Mesoporous cerium oxide for fast degradation of aryl organophosphate flame retardant triphenyl phosphate

Article abstract of DOI:10.1039/c9ra06575j

Cerium oxide nanoparticles were prepared by calcination of basic cerous carbonate (as a precursor) obtained by precipitation from an aqueous solution. Prepared samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), high resolution scanning electron microscopy (HRSEM), BET (Brunauer-Emmett-Teller) surface area and porosity measurement. Prepared cerium oxide was applied as a destructive sorbent for the fast and safe degradation of organophosphorus flame retardant triphenyl phosphate (TPP). It was shown that cerium dioxide was effective in the decomposition of TPP by cleavage of the P-O-aryl bond in the flame retardant molecule. A degradation mechanism for TPP on the ceria surface was proposed. The degradation is governed by conversion of TPP via diphenyl phosphate (DPP) to the final product identified as phenol (Ph). The key parameter increasing the degradation efficiency of CeO₂ is the temperature of calcination. At optimum calcination temperature (500 °C), the produced ceria retains a sufficiently high surface area and attains an optimum degree of crystallinity (related to a number of crystal defects, and thus potential reactive sites). The fast and efficient degradation of organophosphorus flame retardant TPP was observed in a polar aprotic solvent (acetonitrile) that is miscible with water.

Full text of DOI:10.1039/c9ra06575j

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PAPER
Mesoporous cerium oxide for fast degradation of
aryl organophosphate flame retardant triphenyl
phosphate
Cite this: RSC Adv., 2019, 9, 32058
a
b
ab
Marek Dosek,a Jirı Henych
*
Jakub Ederer,
and Pavel Janos
Martin Sˇˇtastny,
´
ˇ
ˇ´
a
ˇ
Cerium oxide nanoparticles were prepared by calcination of basic cerous carbonate (as a precursor)
obtained by precipitation from an aqueous solution. Prepared samples were characterized by X-ray
diffraction (XRD), infrared spectroscopy (FTIR), high resolution scanning electron microscopy (HRSEM),
BET (Brunauer–Emmett–Teller) surface area and porosity measurement. Prepared cerium oxide was
applied as a destructive sorbent for the fast and safe degradation of organophosphorus flame retardant
triphenyl phosphate (TPP). It was shown that cerium dioxide was effective in the decomposition of TPP
by cleavage of the P–O–aryl bond in the flame retardant molecule. A degradation mechanism for TPP
on the ceria surface was proposed. The degradation is governed by conversion of TPP via diphenyl
phosphate (DPP) to the final product identified as phenol (Ph). The key parameter increasing the
degradation efficiency of CeO2 is the temperature of calcination. At optimum calcination temperature
(500 ꢀC), the produced ceria retains a sufficiently high surface area and attains an optimum degree of
crystallinity (related to a number of crystal defects, and thus potential reactive sites). The fast and
efficient degradation of organophosphorus flame retardant TPP was observed in a polar aprotic solvent
(acetonitrile) that is miscible with water.
Received 21st August 2019
Accepted 1st October 2019
DOI: 10.1039/c9ra06575j
rsc.li/rsc-advances
various industries.2–4 TPP is also used as antioxidant and
stabilizer in phenolic, PVC, polyurethane, and other resins, and
1. Introduction
is considered as a neurotoxic and a new type of allergen in
polyvinylchloride gloves7 which can cause allergic contact
dermatitis.8 TPP degradation products and its fate in soil were
investigated by Anderson et al.3 under different conditions
(aerobic and anaerobic) or by Kawagoshi et al.9 in leachate from
sea based solid waste disposal site. The degradation of TPP in
soil and water is relatively a slow process.3 Therefore, the
demands for remediation technologies that would be able to
quickly and efficiently remove surpluses of these pollutants in
the environment increase rapidly nowadays.
The metal oxides are oen used due to their excellent cata-
lytic properties in petroleum, environmental and chemical
industries as catalysts or catalysts support. Mixed metal oxides
are also extensively used as fuel cells, gas sensors, and other
applications. Nanostructured metal oxides can be also applied
as reactive adsorbents as we have shown elsewhere.10 Never-
theless, to the best of our knowledge the decomposition of TPP
by nanocrystalline oxide was not widely investigated.
