Efficient production of hydrogen over supported cobalt catalysts from ethanol steam reforming

Article abstract of DOI:10.1006/jcat.2002.3643

The production of H₂ from the steam reforming of alcohols could favor the use of H₂ as an alternative fuel, removing the difficulty of storage and distribution. The reaction between ethanol and water was studied at 573-723 K at 1 atm over supported Co catalyst. Ethanol steam reforming occurred to a large degree over ZnO-, Sm₂O₃-, and CeO₂-supported catalysts; CO-free hydrogen was produced. Over Co/MgO, the ethanol stream-reforming reaction, the ethanol decomposition to methane, and the ethanol dehydration were also produced. Dehydrogenation of ethanol to acetaldehyde was the main reaction for Co/SiO₂, which showed a selectivity pattern similar to Co/MgO. Co/V₂O₅ required the lowest temperature (623 K) to attain 100% ethanol conversion. Co/ZnO(2) exhibited the highest catalytic performance of the catalysts studied. The extent and degree of ordering of carbon deposits that occurred during reaction depended on the material and the reaction temperature.

Full text of DOI:10.1006/jcat.2002.3643

Journal of Catalysis 209, 306–317 (2002)
doi:10.1006/jcat.2002.3643
Efficient Production of Hydrogen over Supported Cobalt Catalysts
from Ethanol Steam Reforming
Jordi Llorca, Narc ´ı s Homs, Joaquim Sales, and Pilar Ram ı´ rez de la Piscina1
Departament de Qu ´ı mica Inorg a` nica, Facultat de Qu ´ı mica, Universitat de Barcelona, C/Mart ´ı i Franqu e` s 1-11, 08028 Barcelona, Spain
Received July 25, 2001; revised April 17, 2002; accepted April 23, 2002
redox and acid–base properties (7). On the other hand,
The reaction between ethanol and water was studied in the tem- cobalt-based catalysts had been proposed as appropriate
perature range 573–723 K at atmospheric pressure over supported catalysts for this process (5). As a background in the prepa-
cobalt catalysts. Cobalt-loaded catalysts (1%) were prepared by im-
pregnation of Co2(CO)8 on MgO, γ -Al2O3, SiO2, TiO2, V2O5, ZnO,
La2O3, CeO2, and Sm2O3. Ethanol steam reforming occurred to
a large degree over ZnO-, La2O3-, Sm2O3-, and CeO2-supported
catalysts; CO-free hydrogen was produced. Samples were charac-
of these results, we extended the preparation method to
terized after catalytic tests by high-resolution transmission elec-
tron microscopy, electron diffraction, X-ray diffraction, Raman
spectroscopy, X-ray photoelectron spectroscopy, and temperature-
programmed hydrogen reaction. Depending on the support, differ-
ration and behavior of supported cobalt catalysts, we had
reported highly selective synthesis of ethanol from the CO
hydrogenation over Co/ZnO catalyst prepared by impreg-
nation of Co2(CO)8 (8). In this work, and on the basis
several oxide supports, with a wide range of redox and
acid–base properties, which in turn had previously been
studied in the steam reforming of ethanol. The behavior
ent cobalt-based phases were identified: metallic cobalt particles, of catalysts on the title reaction was studied and catalysts
Co2C, CoO, and La2CoO4. The extent and nature of carbon depo- were characterized by transmission electron microscopy
sition depended on the sample and on the reaction temperature. (TEM), electron diffraction (ED), Raman spectroscopy,
ZnO-supported samples showed the best catalytic performances.
Under 100% ethanol conversion, selectivity up to 73.8% to H2 and
X-ray photoelectron spectroscopy (XPS), X-ray diffraction
(
(
XRD), and temperature-programmed hydrogen reaction
TPHR).
2
4.2% to CO2 was obtained.
Key Words: hydrogen; ethanol; steam reforming; supported
cobalt; fuel cells; oxide supports.
ꢀc 2002 Elsevier Science (USA)
EXPERIMENTAL
INTRODUCTION
Preparation of Catalysts
Supported cobalt catalysts with a ca. 1% metal loading
were prepared in a vacuum gas line by impregnation from n-
hexane Co2(CO)8 solutions. The supports used were MgO
The production of H2 from the steam reforming of al-
cohols could favor the use of H2 as an alternative fuel, re-
moving the difficulty of storage and distribution. Although
the steam reforming of methanol has been widely described
2
(
g
prepared by adding ammonia to a MgCl2 solution, 110 m
−
1
2
−1
), γ -Al2O3 (Girdler, 188 m
g ), SiO2 (Degussa-
H u¨ ls, 200 m g ), TiO2 (>80% anatase, Degussa–H u¨ ls,
5 m g ), V2O5 (Merck, 22 m g ), ZnO(1) (Asturienne,
New Jersey, 11 m g ), ZnO(2) (prepared by decom-
(
1–4), fewer studies have dealed with ethanol steam reform-
2
−1
ing (5, 6). However, from an environmental point of view
the use of ethanol is preferred, because ethanol could be
considered a renewable raw material which can be easily
obtained from biomass. Moreover, a high yield of hydro-
gen can be obtained from the above mentioned reaction:
2
−1
2
−1
4
2
−1
2
−1
position of 3ZnO · 2ZnCO3 · 3H2O, 100 m g ), La2O3
2
−1
2
−1
(
(
Merck, 11 m g ), CeO2 (Aldrich, 17 m g ), and Sm2O3
2
−1
Merck, 9 m g ). Prior to impregnation, supports were
partially dehydrated by treatment under high vacuum at
73 K for 16 h. After impregnation and solvent evapora-
CH3CH2OH + 3H2O → 6H2 + 2CO2.
