A simple and convenient approach for preparing core-shell-like silica@nickel species nanoparticles: Highly efficient and stable catalyst for the dehydrogenation of 1,2-cyclohexanediol to catechol

Article abstract of DOI:10.1039/c4dt01476f

A simple and convenient approach denoted as gel-deposition-precipitation (G-D-P) for the preparation of core-shell-like silica@nickel species nanoparticles was studied systematically. Core-shell-like silica@nickel species nanoparticles consisted of a Si-rich core and a Ni-rich shell. The G-D-P process included two steps: one was the deposition-precipitation of nickel over the gelled colloidal silica particle, generating core-shell-like silica@nickel species nanoparticles, and the other was the aging period. It was found that the nickel phyllosilicate layer was formed mainly during the aging period and served as the protective cover to resist against aggregation of the nanoparticles, which could be utilized for regulating the dispersion of nickel over the silica@nickel species nanoparticles. In the present paper, the silica@nickel species nanoparticles were used as the catalysts for preparing catechol via dehydrogenation of 1,2-cyclohexanediol. Their catalytic activity and long-term stability were compared to those of a catalyst prepared by a conventional deposition-precipitation (D-P) approach. The higher activity and better stability of the title reaction over the silica@nickel species nanoparticles catalyst prepared by G-D-P than those over the catalyst prepared by D-P could be due to the higher dispersion of metallic nickel stabilized by the layers of nickel phyllosilicates. Moreover, it was found that the dehydrogenation of 1,2-cyclohexanediol to catechol was a structurally sensitive reaction. This journal is

Full text of DOI:10.1039/c4dt01476f

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A simple and convenient approach for preparing
core–shell-like silica@nickel species nanoparticles:
highly efficient and stable catalyst for the
Cite this: DOI: 10.1039/c4dt01476f
dehydrogenation of 1,2-cyclohexanediol to catechol†
Bao-Hui Chen, Wei Liu, An Li, Ya-Juan Liu and Zi-Sheng Chao*
A simple and convenient approach denoted as gel-deposition–precipitation (G-D–P) for the preparation
of core–shell-like silica@nickel species nanoparticles was studied systematically. Core–shell-like silica@
nickel species nanoparticles consisted of a Si-rich core and a Ni-rich shell. The G-D–P process included
two steps: one was the deposition–precipitation of nickel over the gelled colloidal silica particle, generat-
ing core–shell-like silica@nickel species nanoparticles, and the other was the aging period. It was found
that the nickel phyllosilicate layer was formed mainly during the aging period and served as the protective
cover to resist against aggregation of the nanoparticles, which could be utilized for regulating the dis-
persion of nickel over the silica@nickel species nanoparticles. In the present paper, the silica@nickel
species nanoparticles were used as the catalysts for preparing catechol via dehydrogenation of 1,2-cyclo-
hexanediol. Their catalytic activity and long-term stability were compared to those of a catalyst prepared
by a conventional deposition–precipitation (D–P) approach. The higher activity and better stability of the
title reaction over the silica@nickel species nanoparticles catalyst prepared by G-D–P than those over the
catalyst prepared by D–P could be due to the higher dispersion of metallic nickel stabilized by the layers
of nickel phyllosilicates. Moreover, it was found that the dehydrogenation of 1,2-cyclohexanediol to cate-
chol was a structurally sensitive reaction.
Received 20th May 2014,
Accepted 28th July 2014
DOI: 10.1039/c4dt01476f
www.rsc.org/dalton
core–shell metal@oxide nanoparticles have attracted consider-
1. Introduction
able attraction due to their better property of stability.9
Utilization of nanosized metal particles or clusters in catalysis Usually, some oxides with high chemical and thermal stabi-
is regarded as one of the most important applications of lities, e.g., SiO2,10,11 TiO2,12 Al2O3,13 and Cr2O3,14 serve as
nanomaterials.1–3 The use of metallic nanomaterial catalysts is shells to protect the susceptive metal nanoparticle cores.
tempting due to their high surface areas and small sizes. As is Among these inert oxides, SiO2 is the most studied shell candi-
well known, different nanomaterials with given shapes have date because of its outstanding environmental stability and
different facets and distinctive ratios of the amount of atoms compatibility with other materials, and relative ease in prepa-
on corners and edges to those on facets;4 it would be in favor ration.15 Herein, taking the core–shell metals@SiO2 as
16
of some special catalytic reactions, and could be useful for the examples, e.g., Ni@SiO2
and Au@SiO2,10 the nano-sized
design of highly effective and exclusive catalysts.4,5 Further- metal particle cores, i.e., Ni16 and Au10 cores, are the catalyti-
more, with the reduction of the size of catalyst particles to the cally active components, and SiO2 exists as shells, whose
region of ultimate nanoclusters, some new catalytic properties framework acts as nano-sized reaction vessels, providing a
would be seen, which could not been forecasted by extra- stable environment even at high reaction temperatures.16–18
polation of the behavior known for the larger size.6,7
However, when the core–shell metal@oxide nanomaterials are
However, few metallic nanomaterials are used indepen- used as catalysts, a problem should be noticed: the thick and
dently for catalytic reactions because of their low thermal closed protective inert oxide shells lower the active surface
stability, especially under the reaction temperature.8 Therefore, area and slow down the diffusions of both reactants and pro-
ducts during the catalytic reaction, although they can protect
the susceptive metallic cores.
College of Chemistry and Chemical Engineering, State Key Laboratory of Chemo/
Comparatively, core–shell oxide@metal nanoparticles can
Biosensing and Chemometrics, Hunan University, Changsha, 410082, China.
be regarded as a kind of highly dispersed oxide supported
E-mail: zschao@yahoo.com; Fax: +86-731-88713257; Tel: +86-731-88713257
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt01476f
metal catalyst. Taking the core–shell SiO2@Ni nanoparticles as
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an example, silica cores can act as the support, and the fine However, this route suffers from some drawbacks, i.e., low con-
nickel nanoparticles are deposited over the surface of cores as version (theoretical conversion = 33.3%28) and selectivity (typi-
the shells. Surprisingly, few works have been reported on the cally <60%28), and a rigorous and complex separation route.29
core–shell oxide@metal nanomaterials and their applications Comparatively, dehydrogenation of 1,2-cyclohexanediol pro-
in the catalysis field13,19 relative to those on the core–shell vides another promising reaction pathway for preparing cate-
metal@oxide nanoparticles.13,16–18 It is probably due to the chol because of its high conversion and selectivity, and easy
fact that it is always a challenge to keep the fine metal nano- separation procedures.29 This pathway is derived from cyclo-
particles loaded from aggregation and sintering during the hexene, which is commercial available from several compa-
application of catalytic reactions, if they are directly deposited nies, e.g., Asahi Chemical.30 The starting material 1,2-
over the nano-sized cores.
cyclohexanediol can be achieved by two different strategies:
Core–shell nanomaterials are fabricated usually using a one is the one-pot epoxidation and hydrolysis (dihydroxylation)
two-step approach.20 The first step is the synthesis of colloidal of cyclohexene (see ESI Scheme 1S A†), e.g., ca. 98% yield of
nanoparticle cores in the presence of organic capping agents, 1,2-cyclohexanediol was obtained using a resin-supported sul-
i.e., polymers or surfactants, which keep the nanoparticles fonic acid catalyst.31 The other one is the first oxidation of
from aggregation in solution, and the second one is the cyclohexene, forming cyclohexene oxide,30,32–34 followed by its
growth of shells deposited on the cores. The main issues hydration to obtain 1, 2-cyclohexanediol (see Scheme 1S
associated with this method are the high cost, complex oper- B†).35,36 This strategy, typically, has a yield of ca. 95% on the
ation, and low production efficiency, which restricts its basis of cyclohexane.36,37 It has been reported that, alkali
broader industrial applications.
metal modified Ni-based catalysts are active for preparing cate-
Highly concentrated colloidal silica solutions that are used chol by the dehydrogenation of 1,2-cyclohexanediol,27,29,38,39
in industrial applications since the 1940s are made of discrete e.g., over a Na-doped HZSM-5 supported Ni catalyst,29 95.8%
particles of amorphous silica of relatively uniform particle size selectivity of catechol at a 99.9% of conversion was obtained,
from ca. 5 to 100 nm.21 They seem to be an economic source and the high performance could be retained for as long as
of silica cores for the fabrication of core–shell silica@metal 40 h of on-stream reaction. In the present paper, preparation
nanoparticles. Moreover, the formation of nickel phyllosilicate of catechol via dehydrogenation of 1,2-cyclohexanediol was
is observed in core–shell Ni@SiO2,16 which is generated due to used as the model reaction for studying the core–shell-like silica@
the interaction between silica and nickel compounds. It has nickel species nanoparticles catalysts prepared by G-D–P
been found that, metal phyllosilicates can be used to stabilize method, and their catalytic properties and long-term stability
nanoparticles due to its high thermal stability and reduction were compared to those of a catalyst prepared by a convention-
susceptibility.16,22
al deposition–precipitation (D–P) approach.
