Probing the differential methane decomposition behaviors of n-Ni/SiO2, n-Fe/SiO2 and n-Co/SiO2 catalysts prepared by co-precipitation cum modified St?ber method

Article abstract of DOI:10.1039/c5ra10997c

n-Ni/SiO₂, n-Fe/SiO₂ and n-Co/SiO₂ nano-catalysts were prepared by co-precipitation cum modified St?ber method and applied for thermocatalytic decomposition of methane in order to investigate their thermal stability and activity to produce greenhouse gas free hydrogen and nano-carbon. The mean particles sizes of the produced nano-catalysts obtained from BET analysis are 32.19 nm, 30.26 nm and 49.92 nm, respectively. Temperature programmed methane decomposition were conducted as a preliminary catalytic examination and further isothermal analyses were performed at 700 °C, 600 °C and 500 °C. Production of hydrogen at each experimental temperatures and corresponding carbon yield were measured. Among the three catalysts inspected, n-Ni/SiO₂ was found to be the most efficient one for thermocatalytic methane decomposition. Furthermore, significant catalytic stability was observed with n-Ni/SiO₂ at 500 °C and 600 °C. While, the rapid deactivation of the n-Fe/SiO₂ and n-Co/SiO₂ catalysts are attributed to particle agglomeration and the irregular formation of nano-carbon due to metal fragmentation. Physical and chemical characteristics of the produced nano-catalysts were investigated by N₂ adsorption-desorption measurement (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM) and hydrogen-temperature programmed reduction (H₂-TPR). Produced nano-carbon were inspected with TEM, FESEM and XRD.

Full text of DOI:10.1039/c5ra10997c

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PAPER
Probing the differential methane decomposition
behaviors of n-Ni/SiO , n-Fe/SiO and n-Co/SiO
2
2
2
Cite this: RSC Adv., 2015, 5, 67227
catalysts prepared by co-precipitation cum
modified Stober method
¨
U. P. M. Ashik and W. M. A. Wan Daud*
2 2 2
n-Ni/SiO , n-Fe/SiO and n-Co/SiO nano-catalysts were prepared by co-precipitation cum modified
St o¨ ber method and applied for thermocatalytic decomposition of methane in order to investigate their
thermal stability and activity to produce greenhouse gas free hydrogen and nano-carbon. The mean
particles sizes of the produced nano-catalysts obtained from BET analysis are 32.19 nm, 30.26 nm and
4
9.92 nm, respectively. Temperature programmed methane decomposition were conducted as a
ꢀ
ꢀ
preliminary catalytic examination and further isothermal analyses were performed at 700 C, 600 C and
ꢀ
5
00 C. Production of hydrogen at each experimental temperatures and corresponding carbon yield
2
were measured. Among the three catalysts inspected, n-Ni/SiO was found to be the most efficient one
for thermocatalytic methane decomposition. Furthermore, significant catalytic stability was observed
ꢀ ꢀ
with n-Ni/SiO at 500 C and 600 C. While, the rapid deactivation of the n-Fe/SiO and n-Co/SiO
2 2 2
catalysts are attributed to particle agglomeration and the irregular formation of nano-carbon due to
metal fragmentation. Physical and chemical characteristics of the produced nano-catalysts were
Received 10th June 2015
Accepted 30th July 2015
investigated by N
2
adsorption–desorption measurement (BET), X-ray diffraction (XRD), transmission
DOI: 10.1039/c5ra10997c
electron microscopy (TEM) and hydrogen-temperature programmed reduction (H
2
-TPR). Produced
www.rsc.org/advances
nano-carbon were inspected with TEM, FESEM and XRD.
explore the catalytic application of transition metals with size in
the nano-range. Achieving nano-size is quite challenging and
Introduction
Forthcoming advancements in catalysis are very reliant on the hence there are many techniques were developed. Those
fabrication of catalytic materials with advanced features. methods can be broadly classied as (i) condensation from
Nowadays, investigations on catalyst technology are predomi- vapor, (ii) synthesis by chemical reaction, and (iii) solid-state
nantly focusing on development of smaller materials with effi- processes such as milling. Co-precipitation, a chemical reac-
cient properties like activity, selectivity and stability. The novel tion synthesis was adopted in our study to prepare adequate
advancements in nanotechnology have simplied the produc- metal oxide nano-particles. The inherent surface characteristics
tion of very ne nano-particles with narrow size distributions of such nano-particles are highly desirable for catalysis
1
less than 100 nm. Hence, nano-materials can be dened as purposes. While, those naked nanoparticles are undergoing
those materials having one or more dimension in the nano- easy aggregation at higher temperature results in its faster
meter scale (<100 nm) range. These nano-materials have gained degradation and hence excluded from higher temperature
3
predominant consideration because of the immense changes catalytic applications. We have targeted to safeguard active
that occur in their physical and chemical properties when micro metal phase with stable, inert and porous material by modied
3
particles transform to nanoparticles. The catalytic performance St ¨o ber method. Henceforth, it can effectively prevent higher
of nano-materials is associated with their morphology, size temperature agglomeration and tolerating its higher tempera-
distribution, and electronic properties. It is well known that the ture utilizations.
preparation methods, stabilizer, as well as supports selected,
can individually inuence all of these characteristic properties.
The produced porous and high temperature withstanding
catalysts were used for thermocatalytic decomposition of
2
So far, enormous research efforts have been devoted in order to methane (TCD) to produce hydrogen and nano-carbon, two
cherished products in the eld of environmentally benign
energy and nanotechnology. Hence, there have been intense
Department of Chemical Engineering, University of Malaya, 50603, Kuala Lumpur, research efforts on methane decomposition in recent years as
Malaysia. E-mail: upmashik@gmail.com; ashri@um.edu.my; Fax: +60 379675319;
hydrogen is a relevant raw material in chemical and petroleum
Tel: +60 105023818; +919496844805
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ꢀ
industries. However, a higher temperature (>1200 C) is
necessary in methane decomposition process to achieve a
Experimental section
rational yield. Hence, a variety of metal catalysts have been Co-precipitation method was adopted for preparing ne nano-
studied for methane decomposition with the purpose of sized metal hydroxide precipitate and those hydroxides was
reducing the decomposition temperature as well as the effectively supported with silicate. Initially, nano-sized M–OH
increasing conversion rate. Among them, Ni, Fe and Co based containing suspension was prepared by treating metal nitrate
catalyst gained major attention because of their advantages like with ammonia solution at room temperature. Hence, agglom-
4
–8
availability, low cost, better activity and stability. According eration of metal oxides at comparatively higher temperature
9
,10
to Takenaka et al.,
Ni-based catalyst are very active in a was effectively eluded. The SiO support was fabricated through
2
ꢀ
ꢀ
3
temperature range of 400 C–600 C for methane decomposi- modied St ¨o ber method. It involves the hydrolysis of a mixture
tion. And they found that Ni-based catalysts deactivated of tetraethyl orthosilicate (TEOS) and octadecyl trimethoxy
ꢀ
immediately at temperatures above 600 C. Whereas, Fe-based silane (C18TMS) with aqueous solution of ammonia in the
catalyst are found active at higher temperature as the activation suspension of nano-M–OH.
temperature of the Fe-based catalysts is much higher than that
11–13
of the Ni-based catalysts.
