Investigation of properties of the sonomodified ZnO-CeO2 system

Article abstract of DOI:10.1080/15421406.2018.1542113

The ultrasonic treatment (UST) influence on properties of the ZnO-CeO₂ system (atomic ratios Zn/Ce = 25:75, 50:50, and 75:25) is studied. A decrease of the size of particles of the oxides, the formation of Zn-Ce-O nanocompositions, and an increase in the specific surface area of the samples are established. It is shown that the sonochemical treatment of oxide zinc-cerium mixtures allows one to increase their photocatalytic activity in the safranin T degradation process. A blue shift of the edge absorption in powder-like samples and an increase in the band gap energy (Eg) are fixed by the UV-vis spectroscopy. The catalytic test of the sonochemically treated ZnO-CeO₂ = 50:50 catalyst in the reaction of ethanol selective oxidation shows a possibility of its use in two processes: i) production of acetaldehyde with high selectivity in a low-temperature reaction (235 °C) with the recycle of the raw material, ethanol, and ii) hydrogen production without CO formation at temperatures more than 300 °C.

Full text of DOI:10.1080/15421406.2018.1542113

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Investigation of properties of the sonomodified
ZnO-CeO2 system
V.A. Zazhigalov, O.V. Sachuk, O.A. Diyuk, V.L. Starchevskyy, L.S. Kuznetsova,
M.M. Kurmach & N.D. Shcherban
To cite this article: V.A. Zazhigalov, O.V. Sachuk, O.A. Diyuk, V.L. Starchevskyy, L.S.
Kuznetsova, M.M. Kurmach & N.D. Shcherban (2018) Investigation of properties of the
sonomodified ZnO-CeO2 system, Molecular Crystals and Liquid Crystals, 672:1, 115-122, DOI:
10.1080/15421406.2018.1542113
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 672, NO. 1, 115–122
https://doi.org/10.1080/15421406.2018.1542113
Investigation of properties of the sonomodified
ZnO-CeO2 system
V. A. Zazhigalova, O. V. Sachuka, O. A. Diyuka, V. L. Starchevskyyb,
L. S. Kuznetsovaa, M. M. Kurmachc, and N. D. Shcherbanc
aInstitute for Sorption and Problems of Endoecology of the NASU, Kyiv , Ukraine;bLviv Politechnic
National University, Lviv, Ukraine;cL.V. Pisarzhevskii Institute of Physical Chemistry of the NASU,
Kyiv, Ukraine.
KEYWORDS
ABSTRACT
Sonochemical treatment;
zinc-cerium oxide
composites; nanoparticles;
photocatalyst; ethanol
The ultrasonic treatment (UST) influence on properties of the ZnO-
CeO2 system (atomic ratios Zn/Ce ¼ 25:75, 50:50, and 75:25) is
studied. A decrease of the size of particles of the oxides, the forma-
tion of Zn-Ce-O nanocompositions, and an increase in the specific
surface area of the samples are established. It is shown that the
sonochemical treatment of oxide zinc-cerium mixtures allows one to
increase their photocatalytic activity in the safranin T degradation
process. A blue shift of the edge absorption in powder-like samples
and an increase in the band gap energy (Eg) are fixed by the UV-vis
spectroscopy. The catalytic test of the sonochemically treated ZnO-
CeO2 ¼ 50:50 catalyst in the reaction of ethanol selective oxidation
shows a possibility of its use in two processes: i) production of acet-
aldehyde with high selectivity in a low-temperature reaction (235ꢀC)
with the recycle of the raw material, ethanol, and ii) hydrogen pro-
duction without CO formation at temperatures more than 300ꢀC.
1. Introduction
The oxides ZnO and CeO2 are well-known as two key semiconductors and promising
materials which attracted much attention in recent years for their potential application
for the environment protection, in catalysis with the use of renewable raw materials,
and in the technology of chemical materials [1, 2]. The properties of these individual
oxides can be improved by the addition of a second oxide and the creation of a com-
position of complexes. The various kinds of Zn-Ce-O nanostructures have been synthe-
sized by the use of various traditional methods such as the sputtering, reactive thermal
evaporation, chemical spray technique, pulsed laser deposition, metal-organic vapor
deposition, electrochemical impregnation, and sol-gel method [3–9]. But these techni-
ques are characterized by the some disadvantages, namely, by the use of metal salts as
raw materials, organic solvents, polymers, and surfactants which are potential harmful
pollutants. The next step of the preparation of complex oxide compositions is connected
with a high-temperature treatment accompanied by the aggregation of particles and an
increase of their sizes, and a decrease of the specific surface area. As a result, a decrease
of the catalytic activity is observed. So, the creation of new alternative methods of
CONTACT V. A. Zazhigalov
Slena951@ukr.net
ß 2019 Taylor & Francis Group, LLC

