Topics in Current Chemistry (2019) 377:5
1 3
At least in the case of the cyanide-Pt(111)-modified electrode, it was found that
at least three contiguous atoms at the Pt surfaces are required for the formation of
CO ads , while at least two contiguous atoms are required for the activation pathway of
CO 2 formation [73]. The set of atoms into the circle in Fig. 7c likely are the “active
sites” in the indirect pathway for the methanol electro-oxidation toward CO 2 going
through CO ads . In the cyanide-Pt(111)-modified electrode, those three contiguous Pt
atoms are not available and then the direct pathway takes place. As we can observe
in Fig. 7a, the set of three atoms are a row of atoms and the condition of the three
contiguous atoms is not fulfilled on cyanide-Pt(111)-modified surfaces. This elegant
experiment provides exactly the narrow relationship existing between a specific set
of atoms on the catalyst and the specific reaction pathways.
Similar restriction of at least three contiguous Pt atoms was also observed for
formic acid electro-oxidized to CO 2 with going through CO ads on cyanide-Pt(111)modified electrodes [74]. In conclusion, the mechanisms of methanol electro-oxidation imply that at least two contiguous atoms of Pt are required for the activation
of the reaction pathway of CO 2 formation, and at least three contiguous atoms are
needed for the dehydrogenation of the methanol molecule toward CO ads .
3.4 Electro‑Oxidation of Ammonia
In this reaction, we approach one extreme case in which the electrocatalytic reaction
takes place at a single kind of active site. Interest regarding ammonia electrochemical oxidation is because this toxic gas, or rather, the ammonium sulfate (NH 4 ) 2 SO 4 ,
used worldwide as a fertilizer, is a contaminant of water. As a result, the development of electrochemical sensors and selective catalysts for the degradation of ammonia to a harmless molecule such as N 2 gas are a subject of intense research in electrochemistry [75]. Other interest in ammonia electro-oxidation is because, from the
electrochemical energy conversion point of view, ammonia is a potential candidate
to be used as fuel in direct “ammonia” fuel cells [76, 77]. In extreme alkaline media,
the
standard
potential
for
the
ammonia
oxidation
reaction
NH 3(g) + 3OH
−
(aq) ⇄ 1∕2N 2(g) + 3H 2 O (l) + 3e − isE 0 ≃ −0.770 V SHE
or ∼ 0.055 V RHE
[76]. Then, it is interesting to study catalyst materials on which the selective electrochemical oxidation of ammonia toward N 2 gas is kinetically favored.
In terms of reaction mechanism, the accepted mechanism for the electrochemical oxidation of ammonia in alkaline media was proposed by Gerischer and
Mauerer [78]; according to them, it occurs through a sequential dehydrogenation
of adsorbed ammonia (NH 3, ads ) resulting in NH x species, i.e., NH 2, ads , NH, ads ,
and N ads . The main reaction intermediate is the NH 2 species, which dimerizes
to adsorbed hydrazine-like N 2 H x + y , ads , which might easily oxidize to N 2 . The
complete dehydrogenation of ammonia yields the N ads species which is inert for
the formation of N 2 , but in kinetics it was considered as a catalytic poison [78].
The essence of the Gerischer-Mauerer mechanism has been corroborated [79, 80]
by using differential electrochemical mass spectrometry (DEMS) at Pt in alkaline solution, in which N 2 , NO 2 and NO [79] (byproduct of ammonia oxidation)
were identified. In relation to the adsorbed species, Matsui et al. [81] by in situ
Reprinted from the journal
92
1 3
At least in the case of the cyanide-Pt(111)-modified electrode, it was found that
at least three contiguous atoms at the Pt surfaces are required for the formation of
CO ads , while at least two contiguous atoms are required for the activation pathway of
CO 2 formation [73]. The set of atoms into the circle in Fig. 7c likely are the “active
sites” in the indirect pathway for the methanol electro-oxidation toward CO 2 going
through CO ads . In the cyanide-Pt(111)-modified electrode, those three contiguous Pt
atoms are not available and then the direct pathway takes place. As we can observe
in Fig. 7a, the set of three atoms are a row of atoms and the condition of the three
contiguous atoms is not fulfilled on cyanide-Pt(111)-modified surfaces. This elegant
experiment provides exactly the narrow relationship existing between a specific set
of atoms on the catalyst and the specific reaction pathways.
Similar restriction of at least three contiguous Pt atoms was also observed for
formic acid electro-oxidized to CO 2 with going through CO ads on cyanide-Pt(111)modified electrodes [74]. In conclusion, the mechanisms of methanol electro-oxidation imply that at least two contiguous atoms of Pt are required for the activation
of the reaction pathway of CO 2 formation, and at least three contiguous atoms are
needed for the dehydrogenation of the methanol molecule toward CO ads .
3.4 Electro‑Oxidation of Ammonia
In this reaction, we approach one extreme case in which the electrocatalytic reaction
takes place at a single kind of active site. Interest regarding ammonia electrochemical oxidation is because this toxic gas, or rather, the ammonium sulfate (NH 4 ) 2 SO 4 ,
used worldwide as a fertilizer, is a contaminant of water. As a result, the development of electrochemical sensors and selective catalysts for the degradation of ammonia to a harmless molecule such as N 2 gas are a subject of intense research in electrochemistry [75]. Other interest in ammonia electro-oxidation is because, from the
electrochemical energy conversion point of view, ammonia is a potential candidate
to be used as fuel in direct “ammonia” fuel cells [76, 77]. In extreme alkaline media,
the
standard
potential
for
the
ammonia
oxidation
reaction
NH 3(g) + 3OH
−
(aq) ⇄ 1∕2N 2(g) + 3H 2 O (l) + 3e − isE 0 ≃ −0.770 V SHE
or ∼ 0.055 V RHE
[76]. Then, it is interesting to study catalyst materials on which the selective electrochemical oxidation of ammonia toward N 2 gas is kinetically favored.
In terms of reaction mechanism, the accepted mechanism for the electrochemical oxidation of ammonia in alkaline media was proposed by Gerischer and
Mauerer [78]; according to them, it occurs through a sequential dehydrogenation
of adsorbed ammonia (NH 3, ads ) resulting in NH x species, i.e., NH 2, ads , NH, ads ,
and N ads . The main reaction intermediate is the NH 2 species, which dimerizes
to adsorbed hydrazine-like N 2 H x + y , ads , which might easily oxidize to N 2 . The
complete dehydrogenation of ammonia yields the N ads species which is inert for
the formation of N 2 , but in kinetics it was considered as a catalytic poison [78].
The essence of the Gerischer-Mauerer mechanism has been corroborated [79, 80]
by using differential electrochemical mass spectrometry (DEMS) at Pt in alkaline solution, in which N 2 , NO 2 and NO [79] (byproduct of ammonia oxidation)
were identified. In relation to the adsorbed species, Matsui et al. [81] by in situ
Reprinted from the journal
92