1 3
Topics in Current Chemistry (2019) 377:5
In conclusion, the electro-oxidation of ammonia at Pt surfaces is, at least to the
knowledge of these authors, the most sensitive electrocatalytic reaction to the structure of platinum, so that the electro-oxidation of ammonia has served as a guide for
the characterization of shape-controlled Pt nanoparticles with (100) preferential orientation [85]. The electrochemical reduction of nitrite ( NO
−
2 ) toward N 2 is another
electrocatalytic reaction that preferentially takes place on the (100) facet of the Pt
[86], whose reaction mechanism (in terms of intermediates of reactions) has been
claimed to have some similarity with the oxidation mechanism of NH 3 to N 2 [87].
Returning to the reaction mechanism of ammonia electro-oxidation on Pt(100) in
alkaline media, recently, Katsounaros et al. [80] proposed that the dimerization of
NH, ads species to N 2 H 2, ads ones, being the last of which is dehydrogenated to N 2 ;
and the N ads species serves as a reaction intermediate for the formation of byproducts as is the NO ads species. Both N ads and NO ads act as catalyst poisons.
3.5 Asymmetric Electrocatalysis of the Glucose Oxidation in Intrinsically Chiral Pt
Surfaces
As already reported above, the surface of the catalyst might consist of different
active sites. The sophistication in heterogeneous catalysis and electrocatalysis is
upgraded when enantioselective reactions are proposed to the catalyst surface. Comprehensive reviews on the origin of the chiral recognition and the enantioselectivity
involving asymmetric catalyst surfaces have been published [88, 89]. Briefly, for fcc
(face-centered cubic) metal lattices, the chiral recognition and the possible enantioselectivity are characteristics intrinsically linked to the kink sites. From the surface
chemistry point of view, the elegance of the kinked surfaces lies in its intrinsic chiral
character, and no matter the width of its terraces, all the kinked surfaces are intrinsically chiral [90]. The chiral surfaces cannot overlap with their mirror image. The
kinked surfaces appear at the intercept of the three different basal planes, namely,
the (111), (110) and (100), as shown in Fig. 8. Experimentally, they are obtained by
cutting the surface of a stepped surface (intercept of two basal planes) with respect
to the third basal plane [91] and the chirality obeys the follow condition h ≠ k ≠ l and
h × k × l ≠ 0, in which the (hkl) are the Miller indices [92]. One pair of ideal (643)
R&S
chiral faces (enantiomers) of the metals of fcc lattices is displayed in Fig. 10. The R
(from the Latin rectus) and S (from the Latin sinister) terminology design the clockwise (111) → (100) → (110) and counter-clockwise (111) ← (100) ← (110) sequence
of exposed planes on a surface (similar to the Cahn-Ingold-Prelog rules), taking
into account the priority of the planes on the basis of its packing density ρ (hkl) , i.e.,
ρ (111 ) > ρ (100) > ρ (110) [93].
Chiral properties, i.e., the chiral recognition and the enantioselectivity on solid
catalysts, very often, is introduced by attachment of chiral compounds on nonchiral substrates [89], as the hybrid systems. The disadvantage is that the chiral
modifiers may be leached from the solid surfaces, which would deactivate the
chiral properties. The enantioselective recognition has also been obtained by generation of cavities in mesoporous Pt, by electrochemical reduction of Pt ions in
presence of a “self-assembled liquid crystal” phase and chiral template molecules
Reprinted from the journal
95
Topics in Current Chemistry (2019) 377:5
In conclusion, the electro-oxidation of ammonia at Pt surfaces is, at least to the
knowledge of these authors, the most sensitive electrocatalytic reaction to the structure of platinum, so that the electro-oxidation of ammonia has served as a guide for
the characterization of shape-controlled Pt nanoparticles with (100) preferential orientation [85]. The electrochemical reduction of nitrite ( NO
−
2 ) toward N 2 is another
electrocatalytic reaction that preferentially takes place on the (100) facet of the Pt
[86], whose reaction mechanism (in terms of intermediates of reactions) has been
claimed to have some similarity with the oxidation mechanism of NH 3 to N 2 [87].
Returning to the reaction mechanism of ammonia electro-oxidation on Pt(100) in
alkaline media, recently, Katsounaros et al. [80] proposed that the dimerization of
NH, ads species to N 2 H 2, ads ones, being the last of which is dehydrogenated to N 2 ;
and the N ads species serves as a reaction intermediate for the formation of byproducts as is the NO ads species. Both N ads and NO ads act as catalyst poisons.
3.5 Asymmetric Electrocatalysis of the Glucose Oxidation in Intrinsically Chiral Pt
Surfaces
As already reported above, the surface of the catalyst might consist of different
active sites. The sophistication in heterogeneous catalysis and electrocatalysis is
upgraded when enantioselective reactions are proposed to the catalyst surface. Comprehensive reviews on the origin of the chiral recognition and the enantioselectivity
involving asymmetric catalyst surfaces have been published [88, 89]. Briefly, for fcc
(face-centered cubic) metal lattices, the chiral recognition and the possible enantioselectivity are characteristics intrinsically linked to the kink sites. From the surface
chemistry point of view, the elegance of the kinked surfaces lies in its intrinsic chiral
character, and no matter the width of its terraces, all the kinked surfaces are intrinsically chiral [90]. The chiral surfaces cannot overlap with their mirror image. The
kinked surfaces appear at the intercept of the three different basal planes, namely,
the (111), (110) and (100), as shown in Fig. 8. Experimentally, they are obtained by
cutting the surface of a stepped surface (intercept of two basal planes) with respect
to the third basal plane [91] and the chirality obeys the follow condition h ≠ k ≠ l and
h × k × l ≠ 0, in which the (hkl) are the Miller indices [92]. One pair of ideal (643)
R&S
chiral faces (enantiomers) of the metals of fcc lattices is displayed in Fig. 10. The R
(from the Latin rectus) and S (from the Latin sinister) terminology design the clockwise (111) → (100) → (110) and counter-clockwise (111) ← (100) ← (110) sequence
of exposed planes on a surface (similar to the Cahn-Ingold-Prelog rules), taking
into account the priority of the planes on the basis of its packing density ρ (hkl) , i.e.,
ρ (111 ) > ρ (100) > ρ (110) [93].
Chiral properties, i.e., the chiral recognition and the enantioselectivity on solid
catalysts, very often, is introduced by attachment of chiral compounds on nonchiral substrates [89], as the hybrid systems. The disadvantage is that the chiral
modifiers may be leached from the solid surfaces, which would deactivate the
chiral properties. The enantioselective recognition has also been obtained by generation of cavities in mesoporous Pt, by electrochemical reduction of Pt ions in
presence of a “self-assembled liquid crystal” phase and chiral template molecules
Reprinted from the journal
95