116
V. A. ZAZHIGALOV ET AL.
preparation of Zn-Ce-O nanoparticles is an actual problem. In [10, 11], the successful
use of a mechanochemical treatment for the preparation of Zn-Ce-O nanocompositions
was reported, and the obtained mesoporous samples had photocatalytic activity in the
dye degradation process.
In this study, the possibility to apply the sonochemistry as a next alternative method
to prepare nanocomposites of the ZnO-CeO2 system (Zn/Ce ¼ 25:75, 50:50, and 75:25)
from oxides is examined. The physical-chemical properties of prepared samples and
their photocatalytic and catalytic activities toward the decomposition of Safranin T and
a partial ethanol oxidation, correspondingly, are studied.
2. Experimental
2.1. Materials and Synthesis
The samples of the zinc-cerium oxide system with molar ratios ZnO/CeO2 ¼ 25:75,
50:50, and 75:25 were prepared by the mixing of powders of the oxides ZnO and CeO2
(all reagents marked “pure”). The sonochemical treatment of the samples (total mass
was 10 g) was conducted in an aqueous medium (80 ml) during 1 h in an ultrasonic dis-
perser UZDN operating in the acoustic cavitation mode at a frequency of 22 kHz and
an energy load of 3 W/cm2. The temperature was stabilized at 80 ꢀC by the circulation
of cold water around a reactor. The obtained suspensions were dried at 120 ꢀC in air.
2.2. Methods of analysis
The as-prepared ZnO-CeO2 powders were examined by the X-ray diffraction analysis
on a diffractometer D8 Advance (Bruker), by using the Cu Ka radiation with 0.05ꢀ step
size over the 2H interval from 10ꢀ to 80ꢀ. The average size (L) of crystallites in the
powders for the most intense reflections was estimated from the Scherrer relation
Kk
L ¼
;
(2.1)
b cos H
where L is the size of crystallites (nm), K ¼ 0.9 is the shape factor, the ‘uKa wavelength
kꢁ0.154 nm, b is the full peak width at half maximum intensity (radians), and H is the
Bragg (diffraction) angle.
The specific surface area (SBET) was determined from nitrogen adsorption isotherms
measured at 77 K using a Quantachrome NOVA-2200e instrument. Before the measure-
ments, each sample was degassed under a vacuum of 10ꢂ5 Torr at 453 K during 20 h.
The FT-IR spectra were recorded on a Spectrum-One spectrometer (Perkin-Elmer) in
a region 400-4000cmꢂ1 (mass ratio sample/KBr ¼ 1:20). Thermogravimetric analysis
(TG-DTA) was carried out in the interval 20-900 ꢀC in air with a rate of temperature
increase of 10ꢀ/min on a Derivatorgaph – Q equipment. The band gap was calculated,
by basing on the absorbance spectrum of the samples according to the relation
1240
Eg ¼
;
(2.2)
k
where Eg is the band gap (eV), and the k is the wavelength (nm) of the absorption
edges in the spectrum. The photocatalytic activity was evaluated by measuring the

