Steam reforming, partial oxidation and oxidative steam reforming for hydrogen production from ethanol over cerium nickel based oxyhydride catalyst

Article abstract of DOI:10.1016/j.apcata.2015.10.035

H₂production from ethanol was studied on a cerium nickel based (CeNi₁O_Y) catalyst in presence of water (SRE), in presence of oxygen with different concentrations (POE) and in presence of oxygen and water (OSRE). The influence of different parameters was analyzed, such as the reaction temperature, reactants concentration (oxygen/ethanol/water), and in situ pre-treatment in H₂. At low temperature a high activity i.e., ethanol conversion and H₂formation can be obtained when the solid is previously in situ treated in H₂at 250 °C i.e., in the oxyhydride form. Adding O₂allows increasing ethanol conversion at low temperature. Different physicochemical techniques, including Inelastic Neutron Scattering (INS), were used to characterize the catalyst. An active site based on the formation of anionic vacancies and a mechanism involving a heterolytic abstraction of a hydride species from ethanol are envisaged.

Full text of DOI:10.1016/j.apcata.2015.10.035

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Applied Catalysis A: General xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Steam reforming, partial oxidation and oxidative steam reforming for
hydrogen production from ethanol over cerium nickel based
oxyhydride catalyst
Cyril Pireza,b,1, Wenhao Fanga,b,1, Mickaël Caprona,b, Sébastien Paulb,c, Hervé Jobicd,
a,b,e
a,b,∗
Franck Dumeignil
, Louise Jalowiecki-Duhamel
a
Université Lille Nord de France, 59000 Lille, France
Centre National de la Recherche Scientifique UMR 8181, Unité de Catalyse et Chimie du Solide, UCCS, 59655 Villeneuve d’Ascq Cedex, France
Ecole Centrale de Lille, 59655 Villeneuve d’Ascq, France
IRCELyon Institut de Recherches sur la Catalyse et I’Environnement de Lyon, 69626 Villeurbanne Cedex, France
Institut Universitaire de France, Maison des Universités, 103 Boulevard Saint-Michel, 75005 Paris, France
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2015
Received in revised form 12 October 2015
Accepted 20 October 2015
Available online xxx
H2 production from ethanol was studied on a cerium nickel based (CeNi O ) catalyst in presence of
1
Y
water (SRE), in presence of oxygen with different concentrations (POE) and in presence of oxygen and
water (OSRE). The influence of different parameters was analyzed, such as the reaction temperature,
reactants concentration (oxygen/ethanol/water), and in situ pre-treatment in H2. At low temperature a
high activity i.e., ethanol conversion and H2 formation can be obtained when the solid is previously in
◦
situ treated in H2 at 250 C i.e., in the oxyhydride form. Adding O2 allows increasing ethanol conversion
Keywords:
Ethanol
Hydrogen
Steam reforming
Partial oxidation
Oxidative steam reforming
at low temperature. Different physicochemical techniques, including Inelastic Neutron Scattering (INS),
were used to characterize the catalyst. An active site based on the formation of anionic vacancies and a
mechanism involving a heterolytic abstraction of a hydride species from ethanol are envisaged.
© 2015 Elsevier B.V. All rights reserved.
1
. Introduction
Nowadays, increasing attention is being paid to pollution-
become closer to reality, so increased attention is focused on hydro-
gen production technologies [12–16].
Hydrogen may be generated from ethanol by different technolo-
gies, as previously described in different reviews [3–11]: steam
reforming (SRE) [Eq. (1)] [17–27], partial oxidation (POE) [Eq. (2)]
[28–38], and oxidative steam reforming (OSRE) [39–47], includ-
ing autothermal reforming (ATRE) [48–51] reactions. Moreover,
the different technologies have been already compared on some
catalysts [50–52]. The endothermic SRE reaction extracts more
hydrogen atoms from ethanol and water molecules and provides
a high hydrogen yield, which represents an important advan-
tage in hydrogen-production applications. However, SRE is highly
endothermic reaction and therefore, high operation temperatures
are necessary, so additional energy supply is needed, which leads
to high capital and operation costs as well as environmental degra-
dation. The reforming reaction is generally carried out at high
related environmental and public health problems, it is important
to find a way for obtaining “green” energy. A hydrogen economy
coming from renewable energy sources would lead to high bene-
fits at the world level [1–2]. Today hydrogen is mainly produced
from fossil fuels and while fossil fuel is not a sustainable energy
source, one highly attractive route for hydrogen production is
catalytic transformation of bio-ethanol, obtained from transforma-
tion and fermentation of biomass. In theory, hydrogen production
from biomass or biomass derived liquids can be a neutral carbon-
emission process since all carbon dioxide produced can be recycled
back to plants, and because of its low toxicity and ease of deliver-
ability, ethanol lends itself very well to a distributed-production
strategy [3–11]. Besides, the wide-spread application of fuel cells
◦
temperature (>600 C). An alternative approach is partial oxidation
of ethanol (POE), an exothermic reaction that exhibits fast start
up and response times while potentially offering a more compact
reactor design, desirable features for mobile fuel cell applications.
Therefore, one alternative way of supplying heat to the reforming of
ethanol system is to add oxygen or air to the feedstock (OSRE, ATRE)
∗ Corresponding author at: UCCS, CNRS, 59655 Villeneuve d’Ascq Cedex, France.
Fax: +33 (0) 3 20 33 65 61.
E-mail address: louise.duhamel@univ-lille1.fr (L. Jalowiecki-Duhamel).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.apcata.2015.10.035
0
926-860X/© 2015 Elsevier B.V. All rights reserved.
