Highly stable nano Ce-La catalyst for hydrogen production from bio-ethanol

Article abstract of DOI:10.1039/c4ra12679c

The catalytic activity of a metal free nano particle, whose size ranged from 1-10 nm, was studied according to HRTEM and DLS results. The effect of reaction temperature (300-600°C) on the catalytic activity of the prepared catalysts was studied in a flow system under atmospheric pressure with an ethanol-water molar ratio of 1:10. Selectivity was calculated for the products H₂, CO, CO₂ and CH₄, as well as the intermediates C₂H₆, C₂H₄, CH₃CHO and CH₃COCH₃, at different reaction temperatures. It was found that the addition of La increased the activity and stability of the Ce catalyst. H₂ was produced at the lowest temperature (300°C) over Ce-La_0.2 & Ce-La_0.5 catalysts. Complete conversion of ethanol with high hydrogen selectivity was obtained at 500°C over Ce-La_0.2 catalysts. Further increases of La content lead to decrease in the catalyst activity in accordance with the decrease of surface area to 9.6 m² g^-1. To study the stability of the prepared catalyst, the ESR reactions of Ce-La_0.2 and Ce-La₁ catalysts were investigated at 500°C for 15 h. It was found that the Ce-La_0.2 catalyst is more stable than the Ce-La₁ catalyst over entire 15 h. Finally, we can say that lanthanum oxide (La₂O₃) is particularly suitable as a modifier due to its effectiveness in the prevention of sintering and improvement of thermal resistance at high temperatures. This oxide is also able to gasify the coke through the formation of La₂O₂CO₃ oxy-carbonate.

Full text of DOI:10.1039/c4ra12679c

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Highly stable nano Ce–La catalyst for hydrogen
production from bio-ethanol
Cite this: RSC Adv., 2015, 5, 4292
Dalia R. Abd El-Hafiz, Mohamed A. Ebiad,* Radwa A. Elsalamony
and Lamia. S. Mohamed
The catalytic activity of a metal free nano particle, whose size ranged from 1–10 nm, was studied according
ꢀ
to HRTEM and DLS results. The effect of reaction temperature (300–600 C) on the catalytic activity of the
prepared catalysts was studied in a flow system under atmospheric pressure with an ethanol–water molar
2 2 4
ratio of 1 : 10. Selectivity was calculated for the products H , CO, CO and CH , as well as the intermediates
C
H
2 6
, C
H
2 4
3 3 3
, CH CHO and CH COCH , at different reaction temperatures. It was found that the addition of
ꢀ
La increased the activity and stability of the Ce catalyst. H
2
was produced at the lowest temperature (300 C)
over Ce–La0.2 & Ce–La0.5 catalysts. Complete conversion of ethanol with high hydrogen selectivity was
ꢀ
obtained at 500 C over Ce–La0.2 catalysts. Further increases of La content lead to decrease in the
2
ꢁ1
catalyst activity in accordance with the decrease of surface area to 9.6 m
the prepared catalyst, the ESR reactions of Ce–La0.2 and Ce–La
for 15 h. It was found that the Ce–La0.2 catalyst is more stable than the Ce–La
Finally, we can say that lanthanum oxide (La ) is particularly suitable as a modifier due to its
effectiveness in the prevention of sintering and improvement of thermal resistance at high temperatures.
This oxide is also able to gasify the coke through the formation of La CO oxy-carbonate.
g
. To study the stability of
ꢀ
1
catalysts were investigated at 500
C
Received 19th October 2014
Accepted 21st November 2014
1
catalyst over entire 15 h.
2 3
O
DOI: 10.1039/c4ra12679c
www.rsc.org/advances
O
2 2
3
13,14
performance of Pt electrodes in fuel cell systems.
Moreover,
1
. Introduction
steam reforming is a complex reaction and several side reac-
The reduction of greenhouse gas emissions from the exhaust of tions can take place, forming undesired by-products (such as
4
vehicles is one of the most important challenges, and it has acetaldehyde, ethylene, methane and acetone) that lower the
recently attracted increasing attention. Hydrogen vehicles will overall H
yield and can get converted to coke deposits, result-
2
15
provide higher vehicle efficiency along with signicant reduc- ing in catalyst deactivation. Therefore, reducing the undesired
1
–3
tions in emissions. Thus, hydrogen is a promising alternative byproducts is a very important issue in low-temperature
fuel and has been an energy carrier for decades; it has been used hydrogen production.
for the generation of electricity in fuel cells and as fuel for
transportation without CO emissions. Currently, the produc- ethanol is the use of appropriate catalysts. Several types of
Accordingly, the major challenge in steam reforming of bio-
2
4,5
tion of hydrogen is mainly obtained from fossil fuels: 48% supported transition and noble metals have been studied as
from natural gas, 30% from oil-derived naphtha and 18% from active and selective catalysts in the ESR system, but improve-
coal, which results in the co-production of CO
hydrogen market requires its production to be performed in a deposition.
sustainable way from renewable raw materials. Thus, numerous pure cerium dioxide is highly discouraged because its thermal
. The future ment of their stability is required due to sintering and carbon
2
16,17
Despite its widespread applications, the use of
4
,6–11
researchers
ethanol as a way to replace fossil fuels.
