38
confirmed to be due to anion desorption by in situ infrared spectroscopy [5]. Thus,
Pt(111) adsorbs only about 2/3 of the full monolayer coverage. Impedance measurements have shown that hydrogen adsorption proceeds beyond the onset of hydrogen
evolution region [6].
Voltammetry profiles together with theoretical studies revealed that the most
energetically suitable site for H adsorption is the hollow site outlined by four Pt
atoms arranged in a quadratic fourfold symmetry, identical to the atomic arrangement of the (100) site. Much less suitable is the threefold hollow site composed of
three Pt atoms in a triangle, as in (111) arrangement. The (110) surface contains
similar sites as the (111) surface, because this low-index plane can be described as
a high- index surface composed of two (111) atoms in the terrace and a monoatomic
step of (111) orientation, that is Pt(110) = 2(111) × (111). These sites also exist in
the high- index stepped surfaces.
Adzic’s group performed a systematic evaluation of the hydrogen adsorption
region (0.05 < E < 0.6 V) on 15 single-crystal surfaces [6–8]. The authors concluded
that the sites similar to (100) and (111) also exist on the stepped surfaces and are
more energetically favorable, because of the low-coordination number of the Pt
atoms in them. Thus, hydrogen adsorption commences in a broad region on terrace
sites composed of (100) and (111) orientation and in a relatively narrow region on
step sites. The adsorption of hydrogen on Pt(100) occurs in the most positive region,
around 0.3 V. Further negative, around 0.15 V, another narrow band is obtained,
which is associated to H UPD on (110) steps and terraces. Interestingly, LEED studies
of UHV- prepared Pt(100) has shown that the surface is reconstructed into a more
stable (5 × 20) lattice having hexagonal symmetry [9]. The voltammetry profiles,
however, show that Pt(100) has considerably different shape, suggesting that the
surface retains its square symmetry. As seen in Fig. 5.3, a single sweep beyond 0.8
V completely changes the voltammogram shape, producing a profile that resembles
the CV of Pt (11 1 1) = 6(100) × (111) surface, Fig. 5.3 [10].
The shape of H UPD strongly depends on the nature of the anion present (Fig. 5.4).
In a perchloric acid solution, hydrogen adsorption proceeds in an energy region that
is wider than 0.6 V, as there is no interference from the non-specifically adsorbed
perchlorate anions. In more specifically adsorbed anions of sulfuric acid, the voltammetry profile is essentially similar to that in perchlorate, but occurs in a narrower
range. Voltammetry profile changes considerably in specifically adsorbed ions like
carbonate (Fig. 5.4) or chloride solutions that shift hydrogen adsorption closer to
the hydrogen evolution region. The intensified peaks correspond to simultaneous
desorption of anions and adsorption of hydrogen on the freed sites [6, 11, 12].
Nanoparticle surfaces have received a particular attention in recent years.
Interestingly, the voltammetry profile of H UPD is very similar to that of polycrystalline
Pt. As the shape of the nanoparticle is often dictated by the surface energy, it is
expected that the quasi-spherical nanoparticle resembles the shape of a cuboctahedron consisting of (111) and (100) faces as well as sides and vertices [13]. The
distribution of hydrogen adsorption on a cuboctahedron nanoparticle is explained
5 Important Electrosorption Reactions
confirmed to be due to anion desorption by in situ infrared spectroscopy [5]. Thus,
Pt(111) adsorbs only about 2/3 of the full monolayer coverage. Impedance measurements have shown that hydrogen adsorption proceeds beyond the onset of hydrogen
evolution region [6].
Voltammetry profiles together with theoretical studies revealed that the most
energetically suitable site for H adsorption is the hollow site outlined by four Pt
atoms arranged in a quadratic fourfold symmetry, identical to the atomic arrangement of the (100) site. Much less suitable is the threefold hollow site composed of
three Pt atoms in a triangle, as in (111) arrangement. The (110) surface contains
similar sites as the (111) surface, because this low-index plane can be described as
a high- index surface composed of two (111) atoms in the terrace and a monoatomic
step of (111) orientation, that is Pt(110) = 2(111) × (111). These sites also exist in
the high- index stepped surfaces.
Adzic’s group performed a systematic evaluation of the hydrogen adsorption
region (0.05 < E < 0.6 V) on 15 single-crystal surfaces [6–8]. The authors concluded
that the sites similar to (100) and (111) also exist on the stepped surfaces and are
more energetically favorable, because of the low-coordination number of the Pt
atoms in them. Thus, hydrogen adsorption commences in a broad region on terrace
sites composed of (100) and (111) orientation and in a relatively narrow region on
step sites. The adsorption of hydrogen on Pt(100) occurs in the most positive region,
around 0.3 V. Further negative, around 0.15 V, another narrow band is obtained,
which is associated to H UPD on (110) steps and terraces. Interestingly, LEED studies
of UHV- prepared Pt(100) has shown that the surface is reconstructed into a more
stable (5 × 20) lattice having hexagonal symmetry [9]. The voltammetry profiles,
however, show that Pt(100) has considerably different shape, suggesting that the
surface retains its square symmetry. As seen in Fig. 5.3, a single sweep beyond 0.8
V completely changes the voltammogram shape, producing a profile that resembles
the CV of Pt (11 1 1) = 6(100) × (111) surface, Fig. 5.3 [10].
The shape of H UPD strongly depends on the nature of the anion present (Fig. 5.4).
In a perchloric acid solution, hydrogen adsorption proceeds in an energy region that
is wider than 0.6 V, as there is no interference from the non-specifically adsorbed
perchlorate anions. In more specifically adsorbed anions of sulfuric acid, the voltammetry profile is essentially similar to that in perchlorate, but occurs in a narrower
range. Voltammetry profile changes considerably in specifically adsorbed ions like
carbonate (Fig. 5.4) or chloride solutions that shift hydrogen adsorption closer to
the hydrogen evolution region. The intensified peaks correspond to simultaneous
desorption of anions and adsorption of hydrogen on the freed sites [6, 11, 12].
Nanoparticle surfaces have received a particular attention in recent years.
Interestingly, the voltammetry profile of H UPD is very similar to that of polycrystalline
Pt. As the shape of the nanoparticle is often dictated by the surface energy, it is
expected that the quasi-spherical nanoparticle resembles the shape of a cuboctahedron consisting of (111) and (100) faces as well as sides and vertices [13]. The
distribution of hydrogen adsorption on a cuboctahedron nanoparticle is explained
5 Important Electrosorption Reactions