NJC

PAPER

A novel core–shell Pd(0)@enSiO2–Ni–TiO2

nanocomposite with a synergistic effect for
Cite this: New J. Chem., 2022,
46, 16959 efficient hydrogenations†
Vrinda Sharma, Surbhi Sharma, Nitika Sharma, Sukanya Sharma and Satya Paul *

Bimetallic core–shell nano-structured catalysis has sparked great interest due to the multiple
possibilities relating to morphology, atomic arrangement, and composition. In the present study, three
bimetallic core–shell-based nanocatalysts, viz. Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2, and
Pd(0)@Ni–enSiO2–TiO2, were synthesized via an economical and facile method using Pd(0) and amine-
functionalized silica–titania Ni(II) in the shell and core, respectively. The arrangement of Ni(II) was varied
in the three nanocatalysts and their catalytic activities were studied for hydrogenation reactions. The
best results were obtained in the case of Pd(0)@enSiO2–Ni–TiO2 among all the catalysts. The enhanced
catalytic activity of Pd(0)@enSiO2–Ni–TiO2 was demonstrated via surface characterization techniques.
Brunauer–Emmett–Teller (BET) analysis depicted the highest surface area in the case of Pd(0)@enSiO2–
Received 9th June 2022, Ni–TiO2; X-ray photoelectron spectroscopy (XPS) analysis supported the results, as an additional peak from
Accepted 11th August 2022 Ni(III) was observed in the spectrum of the reused catalyst, which suggested the presence of electronic inter-
DOI: 10.1039/d2nj02845j actions between Ni(II) in the core and Pd(0) in the shell. Moreover, Pd(0)@enSiO2–Ni–TiO2 was found to be
highly active for carrying out hydrogenation reactions under mild reaction conditions with good yields and
rsc.li/njc high selectivity, and it can be reused for up to five runs.

1. Introduction high selectivity, higher yield and easy catalyst recovery.9,10

However, due to the high surface energy of free metal nano-
Catalysis plays a key role in various chemical processes and it particles, they tend to agglomerate into large clusters, which
forms the core of incalculable synthetic transformations, from limits their usefulness. Hence, to make metal nanoparticles
research to industrial levels. By using various catalytic reagents thermodynamically stable and catalytically more expedient,11–13
one can elevate the reaction yield and selectivity, and can easily a new and alternative approach, i.e. core–shell nanostructures
organize the reaction conditions.1 Substantial developments have (CSNs), has been gaining interest among researchers. This is the
been made in the fields of heterogeneous and homogeneous class of nanostructures with one component in the core (known
catalysis in the twentieth century, wherein heterogeneous catalysis as the core) covered (either fully or partially) with supporting
aids in the easy removal of the catalysts at the end of the reaction material which forms a boundary around the core and is itself
and homogeneous catalysis achieves higher catalytic activity. known as the shell.14 Many bimetallic CSNs have been reported
Nano-catalysis is the epitome of such catalysis where both to date, wherein two dissimilar metals are immobilized together
branches link, interfere and gather together the most efficient to modify the individual behaviour of the metal nanoparticles,
properties of a catalyst.2–6 introducing short-range electronic effects, a better surface area
Past studies on different catalytic systems have aided scientists ratio and geometric effects on the nano-shell element.15–17
in establishing desirable nano-structures containing transition Various noble metals like Pt, Pd, Ag, and Au have been reported
metals with well-defined sizes and they are recognized as excellent to date to carry out catalytic hydrogenations. Among these,
catalysts.7,8 Miniaturization of a catalyst to the nanoscale leads to palladium has been widely explored due to its high selectivity
an increase in surface area which in turn increases the interaction and catalytic activity.18–20
between the catalyst and the reactant, allowing fast reactions with Currently, core–shell-structured nanocatalysts containing
mesoporous materials either in the core or on the shell have
been attracting considerable attention as they are economical,
Department of Chemistry, University of Jammu, Jammu, 180006, India.
E-mail: paul7@rediffmail.com; Fax: +91-191-2431365; Tel: +91-191-2453969
highly stable, active, selective and durable.21–24 Various meso-
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/ porous materials used up to now include tin oxide,21 silica,22
10.1039/d2nj02845j copper oxide,25 titania,26 nickel oxide,27 and alumina.28

