Nanostructured CeO2 as catalysts for different AOPs based in the application of ozone and simulated solar radiation

Article abstract of DOI:10.1016/j.cattod.2016.04.034

Two CeO₂ catalysts with different morphology (nanocubes and nanorods) were synthetized by hydrothermal treatment, characterized by TEM, XRD, N₂ adsorption-desorption isotherms, XPS and DR-UV–vis spectroscopy and examined for different AOPs (photocatalytic oxidation, catalytic and photocatalytic ozonation) using simulated solar radiation (λ > 300 nm) or visible radiation (λ > 390 nm) from a Xe lamp, and DEET as target compound. Neither significant DEET removal nor mineralization was achieved by solar photocatalysis and the catalysts did not show an important beneficial effect during catalytic ozonation. The best results were obtained in photocatalytic ozonation process being nanorods more active under visible radiation according to its lower band gap energy whereas nanocubes presented the highest intrinsic photocatalytic activity under visible and solar radiation. Photocatalytic ozonation of DEET with CeO₂ catalysts proceeds through a complex reaction mechanism that involves at least ozone-direct reaction that triggers a radical chain mechanism, indirect ozone reactions, ozone photolytic and photocatalytic decomposition and hydrogen peroxide formation-decomposition.

Full text of DOI:10.1016/j.cattod.2016.04.034

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Nanostructured CeO as catalysts for different AOPs based in the
2
application of ozone and simulated solar radiation
∗
E. Mena, A. Rey , E.M. Rodríguez, F.J. Beltrán
Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, Av. Elvas s/n, 06006 Badajoz, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Two CeO catalysts with different morphology (nanocubes and nanorods) were synthetized by hydrother-
2
Received 29 January 2016
Received in revised form 5 April 2016
Accepted 11 April 2016
Available online xxx
mal treatment, characterized by TEM, XRD, N2 adsorption-desorption isotherms, XPS and DR-UV–vis
spectroscopy and examined for different AOPs (photocatalytic oxidation, catalytic and photocatalytic
ozonation) using simulated solar radiation (␭ > 300 nm) or visible radiation (␭ > 390 nm) from a Xe lamp,
and DEET as target compound. Neither significant DEET removal nor mineralization was achieved by solar
photocatalysis and the catalysts did not show an important beneficial effect during catalytic ozonation.
The best results were obtained in photocatalytic ozonation process being nanorods more active under
visible radiation according to its lower band gap energy whereas nanocubes presented the highest intrin-
sic photocatalytic activity under visible and solar radiation. Photocatalytic ozonation of DEET with CeO2
catalysts proceeds through a complex reaction mechanism that involves at least ozone-direct reaction
that triggers a radical chain mechanism, indirect ozone reactions, ozone photolytic and photocatalytic
decomposition and hydrogen peroxide formation-decomposition.
Keywords:
Nanostructured CeO2
DEET
Ozone
Solar light
Photocatalytic ozonation
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
In this sense, cerium oxide CeO2 is a wide band-gap semiconduct-
ing material usually in the range of 2.9–3.2 eV which absorbs light
in the near UV and slightly in the visible region depending on
the morphology and particle size, being its properties as catalyst
and catalytic support highly shape-size dependent [7–9]. In addi-
Water pollution due to specific organic contaminants (phenolic
compounds, aromatic hydrocarbons, pesticides, pharmaceuticals,
etc.) is a growing problem that requires the implementation of
advanced technologies capable of removing not only the initial
compounds but also their metabolites. Among the available tech-
nologies, advanced oxidation processes (AOPs) have demonstrated
to efficiently remove a wide variety of organic pollutants due to the
generation of oxidizing species, mainly hydroxyl radicals, which
non-selectively attack the organic matter [1]. Photocatalytic treat-
ments are promising alternatives due to the possibility of using
natural solar light as a radiation source to minimize treatment
costs. Besides, the combination of photocatalysis with ozone (pho-
tocatalytic ozonation) increases the generation of oxidizing species
leading to higher degrees of mineralization than individual treat-
ments of ozonation or photocatalysis [2–4].
tion, CeO possesses a sufficiently long lifetime of photo-generated
2
electron-hole pairs to trigger photocatalytic reactions in liquid and
gas phase [10]. Bare CeO2 with different morphologies/properties
(microspheres, nanospheres, nanosheets, etc.) has been used as
photocatalyst in the degradation of organic pollutants in water,
most of them belonging to the dyes family [11–15] or phenol and
phenol derivatives [8,16]. These works pointed out that the mor-
phology and crystal size of CeO are highly important in the amount
2
and nature of surface defects, oxygen vacancies, CeO -light inter-
2
action and, definitely, in the behavior of CeO2 as photocatalyst.
