Ni Nanoparticles Supported on Cage-Type Mesoporous Silica for CO2 Hydrogenation with High CH4 Selectivity

Article abstract of DOI:10.1002/cssc.201600710

Ni nanoparticles (around 4 nm diameter) were successfully supported on cage-type mesoporous silica SBA-16 (denoted as Ni@SBA-16) via wet impregnation at pH 9, followed by the calcination-reduction process. The Ni@SBA-16 catalyst with a very high Ni loadin

Full text of DOI:10.1002/cssc.201600710

DOI: 10.1002/cssc.201600710
Communications
Ni Nanoparticles Supported on Cage-Type Mesoporous
Silica for CO Hydrogenation with High CH Selectivity
2
4
[a]
[b]
[b, c]
[a]
Canggih Setya Budi, Hung-Chi Wu, Ching-Shiun Chen,*
Ming Kao*
Diganta Saikia, and Hsien-
[a]
[
3–12]
Ni nanoparticles (around 4 nm diameter) were successfully sup-
ported on cage-type mesoporous silica SBA-16 (denoted as
Ni@SBA-16) via wet impregnation at pH 9, followed by the cal-
cination-reduction process. The Ni@SBA-16 catalyst with a very
high Ni loading amount (22.9 wt%) exhibited exceptionally
high CH selectivity for CO hydrogenation. At a nearly identi-
ticularly on the effect of the support and catalyst.
Among
the various transition metal nanostructures studied, Ni-based
catalysts, particularly with Ni nanoparticles (Ni NPs), are clearly
proven to be highly active in various catalytic hydrogenation
[3,4,7–12]
processes.
Their low-cost and large-scale supplies are
generally acknowledged these catalysts as the most promising
4
2
cal loading amount, the Ni@SBA-16 catalysts with smaller parti-
cle size of Ni NPs surprisingly exhibited a higher catalytic activi-
candidates for the catalytic hydrogenation of CO . However,
2
the nature of nanoparticles with large surface area-to-volume
ratio often has a tendency to the aggregation of particles, re-
sulting in their catalytic performance with uncontrollable and
undesirable effects. To alleviate the aggregation problem, sev-
eral strategies for the preparation of well dispersed Ni NPs
ty of CO hydrogenation and also led to a higher selectivity on
2
CH formation than the Ni@SiO catalysts. This enhanced activi-
4
2
ty of the Ni@SBA-16 catalyst is suggested to be an accumula-
tive result of the advantageous structural properties of the
support SBA-16 and the well confined Ni NPs within the sup-
port; both induced a favorable reaction pathway for high se-
lectivity of CH in CO hydrogenation.
[13–23]
have been applied.
In particular, various types of siliceous
materials, such as mesoporous silica MCM-41, SBA-15, and KIT-
6, and SiO hollow spheres, have been extensively utilized as
4
2
2
[3,4,7,13,14,20–22]
supports for fabrication of highly dispersed Ni NPs.
It has been pointed out that the catalytic performance is
highly dependent on the characteristics of the immobilized Ni
NPs, the structural property of solid support, and the accessi-
bility or transport of reactants on the Ni active sites. Therefore,
it becomes an essential work to search for an appropriate sup-
port with highly dispersed active metal NPs. To the best of our
knowledge, however, Ni NPs incorporated within the cage-type
To date, the hydrogenation reaction of CO to CH (the so-
2
4
called Sabatier reaction) is considered as a crucial process
since the continuous rising of CO concentration as the main
2
contributor to the greenhouse effect has aggressively raised
[
1]
world concern. Apart from this, hydrogenation of CO2 is
widely applied in the production of organic compounds, such
as formic acid, methanol, methyl formate, carbon monoxide,
mesoporous silica support as catalysts for CO hydrogenation
2
[
2]
and hydrocarbons. Consequently, this reaction has been con-
sidered as one of the most feasible strategies to manage and
have not been reported yet.
