Architecture controlled PtNi@mSiO2 and Pt-NiO@mSiO2 mesoporous core-shell nanocatalysts for enhanced p-chloronitrobenzene hydrogenation selectivity

Article abstract of DOI:10.1039/c5ra00429b

Architecture controlled PtNi@mSiO₂ and Pt-NiO@mSiO₂ mesoporous core-shell nanocatalysts were synthesized for selective p-chloronitrobenzene hydrogenation to p-chloroaniline. Tetradecyl trimethyl ammonium bromide (TTAB) capped PtNi nanoparticles (NPs) were coated by SiO₂ through the hydrolysis of tetraethylorthosilicate. The resultant PtNi@SiO₂ core-shell NPs were calcined to remove TTAB to obtain mesoporous Pt-NiO@SiO₂ core-shell nanocatalysts (Pt-NiO@mSiO₂), which were subsequently reduced by hydrogen to form mesoporous PtNi@SiO₂ core-shell nanocatalysts (PtNi@mSiO₂). The relevant characterizations such as XRD, TEM, H₂-TPR, and BET confirm that the PtNi@mSiO₂ NPs consist of PtNi alloy nanoparticle cores and mesoporous SiO₂ shells while the Pt-NiO@mSiO₂ NPs contain Pt-NiO heteroaggregate nanoparticle cores and mesoporous SiO₂ shells. The catalytic results for selective hydrogenation of p-chloronitrobenzene show that the selectivity of p-chloroaniline formation over the PtNi@mSiO₂ and Pt-NiO@mSiO₂ nanocatalysts is significantly improved relative to that of control Pt@mSiO₂ nanocatalysts. Moreover, the PtNi@mSiO₂ and Pt-NiO@mSiO₂ nanocatalysts demonstrate high stability during multiple cycles of catalytic hydrogenation reactions. The enhanced catalytic performance is ascribed to the metal-metal interaction for the PtNi@mSiO₂ catalysts and metal-oxide interaction for the Pt-NiO@mSiO₂ catalysts. This journal is

Full text of DOI:10.1039/c5ra00429b

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PAPER
Architecture controlled PtNi@mSiO2 and Pt–
NiO@mSiO2 mesoporous core–shell nanocatalysts
for enhanced p-chloronitrobenzene hydrogenation
selectivity
Cite this: RSC Adv., 2015, 5, 20238
a
b
a
Hongmei Liu,ab Hongbo Yu, Chunrong Xiong and Shenghu Zhou
*
*
Architecture controlled PtNi@mSiO2 and Pt–NiO@mSiO2 mesoporous core–shell nanocatalysts were
synthesized for selective p-chloronitrobenzene hydrogenation to p-chloroaniline. Tetradecyl trimethyl
ammonium bromide (TTAB) capped PtNi nanoparticles (NPs) were coated by SiO2 through the hydrolysis
of tetraethylorthosilicate. The resultant PtNi@SiO2 core–shell NPs were calcined to remove TTAB to
obtain mesoporous Pt–NiO@SiO2 core–shell nanocatalysts (Pt–NiO@mSiO2), which were subsequently
reduced by hydrogen to form mesoporous PtNi@SiO2 core–shell nanocatalysts (PtNi@mSiO2). The
relevant characterizations such as XRD, TEM, H2-TPR, and BET confirm that the PtNi@mSiO2 NPs consist
of PtNi alloy nanoparticle cores and mesoporous SiO2 shells while the Pt–NiO@mSiO2 NPs contain
Pt–NiO heteroaggregate nanoparticle cores and mesoporous SiO2 shells. The catalytic results for
selective hydrogenation of p-chloronitrobenzene show that the selectivity of p-chloroaniline formation
over the PtNi@mSiO2 and Pt–NiO@mSiO2 nanocatalysts is significantly improved relative to that of
control Pt@mSiO2 nanocatalysts. Moreover, the PtNi@mSiO2 and Pt–NiO@mSiO2 nanocatalysts
demonstrate high stability during multiple cycles of catalytic hydrogenation reactions. The enhanced
catalytic performance is ascribed to the metal–metal interaction for the PtNi@mSiO2 catalysts and
metal–oxide interaction for the Pt–NiO@mSiO2 catalysts.
Received 9th January 2015
Accepted 12th February 2015
DOI: 10.1039/c5ra00429b
www.rsc.org/advances
common methods to synthesize metal@mSiO2 NPs. Our
previous work23 exploited the typical sol–gel method to synthe-
1. Introduction
size size-controlled Pd@mSiO2 nanocatalysts for catalytic
hydrogenation reactions. The Pd@mSiO2 NPs illustrate high
catalytic activity with high catalytic stability for nitrobenzene
hydrogenation.
