Environmental Engineering Reference
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(ii) The maximum faradaic current for formaldehyde oxidation is about twice that
for formic acid oxidation.
(iii) The formaldehyde oxidation current shows a slow decay with time, while the
formic acid oxidation current is fairly stable.
On the other hand, during potentiodynamic formaldehyde oxidation (solid line
in the upper panel of Fig. 13.3b), there is only a small faradaic current at 0.6 V in
the positive-going scan, in contrast to the much higher steady-state value (about
0.55 mA) attained in the potentiostatic experiment.
The CO 2 formation transient (solid line in Fig. 13.6b) exhibits a small m/z ¼ 44
ion current spike upon stepping the electrode potential, which is absent for the corre-
sponding adsorbate stripping trace (see also Fig. 13.2b) and also for formic acid
oxidation. Apparently, formaldehyde oxidation to CO 2 starts instantaneously, similar
to oxidation of a saturated CO adlayer (Fig. 13.2b). Subsequently, the current trace for
CO 2 formation continues to grow, until reaching a maximum after about 4 - 5 minutes.
The current efficiency for CO 2 formation (dashed line in Fig. 13.6c) passes through an
initial maximum of about 33% after about 1 minute, and then decays to a constant
value of about 14%. The maximum CO 2 current efficiency has about the same value
as that reached in the positive-going scan at potentials of 0.65- 0.7 V (Fig. 13.4b),
indicating that the adlayer composition is similar in both cases. The time evolution of
the current efficiency for formaldehyde oxidation to formic acid (solid line in
Fig. 13.6c), simply mirrors that of the current efficiency for CO 2 formation.
Comparison with the formaldehyde adsorbate stripping trace (gray line in
Fig. 13.6b, data from Fig. 13.2b) implies that within the first minute after the potential
step, oxidation of preformed CO ad , which was generated upon interaction of formal-
dehyde with the Pt electrocatalyst at 0.06 V, contributes significantly to the total
faradaic current and to the total amount of CO 2 formed during formaldehyde oxidation
at 0.6 V. Accordingly, the initial increase and later decay in selectivity for CO 2
production during the first 2 minutes can be explained by oxidation of CO ad that
was pre-adsorbed before the potential step. Since the CO ad species initially present
on the surface are oxidized to CO 2 in a two-electron reaction, whereas a four-electron
reaction was assumed when calculating the CO 2 current efficiency, the CO 2 efficiency
calculated during this initial phase is higher than the actual value. Finally, the slight
decreases in both faradaic and m/z ¼ 44 ion currents observed also after 10 minutes
of formaldehyde oxidation at 0.6 V are most likely due to a slight increase in the
steady-state CO ad coverage with time and/or a structural transformation in the result-
ing CO adlayer.
Conversion of the m/z ¼ 44 ion current into a partial faradaic reaction current for
formaldehyde oxidation to CO 2 (four-electron reaction) shows that, under these exper-
imental conditions, formaldehyde oxidation to CO 2 is only a minority reaction path-
way (dashed line in Fig. 13.6a). Assuming CO 2 and formic acid to be the only
stable reaction products, most of the oxidation current results from the incomplete
oxidation to formic acid (dotted line in Fig. 13.6a). The partial reaction current for
CO 2 formation on Pt/Vulcan at 0.6 V is only about 30% of that during formic acid
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