Cobalt catalysts derived from hydrotalcite-type precursors applied to steam reforming of ethanol

Article abstract of DOI:10.1016/j.catcom.2011.04.018

Catalysts derived from Co/Mg/Al hydrotalcite-type precursors modified with La and Ce were characterized by XANES and tested in ethanol steam reforming. The reaction data showed that, with a molar ratio of water:ethanol = 3:1 in the feed, addition of Ce and La favored acetaldehyde production. Increasing the water content (water:ethanol = 5:1) decreased the acetaldehyde formation by favoring the adsorption of water molecules on these samples, enhancing the acetaldehyde conversion.

Full text of DOI:10.1016/j.catcom.2011.04.018

Catalysis Communications 12 (2011) 1286–1290
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Cobalt catalysts derived from hydrotalcite-type precursors applied to steam
reforming of ethanol
a
b
a,
A.F. Lucrédio , J.A. Bellido , E.M. Assaf ⁎
a
Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São Carlense, 400, São Carlos, SP 13560-970, Brazil
Universidade Federal de São João del-Rei, Campus Alto Paraopeba, Rodovia /MG-443, km 07, Fazenda do Cadete, Ouro Branco-MG, Brazil
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalysts derived from Co/Mg/Al hydrotalcite-type precursors modified with La and Ce were characterized by
XANES and tested in ethanol steam reforming. The reaction data showed that, with a molar ratio of water:
ethanol=3:1 in the feed, addition of Ce and La favored acetaldehyde production. Increasing the water content
Received 3 February 2011
Received in revised form 8 April 2011
Accepted 12 April 2011
(water:ethanol=5:1) decreased the acetaldehyde formation by favoring the adsorption of water molecules
Available online 22 April 2011
on these samples, enhancing the acetaldehyde conversion.
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Catalyst
Cobalt
Ethanol
Reforming
1
. Introduction
suffer significant carbon deposition throughout 6 h on stream and the
reaction data showed that the addition of Ce and La resulted in greater
Ethanol steam reforming (ESR, Eq. 1) has been suggested as an
H
2
production at 550 °C.
alternative route for H
2
production, since this reaction predicts a high
Considering the promising results obtained with hydrotalcite-
yield of H
2
at low reaction temperatures.
type precursors, the goal of this paper was to assess the effect of Ce
and La addition on the performance of cobalt catalysts derived from
hydrotalcite-type precursors, Co/Mg/Al, in the ESR reaction.
0
−1
C H OH + 3H O → 6H + 2CO2
ΔH
= +174 kJmol
298K
2
5
2
2
ð1Þ
0
−1
ΔG
= +65:5 kJmol
98k
2
2
. Experimental
2.1. Synthesis
The preparation of the catalysts has already been described in detail
In the literature, Rh, Co and Ni are described as promising metals
for ESR catalysis [1]. Rh is the most active of these, but very costly,
being a noble metal. Cheaper metals such as Ni and Co usually suffer
deactivation by the deposition of carbon or sintering of the active
phase [1]. The stability of such catalysts may be enhanced by careful
choice of the constituents of the support and the method of preparation
in previous papers [6,7].
Briefly, the hydrotalcite-type precursors were prepared in two
ways: the standard Co-LDH (layered double hydroxide) catalyst by
the traditional technique of precipitation of carbonates and the promoted
catalysts, LaCo-LDH and CeCo-LDH, by anion exchange of Mg/Al/Co with
EDTA chelation complexes of La and Ce, to favor the transport of these
cations into the hydrotalcite layers.
[2].
Hydrotalcites are known for their layered structure, which, on
heat treatment, leads to a homogenous dispersion of the active phase
3]. Comas et al. [4] studied the performance of Ni–Al catalysts made
from hydrotalcite-type precursors in the ESR reaction, carried out at
73 K, varying the molar ratio (R) of H O:C OH in the feed from 1
to 6. They reported that H selectivity increased with rising R. The
authors suggested that the increased H selectivity could be related
[
7
2
2
H
5
All hydrotalcites were calcined at 500 °C in air for 15 h. They were
denominated Co, LaCo and CeCo.
2
2
to the fine dispersion of Ni resulting from the method of preparation
from hydrotalcite-type precursors.
2.2. Characterization
Lucrédio et al. [5] tested catalyst precursors composed of Ni/Mg/Al
oxides promoted with La and Ce in ESR reactions. The catalysts did not
X-ray absorption near-edge structure (XANES) spectra at the Co
K-edge were collected at the D04-XAFS1 beam line in the Brazilian
Synchrotron Light Source Laboratory (LNLS), Brazil. The data acquisition
0 t
system comprised three ionization detectors (incident I , transmitted I
⁎
Corresponding author.
E-mail address: eassaf@iqsc.usp.br (E.M. Assaf).
and reference I ). The reference channel was employed primarily for
r
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.018