Furthermore, iron-based catalysts
Chemicals used
are comparatively inexpensive and non-toxic. However, it was
ꢀ
14
All chemicals purchased were used without performing any further
reported that Fe-based catalysts are not active below 650 C. It
is observed that iron-based catalysts produce thin-wall nano-
tubes, which are the most valuable product among carbon
purication. Nickel(II) nitrate hexahydrate (Ni(NO
cobalt(II) nitrate hexahydrate (Co(NO $6H O) and octadecyl
trimethoxy silane (C18TMS) were obtained from Acros Organics.
Aldrich supplied iron(III) nitrate nonahydrate (Fe(NO $9H O)
and tetraethyl orthosilicate (TEOS). Ammonia solution and
ethanol from R&M solutions and 99.999% hydrogen, 99.995%
methane and 99.99% nitrogen were purchased from Linde
Malaysia Sdn. Bhd.
3 2 2
) $6H O),
3
)
2
2
15
nano-bers. Cobalt is adjacent to nickel and iron in periodic
system and it is found procient for methane coupling reac-
3
)
3
2
16,17
tions.
The intention of this work is to implement similar
preparation methodology for the preparation of Ni, Fe and Co-
based catalyst and study the differences in their activity,
accordingly. It was reported that nickel particles larger than
100 nm is incompetent to produce carbon nano laments in
methane decomposition because the produced carbon isolates Preparation of nano-metal/SiO catalyst
2
15
metal from reaction medium rapidly. Hence, a support for the
active phase is necessary in order to prevent its sintering in
hydrocarbon media. However, a wide variety of dissimilar
support materials were investigated to control the catalyst
particle size and dispersion by physical interactions (porous
0
2
.02 mol of metal nitrate was homogeneously dissolved in
00 mL water by sonication. Dissolved metal nitrate was allowed
to precipitate as corresponding hydroxide by the drop-wise
addition of 6 mL of 30% NH3 solution under sonication for
1
h. The resulting suspension was stirred for another 1 h with
magnet at room temperature. Then, centrifuge the solution at
000 RPM for 30 minutes and wash the precipitate two times with
18
support) or chemical interaction (charge transfer effect). Tak-
9
enaka et al. explored inuence of different catalytic supports like
4
+
MgO, Al O , SiO , TiO , ZrO , MgO$SiO , Al O $SiO , H -ZSM-5,
2
3
2
2
2
2
2
3
2
deionized water and one time with ethanol. Aerwards, the
product was dispersed in 100 mL of ethanol and continuously stir
for 15 h with magnet. Sonicate the suspension for 10 minutes and
etc. and concluded that SiO2 as the most efficient catalyst
support for TCD to produce hydrogen and nano-carbon. Hence,
we have selected SiO as the support material for conserving
2
active metal phase. The porous silica support efficaciously
add 4 mL of 8 M NH solution. Add 0.4 mL of TEOS and 0.4 mL
3
C18TMS simultaneously to the basic dispersion under sonication
in order to form silicate support to guard active metal phase,
named modied St ¨o ber method. The resulting mixture further
sonicate for 60 minutes at room temperature. Stir the solution for
provide enough porosity for accessing reactant molecule to the
19
active metal oxide. However, in this paper we have investigated
characteristic properties of n-Ni/SiO , n-Fe/SiO and n-Co/SiO
2
2
2
nano-catalysts and their activity and stability for thermal
decomposition of methane. We have given equal importance to
catalyst preparation and characterization, methane decompo-
sition and characterization of produced carbon. Various tech-
niques were adopted for physical and chemical characterization
of produced nano-catalysts as follows; N adsorption–desorp-
2
tion measurement (BET), X-ray diffraction (XRD), transmission
electron microscopy (TEM) and hydrogen-temperature pro-
further 5 hours over a magnetic stirrer. Subsequently, M–OH/SiO
2
precipitate was separated by centrifugation and dry in an oven at
ꢀ
1
00 C for 15 hours. Separated precipitate was then converted to
ꢀ
metal oxide/SiO by calcination at 450 C for 3 hours. Calcinated
2
samples are named as n-NiO/SiO , n-FeO/SiO and n-CoO/SiO .
2
2
2
Finally, metal oxide/SiO nanocatalysts were reduced to metal/
2
SiO
are named as n-Ni/SiO
Schematic illustration of catalyst preparation is shown in
Scheme 1.
2
by treating with 30% H
2
for 2.5 h and the reduced samples
2
, n-Fe/SiO
2
and n-Co/SiO
2
.
grammed reduction (H -TPR). Furthermore, activity and
2
stability of catalysts were analyzed in a xed bed pilot plant. In
addition, the characterization of the formed nano-carbon bers
and tubes at various temperatures are explained with the help of
TEM, FESEM and XRD.
Characterization techniques
Investigation of physicochemical properties of the catalyst done
by means of different characterization methods such as N
2
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Scheme 1 Schematic illustration of the fabrication of silicate supported nano-metallic catalyst.
adsorption–desorption measurement, X-ray diffraction (XRD), minutes by ushing with helium gas. Upon degassing, the
transmission electron microscopy (TEM), eld emission scan- reductive gas mixture consisting of 5% hydrogen balanced with
ꢁ1
ning electron microscopy (FESEM) and hydrogen temperature nitrogen at a ow rate of 20 mL min streamed through the
ꢀ ꢀ
programmed reduction (H -TPR).
sample. The sample was heated from 200 C to 700 C to obtain
2
Nitrogen adsorption–desorption measurements (BET the TPR proles of the sample.
method) were carried out in Micromeritics ASAP 2020 BET
ꢀ
apparatus at ꢁ196 C. Surface area, pore size distribution and
Catalytic activity
structure, pore volume and the mean particle size were
measured. Samples were previously degased at 180 C for 4 h.
