On the solution combustion synthesis of copper based nanocatalysts for steam methanol reforming: Effect of precursor, ultrasound irradiation and urea/nitrate ratio

Article abstract of DOI:10.1016/j.molcata.2016.05.028

Steam reforming of methanol over CuO/ZnO/Al₂O₃ nanocatalysts synthesized by ultrasound assisted urea-nitrates combustion method was investigated. Four kinds of nanocatalysts were fabricated and their characteristic properties were st

Full text of DOI:10.1016/j.molcata.2016.05.028

Accepted Manuscript
Title: On the Solution Combustion Synthesis of Copper Based
Nanocatalysts for Steam Methanol Reforming: Effect of
Precursor, Ultrasound Irradiation and Urea/Nitrate Ratio
Author: Hossein Ajamein Mohammad Haghighi Reza
Shokrani Mozaffar Abdollahifar
PII:
S1381-1169(16)30210-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.05.028
MOLCAA 9903
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
25-4-2016
29-5-2016
30-5-2016
Please cite this article as: Hossein Ajamein, Mohammad Haghighi, Reza Shokrani,
Mozaffar Abdollahifar, On the Solution Combustion Synthesis of Copper Based
Nanocatalysts for Steam Methanol Reforming: Effect of Precursor, Ultrasound
Irradiation and Urea/Nitrate Ratio, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.05.028
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On the Solution Combustion Synthesis of Copper Based Nanocatalysts for
Steam Methanol Reforming: Effect of Precursor, Ultrasound Irradiation
and Urea/Nitrate Ratio
Hossein Ajamein1,2, Mohammad Haghighi1,1,2, Reza Shokrani1,2, Mozaffar Abdollahifar1,2
1. Chemical Engineering Faculty, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town,
Tabriz, Iran.
2. Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O.Box 51335-1996,
Sahand New Town, Tabriz, Iran.
1
Corresponding author:
Reactor and Catalysis Research Center, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz,
Iran. Email: haghighi@sut.ac.ir, Tel: +98-41-33458096 & +98-41-33459152, Fax: +98-41-33444355, web:
http://rcrc.sut.ac.ir
1

Graphical Abstract
CuO/ZnO/Al2O3 nanocatalysts were synthesized by urea-nitrate combustion method. The
nanocatalysts were characterized by XRD, FESEM, BET, EDX and FTIR analyses. Also
their catalytic properties were studied by their utilization in steam methanol reforming
process. Results show that boehmite as the aluminium precursor leads to smaller particles
with higher specific surface area. Furthermore, application of ultrasound irradiation instead of
conventional mixing for the preparation of primary gel in the combustion method increase the
surface area and dispersion of active sites and decrease the particle size which result better
catalytic performance. Finally, it was studied that lower urea/nitrates ratio leads to lower CO
production as the undesired product.
CuO/ZnO/Al2O3
H2 formation
H2
CO2 formation
CO2
CH3OH
CH2O
CH3O
H
OH
H CH3O
H
HCOO
OH
CH3OH dissociative
adsorption
CH3OH
CH3OH formation
H
H2O
CH3OCHO
Methyl formate
formation
OH
H
H2O dissociative
adsorption
2

Research Highlights
 Successfully synthesis of CuO/ZnO/Al2O3 by the solution combustion method.
 Effect of aluminium precursor between two alternatives: boehmite and aluminium
nitrate.
 Enhancement of catalytic properties by ultrasonic mixing of primary gel.
 Reaching higher methanol conversion by utilizing ultrasound irradiation.
 Reaching higher hydrogen selectivity by utilizing urea/ nitrate ratio=1.
Abstract
Steam reforming of methanol over CuO/ZnO/Al2O3 nanocatalysts synthesized by ultrasound
assisted urea-nitrates combustion method was investigated. Four kinds of nanocatalysts were
fabricated and their characteristic properties were studied by XRD, FESEM, PSD, BET, EDX
and FTIR analyses. Their physicochemical properties proved that using boehmite instead of
aluminum nitrate as the aluminum precursor leads to smaller crystallites and particles, better
dispersion and more surface area and subsequently higher activity in the steam methanol
reforming process. Furthermore, application of sonication for mixing of the primary gel of
combustion method resulted in better dispersion of active phases, smaller and more
homogeneous particles, higher methanol conversion and hydrogen production. Although
enhancement of fuel/nitrates ratio increased dramatically the surface area of synthesized
nanocatalysts but it had no significant effect for improving methanol conversion or hydrogen
production. In a word, CZA-A-CU-UN1 had better physicochemical properties and catalytic
performance for the steam methanol reforming process.
Keywords: Urea-Nitrates Combustion, CuO/ZnO/Al2O3, Methanol, Steam Reforming, Hydrogen.
3

