Effects of the addition of CeO2on the steam reforming of ethanol using novel carbon-Al2O3and carbon-ZrO2composite-supported Co catalysts

Article abstract of DOI:10.1039/d1ra00141h

Novel carbon-Al₂O₃and carbon-ZrO₂composite-supported Co catalysts were prepared using the sol-gel method with polyethylene glycol (PEG) as a carbon source, and the effects of the addition of CeO₂to catalysts on the steam reforming of ethanol were investigated. The reactions were carried out in a fixed bed reactor with H₂O/EtOH = 12 (mol/mol) and a temperature range of 300 °C to 600 °C. The catalyst characterization was performed by XRD, nitrogen adsorption and desorption isotherms, TG-DTA, XRF and TEM. Although the carbon-Al₂O₃composite-supported Co catalysts exhibited a higher conversion of ethanol than the carbon-ZrO₂composite-supported Co catalysts, the effect of the addition of CeO₂was hardly observed for catalysts with Al₂O₃. In contrast to the case of catalysts with Al₂O₃, the effect of the addition of CeO₂to catalysts with ZrO₂on the conversion and the hydrogen yield was observed, and the hydrogen yield at 600 °C exceeded that of catalysts with Al₂O₃. 16Co42C31.5Ce10.5Zr exhibited the highest hydrogen yield of 89% at 600 °C. Fine Co metal species were observed for the used ZrO₂-based catalysts, while Co₃O₄peaks were observed for the used Al₂O₃-based catalysts. The development of the carbon nanotube-like structure with a diameter of 50 nm was observed with particles having diameters of 30 nm to 50 nm, suggesting that the carbon deposition might occur so as not to deactivate the catalyst.

Full text of DOI:10.1039/d1ra00141h

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Effects of the addition of CeO2 on the steam
reforming of ethanol using novel carbon-Al2O3 and
Cite this: RSC Adv., 2021, 11, 8530
carbon-ZrO2 composite-supported Co catalysts†
*
Atsushi Ishihara,
Hiroshi Tsujino and Tadanori Hashimoto
Novel carbon-Al2O3 and carbon-ZrO2 composite-supported Co catalysts were prepared using the sol–gel
method with polyethylene glycol (PEG) as a carbon source, and the effects of the addition of CeO2 to
catalysts on the steam reforming of ethanol were investigated. The reactions were carried out in a fixed
bed reactor with H2O/EtOH ¼ 12 (mol/mol) and a temperature range of 300 ꢀC to 600 ꢀC. The catalyst
characterization was performed by XRD, nitrogen adsorption and desorption isotherms, TG-DTA, XRF
and TEM. Although the carbon-Al2O3 composite-supported Co catalysts exhibited a higher conversion
of ethanol than the carbon-ZrO2 composite-supported Co catalysts, the effect of the addition of CeO2
was hardly observed for catalysts with Al2O3. In contrast to the case of catalysts with Al2O3, the effect of
the addition of CeO2 to catalysts with ZrO2 on the conversion and the hydrogen yield was observed, and
the hydrogen yield at 600 ꢀC exceeded that of catalysts with Al2O3. 16Co42C31.5Ce10.5Zr exhibited the
highest hydrogen yield of 89% at 600 ꢀC. Fine Co metal species were observed for the used ZrO2-based
catalysts, while Co3O4 peaks were observed for the used Al2O3-based catalysts. The development of the
carbon nanotube-like structure with a diameter of 50 nm was observed with particles having diameters
of 30 nm to 50 nm, suggesting that the carbon deposition might occur so as not to deactivate the catalyst.
Received 7th January 2021
Accepted 5th February 2021
DOI: 10.1039/d1ra00141h
rsc.li/rsc-advances
perovskite were also active for SRE.11 The addition of Fe to Co/a-
Al2O3 promoted SRE.14 The addition of Ni and Co to Cu/ZnO/
Al2O3 also promoted SRE.15 The performance of Co/SiO2 for SRE
1. Introduction
Restrictions on the utilization of fossil fuels have begun in
developed countries of Europe and have spread throughout the
world. In such situations, the technology for the utilization of
renewable energy has to be developed urgently and the
reforming of biomass has been one of the candidates.1 Among
such renewable resources, is ethanol obtained from fruits and
grains, and much attention has been focused on its steam
reforming.1,2 While ethanol may be directly utilized for fuel, the
production of hydrogen from ethanol has been one of the most
attractive technologies. The steam reforming of methane is
carried out using supported Ni-based catalysts. However, it is
known in the steam reforming of ethanol (SRE) that the selec-
tivity for hydrogen of Ni catalysts is not so high as that for Co-
based catalysts.1,2 Co catalysts supported on alumina were re-
ported for their high hydrogen yield although they were oen
deactivated by coke deposition. In these reports, it was sug-
gested that Co metal supported on an oxide would be an active
site for SRE.1–17 The promotion effects of Pt, Pd, Ru, and Ir were
observed in SRE using Co/Al2O3, and among them, the addition
of Ru was the most effective.10 Co and Ni catalysts supported on
was higher than that of Co/Al2O3 since Co was not sufficiently
reduced on Co/Al2O3 due to the presence of the strong inter-
action between Al2O3 and Co species.16 The effect of the addi-
tion of Co species to Ni/Al2O3 was conrmed in oxidative SRE
where Ni and Co oxides on alumina were reduced at the lower
temperature and led to more stable activity and yield of
hydrogen.17 As other catalysts, Co/ZnO–Al2O3,18 Co/CeO2,19–21
Co/ZrO2,19,21 Rh/Co/CeO2 or Al2O3,22 K/Co/Al2O3,23,24 Co/CaO–
Al2O3,25 Cu promoted Ni–Co/hydrotalcite26 for SRE, Co/Al2O3
and Co/CeO2Al2O3 for oxidative SRE,27 and Co–CaO/CeO2, ZrO2
and MgO,28 CaO–Ni/Al2O3 (ref. 29) for sorption enhanced SRE
have been reported to have superior activity and selectivity.
