Structured Reactors as an Alternative to Fixed-bed Reactors: Influence of catalyst preparation methodology on the partial oxidation of ethanol

Article abstract of DOI:10.1016/j.cattod.2016.02.061

This work studied the effect of the preparation of monolithic catalysts for partial oxidation of ethanol. Cordierite monoliths coated with 10Ni/CeSiO_x catalyst were prepared by washcoating with two different binders (PVA and Ludox) and a direct sol–gel techniques. Scanning electron microscopy and transmission electron microscopy analyses and adherence tests revealed that structured washcoated monoliths prepared by the slurry with Ludox and direct synthesis using citrate method exhibited excellent dispersion and adherence of catalyst coatings. For the washcoated monolith obtained by slurry with PVA, large agglomerates of catalyst over the wall were observed. The tests with the monolith without catalyst revealed that POX of ethanol only takes place in the presence of the Ni-based catalysts. The monolith catalysts synthesized by slurry with Ludox and direct synthesis using citrate method exhibited a high formation of syngas. However, the preparation method of the monolithic catalysts affects product distribution at low temperature. The addition of Pt to the NiCeSiO_x monolith catalyst synthesized by citrate method significantly improved the partial oxidation of ethanol reaction.

Full text of DOI:10.1016/j.cattod.2016.02.061

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Structured Reactors as an Alternative to Fixed-bed Reactors: Influence
of catalyst preparation methodology on the partial oxidation of
ethanol
Clarissa P. Rodriguesa, Elka Kralevab, Heike Ehrichb, Fabio B. Noronhaa,∗
a National Institute of Technology, Catalysis Division, Av. Venezuela 82, 20081-312 Rio de Janeiro, Brazil
b Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
This work studied the effect of the preparation of monolithic catalysts for partial oxidation of ethanol.
Cordierite monoliths coated with 10Ni/CeSiOx catalyst were prepared by washcoating with two different
binders (PVA and Ludox) and a direct sol–gel techniques. Scanning electron microscopy and transmis-
sion electron microscopy analyses and adherence tests revealed that structured washcoated monoliths
prepared by the slurry with Ludox and direct synthesis using citrate method exhibited excellent dis-
persion and adherence of catalyst coatings. For the washcoated monolith obtained by slurry with PVA,
large agglomerates of catalyst over the wall were observed. The tests with the monolith without catalyst
revealed that POX of ethanol only takes place in the presence of the Ni-based catalysts. The monolith
catalysts synthesized by slurry with Ludox and direct synthesis using citrate method exhibited a high
formation of syngas. However, the preparation method of the monolithic catalysts affects product dis-
tribution at low temperature. The addition of Pt to the NiCeSiOx monolith catalyst synthesized by citrate
method significantly improved the partial oxidation of ethanol reaction.
Received 15 December 2015
Received in revised form 14 February 2016
Accepted 21 February 2016
Available online xxx
Keywords:
Bioethanol
Partial oxidation
Ni/CeSiOx catalyst
Hydrogen production
Monolithic reactors
Structured catalysts
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Several catalysts have been studied for the POX of ethanol
[8–18]. However, the majority of the studies about POX of ethanol
Hydrogen production through ethanol conversion reactions has
been extensively studied in the literature [1–7]. Different tech-
nologies can be used for the production of hydrogen from ethanol,
including steam reforming (SR) (Eq. (1)), partial oxidation (POX)
(Eq. (2)), and oxidative steam reforming (OSR) (Eq. (3)) [5]:
used catalysts in powder form and fixed-bed reactors and then,
hot-spot formation may occurs, which makes the reaction controll
difficult. Alternatively, the monolithic structures can be used as a
support to obtain catalytic reactors with high structural and ther-
mal stability. Monolithic reactors offer the advantage of thin walls,
high geometric surface area with enhanced heat and mass trans-
fer, low pressure drop and rapid response to transient operations
[19–21]. In addition, the easy scale-up and reduced costs of a com-
pact catalyst design contribute to the large potential of monolith
catalysts with respect to their application in hydrogen production
technologies.
Monolithic catalyst is well known from automotive and indus-
trial applications as exhaust gas converter. Structured reactors have
been used for SR and OSR of ethanol [22–25] but their applica-
tion for POX of ethanol is more scarce [19,20]. The extremely low
contact time makes monoliths suitable for POX of ethanol at high
temperature. Hotspots can be eliminated, improving the selectivity
of the reaction. POX of ethanol was carried out on Co3O4/Al2O3 and
NiO/Al2O3 catalysts washcoated on a cordierite monolith [19,20].
After 30 of time on stream (TOS), a large formation of carbon
filaments was observed, which was responsible for catalyst degra-
C2H5OH + 3H2O → 2CO2 + 6H2
(1)
C2H5OH + 1.5O2 → 2CO2 + 3H2
C2H5OH + (3 − 2x)H2O + xO2 → 2CO2 + (6 − 2x)H20 < x < 0.5
(2)
(3)
The partial oxidation (POX) of ethanol is an exothermic reaction
that exhibits quick start-up and response times. Furthermore, the
POX reactor is more compact than that of a steam reformer since the
indirect addition of heat is not needed. Therefore, POX of ethanol
provides the desired features for vehicle fuel cell applications [8].
