104
behavior shown in Fig. 8.2, DFT calculations were performed to study the following
elementary steps:
O
O O
2 → +
(8.1)
O H
OH
+ →
(8.2)
These two steps are the most activated versions of O-O bond scission and O-H
bond formation, respectively.
In accord with the Sabatier principle, the volcano-type dependence of ORR
activity on ε d, as seen in Fig. 8.2, is a consequence of two opposing demands on the
catalyst connected via a common intermediate. Pt(111) lies close to the intersection
of the two E a trend lines. The only Pt monolayer in the vicinity of the intersection of
the two trend lines is Pt ML /Pd(111), indicating it is a good ORR catalyst. Indeed,
according to experiments, Pt ML /Pd(111) performs better than all the other Pt
monolayers and even shows ca. 30% increase in current density compared to
Pt(111). The binding energy of O appears to be a good reactivity descriptor for the
ORR on these surfaces. More recently, detailed theoretical calculations of free
energy changes in all possible ORR elementary steps, by Nørskov and co-workers,
suggest that OH removal from Pt and from supported-Pt ML surfaces and O-O bond
scission are the limiting factors to the ORR performance of these catalysts [6].
An additional factor favoring Pt ML /Pd(111) is the reduced binding energy of OH
on the Pt ML surface compared to Pt(111) and Pd(111), since high OH coverage may
inhibit the ORR. Voltammetry and in situ X-ray absorption near-edge structure
spectroscopy (XANES) [7] have confirmed that Pt-OH adsorption occurs at more
positive potentials on carbon-supported Pt ML /Pd nanoparticles (Pt ML /Pd/C) than on
Pt/C, which in turn occurs at a higher potential than Pd-OH on Pd/C, in line with the
calculations. It indicates that catalyst performance may be further improved if ways
to destabilize OH could be devised.
One possibility would be to replace some of the Pt atoms on the surface with
atoms of other metals that oxidize more easily than Pt. At low potentials, these metal
atoms should attract oxygen-containing species (e.g. O and OH) to themselves and,
through the electronic modification of the surface plus the enhanced lateral repulsion
among OH groups, destabilize OH on adjacent Pt sites and decrease the lifetime and
coverage of OH on those sites.
Mixed Pt-Ru and Pt-Ir monolayers of varying compositions were tested on
Pd(111) for the ORR reactivity of these surfaces by performing rotating disk
experiments [8]. The kinetic current density is observed to increase substantially
with the Ru mole fraction up to a maximum at a Pt:Ru ratio of 4:1, after which it
decreases as the Pt content of the surface diminishes. Similar results were obtained
for (Pt-Ir) ML /Pd(111) (Fig. 8.3). In fact, the kinetic current density for the Pt:M = 4:1
composition is significantly enhanced compared to that on the Pt ML /Pd(111) surface,
which already has a higher ORR activity than Pt(111).
8 Catalytic Properties of Pt Monolayer Electrocatalysts
behavior shown in Fig. 8.2, DFT calculations were performed to study the following
elementary steps:
O
O O
2 → +
(8.1)
O H
OH
+ →
(8.2)
These two steps are the most activated versions of O-O bond scission and O-H
bond formation, respectively.
In accord with the Sabatier principle, the volcano-type dependence of ORR
activity on ε d, as seen in Fig. 8.2, is a consequence of two opposing demands on the
catalyst connected via a common intermediate. Pt(111) lies close to the intersection
of the two E a trend lines. The only Pt monolayer in the vicinity of the intersection of
the two trend lines is Pt ML /Pd(111), indicating it is a good ORR catalyst. Indeed,
according to experiments, Pt ML /Pd(111) performs better than all the other Pt
monolayers and even shows ca. 30% increase in current density compared to
Pt(111). The binding energy of O appears to be a good reactivity descriptor for the
ORR on these surfaces. More recently, detailed theoretical calculations of free
energy changes in all possible ORR elementary steps, by Nørskov and co-workers,
suggest that OH removal from Pt and from supported-Pt ML surfaces and O-O bond
scission are the limiting factors to the ORR performance of these catalysts [6].
An additional factor favoring Pt ML /Pd(111) is the reduced binding energy of OH
on the Pt ML surface compared to Pt(111) and Pd(111), since high OH coverage may
inhibit the ORR. Voltammetry and in situ X-ray absorption near-edge structure
spectroscopy (XANES) [7] have confirmed that Pt-OH adsorption occurs at more
positive potentials on carbon-supported Pt ML /Pd nanoparticles (Pt ML /Pd/C) than on
Pt/C, which in turn occurs at a higher potential than Pd-OH on Pd/C, in line with the
calculations. It indicates that catalyst performance may be further improved if ways
to destabilize OH could be devised.
One possibility would be to replace some of the Pt atoms on the surface with
atoms of other metals that oxidize more easily than Pt. At low potentials, these metal
atoms should attract oxygen-containing species (e.g. O and OH) to themselves and,
through the electronic modification of the surface plus the enhanced lateral repulsion
among OH groups, destabilize OH on adjacent Pt sites and decrease the lifetime and
coverage of OH on those sites.
Mixed Pt-Ru and Pt-Ir monolayers of varying compositions were tested on
Pd(111) for the ORR reactivity of these surfaces by performing rotating disk
experiments [8]. The kinetic current density is observed to increase substantially
with the Ru mole fraction up to a maximum at a Pt:Ru ratio of 4:1, after which it
decreases as the Pt content of the surface diminishes. Similar results were obtained
for (Pt-Ir) ML /Pd(111) (Fig. 8.3). In fact, the kinetic current density for the Pt:M = 4:1
composition is significantly enhanced compared to that on the Pt ML /Pd(111) surface,
which already has a higher ORR activity than Pt(111).
8 Catalytic Properties of Pt Monolayer Electrocatalysts