Enhancing performance of Ni/La2O3 catalyst by Sr-modification for steam reforming of toluene as model compound of biomass tar

Article abstract of DOI:10.1039/c4ra16983b

Steam reforming of biomass tar with toluene as the model compound was studied using Sr-doped Ni/La₂O₃ catalysts prepared using two methods, i.e. co-impregnation of Sr and Ni on La₂O₃ support (Ni-Sr/La₂

Full text of DOI:10.1039/c4ra16983b

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Enhancing performance of Ni/La2O3 catalyst by Sr-
modification for steam reforming of toluene as
model compound of biomass tar
Cite this: RSC Adv., 2015, 5, 17834
*
Usman Oemar, Ming Li Ang, Kus Hidajat and Sibudjing Kawi
Steam reforming of biomass tar with toluene as the model compound was studied using Sr-doped Ni/La2O3
catalysts prepared using two methods, i.e. co-impregnation of Sr and Ni on La2O3 support (Ni–Sr/La2O3
catalyst) and sequential impregnation of Sr on Ni/La2O3 catalyst (SNL catalyst), which were then calcined
at various temperatures (500, 700, and 900 ꢀC). These two types of catalysts are found to possess better
catalytic performance than the undoped Ni/La2O3 catalyst at the same calcination temperature due to
the presence of Sr, which helps in water adsorption at low steam/carbon (S/C) ratio. Moreover, the
catalytic performance for catalysts calcined at various temperatures decreases following this trend: 500
ꢀC > 700 ꢀC > 900 ꢀC due to lower BET surface area and lower surface active metal available for
reaction. In addition, it is also observed that the Sr/Ni/La2O3 catalyst has better performance than the
Ni–Sr/La2O3 catalyst at the same calcination temperature. Further characterization results suggest that in
the Ni–Sr/La2O3 catalyst, the Sr is present between Ni and La2O3 support. On the other hand, Sr in the
Sr/Ni/La2O3 catalyst is thought to be located on the surface of Ni/La2O3 due to the preparation method.
This study shows that more Sr on the catalyst surface has better catalytic activity and stability in steam
reforming of toluene.
Received 24th December 2014
Accepted 26th January 2015
DOI: 10.1039/c4ra16983b
www.rsc.org/advances
In secondary methods, pretreatment, either chemical or phys-
ical, is performed downstream of the gasier. Both primary and
Introduction
secondary methods require the use of a catalyst.
The need for a renewable alternative of energy sources is
increasingly urgent due to the rapid depletion of fossil fuel
reserves and the costly price tag of crude oil. The utilization of
biomass has been receiving more attention as an abundant and
cheap source of renewable energy. Among them, biomass
gasication has garnered considerable attention for hydrogen/
synthesis gas production, which can then be used for Fischer
Tropsch processes, syntheses of methanol and electricity
production in the case of a fuel cell.1–4
The main challenge in biomass gasication technology is the
removal of tar, which is a mixture of various condensable
hydrocarbon compounds containing toluene as the major
component, followed by naphthalene.3 Furthermore, tar can
polymerize to form more complex structures and aerosols. Tar
can condense on particulate lters, heat exchangers, and
engines, resulting in plugging and attrition. This further causes
a decrease in efficiency of the process operations and an
increase in maintenance and operating costs. Tar removal
methods can be classied as primary or secondary. Primary
methods include all measures taken in the gasication reactor
to prevent tar from being formed in the gasier or to convert it.
Various metal catalysts have been investigated, such as Ni5–12
Co,13–16 Fe,9–12,16–18 and noble metal19 for the steam reforming of
biomass tar using toluene as the model compound. Among
those metal catalysts, Ni catalyst has been used extensively not
only in biomass tar conversion, but also in other reactions such
as reforming of hydrocarbon20–25 and water gas shi26,27 reac-
tions because it shows comparable activity and stability with the
expensive noble metal catalysts such as Rh, Ir, Pt, and Pd.
However, Ni catalyst is susceptible to deactivation mainly
through carbon deposition on the surface of the Ni catalyst and
sintering of the Ni0 metallic species. The agglomeration of the
Ni0 particles occurs due to the typical reaction temperature (800
ꢀC), which is higher than its Tammann temperature (above
which Ni sintering occurs).28 On the other hand, the undesirable
coke deposition is due to the high temperature adsorption and
dissociation of tars, and unsaturated hydrocarbons on the
surface of the catalyst. It has been reported that coke formation
can be minimized in various manners using catalyst modica-
tion. One of the modications is to introduce redox29,30 or
oxygen storage properties into the catalyst.31,32 Some metals
such as Cu and Fe are well-known to have reduction–oxidation
(redox) properties, which aid in the reduction of carbon depo-
sition. Tomishige et al. reported the use of Fe in Ni–Fe/a-Al2O3
catalyst in the steam reforming of biomass tar.29 The addition of
Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive 4, Singapore 117585. E-mail: chekawis@nus.edu.sg;
Web: http://www.nus.edu.sg; Fax: +65-6779 1936; Tel: +65-6516 6312
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Fe to Ni/a-Al2O3 enhanced catalytic activity by the formation of (DI water). The La2O3 (Aldrich) was then added into the solu-
Ni–Fe alloy (conversion increase from 64% to 86%) and catalytic tion. The mixture was vigorously stirred in a water bath of 80 ꢀC
stability due to the presence of Fe on the catalyst surface, which until it dried. Further, drying was performed at 100 ꢀC overnight
increases the oxygen storage/vacancy of the catalyst.29–32 Kawi in an oven before it was subsequently calcined at 500 ꢀC, 700 ꢀC,
et al.9 reported that the addition of CeO2 to the Ni/CaO–Al2O3 or 900 C for 4 hours.