The sol–gel23 and deposition–precipitation (D–P)24 methods
are two important approaches for the preparation of dispersed
metal catalysts. The sol–gel approach possesses the advantage
in stabilizing the active phase supported;25 the deposition–
precipitation (D–P) method exhibits many advantages in the
2. Experimental section
2.1. Catalysts preparation
synthesis of high metal loaded catalysts possessing small metal Nickel nitrate (A.R; Xilong Chemical Co., China), urea (A.R;
particles with a rather narrow size distribution.24 In this work, Kemiou Chemical Co., China) and colloidal silica solution
by combining the merits of the sol–gel and deposition–precipi- (commercial grade, SiO2·nH2O, 30.0 wt%, Na2O type, pH ≈ 10)
tation methods, we report a strategy for preparing core–shell- were employed to prepare the core–shell-like silica@nickel
like silica@nickel species nanoparticles by a simple and con- species nanoparticles. The colloidal silica solution was first
venient method named gel-deposition–precipitation (G-D–P). dispersed into a 7.5 wt% aqueous solution of urea, with a col-
This process is composed of the first gelation (G) of the col- loidal silica solution/urea = 1 in weight ratio, at room tempera-
loidal silica solution, followed by the deposition–precipitation ture with stirring. Then, a calculated amount of 2.5 wt%
(D–P) of nickel over the gelled silica nanoparticles. This aqueous solution of nickel nitrate salt was added into this
method uses the colloidal silica particles directly as the source homogeneous solution. The resultant mixture was heated to
of the silica cores, and nickel phyllosilicates as the protective 95 °C within 20 min, and stirred for a chosen time (0.17–48 h)
coat to stabilize the nanoparticles, which demonstrates the at this temperature point, when the colloidal silica solution
characteristics of low cost, small operation complexity, and was gelled and nickel nitrate salt was deposited-precipitated.
high production efficiency. In addition, interestingly, the dis- This process is called the gel-deposition–precipitation (G-D–P)
persion degree of nickel over the core–shell-like silica@nickel approach by us. After it cooled to room temperature, the so-
species nanoparticles can be controlled easily by regulating prepared mixture was recovered by a separation process,
the preparation (G-D–P) time.
washed repeatedly with de-ionized water and dried at 80 °C for
Catechol has wide applications in many fields, e.g., 12 h. The resultant powder was further calcined in air at
medicines, pesticides, dyes, spices, and chemical auxiliary 450 °C for 4 h, forming the core–shell-like silica@nickel
agents.26,27 The current industrial production scheme for cate- species nanoparticles, denoted as Si–XNi(Y). Herein, X refers
chol is the hydroxylation of phenol with hydrogen peroxide.28 to the theoretical nickel weight loading (named as NiT, wt%)
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when overall the nickel nitrate and colloidal silica particles in weight ratio of 12.5 at room temperature with stirring. Then, a
solution have been deposited–precipitated/and gelled, and its calculated amount of 2.5 wt% aqueous solution of nickel
value equals to 10–50 (wt%); Y refers to the G-D–P time nitrate salt was added, and the resultant mixture was heated to
expressed in hours, and its value equals to 0.17–48 (h). For 95 °C, and stirred for an additional 4 h. After it cooled to the
example, Si–20Ni(4) represents the catalyst that was prepared room temperature, the precipitate was recovered by separation,
according to a 20 wt% of theoretical loading of Ni in catalyst washed repeatedly with de-ionized water and dried at 80 °C for
and via 4 hours of G-D–P. Moreover, to figure out the effect of 12 h. Finally, the resultant powders were calcined in air at
calcination, the catalysts prepared via the above procedure but 450 °C for 4 h, generating the nickel loaded silica catalyst,
without the step of calcination at 450 °C were also employed which was denoted as Si–20Ni(4)–DP, where the content of
in the studies, and these catalysts were defined as uncalcined Ni in the catalyst is 20 wt%, and the preparation (D–P) time
Si–XNi(Y).
However, the actual Ni and silica weight loadings remark-
is 4 h.
According to a previous paper,29 the 2Na/Si–XNi(Y) and
ably depend on the G-D–P time. Thus, after the G-D–P pro- 2Na/Si–20Ni(4)–DP catalysts were prepared by a impregnation
cedure, the actual loadings of Ni and silica, namely, NiA and method using the prepared Si–XNi(Y) and Si–20Ni(4)–DP cata-
SilicaA respectively, can be lower than the theoretical loadings lysts above as the support and calculated amount of aqueous
of Ni and silica, namely, NiT and SilicaT respectively. In this solution of sodium sulfate to provide the precursor salt, where
paper, the NiA and SilicaA values were determined by atomic the Na content was uniformly 2 wt% in catalysts.
absorption spectroscopy (AAS, see section 2.2). Moreover, the
differences between the NiT and NiA values, and between the
SilicaT and SilicaA values, were evaluated by the yields of
2.2. Catalysts characterization
Ni (YNi) and silica (YSilica) loadings, respectively, which refer also Fourier transform infrared (FT-IR) spectroscopy in KBr was
to the recovery rates of Ni and silica, respectively, after the recorded by utilizing a Varian 3100 spectrometer. Each spec-
G-D–P procedure. The calculations of NiT, NiA, SilicaT, SilicaA, trum was recorded as a result of the accumulation of 32 scans
YNi and YSilica values are expressed as follows:
at a resolution of 2 cm−1. X-ray diffraction (XRD) was per-
formed using the Bruker D8-Advance X-ray diffraction equip-
ment, under the following conditions: Cu target Kα ray (λ =
1.54187 Å); scanning voltage, 40 kV; scanning current, 40 mA;
scanning speed, 0.5 s, scanning step, 0.02°. N2-absorption/
desorption isotherm was carried out on a Quantachrome Auto-
sorb-1 instrument at liquid–N2 temperature. The BET method
was used to calculate the specific surface area, and the average
pore size was determined by the Barrett–Joyner–Halenda (BJH)
method. Field-emission scanning electron microscopy
(FE-SEM) was performed with a JSM 6700F scanning electron
microscope. Transmission electron microscopy (TEM) and
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) were performed with a Tecnai F20
electron microscope (Philips Analytical) with an EDAX-Genesis
type energy dispersive X-ray spectrometer (EDS) operated at an
acceleration voltage of 200 kV. Temperature-programmed oxi-
dation (O2-TPO) and temperature-programmed desorption of
H2 (H2-TPD) were performed using a Quantachrome Autosorb-1
WNi;T
WNi;T þ WSilica;T
NiTðwt%Þ ¼
ꢀ 100%
WSilica; T
WNi; T þ WSilica; T
SilicaTðwt%Þ ¼
ꢀ 100%
WNi; A
Wcatalyst; A
NiAðwt%Þ ¼
ꢀ 100%
WSilica; A
Wcatalyst; A
SilicaAðwt%Þ ¼
ꢀ 100%
NiA
NiT
YNið%Þ ¼
ꢀ 100%
SilicaA
SilicaT
YSilicað%Þ ¼
ꢀ 100%
where WNi,T refers to the weight of Ni in the solution of nickel instrument equipped with a thermal conductivity detector
nitrate used for the preparation of catalyst; WSilica,T is the (TCD). The reduction extent of nickel was estimated by
weight of silica in the colloidal silica solution employed in the O2-TPO, and the number of surface nickel atoms available was
preparation of the catalyst; WNi,A and WSilica,A are the actual assessed by H2-TPD.40 The dispersion degree of metallic nickel
weights of Ni and silica, respectively, determined by AAS, in was identified according to the formula for the calculation
the catalyst; and Wcatalyst stands for the weight of the catalyst reported previously,40 which assumed an adsorption stoichi-
used in the AAS determination.
ometry of one hydrogen atom per surface metallic nickel atom.