1 Introduction
Methanol, as one of the popular chemical compounds in many large scale industries, is
mainly produced from synthesis gas (mixture of CO, CO2 and H2) [1-3]. Its advantages such
as no strong C-C bond, easy storage and transportation as well as conversion to hydrogen at
low temperatures; causes to nominate as an attractive choice for production of hydrogen
especially for small scale applications and automobiles [4-6]. Three common pathways are
suggested to convert methanol to hydrogen including partial oxidation, decomposition and
steam reforming [7-9]. Steam methanol reforming (SMR) is a promising and efficient process
for conversion of methanol to the hydrogen-rich product at low temperatures [10-12]. Two
kinds of catalysts are nominated for the SMR process according to their reaction temperature:
Cu-based and group 8-10 metals based catalysts [13-15]. Cu-based catalysts due to their
lower reaction temperature and higher activity are more desirable [16, 17]. The activity of
these catalysts can be influenced by preparation parameters such as promoter type,
composition, synthesis method and operational parameters such as temperature, pressure,
feed ratio and reactor type. Different promoters were applied to investigate their influence on
activity, hydrogen and carbon monoxide selectivity and catalyst stability [18-21]. Al2O3 has
been proved to have efficient effect on thermal and mechanical stability and enhancement of
surface area. However, its excessive amounts have negative influence which should be
controlled by using its optimum content [22]. ZnO increases the dispersion of Cu species and
catalyst reducibility [23]. CeO2 and ZrO2 increase the activity of Cu-based catalysts while
zirconia increases its stability, too [16, 24, 25].
Various synthesis methods such as co-precipitation [23, 26], impregnation [27], hydrothermal
[28] and homogeneous precipitation [29] has been investigated to study their influence on the
catalytic performance of fabricated catalysts. Furthermore, in our previous works, the copper-
4

based nanocatalysts were synthesized by the novel urea-nitrate combustion method [22, 24].
The solution combustion synthesis method is a novel, fast and simple synthesis method which
is applied for fabrication of different nanomaterials. In this method, a viscous solution of an
oxidizer which is usually the nitrate form of precursor and an organic fuel such as urea,
glycine, citric acid and etc. are prepared and heated. The solution starts to foam and burn
until the final product is synthesized in a short time [30-32]. Several parameters such as pH,
type of fuel, fuel/oxidizer ratio, ignition temperature and combustion environment have
significant influence on the properties of resulted powders [33-36]. The fuel/oxidant ratio is
known as one of the most influential parameters on properties of synthesized powders by the
solution combustion method. This parameter can also influence the catalytic performance of
fabricated nanocatalysts due to their effect on producing micro and nano pores which
facilitates the accessibility of reactants to active sites. Also, applying proper amount of fuel
can affect quantity of gaseous combustion products which can be effective on physical
properties of synthesized nanocatalysts such as morphology, particle size and surface area
[31, 37-39]. In addition, the mixing procedure of primary solution of precursor nitrates and
fuel can be effective [40].
Recently, great attention has been paid to utilization of sonication as a powerful source of
energy for preparation of nanomaterials [41-43]. Smaller particle size, enhancement of
surface area and increase of catalytic properties are the great advantages of ultrasound
assisted catalysts [44-46]. Unfortunately, few studies were performed to investigate
application of ultrasonic radiation during the solution combustion synthesis method.
Therefore, in this paper, four kinds of CuO/ZnO/Al2O3 nanocatalysts with similar
composition were synthesized by the solution combustion method. Boehmite and aluminium
nitrate were applied as the aluminium precursor for fabrication of these nanocatalysts. In
addition of studying the aluminium precursor effect, the influence of application of sonication
5

and fuel/nitrate ratio on the physicochemical and catalytic properties of synthesized
nanocatalysts was studied. For this purpose, various characteristic analyses such as X-ray
diffraction, field emission scanning electron microscopy (FESEM), energy dispersive X-ray
(EDX), Brunauer–Emmett–Teller analysis (BET), and Fourier-transformed infrared (FTIR)
techniques were used. Finally, the prepared nanocatalysts were tested in a fixed bed reactor
system for evaluation of their catalytic performance.
2 Materials and Methods
2.1 Materials
Copper nitrate trihydrate (Cu(NO3)2.3H2O, Merck, extra pure), zinc nitrate hexahydrate
(Zn(NO3)2.6H2O, Merck, extra pure), aluminium nitrate nonahydrate (Al(NO3)3. 9H2O,
Merck, extra pure) and Al(OH)3 (Merck) were applied as the precursors for fabrication of
CuO/ZnO/Al2O3 catalysts. All of the reagents were used without further purification. Urea as
a fuel in combustion method was supplied by Merck.
2.2 Nanocatalysts Preparation and Procedure
Figure 1 depicts the fabrication method which was applied for the preparation of
CuO/ZnO/Al2O3 catalysts. This methodology is divided to three main steps: precursor
preparation, synthesis via urea- nitrate combustion as well as heat treatment and catalyst
forming. In the first stage, an aqueous solution of copper nitrate trihydrate and zinc nitrate
hexahydrate in deionized water was prepared. Then, if the aluminium based precursor was
nitrate, its precursor was added to the resulted solution. On the other hand for production of
boehmite-based alumina, aluminium hydroxide heated at 400ºC for 4 hours and added to the
solution simultaneously with urea. This mixing step was performed by mechanical or
ultrasound assisted mixer for 45 minutes. Figure 2 illustrates the ultrasound assisted mixing
6