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Paper NJC

Mesoporous materials not only favour the diffusion and ethanol (40 mL) were stirred in a round-bottom flask (100 mL)
adsorption of the substrate on the catalytic surface but also for 1 h at room temperature. A mixture of ethanol : water (5 : 3,
reduce sintering. Thus, these aspects of CSNs have motivated us 8.4 mL) was then added to the pre-sol and the reaction mixture
to synthesize three sustainable core–shell-based bimetallic was stirred for 4 h to form the gel. Powdered titanium dioxide
heterogenous catalysts comprising Pd nanoparticles immobi- (TiO2) was obtained by drying the gel at 373 K for 1 h followed
lized on an amine-functionalized silica–titania–nickel core, viz. by calcination at 673 K. For the preparation of the Ni–TiO2 core,
Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2 and Pd(0)@Ni– TiO2 (1 g) was dispersed in water (20 mL) and stirred for 1 h
enSiO2–TiO2. In these catalysts, the position of the nickel(II) followed by the addition of nickel acetate (0.225 g) and stirring
nanoparticles has been varied to compare their catalytic activity. was continued for another 4 h. White precipitates of Ni–TiO2
Firstly, we synthesized three core materials, viz. titania–nickel, were filtered and washed with deionized water (5  5 mL),
nickel–titania and silica–titania. In the next step, the core ethanol (5  5 mL) and acetone (5  5 mL) followed by drying at
materials were modified to increase their stability, wherein 60 1C for 6 h.
titania–nickel and nickel–titania cores were separately supported
on silica, whereas in the case of silica–titania, nickel(II) nano- Synthesis of the Ni–SiO2–TiO2 core
particles were directly immobilized on it. The three synthesized To a solution of TiO2 (1 g) in water (15 mL) in a round-bottom
support materials were further functionalized with ethylene flask (100 mL), tetraethyl orthosilicate (3 mL), aq. NH4OH
diamine (en), which provides better binding sites for the metal (3 mL) and ethanol (20 mL) were added and the reaction mixture
nanoparticles, thereby reducing the leaching phenomenon. was stirred at 80 1C for 8 h. SiO2–TiO2 composite was obtained as
Finally, the shell around the three functionalized support materials a grey precipitate which was filtered, washed with deionized
was developed by immobilization of palladium nanoparticles water (5  5 mL), ethanol (5  5 mL) and acetone (5  5 mL) and
to obtain Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2 and dried at 60 1C. To prepare Ni–SiO2–TiO2, 1 g of SiO2–TiO2 was
Pd(0)@Ni–enSiO2–TiO2. Then, the three synthesized catalysts added to an aqueous solution of nickel acetate (0.225 g in 30 mL
were tested for the hydrogenation of nitro-arenes, quinoline of water) and the reaction mixture was stirred at room tempera-
and a,b-unsaturated carbonyl compounds using molecular ture for 2 h. The obtained solid was filtered, washed with
hydrogen (balloon). Among the three catalysts, Pd(0)@- deionized water (5  5 mL), ethanol (5  5 mL) and acetone
enSiO2–Ni–TiO2 gave the best results in terms of catalytic activity, (5  5 mL) and then dried at 60 1C in an oven for 6 h.
selectivity and yield. Surface characterization techniques, i.e. BET
and XPS, provided sufficient further evidence for the enhanced Synthesis of SiO2–TiO2–Ni and SiO2–Ni–TiO2 composites
catalytic activity of Pd(0)@enSiO2–Ni–TiO2. The best catalyst was TiO2–Ni/Ni–TiO2 composite (1 g) was dispersed in a mixture of
fully characterized using thermogravimetric analysis (TGA), tetraethyl orthosilicate (3 mL), NH4OH (3 mL), water (15 mL) and
Fourier-transform infrared spectroscopy (FT-IR), field emission ethanol (20 mL) and then stirred for 8 h at 80 1C. Hydrolysis of
gun scanning electron microscopy (FEG-SEM), energy dispersive tetraethyl orthosilicate with NH4OH led to the formation of silica
X-ray spectroscopy (EDX), high resolution-transmission electron which was deposited on the TiO2–Ni/Ni–TiO2 core. Finally, the
microscopy (HR-TEM), X-ray diffraction (XRD) and inductively prepared composite was filtered, washed with deionized water
coupled plasma atomic emission spectroscopy (ICP-AES). (5  5 mL), ethanol (5  5 mL) and acetone (5  5 mL) and dried
under vacuum for 12 h.

2. Experimental Synthesis of amine-functionalized SiO2–TiO2–Ni, SiO2–Ni–TiO2,

and Ni–SiO2–TiO2
For the synthesis of core–shell nanostructured bimetallic hetero-
SiO2–TiO2–Ni or SiO2–Ni–TiO2 or Ni–SiO2–TiO2 (1 g), ethylene
geneous catalysts, three core materials were prepared which were
diamine (0.3 mL) and ethanol (15 mL) were placed in a round-
encapsulated with a Pd(0) shell to produce Pd(0)@enSiO2–TiO2–
bottom flask (100 mL), and the resulting mixture was stirred for
Ni, Pd(0)@enSiO2–Ni–TiO2 and Pd(0)@Ni–enSiO2–TiO2.
8 h at 80 1C. The solid obtained was filtered and washed with
deionized water (5  5 mL), ethanol (5  5 mL) and acetone
Synthesis of the TiO2–Ni core
(5  5 mL) and then dried under vacuum for 12 h to get enSiO2–
A solution of nickel acetate (0.225 g) in water (50 mL) was TiO2–Ni, enSiO2–Ni–TiO2 and Ni–enSiO2–TiO2 supports.
stirred for 4 h at room temperature. To this reaction mixture,
a solution of tertiary titanium butoxide (6.8 mL) in ethanol Preparation of palladium(0) encapsulated on amine-
(40 mL) was added and stirring was continued for another 6 h. functionalized SiO2–TiO2–Ni, SiO2–Ni–TiO2, and Ni–SiO2–TiO2
The TiO2–Ni composite was filtered, washed with deionized [Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2, and
water (5  5 mL), ethanol (5  5 mL), and acetone (5  5 mL) Pd(0)@Ni–enSiO2–TiO2]
and finally dried under vacuum. To a round-bottom flask (100 mL), enSiO2–TiO2–Ni or enSiO2–
Ni–TiO2 or Ni–enSiO2–TiO2 (1 g), ethanol (20 mL) and palla-
Synthesis of the Ni–TiO2 core dium acetate (0.06 g) were added and the reaction mixture was
A sol–gel method was employed for the preparation of TiO2. stirred for 2 h at room temperature. Then an aqueous solution
To obtain the pre-sol, tertiary titanium butoxide (6.8 mL) and of NaBH4 (0.5 g in 5 mL water) was added to the reaction