On the other hand, CeO2 has demonstrated to be catalytically
active during ozonation of organic pollutants of different nature
such as phenol [17], oxalic and oxamic acids [18] or bezafibrate [19].
Main reaction pathways proposed are related to the adsorption and
subsequent decomposition of ozone into hydroxyl radicals, being
the presence of Ce(III)/oxygen vacancies in the surface the main
responsible species of the catalytic activity [17,18].
For the photocatalytic processes, much of the research effort
is focused on the development of photocatalysts capable of effec-
tively absorbing visible light in order to increase the fraction of
usable solar radiation compared to the commonly used TiO2 [5,6].
As far as we know, the combination of photocatalytic oxidation
with ozone using CeO2 as photocatalyst has not been previously
reported but it is expected to take benefits of both processes (pho-
∗ Corresponding author.
E-mail address: anarey@unex.es (A. Rey).
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0
920-5861/© 2016 Elsevier B.V. All rights reserved.
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tocatalysis and catalytic ozonation) together with the synergism
observed during photocatalytic ozonation treatment [2]. Therefore,
the aim of this work focuses on the application of two nanostruc-
tured CeO2 catalysts with different morphology (nanocubes and
nanorods) in ozone and photocatalysis based AOPs (photocataly-
sis, catalytic and photocatalytic ozonation) using simulated solar
radiation (300–800 nm) and visible radiation (390–800 nm) from
a Xe lamp, and N,N-diethyl-meta-toluamide (DEET) as target com-
pound. DEET is a common insect repellent found in different aquatic
environments [20], that presents a low reactivity with ozone [21],
facilitating the comparison between the different processes.
5
4
3
2
1
0
1
00
NR
NC
80
6
0
0
4
20
2
. Experimental
0
800
300
400
500
600
700
2.1. Catalysts preparation
Wavelength (nm)
Two CeO2 catalysts were synthetized by hydrothermal treat-
Fig. 1. UV–vis characteristics of the photocatalytic system applied (spectral irradi-
ance of the Xe lamp in the solar simulator, transmittance of the UV filter and UV–vis
absorbance of the CeO2 catalysts).
ment according to a previous work [22]. Briefly, Ce(NO ) ·6H O
3
3
2
was dissolved in 6 M NaOH solution and then transferred to a
◦
1
25 mL autoclave (filled at 75%) and heated during 24 h at 100 C for
◦
−1
the reactor was loaded with 500 mL of 5 mg L of DEET aqueous
nanorods (NR) or 180 C for nanocubes (NC). After the hydrothermal
treatment, the autoclave was cooling down to room tempera-
ture and then, the precipitates were separated by centrifugation,
−1
solution (pH0 = 6). The catalyst was then added (0.25 g L ) and the
suspension was magnetically stirred in the dark for 30 min before
◦
−1
switching on the lamp and feeding ozone to the reactor (15 L h gas
washed sequentially with water and ethanol and dried at 100 C
−
1
overnight.
flow rate, 10 mg L O3, with an ozone generator (Labor-Ozonisator
from Sander) fed with pure oxygen from a cylinder). The tem-
◦
2
.2. Characterization
perature in all the experiments was maintained at 35 C. Besides
photocatalytic ozonation (CeO /O /h␯), blank experiments of pho-
2
3
CeO2 catalysts were characterized by different techniques. X-
ray diffraction (XRD) patterns were recorded using a powder
tolysis (h␯), single ozonation (O3), photolytic ozonation (O3/h␯),
adsorption (CeO2), photocatalysis (CeO2/h␯) and catalytic ozona-
tion (CeO2/O3) were carried out for comparative reasons. Most of
the experiments were triplicated. Samples were withdrawn from
the reactor at different interval times, filtered with 0.2 ␮m PTFE
membranes and analyzed.