A.F. Lucrédio et al. / Catalysis Communications 12 (2011) 1286–1290
1287
WHSV=14.9 h− ) for 6 h on stream, and the other with H
1
O:
(WHSV=20.2 h ) for 3 h on
O: ethanol feed solution was driven by a piston pump, the
Table 1
Comparison between composition obtained by XPS (surface) and by EDS (bulk) .
2
a
−1
−1
ethanol=5:1, flowing at 3.3 mLh
stream. The H
flow of ethanol being fixed at 1.3 mLh . The feed was vaporized in a
preheating chamber (180 °C) before entering the reactor. At the end of
the catalytic test, the feed was stopped and the system cooled with a
Catalyst Molar composition (XPS)
Molar composition (EDS)
2
−
1
Co
Mg
Al
La
Ce
Co
Mg
Al
La
Ce
Co
LaCo
CeCo
0.11 0.67 0.22
0.17 0.67 0.12 0.04
0.17 0.60 0.19
–
–
–
0.07 0.67 0.26
0.08 0.76 0.15 0.01
–
–
–
2
flowof pure N . During thereaction, the liquid products werecondensed
–
0.04 0.08 0.75 0.15
–
0.02
in a cold trap kept at 0 °C. At the end of reaction, the condensed liquid
a
Lucrédio et al.[7].
internal calibration of the edge positions, against a pure metal foil.
Nitrogen filled theI , I and I chambers, and a Si (1 1 1) single crystal was
0
t
r
used as monochromator. The Athena computer package was used to
analyze the XAS data.
2
.3. Catalytic tests
The catalytic tests were performed at atmospheric pressure and
5
50 °C, in a fixed-bed tubular quartz micro reactor. Theunused reactants
and gaseous reaction products were analyzed in-line by gas chroma-
tography (Varian, Model 3800), as described elsewhere [5].
Prior to the reaction, 150 mg of the catalyst, sieved in the range
6
0–100 mesh and freshly calcined, was introduced into the reactor and
−
1
2
activated in situ by reduction in flowing H (50 mLmin ) at 550 °C
(
heating rate 10 °Cmin−1) for 1 h and then purged at 550 °C in a flow of
2
pure N . The reaction was started in a hydrogen-free feed of water and
ethanol. The catalysts were tested with two feeds: one witha molar ratio
−
1
2
H O: ethanol=3:1, flowingat 2.5 mLh (weighthourly space velocity,
Fig. 1. Normalized XANES spectra for (A) reference substances and (B) the catalyst
samples Co, CoLa and CoCe compared with reference compound Co
3
O
4
.
Fig. 2. Gaseous composition of products of ESR on (A) Co, (B) CoCe and (C) CoLa catalysts.

