Synthesis of CuO/ZnO/Al2O3/ZrO2/CeO2 nanocatalysts: Via homogeneous precipitation and combustion methods used in methanol steam reforming for fuel cell grade hydrogen production

Article abstract of DOI:10.1039/c6ra03858a

Homogeneous precipitation and urea-nitrate combustion methods have been comparatively investigated for the synthesis of CeO₂ promoted CuO/ZnO/Al₂O₃/ZrO₂ nanocatalysts. The catalytic performance of the samples wa

Full text of DOI:10.1039/c6ra03858a

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Synthesis of CuO/ZnO/Al2O3/ZrO2/CeO2 Nanocatalysts via
Homogeneous Precipitation and Combustion Methods Used in Methanol
Steam Reforming for Fuel Cell Grade Hydrogen Production
Saeed Khajeh Talkhoncheh1,2, Mohammad Haghighi*,1,2, Shahab Minaei1,2, Hossein Ajamein1,2, 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.
*
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

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Abstract
Homogenous precipitation and urea-nitrate combustion methods have been comparatively
investigated for synthesis of CeO2 promoted CuO/ZnO/Al2O3/ZrO2 nanocatalysts. The
catalytic performance of the samples is studied in the methanol steam reforming process for
fuel cell grade hydrogen production. The physicochemical properties of the prepared
nanocatalysts are characterized by XRD, FESEM, PSD, EDX, BET and FTIR analyses. XRD
analysis shows that the homogenous precipitation method and CeO2 addition improves the
dispersion and decreases the relative crystallinity of CuO and ZnO species. FESEM images
illustrates that the homogenous precipitation method is capable for synthesis of smaller
particles than the urea-nitrate combustion method. Addition of ceria to fabricated catalysts
decreases the particles size and enhances the surface homogeneity. EDX analysis
demonstrates that Cu and Zn elements are homogenously dispersed on catalyst surface which
is synthesized by the homogenous precipitation method. The catalytic performance shows
that applying the homogenous precipitation method improves activity while the ceria addition
leads to lower CO selectivity.
Keywords: CuO/ZnO/Al2O3/ZrO2/CeO2, Urea-Nitrates Combustion, Homogeneous Precipitation,
Methanol Steam Reforming, Hydrogen.
1

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1 Introduction
In recent years, fuel cells have been nominated as a promising alternative for the internal
combustion engine while the hydrogen supply as their feed is not solved yet 1, 2. Hydrogen
3-5
can be produced from a variety of feedstock such as methane, methanol, ethanol and etc.
.
But, it should be noted that application of fuel cells in vehicles forces us to produce on board
hydrogen. Among various alternatives, methanol due to its advantages such as high H/C ratio,
no C-C bond and low operation temperature is mostly attracted 6-10. Hydrogen can be
produced via methanol feed through four catalytic pathways 11, 12
 Methanol decomposition.
:
 Steam reforming of methanol.
 Partial oxidation of methanol.
 Oxidative steam reforming of methanol.
Among these methods, steam reforming of methanol (SRM) with high hydrogen yield and
low CO production is more desired 13, 14. The catalysts of this process are divided in two
groups: copper based catalysts and group VIII metals 15-17. Copper based catalysts as the most
appropriate catalysts for the steam reforming of methanol reaction are more active and
selective to hydrogen 18, 19
.
In order to improve the activity and reducibility of copper based catalysts, the most of
investigations have been focused on employing the proper promoter and the synthesis method
20-22. ZnO is known as the most desired promoter for enhancement of reducibility and
dispersion of Cu species 23, 24. Al2O3 as a structure promoter has positive effect on reactivity
of the CuO/ZnO catalysts 14, 25. It was added to the catalysts to increase the surface area,
mechanical resistance and thermal stability 8, 26. However, alumina at high loading has a
negative effect on the activity and hinders the reforming reaction 8, 27. In addition, other
promoters such as CeO2 and ZrO2 were interestingly studied due to their positive influence on
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the steam reforming of methanol process 22. The improvement in activity, stability and
dispersions of CuO and ZnO, as well as CO selectivity reduction are some of the significant
promoting effect of CeO2 8, 22. Furthermore, CeO2 prevented reducibility of the SRM catalyst
8. ZrO2 is introduced as a reducing agent of the active phase and improves dispersion of active
sites. It also can diminish CO yield and delays catalyst deactivation 8, 26-29
.