apparatus. Sonication was carried out on a SONOPULS HD 3200, 130 mm diameter Ti horn,
95 Watt. The Ti horn was inserted 1 cm below the slurry and high intensive irradiation was
applied to the solution. Two urea/nitrate ratios as 1 and 2 were considered. In the second
stage, the urea-nitrate combustion synthesis was done by an open muffle furnace at 400ºC. It
is worth noting that the aqueous solution was preheated over a hot plate at 80ºC to form a
viscous gel before the main combustion reaction. After all in the third stage, the resulted
powder catalyst was calcined at 400ºC for 3 hours under air flow and later was formed to use
in a laboratory reactor. Due to different methodology and aluminium precursor used for
preparation of the catalysts, four different samples were achieved which are mentioned in
Table 1. In this table, samples were labelled by some abbreviated names to facilitate their
calling in the paper. “A” and “N” mean boehmite and nitrate as the aluminium precursor,
respectively. In addition, UN1 and UN2 indicate urea/nitrate ratio of 1 and 2, respectively.
The samples which were synthesized by applying ultrasound irradiation are nominated by
addition of “CU” in their abbreviated names.
(Figure 1)
(Figure 2)
(Table 1)
2.3 Nanocatalysts Characterization Techniques
X-ray diffraction (XRD) analysis was performed by a D5000 Siemens X-ray diffractometer
using Cu Kα radiation (0.154056 nm). Its working voltage and the current were 30 kV and 40
mA, respectively. The Scherrer equation was applied to calculate the average crystallite size
of samples [47]. The morphology of synthesized nanocatalysts was observed by HITACHI
4160-s field emission scanning electron microscope (FESEM). EDX and dot maps were
7

conducted for elemental analysis (VEGA II, TESCAN). The FT-IR spectra were acquired by
a Unicam 4000 FTIR spectrometer using KBr plate in the range of 400-4000 cm-1. The
Brunauer-Emmett-Teller (BET) surface area measurements were calculated using nitrogen
adsorption at 77 K and desorption at room temperature by a Quantachrome ChemBET 3000
analyser while the samples were degassed at 200°C for 30 min before the measurement.
2.4 Experimental Setup for Catalytic Performance Test
The prepared catalysts were tested in a laboratory catalysis system for the steam methanol
reforming process. Figure 3 illustrates schematically the laboratory catalysis setup which
consists of feed gas cylinders, mass flow controllers, a saturator, a reactor and a gas
chromatograph system. 400 mg of the fabricated catalyst was loaded into a U-shaped fixed
bed reactor (5mm i.d.). The reactor was set in a temperature controlled furnace. Before the
main reaction performance, the catalysts were reduced under H2/Ar stream at 300ºC for 3
hours. Then, the feed containing water to methanol ratio of 1.5 and argon as career gas was
inserted to the reactor from a saturator. The product was analysed by a gas chromatograph
device (GC Chrom, Teif Gostar Faraz, Iran) which was equipped with an Agilent J&W HP-
PLOT-U column to separate the gases and FID and TCD detectors for identification of
components. The inner diameter of the column is 0.32 mm and has 30 meter length. Its
temperature limit range is -60-190˚C. As the gaseous products (CO and CO2) exit the column
in 4 minutes at 40 ˚C, a temperature program was proposed to reduce the time of the
methanol existence. It started from 40˚C and stayed in this temperature for 5 minutes to
ensure that CO and CO2 exit completely from the column. Then, the GC temperature rise to
reach 170˚C and stayed in this temperature for 5 minutes and after existence of methanol, the
temperature program ended by reducing to 40˚C.
The conversion of methanol and selectivity of the products have been defined as follows:
8

F
-F
CH3OHout
CH3OHin
XCH OH % =
100
100
100
100
(1)
(2)
(3)
(4)
3
F
CH3OHin
FH
2
out
SH % =
2
F
 F
 F
H2
COout
CO2
out
out
F
COout
SCO% =
F
 F
 F
H2
COout
CO2
out
out
F
CO2
out
SCO % =
2
F
 F
 F
H2
COout
CO2
out
out
Where Fi is the flow rate of component i in the gaseous effluent.
(Figure 3)
3 Results and Discussions
3.1 Nanocatalysts Characterization
3.1.1 XRD Analysis
Figure 4 shows the X-ray diffraction patterns of synthesized CuO/ZnO/Al2O3 catalysts.
Significant peaks of monoclinic CuO (JCPDS 01-080-1268) can be observed at 2θ=35.5˚,
38.8˚ and 48.8˚. The existence of ZnO peaks in some samples is a little difficult due to its
overlapped with CuAl2O4 and ZnAl2O4 peaks. The main characteristic peaks of hexagonal
ZnO (JCPDS 01-076-0704) appear at 2θ= 31.7˚, 34.4˚, 36.2˚ and 56.5˚. The peaks of zinc
aluminate (JCPDS 00-005-0669) and copper aluminate (JCPDS 01-071-0966) are placed at
2θ=31.2˚, 36.8˚, 65.2˚ and 2θ=31.3˚, 36.9˚, 65.3˚, respectively [48, 49]. Therefore, most of
zinc oxide, zinc aluminate and copper aluminate peaks are overlapped. However, there are
some peaks can be helpful to prove existence of spinels. For example, at 2θ=65.2 only
ZnAl2O4 and CuAl2O4 have the characteristic peak which is detectable in samples CZA-N-C-
9

