Highly active and stable Co/La0.7Sr0.3AlO3-δ catalyst for steam reforming of toluene

Article abstract of DOI:10.1016/j.cattod.2015.08.059

We investigated steam reforming of toluene as a model compound of aromatic hydrocarbons included in biomass tar over Co supported La_0.7Sr_0.3AlO_3-δ (LSAO), perovskite oxide. Ni-supported LSAO catalyst has shown high activity and coke resistance from the redox property of lattice oxygen in/on the LSAO support. Co is known as an active metal for this reaction, so Co/LSAO catalyst was investigated in this work. Co/LSAO catalyst, which showed high steady-state activity and stability, was characterized using H₂¹⁸O isotopic transient response tests, STEM, FT-IR, Arrhenius plot and partial pressure dependence to elucidate high and stable catalytic activity. In situ FT-IR measurements revealed that reaction intermediates on Co/LSAO desorbed at 873 K or lower temperatures. Although redox property of lattice oxygen did not change at around 848 K based on isotopic transient tests, the Arrhenius plots indicate that the rate-determining step changed at around 848 K because of reaction intermediate decomposition desorption. Fast reaction and desorption of absorbed intermediates on Co/LSAO enable catalytic stability during toluene steam reforming.

Full text of DOI:10.1016/j.cattod.2015.08.059

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Highly active and stable Co/La Sr AlO catalyst for steam
3−ı
0.7 0.3
reforming of toluene
Kent Takise , Takuma Higo , Daiki Mukai , Shuhei Ogo , Yukihiro Sugiuraa,b,
a
a
a
a
a,∗
Yasushi Sekine
Waseda University, Department of Applied Chemistry, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
a
b
Central Technical Research Laboratory, JX Nippon Oil and Energy Corp., 8 Chidoricho, Naka-ward, Yokohama, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We investigated steam reforming of toluene as a model compound of aromatic hydrocarbons included in
biomass tar over Co supported La0.7Sr0.3AlO3−ı (LSAO), perovskite oxide. Ni-supported LSAO catalyst has
shown high activity and coke resistance from the redox property of lattice oxygen in/on the LSAO support.
Co is known as an active metal for this reaction, so Co/LSAO catalyst was investigated in this work. Co/LSAO
catalyst, which showed high steady-state activity and stability, was characterized using H218O isotopic
transient response tests, STEM, FT-IR, Arrhenius plot and partial pressure dependence to elucidate high
and stable catalytic activity. In situ FT-IR measurements revealed that reaction intermediates on Co/LSAO
desorbed at 873 K or lower temperatures. Although redox property of lattice oxygen did not change at
around 848 K based on isotopic transient tests, the Arrhenius plots indicate that the rate-determining
step changed at around 848 K because of reaction intermediate decomposition desorption. Fast reaction
and desorption of absorbed intermediates on Co/LSAO enable catalytic stability during toluene steam
reforming.
Received 29 June 2015
Received in revised form 13 August 2015
Accepted 26 August 2015
Available online xxx
Keywords:
Perovskite oxide
Steam reforming of toluene
Co catalyst
Hydrogen production
Surficial adsorption property
Stable catalytic activity
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
catalysts are investigated more extensively [27] because Ni is suit-
able for industrial use because of its low cost and high activity.
Conversely, activity of Ni-supported catalyst is easily deactivated
by coke formation during the reaction [28,29]. Generally, Co-
supported catalyst generates less coke and methane as byproducts
during steam reforming reaction of heavy hydrocarbons [17–23]
Biomass has been attractive as an alternative resource to fos-
sil fuels because of its wide distribution, carbon-neutrality, and
renewability. Biomass gasification is a promising technology for
producing H -rich synthesis gas (H + CO) in terms of economic and
2
2
ergonomic efficiency. However, the products contain a consider-
able amount of tar, which causes plugging of the reactor. Biomass
tar consists mainly of aromatic hydrocarbons and oxygenates. A
novel method for removing these heavy aromatics is being eagerly
sought. Steam reforming reaction of aromatics is receiving increas-
ing attention because the produced H2 is applicable not only as
a chemical feedstock for ammonia synthesis and petroleum refin-
ing, but also as a renewable secondary energy source for operating
fuel cells. Furthermore, steam reforming of aromatics can produce
H2 from unconventional resources such as oil shale, oil sands, and
peat.
and oxygenates [30–35]. Wang et al. reported that Co/␣-Al O3
showed higher toluene conversion and less carbon deposition com-
2
pared to Ni/␣-Al O3 in steam reforming of tar and toluene [19].