Bruker D8 Advance XRD diffractometer with a Cu K␣ radiation
◦ ◦
␭ = 0.1541 nm). The data were collected from 2␪ = 20 –80 at
(
−1
a scan rate of 0.02 s . A Tecnai G2 20 (FEI Company) trans-
mission electron microscope at 200 kV was used to study the
crystallinity and morphology of the samples. Textural properties
DEET concentration was analyzed by HPLC-DAD (Hitachi, Elite
LaChrom) using a Phenomenex C-18 column (5 ␮m, 150 mm long,
were analyzed by nitrogen adsorption–desorption isotherms at
◦
−1
−
196 C using an Autosorb-1 apparatus (Quantachrome). The sam-
3 mm diameter) and 0.6 mL min of acetonitrile-acidified water
◦
−2
ples were outgassed at 250 C for 12 h under vacuum (<10 mbar).
Surface characterization was performed by X-ray photoelectron
spectroscopy. XPS spectra were obtained with a K␣ Thermo Sci-
entific apparatus with Al K␣ (h␯ = 1486.68 eV) X-ray source using
a voltage of 12 kV under vacuum (2 × 10 mbar). Binding energies
were calibrated relative to the C1s peak at 284.6 eV and spectra
deconvolution was accomplished using XPS-Peak 4.1 software. Dif-
fuse reflectance UV–vis spectroscopy (DR-UV–vis) measurements
were performed with an UV–vis-NIR Cary-5000 spectrophotometer
(0.1% formic acid) as mobile phase (30–70 v/v, isocratic). Iden-
tification and quantification were carried out at 220 nm. Total
organic carbon (TOC) was measured using a Shimadzu TOC-VSCH
analyzer. Aqueous ozone was photometrically determined by the
indigo method at 600 nm [24], in a UV–vis spectrophotometer Evo-
lution 201 (Thermospectronic) and ozone in the gas phase was
continuously monitored by an online analyzer (Anseros Ozomat
GM-6000Pro). Hydrogen peroxide concentration was photomet-
rically determined by the cobalt/bicarbonate method, at 260 nm
using the same spectrophotometer [25]. Short-chain organic acids
were analyzed by ion chromatography with chemical suppression
(Metrohm 881Compact Pro) and a conductivity detector, using a
−7
(
Varian-Agilent Technologies) equipped with an integrating sphere
device. Band gap values of the samples were obtained using Tauc’s
equation, assuming an indirect band gap semiconductor behavior
◦
[
23].
MetroSep A sup 5 column (250 mm long, 4 mm diameter) at 45 C
−
1
and 0.7 mL min of Na CO3 from 0.6 to14.6 Mm in 50 min (10 min
2
2.3. Catalytic activity measurements
post-time for equilibration) as mobile phase.
Photocatalytic experiments were carried out in a solar simula-
3. Results and discussion
tor (Suntest CPS, Atlas) provided with a 1500 W Xe lamp operated
−
2
at 550 W m . In some cases light transmission was restricted to
> 390 nm by means of a modified polyester cut-off filter (Edmund
3.1. Photocatalysts characterization
␭
Optics) for visible experiments whereas a window glass filter was
used for simulating the spectrum of solar radiation reaching the
Earth’s surface (␭ > 300 nm). The spectral irradiance of Xe lamp
using the window glass filter, and the transmittance of the polyester
filter are represented in Fig. 1. The experiments were carried out
in semi-batch mode (batch with respect to the liquid phase and
continuous with respect to the gas phase) using a borosilicate glass-
made round flask provided with gas inlet, gas outlet and a liquid
sampling port. In a typical photocatalytic ozonation experiment,
The formation of nanostructured CeO2 with NC or NR morphol-
ogy was confirmed by TEM as can be observed in Fig. 2(A). From
the NC images, also some rectangular prism particles have been
detected (a minority in the images analyzed). The size of NC and NR,
determined from several TEM representative images (not shown),
is given in Table 1. A size distribution between 25 and 100 nm was
obtained for NC whereas the NR presented a thickness of ∼7.2 nm
and lengths between 40 and 200 nm. NC and NR showed {100} and
{110} + {100} exposed facets, respectively, which is in accordance
Please cite this article in press as: E. Mena, et al., Nanostructured CeO2 as catalysts for different AOPs based in the application of ozone
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Fig. 2. Structural characterization of the CeO2 nanostructured catalysts: (A) TEM images and (B) XRD patterns.