Steam reforming of bio-ethanol (C H OH + 3H O / 6H + characteristics. The modication of ceria with other metals, like
have studied the catalytic reforming of bio- stability is poor because it undergoes sintering at high
temperatures, thus losing its crucial oxygen storage and release
2
5
2
2
4
+
4+
4+
3+
2
CO ) over supported metal oxide leads to high H yields at Zr , Hf , Ti , and La , is a key issue to overcome the afore-
2
2
12
reasonable costs. However, the endothermic nature of this said drawbacks. Lanthanum oxide (La O ) is particularly suit-
2 3
ꢀ
ꢁ1
reaction (DH298 K ¼ 174.4 kJ mol ) requires high temperatures able as a modier due to its effectiveness in preventing
(
700–1000 K) to achieve good ethanol conversion. This high sintering and improving thermal resistance at high tempera-
18,19
temperature favors the reverse water–gas shi reaction (CO
2
+
tures.
This oxide is also able to gasify the coke through the
CO oxy-carbonate species.
Ceria nano-particles are synthesized by various aqueous
H
2
/ 4CO + H
2
O) and thus generates CO, which degrades the formation of La
2
O
2
3
20
solution methods such as hydrothermal synthesis, precipita-
Egyptian Petroleum Research Institute, Cairo, 11727, Egypt. E-mail: m_a_ebaad@
yahoo.com; Fax: +20 222747433; Tel: +20 01274445773
21
22
23
tion, forced hydrolysis, and electrochemical synthesis.
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However, ceria nano-particles prepared by aqueous solution nitrogen through an airbrush, using air as a carrier. Then, the
methods exhibit hard agglomeration between adjacent particles frozen droplets were placed under vacuum for about 36 h.
ꢀ
due to hydrogen bonds and a huge capillary force during Finally, the catalysts were calcined in air ow at 500 C for 4 h.
24
drying. Freeze-drying technology is an advanced technology
for metal oxide preparation; it imparts two main advantages: it
endows materials with higher surface area and improves the
2
.2. Catalyst characterization
25
uniformity of distribution. The process is simple to operate,
Thermal gravimetric analysis (TGA). Thermal gravimetric
and it has been used in the preparation of ultra-ne metal analysis (TGA) was carried out using a SETARAM Labsys TG-
26
powders (i.e., it is easy to control the nucleation), and catalytic DSC16 equipment in the temperature range from room
27
28
ꢀ
materials. Xiaoli Xi, in her earlier research, used this method temperature to 1000 C under a nitrogen ow to follow the
to prepare a series of nano rare-earth tungsten powders thermal stability of the prepared catalyst.
29–31
(
W–La O , W–Y O , W–CeO ) and nickel powders.
By spraying
a solution or a powder suspension into liquid nitrogen, the drops (XRD) was carried out by a Shimadzu XD-1 diffractometer using
granules) were instantaneously frozen. In a subsequent freeze- a Cu-target and Ni-ltered radiation to trace the various changes
X-ray diffraction analysis (XRD). X-ray Diffraction Analysis
2
3
2
3
2
(
drying cycle, the granules were dried by the sublimation of the in the crystalline structure and the different phases that occur
ice without any segregation effects, as in the case of conventional during preparation. The sample powders were packed in a glass
drying in the air. The process resulted in the formation of holder during the measurement of the diffraction intensity with
ꢀ ꢀ
spherical and free owing granules with optimal homogeneity.
a step scanning in the 2q range between 4 and 90 . The phase
Thus, in this work, a smart ultra-ne Ce–Lax catalyst identication was made by comparing the results with the Joint
prepared by the spray freeze-drying method, without metal Committee on Powder Diffraction Standards (JCPDS). The
addition, was used in ethanol steam reforming reaction to crystalline size, DXRD, was calculated according to the Scherrer
produce hydrogen. Three CeO
2
La
x
O
1.5 solid solutions were equation: DXRD ¼ 0.9l/B cos q, where l is the wavelength of the
prepared with different La content (x ¼ 0.2, 0.5 and 1). The radiation, and q is the diffraction angle. B is the corrected half-
prepared catalysts were investigated using XRD, TGA, BET width of the diffraction peak. The reection from the (111)
surface area, DLS techniques and HRTEM. The catalysts were plane was used to calculate the crystallite size. The geometrical
tested in a bio-ethanol steam reforming reaction, to observe specic surface area (SXRD) was also determined, assuming
3
increase in ethanol conversion and hydrogen yield and decrease spherical particles, according to the equation S
in CO by-product and coke deposition.
