107
8.1.3 Effects of Composition, Shape, and Size of Cores
In this section, the types of core-shell interactions that affect the activity and stability of Pt monolayer electrocatalysts are listed. Some effects are described in more
details than the others. In the following subsections, the syntheses and properties of
several types of new and novel electrocatalysts are described.
Origins of the Effects of Composition, Shape and Size of Cores Monolayer of Pt
atoms on a foreign metal surface will undergo compressive or tensile strain,
depending on the difference in the atomic radii of Pt and the other metal [4, 11].
Thus, Pt atoms deposited on an Ru substrate would have a large compressive strain,
but they would have only a small compressive strain on Pd and a tensile strain when
deposited on Au. In addition to the strain effect, the electronic couplings between
Pt ML and its supporting substrates also affects the electronic property of surface Pt
atoms. Both the surface strain and electronic modification generate a d-band center
shift of the Pt monolayer [6, 12].
O/OH Binding The studies of Pt monolayer electrocatalysts confirmed and elucidated the role of OH or O adsorption on the ORR kinetics [3]. This is in agreement
with the density functional theory (DFT) calculations showing a strong correlation
between the position of d-band center and the binding energies of small adsorbates
on strained surfaces and metal overlayers; the latter has a direct impact on the catalytic activity of the reaction [6]. For Pt ML electrocatalysts, there is a volcano-type
dependence of the ORR activity on the position of d-band center [Fig. 8.2].
OH Coverage/OH-OH Repulsion Energy The work on Pt ML electrocatalysts
revealed a new factor playing a role in determining activity for the ORR, which
involves the OH-OH repulsion between OH moieties on Pt and other metals in the
surface layer. In addition to the strength of O/OH binding with Pt, the coverage of
OH is another important factor affecting the activity of Pt ML electrocatalysts; the
high OH coverage on Pt is known to inhibit the ORR. The data for Pt ML
electrocatalysts, with an additional metal mixing with Pt to form a mixed monolayer, shows the interaction between the Pt and the other metal in the shell induces
a change of lateral repulsion energy of adsorbed OH or O and thus changes the OH
coverage on Pt [12].
Figure 8.5 depicts an experiment on a Pt-M mixed layer [8]. The lateral repulsion
energy between adsorbed OH or between adsorbed OH and O changes when a
different M is used. A very good linear correlation is found between the measured
kinetic current densities and the effective repulsion energy between two OH(a)s or
an O(a) and an OH(a) calculated from the first principles. The strong repulsion
between these two adsorbates destabilizes the OH adsorption on Pt and accordingly
reduces the OH coverage. Such good linear correlation suggests that the
destabilization of OH on Pt and the resulted reduced OH coverage, due to the influence from the second metal, is responsible for the enhanced ORR kinetics.
8.1 Oxygen Reduction Reaction (ORR)
8.1.3 Effects of Composition, Shape, and Size of Cores
In this section, the types of core-shell interactions that affect the activity and stability of Pt monolayer electrocatalysts are listed. Some effects are described in more
details than the others. In the following subsections, the syntheses and properties of
several types of new and novel electrocatalysts are described.
Origins of the Effects of Composition, Shape and Size of Cores Monolayer of Pt
atoms on a foreign metal surface will undergo compressive or tensile strain,
depending on the difference in the atomic radii of Pt and the other metal [4, 11].
Thus, Pt atoms deposited on an Ru substrate would have a large compressive strain,
but they would have only a small compressive strain on Pd and a tensile strain when
deposited on Au. In addition to the strain effect, the electronic couplings between
Pt ML and its supporting substrates also affects the electronic property of surface Pt
atoms. Both the surface strain and electronic modification generate a d-band center
shift of the Pt monolayer [6, 12].
O/OH Binding The studies of Pt monolayer electrocatalysts confirmed and elucidated the role of OH or O adsorption on the ORR kinetics [3]. This is in agreement
with the density functional theory (DFT) calculations showing a strong correlation
between the position of d-band center and the binding energies of small adsorbates
on strained surfaces and metal overlayers; the latter has a direct impact on the catalytic activity of the reaction [6]. For Pt ML electrocatalysts, there is a volcano-type
dependence of the ORR activity on the position of d-band center [Fig. 8.2].
OH Coverage/OH-OH Repulsion Energy The work on Pt ML electrocatalysts
revealed a new factor playing a role in determining activity for the ORR, which
involves the OH-OH repulsion between OH moieties on Pt and other metals in the
surface layer. In addition to the strength of O/OH binding with Pt, the coverage of
OH is another important factor affecting the activity of Pt ML electrocatalysts; the
high OH coverage on Pt is known to inhibit the ORR. The data for Pt ML
electrocatalysts, with an additional metal mixing with Pt to form a mixed monolayer, shows the interaction between the Pt and the other metal in the shell induces
a change of lateral repulsion energy of adsorbed OH or O and thus changes the OH
coverage on Pt [12].
Figure 8.5 depicts an experiment on a Pt-M mixed layer [8]. The lateral repulsion
energy between adsorbed OH or between adsorbed OH and O changes when a
different M is used. A very good linear correlation is found between the measured
kinetic current densities and the effective repulsion energy between two OH(a)s or
an O(a) and an OH(a) calculated from the first principles. The strong repulsion
between these two adsorbates destabilizes the OH adsorption on Pt and accordingly
reduces the OH coverage. Such good linear correlation suggests that the
destabilization of OH on Pt and the resulted reduced OH coverage, due to the influence from the second metal, is responsible for the enhanced ORR kinetics.
8.1 Oxygen Reduction Reaction (ORR)