ꢀ
catalyst resulted in less carbon formation compared to the
For sequential impregnation catalyst, the Ni/La2O3 catalyst
unpromoted catalysts. A similar conclusion was reported by was rst prepared according to the above procedure but with
Atong et al.33 for the substitution of Ce into LaNiO3 perovskite- calcination duration of 2 hours. The calcined Ni/La2O3 catalyst
type oxide, Rirksomboon et al.34 for the addition of Mn to Ni/ was then impregnated with solution containing Sr(NO3)2 dis-
CeO2–ZrO2 catalyst, Zhang et al.35 for the addition of CeO2 to Ni/ solved in DI water. The dry catalyst was calcined for another 2
Al2O3 catalyst, and Brown et al.36 for the addition of CeO2 to Ni/ hours. The catalysts are designated as NSL 500, NSL 700, and
olivine catalyst. Another option is to use basic metal as the NSL 900 catalysts for co-impregnation Ni–Sr/La2O3 catalysts
promoter for the catalyst.37–44 Krause et al.43 reported that higher calcined at 500 C, 700 C, 900 C, respectively, and SNL 500,
ꢀ
ꢀ
ꢀ
basicity of catalyst is a favorable property for tar decomposition, SNL 700, and SNL 900 catalysts for sequential impregnation of
ꢀ
ꢀ
resulting in the enhancement of catalyst stability. Choong Sr/Ni/La2O3 catalysts at 500 C, 700 ꢀC, 900 C, respectively.
et al.44 also reported that the addition of CaO to Ni catalyst
promoted water adsorption and provided an abundance of X-ray diffraction (XRD)
adsorbed OH groups to facilitate the C–C break in Ni catalyst,
resulting in higher ethanol conversion in steam reforming of
ethanol. The last option to reduce coke deposition is by oper-
A plastic sample holder was lled with the powdered catalyst
sample before its surface was pressed at. Subsequently, the
XRD prole was obtained using a Shimadzu XRD-6000 Diffrac-
ating the reaction at high reaction temperatures and excess
tometer using a Cu target Ka-ray (40 kV and 30 mA) as the X-ray
steam-to-carbon ratio.45
source. A scanniꢁn1g range (2q) from 10ꢀ to 80ꢀ, with a scanning
Our previous study11,12 showed that the substitution of a
speed of 2ꢀ min and a step function of 0.02, was used. The
small amount of Sr with La in the LaxSr1ꢁxNi0.8Fe0.2O3 (LSNFO)
receiving slit was set at 0.3 mm while the scattering and diver-
catalyst showed good catalytic activity and stability, especially at
gence slits were set at 1 mm.
a low amount of steam (steam/carbon ¼ 1). It is found that Sr in
LSNFO catalyst can adsorb and desorb more water at higher
Hydrogen temperature-programmed reduction (H2-TPR)
temperature than La. However, only La can activate the water.
TPR analysis was performed using a Thermo Scientic TPDRO
The synergetic effect of Sr and La helps to enhance the catalyst
1100 system equipped with a Thermal Conductivity Detector
(TCD). 50 mg of fresh catalyst was rst degassed at 300 ꢀC for 30
stability. Sekine et al. also reported that the Sr doping in the Ni/
La0.7Sr0.3AlOd catalyst helps to improve the catalytic activity and
minutes with nitrogen gas ushing. This was followed by reduc-
reduce carbon formation due to lattice oxygen of the cata-
tion under 5% H2/N2 gas mixture at a ow rate of 30 ml minꢁ1
lyst.46–49 The similar conclusion of the Sr role on the enhance-
at which the catalyst samples were heated from room temper-
ment of catalytic performance was reported in dry CO2
ature to 1000 ꢀC at a heating rate of 10 ꢀC minꢁ1. The TPR
reforming of methane (DRM) reaction.20,22,50
proles were obtained by monitoring the changes in the signal
Considering the important role of Sr at low steam amount in
readings of the TCD against temperature. The TCD signal was
steam reforming of biomass tar, the catalyst development
initially calibrated by measuring the peak area observed for
should be designed by depositing more Sr on the catalyst
each injection of known volume of H2.
surface. The higher amount of Sr on the catalyst surface is
In order to calculate the number of active sites and amount
expected to enhance not only catalyst stability, but also catalyst
of metal dispersion, pulse titration was performed on the
activity. Hence, in this study, two types of catalysts (Sr/Ni/La2O3
Thermo Scientic TPDRO 1100 system according to the method
and Ni–Sr/La2O3 catalysts) were prepared using two conven-
tional impregnation methods, i.e. co-impregnation and
sequential impregnation to validate the hypothesis. The cata-
reported in recent literature.51 The H2-TPR was initially per-
formed using 50 mg of sample up to 600 ꢀC, followed by cooling
down to room temperature. Puried N2O gas was then intro-
lytic performance of the catalysts in terms of activity and
duced to the system by pulseꢀinjection until saturation. The H2-
stability is studied for steam reforming of toluene. Various
TPR was repeated up to 600 C. The number of active sites can
characterizations of fresh, reduced, and spent catalysts are
be calculated from H2-TPR aer N2O chemisorption, while the
dispersion can be calculated by comparing the second TPR to
the rst TPR results.
performed to show the crucial location of Sr on the catalyst for
obtaining higher catalyst performance.
