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Highly Dispersed Supported Metal Catalysts Prepared via In-Situ Self-Assembled Core-Shell
Precursor Route
Liuye Mo, a Eng Toon Saw, a Yonghua Du, b Armando Borgna, b Ming Li Ang, a Yasotha Kathiraser, a
Ziwei Li, a Warintorn Thitsartarn, c Ming Lin, c Sibudjing Kawi a*
a Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
117576
b Institute of Chemical & Engineering Sciences (ICES), A*STAR, 1 Pesek Road, Jurong Island,
Singapore 628733
c Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore 117602
Corresponding author:
Tel: (65) 6516 6312
Fax: (65) 6779 1936
E-mail: [email protected]
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ABSTRACT
Preparation of highly dispersed supported metal catalysts, which has high activity and selectivty, is
still one of the main challenges for catalysis science. Highly dispersed supported metal catalysts via
an economical incipient wetness impregnation method by impregnation of a small amount of oleic
acid mixed with metal nitrate on catalyst support is reported herein. XRD, TEM(STEM), SEM, XPS,
FTIR and EXAFS results revealed that the small amount of oleic acid and metal nitrate solutions
would In-situ self-assemble to form core-shell precursors (metal nitrate species as core and metal
oleate as shell), and metal agglomeration during heat treatment was drastically prevented by the steric
hindrance of the shell structure. Therefore, the novel and facile method was named as in-situ self-
assembled core-shell precursor route. The effectiveness of this method was attested by copper and
nickel on silica catalysts. Compared to the conventional preparation method, the as-prepared highly
dispersed metal catalysts of Cu/SiO2 and Ni/SiO2 exhibited excellent catalytic activities for water gas
shift reaction and carbon dioxide reforming of methane at high temperatures.
Keywords: self-assembled core-shell, oleic acid, highly dispersed supported metal catalyst, water gas
shift reaction, dry reforming of methane
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1. Introduction
Catalysis is an innovative and powerful technology to solve many of today’s energy and
environmental issues. Particularly, supported metal catalysts hold great importance in catalysis
researches and industrial applications [1-4]. Highly dispersed supported metal catalysts are greatly
sought after since a decrease in the supported particle size is accompanied by an increase in the surface
area of active phase and a consequent enhancement in the catalytic activity. In the past decades,
tremendous advances have been achieved in the preparation of highly-dispersed metal catalysts using
different methodologies, such as sol-gel method [5-11], co-precipitation or deposition precipitation
method [12-19] and ion exchange or strong electrostatic adsorption (SEA) [20, 21]. Although all of the
above-mentioned methods have been successfully adopted in some applications, numerous
disadvantages such as high cost and difficulty in scaling up hindered application of these methods in the
industrial setting.
Impregnation method, in particular incipient wetness impregnation, is highly desirable due to its
practical simplicity and its minimal generation of waste streams. In a bid to improve the metal
dispersion on support, various metal precursors and organic metal compounds (acetate [22], citrate [23,
24] and acetylacetonate [25, 26]), have been utilized. Recently, Ni(1,5-cyclooctadiene)2 was reacted
with octylsilane to form nickel-silicide colloids [NixSi−C8H17] in the presence of H2, and was used as a
precursor to prepare highly dispersed supported nickel catalysts[27]. Besides, a versatile method
involving a surface organometallic chemistry (SOMC) route that uses mono(η3-allyl)nickel complex as
precursors to prepare supported small size nickel nanoparticles (ca. 1–3 nm) with narrow size
distribution was reported [28]. However, the organometallic compounds mentioned above are not only
costly but also have low solubility in aqueous solution and/or highly sensitive to moisture or air,
rendering them difficult to be applied in industry. Due to their inherent advantages, such as low cost,
high solubility in water and ease of anion removal, metal nitrates are widely utilized as precursors for
the preparation of supported metal catalysts. However, the drawback of using metal nitrate during the
preparation of supported metal catalyst lies in the wide distribution of metal particle size which falls