UN1, CZA-A-C-UN1 and CZA-A-CU-UN1. While for the sample CZA-A-CU-UN2, this
peak cannot be detected which means in this sample, spinels were not formed. So, it can be
concluded that enhancement of urea/nitrate ratio from 1 to 2 leads to prevent the spinel
formation. This could be because of decrease of combustion temperature by enhancement of
fuel content. In other hand, the source of alumina and application of ultrasound power for
mixing of primary gel had no significant effect on formation of zinc and copper aluminate.
However, it is worth noting that application of boehmite source instead of nitrate source
decreased the intensity of zinc and copper aluminate peaks by comparing the CZA-N-C-UN1
and CZA-A-C-UN1 samples. The existence of copper and zinc aluminate species inherently
have no bad effect on synthesized nanocatalysts properties. But, due to their high reduction
temperature, most of them were not reduced for carrying out the reforming reaction [50].
Comparison of CZA-A-CU-UN1 and CZA-A-CU-UN2 samples shows that enhancement of
urea/nitrate ratio from 1 to 2 increased the intensity of CuO peaks. So, higher fuel/nitrates
ratio led to larger crystallites. This conclusion is in accordance of Ahmed [51] and Merino
[31] works which claimed that enhancement of fuel/oxidizer ratio leads to higher metal oxide
crystallite size. Therefore, decrease of the urea/nitrates ratio results in smaller and highly
dispersed CuO and ZnO crystallites. By comparing CZA-A-C-UN1 and CZA-A-CU-UN1, it
can be concluded that application of ultrasonic mixing in primary gel formation also led to
smaller crystallite and good dispersion. Finally, it can be concluded that CZA-A-CU-UN1
has smaller CuO and ZnO crystallites with better dispersion which could be led to better
catalytic performance.
Table 1 lists some physicochemical properties of fabricated CuO/ZnO/Al2O3 catalysts. It can
be mentioned the relative crystallinity as a quantitative parameter for comparing the
crystallinity of species. The relative crystallinity of CuO species in CZA-A-CU-UN2 is the
highest while its minimum value refers to CZA-A-CU-UN1. As the XRD patterns also show
10

qualitatively enhancement of urea/nitrate ratio increased the crystallinity of CuO crystallites.
The relative crystallinity of ZnO crystallites cannot be calculated due to the overlap its
characteristic peaks with the aluminate spinels ones. The crystallite sizes were calculated by
Scherer’s equation for CuO and ZnO crystallites. It can be demonstrated that the average
crystallite size of CuO and ZnO species are about 18 and 20.5 nm, respectively. It proved that
the CuO and ZnO phases in fabricated nanocatalysts are nanocrystalline. No data were
reported for relative crystallinity and crystallite size of Al2O3 species because their peaks
cannot be detected in XRD patterns due to its good dispersion in nanocatalysts matrix.
(Figure 4)
3.1.2 FESEM Analysis
Figure 5 illustrates the FESEM micrographs of synthesized nanocatalysts. In all samples, the
size of particles is nanometric. It seems that the particles in samples which have used
boehmite as the alumina precursor are more homogeneous. Also, application of ultrasound
power for mixing the primary gel results in smaller and more homogeneous particles. The
fuel/nitrates ratio has no significant influence on particle size of fabricated nanocatalysts but
it affects crucially on combustion pores which describes in following section.
(Figure 5)
3.1.3 PSD Analysis
The size distribution analysis for fabricated nanocatalysts is presented in Figure 6. This
analysis is performed by applying FESEM images and using ImageJ software [52]. The
analysis shows that the particle size of CZA-N-C-UN1 sample is mostly about 25-40 nm
11

which is very similar to results of CZA-A-C-UN1 sample. But, application of ultrasonic
mixing of primary gel led to significant difference in the particle size of CZA-A-CU-UN1
and CZA-A-CU-UN2 samples. In the CZA-A-CU-UN2 sample, the average particle size is
about 22.8 nm and most of particles are in the range of 15-30 nm. The average size of
particles in the CZA-A-CU-UN1 sample is about 17.5 nm and most of particles size are about
5-20 nm. Therefore, it seems that application of ultrasonic mixing in the synthesis these
nanocatalysts led to smaller size of particles. Also, enhancement of urea/nitrate ratio decrease
the particles size of fabricated nanocatalyst which will lead to higher surface area which
could be resulted higher methanol conversion through the steam reforming of methanol
reaction. In addition, applying different precursors for alumina has no significant effect on
the particle size distribution.
(Figure 6)
3.1.4 Combustion Pore Size Analysis
The solution combustion method has a significant effect on catalytic performance of
fabricated catalysts due to enhancement of reactants accessibility to active phases by creating
some meso pores in nanocatalysts structure. Due to the combustion reaction happened for the
primary gel which leads to sudden production of huge bulk of combustion gases, some meso
pores are created in the resulted nanocatalysts. These pores are illustrated in FESEM images
in Figure 7. It was previously proven by Palneedi that enhancement of fuel/oxidizer ratio
causes production of more combustion pores due to more combustion gases produced [53].
Therefore, it can be seen that these pores are more in images of the CZA-A-CU-UN2 sample.
But, it is interesting to note that these pores were created more in samples applied ultrasonic
mixing which can be due to homogeneous mixing of fuel and nitrates species in the primary
12