4
Recently, we studied the behavior of several oxides in tion, the resulting samples were dried at room temperature
−
5
the steam reforming of ethanol. Some of them have shown until a total pressure of ca. 10 mbar was reached. Chem-
good performances for the production of CO-free hydro- ical analyses performed by inductively coupled plasma
gen, and their catalytic behavior has been related to their atomic absorption revealed that the cobalt contents for all
samples were in the range 1.1–1.2 wt%. A sample con-
1
2 4
taining the La CoO phase was prepared by the citrate
method (9).
To whom correspondence should be addressed. Fax: 34-93-4907725.
E-mail: pilar.piscina@qi.ub.es.
306
0
ꢀ
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.

CO-FREE HYDROGEN FROM ETHANOL STEAM REFORMING
307
Catalytic Tests
modes (SAED). Samples were deposited on copper grids
withaholeycarbon-filmsupport. Theinstrumentusedwasa
Philips CM-30 electron microscope equipped with a LaB6
source and working at 300 kV, with a point-to-point res-
olution of 0.19 nm. Magnification and camera constants
werecalibratedusingappropriatestandardsunderthesame
electron-optical conditions. In some cases, Fourier trans-
formed images were obtained by using local software on
digitalized parts of the negatives.
XRD profiles were collected at a step width of 0.02 de-
grees and by counting 10 s at each step using a Siemens
D-500 X-ray diffractometer equipped with a Cu target and
a graphite monochromator.
Catalytic studies of ethanol steam reforming were per-
formed at atmospheric pressure in a U-shaped quartz re-
actor (5-mm internal diameter). The catalyst (100 mg) was
charged for each of the reaction tests and diluted with in-
active SiC, giving a catalyst bed volume of 0.6 ml. The feed
of the reactants comprised a gaseous mixture of ethanol,
water, and argon. Ar (purity 99.9995%) was supplied by a
mass flow controller. A constant mixture of EtOH : H2O =
2
0 : 80 by volume (EtOH : H2O ∼ 1 : 13 molar basis, HPLC
purity grade) was supplied by a Gilson 307 Piston Pump,
and the mixture was vaporized at 453 K and mixed with the
Ar flow before entering the reaction chamber.
TPHR was followed using a Balzers QMS 200 quadru-
pole mass spectrometer. The experiments were performed
over postreaction samples pretreated in He at 873 K by
The temperature of the catalyst was raised to 573 K under
Ar. Then the EtOH + H2O mixture was introduced and the
temperature increased to that of the catalytic test. All cata-
lysts were tested from 573 to 723 K, with the temperature
increased stepwise from 573 to 623, 673, and 723 K. The
catalystwasheldateachtemperaturefor2h(573and623K)
and 20 h (673 and 723 K). At the end of the catalytic test
the flow of EtOH + H2O was stopped and the catalyst was
cooled under Ar stream and stored for characterization.
Over ZnO-supported catalysts, additional catalytic tests
were carried out at 723 K, with variation in the contact
time and/or the Ar/(EtOH + H2O) ratio. In this case, prod-
ucts were analyzed 1 h after each change in the reaction
conditions.
The analysis of the reactants and all the reaction prod-
ucts was carried out online by gas chromatography (Varian
apparatus). Inside an automated injection valve, the sample
was divided into two aliquots which were then analyzed in
a different way in order to obtain accurate, complete quan-
tification of the reaction products. One of the aliquots was
used to analyze hydrogen, carbon monoxide, carbon dio-
xide, methane, and water. Argon was used as a carrier gas
−
1
using a constant flow of H2 diluted in He (20 ml min ) at
a heating rate of 2 K min up to 873 K.
−
1
XPspectrawererecordedwithaPerkin–ElmerPHI-5500
spectrometer equipped with an Al X-ray source and a hemi-
spherical electron analyzer. The X-ray source was operated
at 12.4 kV. The binding energy (BE) reference was taken
at the Zn 2p peak from ZnO at 1022.0 eV. The accuracy of
the binding energy was within 0.1 eV.
Raman spectroscopy was performed with a Jobin Yvon
T64000 instrument using an Ar ion laser as an illumination
source (514.5 nm) and a CCD detector cooled at 140 K.
The Raman instrument was coupled to a standard Olympus
microscope (×50 magnification) and the collection optics
system was used in the backscattering configuration. The
laser power at the sample was limited to 3 mW in order to
avoid laser heating effects.