In addition to promoter effects, the physicochemical properties of the employed catalyst in
each process could be affected by the synthesis method 30-32. The conventional precipitation
and impregnation are known as the common methods for synthesis of copper based catalysts.
Many researches were performed to compare these methods with other novel fabrication
pathways 21. One of the new techniques that has been recently applied for the catalyst
synthesis is the urea-nitrate combustion (UNC) method. Unlike conventional precipitation
and impregnation methodologies, the UNC method is fast and simple due to elimination of
some time-consuming steps such as washing, filtering and drying 33. This method has two
agents: an oxidizer agent such as nitrate and sulfates salts and an organic fuel likes glycine,
citric acid and urea. Rapid and facile synthesis as well as producing homogenous powder by
cheap precursors are the advantages of the UNC method 34-36. On the other hand, some works
were interested on improvement of the conventional precipitation method by altering the
original pathway. The homogeneous precipitation (HP) method is known as an effective
synthesis method for production of highly dispersed active sites. Also, results have shown
37,
that high crystalline degree can be achieved by this method
38. Comparing the
homogeneous precipitation method with co-precipitation method shows that nanocatalysts
synthesized by the homogeneous precipitation method are more active because of its highly
dispersion and smaller particles of active phases 37.
Although so many works were performed to study the characterizations of catalysts
fabricated by homogeneous precipitation and urea-nitrates combustion methods individually,
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but there are a few researches to compare these methods. Therefore, the aim of this paper is
to compare the characteristic and catalytic properties of CuO/ZnO/Al2O3/ZrO2 nanocatalysts
synthesized by urea- nitrates combustion and homogeneous precipitation methods. Moreover,
the fabricated catalysts were promoted by ceria (10% wt) to investigate its influence on
characterization of prepared nanocatalysts. The characteristics of fabricated catalysts were
evaluated by XRD, FESEM, PSD, EDX, BET and FTIR analyses. The catalytic performance
of CuO/ZnO/Al2O3/ZrO2 (CZAZ) and CuO/ZnO/Al2O3/ZrO2/CeO2 (CZAZC) nanocatalysts
were studied by a fixed bed reactor through the steam reforming of methanol reaction.
2 Materials and Methods
2.1 Materials
Nitrates solution used as nanocatalyst precursor was applied by mixing of Cu(NO3)2.3H2O,
Zn(NO3)2.3H2O, Zr(NO3)4.5H2O, Ce(NO3)3.6H2O which were supplied by Merck Co.,
Germany. Aluminium hydroxide and Al(NO3)3.9H2O were used as alumina precursor in the
UNC and HP methods, respectively. These precursors as well as urea as the precipitant in the
HP method and fuel for the UNC method were supplied by Merck Company, Germany.
Distilled water was used for nanocatalyst preparation stages. All of the reagents were used as
received without further purification.
2.2 Nanocatalysts Preparation and Procedures
To investigate the effect of preparation method in synthesis of CeO2 promoted
CuO/ZnO/Al2O3/ZrO2 nanocatalysts, two samples with different compositions were
synthesized via urea-nitrate combustion (UNC) and homogeneous precipitation (HP)
methods. In this paper, CuO/ZnO/Al2O3/ZrO2 and CuO/ZnO/Al2O3/ZrO2/CeO2 nanocatalysts
were named as CZAZ and CZAZC, respectively. UNC and HP after the name of the
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catalysts were referred to urea-nitrate combustion and homogeneous precipitation,
respectively.
In the UNC method (Figure 1s in supplementary information), an aqueous solution of Cu, Zn,
Zr, Ce nitrates was made with desired composition. Aluminium hydroxide (Al(OH)3) was
heated under air flow at 400ºC for 4 h to produce boehmite (AlOOH) as the Al precursor. In
the next step, the nitrate solution of precursors beside boehmite and urea as the combustion
fuel were mixed for 45 min. The urea/nitrates ratio was determined 3. The resulted solution
was heated at 80ºC; to evaporate excess water and form a viscous gel. The gel put in a muffle
furnace where started to boil and froth at 400ºC and formed as black powder. Finally, the
resulted powder was calcined at 350ºC for 4 h under air flow.