Core–shell nanoparticles (NPs) have received intense interest
due to their wide applications in catalysis,1–5 hydrogen absorp-
tion6,7 and biological sciences.8,9 Among the various core–shell
structures, those with metal nanoparticle cores and meso-
porous oxide shells have attracted particular interest in
heterogeneous catalysis because the mesoporous oxide shells
can easily transport reactants/products and the catalytically
active metal cores resist sintering due to encapsulation by oxide
shells.10 For example, mesoporous Pt@SiO2 core–shell NPs
(Pt@mSiO2) developed by Somorjai and co-workers exhibit
Although Pd@mSiO2 nanocatalysts demonstrate a good
catalytic performance in nitrobenzene hydrogenation,23 they
show poor p-chloroaniline (p-CAN) selectivity in p-chloroni-
trobenzene (p-CNB) hydrogenation,24 where signicant forma-
tion of de-chlorination products is observed. To improve
selectivity for functionalized nitroaromatics hydrogenation,
bimetallic catalysts are developed.25,26 For example, the
poly(N-vinyl-2-pyrrolidone) (PVP) anchored bimetallic Pd–Ru
catalysts exhibit p-CAN selectivity of 94% while the major
product over PVP–Pd catalysts is de-chlorination product of
aniline (AN).27 The selectivity improvement over PVP–Pd–Ru
catalysts is ascribed to the synergic effect between Pd and Ru.
Another way to improve the catalytic selectivity for hydro-
genation of functionalized nitroaromatics is to increase the
metal–support interaction.28–30 Corma and co-workers pre-
treated Pt/TiO2 catalysts by high temperature H2 for selective
hydrogenation, and the selectivity improvement over pretreated
Pt/TiO2 for functionalized nitroaromatics hydrogenation is
ꢀ
thermal stability up to 750 C in air and catalytic stability for
high temperature CO oxidation reaction.11
Currently, various porous metal@oxide core–shell NPs,12–16
such as Au@mSiO217,18 and Pd@CeO219,20 have been successfully
synthesized. Among the oxide shell materials, SiO2 is widely used
as shell materials due to easy availability, high surface area and
thermal stability. The Stober method21 and sol–gel process22 are
¨
aNingbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo, Zhejiang 315201, P. R. China. E-mail: zhoush@nimte.ac.cn; Fax:
+86 574 86685043; Tel: +86 574 86696927
bCollege of Materials and Chemical Engineering, Hainan University, Haikou, Hainan
570228, P. R. China. E-mail: bearcr_82@hotmail.com
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ascribed to the formation of close contact Pt/TiOx structure (TEOS, AR) were purchased from Aladdin. Tetradecyl trimethyl
induced by H2 treatment.30 Dai and co-workers rst transformed ammonium bromide (TTAB, AR) and p-chloronitrobenzene
AuNi NPs into close contact Au–NiO heteroaggregate nano- (p-CNB, AR) were purchased from Shanghai Chemical Reagent
catalysts for low temperature CO oxidation, which resist sinter- Company. All reagents were used without further purication.
ing upon calcination at high temperatures.31 Our group
exploited the same strategy to prepare Pt–MxOy/Al2O3 (M ¼ Ni,
Synthesis
Fe and Co) heteroaggregate nanocatalysts32 and Pd–NiO@mSiO2
nanocatalysts24 for p-CNB hydrogenation. The Pt–MxOy/Al2O3
Synthesis of TTAB-capped PtNi alloy NPs. In a typical
nanocatalysts show enhanced catalytic performance for p-CNB synthesis, 8.1 mg of Ni(acac)2 were dissolved in 85 mL of
hydrogenation relative to the Pt/Al2O3 catalysts, but the catalytic deionized water under a magnetic stirring in a 250 mL three-
activity decreases dramatically from cycle to cycle. The necked round-bottomed ask at room temperature. The
Pd–NiO@mSiO2 NPs illustrate the improvement of p-CAN mixture was then treated by ultrasonic for 5 minutes to totally
selectivity and stability, but the selectivity is still not satised.
dissolve Ni(acac)2. The resultant solution was charged with
These above ndings encourage us to combine the concept 3.0 mL of 10.0 mM H2PtCl6 aqueous solution and 0.840 g of
of mesoporous metal@oxide architecture for catalytic stability TTAB. The obtained mixture was stirred at room temperature
improvement with the concept of the bimetallic and metal– for about 15 minutes, and 5.0 mL of 528 mM ice-cooled NaBH4
oxide heteroaggregate architecture for selectivity improvement solution was then injected into the mixture through a syringe.
to design highly selective and stable nanocatalysts for p-CNB The syringe was kept in the system to release the generated H2,
hydrogenation. Here, we rst synthesized PtNi@mSiO2 and and then the syringe was pulled out to make the ask a closed
Pt–NiO@mSiO2 mesoporous core–shell nanocatalysts using the system. The above solution was further stirred for 2.5 hours at a
sol–gel method, and systematically investigated the effects of low speed at room temperature to obtain TTAB-capped PtNi
active component architectures on the catalytic performance for alloy NPs. Finally, the TTAB-capped PtNi NPs were collected by
p-CNB hydrogenation. The schematic demonstration of the centrifugation with acetone washing and subsequent drying in
syntheses of PtNi@mSiO2 and Pt–NiO@mSiO2 nanocatalysts is air for future characterization.
shown in Scheme 1. The PtNi@mSiO2 and Pt–NiO@mSiO2
Synthesis of PtNi@SiO2 core–shell NPs. PtNi@SiO2 core–
nanocatalysts illustrate signicantly improved p-CAN selectivity shell NPs with ꢁ17 nm thickness of silica shells were synthe-
with the suppression of the de-chlorination product formation sized by a modied sol–gel method.24 The above as-synthesized
and high stability during cycle-to-cycle hydrogenation reactions PtNi alloy colloid containing 0.0077 mmol Pt and 0.0077 mmol
due to their unique architectures.