1288
A.F. Lucrédio et al. / Catalysis Communications 12 (2011) 1286–1290
Table 2
programmed reduction (TPR) and X-ray diffraction (XRD) analysis
Ethanol conversion and selectivity of acetaldehyde.
reported in these previous studies, the Co and Mg species are interacted
strongly, probably in the form of a solid solution of CoO–MgO doped
with Al3± cations [7,8]. In the Ce and La promoted samples, the
H
2
O: ethanol
Time on Catalyst Acetaldehyde
stream (mol/mol EtOH converted
h)
Ethanol
conversion
(%)
)
(
2 3 2 2
analyses indicated the presence of Ce O /CeO and La O3, with no
3
(
:1
Co
0.0040
0.0110
0.0476
0.0155
0.0005
0.0009
97
84
63
85
89
89
interaction with the support. The TPR results for the samples CoLa
and CoCe showed reduction peaks shifted to lower temperatures,
indicating that the presence of Ce and La had weakened the Co-
support interaction [7,8].
Fig. 1 presents the XANES spectra of the catalysts. For comparison,
Fig. 1A shows the normalized Co K-edge XANES spectra of related
feed rate: 2.5 mLh−1) 6 h
CoCe
CoLa
Co
CoCe
CoLa
5
(
:1
feed rate: 3.3 mLh−1) 3 h
reference materials: Co metal foil, CoAl
shows the spectra of the oxide samples Co, CoLa and CoCe, plotted
together with that of the reference compound Co
2 4 3 4
O , Co O and CoO. Fig. 1B
products were collected and analyzed by gas chromatography (Hewlett
Packard 5890), with an HP-FFAP capillary column (25 m×0.2 mm i.d.)
and FID detector. During the sample preparation, the solution was kept in
a temperature lower than 10 °C, to avoid any liquid product evaporation.
The total ethanol conversion during the test was taken as the
total volume of ethanol fed to the reactor minus the total volume
of condensed ethanol, expressed as a percentage of the total fed
3 4
O .
In Fig. 1A, a weak pre-edge peak (α) can be seen in the spectra
for the reference substances, around 7.71 keV, which is attributed
to the 1s–3d transition. The pre-edge region provides information
about local geometry around the absorbing atom and depends on
its oxidation state and bond characteristics. The pre-edge intensity
is related to the symmetry and to the occupancy of the 3d shell. In
the case of tetrahedral coordination, the 1s–3d transition, normally
dipole-forbidden, becomes partially dipole-allowed due to the asym-
metry and results in a weak pre-edge peak. With octahedral coordina-
tion, the pre-edge peak is absent or less intense, only appearing if
thermal vibrations momentarily eliminate the center of symmetry and
2 4 2 2 4
volume. The selectivity of each product (i=H , CH , CO , CO or C H )
was calculated by the following equation:
Selectivity of i ¼ Molar flow of iproduced=Molar flow of ethanol converted
ð2Þ
3 4
allow the 1s–3d transition [9,10]. Co O has a spinel structure, with a
2+
third of the cobalt ions (Co ) occupying tetrahedral sites and two-
thirds (Co3 ) occupying octahedral sites. The pre-edge feature repre-
sents a superposition of the more intense tetrahedral peak and the
+
3
. Results
The characterization of these catalysts is presented elsewhere
weaker octahedral peak. CoAl
2
O
4
also has a spinel structure, but here all
the cobalt ions (Co2 ) occupy only tetrahedral sites, and the normalized
absorption pre-edge (α) is more intense than that of Co . CoO has only
+
by the authors [7,8]. Table 1 reproduces the molar composition of
the catalysts [7] and according to this table, the concentrations of
Co, La and Ce are higher on surface. According to the temperature-
3 4
O
octahedral sites for Co, and therefore the pre-edge (α) feature is very
Fig. 3. Production of (A) H
2
, (B) CH
4
, (C) CO
2
and (D) CO in the ESR reaction with feed molar ratio=3:1.