In the HP method (Figure 2s in supplementary information), an aqueous solution of Cu, Zn,
Zr, Ce and Al nitrate precursors was mixed with urea (urea/nitrates=20 molar ratio). The urea
was hydrolysed by heating the mixture to 90ºC for 24 h to release the precipitants agent
(hydroxide ions). The precipitants were washed by deionized water and dried under air flow
at 80ºC for 24 h. Samples were calcined at 350ºC for 5 h. Finally, the prepared nanocatalysts
from both methods were shaped for catalytic performance studies toward methanol steam
reforming.
2.3 Nanocatalysts Characterization Techniques
X-ray diffraction (XRD) measurement was used to study crystalline structure of synthesised
nanocatalysts. It was carried out using a D5000 Siemens X-ray diffractometer with a Cu Kα
radiation source (0.154056 nm) which was operated at 30 kV and 40 mA. The resulted
patterns were compared with the Joint Committee on Powder Diffraction Standards (JCPDS)
to identify the crystallite phases of metal oxides. Moreover, the crystallite size was calculated
by the Scherrer equation as follows:
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K 
 cos()
 
Where τ is the mean size of the ordered crystallite. K, λ and θ are the shape factor, X-ray
wavelength and the Bragg angle, respectively. β is known as the line broadening at half the
maximum intensity (FWHM), after subtracting the instrumental line broadening.
Furthermore, for more statistical analysis of XRD patterns the relative crystallinity was
calculated by selection of a single peak of the specific material. The highest intensity of the
selected peak was assumed 100% of relative crystallinity and the others which surely had
lower intensity were calculated compared to the basis value.
The surface morphology of CZAZ and CZAZC nanocatalysts were studied by field emission
scanning electron microscope (FESEM, HITACHI S-4160). Before each test for more
accurate images, the samples were coated by an ultrathin layer of gold to increase the
conductivity of the surface. The particle size distribution histograms were depicted by ImageJ
software by applying the FESEM images. EDX and dot mapping were conducted for surface
elemental analysis (VEGA II, TESCAN). The specific surface area measurements were
investigated using nitrogen adsorption at 77 K and desorption at room temperature by a
Quantachrome ChemBET 3000 analyser. Before each measurement, the samples were
degassed at 200°C for 30 min. The FTIR spectra were acquired for surface functional groups
by means of a Unicam 4000 FTIR spectrometer, using KBr plate in the range of 400-4000
cm-1.
2.4 Experimental Setup for Catalytic Performance Test
A schematic flowchart of the experimental setup for catalytic performance test toward the
methanol steam reforming reaction is illustrated in Figure 1. The setup consists of a gas
feeding section, a fixed bed reactor and an analytical section. Methanol and water mixture
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were feed to the reactor by an argon stream through a saturator. The flow rate of this stream
was 70 ml/min which was controlled by a mass flow controller. The H2O/MeOH ratio was
considered 1.5 in the feed stream. The steam reforming of methanol reaction was carried out
in a U-shape Pyrex tube packed bed reactor (ID=8 mm and length=32 cm) with 0.4 g of
nanocatalyst loading. The heat of the reaction was provided by placing the reactor inside an
electrical furnace equipped with an electronic temperature controller. Prior to the catalytic
and stability tests, the nanocatalyst was reduced under a stream of hydrogen with a flow rate
of 100 ml/min at 300ºC under atmospheric pressure for 3 h. Experiments were carried out
over the temperature range of 200-300ºC to evaluate catalytic performance at atmospheric
pressure. The gas hourly space velocity (GHSV) was set at 10000 cm3gcat-1h-1 as the literature
reported for the best efficiency 39-41. An on-line gas chromatography (GC Chrom, Teif Gostar
Faraz, Iran) equipped with Plot-U Column (Agilent Technologies) as well as TCD and FID
detectors were used to analyse reactor products. Argon was used as the carrier gas in the GC
column.
The conversion of methanol and selectivity of the products have been defined as follows:
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.
