Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of Nitroarenes

Article abstract of DOI:10.1007/s10562-020-03127-w

Abstract: In this study, we synthesized a robust and sustainable Pd/SiO₂ nanospheres catalyst. Further, its catalytic activity was demonstrated for the direct reductive coupling of nitroarenes under mild conditions. While the reaction with Pd nanoparticles on other supporting materials such as modified carbon materials and TiO₂, under similar conditions, resulted formation of amines exclusively. Therefore, it was confirmed that the SiO₂ was found to be the best supporting material towards the selective reductive coupling of nitroarenes. Also, the catalyst could be recycled up to five cycles with a marginal loss of product yield (< 2% yield). Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03127-w

Catalysis Letters
https://doi.org/10.1007/s10562-020-03127-w
Palladium Nanoparticles on Silica Nanospheres for Switchable
Reductive Coupling of Nitroarenes
Bhairi Lakshminarayana1 · Arun Kumar Manna1 · G. Satyanarayana1 · Ch. Subrahmanyam1
Received: 19 November 2019 / Accepted: 26 January 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
In this study, we synthesized a robust and sustainable Pd/SiO2 nanospheres catalyst. Further, its catalytic activity was dem-
onstrated for the direct reductive coupling of nitroarenes under mild conditions. While the reaction with Pd nanoparticles
on other supporting materials such as modifed carbon materials and TiO2, under similar conditions, resulted formation
of amines exclusively. Therefore, it was confrmed that the SiO2 was found to be the best supporting material towards the
selective reductive coupling of nitroarenes. Also, the catalyst could be recycled up to fve cycles with a marginal loss of
product yield (<2% yield).
Graphic Abstract
Keywords Heterogeneous catalysis · Hydrogenation of nitroarenes · Nanocatalysis
1 Introduction
Aromatic azo and azoxy compounds are vital and valuable
Electronic supplementary material The online version of this
ingredients for the synthesis of organic dyes, pigments,
article (https://doi.org/10.1007/s10562-020-03127-w) contains
food additives, indicators, optical storage media, therapeu-
tic agents and other drug derivatives [1–3]. Direct reductive
coupling of nitroarenes is a challenging task to produce azo
and azoxy aromatic compounds with controlled selectiv-
ity (without forming amines) at ambient temperature [3].
In general, industrially the azo and azoxy compounds are
synthesized by diazotization (i.e. the coupling of diazonium
supplementary material, which is available to authorized users.
* G. Satyanarayana
gvsatya@iith.ac.in
* Ch. Subrahmanyam
csubbu@iith.ac.in
1
Department of Chemistry, Indian Institute of Technology
Hyderabad, Kandi, Sangareddy, Telangana 502285, India
Vol.:(0123456789)
1 3

