Oxygen vacancy-rich mesoporous silica KCC-1 for CO2 methanation

Article abstract of DOI:10.1016/j.apcata.2016.12.023

Mesoporous silica KCC-1 was successfully synthesized by microemulsion system coupled with microwave-assisted hydrothermal method. Mesoporous silica KCC-1 exhibited spherical morphology surrounded with dendritic fiber with the particle size of 200–400?nm and BET surface area of 773?m²/g. Mesoporous silica KCC-1 has significantly higher number of basicity and oxygen vacancy than those of MCM-41 and SiO₂which directly correlated with the catalytic performance of the catalyst. The activity of mesoporous silica KCC-1 in CO₂methanation is five-fold higher than MCM-41 with the yield of CH₄reached 38.9% at 723?K.

Full text of DOI:10.1016/j.apcata.2016.12.023

Accepted Manuscript
Title: Oxygen Vacancy-Rich Mesoporous silica KCC-1 for
CO Methanation
2
Author: <ce:author id="aut0005"
author-id="S0926860X16306275-
9
a626b891612c3035bff03bc105dd276"> M.Y.S.
Hamid<ce:author id="aut0010"
author-id="S0926860X16306275-
dfbe72e6d809125765421b7ae5982ff3"> M.L.
Firmansyah<ce:author id="aut0015"
author-id="S0926860X16306275-
4
356f8f462d1fdde820164592a4bf3a4"> S.
Triwahyono<ce:author id="aut0020"
author-id="S0926860X16306275-
1
a5cc9365ad5cd3117322216320178f4"> A.A. Jalil<ce:author
id="aut0025" author-id="S0926860X16306275-
8c4166517ea6e9d15b365de64af5e2d"> R.R.
5
Mukti<ce:author id="aut0030"
author-id="S0926860X16306275-
0
61252f7b33cf2504921c11b52bf99c9"> E.
Febriyanti<ce:author id="aut0035"
author-id="S0926860X16306275-
e70d85185461bdb33c05c1f7a3b80f87"> V. Suendo<ce:author
id="aut0040" author-id="S0926860X16306275-
5
47e28b811c1c51d7534c25a0c876c80"> H.D.
Setiabudi<ce:author id="aut0045"
author-id="S0926860X16306275-
b7458861f3badc9f6fe340b67c371937"> M.
Mohamed<ce:author id="aut0050"
author-id="S0926860X16306275-
6
2f618871a10091052fd3197cd8b8e3c"> W.
Nabgan
PII:
S0926-860X(16)30627-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.12.023
APCATA 16108
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
12-8-2016
6-12-2016

Accepted date:
29-12-2016
Please cite this article as: M.Y.S.Hamid, M.L.Firmansyah, S.Triwahyono, A.A.Jalil,
R.R.Mukti, E.Febriyanti, V.Suendo, H.D.Setiabudi, M.Mohamed, W.Nabgan, Oxygen
Vacancy-Rich Mesoporous silica KCC-1 for CO2 Methanation, Applied Catalysis A,
General http://dx.doi.org/10.1016/j.apcata.2016.12.023
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CLEAN REVISED MANUSCRIPT
Oxygen Vacancy-Rich Mesoporous silica KCC-1 for CO
2
Methanation
a
b
b
a,c
d
d
M.Y.S. Hamid , M.L. Firmansyah , S. Triwahyono *, A.A. Jalil , R.R. Mukti , E. Febriyanti ,
d
e
a
a
V. Suendo , H.D. Setiabudi M. Mohamed , W. Nabgan
aDepartment of Chemical Engineering, Faculty of Chemical and Energy Engineering, Universiti
Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
bDepartment of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM
Johor Bahru, Johor, Malaysia.
cCentre of Hydrogen Energy, Insitute of Future Energy, Universiti Teknologi Malaysia, 81310
UTM Johor Bahru, Johor, Malaysia
dPhysical and Inorgnic Chemistry Division, Faculty of Mathemtics and Natural Sciences, Institut
Teknologi Bandung, 40132 Bandung, Indonesia
eFaculty of Chemical & Natural Resources Engineering, Universiti Malaysia Pahang,
Lebuhraya Tun Razak, 26300 Gambang, Pahang, Malaysia.
*
Corresponding author:
Sugeng Triwahyono (PhD)
Department of Chemistry, Faculty of Science
Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia.
Tel: +60-7-5534146
Fax: +60-7-5566162
Email: sugeng@utm.my
1

GRAPHICAL ABSTRACT
RESEARCH HIGHLIGHT





KCC-1 was successfully prepared by microemulsion system for CO methanation
2
KCC-1 possessed unique spherical morphology covered with dendritic fiber
KCC-1 exhibited higher basicity and oxygen vacancy than those of MCM-41 and SiO
2
The CO
2
methanation activity followed the order of KCC-1>MCM-41>SiO
2
The oxygen vacancy is directly correlated with the catalyst performance
Abstract
Mesoporous silica KCC-1 was successfully synthesized by microemulsion system coupled with
microwave-assisted hydrothermal method. Mesoporous silica KCC-1 exhibited spherical
morphology surrounded with dendritic fiber with the particle size of 200-400 nm and BET
2