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(Figure 1)
3 Results and Discussions
3.1 Nanocatalysts Characterization
3.1.1 XRD Analysis
The XRD patterns of the CZAZ-UNC, CZAZ-HP, CZAZC-UNC and CZAZ-HP
nanocatalysts are shown in Figure 2. As represented, most of the known peaks are at 2θ=30-
40˚. The index peaks of monoclinic CuO (JCPDS 01-080-1268) at 2θ=35.5˚, 38.8˚ and 48.8˚,
hexagonal ZnO (JCPDS 01-076-0704) at 2θ= 31.7˚, 34.4˚, 36.2˚ and 56.5˚ and tetragonal
ZrO2 (JCPDS 01-080-0784) at 2θ= 30.2˚ and 35.3˚ were well seen in CZAZ-UNC and
CZAZC-UNC nanocatalysts. However, in the CZAZ-HP and CZAZC-HP nanocatalysts,
applying the homogenous precipitation method leaded to decrease of peak intensity and the
relative crystallinity of all phases, but improved their dispersion. The index peaks of CeO2
(JCPDS 01-075-0076) are presented at 2θ= 28.8˚, 33.2˚, 47.7˚ and 56.6˚. Application of ceria
in CZAZ samples leaded to reduction of nanocatalysts crystallinity. This decrease in crystal
size is more obvious for the CZAZC-HP nanocatalyst. This is related to the nature of the
homogeneous precipitation method to decrease the crystals size and instead enhancement of
their dispersion which improved by addition of ceria. By a glance at the diffraction patterns
of all samples, no Al2O3 (JCPDS 00-004-0880) phase can be acquired. It can be interpreted
due to its special crystallite state, low loading and high dispersion 14. Studies have confirmed
that spinel compounds are formed at high calcination temperatures (>600ºC), so no form of
copper or zinc aluminate was not detected 42. One of the advantages of the homogenous
precipitation method, as seen in the XRD results, is improving the elements dispersion which
was happened for CuO, ZnO and ZrO2 crystals compared to the UNC method. Table 1
demonstrates the relative crystallinity (RC) and the crystallite size of observed phases.
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According to the results, the highest relative crystallinity of all species can be detected in
samples which were fabricated by the urea-nitrates combustion method. However, it was
predictable according to qualitative interpretation of XRD patterns which described the
crystals with low intensity were formed by the homogeneous precipitation. Also, addition of
ceria to CZAZ samples in both synthesis methods led to lower relative crystallinity of other
species due to overlapping of ceria crystals on other ones. This result was proven by the
crystallite size calculation via Scherrer's equation. According to calculated crystallite size of
CuO species, application of the HP method resulted smaller crystallites than the UNC
method. Furthermore, Scherrer calculation proved the nanocrystallite property of synthesized
catalysts. Small crystallite size in the homogeneous precipitation method can effect on
producing small particles and improving accessibility for methanol and water molecules.
(Figure 2)
(Table 1)
3.1.2 FESEM Analysis
The FESEM images of the synthesized nanocatalysts are shown in Figure 3. Combustion
gases produced through the UNC method create micro pores, which is one of the main
superior of the UNC method to other synthesis methods and facilitate the accessibility of feed
molecules to catalytic sites. But on the other hand, it can be seen that the catalytic particles
produced by the HP method are smaller and it seems that this kind of nanocatalyst has a
higher specific surface area compared to UNC ones. Addition of ceria had a positive effect on
improvement of the homogeneity and reduction of particles size in both HP and UNC
catalysts. However, their effect on HP nanocatalyst was more significant. All FESEM images
proved that the particles of all synthesized catalysts are nanometer. But the particles of
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CZAZC-HP sample are smaller than others. Therefore, it seems that CZAZC-HP sample may
has higher specific surface area and as a result achieves higher activity in catalytic
experiments.
(Figure 3)
3.1.3 PSD Analysis
Size distribution histograms of CZAZC-UNC and CZAZC-HP nanocatalysts which
calculated by ImageJ software, are illustrated in Figure 4. To compare the effect of the
synthesis method on particle size, two ceria promoted CZAZ samples which were fabricated
by the HP and UNC methods were studied by the ImageJ software. The particle size of
CZAZC-UNC and CZAZC-HP nanocatalysts are distributed between 16.9-66.5 nm and 9.8-
54.1 nm, respectively. In CZAZC-UNC nanocatalysts, most of the particles are in the range
30-40 nm and for CZAZC-HP nanocatalysts, they are in the range 20-30 nm. This means that
the HP method is capable to synthesis smaller particles than the UNC method.