ꢀ
Experimental setup. Schematic representation of catalytic
methane decomposition unit is shown in Fig. 1. The xed
catalyst bed reactor constructed with stainless steel (SS310S)
has the following dimension: outer diameter ¼ 6.03 cm, wall
thickness ¼ 0.87 cm and height ¼ 120 cm. A quartz tube (3.56
cm internal diameter, 4 cm outer diameter, and 120 cm height),
obtained from Technical Glass Products (Painesville, USA), was
placed inside the reactor in order to avoid interaction of feed
gas with stainless steel. A quartz frit with 150 mm to 200 mm
porosity was used as the catalyst bed. Temperature was supplied
with a vertically mounted, three-zone tube furnace (model TVS
The surface area was determined according to the standard
Brunauer–Emmett–Teller (BET) method in a relative pressure
range of 0.04–0.2 and the total volume was evaluated from the
amount of adsorbed N at a relative pressure (P/P ) of about
2
o
0.98. The pore diameter distributions were calculated based on
desorption isotherms by the Barrett–Joyner–Halenda (BJH)
method.
PANalytical diffractometer was used to collect X-ray diffrac-
tion (XRD) patterns of the fresh and spent catalysts. Crystal
phase and structure of the nano-catalysts were determined.
X'pert HighScore soware were used for diffractogram evalua-
tion. Diffraction patterns of the samples were recorded with a
Rigaku Miniex with Cu Ka radiation with a generator voltage
and a current of 45 kV and 40 mA, respectively. The average
crystallite size was obtained using the global Scherrer equation
as follows:
12/600, Carbolite, UK).
ꢀ
ꢁ
0
:9l 180
D
avg
¼
bcos q
p
In equation, the average crystallite size, peak length, line
broadening full width at half-maxima aer subtracting the
instrumental line broadening (in radians), and the Bragg's
˚
angle are expressed as Davg (nm), l (1.54056 A), b, and 2q,
respectively. 0.9 is the Scherrer constant.
Transmission electron microscopy (TEM) images of fresh
catalyst and produced nano-carbon were acquired by using FEI
Tecnai™ controlled at an accelerating voltage of 200 keV. Field
emission scanning electron microscopic (FESEM) images of
produced nano-carbon and elemental composition of the cata-
lysts were obtained with FEG Quanta 450, EDX-OXFORD.
2
Temperature-programmed reduction (H -TPR) measure-
ments were carried out using a Micromeritics TPR 2720
analyzer. Typically, 0.03 g of catalyst sample was placed in a U-
tube holder and the sample was rst cleaned at 130 C for 60
ꢀ
Fig. 1 Simplified schematic visualization of methane decomposition
unit.
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Temperature measurements were recorded by using two K- precipitation from the respective metal nitrate solution with
type thermocouples (1/16 in diameter, Omega, USA). The rst 30% NH solution; (b) direct silicate support formation over
3
thermocouple was xed on the exterior surface of the stainless produced nano metal hydroxide with a mixture of C18TMS and
23
steel tube. The second thermocouple was inserted into the TEOS. C18TMS was added to the reaction mixture in the sense
quartz tube momentarily for calibration and removed aerward of increasing the porosity of SiO support. And (c) higher
from the quartz tube prior to testing because its internal copper temperature treatments in order to enhance its porosity. The
2
2
0
ꢀ
material could affect the TCD of methane. In addition, pres- high temperature treatments such as calcination (at 450
C
ꢀ
sure and temperature indicators were placed at different loca- under air) and reduction (at 550 C under 30% H
2
balanced with
tions to control the operating conditions. A two-differential
N
2
) prior to activity examination supposed to remove all organic
0
0
00
pressure transducer (0 H O to 4 H O) was supplied by Sen- moieties and convert metal oxides to their metallic form. The
2
2
socon to measure the pressure drop across the reactor. Mass added C18TMS helps to sparse silica polymerization and
ꢁ
1

ow controllers (Dwyer, USA) in the range of 0–2 L min were produces more pores inside the silica network aer calcination.
used to control the gas ow rates. The outow gas was then The efficient silicate support excellently prevent the particle
cooled down to room temperature by means of an air cooler. agglomeration during those high temperature treatment. A
Solid particles that had sizes greater than 2 nm and high series of characterization were conducted to enlighten the
molecular weight components were separated using two lters characteristics of produced nanostructures. Furthermore,
(38 M membrane, Avenger, USA). A calibrated Rosemount activity and stability were studied for TCD at various tempera-
Analytical X-STREAM (UK) was used as an online analyzer to ture in a xed bed pilot plant.
compute the mole percentage of methane and hydrogen.
Characterization
Temperature programmed methane decomposition
The crystalline structure, size and phase purity of n-Ni/SiO , n-
Fe/SiO2 and n-Co/SiO2 were determined by X-ray diffraction
2
1
g of catalyst was homogeneously distributed over catalyst bed
and purge nitrogen for 30 minutes at room temperature to clean
the furnace and catalyst at ow of 1 L min . Increase the bed
temperature to 550 C with a ramp of 20 C min and pass 30%
H in N feed for 2.5 hours to reduce the metal oxide catalyst to
its metallic form. Then, decrease the furnace temperature to
(
XRD). The patterns of all those three catalysts before and aer
ꢁ
1
ꢀ
reduction at 550 C for 2.5 hours in 30% H
2
/N
2
are presented in
ꢀ
ꢀ
ꢁ1
Fig. 2(a–c). Hence, the apparent crystallite structure of catalyst
has noteworthy impacts on its longevity and activity. The crys-
talline size corresponds to each peak calculated from the full
width at half maximum of diffraction peak using Scherrer
equation is provided near to peaks, those are clearly close to the
mean particle size obtained from nitrogen adsorption–desorp-
tion analysis (furnished in Table 1). The strong diffraction
peaks in the XRD patterns of the catalysts characterize the high
2
2
ꢀ
2
00 C by air cooler under N
2
ꢁ
ow. Pass 99.995% methane with
1
a ow rate of 0.64 L min
decomposition from 200 C to 900 C with ramp of 5 C min
for temperature programmed
ꢀ
ꢀ
ꢀ
ꢁ1
.
