Environmental Engineering Reference
In-Depth Information
based on the fact that the amount of CO oxidizing on the terrace depends on the scan
rate following a diffusion law. The exact reason for this low surface mobility is
not fully clear, but may be related to the strong adsorption of the final CO oxidation
product in alkaline media, i.e., carbonate.
Also, under continuous CO oxidation conditions, alkaline media exhibit a much
higher activity than acidic media. Markovic and co-workers observed a shift of
about 150 mV of the main oxidation wave, and a pre-wave corresponding to CO
oxidation at potentials as low as 0.2 - 0.3 V [Markovic et al., 2002]. Remarkably,
the hysteresis that is so prominently observed in the diffusion-controlled CO oxidation
wave in acidic media (see Fig. 6.9), is no longer present in alkaline media. Markovic
and co-workers also attribute the high activity of alkaline media to a “pH-dependent”
adsorption of OH
ads
at defect/step sites.
6.3 FORMIC ACID OXIDATION
6.3.1 Formic Acid Oxidation on Platinum
Within the general mechanism for the oxidation of C
1
molecules, proposed by
Bagotzsky, formic acid is one of the simplest cases, since it requires only the transfer
of two electrons for the complete oxidation to CO
2
[Bagotzky et al., 1977]. In fact, it
has the same oxidation valency as CO; both require two electrons for complete oxi-
dation to CO
2
. When compared with CO, the reaction mechanism of formic acid is
more complex although the catalysis of the oxidation reaction is much easier. In
fact, formic acid can be readily oxidized at potentials as low as 0.2 V (vs. RHE). Its
reaction mechanism takes place according to the well-established dual path mechan-
ism [Capon and Parsons, 1973a, b]:
active intermediate
E
1
CO
2
b
HCOOH
E
2
. E
1
(6
:
16)
d
poisoning intermediate
E
2
CO
2
Earlier results on the oxidation of formic acid on Pt electrodes have been extensively
reviewed [Parsons and VanderNoot 1988; Jarvi and Stuve, 1998; Sun, 1998; Vielstich,
2003; Feliu and Herrero, 2003]. Here, we will summarize previous results, but will
focus on the most recent results.
The voltammetric profile for formic acid on Pt shows a clear hysteresis (Fig. 6.13).
In the general case, currents in the positive-going scan are lower than in the negative-
going one. Additionally, the hydrogen adsorption states, which appear at low poten-
tials, are clearly blocked, indicating that there is a species adsorbing on the electrode
surface, i.e., the poisoning intermediate. The hysteresis of the voltammetric profile is a
consequence of the accumulation of the poisoning intermediate at low potentials, as
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