In this work, highly dispersed Ni NPs that were well confined
in cage-type mesoporous silica SBA-16 were employed as effi-
utilize the abundance of CO , particularly produced from in-
2
dustrial combustion. Unfortunately, even though the Sabatier
cient catalysts to enhance the activity and selectivity in CO hy-
2
reaction
is
thermodynamically
favorable
(DG8=
drogenation. Furthermore, the effects of particle size and struc-
ꢀ
1
ꢀ130.8 kJmol ), the significant kinetic barrier associated with
tural properties of the catalysts on their selectivity of CH for-
4
the conversion is still the bottleneck for industrial and large-
scale applications. To address this issue, extensive studies have
mation were also emphasized by comparing the results with
[24]
the support SiO with similar Ni loadings. From the geomet-
2
focused on enhancing the activity of CO hydrogenation, par-
rical point of view, cage-type SBA-16 with its unique ordered
2
3
D structure, high surface area, large pore volume, and narrow
pore size distribution provides an ideal support for catalysis,
but it has still gained little attention. The synthesis procedure
of Ni NPs incorporated within the cages of SBA-16 is illustrated
in Scheme 1. The Ni NP-based SBA-16 catalysts were denoted
as Ni(x)@SBA-16, where x represents the weight percentage of
the Ni loading and is determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) to be 5.8, 12.9,
and 22.9 wt% for the present study (see Table 1). The ICP re-
sults indicated that the loading amount of the Ni NPs within
the SBA-16 cages gradually increased with the increase in the
[
a] C. S. Budi, Dr. D. Saikia, Prof. H.-M. Kao
Department of Chemistry
National Central University
Chung-Li, 32054, (Taiwan)
E-mail: hmkao@cc.ncu.edu.tw
[
b] Dr. H.-C. Wu, Prof. C.-S. Chen
Center for General Education
Chang Gung University
2
59, Wen-Hua 1st Rd., Guishan Dist., Taoyuan City 33302, (Taiwan)
[
c] Prof. C.-S. Chen
Department of Pathology
Chang Gung Memorial Hospital
2
+
initial Ni precursor concentrations and were very close to
5
, Fusing St., Guishan Dist., Taoyuan City 33302, (Taiwan)
2
+
the theoretical loading of each Ni concentration. Under alka-
line conditions, negatively charged silanols on the surface of
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cssc.201600710.
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Scheme 1. Illustration of the facile synthesis of Ni NPs within the cage of
SBA-16.
Table 1. Structural and textural properties of Ni(x)@SBA-16.
[
a]
[b]
[c]
[d]
x
a
0
S
BET
2
V
Entrance
size [nm]
Cage size
[nm]
ꢀ1
3
ꢀ1
[d]
[wt%]
[nm]
[m g
]
[cm g
]
0
5
2.9
2.9
14.5
14.0
14.0
13.5
511
366
301
233
0.47
0.42
0.35
0.23
3.7
3.2
3.4
3.0
5.8
5.7
5.5
4.9
Figure 1. (A) Small angle and (B) wide angle XRD patterns of Ni(x)@SBA-16,
where x=(a) 0, (b) 5.8, (c) 12.9, and (d) 22.9 wt%.
.8
1
2
[a] Cell parameter. [b] Surface area. [c] Pore volume. [d] Entrance and cage
sizes obtained from the adsorption and desorption isotherms, respective-
ly, by the Barrett–Joyner–Halenda (BJH) method.
three peaks centered at 2q values of 44.58, 51.88, and 76.48,
which can be indexed as the (111), (200), and (220) planes
owing to the face-centered cubic planes of crystalline Ni metal
(
JCPDS# 87-0712), respectively. The relatively broad peaks of
SBA-16, confirmed by the zeta potential measurements (see
Figure S1, Supporting Information), facilitate the electrostatic
the Ni crystalline planes indicated the nano-sized characteris-
tics of Ni NPs within the cage-type pores of SBA-16. A broad
peak in the 2q range of 10–358 corresponded to the amor-
phous characteristic of mesoporous silica. The average crystal-
lite size D [nm] of Ni NPs within the pores of SBA-16 can be es-
timated according to the Scherrer equation,
2
+
interaction to immobilize the Ni species, which was then
thermally reduced under an Ar/H flow to form Ni NPs within
2
the mesopores. The cage-like geometry of SBA-16 not only
regulates the dispersion and size confinement of the nanopar-
2
+
ticles, but also accommodates the Ni reduction inside the
cages of SBA-16. The present approach provides an effective
pathway in which the active Ni NPs are highly dispersed into
the cage-type pores of the SBA-16 support, even for a high
kl
D ¼
Bcosq
where k is a shape factor with the value of 0.9 for spherical ge-
ometry, l is the wavelength of CuKa (1.5405 ꢁ) as the X-ray
source, B is the full width at the half-maximum (FWHM) of the
peak in radians, and q is the Bragg diffraction angle. The aver-
age particle sizes calculated by the Scherrer equation were ap-
proximately 4.0, 4.1, and 4.3 nm for Ni(x)@SBA-16, where
x equals to 5.8, 12.9, and 22.9 wt%, respectively. The Ni particle
size remained almost unchanged regardless of the Ni loading
amount, suggesting that the cage-type pore of SBA-16 played
a significant role to confine the growth of the Ni NPs during
the thermal reduction process. On the other hand, the average
loading of Ni up to 22.9 wt%. For comparison purposes, SiO
2
2
ꢀ1
(Sigma–Aldrich, surface area=300 m g ) was also employed
as the support to prepare the Ni(x)@SiO catalysts with Ni load-
2
ing amounts of 3.3, 5.6, and 12.0 wt%. All of the Ni(x)@SiO2
catalysts were calcined in air and reduced under H at 5008C
2
for 5 h prior to use.