Isothermal methane decomposition
Catalyst bed was uniformly covered with 0.5 g of catalyst. Pure crystallinity of the metallic catalysts. n-Ni/SiO sample exhibits
2
nitrogen was passed for 30 minutes in order to clean the furnace three major diffraction peaks corresponding to (111), (200) and
ꢁ1
and catalyst at ow rate of 1 L min . Then, system temperature (220) reections of the solid. The diffraction peaks of n-NiO/
ꢀ
ꢀ
ꢁ1
ꢀ
ꢀ
ꢀ
was increased to 550 C with a ramp of 20 C min . Reduction SiO2 are positioned at 2q ¼ 37.2294 , 43.2752 , 62.8456 ,
ꢀ
75.3690 and 79.3668 with d-spacing values 2.415 A˚ , 2.090 A˚ ,
ꢀ ꢀ
of catalyst was conducted at 550 C by passing 30% H
2
in N
2
feed for 2.5 hours. Then, increase/decrease the temperature to 1.477 A˚ , 1.260 A˚ and 1.206 A˚ , respectively. All those peaks are in
reaction temperature under N ow, accordingly. Once desti- good agreement with standard card of cubic NiO with JCPDS no.
2
2
2
nation temperature reached, N ow was replaced with 99.995% 01-073-1523. The peaks of XRD pattern for the n-Ni/SiO are at
ꢁ1
ꢀ
ꢀ
ꢀ
methane with a ow rate of 0.64 L min
methane conversion.
for evaluating 44.4667 , 51.8934 and 76.3347 with d-spacing values 2.037 A˚ ,
1.762 A˚ and 1.247 A˚ , respectively. Positions of these diffraction
peaks are in good agreement with those are given in JCPDS no.
0
1-070-1849 for nickel phase. However, it also shows NiO peaks
Results and discussion
Catalyst preparation
with lower intensity, indicating its incomplete reduction at
mentioned condition. It is believe that the reduction with 30%
ꢀ
Nano-sized Ni, Fe and Co metal oxides were prepared by co- hydrogen for 2.5 hours at 550 C was adequate to convert naked
precipitation method using 30% NH
tating agent. Silicate support was developed by treating the severe conditions for complete reduction. Hence, n-Ni/SiO
metal hydroxide suspension with mixture of C18TMS and TEOS structures exhibit both metallic and metal oxide phases
3 2
solution as a precipi- n-NiO to n-Ni, while the reinforced n-NiO with SiO needs more
2
24
in an alcoholic medium named modied St ¨o ber method. (Fig. 2(a)). While, the intensity of n-NiO phases are found
Hence, the alcoholic medium reduces the agglomeration of abridged aer reduction process. The remaining n-NiO phases
21,22
particle as well as free silicates.
While, we haven't used any in the sample even aer reduction diligently interact with
surfactants in order to prevent agglomeration of metal oxide porous silicate support, which supposed to establish a better
particles. The comprehensive catalyst preparation process catalysis environment results a more stable reaction course
contains three stages as follows: (a) metal hydroxide during the TCD process. However, the incomplete reduction of
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Fig. 2 XRD patterns of (a) n-NiO/SiO
2
and n-Ni/SiO
2
, (b) n-FeO/SiO
2
and n-Fe/SiO
2
and (c) n-CoO/SiO
2
2
and n-Co/SiO . Average crystal sizes
computed by using Scherrer equation are provided.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
n-NiO phases are not retarding activity of n-Ni/SiO during TCD 55.7140 , 59.3773 , 65.2070 , 74.2270 and 77.2868 as shown
2
process as methane itself is acting as an excellent reducing in Fig. 2(c). These peaks are showing similarity with CoO with
agent and hence there is no n-NiO phases were detected in XRD JCPDS no. 01-074-1657. One can see that the intensity of all CoO
patterns aer TCD (Fig. 9(a)). The absence of SiO
pattern can be noticed, it is because of its X-ray amorphous
characteristics and hence it was not detected in XRD. In and n-CoO/SiO
Fig. 2(b), n-FeO/SiO , exhibits peaks centered at 2q values as measurements. The corresponding N2 adsorption–desorption
2
in the XRD peaks verily reduced aer reduction stage.
The porous nature of the prepared n-NiO/SiO
were explored with N adsorption–desorption
2
, n-FeO/SiO
2
2
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
follows: 18.4802 , 30.3579 , 35.7862 , 37.3971 , 43.5235 , isotherms and pore size distributions are presented in Fig. 3(a–
5
ꢀ
ꢀ
ꢀ
3.9752 , 57.5678 , 63.3233 , 71.6516 and 74.7656 . These c), respectively. The pore diameter distributions of the samples
diffraction peaks are very closely similar to that of maghemite-Q considered from desorption division of the isotherm were
Fe with JCPDS no. 00-013-0458. While, most of the FeO calculated using Barrett–Joyner–Halenda (BJH) method. The
peaks were disappeared aer reduction treatment and exhibit single point surface area, Brunauer–Emmett–Teller (BET)
2 3
O
ꢀ
ꢀ
following peaks at 44.6929 and at 65.0464 with d-spacing surface area, pore volume, average pore size and average
˚
˚
values 2.027 A and 1.433 A which corresponds to iron with particle size of all investigated nano-catalysts are shown in
JCPDS no. 01-089-7194. In addition, it shows peaks for non- Table 1. Comparatively lower particle size and higher surface
ꢀ
ꢀ
ꢀ
ꢀ
reduced FeO at 35.1381 , 36.5943 , 42.5573 and 61.7748
area and porosity were observed. Those catalytic activity
with lower intensity. n-CoO/SiO exhibit peaks at following 2q promoting features could be attributed to the presence of sili-
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
values: 18.9856 , 31.3425 , 36.8190 , 38.6456 , 44.8147 , cate support. However, the silicate support is supposed to
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prevent metal particle agglomeration efficiently during high
temperature calcination treatment. The BET surface area of n-
2
ꢁ1
NiO/SiO , n-FeO/SiO and n-CoO/SiO are 93.18 m g , 99.11
2
2
2
2
2
ꢁ1
ꢁ1
m g and 50.06 m g , respectively. One can see that the BJH
pore width distributions are in wide range from 0 nm to 160 nm,
while the majority of the pores of all experimented catalysts are
with a width less than 30 nm. Furthermore, the average BET
pore width is calculated as 9.9 nm, 14.9 nm and 7.5 nm for n-
2 2 2
NiO/SiO , n-FeO/SiO and n-CoO/SiO , respectively. The pores
with higher sizes (50 nm to 150 nm) occurred in the meso-
porous and macroporous region may be because of the forma-
tion of voids due to inter-nanoparticles in contact.