2
surface area of 773 m /g. Mesoporous silica KCC-1 has significantly higher number of basicity
and oxygen vacancy than those of MCM-41 and SiO which directly correlated with the catalytic
performance of the catalyst. The activity of mesoporous silica KCC-1 in CO methanation is
five-fold higher than MCM-41 with the yield of CH reached 38.9 % at 723 K.
2
2
4
Keywords: Mesoporous Silica KCC-1; Microemulsion system; CO methanation; Oxygen
2
Vacancy; Basicity
1
.0
Introduction
The increasing concentration of CO
2
in our environment is one of a major concern, which
mitigation. Many researchers have been discussing on
capturing in terms of various environmental and economic aspects. Although various
techniques have been proposed, their immediate practical applications have drawbacks in terms
of economic factors, safety, efficiency, and dependability. The CO methanation (CO + 4H
O ΔH 298K = -165.0 kJ mol ) has become a promising process to utilize CO
triggered an intensive research of the CO
2
CO
2
2
2
2
↔
o
-1
CH
4
+ 2H
2
2
emission and to produce fuel simultaneously [1-2]. This reaction is highly exothermic and needs
a high-performance catalyst to achieve satisfactory rates and selectivity. Herein, a catalyst
support plays an important role in the catalysis and provides high amount of active sites and
lowering the activation energy for the reaction. Over the years, mesoporous silica has taken a
great interest due to their unique properties [1,3-4]. One of the examples of mesoporous silica is
a mesoporous fibrous silica particle (KCC-1) [3]. KCC-1 possessed unique spherical morphology
surrounded by fibers that resemble a sea urchin [3,5-6]. One of the advantages of this fibrous
morphology is high accessibility of the bulky mass reactant to the active sites, thus increases the
3

rate of reaction and the product formation [7]. In recent time, oxygen vacancy is considered as
one of the reactive sites in the methanation. Oxygen vacancy was reported to greatly increase the
reaction rate by adsorbing and activating the carbon-oxygen bond [8]. Researcher also claimed
that oxygen vacancies were responsible for the formation of surface carbon species, an
intermediate species to form methane [4].
Recently, KCC-1 has used mainly in drug delivery system, adsorbent, hydrogen storage,
and as a chromatography support. In contrast to the conventional mesoporous silica materials
with cylindrical pore such as MCM-41 and SBA-15, fibrous silica possessed unique ability to
further reduce the diffusion limitation and prevent Ostwald ripening if any metal is present on
the catalyst surface due to the presence of dendritic fiber [6]. Therefore, this work assessed the
physicochemical properties and the catalytic performance of the mesoporous silica KCC-1 in
CO
2
methanation along with other silica-based materials.
2
.0
Experimental
Mesoporous silica KCC-1 was prepared by using the microwave assisted hydrothermal
technique. In a typical synthesis, tetraethyl orthosilicate (TEOS, 0.012 mol) was dissolved in a
solution of cyclohexane (0.277 mol) and 1-pentanol (0.013 mol). A solution of cetylpyridinium
bromide (CPB; 0.0026 mol) and urea (0.01 mol) in 30 ml of distilled water was then added to the
first solution. The mixture was stirred for 30 min at room temperature, and the resulting solution
was placed in a teflon bottle followed by exposing to the intermittance MW irradiation (400 W)
for 4 h. Then, the product was allowed to cool at room temperature and the solid product was
4

isolated by centrifugation followed by washing with cool distilled water and acetone. The sample
was then air-dried at 383 K overnight. The as-synthesized catalyst was calcined at 823 K for 6 h.
MCM-41 was synthesized by hydrothermal method. Dodecyltrimethylammonium
chloride (0.70 mol) was dissolved in a solution of NaOH (0.24 mol) and distilled water. Then,
tetraethylortho silicate (TEOS) (1.0 mol) was added to the mixture under vigorous stirring at
room temperature for 1h. The solution was transferred to teflon bottle and heated at 413 K for 4
h. The solid product was isolated by centrifugation at 4000 rpm follwed by washing with
distilled water. The solid was air-dried at 383 K. Finally, the solid product was calcined at 823 K
for 6 h.
The crystalline structure of the catalyst was studied by X-ray diffraction (XRD) recorded
on a Bruker Advance D8 X-ray powder diffractometer (40 kV, 40 mA) using Cu Kα radiation
source with a scan rate of 0.1°. The N adsorption-desorption analysis of the catalysts was
2
carried out by using a Beckman Coulter SA 3100. Prior to the measurement, the catalyst was put
into a sample tube holder, followed by evacuation at 573 K for 1 h. Then, adsorption of nitrogen
was carried out at 77 K. UV diffuse reflectance spectrum (UV-DRS) was recorded over a range
of wavelengths from 200 to 800 nm with an Agilent Cary 60 UV–vis equipped with a PIKE
DiffuseIR reflectance accessory. Surface area was determined by using BET method and pore
size distribution was assessed by using non-local density functional theory (NLDFT) method.
The surface morphology and size of all catalysts were characterized with transmission electron
spectroscopy (TEM) (JEOL JEM-2100F) and field emission scanning electron microscopy
(FESEM) (Zeiss Supra VP35). Pyrrole probed IR spectroscopy was used to characterize the basic
properties of catalyst. The measurements were performed by in situ FTIR on an Agilent Cary
6
40 FTIR spectrometer equipped with a high-temperature stainless steel cell with CaF2
5