RESULTS AND DISCUSSION
˚
Ethanol Steam-Reforming Reaction
and separation was accomplished by using a 5-A molecular
sieve and a 6-m-long Hayesep packed column. Hydrogen
was quantified with a thermal conductivity detector
whereas carbon monoxide, carbon dioxide, and methane
were analyzed by a flame ionization detector after being
passed through a methanizer device, which contained a
nickel catalyst. The detection limit of carbon monoxide was
The ethanol steam-reforming reaction was studied under
a mixture 1 : 13 : 70 EtOH : H2O : Ar (molar ratio), between
573 and 723 K, at atmospheric pressure. Figure 1 shows for
each sample values of ethanol conversion and H2 and CO2
production as a function of reaction temperature and time.
Ethanol conversion and CO2 and H2 production increased
with temperature for all catalysts. The values obtained at
1
7 ppm. The other aliquot was used to analyze hydrocar-
bons and oxygenated products. Helium was used as a car-
rier gas and separation and quantification were attained by
a 100-m-long DB-5 capillary column and a flame ionization
detector. Response factors for all products were obtained
and the system was calibrated with appropriate standards
before each catalytic test.
7
23 K along time show a severe decrease in ethanol con-
version for Co/TiO2 and a slight deactivation for Co/MgO,
Co/SiO2, Co/CeO2, and Co/Sm2O3 samples.
Table 1 shows conversion and selectivity values obtained
after 20 h at 723 K (total reaction time, 50 h). These
data allow evaluation of the extent of the ethanol steam-
reforming reaction, CH3CH2OH + 3H2O → 6H2 + 2CO2,
and that of the other ethanol reactions which took
Catalysts Characterization
Catalysts were studied by TEM techniques, basically in place over the catalysts: mainly, ethanol decomposition
bright field and high resolution, as well as by electron to CH4,CH3CH2OH → CO + CH4 + H2; ethanol dehydra-
diffraction in convergent-beam (CBED) and selected-area tion to ethylene; ethanol dehydrogenation to acetaldehyde;

3
08
LLORCA ET AL.
FIG. 1. Ethanol conversion and H2 and CO2 production over supported cobalt catalysts as a function of temperature and reaction time. Reaction
−
1
conditions: total pressure, 1 atm; EtOH : H2O : Ar = 1 : 13 : 70 (molar ratio), GHSV = 5000 h . (ꢀ) Ethanol conversion; (ꢁ) mol H2/mol EtOH;
ꢂ) mol CO2/mol EtOH.
(
ethanol decomposition to acetone, 2CH3CH2OH →
Negligible steam reforming of ethanol was observed over
CH3COCH3 + CO + 3H2; and water–gas shift reaction, Co/Al2O3. The dehydration of ethanol to ethylene took
CO + H2O → CO2 + H2. Table 1 also lists mean cobalt place to a large extent according to the behavior of Al2O3
particle size as determined by TEM after reaction at under similar conditions and with its acidic characteristics
7
23 K.
(7, 10).

CO-FREE HYDROGEN FROM ETHANOL STEAM REFORMING
309
FIG. 1—Continued
Co/MgO catalyst showed the smallest initial conversion that the main reaction that took place over Co/MgO was
at each temperature (see Fig. 1); this catalyst only gave the dehydrogenation of ethanol to acetaldehyde. However,
around 30% conversion of ethanol at the highest tem- over this catalyst the ethanol steam-reforming reaction,
perature (723 K) studied. The selectivity values indicated the ethanol decomposition to methane, and the ethanol
TABLE 1
Catalytic Performance of Supported Cobalt Catalysts and Mean Particle Size of Cobalt Particles Determined by
TEM after 20 h at 723 K on a EtOH + H2O + Ar Stream (EtOH : H2O : Ar = 1 : 13 : 70 molar ratio) at Atmospheric
Pressure and GHSV = 5000 h−
1
a
Selectivity (%)
Conv.
(%)
dCo
(nm)
Catalyst
H2
CO
CO2
CH4
C2H4
C3H6
CH3CHO
Me2CO
Co/MgO
Co/Al2O3
Co/SiO2
29.3
100
87.0
16.4
100
100
100
85.0
93.7
85.9
55.0
0.8
0.4
—
2.9
—
—
—
—
—
—
—
8.2
0.3
3.9
2.1
0.1
2.1
0.3
1.2
0.7
0.8
1.3
0.1
1.6
2.5
98.8
3.4
13.5
19.9
1.1
0.6
1.1
1.9
7.3
4.2
—
0.2
—
0.3
0.5
0.1
0.1
0.1
0.1
27.6
—
—
—
—
—
—
—
49.8
47.4
53.5
66.0
71.3
63.1
69.6
64.7
37.7
34.4
6.4
4.8
0.2
0.5
0.8
—
Co/TiO2
4.4
—
—
Co/V2O5
Co/ZnO(1)
Co/ZnO(2)
Co/La2O3
Co/CeO2
Co/Sm2O3
16.1
20.8
20.2
21.5
21.1
21.0
2.6
6.1
6.8
12.4
6.4
5.3
3.4
3.6
3.5
6.1
7.5
3.9
a
Water not included.

3
10
LLORCA ET AL.
TABLE 2
Catalytic Performance of Co/ZnO(2) at 723 K and Atmospheric Pressure under a Constant EtOH + H2O Feed
(EtOH : H2O = 1 : 13 Molar Ratio)
a
Selectivity (%)
GHSV
Conv.