2
Controlling coke formation in steam reforming is an important
research topic. Many studies of the subject have been conducted
[36–39]. Rostrup-Nielsen reported partial sulfur poisoning of Ni
inhibits carbon formation without suppressing reforming reac-
tion of methane [36]. Sugisawa et al. described that La addition to
Ni/Al O enhances H O activation ability for steam reforming of n-
2
3
2
dodecane, which suppresses coking on a catalytic surface [37]. For
aromatic hydrocarbons, catalytic decomposition of aromatic rings
is difficult, and carbon deposition is more problematic [38].
Lattice oxygen of the catalyst support can activate a reaction
with the redox property and remove deposited carbon oxidatively
Catalytic steam reforming of aromatic hydrocarbons is per-
formed over many metal-supported catalysts such as Rh [1–4],
Ni [5–16], Co [17–23], and Fe [24–26]. Particularly, Ni-supported
[
1,5–8,40–47]. Oemar et al. reported that La0.8Sr0.2Ni0.8Fe0.2O3
shows high activity and carbon resistance on steam reforming
of toluene because of the great amount of lattice oxygen in/on
perovskite oxide [41]. Previously, we conducted toluene steam
∗ Corresponding author.
E-mail address: ysekine@waseda.jp (Y. Sekine).
http://dx.doi.org/10.1016/j.cattod.2015.08.059
0
920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Takise, et al., Highly active and stable Co/La0.7Sr0.3AlO
Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.059
catalyst for steam reforming of toluene,
3−ı

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2
reforming over Ni/La0.7Sr0.3AlO
can be described as:
(LSAO) [48–52]. The reaction
described [48–50,52]. Isotopic water was introduced with
3−ı
16
18
gas consisting of 1.5%C7H :14.7%H2 O:6.3%H2 O:5%Ar:72.5%He
8
−
1
vol% (total flow rate: 200 mL min ) to replace lattice oxy-
gen in/on the LSAO support by O. After purging in an inert
C7H + 7H O → 7CO + 11H
ꢀH0
= 869.8 kJ mol−1
(1)
(2)
8
2
2
298
18
CO + H O → CO + H
ꢁH0
298
= −41.2 kJ mol−1
gas, the second reaction was conducted in the gas composi-
2
2
2
1
6
tion of 1.5%C7H :21%H O:5%Ar:72.5%He vol% (total flow rate:
2
8
2
In the reaction, lattice oxygen suppresses coke formation and
activates toluene with a redox mechanism [48–52]. According to
an earlier IR study, toluene reacts with lattice oxygen and forms
oxygenate intermediates on LSAO [51]. The redox property of LSAO
supports is substantial for coke removal and activating toluene. As
described above, Co metal was expected to be beneficial for steam
reforming of formed oxygenate intermediates on LSAO. Therefore,
steam reforming of toluene as a model compound of aromatic
hydrocarbons was performed over Co/LSAO catalyst in this work.
In addition, this catalyst has been characterized based on isotope
transient tests, FT-IR, STEM, Arrhenius plot, and partial pressure
dependence. Based on these results, stable catalytic activity and
metal-support interaction are discussed for Co/LSAO.
