Highly dispersed Au, Ag and Cu nanoparticles in mesoporous SBA-15 for highly selective catalytic reduction of nitroaromatics

Article abstract of DOI:10.1039/c4ra10050f

This paper demonstrates a homogeneous dispersion of 4 wt% coinage metal nanoparticles (Au, Ag and Cu) of different morphologies in the pores (～8 nm) of 3-aminopropyltriethoxysilane (APTES) modified mesoporous SBA-15 for selective catalytic reduction of m-dinitrobenzene to phenylenediamine. EDX, elemental mapping and HR-TEM analysis confirmed the uniform dispersal of metal nanoparticles within the mesoporous matrix having lattice fringes with a d-spacing of 0.232 nm for Au (111), 0.23 nm and 0.20 nm for Ag (111) and (200) and 0.25 nm for CuO (111) planes. XPS results illustrated the presence of Au and Ag in their metallic states whereas Cu was oxidized to CuO. XRD, TEM and surface area analysis revealed that formation of metal nanoparticles within the sieves led to a significant change in the surface structural and physicochemical properties. Metal nanospheres with increasing size i.e., ～5 nm (Au) < ～11 nm (Ag) < ～13 nm (Cu) were formed within the channels of SBA-15, while small nanorods (aspect ratio ～2-4 nm) were also formed in the case of Ag and Cu impregnation. The catalytic activity was found to depend on the nature, size and dispersion of metal nanoparticles relative to negligible reactivity of bare SBA-15. Au nanosphere (～5 nm) impregnated SBA-15, having the lowest surface area (292 m² g^-1), exhibited the best catalytic activity for m-dinitrobenzene reduction (k = 1.765 × 10^-1 min^-1) with 89% selectivity to m-phenylenediamine.

Full text of DOI:10.1039/c4ra10050f

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Highly dispersed Au, Ag and Cu nanoparticles in
mesoporous SBA-15 for highly selective catalytic
reduction of nitroaromatics†
Cite this: RSC Adv., 2015, 5, 184
a
b
a
a
*
Shweta Sareen, Vishal Mutreja, Satnam Singh and Bonamali Pal
This paper demonstrates a homogeneous dispersion of 4 wt% coinage metal nanoparticles (Au, Ag and Cu)
of different morphologies in the pores (ꢀ8 nm) of 3-aminopropyltriethoxysilane (APTES) modified
mesoporous SBA-15 for selective catalytic reduction of m-dinitrobenzene to phenylenediamine. EDX,
elemental mapping and HR-TEM analysis confirmed the uniform dispersal of metal nanoparticles within
the mesoporous matrix having lattice fringes with a d-spacing of 0.232 nm for Au (111), 0.23 nm and
0.20 nm for Ag (111) and (200) and 0.25 nm for CuO (111) planes. XPS results illustrated the presence of
Au and Ag in their metallic states whereas Cu was oxidized to CuO. XRD, TEM and surface area analysis
revealed that formation of metal nanoparticles within the sieves led to a significant change in the surface
structural and physicochemical properties. Metal nanospheres with increasing size i.e., ꢀ5 nm (Au) < ꢀ11
nm (Ag) < ꢀ13 nm (Cu) were formed within the channels of SBA-15, while small nanorods (aspect ratio
ꢀ2–4 nm) were also formed in the case of Ag and Cu impregnation. The catalytic activity was found to
depend on the nature, size and dispersion of metal nanoparticles relative to negligible reactivity of bare
Received 9th September 2014
Accepted 14th November 2014
SBA-15. Au nanosphere (ꢀ5 nm) impregnated SBA-15, having the lowest surface area (292 m2
g
ꢁ1),
DOI: 10.1039/c4ra10050f
exhibited the best catalytic activity for m-dinitrobenzene reduction (k ¼ 1.765 ꢂ 10ꢁ1 minꢁ1) with 89%
www.rsc.org/advances
selectivity to m-phenylenediamine.
catalytic applications.7 A review by Lofgreen et al.8 also
summarized the synthetic procedure for formation of imprinted
1. Introduction
silica materials based on the concept of molecular imprinting
involving the assembly of cross linked polymer matrix around
an imprint molecule bonded through covalent/noncovalent
interactions by judiciously chosen functional monomers.