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
117
degree of safranin T degradation, by using a high-intensity Na-lamp (70 W). The rate
constants (Kd) were calculated according to the first-order equation for the dye decolor-
ization. UV-vis spectra were recorded from 240 to 800 nm, by using a UV-2450
Shimadzu instrument. The catalytic activity of samples in the ethanol oxidation was
tested in a flow fixed-bed stainless microreactor at the atmospheric pressure in the tem-
perature interval 25-350 ꢀC. The initial components and reaction products have been
analyzed on-line by the gas chromatography, with flame ionization (FID) and thermal
conductivity detectors (TCD). The reaction of ethanol partial oxidation proceeds with
main products H2 and acetic aldehyde and can be presented by the following equations:
C2H5OH þ 1=2O2 ! CH3CHO þ H2O;
2C2H5OH þ 3O2 ! 4CO þ 6H2;
C2H5OH þ 1:5O2 ! 3H2þ2CO2:
(2.3)
(2.4)
(2.5)
The ethanol conversion, acetaldehyde selectivity, and its yield were calculated accord-
ing to the formulas
À
Á
ð Þ
ð
Þ
‘
in ꢂ‘
out
EtOH
ðEtOÞH
XEtOH
¼
ꢃ 100%;
ꢃ 100%;
(2.6)
(2.7)
‘
in
EtOH
CAc
À
Á
SAc
¼
ð Þ
ð
Þ
‘
in ꢂ CEtOH out
EtOH
XEtOHSAc
YAc
¼
:
(2.8)
100
Here, XEtOH – ethanol conversion %, SAc – selectivity of acetaldehyde %, YAc – acetal-
dehyde yield %, ‘ – initial ethanol molar concentration, ‘ – ethanol
EtOH(in)
EtOH (out)
molar concentration after a reactor, and ‘ – acetaldehyde molar concentration after
Ac
a reactor.
3. Results and Discussion
The XRD patterns of ZnO-CeO2 composites treated by ultrasound are shown in Fig. 1.
It can be seen that the diffractograms of the initial and modified samples show ZnO
peaks, which are indexed to the hexagonal wurtzite phase (PDF card No. 74-534), and
reflections of CeO2, which correspond to the cubic fluorite structure of ceria (JCPDS
No. 34-0394) with maximum reflex from plane (111). It is found that the ultrasonic
treatment leads to an insignificant decrease of the intensity of basic reflexes without
change of the dominant reflex, which can be connected with the isotropic deformation
of CeO2 and indicate a size reduction of particles of the initial oxides.
The calculations of the size (L) of particles demonstrate their decrease by 2-3 times
after the treatment in comparison with initial mixture samples and individual oxides
(Table 1). These results well agree with adsorption data, which show an increase of the
specific surface area (SBen). The change of porous structure parameters and total pore
volume (VR) (Table 1) after the treatment were observed as well.
The FTIR spectra of ZnO-CeO2 compositions after the ultrasonic treatment are
shown in Fig. 2. It is established that, after UST the intensities of stretching and bend-
ing vibrations of the adsorbed water molecules (at 3200 and 1692 cmꢂ1, respectively)

118
V. A. ZAZHIGALOV ET AL.
Figure 1. XRD pattern of ZnO-CeO2 compositions: initial 50:50 – a, after UST 25:75 – b, 50:50 –
c, 75:25 – d