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between 1 to 100 nm. The ease of agglomeration of metal precursors during the drying and calcination
steps has been attributed for the cause of this phenomenon [29]. Interestingly, the de Jong group found
that the metal dispersion could be enhanced by using a NO/He mixture atmosphere during catalyst
calcination [29, 30]. It was revealed that NO could promote nearly complete hydrolysis of metal nitrate
to hydroxynitrates, which possessed a reduced mobility to agglomerate upon decomposition that
occurred between 100 oC and 150 oC [29]. In addition, Stucky et al. also reported another noteworthy
method to prepare oxide-supported metal nanoparticle in which the weak interaction between
presynthesized hydrophobic metal particles and oxides supports was utilized to assemble the
nanoparticles onto the oxides surface [31]. However, this method cannot be used in protic solvents and
large scale production. Hence, there remains a tremendous barrier to design a more general and
simplistic method for preparation of highly dispersed, supported metal catalysts. Precedently, we have
reported a method to prepare highly-dispersed and stable Cu/SiO2 and Ni-La2O3/SiO2 catalysts using the
incipient wetness impregnation (IWI) in the presence of a small amount of fatty acids and metal nitrate
[32, 33]. Herein, XRD, TEM, SEM, XPS, FT-IR and EXAFS techniques are applied to elucidate the
promoting mechanism of oleic acid on the dispersion of Ni/SiO2 and Cu/SiO2 catalysts. Catalytic
activities of the prepared Ni/SiO2 and Cu/SiO2 catalysts were measured using carbon dioxide reforming
of methane and water gas shift reaction, respectively.
2. Experimental Section
2.1 10%Cu/SiO2 catalyst preparation and water gas shift reaction conditions
Cu/SiO2 catalysts promoted with oleic acid (OA) were prepared with incipient wetness impregnation,
with a copper loading of 10wt.%. Firstly, 2.1 g copper nitrate (from Merck) was dissolved in D.I. water,
and then different amount of oleic acid (from Sigma-Aldrich) was added into the solution. At last, 5 g
spherical silica (from Kanto Chemical Co.: specific surface area = 753 m2/g, mean pore size = 7.5 nm)
was introduced into the above solution. The samples were subsequently impregnated at room
temperature for > 6 hours and then dried overnight at 100 oC. The above dried samples were calcined at
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450 oC for 4 hours. The amount of OA was calculated as molar ratio of X = nOA/nCu, and X was adopted
as 0, 0.05, 0.25, 0.5, and 1.0, respectively. And the 10%Cu/SiO2 prepared with different X was
designated as 10%Cu/SiO2/X. To investigate the catalytic performance of the prepared catalysts at 300
oC, the Water Gas Shift Reaction (WGSR) was used as a probe reaction. The feed composition, with a
total flow rate of 50 ml/min, was as follows: 5 mol.% CO, 25 mol.% H2O, and He balance. The effluent
gases were analyzed by an HP-GC equipped with a Hayesep D column. All catalysts were reduced at
300 oC for one hour prior to the onset of WGSR.
2.2 5%Ni/SiO2 catalyst preparation and dry CO2 reforming of CH4 reaction conditions
The 5%Ni/SiO2/X catalysts were prepared similar to those of 10%Cu/SiO2. The X = nOA/nNi ratios
adopted were 0, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0, respectively. Calcination was conducted at
700 oC for 4 hours in air atmosphere.
The dry CO2 reforming of CH4 (DRM) reactions were carried out in a quartz reactor. The catalyst
under study was loaded and packed in the middle of the quartz-tube using quartz wool which was inert
in the reaction. Under atmospheric pressure, the catalyst was heated from the initial room temperature to
a reaction temperature of 700 oC at a rate of 20.0 oC/min in a horizontal electric furnace. The Gas-
Hourly-Space-Velocity (GHSV) was 72000 ml.hr-1.g (cat.)-1. Additionally, high purity feed gases
(99.99%) were controlled by mass flow rate controllers and a molar ratio of CH4/CO2/N2 =1/1/1 was
adopted. Prior to the catalytic reaction, the catalysts were reduced in-situ in a H2 atmosphere (purity =
99.99%) at 700 oC for 1 hour and 0.05 g of catalyst was used in each catalytic test. To remove any
moisture from the effluent gas stream, a cold trap was employed to condense the moisture prior to gas
analysis by an on-line gas chromatograph (GC). Effluent gas from the reactor was analyzed by an
Agilent HP-GC equipped with two columns of 5Å and Porapak Q. The nitrogen in the reactants acted as
an internal standard.