In addition, POX of ethanol is also very attractive for solid oxide
fuel cell (SOFC) because further purification steps are not required,
which reduces the costs of the fuel processor [9,10].
∗
Corresponding author.
E-mail address: fabio.bellot@int.gov.brb (F.B. Noronha).
http://dx.doi.org/10.1016/j.cattod.2016.02.061
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: C.P. Rodrigues, et al., Structured Reactors as an Alternative to Fixed-bed Reactors: Influence of catalyst
preparation methodology on the partial oxidation of ethanol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.061

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dation. Therefore, the development of stable structured catalysts
for POX of ethanol at high temperature is still a challenge.
One crucial point for the development of structured reactors is
the deposition of a suitable and stable catalytic layer on the ceramic
support. The most known method to coat monolithic supports is
the washcoating procedure, by dipping and removing the monolith
from a solution containing the catalyst. It is probably the most ver-
satile of the methods because any kind of solution can be applied,
as suspension, colloidal or sol-gel containning the solid or precur-
sors of catalysts. The characteristic of the final coating depends on
the monolith properties, solution properties and preparation con-
ditions [21,26]. Several parameters must be controlled for obtaining
uniform, reproducible and mechanically stable washcoat; such as:
pH of the solution, particle size, viscosity, slurry concentration,
primers and binders used, etc.
For coating ready-made catalysts on a monolith, a slurry of par-
ticles with controlled size (about 5 ␮m) has to be used, with a
properly solvent and a binder agent. The first step is to ball mill
the ready catalyst to a small size, producing an easily accessible
coat layer. According to the literature [21,27,28], the adhesion of
the washcoated layer on the porous support is improved with the
reduction of particle size down to colloidal dimensions. Another
way to improve the catalyst adhesion is increasing the con-
tact surface between the particles and the support, by binding
agent addition. The binder is preferably colloidal silica or alumina,
depending on the application. Alumina has the advantage that is
more thermostable than silica, because high-temperature applica-
tions silica binder can be lost by steaming. It is worth noting that the
binder must be made by the same material as the support, because
it could influence the physical and acid/base properties of cata-
lysts causing a strong influence on the catalytic performance. The
amount of binder to be used is preferably kept minimal, even 1%wt
of total solids is sufficient to properly attach the ready catalyst to the
support, avoidding the coverage of active phase or channel blocking
[27,29–31].
second one was the co-precipitation by citrate method. Before
incorporation of catalyst into monoliths, they were washed in iso-
propanol, calcined for 2 h at 800 ◦C; and weighted for obtaining the
start pure monolith amount. A commercial 400 cells per square inch
(cpsi) cordierite monolith cylinder with a diameter of 2 cm and a
length of 6 cm from Rauschert was used.
The Ni/CeSiOx catalyst prepared in powder form and two dif-
ferente binders were used to investigate the effect of coating
procedure on the dipcoating preparation method of mono-
lithic catalyst. PVA (polyvinyl alcohol, 95%, hydrolyzed, average
MW = 100.000, Fluka) or Ludox HS-40 (colloidal silica, 40 wt.% in
water, Aldrich) were employed as a binder. For the preparation of
the washcoat slurries, the catalyst in powder form was ground by
hand into coarse powder (particle size < 100 ␮m) in a mortar and
pestle. A suspension with 10 wt.% of catalyst and 5 wt.% of binder in
distilled water was prepared. In the first step, the binder was dis-
solved in water in a 250 mL graduated bottle by stirring smoothly
with a magnetic stir bar at 65 ◦C and 150 rpm for 3 h in a water bath,
and left without stirring overnight (about 16 h). An homogeneous
clear solution was obtained [32]. After dipcoating in the slurry con-
taining the catalyst/binder and drying, the impregnated washcoats
monoliths were calcined at a heating rate of 1.7 ◦C min−1 from room
temperature to 500 ◦C, and kept for 5 h in a muffle furnace. The
coating procedure was finished when the monolith contains 2 g of
catalyst before monolith calcination.
A NiCeSiOx monolith catalyst was also prepared by the citrate
method (NiCeSiOx (CM)). Citric acid was added to an aqueous solu-
tion that contained all the required precursor salts of the support
(NH4)2Ce(NO3)6 and SiO2 (Aerosil 300), in a 9:1CeO2:SiO2 weight
ratio, and of the active phase Ni(NO3)2·6H2O (Sigma Aldrich),
as described elsewhere [9,10]. The final solution was stirred for
10 min, and then the temperature was raised to start boiling,
which was continued for 30 min to form a homogeneous chelate
between the metal cations and the citrate anions. Then the solu-
tion was concentrated by evaporation at 80 ◦C until a viscous liquid
was obtained. After dipcoating the monolith into the suspension
containing the catalyst and drying, the impregnated washcoated
monolith was calcined. The coating procedure was finished when
the monolith contains 2 g of catalyst before monolith calcination.
