1 3
Topics in Current Chemistry (2019) 377:5
3 Determination of Specific Electrocatalytic Sites
3.1 Fingerprints of Step and Terrace Sites on Pt Surfaces as Probed by Hydrogen
Adsorption/Desorption
In Pt surface electrochemistry, the so-called hydrogen region is recognized to be
a fast, surface-limited, faradaic charge transfer reaction as a result of protons discharging to adsorbed hydrogen and its reverse reaction: H
+
+ e
−
+ * active sites ⇄ H ads .
These reactions correspond to the under-potential deposition of hydrogen (H UPD ),
and is a characteristic signature of a few transition metals in the periodic table,
such as Pt, Rh, Pd and Ir. At platinum electrodes in contact with different test
electrolytes, as HClO 4 , H 2 SO 4 , NaOH, those reactions develop well-defined
reversible voltammetric features at potentials several mV higher than hydrogen
evolution which serve as a fingerprint of Pt crystal facets and can be used to identify Pt active sites [26, 27]. In a spectro-electrochemistry study, in the region
of H UPD at 0.1 V RHE , a band at 2080–2095 cm
−1
, attributed to H ads on top sites,
was characterized by surface-enhanced infrared absorption spectroscopy [28]. A
single band, assigned to the Pt-H ads , also has been observed for Pt at the solid/
gas interface [29]. However, other geometries of adsorbed hydrogen at potentials
of H UPD on Pt, as is the hollow sites, have been considered [28]. Then, because
the potentials range of H UPD the surface is predominantly covered with H ads , it is
plausible to assume that the proton discharging onto adsorbed hydrogen involves
at least the reaction of water displacement from the Pt surface at potentials of
H UPD : Pt-[(H 2 O) m ] + H
+
+ e
−
⇄ Pt-H ads + mH 2 O. As the potential increases in the
positive direction, the characteristics of the electrified interface favor the Pt surface interaction with anions. Recently [30], attempts to provide a more complete
description of the possible interfacial events responsible for the features in the
“hydrogen” region include the possible participation of cations interacting with
the Pt surface, and attempts to explain the non-Nernstian pH dependence of the
“hydrogen” peak in voltammograms.
The voltammetric behavior of Pt single crystals in contact with a 0.1 M HClO 4
solution are displayed in Fig. 2, for three single crystal surfaces. The two stepped
surfaces, namely Pt(554) and Pt(544), consist of 9-atom-wide (111) terraces,
which are periodically broken by monoatomic steps with (110) and (100) orientations, respectively. The visible peaks caused by hydrogen adsorption/desorption
at the (110) and (100) steps are distinctly separated by ~ 156 mV. Figure 2 also
shows a hard sphere model for each surface orientation, indicating the position of
the monoatomic steps and two-dimensional (111) domains on the surfaces. The
qualitative difference in the voltammetric profiles in the potential range of ~ 0.06
up to ~ 0.35 V RHE concerns the existence of the remarkable peaks at ~ 0.128
and ~ 0.284 V RHE due to the hydrogen adsorption/desorption at the (110) and
(100) monoatomic steps, respectively. As we can see in the inset of Fig. 2 for the
hard sphere model, the row of monoatomic steps consists of two parts: the top
side and the bottom one, that would correspond to the positive and negative sides
of the dipole steps, following the Smoluchowski model [31]. The reversible peaks
Reprinted from the journal
83
Topics in Current Chemistry (2019) 377:5
3 Determination of Specific Electrocatalytic Sites
3.1 Fingerprints of Step and Terrace Sites on Pt Surfaces as Probed by Hydrogen
Adsorption/Desorption
In Pt surface electrochemistry, the so-called hydrogen region is recognized to be
a fast, surface-limited, faradaic charge transfer reaction as a result of protons discharging to adsorbed hydrogen and its reverse reaction: H
+
+ e
−
+ * active sites ⇄ H ads .
These reactions correspond to the under-potential deposition of hydrogen (H UPD ),
and is a characteristic signature of a few transition metals in the periodic table,
such as Pt, Rh, Pd and Ir. At platinum electrodes in contact with different test
electrolytes, as HClO 4 , H 2 SO 4 , NaOH, those reactions develop well-defined
reversible voltammetric features at potentials several mV higher than hydrogen
evolution which serve as a fingerprint of Pt crystal facets and can be used to identify Pt active sites [26, 27]. In a spectro-electrochemistry study, in the region
of H UPD at 0.1 V RHE , a band at 2080–2095 cm
−1
, attributed to H ads on top sites,
was characterized by surface-enhanced infrared absorption spectroscopy [28]. A
single band, assigned to the Pt-H ads , also has been observed for Pt at the solid/
gas interface [29]. However, other geometries of adsorbed hydrogen at potentials
of H UPD on Pt, as is the hollow sites, have been considered [28]. Then, because
the potentials range of H UPD the surface is predominantly covered with H ads , it is
plausible to assume that the proton discharging onto adsorbed hydrogen involves
at least the reaction of water displacement from the Pt surface at potentials of
H UPD : Pt-[(H 2 O) m ] + H
+
+ e
−
⇄ Pt-H ads + mH 2 O. As the potential increases in the
positive direction, the characteristics of the electrified interface favor the Pt surface interaction with anions. Recently [30], attempts to provide a more complete
description of the possible interfacial events responsible for the features in the
“hydrogen” region include the possible participation of cations interacting with
the Pt surface, and attempts to explain the non-Nernstian pH dependence of the
“hydrogen” peak in voltammograms.
The voltammetric behavior of Pt single crystals in contact with a 0.1 M HClO 4
solution are displayed in Fig. 2, for three single crystal surfaces. The two stepped
surfaces, namely Pt(554) and Pt(544), consist of 9-atom-wide (111) terraces,
which are periodically broken by monoatomic steps with (110) and (100) orientations, respectively. The visible peaks caused by hydrogen adsorption/desorption
at the (110) and (100) steps are distinctly separated by ~ 156 mV. Figure 2 also
shows a hard sphere model for each surface orientation, indicating the position of
the monoatomic steps and two-dimensional (111) domains on the surfaces. The
qualitative difference in the voltammetric profiles in the potential range of ~ 0.06
up to ~ 0.35 V RHE concerns the existence of the remarkable peaks at ~ 0.128
and ~ 0.284 V RHE due to the hydrogen adsorption/desorption at the (110) and
(100) monoatomic steps, respectively. As we can see in the inset of Fig. 2 for the
hard sphere model, the row of monoatomic steps consists of two parts: the top
side and the bottom one, that would correspond to the positive and negative sides
of the dipole steps, following the Smoluchowski model [31]. The reversible peaks
Reprinted from the journal
83