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mixture dropwise for 1 h and stirring was continued for another improve the overall stability of the catalysts. Further, to facil-
8 h. Finally, Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2 itate the binding of Pd(0) on the shell, amine functionalization
and Pd(0)@Ni–enSiO2–TiO2 obtained after filtration were of the core is done using ethylene diamine. Finally, all three
washed with deionized water (5  5 mL), ethanol (5  5 mL) core moieties are encapsulated with a Pd(0) shell. The method
and acetone (5  5 mL) and dried under vacuum for 2 h. for preparation of catalysts and the position of Ni(II) were varied
in the core and their catalytic activity was tested for the
General procedure for the hydrogenation of nitro-arenes, hydrogenation of nitro-arenes, quinoline and a,b-unsaturated
quinoline, and the CQ
QC bond in a,b-unsaturated carbonyl carbonyl compounds. The catalyst with the best results in terms
compounds using Pd(0)@enSiO2–Ni–TiO2 of catalytic activity, selectivity and yield, i.e. Pd(0)@enSiO2–Ni–
To a round-bottom flask (25 mL), Pd(0)@enSiO2–Ni–TiO2 (0.1 g), TiO2, was fully characterized using analytical techniques such
nitro-arene/quinoline/a,b-unsaturated carbonyl compound as BET, XPS, TGA, FT-IR, FEG-SEM, EDX, HR-TEM, XRD and
(1 mmol) and ethanol (5 mL) were added and the reaction ICP-AES.
mixture was stirred in the presence of molecular hydrogen BET analysis was performed for precise measurement of
(balloon) for an appropriate time at 80 1C. Reaction progress surface area as well as the porosity of the synthesized catalysts, viz.
was monitored using thin layered chromatography (TLC). After Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2 and Pd(0)@Ni–
completion of the reaction, ethanol was removed using a rotatory enSiO2–TiO2. Fig. 1 shows the N2 adsorption–desorption iso-
evaporator and the remaining residue was diluted with ethyl therms of the three synthesized catalysts. Also, their surface area,
acetate. The catalyst was filtered and washed with deionized total pore volume and mean pore diameter are presented in
water (5  5 mL), ethanol (5  5 mL) and acetone (5  5 mL) and Table 1. Among the three catalysts, Pd(0)@enSiO2–Ni–TiO2
dried in an oven at 60 1C. The organic layer was washed with showed the highest surface area, which accounted for the
water (3  20 mL) and dried over anhydrous Na2SO4. The solvent increased catalytic performance in hydrogenations. The resulting
was removed under reduced pressure and the product was plot for Pd(0)@enSiO2–TiO2–Ni showed type II isotherm, which
purified by crystallization from petroleum ether and ethyl acetate suggests the mesoporous nature of the catalyst.
in the case of nitro-arenes and a,b-unsaturated carbonyl com- XPS was undertaken to determine the elemental states of the
pounds, and via column chromatography in the case of quino- constituent metals present in the catalyst. The high resolution
line. The structures of the products were confirmed by 1H NMR XPS spectra of Pd 3d and Ni 2p are shown in Fig. 2a and b. The
and 13C NMR spectral data. XPS spectrum of Pd shows peaks at binding energies of 335.1 eV
and 340.4 eV for 3d5/2 and 3d3/2, which confirms the presence of
Pd(0) in the catalyst.29 The spectrum of Ni (Fig. 2b) also shows
3. Results and discussion two peaks at 855.8 eV and 873.2 eV corresponding to 2p3/2 and
2p1/2 of Ni(II). In addition, two satellite peaks were also observed
The synthesis of Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni– at 860.8 eV and 879.6 eV.30
TiO2 and Pd(0)@Ni–enSiO2–TiO2 is illustrated in Scheme 1. The XPS of the reused catalyst (after 5 catalytic runs) is
Firstly, three core materials, viz. TiO2–Ni, Ni–TiO2 and SiO2– represented in Fig. 2c and d. For Pd 3d, the peaks are observed
TiO2, were synthesized via a co-precipitation technique in the at binding energies of 334.5 eV and 339.7 eV, indicating a
case of TiO2–Ni and a sol–gel method for Ni–TiO2 and SiO2– negative shift of 0.7 eV (Fig. 2c). For Ni 2p, peaks for 2p3/2 and
TiO2. In the next step, silica was loaded onto TiO2–Ni and Ni– 2p1/2 at binding energies of 855.7 eV and 873.5 eV with their
TiO2, whereas nickel(II) nanoparticles were immobilized corresponding satellite peaks at 861 eV and 879.8 eV were
directly on SiO2–TiO2. Titania and silica in the core not only observed, indicating the presence of Ni(II) in the reused
help in preventing agglomeration and metal sintering, but also catalyst. However, a low-intensity peak for Ni(III) is also seen

Scheme 1 Synthesis of Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2, and Pd(0)@Ni–enSiO2–TiO2 nanocatalysts.