B. Lakshminarayana et al.
salts with electron-rich aromatic compounds) [2]. This pro-
cess is tedious and unfriendly with the environment, due to
the formation of unstable diazo compounds as waste and
harsh process condition is used in this conversion [2, 4]. In a
green approach, azo compounds can be acquired via aerobic
oxidation of amines by heterogeneous catalysis, however,
this process requires a high amount of supported noble metal
catalysts or expensive oxidants, which constrains broad-
scale applications [5, 6].
organic, inorganic moieties and metal nanoparticles is rela-
tively easy due to the presence of surface silanol groups
(polar functional moiety) [18]. Also, having a huge surface
area, which provides a large number of active sites. In addi-
tion, silica has good physicochemical properties and is inert
under the reaction progression. Moreover, silica has tunable
pore structures [19]. Therefore, based on the above reasons,
silica is considered as the best solid support for the organic
transformations.
Heterogeneous catalysis demonstrates incredible potential
in driving essential organic reactions (i.e. C–C couplings,
reduction and oxidation reactions, etc.) and the innovation
has along these lines pulled in signifcant consideration in
the most recent decades [7–14]. The recent development of
silica-supported metal nanoparticles has further boosted the
fundamental research and industrial potential of heterogene-
ous catalysis, as these inexpensive and eco-friendly materi-
als show the reasonable catalytic performance and stability,
under mild conditions [15, 16]. Silica has a wide range sur-
face area with broad applications in every feld especially
in catalysis. Also, silica-supported heterogeneous catalysts
are more benefcial over traditional homogenous catalysts as
they limit the ecological impact of organic transformations
to a great extent [17]. Silica supported metal nanoparticles
has some additional advantages over other heterogeneous
solid supports. Because, modifcation of silica surface with
Recently, a number of studies have been reported for the
formation of aniline, azoxy and azo compounds via het-
erogeneous catalysis using supported metal nanoparticles
shown in Table 1 [20–32]. The heterogeneous catalysts, such
as, Pd [20, 23], Pd–Pt [21], and Rh [22] nanoparticles on
various supporters such as PVP–iron powder, g-C3N4, Fe3O4
and SiO2 coated Fe3O4, proved to be efective in produc-
ing quantitative yields of anilines and without the formation
of self-coupled products [20–23]. Whereas, Au, Pt and Co
nanoparticles supported by CeO2, TiO2, ZrO2 and Mg–Al
hydrotalcite nano-structures, generated, exclusively the
reductive self-coupled product instead of aniline [24–32].
In this study, for the frst time, we describe Pd nanopar-
ticles supported SiO2 for direct reductive self-coupling of
nitroarenes.
Table 1 Literature for nitroarenes reduction using supported metal nanoparticles
NO2
NH2
O
N
R
R
N
Catalyst
N
N
+
R
R
+
R
R
Condition
1
2
3
4
S. nos
Catalyst (loading mol%)
Conditions
% of Yield
References
2
3
4
1
2
3
4
5
6
7
8
Pd/PVP–iron powder (1 mol%)
Pd–Pt/Fe3O4 (1 mol%)
Fe3O4@SiO2–NH2–Rh@mSiO2
Pd/g-C3N4
Au/Mg–Al hydrotalcite
Au/meso-CeO2 (0.5 mol%)
Au/CeO2 yolk shell nanostructure
Au/meso CeO2
NaBH4, H2O, rt, 45 min
99
96
99
99
1
1
7
–
–
–
–
–
–
–
–
1
–
2
94
99
99
–
–
–
–
–
–
–
98
99
91
1
–
–
98
97
89
90
94
–
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
Present work
NH3BH3, MeOH, rt, 5 min
N2H4·H2O, H2O, 100 °C
HCOOH, H2O, 298 K
2 MPa H2, toluene, 50 °C
5 atm CO, 150 °C, toluene, 5 h
KOH, IPA, 50 °C, under Ar atmosphere
KOH, IPA:H2O (5:1), 1 atm N2, 30 °C, 5 h
5 bar O2, toluene, 100 °C, 22 h
KOH, IPA, 40 °C, under Ar atmosphere, 5 h
4 bar H2, 120 °C
3 MPa H2, NaOH, t-BuOH, 80 °C, 1.5 h
1 bar H2, KOH, p-xylene, 80 °C
hv, NaOH, 2-propanol, 30 °C
9
Au/TiO2 (1 mol%)
Au/ZrO2
Au/CeO2
10
11
12
13
14
15
16
Co–N–C
–
–
–
5
Pt nanowires
Au/organic cage (OC1
NMC–Fe
R
)
–
N2H4·H2O, KOH, toluene, 100 °C
NaBH4, EtOH, rt, 1 h
Trace
92
Pd/SiO2 nano-spheres
5
1 3

Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of…
Fig. 1 Diagrammatic represen-
tation of Pd/SiO2 synthesis
2.2 Synthesis of Pd/SiO2 Nanospheres
Initially, 84 mg of PdCl2 was dispersed in 20 mL of EtOH
in a separate vessel and stirred for 30 min. In another vessel,
500 mg of SiO2 nanospheres were dispersed in 30 mL of
EtOH and added PdCl2 solution dropwise at room tempera-
ture and stirred on the magnetic stirrer. After 1 h, 0.5 mL of
N2H4·H2O was added to the stirred solution and then after
15 min the temperature increased from room temperature
to 70 °C and stirred 12 h. Finally, the formed material was
collected by fltration washing with ethanol and water. The
resultant Pd/SiO2 nanospheres were dried under vacuum
oven for overnight. Figure 1 represents the diagrammatic
representation of Pd/SiO2 nanospheres synthesis.
2.3 Characterization of the Catalyst
Fig. 2 PXRD pattern of Pd/SiO2 nanospheres
2.3.1 XRD Analysis
2 Results and Discussion
The phase, crystalline nature and structural confrmation of
as-prepared Pd/SiO2 nanospheres were attained by powder
X-ray difraction (PXRD) studies. The PXRD pattern of Pd/
SiO2 shown in Fig. 2. The Pd nanoparticles showed the cor-
responding d-spacing values are 2.244 Å, 1.943 Å, 1.374 Å,
1.173 Å and 1.121 Å. These d-spacing values can be attrib-
uted to (111), (200), (220), (311) and (222) lattice planes
of Pd (JCPDS No. 46-1043). The broad amorphous peak at
10°–35° indicates the presence of SiO2 [35]. From the XRD
data analysis, it was found that the Pd nanoparticles were
dispersed/impregnated on the surface of the SiO2.
2.1 Synthesis of SiO2 Nanospheres
In a typical synthesis of SiO2 nanospheres, initially,
200 mL of EtOH and 50 mL of water (4:1) were taken
in a 500 mL round bottom flask on a magnetic stirrer.
In addition, 4.5 mL of NH4OH was added dropwise and
immediately 12.9 mL of tetraethylorthosilicate (TEOS)
was added and stirred for 4 h at room temperature.
Finally, the SiO2 nanospheres were collected by centrifu-
gation and washed with ethanol and water. The formed
wet material was dried under a hot air oven at 60 °C.
1 3

