Photocatalytic reduction of aromatic nitro compounds using CdS nanostructure under blue LED irradiation

Article abstract of DOI:10.1016/j.jphotochem.2013.09.011

The aromatic nitro compounds reduced with a high selectivity to the corresponding amines under blue LED irradiation (3 W) using CdS nanostructure as photocatalyst. The reaction is relatively sensitive to the electron demands of the substituents. The nitro compounds with electron withdrawing groups (CN, COR, NO₂) give higher yields than with the electron donating groups (OMe, Me). In the nitro compounds with low activity, the high yields of corresponding amines were achieved by the addition of ammonium format. The CdS nanostructure showed excellent photocatalytic performance for the reduction of nitrobenzene compared with commercial CdS (Aldrich) under visible LED irradiation. The results demonstrated that CdS nanostructure have potential to provide a promising visible light driven photocatalyst for the selective reduction of nitro compounds to corresponding amines under mild conditions. The excellent reusability of the photocatalyst was examined for six runs.

Full text of DOI:10.1016/j.jphotochem.2013.09.011

Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 7–12
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic reduction of aromatic nitro compounds using CdS
nanostructure under blue LED irradiation
Parvin Eskandaria, Foad Kazemia,b,∗, Zahra Zanda
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-6731, Iran
b Center for Climate and Global Warming (CCGW), Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-6731, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The aromatic nitro compounds reduced with a high selectivity to the corresponding amines under blue
LED irradiation (3 W) using CdS nanostructure as photocatalyst. The reaction is relatively sensitive to the
electron demands of the substituents. The nitro compounds with electron withdrawing groups (CN, COR,
NO2) give higher yields than with the electron donating groups (OMe, Me). In the nitro compounds with
low activity, the high yields of corresponding amines were achieved by the addition of ammonium format.
The CdS nanostructure showed excellent photocatalytic performance for the reduction of nitrobenzene
compared with commercial CdS (Aldrich) under visible LED irradiation. The results demonstrated that CdS
nanostructure have potential to provide a promising visible light driven photocatalyst for the selective
reduction of nitro compounds to corresponding amines under mild conditions. The excellent reusability
of the photocatalyst was examined for six runs.
Received 24 June 2013
Received in revised form
11 September 2013
Accepted 26 September 2013
Available online 8 October 2013
Keywords:
Photocatalysis
CdS nanostructure
Aromatic nitro compounds
LED irradiation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
substances highlight its potential as an efficient method for organic
transformation [7]. Thus, considerable efforts are being paid to
The selective reduction of aromatic nitro compounds to amines
is one of the most important and fundamental transformations
in organic synthesis, because the corresponding aromatic amines
are widely utilized in the dyes, photographic, agricultural, and
pharmaceutical industries [1]. Amines are often obtained by the
catalytic hydrogenation of aromatic nitro compounds and many
reducing agents have been applied for this transformation [2]. How-
ever, the selective reduction of the nitro group in the presence
of other reducible functional groups in a molecule such as car-
bonyl, halide, cyano, and etc. is a challenging task. In addition, the
reduction of aromatic nitro compounds often stops at an interme-
diate stage, producing hydroxylamines, hydrazines, and azoarenes
as side products [3,4]. Due to the high importance of the selective
reduction of aromatic nitro compounds, developing simple, green,
and highly efficient methods is required [5].
the photocatalytic selective reduction of nitro compounds. For
instance, TiO2 [8], N-doped TiO2 [9] or dye-sensitized TiO2 P25
[10,11], and PbBiO2X [12] under UV and visible light irradiation
have been reported for the reduction of aromatic nitro compounds
to corresponding amines.