Further, it has been reported that CeO2–ZrO2 composite-
supported cobalt and rhodium catalysts have higher activity
and selectivity in SRE.20,30–41 For example, when Co(10%)Pt(3%)/
CeO2–ZrO2–Al2O3 was used with a Pd membrane, 60% conver-
sion and 70% hydrogen recovery were achieved at 400 ꢀC.30
RhPd/Ce0.5Zr0.5O2–Al2O3 exhibited high performance and
stability in SRE.31 When the CeO2–ZrO2 composite-supported
Co, Ni and NiCo catalysts were compared in SRE, the NiCo
catalyst exhibited superior performance, while the Co catalyst
was not so active.32 When Co, Fe and Rh were added to Ni/CeO2–
ZrO2 catalysts, NiRh/CeO2–ZrO2 exhibited good performance for
400 h in SRE.33 Bimetallic CoIr/Ce1ꢁxZrxO2 catalysts were also
Division of Chemistry for Materials, Graduate School of Engineering, Mie University,
Mie, Japan. E-mail: ishihara@chem.mie-u.ac.jp
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00141h
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used in the oxidative steam reforming of a bio-butanol raw
For an example, 16Co42C21Ce21Zr 700N2 was prepared as
mixture and the catalyst with the better performance was found follows: a 1-butanol solution of ZB (4.09 g ZB, 15.16 g 1-butanol)
to inhibit the sintering of active metal and the coke forma- in a 200 mL beaker was mixed with ceria (0.53 g). To the
tion.34,35 It has been found that 10%Co/CeO2–ZrO2 exhibited mixture, a 1-butanol solution of cobalt nitrate (3.95 g cobalt
80% of hydrogen yield and 100% of ethanol conversion for nitrate hexahydrate, 9.29 g 1-butanol) was added dropwise at
ethanol steam reforming at 450 ꢀC with a steam-to-carbon ratio 0 ꢀC. Further, 3.89 g of PEG was added and then the mixture was
of 6.5.38,39 Also, 10%Co/10%CeO2–ZrO2 exhibited about 75%ꢀof stirred at 0 ꢀC for 10 h. The mixture was dried at 115 ꢀC for 8 h
stable hydrogen yield and 100% of ethanol conversion at 450 C and the gel obtained was pushed out of a syringe for ceramics to
with the molar ratio of EtOH : H2O ¼ 1 : 10.40 A catalyst with make cylindrical pellets with a diameter of 3 mm. The pellets
ꢀ
9.1 wt% Co and 22.3 wt% Ce supported on nanopowder ZrO2 were calcined under nitrogen atmosphere at 700 C for 3 h to
exhibited the conversion of ethanol 100%, 92% H2, 81% CO2, 6% prepare catalysts that were crushed to particles with sizes 125–
CO and 12% CH4 in SRE with H2O/EtOH ¼ 21 mol/mol at 420 ꢀC.41 355 and 355–600 mm, in a weight ratio of 7 : 3. 16Co63C21A,
In these studies, the conversion of ethanol reached 100% at 16Co42C21Ce21A, 16Co63C21Zr, 16Co42C10.5Ce31.5Zr and
ꢀ
a temperature near 450 C, while the hydrogen yield has not yet 16Co42C31.5Ce10.5Zr were prepared using a similar method.
been determined and there have been differences in results
Sample names were expressed as follows: Co was cobalt, C was
between research groups. Further, the effects of the combination carbon from PEG, Ce was ceria, Zr was zirconia, A was alumina and
and its ratio of CeO2 and other oxides, and the role of CeO2 in these gures before abbreviations represent values of wt%. For example,
catalyst systems have not yet been sufficiently understood.