Experimental
X-ray photoelectron spectroscopy (XPS)
Catalyst synthesis
ꢀ
Prior to XPS analysis, the sample was reduced at 600 C for 1
Sr-doped-Ni/La2O3 catalyst was synthesized by either co- hour, which is the same reduction time as in the reaction study.
impregnation method or sequential impregnation method. The XPS was performed on a Kratos AXIS Ultra DLD using a
For the co-impregnation catalyst, the Ni(NO3)2$6H2O (Merck) monochromatized Al Ka X-ray source (1486.71 eV photons, 5
and Sr(NO3)2 (Sigma Aldrich) were dissolved in deionized water mA, 15 kV) at a constant dwell time of 100 ms. The binding
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energy peaks were tted using the XPSPEAK soware and the obtained based on their volume percentage and the total volu-
calibration of all binding energies was performed by refer- metric ow rate of the product gases.
encing C1s (C–C bond) to 284.5 eV.
TGA-DTA
Results and discussion
The spent catalysts were analysed using a Shimadzu DTG60
TGA-DTA analyser. The spent catalyst was heated from 25 ꢀC to
800 ꢀC at a rate of 10 ꢀC minꢁ1. The weight loss from the spent
catalyst was monitored and recorded continuously.
XRD – catalyst structure analysis
Fig. 1 shows the XRD of fresh Sr-doped Ni/La2O3 catalysts
prepared using either co-impregnation or sequential impreg-
nation method. Ni/La2O3 catalysts calcined at various temper-
atures were also synthesized and characterized for comparison.
It can be seen that Ni/La2O3 500 catalyst shows very broad peaks
and signicant decrease in crystallinity of La2O3 compared to
the La2O3 support because calcination at 500 ꢀC is unable to
completely transform the lanthanum oxide hydroxide to
Water temperature-programmed desorption (TPD-H2O)
To determine the water desorption ability of the catalyst, TPD-
H2O was performed in a xed-bed reactor equipped with a
Carbolite furnace and a Thermostar Mass spectrometer. 50 mg
of fresh catalyst was rst reduced in 80 ml minꢁ1 of H2 at 600 ꢀC
for 30 minutes before the samples were cooled to 50 ꢀC.
Subsequently, deionised water was introduced to the sample
through a Helium gas stream of 80 ml minꢁ1 for 30 minutes
aer which the sample was purged at 100 ꢀC for 20 minutes. The
lanthanum oxide. Ekkehard Fuglein and Dirk Walter52 reported
¨
that lanthanum hydroxide can be obtained by the treatment of
lanthanum oxide with water at ambient temperature under
protective gas. They52,53 also showed that lanthanum hydroxide
will transform to lanthanum oxide in a two step mechanism:
2La(OH)3 / 2LaOOH + 2H2O at a temperature of 250–400 C
and 2LaOOH / La2O3 + H2O at a temperature of 400–560 C.
ꢀ
ꢀ
ꢀ
catalyst sample was then heated to 600 C at a heating rate of
10 ꢀC minꢁ1, in which the effluent gas was continuously
monitored using the mass spectrometer. The changes in the
H2O signal (m/z ¼ 18.0) against temperature was recorded by a
microcomputer.
With increasing calcination temperature, the peaks of La2O3 in
Ni/La2O3 700 catalyst become sharper indicating the increase in
crystallinity, but it is still slightly lower than La2O3 support.
When the Ni/La2O3 catalyst was calcined at 900 ꢀC (Ni/La2O3
900), many peaks such as perovskite, spinel, and NiO peaks (43ꢀ
due to Ni(200)54) can be observed. Ni metal has been reported in
many literature sources to have interaction with La2O3 to form
either LaNiO3 perovskite structure22,55 or La2NiO4 spinel struc-
ture depending on the Ni : La ratio and calcination tempera-
tures.56–58 In addition, the hexagonal phase of La2O3 was also
observed instead of the cubic La2O3 phase, which was used as
the catalyst support. It was reported that the cubic La2O3 phase
can change to hexagonal La2O3 when it was calcined at
Catalytic reaction
Catalytic reactions were carried out in a quartz tube xed bed
micro-catalytic reactor (I.D. 4 mm; length 60 cm), enclosed in a
programmable furnace at atmospheric pressure. Approximately
30 mg of catalyst was used for each reaction run, held in a xed
bed using quartz wool. The inlet of the reactor is connected to a
pre-heating coil at a temperature of 300 ꢀC, which is used to
vaporise the reactants (DI water and toluene). A condenser at a
temperature of 5 ꢀC is tted to the outlet of the reactor to
condense any unreacted water vapour and toluene vapour.
The fresh catalysꢀts were rst reduced under H2 gas
(30 ml minꢁ1) at 600 C for 30 minutes. Subsequently, the H2
was shut off, and the sample was purged by a He gas stream
(120 ml minꢁ1) for 10 minutes. Finally, the reactants streams of
both steam and toluene were introduced into the micro-
catalytic reactor by switching on the pumps of water and
toluene. The steam reforming reaction is performed at a
temperature of 650 ꢀC at atmospheric pressure. The system was
allowed to stabilize for 10 minutes before the start of the
analysis of the product gases. The amount and concentration of
gaseous products were analysed using a gas chromatograph (HP
6890 series) equipped with a packed column (Carboxen-1000,
60/80mesh sizes) and a TCD detector. The total product volu-
metric ow rate was measured using a BIOS digital ow meter.