¼ 6 ꢂ 10 /
XRD
ꢁ
3
rDXRD, where r (g cm ) is the theoretical density. Scherrer
analysis was performed to determine the average crystalline
domain size, which was calculated by the Scherrer formula (D ¼
2. Experimental
0
.89l/b cos q) with the FWHM and a peak position (2q) of
2
.1. Catalyst preparation
several peaks, where D is the crystallite size, l is the wavelength
of the radiation, b is the corrected peak width at half-maximum
intensity, and q is the peak position. The average value is given
as the mean crystallite size.
Three Ce–La
x
(x ¼ 0.2, 0.5 and 1) solid solution catalysts were
prepared by the spray freeze-drying method. Fig. 1 shows the
schematic diagram of the main procedure of the method, which
mainly involves three steps: freeze granulation, vacuum drying
and heat treatment.
Temperature programmed reduction (TPR). Temperature
programmed reduction (TPR) measurements were carried out
to investigate the redox properties (the ease of reducibility of the
metal oxide) over the resultant materials. The experiments were
performed in automatic equipment (ChemBET 3000, Quan-
tachrome). Typically, 100 mg of a pre-calcined sample was
loaded into a quartz reactor and pretreated by heating under an
Using Cerium nitrate (Ce(NO ) $6H O, 99% Fluka) and
3
2
2
lanthanum nitrate (La(NO ) $6H O, 99% Cambrian Chemicals)
as a precursor, three solutions containing 0.05 M ceria with
3
3
2
different lanthanum molar ratios (Ce : La ¼ 1 : 0.2, 1 : 0.5, and
1
: 1) were prepared. The solutions were dispersed in liquid
ꢁ1
ꢀ
inert atmosphere (20 ml min nitrogen) at 200 C for 3 h prior
to running the TPR experiment, and then it was cooled to room
temperature in N
2
. Then, the sample was subjected to a
ꢀ
ꢁ1
ꢀ
constant rate of heat treatment (10 C min to 1000 C) in a gas
ꢁ
1

ow (80 ml min ) of a hydrogen/nitrogen (5/95 vol%) mixture
as the reducing gas. A thermal conductivity detector (TCD) was
employed to monitor the amount of hydrogen consumption.
High resolution transmission electron microscopy (HRTEM)
and energy dispersive X-ray (EDS). High resolution trans-
mission electron microscopy (HRTEM) and energy dispersive X-
ray (EDS) spectroscopy were conducted using a JEOL 2100F TEM
at an accelerating voltage of 200 kV. To prepare the TEM
samples, a dilute particle–ethanol colloidal mixture was ultra-
sonicated for 30 min and a drop of the solution was placed on a
carbon coated Cu TEM grid.
Fig. 1 Spray freeze-drying method.
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The textural properties. The textural properties were deter- quartz particles were used for catalytic tests. Prior to a run, the
ꢀ
mined from the N
2
adsorption–desorption isotherms measured catalyst was in situ reduced at 500 C for 2 h under a hydrogen ow
ꢀ
3
ꢁ1
at the liquid nitrogen temperature (ꢁ196 C) using a NOVA2000 rate of 40 cm min . A water–ethanol mixture (EtOH–H
O ¼
2
gas sorption analyzer (Quanta Chrome Corporation) system. All 1 : 10 mol/mol) premixed in a separate container was fed into a
ꢀ
ꢀ
the samples were degassed at 200 C for 17 h in nitrogen pre-heater made in a SS316 reactor (at 150 C) by a dosing pump
atmosphere prior to adsorption to ensure a dry and clean (Model RP-G6; FMI, USA) with a ow rate of 0.1 ml min . Nitrogen
surface. The adsorption isotherm was constructed as the was used as the carrier gas with a ow rate of 40 cm min
ꢁ1
3
ꢁ1
.