Then, the monolithic catalyst was calcined following the same pro-
cedure previously described.
The aim of this work is to investigate the performance of
Ni/CeSiOx and PtNi/CeSiOx washcoated monolithic catalyst for the
POX of ethanol. The monolithic catalyst was prepared by two dif-
ferent methods: (i) the traditional dip-coating procedure for the
monolith washcoat with addition of two different binders and (ii)
the direct synthesis of the mixed oxide on the monolith itself by
one step citrate route.
A 1Pt10NiCeSiOx monolith catalyst was also prepared by citrate
method (PtNiSiOx (CM)). In this case, an aqueous solution contain-
ing Ni(NO3)2·6H2O (Sigma Aldrich) and 8 wt.% H2PtCl6 in water
solution (Sigma Aldrich) was added to a citric acid aqueous solution
with all the required ions of the support. The preparation method
was similar to the one previously described.
2. Material and methods
2.1. Catalyst preparation
CeSiOx catalyst support was synthesized by co-precipitation
2.2. Adhesion test
method. An aqueous solution of Ce(NO3)3·6H2O (Sigma-
Aldrich)(0.31 mol L−1
)
was added to
a
suspension containing
Catalyst layer adhesion to the monolith ceramic walls was
tested with an ultrassonic bath treatment according to the method
described elsewhere [33], to verify the weight loss caused by
mechanical stress. After 10 min, the sample was dried and the
weight loss measured.
3 g of SiO2 (Aerosil 300) solubilized in a NaOH (Aldrich 98%)
solution (CNaOH = 0.34 mol L−1) with continuous stirring at 840 rpm
for 1 h. Finally, the solvent was removed by filtration and the solid
was washed with hot water until reach pH 7. The solid was dried at
100 ◦C for 16 h and then, calcined at a heating rate of 1.7 ◦C min−1
from room temperature to 500 ◦C, and kept for 5 h in a muffle
furnace. The nominal CeO2:SiO2 weight ratio is 8.6.
2.3. Characterization
The Ni/CeSiOx catalyst in powder form was prepared by incipi-
ent wetness impregnation of the support CeSiOx with an aqueous
solution of Ni(NO3)6H2O (Sigma Aldrich) in order to obtain 10 wt.%
of Ni. After impregnation, the samples were dried and calcined at
the same thermal treatment described for the support. The final
powder catalyst was sized in a particle range of 0.4–0.5 mm.
Monolithic catalysts were prepared using two different meth-
ods. The first one involved the washcoating by slurry, and the
The specific surface area of the powder samples was measured
by N2 adsorption-desorption isotherms at −196 ◦C on an ASAP 2420
Micromeritics instrument. Prior to the measurements, the sam-
ples were dried at 100 ◦C for 24 h, and then, submitted to in situ
treatment under vacuum at 350 ◦C. The specific area was calculated
using the BET methodology.
The chemical composition of powder sample was determined
on S8 Tiger Bruker wavelength dispersive X-ray fluorescence
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3
spectrometer (WD-XRF) with a rhodium tube operated at 4 kW. The
analyses were performed with samples (300 mg) in powder form
using a semi-quantitative method (QUANT-EXPRES/Bruker).
X-ray powder diffraction pattern of the calcined samples were
obtained using a D8 Advance Bruker diffractometer equipped with
a CuK␣ radiation source (1.5406 Å) and a nickel filter, in a 2␪ range
of 10–120 at a scanning rate of 0.02/s and a scan time of 1 s/step.
The X-ray tube voltage and current were set at 40 kV and 40 mA,
respectively. The apparent crystallite size of CeSiOx, NiO and Pt was
calculated by using a Lorentzian curve with FWHM (2␪) defined as
(180/␲)␭/cos(␪)CS L in TOPAS 4.2 (DIFFRAC-TOPAS/Bruker), where
CS L is a macros parameter.
PtNiCeSiOx_CM
NiCeSiOx_CM
* *
.
High-resolution transmission electron microscopy (HR-TEM)
was performed with an aberration-corrected JEM-ARM200F (JEOL,
Corrector: CEOS) 200 kV transmission electron microscope. The
microscope is equipped with a JED-2300 energy-dispersive X-
ray-spectrometer (EDXS) for chemical analysis. The aberration
corrected STEM imaging high-angle annular dark field (HAADF)
and annular bright field (ABF) were performed with a spot size of
approximately 0.13 nm, a convergence angle of 30–36◦, and collec-
tion semi-angles for HAADF and ABF of 90–170 mrad and 11–22
mrad, respectively. For the TEM analysis the sample was deposited
without any pretreatment onto a holey carbon supported Cu-grid
(mesh 300) and then transferred into the microscope.