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◦
and simultaneously to burn a portion of ethanol. POE can be per-
formed with different oxygen concentrations as reported in Eqs. (2)
at 250 C for 10 h. The water/ethanol mixture is pumped (with a
HPLC pump) into a heated chamber and vaporized. In order to ana-
lyze the influence of the concentration of ethanol, different liquid
and (3), and the reaction is exothermic for O /ethanol molar ratio
2
higher than 0.5, while with this ratio the reaction is endothermic.
flows of the ethanol–water mixture were used while the H O/EtOH
2
molar ratio was always kept constant at 3. The liquid flows were
C H5OH + 3H O → 6H + 2CO
(1)
(2)
(3)
2
2
2
2
−1
between 0.01 and 0.10 mL min . The ethanol/water/O /N2 gas
2
−
1
stream (O –N2 flow: 60 mL min ) is then fed to the reactor con-
2
◦
H
2
= +174 KJ mol−1
ꢀ
98
taining the (0.008–0.2 g) of catalyst. It has to be remarked that as
the O –N2 flow is maintained constant, when the EtOH concentra-
2
C H5OH + 1.5O → 3H + 2CO
2
2
2
2
2
tion is increased, the total theoretical flow is also increased. The
gases at the outlet of the reactor were taken out intermittently
with the aid of a sampler directly connected to the system and ana-
lyzed on-line by gas chromatography (TRACE GC ULTRA) equipped
with a thermal-conductivity detector (TCD) and a flame ionization
detector (FID). Solid carbon is formed among the products.
◦
H
2
= −545 KJ mol−1
ꢀ
98
C H5OH + 0.5O → 2CO + 3H
2
2
◦
H
2
= +14 KJ mol−1
For POE reaction, catalytic performances were conducted at
atmospheric pressure with a quartz fixed-bed reactor (inner diame-
ter 10 mm) fitted in a programmable oven, in the temperature range
ꢀ
98
Nevertheless, the major barriers to all of these technologies are
◦
the by-products formation and catalytic deactivation. So, enormous
research efforts have been done to develop cheap, highly active,
selective and resistant catalysts and/or electrocatalysts, as ethanol,
can be sent directly to a fuel cell. The reactions have been studied
over noble metals as well as transition metals with different metals
and numerous supports [5–9], and among them nickel and cerium
based catalysts have been tested [17,18]. OSRE interested the sci-
entists because adding O2 show beneficial effects [53]. Although
the POE reaction could be performed at relatively lower tempera-
ture, the high exothermicity of the reaction [Eq. (2)] could lead to
hot-spots and deactivation of the catalyst. It is therefore of great
interest to compare between SRE, POE and OSRE on exactly the
same catalyst, and analyze the influence of different parameters.
In the laboratory, different nickel based mixed oxides were stud-
ied for hydrogen production from ethanol in SRE [54–58] and in
OSRE [59,60]. It is known that the catalyst preparation method is of
paramount importance, and we previously analyzed its influence
over the cerium nickel based catalyst in SRE activity [56]. Therefore,
it is of great interest to analyze the influence of the presence of oxy-
gen using the same catalyst. Here, we report a comparative study
of 200–500 C. When noted, the catalyst was previously in situ
◦
treated in H2 at 250 C for 10 h. Ethanol is sent via a saturator
and the partial ethanol pressure is controlled using a condensator,
4 mol% of EtOH was sent for almost all the experiments, while
4–20 mol% range of EtOH was studied when precised. It has to be
remarked that as the O –He flow is maintained constant (O –He
2
2
−
1
flow: 60 mL min ), when the EtOH concentration is increased, the
total theoretical flow is also increased. The O /ethanol molar ratio
2
varies between 0.5 and 1.5. The ethanol/O /He gas stream is then
2
fed to the reactor containing 0.2 g of catalyst diluted with SiC, and
sandwiched between two layers of SiC. The gases at the outlet of
the reactor were taken out intermittently with the aid of a sampler
directly connected to the system and analyzed by FID and TCD gas
chromatography. Reaction data were collected for each tempera-
ture. Solid carbon is formed among the products but not quantified.
Appropriate blank runs have shown that under our experimental
test conditions the contribution of the gas phase reaction is negli-
gible.
Reaction data were collected as a function of time and reported
after at least about 5 h when the steady state was obtained, for each
temperature. Catalytic performances were reported by ethanol
on H2 formation activity over the CeNi OY catalyst from ethanol in
1
presence of water (SRE), in presence of oxygen with different con-
centrations (POE) and in presence of oxygen and water (OSRE). The
aim of this work was to develop a highly active, selective, stable
and cost effective catalyst at low temperature whatever the reac-
tion mixture, and to participate to the open debate on active site
and mechanism.
conversion (XEtOH), and products molar composition (C ) (dry basis),
based on the following equations [Eqs. (4) and (5)].
i
nEtOH,in − nEtOH,out
XEtOH
=
× 100%
(4)
(5)
n
EtOH,in
ni
ꢁproductsni
Ci =
× 100%
2
. Experimental
2.3. Catalyst characterizations
2.1. Catalyst preparation
The metal loadings were analyzed by ICP-MS technique from
The mixed oxide, denoted CeNi OY was prepared by coprecip-
CNRS-Service Central d’Analyses and the molar ratio was then
deduced.
1
itation of the corresponding hydroxides from mixtures of cerium
and nickel nitrates (0.5 M) using triethylamine (TEA) as a precipitat-
The BET surface area was measured by N2 physisorption at 77 K
by using a Micromeritics TriStar II 3020 Surface-Area and Porosime-
try analyzer. The sample was previously out-gassed under vacuum
◦
ing agent. After filtration, the solid was dried at 100 C and calcined
◦
in air at 500 C for 4 h. The calcined compound is noted as fresh
◦
catalyst.
at 150 C for 3 h.