3
ꢁ1
ꢀ
volume adsorbed (V cm
pressure P/P , where P is the equilibrium pressure and P
saturated vapor pressure of nitrogen. The BET surface area (Agilent 6890 plus HP, Varian Natural Gas Analyzer type C model
BET) was calculated using the ve-point Brunauer–Emmett– CP-3800). The conversion and selectivity to products formed
Teller (BET) theory. Mean particle size (DBET) was calculated during the reaction were calculated according to a previous
g
) versus the equilibrium relative The reaction temperature was varied from 300 to 600 C. The
is the product stream was analyzed by two gas chromatographs (GC)
o
o
(S
3
2
from the BET data according to DBET ¼ 6/(rSBET).
.3. Catalytic activity
report.
2
3
3
. Results and discussion
Bio-ethanol steam reactions were carried out at atmospheric
pressure in a continuous ow system consisting of a xed-bed
tubular reactor (schematically represented in Fig. 2). 0.5 g of
.1. Catalyst characterization
Energy dispersive X-ray spectroscopy (EDX). Energy disper-
catalyst (size: 50–80 mesh) diluted with 0.5 g of same sized sive X-ray spectroscopy (EDX) pattern (Fig. 3) shows that there
Fig. 2 Schematic diagram of a catalytic flow system.
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Fig. 3 EDX pattern of the prepared Ce–La catalysts.
are peaks at about 1.1 and 4.8 keV corresponding to Ce and
others around 0.8 and 4.6 keV corresponding to La. As stated
before, CeO
drying method with different Ce : La molar ratios (where, x ¼ 0,
.2, 0.5 and 1). From Table 1, it can be found that a mean atomic
2 x
–La O1.5 oxides were prepared by a spray freeze
0
ratio was obtained in excellent agreement with the nominal
composition used in the synthesis. It is worth noting that EDX
shows the elemental analysis of one surface point only; thus, the
average results of ve points were tested, and they are shown in
Table 1.
Thermal gravimetric analysis (TGA). Thermal gravimetric
analysis (TGA) of the dried samples was performed to determine
the correct calcination temperature. The TGA proles (Fig. 4) of
the prepared Ce–La catalysts from room temperature to
x
ꢀ
ꢀ
1
000 C show two steps weight loss: a rst decrease at 100 C
due to the adsorbed water on powder, and then a smaller
decreasing step at 350 C. The total weight losses were 25.27%,
Fig. 4 TGA profiles of the prepared samples.
ꢀ
1
32.08% and 43.77% for Ce–La0.2, Ce–La0.5 and Ce–La , respec-
tively. The weight loss and the corresponding endothermic
reactions suggested the decomposition reactions of the
compound or mixture. Moreover, the small exothermic peak
ꢀ
detected at 300 C may be due to a crystallization phenomenon
33
and due to the formation of a Ce–La solid solution. The
decomposition was complete at 500 C; thus, all the samples
ꢀ
were calcined at this temperature.
X-ray diffraction (XRD). X-ray diffraction (XRD) was used to
Table 1 Ce : La atomic ratio
determine the nature of the phases present in Ce–La (Ce : La
x
atomic ratio 1 : 0, 1 : 0.2, 1 : 0.5 and 1 : 1) samples (Fig. 5). For
all the catalysts, peaks were detected corresponding to the
Atomic%
Element
Ce–La0.2
Ce–La0.5
Ce–La
1
typical face centered cubic (FCC) uorite structure of CeO
2
34
(
JCPDS 00-002-1306). The (111) plane for CeO
2
uorite struc-
La
Ce
O
6.3
33.7
60
12.8
27.2
60
19.6
20.4
60
ꢀ
ture (28.507 in CeO
2
) shied to lower angles as the La content
ꢀ
ꢀ
ꢀ
increased (28.31 , 28.09 and 27.4 for Ce–La0.2, Ce–La0.5 and
La/Ce atomic ratio
0.188
0.471
0.965
Ce–La
, respectively), which is also reected by an increase in
1
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that of the pure La
tion of Ce (0.97) cations into the La (1.10 A) cations to form a
2
O
3
phase. This may refer to the incorpora-
4
+
3+
˚
La–CeO solid solution in a hexagonal phase structure (larger
2
37
particle size). Wilkes et al. reported that in Ce1 La O
ꢁx
x
2ꢁx/2
oxides system, at a low ionic fraction of lanthanum (0 < x< 0.5),
lanthanum dissolved in ceria with lanthanum segregation
occurring on its surface; however, at a high lanthanum content
(
x > 0.9), cerium dissolved in lanthanum with ceria segregation
38
can be predicted. In addition, the calculation of the d/DXRD
ratio (degree of agglomeration of the primary crystallites) indi-
cates that the La ratio <1 decreases the particle agglomeration
(d/DXRD). However, when a high La ratio increases, the particle
agglomeration also increases.