Later on, removal of imprint molecule generate cavity of specic
shape and size. This ease with which the morphology of silica
based materials can be controlled earns them a signicant
advantage over other organic polymer systems that uses toxic
organic solvents. So, silica is most frequently used inorganic
materials. A major amount of research has been dedicated to
the synthesis of advanced nanostructured silica materials like
mesoporous silica nanotubes based on sol–gel process and
silane chemistry.9,10
Among different mesoporous silica materials, SBA-15
exhibited excellent properties to be used for catalytic support
viz., large surface areas, tunable pore diameter, thick framework
walls and high hydrothermal stability.11 Despite the presence of
all these promising properties, bare SBA-15 does not exhibit any
catalytic activity. Thus, incorporation of metal NPs in SBA-15 is
desirable to improve its catalytic efficiency. However, func-
tionalization of mesoporous surface by organic functional
groups like –SH,12 –COOH13 is required for loading of metal to
improve interaction between metal complexes and silica surface
leading to high dispersion of metal NPs within pores. Coinage
Nanosized coinage metal particles have been the centre of
attention in recent years owing to their exceptional physi-
ochemical properties like quantum connement and surface
effects1 which makes them ideal for use in catalysis2 and optical
devices.3 Progressive developments in tuning the shape and size
of nanoparticles (NPs) have increased the chances of optimizing
their geometry which ultimately affects their catalytic efficiency.
Due to presence of highly active centers,4 NPs are thermody-
namically unstable so they are immobilized/supported over
inorganic5 and organic frameworks. Recently, Li et al.6 reported
the formation of Pd NPs over urea based porous organic
frameworks exhibiting high catalytic activity for selective
reduction of nitroarenes and Suzuki–Miyaura cross coupling
reactions. Among the inorganic frameworks, silica based mes-
oporous materials (MMs) display many attractive physico-
chemical properties such as high porosity, large specic surface
area, high mechanical stability and easy surface functionaliza-
tion, etc. which enable them to be used as ideal scaffolds for
aSchool of Chemistry and Biochemistry, Thapar University, Patiala 147004, Punjab,
India. E-mail: bpal@thapar.edu; Fax: +91-175-2364498; Tel: +91-175-2393491
bMaharishi Markandeshwar University, Mullana, Ambala 133207, Haryana, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10050f
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metals like Au,14,15 Ag16,17 and Cu18 have been impregnated on analyze the products. Helium was used as a carrier gas with a
SBA-15 surface by different methods but comparison of their ow rate of 1 ml minꢁ1. Injector was maintained at 240 ꢃC. Oven
morphological features, physicochemical properties and cata- was programmed at 60 ꢃC to 290 ꢃC @ 6 ꢃC minꢁ1 rise of
lytic activity has seldom been reported. Therefore, current temperature.
research deals with the preparation of highly dispersed coinage
metal NPs within channels of SBA-15 for studying their
comparative surface structural morphology and catalytic activity
for dinitrobenzene reduction.
2.3. Catalyst characterization
The prepared catalysts were characterized by powder XRD, XPS,
EDX, elemental mapping, transmission electron microscope
(TEM), BET and Solid state UV-vis absorbance spectra. Powder
X-ray diffraction (XRD) were carried on PANalytical X'pert Pro
2. Materials and methods
˚
diffractometer using Cu Ka radiation (l ¼ 1.54060 A) over the 2q
Hydrogen tetrachloroaurate (HAuCl4$3H2O, 99.99%), silver
nitrate (AgNO3, 99.0%), copper(II) chloride, Pluronic (triblock
copolymer EO20.PO70.EO20), tetraethoxysilane (TEOS), m-dini-
trobenzene (DNB), m-phenylenediamine (PDA), m-nitroaniline
range of 0.5–5ꢃ and 10–80ꢃ. Transmission electron micrographs
were recorded on Hitachi (H-7500) operating at 120 kW. HR-
TEM, EDX and elemental mapping were performed using FEI
Tecnai F20 transmission electron microscope operating at an
accelerating potential of 200 kV. X-ray photoelectron spectros-
copy (XPS) signals were collected on KRATOS-AXIS DLD spec-
trometer (Kratos Analytical, U.K.) using monochromatic Al Ka
radiation at 1486.6 eV operated at 10 kV. Binding energies were
calibrated with C1s binding energy of standard hydrocarbons
(284.6 eV). Solid state UV-visible absorption spectra were
recorded in the range of 400–800 nm using Analytikjena Spe-
cord 205 spectrophotometer. Surface area, pore volume and
pore size distribution were determined using BET surface area
analyzer (BEL Sorp-max) by pretreating 20 mg of all samples for
(NA),
sodium
borohydride
(NaBH4)
and
3-amino-
propyltriethoxysilane (APTES) were obtained from Sigma
Aldrich. All other reagents were obtained from Loba Chemie,
India and used as received without further purication. De-
ionized water was used throughout the experiments which
was obtained using an ultra ltration system (Milli-Q, Millipore)
with measured conductivity above 35 mho cmꢁ1 at 25 C.