Organophosphorus compounds are commonly used as pesti-
cides (parathion-methyl, fenchlorphos), or ame retardants
(triphenyl phosphate, tris(2-chloroethyl) phosphate, tris(2-
ethylhexyl) phosphate).1 The organophosphorus ame retar-
dants (PFRs) are widely used in industrial polymeric materials
such as lubricants, electrical and electronic equipment2 and
hydraulic uids.3,4 Furthermore, PFRs are also used as additive
chemicals because they are not covalently bound to the poly-
meric materials, however they are able to migrate into the
environment5 and most of the ame-retardant products even-
tually become waste. Municipal waste is generally disposed of
by incineration or landlling. Incineration of ame-retardant
products can produce various toxic compounds, including
halogenated dioxins and furans further polluting water, air and
soil.6 In the environment, PFRs may have a direct adverse effect
on aquatic organisms and can contaminate drinking water.4
Triphenyl phosphate (TPP) is representative organophosphorus
compound widely used as a ame retardant component in
Cerium oxide (CeO2) is considered, due to extraordinary
thermal and chemical stability, as the most important rare-
earth oxide. One of the most important applications of cerium
oxide is as a catalyst promoter, catalyst or catalysts supports.11,12
Depending on the intended use, several methods have been
developed for preparing cerium oxide, such as chemical vapor
a
ˇ
´
´ˇ
Faculty of Environment, University of Jan Evangelista Purkyne, Kralova Vysina 7, 400
´
´
96 Ustı nad Labem, Czech Republic. E-mail: jakub.ederer@ujep.cz; Fax: +420-475-284-
158; Tel: +420-475-284-111
b
ˇ
ˇ
Institute of Inorganic Chemistry of the Czech Academy of Sciences, Rez, 25068, Czech
Republic
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deposition,13 electrochemical synthesis,14 template synthesis,15 Aldrich (Steinheim, Germany) as reagent grade chemicals
biological synthesis,16 etc. Li et al.17 compared some of the with purity 99.9%; ammonium bicarbonate, NH4HCO3, 99.5%,
methods for the preparation of CeO2, namely the sol–gel was obtained from the same supplier. The destructive sorbent
method, precipitation method, and homogeneous hydrolysis. for pesticide degradation was prepared by the procedure
However, owing to the advantages of simple process, easy scale- described elsewhere.10 Briey, the carbonate precursor was
up and low cost, the precipitation technique has attracted the prepared by precipitation of an aqueous solution of Ce(NO3)3
most extensive attention; cerium dioxide is most oen prepared (0.2 mol Lꢁ1) with an excess of NH4HCO3 (0.5 mol Lꢁ1) under
by the precipitation of sparingly soluble cerous oxalates or stirring. Aer the precipitation process, the agitation
carbonates and subsequent calcination. Cerium carbonates continued for one more hour and the precipitate was le in the
with variable composition and morphology may be precipitated solution until the next day. The precipitate was separated by
from an aqueous solution of alkaline or ammonium carbonate/ ltration, washed with deionized water and dried overnight at
bicarbonate18,19 or by a gaseous mixture of carbon dioxide and 110 ꢀC. The CeO2 samples were prepared from carbonate
ammonia.
precursor by calcination at selected temperatures for two
Although CeO2 particles prepared by the precipitation tech- hours in a muffle furnace. Titanium dioxide (TiO2, Degusa),
nique have been extensively studied, most of the previous nickel oxide (NiO, nanopowder < 50 nm), zinc oxide (ZnO,
reports were focused on effects of cerium precursors, ligands nanopowder < 100 nm) and iron (II, III) oxide (Fe3O4, nano-
and additives in the reaction media. Less information was re- powder < 50 nm) were purchased from Sigma Aldrich. Eqn (1)–
ported on the inuence of the reaction temperature. However, (5) illustrated reaction during preparation of carbonate
the temperature during calcination plays a crucial role in the precursor and subsequent calcination.
surface properties (specic surface area, the presence of surface
active sites), crystallinity, and other physicochemical
characteristics.17,20,21
In this work, we have tested the unique properties of cerium
(III, IV) dioxides for fast and safe degradation of organophos-
phorus ame retardants in the environment using triphenyl
3ꢁ
Ce(NO3)3$6H2O(s) / Ce(aq)3++3NO(aq) + 6H2O(aq)
,
(1)
(2)
+
+
ꢁ
NH4HCO3 (s) / NH4(aq) + H(aq) + CO3(aq) ,
3+
2Ce(aq) + 2CO3(aq) + nH2O(aq) / Ce2O(CO3)2$nH2O(s)Y,(3)
ꢁ
110 ꢀ
Ce2OðCO3Þ $nH2O
C Ce OðCO Þ
þ nH2O [;
(4)
(5)
phosphate (TPP) as
a model compound. The sorbents
ƒƒ!