2.3 Copper (nickel) oleate synthesis
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The copper oleate synthesis follows the method reported in literature [34]. Typically, 0.85 g (5mM)
CuCl2.2H2O (Fisher Chemical, >99%) was dissolved in a mixture solution containing 6 ml of water, 8
ml of ethanol and 14 ml of hexane. The above solution was added to 3.04 g (10 mM) sodium oleate
(TCI, >97%) under vigorous stirring for 4 h at 70 oC. The organic phase was separated in a separatory
funnel and washed three times with 3 ml of water. Following that, the solvent of hexane and ethanol
were distilled. Finally, the dark blue solid obtained was dried in vacuum. Nickel oleate synthesis
procedures were the same as copper oleate.
2.4 Catalyst characterization
Powder X-Ray Diffraction (XRD) characterizations for the fresh and reduced catalysts were
performed on Shimazu XRD-6000 using a Cu target Kα-ray (40 kV and 30 mA) as the X-ray source.
The XRD analysis was promptly conducted after the 10%Cu/SiO2 and 5%Ni/SiO2 catalysts had
undergone reduction at 300 oC and 700 oC for one hour and the transfer of sample to the XRD chamber
was carried out with minimal exposure to air. Thereafter, the crystalline sizes of metal oxides and metals
were calculated by Scherrer equation.
Transmission Electron Microscopy (TEM) images were taken using a JEOL JEM-2100F. Prior to the
TEM analysis, the catalyst was reduced at 300 oC for copper catalysts and 700 oC for nickel catalysts in
purified H2 for one hour. The samples were dispersed in ethanol solution and ultrasonicated for 30 min.
The above solutions were then pipetted onto the copper grid for TEM observation
Scanning Transmission Electron Microscopy (STEM) was used to capture the catalyst images both
before and after catalytic reaction. The images were acquired by a FEI Titan 80-300 S/TEM which was
operated at 200 KV. In order to minimize contamination during the scanning process, the specimen was
placed in a high-vacuum chamber overnight and treated with oxygen plasma for 1-2 seconds before
observation under electron beams.
The morphology and surface elemental composition on the catalyst was analyzed using a Scanning
Electron Microscope coupled to an Energy Dispersive Spectroscopy (SEM-EDS, Jeol JSM-6700). The
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samples were degassed under vacuum and coated with platinum (about 10 nm thickness) at 20 mA for
120 seconds prior to analysis. The 10%Cu/SiO2/X catalysts were reduced under hydrogen atmosphere at
300 oC for 1 hour before SEM-EDS analysis.
H2-Temperature-Programmed Reduction (H2-TPR) experiments were performed on CHEMBET-3000.
Prior to H2-TPR, the samples were pretreated in He atmosphere at 300 oC for 1 hour. After the sample
was cooled down to room temperature, 5%H2/N2 mixture gas with a flow rate of 30 ml/min was
introduced to the 50 mg catalyst fixed with quartz wool in a U quartz tube (I.D.: 1/4 inch). Upon
stabilization of the baseline, the temperature was increased with a ramping rate of 10 oC/min and the
effluent gas was analyzed by TCD.
X-ray Photoelectron spectroscopy (XPS) analyses were performed on KRATOS AXIS HIS 165.
Anode HT with a voltage of 15 kV, a current of 10 mA and a pass energy of 80 eV was used. C1s (284.5
eV) was used as an internal standard to eliminate the charge effect.
Cu K-edge Extended X-ray absorption fine structure (EXAFS) spectra were recorded at room
temperature in a transition mode at Singapore Synchrotron Light Source (SSLS), XAFCA facility. A Si
(III) double crystal was used to monochromatize the X-rays from the 700Mev electron storage ring. In a
typical experiment, the sample was prepared as a compressed pellet and loaded into a cell. Each sample
was measured 6 times to improve the signal to noise ratio of data and to ensure the data. EXAFS data
were analyzed using the EXAFS program, Winxas. Fourier transformation of k3-weighted EXAFS data
were performed over the range k = 2 -12 Å-1. The bond length/peak position was adopted without phase
correction.