The morphological appearance of the fresh n-NiO/SiO
2
, n-
FeO/SiO and n-CoO/SiO catalysts are exhibited in Fig. 4(a–c).
2
2
Particle size distribution of respective nanoparticles are also
exhibited. From the image, it is clearly visible that the particles
are found uniformly distributed with various shapes covering a
narrow range of sizes. However, crystallite in n-FeO/SiO exhibit
2
more or less circular disc morphology with almost similar
particle size and dispersion, supporting the particle distribu-
tion histogram. While, n-NiO/SiO and n-CoO/SiO can be seen
2 2
agglomerated to form giant particles in some area. This
agglomeration results in structural intricacy leads to difficulties
in reduction, supporting a higher temperature H
2
-TPR curve
(Fig. 6). ImageJ soware was used to measure the actual particle
sizes. The average particle sizes of all three compounds are lying
in the range of 35–50 nm, supporting our BET and XRD report.
EDX mapping report of each samples are shown in Fig. 5,
assuring the presence of Ni, Fe, Co, Si and O elements.
The H
2
-TPR proles of n-NiO/SiO
2 2
, n-FeO/SiO and n-CoO/
SiO are shown in Fig. 6. The H
exhibit a single peak in between 300 C and 692 C can be
2
2
-TPR prole of n-NiO/SiO
2
ꢀ
ꢀ
assigned to the complete reduction of n-NiO species, supporting
2
5,26
previous records.
n-NiO/SiO2 catalyst exhibits only one
ꢀ
ꢀ
hydrogen conception peak in between 297 C and 670 C with a
ꢀ
maximum at 420 C. This individual peak indicates a homo-
geneous metal support interaction between nickel and silicate.
It can be noted that n-NiO/SiO
2
could be reduced in between
ꢀ
300–600
C
supporting previous reports on Ni-based
2
7
ꢀ
compounds.
H
2
-TPR prole of n-NiO/SiO
2
starts from 297 C
and hence it can be assumed there is no silica free dispersed
15
nickel oxide. However, Ermakova et al. reported that silica free
ꢀ
ꢀ
nickel oxide reduces in between 236 C and 246 C. Further-
more, the H -TPR prole of n-NiO/SiO quanties a H2
2
2
ꢁ
1
conception of 330.3 mL gcat . n-NiO/SiO
2
catalyst exhibits a
broad reduction prole irrespective of the conventional metal
28
supported catalysts. However, it can be speculate that this
strong nickel metal and silicate support interaction may results
25
in difficulty in its reduction. Furthermore, the existence of the
reduction peak towards a higher value can be attributed to the
presence of some higher sized n-NiO particles. Moreover, the
denser SiO support may cause difficulty in hydrogen diffusion
2
and NiO reduction, supporting the presence of NiO peaks in the
ꢀ
XRD results aer reduction at 550 C (Fig. 2(a)). H
2
-TPR prole
ꢀ
ꢀ
of n-FeO/SiO
2
exhibits one peak from 264 C to 448 C with a
maximum value at 388 C can be attributed to the reduction of
ꢀ
29
Fe
2
O
3
.
Usually Fe has a lower tendency to form a strong
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Fig. 3
N
2
-adsorption–desportion isotherms of the (a) n-NiO/SiO
2
, (b) n-FeO/SiO
2
and (c) n-CoO/SiO
2
catalysts. The pore diameter distributions
calculated with Barrett–Joyner–Halenda (BJH) method is shown near to each isotherm.
30
interaction with SiO2. However, formation of second peak Catalytic methane decomposition
indicates the noticeable shi of complete reduction of n-FeO/
Preliminary catalytic activity tests were conducted with all nano-
SiO towards a higher temperature values. Reduction of n-CoO/
2
catalysts in order to track the active temperature zone of each
catalyst and results are exhibited in Fig. 7. Temperature pro-
SiO
2
accomplished in three stages. The peak in the temperature
ꢀ
ꢀ
range from 243 C to 360 C can be accredited to double stage
reduction of spinal as Co
SiO
ꢀ
grammed methane decomposition was conducted from 200 C
3
1
3
O
4
to CoO and to Co. While, n-Co/
ꢀ
ꢀ
ꢁ1
to 900 C with a temperature ramp of 5 C min . Temperature
programed methane decomposition results reveal that
ꢀ
2
does not show any peaks above 600 C reveals its weak
2
metal support interaction compared to that in n-NiO/SiO .
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Fig. 4 TEM images and particle size distribution histogram of (a) n-NiO/SiO
2
, (b) n-FeO/SiO
2
2
and (c) n-CoO/SiO . 50 nanoparticle were
considered to plot particle size distribution histogram. ImageJ software was used to measure particle size.
ꢀ
ꢀ
n-Ni/SiO
2
catalyst is really active from 450 C to ꢂ730 C with a methane decomposition equation (CH
4
2
/ 2H + C). The
ꢀ
maximum hydrogen production values of 57.28% at 730 C and maximum hydrogen production percentage was observed in the
undergone fast deactivation upon increasing temperature. very beginning of methane decomposition experiment, just
While, n-Co/SiO exhibits an active zone from 510 C to 645 C. aer the contact of methane with catalyst. Aerwards, the
ꢀ ꢀ
2
The maximum hydrogen production occurred over n-Co/SiO
2
is hydrogen production found decreased gradually with time on
ꢀ
22.3% at 645 C. Furthermore, hydrogen production observed stream according to the performance of catalyst. Fig. 8(a) shows
over n-Fe/SiO was almost negligible when compared with that that, n-Ni/SiO2 catalyst exhibits a wide range of activity with
2
of Ni and Co based catalysts. n-Fe/SiO2 produced 7.8% of hydrogen production from 17% to 65% in the experimented
hydrogen at its most active temperature and the active temperatures. The experiments were extended up to 240
2
temperature range was very narrow. A gradual increase in minutes in order to evaluate the stability of n-Ni/SiO .