Temperature programmed reduction (TPR). Temperature
programmed reduction (TPR) of the prepared catalysts shows a
reduction prole with the occurrence of two peaks, the smaller
ꢀ
one is an intense peak located at a higher temperature (600 C)
due to the bulk reduction of CeO . The H uptake at this
2
2
temperature was decreased by increasing amount of lanthanum
due to the quantities of ceria, and thus the number of its oxygen
vacancies gradually decreased. The second one is a high intense
2 2 x
Fig. 5 XRD patterns of CeO and CeO La O1.5 catalysts.
ꢀ
peak located at a lower temperature (512 C) due to surface
reduction.
3
+
The doping of ceria with lower valence La leads to the
introduction of oxygen vacancies that can be expressed by the
Kr ¨o ger–Vink notation as follows:
the lattice parameter “a”, as reported in Table 1. This shi can
be explained as the lattice expansion of CeO and the formation
of ceria lanthanum solid solutions due to the larger radius of
2
3
+
4+
6
2CeO2
˚
˚
0
x
cc
La (1.10 A) versus Ce (0.97 A). This is an indication of the
La
2
O
3
ƒƒƒ ƒƒ ! 2La þ 3O
o
þ Vo
Ce
increasing number of oxygen vacancies as the La doping
0
where La represents the Ce site occupied by La . As repor-
Ce
4+
3+
35
increases. Moreover, no second phases or additional reec-
tions were found in Ce–La0.2 and Ce–La0.5 catalysts, indicating
the formation of a single phase (solid solution). However, in the
39
2 3
ted by Zhang et al., the increase in the addition of La O into
2
the CeO system would lead to the formation of more oxygen
vacancies due to the charge compensation in the material. The
case of Ce–La
1
catalyst, other lines were observed at 2q ¼
cc
ꢀ
ꢀ
ꢀ
vacancies ðVo Þ with positive charge may attract the doping ions
30.064 , 26.111 and 46.28 (JCPDS 00-002-0688) due to the
0
ðLa
Þ
with negative charge to produce complexes
Ce
2 3
hexagonal phase of La O .
0
00
0 00
0
ð2La V =2 La V La Þ due to electrostatic attractions. In
The XRD peaks for all the samples are considerably broad,
indicating that the samples prepared by spray freezing method
Ce
o
Ce o
accordance, as the La concentration increases, the peak at
ꢀ
36
512 C shis to higher temperatures due to the increase in the
interaction between Ce and La because at low concentrations
the Vo are probably free and easily reducible, while at a high
level of doping, the defects association near the dopants begin
are in the nano-size range. The average crystallite sizes
determined by applying the Scherrer equation to the charac-
cc
2
teristic (111) peak of CeO from the XRD results aer structure
renement are listed in Table 2. In comparison to the pure
ceria, which had a crystalline size of 11 nm, Ce–La had larger
crystallite sizes that increased along with the La content.
An interesting observation noted from the XRD measure-
ments was that when we calculated the lattice parameter of the
4
0
to form.
Moreover, Fig. 6 shows that the lower temperature peak
ꢀ
ꢀ
(
512 C) is accompanied by a shoulder at about 450 C, which
4
+
can be ascribed to the surface oxygen adsorbed on the Ce
vacancies produced due to the replacement of Ce by smaller
valence La . Further increase in the dopant addition (0.5 and 1)
4
+
(
101) peak (representing the La O phase) that appeared in
2 3
3
+
1
Ce–La catalyst, it was found to be 0.42 nm, which is larger than
Table 2 XRD data of the prepared samples
2
ꢁ1
Phase
DXRD nm
a nm
S
XRD m g
DBET nm
d/DXRD
CeO
Ce–La0.2
Ce–La0.5
2
(111) uorite
11.4
17.1
18.7
19.3
41.3
0.542
0.543
0.545
0.554
0.420
367
211
162
130
47
49
41
0.42
0.29
0.22
1.35
Ce–La
1
(111) uorite
257
2 3
(101) hexagonal La O
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isotherm indicates the stage at which the monolayer coverage is
complete, followed by a multilayer adsorption. The rst part of
the isotherm is very pronounced, and consequently the mate-
rials present micro-porosity, as conrmed by t-plot analysis
(Fig. 8). The intercept of the t-plot is almost zero for each sample
and the surface area are comparable to that of the BET surface
area due to the micro-pores, which may indicate that the surface
area of all the samples are micro-porous. All the samples (except
1
Ce–La sample) exhibit closed hysteresis loops with varying
closure pressures depending on the Ce : La ratio. The occur-
rence of a hysteresis loop in type II isotherms has been observed
with the samples having parallel plate pores. However, the
1
Ce–La sample (Fig. 7) shows type II without hysteresis loop, i.e.,
desorption points lie on the same adsorption isotherm. This
may indicate either a nonporous structure or the meso-porosity
of the entire texture. This was also conrmed by a v–t plot
(Fig. 8), whose data is collected in Table 3.
