picoseconds) falling edge, which
makes them useful for suppressing the nonresonant contribution [Bain, 1995] from the
metal surface [Shen, 1989]. The NBVIS and BBIR pulses are made time-coincident
using an optical delay line, and both are made spatially coincident on the electrode sur-
face. Typical pulse energies are 5 mJ in the IR and 20 mJ in the visible. In previous
work, following [Patterson et al., 2005; Lagutchev et al., 2005], we made the BBIR
and NBVIS pulses collinear with a dichroic beam splitter and focused them with a
common CaF 2 lens. This set-up proved lossy and cumbersome, with suboptimal aber-
rent beam focusing, and it did not support IR wavelength tuning below 10 mm owing
to CaF 2 absorption. The new apparatus uses a more efficient design shown in
Fig. 12.3c, where the pulses are independently focused by individual lenses: a glass
lens

for the NBVIS and an antireflection-coated ZnSe lens for the BBIR.