The chromatogram obtained for each reaction run showed
peaks for H2, CO and CO2 product species. The area of each
peak was calculated using the integral function of the offline
soware coupled to the gas chromatograph. Based on the peak
areas, the volume percentage of each product gas was obtained
using the calibration curves that were previously determined.
temperatures above 750 C.59
ꢀ
All the catalysts were then reduced and characterized using
XRD. The results of XRD characterization of Sr-doped Ni/La2O3
catalysts are shown in Fig. 2. The Ni(111) peak at 2q of 44.6ꢀ is
observed for all NSL and SNL catalysts with sharper and more
Fig. 1 XRD of fresh catalysts [(C) Perovskite, (() Spinel, (&) hexagonal
The individual volumetric ow rate of the product gases was La2O3, and (A) NiO].
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Fig. 2 XRD of reduced catalysts [(&) cubic La2O3, (A) hexagonal
La2O3, and (C) Ni].
prominent peaks for catalysts calcined at higher temperatures,
showing that the NSL and SNL catalysts calcined at a higher
temperature have bigger crystal sizes than those calcined at a
lower temperature.60 In addition, it can be seen that reduced Ni/
La2O3 500, 700, and 900 catalysts show similar XRD patterns.
The hexagonal structure of La2O3 is also observed in addition to
the cubic La2O3 structure, which is the structure of the catalyst
support. This result indicates the transformation of cubic La2O3
structure to hexagonal La2O3 structure through the H2 reduc-
tion at 650 ꢀC. However, different patterns are observed for NSL
500, 700, and 900 catalysts. The reduced NSL 500 and 700
catalysts show a similar pattern but higher intensity compared
to the fresh NSL 500 and 700 catalysts, indicating that Sr may
strengthen the interaction between Ni and La2O3. In contrast,
the XRD pattern of reduced SNL 500, 700, and 900 catalysts is
very similar to the one of reduced Ni/La2O3 500, 700, and 900
catalysts, suggesting that Sr on these catalysts does not affect
the interaction between Ni and La2O3. The preparation method
in which Sr was intentionally impregnated aer Ni also
supports this idea.
Fig. 3 TPR of reduced catalysts. (a) Ni/La2O3 catalysts and (b) Sr-
doped Ni/La2O3 catalysts.
the calcinations temperature to 700 ꢀC, the TPR reduction peaks
in Ni/La2O3 700 catalyst are separated clearly into three
temperature zones: low, medium and high. Upon further
increase in calcination temperature, the reduction peaks in the
Ni/La2O3 catalyst are observed only in the intermediate and
high temperature zones. The catalyst having weakly interacted
NiO species generally exist in bigger particle sizes and is
possibly easy to migrate and aggregate during reduction reac-
tion.63,64 In contrast, the catalyst having strong metal–support
TPR – catalyst reducibility analysis
In order to study the reduction properties of the prepared interaction is usually preferred as it has a small particle size
catalyst, the TPR experiment was conducted. The TPR proles of aer reduction.65 However, it also strongly depends on the
Ni/La2O3 500, 700, and 900 catalysts are also provided in Fig. 3a reduction temperature in the study. In addition, the H2
for comparison. The TPR results show 3 peaks for all catalysts, consumption below 600 ꢀC, which is the reduction temperature
which shi to higher temperatures with increasing calcination during the catalytic test, surface Ni amount and dispersion also
temperature. The reduction peaks are commonly categorized decrease with increasing calcination temperature, which is well
into three zones: low temperature zone (300–500 ꢀC) assigned to understood in the literature due to a stronger interaction
the reduction of NiO with a weak interaction with the catalyst between the metal and the support. This result shows that with
support,21 intermediate temperature zone (500 to 600 ꢀC) increasing calcination temperatures, the metal has a stronger
assigned to the reduction of NiO species with medium strength interaction with the support.
interaction with the support,61 and high temperature zone
Fig. 3b shows the TPR prole of Sr-doped Ni/La2O3 catalyst. It
(>600 ꢀC) attributed to the reduction of NiO species with strong can be seen that all peaks for SNL and NSL catalysts shi to
interaction with the catalyst support.62 In this case, the strong higher temperatures due to a stronger interaction between Ni
interaction with the catalyst support can be in the form of and the support. The peak shiing trend can be seen evidently,
compound formation such as perovskite or spinel. For Ni/La2O3 for example, increasing calcination temperature from 500 ꢀC to
ꢀ
ꢀ
500 catalyst, the reduction peaks are observed in the low 700 C, the low reduction temperature peak zone (300–500 C)
temperature zone and high temperature zone. With increasing due to weak interaction between NiO and the support in SNL
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and NSL 500 catalysts shi to a medium reduction temperature
peak zone (500–600 ꢀC) in SNL and NSL 700 catalysts. A similar
ꢀ
ꢀ
trend is observed for catalysts calcined at 700 C and 900 C.
Comparing the TPR prole between SNL and NSL catalysts
calcined at the same temperature, the H2 consumption below
600 ꢀC, which is the reduction temperature during the catalytic
test, amount of surface Ni and dispersion (Table 1) are about the
same, indicating that the amount of Ni metal and its dispersion
on both catalysts are the same. However, the peaks of the NSL
catalysts for any calcination temperature are located at a higher
temperature than those on the SNL catalysts, showing that the
Ni has a stronger interaction with La2O3 support on the NSL
catalyst. For example, the NSL catalyst has a peak at around
ꢀ
ꢀ
250–470 C with the maximum peak at 420 Cꢀ, while the SNL
catalyst also shows a peak at around 250–470 C but with the
maximum peak located at 400 C. This stronger interaction on
ꢀ
NSL catalysts compared to SNL catalysts could indicate that Sr
helps to strengthen the interaction between Ni and La2O3
support on the NSL catalyst, while Sr atoms on the SNL catalyst
are thought to be placed on the surface of the Ni/La2O3 catalyst
due to the preparation method using sequential impregnation.