gel lattice. The combustion pore size distribution for the CZA-A-CU-UN1 sample is analysed
in Figure 8. It can be concluded that most of the pores created in this sample has about 0.6
μm size. The importance of these pores for the synthesized nanocatalysts is more accessibility
of active sites inside the pores for feed reagents. Therefore, it seems that the samples which
have more pores in their structure can perform better in catalytic tests.
(Figure 7)
(Figure 8)
3.1.5 EDX Analysis
Figure 9 shows EDX analysis results of the synthesized CZA nanocatalysts. All the elements
such as copper, zinc and aluminium are recognized in synthesized samples. Therefore,
although other analyses such as XRD could not detect the presence of alumina, the existence
of aluminium can be proved by the EDX technique. It can be observed that in the CZA-N-C-
UN1, CZA-A-C-UN1 and CZA-A-CU-UN2 samples, the surface is covered mostly by Al or
Zn species which is not suitable for the catalytic activity. Therefore, it seems that the CZA-A-
CU-UN1 sample is the only fabricated nanocatalyst which all of components especially
active phases is homogeneously dispersed on its surface. The comparison of the primary gel
and the nanocatalyst surface composition is possible by EDX result which is depicted in
Figure 10. It shows that the surface composition of synthesized nanocatalysts is
approximately in accordance with their primary gels. Among them, the surface composition
of CZA-A-CU-UN1 sample is closer to the primary gel composition. So, it can be
demonstrated that the CZA-A-CU-UN1 sample was well synthesized due to similar
composition of primary gel and final nanocatalyst. Therefore, due to EDX- dot mapping
13

analysis results, it can be predicted that CZA-A-CU-UN1 sample could have better catalytic
performance among other prepared samples.
(Figure 9)
(Figure 10)
3.1.6 BET Analysis
Figure 11 shows the specific surface area of synthesized nanocatalysts. As it can be expected
from FESEM analysis, the highest surface area is related to the CZA-A-CU-UN2 sample. Its
surface area is about 2 times higher than the CZA-A-C-UN1 and CZA-A-CU-UN1 samples.
Two main parameters can affect on the surface area of these synthesized nanocatalysts:
urea/nitrate ratio and alumina precursor. It seems that enhancement of urea/nitrate ratio had
more influence on the increase of specific surface area of the prepared nanocatalysts. In this
case, comparison of CZA-A-CU-UN1 and CZA-A-CU-UN2 samples demonstrates that
enhancement of urea/nitrate ratio from 1 to 2 can nearly double the specific surface area
value. This can be predictable due to FESEM and PSD analyses which indicated smaller
particles of the CZA-A-CU-UN2 sample than the CZA-A-CU-UN1 ones. On the other hand,
it can be noted that applying boehmite precursor instead of nitrate one can also improve the
specific surface area significantly. Therefore, it can be concluded from BET analysis that the
CZA-N-C-UN1 nanocatalyst compared to other ones might have the worst efficiency for the
steam methanol reforming process due to its lower specific surface area.
(Figure 11)
14

3.1.7 FTIR Analysis
Figure 12 shows the FTIR spectra of the synthesized CZA nanocatalysts. The trend of all
samples is similar and notices that their functional groups are same. From the literature, it can
be considered that the bands at 430-675 cm-1 can be assigned to metal oxides. More precisely,
the bands near 435 cm-1 are related to existence of ZnO species [54]. Also, existence of Cu-O
species can be proved due to representing peaks near 495 cm-1 [28]. The peak at 670 cm-1 is
assigned to Al-O stretching vibration especially in its copper aluminate and zinc aluminate
forms which their contents are as follows CZA-N-CU-UN1>CZA-A-C-UN1>CZA-A-CU-
UN1>CZA-A-CU-UN2. This is in accordance of XRD results which no spinels is observed
in XRD patterns of CZA-A-CU-UN2 sample as well as its FTIR spectra. In addition, bands at
3450 cm-1 can be assigned to hydroxide group [55-57] and interlayer water molecules are also
detected by bands at 1640 cm-1 which may be remained from synthesis stage [58-60]. The
peak near 2367 cm-1 is related to carbon dioxide which is trapped after the solution
combustion synthesis of nanocatalysts. Its amount is very little due to low adsorption
tendency of the fabricated nanocatalysts and circumstances during their fabrication which is
mentioned in following section.
(Figure 12)
3.2 Reaction Mechanism of Nanocatalysts Formation
The solution combustion synthesis is known as a very fast reaction between oxidizer and fuel
components. Copper, zinc and aluminium nitrates played the role of oxidizer and in the other
side; urea was applied as the fuel in this research. When the mixture of these nitrates and urea
is heated in muffle furnace at 400ºC, melting and dehydration occur in early minutes of
reaction. Then, the mixture starts to decompose with frothing. In this step, urea converts to
15

