Photocatalytic hydrogenation of azobenzene to hydrazobenzene on cadmium sulfide under visible light irradiation

Article abstract of DOI:10.1039/c7cc08428e

Visible-light irradiation (λ < 600 nm) of commercially-available CdS in alcohol successfully promotes hydrogenation of azobenzene to hydrazobenzene with more than 95% selectivity. This is promoted by strong adsorption of azobenzene to the photoformed zerovalent Cd species adjacent to the surface S vacancies on CdS; this leads to efficient reduction to hydrazobenzene.

Full text of DOI:10.1039/c7cc08428e

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Photocatalytic hydrogenation of azobenzene to hydrazobenzene
on cadmium sulfide under visible light irradiation
Yasuhiro Shiraishi,a,b* Miyu Katayama,a Masaki Hashimotoa and Takayuki Hiraia
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Visible light irradiation (λ <600 nm) of commercially-available CdS sacrificial electron and proton donor. Both systems, however,
in alcohol successfully promotes hydrogenation of azobenzene to promote subsequent reduction of the formed hydrazobenzene
hydrazobenzene with more than 95% selectivity. This is promoted to aniline (Scheme 1). In addition, the former system needs UV
by strong adsorption of azobenzene to the photoformed light for catalyst photoexcitation, and the latter needs a base
zerovalent Cd species adjacent to the surface S vacancies on CdS, (NaOH). Exploring a new catalyst that promotes selective
leading to efficient reduction to hydrazobenzene.
azobenzene-to-hydrazobenzene photoreduction driven by
visible light without noble metal or base is therefore desired.
Here we report that cadmium sulfide (CdS), a common and
inexpensive semiconductor photocatalyst driven by visible
Hydrazobenzenes is one important chemical for synthesis of
pharmaceuticals and dyes.1 It is usually manufactured by
hydrogenation of azobenzene using stoichiometric amounts of
metal reducing reagents such as Zn, Fe, and Mg, along with
concomitant formation of large amounts of inorganic wastes.2
Catalytic hydrogenation on Pd catalysts with H2 has also been
proposed.3 This, however, subsequently reduces the formed
hydrazobenzenes to anilines (Scheme 1), and optimization of
reaction conditions such as temperature and H2 pressure is
necessary. Creating a new method for selective azobenzene-
to-hydrazobenzene reduction under mild conditions is desired
for green hydrazobenzene synthesis.
light irradiation (
λ
<600 nm),7,8 promotes azobenzene-to-
hydrazobenzene reduction with >95% selectivity. UV-vis and X-
ray photoelectron spectroscopy (XPS) analysis revealed that S
vacancies on the CdS surface act as the reduction sites. The Cd
atoms adjacent to the
S vacancies are photoreduced,
producing zerovalent Cd species. Azobenzenes are strongly
adsorbed onto the Cd0 species and successfully reduced to
hydrazobenzene, whereas the formed hydrazobenzenes are
inactive. This thus suppresses further reduction of
hydrazobenzene.
Photoreactions were performed by visible light irradiation
H
(
λ
mL) containing catalyst (50 mg) and azobenzene (50 mol)
>420 nm, Xe lamp) of a 2-PrOH/water mixture (9/1 v/v, 5
NH2
2e−, 2H+
2e−, 2H+
N
N
2
N
N
H
ꢀ
under Ar atmosphere (1 atm) at 303 K with magnetic stirring.