(Figure 4)
3.1.4 EDX Analysis
EDX analysis of CZAZ and CZAZC nanocatalysts are illustrated in Figure 5. All of the
elements used in the fabricating method can be observed clearly, in the EDX spectra. As
seen, Cu as the main active site of the steam methanol reforming process in both CZAZ-HP
and CZAZC-HP nanocatalysts are well dispersed. Also Zn species have very low aggregation
and disperses well on the surface this sample. Due to EDX results the comparison between
parent solutions and surface elements was calculated and are shown in Figure 6. Producing
homogenous powders as a feature of the UNC method, has brought to surface analysis of
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CZAZ-UNC and CZAZC-UNC nanocatalysts does not have a well fit with its composition,
but the results of EDX dot mapping and parent solution have a well accordance for CZAZ-
HP and CZAZC-HP nanocatalysts.
(Figure 5)
(Figure 6)
3.1.5 BET Analysis
The specific surface area of the synthesised nanocatalysts has been illustrated in Table 1. As
resulted in XRD, FESEM and PSD analyses, the crystallite size and particle size of HP-
nanocatalysts compared to UNC-nanocatalysts are smaller and narrower. This result as the
influential parameter is in accordance of the BET analysis that the specific surface area of
HP-nanocatalyst synthesis has the highest specific area. Beside, due to the negative effect of
the UNC method in reducing specific surface area, the alumina effect on the enhancement of
surface area neutralized. This is because of the nature of the UNC method to provide foam
with more micro pores instead of nano pores.
3.1.6 FTIR Analysis
Figure 7 illustrates the FTIR spectra of fabricated nanocatalysts to describe their functional
groups. Generally, it can be understand that the spectra reveals significant peaks for all
samples especially the CZAZC ones. This is due to same functional groups in synthesized
nanocatalysts. As considered in the literature, metal oxides lead to the stretching frequencies
in the range of 480-800 cm-1 33. More precisely, absorption bands around 480 cm-1 and 590
cm−1 can be nominated to Cu-O and Zn-O species, respectively 43, 44. The band at 1640 cm-1
can be assigned to interlayer water molecules 5, 45, 46. Hydroxide group are also detected by
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the broad stretching band around 3450 cm-1 which may be loosely adsorbed by nanocatalysts
47-49. The presence of surface water in nanostructured catalysts is due to adsorption of water
molecules during different synthesis steps 50-52. Urea that was used in both synthesis methods
consists of hydrogen and carbon atoms. So, it can produce carbon dioxide during its reaction
with nitrates or thermally decomposition. Therefore, it is expectable to find out carbonated
species formed by adsorbed CO2 on nanocatalysts surface which was appeared at 1405 cm-1
53-55
.
(Figure 7)
3.2 Catalytic Performance Study toward Methanol Steam Reforming
3.2.1 Methanol Conversion
The catalytic performance of fabricated nanocatalysts by homogeneous precipitation and
urea-nitrates combustion methods were investigated in a fixed bed reactor through the steam
reforming of methanol reaction. The methanol conversion of synthesized nanocatalysts is
shown in Figure 8. Figure 8 (a) illustrates the effect of synthesis method on methanol
conversion of CZAZ nanocatalysts while it was investigated about the ceria promoted CZAZ
nanocatalysts in Figure 8 (b). It can be seen that application of the homogeneous precipitation
method for synthesis of CZAZ or CZAZC nanocatalysts leads to higher conversion.
However, this effect is more significant about the CZAZ samples which reached complete
conversion at about 220˚C. So, it can be concluded that the synthesis method has more
important role on methanol conversion of fabricated nanocatalysts. However, it was
predictable according to prominent characteristic properties of nanocatalysts synthesized by
the homogeneous precipitation. This difference between these two synthesis methods are
more apparent at low temperatures where the role of nanocatalysts are more significant. This
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result is much better than previous works which fabricated this kind of nanocatalyst by
different synthesis methods especially coprecipitation ones. Some examples are listed in
Table 2. It can be seen that all previous works which fabricated the CZAZ or CZAZC
catalysts by the conventional co-precipitation method had lower methanol conversion than
the present work. This difference exists even between the samples prepared by the
impregnation method as a popular and simple synthesis method. So, comparison between
these works and the CZAZ-HP sample shows higher efficiency of this fabricated nanocatalyst
for converting methanol to products especially hydrogen at low temperatures.