urea nitrate, biuret, HNCO and NH3 while the nitrates decompose to their metal hydroxides
and nitric acid as following reaction [61]:
M(NO3)x  xH2O  M(OH)x  xHNO3
The mixture of these different components foams due to generation of gaseous product such
as nitrogen oxides, NH3 and HNCO, from decomposition of primary and intermediates
components. Finally, the resulted combustible foam breaks out due to accumulation of gases
which known as hypergolic gaseous mixture and finally the foam burns and decomposes to
metal oxides at high temperature [30]. The overall reaction of each applied metal nitrate is as
follows:
3Cu(NO3)2 5CH4N2O  3CuO 5CO2 8N2 10H2O
(aq)
(s)
(g)
(aq)
(g)
(g)
3Zn(NO3)2 5CH4N2O  3ZnO 5CO2 8N2 10H2O
(aq)
(s)
(g)
(aq)
(g)
(g)
2Al(NO3)3 5CH4N2O  Al2O3 5CO2 8N2 10H2O
(aq)
(g)
(aq)
(s)
(g)
(g)
Existence of water and carbon dioxide in the synthesized nanocatalysts which is proved by
FTIR analysis is due to their production in this stage of the synthesis pathway. The high
amount of gases produced in the foaming stage and the ignition of hypergolic gaseous
mixture lead to create of pores which was mentioned in section 3.1.6 and facilitates the
accessibility of reactant components to active sites of the fabricated nanocatalysts.
3.3 Catalytic Performance Study toward Methanol Steam Reforming
3.3.1 Methanol Conversion
The performance of the synthesized nanocatalysts for conversion of methanol to products
through the steam methanol reforming process is depicted in Figure 13. It can be observed
that increasing the temperature led to enhancement of the methanol conversion for all
samples. But, the CZA-A-CU-UN1 sample converted more methanol compared to the others.
16

In other side, CZA-N-C-UN1 has the worst performance for converting methanol even at
high temperatures where cannot reached 100% conversion like other samples. This
malfunction of the CZA-N-C-UN1 sample can be predicted from its worse characterization
analyses results such as XRD and BET analyses which demonstrate low specific surface area
and large copper and zinc oxides crystallites size. These undesired physicochemical
properties led to inaccessibility of feed molecules to copper based active sites and
subsequently low methanol conversion. On the other hand, the CZA-A-CU-UN1 sample
causes higher methanol conversion due to its high specific surface area, better copper species
dispersion, lower copper and zinc oxides crystallinity and better interaction of copper and
zinc species. According to methanol conversion results after CZA-A-CU-UN1 sample the
second place can be nominated to CZA-A-C-UN1 while the CZA-A-CU-UN2 sample is
placed after that. Therefore, it can be concluded that using boehmite as the aluminum
precursor has a significant effect on the methanol conversion in the SMR process. Also,
application of ultrasonic mixing instead of typical mechanical mixer in preparation of
primary gel for the combustion synthesis method has an efficient influence on better
methanol conversion. Enhancement of urea/nitrates ratio has no desire effect on the catalytic
performance of the CZA nanocatalyst.
(Figure 13)
3.3.2 Products Selectivity
Figure 14 depicts the products selectivity of fabricated nanocatalysts through the steam
methanol reforming process. It is worth noting that due to the application of the SMR process
for applying the feed stoke of fuel cells, the desire product is hydrogen. But, the amount of
carbon monoxide is also important because of its negative influence on anodic catalysts of
17

fuel cells. Therefore, the best catalyst should be selected from samples which produce higher
hydrogen and less carbon monoxide. Due to endothermic nature of reforming processes,
increase of temperature leads to higher activity. Therefore, study about the catalyst role for
decreasing activation energy and enhancement of the reaction rate is carried out at lower
temperatures [62]. Now by considering these points, it can be concluded that the CZA-A-CU-
UN1 sample has better performance in catalytic experiments. Considering the CZA-N-C-
UN1 sample, it indicates that although this sample did not produce carbon monoxide till
260°C but its hydrogen selectivity is lower than others. This explanation is also correct for
the CZA-A-C-UN1 sample. On the other hand, CZA-A-CU-UN2 sample produced carbon
monoxide even at 220°C and its rate increased severely when temperature enhanced. The
study of the catalytic performance of the CZA-A-CU-UN1 sample consequences that its
hydrogen selectivity is higher than others especially at lower temperatures. Also, its carbon
monoxide selectivity is low enough especially compared with the CZA-A-C-UN1 and CZA-
A-CU-UN2 nanocatalysts. The CO amount increases by enhancement of temperature because
the water gas shift reaction is an exothermic equilibrium reaction and it trends to its reverse
reaction and more Co production by increasing temperature.
(Figure 14)
3.3.3 Time on Stream Performance
Due to better catalytic performance of the CZA-A-CU-UN1 nanocatalyst, its stability at
240°C was analysed; which is illustrated in Figure 15. It can be observed that the
nanocatalyst was approximately stable for 1200 min. Its conversion decreased from 100% to
90% with a gentle slope and remained constantly till 1200 min. The hydrogen and CO
selectivity had no significant change during the test.
18