Raman spectra were acquired on a Labram Infinity HORIBA
JOBIN YVON Raman spectrometer using a visible laser with a wave-
length of ꢂ = 532 nm at room temperature.
2.2. Catalytic performance
For SRE and OSRE reactions, catalytic performances were con-
H -TPR was performed on a Micromeritics Autochem II
2
ducted at atmospheric pressure with a quartz fixed-bed reactor
inner diameter 10 mm or 4 mm according to the mass of cata-
Chemisorption analyzer, and the H consumption was measured by
2
(
a TCD detector. The sample was treated in the 5 vol.% H –95 vol.%
2
−
1
lyst) fitted in a programmable oven, in the temperature range of
Ar mixtures with a flow rate of 30 mL min . The temperature was
◦
◦
◦
−1
5
0–500 C. The catalyst was sieved after preparation and the solid
increased to 1000 C at a heating rate of 10 C min .
with dimension between 250 and 500 ␮m was used for catalytic
test. When noted, the catalyst was previously in situ treated in H2
In situ XRD in H2 was performed on a Bruker D8 Advance
type HT1200N X-ray diffractometer equipped with a fast detec-
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Fig. 2. Conversion of ethanol in SRE (14% EtOH) and gas phase products distribution
◦
at 250 C on the CeNi1OY catalyst (0.2 g) as a function of treatment temperature (TT)
◦
Fig. 1. Conversion of ethanol in SRE and gas phase products distribution at 450
on the CeNi1OY catalyst. Ac: acetaldehyde.
C
in H2. Ac: acetaldehyde, Eac: Ethylacetate.
while CO formation starts rising to about 13%, in agreement with
methane reforming at high temperatures and CO transformation
by a water gas shift at low temperatures. As a result, the yield of
tor type VANTEC with a copper anticathode. A mixed gas of 3 vol.%
◦
−1
H –97 vol.% Ar was employed with a heating rate of 10 C min
2
◦
from room temperature to 450 C.
−
1
◦
hydrogen significantly increases up to 4.6 mol molEtOH at 650 C,
while the catalyst exhibits considerable stability, even if some
carbon is formed [56]. It is important to mention that working
at high temperature water gas shift (WGS) equilibrium does not
Inelastic Neutron Scattering (INS) experiments were performed
using the IN1 BeF spectrometer at ILL (Institut Laue Langevin,
Grenoble). 36 g of solid were placed inside stainless steel contain-
ers, and the treatment in H (10 h) was performed using high purity
2
−1
allow reaching the theoretical value of 6 mol molEtOH . More-
gas. The scattering cross-section is much greater for hydrogen (80
barns) than for other elements (5 barns), therefore, INS emphasizes
motions of hydrogen species.
over, it must be emphasized that the hydrogen production can
be promoted by increasing the water partial pressure and the
◦
H O/C H5OH molar ratio. Biswas et al. obtained at 600 C on a
2
2
Ni/CeO –ZrO catalyst, in a reaction mixture with a high H O/EtOH
2
2
2
3
. Results and discussion
−
1
ratio of 8, the highest value for H2 production (5.8 mol molEtOH
that has been reported in the literature, close to the theoret-
)
Hydrogen production from ethanol SRE, POE and OSRE was
−
1
ical value of 6 mol mol
were CO (0.47 mol mol
(
[18]. The other products formed
EtOH
investigated over a cerium nickel based mixed oxide catalyst
CeNi OY ). The influence of different parameters was analyzed,
−
1
−1
), CH4 (0.33 mol molEtOH ) and CO2
EtOH
(
1
−1
1.15 mol molEtOH ). Therefore close to 75% of H2 were obtained
such as reaction temperature, water–oxygen–ethanol feed compo-
sitions, and in situ pre-treatment in H2.
with 6% of CO, 4% of CH4 and 15% of CO . As a matter of fact,
2
thermodynamics predicts the equilibrium composition of reactants
and products at different temperatures. Moreover, the increase of
3.1. Steam reforming of ethanol (SRE)
the H O/EtOH ratio, as well as the addition of helium as inert gas
2
increases H2 production at low temperatures [61–62]. Besides, it
is noticeable that the experimental results of H2 production are
higher than the ones calculated by the thermodynamic analysis. It
has been reported that there is a region of non-equilibrium in which
The efficiency of the CeNi OY catalyst was thoroughly studied
1
by its capacity to produce hydrogen from SRE (H O/C H5OH = 3)
2
2
in diluted conditions (H O: 9 mol%; C H5OH: 3 mol%) to avoid any
2
2
problems due to volume variation when a high concentration of
higher H concentration can be obtained. Moreover, it has been also
◦
2
gases are produced (Fig. 1). At 450 C, the CeNi OY catalyst (0.05 g)
1
shown that the carbon formation involving different types of car-
bon species must also be taken into account in the thermodynamic
calculation [61].
is able to completely convert ethanol to H , CO2 and CH (almost
2
4
−
1
no CO is observed), with a H yield of 3 mol molEtOH [56]. It has to
2
be noticed that decreasing the ethanol concentration allows still
enhancing ethanol conversion [58]. On the present catalyst, the
Hydrogen production from ethanol in the presence of water
(
H O/C H5OH = 3) was also previously investigated over the series
◦
2 2
activity is high at 450 C and almost no variation is observed on the
of cerium nickel CeNiX OY (0 < x ≤ 5) mixed oxide catalysts in
harsh conditions (14 mol% EtOH) [54,55]. The influence of different
parameters was analyzed, such as reaction temperature, Ni con-
conversion with only a small conversion decrease when increasing
EtOH concentration up to 14% (mol). In the meantime, the products
distribution is influenced by the increase of EtOH concentration,
with a decrease of H2 proportion (among the gas phase products)
tent and in situ pre-treatment in H . A stable activity i.e., ethanol
2
conversion and H2 formation can be obtained at very low reaction
and an increase of CO and CH . However, it has to be noted that
4
◦
temperature (200 C) when the solid is previously in situ treated in
the products distribution is reported as obtained by online analy-
sis, without any correction related to volume variation. The catalyst
mass must be decreased down to 0.008 g to see a decrease of EtOH
conversion, evidencing the high activity of this catalyst, with in
such a case appearance of acetaldehyde (Fig. 1). Numerous dif-
ferent reactions can take place, and have been largely reviewed
5,7–10] however, acetaldehyde can be obtained by ethanol dehy-
drogenation [Eq. (6)] and decomposition of ethanol [Eq. (7)] and/or
acetaldehyde lead to the formation of methane.
