Environmental Engineering Reference
In-Depth Information
perturbation. The final potential for the pulse is selected in such a way that the CO is
completely oxidized, without perturbing the surface order. These experiments showed
that poisoning rate was higher between 0.2 and 0.4 V, and almost negligible for poten-
tials above 0.5 V [Clavilier, 1987; Fern´ndez-Vega et al., 1991]. The poisoning rate
measured by chronoamperometry indicated that the maximum rate is obtained at
0.3 V [Sun et al., 1994]. It has been suggested that the position of this maximum is
influenced by hydrogen and anion adsorption; strong anion or hydrogen adsorption
inhibits the poison formation reaction [Lu et al., 1999]. For that reason, the poisoning
rate diminishes at very low or high potentials, where hydrogen and anions, respect-
ively, are strongly adsorbed on the surface. In fact, the position of the maximum is
close to the potential of zero total charge of the Pt electrode [G ´mez et al., 2000], a
potential for which the coverages of both species are low.
As already mentioned, the poison formation reaction is potential-dependent, and
the poisoning rate for the basal planes is Pt(110) . Pt(100) . Pt(111) [Sun et al.,
1994; Iwasita et al., 1996]. The case of Pt(111) is special, since the poisoning has
been associated with the presence of defects on the surface. Selective covering of
the defects on the Pt(111) electrode by some adatoms prevents the formation of CO
on the electrode surface [Maci´ et al., 1999, 2001; Smith et al., 2000].
The simplest way to study the reaction through the active intermediate is to measure
the currents in absence of the poisoning intermediate. This effect can be achieved in
two different ways: either by eliminating the CO prior to the studies or by suppressing
the formation of CO with some surface modification. The first attempts were carried
out by Clavilier using pulse voltammetry [Clavilier, 1987]. The current measured
just after the pulse is the intrinsic activity of the surface at this potential, i.e., the maxi-
mum activity (current density) that can be achieved in absence of poison. These
studies revealed that the Pt(100) electrode is very active for formic acid oxidation,
its maximum intrinsic activity being around 30 mA/cm
2
at 0.46 V (vs. RHE)
(Fig. 6.15), which means a turnover rate of more than 70 molecules per (100) site
per second under these conditions. This can be compared with the nearly zero current
obtained in the positive-going scan of the voltammetric experiments for which the
surface is completely covered by the poison.
In order to obtain quantitative reaction rates, chronoamperometric experiments are
normally used, because their analysis is always simpler. This strategy was used by
Sun's group [Sun and Yang, 1999]. Assuming that formic acid oxidation can be
described as a process controlled by both charge transfer and mass transport, this
group was able to obtain the rate constant of the reaction from the analysis of the
current - time transient obtained in potentiostatic experiments, as it would proceed
in the absence of poison on the surface. In this way, activation energies and Tafel
slopes were obtained. For Pt(100), a Tafel slope of about 120 mV has been obtained.
Using the other strategy, namely, preventing CO formation by blocking all the defect
sites on a Pt(111) electrode, the same Tafel slope was measured [Maci´ et al., 2001].
Such a Tafel slope indicates that the rate-determining step for the formic acid oxidation
reaction through the active intermediate is the first electron transfer. Since two elec-
trons are exchanged in the oxidation of formic acid, the step that leads to the formation
of the reactive intermediate is the slowest step. Once the active intermediate has
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