Differential Thermal Analysis and Thermal Gravimetry Analysis (DTA-TGA) was carried out using a
Shimadzu DTG-60 thermoanalyzer. The spent catalysts were heated in an atmosphere of air from room
temperature to 800 oC at a heating rate of 10.0 oC/min.
3. Results and discussion
3.1 XRD results
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Fig. S1 (in Supporting Information) displays the XRD patterns of fresh 10%Cu/SiO2/X catalysts. Fig.
S1 shows that the CuO peaks were dramatically diminished when a small amount of OA (X = 0.05) was
introduced, indicating high dispersion of CuO on SiO2. This comes as a surprise since a small amount of
OA could promote the dispersion of CuO despite the insolubility of OA in water. However, at higher
values of X above 0.25, the intensity of CuO peaks increased. The crystalline sizes of CuO are 38.0,
11.9, 6.3, 9.3 and 10.6 nm for X = 0, 0.05, 0.25, 0.5, and 1.0, respectively. The optimal X is 0.25. The
copper crystalline size (dCu) on silica after reduction at 300 oC for one hour is depicted in Fig. 11. The
dCu exhibited significant dependence on X. With the addition of a small amount of OA, dCu decreased
tremendously from 38.3 nm to 11.0 nm as X increased from 0 to 0.05. However, upon a further increase
in X from 0.25 to 1.0, dCu increased from 6.0 nm to 17.6 nm.
Interestingly, the XRD peaks of NiO on 5%Ni/SiO2/X (the 5%Ni/SiO2 catalysts with different X were
abbreviated as 5%Ni/SiO2/X, and X was a molar ratio of nOA/nNi) were almost invisible as X varied in a
wide range of 0.05-1.5 (shown in Fig. S2), displaying a great disparity from the copper catalysts. The
peaks of NiO were evidently observed when the X was > 1.5. Accordingly, the crystalline sizes of
nickel (dNi) on reduced samples were around 3.0 nm for X between 0.05 and 1.5. The dNi increased
slightly from around 3.0 nm to 4.0 nm with the increment in X from 1.5 to 3.0. Comparatively, the
crystalline size of nickel on reduced 5%Ni/SiO2/0 was significantly larger at around 12.0 nm (Fig. S3).
In order to track the evolution in the catalyst composition, the 10%Cu/SiO2/X samples dried at 100
oC were characterized by XRD (shown in Fig. S4). From XRD profiles, it can be seen evidently that
copper hydroxynitrate phase could be detected in the sample without OA. However, no copper
compound phases could be seen for 10%Cu/SiO2/X samples with X ≧0.25. Copper nitrate would
partially hydrolyze to form copper hydroxynitrate and agglomerate into big particles during drying at
100 oC. Subsequently, they would redistribute and form big particles of CuO under high calcination
temperature conditions [29]. XRD results display that OA plays a vital role in preventing copper nitrate
from forming bigger particles on 10%Cu/SiO2/X during the thermal process, thereby achieving high
dispersion of CuO after high temperature calcination.
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3.2 TEM (STEM) results
Fig. 1 shows the TEM image characterizing the Cu particle size distribution of 10%Cu/SiO2/0.25
catalyst after 300 oC reduction. It can be observed that the Cu particles were homogenous and had a
narrow distribution range between 2.0-6.0 nm with a mean size of 3.8 nm. However, the Cu particles of
10%Cu/SiO2/0 catalyst ranged from 2.0 to 200.0 nm, with a mean size of 38.0 nm (Fig. S5).
Fig. 1. TEM image (bright field) and particle size histogram of 10%Cu/SiO2/0.25 catalyst after 300 oC
reduction.