H at adequate temperature. When the CeNi OY [54] or CeNi0.5OY
2
0.4
◦
[
55] catalysts are previously in situ treated in H2 at 200 C, there is
globally an increase of the conversion versus temperature with the
◦
existence of an optimum for a reaction temperature of 250 C. The
◦
influence of the in situ treatment in H at 200 C on the ethanol
2
◦
◦
conversion obtained at 200 C and 250 C has been analyzed versus
the Ni content of CeNiX OY compounds [54]. Moreover, the influ-
ence of the in situ treatment temperature TT has been analyzed
on the CeNi0.7OY compound, and an optimum of activity at 250 C
has been shown for a TT of 270 C of the catalyst [55]. Fig. 2 shows
[
◦
◦
C H5OH → CH CHO + H
(6)
(7)
2
3
2
the high influence of TT on the CeNi OY compound. The reaction
1
C H5OH → H + CH + CO
2
2
4
◦
is performed at a particularly low temperature of 250 C in harsh
◦
By increasing the reaction temperature from 450 C up to
conditions (14 mol% of EtOH) maintaining the molar H O/C H5OH
2
2
◦
6
50 C, CH formation gradually decreases from 20% down to zero,
ratio of 3. Clearly, the conversion and products distribution depend
4
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hence the performance obtained here with a H O/EtOH ratio of 3 is
2
interesting as compared to the literature [58]. At this particularly
low temperature, it could be expected that increasing Ni content
would allow increasing conversion and ameliorating selectivity, if
high dispersion of Ni could be maintained. As a matter of fact, it
has been recently reported, in similar conditions, that total con-
version of EtOH can be obtained with the expected products from
SRE without formation of acetaldehyde on Ni–Mg–Al–O based cat-
alyst involving higher Ni content [58]. Moreover, it has been shown
that conversion of EtOH can be increased and acetaldehyde forma-
tion decreased by increasing Ni content on different nickel based
catalysts (even if in such a case carbon formation increased also)
[
56,58].
◦
Fig. 3. Conversion of ethanol in SRE and gas phase products distribution at 300
C
3
3
.2. Partial oxidation of ethanol (POE)
as a function of time on stream on the CeNi1OY catalyst (0.05 g) in situ pretreated in
H2 at 250 C. EtOH/H2O/N2 = 1/3/96. No CO is observed.
◦
.2.1. Effect of O /EtOH ratio
2
The fresh CeNi OY catalyst has been tested in POE for differ-
1
◦
◦
on the pre-treatment in H2 and an optimum TT of about 250 C is
ent O /EtOH ratios between 200 and 480 C. Fig. 4 shows ethanol
2
◦
obtained. Increasing TT up to 450 C leads to a conversion decrease,
conversion and products distribution versus reaction tempera-
ture obtained with different oxygen/ethanol molar ratios: 0.56,
1 and 1.5. All the curves show the same tendency, the conver-
and much particularly to an increase of carbon formation, even if a
slight increase of H2 can be obtained. Without any pretreatment in
H2 or with a low TT, the conversion remains very low and ethylac-
etate and acetaldehyde are mainly obtained in such conditions.
All the results obtained show the high performance of the
◦
sion increases with the temperature and reaches 100% at 350 C
◦
with a ratio of 1.5 and at 400 C with the other ratios, so at lower
temperature with the highest concentration of O . Ethanol con-
2
CeNi OY catalyst, in particular at low temperature. Numerous
parameters are influencing the results obtained, in complement to
version increases more rapidly when the O /EtOH ratio increases.
1
2
◦
H2 appears at 300 C in the gas phase products, its molar fraction
◦
the already reported water concentration and/or H O/EtOH ratio
increases drastically between 300 and 350 C, and then it increases
2
effects, such as the in situ pre-treatment in H2 that has to be
performed at adequate temperature, as well as the ethanol con-
centration, the highest conversion being obtained with the lowest
ethanol concentration. To be able to report high conversion at low
temperature, low concentration of ethanol is favorable. As shown
slightly when increasing temperature. H formation follows almost
2
the same trend with the O /EtOH ratios, however, it is lower with
2
the highest O /EtOH ratio, and a slight optimum could be seen
2
◦
with the ratio 1 at 440 C, with 68% of H2 among the gas phase
products. Even if ethanol conversion is complete at 350 or 400 C
◦
◦
in Fig. 3, high conversion of EtOH can be obtained at only 300 C in
depending on O2 concentration, oxygen conversion is complete
◦
◦
highly diluted conditions (1% EtOH). After several hours, a stable
EtOH conversion at about 60% is obtained on a low mass of cata-
lyst (0.05 g), with the formation of 75% of H , 10% of CO , 10% of
at 300 C. At low temperature (200–300 C), the main products
observed are acetaldehyde, CO , acetone and ethylacetate, depend-
2
ing on the O2 content; acetaldehyde, being the main product at
2
2
◦
CH4 and 5% of acetaldehyde. No CO and no carbon are observed. It
the lowest temperature analyzed (200 C). The formation of CO2
seems that the present catalyst allows totally transforming CO at
presents an optimum; it reaches about 70% with the highest con-
centration of O2 at 300 C. The high formation of CO2 is coherent
◦
◦
low temperature of 300 C through WGS to H . As it is known that
2
increasing water–ethanol ratio allows obtaining higher H2 yield,
with Eq. (2), however, hydrogen started only to be observed at this
Fig. 4. Ethanol conversion and products distribution in POE over fresh CeNi1OY catalyst (0.2 g) in presence of different O2/EtOH ratios: (ᮀ) 0.56, (ꢀ) 1, (ꢁ) 1.5.
