Environmental Engineering Reference
In-Depth Information
electrolytes with specifically adsorbed anions. They also decrease the effective dielec-
tric constant adjacent to the electrode surface, which favors the adsorption of the rela-
tively nonpolar O
2
molecule.
9.2.2.1 Inhibition Effect of Anions
For an adequate understanding of the
effect of anions on ORR kinetics, it is necessary to obtain information on the adsorbed
anion structure during reaction. Considerable progress in structural determination of
adlayers on electrode surfaces in the last decade and a half has inspired the application
of these techniques to the anion effects on ORR kinetics [Wang et al., 2000].
Obtaining atomic-scale information on the structure of catalytic surfaces during reac-
tion has been one of the major goals of studies in both heterogeneous catalysis and
electrocatalysis. SXS techniques can provide detailed structural information on
long-range ordered adlayers during the course of electrocatalytic reactions.
Obtaining information on the anion inhibition mechanism of the ORR on an atomic
level has been demonstrated by Adzic and Wang, who used SXS to monitor the struc-
ture of adsorbed Br
2
during the course of the ORR [Adzic and Wang, 2000].
Structures of Br
2
adlayers are determined on Pt(111) and Au(100) electrodes in the
absence and in the presence of O
2
reduction in 0.1 M HClO
4
solutions. On
Au(100), the c(
p
p)R458 phases are found above 0.35
and 0.68 V, respectively. On Pt(111), the commensurate (3
3)-Br
2
adlayer is
found between 0.3 and 0.8 V. Sharp peaks are due to phase transitions. As shown
in Figs. 9.8 and 9.9, the c(
p
2
p
)-R458 and c(
p
)-R458 adlayer of Br
2
on Au(100) and the
(3
3)-Br
2
adlayer on Pt(111) inhibit the ORR. The reaction takes place only at
potentials more negative than those of the ordered Br
2
phases. The stability of Br
2
adlayers is monitored in situ by measuring X-ray intensity at a particular position in
reciprocal space corresponding to the diffraction peak for the adlayer as a function
of potential during O
2
reduction. Adsorbed disordered Br
2
anions change the mech-
anism of ORR on Pt(111) from a four-electron into a two-electron reaction.
For Ag(100) in 0.1 M NaClO
4
solutions, the adlayer c(2
2)-Br
2
precludes the
adsorption of O
2
in the bridge state, and the reduction current originates solely from
O
2
adsorbing in the end-on configuration through the four-fold symmetry holes in
the c(2
2)-Br
2
adlayer. The experiment demonstrates the site sensitivity of the
blocking effect. Markovic and co-workers reported the same structure of Br
2
on
Pt(111) in the absence of O
2
and the inhibition of the ORR measured using a rotating
disk electrode. They also studied halide adsorption on Pt(100) and Pt(110) [Markovic
and Ross, 2002].
The activity of Pt for the ORR in H
2
SO
4
solutions is highly dependent on the
structure of electrode surface. Bisulfate anions show the most pronounced effect on
the ORR on Pt(111). The strong adsorption of bisulfate is a consequence of its
structure and its adsorption through the three unprotonated oxygen atoms. The inhi-
bition was ascribed to the same symmetry of the anion and the adsorption sites.
The exact nature of the adsorbed species has not yet been completely determined.
Yeager's group identified it as bisulfate using FTIR spectroscopy [Faguy et al.,
1990], but more recent data [Faguy et al., 1996] are more consistent with an adsorbed
p
2
Search WWH ::
Custom Search