Topics in Current Chemistry (2019) 377:27
1 3
both formate solution and its deuterated counterpart to evaluate photodecomposition to produce H 2 . The yield of H 2 was higher than 90% for the formate solution,
while the deuterated solution displayed a lower H 2 evolution rate, indicating that
the limiting step of the formate oxidation reaction was the generation of holes in
CdS particles (h
+
+ HCO 2
−
→ CO 2
−
+ H
+
), and that water was the source of H
+
for
the H 2 formation. Moreover, they realized that the formation of CO 2
−
(reducing
agent) after the photodecomposition promoted the presence of Cd metal in the
system. Nedoluzhko et al. [87] also reported that Cd
0
was involved in the photoformation of H 2 . In that study, a solution of CdS particles (~ 1.5 µm), FA and
buffer was irradiated under anaerobic conditions. Among evolved gases, the H 2
profile exhibited an induction time and the rate of H 2 evolution increased rapidly.
This turning point was related to the generation of Cd
0
from CdS. The formation
of cadmium (metallic state) was first attributed to the presence of formate; the
authors proposed that two CO 2
−
anion radicals, which acted as reducing agents,
are involved in the reduction process of CdS.
During this process, electrons present in the conduction band reduced water to
form H 2 . However, the authors noted that H 2 gas evolution did not happen at the
beginning of the photoreaction, and they claimed that one equivalent of CO 2
−
participated to reduce the lattice Cd
2+
. Therefore, the proposed mechanism consisted of
trapping of electrons, followed by a reduction.
According to the standard reduction potential values, it was stated that electrons of the CdS conduction band could not reduce the lattice Cd
2+
to Cd
0
. Once a
determined amount of the lattice Cd
2+
was converted to Cd
0
, H 2 gas evolution commenced following the suggested pathways.
Although CdS exhibited photocatalytic activity towards FA decomposition, it was
restricted to CdS in powder form. For instance, Nedoluzhko et al. [87] also observed
that the formation of Cd
0
, which played a pivotal role in the H 2 formation from FA
CdS + 2CO
⋅−
2
→ Cd
0 + S
2− + 2CO 2
CdS + e
−
cb
→
Cd
2+ e
− S
2−
,
Cd
2+ e
− S
2−
+ CO
⋅−
2
→ Cd
0 + CO 2 + S
2−
(1) COOH
− + h
+ → CO
⋅−
2
+ H
+ ,
CO
⋅−
2
+ h
+ → CO 2
2H
+ + 2e
− → H 2
(2) CO
⋅−
2
+ H
+ → CO 2 +
1
2
H 2
H
+ + e
− →
1
2
H 2
206
Reprinted from the journal
1 3
both formate solution and its deuterated counterpart to evaluate photodecomposition to produce H 2 . The yield of H 2 was higher than 90% for the formate solution,
while the deuterated solution displayed a lower H 2 evolution rate, indicating that
the limiting step of the formate oxidation reaction was the generation of holes in
CdS particles (h
+
+ HCO 2
−
→ CO 2
−
+ H
+
), and that water was the source of H
+
for
the H 2 formation. Moreover, they realized that the formation of CO 2
−
(reducing
agent) after the photodecomposition promoted the presence of Cd metal in the
system. Nedoluzhko et al. [87] also reported that Cd
0
was involved in the photoformation of H 2 . In that study, a solution of CdS particles (~ 1.5 µm), FA and
buffer was irradiated under anaerobic conditions. Among evolved gases, the H 2
profile exhibited an induction time and the rate of H 2 evolution increased rapidly.
This turning point was related to the generation of Cd
0
from CdS. The formation
of cadmium (metallic state) was first attributed to the presence of formate; the
authors proposed that two CO 2
−
anion radicals, which acted as reducing agents,
are involved in the reduction process of CdS.
During this process, electrons present in the conduction band reduced water to
form H 2 . However, the authors noted that H 2 gas evolution did not happen at the
beginning of the photoreaction, and they claimed that one equivalent of CO 2
−
participated to reduce the lattice Cd
2+
. Therefore, the proposed mechanism consisted of
trapping of electrons, followed by a reduction.
According to the standard reduction potential values, it was stated that electrons of the CdS conduction band could not reduce the lattice Cd
2+
to Cd
0
. Once a
determined amount of the lattice Cd
2+
was converted to Cd
0
, H 2 gas evolution commenced following the suggested pathways.
Although CdS exhibited photocatalytic activity towards FA decomposition, it was
restricted to CdS in powder form. For instance, Nedoluzhko et al. [87] also observed
that the formation of Cd
0
, which played a pivotal role in the H 2 formation from FA
CdS + 2CO
⋅−
2
→ Cd
0 + S
2− + 2CO 2
CdS + e
−
cb
→
Cd
2+ e
− S
2−
,
Cd
2+ e
− S
2−
+ CO
⋅−
2
→ Cd
0 + CO 2 + S
2−
(1) COOH
− + h
+ → CO
⋅−
2
+ H
+ ,
CO
⋅−
2
+ h
+ → CO 2
2H
+ + 2e
− → H 2
(2) CO
⋅−
2
+ H
+ → CO 2 +
1
2
H 2
H
+ + e
− →
1
2
H 2
206
Reprinted from the journal