(Figure 15)
3.4 Reaction Mechanism of Methanol Steam Reforming
Figure 16 illustrates a proposed reaction mechanism for the steam methanol reforming
process. At first, methanol adsorbs and dissociates on the active phases of CuO/ZnO/Al2O3
nanocatalysts [63]. This reaction is nominated as the rate determining step in the steam
methanol reforming process.
CH3OH → CH3Os + Hs
ΔHº=435.4 kJ.mol-1
(1)
The adsorbed CH3O species can re-dissociate to formaldehyde as follows [64]:
ΔHº=93.1 kJ.mol-1
CH3Os → CH2Os + Hs
(2)
The formed formaldehyde can follow two pathways: reacts with a hydroxyl group or
adsorbed CH3O species [65].
CH2Os + OH- → HCOOs + H2(g)
ΔHº=-60.1 kJ.mol-1
ΔHº=-48.2 kJ.mol-1
(3)
(4)
CH2Os + CH3Os → CH3OCHOs + Hs
The first reaction is one of the main pathways for production of hydrogen while the second
leads to formation of methyl formate. The resulted methyl formate can react with an adsorbed
hydroxyl group and results methanol [66].
CH3OCHOs + OH- → CH3OH + HCOOs
ΔHº=-11.3 kJ.mol-1
(5)
The HCOO species from reaction (3) and (5) dissociates to carbon dioxide.
ΔHº=-45.8 kJ.mol-1
HCOOs → CO2(g) + Hs
(6)
The main pathway for formation of carbon monoxide as the undesired product of the SMR
process is the reverse water gas shift reaction. It is worth noting that the straight water gas
shift (WGS) reaction can also be one of the routes for production of hydrogen [67].
19

CO2(g) + H2(g) → CO(g)+ H2O(g)
ΔHº=41.15 kJ.mol-1
(7)
(8)
Furthermore, coke can be produce by the well-known Boudouard reaction.
ΔHº=-172.5 kJ.mol-1
2CO(g) → C(S) + CO2(g)
The negative effect of produced coke can be resolved by its reaction with the steam as
follows [68]:
H2O(g) + C(S) → CO(g) + H2(g)
ΔHº=131.3 kJ.mol-1
(9)
Therefore, hydrogen produces mainly by reaction (3) and WGS one which are exothermic.
So, as mentioned in section 3.3, increasing temperature can lead the process to prevent
hydrogen production. Also, it should be noted that the production of hydrogen is related to
performance of two endothermic reactions (1) and (2) and two exothermic reactions (4) and
(6) which make the prediction of temperature effects on the hydrogen selectivity difficult. In
other side, RWGS reaction is an endothermic reaction which enhancement of temperature
results to increase the rate of CO formation.
Methanol decomposition is an endothermic reaction and rate determining step of SMR
process [69]. So its activation energy cannot be applied at low temperatures. In this case, the
application of better catalyst leads to reducing of the activation energy. Therefore, choosing
of a good catalyst can result better methanol conversion at low temperatures. Accordingly,
due to better catalytic performance of sample CZA-A-CU-UN1, it seems that application of
ultrasound mixing and boehmite precursor can facilitate the associative adsorption of
methanol molecules on copper active sites. This is in accordance of results from analyses
such as XRD, EDX and BET which interpreted that this nanocatalyst has smaller and more
dispersed particles. The EDX analysis also showed that the copper particles are
homogeneously dispersed on the surface of the nanocatalyst. It is worth noting that
interaction of Cu and Zn facilitates the methanol decomposition while alumina increases the
surface area and makes required space for active sites.
20

(Figure 16)
4 Conclusions
Three crucial parameters on the preparation of CuO/ZnO/Al2O3 nanocatalysts by the urea-nitrates
combustion method were investigated. Firstly, the effect of aluminium precursor was studied by
preparation of SMR nanocatalysts from boehmite and aluminium nitrate. Characteristic analyses
interoperated that using boehmite precursor instead of nitrate form reduces the crystallite size of
copper and zinc species and increases their dispersion and specific surface area. Application of
ultrasound mixing and urea/ nitrates ratio as two synthesis parameters was also probed in this study. It
was concluded that utilizing sonication for mixing of primary gel instead of conventional mechanical
mixing leads to more homogeneous nanocatalysts while it has no effect on its surface area. Also,
increase of urea/nitrates ratio enhances the crystallite size of CuO and ZnO and specific surface area.
The catalytic performance of the fabricated nanocatalysts were tested and finally, it was found out the
CZA-A-CU-UN1 sample has better methanol conversion and higher hydrogen production due to its
smaller crystallites and more dispersed particles.
Acknowledgements
The authors gratefully acknowledge Sahand University of Technology for the financial
support of the research as well as Iran Nanotechnology Initiative Council for complementary
financial supports.
21

