CET Volume 86
DOI: 10.3303/CET2186134
Paper Received: 25 October 2020; Revised: 9 February 2021; Accepted: 18 April 2021
Please cite this article as: Garcia-Casas I., Montes A., Valor D., Pereyra C., Martinez De La Ossa E.J., 2021, Precipitation of Cerium Oxide
Nanoparticles by Sas Process, Chemical Engineering Transactions, 86, 799-804 DOI:10.3303/CET2186134
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 86, 2021
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-84-6; ISSN 2283-9216
Precipitation of Cerium Oxide nanoparticles by SAS Process
Ignacio García-Casas*, Antonio Montes, Diego Valor, Clara Pereyra, Enrique J.
Martínez de la Ossa
Chemical Engineering and Food Technology Department
University of Cadiz. Avda. República Saharaui, s/n, 11510, Puerto Real, Cádiz, SPAIN
ignacio.casas@uca.es
Cerium oxide nanoparticles could put a stop to tooth cavities. These nanoparticles could inhibit the formation
of the biofilm on teeth, known as plaque, created by bacteria. In this work, nanoparticles of cerium oxide were
generated using Supercritical antisolvent (SAS) process followed by a calcination process. The use of
supercritical fluids and particularly SAS process to prepare nanoparticles removes the drawbacks of the
conventional techniques such as excessive use of solvent, thermal solute degradation, high residual solvent
concentration, and mainly difficulty in controlling the particle size and particle size distribution during
processing. In the first step, nanoparticles of cerium oxide have been precipitated by SAS process. In this
preliminary work, the influence of the pressure on the particle size, particle size distribution, morphology, and
specific surface area of these particles have been investigated. Most of the experiments led to successful
precipitation of a cluster of particles in the nanometer range. In general, the morphology was greatly improved
to spherical nanoparticles. Particle size was higher when the pressure was lower. The specific surface area
increases dramatically after the SAS process, from 0.46 m
2/g to a gap between 27 m2/g and 197 m2/g.
1. Introduction
Nanoscience and Nanotechnology are currently one of the main focuses of attention of the scientific, industrial
and business community due to advances in the synthesis and manipulation of materials at the nanoscale,
with multiple applications in the near future. Nanomaterials present characteristics that differentiate them from
macro materials, and thus, the same material at nanoscale modifies its electrical, magnetic, optical, and
catalytic properties, among others, and these properties are used in fields as diverse as electronics,
biomedicine, pharmaceuticals, catalysis, and energy.
In recent years, the study of cerium nanoparticles has expanded beyond their use as a catalyst as Dowding et
al. (Dowding et al., 2013) that used nanoparticles of cerium oxide to accelerate the decay of peroxynitrite. Ta
et al. (Ta et al., 2013) tuned the shape of ceria nanomaterials for different catalytic applications. The use of
cerium dioxide nanoparticles has expanded its use in medical or pharmaceutical fields. Sehar et al. (Sehar et
al., 2021) shows the efficiency of these nanoparticles in decreasing photodegradation and bacterial activity. In
this way antibacterial activity of these particles of ceria against wound pathogens (Sharma et al., 2020) or
Escherichia coli showed by Senthilkumar et al. (Senthilkumar et al., 2019). Cerium nanoparticles have been
also used in advances in tissue engineering (Hosseini & Mozafari, 2020) or in biosynthesis and biomedical
application (Singh et al., 2020). Even, a recent article links nanoceria as an agent for the management of
covid-19 (Allawadhi et al., 2020).
Supercritical CO2 (scCO2) has been widely applied to produce nanoparticles of nutraceuticals,
pharmaceuticals, metallic, polymers, etc. It has low cost, low toxicity and fairly low critical temperature (31.1
˚C) and pressure (73.8 bar), which makes it quite adequate for processing thermolabile solutes in
environmental agreement (Jung & Perrut, 2001). In addition, the use of scCO2 processes provides distinct
advantages, such as greater product quality in terms of purity, more homogeneous dimensional
characteristics, and its properties can also be continuously tuned by altering the experimental conditions
(Cocero et al., 2009).
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SAS process has been already used to precipitate metallic nanoparticles (Rueda et al., 2014) using
micronized magnesium acetate as precursor for the production of magnesium oxide and magnesium hydrate.