Scanning electron microscopy analysis in emission field (FEG-
SEM) was performed in a Quanta 200 microscope (FEI) with
maximum operating voltage of 20 kV. The images were acquired
using EDT detector. Details of operating conditions for images
acquisition, such as pressure, spot size and working distance (WD),
and sample region extension observed are available in the micro-
graphic bar presented here.
*
Ni/CeSiOx
CeSiOx
20 40 60 80 100
2Theta / o
Fig. 1. Diffractograms of the CeSiOx (orange line) and Ni/CeSiOx (green line) pow-
der samples and the NiCeSiOx (gray line) and PtNiCeSiOx (red line) citrate method
powder sample. (*) NiO; (᭹) Pt. (For interpretation of the references to color in this
subtitle, the reader is referred to the web version of this article.)
3. Results and discussions
3.1. Characterization of powder samples
The elemental composition of the calcined CeSiOx support and
Ni/CeSiOx catalysts, their specific surface area and the XRD patterns
were measured in the powder form. The chemical composition of
the support agrees well with the nominal composition, which cor-
responds to 95 wt.% of Ce and 4.3 wt.% of Si. For the catalyst, the
nickel content was about 14 wt.%. The specific surface area of the
nickel catalysts (110 m2g−1) do not change significantly in compar-
ison with the value of the CeSiOx support (127 m2g−1) (Table 1).
As previously shown in the literature [34], doping CeO2 with Si
significantly increased the surface area, indicating high degree of
incorporation of the Si into the CeO2 structure. The diffractograms
of the support and Ni-based catalysts are shown in Fig. 1. All sam-
ples exhibits the lines characteristic of CeO2 with cubic phase [35].
In addition, the diffractogram of Ni/CeSiOx catalyst also shows the
lines of NiO phase (2␪ = 37.3◦; 43.3◦; 62.9◦) [35]. For 10NiCeSiOx
and PtNiCeSiOx powder prepared by citrate method, low intensity
lines corresponding to NiO (2␪ = 37.3◦; 43.3◦) were observed, sug-
gesting the formation of small particles of NiO. The diffraction lines
corresponding to metallic platinum were detected at 2␪ = 39.4◦ and
81.38◦.
2.4. Catalytic tests
The monolithic catalysts were tested for the POX of ethanol at
temperatures between 400 to 800 ◦C and atmospheric pressure. The
operating temperature was regulated by a thermocouple inside the
oven, which was placed closed to the reactor. A thermocouple was
inserted into the center of the monolith to measure the reaction
temperature. The samples were previously reduced under H2/N2
flow (10 vol.%, 100 mL min−1) at 600 ◦C for 1 h, followed by a N2
purge for 30 min at 600 ◦C.
In the standard test, a feed containing an oxygen-to-ethanol
molar ratio of 0.75 was used. Ethanol was pumped at a constant
rate of 10 g h−1 to the evaporator heated at 120 ◦C and then mixed
with a stream of air (290 mL min−1) and N2 (100 mL min−1) at a
GHSV = 50 h−1. The reactant composition in gas flow under normal
conditions was Ethanol:O2:N2 = 16/12/72 vol%. The catalysts were
tested step-wise at selected temperatures, maintaining the reaction
for 2 h at each temperature to check for possible deactivation.
The composition of the inlet and outlet gas mixtures was
frequently analyzed on-line using Shimadzu gas chromatographs
equipped with FID and TCD detectors. The carbon balance was
higher than 95% for all the catalytic systems tested. The products
detected by gas chromatography were hydrogen, methane, car-
bon dioxide, carbon monoxide, ethylene, ethane and acetaldehyde.
Conversion of ethanol was defined as the ratio of the consumed
moles of ethanol to the moles of ethanol in the feed. The prod-
uct distribution was defined as the ratio of moles of one product
i to total moles of products in the effluent flow free of water, as
previously described [16].
The crystallite sizes of the CeSiOx and NiO phases estimated by
Lorentzian curves are reported in Table 1. The Ni/CeSiOx prepared
by incipient wetness impregnation of the support presents small
Table 1
Specific surface area of support and catalysts and crystallite sizes of CeSiOx, NiO and
Pt phases calculated by Scherrer equation.
Sample
Surface area (m2 g−1
)
Crystallite size (nm)
CeSiOx
NiO
Pt
CeSiOx
Ni/CeSiOx
NiCeSiOx (CM)
PtNiSiOx (CM)
127
110
123
123
2.6
2.7
5.4
5.3
–
–
–
–
19.3
27.0
2.3
6.9
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Fig. 2. STEM images of the Ni/CeSiOx powder catalyst. Element mapping of nickel, CeSi mixed oxide and oxygen in powder catalyst. (For interpretation of the references to
color in the text, the reader is referred to the web version of this article.)