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Fig. 5. Ethanol conversion and products distribution in POE over fresh CeNi1OY catalyst (0.2 g) in presence of an O2/EtOH ratio of 1.5. The oven temperature (ꢀ) and the
measured temperature (ꢀ) are also reported in (a).
◦
temperature, may be because the catalyst consumes hydrogen and
get partially reduced. CO is found in much lower concentrations,
whatever the O2 concentration; it presents an optimum at 350 C.
treated and fresh catalyst. At temperatures higher or equal to 350 C
ethanol and O2 conversions and products distribution (unless for
◦
CH ) do not seem to depend on the activation of the catalyst. How-
4
◦
Whatever the O /EtOH ratio, ethylacetate is found at 17% at 200 C
ever, one must note that the conversion being total it may be not
possible to see differences. The main differences in the gas phase
products distribution are observed at low temperature, mainly for
H , CO and CH that are favored at lower temperature on the treated
in H2 catalyst. Clearly, the treatment in H2 at 250 C leads to a ben-
eficial effect on H2 production at low temperature. CH4 and CO are
2
◦
and it decreases with temperature to disappear at 350 C, while
acetone presents an optimum at about 7% at 300 C. Acetaldehyde
and ethylacetate disappear at 400 C while methane is formed at
temperatures higher than 300 C. As discussed for SRE reaction, sev-
◦
◦
2
4
◦
◦
eral competing reactions can exist such as ethanol dehydrogenation
◦
and decomposition. Moreover, in presence of O , partial oxidation
observed at temperatures higher than 250 C. Hence, ethanol [Eq.
2
reactions take place. POE may be even considered as a combination
of partial oxidation to syngas coupled with CO oxidation. Further-
more, oxidative dehydrogenation to acetaldehyde could occur in
parallel with POE, as already previously reviewed [5,7–9].
(7)] and/or acetaldehyde are decomposed to CO and CH4 at lower
temperature over the pretreated in H catalyst. Moreover, CH CHO
2
3
obtained by dehydrogenation of ethanol [Eq. (6)], decreases more
drastically with reaction temperature on the H pretreated catalyst.
2
So the pretreatment in H generates active sites able to perform the
2
corresponding reactions at low temperature. Formation of acetone
3
.2.2. Effect of reactants concentration
and ethylacetate, not reported here, diminish on the H treated cat-
The fresh CeNi OY catalyst has been also tested in POE with the
2
1
◦
alyst. At 300 C their molar fractions is of about 1%, showing that
O /EtOH ratio of 1.5 varying the concentration of EtOH between 4
and 20 mol.%, and consequently also the O2 concentration as here
the O /EtOH ratio is maintained constant (Fig. 5). The conversion of
EtOH is found relatively stable versus EtOH concentration, reaching
almost 60%, however, it is important to note that to get the same
reaction temperature, the oven temperature had to be lowered
2
the active sites related to the formation of these products are sup-
◦
pressed. Besides, over the fresh catalyst, up to 300 C acetaldehyde
2
is observed but not H , so H that should be produced from ethanol
2
2
by dehydrogenation reaction [Eq. (6)] is certainly consumed by the
solid leading to a partially reduced compound, explaining the ben-
◦
eficial effect observed due to the pretreatment in H of the catalyst.
(Fig. 5a). With an oven fixed at 300 C and an EtOH concentra-
2
Hydrogen species already present in the solid allow the H2 forma-
tion (desorption) at low temperature even in presence of oxygen.
The partial oxidation of ethanol (POE) reaction for H2 produc-
tion has been much less reported compared to SRE, and in particular
tion of 4%, there is almost no variation of temperature between the
oven and reaction temperatures. However, there is almost 100 C of
difference between the oven and reaction temperatures when the
EtOH concentration is of 20% (oven temperature at about 220 C in
◦
◦
◦
POE using an O /EtOH = 1.5 [Eq. (2)], some more papers focusing on
order to get a reaction temperature of 300 C). Therefore, the same
2
lower O /EtOH ratios [Eq. (3)]. Depending on the nature of metal
conversion is obtained at the same reaction temperature, however,
with a higher total theoretical flow when EtOH concentration is
increasing. So the catalyst can be seen as more active in high EtOH
concentration in comparison to the results obtained with low EtOH
concentration, while the inverse has been observed previously in
SRE. H2, CH , CO and acetaldehyde increase globally with increasing
EtOH concentration while CO and acetone decreases. Ethylacetate
is found in low quantity.
2
catalyst used, the pretreatment applied and the reaction operating
conditions employed, different products can be observed [5,7–9].