Fig. 2 exhibits STEM images of 5%Ni/SiO2 catalyst reduced at 700 oC for one hour. The nickel
particles dispersed well and homogeneously on the silica support with a narrow distribution between 1.0
and 6.0 nm, and the mean particle was 2.9 nm. The result is in good agreement with the crystalline size
measured with XRD (Fig. S3). In contrast, in the absence of OA, the nickel particles on silica derived a
wide range between 3.0 and 50.0 nm and the mean size was 19.0 nm (Fig. S6).
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Fig. 2. STEM image (Dark field) and particle size distribution histogram of 5%Ni/SiO2/0.5 catalyst
after 700 oC reduction.
3.3 SEM-EDS results
Fig. 3. SEM-EDS images of 10%Cu/SiO2/0
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Fig. 4. SEM-EDS images of 10%Cu/SiO2/0
SEM was conducted to observe the morphologies of the catalysts prepared in presence/absence of
OA. From SEM images, it is evident that the size of micro-spherical silica is between 20-60 m. The
10%Cu/SiO2/0 catalyst exhibited large rod-like particles that agglomerated on the silica outer surface
(Fig. 3). The molar ratio of Si to Cu (nCu/nSi) measured by EDS on the big particle surface was 1.61 (Fig.
3 Left), while it was 0.05 on the smooth surface (Fig. 3 right). The theoretical value calculated was 0.11.
Therefore, the big particles on the silica surface are most likely copper particles although the EDS result
showed that the big particles were not purely copper and had interferences with the silica support. On
the contrary, the surfaces of 10%Cu/SiO2/0.25 catalyst (Fig. 4) were smoother than that of
10%Cu/SiO2/0. The rough surface was analyzed by EDS and showed the nCu/nSi was as low as 0.04 (Fig.
4 left), which suggested that the rough surface was not copper particle and was lower in copper
concentration compared to the smooth surface value (nCu/nSi =0.14). The SEM-EDS results showed that
OA could effectively prevent the copper species from agglomerating to form big particles on the silica
outer surface and promoted the copper species to homogeneously distribute on the silica support.
3.4 H2-TPR results
H2-TPR is a useful technique in characterizing the reduction behaviour of a catalyst, which affects its
catalytic performance. The TPR profiles of 10%Cu/SiO2/X catalysts are illustrated in Fig. 5. The CuO
on 10%Cu/SiO2/0 showed two reduction peaks at 268 and 337 oC. However, on 10%Cu/SiO2/0.05,
there was a single peak centred at 268 oC. With increasing X from 0 to 0.25, the reduction peak shifted
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further to lower temperature at 252 oC together with the appearance of a small shoulder peak at 268 oC.
However, increment in X from 0.25 to 1.5 decreased intensity of the low temperature reduction peak
and increased the intensity of the high reduction peak at 288 oC. It could be observed that catalysts with
bigger CuO crystalline size had higher reduction temperature and these TPR results are well correlated
to XRD results [35, 36]. Furthermore, 10%Cu/SiO2/0 showed two peaks at low temperature and high
temperature which could be ascribed to the reduction of small CuO particles and bigger CuO particles,
respectively. Hence, it can be inferred from the TPR results that OA used in the catalyst preparation
promoted the dispersion and homogeneous distribution of CuO on the silica support whereby the
reduction peaks appeared as single or very near shoulder peaks.
Fig. 6 exhibits H2-TPR curves of 5%Ni/SiO2/0 and 5%Ni/SiO2/0.5. The peaks at low temperature
(417 and 499 oC) and high temperature at > 600 oC could be assigned to the reduction of nickel oxide
species and nickel silicate which had weak and strong interaction with the silica support, respectively
[24]. Low temperature reduction peaks between 417 and 499 oC were evident from the H2-TPR curve of
5%Ni/SiO2/0.5, indicating that there were nickel oxide species interacting weakly with silica or
unsupported free NiO that had no interaction with the support.
50 100 150 200 250 300 350 400 450 500
X=1.5
X=0.05
288 oC268 oC
Hyd
roge
n co
nsum
ptio
n (
a. u
. )
Temperature ( oC )
X=1.0
X=0.5
X=0.25
without OA
CuO
252 oC337 oC
Fig. 5. H2-TPR profiles of 10%Cu/SiO2/X
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0 100 200 300 400 500 600 700 800 900 1000
5%Ni/SiO2 /0.5
609 oC
Hyd
roge
n c
ons
um
ptio
n (
a.u
.)