However, some of the colloidal silica solution was directly Flame atomic absorption spectrometry (AAS) performed by a
dried at 80 °C and calcined in air at 450 °C for 4 h to obtain Varian AA240 apparatus was utilized to determine the elemen-
the SiO2 sample. Moreover, the obtained SiO2 sample was uti- tal compositions of the catalyst. X-ray photoelectron spec-
lized for preparing the Si–20Ni(4)–DP catalyst by a convention- troscopy (XPS) was performed by a PHI Quantum 2000 XPS
al deposition–precipitation (D–P) approach. The SiO2 sample system with a monochromatic Al Kα source and a charge
obtained above (sieved to 100–120 mesh) were first dispersed neutralizer. The binding energy (B.E.) was referenced to the C
into a 7.5 wt% aqueous solution of urea with a liquid–solid 1s peak at 284.8 eV for the surface adventitious carbon.
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2.3. Dehydrogenation of 1,2-cyclohexanediol
3. Results
The reaction was performed by using a fixed-bed, flow-type
stainless reactor (33 cm × 1.5 cm × 0.8 cm), which was
mounted in a tubular electric heater. For a typical reaction,
first, ca. 0.5 g catalyst was filled in the middle of the reactor,
and the upper space of the catalyst bed in the reactor was
filled with inert quartz granulates (20–40 mesh). Before the
reaction, the temperature was increased to 450 °C in 60 min,
and then a flow of H2 was introduced from the upper of
reactor to activate the catalyst. After the H2 activation for 4 h,
the temperature was decreased to 320 °C. Then, the reactant, a
10 wt% 1,2-cyclohexanediol aqueous solution, was pumped
into the reactor at a constant liquid hourly space velocity
(LHSV) of 1.43 h−1, while the flow of H2 was kept to provide a
reduction environment and constant total pressure of 0.1 MPa.
The effluent from the bottom of reactor was passed through a
gas–liquid separator cooled by ice-water.
3.1. Gel-deposition–precipitation (G-D–P) process
Fig. 1a shows the changes in the actual Ni and silica loadings
(NiA and SilicaA, respectively) and yields of Ni and silica load-
ings (YNi and YSilica, respectively) over the Si–XNi(4) catalysts,
as a function of theoretical Ni loading (NiT). These catalysts
are prepared uniformly with the same G-D–P time = 4 h. At the
NiT value below 20 wt%, the YNi value retains a high value
above 99.9%, which indicates that almost all of the Ni species
in the solution have been deposited at relatively low NiT
The catalytic rate for the dehydrogenation of 1,2-cyclohexa-
nediol was measured under reaction conditions as follows:
catalyst weight was in the range of 10–20 mg, total pressure
was 0.1 MPa, reduction temperature was 450 °C, reaction temp-
eratures was 250 °C, and LHSV was 160 h−1. Quantities of cata-
lyst filled in the reactor referred to the same amount of surface
metallic nickel (= 9.9 × 1018 metallic nickel atoms). The reac-
tion conversion was controlled in the range of 4%–13% to
ensure that the experimental results obtained are in the region
of intrinsic kinetics. Moreover, transport artifacts were elimi-
nated by experimental method, leading to undetectable vari-
ations in catalytic reaction rates or selectivities.
The qualitative and quantitative analyses of the liquid pro-
ducts mixture were carried out with a Varian Saturn 2200/CP
3800 GC/MS spectrometer, equipped with a flame ionization
detector (FID) and two CP-Wax 52CB fused silica capillary
columns (15 m × 0.32 mm). It was identified that,29 in addition
to the aimed product catechol, the byproducts cyclopentanone,
cyclopentanol, phenol, cyclohexanone, cyclohexanol, o-
hydroxyl-cyclohexanone, and, in some cases, the unconverted
1,2-cyclohexanediol, were also present in the liquid products
mixture. The quantitative analysis of the gas products mixture
was carried out with a PE Clarus 500 GC, which was equipped
with a thermal conductivity detector (TCD) and a HAYESEP DB
100/120 packed column (30′ × 1/8″ × 0.85″ SS). The conversion
of 1,2-cyclohexanediol (χ) and selectivity towards product (Si)
were defined as follows:
X0 ꢁ X1
χ ðmol%Þ ¼
ꢀ 100%
X0
Yi
Si ðmol%Þ ¼
ꢀ 100%
n
P
Yi
Fig. 1 (a) Changes of actual Ni and silica loadings (NiA and SilicaA,
respectively) and yields of Ni and silica loadings (YNi and YSilica, respect-
ively) over Si–XNi(4) catalysts as a function of theoretical Ni loading
i¼1
where X0 and X1 refer to the initial and final moles of 1,2-cyclo-
hexanediol, respectively; Yi indicated the mole of component
i in the products mixture.
(NiT). (b) Changes of yields of Ni and silica loadings (YNi and YSilica
,
respectively) and pH value over Si–20Ni(Y) catalysts prepared at various
G-D–P time Y (Y = 0.17–48 h).
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values. However, it is found that, when the NiT value is in of stage (1) to 0.4 h of G-D–P, the pH value decreases quickly;
excess of 20 wt%, the YNi value decreases steeply to lower (3) then, a plain stage (pH value = ca. 5) is formed from 0.4 to
values. This result suggests the incomplete deposition of the 2 h of G-D–P; (4) from 2 to 16 h of G-D–P, the pH value
nickel species in the solution at the relatively high NiT values. increases at a relatively mild speed from ca. 5 to 8; (5) the pH
Comparatively, the actual Ni loading (NiA) displays a mono- value increases abruptly from ca. 8 to 10, and then fluctuates
tonic increase (from 5 to 33 wt%) with increasing the NiT around this high value, forming a plain stage at pH value = ca.
value. The result that the yield of Ni loading (YNi) decreases 10. Undoubtedly, these variations in pH values are associated
with the increase of the NiT value at the relatively high NiT with some specific chemical reactions, e.g., the precipitation of
values can be explained by the fact that the higher the nickel Ni and some other reactions during aging of the catalyst,
concentration in solution, the more OH− ions are required for which will be discussed later as a key.
the deposition–precipitation of Ni. However, during the depo-
3.2. Catalysts characterizations
sition–precipitation of Ni, the generation of OH− ions is
limited by the pyrolysis rate of urea.41 Thus, at the relatively Fig. 2 shows the FT-IR spectra of SiO2, uncalcined Si–XNi(4),
high NiT values, the nickel species in the solution cannot be Si–XNi(4), and Si–20Ni(4)–DP catalysts. For the absorption
totally precipitated after only 4 h G-D–P. Moreover, although bands of pristine SiO2 (Curve 2 a-i), the bands at ca. 478, 801,
the actual silica loading (SilicaA) decreases with increasing the 964, 1118, 1280 cm−1 can be associated with the rocking (R)
NiT value, the YSilica value always demonstrates a high value and bending (B) vibrational modes, the four-fold strained
above 98.5%. This result indicates that almost all the colloidal structural ring, as well as the symmetric stretching (AS1) and
silica solutions have been gelled after 4 h of G-D–P.
coupled disorder induced (AS2) modes, respectively;42,43 and
The changes of yields of Ni and silica loadings (YNi and the absorptions at ca. 3445, 3652, and 3739 cm−1 are assigned
YSilica, respectively) of Si–20Ni(Y) catalysts and pH value as a to the H-bonded Si–OH in chain (O–H stretching), stretching
function of G-D–P time are shown in Fig. 1b. These catalysts mode of OH groups expect for isolated and terminal OH, and
are prepared by employing the same nickel concentration in isolated surface silanols (O–H stretching), respectively.42 Both
solutions, which corresponds to a theoretical Ni loading (NiT) nickel hydroxide and nickel phyllosilicate can be formed by
= 20 wt%. One can see that, with increasing the G-D–P time, basification of the nickel salt solution.24 For uncalcined Si–
the YNi and YSilica values first show a steep increase, attaining 10Ni(4) (Curve 2 a-ii), three bands at 666, 1058, and 3640 cm−1
99.9% and 99.5% at the G-D–P time equal to 3 and 2 h, are present, which are related to the tetrahedral Si–O mode,
respectively, and then maintaining these high values when the Si–O stretching, and νOH vibration, respectively, of 1 : 1 nickel
G-D–P time is above these time values. Thus, it can be con- phyllosilicate (Si2Ni3O5(OH)4, also called Ni–lizardite or
cluded that, after 3 h of G-D–P, almost all the colloidal silica nepouite).24,44,45 These bands are present in all of the un-
solution and Ni species in the solution have been gelled/and calcined Si–XNi(4) catalysts, and increase in strength with
deposited–precipitated, and beyond this time, the catalyst is increasing the Ni loading (Curve 2 a-ii to iv, in order).
aging. Nevertheless, it is found that, the gelation of colloidal However, the band at ca. 637 cm−1, a characteristic band of
silica solution is faster, obviously, than the deposition–precipi- nickel hydroxide,24 only can been seen in Curve 2 a-iii and iv.