16Co42C21Ce21Zr is a catalyst having Co metal of 16 wt%, C of
Although supported Co catalysts exhibited high activity in SRE, 42 wt%, CeO2 of 21 wt% and ZrO2 of 21 wt%.
active species may be metallic cobalt or cobalt oxides, and argu-
ments remain. It seems that oxide-supported cobalt species would
be reduced to the metallic state and that the successive interaction
with oxide support may stabilize metallic species, which would lead
2.2. Steam reforming of ethanol using carbon-Al2O3 and
ZrO2 composite-supported Co catalysts with and without CeO2
to a decrease in the activity. In contrast, some metal oxides may Fig. S2† shows an apparatus for the SRE reaction. A xed bed
generate both metal and oxide species in an interface, where metal ow reactor of stainless steel (ID 8 mm) and a back pressure
species may have high dispersion and bring about high activity.
regulator were used. Here, 1 g catalyst was added in the order,
Our group has already reported that Fe and Co catalysts quartz sand, glass wool, catalyst, glass wool and quartz sand
supported on carbon-oxide composites were active for the from the top to the bottom in the center of the reactor. The
Fischer–Tropsch reaction in the presence of solvent42 and Ni condition of the steam reforming was as fꢀollows: the reaction
catalysts were effective for the hydrothermal gasication of phenol temperature was in the range of 300–600 C, the heating rate
dissolved in water.43–45 In the reports, metallic species were main- was 5 ꢀC minꢁ1, LHSV was 48 hꢁ1, the volume of a catalyst was
tained on carbon-coated oxide supports, which inhibited the 0.5 mL and H2O/EtOH was 12 mol/mol. The pressure in the inlet
interaction between oxides and metal species and increased the of the reactor changed depending on the catalysts used and the
activity. Further, we have found that the carbon-oxide composite- temperature, and increased with theꢀprogress of the reaction.
supported Ni and Co catalysts were active for the SRE reaction. The pressure for each catalyst at 600 C is tabulated in Table 1
The Ni/C/Al2O3 catalyst showed high activity and the Co/C/Al2O3 and all data are tabulated in Table S1.† The reaction tempera-
catalyst exhibited a high hydrogen yield for SRE. In the present ture was kept at each tested temperature for 1 h and then
study, we tried to test catalysts consisting of not only Co/C/Al2O3 products were collected every 30 min for 1 h, that is, two times. The
but also Co/C/ZrO2 in SRE and the effects of the addition of CeO2 obtained products were separated in the gas–liquid separator.
to both catalyst systems on SRE were investigated. Only the ZrO2- Gaseous products collected in a Tedlar bag were determined using
based catalyst system exhibited the effects of the addition of CeO2 a gas chromatograph with a thermal conductivity detector (GC-
and that CeO2-modied ZrO2-based catalysts exhibited a higher TCD, Shimadzu GC-8A). Liquid products were determined using
yield of hydrogen as compared to Al2O3-based catalysts.
a gas chromatograph with a ame ionization detector (GC-FID,
Shimadzu GC-2014) to obtain the ethanol conversion.
A liquid product (1 mL) injected into GC-FID by the auto-
sampler (AOC-20i) was determined under the following condi-
tions: injection temperature 250 ꢀC, detector temperature
250 ꢀC, initial column temperature 50 ꢀC for 3 min, nal
2. Experimental
2.1. Preparation of carbon-Al2O3 and ZrO2 composite-
supported Co catalysts with and without CeO2
Starting materials for Al2O3 and ZrO2 were aluminum tri-sec- column temperature 200 ꢀC, pressure 107.8 kPa, ow rate of N2
butoxide (ASB, C12H27Al2O3, Tokyo Kasei) and zirconium(IV) carrier gas 153.7 mL minꢁ1, sprit ratio 200, BP-1 column with
butoxide (ZB, (C4H9O)4Zr, ca. 80% in 1-butanol, Tokyo Kasei). a length 60 m, column diameter 0.25 mm and lm thickness 0.5
CeO2 was a reference catalyst from the Catalysis Society of mm. H2 was determined using GC-TCD under the conditions of
Japan, JRC-CEO-3. A carbon source was polyethylene glycol nitrogen carrier gas, gaseous product sample 0.1 mL, injection
(PEG, (CH2–CH2–O)n, Nakalai Tesque). A cobalt source was temperature 110 ꢀC, detector temperature 110 ꢀC, column
cobalt nitrate hexahydrate (Co(NO3)2$6H2O, Nakalai Tesque). temperature 50 ꢀC, column length of 2 m and column Porapak
The catalyst was prepared using the sol–gel method according T. Carbon monoxide, carbon dioxide and methane were deter-
to the owchart shown in Fig. S1.†
mined using GC-TCD under the conditions of He carrier gas,
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Table 1 EtOH conversion, H2 yield, carbon gas selectivity and inlet pressure in the steam reforming of ethanol at 600 ꢀCa
EtOH conv.
(%)
H2 yield
(%)
CO selct.
(%)
CO2 selct.
(%)
CH4 selct.