A.F. Lucrédio et al. / Catalysis Communications 12 (2011) 1286–1290
1289
weak [11,12]. The oxide catalyst samples Co, CoLa and CoCe showed very
weak pre-edge peaks, weaker than the reference compound Co
Fig. 1B), indicating Co ions in octahedral symmetry, as in CoO.
The XANES profile is affected by the oxidation state, the main
on the work of Mas et al. [18] which studied the ethanol conversion as a
function of H O:ethanol molar ratio. The authors observed a maximum
of ethanol conversion using a H O:ethanol molar ratio of 5:1 and suggest
that there is a competition between ethanol and water adsorption for
the active sites.
O
3 4
2
(
2
absorption edge shifting to higher energy at higher oxidation states.
The main absorption edge, or white line (β), is observed at 7.726–
Table 2 also shows the ethanol conversion rates and the liquid
7
.727 keV in the samples Co, CoLa and CoCe. This is close to the value
2
product collected after 3 h of reaction with feed molar ratio H O:
reported in the literature for CoO. Also, for the catalyst samples, the
white line is appreciably more intense than for the reference compound
ethanol=5:1. The other liquid by-products were observed only in
trace amounts.
3 4
Co O . The white line reflects holes in the d band: the more unoccupied
These data show that, the ethanol conversion was greater on the
the d band, the more intense is the white line absorption [13]. In these
samples, the high intensity of the white line could be due to a strong
interaction between Co and Mg, involving a charge transfer from the d
band of Co to Mg, resulting in lower d occupancy and an increase in the
intensity at the edge. The sample Co exhibited the most intense white
line, possibly indicating a stronger Co–Mg interaction in this material.
This would be consistent with the X-ray photoelectron spectroscopy
2
CoCe and CoLa catalysts than when the feed molar ratio of H O:ethanol
was 3:1, while that on Co was lower. Also, acetaldehyde formation was
(
XPS) results presented elsewhere by the authors, which showed a
charge transfer from Co to Mg in these samples, due to the solid solution
formation as suggested by XRD and TPR results [7,8].
3
.1. Catalytic tests
Fig. 2 plots the composition of the gaseous products against
reaction time, the main products being H
seen in these profiles that the gaseous composition is practically the
same for all catalysts. Note that C was observed at low levels, due
2 4 2 2 4
, CH , CO, CO and C H . It is
2 4
H
to the weak acidity of the catalyst supports, which prevents the
dehydration of ethanol to ethylene [3]. However, the sample CoLa
presented a slight increase in the C
2
H
4
formation with time on stream.
O:ethanol=3:1
Therates of ethanol conversion at feed molar ratio H
2
are presented in Table 2 and a decrease is observed on the addition of the
rare earth metals, in the order: CoNCoCeNCoLa. This behavior is
accompanied by the formation of acetaldehyde, in the liquid effluents,
in the order: CoLaNCoCeNCo. Ethyl ether, acetone, ethyl acetate and
acetic acid were observed in trace amounts. Also, carbon was deposited
only in trace amounts.
The higher formation of acetaldehyde on catalysts CoLa and CoCe
indicates that the basic additives stabilized the formation of adsorbed
acetaldehyde. In the sample CoLa, the La3 is the adsorption site for
oxygen radicals [14] and in the present case it could favor the dissociative
+
OH+O2 ⇆CH
−
CH
O
−
+
−
OH) with
adsorption of ethanol (CH
3
CH
2
3
2
O−
+
−
OH⇆
consequent dehydrogenation to acetaldehyde (CH
3
CH
2
2
−
CH
3
CHO+H
2
+O ) [15].
The sample CoCe presented higher production of acetaldehyde,
when compared to Co, but lower than CoLa. Ceria has high oxygen
storage capacity and oxygen mobility that can favor the oxidation of
−
−
−
−
acetaldehyde to acetate (CH
which can then be decomposed to CH
Fig. 3 shows the selectivity profiles of H2, CH
h on stream. According to the data, the selectivity of H
3
CHO+2O2 ⇆CH
3
–COO
or CO [15,16].
, CO and CO
, CH
+ OH+2e ),
4
and CO
2
4
2
during
and CO
6
2
4
2
are decreasing with the time on stream. Considering the low carbon
deposition and the high values of ethanol conversion presented in
Table 2, it can be inferred that this decreasing may be occurring due to
a change in the selectivity of the reaction by the formation of liquid
products or C
2
H
4
, as observed for the sample CoLa, in Fig. 2.
production follows practically the
Considering the selectivity, H
2
same curve on CoLa and CoCe catalysts, which presented higher H
selectivity than Co. The addition of La and Ce led to a higher CO
2
2
4
selectivity and reduced CH selectivity relative to the Co catalyst, probably
because the presence of the basic promoters Ce and La favored the
activation of water or other oxygen compounds [8,17], thus promoting
the steam reforming of methane (SRM: CH
water gas shift reaction (WGSR: CO+H O→H
To investigate further the role of the water in the catalyzed reactions,
the H O:ethanol molar ratio in the feed was raised from 3:1 to 5:1, based
4
+H
2
O⇆CO+3H
2
) and the
2
2
+CO
2
).
Fig. 4. Gaseous composition of products of ESR on catalysts (A) Co, (B) CoCe and
(C) CoLa, with feed molar ratio of water:ethanol=5:1.
2

1290
A.F. Lucrédio et al. / Catalysis Communications 12 (2011) 1286–1290
strongly inhibited on the catalysts CoLa and CoCe and favored on the Co
catalyst in the presence of excess water. The lower ethanol conversion
on Co catalyst could be due to partial oxidation of the Co sites by the
extra water and the increase in the acetaldehyde production due to a
formation, probably by favoring the adsorption of water under these
conditions.
Acknowledgements
3 2 3 2
favoring of ethanol dehydrogenation (CH CH OH⇆CH CHO+H ) on
the basic sites of the MgAl support.
The authors are grateful to the Brazilian state and federal research
funding agencies, FAPESP, CNPq and CAPES, for the financial support
and to the LNLS (Brazilian Synchrotron Light Source Laboratory) for
the XANES facilities.
Regarding the conditions used, the flow of ethanol was kept
constant and the flow of water accelerated, leading to a shorter
contact time. These conditions seem to have favored the adsorption
of water on the samples CoLa and CoCe, leading to inhibition of
acetaldehyde production [17,19]. The presence of these additives
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