azobenzene
hydrazobenzene
aniline
Commercially-available CdS (Sigma-Aldrich; average diameter,
1
Scheme 1. Consecutive reduction of azobenzene
40 nm, BET surface area, 49 m2 g– ) was used. Scanning
electron microscopy (SEM) images of the sample are shown in
Photocatalysis is one ideal system for this purpose because
it promotes reduction reactions using inexpensive and safe
reducing reagents such as alcohols at room temperature and
atmospheric pressure. Some systems have been employed for
azobenzene reduction. TiO2 loaded with Pt particles (Pt/TiO2)4
and CeO2 loaded with Au particles (Au/CeO2)5,6 promote
Fig. S1 (ESI†). Table 1 summarizes the results obtained by 5 h
photoirradiation with respective catalysts. Absence of catalyst
does not promote the reaction (entry 1). However, as shown
by entry 2, use of CdS promotes almost complete azobenzene-
to-hydrazobenzene transformation with 97% selectivity, where
aniline, a further hydrogenation product of hydrazobenzene, is
produced at only a minor level (3%). Note that other common
visible-light-responsive semiconductors such as Fe2O3,9
photocatalytic azobenzene reduction with alcohol as
a
13
BiVO4,10 WO3,11 Ag3PO4,12 and g-C3N4 (entries 3–7) are
inactive; almost no azobenzene reacts even in 5 h reaction. In
contrast, TiO2 (rutile),14 which absorbs visible light only weakly,
promotes
selective
azobenzene-to-hydrazobenzene
conversion (entry 8), although the azobenzene conversion is
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only 20%. The TiO2, when photoexcited at
λ
>300 nm (entry 9),
promotes almost complete transformation. These indicate that
bare CdS and TiO2 promote selective azobenzene-to-
hydrazobenzene conversion; only CdS is effective under visible
light. As shown by entry 2, the CdS recovered after the
reaction, when reused for further reaction, shows almost the
same activity and selectivity to those obtained by the fresh
one. In addition, inductively-coupled plasma (ICP) analysis of
the solution recovered after the reaction scarcely detected Cd
or S component. These suggest that CdS activated by visible
light promotes selective azobenzene-to-hydrazobenzene
conversion and is reusable at least two times without loss of
activity and selectivity.
60
40
20
0
H2
12
0
6
t / h
Fig. 1 Change in the amounts of azobenzene and products during photoreaction with
Table 1 Results of photocatalytic reduction of azobenzene on the respective catalysts.a
CdS under visible light. Reaction conditions are identical to those in Table 1.
H
NH2
hv (5 h)
N
N
+
N
N
H
catalyst (50 mg)
2-PrOH/water = 9/1 v/v (5 mL)
Ar (1 atm), 298 K
0.05 mmol
1
2
Selectivity / %
Azobenzene
conv / % c
<0.1
99
Entry
Catalyst
Light b
1 c
2 c
1
2
none
CdS
1st reuse d
2nd reuse d
Fe2O3
UV + vis
vis
97
95
97
3
5
3
vis
94
vis
96
3
4
vis
<0.1
<0.1
<0.1
<0.1
<0.1
20
BiVO4
vis
5
WO3
vis
6
Ag3PO4
g-C3N4
vis
7
vis
e
8
TiO2
vis
96
97
84
4
3
e
9
TiO2
UV + vis
vis
99
10
11
Pt (0.5 wt %)/CdS
Pt (1 wt %)/CdS
37
16
vis
0.3
a Reaction conditions: catalyst (50 mg), 2-PrOH/water mixture (9/1 v/v, 5 mL), Ar
(1 atm), time (5 h), temperature (298 K). b UV + vis (λ >300 nm), vis (λ >420 nm). c
Amounts of substrate and products were determined by GC-FID. d Catalysts were
recovered by centrifugation, washed with water, and used for further reactions. e
Japan Reference catalyst (JRC-TIO-6) supplied from the Catalyst Society of Japan.
Average diameter (20 nm), BET surface area (49 m2 g–1).
Fig. 1 shows time-dependent change in the amounts of
azobenzene and products during photoreaction on CdS.
Almost all of azobenzene is transformed to hydrazobenzene
selectively and quantitatively with only a minor amount of
aniline. The amounts of hydrazobenzene and aniline scarcely
change even after prolonged photoirradiation, indicating that
further reduction of hydrazobenzene scarcely occurs (Scheme
Fig. 2 (a) Structure of S vacancies on Wurtzite CdS surface, and proposed mechanism
for the formation of Cd0 species by the photoformed CB electrons. (b) Electronic band
structure of CdS and reduction potential of azobenzene.