Moreover, it is clearly shown in the present study that even on
exactly the same catalyst, different pretreatment and conditions
applied lead to different results evidencing the versatility of a cata-
4
lyst. The POE to H and CO has been reported at low temperatures,
2
2
2
◦
300–400 C) using an O /EtOH molar ratio up to 2 over Ni–Fe cata-
2
(
lysts [28] and Cu/Nb O5 catalyst [32] previously in situ pretreated
2
in H . It has been shown that H2 production can be increased by
2
3
.2.3. Effect of in-situ pretreatment in H2
The influence of the in situ pretreatment in H2 at 250 C of
the CeNi OY catalyst has been studied with the O /EtOH ratio of
the addition of O , and that an optimum can be obtained with an
◦
2
O /EtOH ratio of 1.5 [28]. Very high conversion of ethanol could be
2
1
2
◦
obtained at 300 C (100% on Cu/Nb O5 and 87 % on Ni–Fe), and total
conversion of O2 was observed at lower temperature (200 C on
2
1
.5 (Fig. 6). This treatment leads to a complete ethanol conver-
◦
◦
◦
sion in POE at 300 C, so at 50 C lower than without pretreatment,
Cu/Nb O5). Therefore, the results obtained here are in good agree-
◦
◦
2
and also to the apparition of H2 at 250 C (Fig. 6a), so at 100 C
lower compared to fresh catalyst. Therefore, the activation in H2
has a beneficial effect on ethanol conversion and H2 formation at
low temperature in POE. While O2 conversion is the same for the
ment with previous results obtained on different types of catalysts,
even if products distributions could be different. In particular no
CH4 on the Ni–Fe catalyst, and little formation of CO on Cu/Nb O5
2
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6
◦
Fig. 6. Ethanol conversion and gas phase products distribution in POE with an O2/EtOH = 1.5 over fresh (solid symbol) and treated in H2 at 250 C (hollow symbol) CeNi1OY
catalyst (0.2 g).
1
00
sition of ethanol (and/or acetaldehyde) produces methane, carbon
monoxide and hydrogen [Eq. (7)]. CO can react with water in the
so called WGS reaction.
400
300
200
100
0
80
60
40
20
0
A stable activity i.e., ethanol conversion, and H2 selectivity can
be obtained at very low temperature when the solid is previously
◦
in situ treated in H2 at 250 C. So, a beneficial effect of the H2
pretreatment is also observed as already reported for SRE and POE
reactions. However, it is important to remark that in presence of
◦
O , once the catalytic reaction started at 200 C only a small energy
2
supply was needed. The oven temperature was lowered to about
◦
◦
1
30 C. So the results reported at a reaction temperature of 250 C
2
00
250
300
350
400
450
500
are obtained with a much lower oven temperature, as previously
observed in POE when using a high concentration of EtOH. The
evolution of the reaction temperature is reported versus the oven
temperature i.e., the energy needed to maintain the catalytic reac-
tion.
T (reaction) (°C)
Fig. 7. Ethanol conversion (ꢁ) in the presence of water and oxygen
EtOH/H2O/O2/N2 = 1/3/1.2/N2 and EtOH = 14 mol%), and gas phase products
distribution on CeNi1OY catalyst (0.2 g) in situ treated in H2 at 250 C. H2 (♦), CO2
ꢁ), CH3CHO (ᮀ), CH4 (ꢀ) and CO (᭹), and oven temperature (+).
(
◦
(
The catalytic results could be ameliorated and high activity
has been reported using only a small quantity of catalyst (0.03 g)
[
59,60]. The unique activation phenomenon of the reaction (the
were observed. Subramani et al. proposed that the large amounts
of H2 formation in POE could be explained by the fact that the
acetaldehyde formed by the oxidative dehydrogenation could be
partially oxidized to H2 and carbon oxides [7].
dramatic variation of temperature between the catalyst bed and the
oven) was deeply analyzed previously [59]. Moreover, the remark-
able catalytic stability was attributed to the graphitic filamentous
carbon formed during the reaction. Fig. 8 shows full ethanol con-
version on the CeNi OY catalyst with a H2 formation of about 45%
1
3.3. Oxidative steam reforming of ethanol (OSRE)
(mol%) relative to all the gas phase products (dry basis). The other
products analyzed are mainly CO (41%) and CO (12%) with very low
2
The conversion is largely increased when adding O2 to the
formation of CH4 and CH3CHO (<1%). Continuous complete con-
version of ethanol specifically at 50 C (oven temperature), with
◦
reaction mixture compared to SRE conditions. As an example,
Fig. 7 presents the results obtained in OSRE conditions over the
simultaneously production of H2, is obtained. The catalyst exhibits
remarkable stability after at least 75 h of reaction, even if carbon
deposition is found after the reaction. Therefore, compared to pre-
vious results we reported, the oven temperature has been still
◦
CeNi OY catalyst pretreated in H at 250 C (using harsh conditions:
1
2
1
4 mol% EtOH). For comparison, about 40% of ethanol conversion
◦
was obtained at 250 C in SRE conditions with the same mass of
◦
◦
◦
catalyst (Fig. 2). At a reaction temperature of 250 C, the ethanol
decreased (50 C compared to 60 C previously reported).
conversion is very high at about 75% with about 40% of H in the gas
phase products. At 400 C, ethanol is totally converted. As expected,
Finally, a cerium nickel based catalyst was developed for the
highly efficient sustainable H2 production from ethanol and water
in the OSRE reaction involving only a small energy input. The cata-
lyst is able to continuously completely convert ethanol specifically
at 50 C (oven temperature), and simultaneously produce H2, CO2
and CO. To the best of our knowledge, OSRE at a so low temperature
2
◦
CO2 is found in high quantity while CO presents an optimum at
◦
3
50 C. CH increases with temperature whereas acetaldehyde dis-
4
◦
◦
appears at about 400 C. As already reported, dehydrogenation of
ethanol, produces acetaldehyde and H2 [Eq. (6)] while decompo-
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7
Fig. 8. Ethanol conversion and products distribution in the presence of water and
Fig. 9. Raman analysis of CeNi1OY compound and ceria from ref [60] is reported for
comparison.
oxygen (EtOH/H2O/O2/N2 = 1/3/1.6/1.3; EtOH = 14 mol%) with an oven temperature
◦
◦
fixed at 50 C on CeNi1OY catalyst (0.03 g) in situ treated in H2 at 250 C (the reaction
◦
has been started at 200 C).