(%)
Mol H2/
mol EtOH mol EtOH
Mol CO2/
−
1
(h
)
Ar/EtOH + H2O
H2
CO
CO2
CH4
C2H4
C3H6
CH3CHO
Me2CO
2
,300
,800
,800
,900
2
4
8
12
16
20
24
28
100
100
100
100
100
100
100
100
65.7
68.7
69.5
69.9
71.6
72.0
73.4
73.8
—
—
—
—
—
—
—
—
21.6
21.4
22.6
22.7
22.8
23.2
23.7
24.2
0.8
0.7
0.6
0.5
0.4
0.4
0.3
0.3
1.3
1.1
0.9
1.2
1.2
1.2
0.9
0.8
0.2
0.1
0.1
0.2
0.1
—
1.6
0.6
0.7
0.6
0.5
0.8
0.5
—
8.8
7.4
5.6
4.9
3.4
2.4
1.2
0.9
2.4
2.9
3.2
3.3
3.9
4.1
4.9
5.1
0.8
0.9
1.0
1.1
1.2
1.3
1.6
1.7
3
6
9
1
1
1
2
2,900
6,000
9,000
2,000
—
—
a
Water not included.
dehydration were also produced. The dehydrogenation of condensation (7, 11, 12), several catalytic tests were car-
ethanol to acetaldehyde was also the main reaction ob- ried out with variations in the contact time and/or the
served over MgO, but a maximum conversion value of 6.7% Ar/(EtOH + H2O) ratio. Table 2 reports the catalytic per-
was attained at 723 K and no steam reforming of ethanol formance at 723 K of Co/ZnO(2) under a mixture of
took place (7).
Ar : EtOH : H2O obtained by dilution with Ar of a constant
Dehydrogenationofethanoltoacetaldehydewasalsothe feed of EtOH : H2O = 1 : 13 (molar ratio). Total conversion
main reaction for Co/SiO2, which showed a selectivity pat- was attained in all cases and no carbon monoxide was de-
tern similar to Co/MgO; the conversion values were much tected (detection limit, 17 ppm). Short contact times and
higher than over SiO2 (7).
high Ar/(EtOH + H2O) ratios favored the reforming reac-
Co/V2O5 required the lowest temperature (623 K) to at- tion over the decomposition to acetone, and in the best case
tain 100% ethanol conversion (Fig. 1). Although the de- only 2% of the total products were other than H2 and CO2.
hydration of ethanol to ethylene was the reaction which
In order to gain further insight, two different catalytic
took place to the highest extent, 15.7% of the ethanol tests were carried out with the same catalyst at 723 K, where
reacted over Co/V2O5 was through the steam-reforming either the contact time or the Ar/(EtOH + H2O) ratio was
reaction.
kept constant; in both cases a molar ratio EtOH : H2O =
Co/ZnO(1), Co/ZnO(2), Co/La2O3, Co/CeO2, and 1 : 13 was used (Fig. 2). Figure 2A corresponds to the ex-
Co/Sm2O3 showed selectivity to H2 higher than 60% and periment carried out at a fixed Ar/(EtOH + H2O) molar
to CO2 higher than 20%. Over these catalysts the main ratio of 5 and with variations in the gas hourly space ve-
−
1
−1
reactions were the steam reforming of ethanol and the locity (GHSV) from 30,000 to 1,250 h . Up to 15,000 h
ethanol decomposition to acetone. Co/La2O3, Co/CeO2, the extent of the decomposition of ethanol to acetone de-
and Co/Sm2O3 catalysts showed higher conversion values creases gradually while the extent of the reforming reaction
than La2O3, CeO2, and Sm2O3, respectively. The selectivity increases. At higher GHSV values conversion of ethanol is
patterns of these catalysts were also different from those of not complete and acetaldehyde is produced, in accordance
the oxide supports (7); supported cobalt catalysts showed with a reaction scheme in which ethanol is first dehydro-
higher selectivity to H2 and CO2 and lower selectivity to genated to acetaldehyde. On the other hand, the effect of
ethylene.
the Ar/(EtOH + H2O) ratio is shown in Fig. 2B, where a se-
Co/ZnO(2) exhibited the highest catalytic performance ries of experiments were performed at a fixed GHSV value
−
1
of all catalysts studied (see Fig. 1 and Table 1). In the experi- of 5000 h . As the EtOH + H2O mixture is more diluted
mental conditions used, total conversion of ethanol and the in the carrier, the reforming reaction takes place preferen-
highest values of H2 and CO2 per mole of reacted EtOH tially, and the ethanol decomposition to acetone via aldol
were obtained and no deactivation was observed. When condensation of acetaldehyde is depressed. Again, when
a similar test was carried out over the ZnO(2) support at total conversion of ethanol is not attained, acetaldehyde is
7
23 K, 100% ethanol conversion was also reached but a observed.
lower yield on H2 and CO2 was obtained (7).