Ni (24.0 mL colloid) was mixed with 120 mL deionized water in a
250 mL three-necked round-bottomed ask with a magnetic
stirring at low speeds. The pH value of the solution was adjusted
to 10.5, 10.6 or 10.7 by concentrated ammonia aqueous solution
(25–28 wt%). Aer pH adjustment to a certain value, 600 mL of
TEOS (2.69 mmol, a TEOS/Pt ratio of 350/1) was injected into the
system to coat PtNi alloy NPs with silica layer through the
hydrolysis of TEOS. The resultant mixture was further stirred for
1.5 hours at low speeds. Finally, the PtNi@SiO2 NPs were
collected by centrifuging with acetone washing and following
drying in air for future use.
2. Experimental section
Chemicals
Chloroplatinic acid (H2PtCl6$6H2O), nickel(II) acetylacetonate
(Ni(acac)2, 95%), sodium borohydride (NaBH4, 98%), ammo-
nium hydroxide (NH4OH, AR), and tetraethylorthosilicate
Synthesis of Pt–NiO@mSiO2 and PtNi@mSiO2 mesoporous
core–shell nanocatalysts. Pt–NiO@mSiO2 mesoporous core–
shell nanocatalysts were obtained through calcinations of the
above as-synthesized PtNi@SiO2 NPs in a muffle furnaceꢂa1t
500 ꢀC for 3.0 hours with a ramping rate of 4 ꢀC min
.
PtNi@mSiO2 mesoporous core–shell nanocatalysts were
obtained by H2 reduction of Pt–NiO@mSiO2 nanocatalysts with
a H2 ow rate of 60 mL minꢂ1 for 2.0 hours at 500 C with a
ꢀ
ramping rate of 5 C minꢂ1 in a xed bed reactor.
ꢀ
Synthesis of control Pt@mSiO2 mesoporous core–shell
nanocatalysts. The similar synthetic procedure of Pt–NiO@mSiO2
nanocatalysts was employed to prepare Pt@mSiO2 nanocatalysts
with ꢁ17 nm silica shells. H2PtCl4 aqueous solution (5.2 mL,
10.0 mM), 0.840 g of TTAB and 85 mL of deionized water were
mixed under a magnetic stirring in a 250 mL three-necked round-
bottomed ask at room temperature. Aer stirring for 15 minutes,
ice-cooled NaBH4 (9.0 mL, 250 mM) aqueous solution was injec-
ted into the ask. The syringe was kept in the system to release the
Scheme 1 Schematic demonstration of synthetic procedures for
PtNi@mSiO2, Pt–NiO@mSiO2 and Pt@mSiO2 mesoporous core–shell
nanocatalysts.
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Fig. 1 TEM images of different NPs showing (a), TTAB-capped PtNi NPs, and the insert is the HRTEM image of one particle showing a lattice
spacing of 0.267 nm; (b), TTAB-capped Pt NPs; (c), PtNi@SiO2 NPs synthesized at a pH value of 10.5 and a TEOS/Pt ratio of 350/1; (d) PtNi@SiO2
NPs synthesized at a pH value of 10.6 and a TEOS/Pt ratio of 350/1; (e), PtNi@SiO2 NPs synthesized at a pH value of 10.7 and a TEOS/Pt ratio of
350/1; (f), Pt–NiO@mSiO2 nanocatalysts obtained after 500 ꢀC calcination of PdNi@SiO2 and the insert is the enlarged image of one particle; (g),
PtNi@mSiO2 nanocatalysts obtained after 500 ꢀC calcination and following 500 ꢀC H2 reduction of PtNi@SiO2. (h), Pt@SiO2 NPs synthesized at
pH value of 10.9 and a TEOS/Pt ratio of 200/1; (i), Pt@mSiO2 nanocatalysts obtained after 500 ꢀC calcination of Pt@SiO2. Scale bars of (a) and (b)
are 20 nm; scale bar in the insert of (a) is 2 nm; scale bars of (c)–(i) are 50 nm; scale bar in the insert of (f) is 20 nm.
generated H2, and then the syringe was pulled out to make the radiation in the 2q range from 10ꢀ to 80ꢀ. The transmission
ask a closed system. The above solution was further stirred for electron microscopy (TEM) images of the samples were
8.0 hours at a low speed at room temperature to obtain TTAB- obtained using a JEOL 2100 transmission electron microscope.
capped Pt NPs. The as-synthesized TTAB-capped Pt colloids The thermal property of the samples was measured by using a
(24.0 mL) containing 0.0134 mmol Pt were mixed with 120 mL of Pyris Diamond thermogravimetric analyzer (TGA) in the
deionized water in a 250 mL three-necked round-bottomed ask temperature range from 30 ꢀC to 800 ꢀC at a heating rate of 5 ꢀC
under a magnetic stirring. A certain amounts of concentrated minꢂ1 with a gas ow rate of 50 mL minꢂ1 in an air atmosphere.