B. Lakshminarayana et al.
Fig. 3 IR spectrum analysis of Pd/SiO2 nanospheres
2.3.2 FTIR Spectrum Analysis
intense band SiO2 nanospheres assigned at 1059.3 cm−1,
which indicates the antisymmetric stretching frequency of
Si–O–Si bond [39–41]. Whereas, symmetric stretching fre-
quency of Si–O–Si bond observed at 787.4 cm−1 [40]. A
weakly bonded silanols (Si–OH) on the SiO2 nanospheres
surface observed the vibrational band at 940.3 cm−1 [39].
When Pd nanoparticles were dispersed on the surface of
SiO2 nanospheres, the highest intense Si–O–Si and Si–OH
peaks shifted to lower frequency side i.e., 1050.8 cm−1 and
936.5 cm−1 respectively [39]. The shifting of Si–O–Si and
Si–OH bands indicates that Pd nanoparticles were strongly
adsorbed on the surface of the SiO2 nanospheres [39].
The FTIR spectra of SiO2 nanospheres and Pd on SiO2
nanospheres were scheduled in the range of 660–4000 cm−1
and the spectral data shown in Fig. 3. In SiO2 nanospheres,
the observable peaks were attributed at 3704.8 cm−1
,
1514.0 cm−1, 1059.3 cm−1, 940.3 cm−1 and 787.4 cm−1. The
band due to adsorbed water shows an asymmetric broadband
range from 3540 to 3660 cm−1. This band attribute with
the moisture water which exists everywhere around us [36].
Moisture on SiO2 nanospheres is mainly trapped as Si–OH
by breaking the bonds in the network of SiO2 nanospheres
[37, 38]. Therefore, the resultant stretching frequency of
–OH band in Si–OH observed a sharp peak at 3704.8 cm−1
and the bending mode of –OH band in Si–OH could be
seen at around 1630 cm−1 but which was hindered by many
stretching peaks of Si–O [38]. The broad peaks can be seen
between the ranges of 1400 and 1800 cm−1 which indicates
the stretching bands of Si–O [38]. The main and highest
2.3.3 XPS Analysis
To further analyze the oxidation state and bonding between
the atoms and quantifcations of the elements in the cata-
lyst were characterized by X-ray photoelectron spectros-
copy (XPS). The wide scan XPS spectra of Pd/SiO2 and the
1 3

Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of…
Fig. 4 XPS pattern of Pd/SiO2 nanospheres
elements Pd 3d, Si 2p, O 1s core-level spectra’s are shown in
Fig. 4. In Fig. 4, the characteristic peaks 3d3/2 and 3d5/2 were
showed at 335.4 eV and 342.4 eV and these peaks assigned
to Pd (0) and the new peak at 347.9 eV is a plasmon loss
associated with the peak 335.4 eV [42, 43]. Moreover, the
core level spectra of Si 2p showed the binding energies at
107.6 eV, 109.0 eV and 110.2 eV respectively. These peaks
accounted for the unique chemical state of Si atom (Si4+
from SiO2) [44]. The oxygen atoms surrounded by the Si
atom in SiO2 nanospheres which supplies a certain density
to the Si atom. Therefore, the local position of the Si 2p
shifted to higher binding energy side [44–46]. At the same
time, Si 2s core-level spectra also shifted to higher binding
energy side i.e., 158.8 eV and 160.3 eV. The O 1s core-level
spectrum showed the three components of binding energy
at 538.9 eV, 539.7 eV and 540.4 eV which indicates the
oxygen atoms surrounded by the Si atom [44]. Also, C 1s
1 3