Fig. 3. FEG-SEM images of (a) uncoated cordierite monolith; (b) 10NiCeSiOx coated monolith with citrate method; (c) 10Ni/CeSiOx coated monolith with Ludox; (d)
10Ni/CeSiOx coated monolith with PVA.
ceria crystallite size (about 2.6 nm) and large NiO crystallites size
(27.0 nm). Regarding the NiO crystallite size, the 10NiCeSiOx CM
exhibited smaller NiO crystallite size (2.3 nm) than Ni/CeSiOx. The
addition of Pt increased the NiO crystallite size. However, the citrate
method led to smaller NiO crystallite size.
oxide support (5–10 nm) and large NiO particles (50–80 nm). These
results are in agreement with XRF and XRD experiments.
3.2. Characterization of monolith catalysts
Fig. 2 presents STEM images of Ni/CeSiOx powder catalyst cal-
cined at 500 ◦C. The micrographs clearly show large Ni particles
(red) over the mixed oxide support (Ce-Si green and O blue).
The elemental composition of the sample was confirmed by EDX
measurements using a finely focused beam. According to element
distribution, this catalyst consists of a small particles of Ce-Si mixed
Table 2 presents the results obtained for coating procedure of
monolith using the washcoating with a citrate solution and with
a slurry contaning the ready-made catalyst and binder (Ludox or
PVA). The monoliths were dipped vertically into the citric acid
sol-gel solution and kept for 16 h and into the slurry for 10 min.
This procedure was repeated until the increase in weight of the
monolith achieves 2 g due to the addition of the coating material.
Guiotto et al. [33], suggests that the precursor solution in the direct
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Table 2
Amount of catalyst deposited on monolith after washcoating and the weight loss for the different washcoating procedures employing citric method, slurry with Ludox and
slurry with PVA as a binder.
Sample
% Material washcoated
% weight loss
Citrate method
4.8
0.095
Ludox slurry
4.2
0.086
PVA slurry
3.3
0.558
synthesis procedure needs to wet the supporting surface, beginning
the deposition on the defects. The initial step of the deposition could
be rather slow and difficult, due to preliminary diffusion of the pre-
cursors into the pore of the monolith. For this reason, the immersion
step for citrate method was longer than the slurries washcoating.
After calcination at 500 ◦C, the amount of catalyst deposited dur-
ing washcoating procedure was 0.77; 0.71 and 0.53 g for citrate
method and for slurry with Ludox and PVA, respectively. The first
characterization of the monolithic catalyst is visual. The images of
the monolith obtained by citrate method and Ludox slurry show
an uniform brown collored catalytic coating, while the image of
the monolith prepared with PVA exhibits a non uniform collored
coating distribution with some channels blockage.
The degree of adhesion of the catalytic layer was evaluated
by measuring the sample weight loss after being immersed in
isopropanol and exposed to ultrasonic vibration. After 10 min of
exposure to mechanical vibrations, the washcoated monolith with
citrate method, Ludox and PVA showed 0.095%; 0.086% and 0.558%
of weight loss (Table 2), respectively, that correspond to a loss of 2%
of material deposited for citrate method and Ludox and 17% for PVA.
Similar weight loss is reported in the literature [27] for a monolith
coated with 7% of zeolite BEA. The authors considered this loss very
small, when compared with weight loss occurred for an uncoated
cordierite monolith. PVA presented higher loss of the washcoating
material due to a lower strenght of adhesion of the catalytic layer.
Cordierite monolith before and after washcoating procedure
was sectioned in 2 × 2 cm small pieces, and they were character-
ized in a Quanta 200 microscope (FEI) both by backscattered and
secondary electrons imaging and X-ray energy dispersive spec-
troscopy to investigate the distribution of coatings on monoliths
wall, and the results are shown in Fig. 3. It is clear that the binder
addition in the washcoat slurry cause an accumulation of the coat-
ing material inside the channels by the following order: Citrate
Method (Fig. 3b) <Ludox (Fig. 3c) <PVA (Fig. 3d).
Fig. 4 shows the coating morphologies of the catalytic layers
obtained by the different washcoating methods. Sol-gel solution
allows to penetrate the defects of cordierite ceramic structure
(Fig. 4(d–f)) where the surface cavities (black holes) could be clearly
observed. This result suggests that a homogeneous thin cracked
layer of a mixed oxide was formed over all cordierite support.
For Ludox binder (Fig. 4g-i), it is observed that a thin layer with
small particles was formed on the cordierite surface. For PVA,
Fig. 4(j–l) shows a catalyst aglomeration and large particles of cat-
alyst deposited in the ceramic surface. These results agree very
well with the literature that reported the surface defects could be
observed clearly on the channel walls, when the binder amount is
less than 20%, [13].
In fact, according to Liu et al. [29] the degree of dispersion of
the catalytic material particles in the washcoating solution affects
the distribution of the catalytic layer on the surface of monolith.
In addition, more homogeneous distribution of colloidal particles
may help to form more uniform coating [33]. The slurry with Ludox
led to a more homogeneous coating, with well dispersed particles,
than the slurry with PVA.
