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 409

The electrochemical measurements were performed in parallel to online mass spec-
trometry measurements using a dual thin-layer flow cell [Jusys et al., 1999]. The
second thin-layer compartment was interfaced to the MS inlet through a porous mem-
brane (Scimat, 60 mm thick, 50% porosity), and interconnected via four capillaries
with the first thin-layer compartment. The latter was designed to accommodate a
glassy carbon disk (9 mm in diameter, Sigradur G from Hochtemperatur Werkstoffe
GmbH), on which a thin layer of the carbon-supported Pt catalyst (E-TEK Inc., Pt par-
ticles supported on carbon Vulcan XC 72, 20 wt% Pt, mean Pt particle size 3.0 nm
[Jusys et al., 2003], Pt loading 7 mgcm 22 , catalyst film diameter 6 mm) was deposited
by successive pipetting - drying cycles of aqueous catalyst suspension and Nafion
solution, respectively [Jusys et al., 2003; Schmidt et al., 1998]. Two Pt wires were
used as counter-electrodes in the thin-layer cell. A saturated calomel electrode
(SCE) connected to the outlet of the DEMS cell through the Teflon capillary served
as a reference electrode. All potentials are quoted, however, with respect to that of
the reversible hydrogen electrode (RHE).
The CO 2 signal was calibrated by comparing against the well-known amount of
CO 2 formation produced during oxidation of a saturated CO adlayer (“CO stripping”)
[Jusys et al., 2001], determining the calibration constant K

between the two signals

via the relation

K ¼ zQ MS = Q F (13 : 1)

where Q MS and Q F are the mass spectrometric and the faradaic charge, respectively,
and z ¼ 2 is the number of electrons per CO ad molecule oxidized to CO 2 .
Alternatively, for calibration of the CO 2 production in bulk reactions, the potentiostatic
oxidation of formic acid was used for comparison. In this reaction, CO 2 formation is
the only reaction pathway (two-electron reaction). The methyl formate signal was
calibrated by evaluating the product distribution (CO 2 and formic acid yields)
during methanol oxidation at high catalyst loadings, where formaldehyde production
is negligible [Ota et al., 1984; Childers et al., 1999; Jusys et al., 2003]. The partial
current for formaldehyde formation was calculated from the difference between the
calculated faradaic currents (integrated charges in potentiodynamic measurements)
for CO 2 and formic acid formation on the one hand and the measured faradaic current
(charge) on the other, assuming that these three are the only reaction products [Wang
et al., 2001a; Jusys et al., 2003].
The supporting electrolyte (0.5 M sulfuric acid) was prepared using Millipore Milli
Q water and ultrapure sulfuric acid (Merck, suprapur), and deaerated by high purity Ar
(MTI Gase, N 6.0). For adsorbate stripping experiments, the respective reactant
was pre-adsorbed at a constant electrode potential for about 10 minutes, after inserting
2 mL of 0.5 M H 2 SO 4 solution saturated either with CO (CO ad saturation) or with
a 10% CO in Ar mixture (CO ad submonolayer coverage) (CO: Messer-Griesheim,
N 4.7), or a similar amount of electrolyte containing 0.1 M of the corresponding C 1
species (methanol, formaldehyde, or formic acid) by an all-glass syringe through a
separate port. Subsequently, the cell was carefully flushed (10 minutes) by the support-
ing electrolyte at the adsorption potential, and then the adsorbate stripping experiment