B. Lakshminarayana et al.
Fig. 5 SEM images of Pd/SiO2 nanospheres
peak attributed at 289.3 eV and 291.6 eV which indicates the
carbon atom surrounded by oxygen atoms [44].
nitroarenes 1a (1 mmol) in the presence of various catalysts.
The results are summarized in Table 2. Thus, initially, the
reaction of nitrobenzene 1a was performed with hydrazine
monohydrate as a reductant in the presence of catalyst Pd/
GO and ethanol as solvent at room temperature for 12 h.
However, aniline was formed as the product, in 72% yield
instead of reductive coupled product 3/4 (Table 2, entry
1). Similar results were noted with Pd–Au/TiO2 catalyst
(Table 2, entry 2). Also, the reaction under neat reaction
conditions with hydrazine monohydrate and with various
catalysts (Pd/GO, Pd–Au/TiO2, Pd/TiO2, Au/TiO2, Pd/acti-
vated charcoal and Pd/SiO2 nanospheres), furnished the ani-
line 2a (Table 2, entries 3 to 8). Even when water was used
as the solvent, gave aniline 2a, as an exclusive product in the
presence of Pd–Au/TiO2 (Table 2, entry 9). The same results
were observed by using NaBH4 as the reducing agent and
water as a solvent for 12 h (Table 2, entry 10). Notably, with
the Pd/SiO2 nanospheres catalyst, NaBH4 as a reductant and
in water, direct reductive self-coupling of nitrobenzene to
the azoxybenzene 3a (84%) was noticed along with the for-
mation of aniline 2a as a minor product (Table 2, entry 11).
While the reaction in water and ethanol (1:1) mixture, with
the same catalyst and reductant, gave 78% yield of azoxy-
benzene 3a (Table 2, entry 12). Interestingly, when ethanol
was used as the sole solvent, isolated 3a in 92% (Table 2,
entry 13). On the other hand, at 80 °C, the yield of 3a was
decreased to 75% and without appreciable formation of ani-
line 2a (Table 2, entry 14). No progress was noted in the
2.3.4 SEM Analysis
The FE-SEM images of Pd/SiO2 nanospheres were shown
in Fig. 5. From Pd/SiO2 nanospheres SEM images, we dem-
onstrate that the morphology of SiO2 is a smooth spherical
in shape. Moreover, Fig. 5 reveals that the Pd nanoparticles
are distributed on the surface of SiO2 nanospheres. Also,
some of the Pd nanoparticles are agglomerated. From SEM
images of Pd/SiO2 nanospheres, the average of spherical size
in SiO2 is 410.5 nm. Whereas, the Pd nanoparticles average
particles is approximately 8.3 nm.
2.3.5 TEM Analysis
To further confrmation of morphology and particle size of
SiO2 and palladium, we investigated with transmission elec-
tron microscopy (TEM) shown in Fig. 6. In this TEM analy-
sis, we observed the morphology of SiO2 is a soft spherical
in shape and the average particle size of each spherical is
410 nm. Whereas, the Pd nanoparticles size is 7.5 nm. These
TEM results support with the SEM results.
2.4 Catalytic Performance
To fnd out the optimal reaction conditions, the reactions
were screened for the direct reductive self-coupling of
1 3

Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of…
Fig. 6 TEM images and particle size distribution of Pd/SiO2 nanospheres
absence of palladium catalyst and with SiO2 nanospheres,
even for prolonged reaction time (Table 2, entry 15).
the number of equivalents of NaBH4 (1 to 5 equiv), the
formation of by-product aniline 2a was decreased and the
yield of azoxybenzene 3a was increased. When 1, 2, 3.5
and 5 equivalent of NaBH4 was employed along with Pd/
SiO2 nanospheres and ethanol, the % of yields ratio of ani-
line 2a and azoxybenzene 3a are 33:35, 25:48, 20:56 and
5:92, respectively. From these results, it was concluded that
2.5 Efect of NaBH4 Concentration
In addition, we optimization study was screened with regards
to the equivalents of NaBH4. From Fig. 7, when increasing
1 3