The effects of fabrication of Ni NPs on the mesostructure of
SBA-16 were examined. As shown in Figure 1A, the SBA-16
support shows a very sharp major diffraction peak at 2q value
of 0.748, and two minor diffractions at 2q values of 1.068 and
1.468, which can be indexed as (110), (200), and (211) reflec-
particle size of the Ni(x)@SiO catalysts increased with increas-
tion planes and correspond to the cubic structure with the
2
[
25]
ing the Ni loading amount. As revealed in Figure S2, the parti-
Im3m space group.
After incorporation with Ni NPs, the
cle size of Ni NPs on Ni(x)@SiO was estimated to be 6.5, 7.4,
peak intensity of the (110) diffraction of SBA-16 gradually de-
creased and slightly shifted to a higher 2q value with the in-
crease in Ni loading amount. These observations gave support
that the Ni NPs formed within the SBA-16 gradually occupied
the cage space, which might reduce the contrast of electronic
density between the walls and the pores of the mesoporous
silica support, and thus decreased the intensities of the diffrac-
tion peaks. Figure 1B displays the wide-angle XRD patterns of
Ni(x)@SBA-16 with different Ni loading amounts. Except for
Ni(0)@SBA-16 (i.e., pure silica SBA-16), all the samples exhibited
2
and 8.9 nm for the Ni loading amount of 3.3, 5.6, and
12.0 wt%, respectively (Table 2).
The N adsorption–desorption isotherms and entrance size
2
distribution curves of Ni(x)@SBA-16 are depicted in Figure 2A
and 2B, respectively. All the Ni(x)@SBA-16 samples exhibited
the type IV isotherm characteristic with H2 hysteresis loops
owing to their cage-like pore structures. The increase in the Ni
loading amount led to some changes of the hysteresis loops.
The textural properties of the pristine SBA-16 and Ni(x)@SBA-
&
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effect during the growth of Ni NPs. The energy dispersive X-
ray spectrum (EDS) from the corresponding TEM image (Fig-
ure S3) clearly displayed that the Ni(22.9)@SBA-16 catalyst con-
sisted of O, Si, and Ni elements. The peaks owing to the Cu
and C elements were generated from the sample grid used for
the TEM measurements. In comparison to the literature, the
present synthesis route provided a controllable way of fabrica-
tion of Ni NPs down to 4 nm without the use of additional
polyols delivery agents, carbon templates, reducing agents, or
Table 2. Comparison of Ni particle size in Ni(x)@SBA-16 and Ni(x)@SiO
2
catalysts and their selectivity of CH
4
for CO
2
hydrogenation.
x
Support
Ni NP
size [nm]
Conv.
[%]
Selectivity
of CH [%]
4
[wt%]
3
5
2.0
.3
.6
SiO
SiO
SiO
SBA-16
SBA-16
SBA-16
2
2
2
6.5
7.4
8.9
4.0
4.1
4.3
11.0
10.0
10.5
11.5
10.0
11.2
5
8
12
16
36
53
1
5.8
1
2
2.9
2.9
[3,15–20]
stabilizers.
The hydrogenation of CO was employed to examine and
2
compare the catalytic activity of Ni(x)@SBA-16 and Ni(x)@SiO2
catalysts. The major products detected during CO hydrogena-
2
tion were CO and CH . Figure 4(A–C) shows the dependence
4
of the overall turnover frequency (TOF) of CO hydrogenation
2
and the formation of CO and CH as a function of temperature.