ammonia solution (25–28 wt%) was used to adjust pH value of the The fourier transform infrared (FT-IR) spectra of the samples
solution to 10.9. TEOS (600 mL, 2.69 mmol, a TEOS/Pt ratio of were recorded at a Bruker Tensor 27 spectrophotometer. H2
200/1) was then injected into the solution, and the solution was temperature-programmed reduction (H2-TPR) was performed
further stirred for 1.5 hours. The particles were collected by on a FINESORB3010 instrument. The Pt–NiO samples obtained
centrifugation, and were further calcined to obtain Pt@mSiO2 by calcination of PtNi NPs at 500 ꢀC was pretreated at 200 ꢀC in
nanocatalysts. The collection and calcination procedures are Ar for 1.0 hour prior to H2-TPR measurement, and further
same as those of Pt–NiO@mSiO2 nanocatalysts.
reduced by 1/10 H2/Ar ow (v/v, 50 mL minꢂ1) from room
temperature to 800 ꢀC at a ramping rate of 5 ꢀC minꢂ1
.
Hydrogen consumption was detected by a thermal conductivity
detector (TCD). The actual metal contents of nanocatalysts were
Characterization
X-ray diffraction (XRD) patterns of various samples were recor- identied using a PE Optima 2100DV inductive coupled plasma
ded at a Bruker D8 Advance X-ray diffractometer with a Cu Ka optical emission spectrometer (ICP-OES). The Brunauer–
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Fig. 2 Size distributions of PtNi, Pt, and Pt-containing cores of different core–shell NPs showing (a), PtNi NPs; (b), PtNi@SiO2 NPs; (c), Pt–
NiO@mSiO2 nanocatalysts; (d), PtNi@mSiO2 nanocatalysts; (e), Pt NPs; (f), Pt@mSiO2 nanocatalysts.
Emmett–Teller (BET) surface area, pore size distribution and
the adsorption/desorption isotherms of various samples were
measured by N2 adsorption at 77 K, using a Micromeritics
ASAP-2020 M automatic specic surface area and porous phys-
ical adsorption analyzer.
Catalytic hydrogenation of p-chloronitrobenzene with H2
Hydrogenation of p-CNB with H2 over different Pt-containing
mesoporous core–shell nanocatalysts was carried out at
ambient H2 pressure. Pt–NiO@mSiO2 nanocatalysts (0.186 g,
0.72 wt% of Pt), 0.6 g of p-CNB and 24 mL of absolute ethanol
were mixed in a 50 mL three-necked round-bottomed ask at
room temperature under a magnetic stirring. The ask was rst
purged with high purity H2 three times to get rid of air, and then
the hydrogenation reactions were carried out with a vigorous
magnetic stirring under H2 ambient pressure and a dened
temperature for a desired time. The liquid products were
collected by centrifuging to remove the catalysts and analyzed
online by a gas chromatography equipped with a ame ioniza-
tion detector. For p-CNB hydrogenation over PtNi@mSiO2 and
Fig. 3 XRD patterns of different samples showing (a), PtNi alloy NPs;
Pt@mSiO2 catalysts, 0.179 g of PtNi@mSiO2 (0.75 wt% of Pt)
and 0.098 g of Pt@mSiO2 nanocatalysts (1.37 wt% of Pt) were
used to ensure the same molar number of Pt used in hydroge-
nation over all three catalysts. The reaction conditions were
same as those in hydrogenation over Pt–NiO@mSiO2 catalysts.
(b), PtNi@SiO2 NPs; (c), Pt–NiO@mSiO2 nanocatalysts; (d),
PtNi@mSiO2 nanocatalysts; (e), Pt@mSiO2 nanocatalysts. Vertical lines
indicating PtNi (JCPDS 03-065-2797), Ni (JCPDS 03-065-2865), and
Pt (JCPDS 01-071-6560).
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Fig. 4 TG/DTA curves of as-synthesized PtNi@SiO2 NPs (left panel) and FT-IR spectra of PtNi@SiO2 NPs before and after 500 ꢀC calcination
(right panel).
Cycle-to-cycle
experiments
for
PtNi@mSiO2
and synthesized by reduction of Pt and Ni precursors with NaBH4 in
Pt–NiO@mSiO2 nanocatalysts were performed to investigate the the presence of TTAB. The PtNi@SiO2 NPs were then prepared
catalytic stability. In p-CNB hydrogenation over Pt–NiO@mSiO2 by coating TTAB-capped PtNi NPs with SiO2 through the
nanocatalysts, 0.186 g of nanocatalysts, 0.6 g of p-CNB and hydrolysis of TEOS using TTAB as structure directing agents.
ꢀ
24 mL of absolute ethanol were mixed in a 50 mL three-necked The as-synthesized PtNi@SiO2 NPs were calcined at 500 C to
round-bottomed ask at room temperature under a vigorous remove TTAB to obtain mesoporous Pt–NiO@mSiO2 nano-
ꢀ
magnetic stirring. The hydrogenation reactions were carried out catalysts, and subsequently reduced by H2 at 500 C to obtain
for 2.0 hours under ambient H2 pressure at 45 ꢀC. Aer catalytic mesoporous PtNi@mSiO2 nanocatalysts. The real Pt and Ni
reactions, the nanocatalysts were collected by centrifugation contents in nal products were identied by ICP-OES.
