Methanol, Formaldehyde, and Formic Acid Adsorption/Oxidation on a Carbon-Supported Pt Nanoparticle Fuel Cell Catalyst: A Comparative Quantitative DEMS Study - Fuel Cell Catalysis: A Surface Science Approach - page 417

For saturated CO adlayer oxidation (solid line in Fig. 13.2c), the CO sat coverage
transient shows only a small initial decay of 1 - 2% of the adlayer coverage (a small
decrease in the CO ad coverage at zero time). Then the CO sat coverage decreases
nearly linearly for about 3 minutes down to about 0.8 of its initial value, changes to
a steeper slope in the coverage range from about 0.7 to 0.3 (400 - 700 s after the poten-
tial step), and finally changes slope again to an almost linear decay from about 0.2
down to nearly zero. A subsequent stripping experiment confirmed that the initial,
saturated CO adlayer was almost completely removed after 25 - 30 minutes' oxidation
at 0.6 V (remaining CO ad 3%). The CO ad coverage transients obtained after adsorp-
tion of the three C 1 molecules differ significantly from these characteristics. They
exhibit a pronounced coverage decay in the initial phase (Fig. 13.2b), where almost
all of the adsorbate is removed.
The different characteristics of the adsorbate stripping transients can be qualitat-
ively explained by using the concepts derived for describing the oxidation behavior
of a saturated CO adlayer, after adsorption from CO-containing solution [Gilman,
1963, 1964]. It is generally agreed (for carbon-supported Pt/C catalysts, see
[Friedrich et al., 2000; Maillard et al., 2004a, 2005; Arenz et al., 2005; Andreaus
et al., 2006; Andreaus and Eikerling, 2007]; for smooth Pt electrodes, see [Santos
et al., 1991; Petukhov et al., 1998; Koper et al., 1998; Bergelin et al., 1999;
Korzeniewski and Kardash, 2001; Lebedeva et al., 2002; Housmans et al., 2007])
that electro-oxidation of adsorbed CO on Pt electrodes proceeds via a Langmuir -
Hinshelwood mechanism by reaction between CO ad and neighboring, electrosorbed
OH ad species. In the initial peak, this includes initial (instantaneous) nucleation of
OH ad , possibly on defect sites, reaction with weakly bound CO ad species, and slow
relaxation of the CO adlayer into a more strongly bound state. Progressive nucleation
of OH ad on free Pt sites and reaction with neighboring “strongly bound” CO ad leads to
the main oxidation peak. For all adsorbate stripping transients, the mass spectrometric
measurements clearly reveal a fast onset of CO ad electro-oxidation immediately
after stepping the electrode potential, which supports the proposal of instantaneous
nucleation of OH ad species and reaction on a limited number of defect sites within
the CO adlayer [Korzeniewski and Kardash, 2001]. For the saturated adlayer, the
amount of initial, instantaneous OH ad nucleation is small, as expected for a surface
fully covered by a stable saturated adlayer. It is somewhat higher after formaldehyde
adsorption, in agreement with the slightly lower CO ad coverage. For the other two
reactants, OH ad nucleation is no longer rate-limiting. An increasingly faster onset
of the main oxidation peak for transient C 1 adsorbate oxidation with decreasing
CO ad coverage would be consistent with a Langmuir - Hinshelwood mechanism,
since nucleation of OH ad should become more facile at lower CO ad coverages,
which in turn increases the adlayer oxidation rate and shifts the main oxidation
peak towards the initial spike. The asymmetric shape of the initial peak with a
significant tailing at longer times can be explained by the increasing overlap of the
initial “instantaneous” OH ad nucleation and reaction step [Korzeniewski and
Kardash, 2001], and CO 2 formation via progressive OH ad nucleation and reaction
with CO ad [Lebedeva et al., 2000, 2002; Andreaus and Eikerling, 2007; Housmans
et al., 2007].