On the other hand, over the Co/ZnO(2) catalyst the de-
composition of ethanol to acetone was high. Taking into
account the fact that this reaction results from several
Characterization of Catalysts
TEM studies were carried out on the catalysts after
successive reactions, such as dehydrogenation and aldol being tested for 50 h in a EtOH + H2O + Ar stream with

CO-FREE HYDROGEN FROM ETHANOL STEAM REFORMING
311
the formation of MgO–CoO solid solution is likely to occur
14).
The characterization of Co/SiO2 allowed us to identify
(
CoO particles and abundant carbon deposition on the sil-
ica. Figure 3 shows a bright-field image of this sample along
with selected-area electron diffraction (SAED) patterns
recorded on the silica support and on the metal particles,
which are visible in the bright-field image. Diffuse rings cor-
responding to poorly graphitized carbon are visible in the
SAED pattern recorded on silica, whereas defined rings
corresponding to multiple particles of the cobalt oxide CoO
are identified sitting on the silica support.
High carbon deposition was observed on Co/TiO2; a
10-nm-thick amorphous layer was clearly visible covering
both TiO2 and cobalt-based particles (see inset in Fig. 4).
A lattice-fringe image of this catalyst is shown in the same
˚
figure, where a 3.6-A spacing for C(002), characteristic of
poorly graphitized carbon, is visible. At the same time,
cobalt-containing particles of about 20 nm appeared, like
the one showed in the figure. Convergent-beam electron
diffraction of this individual particle (inset) showed that it
was Co2C carbide. These characterization data are consis-
tent with the severe decrease observed on ethanol conver-
sion after 20 h under reaction at 723 K.
TEM analysis of Co/La2O3 catalyst showed the presence
of aggregates on the La2O3 support. Figure 5A corresponds
to a high-resolution image of one of these aggregates, where
cobalt and the support originated a mixed oxide phase with
a spinel structure, La2CoO4. This new phase consisted of
individual crystals of about 4–6 nm and was located on top
FIG. 2. Catalytic performance of Co/ZnO(2) at 723 K and atmo-
spheric pressure. (A) As a function of GHSV, EtOH : H2O : Ar = 1 : 13 :
7
5
0 molar ratio. (B) As a function of Ar/EtOH + H2O ratio, GHSV =
−1
000 h , EtOH : H2O = 1 : 13 molar ratio. (ꢀ) Ethanol conversion. Se-
lectivities: (ꢃ) H2; (ꢄ) CO2; (ꢅ) CH3CHO; (ꢆ) Me2CO.
Ar/(EtOH + H2O) = 5 molar ratio and 5000 h− GHSV at
increasing temperatures from 573 to 723 K, as detailed
under Experimental. Several phases were systematically
checked for their presence, including carbon (graphite as
well as poorly ordered carbon), metallic cobalt (both cubic
and hexagonal phases), cobalt oxides, cobalt carbides, and
mixed alloys and oxides which could be obtained from
cobalt and the oxide supports.
1
We could not identify any cobalt-based phase or carbon
deposition on Co/MgO and Co/Al2O3 catalysts. Taking into
account the low metal loading used in our study, the entry of
cobalt into the lattice of Al2O3 and theformation of nonre-
ducible aluminate may be favored on Co/Al2O3 (13); this
phase could be not active in the ethanol steam-reforming
FIG. 3. Bright-field transmission electron microscopy image corre-
sponding to the Co/SiO2 catalyst. The two insets correspond to selected-
area electron diffraction patterns recorded on the silica support alone (A)
reaction according to the catalytic results. Over Co/MgO, and on silica-supported cobalt-containing particles (B).

3
12
LLORCA ET AL.
image of an aggregate under two different focus condi-
tions. In Fig. 7A, planes corresponding to the spacing of
cubic Co(111) are seen along with CeO2 (111) planes of the
support. The cobalt particle grew following an epitaxial re-
lationship with the support, as both of them belong to the
Fm3m cubic space group. In Fig. 7B, the cobalt planes are
out of focus but the covering shell is now identified as poorly
˚
ordered carbon and C(002) planes are visible at 3.7 A. Mi-
nor carbon deposition occurred on the surface of the CeO2
support.
Small cobalt particles with cubic structure were also iden-
tified in the Co/Sm2O3 catalyst (see the high-resolution im-
age in Fig. 8), where carbon deposition occurred almost
exclusively on the Sm2O3 support (see inset in Fig. 8).
Both Co/Sm2O3 and Co/CeO2 catalysts showed minor
FIG. 4. Images corresponding to the Co/TiO2 catalyst. A Co2C car-
bide phase is identified by means of convergent-beam electron diffraction.
The lattice-fringe image shows the presence of TiO2 support particles and
cobalt-based particles covered by a poorly graphitic carbon.
of the support. Two insets in Fig. 5A show La2CoO4 parti-
cles with distinctive spacing and reciprocal space patterns as
obtained by Fourier transformed processing. On the other
hand, discrete amorphous carbon deposition was observed
on the La2O3 support, which is visible directly in the high-
resolution image as well as in the Fourier transformed im-
age in another inset of Fig. 5A. In contrast, the La2CoO4
crystals appeared free from any carbonaceous layer. In ad-
dition to La2CoO4, the Co/La2O3 catalyst also contained
metallic cobalt. Figure 5B corresponds to another aggre-
gate of this catalyst, where different nanometer-size cobalt
crystallites are located on top of the aggregate, as evidenced
by Fourier transform image.