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were observed in the spectrum of SiO2–Ni–TiO2 (Fig. 4b).40 This

also confirms the loading of Ni onto the SiO2/TiO2 support.
Furthermore, the FT-IR spectra of enSiO2–Ni–TiO2 show a
characteristic band at 3545 cm 1 corresponding to the N–H
stretching vibration of ethylene diamine (Fig. 4c).41 In the FT-IR
spectrum of Pd(0)@enSiO2–Ni–TiO2 (Fig. 4d), the decrease in
the intensity of the bands confirms that the palladium has been
immobilized onto the enSiO2–Ni–TiO2 composite.
FEG-SEM was performed to determine the physical morphology
and micro-structure of the synthesized catalyst. The SEM images
indicate the uniform distribution of nanoparticles which are self-
assembled to form spherical nanostructures. The spherical mor-
phology of the nanoparticles prevents agglomeration, which results
in an increase in surface area as well as catalytic activity (Fig. 5).
EDX was performed for the qualitative analysis of the
elements present in Pd(0)@enSiO2–Ni–TiO2. The EDX spectrum
Fig. 1 N2 adsorption–desorption isotherms of: (a) Pd(0)@enSiO2–TiO2– confirms the presence of C, N, O, Si, Ni, Ti and Pd (Fig. 6). The
Ni; (b) Pd(0)@enSiO2–Ni–TiO2; and (c) Pd(0)@Ni–enSiO2–TiO2. absence of additional peaks in the spectrum suggests that the
catalyst is chemically pure.
HR-TEM was recorded to gain information about the morpho-
at a binding energy of 858.5 eV in the XPS spectrum of Ni
logical features and particle size of the catalyst. The average size of
(Fig. 2d).31,32 This negative shift in binding energies of Pd and
the nanoparticles, as calculated by plotting a histogram, came out
the presence of Ni3+ in the reused catalyst clearly indicate that
in the range of 13 to 14 nm (Fig. 7b). The SAED pattern (Fig. 7d)
there is a drift of electron density from nickel to palladium. The
shows concentric rings with dots, indicating the polycrystalline
synergistic effect shown by Pd and Ni in Pd(0)@enSiO2–Ni–TiO2
nature of the catalyst, which is in good agreement with literature
accounts for the enhanced catalytic activity of the catalyst.
studies of core–shell-type nanocatalysts.42 Many lattice fringes
TGA plots of Ni–TiO2, SiO2–Ni–TiO2, enSiO2–Ni–TiO2 and
(Fig. 7c) with distances of 0.234 nm, 0.243 nm and 0.203 nm
Pd(0)@enSiO2–Ni–TiO2 are shown in Fig. 3. Initially, a minimal
corresponding to the (111), (001), and (111) planes of palladium,
weight loss (2–3%) up to 100 1C was observed in all cases, which
titania and nickel, respectively, were also observed.43,44 Periodic
could be due to the removal of trapped solvents from the
lattice fringes observed with a spacing of 0.336 nm can be
surface of the catalyst.33 The TGA curves of Ni–TiO2 and
ascribed to the interference lattice fringes of Pd and Ni.30,42
SiO2–Ni–TiO2 show slight weight loss in the temperature range
XRD of Pd(0)@enSiO2–Ni–TiO2 was performed to study the
of 150–450 1C, which can be associated with the loss of water
crystalline nature of the catalyst. In the XRD spectra of the catalyst,
molecules produced by the condensation of terminal hydroxyl
peaks at 25.41, 37.91, 48.131, 551, and 62.41 correspond to the (101),
groups.34 For the enSiO2–Ni–TiO2 core, a decrease in weight of
(004), (200), (211) and (204) planes of titania,45 whereas the weak
14% up to 900 1C was observed, which could be due to the loss
diffraction peaks at 45.31, 681 and 821 correspond to the (111) plane
of the carbon moieties of ethylene diamine.35 However, from
of Ni and the (220) and (331) planes of Pd, respectively, in a face
the TGA curve of Pd(0)@enSiO2–Ni–TiO2, it was observed that
centred cubic crystal (fcc). The weak signals for Ni and Pd in the
the loading of metal nanoparticles lowered the overall thermal
XRD spectra suggest that the metal nanoparticles are well dispersed
stability of the catalyst. This could be attributed to the fact that
on the catalyst surface (Fig. 8).46,47
metal nanoparticles enhance the polymer degradation process
ICP-AES was performed to ascertain the precise elemental
by decreasing its activation energy.36,37
composition of Pd(0)@enSiO2–Ni–TiO2. The amounts of
FT-IR spectra of Ni–TiO2, SiO2–Ni–TiO2, enSiO2–Ni–TiO2 and
nickel and palladium loaded onto the catalyst were found to
Pd(0)@enSiO2–Ni–TiO2 are represented in Fig. 4. In the spec-
be 3.50% (w/w) and 1.80% (w/w), respectively.
trum of Ni–TiO2 (Fig. 4a), three prominent bands at 3490 cm 1,
1640 cm 1, and 1365 cm 1 were observed corresponding to O–
H stretching vibrations of titania, and bending modes of water Catalyst testing for the hydrogenation of nitro-arenes
and Ti–O, respectively.38,39 Bands at 3222 cm 1 and 973 cm 1 The catalytic performance of the three synthesized bimetallic
due to SiO–H and Si–O–Ni stretching vibrations, respectively, catalysts was studied for the hydrogenation of nitro-arenes. To find