tation of Ni. For instance, 56% and 91.8% of colloidal silica This result shows that, the presence of the Ni(II) phase in the
solutions have been gelled at 0.17 and 0.33 h of G-D–P, respect- uncalcined Si–XNi(4) catalysts is mainly 1 : 1 nickel phyllosili-
ively, when only 29.7% and 33.8% of Ni have been deposited– cate at relatively low Ni loading, and a mixture of 1 : 1 nickel
precipitated, respectively. This result indicates that, during the phyllosilicate and nickel hydroxide at relatively high Ni
G-D–P procedure, generally, the colloidal silica solution is loading. Moreover, the wide band at ca. 3313 cm−1 indicates
gelled firstly; then, the nickel is deposited–precipitated gradu- the hydrogen-bonded OH bonds of water.46,47 Note that the
ally over the surface of gelled silica support.
band at ca. 900 cm−1 is always seen over the ill-crystallized
Moreover, the variation of the pH value depended on the nickel phyllosilicate.24 Furthermore, it should be noticed that,
variation of OH− ions in the solution as a function of G-D–P the band at ca. 3640 cm−1 at high Ni loading (Curve 2 a-iii and
time is also present in Fig. 1b. The OH− ions in the solution iv) is probably an integrated absorption, which is ascribed to
are composed of two groups. One is the primary presence of the surface hydroxyls of the nickel hydroxide and nickel phyllo-
OH− ions owing to the fact that sodium hydroxide is employed silicate mixture, because both of them have a similar absorp-
as stabilizer in the colloidal silica solution, which are defined tion of νOH vibration in this absorption interval.24
as free OH− ions. The other group is generated OH− ions due
Compared to uncalcined Si–XNi(4), for Si–XNi(4) (see
to the hydrolysis of urea during the G-D–P. These OH− ions Fig. 2b); (1) the disappearance of the band at ca. 637 cm−1
can be consumed during the deposition–precipitation of Ni indicates the decomposition of nickel hydroxide; (2) the band
over the silica support. In this figure, the variation of the pH at ca. 3640 cm−1 disappears, and a new band emerges at
value is divided into five stages: (1) the pH value decreases ca. 3626 cm−1, which is a characteristic band of 2 : 1 nickel
instantly from ca. 10 to 8 after the addition of nickel nitrate phyllosilicate (Si4Ni3O10(OH)2, also called Ni–talc or willem-
into the mixture of urea and colloidal silica solution, and it is seite);24 (3) the bands at ca. 1058 and 666 cm−1 shift to the
related to the consumption of free OH− ions in the colloidal higher wavenumbers at ca. 1034 and 671 cm−1, respectively,
silica solution with the precipitation of Ni; (2) from the ending and the latter two wavenumbers are assigned to 2 : 1 nickel
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bands at ca. 666 and 3626 cm−1 relative to that of Si–20Ni(4).
This result suggests that, more nickel phyllosilicates are
present over the Si–20Ni(4)–DP catalyst prepared by the con-
ventional D–P method, than the Si–20Ni(4) catalyst prepared
by the G-D–P method depicted in this work.
Fig. 3 shows the FT-IR spectra of uncalcined Si–20Ni(Y) and
Si–20Ni(Y) catalysts. For the uncalcined Si–20Ni(Y) catalysts
(see Fig. 3a), at a low G-D–P time below 2 h, the absorption
bands at ca. 639 and 3640 cm−1 are attributed to nickel hydrox-
ide.24 With increasing the G-D–P time up to 4 h, the band at
ca. 666 cm−1, characteristic of 1 : 1 nickel phyllosilicate,24
emerges and increases in strength with increasing the G-D–P
time further. Comparatively, first, the band at ca. 639 cm−1
Fig. 2 (a) FT-IR spectra of (i) SiO2 and uncalcined Si–XNi(4) catalysts,
X = (ii) 10, (iii) 20 and (iv) 50, respectively. (b) FT-IR spectra of Si–XNi(4)
catalysts, X = (ii’) 10, (iii’) 20 and (iv’) 50, respectively; and (v’) Si–
20Ni(4)–DP.
phyllosilicate;24 (4) the band at ca. 900 cm−1 disappears,
which demonstrates that 2 : 1 nickel phyllosilicates in the Si–
XNi(4) catalyst have relatively good crystallinity.24 It should be
noted that, no obvious bands related to nickel oxides, which
should be formed with the decomposition of nickel hydrox-
ides, are observed in Si–XNi(4). This finding can be due to the
fact that, the bands of nickel oxides overlap with those of SiO2,
which makes the assignment difficult. According to the results
above, during the calcination, nickel hydroxides are decom-
posed; moreover, a 1 : 1 nickel phyllosilicate transforms into
2 : 1 nickel phyllosilicate. Moreover, the FT-IR spectrum of Si–
Fig. 3 (a) FT-IR spectra of uncalcined Si–20Ni(Y) catalysts, Y = (i) 1, (ii)
2, (iii) 4, (iv) 8, (v) 12 and (vi) 24, respectively. (b) FT-IR spectra of Si–20Ni
20Ni(4)–DP is present in Fig. 2b–v′, which shows the stronger (Y) catalysts, Y = (i’) 1, (ii’) 2, (iii’) 4, (iv’) 8, (v’) 12 and (vi’) 24, respectively.
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increases in strength with increasing the G-D–P time, and loading, the presence of the Ni(II) phase is mainly nickel phyl-
attains its maximum at 4 h of G-D–P, followed by a decrease losilicate, whereas a mixture of nickel phyllosilicate and nickel
with the further increased G-D–P time. This result indicates hydroxide exists at relatively high Ni loading. This conclusion
that, at a relatively low G-D–P time, nickel hydroxides exhibit is in agreement with the IR results above.
in the uncalcined Si–20Ni(Y) catalysts as the main form of the
Ni(II) phase; however, they transform gradually into 1 : 1 nickel vi and vii, in order), the intensities of the diffaction peaks
phyllosilicates with the increase in G-D–P time. related to 2 : 1 nickel phyllosilicate increase obviously. This
For Si–20Ni(Y), with increasing the G-D–P time (Curve v, ii,
After the calcination procedure (see Fig. 3b), the disap- finding can be due to the transformation from nickel hydrox-
peared absorption at ca. 637 cm−1 suggests the decomposition ide into nickel phyllosilicate in the aging stage, also in line
of nickel hydroxide, as discussed above (see Fig. 2b). Moreover, with our FT-IR analyses in Fig. 3. Moreover, the diffusion prop-
the absorption bands at 3640 and 1061 cm−1 shift to lower erty of the diffraction reflections of nickel phyllosilicate and
wavenumbers at 3626 and 1056 cm−1, respectively, indicating NiO suggests that, the nickel species have significantly small
the transformation of 1 : 1 nickel phyllosilicate into 2 : 1 nickel particle sizes, and also, are highly dispersed over the silica
phyllosilicate; moreover, the band at ca. 899 cm−1 disappears, support.
which is associated with the improvement of the crystallinity
of the nickel phyllosilicate.24
Furthermore, comparing the XRD pattern of Si–20Ni(4)
(Curve iii) with that of Si–20Ni(4)–DP (Curve viii), the diffrac-
Fig. 4 shows the XRD patterns of SiO2, Si–XNi(4), Si–20Ni(Y), tion peaks associated with nickel phyllosilicate in the former
and Si–20Ni(4)–DP catalysts. For SiO2, only a broad and catalyst is obviously weaker than those in the latter one. This
diffuse diffraction peak at 2θ = ca. 21.7° is seen. This peak is finding shows that more nickel phyllosilicates are present over
related to the typical amorphous character of the SiO2 vitreous Si–20Ni(4)–DP than over Si–20Ni(4), consistent with the above
matrix.46,48 After the loading of Ni, as can be observed in curve IR results (see Fig. 2b).
ii to iv that, another set of dispersed reflections at 2θ = ca.