(%)
Total C gas
(%)
Inlet pressure
(MPa)
Sample name
16Co63C21Al
16Co42C21Ce21Al
16Co63C21Zr
16Co42C10.5Ce31.5Zr
16Co42C21Ce21Zr
16Co42C31.5Ce10.5Zr
100
100
99
96
100
100
80
76
63
61
84
89
12
11
8
9
20
9
71
59
39
52
46
73
17
12
15
11
10
18
100
82
62
72
76
0.39
0.15
1.86
0.68
0.14
0.42
100
a
Conv. ¼ conversion; selct. ¼ selectivity; C gas ¼ CO, CO2 and CH4.
gaseous product sample 0.1 mL, injection temp. of 110 ꢀC, shown in Fig. 2a, the yields of hydrogen for 16Co63C21Al and
detector temp. of 110 ꢀC, column temp. of 150 ꢀC and a packed 16Co42C21Ce21Al increased with increasing temperature and
column of Unibeads C with column length of 3 m.
reached 80% and 76% at 600 ꢀC, respectively, indicating that
there was no positive effect resulting from the addition of
CeO2. In contrast, the yields of hydrogen for 16Co63C21Zr,
16Co42C10.5Ce31.5Zr, 16Co42C21Ce21Zr and 16Co42C31.5-
Ce10.5Zr at 600 ꢀC were 63%, 61%, 84% and 89%, respec-
tively, indicating that catalysts with larger amounts of CeO2
exhibited higher hydrogen yields, which were higher than
those for catalysts with Al2O3. The carbon recoveries of
16Co42C31.5Ce10.5Zr and 16Co63C21Al catalysts reached
100% at 600 ꢀC. The selectivities of CO, CO2 and CH4 for
16Co42C31.5Ce10.5Zr were 9, 73 and 18%, respectively, and
were very similar to those for 16Co63C21Al. As the carbon
recoveries for other catalysts were lower than those for
16Co42C31.5Ce10.5Zr and 16Co63C21Al, it seems that
signicant carbon deposition would have occurred. The
result was conrmed by the high value of the pressure in the
reactor inlet at 600 ꢀC as given in Table 1 and the fact that the
carbon content of the catalyst aer the reaction was higher
than that before the reaction as shown in Table 2.
2.3. Characterization of carbon-Al2O3, ZrO2 and CeO2
composite-supported Co catalysts
A Rigaku Ultima IV was used to obtain X-ray diffraction (XRD)
patterns of crystals of the catalyst, which were measured under
the conditions of Ni-ltered Cu-Ka radiation (l ¼ 0.15418 nm),
0.10 g catalyst on a plate of slide glass, 2q in the range 10ꢀ to 7ꢁ0ꢀ1,
continuous scan, sampling width 0.01ꢀ, scan speed 4ꢀ min
,
radiation slit 1/3ꢀ, radiation column limitation slit 10.00 nm,
scattering slit 1/3ꢀ, detecting slit 0.30 nm, offset angle 0ꢀ,
voltage 40 kV and current 40 mA.
Nitrogen adsorption and desorption were performed using
a BELSORP-mini I-MSP to estimate pore volumes, surface areas
and pore diameters by BET and BJH methods. TG-DTA using
DTG-60AH (Shimadzu) was measured under the following
ꢀ
ꢀ
conditions: temperature from 25 C to 800 C, heating rate of
10 ꢀC minꢁ1, catalysts of 10 mg, a platinum pan, and an air
atmosphere to estimate the content of carbonaceous materials
in fresh and used catalysts. XRF using EDX-720 (Shimadzu) was
measured to estimate the content of inorganic matter in the
catalyst. TEM images were obtained using JEM-1011 (Nihon
Denshi, BEAM current: 60 mA).
As reported in the literature,28 when recycling was repeated,
deactivation was observed. Although the recycling experiments
are not reported in the present study, the change in the inlet
3. Results and discussion
3.1. Ethanol steam reforming using carbon-Al2O3, ZrO2 and
CeO2 composite-supported Co catalysts
Fig. 1 and 2 show the effect of temperature on the conversion of
ethanol and the yield of hydrogen in the steam reforming of
ethanol using carbon-Al2O3–CeO2 and carbon-ZrO2–CeO2
composite-supported Co catalysts, respectively. Table
1
summarizes ethanol conversions, hydrogen yields, carbon
recoveries of CO, CO2 and CH4 against converted ethanol and
the inlet pressure at the reaction temperature of 600 ꢀC. Carbon-
Al2O3 composite-supported Co catalysts exhibited higher
conversions than carbon-ZrO2 composite-supported Co cata-
lysts and the conversion reached 100% at 500 ꢀC, while the
addition of CeO2 hardly affected the conversions for the cata-
lysts with Al2O3. In contrast, the effect of the addition of CeO2
was observed for catalysts with ZrO2 and the conversions
Fig. 1 The effect of temperature on EtOH conversion in the steam
changed depending on the amounts of ZrO2 and CeO2 added. As reforming of ethanol.