In recent years, the search for the design of new visible-light-
driven photocatalysts for organic transformation has significantly
increased because of facile employment of sunlight due to its
role in conservation of the global ecosystem [13]. In this regard,
owing to the suitable band gap of CdS (2.4 eV) corresponding
well with the spectrum of sunlight, the use of CdS is tradition-
ally favored for many applications such as biological sensors [14],
solar cells [15,16], field effect transistors (FET) [17,18], hydrogen
evolution [19,20], and as photocatalyst in environmental purifi-
cation [21]. In addition, the CdS has long been considered as a
photocatalyst in organic transformations. For example, Yanigida
et al. reported the photoreduction of aromatic ketones and olefins
using CdS [22–24] and the first work on photocatalytic reduction
of nitrobenzene to aniline using CdS was reported by Kuchmii et al.
in 1989 [25]. More recently, CdS based graphene nanocomposite
[26,27], commercial CdS [28] and synthesized CdS [29] applied as
visible-light-driven photocatalysts for the selective reduction of
limited nitro compounds under visible light irradiation. The dis-
advantage of pure CdS particles is the high recombination rate of
photogenerated electron-hole pairs and susceptibility to agglom-
eration, restricting its wide application [19,30]. In order to solve
Recently, photocatalysis as a promising and green technique
has received increasing interest and provided an alternative to
the conventional synthetic process [6]. The required mild condi-
tions and the possibility to decrease the production of undesired
∗
Corresponding author at: Department of Chemistry, Institute for Advanced Stud-
ies in Basic Sciences (IASBS), Gava Zang, Zanjan 45137-6731, Iran. Tel.: +98 415 3219;
fax: +98 415 3232.
E-mail addresses: skandari@iasbs.ac.ir (P. Eskandari), kazemi f@iasbs.ac.ir
(F. Kazemi), z zand@iasbs.ac.ir (Z. Zand).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.09.011

8
P. Eskandari et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 7–12
these problems and enhance the photocatalytic activity of CdS
particles, many approaches have been developed, including the
preparation of quantum-sized CdS using various capping agents as
the stabilizer [31] or combination with other components, includ-
ing noble metals, semiconductors [32,33], and carbon materials
[19,26,27,34]. Among different methods, aqueous synthesis of CdS
using short chain thiols provides a useful alternative route to the
preparation of CdS nanostructures. It is well known that charac-
ters of surface of nanoparticles such as excitation lifetimes, flat
bond redox potentials, stability, solubility, and surface functional-
ity can be influenced by end-chain substituents on chemisorbed
thiols [35]. However, to our best knowledge, there has been
little research investigating the reduction of nitro compounds
using simply synthesized CdS nanostructure. This prompted us
to undertake an investigation featuring the preparation of CdS
nanostructure via a simple precipitation method using mercap-
toethylamine hydrochloride (MEA) as a capping reagent [36]. MEA
as an amine terminated thiol was selected with the expectation
that the nanostructure surface would be positively charged at
neutral pH and offer a strong binding interaction to the polar
molecules [35]. The synthesized CdS was characterized by dif-
fuse reflectance spectroscopy (DRS) technique, X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), TGA, and FTIR (see Supplementary Figs. S1–S5).
Recently, light emitting diodes (LEDs) with a low electrical
power requirement in the visible spectra offer a promising replace-
ment for conventional light sources in many applications [37–39].
Using LED lamps as a light source are associated with several advan-
tages such as high photon efficiency, low voltage electrical power
source, power stability, emission in broader spectral wavelength,
and no need for cooling during long time operation for complete
photocatalytic reactions [39]. Synthesized sample was utilized for
the selective photoreduction of a wide range of aromatic nitro com-
pounds to their corresponding amines under blue LED (3 W) lamp
irradiation under very simple and mild experimental condition. The
results demonstrated that the CdS nanostructure can be utilized
as an efficient visible light driven photocatalyst for the selective
reduction of aromatic nitro compounds to corresponding amines.
The high photo activity of prepared sample attributed to the effect
of MEA on CdS size reduction and its significant influence on the
surface functionality and stability of CdS nanophotocatalyst during
recycled experiments.
stirring. This mixture was stirred at room temperature for 45 min.
During the reaction, the color of mixture gradually turned to pale
yellow. After 1 h stirring, the precipitates were centrifuged and
rinsed with deionized water several times, then dried in a desic-
cator for 48 h. Deionized water used for the preparation of CdS,
was boiled and sparged continuously with argon while cooling to
room temperature.
2.3. Photocatalytic activity
The photocatalytic reduction of nitro compounds to their cor-
responding amines was performed using CdS nanopowder under
visible blue LED irradiation. Typically, 20 mg of the photocatalyst
was added to the 5 ml of 1 × 10−2 M nitrocompounds solution in
isopropanol. After that, the mixture was sunicated for 5 min to
obtain a homogeneous dispersion of CdS nanostructure in i-PrOH,
then purged argon (5 min) and then the vial was sealed up with a
rubber stopper. The mixture was stirred magnetically during reac-
tion (20 h) and illuminated with a visible blue LED (3 W) irradiation.