Jiang et al. (Jiang et al., 2014) utilized the same process to precipitate a series of manganese–cerium oxide
(MnOx– CeO2) catalysts used for low temperature selective catalytic reduction (SCR) of NOx with NH3. Franco
et al. (Franco et al., 2018; Palma et al., 2017) precipitated the same particles, Ce(acac)3 using SAS process.
They studied the effect of concentration and flow rate on particle size and granulometric distribution. Its TEM
images showed a particle size around 50nm. Finally, these nanoparticles were used as catalyst support for
water shift reaction.
In the present work, the direct influence of pressure on the morphology, size and distribution of nanoparticles
were studied. At the same time, the greatest significance in the results obtained is reached in the specific
surface area of the nanoparticles. The remarkable increase in its specific surface area after using the SAS
technique, and the possibility of using supercritical impregnation techniques, make this technique a very useful
method to functionalize this kind of nanoparticles.
2. Material and methods
2.1 Materials
Cerium(III) acetylacetonate hydrate (Ce(C5H7O2)3 · xH2O) and ethanol absolute were supplied by Sigma-
Aldrich (Spain). Carbon dioxide (99.8%) was purchased from Abello-Linde S.A. (Barcelona, Spain).
2.2 Supercritical particle formation
Experiments were carried out in a lab-scale high-pressure equipment (SAS200) provided by Thar
Technologies (Pittsburgh, PA, USA) (Figure 1). The SAS 200 pilot plant includes two high-pressure pumps to
pump the CO2 (P1) and the solution (P2); a stainless steel precipitator vessel (V1) (0.5 L volume) where the
powder is gathered, composed by two main parts, a cylinder body and the frit, all enclosing by an electrical
heating jacket (V1-HJ1); an automated high-precision back-pressure regulator (ABPR1) connected to a
controller; a jacketed (CS1-HJ1) stainless steel cyclone separator (CS1) (0.5 L volume). SAS was used to
precipitate nanoparticles of Ce(acac)3. In this technique, CO2 was pumped into the vessel and when
supercritical conditions were achieved, the cerium solution was pumped into the precipitator vessel by a
nozzle. The small drops of solvent were dissolved by the supercritical CO2, causing supersaturation of the
liquid solution and consequent precipitation in the form of a powder that accumulated on the internal wall of
the vessel.
Figure 1. Schematic diagram SAS200 lab-scale plant
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In this work, Ce(acac)3 was dissolved in ethanol at concentrations of 3-6 mg/mL. Injection flow rates of 2-6
mL/min for scCO2 were used. The conditions of CO2 flow rate at 20 g/min and a drying time of 30 min were
kept constant. These particles were then calcined in a muffle oven at 673 K for 2 h, with a heating ramp of 10
K/min.
2.3 Morphology and particle size
Nanoparticles of Ce(acac)3 were analysed by SEM (FEI Teneo) in order to analyse the morphology and
structure of the nanoparticles. The particle size was analysed by a dynamic light scattering (DLS) with
Zetasizer Nano Z.
2.4 X-Ray Diffraction
X-ray diffraction (XRD) analysis was performed on a Bruker D8 advance diffractometer to establish the identity
and purity of the synthesized samples of the oxide nanoparticles obtained by SAS and calcined process. The
diffraction design was measured with CuKα radiation (40Kv, 40mA), 2Ɵ angle range from 20º to 75º with a
step size of 0.02º and 1 s as step time.
2.5 Nitrogen Physisorption
3. Specific surfaces areas and porosity were measured by ASAP 2420 system. The physical principle of
measurement is the adsorption of molecules of a gas in a solid sample. In this study, nitrogen adsorption-
desorption isotherms were recorded at 77.35 K. The nanoparticles of Ce(acac)3 and CeO2 powder was pre-
treated at 393 K for 3 hours. The specific surface area was determined by the BET method. Microporosity was
calculated by t-plot method.