−1
00 mL min ). Products of the second reaction were detected
using a quadrupole mass spectrometer (Q-Mass, HPR20; Hiden
Analytical Ltd.). The observed signals of m/e were 2(H ), 4(He),
2
18
18
1
4
5(CH ), 18(H O), 20(H O), 28(CO), 30(C O), 40(Ar), 44(CO ),
O), 48(C O ), 78(C H ), and 91(C7H ). The effect of
Ar (m/z = 20) signal to H2 O was negligible [48–50,52]. The lat-
4
O
2
2
2
16 18
18
6(C
2 6 6 8
2+
18
18
tice oxygen release rate was calculated with signals of 30(C O),
16 18
18
4
6(C O O), and 48(C O ).
2
2.4. Catalyst structure analysis
X-ray diffraction measurements (RINT-2000; Rigaku Corp.)
were conducted to ascertain the perovskite-type structure of
the obtained support using Cu K␣ X-ray radiation of 40 kV at
2
. Experimental
2
0 mA. Results confirmed that all resulting catalyst supports had a
perovskite structure. The supported Co metal particle size was mea-
sured using a scanning transmission electron microscopy (STEM;
HF-2210; Hitachi Ltd.). More details related to STEM measurement
have been explained in our earlier reports [50,52]. Metal particle
diameter was obtained using hemisphere approximate measuring
major axis and minor axis of ellipsoid-shaped particles. The most
frequent value in a distribution chart of particle diameters (more
than a hundred) was regarded as a mean particle size. Subsequently,
a metallic surface area and a metal-support perimeter of the one
hemisphere were calculated with the mean particle size obtained.
After that, such values of a hemisphere were integrated to total
metallic surface area and metal-support perimeter depending on
loaded amount of the active metal.
2
.1. Catalyst preparation
Perovskite-type oxide, La0.7Sr0.3AlO
(LSAO) for catalyst sup-
3−ı
port was prepared using the citric acid complex method. More
details related to the citric acid complex method were explained
in previous reports [48,49,51]. The obtained LSAO support was
impregnated with a solution of cobalt (II) nitrate (Kanto Chemical
Co. Inc.). Subsequently, it was dried and calcined at 1073 K for 1 h.
The Co impregnation was conducted likewise a Ni impregnation
method explained elsewhere [48,49,51]. The size of the obtained
catalyst was adjusted to 250–500 ␮m with sieving.
2.2. Activity test
2.5. Fourier transform infrared (FT-IR) measurement
Catalyst activity tests were conducted in a tubular reac-
tor, in which 25 mg of catalyst was charged into a fixed bed
Adsorbing features of toluene and reaction intermediates on
and diluted with SiO . After pre-reduction at 1073 K, toluene
2
the Co/LSAO catalyst were examined using a Fourier transform
infrared spectrometer (FT/IR-6100; Jasco Corp.). A previous report
has described details of the procedures of this measurement [51].
The Co/LSAO catalyst was shaped to a 20 mm␾ disk. After pre-
reduction at 1073 K and background measurement at 323 K, the
steam reforming was conducted at 873 K in the gas con-
sist of 1.5%C7H :21%H O:5%Ar:72.5%He vol% (total flow rate:
8
2
−
1
2
00 mL min ) for 180 min. Detailed information related to activity
tests was presented in previous reports [48–50,52]. In such experi-
mental conditions, the effect of mass transportation was negligible,
as confirmed by our preliminary experiments. The product gas of
this reaction was measured using GC-FID (GC-8A; Shimadzu Corp.)
−1
feed gas was introduced through a bubbler within 2 mL min
(
[
5
gaseous) of toluene feed rate, then it was purged with N2 gas
51]. Subsequently, the IR cell was heated and kept at 373, 473,
73, 673, 773, and 873 K for 10 min independently in N2 gas. After
each heating operation, the cell was cooled to 323 K. Then the IR
spectrum was measured. Figures in a previous report present a
comprehensive view of the relevant spectra [51].
and GC-TCD (GC-8A; Shimadzu Corp.). Toluene conversion and H
2
−
1
yield were defined with a toluene feed rate (mmol s ) and the
−
1
following formation rate of products: r (mmol s ) for CO, r
CO
CH4
−
1
−1
−1
(
mmol s ) for CH , r
(mmol s ) for CO , rH (mmol s ) for H2.