After the reaction, for removing the photocatalyst particles com-
pletely, the mixture was centrifuged, and the remaining solution
was analyzed with thin layer chromatography (TLC), and by Varian
gas chromatograph (CP-3800). Conversion of nitro, yield of amine,
and selectivity for amine were defined as follows:
ꢀ
ꢁ
(C0 − Cnitro
)
conversion (%) =
× 100
C0
Camine
yield (%) =
× 100
C0
ꢀ
ꢁ
Camine
selectivity (%) =
× 100
(C0 − Cnitro
)
where C0 is the initial concentration of nitro compound and Cnitro
and Camine are the concentration of the substrate nitro and the cor-
responding amine respectively, after the photocatalytic reaction.
3. Result and discussion
Since nanoparticles size directly affects exciton energies and
electron–hole redox properties, it is important to well control the
size of these photocatalytic nanoparticles. Also, the decrease in
particles size to nanometer scale of CdS increases surface to vol-
ume ratio leading to an enhancement of the electron and holes
surface access thereby enhances their photocatalytic activity for
efficient oxidation and reduction processes respect to bulk mate-
rial. Although several different methods have been developed
to obtain nanostructures, convenient precipitation method was
adopted to the synthesis of CdS nanostructures under mild con-
ditions. MEA was used as a capping reagent and surface stabilizing
ligand. Use of the MEA caused polar surface and enhanced the
adsorption of compounds in the surface of photocatalyst. Moreover,
CdS Q-dots synthesized by MEA derivatives as stabilizer reagent
utilized as photocatalyst for the reduction of aryl azides to the cor-
responding arylamines in homogeneous solution [35]. However,
using heterogeneous CdS photocatalyst allows much easier sepa-
ration of the photocatalyst during product isolation. To evaluate the
photocatalytic activity of the prepared nanostructure, reduction of
nitro compound was performed under the blue LED (ꢀ ≥ 420 nm)
irradiation. The photocatalytic activity of the prepared CdS nano-
structure is initially examined by reduction of nitrobenzene to
aniline under blue LED (3 W) irradiation. In order to determine
the optimal experimental conditions, first the impact of solvents
EtOH and i-PrOH was examined; it was found that reaction in
2. Experimental
2.1. Materials
Nitro compounds, sodium sulfide (Na2S), and ammonium for-
mate (HCO2NH4) were purchased from Merck Co. Cadmium acetate
dehydrate Cd (OAc)2·2H2O, mercaptoethylamine hydrochloride
(MEA), and commercial CdS were supplied by Aldrich Co. Iso-
propanol was purchased from J.T. Baker Co. All other chemicals
were used as received without further purification. Deionized
water was used in all experiments.
2.2. Preparation of CdS nanostructures
To synthesize CdS nanostructure, aqueous solutions of cadmium
acetate and sodium sulfide (0.1 M) were prepared by dissolving
the respective salts in deionized water. 10 ml aqueous solution of
mercaptoethylamine hydrochloride (MEA) as a capping reagent,
containing 2 g MEA, was prepared and added to the 20 ml of cad-
mium acetate solution at room temperature. The resulting solution
was stirred for 10 min, and then 20 ml of freshly prepared Na2S
solution was added dropwise to the mixture in 15 min under

P. Eskandari et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 7–12
9
Table 1
Optimization of the photocatalytic reduction condition of nitro benzene with CdS nanostructure under blue LED (3 W) irradiation and argon atmosphere, irradiation time:
20 h.
Entry
Nitrobenzene (M)
Solvent
CdS (mg)
Aniline (%)a
Conversion (%)a
Selectivity (%)
1
2
3
4
0.01
0.01
0.01
0.01
0.001
0.01
0.01
0.01
EtOH
20
40
20
10
20
100
0
14
52
55
25
48
0
33
90
77
50
80
0
42
57
71
50
60
0
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
5
6b
7
0
0
0
0
0
0
8c
20
a
Determined by gas chromatography.
In the dark.