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Fig. 2 The effect of temperature on H2 yield in the steam reforming of ethanol using (a) carbon-alumina composite-supported cobalt catalysts
and (b) carbon-zirconia composite-supported cobalt catalysts.
pressure with increasing temperature as shown in Table S1† formation reaction seems to proceed rapidly aer the C–C bond
could be referred to in orderꢀto predict the reactivity for this type scission of ethanol.38,39
of catalyst. At 300 and 400 C, the pressure was rather low for
each catalyst because of the low activity. At 500 ꢀC, the pressure
3.2. Characterization of carbon-Al2O3-ZrO2-CeO2 composite-
increased rapidly because the steam reforming started but the
formation of coke and less reactive intermediates would be
faster than CO and CO2 formation. However, at 600 ꢀC the
pressure decreased in some catalysts because the oxygen from
water started to act for the removal of coke and less reactive
intermediates and the formation of CO and CO2 on the cata-
lysts. The results suggested that the coke and the less reactive
intermediate could be removed by the appropriate oxidation to
disperse metal species again and the reduction to obtain a clean
metal surface on a catalyst.
supported Co catalysts
N2 adsorption and desorption isotherms were determined to
estimate the pore structure of the carbon-oxide composite-
supported cobalt catalysts and the results are shown in Table
3. Surface areas (SA) were estimated by the BET and BJH
methods. Since only mesopores having pore diameters larger
than 3.3 nm are estimated by the BJH method, the difference
between BET-SA and BJH-SA could likely be derived from the
existence of micropores in the estimation of BET-SA. The cata-
lysts with Al2O3 included signicant amounts of not only mes-
opores but also micropores. The catalysts with ZrO2 also
included not only mesopores but also micropores, however, the
values of surface area were much smaller than those of catalysts
with Al2O3. The addition of CeO2 decreased the values of BET-SA
for both catalysts with Al2O3 and ZrO2, indicating that CeO2
would not have micropores. Most of the pore volumes were
made up of mesopores and the contribution of micropores to
pore volume was small, especially for catalysts with CeO2. All
the catalysts exhibited increases in BET-SA, TPV, BJH-SA, and
BJH-PV aer the reaction, indicating that signicant coke
formation would occur and that not only new mesopores but
also new micropores could be formed by carbonaceous mate-
rials. This was conrmed by the increase of the carbon content
in the elemental analysis, which was estimated from the results
of TG-DTA and XRF measurements shown in Table 2. Graphite-
like signals were also observed in the XRD patterns shown in
Fig. 3 and most of the catalysts showed increases in these
signals aer the reaction. Further, the catalysts with CeO2 and
According to the previous reports of SRE catalyzed by Co/
CeO2–ZeO2, the effect of the feed concentration on the activity
and the hydrogen yield would be rather low and the H2
Table 2 Elemental analysis for carbon-oxide composite-supported
metal catalysts by XRF and TG-DTAa
Weight ratio (wt%)
Al catalyst
Co
C
CeO2
Al2O3
16Co63C21Al*b
48.17
42.97 (Co3O4)
18.5
17.45
36.46
1.00
0
0
34.38
20.57
37.76
16.16
16Co63C21Al*a
16Co42C21Ce21Al*b
16Co42C21Ce21Al*a
43.09
23.19
27.10 (Co3O4)
33.54
Weight ratio (wt%)
Zr catalyst
Co
C
CeO2
ZrO2
16Co63C21Zr*b
32.06
15.97
25.44
10.39
21.70
8.41
28.11 (Co3O4)
11.5
34.93
69.29
12.76
63.24
7.92
67.68
0.10
53.95
0
0
33.01 ZrO2 exhibited signicant development of carbon nanotubes
16Co63C21Zr*a
14.74
with diameters of 50 nm as shown in the TEM images of Fig. 4.
16Co42C10.5Ce31.5Zr*b
16Co42C10.5Ce31.5Zr*a
16Co42C21Ce21Zr*b
16Co42C21Ce21Zr*a
16Co42C31.5Ce10.5Zr*b
16Co42C31.5Ce10.5Zr*a
14.5
5.77
28.65
11.86
48.36
23.49
47.65
20.60
39.48
12.03
XRD patterns of carbon-Al2O3 composite-supported Co
catalysts before reaction are shown in Fig. 3a. Catalysts with
Al2O3 exhibited Co metal, while signals of Al2O3 were not
23.43 observed in XRD measurement, indicating that Al2O3 would
11.40
also form small particles in the presence of carbon as it was
a
made by the sol–gel method. It has been proposed that the
b: fresh catalyst, a: used catalyst.