Fig. 5 shows the N2-adsorption/desorption isotherms of
28.6°, 34.5°, 60.7°, in addition to the broad diffraction peak of SiO2, Si–XNi(4), Si–20Ni(Y) and Si–20Ni(4)–DP catalysts. Firstly,
silica, whose strength increase with the Ni loading, are also all of the isotherms exhibit obvious hysteresis loops at P/Po >
observed in XRD. This phenomenon indicates the generation 0.4, demonstrating the presence of mesopores.49 The begin-
of 2 : 1 nickel phyllosilicate, combined with the IR analyses ning of the desorption branch at P/Po = ca. 0.4 is intrinsic to
above.24,44 Moreover, the reflections with respect to NiO at 2θ = the use of nitrogen as a result of the instability of the meniscus
ca. 37.1°, 43.2°, 62.8° can also be indistinctly identified, when of the condensed nitrogen within the capillary pores.50–52 In
the Ni loading is above 20 wt% (Curve iv). NiO can be gener- the isotherm of pristine SiO2, a relatively unobvious hysteresis
ated by the decomposition of nickel hydroxide upon calcina- loop belonging to type H3 or H4 is observed, suggesting
tion. These results above indicate that, at relatively low Ni complex and irregular shapes, e.g., slit-like pores with uniform
Fig. 5 N2-adsorption/desorption isotherms of (i) SiO2; Si–XNi(4), X = (ii)
Fig. 4 XRD patterns of (i) SiO2; Si–XNi(4), X = (ii) 10, (iii) 20 and (iv) 50,
10, (iii) 20, and (iv) 50, respectively, Si–20Ni(Y), Y = (v) 2, (vi) 12 and (vii)
respectively, Si–20Ni(Y), Y = (v) 2, (vi) 12 and (vii) 24, respectively, and
24, respectively, and (viii) Si–20Ni(4)–DP.
(viii) Si–20Ni(4)–DP.
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size.53 Frequently, the H3 and H4 type loops are formed by the ∼2.9 nm, respectively). The total pore volume increases firstly
aggregation or agglomeration of plates or edged particles like from 0.449 ml g−1 for Si–20Ni(2) to 1.003 ml g−1 for Si–20Ni(12),
cubes.54 For Si–XNi(4) catalysts, the isotherm of Si–10Ni(4) and then, it decreases slightly to 0.920 ml g−1 for Si–20Ni(24).
(Curve ii) resembles the type H2 hysteresis loop, probably These results above can be due to the transformation from
associated with the ink bottle-shaped pore structure. However, nickel hydroxide to nickel phyllosilicate during the aging
when the loading of Ni increases, the hysteresis loop first elev- stage. Moreover, Si–20Ni(4)–DP presents the bigger specific
ates in magnitude (Si–20Ni(4), Curve iii) because of the surface area, average pore size, and total pore volume, relative
increase in the formation of mesoporosity by Ni loading, and to the SiO2 support, due to the loading of Ni. Comparing the
then, it is followed by an obvious decrease in magnitude (Si– texture properties of Si–20Ni(4)–DP with those of Si–20Ni(4),
50Ni(4), curve iv), probably due to the aggregation or agglo- the former catalyst exhibits the bigger average pore size and
meration of the nickel species particles at high Ni loading.
total pore volume, but lower specific surface area, than the
For the Si–20Ni(Y) catalysts, with increasing the G-D–P latter one.
time, the hysteresis loops of the isotherms change from H2-
Fig. 6 shows the bright-field TEM images of Si–20Ni(Y) cata-
like type at the G-D–P time below 4 h (Curve v and iii) to H1- lysts prepared at G-D–P time equal to 4 and 24 h, to study the
like type at the D–P-G time in excess of 4 h (Curve vi and vii). change of morphology of the as-prepared catalysts. Generally,
The H1 type hysteresis loop is associated with a typical cylind- nickel phyllosilicate is prepared by the basification of nickel
rical-shape pore structure,54 which is formed by aggregates salt onto
a silica surface under a hydrothermal con-
(consolidated) or agglomerates (unconsolidated) of spheroidal dition.16,24,41 In a basic aqueous solution, nickel salt forms the
particles. Moreover, the isotherm of Si–20Ni(4)–DP is present nickel hydroxide species, which subsequently react with silicic
in curve viii, exhibiting the similar H4 type hysteresis loop to acid to generate nickel phyllosilicate through a heteroconden-
SiO2, although its hysteresis loop is obviously bigger than that sation/polymerization reaction.41 Kim et al. reported the
of SiO2 in magnitude.
Pd@SiO2-Niphy nanostructure prepared via this formation
The texture properties of these catalysts, containing specific mechanism,55 over which needle-like, branched nickel phyllo-
surface area, total pore volume and average pore size, are sum- silicates were formed by the dissolution of a nickel salt and
marized in Table 1. For the Si–XNi(4) catalysts, with the the simultaneous condensation with the SiO2 shell of the
increase of Ni loading, the total pore volume and average pore core–shell Pd@SiO2 nanoparticle. Park et al. prepared a nickel
size decrease monotonically (from 0.579 ml g−1 and 6.3 nm for phyllosilicate/SiO2 nanostructure by the hydrothermal treat-
Si–10Ni(4) to 0.267 ml g−1 and 2.5 nm for Si–50Ni(4), respect- ment of a core–shell Ni@SiO2 nanoparticle.16 The generated
ively). This result can be caused by the probable partial occu- nickel phyllosilicate layer also exhibited a needlelike branched
pation of silica channels by fine particles of Ni species. morphology, and was anchored on the outside surface of the
However, the specific surface area increases first from 250 m2 nanostructure.16 In the present work, as discussed in the IR
g−1 for Si–10Ni(4), to 277 m2 g−1 for Si–20Ni(4), due to the and XRD analyses above, over the Si–20Ni(Y) catalysts, nickel
generation of mesoporosity with the doping of Ni, and then, it phyllosilicates increase in concentration with increasing the
is followed by a decrease (to 259 m2 g−1 for Si–50Ni(4)), prob- G-D–P time. In the present figure, one can see that, some
ably ascribed to the aggregation or agglomeration of the Ni nano-sized spherical particles with needle-like branches exist
species particles at high Ni loading. For the Si–20Ni(Y) cata- on both the images of Si–20Ni(4) and that of Si–20Ni(24),
lysts, with the increase of G-D–P time, the specific surface area which can be distinguished due to the high density of nickel
and average pore size increase uniformly (to ∼298 m2 g−1 and phyllosilicate compared to that of silica.55 These branches are
ca. 5–30 nm in length and 1–2 nm in thickness. Moreover, it is
found that, more needle-like branches are observed clearly
over Si–20Ni(24) than over Si–20Ni(4). In addition, these gener-
ated branched nickel phyllosilicates are mainly distributed
Table 1 Textural properties, dispersions of metallic nickel, and turnover
frequencies of various catalysts
a
over the edges of the nanoparticles (see the red ring in Fig. 6 ii-2).
SBET
Vpore,t
Dave
(nm)
Nidis
(%)
TOFini
Catalyst
(m2 g−1
)
(cm3 g−1
)
(h−1
)
Thus, we proposed that the generated nickel phyllosili-
cates are mainly located on the external surface of the nano-
particles. The XPS results support this assumption (see ESI
Fig. 1S†). With increasing the G-D–P time, the mean diameter
of the nanoparticles increases from ca. 25 to ca. 35 nm (see
ESI Fig. 2S†).
Fig. 7 shows the HAADF-STEM images and cross-sectional
compositional line profiles of Si–20Ni(Y) catalysts, choosing
the G-D–P time equal to 4 and 24 h as those in Fig. 6 above, to
unambiguously confirm the morphologies and distributions
of Si and Ni atoms in the particles of silica–nickel nanoparti-
cles. In Fig. 7 i-1 and 7 ii-1, the bright floccules over the
lower contrast spherical particles can be related to the nickel
SiO2
139
250
277
259
241
298
298
266
0.240
0.579
0.484
0.267
0.449
1.003
0.920
0.671
5.7
6.3
2.6
2.5
2.0
2.6
2.9
2.8
—
—
—
Si–10Ni(4)
Si–20Ni(4)
Si–50Ni(4)
Si–20Ni(2)
Si–20Ni(12)
Si–20Ni(24)
Si–20Ni(4)–DP
20.7
10.7
7.0
11.0
20.9
26.6
6.5
0.22
—
0.23
0.37
0.45
0.14
Notes: SBET: specific surface area; Vpore,t: total pore volume; Dave
:
average pore size; Nidis: Ni dispersion measured by H2 chemisorption;
TOFini: initial TOF for the dehydrogenation of 1,2-cyclohexanediol
calculated at 250 °C of reaction temperature and 160 h−1 of liquid
hourly space velocity (LHSV). a 2 wt% of sodium sulfate is added as
promoter.
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Fig. 6 Bright-field TEM images of Si–20Ni(Y) catalysts with different magnification (1 and 2), Y = (i) 4 and (ii) 24, respectively; typical embracement
effect of nickel phyllosilicates phase around the nanoparticles is observed (for example, see the red rings).
Fig. 7 (1) HAADF-STEM micrographs and (2) cross-sectional compositional line profiles drawn along the row of the bright atomic columns on the
nanoparticles of Si–20Ni(Y) catalysts, Y = (i) 4 and (ii) 24, respectively.
