159
and slowing the kinetics of oxygen reduction. Additionally, the slowness of O/OH
hydrogenation also impedes the adsorption of O by occupying the adsorption sites
with O/OH. Pd/Pt(111) with ε d lying in the middle and therefore forming a moderate bond with the adsorbates may be a good catalyst for the ORR.
When the half-wave potentials obtained experimentally are plotted against the
calculated d-band center, a volcano plot is observed for the ORR activity of Pd/M
vs. d-band of Pd (Fig. 10.2). Pd/Pt(111) that has a moderate ε d value and therefore
moderate activity is the best catalyst for ORR.
The tensile strain tends to shift ε d to a higher value, whereas compressive strain
has the opposite effect. A Pd monolayer on a Au(111) surface has a large tensile
strain (4.8%). Accordingly, the Pd d-band lies closer to the Fermi level than does
that of Pd(111); consequently, the kinetics of ORR is slower because of the very
strong Pd–O bonding. A similar phenomenon was found on Pt/Au(111) [5]. For a
Pd monolayer on Pt(111), the small tensile strain (0.4%) is expected to result in a
higher-lying ε d compared with that of pure Pd(111) considering that the lattice constant of Pd is a little smaller than that of Pt. However, as Fig. 9.2 reveals, a lowerlying ε d was observed for Pd/Pt(111). This discrepancy reflects the fact that the
position of the d-band depends both on the amount of strain and the electron distribution between the Pd monolayer and its substrates (ligand effect) [6]. For Pd/
Pt(111), electron transfer from Pt to the Pd monolayer seems to dominate, causing
the downshift of ε d , which makes Pd/Pt(111) a better catalyst than Pd(111) for ORR
because it binds O less strongly. These data provide guidance for designing Pd-based
alloy catalysts. The alloying element should facilitate the decrease in energy of the
d-band center. A PdCo alloy is, for example, such a system.
Kolb et al. [3] reported electrochemical study of hydrogen adsorption and formic
acid oxidation on pseudomorphic monolayer of Pd on Au(111), Pt(111), PtRu(111),
and Rh(111). The results are in agreement with the d-band model from Nørskov
et al. [4]. The adsorption behavior and the catalytic activity were determined for
formic acid electrooxidation on a pseudomorphic palladium monolayer on various
single-crystal electrodes. The main origin of a shift in the d-band center ε d is a
change in the interatomic distances within an overlayer. If the d-band of a metal is
more than half-filled, an expanded pseudomorphic monolayer will lead to an upshift
of ε d owing to band narrowing and energy conservation [7]. A plot of the hydrogen
desorption potentials versus the shift of the d-band center shows a linear correlation
as theoretically predicted [7].
The data also emphasize the ease of tuning reaction rates by the electrochemical
modification of catalysts.
References
and slowing the kinetics of oxygen reduction. Additionally, the slowness of O/OH
hydrogenation also impedes the adsorption of O by occupying the adsorption sites
with O/OH. Pd/Pt(111) with ε d lying in the middle and therefore forming a moderate bond with the adsorbates may be a good catalyst for the ORR.
When the half-wave potentials obtained experimentally are plotted against the
calculated d-band center, a volcano plot is observed for the ORR activity of Pd/M
vs. d-band of Pd (Fig. 10.2). Pd/Pt(111) that has a moderate ε d value and therefore
moderate activity is the best catalyst for ORR.
The tensile strain tends to shift ε d to a higher value, whereas compressive strain
has the opposite effect. A Pd monolayer on a Au(111) surface has a large tensile
strain (4.8%). Accordingly, the Pd d-band lies closer to the Fermi level than does
that of Pd(111); consequently, the kinetics of ORR is slower because of the very
strong Pd–O bonding. A similar phenomenon was found on Pt/Au(111) [5]. For a
Pd monolayer on Pt(111), the small tensile strain (0.4%) is expected to result in a
higher-lying ε d compared with that of pure Pd(111) considering that the lattice constant of Pd is a little smaller than that of Pt. However, as Fig. 9.2 reveals, a lowerlying ε d was observed for Pd/Pt(111). This discrepancy reflects the fact that the
position of the d-band depends both on the amount of strain and the electron distribution between the Pd monolayer and its substrates (ligand effect) [6]. For Pd/
Pt(111), electron transfer from Pt to the Pd monolayer seems to dominate, causing
the downshift of ε d , which makes Pd/Pt(111) a better catalyst than Pd(111) for ORR
because it binds O less strongly. These data provide guidance for designing Pd-based
alloy catalysts. The alloying element should facilitate the decrease in energy of the
d-band center. A PdCo alloy is, for example, such a system.
Kolb et al. [3] reported electrochemical study of hydrogen adsorption and formic
acid oxidation on pseudomorphic monolayer of Pd on Au(111), Pt(111), PtRu(111),
and Rh(111). The results are in agreement with the d-band model from Nørskov
et al. [4]. The adsorption behavior and the catalytic activity were determined for
formic acid electrooxidation on a pseudomorphic palladium monolayer on various
single-crystal electrodes. The main origin of a shift in the d-band center ε d is a
change in the interatomic distances within an overlayer. If the d-band of a metal is
more than half-filled, an expanded pseudomorphic monolayer will lead to an upshift
of ε d owing to band narrowing and energy conservation [7]. A plot of the hydrogen
desorption potentials versus the shift of the d-band center shows a linear correlation
as theoretically predicted [7].
The data also emphasize the ease of tuning reaction rates by the electrochemical
modification of catalysts.
References