Table 1 Surface area, total pore volume, and mean pore diameter data for the three synthesized catalysts

Entry Catalyst Surface area (m2 g 1) Total pore volume (cm3 g 1) Mean pore diameter (nm)
1 Pd(0)@enSiO2–TiO2–Ni 6.82 0.0610 35.95
2 Pd(0)@enSiO2–Ni–TiO2 10.4 0.1500 5.75
3 Pd(0)@Ni–enSiO2–TiO2 3.74 0.0045 4.82

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Fig. 2 XPS spectra of: (a) fresh Pd; (b) fresh Ni; (c) reused Pd (after 5 runs); and (d) reused Ni (after 5 runs) in Pd(0)@enSiO2–Ni–TiO2.

Fig. 3 TGA plots of: (a) Ni–TiO2; (b) SiO2–Ni–TiO2; (c) the enSiO2–Ni–
TiO2 core; and (d) Pd(0)@enSiO2–Ni–TiO2.

out the most efficient catalyst, hydrogenation of 4-nitroaniline

(1 mmol) in ethanol (5 mL) using molecular hydrogen (balloon)
at 80 1C was carried out in the presence of the three separate Fig. 4 FT-IR spectra of: (a) Ni–TiO2; (b) SiO2–Ni–TiO2; (c) the enSiO2–
Ni–TiO2 core; and (d) Pd(0)@enSiO2–Ni–TiO2.
synthesized catalysts. Among Pd(0)@enSiO2–TiO2–Ni, Pd(0)@-
enSiO2–Ni–TiO2 and Pd(0)@Ni–enSiO2–TiO2, Pd(0)@enSiO2–Ni–

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Fig. 5 SEM micrographs of Pd(0)@enSiO2–Ni–TiO2.

Fig. 7 (a–c) HR-TEM images of Pd(0)@enSiO2–Ni–TiO2 and (d) the SAED

pattern.

Fig. 6 The EDX spectrum of Pd(0)@enSiO2–Ni–TiO2.

TiO2 gave the best results in terms of time and yield (entries 2–4,
Table 2). The higher catalytic activity was due to the high surface
area of Pd(0)@enSiO2–Ni–TiO2 in comparison to the other two
catalysts. Moreover, the synergistic effect between Ni in the core
and Pd on the shell also contributed to the higher activity of the Fig. 8 The XRD spectrum of Pd(0)@enSiO2–Ni–TiO2.

catalyst. Thus, Pd(0)@enSiO2–Ni–TiO2 was selected as a competent

catalyst for carrying out the hydrogenation of nitro-arenes. in the yield of the product above 80 1C (entries 1–5, Table 3).
To further explore the scope and catalytic performance of Lastly, the effect of catalyst amount on the hydrogenation of
the developed catalyst, various reaction parameters, such as nitro-arenes was examined. Different amounts of catalyst, i.e.,
solvent, temperature and amount of catalyst, were optimized 0.05 g, 0.1 g and 0.2 g (entries 3, 5–6, Table 2), were used to
using the hydrogenation of 4-nitroaniline as a model reaction. carry out the hydrogenation of 4-nitroaniline and the results
Firstly, the effect of different solvents, viz. water, ethanol, indicated that 0.1 g of catalyst gave the maximum yield in the
toluene, acetonitrile, isopropanol and ethanol–water (1 : 1), minimum reaction time (entry 3, Table 2). Also, it was observed
was studied. Ethanol gave the best results in terms of selectiv- that there was no significant change in the reaction time or
ity, yield and reaction time among all the solvents (entry 4, yield of the product on increasing the amount of catalyst
Table 3). Afterwards, to analyse the effect of temperature on beyond 0.1 g. Hence, the optimal amount of catalyst for
hydrogenation, the test reaction was carried out at different carrying out the hydrogenation of nitro-arenes was set at 0.1 g.
temperatures, viz. room temperature, 40 1C, 60 1C, 80 1C and After optimizing the reaction conditions, the scope of the
100 1C. It was observed that the rate of reaction increases with developed catalyst was explored using a range of nitro-arenes
an increase in temperature and the optimal reaction tempera- bearing electron-donating and electron-withdrawing groups,
ture was found to be 80 1C as there was no significant increase and excellent results were obtained (Table 4). Desirable

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Table 2 Optimization of the catalyst amount for the hydrogenation of nitro-arenes and a,b-unsaturated carbonyl compounds and a comparison of the
catalytic activities of the three synthesized core–shell-based heterogeneous catalysts