ꢀ
with absolute ethanol washing twice and drying at 75 C in an
TEM images of various samples are shown in Fig. 1. PtNi NPs
oven for 1.0 hours. The dried nanocatalysts were then used in in Fig. 1a basically display a spherical shape while individual Pt
the next cycle experiment. Due to the catalyst loss in the NPs in Fig. 1b exhibit an irregular shape. The lattice spacing of
recovery process, the weight of p-CNB and ethanol in the next PtNi NPs in the insert of Fig. 1a is 0.267 nm, which is consistent
cycle decreased accordingly to keep the ratios of p-CNB/catalyst with the lattice spacing of tetragonal PtNi alloy (110) planes.
and p-CNB/solvent same as those in the rst cycle. In cycle-to- Fig. 1c–e show TEM images of PtNi@SiO2 NPs. The core–shell
cycle hydrogenation experiments over PtNi@mSiO2 nano- structure with PtNi nanoparticle cores and silica shells is clearly
catalysts, 0.179 g of catalysts were used in the rst cycle, and observed in these gures. The thicknesses of silica shells of
other reaction conditions were same to those of Pt–NiO@mSiO2
catalysts. Five cycles of hydrogenation were carried out for both
catalysts due to the catalyst loss.
3. Results and discussion
In the syntheses of Pt–NiO@mSiO2 and PtNi@mSiO2 meso-
porous core–shell NPs, TTAB-capped PtNi alloy NPs were rst
Fig. 6 N2 adsorption/desorption isotherms and pore size distribution
Fig. 5 H2-TPR of Pt–NiO NPs obtained by calcination of PtNi NPs at (inset) showing (a), Pt@mSiO2 nanocatalysts; (b), Pt–NiO@mSiO2
500 ꢀC.
nanocatalysts; (c), PtNi@mSiO2 nanocatalysts.
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Table 1 BET surface areas, pore volumes, and pore sizes of Pt@mSiO2,
Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts
particle size in Fig. 2f), conrming that the cores of
metal@mSiO2 structures can resist sintering due to the encap-
sulation of metal particles by oxide shells.
BET surface area
Pore volume
Pore size
(nm)
(m2 gꢂ1
)
(cm3 gꢂ1
)
XRD patterns of various samples are illustrated in Fig. 3. As
shown in Fig. 3a, the broad diffraction at 2q degree of 41.8ꢀ is in
good agreement with the (111) diffraction of tetragonal PtNi
phase, suggesting the formation of PtNi alloy. Due to their small
size, the PtNi alloy NPs (4.4 nm) illustrate the weak (111)
diffractions without any diffractions at high 2q degrees. Aer
coating by silica, the PtNi@SiO2 NPs in Fig. 3b show broad
amorphous silica diffraction at 2q degree around 22ꢀ, and the
(111) diffraction of PtNi alloy is very weak due to the low crys-
tallinity of PtNi cores in the PtNi@SiO2 NPs. Aer calcination of
PtNi@SiO2 NPs at 500 ꢀC, the Pt diffractions of Pt–NiO@mSiO2
nanocatalysts in Fig. 3c is clearly observed, and the diffractions
of NiO of Pt–NiO@mSiO2 is not observed possibly due to the
formation of amorphous NiO phase. The existence of Ni in the
Pt–NiO@mSiO2 nanocatalysts is evidenced by ICP-OES analysis,
in which the ratio of Pt/Ni is close the 1/1 Pt/Ni ratio of the
starting materials. Fig. 3d illustrates the XRD pattern of
PtNi@mSiO2 nanocatalysts obtained by reduction of
Pt–NiO@mSiO2 nanocatalysts with H2 at 500 ꢀC. The
PtNi@mSiO2 nanocatalysts exhibit weak PtNi diffractions aer
reduction with H2. However, the diffractions are slightly shied
to low 2q degrees, possibly due to the formation of PtNi alloy
with composition ingredients inside the particles. The XRD
patterns of Pt@mSiO2 nanocatalysts are shown in Fig. 3e. The
apparent Pt diffractions in Fig. 3e conrm that the major phase
Samples
Pt@mSiO2
Pt–NiO@mSiO2
PtNi@mSiO2
656
786
753
1.1
1.2
1.0
2.0
2.0
2.0
PtNi@SiO2 NPs synthesized at the pH values of 10.5 in Fig. 1c,
10.6 in Fig. 1d and 10.7 in Fig. 1e are ꢁ16 nm, ꢁ17 nm and
ꢁ18 nm, respectively. In addition, some empty silica particles
are observed at a lower pH value (Fig. 1c) while some particles
with multiple cores are present at a higher pH value (Fig. 1e).
This phenomenon is similar to the observation in our previous
work,24 conrming that fast hydrolysis at higher pH values
results in the formation of core–shell NPs with multiple cores
and slow hydrolysis at lower pH values results in the formation
of empty silica particles.