Fig. 5 shows in detail one cordierite defect, which was wash-
coated with the slurry containing Ludox binder. According to
literature [29], the penetration of small particles into monolith cavi-
ties leads to increase in the adherence of catalyst coating. This result
is in agreement with the data of adhesion test (Table 2). Smaller
weigth losses were observed for citrate method and Ludox slurry
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Fig. 4. FEG-SEM images of pure Cordierite monolith (200× (a); 500× (b); 20,000× (c)) and NiCeSiOx citrate method monolith (200x (d); 500× (e); 20,000× (f)) and Ni/CeSiOx
Ludox slurry monolith (200x (g); 500× (h); 20,000× (i)) and Ni/CeSiOx PVA slurry monolith (200x (j); 500× (k); 20,000× (l)). (pressure: 130 Pa; spot size: 3.5; WD: 13.8 mm).
washcoated monoliths, that exhibits smaller particles in the coat-
ing layer. On the other hand, the large particles formed using PVA
as a binder decreases the strenght of adhesion layer; forming an
irregular coating and lowering the interaction among the particles
in the slurry.
The spatial distribution of the coating elements within the
monolith channel is also investigated by EDX mapping. It is impor-
tant to stress that these areas were selected at positions far away
from the meniscus, which develops at the corners of the monolith
channels. At these positions the action of the suface tension forces
that appear during the drying step of the slurry tends to concen-
trate the catalyst powders at values above the average on the rest
of monolith walls [31]. The differences in the coated surface mono-
liths with various washcoating procedures are shown in Fig. 6. In
principle, the direct synthesis (Fig. 6a) resulted in a more uniform
and complete covering of the monolith. For Ludox slurry coating
(Fig. 6b) it is possible to see the effect of surface tension forces, due
to a large concentration of catalyst coating present at the channel
outlet and the wall corner. The PVA slurry coating (Fig. 6c) exhibits
the most heterogeneous catalyst layer, revealing the presence of
regions (dark parts) with a little content of catalyst particles.
The element mapping of the monolith prepared by citrate
method is presentedin Fig. 7, which shows the pixels corresponding
to aluminium (red) and magnesium (orange) from the monolithic
support; to cerium (yellow), silicium (pink) and nickel (green)
from the catalyst coating. From these results it is clear that the
layer deposited on the surface is less compact and very thin, due
to intense pixels to aluminium and magnesium elements from
cordierite. Silicium signal shows two magnitude, one intense cor-
responding to the monolith surface composition, and other weak
corresponding to the silicium element present in the mixed oxide.
It is also interesting to note that there is an overlapping of nickel
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Fig. 5. Detail of cordierite cavity: penetration of catalyst small particles from the
slurry with Ludox binder.
and cerium signals, suggesting that NiO layers are in contact with
the surface of CeSiOx all over the monolith area.
The elements distribution for Ni/CeSiOx slurry Ludox coated
monolith for one region with a good channel coverage by the
catalyst particles is presented in Fig. 8. Unlike the previous sam-
ple, this one features large yellow, pink and green area where
the signals overlap; corresponding to cerium, silicon and nickel
elements present in the initial powder catalyst. Concerning the
cordierite support, signals with weak intensity are observed for alu-
minium and magnesium elements, suggesting a good dispersion of
Ni/CeSiOx catalyst over the monolith surface.
The element mapping of Ni/CeSiOx slurry PVA monolith is
presented in Fig. 9. Some regions of the surface only show pix-
els corresponding to cordierite support elements (Al (red); Mg
(orange); Si (pink)). It is possible to identify fewer black areas
that correspond to CeSiOx aggregated particles of the catalyst. This
results differs significantly from that ones corresponding to a more
intermixed and homogeneous distribution of the others two types
of washcoating procedures.
The element mapping of the PtNiCeSiOx monolith is presented
in Fig. 10, which shows the pixels corresponding to aluminium (red)
and magnesium (orange) from the monolithic support; to cerium
(yellow), silicium (pink), nickel (green) and platinum (light cyan)
from the catalyst coating. From these results it is clear that the layer
deposited on the surface is more compact, due to less intense pixels
related to aluminium and magnesium elements from cordierite.
Silicium signal shows an intense magnitude corresponding to the
element present in the mixed oxide because this is overlapping of
cerium signal. It is also interesting to note that the nickel signal
shows evidence that the NiO layer is in contact with the surface of
CeSiOx all over the monolith area. Less intense pixels to platinum is
observed, appearing as large, isolated, aggregates. The dimension
of the individual crystallites of metallic paltinum phase, estimated
by Lorentzian curve, was about 19 nm (Table 1).
Fig. 6. FEG-SEM images of (a) 10NiCeSiOx citrate method; (b) 10Ni/CeSiOx Ludox
slurry; (c) 10Ni/CeSiOx PVA slurry monolith (Amplification: 60×).
