Environmental Engineering Reference
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calculations. An alternative explanation for this behavior is given below. Similar beha-
vior has been observed with Sb, Te, As, Sn, and Pb on Pt(111).
If we recall now what has been discussed before about the different abilities of ada-
toms to decorate step sites and the relation to the induced surface dipole, a correlation
between these effects and electrocatalytic behavior becomes clear: the adatoms that
decorate steps on Pt(111) are the same ones that give the extended electrocatalytic
effect. More insight into this can be obtained from study of the poisoning reaction
on adatom-modified stepped surfaces [Macia et al., 1999, 2001]. With sufficiently
wide terraces, the amount of poison remains essentially constant until Bi completely
covers the step. Once the step sites are completely covered and Bi starts to deposit on
the terraces, a strong inhibition of the poisoning reaction is observed. This result was
interpreted as evidence that poison formation only takes place on step sites, and when
those are completely blocked by adatoms, the poisoning reaction no longer takes place
on the modified surface. This argument was also used to explain the “long-range
effect” observed for Bi on Pt(111) surfaces [Macia et al., 2001]. It was considered
that, even on the most perfect surface, there are always a certain number of defects.
These defects would be, under this hypothesis, the only active sites for poison for-
mation. The amount of poison is not limited to the step sites, because of the significant
mobility of adsorbed CO, which can migrate from the step where it is formed to the
adjacent terrace. Then, when the Pt(111) surface is modified with a small amount
of an electropositive adatom, defect sites will be preferentially covered, rendering a
surface inactive for the poison formation reaction. On the other hand, electronegative
adatoms such as Se and S will randomly cover both defect and terrace, giving a
third-body effect.
A different mechanism seems to operate in the case of poison formation from
methanol [Herrero et al., 1993]. In this case, modification of the Pt(111) surface by
Bi deposition only causes a linear decrease in the amount of poison formed, indicating
the existence of a mere third-body effect. Complete inhibition of the poisoning reac-
tion is achieved for u
Bi
. 0.23, i.e., before the surface is completely covered. This
suggests the existence of ensemble requirements for this reaction, which need enough
free contiguous Pt sites to take place.
7.6.3 Electrocatalysis of the Direct Reaction
In the previous section, it has been shown how it is possible to isolate and measure the
amount of poison formed from formic acid dissociation by transferring the poisoned
electrode to a solution free of formic acid. Separation of the direct path from the poi-
soning reaction is more difficult, since this reaction can only be studied in solutions
containing formic acid, and hence both reaction pathways will take place simul-
taneously in this case. However, the capacity of Bi to virtually stop the poisoning reac-
tion on Pt(111) for any coverage higher than 0.04 allows one to study the effect of
surface modification on the direct formic acid oxidation without the interference of
the poisoning reaction. Figure 7.14a plots the maximum current density measured
in the anodic scan during formic acid oxidation as a function of adatom coverage
on Pt(111). The curve features a roughly linear increase at low coverages and a
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