ðsÞ
2
3
ðgÞ
2
2 ðsÞ
prepared by our developed procedure are inexpensive and
efficient in degradation of organophosphorus compounds.9–11
As we have shown,10 nanocrystalline cerium oxide can be used
for degradation of organophosphorus pollutants, but it can be
also used in catalysis,22 fuel cells,11,23 as polishing material24,25
and in biomedical applications.26 The precipitated ceria
samples were annealed at different temperatures (200; 500;
200ꢁ1100ꢀC
Ce2OðCO3Þ
2CeðIII;IVÞ
O
2ꢁxðsÞ þ 2CO2ðgÞ
[
ƒƒƒƒ!
2ðsÞ
2.2. Characterization methods
800; and 1100 ꢀC) and characterized by scanning electron The diffraction pattern was collected with diffractometer Bruker
microscopy (SEM), Brunauer–Emmett–Teller (BET) surface D2 equipped with conventional X-ray tube (Cu Ka radiation, 30
area, X-ray diffraction (XRD) and infrared spectroscopy (FTIR). kV, 10 mA) and the LYNXEYE 1-dimensional detector. The
The main TPP degradation products diphenyl phosphate primary divergence slit module width 0.6 mm, Soler Module 2.5,
(DPP) and phenol (Ph) were identied and quantied using Airscatter screen module 2 mm, Ni Kbeta-lter 0.5 mm, step
high-performance liquid chromatography with DAD detection 0.00607ꢀ and time per step 0.3 s were used. Qualitative analysis
(HPLC-DAD). The mechanism of the degradation of TPP on the was performed with the DiffracPlus Eva soware package
ceria in a polar aprotic solvent (acetonitrile) was proposed and (Bruker AXS, Germany) using the ICDD PDF database.
discussed.
The surface area of the sample was determined from
nitrogen adsorption–desorption isotherm at liquid nitrogen
temperature uꢀsing a Coulter SA3100 instrument with 15 min
outgas at 150 C. The Brunauer–Emmett–Teller (BET) method
was used for surface area calculation.
2. Materials and methods
2.1 Chemicals and preparation of sorbents
All chemicals were obtained from commercial sources and
The FTIR spectra of as prepared CeO2 samples were
were used without further purication. Triphenyl phosphate measured by diffuse reectance Nicolet Impact 400D spectro-
(TPP), diphenyl phosphate (DPP) and phenol (Ph) were photometer (Thermo Nicolet, Madison) in diffuse reectance
purchased from Sigma Aldrich (Steinheim, Germany). All mode in the 4000–500 cmꢁ1 range at 128 scans per spectrum
purchased chemicals were of analytical grade (p.a.). TPP stock and expressed in a transmittance mode (%T). Raw FTIR data
solution (5 g Lꢁ1), DPP and Ph stock solutions (equimolar to were processed utilizing the OMNIC 7.3 soware.
TPP) were prepared in acetonitrile (ACN). HPLC-grade organic
High-resolution scanning electron microscopy (HRSEM)
solvents and deionized water from a GORO Pharmapur system analysis was conducted on an FEI Nova NanoSEM scanning
(Goro, Prague, Czech Republic) were used to prepare the electron microscope equipped with an Everhart-Thornley
solutions including mobile phases for liquid chromatography. detector (ETD), Through Lens detector (TLD), and accelerating
Cerous nitrate, Ce(NO3)3$6H2O was obtained from Sigma- voltage 4–30 kV. The samples were plated on the carbon holder.