Table 3 depicts the physical properties of catalysts. It shows
Fig. 6
2 2 x
H consumption profiles during TPR of CeO La O1.5 catalysts.
that surface area, pore volume and pore radius decrease by the
3
+
incorporation of a trivalent cation (La ) into the CeO lattice,
2
which may be attributed to the incorporation of lanthanum into
the pore systems, although it may not block the pores. This was
also conrmed by the BJH pore size distribution (Fig. 9), which
shied to a lower pore diameter as the La content increased.
The decrease of surface area can be conrmed by the increase of
the crystallite size, as shown from the XRD data. These results
are nearly similar to those recently reported by Petallidou and
causes an increase in the intensity of this shoulder (which
becomes a peak at 500 C with a shoulder at 530 C in the case of
ꢀ
ꢀ
Ce–La
1
). The presence of this new peak at a lower temperature
ꢀ
4+
3+
(500 C) can conrm a higher Ce replacement by La ions,
2
ꢁ
which promotes the diffusion of O anions within the lattice,
facilitating the bulk and surface reduction at lower
41
temperatures.
It can be observed from the XRD analysis that the expansion
42
Efstathiou.
The geometrical surface areas (SXRD) of the prepared oxides
calculated from the XRD data are considerably higher than that
obtained from the BET analysis (Table 3). This variation can be
related to the fact that SXRD is calculated based on the diffrac-
tion peaks of crystallites with the initial Ce–La solid solution
3
+
of CeO2 lattice due to La doping lead to lattice strain and
formation of free volume, which can facilitate the equilibrium
of oxygen atoms between the surface and bulk, promoting the
surface-bulk oxygen transport. This phenomenon is responsible
17
for the observed reduction behavior.
composition, whereas S
the sample. Furthermore, solid solutions may contain small
agglomerates and grain boundary interfaces, which are not
corresponded to all the particles in
BET
Texture properties (BET). Specic surface area, pore volume
and average pore diameter play an important role in the
performance of the catalysts, especially in the high space
velocity catalytic reaction. Thus, the catalysts must have prop-
erties, such as a large specic surface area, pore volume and a
suitable pore diameter. Signicant information about the
textural properties of the Ce–La solid solution catalysts were
obtained by the analyses of nitrogen adsorption–desorption
isotherms, which are collected in Table 3.
Fig. 7 shows that all the samples show a type II isotherm
according to the Brunauer classication, indicating that the
adsorption proceeds by an unrestricted monolayer-multilayer
adsorption on micropores. The inection point or knee of the
43
2
available to N gas during the BET analysis. We also believe that
some internal porosity associated with the grain boundary
regions have an inuence on the obtained results.
Dynamic light scattering (DLS). Fig. 10 shows the particle
x
size distribution measured for Ce–La with different Ce : La
ratio. The obtained results indicate that the spray freezing
method obtains catalysts with particle sizes in the nano-scale
(ranged 18–50 nm), and the distribution of the powders was
well-proportioned, except in the case of Ce–La
1
.
28
In the process of spray freezing, the droplets were sprayed
ꢀ
into liquid nitrogen (ꢁ196 C). Due to the rapid cooling rate, the
Table 3 Textural properties of the prepared catalysts
2
ꢁ1
2
ꢁ1
3
ꢁ1
3
ꢁ1
2
ꢁ1
2
ꢁ1
˚
A
S
BET m g
S
XRD m g
V
P
cm g
r
P
V
Micro cm g
r
Micro m g
SEx. area m g
CeO
Ce–La0.2
Ce–La0.5
2
87.8
74.7
72.3
9.6
367
211
162
130
0.086
0.084
0.062
0.030
23.91
18.45
18.50
11.38
0.025
0.017
0.019
0.000
52.59
36.93
38.27
0.00
35.2
36.7
36.0
9.6
Ce–La
1
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Fig. 7
2
N adsorption–desorption isotherms of the prepared catalysts.
Fig. 8 t–v Plot of the prepared catalysts.