Under air atmosphere.
b
c
Table 2
Photocatalytic reduction of nitrobenzene derivatives to corresponding amines via CdS nanostructure.a
NH2
NO2
CdS nanostructure (0.02 g)
i -PrOH, blue LED (3W)
( HCO2NH4) c
R
R
.
Entry
1
Starting material
Product (%)
Yield (%)b,c
90
Conversion (%)b,c
100
Selectivity (%)c
90
NH2
NH2
NO2
NC
NC
NO2
2
3
97
90
100
100
97
90
O2N
NO2
O2N
NH2
Cl
O
Cl
O
NH2
NO2
H3C
H3C
4
5
98
80
100
80
98
NO2
NH2
100
NO2
NH2
6
7
55(97)
77(100)
50(100)
71(97)
NO2
NO2
50 (100)
100(100)
NO2
NO2
NH2
NH2
8
47(100)
64(100)
73(100)
CH3
CH3
NO2
NH2
9
40 (100)
40 (100)
86(100)
47(100)
46(100)
85(100)
H3C
H3C
NH2
NO2
10
OMe
OMe
NH2
NO2
11d
19(23)
21(25)
90(92)
a
Photocatalyst, 20 mg; nitro compound alcoholic solution (1 × 10−2 M), 5 ml; irradiation with blue LED (3 W), 80 Lumen; irradiation time, 20 h.
Determined by gas chromatography.
Numbers in parentheses represent results in the presence of 50 mg of the HCO2NH4.
In the presence of commercial CdS (Aldrich).
b
c
d

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P. Eskandari et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 7–12
Fig. 1. Photocatalytic reduction of nitro compounds in the presence of HCO2NH4.
i-PrOH gives better results in term of product yields and selectivity
to amine (Table 1, entry 3).
Indeed, HCO2NH4 can be oxidized by the photoexcited holes of
the CdS photocatalyst, resulting in the formation of •CO2 radicals
−
Notably, by increasing the amount of the CdS photocatalyst
more than 20 mg, the nitrobenzene conversion increased, while
aniline selectivity decreased. On the basis of these results, the 20 mg
was chosen as optimum amount of CdS photocatalyst (Table 1,
entry 3). The reduction of nitrobenzene in low concentration
caused a high conversion but selectivity is lower than concentration
1 × 10−2 M (Table 1, entry 5). To ensure that CdS powder and light
together are required for reduction, several control experiments
were carried out. The obtained results indicate that the reduction
of nitro compounds to amines is attributed to the photocatalytic
activity of the CdS nanophtocatalyst (Table 1, entries 6, 7). In addi-
tion, photocatalyticactivitywas observedonly in argon atmosphere
(Table 1, entry 8). To demonstrate the general applicability of the
CdS nanopowder for the reduction of aromatic nitro compounds
and the scope of the process, various aromatic nitro compounds
were investigated.
As shown in Table 2, the photocatalytic reduction of various
aromatic nitro compounds including electron-rich and electron-
deficient substrates proceeded readily at simple experimental
condition to afford well to excellent yields of the corresponding
amines (Table 2). Chemical yields depend strongly on the elec-
tron withdrawing or electron donating nature of the functional
groups in the aromatic rings. For example, nitro compound with
electron withdrawing groups (CN, NO2, COR) have higher con-
version and more selectivity than those with electron donating
substituents (Me, OMe). During photocatalytic reduction of the
nitro compounds, the cyano and carbonyl functionalities present in
the aromatic ring remained unaffected (Table 2, entries 1, 4). Also,
it can be seen that 3-chloronitrobenzene was reduced to the cor-
responding aniline in excellent yield without any dehalogenation
which was often encountered with procedures such as hydrogena-
tion (Table 2, entry 3).
[28]. It has been reported that •CO2− radicals have strong reductive
powers [E0(CO2/•CO2−) = −1.9 V vs NHE] [40] and are able to reduce
nitro compounds. Therefore, nitro compounds might be reduced to
corresponding amines by both photoinduced electrons from the
conduction bond of the light activated CdS and •CO2− in the exper-
imental condition (Fig. 1). This can explain why the synthesized CdS
photocatalyst shows excellent photocatalytic activity for the reduc-
tion of nitro compounds to corresponding amines in the presence
of HCO2NH4 under visible LED irradiation.