On the other hand, it can be understood that addition of ceria had no significant influence on
enhancement of methanol conversion while in some cases it partially decreased the
conversion values. Some reasons can be counted for this behaviour of ceria in CZAZ
8
nanocatalysts. As mentioned in previous works, CeO2 improves methanol conversion by
ameliorating the dispersions of CuO and ZnO in the nanocatalysts, but the steam methanol
reforming reaction was clearly weakened as CeO2 was doped. The interactions between CeO2
and Al2O3 have negative effect on the CeO2 and ZnO interaction as an effective parameter in
8
methanol reaction . Therefore, the methanol conversion values of CZAZC nanocatalysts are
lower than CZAZ samples due to negative effect of ceria on ZnO as an effective parameter on
methanol conversion. On the other side, the interaction between ceria and zirconia can
influence on good interaction between CuO and ZnO as the main active phases of the SRM
reaction to decrease the methanol conversion 56.
(Figure 8)
(Table 2)
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3.2.2 Products Selectivity
Figure 9 depicts the effect of synthesis method on products selectivity. High H2 selectivity
and low CO selectivity are two important parameters for selecting suitable steam methanol
reforming catalysts. A glance view demonstrates that all the samples have suitable
performance for production of hydrogen (around stoichiometric value) as the main desired
product. At higher temperatures the amount of CO increases because the reverse water-gas
shift reaction proceeds. In this research, it is observed that the addition of CeO2 reduces CO
8
selectivity according to previous works . It is worth nothing that selection of a good catalyst
for the steam reforming of methanol reaction should be done with regard to methanol
conversion and products selectivity. It means that a catalyst must be chosen which reached its
complete conversion at lower temperature and appropriate selectivity of products. So,
according to methanol conversion results it can be concluded that the CZAZC-HP
nanocatalyst which reached its complete conversion at 240˚C is the favorite sample in this
work. This choice is approved in terms of hydrogen and carbon monoxide selectivity.
Therefore, it can be concluded that the samples prepared by the homogeneous precipitation
and promoted by the ceria has better performance for production of fuel cell grade hydrogen
with low amount of CO from the steam methanol reforming process.
(Figure 9)
3.2.3 Time on Stream Performance
The result of the stability tests for CZAZC-HP nanocatalyst for 1200 min is shown in Figure
10. As seen the methanol conversion remained fixed for 1200 min. Also, the CO and H2
selectivity approximately stayed constant. The fine particle size distribution, well dispersion
and high surface area of the CZAZC-HP nanocatalyst have improved the stability of the
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nanocatalyst. Also, CeO2 is able to oxidize the deposited carbon on nanocatalyst surface. The
selectivity of the products has nearly stayed constant.
(Figure 10)
4 Conclusions
The effects of preparation method and promoter addition on physicochemical and catalytic
properties of CuO-ZnO-Al2O3-ZrO2 (CZAZ) nanocatalyst were studied. XRD patterns
showed that the homogeneous precipitation method improved the Cu dispersion compared to
the urea-nitrate combustion method. FESEM analysis indicated that HP-nanocatalysts had
more homogeneous and smaller particles than UNC-nanocatalysts. EDX dot mapping
analysis showed good dispersion of Cu and Zn elements of the HP-nanocatalysts. FTIR
analysis depicted that metal oxides were formed during the both preparation methods and no
annoying functional groups found on synthesized nanocatalysts. Catalytic performance tests
demonstrated the HP-nanocatalysts due to their characteristic advantages had low CO
selectivity and high methanol conversion. Moreover, CeO2 addition decrease CO selectivity
and modify Cu dispersion, however, CeO2 affected the interaction between ZnO and CuO,
and reduced methanol conversion.
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.
