Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 175

may be related to the formation of “activated water” or surface-bonded hydroxyl that
can oxidize CO. Note that this would correspond to a potential some 0.8 V negative of
Au surface oxidation. Such small features are generally observed on Au single-crystal
electrodes [Hamelin, 1996; Hamelin and Martins, 1996], although they have never
been unequivocally been identified with OH ads formation. On Au(100), surface recon-
struction is known to occur in this potential region [Kolb, 1996], but whether it is
associated with concomitant OH adsorption remains an open question. Using
surface-enhanced Raman spectroscopy (SERS), Li and Gewirth [2003] observed a
spectral feature at 790 cm 21 at potentials above 0.7 V (vs. RHE), which they ascribed
to a bending vibration of surface-bonded OH, lending credibility to the claim by
Markovic and co-workers that some level of OH formation may occur well below
the Au surface oxidation potential (1.2 - 1.3 V).
DFT calculations showed that a reaction between surface-bonded CO and OH on
Au(110) has a low activation barrier (approximately 0.2 eV) whereas the same reaction
on Pt(111) has a much higher barrier [Shubina et al., 2004]. Both on Au and on Pt, the
resulting COOH is relatively strongly bonded. This is evidence in favor of Weaver's
model for CO oxidation on Au, in which adsorbed CO reacts directly with nearby
water to form adsorbed COOH.
6.2.4 Carbon Monoxide Oxidation in Alkaline Solution
Alkaline solutions are generally known to lead to better catalytic activities than acidic
solutions for many relevant electrode reactions. However, owing to the paucity in the
development of suitable electrolyte materials, such as alkaline membranes, there has
been much less fundamental work in the area of fuel cell catalysis in alkaline media.
Nevertheless, there are a few hopeful developments in new alkaline polymer mem-
branes [Varcoe and Slade, 2005] that are currently stirring up interest in studying
fuel cell catalytic reactions in alkaline solution.
Spendelow, Wieckowski, and co-workers have published results on the mechanism
of CO oxidation on a Pt(111) electrode in 0.1 M KOH [Spendelow et al., 2004, 2006].
In 0.1 M NaOH, the CO stripping potential lies about 100 mV lower than in 0.1 M
H 2 SO 4 , on the reversible hydrogen potential scale. On slightly disordered Pt(111),
a new CO oxidation peak appears between 0.4 and 0.6 V, whereas the one correspond-
ing to well-ordered Pt(111) remains at about 0.8 V but loses intensity. The low poten-
tial oxidation peak is ascribed to small Pt islands, the edges of which are supposed to
adsorb OH at low potential. Spendelow, Wieckowski, and co-workers attribute the
higher CO electro-oxidation activity in alkaline media to the higher affinity of the
Pt(111) surface, and especially the defects, for the adsorption of OH. This should
be more than a trivial pH effect, as that has been taken into account by referring all
the potentials to the RHE scale. They also conclude that CO oxidation in alkaline
media follows the standard Gilman - Langmuir - Hinshelwood mechanism, in both
potential regions.
More recently, Garc´a and Koper studied CO adlayer stripping on a series on
stepped Pt electrodes in alkaline media [Garc´a and Koper, 2008]. They found that,
in contrast to acidic media, CO mobility on the (111) terraces is very low, a conclusion