Therefore, the characterization experiments of monolithic cat-
alysts revealed that: (i) the amount of catalyst deposited on the
monoliths and the degree of adhesion of the catalytic layer are
determined by the preparation procedure employed; and (ii) the
distribution of the catalytic layer on the surface of monolith
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Fig. 7. Element mapping of aluminium, magnesium, cerium, silicium and n ickel on NiCeSiOx citrate method monolith. (For interpretation of the references to color in the
text, the reader is referred to the web version of this article.)
strongly depend on the type of the binder used for preparation of
washcoted slurry.
Since the monolithic catalysts prepared by the citrate and slurry
with Ludox methods exhibited a more uniform, stable and complete
covering of the monolith, they were tested on the POX of ethanol
reaction.
the conditions identical to those used in catalytic POX of ethanol.
Ethanol conversion and products distribution for the non-catalytic
oxidation of ethanol at different reaction temperatures are shown
in Fig. 11. At 400 ◦C, ethanol conversion is low (<10%) and ethanol
is only dehydrogenated to acetaldehyde and hydrogen (Eq. (4))
in the absence of catalyst. Increasing the temperature to 500 ◦C
significantly increased ethanol conversion that achieved 100%. In
addition, acetaldehyde is no longer observed and H2, CO, and CH4
were the main products formed. These results suggest that the
homogeneous gas-phase reaction at low temperature is character-
ized by the dehydrogenation of ethanol to H2 and acetaldehyde (Eq.
(4)), which is decomposed to CO and CH4 at temperatures above
3.3. Catalystic tests
For a better understanding of the catalytic processes occur-
ring on monolith catalysts, ethanol conversion reaction was first
performed for the monolith without any active materials under
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Fig. 8. Element mapping of aluminium, magnesium, cerium, silicium and nicke l on Ni/CeSiOx slurry Ludox monolith. (For interpretation of the references to color in the
text, the reader is referred to the web version of this article.)
500 ◦C (Eq. (5)). Similar results were observed in the literature [20]
employing cordierite monolith without catalyst layer, evidencing
that the homogeneous gas phase reaction comprises the decompo-
sition of ethanol. Only trace amount of CO2 was detected, indicating
that the water gas shift or combustion reactions does not occur in
a significant extent in the absence of catalyst. Therefore, the POX
of ethanol reaction did not take place in the whole range of studied
temperatures on the pure monolith.
The ethanol conversion and products distribution as a function of
reaction temperature for monolithic catalysts prepared by citrate
and slurry with Ludox methods are shown in Fig. 12(a) and (b),
respectively. For both monoliths, H2, CO2, CO and CH4 were the
main products formed, indicating that POX of ethanol, water gas-
shift and acetaldehyde decomposition reactions take place.
Increasing the reaction temperature increased ethanol conver-
sion that was complete at 600 ◦C. Furthermore, the amount of H2
and CO formed increased whereas the production of CO2, acetalde-
hyde and ethylene decreased. At 600 ◦C, acetaldehyde and ethylene
were no longer observed, and methane achieved a maximum for-
mation. Further increase in the temperature favors the POX of
ethanol reaction. The preparation method of the monolithic cata-
C2H5OH(g) → C2H4O(g) + H2(g)
C2H4O(g) → CH4(g) + CO(g)
(4)
(5)
In order to verify the potential of the monoliths washcoated as a
catalyst for syngas production, the POX of ethanol was performed.
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Fig. 9. Element mapping of aluminium, magnesium, cerium, silicium and nick el on Ni/CeSiOx slurry PVA monolith. (For interpretation of the references to color in the text,
the reader is referred to the web version of this article.)
lysts affects product distribution at low temperature (500–600 ◦C).
The monolithic catalyst prepared by citrate method exhibited
a higher formation of H2 and lower production of methane at
this temperature range, indicating that POX of ethanol reaction
is favored over this monolithic catalyst. This is likely due to the
smaller NiO crystallite size of this catalyst as revealed by XRD.
Moreover, the addition of catalyst to the monolith increased the
formation of H2, CO and CO2, promoting the POX of ethanol and
WGS reactions instead of the decomposition reaction and inhibited
the formation of acetaldehyde. Mattos and Noronha [8,12] reported
that the POX of ethanol in the absence of catalyst led mainly to
the formation of acetaldehyde, methane and ethylene. When the
reaction was carried out over Pt/CeO2 catalyst, the formation of
acetaldehyde significantly decreased whereas the production of H2
and CO increased. Based on TPD, TPSR and IR experiments, a mech-
anism for the POX of ethanol reaction was proposed. According
to this mechanism, ethanol adsorbs as ethoxy species that can be
dehydrogenated to acetaldehyde. This can be decomposed to CH4
and CO or it can oxidized to acetate species by oxygen from the feed
or the ceria support. The acetate species can be decomposed to CH4,
CO and/or oxidized to CO2 via carbonate species. Our results are in
agreement with the mechanism proposed in the literature.