4
The Ni NPs on SBA-16 induced an apparently higher level of
catalytic activity for CO hydrogenation than the Ni NPs on
2
SiO2 at all the temperatures investigated. As seen in Fig-
ure 4(A–C), at nearly identical Ni loading amounts, Ni(x)@SBA-
16 (x=5.8 and 12.9) exhibited higher TOF values than
Ni(x)@SiO (x=5.6 and 12.0) at any given temperature range,
2
respectively. The significant difference in their catalytic activi-
ties might be attributed to the difference in the particle size of
Ni NPs (Table 2) and their dispersion within the support. The
small size (~4 nm) of Ni NPs may significantly enhance the cat-
alytic efficiency of CO2 hydrogenation. It is also worthy of
noting that smaller sized Ni NPs with narrow particle size distri-
bution were highly dispersed in the mesoporous silica SBA-16
Figure 2. (A) Nitrogen adsorption–desorption isotherms and (B) entrance
size distribution curves of Ni(x)@SBA-16 with different weight percentages
of Ni loading amount.
16 samples are summarized in Table 1. A significant decrease
in the Brunauer–Emmett–Teller (BET) surface area and pore
volume of Ni(x)@SBA-16 was noticed when the loading
amount of Ni NPs was increased. Nevertheless, the relatively
high surface area of Ni(x)@SBA-16 could promote higher per-
formance for its catalytic activity.
[24]
than in the support SiO2. Moreover, the support SBA-16 ex-
hibits a well-ordered 3D cubic mesostructure, whereas the
support SiO is basically amorphous in nature. The fundamen-
2
tal difference in particle size and distribution of Ni NPs as well
as the characteristics of the support may play a key role in the
selectivity and reaction pathway.
The TEM images (Figure 3) of Ni(22.9)@SBA-16 confirmed
that the mesostructure of the support was well maintained
after incorporation of Ni NPs. As shown in the inset of
Figure 3, the Ni NPs were highly dispersed within the cage
pores of the SBA-16 support with particle size around 4 nm for
a high Ni loading of 22.9 wt%, which was consistent with the
particle size estimated by XRD. Since there was no particle ag-
gregation found outside the support, it should be noted that
the cage-type pores of SBA-16 might facilitate the confinement
Figure 4D shows the selectivity of CH4 formation for all
Ni(x)@SBA-16 and Ni(x)@SiO2 catalysts, revealing that
Ni(x)@SBA-16 exhibited a higher selectivity of CH4 than
Ni(x)@SiO . It clearly demonstrated that the highest Ni loaded
2
catalyst Ni(22.9)@SBA-16 exhibited the highest selectivity of
CH formation. It was also definitely proved that the particle
4
size of Ni NPs affected the activity and selectivity of CH forma-
4
tion. At nearly identical Ni loading amounts, the smaller parti-
cle size of Ni(12.9)@SBA-16 (about 4.1 nm) could induce the se-
lectivity of CH4 three times higher than that of the
Ni(12.0)@SiO2 catalyst with a larger Ni particle size (about
8.9 nm). Several transition metals, such as Ni, Ru, Rh, Pd, Pt, Cu,
and bimetallic alloys, have been used as active catalysts in CO
2
[4–7,26,27]
hydrogenation for the formation of CO and CH4.
The
effect of the metal particle size has been frequently considered
as one of the key factors in controlling the CO/CH selectivity
4
during CO hydrogenation. It has been shown that the CO/CH
2
4
selectivity of CO hydrogenation is sensitive to the particle size
2
[
4–6]
of metal NPs for Ni-, Ru-, and Pd-based catalysts.
According
[5,6,24]
to the literature,
it is believed that smaller particle sizes of
metal NPs can lead to a high rate of CO formation. On the
Figure 3. The TEM images of Ni(22.9)@SBA-16. The particle size distribution
is shown in the inset.