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
119
Table 1. Size of crystallites and the porous structure of samples after UST
Sample
L, nm
SBen, m2/g
VRꢃ10-2, cm3/g
ZnO
CeO2
50:50 initial
25:75
50:50
75:25
80
57
60
32
29
21
4
3
2
3
4
8
8
5
1
4
5
6
L – average crystallite size; SBen – specific surface area, and VR – total pore volume of powders, respectively.
decrease, which is caused by a decrease of the content of surface groups jy- as absorp-
tion centers. The initial sample Zn/Ce ¼ 50:50 (Fig. 2, a) demonstrates a peak at
613 cmꢂ1, which is attributed to the characteristic absorption band of the ZnO bond,
and a broad peak at 1195 cmꢂ1, which corresponds to the Ce-O-Ce bond. It is found
that the sonochemical modification accompanied by a shift of these absorption bands in
the short-wave region up to 609 and 1177 cmꢂ1, respectively, and the appearance of the
absorption band of ‘e-j bonds at 970 cmꢂ1, which is caused by the ‘e-j bond redistri-
bution as result of the treatment (Fig. 2, b-d).
The study of the thermal stability of oxide Zn/Ce composites show that the DTA-
curve of the initial sample Zn/Ce ¼ 25:75 has two endothermic effects, which corres-
pond to the water elimination process – at 70-180 ꢀC and 190-240 ꢀC with a total weight
loss of 8-10%. It is found that the sonochemical treatment leads to changes in DTA-TG
curves. So, for the sample Zn/Mo ¼ 25:75, the presence of two thermal effects on the
DTA-curve is observed. The first endothermic peak, which corresponds to the water
desorption, is observed at 95-210 ꢀC and its accompanied by an insignificant weight loss
(0.2%). The second exothermic effect at 252-340ꢀ‘, which can be attributed to the crys-
tallization process of CeO2, and some loss of weight (0.5%) are observed. The thermal
treatment of the sample Zn/Ce ¼ 50:50 after the ultrasound modification shows that the
dehydration process proceeds by one step before 163ꢀ‘ in contrast to the initial com-
position, where this process proceeds in two stages in the interval 70-240 ꢀC. These
changes can be related to the formation of a homogeneous porous structure of the sam-
ple after the treatment. In addition, two exothermic effects at 460 and 660ꢀ‘, whose
presence is associated with the Znj crystallization process and polymorphous transfor-
mations, respectively (without weight loss) are observed on the DTA-curve. Similar
changes are characteristic of the sample Zn/‘e ¼ 75:25.
In order to evaluate the oxide Zn/‘e system photocatalytic activities before and after
UST, the degradation of dye Safranin T in an aqueous solution are studied. The results
obtained at the visible light irradiation of a dye solution in the presence of sonochemi-
cally treated Zn-Ce-O photocatalysts demonstrate a decrease of the absorption max-
imum intensity at 520 nm, which indicates an increase of the photocatalytic activity of
treated samples in comparison with the initial composition. The results of calculations
of the degradation rate constants presented in Table 2 confirm this fact.
The optical properties of compositions, which were studied by the UV-vis spectros-
copy, show that the absorption edge of the initial sample Zn/Ce ¼ 50:50 is at 396 nm
corresponding to the band gap energy equal to 3.13 eV. After the sonochemical treat-
ment of samples, a blue shift of the absorption edge is observed, and the calculated
band gap energies for treated samples have the values slightly lower in comparison with

120
V. A. ZAZHIGALOV ET AL.
Figure 2. FTIR spectra of the initial sample Zn/Ce ¼ 50:50 – a, and compositions Zn/Ce ¼ 25:75 –
b, 50:50 – c, 75:25 – d after UST
Table 2. Results of optical and photocatalytic researches of the sonomodified ZnO-CeO2 system
Atomic ratio Zn/Ce
k, nm
Eg, eV
Kdꢃ104, c-1
ZnO
CeO2
initial (50:50)
25:75
50:50
75:25
382
392
396
393
392
391
3.24
3.16
3.13
3.15
3.16
3.17
2.2
0.1
0.2
1.4
3.3
3.7
k - absorption edge, Eg – band gap energy, Kd - degradation rate constant of a safranin T water solution
the band gap value of pure zinc oxide (Table 2). These values are close to Eg of ‘eO2.
The data presented in Table 2 show that, in the series Zn/‘e ¼ 25:75, 50:50, 75:25 of
treated samples, an insignificant increase of the band gap energy from 3.13 up to 3.17 is
observed, which can indicate the quantum-dimensional effect of ZnO. We observe that
a shift of the absorption band in the short-wave region and an increase of the band-gap
of composites proceed with an increase of the zinc oxide content in a mixture. Such
changes can be connected with the oxide dispersion, accumulation of an electronic
density excess on the crystal surface, and formation of hydrogen bonds with solvent
molecules as a result of the treatment in a water solution.
The sample ZnO-CeO2 ¼ 50:50 after the ultrasonic treatment was tested in the partial
oxidation of ethanol (POE). It is well known that hydrogen may be generated from
ethanol by different ways, namely, by the steam reforming (SRE), partial oxidation
(POE), and oxidative steam reforming (OSRE). Generally, Ce- and Ni-containing cata-
lysts are used for the H2 production from ethanol [12–14]. SRE is a highly endothermic
reaction. Therefore, the high operation temperatures are used, and an inert medium
should be introduced, which leads to high capital and operation costs, as well as to
problems with the environmental protection. An alternative approach is the partial oxi-
dation of ethanol (POE), where the reaction can be realized at relatively lower tempera-
tures. But it is worth to note that, before the POE reaction, the catalysts must be

