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

Inhibition of H 2 evolution was reported for stationary single crystal electrodes for
As/Pt(111) [136], Bi/Pt(100), Sb/Pt(100), Bi/Pt(111) and Sb/Pt(111) [137] surfaces. For
the Bi/Pt(111) system, the polarization curves have slopes of –30 to -35mV for _ Bi < 0.04
(0.02), and for 0.14(0.07) < _ Bi < 0.33(0.18), the slopes are –35 and –54 mV at small and
high overpotentials, respectively [137] (Fig. 16.13). For Pt(100), a linear decrease of the
H 2 evolution current as a function of coverage has been observed up to _ Bi = 0.35(0.18),
and a steep decrease has been observed above it. For Pt(111), a large decrease of H 2
evolution was observed at small Bi coverages up to 0.1(0.03), and a linear decrease was
seen for larger coverages. A similar observation was made for polycrystalline Pt with Pb
[132]. Small changes of Tafel slope for low Bi coverages were explained by a
hypothetical mechanism in which a slow recombination and an ion-plus-atom reaction
rate determining steps occur in parallel. Inhibition effects of As, which is a prototype
catalytic poison, have been reported for polycrystalline electrodes [138,139] and for a
Pt(111) surface [136]. Surprisingly small inhibition was observed up to _ As = 1/3(1/6) for
the latter system.
The mechanism of significant H 2 evolution on Pt with large coverage of inhibiting
metal adatoms is not clear. For Bi on Pt(100), the existence of the c(2 x 2) Bi adlayer was
assumed, and it was proposed that H 2 evolution takes place through four-fold symmetry
holes in the adlayer [137]. This adlayer exists at Pt(100) [68], but its structure still needs
to be verified during H 2 evolution. In situ verification of the adlayer structure during the
course of H 2 evolution has been obtained by SXS for Pt(111) with a Tl adlayer [140]. Fig.
15 shows data for Tl adlayers on Pt(111) in solutions of two different pH values. A
significant H 2 evolution current flows at Pt(111) despite the close packed configuration of
27