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Figure 4. Comparison of the TOF values in CO
drogenation, (B) CO formation, and (C) CH formation. The H
lysts. (D) Comparison of CH selectivity on the Ni(x)@SBA-16 and Ni(x)@SiO
2
hydrogenation for the Ni(x)@SBA-16 and Ni(x)@SiO
2
2
catalysts as a function of temperature: (A) overall CO hy-
ꢀ
1
4
2
/CO stream with a 1:1 ratio at a total flow rate of 100 mLmin was passed over 50 mg of cata-
2
4
2
catalysts at 4008C. Samples notations are same as (A) for (B) and (C).
other hand, the selectivity for CH formation can be greatly en-
selectivity can also be attributed to the highly 3D ordered
cage-type mesoporous structure of SBA-16. A possible explana-
4
hanced with larger particle sizes of metal NPs.
The selectivity of CH on the Ni(x)@SBA-16 and Ni(x)@SiO2
tion for the difference in selectivity of CO and CH is given and
4
4
catalysts at 4008C was compared and the results are listed in
illustrated in Figure S4. In the present case, the mesopores of
the SBA-16 support were partially filled by the Ni NPs, which
have an average diameter of around 4 nm. The arrangement
of these partially filled mesopores might allow them to serve
as molecular sieves, and thus have some influence on the se-
Table 2. The total conversions in the CO reduction reactions,
2
for the purpose of comparison, were controlled at a similar
conversion (10–11.5%). When the particle size of Ni NPs in
Ni(x)@SiO changed from 6.5 to 8.9 nm, the selectivity of CH4
2
increased from 5% to 12% with the increase in the Ni loading
amount. This trend should undoubtedly be in accordance to
lectivity of CO and CH . By contrast, such a molecular sieving
4
effect is not present for the case of Ni(x)@SiO , because the
2
[
5,6,24]
the results reported in the literature.
However, the
SiO support is basically amorphous in nature (Figure S5).
2
Ni(x)@SBA-16 catalysts, with the Ni loading amount ranging
from 5.8 to 22.9 wt%, effectively controlled the small sized Ni
NPs down to around 4 nm, leading to the enhanced selectivity
The H temperature-programmed desorption (H -TPD) mea-
2
2
surement was carried out to further investigate the CH selec-
4
tivity. The H -TPD results of Ni(x)@SBA-16 and Ni(x)@SiO cata-
2
2
of CH . The highest Ni loaded catalyst Ni(22.9)@SBA-16 exhibit-
lysts are shown in Figure S6. In our previous study, we have
found that the Ni catalysts provide stronger H2 adsorption,
4
ed the highest selectivity of CH formation (up to 53%) when
4
the CO2 conversion was controlled to be 11.2% at 4008C
leading to enhancement in H coverage and possibility of the
2
(
Table 2). However, there was no report in the literature that
formation of formate species as intermediate in the CH pro-
4
[24]
the small particle of transition metal-based catalysts could pro-
duction. As seen in Figure S6, H desorbed from Ni(x)@SBA-
2
vide high efficiency for CH formation, as observed in the pres-
16 around 908C. However, the intensity of H desorption was
4
2
ent Ni(x)@SBA-16 catalysts. It is believed that apart from Ni
nanoparticles loading and particle size, metal- support interac-
tions, and composition of support play an important role in
too weak to detect in the mass spectrometer when the same
H -TPD experiment was conducted on the Ni(x)@SiO catalysts.
2
2
The possible reason for the high H coverage on Ni(x)@SBA-16
2
[
28,29]
the CH selectivity.
is given based on the different behavior of H between the
4
2
From the N2 adsorption–desorption and TEM results we
have found that Ni(x)@SBA-16 catalyst has large specific sur-
face area, highly dispersed Ni species, narrow pore size distri-
bution, and highly ordered mesoporous structure. The large
specific surface area helps the adsorption and activation of the
reactant molecules, such as CO and H , and thus enhances the
SiO and SBA-16 supports, which are compared in Figure S7.
2
As seen in Figure S7, the H signal on SBA-16 is apparently
2
broader than that on SiO , implying that the cage-type ordered
2
mesopores in SBA-16 might lead to a broader distribution of
diffusion rate for H molecules. In other words, H molecules
2
2
might easily remain inside the cage-type mesopore of SBA-16,
thus lead to enhancement in the surface coverage on
Ni(x)@SBA-16. The higher H2 coverage of the Ni surface on
2
2
CO hydrogenation. The highly dispersed Ni species provide
2
more active centers for the CO hydrogenation. The higher CH
2
4
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Ni(x)@SBA-16 may result in higher CH selectivity in compari-
support; both induced a favorable reaction pathway for high
selectivity of CH in CO hydrogenation.