Temperature ( oC )
417 oC499 oC
5%Ni/SiO2 /0
Fig. 6. H2-TPR profiles of 5%Ni/SiO2/X
3.5 FT-IR results
2000 1800 1600 1400 1200 1000 800 600
1.5
1.0
0.5
0.25
0.05
Ads
orpt
ion(
a.u.
)
Wavenumber(cm-1)
1460 cm-1
1719
0
X=nOA
/nCu
1565
Fig. 7. FT-IR spectra obtained from uncalcined samples of 10%Cu/SiO2/X dried at 100 oC.
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In order to further elucidate the promoting effect of OA, the 10%Cu/SiO2/X samples dried at 100 oC
were measured by FT-IR. From Fig. 7, the peak at 1719 cm-1 which can be assigned C=O stretch of free
oleic acid appeared for X > 0.5. The intensities of characteristic carboxylate peaks at 1565 and 1460 cm-
1, ascribed to antisymmetric stretch of COO- and symmetric stretch of COO- [34, 37], increased with X.
The carboxylate group may combine with a metal cation in either a monodentate, bidentate (chelating),
or bridging mode. The coordination type can be interpreted based on the wavenumber separation (△)
between the antisymmetric, υas(COO−), and the symmetric, υs(COO−), stretching bands. For △>200
cm−1, a monodentate ligand is expected, and for △<110 cm−1, a bidentate. For a bridging ligand, △ lies
somewhere between 140 and 200 cm−1 [38]. In this case, △=1565-1460=105, is smaller than 110 cm-1
indicating that oleic acid is bound in a bidentate mode, with both oxygen atoms symmetrically
coordinated to the surface.
3.6 XPS results
Fig. 8. Cu 2p3/2 XPS spectra of uncalcined 10%Cu/SiO2/X and copper oleate samples.
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XPS is a sensitive and effective technique to characterize surface species of a solid. Fig. 8 displays
binding energy of Cu2p3/2 of uncalcined 10%Cu/SiO2/X and copper oleate samples. The sample without
OA showed two Cu 2p3/2 peaks at 935.3 eV and 933.0 eV. The peak at 935.3 eV could be ascribed to the
Cu2+ chemical shift in copper nitrate [39]. The peak at 933.0 may be attributed to the chemical shift of
Cu2+ in copper hydroxyl nitrate, which was formed during drying at 100 oC (Fig. S4). With increasing X
from 0 to 0.25, two peaks merged into one peak centered at 934.7 eV, which is 0.2 eV higher than that
of copper oleate. Further increment of X to 1.0 resulted in the decrease of binding energy to 934.5,
which is the binding energy of copper oleate. The XPS results postulated that the particles of copper
nitrate species were covered by a layer of copper oleate compounds at X≧0.25 and this corresponds
well with the FT-IR results. The XPS results of uncalcined 5%Ni/SiO2 samples also drew a similar
conclusion with that of uncalcined 10%Cu/SiO2 samples (Fig. S7).
3.7 EXAFS results
0 1 2 3 4 5 6 7 8
X=1.0
1.2 1.3 1 .4 1 .5 1 .6 1 .7 1 .8 1 .9
R (a ngs trom )
1.53
1.52
1.50
1.43 Cu-O
FT
ma
gn
itud
e
R(Angstrom)
Cu-O
Cu-Si
2.72
X=0
X=0.25
Copper oleate
Fig. 9. EXAFS of uncalcined 10%Cu/SiO2/X and copper oleate samples.