windows. Prior to the measurements, all samples were activated at 673 K for 1 h. Then, the
activated catalyst was exposed to 4 Torr of pyrrole at room temperature for 15 min, followed by
outgassing at room temperature for 15 min. All spectra were recorded at room temperature with a
spectral resolution of 8 cm−1 with three scans. Electron spin resonance (ESR) spectrometer
(JEOL JES-FA100) was used to observe the formation of electron holes or unpaired electrons.
The catalyst was evacuated at 673 K for 1 h. All signals were recorded at room temperature.
29
While, the Si MAS NMR spectra were obtained on a Bruker Solid NMR (Bruker Avance 400
MHz) spectrometer. The 29Si MAS NMR signal of tetramethylsilane (TMS) was used as a
chemical-shift reference. The spectra were recorded using 4 ms radio frequency pulses, a recycle
delay of 60 s and spinning rate of 7 kHz using a 4 mm zirconia sample rotor.
CO
23–773 K. Initially, 0.2 g of catalyst were treated in an air stream at 823 K for 1 h followed by a
hydrogen stream for 3 h and cooled down to the desired reaction temperature in a hydrogen
stream. After the temperature was stable, a mixture of H and CO was fed into the reactor at a
mass ratio of 4. The composition of the outlet gases
2
methanation was carried out in a fixed-bed quartz reactor at temperature range of
4
2
2
-1
-1
GHSV of 50,000 mL g-cat h and H
2
/CO
2
was analyzed by an on-line gas chromatograph (7820 N Agilent Gas Chromatograph) equipped
with a TCD detector. The moisture trap was installed at the outlet gas line of the reactor to
prevent moisture from entering to the gas chromatograph. To determine the activity and
selectivity, the sampling was done after 1 h of steady-state operation at each temperature. The
conversion of CO
2
, selectivity of methane, yield of methane and rate of methane formation were
calculated by the following equations:
푀
+ 푀퐶푂
퐶퐻4
푋퐶푂2
=
(1)
푀
+ 푀퐶퐻4 + 푀퐶푂
퐶푂ꢀ
6

푀
퐶퐻4
푆퐶퐻4
푌
=
(2)
푀
+ 푀퐶푂
퐶퐻4
= 푋퐶푂ꢀ푥 푆퐶퐻4
(3)
(4)
퐶퐻
4
푀퐶퐻4
−
2 −1
푅푎푡푒 (푚푚표푙 푚 푠 ) =
푆퐴 푥 푊 푥 푡
Where, 푋퐶푂ꢀis the conversion of carbon dioxide, 푆퐶퐻4is the selectivity of methane YCH4 is the
yield of methane and M is the mole number of particular chemicals. The CH formation rate was
expressed as mole number of CH formed (µmol CH ) divided by surface area, SA (m g ),
catalysts weight, W (g-cat) and time, t (s).
4
2
-1
4
4
3
3
.0
.1
Results and discussion
Physical properties of the catalysts
Figure 1A shows x-ray diffraction (XRD) patterns of the catalysts. For MCM-41 (Fig.
A-b), the pattern revealed three peaks at 2θ = 2.2, 3.8 and 5.5° which indexed as (100), (110)
1
and (200) reflections. These peaks attributed to the typical 2D hexagonally ordered of
mesostructured (p6mm) with uniform mesopore packing [9-10]. While, mesoporous silica KCC-
1
and SiO showed the absence of diffraction peak at 2θ < 10° and the presence of small and
2
broad diffraction peak centered at 2θ ≈ 23° for all catalysts (inset figure) indicating the presence
of amorphous silica phase in the catalysts (JCPDS No. 29-0085) [11].
Mesoporous silica KCC-1 and MCM-41 exhibited a similar type IV isotherm with type
H1 hysteresis loop which indicated to the presence of mesopores. While, SiO showed a type II
2
7