Table 1
Ni content, specific surface area and crystallites size of CeNi1OY catalyst.
−1
)
Catalyst
Ni (wt.%)
20
SBET (m2
g
d CeO2 (nm)
d NiO (nm)
CeNi1OY
94
5
10
has not been reported before. Sato et al. reported OSRE at very low
◦
temperature (100 C) on Ni/Ce0.5Zr0.5O and Ni/CeO catalysts [40].
2
2
They have shown that reduced Ni/Ce0.5Zr0.5O repeatedly triggered
2
◦
OSRE of ethanol at 100 C even in the presence of a stoichiometric
excess of steam. Interestingly, the furnace heater was switched off
after the reaction mixture was fed. However, the conversions and
H2 formation rates were measured after 30 min, but in repeated
cycles.
In the OSRE process, the endothermic SRE is assisted by the
exothermic POE reaction. The competing reactions such as dehy-
drogenation of ethanol to acetaldehyde [Eq. (6)], and ethanol
Fig. 10. XRD in H2 (10 h in H2 flow) at different reduction temperatures for CeNi1OY
◦
◦
◦
◦
◦
◦
catalyst. (a) 25 C, (b) 200 C, (c) 250 C, (d) 266 C, (e) 300 C, (f) 450 C. CeO2 ( ),
decomposition to CH , CO and H2 [Eq. (7)] could also occur. It has
4
0
NiO (ꢁ), Ni (᭹).
been reported that OSRE could occur to produce H2 and CO/CO2
◦
as major products with maximum H2 yield above 600 C, if CH4
is formed as an intermediate. Therefore, usually, the OSRE pro-
cess has been studied at high temperatures and low O /EtOH ratios
2
[
50–52]. However, a catalyst that has poor selectivity for ethanol
decomposition to CH4 could produce H2 and CO2 at relatively low
temperature, because thermodynamic analyses also indicated that
when acetaldehyde is formed as an intermediate, it could be read-
ily converted into H2 and CO2 via oxidative steam reforming; this
reaction being highly favorable would go to completion even at
low reaction temperatures. Selected best performing catalysts for
◦
the low temperature OSRE (300–450 C) have been proposed, and it
has been suggested that these catalysts proceed without involving
CH4 as intermediate [7].
Fig. 11. TPR profile of CeNi1OY compound.
3.4. Characterizations
insertion of Ni into the ceria phase (Ni species replacing some Ce
species), in agreement with the presence of a solid solution [67]. Ion
sputtering followed by XPS analysis allowed estimating the size of
NiO nanoparticles (2 nm) co-existing in the compound with some
larger nanoparticles evidenced by XRD and microscopy [63]. As a
matter of fact, XRD allowed an estimation of the crystallite size at
It is well known that physicochemical properties of a catalyst
play an important role in the evolution of surface reactions. The
main factor leading to a high activity i.e., ethanol conversion, and
H2 selectivity at very low temperature is the in situ activation of
◦
the solid in H2 in a temperature range between 200 and 300 C.
Different physicochemical techniques, including TPR, TGA, XRD,
Raman, XPS, ion sputtering, and TEM, were used to characterize
the CeNiX OY catalysts [63,70–72]. A relatively high surface area
about 5 nm for CeO like species and at about 10 nm for NiO species
2
for the calcined compound (Fig. 10, Table 1). The TPR profile in H of
2
the catalyst is shown in Fig. 11. A first reduction temperature peak
◦
is obtained for the CeNi OY catalyst (Table 1). A cerium nickel
is observed at 266 C. This peak is more intense for a low Ni content
1
solid solution and highly dispersed nickel oxide with ceria can be
obtained for the compound. The Raman spectroscopy results show
a strong frequency shift and broadening for the first-order F ceria
on CeNiX OY catalysts and has been attributed to the reducibility of
Ni species in solid solution and/or small NiO nanoparticles [72]. The
◦
second peak at about 370 C that increases with the Ni content on
2g
−1
peak located near 460 cm
related to fluorite nano-crystalline
CeNiX OY catalysts is attributed to larger NiO nanoparticles (10 nm)
CeO2 (Fig. 9) [64–66]. This behavior is interpreted as a result of the
visible by XRD, in good agreement with literature [72].
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◦
Fig. 12. INS spectrum of CeNi1OY treated in H2 at 250 C (after subtraction of the
Scheme 1. XM– M active site generated in H2 on cerium and nickel solid solution
Y
ꢂ
◦
spectrum obtained on CeNi1OY treated in vacuum at 200 C).
with oxyhydride formation and ethanol activation by heterolytic abstraction of a
n+
2+
ı+
m+
4+
3+
hydride species. (with ᮀ: anionic vacancy, and Ni = Ni , Ni and Ce = Ce , Ce ).
As an example a M– M is represented.
3
3
ꢂ
The Ni species from the solid solution and/or small NiO particles
are first concerned by the reduction when increasing tempera-
ture [Eq. (8)], and then those of the larger NiO particles but with
the simultaneous re-oxidation of a part of these species by reduc-
to accept hydrogen and to allow its mobility can be an important
factor. By analogy to the heterolytic dissociation of H2, heterolytic
dissociation of ethanol can be envisaged on a low coordination site
involving anionic vacancies as it has already been proposed for
oxidative dehydrogenation of propane [71] and methane activation
[72] on CeNiX OY compounds. The active nickel species belonging
to the small particles and/or to the solid solution, participating
actively to the catalytic reaction, present the characteristic of being
able to be reduced and re-oxidized easily and reversibly (redox pro-
cess), allowed by their strong interaction with Ce species. As an
example, Scheme 1 presents such a site that can be obtained in the
solid solution of cerium and nickel, where the Ni cation is replac-
ing a Ce cation inside the ceria phase. The active site is modeled by
an ensemble of two cations (nickel–cerium) in strong interaction,
4
+
3+
tion of the Ce ions in their vicinity into Ce species [Eq. (9)] as
the existence of a redox system was established [63,70,71]. For an
◦
in situ treatment temperature up to 300 C in H , metallic Ni is
2
not observed by in situ XRD in H2 (Fig. 10), therefore a “partially
reduced” solid is obtained, with mainly the generation of anionic
vacancies, which number is increased by the H2 treatment.