4
CO2
2
2
rCO + r
+ r
CH4
CO2
Toluene conversion (%) = C7H8
× 100
(3)
(4)
2
.6. Evaluating the reaction mechanism on Co supported LSAO
feed rate × 7
× 100
catalyst
rH2
H2 yield (%) = C7H
feed rate × 18
8
The apparent activation energy of toluene steam reforming on
After the reaction for 180 min, the amount of deposited carbon
was measured using temperature programmed oxidation (TPO)
measurements using a thermogravimetry. The temperature was
Co/LSAO was estimated using Arrhenius plots. Activity tests were
conducted at temperatures of 723–923 K. The loaded amount of
catalyst was decreased to 10 mg for obtaining kinetic values. The
reaction rate was calculated from the formation rate of CO, CO2,
and CH4 analyzed with GC-FID (GC-8A; Shimadzu Corp.).
−1
increased from 298 K to 1173 K at 10 K min in the gas composition
−
1
of 90%N :10%O vol% (total flow rate: 100 mL min ).
2
2
Partial pressure dependences of the reaction rates on H O
2
2
.3. Isotopic transient response test
and toluene on the Co/LSAO catalyst were measured at each
temperature: 748 K and 898 K. The feed gas composition was
C7H :H O:Ar:He = 1.5:(15, 21 or 27):5:(78.5, 72.5 or 66.5) vol%
To evaluate the lattice oxygen mobility on the Co/LSAO
8
2
−
(total flow rate: 200 mL min ) and C7H :H O:Ar:He = (1, 1.5
8 2
1
catalyst, isotope transient tests were conducted. Details of
the procedures used for isotopic transient tests have been
−
1
or 2):21:5:(73, 72.5 or 72) vol% (total flow rate: 200 mL min
)
Please cite this article in press as: K. Takise, et al., Highly active and stable Co/La0.7Sr0.3AlO
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3−ı

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Table 2
3
100
Redox properties of Co/LSAO catalysts. Reaction conditions: S/C = 2.0, W/F = 0.393 g-
−
1
5
1
1
5
wt% Co/LSAO
0wt% Co/LSAO
5wt% Co/LSAO
wt% Ni/LSAO
cat h mol
.
8
6
4
2
0
0
0
0
0
Catalyst
Metal
loading
Reaction
temperature
(K)
Release rate of
lattice oxygen
Release amount
of lattice oxygen
(␮mol)
−
1
(
wt%)
(␮mol s
)
Co/LSAO
Co/LSAO
Co/LSAO
Co/LSAO
Co/LSAO
5
873
873
873
823
898
1.09
2.93
2.08
1.78
2.88
18.9
59.3
38.6
23.9
46.2
10
15
10
10
undetectable thanks to the high coke suppression ability of the
catalyst. The amounts of coke on 10 wt% and 15 wt% Co/LSAO
−
1
−1
were, respectively, 22.1 mg g-cat and 8.9 mg g-cat . Amounts of
formed coke stayed low on Co/LSAO catalysts in spite of increas-
ing of metal loading. Therefore, it can be said that Co metal hardly
forms coke during this reforming reaction. Ni/LSAO forms much
coke on the catalyst after the reaction, and the amounts are 57 mg g-
0
50
100
150
200
Time on stream / min
Fig. 1. Toluene conversions on Co/LSAO catalysts: 5 wt% Co/LSAO, 10 wt% Co/LSAO,
and 15 wt% Co/LSAO. Reaction conditions: 873 K reaction temperature, S/C = 2.0,
W/F = 0.393 g-cat h mol . Results for Ni/LSAO [52] are also plotted for better com-
prehension.
−
1
−1
for 10 wt% Ni/LSAO and
cat
for 5 wt% Ni/LSAO, 450 mg g-cat
−1
−1
1
000 mg g-cat for 15 wt% Ni/LSAO [48]. Better stability of the cat-
alytic activity on Co/LSAO than on Ni/LSAO is confirmed by other
methods in latter sections.
independently. Reaction rates were calculated from the formation
3.2. Redox property and catalytic structure of Co/LSAO
rates of CO, CO , and CH4 analyzed with GC-FID (GC-8A; Shimadzu
2
Corp.).
