Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 185

The first part of the mechanism is a sequential reaction yielding formic acid, and
from that point the typical dual path mechanism for formic acid occurs. In fact, it
has been proposed that the mechanisms of formic acid and methanol oxidation consist
of the same dominating elemental steps [Okamoto et al., 2005]. However, experiments
have revealed that the mechanism is much more complicated than that.
The initial DEMS studies showed the first differences [Willsau and Heitbaum,
1986]. The surface was initially covered with 13 CO coming from 13 CH 3 OH dis-
sociation. Next, the solution was exchanged with one containing 12 CH 3 OH and
the oxidation of methanol was followed with voltammetry. The CO 2 thus formed,
analyzed with a mass spectrometer, revealed that 12 CO 2 and 13 CO 2 were formed
simultaneously, i.e., the oxidation of methanol only took place when the CO present
on the electrode surface oxidized. As already mentioned, the same experiment with
formic acid detected the formation of 12 CO 2 prior to 13 CO 2 . This fact indicates that
steric requirements for methanol to oxidize are higher than those needed for formic
acid oxidation, since formic acid oxidation can take place on a surface pre-covered
with the poisoning intermediate. In fact, it has been proposed that the formation of
CO from methanol requires at least three contiguous Pt sites [Cuesta, 2006].
Another important difference in the poison formation reaction is observed when
studying this reaction on Pt(111) electrodes covered with different adatoms. On
Pt(111) electrodes covered with bismuth, the formation of CO ceased at relatively
high coverages only when isolated Pt sites were found on the surface [Herrero
et al., 1993]. For formic acid, the formation takes place only at defects; thus, small
bismuth coverages are able to stop poison formation [Herrero et al., 1993; Maci´
et al., 1999]. Thus, an ideal Pt(111) electrode would form CO from methanol but
not from formic acid. This important difference indicates that the mechanism proposed
in (6.17) is not valid. It should be noted that the most difficult step in the oxidation
mechanism of methanol is probably the addition of the oxygen atom required to
yield CO 2 . In the case of formic acid, this step is not necessary, since the molecule
has already two oxygen atoms. For that reason, the adatoms that enhance formic
acid oxidation, such as bismuth or palladium, do not show any catalytic effect for
methanol oxidation.
The FTIR studies revealed that the formation of CO 2 is only detected when the
CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism
has only one path, with CO as the CO 2 -forming intermediate [Chang et al., 1992;
Vielstich and Xia, 1995]. This has two important and practical consequences. First,
methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation,
mainly ruthenium. Second, since the steric requirements for CO formation from
methanol are quite high, the catalytic activity of small (,4 nm) nanoparticles
diminishes [Park et al., 2002].
Aside from CO, other intermediate species have been detected. The formation of
formic acid was detected by DEMS [Jusys and Behm, 2001; Wang and Baltruschat,
2007], whereas formaldehyde was found by fluorescence and DEMS [Korzeniewski
and Childers, 1998; Jusys and Behm, 2001; Wang and Baltruschat, 2007]. The
presence of formic acid clearly indicates that the mechanism should always have a
parallel path, although its contribution to the total CO 2 could be minor. In fact, only