isotherm with type H3 hysteresis loop, which associated to the presence of material with random
network of pores or macroporous material (Figure 2) [12]. Mesoporous silica KCC-1 and MCM-
4
1 isotherms showed two steps of capillary condensation. The first capillary condensation step
was observed at P/Po = 0.3 which attributed to the intraparticle porosity on the materials. In this
step, MCM-41 showed a higher N uptake which could be assigned to the presence of higher
2
uniform intraparticle pores than mesoporous silica KCC-1. The second step of capillary
condensation was observed at P/Po = 0.9 which ascribed to the presence of large pore or
irregular stacking of the materials. However, in this step MCM-41 showed a small increase in the
N
2
uptake indicating the minimum presence of interparticle porosity. On the other hand, SiO
2
showed a small capillary condensation step with gradual slope in the middle and steep slope at
higher partial pressure. This is indicating the presence of pores with multidimensional cylindrical
channels [13-14]. In accordance with the isotherms results, pore distribution from non-local
density functional theory (NLDFT) method confirmed the presence of mesopore in the catalysts.
Mesoporous silica KCC-1 showed a narrow pore distribution in the range of 4-6 and 20-25 nm,
which could be attributed to the pores from the surfactant self-assembly and the distance between
mesoporous silica KCC-1 fibers, respectively. Intraparticle porosity of MCM-41 was presented
in the range of 2-4 nm. The pore distribution of SiO was much lower compared to other catalyst
2
probably due to amorphous structure of SiO
2
. Mesoporous silica KCC-1 possessed highest pore
2
2
volume despite that the surface area (773 m /g) was smaller than MCM-41 (1052 m /g) as listed
in Table 1. This could be probably caused by the presence of large pores from the distance
between mesoporous silica KCC-1 fibers.
Microscopic FESEM and TEM images of the catalysts are shown in Figure 3.
Mesoporous silica KCC-1 showed uniform microsphere (200-400 nm) with well-defined fibrous
8

morphology. TEM images of mesoporous silica KCC-1 showed the presence of irregular fiber
with 10-20 nm distances between each fiber. While, MCM-41 elucidated the rod-shaped
structure with irregular crystal particle size in average 500 nm. While SiO exhibited an
2
abundance of micro granules particles that interconnected to each other with average particle size
about 20 nm.
3
.2
Basic properties of the catalysts.
-1
IR results showed that all catalysts possess an absorbance band at 3740 cm ascribed to
the silanol groups (SiOH) with bending vibration on the external surface (figure not shown).
Figure 1B shows the IR spectra of pyrrole adsorbed on the catalysts. Stretching band at 3650–
-1
3
250 cm can be assigned to the N–H stretching vibrations of pyrrole molecules interacting with
the framework oxygen by hydrogen boding and interacting via aromatic system with the non-
framework cations which occurred simultaneously and influence each other [4, 15-17]. The
-1
concentration of basic sites was determined based on the intensity of the peaks at 3440 cm for
-1
-1
mesoporous silica KCC-1, 3463 cm for MCM-41, and 3470 cm for SiO
KCC-1 showed the highest peak intensity indicating the presence of highest concentration of
basic sites which followed by MCM-41 and SiO (Table 1). The shifting of the IR band assigned
2
. Mesoporous silica
2
to N-H stretching vibration in the gas phase to a lower wave number upon the interaction of the
H atom in pyrrole with a basic site could be associated to the strength of the basic sites [2, 15].
As depicted in Figure 1B, mesoporous silica KCC-1 showed the shifting of N–H stretching
-1
-1
-1
vibration Δv(NH) = -90 and -60 cm ; Δv(NH) = -67 cm for MCM-41 and Δv(NH) = -56 cm
for SiO . Thus, the basic strength of the catalysts is following the order of mesoporous silica
KCC-1 > MCM-41 > SiO
2
2
.
9

UV DRS analysis showed a similar pattern for all catalyst with two peaks at around 230
-1
-1
-1
and 315 cm as shown in Figure 4A. The band at 227 cm for KCC-1, 230 cm for MCM-41
-1
and 233 cm for SiO
2
can be attributed to neutral oxygen which include simple oxygen vacancy
-1
(
≡Si−Si≡) and twofold-coordinated silicon (−O−•Si•−O−). While the band around 315 cm can
be related to light-emitting non-bridging oxygen hole center [18]. This indicated the presence of
oxygen vacancy in the catalyst. The presence of oxygen vacancy in the catalysts was also
measured by using electron spin resonance (ESR) spectroscopy. Figure 4B shows the ESR
signals of the catalysts after evacuated at 673 K. The presence of the oxygen vacancies in the
catalysts was indicated by the presence of g-values at 1.99, 1.95, and 1.91 [19-21]. These g-
values were attributed to the structure vibration of non-bridging oxygen hole centers (NBOHCs)
in silica. These sites transform bulk SiO into SiOx (x < 2) which resulted in the formation of
2
dangling bonds (E’ centers) or non-bridging oxygen hole (NBOHCs) with unpaired electrons as
magnetic sites [18, 21]. This radical sites of silica; ≡Si–O• (NBOHC) and ≡Si• (E’ center) may
act as catalytic active sites for the CO methanation. Based on Figure 4B, mesoporous silica
2
KCC-1 revealed higher intensity of g-value at 1.99 which associated to the paramagnetic oxygen
vacancies (O2-) species [20, 23-24]. Therefore, it is envisaged that the mesoporous silica KCC-1
will have a higher activity in CO
2
methanation than those of MCM-41 and SiO (Table 1). To
2
assess whether the activity of the mesoporous silica KCC-1 catalysts arises from stable oxygen
vacancy sites, ESR studies was also performed on the spent catalyst. Figure 4C shows the fresh
and spent mesoporous silica KCC-1 catalysts after evacuated at 673 K. The result indicated the
presence of paramagnetic oxygen vacancies at g-value 1.99 [20, 23-24] dropped around 20%.
This shows that the oxygen vacancy can be recycled during the reaction. A study on oxygen
1
0

