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

een clearly determined so far. In most studies, the adlayer structures were derived from
coverages obtained from voltammetry. It is, however, necessary to determine adlayer
structures during the course of HCOOH oxidation because of the sensitivity of adatom
ordering to the interaction with other adsorbates. For instance, in situ x-ray scattering
techniques show that the ordered (3 x √3) Pb adlayer on Pt(111) vanishes during HCOOH
oxidation, but the Pb coverage remains unaffected [49]. Further work is needed to
determine whether the adlayer structure has an influence on the magnitude of the
catalytic effect, or whether only the adatom coverage is important.
An extension of the UPD modification of the electrocatalytic properties of
electrode surfaces is a process described as an “irreversible” adsorption of metal adatoms
on electrode surfaces. These adsorbates can be obtained by adsorption from cationcontaining
solution without the application of external potential. Janssen and Moolhyusen
[50] demonstrated that immersion of clean Pt surfaces in solution containing metal salts,
and subsequent emersion and removal of the excess solution, could produce adlayers of
metal adatoms. Clavilier and coworkers [46] used this procedure with several metal
adsorbates on Pt single crystal electrodes. These “preadsorbed” layers are quite stable. It
is, however, necessary to control the electrode potential, since the adlayer coverage is
potential independent only within certain potential limits. In some cases, as for Bi on Pt,
it is believed that the adsorbate changes oxidation states with potential and that therefore
both species interact with a surface forming adlayer [51]. Ex situ XPS data show that the
charge should be attributed to OH - adsorption rather than to a change in the valence state
of adsorbed Bi [52].
12