Topics in Current Chemistry (2019) 377:5
1 3
(as is the case of the perchlorate anion) [98]. Intuitively, the expected results would
be of loss in ability of chiral recognition in the presence of strong adsorbing anions
because this species could block the “active sites” [100], but it is known that anions, as is the (bi)sulfate anions, adsorb at higher potentials at the (111) terrace sites
[101]. This fact can explain why there is no inhibition of the hydrogen region in the
voltammogram of the Pt(643)
R&S
surfaces in the presence of glucose in sulfuric acid
(Fig. 11). Moreover, the kink stability is an incognita because surface reconstruction
would modify the surface irreversibly. Indeed, glucose oxidation is too complex a
problem to facilitate the understanding of enantiomeric reactivity [102] and clearly
more work is needed to disentangle this problem using pure enough isomers (the
key step) with low molecular weight.
4 Concluding Remarks and Prospects
This report illustrates some examples aimed at determining the specific active sites
in electrocatalytic reactions which preferentially take place on specific facets of a
catalyst consisting of non-equivalent sites. The determination of the active sites for
the CO electro-oxidation requires a precise specification of the experimental conditions employed, especially because the catalytic activity of the Pt toward CO electro-oxidation deeply depends on the history of the CO adlayer. Then, for the cases in
which a CO adlayer is deposited under potential control, i.e., in the hydrogen region,
the main characteristic of the electro-oxidation of CO on Pt catalyst is that the reaction preferentially takes place at the (111) plane of defected surfaces. The preferred
reaction on (111) terraces of the defected surfaces combines the action of steps in
modifying the catalytic properties at (111) terraces, and the most active sites (that
are locally concave structures) and the lowest active ones (that are locally convex
structures) are facets of similar local structure at the Pt surfaces. The CO ads is a reaction intermediate formed during the oxidation of methanol. The set of atoms able
to activate the direct reaction pathway of methanol electro-oxidation toward CO 2
involve three contiguous atoms. Both surface defects and (111) terraces can provide
this set of contiguous atoms. The active sites for a more complex reaction, such as
glucose oxidation, cannot be determined with apparent simplicity like in the case
of CO electro-oxidation. However, it is possible to note that the introduction of the
kinks sites is responsible for the appearance of chiral recognition of glucose on
kinked surfaces. On the other hand, the determination of the active sites in ammonia
oxidation (and also other nitrogen-containing species, as nitrite reduction) is easily
characterized as being (100) square symmetry, because the extreme difference of
electrochemical reactivity among the different sites. Moreover, at this type of site,
the reaction is entirely selective to the N 2 formation, which is the desired product.
This paper is only a step towards determining of the active site on the surfaces of
catalysts in a macroscopic scale. It is mainly centered around Pt(111) and its vicinal surfaces. In this sense, there is a lot of work still necessary on single crystal
vicinal to the other two basal planes to make breakthroughs in the identification
of active sites and progress forward to incorporate more complex situations, e.g.,
those involving shape-controlled nanoparticles. The macroscopic crystals at some
Reprinted from the journal
98
1 3
(as is the case of the perchlorate anion) [98]. Intuitively, the expected results would
be of loss in ability of chiral recognition in the presence of strong adsorbing anions
because this species could block the “active sites” [100], but it is known that anions, as is the (bi)sulfate anions, adsorb at higher potentials at the (111) terrace sites
[101]. This fact can explain why there is no inhibition of the hydrogen region in the
voltammogram of the Pt(643)
R&S
surfaces in the presence of glucose in sulfuric acid
(Fig. 11). Moreover, the kink stability is an incognita because surface reconstruction
would modify the surface irreversibly. Indeed, glucose oxidation is too complex a
problem to facilitate the understanding of enantiomeric reactivity [102] and clearly
more work is needed to disentangle this problem using pure enough isomers (the
key step) with low molecular weight.
4 Concluding Remarks and Prospects
This report illustrates some examples aimed at determining the specific active sites
in electrocatalytic reactions which preferentially take place on specific facets of a
catalyst consisting of non-equivalent sites. The determination of the active sites for
the CO electro-oxidation requires a precise specification of the experimental conditions employed, especially because the catalytic activity of the Pt toward CO electro-oxidation deeply depends on the history of the CO adlayer. Then, for the cases in
which a CO adlayer is deposited under potential control, i.e., in the hydrogen region,
the main characteristic of the electro-oxidation of CO on Pt catalyst is that the reaction preferentially takes place at the (111) plane of defected surfaces. The preferred
reaction on (111) terraces of the defected surfaces combines the action of steps in
modifying the catalytic properties at (111) terraces, and the most active sites (that
are locally concave structures) and the lowest active ones (that are locally convex
structures) are facets of similar local structure at the Pt surfaces. The CO ads is a reaction intermediate formed during the oxidation of methanol. The set of atoms able
to activate the direct reaction pathway of methanol electro-oxidation toward CO 2
involve three contiguous atoms. Both surface defects and (111) terraces can provide
this set of contiguous atoms. The active sites for a more complex reaction, such as
glucose oxidation, cannot be determined with apparent simplicity like in the case
of CO electro-oxidation. However, it is possible to note that the introduction of the
kinks sites is responsible for the appearance of chiral recognition of glucose on
kinked surfaces. On the other hand, the determination of the active sites in ammonia
oxidation (and also other nitrogen-containing species, as nitrite reduction) is easily
characterized as being (100) square symmetry, because the extreme difference of
electrochemical reactivity among the different sites. Moreover, at this type of site,
the reaction is entirely selective to the N 2 formation, which is the desired product.
This paper is only a step towards determining of the active site on the surfaces of
catalysts in a macroscopic scale. It is mainly centered around Pt(111) and its vicinal surfaces. In this sense, there is a lot of work still necessary on single crystal
vicinal to the other two basal planes to make breakthroughs in the identification
of active sites and progress forward to incorporate more complex situations, e.g.,
those involving shape-controlled nanoparticles. The macroscopic crystals at some
Reprinted from the journal
98