higher yield of hydrogen and conversion would be derived from
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Table 3 N2 adsorption and desorption measurements for carbon-oxide composite-supported metal catalystsa
BET SA*1
Total PV*2
Avg. PD*3
(nm)
BJHSA*1
BJHPV*2
BJHPD*3
(nm)
Sample name
(m2 gꢁ1
)
(cm3 gꢁ1
)
(m2 gꢁ1
)
(cm3 gꢁ1
)
16Co63C21Al*b
216
238
124
227
120
216
91
155
88
141
46
0.32
0.41
0.29
0.52
0.21
0.58
0.15
0.38
0.16
0.43
0.15
0.42
6.0
7.0
9.2
9.2
6.9
10.6
6.6
9.7
7.5
96
156
83
145
76
139
82
113
82
104
55
0.26
0.36
0.25
0.48
0.18
0.53
0.14
0.35
0.16
0.41
0.15
0.40
3.7
3.7
10.7
3.7
3.7
3.7
3.7
3.7
3.7
3.7
9.2
10.6
16Co63C21Al*a
16Co42C21Ce21Al*b
16Co42C21Ce21Al*a
16Co63C21Zr*b
16Co63C21Zr*a
16Co42C10.5Ce31.5Zr*b
16Co42C10.5Ce31.5Zr*a
16Co42C21Ce21Zr*b
16Co42C21Ce21Zr*a
16Co42C31.5Ce10.5Zr*b
16Co42C31.5Ce10.5Zr*a
12.2
13.0
12.8
131
108
a
SA: surface area, PV: pore volume, PD: pore diameter, b: fresh catalyst, a: used catalyst.
the initial reduction of Co species to Co metal in the prepara- CeO2. Co3O4 was also observed for 16Co42C31.5Ce10.5Zr,
tion.1 With the addition of CeO2, Co metal was also observed probably because of the high ability of oxygen transfer by a large
while signals of Al2O3 were not observed. As signals of cubic amount of CeO2. When CeO2–ZrO2 supports were prepared by
CeO2 were observed, this indicated that the particle size of Al2O3 the coprecipitation method, CeO2–ZrO2 mixed oxides were
prepared by the sol–gel method with aluminum alkoxide was formed, signals of mixed oxides near cubic CeO2 signals were
much smaller than that of CeO2. XRD patterns of carbon-ZrO2 observed, and single ZrO2 signals were not observed.20,32,33,38,39,47
composite-supported Co catalysts before the reaction are also In our present study, ZrO2 was made by the hydrolysis of
shown in Fig. 3a. In contrast to the Al2O3-based catalysts, crys- zirconium butoxide but signals of ZrO2 were observed because
tals of tetragonal ZrO2 as well as Co metal were observed in the CeO2 was directly used and the CeO2–ZrO2 mixed oxide was not
catalysts with ZrO2. It is known that the monoclinic ZrO2 phase formed.
is stable at temperatures of 500 C and higher,20,39,46 while the
XRD patterns of carbon-Al2O3 and carbon-ZrO2 composite-
ꢀ
tetragonal ZrO2 phase also exists in the presence of other metal supported Co catalysts aer reaction are shown in Fig. 3b.
oxides34,46 and at the calcination temperature of 400 C.33,46 In Although the accumulation of carbon may occur on catalysts
ꢀ
our present case, there would be such an effect of coexistence. with Al2O3, the development of carbon crystals was inhibited
Further, the signicant accumulation of crystals of carbon was probably because of the presence of the high surface area of
also observed in 16Co63C21Zr before the reaction, indicating Al2O3 where amorphous carbon would be accumulated. The
that the crystallization of carbon would proceed on Co metal. presence of CeO2 does not seem to play a role in providing
With the addition of CeO2, this development of carbon crystals oxygen to carbon, probably because the interaction between
was inhibited probably because oxygen atoms of PEG would be cobalt and CeO2 may be weak on the Al2O3-based catalyst. In
appropriately provided from CeO2 to carbon species developed contrast to the catalysts with Al2O3, ZrO2-based catalysts
on Co metal. This was also supported by the results from the exhibited the effects of CeO2, which provided oxygen from water
elemental analysis in Table 2, where 16Co63C21Zr before use to carbon and inhibited the signicant accumulation of carbon.
included about 34 wt% of carbon although other catalysts with However, total amounts of carbon for catalysts with ZrO2 were
CeO2 decreased the carbon content by increasing the amount of larger than those of Al2O3 as shown in Table 2 and the
Fig. 3 XRD patterns of carbon-oxide composite-supported cobalt catalysts (a) before use and (b) after use.
