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

A third-body effect, which is equivalent to an “ensemble” effect in heterogeneous
catalysis, is based on the role of metal adatoms in blocking the surface sites for a side
reaction that generates poisoning species, or in blocking the adsorption of the inhibiting
species, which requires more than one surface site for adsorption. The reaction in the
main pathway can proceed at the unoccupied sites that now form smaller ensembles. In
this model, adatoms do not enhance the rate on the uncovered substrate sites. In
<strong>electrocatalysis</strong>, the effect was proposed by Conway et al. [31] to interpret the effect of
acetonitile on formic acid oxidation on Pt. The third body effect is operative for any
adsorbate statistically distributed on the surface that is not desorbed upon adsorption of
reacting species and intermediates of reactions.
ii)
Bifunctional mechanism
The bifunctional mechanism, often used in catalysis, has been considered in
<strong>electrocatalysis</strong> by Cathro [32] and Shibata and Motto [33]. The surface is considered to
have two types of sites that have distinct roles in the reaction. The substrate, such as Pt,
breaks the bonds in organic molecules upon adsorption, while the adatom can adsorb
oxygen-containing species that can oxidize strongly adsorbed intermediates such as CO.
Oxygen adsorption on adatoms is expected to occur at potentials negative of oxygen
adsorption on Pt. This facilitates CO oxidation at potentials more negative than on bare
Pt, thus freeing the surface for the main reaction.
iii)
Electronic effects
Electronic effects can result from the modifications of the properties of uncovered
near-neighbor sites and next near-neighbor sites. The adatoms modified in the interaction
with substrates, may, per se, take part in reactions.
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