The redox property of lattice oxygen is important when using
LSAO as a catalytic support [48–52]. For this reason, isotopic tran-
sient response tests were conducted over each Co/LSAO catalyst
under the same condition to activity tests: the reaction temperature
was 873 K. Fig. 2 shows the transient response curves of 18O com-
pounds on these catalysts. The resultant release rates and release
amounts of lattice oxygen are summarized in Table 2. The release
3
. Results and discussion
3.1. Catalytic activity and coke formation on Co/LSAO
Toluene steam reforming was conducted over 5, 10 and
5 wt% Co/LSAO catalysts under the following conditions: GHSV
1
−
1
−1
−1
rates of lattice oxygen were 1.09 ␮mol s , 2.93 ␮mol s
and
.08 ␮mol s on 5 wt%, 10 wt% and 15 wt% Co/LSAO, respectively.
Simultaneously, release amounts of lattice oxygen were 18.9 ␮mol,
9.3 ␮mol and 38.6 ␮mol on 5 wt%, 10 wt% and 15 wt% Co/LSAO,
respectively. In the case of 5 wt% Ni/LSAO catalyst, the release
ca. 12,000 h , S/C = 2.0, reaction temperature 873 K. Fig. 1 shows
time course of the catalytic activities for these three catalysts,
and Table 1 shows the H2 yield and carbon deposition after the
reaction over these catalysts. For better comparison, the catalytic
activity of Ni/LSAO under the same reaction conditions [52] is also
depicted in Fig. 1. According to these results, Ni/LSAO was deacti-
vated during 180 min of toluene steam reforming, whereas Co/LSAO
catalysts show stable activity in this reaction. Particularly, 10 wt%
Co/LSAO performed higher steady-state activity compared to the
previously reported Ni/LSAO catalyst [52]. In addition, hydrogen
yields were also higher on Co/LSAO catalysts because methane
formation as a byproduct occurred only slightly on Co/LSAO. Here-
inafter, the stable catalytic activities of Co/LSAO catalysts are
examined.
−
1
2
5
−
1
rate and release amount of lattice oxygen were 2.66 ␮mol s and
1.4 ␮mol, respectively, which were reported in elsewhere [52].
7
Also, increasing of oxygen release rate brought higher activity and
stability on the Ni/LSAO [52]. From obtained results, Co/LSAO cat-
alysts showed high redox properties and there was a correlation
between the redox properties and catalytic activities/selectivities
like as Ni/LSAO catalyst.
Catalyst structures were investigated for 5 wt%, 10 wt% and
1
5 wt% Co/LSAO. Fig. 3 shows STEM images of these cata-
lysts, on which the location of Co metals was confirmed using
EDX (not shown). Table 3 shows the calculated mean parti-
cle size, metallic surface area, and the metal-support perimeter
of Co/LSAO catalysts from such measurements. The mean par-
ticle sizes of Co were 17.5 nm, 17.5 nm and 32.5 nm on 5 wt%,
10 wt% and 15 wt% Co/LSAO, respectively. The calculated metal-
lic surface areas of 5 wt%, 10 wt% and 15 wt% Co/LSAO were,
Table 1 presents the amounts of carbon deposition after 180 min
of the reaction. On 5 wt% Co/LSAO catalyst, deposited carbon was
Table 1
Catalytic activities and the amount of carbon deposition during steam reforming
on Co/LSAO catalysts. Reaction conditions: 873 K reaction temperature, S/C = 2.0,
−1
W/F = 0.393 g-cat h mol
.
Catalyst
Metal loading
Toluene
conversion (%)
H2 yield
(%)
CO yield
(%)
(wt%)
Table 3
Catalytic structures of Co/LSAO catalysts. Calcination temperature of supported
metal: 1073 K.