ꢀ
ꢀ
hydrogen percentage was observed above 800 C with all Maximum hydrogen production were observed at 700 C with n-
experimented catalysts can be attributed to the endothermic Ni/SiO catalyst, while catalyst was deactivated rapidly and turn
2
nature of methane decomposition reaction. Based on the out to be completely inactive within 100 minutes of experiment.
results of temperature programmed methane decomposition, This fast deactivation may be attributed to its thermal degra-
3
isothermal methane decomposition experiments were con- dation at higher temperature. It is interesting to notice that, n-
ducted for detailed catalysis evaluation at temperatures like Ni/SiO maintain its activity even aer 240 minutes of experi-
2
ꢀ
ꢀ
ꢀ
ꢀ
5
00 C, 600 C and 700 C.
mental duration with a very low catalytic deactivation at 600 C
ꢀ
Fig. 8 shows the changes in hydrogen production percentage and 500 C. However, minimum deactivation was observed at
with time on stream for the TCD over n-Ni/SiO
n-Co/SiO
temperature on hydrogen production, activity and temperature and supported Ni-based catalyst prepared by conventional
sustainability of each catalyst are analyzed. All isothermal methods at a temperature range of 500 C–700 C,
ꢀ
2
, n-Fe/SiO
2
and 500 C. Furthermore, it is worth pointing out that the experi-
ꢀ ꢀ ꢀ
catalysts at 500 C, 600 C and 700 C. Inuence of mented n-Ni/SiO catalyst is noticeably superior to the naked
2 2
ꢀ
ꢀ
32–36
clearly
examinations were conducted with 99.995% methane. Rose- demonstrate the advantage of co-precipitation cum modied
mount Analytical X-STREAM detected only hydrogen and St ¨o ber method for preparation of n-Ni/SiO catalyst. Undesir-
2
ꢀ
methane as gaseous products as indicated in the balanced ably, n-Fe/SiO
2
catalyst was active at 700 C only. Furthermore,
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Fig. 5 EDX mapping and elemental composition of (a) n-NiO/SiO
2
, (b) n-FeO/SiO
2
2
and (c) n-CoO/SiO .
ꢀ
ꢀ
the initial hydrogen production was very less (12.2%) compared temperatures like 600 C and 500 C as observed in temperature
to n-Ni/SiO
2
catalyst and reached negligible value by 1 hour of programmed methane decomposition (Fig. 7). n-Co/SiO
2
given
ꢀ
ꢀ
stream. It was completely inactive at experimented moderate initial hydrogen production at 700 C and 600 C.
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that the initial catalytic activity and deactivation rate increases
as increasing decomposition temperature, indicates the
temperature sensitivity of TCD process. However, it is worth to
note that the isothermal methane conversion over all experi-
mented catalyst clearly follows the hydrogen production
percentage and active temperature zone revealed by tempera-
ture programmed methane decomposition (Fig. 7). Hence,
temperature programmed methane decomposition can be
considered as an efficient step in order to identify catalytically
active temperature zone of any catalyst.
Characterization of produced nanocarbon
Fig. 9 displays the XRD patterns of n-Ni/SiO , n-Fe/SiO and n-
2
2
Co/SiO catalysts aer TCD process at experimented tempera-
2
ꢀ ꢀ
tures. XRD peaks for n-Fe/SiO
were omitted as carbon production was negligible at those
temperatures. Graphitic carbon produced over n-Ni/SiO cata-
2
aer TCD at 500 C and 600 C
Fig. 6
2
H -TPR profile of n-NiO/SiO
2
, n-FeO/SiO
2
and n-CoO/SiO
2
.
2
ꢀ
lyst can be identied by the diffraction peaks at 2q ¼ 26.26 and
ꢀ
4
4.45 as indicated in Fig. 9(a). Those peaks are in good
agreement with the JCPDS no. 98-005-3781 for graphite.
However, reduced Ni-phases can be recognised at peaks 2q ¼
ꢀ ꢀ ꢀ
4.5 , 51.83 and 76.28 , conrmed with Ni peaks in JCPDS no.
4
0
1-070-1849. n-Fe/SiO
carbon as the peaks observed at 2q ¼ 26.38 and 44.39 in
2 2
and n-Co/SiO produced graphitic
ꢀ
ꢀ
Fig. 9(b and c), are in good agreement with JCPDS no. 00-041-
1487. Furthermore, the presence of iron carbide (Fe
3
C) can be
identied over n-Fe/SiO
2
catalyst aer TCD (Fig. 9(b)) with the
ꢀ
ꢀ
ꢀ
ꢀ
peaks at 2q ¼ 37.76 , 45.01 , 70.88 and 78.65 , correspond to
the peaks in JCPS no. 00-034-0001. The intensity of peaks
corresponds to graphite produced over n-Fe/SiO and n-Co/SiO2
2
2
are not high as those observed with n-Ni/SiO . However, it is
observed that the graphitization intensity of produced nano-
carbon improved as increasing decomposition temperature. It
is clear from the amendment of the carbon peaks to a higher
values in Fig. 9(a and c) in a similar manner to those produced
over Ni impregnated zeolite catalyst as observed by Nasir Uddin
Fig. 7 Hydrogen production in percentage during temperature pro-
grammed methane decomposition over 1 g of n-Ni/SiO , n-Fe/SiO
2
ꢀ
2
and n-Co/SiO catalysts. Temperature range 200–900 C, flow rate
2
ꢁ
1
0
.64 L min .
40
et al.
Enormous quantity of carbon were formed over n-Ni/SiO2
compared to n-Co/SiO and n-Fe/SiO . The carbon yield
percentage over each catalyst at respective temperatures were
While, catalytic stability was pitiable and dip to 5% within 10
2
2
minutes aer methane stream reached the catalyst, similar
37
rapid catalytic deactivation were observed by Lee et al. over Co-
based catalysts. It is reported that higher methane decomposi-
tion to hydrogen and nano-carbon occurs over coalesced metal
particles, while it continues until the crystal size of sintered
41,42
calculated with the following equation
and the results are
depicted in Fig. 10. The carbon yield of the catalysts was eval-
uated based on the extent of methane conversion against time
ꢁ1
38
on stream at a CH ow rate of 0.64 L min . Carbon deposition
4
particle favor the nano-carbon growth. It is worth to notice
that the Ni-particle agglomeration is very less aer methane
decomposition as shown in TEM images (Fig. 11), results in
higher hydrogen production (Fig. 8(a)) as well as huge carbon
ꢀ
ꢀ
period was 5 hours for n-Ni/SiO at 500 C and 600 C, while all
2
other percentages are up to the complete deactivation of
respective catalyst.
yield (Fig. 10). However, the lower activity of n-Co/SiO (Fig. 8(c))
weight of deposited carbon on the catalyst
Carbon yield ð%Þ ¼
2
and n-Fe/SiO
2
(Fig. 8(b)) may be attributed to the sintering of
weight of metal portion
metal particle to giant sizes which exceed the critical size for
ꢃ 100
carbon nano-lament growth, as observed in Fig. 12 and 13,
39
respectively. Furthermore, Ashok et al. reported that metal
Fig. 10 comprises the comparison of produced carbon yield
over each experimented catalyst which reveals that n-Ni/SiO2
particles of very large size were unable to grow carbon nano-

laments. Among the studied catalysts, n-Ni/SiO2 catalyst is
produced very high quantity of nano-carbon compared to n-Co/
SiO and n-Fe/SiO . n-Ni/SiO produced 4947.3% of carbon at
2 2 2
superior to Fe and Co based catalyst by all aspects. It is observed
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2 2 2
Fig. 8 Hydrogen formation percentage during isothermal methane decomposition over (a) n-Ni/SiO , (b) n-Fe/SiO and (c) n-Co/SiO catalysts
ꢁ1
at different temperature. Flow rate ¼ 0.64 L min and catalyst weight ¼ 0.5 g.
