1 3
Topics in Current Chemistry (2018) 376:42
urea molecular. Whereas, the bridging hydroxyl group is thought to participate in
this process by supplying protons to the amino group that is linked to the nickel
site. Figure 4b gives a simulation of urea decomposition catalyzed by urease at the
active site of the enzyme, which can exactly explain why Ni element promotes the
urea electro-oxidation reaction. Actually, almost all of these catalysts towards urea
electro-oxidation are based on nickel element. These nickel-containing catalysts
have one thing in common during the catalyzing process, namely that the onset oxidation potential (OOP) of urea is exactly consistent with that of NiOOH generated
from Ni(OH) 2 . This phenomenon makes it clear that NiOOH instead of metallic Ni
is the active site for urea electro-oxidation. Combined with in situ Raman spectroscopy and in situ X-ray diffraction patterns by Vedharathinam et al. [70–73], it can
be preliminarily presumed that the electro-oxidation reaction mechanism of urea on
nickel-based electrode surface is the electro-oxidation (E)-chemical oxidation (C)
mechanism. As shown in Eqs. (7) and (8), Ni(OH) 2 loses electrons to form NiOOH
at first, and then urea undergo chemical oxidation to form CO 2 and N 2 while NiOOH
is electro-reduced to form Ni(OH) 2 .
In order to further understand the electrochemical oxidation process, Botte et al.
studied the various mechanisms of urea decomposition into HNCO, NCO
–
, NH 3 ,
CO 2 , and N 2 , and adopting density functional theory (DFT) to forecast the product
of urea decomposition using NiOOH as active catalyst [74]. The reaction process
and relative rate-determination steps suggest three possible mechanisms for urea
electro-oxidation in basic media. Among three reaction pathways, urea molecules
are adsorbed on NiOOH through the same way of “bridge” connections in the first
step, as shown in Fig. 5. The proposed connection method is based on the binding mode of urea decomposition catalyzed by urease. The Ni atom in the urease
molecule is connected to the N atom or O atom in urea molecule, and the O atom
of the urease molecule is linked to the C atom of urea molecule. This structure was
confirmed as the most possible adsorption of urea on the catalyst surface. By comparison with the rate constants and free energies for each step of the corresponding
(7)
Electro − oxidation (E) ∶ Ni(OH) 2 + OH
− ⇄ NiOOH + H 2 O + e
−
(8)
Chemical oxidation (C) ∶ CO
NH 2
2
+ 6NiOOH + H 2 O → N 2 + CO 2 + 6Ni(OH) 2
Fig. 5 Optimal construction
of bridge-connected urea on
NiOOH (N2 atom was among
H4 and H3) Reproduced with
permission from Ref. [74]
Reprinted from the journal
51
Topics in Current Chemistry (2018) 376:42
urea molecular. Whereas, the bridging hydroxyl group is thought to participate in
this process by supplying protons to the amino group that is linked to the nickel
site. Figure 4b gives a simulation of urea decomposition catalyzed by urease at the
active site of the enzyme, which can exactly explain why Ni element promotes the
urea electro-oxidation reaction. Actually, almost all of these catalysts towards urea
electro-oxidation are based on nickel element. These nickel-containing catalysts
have one thing in common during the catalyzing process, namely that the onset oxidation potential (OOP) of urea is exactly consistent with that of NiOOH generated
from Ni(OH) 2 . This phenomenon makes it clear that NiOOH instead of metallic Ni
is the active site for urea electro-oxidation. Combined with in situ Raman spectroscopy and in situ X-ray diffraction patterns by Vedharathinam et al. [70–73], it can
be preliminarily presumed that the electro-oxidation reaction mechanism of urea on
nickel-based electrode surface is the electro-oxidation (E)-chemical oxidation (C)
mechanism. As shown in Eqs. (7) and (8), Ni(OH) 2 loses electrons to form NiOOH
at first, and then urea undergo chemical oxidation to form CO 2 and N 2 while NiOOH
is electro-reduced to form Ni(OH) 2 .
In order to further understand the electrochemical oxidation process, Botte et al.
studied the various mechanisms of urea decomposition into HNCO, NCO
–
, NH 3 ,
CO 2 , and N 2 , and adopting density functional theory (DFT) to forecast the product
of urea decomposition using NiOOH as active catalyst [74]. The reaction process
and relative rate-determination steps suggest three possible mechanisms for urea
electro-oxidation in basic media. Among three reaction pathways, urea molecules
are adsorbed on NiOOH through the same way of “bridge” connections in the first
step, as shown in Fig. 5. The proposed connection method is based on the binding mode of urea decomposition catalyzed by urease. The Ni atom in the urease
molecule is connected to the N atom or O atom in urea molecule, and the O atom
of the urease molecule is linked to the C atom of urea molecule. This structure was
confirmed as the most possible adsorption of urea on the catalyst surface. By comparison with the rate constants and free energies for each step of the corresponding
(7)
Electro − oxidation (E) ∶ Ni(OH) 2 + OH
− ⇄ NiOOH + H 2 O + e
−
(8)
Chemical oxidation (C) ∶ CO
NH 2
2
+ 6NiOOH + H 2 O → N 2 + CO 2 + 6Ni(OH) 2
Fig. 5 Optimal construction
of bridge-connected urea on
NiOOH (N2 atom was among
H4 and H3) Reproduced with
permission from Ref. [74]
Reprinted from the journal
51