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DOI: 10.1039/C6RA03858A
Figures Caption
Figure 1. Experimental setup for activity test of CuO/ZnO/Al2O3/ZrO2 and
CuO/ZnO/Al2O3/ZrO2/CeO2 nanocatalysts used in hydrogen production via
methanol steam reforming. ..............................................................................22
Figure 2. XRD patterns of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c)
CZAZC-UNC and (d) CZAZC-HP..................................................................23
Figure 3. FESEM images of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP,
(c) CZAZC-UNC and (d) CZAZC-HP. ...........................................................24
Figure 4. Surface particle size distribution histogram of synthesised nanocatalysts: (a)
CZAZC-UNC and (b) CZAZC-HP..................................................................25
Figure 5. EDX analysis of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c)
CZAZC-UNC and (d) CZAZC-HP..................................................................26
Figure 6. Parent solution vs. surface chemical analysis of synthesised nanocatalysts:
CZAZ-UNC, CZAZ-HP, CZAZC-UNC and CZAZC-HP. .............................27
Figure 7. FTIR spectra of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c)
CZAZC-UNC and (d) CZAZC-HP..................................................................28
Figure 8. Influence of synthesis method and CeO2 addition on methanol conversion over
synthesised nanocatalysts: (a) CZAZ-UNC vs. CZAZ-HP and (b) CZAZC-
UNC vs. CZAZC-HP. ......................................................................................29
Figure 9. Influence of synthesis method and CeO2 addition on products selectivity over
synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c) CZAZC-UNC
and (d) CZAZC-HP..........................................................................................30
Figure 10. Time on stream performance of CZAZC-HP nanocatalyst..............................32
Tables Caption
Table 1. Chemical composition, specific surface area and structural properties of
synthesized nanocatalysts. ...............................................................................21
Table 2. Catalytic performance comparison of promoted CuO/ZnO/Al2O3 catalysts
synthesized by different methods.....................................................................21
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Tables and Figures
Table 1. Chemical composition, specific surface area and structural properties of synthesized
nanocatalysts.
Relative Crystallinitya
CuO ZnO Al2O3 ZrO2 CeO2 CuOc ZnOd Al2O3e ZrO2 CeO2
Crystallite sizeb (nm)
Synthesis Cu Zn Al Zr Ce
Method
BET
Nanocatalyst
(%) (%) (%) (%) (%) (m2g-1)
f
g
CZAZ-UNC
CZAZ-HP
Combustion 40 30 10 20
Precipitation 40 30 10 20
0
0
26.0
132.9
26.7
100
34.2
55.4 88.2
10.9 11.8
100
0.0
0
0
0
0
98.9
11.2
100 100
0.0 50.0
-
-
15.6 15.9
8.4
16.5 17.2
-
-
-
-
9.8
-
8.7 30.7
-
-
-
CZAZC-UNC Combustion 30 30 10 20 10
CZAZC-HP Precipitation 30 30 10 20 10
149.9
-
-
-
-
a. Relative crystallinity: XRD relative peak intensity.
b. Crystallite size estimated by Scherre's equation.
c. Crystallite phase: monoclinic (JCPDS: 01-080-1268, 2θ = 35.5, 35.6, 38.8, 38.9, 48.8, 61.6, 68.2)
d. Crystallite phase: hexagonal (JCPDS: 01-076-0704, 2θ = 31.7, 34.4, 36.2, 47.5, 56.5, 62.8, 67.9, 69.0)
e. Crystallite phase: cubic (JCPDS: 00-004-0880, 2θ = 37.4, 39.7, 42.8, 45.8, 67.3)
f. Crystallite phase: cubic (JCPDS: 01-080-0784, 2θ = 30.2, 34.6, 35.3, 43.0, 50.3, 50.7, 53.9, 59.4, 60.2)
g. Crystallite phase: cubic (JCPDS: 01-075-0076, 2θ = 28.8, 33.2, 47.7, 56.6, 59.4, 69.8, 77.1, 79.5, 88.9)
Table 2. Catalytic performance comparison of promoted CuO/ZnO/Al2O3 catalysts synthesized by
different methods.
Composition
(wt.%)
T
Methanol
Conversion
92.4
Sample
Synthesis Method
Reference
(˚C)
270
270
270
260
270
350
240
280
220
260
8
CuO/ZnO/ZrO2/Al2O3
CuO/ZnO/CeO2/ZrO2/Al2O3
CuO/ZnO/ZrO2/Al2O3
CuO/ZnO/CeO2/ZrO2
CuO/ZnO/ZrO2/Al2O3
CuO/ZnO/CeO2/Al2O3
CuO/ZnO/CeO2/ZrO2/Al2O3
CuO/ZnO/CeO2/ZrO2/Al2O3
CuO/ZnO/ZrO2/Al2O3
CuO/ZnO/ZrO2/Al2O3
Co-precipitation
Co-precipitation
Co-precipitation
Co-precipitation
Impregnation
30/30/30/10
30/30/9/27/10
30/40/20/10
45/20/17.5/17.5
15/15/10/60
10/10/40/40
30/30/10/20/10
30/30/10/20/10
40/30/20/10
40/30/20/10
8
89.4
80
100
70
96
100
100
100
57
56
27
58
Impregnation
Homogeneous Precipitation
Urea-nitrates Combustion
Homogeneous Precipitation
Urea-nitrates Combustion
Present Study
Present Study
Present Study
Present Study
100
21

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PI-02
Ar
MFC (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
MFC (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 1. Experimental setup for activity test of CuO/ZnO/Al2O3/ZrO2 and CuO/ZnO/Al2O3/ZrO2/CeO2
nanocatalysts used in hydrogen production via methanol steam reforming.
