Topics in Current Chemistry (2019) 377:5
1 3
steps were selectively blocked by CO adsorption. In this case, in comparison to the
blank cyclic voltammogram (red line), the reversible pair of peaks at ~ 0.264 V are
fully blocked for hydrogen adsorption, and only the sites at the (111) terraces were
free to adsorb hydrogen (Fig. 5, olive line). The oxidation of the CO only on (110)
steps develops a peak at ~ 0.79 V. On the other hand, in the oxidation of full-coverage
CO, where presumably all the kinds of sites were occupied by CO, the voltammetry
develops multiple CO oxidation peaks. In the case of CO electro-oxidation on kinked
Pt surfaces in alkaline media, three oxidation peaks arise in the CO stripping voltammogram [53]. For full coverage (Fig. 5), one prominent peak appears at ~ 0.58 V,
and on the basis of partial CO stripping, it is due to the CO electro-oxidation along
the (111) terrace sites of the Pt(554) surface. The position of this peak is extremely
sensitive to the surface orientation (and flame annealing preconditioning). Then, in
Fig. 5, the potential required for the CO electro-oxidation involving the (111) terraces
is lower than that required for the CO oxidation at the (110) steps or top side of the
step sites. Therefore, it is clear that the electro-oxidation of CO on Pt surfaces is a
reaction which preferentially takes place at (111) terrace sites. Even the kink sites are
less catalytically active toward CO electro-oxidation than the (111) terrace sites of
the kinked Pt surfaces [51]. The identification of the (111) terraces of the stepped Pt
surfaces as being the most active sites also has been possible by in situ FTIR analysis [51, 52, 54]. On the other hand, on the basis of theoretical (coordination-activity
plot) modeling (and experimental data), Calle-Vallejo et al. [55] suggested that the Pt
sites become activated at the lowest potentials are located at convex structures, being
OH ads at the top side of the steps, whereas the CO ads were at the (111) domains from
the upper side of the (111) terraces (close to the step occupied with OH ads ). Preferential CO electro-oxidation at the (111) terraces has also been observed on shapecontrolled Pt nanoparticles (size ~ 8.5 nm) in alkaline media [56], and on commercial
carbon-supported Pt nanoparticles (mean size 1.81 nm) in acid [57].
However, despite the higher catalytic activity along the (111) terraces of the
stepped Pt surfaces, the catalytic activity of the Pt(111) electrode is lower than that
observed for stepped surfaces in similar conditions. This can suggests that the break
of the (111) terraces by steps induces variation in catalytic activity along the (111)
terraces, not present in the “infinite” Pt(111) surface. In this way, on the basis of
sequential sites for the CO adsorption and the oxidation at stepped Pt surfaces, it
was suggested that there is a hierarchical energy gradient along the terraces [51]. In
this sense, in terms of site occupancy, the top side of the step sites represents the set
of sites that are preferentially occupied by CO adsorption, i.e., the set of sites to be
first occupied, which also are the same sites which were the last released after CO
electro-oxidation, in this particular case. At the solid/gas interface, in an experiment
in which the CO ads was forced to shift from steps to terraces and vice versa, as a
function of a local thermal perturbation, it was observed that CO occupies preferentially the top of the steps, where it binds more strongly than on (111) terraces [58].
The set of sites close to the step sites from the bottom side (or terrace sites—see
hard sphere model in Fig. 5) are concave sites and are the set of sites to be the last
occupied by CO adsorption; but during the oxidation of the CO adlayer, these sites
are released first [51]. Then, the most catalytically active sites correspond to the
locally concave structures and the lowest catalytically active ones correspond to the
Reprinted from the journal
88
1 3
steps were selectively blocked by CO adsorption. In this case, in comparison to the
blank cyclic voltammogram (red line), the reversible pair of peaks at ~ 0.264 V are
fully blocked for hydrogen adsorption, and only the sites at the (111) terraces were
free to adsorb hydrogen (Fig. 5, olive line). The oxidation of the CO only on (110)
steps develops a peak at ~ 0.79 V. On the other hand, in the oxidation of full-coverage
CO, where presumably all the kinds of sites were occupied by CO, the voltammetry
develops multiple CO oxidation peaks. In the case of CO electro-oxidation on kinked
Pt surfaces in alkaline media, three oxidation peaks arise in the CO stripping voltammogram [53]. For full coverage (Fig. 5), one prominent peak appears at ~ 0.58 V,
and on the basis of partial CO stripping, it is due to the CO electro-oxidation along
the (111) terrace sites of the Pt(554) surface. The position of this peak is extremely
sensitive to the surface orientation (and flame annealing preconditioning). Then, in
Fig. 5, the potential required for the CO electro-oxidation involving the (111) terraces
is lower than that required for the CO oxidation at the (110) steps or top side of the
step sites. Therefore, it is clear that the electro-oxidation of CO on Pt surfaces is a
reaction which preferentially takes place at (111) terrace sites. Even the kink sites are
less catalytically active toward CO electro-oxidation than the (111) terrace sites of
the kinked Pt surfaces [51]. The identification of the (111) terraces of the stepped Pt
surfaces as being the most active sites also has been possible by in situ FTIR analysis [51, 52, 54]. On the other hand, on the basis of theoretical (coordination-activity
plot) modeling (and experimental data), Calle-Vallejo et al. [55] suggested that the Pt
sites become activated at the lowest potentials are located at convex structures, being
OH ads at the top side of the steps, whereas the CO ads were at the (111) domains from
the upper side of the (111) terraces (close to the step occupied with OH ads ). Preferential CO electro-oxidation at the (111) terraces has also been observed on shapecontrolled Pt nanoparticles (size ~ 8.5 nm) in alkaline media [56], and on commercial
carbon-supported Pt nanoparticles (mean size 1.81 nm) in acid [57].
However, despite the higher catalytic activity along the (111) terraces of the
stepped Pt surfaces, the catalytic activity of the Pt(111) electrode is lower than that
observed for stepped surfaces in similar conditions. This can suggests that the break
of the (111) terraces by steps induces variation in catalytic activity along the (111)
terraces, not present in the “infinite” Pt(111) surface. In this way, on the basis of
sequential sites for the CO adsorption and the oxidation at stepped Pt surfaces, it
was suggested that there is a hierarchical energy gradient along the terraces [51]. In
this sense, in terms of site occupancy, the top side of the step sites represents the set
of sites that are preferentially occupied by CO adsorption, i.e., the set of sites to be
first occupied, which also are the same sites which were the last released after CO
electro-oxidation, in this particular case. At the solid/gas interface, in an experiment
in which the CO ads was forced to shift from steps to terraces and vice versa, as a
function of a local thermal perturbation, it was observed that CO occupies preferentially the top of the steps, where it binds more strongly than on (111) terraces [58].
The set of sites close to the step sites from the bottom side (or terrace sites—see
hard sphere model in Fig. 5) are concave sites and are the set of sites to be the last
occupied by CO adsorption; but during the oxidation of the CO adlayer, these sites
are released first [51]. Then, the most catalytically active sites correspond to the
locally concave structures and the lowest catalytically active ones correspond to the
Reprinted from the journal
88