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Extended X-ray Absorption Fine Structure (EXAFS) was used to characterize the interaction between
copper nitrate species and silica support on the samples of 10%Cu/SiO2/X dried at 100 oC (Fig. 9). The
Cu-O distance shifted from 1.53 Å to 1.50 Å as X was increased from 0 to 1.0. In contrast, copper
oleate had a Cu-O bond distance of 1.42Å (Fig. 9 inset). The EXAFS results revealed that most of the
copper atoms on 10%Cu/SiO2/1.0 did not form copper oleate compound although copper oleate was the
only compound as detected by XPS (Fig. 8). Therefore, EXAFS and XPS results reveal that core-shell
particles would in situ self-assemble on 10%Cu/SiO2/X (X ≧ 0.25) with copper nitrate species as core
and copper oleate compounds as shell. Other important fact which could be derived from Fig. 9 is that
the peak intensity at 2.72 Å decreased when X was increased to 0.25 but completely vanished when X
was further increased to 1.0. The peak at 2.72 Å could be ascribed to Cu-Si distance, indicating that
Cu(Ⅱ) was bonded to silica surface [40]. Therefore, as X increased, the interaction between copper
nitrate species decreased.
Fig. 10. The proposed promotion mechanism of oleic acid on highly dispersed supported metal
catalysts preparation.
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From XRD, TEM (STEM), H2-TPR, SEM-EDS, FT-IR, XPS and EXAFS results, the promotional
effect of OA was proposed in Scheme 1. In the case of X=0 (Fig. 10 (a)), although the particles interact
with the silica via hydrogen bond or coordination bond, the metal nitrate species form big particles and
distribute inhomogeneously. Druing thermal treatment, the remaining big metal nitrate species particles
would further agglomerate to form big metal oxide particles. If an appropriate amount of OA was added,
core-shell (metal oleate layer on metal nitrate species) particles would in situ self-assemble and interact
weakly with the support (Fig. 10 (b)). The agglomeration of metal nitrate species particles during
calcination or drying process was hindered by OA due to its long aliphatic carbon chain that poses a
strong steric hindrance – an important factor required for nanocrystal synthesis. However, too much OA
imposed a negative effect to the metal crystalline size as shown in XRD results for both copper and
nickel catalysts. As shown in Fig. 10 (c), in the presence of excess OA, OA would assemble on the
hydrophilic surface of silica via hydrogen bond with carboxylic group and form ester as the temperature
increased to > 100 oC [36]. Then, core-shell particles (metal oleate as the shell structure) would be
uplifted and floated on the hydrophobic layer of OA. As a consequence, the metal nitrate species would
distance away from the support surface, thereby decreasing the interaction with the support. This leads
to increased mobility of the particles on OA layer which would easily migrate and combine to form
bigger particles during calcination. However, another possible reason which could not be ruled out was
that excess OA could occupy the pores in the support, and hinder the diffusion of metal nitrate into the
pores, which would then agglomerate out of the pores. The 10%Cu/SiO2 catalysts were sensitive to X
but not for 5%Ni/SiO2 catalysts. This could be attributed to the strong interaction between nickel oxide
and silica compared to that of copper oxide and silica, leading to easier formation of more stable nickel
silicates in the former case [23, 24]. Therefore, high dispersion of nickel could be achieved in a wide
range of nOA/nNi ratio from 0.05 to 1.5 (Fig. S3).
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Fig. 11. Relationship between X, crystalline size of Cu and catalytic activities for WGSR.
3.8 WGSR activities on 10%Cu/SiO2 catalysts
The water-gas shift reaction (WGSR, CO + H2O → H2 + CO2) is important in upgrading H2-rich fuel
gas streams for fuel cell and other applications [42-45]. Water gas shift reaction was used as a probe
reaction to test the catalytic performances of 10%Cu/SiO2 catalysts prepared in the presence/absence of
OA. Fig. 11 describes the relationships of catalytic activity (in terms of CO conversion), X and
crystalline size of copper (dCu). As X is increased from 0 to 0.05, CO conversion (XCO) improved
drastically from ~20% to ~37%. However, upon a further increase in X from 0.25 to 1.0, XCO decreased
from ~47% to ~38%. Thus, it can be observed that the catalytic activities of 10%Cu/SiO2/X catalysts
can be well correlated with dCu, i.e. smaller dCu gives higher WGSR activity.
3.9 DRM activities on 5%Ni/SiO2 catalysts and spent catalysts characterization
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Fig. 12. The catalytic activities of 5%Ni/SiO2/0.5 (square symbol) and 5%Ni/SiO2/0 (triangle symbol)
for DRM reaction.