ꢀ
6
00 C during 5 hours of experiment. Hence, it was not deac- dip of formed nano-carbon. Very low carbon yield observed over
ꢀ
tivated during the experimented period. While, it produced n-Co/SiO at 600 C can be attributed to the complete encap-
1
2
ꢀ
372.6% nano-carbon at 700 C before it deactivated in 100 sulation of catalyst with produced nano-carbon and heavy
minutes. However, 105.2% and 144.6% nano-carbon were agglomeration of catalyst as shown in Fig. 12(b). However, pear
formed over n-Co/SiO catalyst before its complete deactivation or diamond shaped metals with its sharp tail inserted to the
2
ꢀ
ꢀ
at 700 C and 600 C, respectively. Whereas, it was very low at produced nano-tubes can be seen in Fig. 11(a–c) and in
ꢀ
45
6
00 C as n-Co/SiO undergone fast deactivation. Very regret- Fig. 12(c), following tip-growth carbon formation mechanism,
table performance was shown by n-Fe/SiO
04.4% of nano-carbon at 700 C. Though, n-Fe/SiO was almost Ni/SiO catalyst also produces different types of nano-carbons
inactive at 500 C and 600 C as observed in temperature pro- as follows: sh-bone nanocarbon, carbon nano tubes with
grammed methane decomposition (Fig. 7). The observed open end, carbon nano tubes with closed end and carbon
carbon yield is outstanding compared to many other available nanotube with Ni particle embedded in it. Such varieties of
2
46–48
2
, which produced which is reinforcing many previous works.
Furthermore, n-
ꢀ
1
2
2
ꢀ
ꢀ
41
results over Ni-based catalyst. However, the performance of n- nano-carbons were absent with n-Fe/SiO
Co/SiO and n-Fe/SiO are pitiable. Likewise, such disgraceful nano-tubes, irregular carbon formulation was observed over n-
results were reported by Zadeh and Smith over Co-based Co/SiO
catalysts. This deprived catalyst performance can be attributed metal particle fragmentation which maintains the availability of
2
. In addition to carbon
2
2
43
2
2
and n-Fe/SiO , could be attributed to the occurrence
49
to the faster particle agglomerations and complete catalyst more active metal phases. The availability of such higher
encapsulation with produced carbon as shown in TEM images active metal phases because of the diffusion of supersaturated
(Fig. 12 and 13).
nano-carbon results in the formation of more nano-carbon
Fig. 11(a–c) displays TEM images of produced nano-carbon around the catalyst particles by methane decomposition
ꢀ ꢀ ꢀ
over n-Ni/SiO catalyst at 700 C, 600 C and 500 C, respec- which leads to its complete encapsulation. Furthermore, the
2
tively. However, TEM images of produced nano-carbon over n- carbon diffusion occurred may be attributed to the less effective
Co/SiO and n-Fe/SiO exhibited in Fig. 12 and 13, respectively. interaction between Co and Fe metals with the silicate supports
TEM images (Fig. 11 and 12) elucidate that n-Ni/SiO and n- or their incomplete shielding which results in the domination
2
2
2
Co/SiO mainly produce carbon nano-tubes, while small quan- of Co and Fe metal phases at their surface. It is obvious from the
2
tity of nano-bers were also identied. Hence, nano-tube can be displayed TEM images (Fig. 12 and 13) that n-Co/SiO and n-Fe/
2
recognized with the presence of a hallow cavity, though it is SiO catalysts were rapidly agglomerated and encapsulated with
2
44
absent with nano-bers. Ni and Co metals can be seen at the produced carbon aer methane came in contact with it and
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Fig. 9 XRD patterns of (a) n-Ni/SiO
2
(b) n-Fe/SiO
2
and (c) n-Co/SiO
2
after isothermal methane decomposition at different temperature. Peaks
corresponds to graphitic carbon, Ni, FeO, Fe
3
C and Co are indicated.
hence loose its activity completely. However, such metal particle
fragmentation is absent with n-Ni/SiO because of the efficient
2
interaction between Ni metal phase and silicate supports results
in the enhanced activity and stability. It is worth to note that
nano-carbon with larger diameter were formed over n-Co/SiO
2
ꢀ
ꢀ
at 600 C and over n-Fe/SiO at 700 C may be attributed to the
2
formation of carbon over agglomerated larger catalysts parti-
cles, and hence encapsulated by carbon leads to their faster
31
deactivation. Similar result was already reported by Jana et al.
over the spinel catalysts. It was reported that the outer diameter
of the carbon nanotubes greatly depend on the size of catalyst
particles. Hence, larger particles produce carbon nanotubes
50
15
with larger diameter. Furthermore, Ermakova et al. reported
such a speedy catalyst encapsulation with carbon over Fe-based
catalyst. However, there is no such agglomeration or encapsu-
2
lation can be seen with n-Ni/SiO (Fig. 11) which endure a longer
activity and produces a huge carbon deposition at all their
experimented temperatures. The formation of nano-carbon over
Fig. 10 Comparison of calculated carbon yield in percentage
produced over respective catalyst at 700 C, 600 C and 500 C.
ꢀ
ꢀ
ꢀ
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decomposition depend on the catalysts, catalytic characteristics
and operating parameters.
FESEM images of produced nano-carbon over n-Ni/SiO2- at
respective temperature are displayed in Fig. 14(a–c) and the
diameter distribution histogram in Fig. 14(d). FESEM images
disclose that the produced nano-carbons have smooth elliptical
shape with diameter covering a range of 5 nm to 145 nm. It is
very difficult to compute the actual length of the carbon nano-
carbon as they exist in an interweaving manner. However, it
can speculate that the length exceeds some micrometers. The
brighter spot observed in Fig. 14(a–c) at the tips of carbon nano-
tubes are Ni-metal particle and it is worth to note that the
diameter of nano-carbon are similar to that of Ni-particles. The
diameter of nano-carbons were measured with ImageJ soware.