XPS of reduced catalysts
Fig. 4a shows the XPS Ni 3p prole of reduced undoped and Sr-
doped Ni/La2O3 catalysts. The XPS Ni 2p prole is not shown
here because the Ni 2p3/2 peak overlaps with the La 3p3/2 peak at
around 853 eV. It can be seen that the main deconvoluted peak
at around 65.8 eV in Ni/La2O3 500 catalyst shis to a lower
binding energy for the Ni/La2O3 700 catalyst (65.3 eV) and
considerably lower binding energy for the Ni/La2O3 900 catalyst
(65.1 eV) due to stronger interaction between Ni and the La2O3
support. The similar peak location is observed for SNL catalysts.
The binding energy for the SNL 500 catalyst is located at 65.8 eV,
and it shis to a lower binding energy (65.3 eV) for the SNL 700
catalyst due to stronger interaction between Ni and the La2O3
support. The peak shiing is more intense for the SNL 900
catalyst, which has binding energy around 65.1 eV due to a
much stronger metal–support interaction. The same peak
position between Ni/La2O3 catalysts and SNL catalysts shows
Fig. 4 XPS of reduced Sr-doped Ni/La2O3 catalysts for (a) Ni 3p and (b)
Sr 3d.
that Sr does not signicantly affect the electronic state of Ni on
La2O3 support. Similarly, the peak location in the NSL catalysts
decreases in the following order: NSL 500 catalyst (66.3 eV) >
NSL 700 catalyst (65.4 eV) > NSL 900 catalyst (65.3 eV), but the
peak position is located at a considerably higher binding
energy, showing that the Ni in the NSL catalysts has transferred
Table 1 Physicochemical properties of the prepared catalysts
Content (mmol gꢁ1
)
H2 consumptiona
BET surface
area (m2 gꢁ1
Amount of surface Nib
Dispersionc
(%)
Catalysts
Ni
Sr
(mmol gꢁ1
)
)
(mmol gꢁ1
)
Ni/La2O3 500
Ni/La2O3 700
Ni/La2O3 900
NSL 500
NSL 700
NSL 900
SNL 500
SNL 700
SNL 900
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
—
—
—
0.57
0.57
0.57
0.57
0.57
0.57
324.1
215.0
189.1
512.2
260.6
130.6
515.3
269.5
196.2
15.19
10.43
5.07
16.90
12.08
4.65
17.81
12.33
2.76
33.9
19.3
12.1
76.3
33.7
12.3
76.8
34.1
16.1
10.5
8.9
6.4
14.9
12.9
9.4
14.8
12.7
8.2
a
b
H2 consumption below 600 ꢀC in TPR proles shown in Fig. 3. Amount of surface Ni was calculated by H2-TPR aer N2O chemisorption.
c
Calculated by comparing TPR-H2 before and aer N2O pulse titration up to 600 ꢀC.
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more electrons than the Ni in the SNL catalyst as an indication
of stronger interaction of Ni with either Sr or La in NSL catalysts.
This result also shows that with increasing calcination
temperature, the interaction between Ni and the support
becomes stronger, which is a similar observation with XRD and
TPR results.
The XPS Sr 3d proles of reduced Sr-doped Ni/La2O3 cata-
lysts are shown in Fig. 4b. The binding energy of the NSL
catalysts shis to a higher binding energy from NSL 500 to NSL
700 to NSL 900 catalysts, suggesting signicant transfer of
electrons for Sr in the catalyst calcined at a higher tempera-
ture. Because Sr has higher electronegativity than La and Ni,
the electron transfer should be from Sr to La and/or Ni.
However, the binding energy of the SNL catalysts is observed to
be constant at 133 eV, which is due to SrO binding energy,66
indicating that there is no electron transfer from Sr. This also
indicates that the Sr is only located on the catalyst surface
without any strong interaction with either Ni and La2O3
support. This observation is in-line with observations in the
TPR and XRD experiments.
Catalytic activity and stability
Fig. 5 shows the catalytic activity of Sr-doped Ni/La2O3 catalysts
prepared by either co-impregnation (NSL catalysts) or sequen-
tial impregnation (SNL catalysts) and calcined at various
temperatures compared to the undoped Ni/La2O3 catalysts
calcined at various temperatures. It can be seen that the cata-
lytic activity of NSL and SNL catalysts is consistently higher than
that of undoped Ni/La2O3 catalysts calcined at similar temper-
atures because of the higher amount of surface Ni in the Sr-
doped Ni/La2O3 catalysts (Table 1). Comparing the effect of
calcinations temperature on catalytic performance, one can
clearly see that the catalytic performance of undoped Ni/La2O3
catalysts decreases in the order of 500 ꢀC > 700 ꢀC > 900 ꢀC, due
to a lower amount of surface Ni metal on the catalyst for the
higher calcination temperature (Table 1) because higher calci-
nation temperature can lead to stronger interaction between
metals and the support, resulting in higher difficulty for
reduction. This result is also in-line with H2 consumption of the
catalysts from the TPR result.
Fig. 5 Catalytic activity of Sr-doped Ni/La2O3 catalysts at 650 ꢀC and S/C ¼ 1. Reaction condition: toluene 188 mmol minꢁ1; steam 1316 mmol
minꢁ1; He 5357 mmol minꢁ1; W ¼ 30 mg.