In order to study the role of the La2CoO4 phase identified
in the Co/La2O3 catalyst in the ethanol steam-reforming
reaction, we synthesized a new mixed oxide catalyst which
contained the phase La2CoO4. The catalytic performance
of this sample toward ethanol steam reforming was poorer
than that of Co/La2O3, where cobalt particles were identi-
fied after reaction.
OnCo/V2O5 vesicular, amorphouscarboncoveredallthe
V2O5 support, as illustrated in Fig. 6, which corresponds to
a bright-field image. In some areas the vesicular carbon con-
tained cobalt particles of about 2–3 nm with a cubic struc-
ture, as shown in the inset of Fig. 6, and the empty vesicles
marked by white arrows may have been once occupied by
such cobalt particles.
TEM analysis of Co/CeO2 catalyst revealed the pres-
ence of aggregates on the surface of CeO2 with a layered
structure, with a more electron-dense core covered by a
shell. Lattice-fringe imaging of the aggregates showed that
a metallic cobalt core with cubic structure was covered
FIG. 5. High-resolutiontransmissionelectronmicroscopyimagescor-
responding to the Co/La2O3 catalyst and derived Fourier transformed pat-
terns. La2CoO4 spinel (A) and cobalt metal (B) particles are located in
by amorphous carbon. Figure 7 shows a high-resolution aggregates over La2O3 hexagonal crystals.

CO-FREE HYDROGEN FROM ETHANOL STEAM REFORMING
313
FIG. 8. Lattice-fringe images corresponding to the Co/Sm2O3 cata-
lyst. Metal cobalt particles about 2 nm in size are supported on Sm2O3,
as shown in inset A. Arrow marked as C in the low-magnification inset
corresponds to carbon deposition on the support.
FIG. 6. Co/V2O5 catalyst. Extended carbon deposition is evident
from transmission electron microscopy analysis (see, for example, inset
A). Individual, 2- to 3-nm-size metal cobalt particles are present, which in
turn are also covered by amorphous carbon.
at different temperatures was done. On these samples, Co
particles covered by highly disordered carbon phases and
carbon deposition on the ZnO supports were identified.
The morphology and extension of carbonaceous phases de-
pended on the ZnO support and on the reaction tempera-
ture. It appears that carbon deposition on the high-surface
ZnO-supported cobalt catalysts, Co/ZnO(2), was less pro-
nounced than on the low-surface one, Co/ZnO(1). On the
other hand a higher reaction temperature produced higher
carbon deposition with more ordered structure.
deactivation under reaction at 723 K. However, over
Co/CeO2 carbon deposited preferentially on metal cobalt
particles whereas over Co/Sm2O3 carbon deposition took
place largely on the support. This is in accordance with the
higher deactivation and carbon deposition of Sm2O3 with
respect to CeO2 under similar reaction conditions (7).
Taking into account the fact that ZnO-supported cata-
lysts showed the best catalytic performances, a more com-
plete characterization of these samples after catalytic tests
Figure 9 displays a lattice-fringe image of the Co/ZnO(2)
catalyst along with a low-magnification view. Metallic
FIG. 7. High-resolutiontransmissionelectronmicroscopyimagescor-
responding to the Co/CeO2 catalyst in profile view. Figures A and B were
obtained consecutively on the same area but under different focus con-
ditions. A single metal cobalt particle exhibiting (111) planes has grown
epitaxially on the CeO2 support (A). On the other hand, poorly ordered
carbon has developed onto the metal cobalt particle (B).
FIG. 9. Transmission electron microscopy images corresponding to
Co supported on high-surface ZnO, Co/ZnO(2). Nanometer-size metal
cobalt particles are well dispersed on ZnO. Highly disordered carbon is
also present, as deduced from lattice-fringe imaging (inset A).

3
14
LLORCA ET AL.
was the formation of carbon filaments containing cobalt
particles much bigger than the metallic cobalt which re-
mained in contact with the support: ca. 15–30 vs 3 nm (see
inset in Fig. 11). The selected-area electron diffraction pat-
tern included in the same figure shows single spots cor-
responding to a single metallic cobalt particle with cubic
structure along with rings due to carbon. Now carbon rings
˚
are well-defined and constrained at 3.47 and 2.10 A, which
corresponds to the C(002) and C(100), C(101) planes, re-
spectively. Thesevaluesindicateagraphiticstructure, which
˚
can range from 3.44 A (interlayer spacing of randomly
˚
stacked graphitic layers) to 3.35 A (fully ordered hexag-
onal graphite) (16). Similar spacing was obtained by X-ray
diffraction at 2θ ∼ 25.7 and 43.6 degrees, which corresponds
˚
to 3.46 and 2.08 A, respectively.