Dry reforming of methane (DRM) using carbon dioxide is considered to be an important reaction for
both the chemical industry and environment since it utilizes both greenhouse gases whilst producing the
valuable chemical precursor syngas [46-51]. Ni/SiO2 catalyst with high metal dispersion on silica
support can present itself as a good catalyst for DRM [24, 26, 52]. Therefore, the DRM reaction was
chosen as a probe reaction to test the Ni/SiO2 catalyst prepared with our methodology. Fig. 12 displays
the catalytic activities of 5%Ni/SiO2 catalysts with or without OA for at 700 oC with GHSV = 72000
ml.g(cat)-1.h-1. The catalyst of 5%Ni/SiO2/0.5 was relatively stable for 100 hours of reaction on stream,
while the conversions of CH4 and CO2 dropped slightly from ~65% and ~75% to ~60% and ~70%,
respectively. In contarast, rapid deactivation was observed with the 5%Ni/SiO2 catalyst without OA and
carbon deposition blocked the catalyst bed after one hour of reaction.
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Fig. 13. TEM images of the spent 5%Ni/SiO2/0 (left) and 5%Ni/SiO2/0.5 (right) catalysts used for
DRM reaction.
The spent catalyst of 5%Ni/SiO2/0 and 5%Ni/SiO2/0.5 were analyzed by DTA-TGA. The carbon
deposition rate was as high as 76.9 mg/g(cat.).h on the spent 5%Ni/SiO2/0 catalyst and the carbon
combustion temperature was ~660 oC (Fig. S8). The carbon morphology on the spent 5%Ni/SiO2/0
catalyst was characterized by TEM (Fig. 13, left). Carbon nanotubes were formed on the spent
5%Ni/SiO2/0 catalyst. Therefore, the deactivation of catalyst of 5%Ni/SiO2/0 was clearly due to its
severe carbon deposition. On the other hand, negligible carbon was detected on the spent
5%Ni/SiO2/0.5 catalyst as measured by DTA-TGA (Fig. S9) and TEM (Fig. 13 (right)) despite
observation of several long carbon nanotubes on the spent catalyst by STEM (Fig. S10). These
nanotubes could have been produced by the unsupported nickel particles or by those particles which had
less interaction with the support. The spent catalyst had a mean nickel particle size of 3.8 nm (Fig. S10),
which was marginally bigger than that of the freshly reduced sample. The spent 5%Ni/SiO2/0.5 catalyst
was analyzed by XRD and the crystalline size of nickel increased slightly from 2.7 nm to 3.9 nm, which
showed that the nickel particles were highly stable in catalytic activity during the 100 hours of reaction.
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21
Highly nickel dispersion of 5%Ni/SiO2/0.5 catalyst should be the major attribute for low carbon
formation on the catalyst [24, 26, 53].
4. Conclusions
A novel, simple and large-scale impregnation method has been successfully devised to prepare highly
dispersed supported metal catalysts via in-situ self-assembled core-shell precursor route for sustainable
hydrogen production. The core-shell precursors formed easily through in-situ self-assembly during the
impregnation of metal precursor with a small amount of OA (oleic acid). The carboxylic group of OA
chelated to metal ion as bidentate type oleate outside of metal nitrate species particles (core-shell)
prevented the particles of metal nitrate species and metal oxide from agglomeration during thermal
treatment at high temperature. High catalytic activities and stabilities were exhibited by the catalysts
prepared in the presence of OA as compared to the catalysts prepared with conventional impregnation.
The novel method reported in this communication can also be used effectively to prepare metal catalysts
besides copper and nickel metals (Fig. S11, S12). Besides, it can also be used to prepare mesoporous
material (i.e. SBA-15 (Fig. S13)) supported metal catalysts, supported bi-metallic catalysts or metal
catalysts doped with metal oxides. Utilization of other metal oxides, like ceria, alumina etc, to support
metals are currently explored.
Acknowledgements
The authors gratefully thank the National University of Singapore and NEA (NEA-ETRP Grant
No.1002114, RP No. 279-000-333-490) for generously supporting this work.
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22
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