In addition, the effect of Pt addition on the performance of
NiCeSiOx CM monolithic catalyst (Fig. 12a) for POX of ethanol
is investigated in the temperature range of 500–700 ◦C, and the
results are presented in Fig. 13. The PtNiCeSiOx monolithic catalyst
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Fig. 10. Element mapping of aluminium, magnesium, cerium, silicium, nickel and platinum on Pt NiCeSiOx monolith. (For interpretation of the references to color in the text,
the reader is referred to the web version of this article.)
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100
80
60
40
20
0
C.P. Rodrigues et al. / Catalysis Today xxx (2016) xxx–xxx
ethanol
ethanol
H2
100
H2
CO
CO
80
60
40
20
0
CO2
CH4
CO2
CH4
ethylene
ethylene
acetaldehyde
acetaldehyde
400 500 600 700 800
Tmonolith / oC
500
600
monolith
700
T
/°C
Fig. 11. Ethanol conversion and products distribution as a function of reaction
temperature for POX of ethanol reaction over blank monolith (T = 400–8 00 ◦C;
O2/EtOH=0.75; P = 1 bar; GHSV = 50 h−1).
Fig. 13. Ethanol conversion and products distribution as a function of reaction
temperature for PtNiCeSiOx CM for POX of ethanol reaction (T = 600–8 00 ◦C;
O2/EtOH = 0.75; P = 1 bar, GHSV = 50 h−1).
prepared by citrate method exhibited complete ethanol conversion
at all reaction temperatures, and the main products were H2, CO,
CO2 and CH4. Increasing the temperature, significantly increased
the formation of H2 and CO whereas the production of methane
and CO2 decreased. Furthermore, acetaldehyde and ethylene were
not formed even at 500 ◦C. A comparison between the product dis-
tribution obtained for the monometallic (Fig. 12a) and bimetallic
(Fig. 13) monolithic catalysts reveals that Pt addition promoted the
POX of ethanol reaction, reduced the production of methane and
inhibited the formation of acetaldehyde. In addition, the highest
syngas production occurs at high temperature (700 ◦C).
The type of the metal affects signficantly the mechanism of POX
of ethanol [12–14]. Ru/Y2O3 catalyst showed a highest selectiv-
ity to syngas than Pd/Y2O3 catalyst for POX of ethanol at 800 ◦C.
Oxidative dehydrogenation of ethanol to acetaldehyde was favored
over Co/CeO2 catalyst whereas ethanol decomposition took place
over Pd/CeO2 catalyst during POX of ethanol at low tempera-
ture (300 ◦C). These results were explained based on the reaction
mechanism previously proposed [8]. Recently, we studied the
effect of Pt addition to a Ni/CeO2 catalyst for the low tempera-
ture steam reforming (LTSR) [36]. Mechanistic studies using mass
spectroscopy and DRIFTS techniques revealed that Pt enhanced
the decomposition rate of acetaldehyde, which could explain the
absence of acetaldehyde formation on the bimetallic monolithic
catalyst. Furthermore, the segregation of Pt on the surface of Ni
particles promoted the decomposition of dehydrogenated and
acetate species to hydrogen, methane, CO and carbonate species
at low temperature. This might favor the syngas production at high
temperature.
4. Conclusions
Ni/CeSiOx monolithic catalysts were successfully prepared
employing the traditional dip-coating procedure with addition
of Ludox binder and the direct synthesis of the mixed oxide in a
single step through a citrate route. Both procedures resulted in
an uniform and homogeneous catalyst layer with high adherence.
The FEG-SEM microscopy revealed a good adherence and catalyst
distribution over the monolith surface. For the washcoated mono-
lith obtained by slurry with PVA, large agglomerates of catalyst
over the wall were observed. The tests with the monolith without
catalyst revealed that the homogeneous gas-phase reaction at low
temperature is characterized by the dehydrogenation of ethanol
to H2 and acetaldehyde, which is decomposed to CO and CH4 at
high temperatures. The monolith catalysts synthesized by slurry
with Ludox and direct synthesis using citrate method exhib-
ited a high formation of syngas, indicating that POX of ethanol
only takes place in the presence of the Ni-based catalysts.
ethanol
100
80
60
40
20
0
100
(b)
(a)
H2
CO
80
60
40
20
0
CO2
CH4
ethylene
acetaldehyde
500
600
monolith
700
500
600
700
T
/°C
T
/°C
monolith
Fig. 12. Ethanol conversion and products distribution as a function of reaction temperature for monolithic catalysts prepared by (a) citrate method and (b) washcoating with
Ludox slurry for POX of ethanol reaction (T = 500–700 ◦C; O2/EtOH = 0.75; P = 1 bar, GHSV = 50 h−1).
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Acknowledgements
Financial support of this work was provided by the DFG (GZ
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Please cite this article in press as: C.P. Rodrigues, et al., Structured Reactors as an Alternative to Fixed-bed Reactors: Influence of catalyst
preparation methodology on the partial oxidation of ethanol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.02.061