The photoactivity of CdS nanostructure in the photoreduc-
tion of nitrobenzene was compared to that of commercial CdS
(Aldrich) under identical conditions (with and without the addition
of HCO2NH4). It was observed that the synthesized CdS nano-
structure is more efficient than commercial CdS (Aldrich) (Table 2,
entries 6, 11). A time course study of the photoreduction of 1,2-
di-nitrobenzene to corresponding aniline with CdS nanostructure
showed an increase in the rate of aniline formation from begin-
ning of irradiation with blue LED lamp (Fig. 2.). This study indicates
that no catalyst activation occurred during photocatalytic process
as reported in the photoreductions of nitrobenzenes to anilines via
dye-sensitized TiO2 and metal salts [12].
The recyclability of CdS photocatalyst was also examined by the
photocatalytic reduction of 3-nitroacetophenone as a test model.
It is interesting that using HCO2NH4 significantly enhanced the
reduction of nitro compounds with low activity (Table 2, entries
6–10). Generally, HCO2NH4 used as a hole scavenger for the reduc-
tion of organic compounds [26–28,35]. In the presence of HCO2NH4
and absence of CdS, in a blank experiment, irradiation of an alco-
holic solution of nitrobenzene led no reduction product, indicating
that CdS is essential for the reduction of nitro compound. On the
other hand, without irradiation under blue LED, no amine was
detected too.
Fig. 2. Time course study of the reduction of 1,2-di-nitrobenzene to corresponding
aniline via CdS nanostructure.

P. Eskandari et al. / Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 7–12
11
4. Conclusions
It is evident that the CdS nanostructure prepared by MEA as a
capping agent is an effective and reusable heterogeneous photo-
catalyst for the photoreduction of aromatic nitro compounds to
their corresponding amines under blue LED (3 W) lamp irradia-
tion. As synthesized CdS photocatalyst is capable of reducing a
wide scope of nitro compounds, the efficient reduction of nitro
compounds with low activity was also achieved by the addition of
HCO2NH4. The reducible functionalities such as carbonyl and cyano
groups remained intact under the reaction condition. The excel-
lent reusability for six runs shows the good stability of synthesized
sample. The time course study indicates that no catalyst activa-
tion occurred during photocatalytic process. In comparison with
commercial CdS (Aldrich), the prepared CdS nanostructure shows
superior photocatalytic activity. The high photoactivity of the CdS
nanostructure under visible light irradiation can be attributed to
the synergy effect of its high surface area, high photostability, and
surface functionality.
Fig. 3. Representation of 3-amino acetophenone yields at different reuses of the
photocatalyst.
After the first use of CdS nanostructure in the photoreduction, the
photocatalyst was isolated by centrifuge, washed thoroughly with
EtOH or i-PrOH, and then dried in a desiccator at room temperature.
The recycled catalyst showed excellent efficiency in 6 subsequent
reaction runs (Fig. 3).
The photocorrosion is often a problem to prevent use of CdS
photocatalyst (CdS + h+ → Cd2+ + S) [28]. The amount of Cd2+ is
determined by atomic absorption spectroscopy (AAS) (less than the
detection limit of 1 ppm), demonstrating that the photocorrosion
of CdS nanostructure was negligible. In addition, the XRD analy-
sis of CdS photocatalyst after the photoreduction was used for the
investigation of the stability of CdS that clearly showed no change
in the crystal structure under the experimental condition (Fig. 4).
The XRD patterns of the catalyst can be well indexed to cubic zinc
blend CdS (JCPDS card no. 80-0019).
Acknowledgment
The authors gratefully acknowledge the support provided by the
Institute for Advanced Studies in Basic Sciences (IASBS).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.jphotochem.2013.09.011.
References
The high efficiency of CdS nanostructure as a photocatalyst is
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NHE [41] and much negative potential of excited CdS nanostructure
(CB: −0.75 V vs NHE) [42] clearly recognized the thermodynamic
aspect of electrons realizing from CdS conduction bond to the aro-
matic nitro compounds. In addition, using thiols as a capping agent
is a proper way for passive surface dangling bonds and retard-
ing recombination of photogenerated electron–hole pairs. It also
provides stability and surface functionality of CdS nanostructure
[43].
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