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phyllosilicate phase, which refer to the needle-like thin that, two maxima in the Si signals are discernible at ca. 25 and
branches observed in the bright-field TEM images (Fig. 6). 41 nm, which can be attributed to the two Si-rich cores of the
From the results obtained, more bright floccules are observed twin-particle chosen, and three smaller maxima in the Ni
over Si–20Ni(24) (Fig. 7 ii-1) than over Si–20Ni(4) (Fig. 7 i-1) signals are observed at ca. 20, 34 and 44 nm, which can be
due to the increased formation of nickel phyllosilicates with assigned to the three Ni-rich shells of the twin-particle. More-
increasing the G-D–P time. This result is in line with the over, this demonstrates that the first particle shows a higher
FT-IR, XRD, and TEM analyses above. First, a simply spherical Si/Ni atomic signal ratio than the second one. This result can
particle with few flocculence-like nickel phyllosilicates in Si– be explained by the fact that, the first particle possesses a
20Ni(4) (Fig. 7 i-1) is chosen to analyze the distributions of Si smaller surface/volume ratio than the second one because of
and Ni atoms by the compositional line profile (Fig. 7 i-2). its bigger particle size; therefore, it is probable that a thinner
This nano-sized silica–nickel composite shows a diameter of layer of Ni is deposited over the former particle than the latter
ca. 23 nm. It is interesting to note that, this as-obtained nano- one. Moreover, it is found that, these core–shell-like silica@
particle is not separated simply as a pure silica core and a pure nickel species nanoparticles are easily deformed, and some
nickel species shell, but as a Si-rich core and a Ni-rich shell. dark dots are formed over the surface of the nanoparticles
Firstly, the existence of the nickel phyllosilicate compound, when the electron beam directly irradiates for a long time
which includes both silicon and nickel elements, obscures the during the TEM and STEM measurements (see ESI Fig. 3S†). It
boundary of the Si-core and Ni-shell. This fact impedes the for- can be related to the formation of metallic nickel nanoparti-
mation of the pure silica core and nickel species shell. More- cles due to the reduction of the Ni species by high energy
over, during the G-D–P process, the gelation of the colloidal electrons.16
silica solution is remarkably quicker than the deposition–
Fig. 8 shows the FE-SEM images of Si–20Ni(4) and reduced
precipitation of nickel. Consequently, during the deposition– Si–20Ni(4) catalysts. The Si–20Ni(4) catalyst (Fig. 8a) shows the
precipitation of nickel, some nickel cations probably dip into the spherical shape particles, exhibiting a narrow size distribution
pores of the silica particles and are subsequently precipitated of 20–40 nm with an average diameter of ca. 30 nm. This
in these pores. This also impedes the formation of the pure observation is consistent with the results obtained in the TEM
silica core and nickel species shell. However, in Fig. 7 i-2, the and HAADF-STEM images. These spherical particles accumu-
layer at a depth of ca. 5 nm from the outermost layer can be late together, generating the obvious mesoporosity. As dis-
assigned to a Ni-rich layer, whereas the distribution of Si is cussed above, these mesoporous structures are characterized
more diffused, proximately in the depths of ca. 5–11.5 nm in the N2 absorption/desorption isotherms as H1/or H2 type
from the outmost layer. Within the nanoparticle, the Ni-rich hysteresis loops, which are related to the cylindrical or ink
layer is composed of nickel oxides formed by the decompo- bottle-shaped pore structures. After reduction (Fig. 8b), the
sition of nickel hydroxides, and nickel phyllosilicates gener- spherical particle morphology of Si–20Ni(4) is maintained to a
ated mainly during the aging procedure; and the Si-rich layer large extent. This suggests that no obvious aggregation and
exists in the form of silica and also nickel phyllosilicates. sintering of nickel metal particles are present after the
Nevertheless, this nickel–silica composite is defined as a kind reduction process.
of core–shell-like silica@nickel species nanoparticle.
Fig. 9 shows the H2-TPR profiles of XNi–Si(4), 20Ni–Si(Y),
Moreover, a twin-particle (with diameters equal to ca. 22 and 20Ni–Si(4)–DP catalysts. In the 10Ni–Si(4) catalyst, a much
and 13 nm, respectively) surrounded by obvious flocculent wider range from 300 to 780 °C with an asymmetric peak at
nickel phyllosilicates in Si–20Ni(24) (Fig. 7 ii-1) is selected, 618 °C is observed, which consists of two components with
and its distributions of Si and Ni atoms are also analyzed via maxima at 475 and 618 °C, respectively. According to previous
the compositional line profile (Fig. 7 ii-2). The result shows papers,24,56 these two peaks are attributed to the reduction of
Fig. 8 FE-SEM images of (a) Si–20Ni(4) and (b) reduced Si–20Ni(4) catalysts; reduced temperature = 450 °C (4 h).
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characterized by a single TPR peak at 300–400 °C.56 Hence, the
small peak at 334 °C can be attributed to the reduction of free
nickel oxide. The peak at 494 °C can be deconvoluted into two
peaks at 419 and 513 °C, which may be related to the reduction
of nickel oxide and nickel phyllosilicate, respectively. More-
over, one can obtain that, the area ratio between nickel phyllo-
silicate and nickel oxide over Si–20Ni(4)–DP is clearly higher
than that over Si–20Ni(4). This result indicates that the con-
centration of nickel phyllosilicates in the former catalyst is
higher than the latter one, which is in line with the FT-IR and
XRD analyses. However, interestingly, the main reduction peak
of Si–20Ni(4)–DP (494 °C) is clearly lower than that of Si–20Ni(4)
(574 °C), which may be caused by a weaker interaction
between the nickel species and the silica support over the
former catalyst than the latter one. These results above indi-
cate that, Si–20Ni(4)–DP probably possesses a lower dispersion
of nickel, relative to Si–20Ni(4), because of the existence of free
nickel oxides and the weak interactions of nickel species and
silica support.
3.3. Discussion on the formation of the core–shell-like
silica@nickel species nanoparticle
Fig. 9 H2-TPR profiles of various catalysts: Si–XNi(4), X = (i) 10, (ii) 20,
and (iii) 50, respectively; 20Ni–Si(Y), Y = (iv) 2, (v) 12 and (vi) 24, respect-
ively; and (vii) Si–20Ni(4)–DP.
As is well known, silica exhibits its zero charge point at pH
value = 3. In the preparation of core–shell-like silica@nickel
species nanoparticles, a colloidal silica solution (Na2O type)
supported nickel oxide (475 °C) and that of nickel phyllosili- with a high pH value (ca. 10) is used, well above the zero
cate (618 °C), respectively. In the present work, combining the charge point. Therefore, the silica surface is negatively
above TEM (Fig. 6) and HAADF-STEM (Fig. 7) analyses, we
charged, facilitating the adsorption of nickel cations. Water-
propose that the peak at 475 °C should be assigned, more borne colloids own a surrounding “cloud” of water and electro-
accurately, to the nickel oxides in the Ni-rich shells of silica@ lytes, which is called the “double-layer,” including a layer of
nickel species nanoparticles, and the peak at 618 °C to the
bound counter ions called the Stern layer, and a layer of
nickel phyllosilicates distributed mainly over the periphery of diffuse cations and anions noted as the Guoy layer.57 There are
the silica@nickel species nanoparticles. With increasing Ni two types of water present in a colloidal silica solution called
loading (Curve i to iii, in order), the main reduction peak
as “bound” water and “free” water. The former is bounded by
shifts to the lower temperature from 618 °C for 10Ni–Si(4) an electrolyte near the silica surface through hydrogen
(Curve i) to 514 °C for 50Ni–Si(4) (Curve iii). This finding can bonding, inhibiting the gelation of the colloidal silica solu-
tion.57 In a basic aqueous solution, nickel hydroxide is the
be ascribed to the increased concentration of nickel oxides
within the silica@nickel species nanoparticles with increasing most stable compound among the nickel aqua species when
the Ni loading, in line with the XRD results above.
the temperature ranges from 25 to 100 °C,16 although nickel
However, for the 20Ni–Si(Y) catalysts, with increasing G-D–P
phyllosilicate can be also generated via precipitation of nickel
time (Curve iv, ii, v, vi, in order), the main reduction peak onto the surface of the silica by basification of the nickel(II)
shifts obviously to the higher temperature, from 534 °C for solution.24 The colloidal silica solution then loses its stability,
20Ni–Si(2) to 616 °C for 20Ni–Si(24). As confirmed by FT-IR
and XRD analyses, nickel phyllosilicates increase in amount aggregation of colloidal particles. During the G-D–P procedure
which is controlled in terms of electrolyte-pH, as a result of
with increasing the G-D–P time. Moreover, it is known that24
supported nickel phyllosilicate is more difficult to reduce than
a
in the present work, nickel nitrates react first with the stabil-
izer, sodium hydroxide in the solution, generating nickel
unsupported ones. Furthermore, the nickel species are hydroxides. This fact makes the pH value of solution decrease
reduced at a higher temperature due to the interaction with largely, and thus promotes the aggregation of colloidal silica
the support, as often seen for supported oxides catalysts.24 In
particles. Moreover, the high level of electrolytes, herein the
this work, for the 20Ni–Si(Y) catalysts, the increased reduction nickel nitrates, leads to the increase in the rate of coagulation
temperature with increasing G-D–P time may be due to both of the colloidal silica particles as a result of the decrease in
the increase in the formation of nickel phyllosilicates and the
thickness of the “bound” water layer, resulting in closer
enhancement of the interaction between the nickel species approaching particles.57
and silica support.
