51
5.3.3 Oxygen Reduction Reaction on Surfaces Modified by
UPD Adlayers
A remarkable effect is produced by UPD metal adlayers on the ORR on several
electrode surfaces. Oxygen reduction on Au in alkaline solution is catalyzed causing
a change of the reaction mechanism involving a 2e-reduction to a 4e-reduction in
the presence of Pb, Bi, or Tl monolayer on the surface. This is an important effect
that will help in designing new catalysts and in other various applications of bimetallic surfaces.
Figure 5.11 shows rotating disk-ring measurements of O 2 reduction on Au and
Au/Pbad. The upper panel shows the UPD of Pb on Au in NaOH solution. Doubling
of the diffusion-limited current means that four electrons are exchanged in the reac0.2
0.1
0.0
–0.1
0.2
0.0
2
4
–0.4
0.0
0.4
E / V vs SHE
-I
O / mAcm –2
i / mAcm –2
I
R / mA
Au /Pb
Au /Pd ad
Au /Pd ad
Au
Au
Au
900 rpm
900 rpm
Fig. 5.11 Rotating disk-ring measurement of the ORR on Au and Au covered by Pb adalayer.0.1
M NaOH with 1 mM HPbO2
−
; sweep rate 50 mV/s. Upper panel, UPD of Pb in the same solution.
ID, IR, disk and ring current, respectively. (From [35] with permission)
5.3 Underpotential Deposition (UPD) of Metals and Catalytic Properties of Surfaces…