Due to their perfect core–shell structure and acceptable silica
shell thickness for catalysis, the PtNi@SiO2 NPs synthesized at a
pH value of 10.6 and a TEOS/Pt ratio of 350/1 were chosen for
subsequent treatment under different conditions. Fig. 1f shows
the TEM image of Pt–NiO@mSiO2 nanocatalysts, which is
ꢀ
obtained by calcination of PtNi@SiO2 NPs at 500 C. The mes-
oporous silica shells are clearly observed in the enlarged TEM
image in the insert. Fig. 1g shows the TEM image of
PtNi@mSiO2 nanocatalysts ꢀ obtained by reduction of
Pt–NiO@mSiO2 by H2 at 500 C. As shown in Fig. 1g, the mes-
oporous core–shell structures are intact, conrming the struc-
ture maintenance aer subsequent H2 reduction at 500 ꢀC. The
TEM images of Pt@SiO2 NPs before and aer calcination at
500 ꢀC are illustrated in Fig. 1h and i, respectively. The
Pt@mSiO2 nanocatalysts were synthesized by calcination of as-
synthesized Pt@SiO2 NPs at 500 ꢀC. To ensure that the silica
shell thickness of Pt@mSiO2 NPs is close to those of
Pt–NiO@mSiO2 and PtNi@mSiO2 NPs, the synthetic parame-
ters of a pH value of 10.9 and a TEOS/Pt ratio of 200/1 were used
to obtain Pt@SiO2 NPs with ꢁ17 nm silica shell.
ꢀ
of Pt cores still maintain a metallic state upon 500 C calcina-
tion in air, which is consistent with the literature.33
TG/DTA analysis of PtNi@SiO2 NPs and FT-IR spectra of
PtNi@SiO2 NPs before and aer calcination at 500 ꢀC are shown
in Fig. 4. From the le panel of Fig. 4, it is clear observed that
TTAB starts to remove at ꢁ130 ꢀC and is completely removed at
ꢁ450 ꢀC. Therefore, the calcination at 500 ꢀC is applied to
remove TTAB in PtNi@SiO2 NPs to obtain Pt–NiO@mSiO2
nanocatalysts. In the right panel of Fig. 4, the absorption peaks
of antisymmetric and symmetric stretching modes of methylene
of TTAB34 in PtNi@SiO2 NPs before calcination are observed at
2922 cmꢂ1 and 2856 cmꢂ1, respectively. In contrast, these
absorption peaks in the PtNi@SiO2 NPs aer calcination at
The size distributions of various NPs are shown in Fig. 2. The
average particle sizes of PtNi NPs in Fig. 2a, the cores of
PtNi@SiO2 NPs in Fig. 2b, the cores of Pt–NiO@mSiO2 nano-
catalysts in Fig. 2c and the cores of PtNi@mSiO2 nanocatalysts
in Fig. 2d are 4.4, 4.3, 4.4 and 4.2 nm, respectively. The close
particle sizes are also found for Pt NPs (ꢁ5.0 nm size in Fig. 2e)
and the Pt cores of Pt@mSiO2 nanocatalysts (ꢁ5.1 nm Pt
Table 3 Product distributions in p-CNB hydrogenation over Pt-con-
taining core–shell mesoporous nanocatalysts with a SiO2 shell thick-
ness of ꢁ17 nma
Selectivity (%)
Catalyst
Conv. (%)
p-CAN
AN
p-AP
Others
Pt@mSiO2
Pt–NiO@mSiO2
PtNi@mSiO2
97.8
87.9
96.1
77.6
95.3
97.1
20.2
4.7
1.3
0
0
0.6
2.2
0.0
1.0
Table 2 The real loadings of Pt and Ni in Pt-containing core–shell
mesoporous nanocatalysts by ICP-OES
Sample
Pt loading (wt%)
Ni loading (wt%)
a
Reaction conditions: reaction temperature-45 ꢀC; ambient H2
pressure; p-CNB-0.6 g; reaction time-2.0 hours; ethanol-24 mL;
Pt@mSiO2-0.098 g (Pt loading-1.37 wt%); Pt–NiO@mSiO2-0.186 g (Pt
loading-0.72 wt%); PtNi@mSiO2-0.179 g (Pt loading-0.75 wt%); speed
of agitation-500 rpm.
Pt@mSiO2
Pt–NiO@mSiO2
PtNi@mSiO2
1.37
0.72
0.75
NA
0.17
0.19
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500 ꢀC disappear, conrming the absence of TTAB in the
Pt–NiO@mSiO2 nanocatalysts.
p-CNB hydrogenation with H2 was carried out under ambient
H2 pressure and reaction temperature of 25–55 ꢀC. Different
H2-TPR curve of Pt–NiO NPs obtained by calcination of PtNi weights of core–shell nanocatalysts with different Pt loadings
NPs at 500 ꢀC is shown in Fig. 5. Since XRD patterns of were applied to ensure the same molar number of Pt used in
Pt–NiO@mSiO2 in Fig. 3c and Pt@mSiO2 in Fig. 3e have hydrogenation. The major products are p-CAN with
conrmed the metallic state of Pt species and it is well known some de-chlorination products including AN and p-amino-
that Pt is in metallic state upon calcination at high tempera- phenol (p-AP).