ꢃ
2.1. Preparation of coinage metal incorporated ap-SBA-15
Mesoporous SBA-15 was prepared by method reported by Zhao
et al.19 For the synthesis of SBA-15, 4ꢃg of Pluronic 123 was
suspended in 120 ml of 2 M HCl at 40 C under stirring. Upon
dissolution of the polymer a clear solution was obtained, 0.0ꢃ41
mol of TEOS was added and the contents were stirred at 40 C
for 24 h. The solution thus obtained was transferred into Teon
autoclave and kept at 95 ꢃC for 3 days. It was ltered, dried and
calcined at 550 ꢃC for 8 h resulting in SBA-15. For surface
functionalistaion of SBA-15, 2 g of prepared SBA-15 was sus-
pended in 100 ml of 2 wt% of ethanolic solution of APTES under
stirring for 3 h. It was then, ltered, washed with ethanol, dried
ꢃ
2 h at 200 C under vacuum.
3. Results and discussion
3.1. Optical properties
Fig. 1 revealed an absorption band at 518 nm for Au/ap-SBA-15
conrming the presence of Au in the samples20,21 while no
absorption band for bare SBA-15 was observed. Presence of a
broad but weak band at 480 nm for Ag/ap-SBA-15 was found to
be consistent with surface plasmon resonance band of large
sized AgNPs.22,23 A broad band centered at 756 nm for Cu/ap-
SBA-15 attributed to d–d transitions of Cu2+ ions in a pseudo-
octahedral ligand oxygen environment indicating the presence
of CuO particles.24 A change in color of samples was observed
ꢃ
at 60 C and was designated as ap-SBA-15.
Metal deposition was carried out by stirring 0.5 g of ap-SBA-
15 with 101.5 ml, 185.2 ml and 314.9 ml of 1 mM HAuCl4$3H2O,
AgNO3 and CuCl2$2H2O solutions respectively for 2 h. The
contents were ltered, rinsed three times with deionized water
and dried at 60 ꢃC. The resulting solid was calcined at 350 ꢃC for
3 h. The corresponding catalysts were named as M/ap-SBA-15
(where M is Au, Ag or Cu). The outline for the preparation of
noble metal (Au, Ag and Cu) incorporated SBA-15 catalysts is
shown in ESI-Scheme 1.†
2.2. Catalytic activity
Catalytic activity of the prepared catalysts was investigated
using reduction of m-dinitrobenzene (DNB) by mixing catalyst
(5 mg), substrate (5 mM) in ethanol and NaBH4 (0.5 M) in a
10 ml round bottom ask with constant stirring at 298 K. The
progress of the reaction was monitored by HPLC (Agilent, 1120
compact LC using C-18 column) at 254 nm using MeOH : H2O
(70 : 30) as mobile phase with ow rate of 1 ml minꢁ1. GC-MS
using Shimadzu, GC-2010 and GC-MS-QP 2010 plus with RTX-
Fig. 1 Solid state absorption spectra of (a) bare SBA-15, (b) Au/ap-
5Sil-MS column (30 mm ꢂ 0.25 mm ꢂ 0.25) was used to SBA-15, (c) Ag/ap-SBA-15 and (d) Cu/ap-SBA-15 catalysts.