4
son to Ni(x)@SiO2.
4
2
Figure S8 shows the CH selectivity versus CO conversion
4
2
for the Ni(x)@SBA-16 catalysts at 2758C. As seen in Figure S8,
the selectivity of CH decreased when the conversion was de- Experimental Section
4
creased, and its value approached zero at zero conversion, in-
Synthesis of mesoporous silica SBA-16: The synthesis of cage-
type mesoporous silica SBA-16 was first carried out by following
dicative of a typical stepwise reaction in which CO was an in-
[
30]
[25,31]
termediate in the process of CH formation. The formation
the previous procedures reported in the literature.
First, 0.93 g
4
of CO and CH4 in the CO2 hydrogenation occurred on the
Ni@SBA-16 catalysts was suggested to follow the consecutive
of Pluronic F127 (Sigma–Aldrich), 0.19 g Pluronic P123 (Sigma–
Aldrich), and 4 g KCl were dissolved in 60 mL of 2m HCl aqueous
solution and vigorously stirred at 358C for 4 h. The silica precursor,
TEOS (tetraethyl orthosilicate), was then added into the solution
and the stirring was continued for 20 h. The reaction mixture was
then hydrothermally aged at 1008C for 24 h. The molar ratio of the
reaction mixture for the synthesis of SBA-16 was
pathway, where the CH formation is mainly derived from the
4
[
24,30]
[24]
CO hydrogenation.
In our previous study, the formate
(HCOO) species is attributed to be intricately involved in CO2
hydrogenation with the Ni catalysts, regardless of the Ni load-
ing and particle size. The reaction rate for the hydrogenation
of the HCOO intermediate to CH likely depends on the H cov-
1
:0.0016:0.0037:2.7:4.4:144 for TEOS/P123/F127/KCl/HCl/H O, re-
2
4
2
spectively. The precipitate was then separated by filtration, washed
with deionized water, and dried at 708C. The template removal
was carried out by calcination in air at 5508C for 6 h.
erage of the Ni surface. The low H coverage on the Ni parti-
2
cles usually leads to the quick formation of CO from the HCOO
intermediate and poor formation rate of CH . In this study, the
4
Synthesis of Ni NPs incorporated SBA-16 and SiO : The Ni NPs
2
Ni(x)@SBA-16 catalysts have more active sites for H adsorption
2
incorporated into SBA-16 with various loading amounts were pre-
pared by a wet impregnation technique followed by the thermal
reduction method. Typically, 100 mg of SBA-16 solid support was
added to aqueous solutions (10 mL) of Ni(NO ) ·6H O of different
than the Ni(x)@SiO catalysts, resulting in a higher H coverage
2
2
on the Ni surface, further enabling the high selectivity of CH4
formation.
3
2
2
As seen in Figure 4A, the turnover frequency (TOF) clearly
depends on the loading of Ni NPs. In fact, the reaction rate in-
creased with increasing the loading amount of the Ni NPs in
SBA-16. However, the Ni(22.9)@SBA-16 exhibited a lower TOF
value than other Ni(x)@SBA-16 samples, because it had the
largest Ni surface area. This implied that some of the Ni NPs in
Ni(22.9)@SBA-16 might be located outside the mesopore,
which were not as catalytically effective as the ones inside the
mesopore. Furthermore, the blockage problem might also
occur in the case of SBA-16 with a high loading of Ni NPs,
which was evidenced by the significant decrease in the surface
area of Ni(22.9)@SBA-16. As a result, the Ni(5.8)@SBA-16
sample exhibited the largest TOF value among the Ni(x)@SBA-
concentrations (0.010, 0.025, and 0.050m). A certain amount of
0.1m NaOH aqueous solution was then added dropwise into the
solution to control the solution pH in the range of 3 to 9. It was
found that a pale green precipitate was not well formed unless by
keeping the solution pH to be 9. A pale green precipitate was col-
lected and then thermally treated at 6008C for 6 h under a flow of
Ar/H (95%/5%). The resulting dark gray powder was then kept
2
under vacuum to avoid oxidation by air. Meanwhile, the Ni@SiO2
catalysts were prepared by impregnating 1 g SiO (Sigma–Aldrich,
2
2
ꢀ1
300 m g ) with 20 mL of aqueous Ni(NO ) ·6H O to prepare 3.3,
3
2
2
5.6, and 12.0 wt% Ni catalysts. All of the Ni@SiO catalysts were cal-
2
cined in air and reduced under H at 5008C for 5 h before use.