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Fig. 4 TEM images of used catalysts, (a) 16Co63C21Zr, (b) 16Co42C21Ce21Zr and (c and d) 16Co42C31.5Ce10.5Zr.
signicant development of carbon crystals was observed for aer the reaction shown in Table 2 decreased in comparison
catalysts with ZrO2 probably because of the smaller surface area with those for other ZrO2-based catalysts.
of ZrO2-based catalysts. The addition of CeO2 decreased the
TEM images of used catalysts, 16Co63C21Zr, 16Co42C21-
development of crystallization of carbon as shown in Fig. 3b, Ce21Zr and 16Co42C31.5Ce10.5Zr, are shown in Fig. 4.
although crystals of carbon somewhat increased for 16Co63C21Zr included large amounts of carbon aer the reac-
16Co42C30.5Ce10.5Zr. As the carbon content of 16Co42C30.5- tion while its shape of carbon did not seem to be carbon
Ce10.5Zr aer the reaction shown in Table 2 decreased, the nanotubes as shown in Fig. 4a. In contrast to 16Co63C21Zr,
effect of CeO2 was observed. However, the small surface area of catalysts with CeO2 generated carbon nanotubes with diameter
this catalyst may develop the crystallization of carbon species. of about 50 nm. The slightly smaller size of the catalyst particles
Signals of Co metal were observed for ZrO2-based catalysts while seemed to be located at the end of the carbon nanotubes.
Co3O4 signals were observed for Al2O3-based catalysts. Since Although the carbon deposition might occur in this type of
both signals of Co metal and Co3O4 were very small, it is likely reaction, it is likely that the formation and the development of
that most of the active Co species that were not detected by XRD this type of carbon nanotube would prevent the deactivation of
would have been dispersed in both catalyst systems.
the catalysts and even increase their pore volumes and surface
The elemental analyses of components of catalysts including areas as shown in Table 3. It was reported that 16Co63C21Al
carbon content were performed by XRF and TG-DTA and the had metal species with diameters of 10–30 nm surrounded by
results are shown in Table 2. Calcination under nitrogen materials like carbon nanotubes with 100 nm length and 10 nm
atmosphere changed the original composition of metal, carbon width.1 Similar carbon nanotube-like materials were also re-
and oxide, which appeared for the catalysts since most of the ported in SRE over the Co/ZrO2 catalyst although it exhibited
organic parts of PEG were lost through the thermal decompo- deactivation aer 24 h at 450 ꢀC.40 The diameters of the carbon
sition. Specically, the presence of CeO2 converted the carbon materials changed in the range from 20 nm to 150 nm,
species and the carbon content decreased signicantly with depending on the size of the Co particles. When CeO2 was
increasing the CeO2 content. For all the catalysts, signicant added,40 the deactivation was inhibited and the carbon mate-
amounts of carbon were accumulated aer the reaction and the rials were hardly observed aer the reaction. Although carbon
extent of the carbon accumulation was higher for the ZrO2- materials were observed for the Co catalysts with ZrO2 and CeO2
based catalysts than for the Al2O3-based catalysts. ZrO2-based in our present study, it seems that carbon materials formed in
catalysts had smaller surface areas and therefore the dispersion the reaction under the low temperature would not damage the
itself of active Co species may be lower than those for Al2O3- catalyst. This would also be supported by the fact that the
based catalysts, which led to the lower conversions for ZrO2- complete material balance in carbon was obtained at 600 ꢀC as
based catalysts and even to the lower recovery of carbon at shown in Table 1. Further, the inlet pressure at 600 ꢀC was lower
ꢀ
600 ꢀC as shown in Table 1. On the other hand, the presence of than that at 500 C in several catalysts as shown in Table S1,†
CeO2 improved the mobility of oxygen, the carbon recovery for suggesting that surplus carbonaceous materials would be
16Co42C31.5Ce10.5Zr reached 100% and its carbon content removed with the appropriate progress of the reforming
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3Co0; 2CeIVðO0:5
reaction. As the carbon-ZrO2 composite-supported Co catalyst
without CeO2 (16Co63C21Zr) showed signals of graphitic
carbon in XRD before the reaction, the graphitic carbon may
exist without CeO2 and lead to the deactivation. Metal sizes for
the ZrO2-based catalyst would be larger than those for Al2O3-
based catalysts, which led to the low dispersion of cobalt species
and the lower conversion, especially at the lower temperature.
However, it seems that with the help of CeO2 at higher
temperatures, the improvement of the oxygen mobility would
have brought about an increase in the activity and the yield of
hydrogen.
It was assumed from previous reports of Al2O3-supported Co
catalysts that the presence of large amounts of Co metal would
be needed to obtain the high activity and to inhibit the coke
formation in the steam reforming of ethanol.1,48,49 Further, XRD
signals of both Co metal for ZrO2-based and Co3O4 for Al2O3-
based catalysts were very small and these systems exhibited
high activity and hydrogen yield at the higher temperature,
suggesting that small particles of metallic Co would exist and
play the role of a major active species in the catalyst.