Fig. 11 TEM images of produced nano-carbon over n-Ni/SiO
2
at (a)
The average diameter calculated from 50 nano-carbons were
ꢀ ꢀ ꢀ
700 C, (b) 600 C and (c) 500 C.
ꢀ
3
6
5.75 ꢄ 7.8 nm, 52.64 ꢄ 11.5 nm and 56.34 ꢄ 15.2 nm at 700 C,
ꢀ
ꢀ
00 C and 500 C, respectively. These results are in well
consistent with the Ni-crystallite size calculated from XRD
patterns using Scherrer equation. Hence, calculated crystallite
ꢀ
ꢀ
sizes are 34.2 nm, 47.03 nm and 50.22 nm at 700 C, 600 C and
5
ꢀ
00 C, respectively. This strong consistency between carbon
diameter and Ni-crystallite size are clearly reveals the depend-
54,55
ability between them, supporting previous reports.
Furthermore, diameter distribution histogram (Fig. 14(d))
reveals that the diameters of the most of produced nano-
carbons are in between 40 nm and 80 nm. It is clear from the
FESEM images that the morphology and quality of produced
nano-carbons are almost similar at all experimented tempera-
ture over n-Ni/SiO
at 700 C produced more nano-carbon with very low (<40 nm)
diameter compared to 600 C and 500 C. Furthermore, nano-
carbon with open end, closed end and with metal particle at
the dip also can clearly found in the FESEM images, seconding
the TEM images shown in Fig. 11.
2
catalyst. However, methane decomposition
ꢀ
ꢀ
ꢀ
Fig. 12 TEM images of produced nano-carbon over n-Co/SiO
2
at (a)
ꢀ ꢀ ꢀ
700 C, (b) 600 C and (c) 500 C.
Conclusion
Thermocatalytic decomposition of methane conducted over n-
Ni/SiO , n-Fe/SiO and n-Co/SiO catalysts to produce hydrogen
2
2
2
and nano-carbon. Active metallic phases of catalysts were
prepared by co-precipitation method from corresponding metal
nitrates. Modied St ¨o ber method was adopted to develop a
safeguard support for active nano-metal with silicate using
TEOS and C18TMS as silicate precursors. During methane
2
decomposition catalytic activity examination, n-Ni/SiO catalyst
exhibited an outstanding performance compared to n-Fe/SiO2
ꢀ
Fig. 13 TEM images of produced nano-carbon over n-Fe/SiO
2
at 700 C.
and n-Co/SiO catalysts. The poor performance of n-Fe/SiO and
2
2
2
n-Co/SiO catalysts were attributed to the formation of giant
n-Ni/SiO
silicate support and hence metal is detached from support.
2
occurred at the interface between the Ni particle and metal particles with unfavorable crystal size for growth of nano-
51
carbon. Furthermore, formation of irregular shaped nano-
However, Ni-particle maintained its activity at the surface of carbons over Fe and Co-based catalyst because of the metal
growing carbon laments which results in the longer activity of particle fragmentation also retarding their activity. While, such
n-Ni/SiO . Furthermore, almost similar graphite formation was defects were absent with n-Ni/SiO catalyst, which produce ne
2
2
observed with n-Co/SiO , while the carbon deposited on it carbon nano-tubes with active metal at the tip. Maximum
2
ꢀ
encapsulate active metal face and hence results in its rapid hydrogen production over n-Ni/SiO was 64.4% at 700 C, while
2
52
53
deactivation. In accordance to previous reports, our results minimum deactivation aer 240 minutes of examination was
also reveals that stable catalytic performance and catalytic found at 500 C. A variety of nano-carbons were formed over n-
ꢀ
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ꢀ ꢀ ꢀ
2
Fig. 14 FESEM images of produced nano-carbon over n-Ni/SiO catalyst at (a) 700 C, (b) 600 C and (c) 500 C. (d) Nano-carbon diameter
distribution histogram. Diameter of 50 nano-carbons were measured with ImageJ software.
´
˜
Ni/SiO2 catalyst. According to our experimental results, the
performance of analyzed catalysts in terms of its stability and
4 A. F. Cunha, J. J. M. Orfao and J. L. Figueiredo, Int. J.
Hydrogen Energy, 2009, 34, 4763–4772.
activity follow this order n-Ni/SiO
Considering the abundance and cheap rate of nickel precursors
as well as considerably simple and room temperature catalyst
2
> n-Co/SiO
2
> n-Fe/SiO
2
.
5 A. Venugopal, S. Naveen Kumar, J. Ashok, D. Hari Prasad,
V. Durga Kumari, K. B. S. Prasad and M. Subrahmanyam,
Int. J. Hydrogen Energy, 2007, 32, 1782–1788.
production method, the nano-structured n-Ni/SiO
co-precipitation cum modied St ¨o ber method is a kind of
promising material for the production of GHG free H through
the catalytic decomposition of methane.
2
prepared by
6 U. P. M. Ashik, W. M. A. Wan Daud and H. F. Abbas,
Renewable Sustainable Energy Rev., 2015, 44, 221–256.
7 A. E. Awadallah, A. A. Aboul-Enein and A. K. Aboul-Gheit,
Energy Convers. Manage., 2014, 77, 143–151.
2
8
B. Dou, C. Wang, Y. Song, H. Chen and Y. Xu, Energy Convers.
Manage., 2014, 78, 253–259.
Acknowledgements
9
S. Takenaka, S. Kobayashi, H. Ogihara and K. Otsuka, J.
Catal., 2003, 217, 79–87.
The authors gratefully acknowledge nancial support from the
postgraduate Research Fund (UM.C/HIR/MOHE/ENG/11),
University of Malaya, Malaysia. Authors would like to extent
thanks to both reviewers for their constructive comments and
suggestions. Help from Mr Zaharudin bin Md. Salleh, Science
Officer Assistant, Department of Geology, University of Malaya
for XRD analysis is also sincerely acknowledged.
1
1
1
0 S. Takenaka, H. Ogihara, I. Yamanaka and K. Otsuka, Appl.
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1 S. Takenaka, M. Serizawa and K. Otsuka, J. Catal., 2004, 222,
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2 S. Karnjanakom, G. Guan, B. Asep, X. Du, X. Hao, C. Samart
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