Environmental Engineering Reference
In-Depth Information
OH/oxide species. At potentials anodic of 1 V, incomplete oxidation of formaldehyde
to formic acid is activated, while methanol oxidation is almost completely hindered.
This reflects an easier oxidation of the C - H group in the aldehyde than in the alcohol.
For the negative-going scan, where the CO
ad
coverage at the onset of oxidation is
negligible, there is no such double-peak structure in the current efficiency.
The oxidation transients, recorded after stepping the potential from 0.16 to 0.6 V,
show similar characteristics. At 0.6 V, the steady-state faradaic current decreases in the
order formaldehyde . formic acid . methanol. A higher CO
2
efficiency in the initial
stage of the transient is attributed to the oxidation of CO
ad
that was formed while hold-
ing the electrode at 0.16 V. The current efficiencies and product distributions follow
the trends observed for potentiodynamic oxidation, with a significantly higher
steady-state current at 0.6 V for formaldehyde oxidation than for formic acid and
methanol oxidation. The steady-state current efficiency for CO
2
formation is much
lower during formaldehyde formation (8%) than for methanol oxidation (50% CO
2
current efficiency and 10% formic acid current efficiency). For all three reactants,
the steady-state currents at 0.6 V are between those obtained for potentiodynamic
oxidation in the positive-going and negative-going scans at the same potential,
and the same is true also for the CO
2
current efficiencies during methanol and
formaldehyde oxidation. This was explained by a reaction-dominating role of the
adlayer during the reaction, with higher adlayer coverage during the positive-going
scan and lower coverage during the negative-going scan compared with the steady-
state situation.
The results have been compared with the earlier proposal of a dual-pathway mech-
anism for C
1
oxidation, and, together with previous experimental and theoretical
results, summarized in a comprehensive reaction scheme that explicitly includes
also the (reversible) exchange between adsorbed species, dissolved product species
in the catalyst layer, and similar species in the bulk electrolyte. The traditional dual-
pathway mechanism, where both the direct and indirect pathways lead to CO
2
formation, has beenextended by adding a third pathway that accounts for formation
and desorption of incomplete oxidation products. In the mechanistic discussion, we
have focused on the role in and contribution to the C
1
oxidation process of the
formation/desorption and re-adsorption plus further oxidation of incomplete
oxidation products. This not only leads to faradaic currents exceeding that for CO
2
formation, but may result in additional CO
ad
and CO
2
formation, via adsorption and
oxidation of the incomplete oxidation products.
Finally, we have discussed the effect of incomplete C
1
oxidation product formation
for fuel cell applications and the implications of these processes for reaction modeling.
While for standard DMFC applications, formaldehyde and formic acid formation will
be negligible, they may become important for low temperature applications and for
microstructured cells with high space velocities. For reaction modeling, we have
particularly stressed the need for an improved kinetic data base, including kinetic
data under defined reaction and transport conditions and kinetic measurements on
the oxidation of C
1
mixtures with defined amounts of formaldehyde and formic
acid, for a better understanding of cross effects between the different reactants at an
operating fuel cell anode.
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