Fig. 9 Pore size distribution of the prepared catalysts.
chances of nucleation were greatly improved but the growth rate
decreased, resulting in the solute precipitating in the form of
ultrane grained salts. During the rise of temperature (the
process when the frozen droplets were taken out of the liquid
nitrogen before the rst step of freeze drying), the crystal
transformation of ice (ice sublimation) resulted in some
aggregation of ultrane grained salts. When the ice sublimated,
the ultra-ne grain salts turned into precursors, conrming the
broad particle size distribution of all the samples.
3
+
La , as indicated by the XRD data. Moreover it is noticeable
that high concentration of La in Ce–La allows some desegre-
1
gation of the material showing a non-uniform particle size
distribution pattern. This data were also conrmed by XRD.
TEM technique. TEM images obtained visual impressions of
the morphology of the catalysts prepared by the spray freeze-
drying method (Fig. 11 and 12). Fig. 11 indicates that the
overall particle shape was non-spherical and appeared slightly
aggregated for cerium oxide. The non-spherical shape of
The incorporation of La onto CeO lattice led to an increase
2
in the particle size distribution due to the high atomic radius of
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Fig. 10 Particle size distribution of the prepared catalysts.
2 x
Fig. 12 HRTEM images of the CeO La O1.5 samples.
Experimental section). As seen from Fig. 14, 92% of ethanol
conversion with 27% hydrogen selectivity was obtained at a very
ꢀ
low temperature (300 C) in the presence of a Ce–La0.2 catalyst;
this percentage increases by increasing reaction temperature,
ꢀ
until it approaches a 100% conversion with 75% H
2
(at 600 C)
over the same catalyst. Moreover, an increase in the La content
leads to a decrease of both ethanol conversion and hydrogen
selectivity. This can be related to the increase of the particle
size, decrease in the BET surface area and increase of OSC by
the increase of La content, as shown by the catalyst
characterization.
Fig. 14 shows the product distribution during ethanol steam
reforming over the prepared Ce–La
x
solid solution. From this

gure, we can see that the mechanism of ESR over those cata-
lysts contain a small amount of La that is completely different
from the catalyst containing high amounts of La. In the case of
Ce–La0.2 and Ce–La0.5 catalysts, the main reaction products
2
Fig. 11 HRTEM image of the CeO sample.
particles for the coarse mode may be due to the fact that the were H
2
and CO
and acetone (less than 10%) were also produced.
By increasing the reaction temperature, the selectivity to H
The incorporation of La (Ce : La atomic ratio ¼ 1 : 0.2 and increased and the selectivity to CO passed through a
: 0.5) led to an increase in particle size and a decrease in the maximum. The selectivity to CO and CH were slightly
2
. The trace amounts of CO, acetaldehyde,
particles consisting of a single crystal or a large number of ethylene, CH
4
44
crystals tend to be spheroidal.
2
3
+
2
1
4
degree of agglomeration, as clearly shown by Fig. 12. However, increased, while the selectivities to ethylene, acetaldehyde and
CeO La O exhibits very large particle size with a high degree acetone formation were negligible. However, in the case of the
2
1 1.5
of agglomeration (Fig. 12a). More magnied images of this Ce–La catalyst, the main reaction products were CO , acetal-
1
2
sample (Fig. 12b) show the formation of Ce–La nanotube, with dehyde and acetone at lower reaction temperatures, while at
tube length ranging from 50 to 100 nm and a uniformly higher reaction temperatures, the main products were CO , H
distributed diameter around 3 nm. These data agree well with and CH . This may be attributed to the differences in the
the XRD and DLS data. structure and morphology of Ce–La , as shown from the char-
Catalytic activity. Fig. 13 reports the bio-ethanol conversion acterization data.
and hydrogen selectivity during ESR over the prepared Ce–Lax It is worth noting that the selectivity to CO over all the
2
2
4
1
catalysts with different Ce : La atomic ratio as a function of the prepared catalysts was lower than 2%, especially at lower reac-
reaction temperature. Generally, it is important to note that the tion temperatures over Ce–La0.2 and Ce–La0.5 catalysts, which
low gas production (small gas yield), as shown in Fig. 14, can be means that these catalysts can be directly used for fuel cell
45
attributed to the injection of small ethanol concentrations (see application.
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x
Fig. 13 Ethanol conversion and hydrogen selectivity during ESR over Ce–La catalysts.
x
Fig. 14 product distribution during ESR over Ce–La catalysts.