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
121
Figure 3. Temperature dependence of the ethanol conversion and selectivity of the formation of
reaction products on the sonomodified ZnO-CeO2 ¼ 50:50 catalyst
previously reduced, and the in situ treatment by H2 should be used [14]. In our case
for the ZnO-CeO2 catalyst prepared by the sonochemistry, any preliminary H2-treat-
ment was used before the catalytic POE reaction.
The data presented in Fig. 3 show the ethanol conversion and the selectivity concern-
ing the products in the temperature interval 235-300 ꢀC on the ZnO-CeO2 ¼ 50:50 sam-
ple after the ultrasonic treatment. The obtained results demonstrate that the main
product of the POE reaction at 235 ꢀC is acetic aldehyde, which is formed with a select-
ivity equal to 96%. Its amount is reduced, as the temperature increases, and its complete
disappearance occurs at 300 ꢀC. At the reaction temperature equal to 270 ꢀC, the appear-
ance of H2 was observed, and its amount increases with the temperature.
So, the sample obtained by the sonochemical treatment demonstrates a possibility of
its use in two processes on the base of a renewable raw material such as ethanol. The
first process is the preparation of acetaldehyde with the high selectivity at a low ethanol
conversion and with the possibility of the raw ethanol recycling, which proceeds at a
low temperature of the reaction.
Second, the hydrogen production at a medium temperature with essential high select-
ivity and the possibility to use this hydrogen directly in fuel cells are related to the
absence of CO in reaction products. The possibility of the stable operation of the cata-
lyst during 18 h at 300 ꢀC without changes in the distribution of reaction products
is shown.
4. Conclusion
A decrease of the size of particles of the initial oxides, increase of the specific surface
area, and the formation of Zn-‘e-O nanocomposites are observed as a result of the
ultrasonic treatment of ZnO-CeO2 powder mixtures. The prepared nanostructured
ZnO-CeO2 compositions possess a higher photocatalytic activity than the initial sample
in the reaction of degradation of Safranin T dye in a water solution. The new by-func-
tional catalyst for the renewable raw material (ethanol) use is obtained by UST of this

122
V. A. ZAZHIGALOV ET AL.
composition. At a low reaction temperature, this catalyst allows one to obtain acetalde-
hyde with high selectivity, and the process with a recycle of the raw material can be
realized. Practically pure hydrogen (without CO traces) from ethanol (at its 100-% con-
version) in air can be obtained at reaction temperatures more than 300 ꢀC, which can
be directly used in fuel cells. These catalytic results were obtained without any add-
itional operation (catalyst pretreatment by H2), which is necessary, according to litera-
ture data, for the activation of the ZnO-CeO2 system.
Acknowledgement
This work was financially supported by NASU Programs: Fundamental Research “New
Functional Substances and Materials for Chemical Engineering” (project 7-17/18) and Program
for Young Scientists (project 41- “Synthesis of new nanodispersed photoacatalysts of environmen-
tal protection processes”).
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