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Fig. 5 also shows that the SNL catalysts prepared using the Sr in SNL catalysts. The difference can be explained by the
sequential impregnation method always exhibit higher activity absence of the Sr-incorporated perovskite structure in the SNL
than NSL catalysts prepared using the co-impregnation method catalyst. The shiing of peaks to higher temperature in the SNL
for the same calcination temperature. Because the amount of catalysts is even more intense as the Sr in the SNL catalysts was
metal surface area is similar for NSL and SNL catalysts calcined intentionally impregnated aer Ni such that more Sr is present
at the same temperature, the sole difference lies in the prepa- on the surface to adsorb and desorb water to enhance the cata-
ration method, resulting in the disparity in the location of Sr. lytic activity and stability of the SNL catalysts. This result shows
The above characterization results and the preparation method that a higher amount of strongly adsorbed water due to the
(sequential impregnation) suggest that all Sr is on the catalyst presence of a greater amount of Sr on the surface results in
surface without strong interaction with Ni for the SNL catalysts enhancement of catalytic performance in the SNL catalyst.
because Sr was impregnated at the last step using the sequential
impregnation method. However, for NSL catalysts, some of the
Sr is located between the Ni and the La2O3 support, resulting in
stronger interaction between the Ni and La2O3 support. Our
Carbon formation on spent catalysts
The main problem of catalyst deactivation in steam reforming
previous study11,12 reported that Sr can adsorb and desorb more
water at higher temperature than La, but only La can activate
the water. Therefore, this study shows that the crucial role of Sr
to strongly adsorb and desorb water can be enhanced by
arranging Sr on the catalyst surface.
of the toluene reaction is high carbon deposition on the catalyst
surface from toluene decomposition. It is, therefore, important
to characterize the spent catalyst using various methods to
observe the type and amount of carbon deposition. Fig. 7 shows
the FESEM images of spent NSL 500 and SNL 500 catalysts aer
reaction for 8 h. It can be seen that the spent NSL 500 catalyst
clearly produces lamentous carbon, which is graphitic carbon
that is difficult to oxidize at low temperature. Although this type
of carbon did not result in deactivation of the nickel surface, it
blocked the reactor and caused catalyst destruction.67 However,
it is hard to nd any lamentous carbon on spent SNL 500
catalyst, indicating that the SNL 500 catalyst has higher stability
than the NSL catalyst.
The quantitative analysis of carbon deposition on spent Ni/
La2O3, NSL and SNL catalysts was performed using TGA anal-
ysis, and the results are shown in Fig. 8a. It can be seen that the
amount of carbon deposition on spent Ni/La2O3 catalysts is
considerably lower than either NSL or SNL catalysts due to its
lower catalytic performance. However, the carbon formation on
SNL catalysts is considerably lower than the one on spent NSL
catalysts, even though the catalytic activity of the SNL catalysts
is higher than that of the NSL catalysts. This result indicates
that by allocating more Sr on the catalyst surface (SNL catalyst),
it can not only enhance the activity, but also signicantly reduce
the carbon formation. In addition, the order of carbon forma-
tion rate in those catalysts calcined at various temperatures
(NSL/SNL 500 z NSL/SNL 700 > NSL/SNL 900) also follows the
order of catalytic performance temperatures (NSL/SNL 500 >
NSL/SNL 700 > NSL/SNL 900), showing that the carbon depo-
sition rate is highly dependent on the catalytic activity.
Water desorption (TPD-H2O)
In order to understand the characteristics of NSL and SNL cata-
lysts, which have higher catalytic performance than Ni/La2O3
catalysts, TPD-H2O was performed on reduced catalysts, and the
result is shown in Fig. 6. It can be seen that one peak at around
300–400 ꢀC due to desorption of water from La2O3 is observed for
the Ni/La2O3 catalysts. This peak shis to a higher temperature
for catalysts calcined at lower temperature as the La is not
strongly interacted with the Ni, and hence it has stronger
desorption capacity. For NSL and SNL catalysts, two peaks are
observed for all catalysts. The peak at lower temperature can be
attributed to the water desorption from La2O3, while the one at
higher temperature is due to water desorption from SrO.11 In
addition, both peaks shi to higher temperature for catalysts
calcined at lower temperature as the La and Sr are not strongly
interacted with the Ni. The water desorption from La2O3 and SrO
in La0.8Sr0.2Ni0.8Fe0.2O3 perovskite oxide catalyst was reported at
around 270–350 ꢀC and 400–500 ꢀC, respectively.12 Compared to
this literature, the peaks in SNL catalysts appear at a higher
temperature, showing the stronger adsorption of water on La and
Fig. 6 TPD-H2O profiles of reduced Sr-doped Ni/La2O3 catalysts.
17840 | RSC Adv., 2015, 5, 17834–17842
Fig. 7 FESEM images of spent SNL 500 and NSL 500 catalysts.
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catalyst/SNL). The SNL catalysts exhibit higher performance
than the NSL catalysts calcined at the same temperature due to
the presence of more Sr on the catalyst surface for SNL catalysts
because Sr was impregnated aer Ni during the sequential
impregnation method. The characterization using TPR, XRD,
XPS, and TPD-H2O suggests that some of the Sr in the NSL
catalysts is located between Ni and La2O3 support and affects
the electronic state of Ni. On the other hand, the preparation
method and characterization data suggest that Sr in the SNL
catalysts is located on the catalyst surface without strong
interaction with Ni. This study shows that more Sr on the
catalyst surface induces better catalytic activity and stability in
steam reforming of toluene.
Acknowledgements
The authors acknowledge nancial support from National
University of Singapore and NEA-ETRP (Project no. 1002 114,
Research Grant no. 279-000-333-490). Usman Oemar sincerely
thanks Dr Yasotha Kathiraser and Dr Jangam Ashok for tech-
nical support and discussions.
Notes and references
1 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106,
4044.