Interestingly, in this work, we can accurately relate these
above assumptions to the changes in actual yield of silica
Comparatively, the 20Ni–Si(4)–DP catalyst exhibits a peak at
494 °C with a shoulder peak at 334 °C. Typically, pure NiO is loading and pH value versus the G-D–P time (see Fig. 1b).
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At the beginning of G-D–P process (stage 1), the pH value in the TEM images and floccules in the STEM images are
decreases abruptly from ca. 10 to 8. This phenomenon can be associated with the nickel phyllosilicates phase. Nickel phyllo-
due to the consumption of the free stabilizer sodium hydrox- silicates are distributed over the outside surface of spherical
ides in the solution by nickel nitrates. From stage 1 to 0.4 h of silica@nickel species nanoparticles, and subsequently
G-D–P (stage 2), the pH value decreases at a quick rate. Hence, embrace them (see the red rings in Fig. 6). Furthermore, the
the stabilization of the colloidal silica solution is destroyed nickel phyllosilicates phase around the silica@nickel species
largely, and the colloidal silica particles aggregate rapidly. One nanoparticles probably brings an effect of segregation among
can observe that, the actual yield of gelation of silica increases these nanoparticles, thus benefitting the stability and dis-
steeply in this period, and above 96% of colloidal silica solu- persion of nanoparticles. We can see that, in Fig. 8, the mor-
tions are gelled after 0.4 h of G-D–P.
phology of the Si–20Ni(4) nanoparticle is retained to a large
The [Ni(OH2)6]2+ species exists in a nickel nitrate aqueous extent after the reduction procedure.
solution with a stronger acidity (pK1 = 6.5–10.2, I > 0) than
We can also associate the experimental results above with
silicic acid.58,59 In previous literature,41,51 the nature of the the changes of actual yield of Ni loading and pH value versus
Ni(II) species in silica and siliceous materials supported nickel the G-D–P time (see Fig. 1b). One can see that, a plain stage is
prepared by the deposition–precipitation (D–P) approach have formed from 0.4 to 2 h of G-D–P when the pH value is equal to
been investigated systematically. During the D–P of Ni, there is ca. 5 (stage 3); moreover, the yield of actual Ni loading
competition between the formation of nickel hydroxide and increases steeply. The formation of the plain stage can be
that of nickel phyllosilicate.41 Nickel hydroxide is formed caused by the formation of a dynamic equilibrium between
through an infinite olation-type condensation of [Ni- the generation of hydroxide ions by the hydrolysis of urea, and
(OH)2(OH2)4]0 precursors.24,41,51 Comparatively, nickel phyllo- the consumption of hydroxide ions for the formation of nickel
silicate can be generated via two different mechanisms24,41
:
hydroxides and nickel phyllosilicates. At 2 h of G-D–P, an
One is the deprotonation of aquanickel cations by hydroxide actual yield of Ni loading = ca. 92% is obtained, and after that,
ions, followed by a heterocondensation and polymerization the pH value begins to increase at a relatively mild speed from
reaction with the silicic species in aqueous solution; the other ca. 5 to 8 (stage 4). It is suggested that, this process probably
is a Ni(OH)2 depolymerization/Ni–O–Si heterocondensation/ corresponds to the transformation from nickel hydroxides to
polymerization reaction. In both mechanisms of the gen- nickel phyllosilicates with the depolymerization of nickel
eration of nickel phyllosilicate, a prerequisite that should be hydroxides and dissolution of the silica framework,24 which
noted: the silicic species in aqueous solution must be mono- consumes some hydroxides ions. In addition, at the stage 4,
meric before the formation of nickel phyllosilicate. Thus, in the generation of hydroxide ions seem quicker than its con-
the preparation of silica supported nickel, the generation sumption; therefore, the pH value increases mildly. After ca.
of nickel hydroxide is kinetically favored than that of nickel 20 h of G-D–P, the pH value increases abruptly from ca. 8 to
phyllosilicate, especially at high Ni loading.24,41 It is due to the 10, and then fluctuates around these high values (stage 5).
restriction of the amount of monomeric silicic species in This consequence can be explained by the fact that, at this
aqueous solution, which is limited to the dissolution of the moment, almost all the nickel hydroxides have been trans-
silica framework.41 However, at the aging stage, nickel hydrox- formed into nickel phyllosilicates, or that the generated nickel
ide can transform into nickel phyllosilicate via the second phyllosilicates embrace the nanoparticles, and therefore prohi-
mechanism with the gradual dissolution of the silica bit the further transformation of nickel hydroxides to nickel
framework.
phyllosilicates. Moreover, the mean diameter of silica@nickel
In this work, we provide experimental evidence for the species nanoparticles detected by FE-SEM also changes with
assumptions depicted above. For the Si–XNi(4) catalysts (see the G-D–P time, which can also be well explained (see ESI S2†).
Fig. 2 and 4), at relatively low Ni loading, the Ni(II) phase is
Based on the discussion above, the formation mechanism
mainly nickel phyllosilicates, and then with increasing Ni of core–shell-like silica@nickel species nanoparticles is pro-
loading, the ratio of nickel hydroxides/nickel phyllosilicates posed in Scheme 1, and it is depicted as follows. During the
increases, due to the limitation of the dissolution of the silica G-D–P procedure, the colloidal silica solution is gelled quickly,
framework. For the Si–20Ni(Y) catalysts (see Fig. 3 and 4), with and nickel salts are gradually deposited–precipitated over the
increasing the G-D–P time, nickel hydroxides are transformed gelled spherical SiO2 nanoparticles. At the aging stage, nickel
into nickel phyllosilicates because of the gradual dissolution hydroxides transform into nickel phyllosilicates with the dis-
of the silica framework. Moreover, after the dissolution of the solution of the silica nanoparticle matrix. However, limited to
silica framework into monomeric silicic species, and the move- the dissolution rate of the silica framework, nickel oxides are
ment of these soluble monomeric silicic species in aqueous exhibited as the main Ni phase at the relatively high Ni
solution, it is reasonable to deduce that, the reaction between loading and short G-D–P time, whereas nickel phyllosilicates
the depolymerized nickel hydroxides and soluble monomeric are the main Ni phase at the relatively low Ni loading and long
silicic species should occur primarily over the interface G-D–P time. After calcination, the core–shell-like silica@nickel
between the precipitated nickel hydroxides and aqueous solu- species nanoparticles are generated. Moreover, nickel phyllo-
tion. The experimental evidence in TEM (Fig. 6) and STEM silicates, with a morphology of needle-like branches observed
(Fig. 7) supports this deduction. The needle-like, thin branches in the TEM images and a morphology of floccules seen in the
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Scheme 1 Proposed mechanism for the preparation of core–shell-like silica@nickel species nanoparticle by the G-D–P approach.
STEM images, are formed over the outside of core–shell-like
silica@nickel species nanoparticles, and subsequently enwrap
the nanoparticles.
3.4. Catalytic dehydrogenation of 1,2-cyclohexanediol to
catechol
For the dehydrogenation of 1,2-cyclohexanediol to catechol, in
addition to the aimed catechol, the main byproducts are
phenol and o-hydroxyl-cyclohexanone. In our previous work,29
we studied systematically the reaction mechanism and the role
of the Na promoter for the dehydrogenation of 1,2-cyclohexa-
nediol to catechol over a nickel-based catalyst. The results
obtained exhibited that,29 catechol was generated over the Na-
modified Ni metal complexity, o-hydroxyl-cyclohexanone was
an important reaction intermediate formed on the unreduced
or reduced Ni, and phenol, as the main byproduct, was con-
cerned with the pyrolysis of catechol. In the present work, the Fig. 10 Catalytic performances of 2Na/Si–XNi(4) catalysts as a function
of X (X = 10–50); reduction temperature = 450 °C, reaction temperature
core–shell-like silica@nickel species nanoparticles prepared by
= 320 °C, LHSV = 1.42 h−1
.