ƒƒƒƒƒƒƒ!Þ
CoIðCH3Þ
Co0ðCOÞ þ 3CoIðHÞ þ 2CeIII
(6)
Ideal reaction routes of steam reforming of ethanol catalyzed
by a carbon-ZrO2-CeO2 composite-supported Co catalyst can be
described by eqn (1)–(6) and Fig. 5. Ethanol can be dehydro-
genated to acetaldehyde on Co0 in eqn (1). Further, the oxidative
addition of the C–H bond for acetaldehyde may occur to form
CoI(CH3CO) and CoI(H) in eqn (2). Also, in eqn (2), two atoms of
Co are described expediently while one Co atom may react to
give the CoII(CH3CO)(H) species. C–C bond scission may occur
with the help of Co0 to form CoI(CH3) and Co0(CO) in eqn (3). In
eqn (4), an oxygen atom could be given from 2CeIV(O0.5) to
Co0(CO) to form carbon dioxide, Co0 and 2CeIII. In this equa-
tion, an intermediate CoIIO(CO) may exist, although it was not
drawn.50 As in eqn (5), H2O could react on CeIII to give CeIV(O0.5
)
and H atoms, which would react with CoI(H) to form H2 and
Co0. CoI(CH3) could react with Co0 and CeIV(O0.5) to give
Co0(CO), CoI(H) and CeIII. It seems that Co0 metal and CeO2
could be closely supported on ZrO2 with a small surface area
and that this close position of Co and CeO2 would enable the
reactions of eqn (4) and (5) to proceed. When Al2O3 was used,
the distance between CeO2 and Co0 metal would be too far for
active species to interact with each other because of the large
surface of Al2O3. In the case of CeO2 only as a support, the
surface area of CeO2 may be too small to increase the activity.
When CeO2 with a high surface area was used, a similar high
hydrogen yield was reported.41 Further, when ZrO2 was used
without CeO2, the initial high activity similar to that of the
CeO2–ZrO2 system was shown.40 However, there would be the
late transfer of the oxygen of water to carbonyl species to form
carbon dioxide. Although the steam reforming of formed
Co0
CH CH OH
3
CH CHO þ H
(1)
(2)
(3)
(4)
ƒ!
2
3
2
Co0
CH CHO
3
CoIðCH COÞ þ CoIðHÞ
ƒ!
3
Co0
CoIðCH COÞ
CoIðCH Þ þ Co0ðCOÞ
ƒ!
3
3
2CeIVðO0:5
Co0ðCOÞ
Þ CO þ Co0 þ 2CeIII
ƒƒƒƒƒ!
2
2CeIII þ H2O þ 2CoIðHÞ/2Co0 þ 2CeIVðO0:5Þ þ 2H2 (5)
Fig. 5 Ideal reaction routes in the steam reforming of ethanol catalyzed by Co/CeO2–ZrO2.
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methane at 500 ꢀC and lower than 500 ꢀC was approached 100% for all the catalysts at 600 ꢀC. Hydrogen yields
proposed,21,26,28,31,33,51 it is likely that the methyl group on Co at 600 ꢀC decreased in the order of 16Co42C31.5Ce10.5Zr >
(CoI(CH3)) in eqn (6) would react further before methane 16Co42C21Ce21Zr
>
16Co42C21Ce21Zr
¼
16Co63C21Zr.
formation because methane is difficult to convert once it is Further hydrogen yields for 16Co42C31.5Ce10.5Zr and
formed. The hydrogen transfer from CoI(CH3) to Co0 and the 16Co42C21Ce21Zr were 89% and 84%, respectively, higher than
oxygen transfer from CeIV(O0.5) to Co(C) species would produce those for Al2O3-based catalysts at 600 C. ZrO2-based catalysts
ꢀ
Co0(CO), CoI(H) and CeIII. When this oxygen transfer is late, accumulated larger amounts of coke, while the accumulation of
carbonaceous species on Co may polymerize, which would lead coke brought about the increase in the surface areas and pore
to coke formation and successive deactivation. It seems that the volumes, which would inhibit the deactivation of catalysts.
presence of CeIV(O0.5) next to Co species could properly inhibit 16Co42C31.5Ce10.5Zr and 16Co42C21Ce21Zr exhibited the
the coke formation and the deactivation and that the appro- formation of carbon nanotubes with diameters of about 50 nm,
priate use of ZrO2 support could form such conguration of which would contribute to the increase in the surface areas and
CeO2 and Co species. The reaction of CoI(CH3) with CoI(H) to pore volumes.
form methane could always occur with some probability while
the congurations of Co and CeO2 species of such sites for
methane formation may be different from those of sites for
Conflicts of interest
steam reforming.
There are no conicts to declare.
It was reported that surface acetate species observed in FT-IR
measurements of ethanol TPD could rst evolve to mono-
dentate carbonate as an intermediate, then dissociate into
Acknowledgements
CO2.40 When water was added to facilitate ethanol conversion, The authors thank Mr Yuuta Miki, Mr Akira Andou and Dr
the surface acetate species peaks disappeared at lower Hiroyuki Nasu for their helpful works.
temperatures. When FT-IR spectra of adsorbed methanol were
measured at temperatures lower than the reaction tempera-
ture,32 methoxy and formate species were observed. Therefore,
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© 2021 The Author(s). Published by the Royal Society of Chemistry
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