2 J. Han and H. Kim, Renewable Sustainable Energy Rev., 2008,
12, 397.
3 H. de Lasa, E. Salaices, J. Mazumder and R. Lucky, Chem.
Rev., 2011, 111, 5404.
4 P. R. Buchireddy, R. M. Bricka, J. Rodriguez and W. Holmes,
Energy Fuels, 2010, 24, 2707.
5 T. Kimura, T. Miyazawa, J. Nishikawa, S. Kado, K. Okumura,
T. Miyao, S. Naito, K. Kunimori and K. Tomishige, Appl.
Catal., B, 2006, 68, 160–170.
Fig. 8 (a) Carbon formation rate on spent Sr-doped Ni/La2O3 cata-
lysts. (b) Carbon-based yield of reaction products from Sr-doped Ni/
La2O3 catalysts.
6 Z. A. El-Rub, E. A. Bramer and G. Brem, Ind. Eng. Chem. Res.,
2004, 43, 6911.
7 D. Li, M. Koike, J. Chen, Y. Nakagawa and K. Tomishige, Int.
J. Hydrogen Energy, 2014, 39, 10959.
8 C. Phuhiran, T. Takarada and S. Chaiklangmuang, Int. J.
Hydrogen Energy, 2014, 39, 3649.
9 J. Ashok and S. Kawi, Int. J. Hydrogen Energy, 2013, 38, 13938.
10 U. Oemar, P. S. Ang, K. Hidajat and S. Kawi, Int. J. Hydrogen
Energy, 2013, 38, 5525.
11 U. Oemar, M. L. Ang, W. F. Hee, K. Hidajat and S. Kawi, Appl.
Catal., B, 2014, 148–149, 231.
12 U. Oemar, M. L. Ang, K. Hidajat and S. Kawi, AIChE J., 2014,
60, 4190.
13 C. Soongprasit, D. D. Aht-Ong and D. Atong, Adv. Mater. Res.,
2013, 747, 690.
14 M. Koike, Y. Hisada, L. Wang, D. Li, H. Watanabe, Y. Nakagawa
and K. Tomishige, Appl. Catal., B, 2013, 140–141, 652.
15 K. Soongprasit, D. Aht-Ong, V. Sricharoenchaikul and
D. Atong, Top. Catal., 2013, 56, 339.
The carbon-conversion yield of deposited carbon compared
to the CO and CO2 produced from reaction and unreacted
toluene is shown in Fig. 8b. The result clearly shows that the low
amount of deposited carbon on Ni/La2O3 catalysts is due to low
catalytic performance of the catalysts, which can be seen from
the high amount of unconverted toluene. However, the low
amount of deposited carbon on the SNL catalysts and low
amount of unconverted toluene show that SNL catalysts can
enhance the catalytic activity and reduce the carbon formation
at the same time. On the other hand, the high amount of
deposited carbon on NSL catalysts and relatively low amount of
unconverted toluene show that NSL catalysts only enhance the
catalytic activity without suppressing the deposited carbon. As a
result, the deposited carbon on NSL catalysts increases.
Conclusions
Two types of catalysts were prepared using co-impregnation of 16 L. Wang, Y. Hisada, M. Koike, D. Li, H. Watanabe,
Sr and Ni on La2O3 support (Ni–Sr/La2O3 catalyst/NSL) and
sequential impregnation of Sr on Ni/La2O3 catalyst (Sr/Ni/La2O3
Y. Nakagawa and K. Tomishige, Appl. Catal., B, 2012, 121–
122, 95.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 17834–17842 | 17841

View Article Online
RSC Advances
Paper
¨
¨
17 A. Laobuthee, C. Veranitisagul, W. Wattanathana, 43 T. Viinikainen, H. Ronkkonen, H. Bradshaw, H. Stephenson,
N. Koonsaeng and N. Laosiripojana, Renewable Energy,
2015, 74, 133.
S. Airaksinen, M. Reinikainen, P. Simell and O. Krause, Appl.
Catal., A, 2009, 362, 169.
18 N. Laosiripojana, W. Sutthisripok, S. Charojrochkul and 44 C. K. S. Choong, L. Huang, Z. Zhong, J. Lin, L. Hong and
S. Assabumrungrat, Fuel Process. Technol., 2014, 127, 26. L. Chen, Appl. Catal., A, 2011, 407, 155.
19 K. Tomishige, T. Miyazawa, M. Asadullah, S.-I. Ito and 45 D. Swierczynski, C. Courson and A. Kiennemann, Chem. Eng.
K. Kunimori, Green Chem., 2003, 5, 399. Process., 2008, 47, 508.
20 K. Sutthiumporn, T. Maneerung, Y. Kathiraser and S. Kawi, 46 Y. Sekine, D. Mukai, Y. Murai, S. Tochiya, Y. Izutsu,
´
´
Int. J. Hydrogen Energy, 2012, 37, 11195.
21 U. Oemar, K. Hidajat and S. Kawi, Appl. Catal., A, 2011, 402,
176.
22 K. Sutthiumporn and S. Kawi, Int. J. Hydrogen Energy, 2011,
36, 14435.
23 K. Yoshida, N. Begum, S. Ito and K. Tomishige, Appl. Catal.,
A, 2009, 358, 186.
24 L. Mo, K. K. M. Leong and S. Kawi, Catal. Sci. Technol., 2014,
4, 2107.