Co/ZnO(1) catalyst was also characterized by TEM be-
fore catalytic reaction. We failed to identify any cobalt-
based phase according to the preparation method of cata-
lysts, which were tested after impregnation of Co2(CO)8
without previous calcination and/or reduction steps. This
FIG. 10. Catalyst Co/ZnO(1) after reaction at 723 K. The occurrence
of 3- to 4-nm-size metal cobalt particles through all the catalyst is clear
from TEM. In all cases, metal cobalt particles appear covered by a poorly
ordered carbonaceous phase. As is observed from Fourier transformed indicates that phases characterized after reaction were
patterns, carbon deposition takes place to a larger extent on cobalt (111)
planes (see inset).
formed under reaction conditions and, according to the re-
sults, depended on the support characteristics. On some
catalysts we detected cobalt particles after reaction. These
catalysts showed a significant enhancement of the catalytic
cobalt particles with a cubic structure are located out of the
performanceinthesteamreformingofethanol(seeTable1)
support and are surrounded by amorphous carbon. Carbon
when compared with the corresponding supports (7). How-
deposition took place also on the ZnO support, but not in
an organized way.
ever, from our results it was not possible to establish a re-
lation between catalytic performance and cobalt particle
Co/ZnO(1) catalyst was characterized after reaction at
size.
7
23 and 873 K. Figure 10 corresponds to the characteri-
zation of Co/ZnO(1) after reaction at 723 K. The high-
resolution image shows a single metallic cobalt particle with
a cubic structure surrounded by a poorly ordered carbona-
ceous phase with an onion-shell-type structure. The Fourier
transformed image in the inset contains spots correspond-
˚
ing to Co(111) planes at 2.06 A, as well as a diffuse ring at
˚
3
.72 A corresponding to the (002) Miller indices of carbon.
˚
A C(002) spacing of 3.74 A has been reported in the litera-
ture to represent a metastable carbon-rich phase related to
the removal or formation of bonds or linkages during poly-
mer carbonization (15). The diffuse ring is more dense in
the direction of the Co(111) spots, indicating that the (002)
crystallographic planes of carbon initially developed on the
(
111) planes of metallic cobalt. It appears that the growth
of carbon was associated with the separation of metallic
cobalt particles from the support.
When the reaction was carried out at 873 K over
Co/ZnO(1), a strong change in the catalyst structure took
place. Two different phenomena occurred. On one hand
there was an abundant carbon deposition all over the cata-
lyst with a more ordered structure and graphitic character
with respect to the catalyst after reaction at 723 K. This is
FIG. 11. Catalyst Co/ZnO(1) after reaction at 873 K. The size distri-
bution of metal cobalt particles ranges from 3–4 nm up to 15–30 nm. Small
metal cobalt particles are located on ZnO, whereas larger cobalt particles
˚
shown in Fig. 11, where a C(002) spacing of 3.46 A is calcu-
lated directly from lattice-fringe imaging. In addition, there are located inside carbon filaments.

CO-FREE HYDROGEN FROM ETHANOL STEAM REFORMING
315
Analysis of the profile corresponding to the catalyst after
reaction at 723 K (Fig. 13, spectrum a) indicates the pres-
ence of four different types of surface carbon, with C 1s
binding energies located at 284.4 (35%), 285.6 (38%), 286.8
(
17%), and 289.8 (10%) eV. In contrast, the C 1s profile
recorded for the catalyst after reaction at 873 K (Fig. 13,
spectrum e) can be deconvoluted into two bands, centered
at 284.4 eV (71%) and 285.5 eV (29%). C 1s BE values
taken from the literature (18, 19) allow identification of the
signalarisingat284.4eVasduetographeneorgraphiticcar-
bon, whereas C 1s BE at ca. 285.6 eV is ascribed to aliphatic
polymers. On the other hand, C 1s BE at 286.8 eV is typical
of alcohol and keto surface groups, and that at 289.9 eV
corresponds to carbonate species.
FIG. 12. Evolution of CH4 obtained from temperature-programmed
hydrogen reaction of the carbonaceous deposits formed on the Co/ZnO(1)
catalyst. (a) After reaction at 723 K; (b) after reaction at 873 K.
Figure 13 also shows XP spectra in the C 1s region
of Co/ZnO(1) catalyst after reaction at 723 K and ulte-
rior H2 treatment at different temperatures, from 723 to
8
73 K (spectra b–d). An increase in thermal treatment with
More information about the nature of the carbona-
ceous deposits formed over Co/ZnO(1) was obtained
by temperature-programmed hydrogen reaction (TPHR),
X-ray photoelectron spectroscopy (XPS), and Raman
spectroscopy.
hydrogen produced the progressive diminution of bands at
higher BE values. After a treatment under H2 at 873 K,
the C 1s spectrum profile (spectrum d) is similar to that
obtained after the ethanol steam-reforming reaction at
8
73 K (spectrum e). The spectrum mainly shows a band
TPHR was performed over the Co/ZnO(1) catalyst after
reaction at 723 and 873 K. Both experiments were followed
from 500 to 873 K. The evolution of methane, which was
the only significant product evolved, is depicted in Fig. 12.
The TPHR profile depended on the reaction temperature.