22

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CuO
ZnO
Al2O3
ZrO2
CeO2
(a) CZAZ-UNC
(b) CZAZ-HP
(c) CZAZC-UNC
(d) CZAZC-HP
Reference patterns
CuO (Monoclinic, 01-080-1268)
ZnO (Hexagonal, 01-076-0704)
Al2O3 (Cubic, 00-004-0880)
ZrO2 (Tetragonal, 01-080-0784)
CeO2 (Cubic, 01-075-0076)
20
30
40
50
60
70
80
90
2θ (degree)
Figure 2. XRD patterns of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c) CZAZC-UNC
and (d) CZAZC-HP.
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(a) CZAZ-UNC
300nm
(b) CZAZ-HP
500nm
500nm
500nm
300nm
(c) CZAZC-UNC
300nm
(d) CZAZC-HP
300nm
500nm
Figure 3. FESEM images of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c) CZAZC-UNC
and (d) CZAZC-HP.
24

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DOI: 10.1039/C6RA03858A
70
60
50
40
30
20
10
0
(a) CZAZC-UNC
Min: 16.9 nm
Max: 66.5 nm
Average = 31.0 nm
Standard deviation: 10.1
46.1 nm
39.2
30.8
100nm
14.0
11.2
3.5
1.4
0.0
200nm
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Particle size (nm)
70
60
50
40
30
20
10
0
Min: 9.8 nm
Max: 54.1 nm
Average = 24.0 nm
(b) CZAZC-HP
Standard deviation: 9.6
29.2 nm
44.7
100nm
17.0
17.0
12.8
7.4
1.1
0.0
200nm
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Particle size (nm)
Figure 4. Surface particle size distribution histogram of synthesised nanocatalysts: (a) CZAZC-UNC and
(b) CZAZC-HP.
25

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DOI: 10.1039/C6RA03858A
Zn
Cu
Al
(a) CZAZ-UNC
25.0µm
25.0µm
25.0µm
Cu-Zn-Al-Zr-O
Ce=0
Zr
25.0µm
25.0µm
25.0µm
keV
keV
keV
keV
Zn
(b) CZAZ-HP Cu
Al
25.0µm
25.0µm
25.0µm
Cu-Zn-Al-Zr-O
Ce=0
Zr
25.0µm
25.0µm
25.0µm
(c) CZAZC-UNC
Zn
Ce
Cu
Al
25.0µm
25.0µm
25.0µm
Cu-Zn-Al-Zr-Ce-O
Zr
25.0µm
25.0µm
25.0µm
(d) CZAZC-HP
Zn
Ce
Cu
Al
25.0µm
25.0µm
25.0µm
Cu-Zn-Al-Zr-Ce-O
Zr
25.0µm
25.0µm
25.0µm
Figure 5. EDX analysis of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c) CZAZC-UNC
and (d) CZAZC-HP.
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Cu
Zn
Al
Zr
Ce
120
100
80
60
40
20
0
CZAZC Nanocatalyst
CZAZ Nanocatalyst
CZAZ
Precursors
CZAZ
UNC
CZAZ
HP
CZAZC
Precursors
CZAZC
UNC
CZAZC
HP
Nanocatalyst
Figure 6. Parent solution vs. surface chemical analysis of synthesised nanocatalysts: CZAZ-UNC, CZAZ-
HP, CZAZC-UNC and CZAZC-HP.
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(a) CZAZ-UNC
(b) CZAZ-HP
(c) CZAZC-UNC
(d) CZAZC-HP
400
4000
900
3500
1400
3000
1900
2500
2400
2000
2900
1500
3400
1000
3900
500
Wavenumber (cm-1)
Figure 7. FTIR spectra of synthesised nanocatalysts: (a) CZAZ-UNC, (b) CZAZ-HP, (c) CZAZC-UNC
and (d) CZAZC-HP.
28