Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 160

Similar considerations apply to the oxidation of ethanol, although one must take into
account the breaking of the C22C bond, and the formation and involvement of differ-
ent intermediates and side products. Ethanol oxidation will be the topic of Section 6.5.
Section 6.6 will briefly summarize our main conclusions and discuss the relation of
our “surface science approach” to real catalysts.

6.2 CARBON MONOXIDE OXIDATION

6.2.1 Carbon Monoxide Oxidation on Platinum

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most
severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly
bonded intermediate in methanol (and ethanol) oxidation. It is also a side product
in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as
such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electro-
oxidation is one of the most intensively studied electrocatalytic reactions, and
there is a continued search for CO-tolerant anode materials that are able to either
bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced
overpotential.
There are essentially two ways to study electrochemical CO oxidation. The first
one, called stripping, involves pre-adsorbing a (saturated) monolayer of CO and
then oxidizing it from a solution that does not contain CO, in either a voltammetric
scan (stripping voltammetry) or a potential-step experiment (stripping chronoampero-
metry). Chronoamperometry has the advantage of studying the system at a fixed poten-
tial, and is generally the preferred method for investigating kinetics. The second way to
study electrochemical CO oxidation is continuous or bulk CO oxidation, that is, with a
fixed (often saturated) concentration of CO in solution. We will discuss here mainly
CO stripping chronoamperometry on platinum single-crystal electrodes, as these
experiments are relatively straightforward to interpret (Section 6.2.1.1). Stripping vol-
tammetry of CO on platinum will be discussed in Section 6.2.1.2. Section 6.2.1.3 will
deal with continuous CO oxidation.
All experiments to be described below are interpreted on the basis of the
Langmuir - Hinshelwood (LH) mechanism for CO electro-oxidation suggested
by Gilman more than 40 years ago [Gilman, 1964]. According to Gilman's
model, water needs to be activated on a free site on the surface, leading to surface-
bonded OH:

H 2 O þ ! OH ads þ H þ þ e

(6 : 1)

where the denotes a free site on the platinum surface. Note that we write this reaction
as reversible. The surface-bonded OH is the oxygen donor reacting with surface-
bonded CO to form CO 2 :

CO ads þ OH ads ! CO 2 þ H þ þ e þ 2 (6 : 2)