defect recyclability was also done by previous research on tungsten oxide and reported that the
oxygen vacancies were indeed recyclable [25].
All catalysts were also characterized by solid-state 29Si MAS NMR spectroscopy to
determine the type of environment of Si atoms (Figure 5). Mesoporous silica KCC-1 and MCM-
4
1 possessed two high peaks at -102 and -112 ppm confirming the (≡SiO)
3
Si and (≡SiO) Si
4
species are the most dominant components in the catalysts [26–27]. However, overlapping
resonances were observed in mesoporous silica KCC-1 catalyst at -102 ppm and -106 ppm.
Lippman et al., (1980) had reported that different geometry of SiO tetrahedra can caused the
29
slight shift solid-state Si NMR spectra [28]. Therefore, the resonance line at -102 ppm may be
attributed to the bridging tetrahedra (≡SiO) Si and a line at -106 ppm may be attributed to the
non-bridging tetrahedra (≡SiO) Si [29]. The presence of the signal at -106 ppm provided a
3
3
structural defect by oxygen-excess center known as non-bridging oxygen. The presence of
paramagnetic mesoporous silica KCC-1 catalyst caused the resonance of NMR shifting and line
broadening [30]. A weak (≡SiO)
KCC-1 and MCM-41. On the contrary, the spectrum of SiO
to -120 ppm with low intensity of all SiO tetrahedral species, which could be attributed to the
non-uniform framework of SiO .[30] The ratio (≡SiO) Si/(≡SiO)4Si indicates the extent of
silanol condensation, which is inversely proportional with the degree of framework condensation
31]. The ratio was found for mesoporous silica KCC-1 = 0.8; MCM-41 = 0.9; and SiO = 1.0.
2
Si signal at −93 ppm was also observed for mesoporous silica
2
showed a broad peak between -78
2
3
[
2
Based on those ratios, it can be concluded that the modification of silica morphology
significantly altered the degree of framework condensation.
CO
2
can be used as probe molecule in FTIR to give more understanding on how the
species change and adsorption capacity of the catalyst. Figure 6A and 6B shows series of FTIR
1
1

spectra of adsorbed CO
with 4 Torr of CO and heated from 323 to 623 K with 50 K increment. CO
like species when interact with surface oxygen vacancy. Both catalysts revealed peak at 1631
2
on mesoporous silica KCC-1 and MCM-41 respectively after exposure
2
2
formed carbonate-
-1
cm which can be attributed to symmetric O=C=O stretching modes of bridged carbonyl [32]. At
room temperature, the adsorption band for mesoporous silica KCC-1 was higher than MCM-41
which shows higher CO
2
accessibility to active sites on mesoporous silica KCC-1. As the
temperature increased from room temperature to 623 K, the intensity of 1631 peak decreased for
-1
both catalyst. New peaks were observed at 2360 and 2340 cm which attributed to linear
O=C=O band. Linear O=C=O was also acknowledged as intermediate for CO methanation [4].
2
The peak revealed at temperature 473 and 573 K for mesoporous silica KCC-1 and MCM-41
respectively and becomes more prominent at higher temperature. The splitting of the O=C=O
band might be caused by the repulsive interactions of two linear CO
active sites of silica site or physisorption on different active sites [33]. Therefore, having more
adsorbed CO would be an advantage in order to achieve higher catalytic activity. SiO in the
adsorption which explained the low activity of the
2
molecules on the same
2
2
other hand showed no significant peak of CO
catalyst.
2
The changes in the hydroxyl vibration region caused by adsorption of molecular
hydrogen for the temperature range of 173–573 K was shown in Fig. 6C. The adsorption of
molecular hydrogen on the mesoporous silica KCC-1 catalyst at 173 K interacted with the acidic
-1
bridging hydroxyl groups thus evolving a peak around 3610 cm . However, the peak disappear
-1
as the temperature increase The intensity of OH stretching band at 3740 cm increased and a
-1
new peak emerged at 3715 cm . An increase in the adsorption temperature to 223 K intensified
-1
the band at 3740 cm due to an increase in the amount of adsorbed molecular hydrogen while
1
2