Selective hydrogenation of CQC in a,b-unsaturated

Hydrogenation of nitro-arenesa carbonyl compoundsa
Entry Catalyst Amount (g) Time (h) Yieldb (%) Time (h) Yieldb (%)
1 No catalyst — 5.0 NR 5.0 NR
2 Pd(0)@enSiO2–TiO2–Ni 0.1 1.75 75 1.75 77
3 Pd(0)@enSiO2–Ni–TiO2 0.1 1.25 88 1.50 85
4 Pd(0)@Ni–enSiO2–TiO2 0.1 2.0 60 2.0 65
5 Pd(0)@enSiO2–Ni–TiO2 0.05 1.25 75 1.50 75
6 Pd(0)@enSiO2–Ni–TiO2 0.2 1.25 89 1.50 86
a
Reaction conditions: 4-nitroaniline/3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (1 mmol), ethanol (5 mL), molecular hydrogen (balloon), and
catalyst at 80 1C. b Isolated yield.

products were obtained for halogenated nitro-arenes and no indicated by the XPS data. Thus, Pd(0)@enSiO2–Ni–TiO2, was
evidence of dehalogenation was seen (entries 2c–2e, Table 4). selected to carry out the hydrogenation of the CQC double
Furthermore, the order of reactivity for o-, m- and p-nitroaniline bond in a,b-unsaturated carbonyl compounds.
was found to be p 4 o 4 m (entries 2f–2h, Table 4). A The reaction conditions were optimized by considering
satisfactory yield was obtained in the case of ortho- various parameters, such as solvent, temperature and amount
nitroaniline regardless of the steric hindrance offered by the of catalyst. For this, the hydrogenation of 3-(4-methoxyphenyl)-
amino group at the ortho position. The catalytic activity of 1-phenylprop-2-en-1-one was carried out under different reaction
Pd(0)@enSiO2–Ni–TiO2 was also tested for quinoline, and conditions and the best results in term of selectivity, yield and
1,2,3,4-tetrahydroquinoline was obtained in good yield (entry reaction time were given by ethanol at 80 1C (entry 4, Table 3)
4a, Table 4). with 0.1 g of the catalyst (entry 3, Table 2). It is relevant to
mention that no traces of any other unsaturated or saturated
Catalyst testing for the selective hydrogenation of the CQ
QC alcohols were observed in the product, which suggests that the
double bond in a,b-unsaturated carbonyl compounds catalyst is highly selective for reducing the CQC bond in a,b-
The proficiency of the synthesized catalysts for the hydrogena- unsaturated carbonyl compounds, keeping the carbonyl group
tion of nitro-arenes encouraged us to explore their catalytic intact. After selecting the appropriate conditions for hydrogena-
activity for the selective hydrogenation of the CQC double tion, i.e., ethanol as the solvent, 0.1 g of catalyst and 80 1C as the
bond in a,b-unsaturated carbonyl compounds. To evaluate optimal temperature, the substrate scope was studied (Table 5).
the catalytic activity of Pd(0)@enSiO2–TiO2–Ni, Pd(0)@enSiO2– Excellent yields were obtained for both electron-donating and
Ni–TiO2 and Pd(0)@Ni–enSiO2–TiO2, a test reaction was carried electron-withdrawing groups containing a,b-unsaturated carbo-
out using molecular hydrogen (balloon) as the source of nyl compounds.
hydrogen and 3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one as
the model substrate. Pd(0)@enSiO2–Ni–TiO2 again gave the Proposed mechanism for the hydrogenation of nitro-arenes
best results among the three synthesized catalysts (entry 3, A plausible mechanism for the reduction of nitro-arenes using
Table 2). This can be attributed to the presence of electronic Pd(0)@enSiO2–Ni–TiO2 is represented in Fig. 9. XPS analysis
synergism between the Pd on the surface and Ni in the core, as has shown that there is a drift of electron density from Ni in the

Table 3 Effects of different solvents and reaction temperatures on the hydrogenation of nitro-arenes and CQC in a,b-unsaturated carbonyl
compounds using Pd(0)@enSiO2–Ni–TiO2a

Hydrogenation of Selective hydrogenation of CQC in a,b-unsaturated

nitro-arenes carbonyl compounds
Entry Solvent Temperature (1C) Time (h) Yieldb (%) Time (h) Yieldb (%)
1 Ethanol RT 1.25 NR 1.50 NR
2 Ethanol 40 1.25 50 1.50 60
3 Ethanol 60 1.25 70 1.50 75
4 Ethanol 80 1.25 88 1.50 85
5 Ethanol 100 1.25 88 1.50 85
6 Water 80 1.5 80 1.75 80
7 Toluene 80 2 75 2 75
8 Isopropanol 80 2.15 70 1.75 65
9 Acetonitrile 80 1.5 85 2.15 80
10 Ethanol : water (1 : 1) 80 1.5 85 1.75 80
a
Reaction conditions: 4-nitroaniline/3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (1 mmol), catalyst (0.1 g), solvent (5 mL), and molecular
hydrogen (balloon) at different temperatures. b Isolated yield.