Table 1
Properties of the CeO2 nanostructured catalysts.
aSize (nm)
25–100
Exposed facets CeO2 crystalline phase
SBET (m2
14
g
−1
)
Vp (cm3
0.185
g
−1
)
bCe(III) (%, at.) bCe(IV) (%, at.) Eg (eV)
Catalyst Synthesis
NC
Hydrothermal
{100}
Cubic
12.6
87.4
3.32
NaOH 6 M
◦
1
80 C, 24 h
NR
Hydrothermal
(7.2 ± 1.1) x (40–200) {110} + {100}
Cubic
109
0.522
23.6
76.4
3.07
NaOH 6 M
◦
1
00 C, 24 h
a
Size of the main dimensions by TEM analyses.
Calculated from Ce 3d XPS deconvolution.
b
with previous reports [22]. On the other hand, XRD patterns shown
in Fig. 2(B) confirmed the formation of pure CeO2 cubic phase (flu-
orite structure, JCPDS 34-0394) in both NC and NR samples [22].
The porosity of the catalysts was evaluated by means of N2
adsorption-desorption isotherms. BET surface area and total pore
volumes are summarized in Table 1. Previous reports have shown
that these materials are free of a developed pore structure except
general random stacking of nanoparticles [22,26]. Due to its mor-
phology and lower crystal size, NR presented higher SBET and pore
volume values than NC. With these characteristics, it is reasonable
to assume that the powder dispersion of these materials (NR and
NC) can be illuminated without internal shading from pores. Thus,
the surface area exposed to light (SBET in this case) should be taken
into account since it can affect the photocatalytic behavior to some
extent.
One of the main characteristics of CeO2 catalysts is their oxy-
gen and electron mobility due to the presence of surface defects,
reduced states of Ce(III) or oxygen vacancies in the CeO2 struc-
ture. High-resolution XPS spectra corresponding to Ce 3d spectral
region of NC and NR samples have been plotted in Fig. 3. From this
figure, a slight shift of Ce 3d peaks to lower binding energies in NR
with respect to NC sample is noticed. This has been attributed to
an increase of the electron density around Ce nucleus, suggesting
the existence of more Ce(III) ions on the surface of NR sample [26].
In addition, this has been confirmed by the higher intensity of the
peaks corresponding to Ce(III) species (v , v’, u , u’) in NR catalyst
0 0
[18,26,27], leading to a higher percentage of surface Ce(III) in NR
compared to NC sample (see Table 1).
Finally, optical properties of the catalysts have been studied
by DR-UV–vis spectroscopy. Fig. 1 shows the absorbance UV–vis
spectra of NC and NR where a noticeable shift to higher wave-
length absorption for NR sample is observed, which suggests this
catalyst can use a higher fraction of visible radiation to trigger
photocatalytic reactions. The optical energy band gap values (Eg)
calculated assuming indirect band gap behavior for both catalysts
are also summarized in Table 1. A value of 3.32 eV was obtained
for NC, somewhat higher to that reported for bulk CeO2 (around
3.19 eV) due to smaller particle sizes [28]. On the contrary, a notice-
able lower Eg value was calculated for NR indicating a clearly shift
to higher absorption wavelengths as deduced from Fig. 1. This
effect may be attributed to the lattice defects and oxygen vacancies
present in the ceria NR [28], in agreement with the results obtained
by XPS analyses.
3.2. Catalytic activity
The activity of the CeO2 catalysts was evaluated in DEET
degradation through AOPs of photocatalytic oxidation, catalytic
ozonation and photocatalytic ozonation in aqueous solution using
visible and simulated solar radiation in photocatalytic systems. The
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u0
u'
v'
v0
NR
Fig. 5. TOC removal with the CeO2 catalysts and O3 processes.
