Significant enhancements in the electrocatalytic oxidation of HCOOH are obtained on deliberately
stepped platinum surfaces with (111) terraces and (100) monatomic steps when bismuth is deposited on
terrace and/or step sites. The bismuth-modified surfaces with higher step densities/narrower terrace widths
exhibit higher catalytic activity in the oxidation of HCOOH than surfaces with lower step densities/wider
terrace widths, behavior which is opposite to that exhibited by the analogous bare (unmodified) surfaces.
For Bi/Pt(544) and Bi/Pt(755), with nine- and six-atom wide (111) terraces, respectively, maximum catalytic
behavior occurs when bismuth is deposited on all step sites and partially deposited on the terraces. On
the other hand, for Bi/Pt(211) and Bi/Pt(311), with three- and two-atom wide (111) terraces, respectively,
maximum catalytic behavior occurs when bismuth is deposited only on the steps, suggesting that a critical
ensemble (critical size), two atoms wide, enhances the reactive intermediate pathway for the complete
oxidation of HCOOH over the poisoning reaction pathway. Moreover, the potential for the onset of oxidation
of HCOOH shifts to less positive potentials as the terrace width of the bismuth-modified surfaces becomes
narrower, suggesting a diminution in the activation energy barrier as the ensemble size becomes smaller.
Contrary to the behavior on the stepped surfaces, bismuth-modified, polyoriented spherical platinum
electrodes, which contain a random distribution of (111), (100), and (110) faces, exhibit a continuous rise
in catalytic activity for HCOOH oxidation with bismuth coverage, even at bismuth coverages approaching
100%.
The voltammetric profile in sulfuric acid electrolytes of high
coverage bismuth monolayers on Pt(111) having
a fractional coverage (θ) of 0.39 exhibits two pairs of peaks located
positive and negative, respectively, of
the redox peak associated with the 0.33 coverage Bi/Pt(111)
adlayer. The ΔE
fwhm (full width at
half-maximum)
values for these peaks are 10 and 20 mV, respectively, suggesting that
the redox events involve significant
near-neighbor interactions and may correspond to phase transitions
within the bismuth adlayer. The formal
potentials of these redox peaks shift by 65 mV per pH unit, suggesting
that two protons are involved in the
two-electron redox reaction of the bismuth adatoms. This provides
evidence that the two pairs of redox
peaks could correspond to the abrupt formation of
[Bi(OH)2]ad and [BiO]ad
from Biad. Upon continuous
potential scanning in sulfuric acid electrolyte, these peaks decay
while two new peaks grow in, with an
isopotential point forming between the two anodic peaks. The new,
latter peak corresponds to that found for
the 0.33 coverage Bi/Pt(111) adlayer. The peaks that grow in
upon potential scanning could arise from the
interconversion of the hydroxide and oxide network to a stable,
long-range hydrogen-bonded (bi)sulfate network
upon bismuth oxidation. The peak associated with the 0.33 coverage
Bi/Pt(111) adlayer could then be ascribed
to the abrupt formation of this (bi)sulfate network upon bismuth
oxidation. In perchloric acid electrolytes
the two pairs of high coverage bismuth redox peaks are also present,
but one increases monotonically while
the other decreases upon potential scanning, without the appearance of
any new peaks. The high coverage
peaks are thus associated with the formation of a long-range hydroxide
and oxide network upon bismuth
oxidation. The presence of halide anions (Cl-,
Br-, I-) dramatically affects the
voltammetric profile of
Bi/Pt(111). Initially, chloride appears to form a complex
with the bismuth adlayer. Upon potential scanning,
the redox peak ascribed to the chloride complex decays while that
associated with the stable bismuth adlayer
reemerges. Bromide and iodide anions both cause the complete
desorption of the bismuth adlayer, suggesting
that they have significantly stronger bonding interactions to
Pt(111) than does bismuth.
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