K. Sekiguchi, N. Hosomura, H. Arai, E. Kikuchi and
Y. Sugiura, Appl. Catal., A, 2013, 451, 160.
47 D. Mukai, S. Tochiya, Y. Murai, M. Imori, T. Hashimoto,
Y. Sugiura and Y. Sekine, Appl. Catal., A, 2013, 453, 60.
48 D. Mukai, S. Tochiya, Y. Murai, M. Imori, Y. Sugiura and
Y. Sekine, Appl. Catal., A, 2013, 464–465, 78.
49 Y. Sugiura, D. Mukai, Y. Murai, S. Tochiya and Y. Sekine, Int.
J. Hydrogen Energy, 2013, 38, 7822.
50 G. Valderrama, A. Kiennemann and M. R. Goldwasser, J.
Power Sources, 2010, 195, 1765.
25 X. Wu and S. Kawi, Energy Environ. Sci., 2010, 3, 334.
26 E. T. Saw, U. Oemar, X. R. Tan, Y. Du, A. Borgna, K. Hidajat 51 S. Tada, M. Yokoyama, R. Kikuchi, T. Haneda and
and S. Kawi, J. Catal., 2014, 314, 32. H. Kameyama, J. Phys. Chem. C, 2013, 117, 14652.
27 M. L. Ang, U. Oemar, E. T. Saw, L. Mo, B. H. Chia and S. Kawi, 52 E. Fuglein and D. Walter, Z. Anorg. Allg. Chem., 2006, 632,
¨
ACS Catal., 2014, 4, 3237.
2154.
28 D. L. Trimm, Catal. Today, 1999, 49, 3.
29 L. Wang, D. Li, M. Koike, S. Koso, Y. Nakagawa, Y. Xu and
K. Tomishige, Appl. Catal., A, 2011, 392, 248.
30 M. Koike, C. Ishikawa, D. Li, L. Wang, Y. Nakagawa and
K. Tomishige, Fuel, 2013, 103, 122.
53 A. Neumann and D. Walter, Thermochim. Acta, 2006, 445,
200.
54 M. A. Gondal, T. A. Saleh and Q. A. Drmosh, Appl. Surf. Sci.,
2012, 258, 6982.
55 C. Batiot-Dupeyrat, G. Valderrama, A. Meneses, F. Martinez,
¨
31 C. Li, D. Hirabayashi and K. Suzuki, Appl. Catal., B, 2009, 88,
351.
J. Barrault and J. M. Tatibouet, Appl. Catal., A, 2003, 248, 143.
56 Y. Kathiraser, W. Thitsartarn, K. Sutthiumporn and S. Kawi,
32 J. Nishikawa, K. Nakamura, M. Asadullah, T. Miyazawa,
J. Phys. Chem. C, 2013, 117, 8120.
K. Kunimori and K. Tomishige, Catal. Today, 2008, 131, 146. 57 B. S. Liu and C. T. Au, Appl. Catal., A, 2003, 244, 181.
33 K. Soongprasit, D. Aht-Ong, V. Sricharoenchaikul and 58 W. D. Zhang, B. S. Liu, C. Zhu and Y. L. Tian, Appl. Catal., A,
D. Atong, Curr. Appl. Phys., 2012, 12, S80.
34 A. Bampenrat, V. Meeyoo, B. Kitiyanan, P. Rangsunvigit and 59 N. Imanaka, T. Masui and Y. Kato, J. Solid State Chem., 2005,
T. Rirksomboon, Appl. Catal., A, 2010, 373, 154. 178, 395.
35 Y. Lu, S. Li, L. Guo and X. Zhang, Int. J. Hydrogen Energy, 60 W. Lin, H. Cheng, L. He, Y. Yu and F. Zhao, J. Catal., 2013,
2010, 35, 7161. 303, 110.
36 R. Zhang, Y. Wang and R. C. Brown, Energy Convers. Manage., 61 A. Jones and B. D. Mc Nicol, Temperature-programmed
2005, 292, 138.
2007, 48, 68.
Reduction for Solid Materials Characterization, Marcel
Dekker, New York, 1986.
62 G. B. Sun, K. Hidajat, X. S. Wu and S. Kawi, Appl. Catal., B,
2008, 81, 303.
37 D. Li, L. Wang, M. Koike, Y. Nakagawa and K. Tomishige,
Appl. Catal., B, 2011, 102, 528.
38 K. Nakamura, T. Miyazawa, T. Sakurai, T. Miyao, S. Naito,
N. Begum, K. Kunimori and K. Tomishige, Appl. Catal., B, 63 J. Gao, Z. Hou, J. Guo, Y. Zhu and X. Zheng, Catal. Today,
2009, 86, 36. 2008, 131, 278.
39 D. Swierczynski, S. Libs, C. Courson and A. Kiennemann, 64 Z. Xu, Y. M. Li, J. Y. Zhang, L. Chang, R. Q. Zhou and
Appl. Catal., B, 2007, 74, 211. Z. T. Duan, Appl. Catal., A, 2001, 210, 45.
40 J. S. Lisboa, D. C. R. M. Santos, F. B. Passos and 65 Y. H. Hu and E. Ruckenstein, Catal. Lett., 1997, 43, 71.
´
´
F. B. Noronha, Catal. Today, 2005, 101, 15.
41 F. Bimbela, D. Chen, J. Ruiz, L. Garcia and J. Arauzo, Appl.
Catal., B, 2012, 119–120, 1.
42 A. Carrero, J. A. Calles and A. J. Vizcaino, Chem. Eng. J., 2010,
163, 395.
66 Handbook of X-ray Photoelectron Spectroscopy, ed. J. F.
Moulder, J. Chastain and R. C. King, Perkin-Elmer, 1992.
67 A. P. E. York, T. Xiao, M. L. H. Green and J. B. Claridge, Catal.
Rev., 2007, 49, 511.
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