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with metal loading from white colored bare SBA-15 to deep red, amorphous walls of SBA-15. In addition to the broad band, four
dark gray and light green for gold, silver and copper respec- distinct diffractions at 38ꢃ, 44.3ꢃ, 64.3ꢃ, and 77.3ꢃ correspond-
tively, indicating the inclusion of metallic species in the mes- ing to (111), (200), (220) and (311) planes of nanocrystalline Au
oporous support.
[JCPDS: 04-0784] were observed. In Ag/ap-SBA-15 four distinct
diffractions at 38.3ꢃ, 44.3ꢃ, 64.6ꢃ, 77.5ꢃ for (111), (200), (220) and
(311) lattice planes conrmed the presence of Ag NPs [JCPDS:
04-0783]. However, in case of Cu/ap-SBA-15 sharp diffraction
peaks were observed at 35.6ꢃ, 38.8ꢃ, 48.5ꢃ, 61.5ꢃ, 66ꢃ that could
be assigned to CuO indicating the presence of comparatively
large metallic crystallite size in the catalyst [JCPDS-48-1548].
TEM images of M/ap-SBA-15 catalysts (Fig. 3b–d) displayed
well dened hexagonal symmetry of mesoporous channels with
pore diameter ꢀ8 nm indicating the preservation of hexagonal
structure of SBA-15 (Fig. 3a). Nanospheres (NS) were observed
with Au loading where as loading of Ag and Cu resulted in the
formation of small nanorod (NR) of aspect ratio of 2–4 nm along
with NS depicted as dark structures homogeneously dispersed
against light background of SBA-15 matrix. The average particle
size of NS for Cu, Ag and Au was found to be 13, 11 and 5.5 nm
respectively (ESI-Fig. 1†). Moreover, STEM images and corre-
sponding elemental mapping of the M/ap-SBA-15 catalysts
(Fig. 4a–f) demonstrated the homogeneous dispersion of
metallic NPs over the mesoporous support. HR-TEM micro-
graphs and the corresponding EDX spectra of various catalysts
were depicted in Fig. 5a–c. The HR-TEM images revealed that
3.2. Surface structural morphology
Low angle powder XRD patterns of prepared materials (Fig. 2a)
displayed characteristic diffraction peaks at 0.8, 1.5 and 1.8ꢃ
indexed to 100, 110 and 200 planes corresponding to 2D
hexagonal symmetry19 of SBA-15. This signied the retention of
long range ordering of mesochannels even aer functionaliza-
tion with APTMS and metal loading. However, observed
decrease in the intensity of peaks for 110 and 200 planes was
due to graing of metal precursor on the surface of function-
alized SBA-15 partially lling the pores. Takai et al.16 and Zhang
et al.25 also observed decrease in diffraction intensities due to
incorporation of Pt and Ag ions within SBA-15. Structural
parameters viz. unit cell parameter (a0), pore wall thickness and
d-spacing (Table 1, ESI-Fig. 3†) were found to increase for M/ap-
SBA-15 indicating that metal has been incorporated in the
sieves. For Ag/ap-SBA-15 and Cu/ap-SBA-15, peak corresponding
to 100 plane is shied to lower angle from 0.8ꢃ to 0.7ꢃ due to the
development of strain that resulted from inclusion of compar-
atively large sized Ag and Cu NPs within the mesoporous sieves
indicating increase in structural parameters. Similar results
have also been reported by Gu et al.5 for highly dispersed Cu
species within SBA-15. Wide angle XRD patterns (Fig. 2b)
26
exhibited a broad band at 2q ¼ 22ꢃ
characteristic of the
Fig. 2 (a) Low angle and (b) wide angle XRD patterns of SBA-15, ap- Fig. 3 TEM images of (a) SBA-15, (b) Au/ap-SBA-15, (c) Ag/ap-SBA-15
SBA-15 and M/ap-SBA-15 catalysts. and (d) Cu/ap-SBA-15 catalysts.