Co/LSAO
Co/LSAO
Co/LSAO
5
10
15
40.9
61.3
48.0
50.0
59.3
54.1
14.9
26.2
17.5
Catalyst
Metal
loading
(wt%)
Metal average
particle size
(nm)
Metallic
Metal-support
perimeter
(10 m g
surface area
2
−1
8
−1
)
CH4 yield
CO2 yield
(%)
Carbon deposition
(mg g-cat
Selectivity to
carbon (%)
(m
g
)
−1
(
%)
)
Co/LSAO
Co/LSAO
Co/LSAO
5
10
15
17.5
17.5
32.5
1.43
2.87
2.31
1.64
3.27
1.42
0.0
0.0
0.0
26.0
35.1
30.5
N.D.
22.1
8.9
–
0.049
0.025
Particle size: mode diameter.
Please cite this article in press as: K. Takise, et al., Highly active and stable Co/La0.7Sr0.3AlO
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3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
3.0
(
a)
(b)
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
0
20
40
60
80
100
Time on stream / sec
Time on stream / sec
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
(
c)
0
20
40
60
80
100
Time on stream / sec
Fig. 2. Formation rates of 18O compounds on (a) 5 wt% Co/LSAO, (b) 10 wt% Co/LSAO and (c) 15 wt% Co/LSAO. Reaction conditions: 873 K reaction temperature, S/C = 2.0,
W/F = 0.393 g-cat h mol 1, ꢀ: C16
−
O
18
O, ᭿: C O, ꢁ: C O2, ꢀ: H2 O.
18
18
18
2
−1
2
−1
2
−1
respectively, 1.43 m g , 2.87 m g
and 2.31 m g . Addition-
3.3. Surficial adsorbents and stable catalytic activity on Co/LSAO
ally, the metal–support perimeters of 5 wt%, 10 wt% and 15 wt%
8
−1
8
−1
Co/LSAO were, respectively, 1.64 × 10 m g , 3.27 × 10 m g and
The Co-supported LSAO catalysts were not deactivated during
toluene steam reforming. Its stability might be related to coke for-
mation, catalytic structures, or redox characteristics. Our previous
report described that toluene adsorbed and converted to reaction
intermediates on LSAO support [51]. Accordingly, the adsorption
8
−1
1
.42 × 10 m g . These results show that 10 wt% Co/LSAO has the
largest Co surface area and the longest Co-support perimeter.
Therefore, 10 wt% Co/LSAO showed better redox property and cat-
alytic activity compared to 5 wt% and 15 wt% Co/LSAO.
Fig. 3. STEM images of Co/LSAO catalysts: (a) 5 wt% Co/LSAO, (b) 10 wt% Co/LSAO and (c) 15 wt% Co/LSAO.
Please cite this article in press as: K. Takise, et al., Highly active and stable Co/La0.7Sr0.3AlO
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3−ı

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5
1.5
1.0
0.5
0.0
-
1
31.6 kJ mol
-
1
131.6 kJ mol
-
-
-
-
-
0.5
1.0
1.5
2.0
2.5
8
48 K
1
.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40
3
-1
1
/T / 10 K
Fig. 5. Arrhenius plots for Co/LSAO catalyst.
respectively in the higher temperature range and in the lower tem-
perature range. Fig. 6 presents transient response curves for 18
O
compounds in both regions. Additionally, Table 2 explains release
rates and amounts of lattice oxygen calculated from such response
curves in both temperature ranges. The lattice oxygen release rate
−
1
−1
was 2.88 ␮mol s
and 1.78 ␮mol s in the higher temperature
range and in the lower temperature range, respectively. Further-
more, the release amount of lattice oxygen was 46.2 ␮mol in the
higher temperature range and 23.9 ␮mol in the lower tempera-
ture range. Specifically, the calculated values differed between two
regions. However, Fig. 6 depicts that Co/LSAO had redox property
both below and above 848 K. Consequently, the change of apparent
activation energy between higher and lower temperature ranges
was not attributable to whether the surface redox reaction partic-
ipates the total reaction or not.