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Table 4 The Pd(0)@enSiO2–Ni–TiO2-catalysed selective hydrogenation Table 5 The Pd(0)@enSiO2–Ni–TiO2-catalysed selective reduction of the
of substituted nitro-arenes and quinoline in ethanol using molecular CQC bond in a,b-unsaturated carbonyl compounds using molecular
hydrogena hydrogen (balloon)a

a
Reaction conditions: a,b-unsaturated carbonyl compound (1 mmol),
molecular hydrogen (balloon), Pd(0)@enSiO2–Ni–TiO2 (0.1 g), and
ethanol (5 mL) at 80 1C. b Isolated yield.

a
Reaction conditions: nitro-arene/quinoline (1 mmol), molecular
hydrogen (balloon), Pd(0)@enSiO2–Ni–TiO2 (0.1 g), and ethanol (5 mL)
at 80 1C. b Isolated yield. c Column chromatography yield.

core to the Pd in the shell, which results in an electronically

rich environment around the Pd atoms and hence facilitates
the interaction of H2 with the catalyst, forming Pd–H bonds.
Subsequently, the catalyst reacts with the nitro compound
Fig. 9 Plausible mechanism for the hydrogenation of nitro-arenes using
followed by the transfer of a hydride ion from palladium to Pd(0)@enSiO2–Ni–TiO2.
the nitro group, which results in the elimination of a water
molecule and the formation of substituted nitrosobenzene (I).
catalytic activity. Usually, separation of the catalyst from the
The substituted nitrosobenzene (I) again reacts with the Pd–H
reaction mixture is a tedious task, but due to the heterogeneous
bonds present on the surface of the catalyst, leading to the
nature of Pd(0)@enSiO2–Ni–TiO2, it can be recovered by simple
formation of substituted phenylhydroxylamine (II). Finally, it
filtration. Furthermore, to study the reusability of the catalyst,
interacts with Pd–H, where an H atom is again transferred from
we carried out hydrogenation in the cases of entry 2h, Table 4
the metal to the N of hydroxylamine to give the final reduced
and entry 6a, Table 5. The catalyst was filtered, washed with
product (III) and regenerating the catalyst.
deionized water (5  5 mL), ethanol (5  5 mL) followed by
acetone (5  5 mL) and dried. The dried catalyst was reused for
Recycling of the catalyst the next cycle and the process was repeated for five consecutive
A crucial factor for the practical applicability of a heteroge- runs. The recyclability study suggests that the catalyst could be
neous catalyst is its recovery and recyclability without losing its reused for up to five catalytic cycles without loss of catalytic

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NJC Paper

Table 6 Recyclability of Pd(0)@enSiO2–Ni–TiO2 for the hydrogenation of nitro-arenes and a,b-unsaturated carbonyl compoundsa

Hydrogenation of nitro-arenes Selective hydrogenation of CQC in a,b-unsaturated carbonyl compounds

Catalytic run Catalyst amount (g) Yieldb (%) Catalyst amount (g) Yieldb (%)
1 0.1 88 0.1 85
2 0.094 88 0.096 85
3 0.090 85 0.092 83
4 0.087 80 0.086 80
5 0.084 80 0.082 78
a
Reaction conditions: 4-nitroaniline/3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one (1 mmol), molecular hydrogen (balloon), ethanol (5 mL), and
catalyst at 80 1C. b Isolated yield.

Table 7 Comparison of the catalytic activity of Pd(0)@enSiO2–Ni–TiO2 with some reported catalytic systems for the hydrogenation of nitroarenes and
a,b-unsaturated carbonyl compounds

Entry Catalyst used Reaction conditions Source of hydrogen Time (h) Yield (%) Ref.
1. Pd/Fe3O4-1 Nitrobenzene, THF, RT H2 balloon 3 97 48
2. Pd NPs Nitrobenzene, methanol, 50 1C H2 (8 bar) 3 96 49
3 PdCu/graphene 4-Nitroaniline, EtOH : H2O, 50 1C NaBH4 1.5 91 50
4 SNTs/Pd–Fe/NC Nitrobenzene, H2O : EtOH (2 : 3), RT NaBH4 0.58 99 20
5 Cu6/7Co1/7Fe2O4–G Nitrobenzene, H2O : EtOH (1 : 1), 70 1C NaBH4 0.41 95 51
6 ASNTs@Pd Nitrobenzene, H2O : EtOH (1 : 9), RT NaBH4 0.08 95.2 19
7 Pd/PSi–Al2O3 1,3-Diphenylprop-2-en-1-one, AcOEt, 50 1C H2 24 93 52
8 HAP-Pd0 1,3-Diphenylprop-2-en-1-one, CH3CN, 80 1C HCOONH4 7 87 53
9 Pd NPs 1,3-Diphenylprop-2-en-1-one, EtOH, RT H2 6 94 54
10 Pd(0)@enSiO2–Ni–TiO2 4-Nitroaniline, EtOH, 80 1C H2 balloon 1.25 88 This work
11 Pd(0)@enSiO2–Ni–TiO2 Nitrobenzene, EtOH, 80 1C H2 balloon 1.5 88 This work
12 Pd(0)@enSiO2–Ni–TiO2 1,3-Diphenylprop-2-en-1-one, EtOH, 80 1C H2 balloon 2 80 This work

activity or selectivity (Table 6). This could be attributed to the 4. Conclusions