tures, the hydrogen absorption in Fig. 5 is mainly due to the
Table 3 summarizes the comparison of catalytic perfor-
reduction of NiO. However, the hydrogen absorption around mance over Pt@mSiO2, Pt–NiO@mSiO2 and PtNi@mSiO2
room temperature in Fig. 5 could be from the reduction of nanocatalysts with a SiO2 shell thickness of ꢁ17 nm. The
surface Pt oxides,35 which is formed due to the high activity of Pt@mSiO2 nanocatalysts exhibit a 97.8% of p-CNB conversion
surface Pt atoms. Moreover, the presence of Pt in Pt–NiO will with a 77.6% of p-CAN selectivity while the selectivity of de-
facilitate the reduction of NiO by hydrogen spillover effect, chlorination product of AN is 20.2%. The 77.6% of p-CAN
resulting in the reduction of NiO at lower temperatures.36 From selectivity over Pt@mSiO2 is similar to those of reported Pt
the Fig. 5, it is concluded that the complete reduction of NiO colloid and carbon nanotube supported Pt catalysts at ambient
with H2 is around 500 ꢀC, and thus the reduction of H2 pressure.25,38 In contrast, the Pt–NiO@mSiO2 nanocatalysts
Pt–NiO@mSiO2 nanocatalysts at 500 ꢀC is applied to obtain illustrate an improved p-CAN selectivity of 95.3% with a slightly
PtNi@mSiO2 nanocatalysts.
decreased p-CNB conversion. The PtNi@mSiO2 nanocatalysts
The textural properties of Pt–NiO@mSiO2, PtNi@mSiO2 and maintain the high p-CNB conversion with a signicantly
Pt@mSiO2 nanocatalysts were characterized by N2 adsorption/ improved p-CAN selectivity of 97.1%. This study clearly
desorption experiments. Fig. 6 presents the N2 adsorption/ demonstrates that the PtNi bimetallic and Pt–NiO hetero-
desorption isotherms and the pore size distributions of aggregate structures could signicantly enhance the p-CAN
different Pt-containing nanocatalysts. The typical type IV selectivity, and basically maintain the high catalytic activity of
isotherms37 are observed for all samples, conrming the pres- Pt catalysts for p-CNB hydrogenation.
ence of mesopores resulted from TTAB removal at 500 ꢀC
Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts were chosen
calcination. Moreover, the insert in Fig. 6 suggests that the pore for cycle-to-cycle hydrogenation experiments. In the experi-
size of mesopores in these samples is around 2 nm with some ments, no additional fresh catalysts were introduced into the
large pores possibly formed by connection of particles. Table 1 system, and due to the loss of catalysts in the recovery process
shows BET surface areas, pore volumes and pore sizes of the weights of p-CNB and ethanol decreased to keep the same
Pt-containing samples. These materials exhibit large BET ratios of p-CNB/catalyst and p-CNB/solvent as those in the rst
surface areas, further conrming the thermal stability upon cycle. Table 4 summarizes the catalytic results in ve cycles of
high temperature treatment.
reactions. The Pt–NiO@mSiO2 nanocatalysts illustrate a good
The actual metal loadings by ICP-OES measurement for catalytic stability with an average 88.8% of p-CNB conversion
Pt–NiO@mSiO2, PtNi@mSiO2, and Pt@mSiO2 nanocatalysts are and 96.7% of p-CAN selectivity. The PtNi@mSiO2 nanocatalysts
presented in Table 2. The real Pt loadings of Pt@mSiO2, show an even better result with an average 97.0% of p-CNB
Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts are 1.37 wt%, conversion and 97.6% of p-CAN selectivity.
0.72 wt%, and 0.75 wt%, respectively, while the real Ni loadings
The Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts not only
in Pt–NiO@mSiO2 and PtNi@mSiO2 are 0.17 wt% and 0.19 wt%, exhibit high activity and high selectivity, but also illustrate good
respectively. According to the ICP-OES measurements, the catalytic stability during multiple cycles of catalytic reactions.
molar ratios of Pt/Ni in the Pt–NiO@mSiO2 and PtNi@mSiO2 The selectivity enhancement over PtNi@mSiO2 is ascribed to
nanocatalysts are 1/1.27 and 1/1.19, respectively, which is close the synergistic effect between Pt and Ni, which has been docu-
to the starting 1/1 ratio.
mented in other systems.39 The selectivity enhancement over
Table 4 Cycle-to-cycle reactions of p-CNB hydrogenation over Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalystsa
Pt–NiO@mSiO2 catalyst
PtNi@mSiO2 catalyst
Weight (g)
Weight (g)
p-CAN
Conv.
(%)
p-CAN
selectivity (%)
Cycle index
Catalysts
p-CNB
Conv. (%)
selectivity (%)
Catalysts
p-CNB
1
2
3
4
5
0.186
0.153
0.124
0.106
0.086
0.600
0.495
0.400
0.342
0.277
87.9
85.1
92.6
88.2
90.1
95.3
98.2
96.9
95.4
97.5
0.179
0.141
0.119
0.097
0.070
0.600
0.473
0.399
0.326
0.236
96.1
91.9
99.7
100
97.1
96.3
97.6
98.3
98.5
97.4
a
Reaction conditions: reaction temperature-45 ꢀC; atmospheric H2 pressure; reaction time-2.0 hours; speed of agitation-500 rpm; 24 mL of ethanol
was used as solvent in the rst cycle, and the ratio of ethanol/p-CNB in the following cycles was kept same as that of the rst cycle.
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Fig. 7 Effect of reaction time on catalytic performance over Pt–NiO@mSiO2 (left panel) and PtNi@mSiO2 nanocatalysts (right panel). Reaction
conditions: reaction temperature-45 ꢀC; ambient H2 pressure; Pt–NiO@mSiO2-0.186 g; PtNi@mSiO2-0.179 g; p-CNB-0.6 g; 24 mL of ethanol;
speed of agitation-500 rpm.