To elucidate adsorption properties and changed apparent acti-
vation energy, partial pressure dependences of toluene and steam
were analyzed on 10 wt% Co/LSAO catalyst. Resultant data are
shown for the higher temperature region and the lower temper-
ature region in Fig. 7. The reaction rate of toluene steam reforming
(rSR) is defined as follows:
Fig. 4. IR spectra during temperature programmed desorption of toluene on (a)
5
wt% Co/LSAO and (b) 5 wt% Ni/LSAO [51] at temperatures of 323–873 K.
property was investigated on Co/LSAO catalyst using FT-IR mea-
surements. This measurement was conducted using 5 wt% Co/LSAO
for comparison with 5 wt% Ni/LSAO. The resultant spectra obtained
at temperatures of 323–873 K are depicted in Fig. 4. Two peaks
−1
were observed at 1400 and 1478 cm
on the Ni/LSAO catalyst
from 573 K to 873 K. These peaks are assigned to COO asymmet-
ric stretching and C C vibration [51]. These results suggested that
toluene was adsorbed and converted to oxygenate intermediates
on LSAO support. On the Co/LSAO catalyst, similar peaks to the case
a
b
rSR = k[C7H ] [H O]
(5)
−
1
8
2
of Ni/LSAO catalyst [51] were observed at 1398 and 1471 cm at
temperatures of 573–773 K. Possibly, the same oxygenate interme-
diates were formed on Co/LSAO catalyst to Ni/LSAO. On the other
hand, peaks indicating adsorbed reaction intermediates existed on
Ni/LSAO, although those peaks disappeared on Co/LSAO at the reac-
tion temperature of 873 K. Thereby, intermediate adsorbents are
more reactive and easy to desorb on Co/LSAO catalysts so that
Co/LSAO shows stable catalytic activities.
In the higher temperature region, this reaction rate depended
on toluene partial pressure i.e., a = 0.6 and b = −0.13. As described
above, adsorbed reaction intermediates are highly reactive and
desorbed quickly at high temperatures. Therefore, the decom-
position of toluene to reaction intermediates is probably the
rate-determining step because the decomposition desorption of
intermediates is rapid.
In the lower temperature region, the reaction rate relied both
on the partial pressures and particularly on steam partial pres-
sure i.e., a = 0.21 and b = 0.59. Below 848 K, reaction intermediates
remained on catalytic surface, and reaction and desorption of
3
.4. Reaction mechanism of toluene steam reforming on Co/LSAO
FT-IR measurements revealed adsorption properties on Co/LSAO
catalyst, which are important for consideration of stable catalytic
activity during toluene steam reforming. The reaction mechanism
of surficial adsorbent on Co/LSAO was studied in greater detail
with Arrhenius plots, transient response tests, and partial pres-
sure dependences. Fig. 5 presents Arrhenius plots for toluene steam
reforming over 10 wt% Co/LSAO. The apparent activation energy
changed at around 848 K. The obtained apparent activation energy
adsorbed intermediates requires activation of H O. This activation
might be the rate-determining step. Consequently, partial pressure
2
of H O strongly affects the reaction rate of toluene steam reform-
2
ing at lower temperature. From Arrhenius plots and partial pressure
dependence of toluene and H O, desorption rate of adsorbed inter-
2
mediates exceeds the formation rate of them above 848 K, and
intermediate adsorbents were not detected at 873 K, the reaction
temperature.
Altogether, the reaction mechanism on Co/LSAO catalyst is por-
trayed in Fig. 8. The rate-determining step changed at around
848 K. At temperatures higher than 848 K, no peak assigned to
−1
was 31.6 kJ mol in the higher temperature region (above 848 K)
−1
and 131.6 kJ mol in the lower temperature region (below 848 K).