-1
-1
eliminating peak at 3715 cm . At 248 K, the band at 3740 cm has been partially eliminated,
-1
and new broad bands at 3680 cm appeared. Further heating to 573 K caused the bands of the
-1
-1
perturbed silanol groups at 3740 cm to disappear, and the bands at 3680 cm to further
intensify. The evolution of the absorbance bands maybe due to the alteration of the physical
adsorption of molecular hydrogen at low temperature to the hydrogen bonded OH groups at
relatively higher temperature [34]. Previous research by Vitiello M. et al., revealed that
dissociation of hydrogen in the following defect sites: Si singly occupied sp3 dangling bonds (E’
centers), ≡Si*; nonbridging oxygen centers (NBO), ≡Si-O*; divalent Si=Si; and neutral oxygen
vacancies, ≡Si-Si≡ [35]. Thus, increase in oxygen vacancy and surface defect will enhance
hydrogen activation in this process
The experiments of ESR adsorbed CO
vacancies in adsorption of CO . Figure 7 shows the ESR signal of all catalysts, where all
catalysts were evacuated at 673 K followed by reduction with hydrogen and heated in the
presence of CO to study the interaction between CO molecule and oxygen vacancies. The CO
pre-adsorbed mesoporous silica KCC-1 was then subjected to hydrogen gas exposure to study
the effect of hydrogen in the CO methanation. The true line represents the evacuated catalyst
after reduction with hydrogen. Comparing the true lines, the intensity of KCC-1 was highest,
followed by MCM-41 and lastly SiO which indicated abundance of oxygen vacancies on
mesoporous silica KCC-1. Introduction of CO influenced the decrease of the 1.99 g-values
intensity at temperature 373 K for all catalysts as the oxygen vacancies were filled with oxygen
atom from the gaseous CO [13]. The oxygen vacancies participate in the catalytic process by
storing and releasing oxygen and/or enhancing the dispersion of supported metal. The same
results were obtained by Wang et al., where the oxygen vacancy of CeO level remain high at
2
were also done to understand the role of oxygen
2
2
2
2
2
2
2
2
2
1
3

2
93 K and only shows decrease trend between 373 to 473K [8]. SiO peak was almost not visible
2
at temperature of 473 K. The peak for MCM-41 and mesoporous silica KCC-1 decreased by 19
and 27.5% respectively when heated up to 473 K. The higher reduction of g-value of 1.99 peak
shows more oxygen from gaseous CO
is consistent with FTIR study of adsorbed CO
potential towards CO adsorption thus increasing the chances for reaction to occur.
2
occupied the oxygen vacancies. This result of ESR CO
2
2
where mesoporous silica KCC-1 shows higher
2
3
.3
Catalytic testing
To evaluate the catalytic activity of the catalysts, CO
temperature range of 423 – 773 K. As shown in Figure 8A, the rate of CH
catalysts increased markedly with increasing of the reaction temperature. At 723 K, the rate of
2
methanation was carried out at
4
formation for all
-8
-8
CH
.4x10 mmol m s , respectively. However, the rate of CH
change at relatively higher reaction temperature. The product distribution of CO
4
formation for mesoporous silica KCC-1, MCM-41 and SiO
2
was 9.2x10 , 6.8x10 and
formation did not significantly
methanation by
-8
-2 -1
2
4
2
different type of silica catalysts were shown in Figure 8B. At temperature lower than 523 K, the
methanation was inactive for all catalysts. The selectivity towards methane for all catalysts
increased as the reaction temperature increase. MCM-41 and mesoporous silica KCC-1 showed
CH
4
selectivity of 72% and 84% respectively at 773 K. SiO
2
exhibited non-significant
conversion despite having high selectivity. The selectivity of CH
4
proceeded at slower increment
at higher temperature which followed the same trend of reaction rate data (Figure 8A). This may
be due to the reversed water gas shift reaction (RWGS) which takes place with a small
contribution of steam reforming (SR) of methane to form CO [36,37]. The high rate of CH
4
formation of mesoporous silica KCC-1 could also be attributed to the smaller activation energy
1
4

of the catalyst in the CO
was 47.14 kJ/mol which is more than 40% lower than those of MCM-41 (85.53 kJ/mol) and SiO
109.65 kJ/mol) (Table 1).
2
methanation. In fact, the activation energy of mesoporous silica KCC-1
2
(
Figure 8C shows the plots of the dependence of catalytic activity towards basicity and
oxygen vacancy of the catalysts. For all catalysts, the ratio of CH formation rate and number of
oxygen vacancy did not exhibit significant difference, indicating the activity of the catalysts is
determined by the number of oxygen vacancy. Thus, it is suggested that the higher rate of CH
formation over the mesoporous silica KCC-1 might be caused by the presence of large number of
oxygen vacancies that can promote the CO adsorption followed by transformation of CO to
form CH , subsequently [4,37]. Study by Singh and Polshettiwar also showed a similar pattern in
CO adsorption where KCC-1 resulted in a superior capture capacity compared to MCM-41. It is
due to nature of fibrous morphology which gave higher accessibility of CO in capturing process
38]. The dissociation of hydrogen in the other hand, the hydrogen bonding can be cleaved by
4
4
2
2
4
2
2
[
metal and strong oxidant. Thus, silica, which is a strong oxidant has the ability to dissociate
hydrogen molecule. Previous research has also discussed the rate determining step in
methanation process and proposed that the rate-determining step was either the formation of the
CHxO intermediate or the CO dissociation to form surface carbon [39]. The stability of
mesoporous silica KCC-1 towards CO
2
methanation reaction was conducted at 723 K for 90
hours. Figure 8D shows mesoporous silica KCC-1 maintained high activity throughout the
-
8
-2 -1
experiment. The rate of CH
of CH at the average of 83%. The ability of the mesoporous silica KCC-1 to remain active after
0 hours might be caused by properties of mesoporous silica KCC-1 that prevent the Ostwald
ripening deactivation [40]. Mesoporous silica KCC-1 had been proven to be stable at temperature
4
formation remained around 9.0 x10 mmol m s , with selectivity
4
9
1
5