B. Lakshminarayana et al.
Table 2 Optimization for the switchable hydrogenative coupling of nitrobenzene
NO2
NH2
O
N
Catalyst
N
N
N
+
+
reductant, solvent,
room temperature, 1-12h
1a
2a
3a
4a
Entry
Catalyst
Reductant
Solvent
Yield (%)
2a
3a
4a
1
2
3
4
5
6
7
8
PdO/GO
Pd–Au/TiO2
PdO/GO
Pd–Au/TiO2
Pd/TiO2
Au/TiO2
Pd/activated charcoal
Pd/SiO2 nanospheres
Pd–Au/TiO2
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
N2H4·H2O (0.5 mL)
NaBH4 (5 equiv)
NaBH4 (5 equiv)
EtOH
EtOH
Neat
Neat
Neat
Neat
Neat
Neat
Water
Water
Water
Water:EtOH (1:1)
EtOH
EtOH
72
85
75
96
82
80
46
35
84
76
15
10
5
–
–
–
–
–
–
–
–
–
–
84
78
92
75
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
9
10
11
12
13
14a
15b
Pd–Au/TiO2
Pd/SiO2 nanospheres
Pd/SiO2 nanospheres
Pd/SiO2 nanospheres
Pd/SiO2 nanospheres
SiO2 nanospheres
NaBH4 (5 equiv)
NaBH4 (5 equiv)
NaBH4 (5 equiv)
Trace
–
NaBH4 (5 equiv)
EtOH
Bold highlighted in entry 13 indicates the highly active reaction condition (standard reaction condition)
Reaction conditions: nitrobenzene 1a (1 mmol), reductant (5 mmol), catalyst (1 mol%), solvent (1 mL) at rt. Isolated yields of products 2a and
3a
aReaction conducted at 80 °C
bThe reaction carried out up to 24 h
5 equiv of NaBH4 is the optimized amount for the direct
hydrogenative coupling of nitroarenes to form azoxyben-
zene 3a.
With these optimized conditions in hand (Table 2, entry
13), we amplifed the scope of the reaction and explored
with various substituted nitroarenes 1a–q, for the formation
of hydrogenative coupled azoxybenzene products. The reac-
tion was quite smooth and furnished the azoxybenzenes 3.
The process showed broad functional group tolerance, for
example, the reaction was amenable to nitrobenzenes hav-
ing functional groups (H, Me, Br, Cl, F, NH2, CH2OH) and
aforded the corresponding azoxybenzenes [3a (92%), 3b
(73%), 3c (94%), 3j (20%) and 3m (83%)] (Table 3). Par-
ticularly, the reaction was found to show excellent selectiv-
ity and tolerance with Br, Cl and F functionalities present
the benzene ring of 1 [3d (65%), 3e (86%), 3f (88%), 3g
(92%), 3h (48%) and 3i (52%)] (Table 3). The low yield in
case of 3j may be attributed to slow reaction rate due to the
presence of a strong electron-donating amino group of 1m.
While nitrobenzaldehydes 1k–m, furnished azoxybenzenes/
Fig. 7 Number of equivalents of NaBH4 vs the % of products 2a and
3a yields. Reaction conditions: nitrobenzene 1a (1 mmol), NaBH4,
Pd/SiO2 nanosphere (1 mol%), EtOH (1 mL) at rt. Isolated yields of
products 2a and 3a
1 3

Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of…
NO2
O
N
Table 3 Pd/SiO2 nanospheres
R
R
Pd/SiO2 nanospheres
N
N
N
catalyzed direct reductive
coupling of nitroarenes
+
R
R
R
NaBH4, EtOH, rt, 1-3 h
1
3
4
Entry
1
3
4
NO2
O
1
N
-----
4a
N
(0%)
1a
3a
(92%)
NO2
O
N
2
Me
-----
4b
N
Me
(0%)
Me
1b
3b
(73%)
NO2
Me
3
O
N
N
-----
4c
Me
Me
1c
(0%)
3c
(94%)
NO2
F
O
N
4
F
N
F
-----
4d
1d
NO2
(0%)
3d
(65%)
Cl
5
O
N
N
-----
Cl
Br
Cl
1e
4e (0%)
3e
(86%)
NO2
O
N
6
-----
N
Br
Br
4f
(0%)
(0%)
Br
1f
NO2
3f
(88%)
7
O
N
-----
N
4g
Br
Br
1g
3g
(92%)
Br
Br
NO2
8
O
N
Br
Br
N
-----
Br
4h
(0%)
Br
3h
(48%)
1h
NO2
OH
9
Br
O
N
-----
CHO
HO
N
4i
(0%)
1i
Br
Br
3i
(52%)
NO2
O
N
10
11
H2N
N
NH2
-----
NH2
4j
(0%)
3j
(20%)
O
1j
NO2
HO
HO
CHO
N
N
N
N
OH
OH
1k
NO2
4k
(28%)
3k
(55%)
-----
12
13
N
OH
OH
HO
HO
N
CHO
3l
(0%)
4l
(97%)
1l
NO2
O
N
OH
N
N
N
HO
CHO
1m
4m (37%)
3m
(59%)
azobenzenes and with relatively higher reaction rates. In
addition, the aldehyde group also was reduced, which is
usual in the presence of NaBH4. This may be due to the
electron-withdrawing ability of aldehyde moiety. The pro-
tocol was also successful with 4-acylnitrobenzene 1o, exclu-
sively aford secondary alcohol azobenzene 3o, in 90% yield.
1 3

B. Lakshminarayana et al.
Table 3 (continued)
Entry
1
3
4
OH
NO2
14
O
N
-----
N
4n
(0%)
OH
OH
3m
(83%)
1n
OH
Me
NO2
15
-----
3o
N
(0%)
N
Me
HO
O
Me
OH
4o
(90%)
1o
NO2
OH
16
17
N
N
-----
COCl
1p
3p
(0%)
4m
(70%)
O
O
NO2
O
O
O
N
N
N
N
O
O
O
O
1q
O
O
3q
4q
(43%)
(56%)
Reaction conditions: nitroarenes 1a–q (1 mmol), NaBH4, Pd/SiO2 nanospheres (1 mol%), EtOH
(1 mL) at rt. Isolated yields of products 3a–q/4q
On similar grounds, para-nitrobenzoyl chloride 1p generated
exclusively 4m. The substrate nitrocinnamic acid ester 1q
was also found amenable and furnished a mixture of azoxy-/
azo-cinnamic acid esters 3q/4q.
NO2
O
N
Pd/SiO2 nanospheres
NaBH4, EtOH, rt, 3 h
Br
N
Br
Br
1f
3f
2.6 Recyclability Test
To check the sustainability of catalyst retains its activity,
recyclability was also tested. Catalyst recovered by using
centrifugation, ethanol washing, treatment with water and
acetone and drying (vacuum oven at 60C for overnight)
steps. Thus, the recovered Pd/SiO2 nanospheres catalyst was
then subjected to the next catalytic cycles. The recovered
catalyst was found to be active without an appreciable reduc-
tion in the product 3f yield, under the established conditions.
Thus, based on the above results it was confrmed that the
Pd/SiO2 nanospheres catalyst is having enough stability and
can be reused (Fig. 8).
2.7 Mechanism
Fig. 8 Recyclability test of the catalyst Pd/SiO2 nanospheres. Reac-
tion conditions: 3-bromo nitrobenzene 1f (1 mmol), NaBH4, Pd/SiO2
nanospheres (1 mol%), EtOH (1 mL) at rt. Isolated yields of products
3f
The plausible reaction pathway for the conversion of
nitrobenzene 1a into azoxybenzene 3a and azobenzene
4a is shown in Fig. 9. Initially, the adsorbed nitrobenzene
1a on the metal surface would undergo partial reduction
1 3

Palladium Nanoparticles on Silica Nanospheres for Switchable Reductive Coupling of…
NH2
NO2
O
N
Pd/SiO2 nanospheres
EtOH
N
N
N
NaBH4
+
+
+
1a
2a
3a
4a
NO2
NO
Reduction
A
1a
O
N
N
Step I
N
N
Reduction
3a
4a
NHOH
B
Reduction
Step II
NH2
Step III
Reduction
2a
Fig. 9 The reaction pathway for the reductive coupling of nitrobenzene
via a two-hydride ion transfer and furnish nitrosobenzene
A and N-phenyl hydroxylamine B. The nitrosobenzene A
and N-phenyl hydroxylamine B would combine and aford
azoxybenzene 3a (step I, Fig. 9). Reduction of azoxybenzene
3a gives azobenzene 4a (step II, Fig. 9). Further, azobenzene
4a would reduce to produce aniline 2a (step III, Fig. 9).
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