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Figures Caption
Figure 1. Preparation steps of CuO/ZnO/Al2O3 nanocatalyst via urea-nitrate combustion
method..............................................................................................................27
Figure 2. Ultrasound assisted mixing system for synthesis of CuO/ZnO/Al2O3
nanocatalyst via urea-nitrate combustion method............................................28
Figure 3. Experimental setup for performance test of CuO/ZnO/Al2O3 nanocatalysts used
in hydrogen production via methanol steam reforming...................................28
Figure 4. XRD patterns of synthesized CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-
UN1, (b) CZA-A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1....29
Figure 5. FESEM images of CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1, (b)
CZA-A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1...................30
Figure 6. Size distribution histogram of CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-
UN1, (b) CZA-A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1....31
Figure 7. Combustion pore of CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1, (b)
CZA-A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1...................32
Figure 8. Combustion pore size distribution histogram of CuO/ZnO/Al2O3 nanocatalyst
(CZA-A-CU-UN1)...........................................................................................33
Figure 9. EDX analysis of CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1, (b) CZA-
A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1. ...........................34
Figure 10. Gel vs. CuO/ZnO/Al2O3 nanocatalysts surface analysis: CZA-N-C-UN1, CZA-
A-C-UN1, CZA-A-CU-UN2 and CZA-A-CU-UN1. ......................................35
Figure 11. BET analysis of CuO/ZnO/Al2O3 nanocatalysts: CZA-N-C-UN1, CZA-A-C-
UN1, CZA-A-CU-UN2 and CZA-A-CU-UN1................................................35
Figure 12. FTIR spectra of CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1, (b) CZA-
A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1. ...........................36
Figure 13. Methanol conversion over CuO/ZnO/Al2O3 nanocatalysts: CZA-N-C-UN1,
CZA-A-C-UN1, CZA-A-CU-UN2 and CZA-A-CU-UN1. .............................37
Figure 14. Products selectivity over CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1,
(b) CZA-A-C-UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1. ............38
Figure 15. Time on stream performance of CuO/ZnO/Al2O3 nanocatalyst (CZA-A-CU-
UN1).................................................................................................................39
Figure 16. Reaction mechanism for the formation of hydrogen via methanol steam
reforming over CuO/ZnO/Al2O3 nanocatalyst.................................................39
26

(a) Precursor preparation
Precursors
Al(NO3)3.9H2O
Aq. solution of Cu, Zn and Al nitrates Heating at 400 C under air flow for
Combustion fuel: Urea Zn(NO3)2.3H2O Cu(NO3)2.3H2O
Al(OH)3
in appropriate ratios
4 h at a heating rate of 1-2 C/min
AlOOH
Ultrasound assisted mixing for 45 min
Urea/Nitrates = 1 and 2
(b) Synthesis via urea-Nitrate combustion
Heating over a hot plate at 80 C to evaporate excess water and form viscose gel
Heating in an open muffle furnace maintained at 400ºC
Gel boiling with frothing and foaming to form combustion powder
(c) Heat treatment and catalyst forming
Calcination at 400 C for 3 h under air flow
Catalyst forming/shaping: CuO/ZnO/Al2O3
CZA-N-C-UN1, CZA-A-C-UN1, CZA-A-CU-UN2, CZA-A-CU-UN1
Figure 1. Preparation steps of CuO/ZnO/Al2O3 nanocatalyst via urea-nitrate combustion method.
27

Pressure
Gage
Pressure
Regulator
Gas Injection: Ar
Needle
Valve
Sonication:
90 W
45 min
Sonication Generator
Cu(NO3)2.3H2O
Zn(NO3)2.3H2O
Al(NO3)3.9H2O
AlOOH
Synthesis
Vessel
De-ionized water
Ar
Heating Magnetic
Stirrer Plate
Ultrasonic
Booster
Figure 2. Ultrasound assisted mixing system for synthesis of CuO/ZnO/Al2O3 nanocatalyst via urea-
nitrate combustion method.
PI-02
Ar
FM (02)
Notations
:
C:
Cylinder
PRV: Pressure Regulator Valve
R: Reactor
TIC: Temp. Indicator & Controller
3WV: 3-Way Valve
GC: Gas Chromatography
MFC: Mass Flow Controller
PI:
Pressure Indicator
PI-01
Ar
Ar
FM (01)
Ar+CH3OH
Feed
to reactor
Feed
to GC
Feed
to GC
Ice Bath
Ar
Vent
CH3OH
Reactor
outlet
Reactor
inlet
Saturator
PRV-02
PRV-01
Cooler
Ar
Vent
Air
Electrical heating
H2
Products
to GC
furnace with
temperature
controller
Vent
Temperatur
e Controller
Controlled
power supply
Ar
Ar
GC
Ar Air H2
TIC-01
R-01
Furnace/Reactor
C-1
C-2
C-3
C-5
C-4
Figure 3. Experimental setup for performance test of CuO/ZnO/Al2O3 nanocatalysts used in hydrogen
production via methanol steam reforming.
28

CuO
CuAl2O4
ZnO
ZnAl2O4
(a) CZA-N-C-UN1
(b) CZA-A-C-UN1
(c) CZA-A-CU-UN2
(d) CZA-A-CU-UN1
Reference patterns
CuO (Monoclinic, 01-080-1268)
CuAl2O4 (Cubic, 01-071-0966)
ZnO (Hexagonal, 01-076-0704)
ZnAl2O4 (Cubic, 00-005-0669)
Al2O3 (Cubic, 00-004-0880)
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2θ (degree)
Figure 4. XRD patterns of synthesized CuO/ZnO/Al2O3 nanocatalysts: (a) CZA-N-C-UN1, (b) CZA-A-C-
UN1, (c) CZA-A-CU-UN2 and (d) CZA-A-CU-UN1.
29