strong binding of palladium nanoparticles with the NH2 group
of ethylene diamine, which prevents the agglomeration of the In conclusion, we have developed environmenally benign core–
catalyst and increases its stability. shell-based bimetallic heterogeneous nanocatalysts, Pd(0)@enSiO2–
To check the leaching of Pd(0) in the developed catalyst, a TiO2–Ni, Pd(0)@enSiO2–Ni–TiO2, and Pd(0)@Ni–enSiO2–TiO2,
hot filtration test was performed using the model reaction using a simple and economical procedure. The catalytic activ-
(entry 2h, Table 4) under the optimized reaction conditions ities of the catalysts were evaluated for the hydrogenation of
until the conversion reached 50% (0.62 h). After this, the nitro-arenes, quinoline, and the CQC bond in a,b-unsaturated
catalyst was removed and the reaction was allowed to run up carbonyl compounds using molecular hydrogen (balloon).
to 1.25 h. No further conversion of the reactant was observed Among the three synthesized catalysts, Pd(0)@enSiO2–Ni–TiO2
after the removal of the catalyst, which excludes the possibility exhibited high stability, selectivity, and catalytic activity. XPS
of leaching. Also, ICP-AES of the reused catalyst (after 5 runs) spectra of fresh and reused catalyst gave strong evidence of
was recorded and the results were found to be in good agree- electronic interactions between Ni (core) and Pd (shell). This
ment with the hot filtration test. There was a negligible electronic synergism between the metals is the principle
decrease in Pd content [3.50% (w/w, fresh) to 3.42% (w/w, after behind the enhanced catalytic activity of the synthesized cata-
5 runs)], which suggests the high stability of the catalyst under lysts. Comparative analysis of the present protocol with earlier
the reaction conditions. reported Pd-based catalytic systems for carrying out hydrogena-
tion reactions revealed a slightly lower yield of the corres-
Comparison of Pd(0)@enSiO2–Ni–TiO2 with other reported ponding hydrogenated products. However, a shorter reaction
catalytic systems time, mild reaction conditions, the use of a green solvent
(ethanol), and the use of molecular hydrogen as the reducing
In order to establish the uniqueness of the present protocol, a
agent instead of other reducing agents (NaBH4, formic acid,
comparison of the synthesized catalyst for hydrogenation of
etc.) makes this approach more effective for carrying out
nitroarenes and a,b-unsaturated carbonyl compounds with
hydrogenation reactions.
an earlier reported palladium-based catalyst is shown in
Table 7. The use of a greener source of hydrogen i.e., molecular
hydrogen, benign reaction conditions, shorter reaction time and Author contributions
recyclability (up to 5 runs) are the major advantages of
our catalytic system compared to earlier reported methods, where Vrinda Sharma: conceptualization; data curation; formal ana-
reducing agents such as NaBH4 and formic acid have been used. lysis; investigation; methodology; software. Surbhi Sharma:

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2022 New J. Chem., 2022, 46, 16959–16969 | 16967
Paper NJC

data curation; formal analysis; investigation; software; funding 16 J. R. Kitchin, J. K. Nrskov, M. A. Barteau and J. G. Chen,
acquisition. Nitika Sharma: formal analysis; investigation. Phys. Rev. Lett., 2004, 93, 156801.
Sukanya Sharma: formal analysis; investigation. Satya Paul: 17 M. Mavrikakis, B. Hammer and J. K. Norskov, Phys. Rev.
conceptualization; data curation; formal analysis; investigation; Lett., 1998, 81, 2819.
methodology; project administration; software; supervision; 18 D. Wang, J. Liu, J. Xi, J. Jiang and Z. W. Bai, Appl. Surf. Sci.,
validation; visualization. 2019, 489, 477–484.
19 J. Liu, J. Hao, C. Hu, B. He, J. Xi, J. Xiao, S. Wang and
Z. W. Bai, J. Phys. Chem. C, 2018, 122, 2696–2703.
Conflicts of interest 20 N. Zhang, Y. Qiu, H. Sun, J. Hao, J. Chen, J. Xi, J. Liu, B. He
and Z. W. Bai, ACS Appl. Nano Mater., 2021, 4, 5854–5863.
The authors declare no conflicts of interest. 21 K. Yu, Z. Wu, Q. Zhao, B. Li and Y. Xie, J. Phys. Chem. C,
2008, 112, 2244–2247.
22 F. Grasset, N. Labhsetwar, D. Li, D. C. Park, N. Saito,
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We thank the Head, SAIF, IIT Bombay for assistance with FEG- J. Portier and J. Etourneau, Langmuir, 2002, 18, 8209–8216.
SEM, HR-TEM, and ICP-AES analysis; Director, SAIF, Panjab 23 K. P. Velikov, A. Moroz and A. V. Blaaderen, Appl. Phys. Lett.,
University, Chandigarh for assistance with XRD and EDX 2002, 80, 49–51.
studies; and Head, SAIF, IIT Kanpur for assistance with XPS 24 H. Amouri, C. Desmarets and J. Moussa, Chem. Rev., 2012,
analysis and Department of Chemistry, University of Jammu for 112, 2015–2041.
assistance with TGA, FT-IR, and BET studies. Financial assis- 25 L. Kong, W. Chen, D. Ma, Y. Yang, S. Liu and S. Huang,
tance from the Department of Science & Technology under J. Mater. Chem., 2012, 22, 719–724.
PURSE programme; RUSA 2.0 and S. S. (University Research 26 K. S. Mayya, D. I. Gittins and F. Caruso, Chem. Mater., 2001,
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27 L. Wang, Z. Lu, Q. Cheng and L. Liu, Anal. Lett., 2015, 48,
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