Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 531

15.5.3 Oxidation of C 1 Molecules: CO, HCOOH, and CH 3 OH
15.5.3.1 Oxidation of Adsorbed CO The electro-oxidation of CO has been
extensively studied given its importance as a model electrochemical reaction and its
relevance to the development of CO-tolerant anodes for PEMFCs and efficient
anodes for DMFCs. In this section, we focus on the oxidation of a CO ads monolayer
and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable
advantage of CO ads electro-oxidation as a model reaction is that it does not involve
diffusion in the electrolyte bulk, and thus is not subject to the problems associated
with mass transport corrections and desorption/readsorption processes.
It is now widely accepted that CO ads electro-oxidation proceeds via the L - H
mechanism, which includes the reaction steps of CO adsorption, water splitting
(15.18), and CO ads þ OH ads recombination (15.19) on two adjacent sites to yield CO 2 :

H 2 O þ Pt ! Pt OH ads þ H þ þ e (15 : 18)

CO ads þ OH ads ! COOH ads ! CO 2 þ H þ þ e (15 : 19)

Owing to the high adsorption energy of CO on Pt, high surface coverages of CO ads are
formed at adsorption potentials in the H UPD region [Cuesta et al., 2006].
The COOH ads species was proposed by Gilman [1964] as an intermediate in
CO ads þ OH ads recombination (15.19), and the feasibility of this reaction scheme
has recently been validated by DFT calculations [Anderson and Neshev, 2002;
Saravanan et al., 2003; Shubina et al., 2004; Filhol and Neurock, 2006]. Formation
of COOH ads species was suggested by Zhu and co-workers based on the results of atte-
nuated total reflection (ATR)-FTIR spectroscopy [Zhu et al., 1999]. CO ads þ OH ads
recombination has been suggested to be controlled by the number of active sites
that are able to dissociate water molecules [Maillard et al., 2004a; Andreaus et al.,
2006]. Lebedeva and co-workers have shown that on Pt single crystals, OH ads species
are preferentially formed at monoatomic steps [Lebedeva et al., 2002a, b]. An increase
in step density strongly enhances the electrocatalytic activity of Pt, and the reaction rate
increases linearly with defect density [Petukhov et al., 1998; Lebedeva et al., 2002b],
confirming that steps of (110) configuration are indeed the active sites for CO ads
electro-oxidation.
Typical CO ads stripping voltammograms on Pt/GC electrodes with different mean
particle sizes are presented in Fig. 15.8. The position of the main CO ads stripping peak
for ¯ N ¼ 3.2 nm (E ¼ 0.86 V vs. RHE) is in good agreement with that for commercial
30 wt% Pt/C (E-Tek) [Maillard et al., 2002; Guerin et al., 2004]. As the particle size
decreases, both the onset and the peak are shifted positively, and the peak becomes
broader, with tailing on the descending slope [Friedrich et al., 1998, 2000;
Cherstiouk et al., 2003b; Maillard et al., 2004a, b, 2005, 2007b; Mayrhofer et al.,
2005b]. The peak shift is very systematic, scaling with 1/d N and providing clear
evidence that particle edges cannot be the active sites for CO monolayer electro-
oxidation. Indeed, if this were the case, then the overpotential should have diminished
with decreasing particle size, since the fraction of edges increases as the particle size