2
Catalytic CO2 hydrogenation: Catalytic performances of
Ni(x)@SBA-16 samples were investigated under catalytic hydroge-
nation of CO . All CO hydrogenation reactions were performed in
16 samples.
2
2
In summary, Ni NPs incorporated into cage-type mesoporous
a fixed-bed reactor (0.95 cm inner diameter) at atmospheric pres-
sure. A thermocouple connected to a PID (proportional–integral–
derivative) temperature controller was placed on top of the cata-
silica support SBA-16 with various loading amounts were suc-
cessfully prepared through a facile wet impregnation method.
The mesoporous silica SBA-16 with 3D ordered cage-type mes-
opores were demonstrated to be an ideal support for the
growth of well confined Ni NPs with a very small particle size
lyst bed. Catalyst samples (50 mg) were used in all of the CO hy-
2
drogenation reactions, which were conducted by treating the cata-
lyst with a stream of H /CO . The conversion rate of the reaction
2
2
was maintained at less than 10% to ensure that the conditions
were similar to differential conditions. All products were analyzed
via gas chromatography (GC) through a 12 ft Porapak-Q column.
The GC was equipped with a thermal conductivity detector (TCD).
The TOF values were calculated using the following formula:
(
around 4 nm), which were highly dispersed within the support
without aggregation. Furthermore, the particle size of Ni NPs
in the Ni(x)@SBA-16 catalysts could be well maintained even
though the Ni loading amount was increased up to 22.9 wt%,
a remarkably high loading of Ni on the support as compared
to the values reported in the literature. At a nearly identical
loading amount, the Ni(x)@SBA-16 catalysts with smaller parti-
cle size of Ni NPs surprisingly exhibited a higher catalytic activi-
ꢀ
1
TOF=(conversionꢂCO
flow
rate [mLs ]ꢂ6.02ꢂ
10 moleculesmol )/(24400 mLmol ꢂNi sites). Carbon balance
data were measured from the peak areas corresponding to CH
2
23
ꢀ1
ꢀ1
4
and CO in the chromatograms throughout the experiment over
the temperature range, which were good at approximately 95–
ty of CO hydrogenation and also led to a higher selectivity on
2
1
00%. The catalytic performance affected by the size of Ni NPs,
CH formation than the Ni(x)@SiO catalysts. This enhanced ac-
4
2
loading amounts and the textural properties of the solid support
were investigated by this method.
tivity of the Ni(x)@SBA-16 catalyst was suggested to be an ac-
cumulative result of the advantageous structural properties of
the support SBA-16 and the well confined Ni NPs within the
Characterizations: Wide-angle XRD patterns were obtained with
a powder X-ray diffractometer (Lab-X XRD-6000 SHIMADZU) using
ChemSusChem 2016, 9, 1 – 7
www.chemsuschem.org
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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Communications
monochromatic CuKa radiation (l=1.5406 ꢁ). Small-angle X-ray
scattering (SAXS) measurements were performed on Wiggler-A
beamline (l=1.321712 ꢁ) facility at National Synchrotron Radiation
Research Center (NSRRC), Taiwan. Nitrogen adsorption–desorption
isotherms were obtained with a surface area and porosity analyzer
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The financial support of this work by the Ministry of Science and
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Keywords: carbon dioxide · hydrogenation · mesoporous
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7511.
Published online on && &&, 0000
&
ChemSusChem 2016, 9, 1 – 7
www.chemsuschem.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATIONS
Battling CO with Ni: Nickel nanoparti-
C. S. Budi, H.-C. Wu, C.-S. Chen,* D. Saikia,
H.-M. Kao*
2
cles are incorporated into a cage-type
mesoporous silica support (SBA-16) with
various loading amounts through
a facile wet impregnation method. The
Ni@SBA-16 catalyst with a very high
nickel loading (22.9 wt%) exhibits ex-
ceptionally high methane selectivity for
carbon dioxide hydrogenation.
&& – &&
Ni Nanoparticles Supported on Cage-
Type Mesoporous Silica for CO2
Hydrogenation with High CH4
Selectivity
ChemSusChem 2016, 9, 1 – 7
www.chemsuschem.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