Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 386

conditions of the CO adsorbate resulting from the methanol decomposition process. In
the forward direction, starting at negative potential, the CO SFG amplitude increases,
reaching a maximum at about 0.3 V, and then disappearing at about 0.5 V. In the back
scan, CO reappears at about 0.47 V and reaches a maximum at about 0 V. The CO fre-
quency shift reflects Stark tuning and intermolecular interactions among the coadsor-
bates. For kinetic studies (Fig. 12.13e - g), the potential was first fixed at 0.75 V, where
CO was removed rapidly while the bulk methanol oxidation activity is low to minimize
depletion of methanol in the solution gap (Fig. 12.13g). Jumping to a lower potential
(0.20 V) where the CO adlayer is stable, SFG monitored the CO build-up from metha-
nol. Figure 12.13e shows that the build-up occurs on the 20 s time scale, and reaches its
maximum at steady state. The CO frequency shift tracks the amplitude (Fig. 12.13f ).
As CO is deposited on the bare electrode and coverage grows, the CO frequency blue-
shifts as a result of interactions among coadsorbates [Severson et al., 1995]. The CO
frequency closely reflects the coverage dependence, suggesting that CO is formed ran-
domly and is distributed uniformly instead of being arranged in dense patches.
Surface reactions that give rise to the electric current measured depend on the elec-
trode potential range and the type of measurements reported. The dominating reactions
contributing to the voltammetric data in Fig. 13a - d are chemisorbed CO oxidation,
oxidation of methanol to CO 2 (on both scans), and chemisorbed CO formation on
the reverse run:

CO þ H 2 O ! CO 2 þ 2H þ þ 2e (12 : 7)

CH 3 OH þ H 2 O ! CO 2 þ 6H þ þ 6e (12 : 8)

CH 3 OH ! CO þ 4H þ þ 4e (12 : 9)

The drop of the voltammetric current is associated with Pt surface oxidation, and the
drop on the negative-going run is due to Reaction (12.9) (surface poisoning by CO)
and the Tafelian kinetics of Reaction (12.8). Further, the shift between curves in
Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol
oxidation occurs with zero or low level atop CO surface intermediate. The amplitudes
on Fig. 12.13 on both scans nearly equal to each other indicate a high level of prefer-
ential (111) crystallographic orientation of the polycrystalline Pt surface used for this
work, as inferred from data in [Adzic et al., 1982].

12.4.4 Decomposition of Formic Acid on Pt(111)

While very limited data are presented here, the kinetics of adsorption/decomposition
of formic acid molecules [Rice et al., 2002] have been measured by BB-SFG, as
shown in Fig. 12.14. A Pt(111) electrode and a 0.1 M H 2 SO 4 electrolyte containing
0.1 M formic acid were used. The families of spectra at 20.200, 20.025, and 0.225
V vs. Ag/AgCl were collected as functions of decomposition time. The electrode
potential was initially held at 0.75 V to produce a clean Pt(111) surface, and was
next switched to monitor the CO uptake. Starting at 0s, where CO adsorption (from
HCOOH decomposition) had not yet begun, the potentiostatic experiment lasted
until about 500 s of the progress of reaction. The spectral position is typical of the