Table 1 Physiochemical parameters for pure SBA-15 and M/ap-SBA-15 catalysts
d-Spacing,
d100 (nm)
Unit cell parameter,
a0 (nm)
Wall thickness,
Surface area
m2 gꢁ1
Pore volume
Pore diameter
(nm)
(cm3 gꢁ1
)
a
Sample
dwb (nm)
SBA-15
Ap-SBA-15
Au/ap-SBA-15
Ag/ap-SBA-15
Cu/ap-SBA-15
11
12.70
13.46
12.82
13.33
14.18
1.70
1.8
1.71
1.78
1.90
664
1.3317
0.7132
0.6031
1.2846
1.33
8.02
7.64
8.24
7.39
7.48
11.66
11.11
11.55
12.28
373.15
292.67
694.84
714.81
a
b
a0 ¼ 2/31/2d100
.
dw ¼ a0 ꢁ d100
.
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Fig. 5 HR-TEM images and EDX spectra of (a) Au/ap-SBA-15, (b) Ag/
ap-SBA-15 and (c) Cu/ap-SBA-15 catalysts.
Fig. 4 STEM images and elemental mapping of (a and b) Au/ap-SBA-
15, (c and d) Ag/ap-SBA-15 and (e and f) Cu/ap-SBA-15 catalysts.
exhibited binding energy of 367.9 eV and 373.5 eV corre-
sponding to Ag 3d5/2 and Ag 3d3/2 indicating Ag0 state and is in
particles exhibited single crystalline structures with lattice consonance with the reported data29,30 of metallic Ag (Fig. 6b).
fringes giving d-spacing of 0.232 nm which is close to the However, in Cu/ap-SBA-15 (Fig. 6c), the Cu 2p3/2 peak appeared
spacing of 0.236 nm for Au (111) planes (Fig. 5a, marked at binding energy of 932.5 eV implying the presence of either
section of ESI-Fig. 6a†), 0.23 nm and 0.20 nm for Ag (111) and Cu0 or Cu+1 species31 suggesting that Cu may have been oxidized
(200) planes (Fig. 5b, marked section of ESI-Fig. 6b†). However, to Cu+1 by oxygen at high temperature. Moreover, being highly
lattice fringes giving d-spacing of 0.25 nm were observed for susceptible to oxidation, there is a possibility that it may further
Cu/ap-SBA-15 catalyst in agreement with that of crystalline reoxidise back to Cu+2 species (as revealed in XRD studies).
CuO (111) plane (Fig. 5c, marked section of ESI-Fig. 6c†)
Nitrogen adsorption and desorption isotherms of prepared
revealing that metallic CuNPs may not be formed instead, materials (Fig. 7a) exhibited typical type IV isotherms with H1
highly dispersed and crystalline CuO may be formed on the hysteresis curve characteristic of MMs.32 The BET prole of SBA-
surface of the SBA-15 support as a result of oxidation of Cu in 15 represented three well distinguished regions with monolayer
consonance with wide angle powder XRD studies. The corre- multilayer adsorption (p/p0 ¼ 0–0.5), capillary condensation (p/
sponding EDX analysis further conrmed the presence of Au, p0 ¼ 0.5–0.8) and multilayer adsorption on the outer surface (p/
Ag and CuO on the mesoporous support with Au, Ag and Cu p0 ¼ 0.8–1.0). Moreover, a sharp inexion was observed at
showing 3.9, 4.25 and 0.29 wt% loading. Moreover, change in relative pressure in the range of 0.5–0.8 indicative of uniform
the color of prepared samples in comparison to bare SBA-15 pore size distribution of SBA-15.26,33 The nitrogen adsorption–
illustrated the deposition of metallic species on the meso- desorption isotherm of ap-SBA-15 showed a single step
porous support.
adsorption–desorption branch implying no obstruction or pore
The incorporation of metallic species within SBA-15 and the blockage of silica by organic functionalities. Further observed
elemental state of the nanocomposite materials was analyzed decrease of surface area is due to presence of thin layer of
using XPS (Fig. 6). Au/ap-SBA-15 catalyst (Fig. 6a) showed a APTES over SBA-15 (Table 1, ESI-Fig. 2†). Similar ndings have
doublet at 83.4 eV and 87.3 eV corresponding to Au 4f7/2 and Au been reported by Fattori et al.34 for surface functionalisation of
4f5/2 peaks respectively, implying the presence of metallic Au0 SBA-15 with viologen. The shiing of the inexion point to the
within the catalyst.27,28 Moreover, Ag/ap-SBA-15 catalyst lower pressure (p/p0 ꢀ 0.5) further signied the decrease in pore
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Fig. 6 XPS spectra of (a) Au/ap-SBA-15, (b) Ag/ap-SBA-15 and (c) Cu/ap-SBA-15 catalysts.