After reaction at 723 K (profile a), three peaks with max-
ima at 680 K (14% of total integrated area), 770 K (68% of
total integrated area), and 830 K (18% of total integrated
area) can be distinguished. The very broad signal centered
at 680 K indicated the presence of relatively reactive carbon
deposits on the catalyst, which may be identified as carbon
in the β state (17). The other two signals, at 770 and 830 K,
indicated the presence of amorphous carbon, which can be
correlated with the poorly structured carbon deposition ev-
idenced by TEM for this sample (Fig. 10).
located at 284.4 eV, associated with graphene or graphitic
carbon. The band located at 285.5 eV significantly dimin-
ished after a H2 treatment at 873 K, and it may be re-
lated to the carbon species showing a maximum of CH4
evolution in the TPHR experiment at 770–830 K. As
discussed above, the graphene/graphitic species respon-
sible for the band at 284.4 eV will react with hydrogen
at a higher temperature and is not visible in the TPHR
experiment.
The TPHR profile of catalyst after reaction at 873 K
(
profile b in Fig. 12) only showed a signal centered at 825 K,
corresponding to the best structured amorphous carbon
already shown by the sample after reaction at 723 K. Tak-
ing into account the temperature interval of TPHR exper-
iments (500–823 K), these experiments did not show more
orderedcarbon, suchasthefilamentousstructuresobserved
by TEM in this sample (Fig. 11). Filamentous and platelet
carbon exhibit peak temperatures of about 875 and 1120 K,
respectively (17).
Figure 13 displays the X-ray photoelectron spectra reco-
rded in the C 1s region over the Co/ZnO(1) catalyst after
reaction at 723 and 873 K (spectra a and e, respectively).
The C/Zn atomic ratio calculated for the catalyst reacted at
FIG. 13. X-rayphotoelectronspectraintheC1sregioncorresponding
to the Co/ZnO(1) catalyst. (a) After reaction at 723 K; (b), after (a),
H2 treatment at 723 K; (c) after (b), H2 treatment at 800 K; (d) after (c),
8
73 K is about 20 times greater than that corresponding to
the catalyst after reaction at 723 K, in accordance with a ma-
jor carbon deposition in the former, as evidenced by TEM. H2 treatment at 873 K; (e) after reaction at 873 K.

3
16
LLORCA ET AL.
(
22), suggesting that a very poorly graphitized structure was
developed on the ZnO-supported catalyst in our experi-
ment. On the other hand, when the reaction temperature
was increased from 723 to 873 K, a certain ordering of the
carbonaceous deposit took place with respect to the struc-
ture, resulting from reaction at 723 K, as deduced from an
increase of the La value. This is also reflected by an increase
in the frequency of the D and G lines, as well as by a de-
crease in their bandwidth; see Table 3).
Raman results are in close agreement with TEM data
discussed above and illustrated in Figs. 10 and 11 and
FIG. 14. Raman spectra in the carbon region obtained for catalyst with XP spectra and TPHR experiments. The higher the
Co/ZnO(1) after reaction at different temperatures.
ethanol steam-reforming reaction temperature, the larger
the amount of carbon deposits with increasing ordering.
Finally, in Fig. 14 and Table 3 also appear the Raman
results for the Co/ZnO(1) catalyst, characterized after a
separate catalytic test carried out at 673 K for 50 h. The
characteristics of the bands and the La value obtained con-
firm that a lower amount of carbonaceous deposit with a
lower order is obtained after a catalytic test at a lower
temperature.
The Raman spectra in the range 1200–1700 cm−1 of the
sample Co/ZnO(1) after reaction at 723 and 873 K are
presented in Fig. 14. Two distinctive bands are visible in
−
1
each spectrum, at 1339–1345 (D line) and 1592–1597 cm
G line). These two bands are very sensitive to the extent
(
of two-dimensional graphitic ordering and have been used
extensively to study disorder in carbon deposits (20). The
bands in our spectra are relatively broad, indicating a large
degree of interstitial disorder, that is, along the c-axis be-
tween the in-plane direction. The spectroscopic parameters
obtained after curve fitting for each spectrum are compiled
in Table 3.
A quantitative characterization of the degree of disor-
der in carbonaceous materials can be made on the basis
of the integrated intensity of the D and G lines, as well
as their relative width and frequency. The microcrystalline
planar size La shows a linear relationship with the ratio of
the integrated intensity of G vs D line: La = 44(IG/ID) (21).
When the ethanol-reforming reaction is carried out over
the Co/ZnO(1) catalyst, the resulting La value is about 0.9
nm at 723 and 1.4 nm at 873 K. Both values indicate a large
degree of disorder compared to values reported in the liter-
ature for other cobalt-based catalysts in different reactions
CONCLUSIONS
A positive effect of cobalt addition to several oxides in
the catalytic production of hydrogen from ethanol aque-
ous solutions has been shown. Catalysts containing cobalt
particles showed a significant enhancement of the catalytic
performance in the steam reforming of ethanol compared
with the corresponding supports. The extent and degree of
ordering of carbon deposits that occur during reaction de-
pend on the material and the reaction temperature. The
higher the temperature, the higher the extension and order
of the carbonaceous phase.
ACKNOWLEDGMENTS
We thank CICYT (MAT1999-0477) and Generalitat de Catalunya (1999
SGR-00044) for financial support. J. Ll. is grateful to MCYT for a Ram o´ n
y Cajal Research Program.
TABLE 3
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