Ce(III) Ce(IV)
NR 23.6% 76.4%
NC 12.6% 87.4%
results obtained for DEET degradation are shown in Fig. 4. Some
blank experiments are also included for comparative purposes. As
reported in a previous work, at the conditions applied no DEET
degradation by direct photolysis was observed [29]. The adsorption
capacity of both catalysts was also negligible being DEET removal
lower than 10%. In addition, in spite of the optical properties of
the catalysts, no significant DEET removal was achieved by photo-
catalysis regardless of the radiation used (visible or solar). These
results differ from those previously reported using different CeO2
catalysts, although in those cases photosensitizing dyes were used
as target compounds [11–15]. On the other hand, the highest DEET
degradation rate was obtained for ozone-based systems. Thus, sin-
gle ozonation led to a complete removal of DEET in less than 60 min.
A similar DEET evolution was observed during catalytic ozona-
tion with respect to ozonation regardless of the catalyst used. This
points out that these materials do not show any beneficial effect
on DEET removal when combined with ozone in the dark, at the
conditions used in this work.
NC
920
910
900
890
880
870
Binding energy (eV)
Fig. 3. High-resolution XPS spectra and deconvolution of the Ce 3d spectral region
of the CeO2 nanostructured catalysts.
However, the impact of the radiation (visible and solar) on the
efficacy of ozone-based processes was significant and, in general,
higher degradation rates were observed when solar light was used
(Fig. 4(B)). In this sense, DEET was completely removed after 45 and
3
0 min when ozone was combined with visible and solar radiation,
respectively, being these periods clearly lower than that required
by single ozonation (60 min). It has been previously demonstrated
that ozone can undergo direct photolysis under solar radiation pro-
moting the formation of some reactive oxygen species capable of
oxidizing the organic matter in aqueous solution [30,31]. Neverthe-
less, the interaction between visible radiation and ozone is unclear
and will be investigated in future works. Finally, according to
Fig. 4(A), the addition of NC and NR catalysts to the ozone + radiation
system had almost no effect on DEET degradation rate. Only a slight
beneficial effect was observed when visible light was used, regard-
less of the type of catalyst.
Main differences between ozone-based treatments were
observed in terms of DEET mineralization (i.e. transformation into
CO , H O and inorganic compounds). Fig. 5 shows the percent-
2
2
age of mineralization during ozone-based treatments at 120 min
reaction time. As observed, ozone alone gave rise to only a 19%
mineralization due to the formation of intermediate compounds
refractory to ozone direct reaction (oxalic, formic and acetic acids
were detected in the reaction medium) [32]. The presence of CeO2
catalysts slightly increased (by 5%) the final mineralization, which
could be attributable to a positive effect of cerium oxide on the
elimination of some intermediate compounds formed during DEET
ozonation. The combination of ozone and radiation gave rise to 29%
Fig. 4. Evolution of normalized DEET concentration during different treatments
applied under visible (A) and solar (B) simulated radiation (maximum standard devi-
ation of the normalized DEET concentrations in 3 repeated runs 0.03, not shown).
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Fig. 6. Evolution of DEET converted, H2O2 formed and O3 dissolved during the O3 treatments applied using CeO2-NC catalyst and simulated solar light.
Table 2
−1
−2
m g) have been calculated. The val-
constants (kTOC /SBET , min
Pseudo-first order apparent rate constant for TOC elimination during photocatalytic
ozonation runs.
ues obtained (Table 2) confirm the highest photocatalytic activity of
ceria NC. It is also noticeable that solar UV radiation did not improve
the mineralization rate when NR is used probably due to an increase
of the recombination rate in the semiconductor.
3
−1
4
−1 −2
m
g) R2
Catalyst Radiation kTOC × 10 (min
)
kTOC/SBET × 10 (min
NC
Visible
Solar
Visible
Solar
7.2
11.8
11.2
11.5
5.1
8.4
1.1
1.1
0.98
0.96
0.95
0.94
NR
3
.3. Considerations on the reaction mechanism of DEET
photocatalytic ozonation with CeO2 catalysts and solar radiation
and 47% of TOC removal under visible and solar light, respectively.