3.3. Catalytic activity
The reduction of m-DNB (Scheme 1) showed linear plots
(Fig. 8a) for change in concentration of m-DNB (ꢁln C/C0 versus
Scheme 1 Selective reduction of m-dinitrobenzene to m-phenyl-
enediamine and m-nitroaniline by M/ap-SBA-15 catalysts where M is
Au, Ag or Cu.
Fig.
7 (a) N2 adsorption–desorption isotherm and (b) pore size
distribution of M/ap-SBA-15 catalysts.
diameter26 (Fig. 7b). However, gold doped ap-SBA-15 repre-
sented a single step capillary condensation indicating presence
of uniform mesopores but a two step capillary evaporation with
pronounced tailing at p/p0 ꢀ 0.4 suggested the plugging of the
pores with Au during loading, consistent with pore blocking
effect35,36 resulting in decrease of surface area and pore volume.
However, a slight increase in the pore diameter was observed
due to the encapsulation of metal NPs within the sieves at high
calcination temperature. This leads to the development of
strain resulting in decrease in surface area but increase in pore
diameter. On the other hand, Ag and Cu incorporated ap-SBA-15
exhibited isotherms similar to that of bare SBA-15 with an
unexpected increase in surface area possibly due to deposition
of some large Ag and Cu species onto the surface of pores as can
be seen in TEM images.
Fig. 8 (a) Time course graph of reduction of m-DNB (ꢁln C/C0) by
SBA-15 and M/ap-SBA-15 catalysts and (b) product distribution of
m-DNB reduction (5 mM) to m-PDA and m-NA by SBA-15 and
M/ap-SBA-15 catalysts.
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time) for all metal loaded catalysts indicating pseudo rst order
kinetics. However, no change in concentration of m-DNB was
observed with bare SBA-15. With metal loading catalytic activity
improved abruptly in comparison to SBA-15 indicating that
metal NPs were real active sites. However, the order of catalytic
activity was found to be dependent upon nature of metal, size
and metallic dispersion which inturn rely on the synthetic
conditions. During the synthesis, surface of SBA-15 was func-
tionalized with APTES, followed by metal deposition. As a result,
the amine groups present on the silica surface interacted with
metal species (Au/Ag/Cu) leading to uniform dispersion of
metal throughout the pore channels.37–42 APTES acted as stabi-
lizing agent by improving the interaction between metal and
silica surface thus preventing aggregation of metal NPs. Aer
metal impregnation the prepared materials were calcined to
reduce metal (Au/Ag/Cu) and remove organic moieties resulting
in the formation of highly dispersed metal NPs within meso-
porous silica43 (ESI-Scheme 1†). Among all the prepared cata-
lysts, Au and Ag were reduced to metallic state with Au
exhibiting the smallest particle size whereas Cu being suscep-
tible to oxidation formed large CuO NPs (as revealed by wide
angle XRD and EDX analysis). Thus, Au with smallest particle
size and better metallic dispersion produced greater number of
active metal sites resulting in easy accessibility of reactants and
enhanced reaction rate and selectivity. Au/ap-SBA-15 exhibited
the highest selectivity (89%) for m-PDA (Fig. 8b) with m-NA as an
intermediate as conrmed by HPLC (ESI-Fig. 4†) and GC-MS
(ESI-Fig. 5†) analysis. Rojas et al.44 also reported the inuence
of particle size and metal dispersion on catalytic activity of
supported platinum catalyst for the reduction of m-DNB.
However, lower catalytic activity for Ag/ap-SBA-15 in comparison
to Au/ap-SBA-15 can be explained on the basis of comparatively
large sized NPs embedded on the external surface resulting in
lesser number of exposed active sites and hence decreased
reaction rate. In case of Cu/ap-SBA-15, Cu being highly reactive
got oxidized to large CuO NPs, which present on the external
surface does not allow reactant molecules to access all the
reaction sites resulting in lower catalytic activity.
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In summary it is represented that mesoporous SBA-15 having
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Acknowledgements
Financial support from the Department of Science & Tech-
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