1 3
Topics in Current Chemistry (2019) 377:5
locally convex structures of the same local structure present at the Pt surfaces [51,
52] (see inset in Fig. 5). This means that the defects/steps do not partake in chemistry by themselves, but step sites act by modifying the catalytic properties of their
neighboring sites. This seems to be a general characteristic of the sites vicinal to
Pt(111) toward CO electro-oxidation. Recently, we have reported a case in which the
activation pathway of CO oxidation on Pt was inhibited as the (111) planes become
defect-rich. In this case, the CO adlayer was deposited during cooling of the Pt(111)
surface in a CO atmosphere [44]. This means that the active site designation for the
CO electro-oxidation requires a precise specification of the experiments.
In electrocatalysis of CO stripping experiments, after CO ads oxidation at the most
active sites, CO molecules do not occupy them again, because the CO ads behaves
as an “immobile” species during the process. This is a general characteristic of CO
stripping on Pt electrodes in entire pH range [25, 51, 52]. Then, based on these
arguments, in terms of site occupancy, the set of the most catalytically active sites
become occupied only under conditions of full CO coverage or if there is CO in
the solution side. If we consider the (111) terraces and steps/defects, the strength of
CO adsorption at the specific sites is a parameter which must be considered in the
kinetic analysis of the preferential electrocatalytic oxidation of CO, and this preference is directly linked to the local arrangement of atoms at the catalyst surface. In
this regard, the influence of (100) steps in improving the catalytic activity of (111)
terraces is lower in comparison to the catalytic shift in (111) terraces in the presence
of (110) steps, despite the intrinsic catalytic activity of the (110) and (100) steps
being very similar to each other [25].
In conclusion, the most active sites toward CO electro-oxidation at the Pt(111)
vicinal surfaces consists of those located on the (111) terraces. A descriptive experiment which elucidates this issue starts by decoration of the (110) steps by
13
CO
leaving all the (111) terraces sites of a Pt(332) surface free for electro-oxidation of
the CO ads coming from (10
−3
to 10
−1
M) ethanol dissociation [59]. It was shown
that
13
CO ads previously attached at the (110) steps oxidized at potentials higher than
the electro-oxidation of CO ads from ethanol dissociation at the (111) terraces of
the stepped Pt surface. Again, a series of experiments including cyclic voltammetry (partial CO adlayer stripping), chronoamperometry (potential steps) and in situ
FTIR employing well-defined Pt-based electrodes, indicate that CO ads behaves as an
immobile species during its oxidation [52, 60]. The oxidation of the CO adlayer initiates at sites at the bottom side of the steps, that belong to the (111) terraces. It is
in this sense that we propose that the (111) domains of the stepped surfaces contain
the most active sites toward CO electro-oxidation. The reaction proceeds along the
(111) terraces, which became free for H UPD , and the last set of sites at which the
CO ads was oxidized are the step sites, which were finally accessible to H UPD .
3.3 Pathways of Methanol Electro‑Oxidation Toward Carbon Dioxide
The main reason why the electrochemical oxidation reaction of methanol is so
widely studied is because of its potential use in low-temperature fuel cells [61]. In
terms of thermodynamics, the standard potential for the reaction CH 3 OH (l) + H 2 O (l)
Reprinted from the journal
89
Topics in Current Chemistry (2019) 377:5
locally convex structures of the same local structure present at the Pt surfaces [51,
52] (see inset in Fig. 5). This means that the defects/steps do not partake in chemistry by themselves, but step sites act by modifying the catalytic properties of their
neighboring sites. This seems to be a general characteristic of the sites vicinal to
Pt(111) toward CO electro-oxidation. Recently, we have reported a case in which the
activation pathway of CO oxidation on Pt was inhibited as the (111) planes become
defect-rich. In this case, the CO adlayer was deposited during cooling of the Pt(111)
surface in a CO atmosphere [44]. This means that the active site designation for the
CO electro-oxidation requires a precise specification of the experiments.
In electrocatalysis of CO stripping experiments, after CO ads oxidation at the most
active sites, CO molecules do not occupy them again, because the CO ads behaves
as an “immobile” species during the process. This is a general characteristic of CO
stripping on Pt electrodes in entire pH range [25, 51, 52]. Then, based on these
arguments, in terms of site occupancy, the set of the most catalytically active sites
become occupied only under conditions of full CO coverage or if there is CO in
the solution side. If we consider the (111) terraces and steps/defects, the strength of
CO adsorption at the specific sites is a parameter which must be considered in the
kinetic analysis of the preferential electrocatalytic oxidation of CO, and this preference is directly linked to the local arrangement of atoms at the catalyst surface. In
this regard, the influence of (100) steps in improving the catalytic activity of (111)
terraces is lower in comparison to the catalytic shift in (111) terraces in the presence
of (110) steps, despite the intrinsic catalytic activity of the (110) and (100) steps
being very similar to each other [25].
In conclusion, the most active sites toward CO electro-oxidation at the Pt(111)
vicinal surfaces consists of those located on the (111) terraces. A descriptive experiment which elucidates this issue starts by decoration of the (110) steps by
13
CO
leaving all the (111) terraces sites of a Pt(332) surface free for electro-oxidation of
the CO ads coming from (10
−3
to 10
−1
M) ethanol dissociation [59]. It was shown
that
13
CO ads previously attached at the (110) steps oxidized at potentials higher than
the electro-oxidation of CO ads from ethanol dissociation at the (111) terraces of
the stepped Pt surface. Again, a series of experiments including cyclic voltammetry (partial CO adlayer stripping), chronoamperometry (potential steps) and in situ
FTIR employing well-defined Pt-based electrodes, indicate that CO ads behaves as an
immobile species during its oxidation [52, 60]. The oxidation of the CO adlayer initiates at sites at the bottom side of the steps, that belong to the (111) terraces. It is
in this sense that we propose that the (111) domains of the stepped surfaces contain
the most active sites toward CO electro-oxidation. The reaction proceeds along the
(111) terraces, which became free for H UPD , and the last set of sites at which the
CO ads was oxidized are the step sites, which were finally accessible to H UPD .
3.3 Pathways of Methanol Electro‑Oxidation Toward Carbon Dioxide
The main reason why the electrochemical oxidation reaction of methanol is so
widely studied is because of its potential use in low-temperature fuel cells [61]. In
terms of thermodynamics, the standard potential for the reaction CH 3 OH (l) + H 2 O (l)
Reprinted from the journal
89