As shown in Fig. 2b, valence band (VB) and conduction
band (CB) of CdS lie at 1.88 V and
–0.52 V (vs. NHE),
respectively, with the band gap energy 2.4 eV (514 nm).7 It is,
however, reported that reduction potential of azobenzene lies
1). As shown in Fig. S2 (ESI†), photoirradiation of a 2-PrOH
at
–
1.1 V (vs. NHE),5 more negative than the CdS CB. This
solution with CdS and hydrazobenzene gives only a minor
amount of aniline, again indicating that subsequent reduction
of hydrazobenzene scarcely occurs. During the reaction of
azobenzene, formation of H2 was not detected by GC analysis.
In addition, as shown by green symbols in Fig. 1, amount of
acetone formed is similar to the sum of the amounts of
hydrazobenzene (blue) and aniline (black). These data indicate
suggests that reduction of azobenzene by the CB electrons on
CdS is thermodynamically unfavourable. As shown in Fig. 2a,
CdS has a hexagonal wurtzite structure and usually contain a
large number of surface S vacancies.15,16 These S vacancies play
a crucial role in the photocatalytic azobenzene reduction. On
the S vacancies, low-coordinated Cd atoms are exposed on the
that
H atoms of 2-PrOH are used quantitatively for
surface. The donor levels of these Cd atoms lie at ca. 0.1–0.4
eV below the CB bottom.17,18 They, thus, act as the trapping
sites for the CB electrons. As shown in Fig. 2a, the trapped
hydrogenation of azobenzene.
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electrons reduce these Cd atoms and produce Cd0 species via CdS, when photoirradiated by visible light in a 2-PrOH solution
aggregation, although structure of the species has not been under Ar (Fig. 3b), shows a new component assigned to Cd0
clarified.19,20 As shown in Fig. 2b, electron-deficient N=N bond (blue).25 This indicates that the Cd0 species are indeed
of azobenzene is adsorbed onto the Cd0 via the electron produced by the photoformed CB electrons (Fig. 2a). In
transfer. This decreases the reduction potential of azobenzene contrast, as shown in Fig. 3c, azobenzene, when adsorbed onto
and successfully promotes the reduction to hydrazobenzene.
the photoirradiated CdS, creates additional component at
The S vacancies as the reduction sites for azobenzene are higher binding energy (green), together with a decrease in the
confirmed by loading of metal particles onto CdS. Pt particles Cd0. This indicates that the surface Cd0 are oxidized via
were loaded by the photodeposition method with electron donation to the adsorbed azobenzene. As shown in
H2PtCl6·6H2O as a precursor, where Pt particles are created on Fig. S3 (ESI†), XPS (S 2p) of CdS reveals that photoirradiation
the surface vacancies.21,22 As shown in Table 1 (entries 10, 11), creates an S component charged more negatively at lower
CdS loaded with 0.5 or 1 wt % Pt show decreased azobenzene binding energy. This is due to the formation of adjacent Cd0 by
conversion, and the decrease becomes more apparent with photoreduction. Addition of azobenzene to this sample
the amount of Pt. This indicates that the surface S vacancies recovers the S component at higher binding energy due to the
indeed act as the active sites for azobenzene reduction.
electron donation from Cd0 to the adsorbed azobenzene.
These are consistent with the Cd XPS data.
3d 5/2
CdS/hydrazobenzene
CdS
1
0.5
0
3d 3/2
Cd2+
405.0
Cd2+
411.7
30
20
10
0
a
b
c
CdS/azobenzene
azobenzene
620 nm
hydrazobenzene
410
400
300
400
500
600
700
800
Wavelength / nm
Cd0
404.3
Cd0
411.0
Fig. 4 (Solid lines) Absorption spectra of CdS and the sample obtained after visible light
irradiation in a 2-PrOH/water (9/1 v/v) mixture under Ar for 6 h followed by adsorption
of azobenzene or hydrazobenzene. The spectra for azobenzene or hydrazobenzene in a
2-PrOH/water mixture (1 mM) are also shown. (Dashed line) difference spectra
obtained by subtraction of CdS (black) from azobenzene-adsorbed CdS (red). (Circles)
Action spectrum for hydrazobenzene formation obtained by the photoreactions of
azobenzene on CdS. The apparent quantum yields for hydrazobenzene formation was
determined with the equation: ΦAQY (%) = ([hydrazobenzene formed (ꢀmol)] ×2) /
[photon number entering into the reaction vessel (ꢀmol)]) × 100.