as high as 1173 K thus reaction at 723 K might possess less effect toward sintering of the catalyst
10]. ESR result for spent catalyst also proved the stability of the catalyst with the reusability of
the oxygen vacancies.
[
The mass transfer in a porous catalysts is a one of the main factors contribute to the
activity of the catalysts. The reaction rate is influenced by inter and intra-particle diffusion
limitation of the pore size of the catalysts [40]. To estimate the influence of external and internal
pore diffusions, Mears (NM) and Weisz–Prater (NWP) criteria were used, respectively. External
mass transfer and internal diffusion were evaluated according to Mears (Equation 5) and Weisz-
Prater criteria (Equation 6) [40].
ꢅ푏ꢆ
ꢂ−푟
ꢇ푝퐿 ꢈ 푛
ꢉ
ꢃꢄ
ꢀ
푁 =
< 0.ꢊ5
(5)
(6)
ꢁ
푘 퐶
ꢃ
ꢃꢄ
ꢀ
ꢀ
ℜ ꢈ
ꢉ
푁ꢋ푃 = 퐶ꢆ퐷 < ꢊ
ꢌ푓푓
ꢏꢐꢑ
-3
-1
Where, , (ꢍꢎ퐶푂ꢀ ) is the highest reaction rate of CO (mol m -cat s ), n is the reaction
2
order, R
is the gas-solid mass transfer coefficient (ms ), ℜ is the reaction rate of CO
of the catalyst, C is the surface concentration of CO and Deff is the pore diffusion. However, all
catalysts showed Mear’s (N ) value lower than 0.15 and Weisz-Prater’s (NWP) value for all
p
is the catalyst radius, pL is the bulk density, ꢒ퐶푂ꢀ is the concentration of gas, solid and
-1
k
c
2
per unit volume
s
2
M
catalysts was smaller than 0.1, which indicated the absence of external diffusion and pore
diffusion limitation, respectively.
Mesoporous silica KCC-1 showed highest catalytic performance compared with bare Si-
based materials such as MCM-41, mesostructured silica nanoparticle, rice husk ash and SiO
Table. 2) [13, 41-44]. However, mesoporous silica KCC-1 exhibited almost similar CO
conversion compared with metal (Pd, Ni or Co) supported on Si-based materials. For metal
2
(
2
1
6

supported on Si-based catalysts, the metal supposedly acts as active species for adsorption and
dissociation of CO by providing an electron deficient species [45]. Conversely, the metal seems
2
not indispensable for mesoporous silica KCC-1 since it already possesses a sufficient number of
oxygen vacancy on the framework which will act as electron deficient species to
adsorb/dissociate CO . In fact, mesoporous silica KCC-1 could exhibit comparable performance
2
with metal coupled with silica-based materials. Therefore, mesoporous silica KCC-1 with fibrous
silica morphology structure outwits other silica-based materials with ordered mesoporous
structure and amorphous structure
3
.4
Mechanistic Study
The reaction mechanism and pathways of CO methanation did not reach consensus.
2
However, the general agreed idea were CO adsorbed onto catalyst support and dissociation of
2
hydrogen on the metal sites [46]. Other than metals, strong oxidant such as silica was also able to
cleave the hydrogen bonding to produce molecular hydrogen. Previous study also suggested that
radical sites of silica or also known as E’ centre was one of the dissociation sites on silica based
catalyst [47]. In this study, ESR and FTIR studies were used to identify the possible mechanism
of CO
2
methanation over mesoporous silica KCC-1.
ESR studies of adsorbed CO were discussed in previous section (Figure 7C) where the
ESR signal at g = 1.99 showed decreasing trend with the increasing temperature. This suggested
the adsorption of CO at the oxygen vacancies sites increase with increase of temperature. The
pre-adsorbed CO on mesoporous silica KCC-1 was then subjected to hydrogen gas exposure
2
2
2
and heated from 323 to 473 K. As shown in Figure 7D, the ESR signal at g-value = 1.99
continued to decrease, however, only by 9% when heated up to 423 K. These phenomena might
1
7

be caused by small amount of electron which originated from dissociation of hydrogen, filled the
oxygen vacancy. This result was similar to a research on Ir/Pt-HZSM5 where the oxygen
vacancy signal decreased when hydrogen was introduced and heated [48]. When the temperature
was increased to 473 K, the signal increased slightly which might be from desorption CH
product or CO by-product thus leaving the oxygen vacancy unoccupied.
4
As shown in Scheme 1, oxygen atom from gaseous CO was expected to fill the
2
vacancies located at the surface of mesoporous silica KCC-1 due to the nature of basicity of the
catalyst. This would result in formation of carbonyl species. A recent study has shown that
surface defects, such as oxygen vacancy, play an important role because CO adsorbed in the
2
oxygen vacancy can easily dissociate to form CO [8]. At temperature lower than 473 K, only
bridged carbonyl was formed while at temperature higher than 473 K, both bridged and linear
carbonyl were formed as evidenced by CO
2
probed FTIR study. The peak corresponded to linear
formation started to increase
carbonyl started to emerge at 473 K where the rate of CH
4
significantly. Thus, it was suggested that the linear carbonyl was the precursor to the
methanation reaction on mesoporous silica KCC-1. Hydrogen on the other hand, would
dissociate at the E’ centers to produce molecular hydrogen. The cycle was continued with
hydrogenation of carbonyl species to form methane and water. Hydrogen probed FTIR study
reveals that the adsorption of molecular hydrogen differ at low and high temperature. The
interaction of hydrogen would play role to production of H
2
O, CH and small amount of by-
4
product CO by dissociation and further hydrogenation of CO species. The similar method was
also proposed for MgO catalyst by Kim et al., where they suggested the steps of methanation
process can be concluded by the chemisorption of CO
2
on the catalyst surface, hydrogenation of
oxygen to produce water and lastly further hydrogenation of carbon to produce methane [49].
1
8