G-D–P and the contrastive nickel-based catalyst prepared by
conventional D–P have been used as catalysts for the dehydro-
genation of 1,2-cyclohexanediol to catechol. Before the reac-
tions, 2 wt% of Na is added into the catalysts as the promoter,
and all the catalysts are uniformly reduced by hydrogen at
450 °C for 4 h.29
lysts are shown in Table 1. First, it is found that, the metallic
nickel dispersion decreases with increasing the Ni loading,
from 20.7% for Si–10Ni(4) to 7.0% for Si–50Ni(4). This can be
due to the aggregation or agglomeration of the nickel species
particles at the high loading of nickel. However, it is interest-
ing for the Si–20Ni(Y) catalysts that, with increasing the G-D–P
time, the dispersion of metallic nickel decreases firstly and
obtains its minimum at 4 h of G-D–P time (10.7%), followed
by a monotonic increase after 4 h of G-D–P time. As discussed
in Fig. 1b above, more than 98.6% of the colloidal silica solu-
tion has been gelled at G-D–P time >1 h. However, only 68.2%
and 92.0% of nickel are deposited–precipitated at 1 and 2 h of
G-D–P time, respectively, and more than 99% yields of depo-
sition–precipitation of nickel are obtained only after 4 h of
G-D–P time. According to the FT-IR, XRD, and TEM results
above, nickel phyllosilicates are formed mainly during the
aging period, which envelope the core–shell-like silica@nickel
species nanoparticles and increase in concentration with
increase in G-D–P time. Based on these conclusions, it is
obtained that, (1) Si–20Ni(1) and Si–20Ni(2) catalysts have a
higher dispersion of metallic nickel than Si–20Ni(4), which is
probably due to the lower actual Ni loadings over the former
two catalysts than the latter one; (2) after 4 h of G-D–P, the dis-
Fig. 10 shows the effect of Ni loading on the catalytic pro-
perties of 2Na/Si–XNi(4) catalysts (X = 10–50). With an increase
in the Ni loading, the conversion increases firstly, and then
changes very slightly, retaining a high value above 99.9% when
the Ni loading is higher than 10 wt%. Moreover, when the Ni
loading increases, the selectivity of catechol demonstrates first
an increase, attaining its maximum ca. 94.4% at 20 wt% of Ni
loading, and then it decreases. Comparatively, the selectivities
of o-hydroxyl-cyclohexanone and phenol show very low values
(<0.5%) when the Ni loading is below 30 wt%. However, they
increase dramatically when the Ni loading is above 30 wt%.
This consequence can be caused by the aggregation and
agglomeration of the nickel species particles at the relatively
high Ni loading, which is probably unfavorable for the main
reaction, but accelerates the formation of byproducts. Anyway,
there exists an optimal Ni loading for the dehydrogenation of
1,2-cyclohexanediol at ca. 20 wt%.
The dispersions of metallic nickel and initial surface-atom-
based reaction rates (turnover frequencies, TOF) of various cata-
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persion of metallic nickel increases with increasing the G-D–P
time, which can be related to the increase in the formation of
nickel phyllosilicates. This result proves that, the nickel phyllo-
silicate layers formed during the G-D–P procedure is beneficial
to the dispersion of nickel, because they distribute over the
periphery of the core–shell-like silica@nickel species nanopar-
ticles, and therefore bring an effect of segregation among the
nanoparticles. Moreover, the dispersion of metallic nickel of
Si–20Ni(4)–DP is also present in Table 1, which shows an
obviously lower dispersion of metallic nickel (6.5%) than that
of Si–20Ni(4) (10.7%). This result suggests that the catalyst pre-
pared by the G-D–P method depicted in this work owns a
higher dispersion of metallic nickel than that prepared by the
conventional D–P method.
Moreover, the initial TOF values are normalized by the
number of exposed surface metallic nickel atoms measured by
H2 chemisorption uptake before the catalytic measurements.
The conversion of the reaction is controlled, especially in the
range of 4%–13% for ensuring the experimental results are
obtained in the intrinsic kinetics region. Here, four Si–20Ni(Y)
catalysts (Y = 2, 4, 12, and 24) are tested. The catalyst weight
(10–20 mg) adopted is based on the same amount of surface
metallic nickel atoms = 9.9 × 1017. Then, an important con-
clusion can be drawn that, the initial TOF value over the
surface reduced nickel metal atoms increases with the dis-
persion of metallic nickel. One can also see that, the TOF
value for 2Na/Si–20Ni(4) (0.22 h−1) is obviously higher than
that for 2Na/Si–20Ni(4)–DP (0.14 h−1), indicating the higher
catalytic efficiency of the former catalyst than the latter one.
These results also indicate that the dehydrogenation of 1,2-
cyclohexanediol to catechol over Na-doped nickel catalysts is a
structurally sensitive reaction.60 For this catalytic reaction, the
coordinately unsaturated surface metallic nickel atoms are
more active than those in the low-index surface planes predo-
minately exposed on larger nickel crystallites.
The comparison of the catalytic property of the 2Na/Si–20Ni(4)
catalyst prepared by G-D–P and the 2Na/Si–20Ni(4)–DP cata-
lyst prepared by conventional D–P is studied. As shown in
Fig. 11, the 2Na/Si–20Ni(4) catalyst has an obviously better cata-
lytic performance than the 2Na/Si–20Ni(4)–DP catalyst in the
addressed reaction. For 2Na/Si–20Ni(4), the conversion of reac-
tant can be maintained stably at above 99.9% within 300 h, fol-
lowed by a slight decrease to 98.8% after 432 h. The catechol
selectivity achieves its maximum at 98.5% after an upward ten-
dency within the first 200 h, and then it decreases gradually to
below 90% after 324 h. As a result, the high performance with
a yield of aimed catechol above 90% can be obtained for as
long as 324 h of on-stream reaction. In comparison, for 2Na/
Si–20Ni(4)–DP, a lower catechol selectivity at 90.5% is obtained
Fig. 11 Catalytic performance of 2Na/Si–20Ni(4) catalyst as a function
of time on stream (TOS); reduction temperature = 450 °C, reaction
temperature = 320 °C, LHSV = 1.42 h−1
.
20Ni(4) catalyst prepared by the G-D–P approach, the better
selectivity toward the aimed product catechol and the better
stability for the model reaction can be explained by the higher
dispersion of metallic nickel over the special core–shell-like
silica@nickel species nanoparticles stabilized by outer nickel
phyllosilicates layers. In addition, we can compare the Si–20Ni(4)
catalyst prepared by the G-D–P approach described in this
paper with the Si–20Ni(4)–DP catalyst prepared by convention-
al D–P approach. As shown in the IR (Fig. 3) and XRD (Fig. 2)
results above, the nickel phyllosilicates over Si–20Ni(4)–DP are
more than Si–20Ni(4) in concentration. However, in the H2-
TPR profiles (Fig. 9), we find the existence of free nickel oxide
compounds and weak interactions between nickel species and
silica support over the Si–20Ni(4)–DP catalyst. As a result, the
dispersion of metallic nickel over the Si–20Ni(4)–DP catalyst is
clearly lower than the Si–20Ni(4) catalyst (Table 1). This result
suggests that, the G-D–P method provides an effective
approach for preparing highly dispersed and stable catalysts. It
is also reasonable to deduce that, the highly dispersed nickel
species is necessitated for obtaining optimum performance in
the dehydrogenation of 1,2-cyclohexanediol to catechol.
4. Conclusions
at 31 h. Furthermore, a severe loss of activity (<80%) is A simple approach named the gel-deposition–precipitation
observed after only 100 h of time on-stream. As discussed (G-D–P) approach is developed to prepare core–shell-like silica@
above, the dispersion of metallic nickel over Si–20Ni(4) is nickel species nanoparticles using a commercial grade col-
higher than that over Si–20Ni(4)–DP. Moreover, it has been loidal silica solution directly as the source of silica cores. The
proposed in previous studies that,22,46,61 the formation of core–shell-like silica@nickel species nanoparticles can serve as
hard-reducible metal silicates contributes largely to the stabi- a highly dispersed and stable catalyst. This approach can be
lity of metal supported catalysts. In this work, for the 2Na/Si– readily extended to the production of other similar catalyst
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Acknowledgements
We are grateful for the financial support from the Project 25 G. Dıaz, R. Perez-Hernandez, A. Gomez-Cortes, M. Benaissa,
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