4. Results and discussion
4.1 Study of the precipitated nanoparticles
In this preliminary work, six experiments were carried out to study the effect of the pressure in the formation of
Ceria nanoparticles. In a previous work (Yeo et al., 2000) determined that the critical point of the mixture CO2
+ ethanol for XA concentrations of 0.956 is 77.73 bar at 310.58 K, while for an XA of 0.938, the critical point is
86.35 bar at 318.24 K. To be sure that the experiments would be performed above MCP, the minimum
pressure and temperature conditions were fixed at 100 bar and 308 K. The Table 1 shows the experiments
and the average size obtained. The nanoparticles show an average size between 133-328 nm. It was
observed that in the three pairs of experiments (1-2, 3-4, 5-6), each pair at the same conditions, varying only
the pressure, shows that with increasing pressure, the average size of the nanoparticles decreases.
Table 1.Experiments carried out by SAS process. Exp (1-2) 308 K, 3 mg/mL, 6 mL/min; Exp (3-4) 308 K, 6
mg/min, 2 mL/min; (5-6) 323 K, 3 mg/mL, 2 mL/min
The transmission electron microscopy shows in all experiments a cluster of spherical nanoparticles. In Figure
2A and 2B, it can be observed the nanoparticles precipitated in experiment 2 at 180 bar, experiment 1, at 100
bar and the raw Ce(acac)3 before SAS process (Figure 2C), that shows nanoparticles but also sticks particles
of around 1 µm.
Experiments Pressure (bar) Particle Size (nm)
1 100 156 ± 46
2 180 133 ± 64
3 100 328 ± 67
4 180 198 ± 72
5 100 281 ± 91
6 180 230 ± 64
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(A) Exp.2 (B) Exp.1 (C) Raw
Figure 2. TEM images of Experiments 1, 2 and Raw Ce(acac)3
4.2 Samples Characterization
XRD analysis was presented in Figure 3. The diffractogram shows four peaks that are labeled and can be
identified and indexed as the face-centered cubic phase (Goharshadi et al., 2011). The clear strong and sharp
diffraction peaks denote the good crystallization of the sample. No extra peaks were detected in the XRD,
indicating a good purity of the prepared ceria nanoparticles.
Figure 3. XRD analysis of experiment 4 calcined
In the physisorption study (Table 2), it can be observed that for all experiments the specific surface area
increases very significantly after precipitating the cerium compound with the SAS technique. The commercial
Ce(acac)3 has a specific surface area of 0.43 m2/g, while the nanoparticles obtained range from 27 m2/g in
experiment 6 to 192 m2/g in experiment 4. In this case, it seems that the operating pressure, always under
supercritical conditions, does not clearly affect the specific surface area of the nanoparticles.
0
100
200
300
400
500
600
700
800
20 30 40 50 60 70 80
In
te
ns
it
y
(a
.u
.)
2ϴ(°)
802
On the other hand, it was studied the specific surface of the nanoparticles of cerium dioxide obtained in
experiments 3 and 4 after calcination. Both experiments show a reduction of the specific surface area after
calcination. Experiment 3 reduces its specific surface area from 66 m2/g to 25 m2/g, while experiment 4
reduces it from 192 m2/g to 43m2/g. As can be expected, the average pore size in the supercritical Ce(acac)3
tests decreases, which favors the increase of the specific surface area. Similarly, the average pore size
increases in experiments 3 and 4, once oxidized, from 10.04 nm to 23.67 nm and from 5.92 nm to 18.51 nm
respectively, which leads to a reduction of their specific surface area.
Table 2. Physisorption results for SAS experiments of Ce(acac)3 and CeO2
Experiments Pressure (bar) Specific Surface Area (m2/g) Average Pore Size (nm)
Raw 0.43 26.61
1 100 141 11.31
2 180 166 12.20
3 100 66 10.04
4 180 192 5.92
5 100 113 13.21
6 180 27 13.17
3 Calcined 25 23.67
4 Calcined 43 18.51
5. Conclusions
In this preliminary work the formation of cerium dioxide particles using the SAS technique and the influence of
pressure on the average size of the obtained nanoparticles was achieved with a wide range of conditions,
always under supercritical conditions. These nanoparticles showed a decrease in the average size by
increasing the pressure. The exponential increase of the specific surface area for both Ce(acac)3 and CeO2
nanoparticles, with respect to commercial Ce(acac)3, opens the possibility of functionalizing these
nanoparticles by impregnating them due to the significant increase of the specific surface area.
Acknowledgments
We gratefully acknowledge the Spanish Ministry of Economy Industry and Competitiveness (Project
CTQ2017-8661-R) for financial support.
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