Therefore, compared to visible the effect of the entire solar spec-
trum (including wavelengths between 300 and 390 nm) is much
higher. However, the best results were obtained when photocat-
alytic ozonation was applied, being NR the most active under visible
radiation (68% TOC removal), likely due to its lower band gap energy
as a consequence of higher Ce(III) surface ratio, whereas NC pre-
sented the highest photocatalytic activity under solar radiation
According to the above results, photocatalytic ozonation of DEET
using the CeO2 catalysts studied here, may proceed through a com-
plex mechanism with the expected participation of ozone (direct
•
reaction), and/or HO radicals formed from ozone decomposition
•
(indirect reactions). Ozone can decompose into HO by several
pathways: (1) in the dark, (2) by interaction with light, (3) by inter-
action with the catalyst, and (4) by interaction with both light and
the catalyst. To discuss the possible contributions of each pathway
Fig. 6 shows the evolution of DEET eliminated, dissolved ozone and
hydrogen peroxide formed during ozone-based treatments using
NC CeO2 and simulated solar radiation.
(
80% TOC removal). This is in agreement with the highest energy of
{
100} facets (thermodynamically more unstable) predominant in
NC with respect to NR, allowing higher photocatalytic activity for
this material due to once excited, the photoinduced holes exhibit
superior oxidizing ability, according to previously reported results
Regarding to ozone-direct reactions,
a
low reactiv-
ity of DEET towards ozone has been previously reported
−
1
−1
s [21]). Taking into account this value,
[
26].
These results have also been confirmed by calculating the
(kO3-DEET = 0.123 M
at the conditions used in this work, the reaction between ozone
and DEET progress into the slow kinetic regime [32,33], being
DEET degradation rate by direct ozone reaction given by Eq. (2):
pseudo-first order apparent rate constant of TOC removal, kTOC
,
during photocatalytic ozonation treatment, according to Eq. (1):
ꢀ
ꢁ
dTOC
dCDEET
dt
−
= kTOC TOC
(1)
−
= kO3−DEET CDEET CO3
(2)
dt
where TOC is expressed in mg L−1, t in min and kTOC in min−1
.
where CDEET is DEET molar concentration and C is dissolved ozone
O3
Results are summarized in Table 2 together with the correlation
coefficient. Taking into account the different irradiated surface area
of the two catalysts (see Table 1), normalized to SBET apparent rate
molar concentration. The expected evolution of DEET eliminated
during ozonation in the dark considering only its reaction with
ozone is shown in Fig. 6. As it is observed, the experimental conver-
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sion rate was much higher than that predicted by Eq. (2), indicating
that species different than O3 are the main responsible for the
degradation of the target compound. However, it is important to
that involves at least an ozone-direct reaction that triggers a radical
chain mechanism, indirect ozone reactions, ozone photolytic and
photocatalytic decomposition and hydrogen peroxide formation-
decomposition. The importance of the different processes and the
effect of visible radiation will be the subject of a future work.
◦
notice that kO3-DEET value was reported at 20 C whereas the tem-
◦
perature during the experiments reaches 35 C. Also, due to the low
value of kO3-DEET some uncertainty on its determination should not
be disregarded.
Acknowledgements
During all ozone-based treatment initial pH evolved from 6
to 4.5-4. At these pH values, at the beginning of the experiment,
some ozone decomposition can be promoted by hydroxide anion
although it likely becomes negligible when pH decreases [32].
Therefore, during DEET degradation by single ozonation in the dark
it can be hypothesized that direct interaction between DEET-O3
acts as a trigger of a chain mechanism that leads to the formation
Authors thank the Spanish MINECO/European Feder Funds
(CTQ2012-35789-C02-01) and Junta de Extremadura (Ayuda Exp.
GR15-033) for economic support. E. Mena thanks the Consejería de
Empleo, Empresa e Innovación (Junta de Extremadura) and Euro-
pean Social Fund for her FPI grant (Ref. PD12059).
•
of ozonide and/or HO radicals, as has been previously reported for
References
•
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[
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[
tion treatment of DEET under visible and simulated solar radiation.
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Please cite this article in press as: E. Mena, et al., Nanostructured CeO2 as catalysts for different AOPs based in the application of ozone
and simulated solar radiation, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.034