410
400
Cd0–azobenzene
404.8
Cd0–azobenzene
411.5
As shown in Fig. 4 (red), the obtained azobenzene-
adsorbed CdS shows red-shifted absorption at
compared to bare CdS (black), although azobenzene itself
absorbs light at <570 nm (orange). This new band originates
form the interfacial charge transfer (IFCT)26 from the CdS VB to
λ >600 nm as
410
400
Binding energy / eV
λ
Fig. 3 XPS chart (Cd 3d region) of (a) CdS, (b) CdS obtained after visible light irradiation
in
a
2-PrOH/water mixture, and (c) the sample obtained after adsorption of the adsorbed azobenzene, as is also observed for
nitrobenzene-adsorbed TiO2.22 The absorption maximum of
this band, determined by subtraction of the spectrum for bare
CdS, is ca. 620 nm (2.0 eV). The donor level of this transition
azobenzene onto the photoirradiated CdS (b).
Azobenzene adsorption onto the Cd0 species photoformed
on the S vacancies is confirmed by XPS analysis. CdS powders,
when photoirradiated by visible light in a 2-PrOH/water
mixture (9/1 v/v) under Ar, turns brownish, indicative of a
formation of Cd0. Exposure of the solution to air immediately
recovers the original bright yellow color. This indicates that, as
shown in Fig. 2a, the Cd0 species formation occurs
calculated from the CdS VB is therefore
shown in Fig. 2b, this donor level is similar to that of surface S
vacancies ( 0.42 to 0.02 V vs NHE). This suggests that the
–0.12 V (vs NHE). As
–
–
red-shifted absorption is indeed due to the IFCT transition
from CdS VB to the adsorbed azobenzene. Electron donation
from the Cd0 species to the electron-deficient N=N bond of
azobenzene may lead to strong adsorption and facilitate the
IFCT transition. Similar strong interaction with the Cd0 species
is observed for electron-deficient C=O, C=C,19,27 or C=N
groups.20 As shown in Fig. 2b, the donor level of this IFCT
reversibly.23,24
Azobenzene,
when
added
to
the
photoirradiated brownish CdS suspension under Ar, leads to a
formation of bright orange powders. This color does not
change even after exposure to air. Fig. 3 shows XPS (Cd 3d
level) of CdS during the course of these sequence. CdS itself
(Fig. 3a) shows a single component assigned to Cd2+ (red). The
transition (
–
potential of azobenzene (
0.12 V) is more positive than the reduction
1.1 V). The strong Cd0
azobenzene
–
–
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interaction may lead to a decrease in its reduction potential
and, hence, facilitates efficient reduction of azobenzene.
As shown in Table 1 (entries 8, 9), TiO2 is also active for
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–
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2-PrOH/water mixture under Ar, shows red-shifted absorption,
as observed for CdS (Fig. 4), although other semiconductors do
not. It is well known that TiO2 contains a large number of
surface oxygen vacancies, where neighboring Ti4+ act as
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observed red-shifted absorption may thus be due to the IFCT
transition from TiO2 VB to the azobenzene adsorbed onto the
Ti3+. This allows efficient azobenzene reduction on TiO2.
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hydrazobenzene during photoreaction of azobenzene on CdS
measured by monochromated light irradiation. The apparent
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–
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ꢀ
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This work was partly supported by PRESTO (JPMJPR1442)
from JST.
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Manuscript title: Photocatalytic hydrogenation of azobenzene to hydrazobenzeneViewoAnrticle Online
DOI: 10.1039/C7CC08428E
cadmium sulfide under visible light irradiation
Authors: Yasuhiro Shiraishi,* Miyu Katayama, Masaki Hashimoto and Takayuki Hirai
Graphical Abstract
visible light (~600 nm)
S vacancy
Visible light irradiation of commercially-available CdS in alcohol successfully promotes
selective azobenzene-to-hydrazobenzene reduction, where surface S vacancies play a crucial
role for activity and selectivity.