Ni2 + O2− + H → Ni + H O + ᮀ
+
0
(8)
2
2
(
with ᮀ: anionic vacancy)
+
0
3+
2
Ce4 + Ni ↔ 2Ce + Ni2+
(9)
◦
Moreover, during the activation treatment in H2 at 250 C, the
anionic vacancy is filled with hydride species by heterolytic split-
ting of H2 [Eq. (10)].
X
Y
which can be also generally expressed by Ni– M (where x and y are
the unsaturation degrees of each cation), as it has been proposed
previously on Ni based catalysts [69]. The possible mechanism for
ethanol transformation can be envisaged taking into account the
O2
−
M
n+
ᮀ + H → OH M
−
n+
−
H
(10)
2
X
Y
Ni– M ensemble. For example, with a lower number of anionic
As shown by INS (Fig. 12). Two new intense and large bands at
about 460 and 870 cm are observed on the solid treated in H , due
to the insertion of different hydrogen species. The hydrogen species
of the hydride nature related to the peak at about 460 cm appear
1
1
vacancies on the site ( Ni– M), acetaldehyde and H [Eq. (6)] can
−1
2
2
be produced by heterolytic abstraction of hydrogen from ethanol
[
Eq. (11)] [59]. After in situ pretreatment in H2 the solid is in
−
1
the oxyhydride form, it is partially reduced with the presence of
occluded hydrogen species which allow H2 formation at low tem-
◦
−1
clearly after a treatment in H2 at 250 C [68]. The band at 870 cm
has been previously attributed to hydrogen species related to the
presence of metallic Ni [68]. However, it has to be recalled that
metallic Ni is not observed by in situ XRD in H2 at this tempera-
ture (Fig. 10). Therefore, during the activation treatment in H2 in
X
Y
perature even in presence of O [Eq. (12)]. Each elementary Ni– M
2
ensemble is associated with a particular reaction. Depending on the
unsaturation degree of the active site, conversion of ethanol can
3
1
lead to different products. The Ni– M site can lead to the formation
of H , CO and CH [Eq. (7)] [59]. This model also presents the advan-
temperature, the CeNi OY solid becomes a hydrogen reservoir with
1
2
4
hydrogen species stored in the bulk and at the surface of the solid;
anionic vacancies are created, able to accept hydride species with
the formation of an oxyhydride [69].
tage to be in good agreement with the synergetic effect observed
when several cations with strong interactions are in presence in a
mixed oxide.
O2
−
M
n+
ᮀ + C H OH → OH M
−
n+
−
H
+ C H O
(11)
(12)
3
.5. Modeling
2
5
2
4
−
n+
−
−
n+
−
OH M
H
+ C H5OH → OH M
H
+ H + C H O
2
2
2
4
Particular active sites were modeled, involving anionic vacan-
cies, hydrogen species and cations in strong interaction whatever
in the solid solution or at the NiO and ceria, and/or solid solution
By using the exothermic reaction between hydride species of
the solid and O2 [Eq. (13)] (chemical energy) together with the
exothermic reaction between ethanol and O2 (partial oxidation),
the reaction can be performed at particularly low temperature.
(
with Ni) interfaces. Different kinds of active sites which differ from
each other in terms of the environment of Ni can exist [69]. There-
fore, H production from ethanol in presence of water (and oxygen)
2
1
2
H + O2 → OH−
−
(13)
certainly involves particular Ni species in strong interaction with
Ce species. It is clearly shown by INS that the activation treatment
in H2 allows the cerium nickel based mixed oxide to become an
oxyhydride in agreement with previous results reporting that this
catalyst is able to accept large quantities of hydrogen in the bulk
and at the surface of the solid by the presence of anionic vacan-
cies [70]. Taking into account that dehydrogenation step requires
abstraction of hydrogen species from alcohol, the ability of the solid
Moreover, it has been proposed that, depending on the O2 con-
centration, the hydride can react and lead to CH4 formation [59].
In such a case partial oxidation of methane leading to H2 and CO
cannot be discarded [73]. However, no CH4 is observed with the
high O /EtOH ratio (Fig. 8), in agreement with easy acetaldehyde
2
transformation in the presence of water and oxygen [Eq. (14)] [7].
Please cite this article in press as: C. Pirez, et al., Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.035

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9
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◦
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. Conclusion
Interesting hydrogen production from ethanol has been
obtained in SRE, POE, and OSRE at very low temperature on the
[
[
CeNi OY catalyst by applying accurate activation of the catalyst in
1
H2 and by varying reactants concentration. The treatment of the
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W. Fang gratefully acknowledges a grant from Erasmus Mundus
Tandem. C. Pirez gratefully acknowledges a grant from French min-
istry. The authors thank ILL (Institut Laue Langevin, France) for
supporting the INS experiments and the help of ILL group. The
authors would like to thank Ms. L. Burylo (for XRD), Mr. O. Gar-
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Unité de Catalyse et Chimie du Solide. Chevreul institute (FR 2638),
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Please cite this article in press as: C. Pirez, et al., Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.035