H /CO ratios (Fig. 15) suggest that the reaction that takes acetaldehyde (produced in large amounts in the liquid product,
2
2
ꢀ
place over Ce–La0.2 and Ce–La0.5 is indeed ethanol steam not shown here) at reaction temperatures lower than 500 C
reforming reaction (reaction 1), because the experimental indicate the predomination of reactions that lead to acetone
values are approximately close to the theoretical ones (z3). The (2C
2
H
5
OH + H
2
O / CH
3
COCH
3
+ CO
) formation, in addition to the
/ CH + H O), which was
and CO. Moreover, at reaction
2 2
+ 4H ) and acetaldehyde
high H
2
/CO
2
value in addition to the appearance of CH
4
and CO (CH
3
CH OH / CH
2
3
CHO + H
2
ꢀ
at reaction temperatures >500 C indicate that the ethanol methanation reaction (CO + 3H
2
4
2
decomposition and the reverse WGS reaction (CO + H
2
O 4 CO
2
conrmed by the absence of H
2
ꢀ
+
H ) were also taking place at higher reaction temperatures. temperatures >500 C, acetaldehyde steam reforming (CH CHO
2
3
However, over Ce–La catalyst, H /CO is 0 3, which indicates + 3H O / 5H + 2CO ) and its decomposition (CH CHO / CH
4
1
2
2
2
2
2
3
2 2 4
that the ESR reaction does not occur completely over this + CO) were favorable to produce H , CO , CH and CO. The CO
catalyst. The large amount of CO
2
,
CH
4
,
acetone and product was consumed in the methanation reaction, which
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selectivity to CO
2
and CH
4
slightly increased with time, maybe
owing to the gasication of the deposited carbon. The lower
stability of Ce–La with respect to the other one can be attrib-
1
uted to the large amount of carbon deposit on its surface
because of its high selectivity to acetone and ethylene, and
acetone being further polymerized to form coke. This data can
be conrmed by the characterization of spent catalysts using
TGA, XRD and HRTEM techniques.
TGA results (Fig. 17) reveal that there is a signicant decrease
ꢀ
in the %C by about 3% (at 250 C) on the Ce–La catalyst and
5
0
.2
ꢀ
% (at 400 C) on the Ce–La1 catalyst. The lower burning
temperature over the two catalysts is attributed to the highly
49,50
reactive deposited carbon.
This also indicates that Ce–La
x
catalysts prepared by a spray freeze-drying method can be
ꢀ
regenerated at about 400 C in an oxygen atmosphere to restore
its original activity.
XRD proles of spent catalysts (Fig. 18) show that the solid
solution structures are well preserved for all the samples aer
Fig. 15
2 2 x
H /CO ratios during ESR over Ce–La catalysts.
explains the absence of CO and large amounts of CH
4
. The very
large amounts of CO produced when Ce–La is used as a
2
1
catalyst can be attributed to the large amounts of carbon
deposits due to ethanol decomposition, this carbon convert to
4
6,47
CO
2
by the effect of La
2
O
3
. Many authors
reported that La
2
O
3
3
generates surface oxygen by the formation of La O CO
2
2
carbonate, which is active for reacting with the surface carbon
converting it to CO
2
.
Catalytic stability test. To study the effect of the La content
on the stability of the prepared catalysts, the ethanol steam
reforming reaction was studied over 100 mg Ce–La0.2 and
ꢀ
Ce–La
1
catalysts at 500 C for 15 h. The Ce–La0.2 catalyst was
found to be considerably more stable, presenting a more
promising behavior than Ce–La , as shown in Fig. 16. Ce–La
1
0.2
catalyst lost only about 0.5% of its activity; the hydrogen
selectivity was stable and very high at about 60%, which is
48
close to the theoretical highest selectivity to hydrogen. The
selectivity to CO was neglected over the two catalysts and the Fig. 17 TGA of spent catalysts.
1
Fig. 16 Variation of ethanol conversion and product selectivity with reaction time over Ce–La0.2 and Ce–La catalyst.
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conversion, high hydrogen yield with very low CO selectivity and
high stability with very low carbon deposition.
When considering the high hydrogen production with low
CO, less than 1%, to be used in a fuel cell, Ce–La0.2 was found to
be the most selective catalyst, giving 57% H
2
and 0.92% CO at a
ꢀ
reaction temperature of 500 C. Studying the effect of La content
on the stability of the catalysts reveals that an increase in the La
content decreases the catalyst stability to a small extent. The
characterizations of the used catalysts show that the deactiva-
tion process is attributed to the carbon deposited, and not to
the sintering of the catalysts. Moreover, it reveals that the
deposited carbon is an amorphous one. Thus, we can regen-
erate the catalysts to restore its catalytic activity by burning the
ꢀ
carbon deposit at about 400 C in oxygen atmosphere.
Fig. 18 XRD of spent catalysts.
Acknowledgements
The authors gratefully acknowledge the nancial supports from
the Science & Technology Development Fundation in Egypt (ID:
4811).
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