4
.0
Conclusions
In conclusion, silica fibrous morphology greatly affected the catalytic performance of
mesoporous silica KCC-1 in the CO
2
methanation which may be due to the presence of abundant
adsorption/dissociation. The presence of oxygen vacancy
oxygen vacancy facilitating the CO
2
facilitates the formation of bridged carbonyl only at temperature lower than 473 K while both
bridged and linear carbonyl at temperature higher than 473 K. Linear carbonyl was suggested to
be main route for the formation of methane as it corresponded to the increase in catalytic activity
when linear carbonyl was present. Hydrogenation of these surface carbonyl then form water and
methane molecule. Mesoporous silica KCC-1 also has a tremendous potential to be an effective
catalyst in general heterogeneous catalysis by providing low mass transfer limitation and
activation energy. Mesoporous silica KCC-1 also shown good stability throughout 90 hours of
reaction due to the resistance towards Ostwald ripening.
Acknowledgements
A part of this work was supported by the Nippon Sheet Glass Foundation for Materials Science
and Engineering, Japan (No. 4B181) and Ministry of Higher Education, Malaysia through
Fundamental Research Grant Scheme (No. 4F781) and MyPhD Scholarship for M.Y.S. Hamid.
Justification for publication
1
9

Silica-based catalyst has been a popular choice for catalysis due to their advantages such as
modifiable and high thermal stability. Mesoporous silica KCC-1 emerged as a new morphology
silica material having a unique spherical shape surrounded with fiber. Several application of
KCC-1 have been examined such as in drug delivery, photocatalyst, heavy metal removal and
CO capturing. In this report, we presented another application of mesoporous silica KCC-1 in
2
the carbon dioxide methanation. Based on ESR and FTIR analyses, KCC-1 possesses a large
number of oxygen vacancy and basic sites which are the most important active sites for
methanation process. The number of oxygen vacancy and basic sites are larger than those of
MCM-41 and SiO
2
. As a result, the catalytic activity of CO methanation followed the order of
2
KCC-1 > MCM-41 > SiO
2
.
Mesoporous silica KCC-1 is a promising material for the hydrogenation of CO and general
2
catalysis which the analyses and utilization of KCC-1 fit into the scope of this journal. In fact,
mesoporous silica KCC-1 has a great potential to be an effective catalyst for general reaction due
to the properties of KCC-1 such as modifiable, large surface area, high accessibility and high
thermal stability.
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.
CAPTION TO FIGURES
Figure 1
(A) XRD pattern and (B) Pyrrole adsorbed IR spectra of (a) Mesoporous
silica KCC-1, (b) MCM-41, (c) SiO2.
Figure 2
2
N adsorption–desorption isotherm and pore size distribution of all
catalysts.
Figure 3
Figure 4
FESEM and TEM images of all catalysts.
(A) DR UV spectra (B) ESR spectra catalysts (C) ESR spectra of fresh
and spent mesoporous silica KCC-1
Figure 5
Figure 6
29Si NMR spectra of all catalysts with Gaussian curve-fitting.
CO
at (a) room temperature (b) 323 K (c) 373 K (d) 423 K (e) 473 K (f) 523 K
g) 573 K and (h) 623 K (C) IR spectra of hydrogen adsorbed on activated
2
adsorbed IR spectra of (A) Mesoporous silica KCC-1, (B) MCM-41
(
meseporous silica KCC-1 when the catalysts were exposed to 13.3 kPa
hydrogen at (b) 173 K, followed by heating at (c) 198 K, (d) 223 K, (e)
2
5
48 K, (f) 273 K, (g) 303 K, (h) 323 K, (i) 373 K, (j) 423 K, (k) 473 K, (l)
23 K and (m) 573 K. (a) Before exposure to hydrogen.
Figure 7
ESR spectra for all catalyst. The molecule adsorb onto the catalyst are
identified in the parentheses and heated at (a) 323 K (b) 373 K (c) 423 K
(d) 473 K. The true line represent the evacuated spectra after reduction
with hydrogen
2
4

Figure 8
(A) Rate of CH
4
formation (B) Product Selectivity (C) Stability test for
Mesoporous Silica KCC-1 (D) Dependence study of catalysts.
Scheme 1 Possible reaction mechanism of CO2 methanation over mesoporous silica KCC-1
2
5

Figure 1
2
6

Figure 2
2
7

Figure 3
2
8