Arrhenius plots showed that the rate-determining step changed
at around 848 K. Therefore transient response tests were conducted
Please cite this article in press as: K. Takise, et al., Highly active and stable Co/La0.7Sr0.3AlO
Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.08.059
catalyst for steam reforming of toluene,
3−ı

G Model
CATTOD-9849; No. of Pages7
ARTICLE IN PRESS
6
K. Takise et al. / Catalysis Today xxx (2015) xxx–xxx
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.0
(
a)
(b)
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
0
20
40
60
80
100
Time on stream / sec
Time on stream / sec
1
8
−1
Fig. 6. Formation rate of O compounds on 10 wt% Co/LSAO. Reaction conditions: (a) 898 K and (b) 823 K reaction temperature, S/C = 2.0, W/F = 0.393 g-cat h mol , ꢀ:
16
18
18
18
18
C
O
O, ᭿: C O, ꢁ: C O2, ꢀ: H2 O.
1
1
0
.5
.0
.5
.0
1.5
1.0
0.5
0.0
as the rate-determining step because the decomposition desorp-
tion of intermediates is fast.
At temperatures lower than 848 K, peaks attributable to
adsorbed oxygenate intermediates remained on the catalyst sur-
face from in situ FT-IR measurements. Furthermore, the reaction
(
a-1)
a = 0.60
(b-1)
b = -0.13
rate depends on H O rather than toluene in such a low tempera-
2
ture area. These results suggest that the rate-determining step is
dissociation of H O. Then, dissociated species from H O such as
2
2
lattice oxygen and hydroxyl group reacts with adsorbed oxygenate
intermediates on the Co-support interface or Co surface. At the
reaction temperature of 873 K, the formed reaction intermediates
react rapidly on Co/LSAO. Such a desorption property is important
for the stability of the catalytic activity.
0
-
6
-5.5
-5
-4.5
-4
-2 -1.8 -1.6 -1.4 -1.2 -1
ln P (C H ) / -
ln P (H O) / -
7
8
2
0
.0
0.0
(
a-2)
(b-2)
-
0.2
0.4
0.6
0.8
1.0
1.2
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-
-
-
-
-
4. Conclusion
b = 0.59
Co/LSAO catalysts were not deactivated during 180 min of
toluene steam reforming reaction. Higher toluene conversion and
stable catalytic activity were achieved on 10 wt% Co/LSAO. FT-
IR measurement revealed that adsorbed reaction intermediates
on LSAO support were highly reactive and desorbed easily on
Co/LSAO. This adsorption property prevented the deactivation
of catalytic activity by strong adsorption of reaction intermedi-
ates. Arrhenius plots indicated that the rate-determining step was
changed at around 848 K on Co/LSAO. At temperatures higher
than 848 K, the decomposition reaction of toluene to reaction
intermediates was regarded as the rate determining step because
desorption of intermediate adsorbents occurs quickly. Below 848 K,
a = 0.21
-6
-5.5
-5
-4.5
-4
-2 -1.8 -1.6 -1.4 -1.2 -1
ln P (C H ) / -
ln P (H O) / -
7
8
2
Fig. 7. Partial pressure dependences of toluene and H2O over 10 wt% Co/LSAO in
each temperature range: higher and lower than 848 K. Higher temperature range:
a-1) toluene and (b-1) H2O at 898 K. Loaded amount of catalyst: 10 mg. Lower tem-
perature range: (a-2) toluene and (b-2) H2O at 748 K. Loaded amount of catalyst:
5 mg.
(
2
the rate-determining step was activation of H O. This activation
2
adsorbed oxygenate intermediates was observed according to FT-
IR measurements. Moreover, the reaction rate depended mainly on
toluene partial pressures. Therefore, conversion of toluene to such
oxygenate intermediates (shown in the circle in Fig. 8) is regarded
was necessary for the reactive desorption of intermediate adsor-
bents. Changing of the rate-determining step was attributable to
the desorption property of surficial adsorbents on Co/LSAO. In con-
clusion, high catalytic stability on Co/LSAO was accomplished with
the promotion of reactive desorption of the adsorbed reaction inter-
mediates on the catalytic surface.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.08.
0
59.
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catalyst for steam reforming of toluene,
3−ı