Topics in Current Chemistry (2019) 377:27
1 3
As for noble-metal-free photocatalysts, some nice works can also be found in
the literature. For instance, Clarizia et al. [82] reported on novel nano-TiO 2 photocatalytic system based on the solar reforming of FA in presence of cupric ions
and chlorides, in a study in which the effect of the pH values, initial concentration
of FA, chloride and cupric ion in the H 2 production ability was investigated. The
H 2 production was suggested to proceed via the following steps:
• Step 1: reduction of cupric ions to cuprous with a simultaneous oxidation of
formic acid.
• Step 2: photolysis of some chloride complexes of cuprous ions.
Dong et al. [83] investigated the engineering of binary CuO/TiO 2 heterojunction nanofibers prepared from electrospinning and followed calcination treatment.
It was found that, after irradiation for 30 min during the photodecomposition of
FA, the binary heteroconjunction (CuO/TiO 2 ) changed to a heteroconjunction
formed by four components (Cu/Cu 2 O/CuO/TiO 2 ) originated by a photo-assisted
recrystallization reaction (Fig. 7), enhancing the separation of electron and hole
pairs. This aspect is crucial, because the production of H 2 takes place in a twosteps photoreaction: the generation of H
+
via photocatalytic oxidation of HCOOH
by the holes in the valence band, and formation of H 2 via photocatalytic reduction
of H
+
with photoinduced electrons in the conduction band. Then, as the oxidation reaction by the holes occurs before the reduction, the lifetime of the electrons should be longer than that of the holes. In this particular case, HCOOH
can be oxidized to H
+
by the holes accumulated in the valence band of CuO,
because of their higher potential (∼ + 2.05 V) than those of the pair CO 2 /HCOOH
(∼ − 0.61 V). However, the position of the conduction band of CuO is below the
reduction potential of the pair H
+
/H 2 (∼ + 0.36 V and ∼ − 0.42 V, respectively),
meaning that the reduction of H
+
to H 2 by the photoinduced electrons is not thermodynamically favored. For that reason, Cu 2 O and Cu species, which are visiblelight responsive and inexpensive co-catalysts, respectively, are formed from the
reduction of CuO. As a result, the quaternary multi-heterojunction photocatalysts
(Cu/Cu 2 O/CuO/TiO 2 ) displayed 40 times higher production of H 2 from FA than
their pure TiO 2 nanofibers counterpart.
Cu (II)
e
−
CB
→ Cu (I)
e
−
CB
→ Cu (0)
HCOOH
h
+
VB
→ CO
⋅−
2
+ 2H
+
CuCl
−
2
+ H
+
h
→ Cu
2+ + 2Cl
− + e
−
s
,
CuCl
2−
3
+ H
+
h
→ Cu
2+ + 3Cl
− + e
−
s
.
204
Reprinted from the journal
1 3
As for noble-metal-free photocatalysts, some nice works can also be found in
the literature. For instance, Clarizia et al. [82] reported on novel nano-TiO 2 photocatalytic system based on the solar reforming of FA in presence of cupric ions
and chlorides, in a study in which the effect of the pH values, initial concentration
of FA, chloride and cupric ion in the H 2 production ability was investigated. The
H 2 production was suggested to proceed via the following steps:
• Step 1: reduction of cupric ions to cuprous with a simultaneous oxidation of
formic acid.
• Step 2: photolysis of some chloride complexes of cuprous ions.
Dong et al. [83] investigated the engineering of binary CuO/TiO 2 heterojunction nanofibers prepared from electrospinning and followed calcination treatment.
It was found that, after irradiation for 30 min during the photodecomposition of
FA, the binary heteroconjunction (CuO/TiO 2 ) changed to a heteroconjunction
formed by four components (Cu/Cu 2 O/CuO/TiO 2 ) originated by a photo-assisted
recrystallization reaction (Fig. 7), enhancing the separation of electron and hole
pairs. This aspect is crucial, because the production of H 2 takes place in a twosteps photoreaction: the generation of H
+
via photocatalytic oxidation of HCOOH
by the holes in the valence band, and formation of H 2 via photocatalytic reduction
of H
+
with photoinduced electrons in the conduction band. Then, as the oxidation reaction by the holes occurs before the reduction, the lifetime of the electrons should be longer than that of the holes. In this particular case, HCOOH
can be oxidized to H
+
by the holes accumulated in the valence band of CuO,
because of their higher potential (∼ + 2.05 V) than those of the pair CO 2 /HCOOH
(∼ − 0.61 V). However, the position of the conduction band of CuO is below the
reduction potential of the pair H
+
/H 2 (∼ + 0.36 V and ∼ − 0.42 V, respectively),
meaning that the reduction of H
+
to H 2 by the photoinduced electrons is not thermodynamically favored. For that reason, Cu 2 O and Cu species, which are visiblelight responsive and inexpensive co-catalysts, respectively, are formed from the
reduction of CuO. As a result, the quaternary multi-heterojunction photocatalysts
(Cu/Cu 2 O/CuO/TiO 2 ) displayed 40 times higher production of H 2 from FA than
their pure TiO 2 nanofibers counterpart.
Cu (II)
e
−
CB
→ Cu (I)
e
−
CB
→ Cu (0)
HCOOH
h
+
VB
→ CO
⋅−
2
+ 2H
+
CuCl
−
2
+ H
+
h
→ Cu
2+ + 2Cl
− + e
−
s
,
CuCl
2−
3
+ H
+
h
→ Cu
2+ + 3Cl
− + e
−
s
.
204
Reprinted from the journal