electrocatalysis on surfaces modified... - Brookhaven National ...

where E 2 > E 1 . An indication for the dual path mechanism is provided by isotopelabeling
experiments of Heitbaum et al. [19, 20]. Strong evidence for the reactive
intermediate branch comes from infrared spectroscopy data showing generation of CO 2 at
potentials where CO is not oxidized [21]. On the basis of a kinetic isotope effect for the
oxidation of HCOOH and DCOOH on Pt(111) and Pt(100), it was concluded that the
rate determining step is H-C bond scission [22].
The nature of the strongly bound intermediate in the oxidation of formic acid,
methanol, and formaldehyde has been a long-disputed issue. CO appears to be the
principal poisoning adsorbate, while other adsorbates, such as COH and CHO, are
difficult to detect. These species appear less stable than CO and may be slow-reacting
intermediates [23]. In the oxidation of HCOOH on Pt, COH and/or CHO were proposed
as poisoning species on the basis of coulometric determination of a number of electrons
per site (e.p.s.) used for the oxidation of adsorbate [18], differential electrochemical mass
spectrometry (DEMS) [19], and radiotracer measurements [24]. Bewick and coworkers
[25] employed electrochemically modulated IR spectroscopy (EMIRS), which clearly
identified CO as the poisoning species. This was strongly supported by further work [26].
Two e.p.s. were found for several single crystal surfaces which also suggest that CO is a
poison [23,27]. More details on this topic can be found in reviews in references
[28,29,30].
The mechanism of the catalytic action of metal adlayers in the oxidation of
organic molecules has been interpreted by using several models including a “third body”
effect, an electronic effect, and the bifunctional catalyst mechanism.
(i)
Third-body effect
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