Pt–NiO@mSiO2 can be assigned to the strong interaction at 45 ꢀC are shown in Fig. 7. In the le panel of Fig. 7, it is
between Pt and NiO, and the literature has reported that the observed that p-CNB conversions over Pt–NiO@mSiO2 nano-
metal–oxide strong interaction can transform non-selective catalysts increase from 41% at 0.5 hours to 100% at 3.0 hours
catalysts into selective catalysts in high temperature H2 and p-CAN selectivity increases from 87.6% at 0.5 hours to 100%
pretreated Pt/TiO2 catalysts for hydrogenation of functionalized at 3.0 hours. Similarly, the p-CNB conversion and p-CAN selec-
nitroaromatics.30 In our previous work of Pt–MxOy/Al2O3 (M ¼ tivity over PtNi@mSiO2 nanocatalysts follows the similar
Ni, Co and Fe) for p-CNB hydrogenation,32 the Pt–MxOy/Al2O3 trends. The p-CAN selectivity increases with the p-CNB conver-
catalysts illustrate high activity and high p-CAN selectivity at sion increasing, and this phenomenon is consistent with the
elevated H2 pressures, but shows a poor catalytic stability, in literature.40–42 The effects of reaction temperatures on catalytic
which the p-CNB conversion is 35.8% aer 4 cycles of reaction. performance over Pt–NiO@mSiO2 nanocatalysts are illustrated
In this study, the stability enhancement over PtNi@mSiO2 and in Fig. 8. It is obviously seen that p-CNB conversion increase
Pt–NiO@mSiO2 nanocatalysts for p-CNB hydrogenation is due with reaction temperatures increasing and p-CAN selectivity
to the mesoporous core–shell structures, which can easily increase with p-CNB conversions increasing. The p-CNB
transport reactants/products and prevent active components conversions increase from 60.2% at 25 ꢀC to 93.7% at 55 ꢀC, and
ꢀ
from sintering and dissolution.
The effects of reaction time on p-CNB conversion and p-CAN 98.3% at 55 C.
selectivity over Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts
p-CAN selectivity accordingly increases from 86.3% at 25 C to
ꢀ
4. Conclusions
In summary, Pt–NiO@mSiO2 and PtNi@mSiO2 mesoporous
core–shell nanocatalysts were successfully synthesized for
selective p-CNB hydrogenation. The relevant characterizations
conrm that the Pt–NiO@mSiO2 nanocatalysts consist of
Pt–NiO heteroaggregate nanoparticle cores and mesoporous
silica shells, and the PtNi@mSiO2 nanocatalysts contain PtNi
alloy nanoparticle cores and mesoporous silica shells. Both
Pt–NiO@mSiO2 and PtNi@mSiO2 show superior catalytic
performance relative to Pt@mSiO2. The p-CAN selectivity over
Pt–NiO@mSiO2 and PtNi@mSiO2 is signicantly enhanced
with maintaining the high activity of Pt catalysts. Moreover, the
Pt–NiO@mSiO2 and PtNi@mSiO2 nanocatalysts show high
catalytic stability during cycle-to-cycle experiments. The cata-
lytic performance enhancement over Pt–NiO@mSiO2 and
PtNi@mSiO2 nanocatalysts is ascribed to their unique archi-
tectures, in which the bimetallic PtNi and Pt–NiO hetero-
aggregate structures enhance the p-CAN selectivity and
mesoporous core–shell structure enhance the catalytic stability.
Due to mesoporous core–shell structures, the catalytically active
cores (PtNi alloy or Pt–NiO heteroaggregate NPs) are protected
by mesoporous silica shells and the core–shell structures
prevent the cores from sintering and dissolution, thus
Fig. 8 Effect of temperatures on catalytic performance over Pt–
NiO@mSiO2 nanocatalysts. Reaction conditions: ambient H2 pressure;
Pt–NiO@mSiO2-0.186 g; p-CNB-0.6 g; 24 mL of ethanol; reaction
time-2.0 hours; speed of agitation-500 rpm.
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improving their catalytic stability and efficiency. The synthetic
method could be extended to synthesize other mesoporous
core–shell structures. Importantly, this study provides a catalyst
design using the mesoporous core–shell structure to improve
the catalytic stability and bimetallic or metal–oxide hetero-
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Conflicts of interest
The authors declare no competing nancial interest.
12 S. Poovarodom, J. D. Bass, S. J. Hwang and A. Katz,
Investigation of the core–shell interface in gold@silica
nanoparticles: A silica imprinting approach, Langmuir,
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Acknowledgements
S. Zhou thanks the Ministry of Science and Technology of China
(Grant no. 2012DFA40550) for nancial supports. H. Yu thanks
the nancial support from Ningbo Municipal Natural Science
Foundation (Grant No. 2013A610038). H. Liu thanks Hainan
University for nancial supports of development plan of
outstanding graduate thesis.
14 P. Du, Y. H. Cao, D. Li, Z. Y. Liu, X. G. Kong and Z. C. Sun,
Synthesis of thermally stable Ag@TiO2 core–shell
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