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2.3. Degradation of organophosphorus ame retardants
For testing of stoichiometric degradation of TPP a new testing
procedure was developed, because the degradation procedure
published elsewhere10 was found to be inappropriate in our
case. 800 mg of each tested sorbent was weighted into the 25 mL
glass vials with magnetic stirrer, and 4 mL of acetonitrile (ACN)
and 12 mL of TPP stock solution in ACN (5 g Lꢁ1) was added. In
corresponding time intervals (0.5; 8; 16; 32; 64; 96 and 128
minutes) 0.5 mL aliquots were taken and added into 4 mL vials
containing 0.5 mL of formic acid (1 : 1; v/v) and lled with
mobile phase (75% methanol and 25% formic acid (1%)). Vials
were centrifuged at 4000 RPM for 4 min and the supernatant
was transferred into the 25 mL volumetric ask. The extraction
process of degradation products of TPP from sorbents surfaces
with a mobile phase was done three times. All the supernatants
were combined into one volumetric ask, made up to the mark
with the mobile phase, and analyzed immediately by liquid
chromatography (HPLC). Preparation of standard solution of
TPP, DPP, and phenol were made by the same procedure
without adding the sorbent. All experiments were performed at
ꢀ
a laboratory temperature 22 ꢂ 1 C.
2.4. Analysis and data evaluation
Ultra-high performance liquid chromatography (U-HPLC) Dio-
nex UltiMate 3000 system was used to determine the TPP
concentrations and for the detection of degradation products.
The liquid chromatograph consisted of the high-pressure
pump, the Rheodyne 7125 injection valve with 20 mL sampling
loop, vacuum degasser, and the variable wavelength DAD
detector operating at a wavelength of 261 nm (absorption
maximum for TPP), 265 nm (absorption maximum for DPP) and
275 nm (absorption maximum for Ph). Chromatographic
separations were carried out on the Phenomenex column 150 ꢃ
4.6 mm packed with pentauorophenyl stationary phase
Kinetex, PFP, 5 mm (USA) with mobile phase consisting of 75%
methanol and 25% formic acid (1%) (v/v); the ow rate was 1
mL minꢁ1. MS Excel and OriginPro 8.5 (OriginLab Corp., USA)
soware were used for calculations and data evaluations.
Fig. 1 XRD patterns of cerium carbonate (precursor) and cerium
oxides annealed at 200, 500, 800, 1100 ꢀC and cerium oxide 00-004-
0593 (from top to bottom).
Kl
a ¼
(6)
b cos q
where K is the shape factor, l is the wavelength of the applied
radiation, b is the broadening of the diffraction line, and q is the
diffraction angle. The Scherrer Calculator from the X-Pert
HighScore Plus SW package was used for these calculations.
Using the X-ray diffraction (XRD) analysis, a crystalline phase
of the Ce2O(CO3)2$nH2O type was identied (see eqn (3)).
Destructive sorbents were prepared from this carbonate
precursor by calcination at pre-determined temperatures in the
ꢀ
range of 200–1100 C for 2 h; possible chemical reactions are
3. Results and discussion
3.1 Characterization of prepared sample
described by eqn (5), which suggests that a non-stoichiometric
cerium oxide (CeO2ꢁx) may occur as a result of the changes of
the surface to volume ratio.27 The increasing annealing
temperature cause the increase of nanocrystallites size, and
decrease in the surface-to-volume ratio of nanocrystallites. It
reduces the formation energy of oxygen vacancies positioned
close to a crystallite surface. Vacancy creation leads to a change
in electronic density since the electrons that are le behind are
led to oxygen atom occupy 4f electron energy levels of cerium
ions in the vicinity of a newly created vacancy and reduce Ce4+
ions to the Ce3+ state.28,29
SEM images of the ceria powders prepared by the chemical
precipitation method with varꢀious calcination temperature
namely 200; 500; 800; and 1100 C are shown in Fig. 2.
From the SEM images, it was found that all the nanoparticles
exhibit a ake-like morphology and clusters of thin plates of
Fig. 1 shows the results of powder X-ray diffraction (XRD). The XRD
analysis identied a single crystalline phase in the calcination
products obtained in the temperature range of 200–1100 ꢀC. In all
samples was identied cerium dioxide with its characteristic face-
centered cubic uorite-type structure with corresponding diffrac-
tion lines (111), (200), (220), (311), (222), (400), (331) and (420)
located at 28.535ꢀ, 33.080ꢀ, 47.495ꢀ, 56.348ꢀ, 59.102ꢀ, 69.427ꢀ,
76.710ꢀ, 79.073ꢀ and 88.447ꢀ (ICDD PDF 34-0394). From the indi-
vidual diffractograms are evident constriction and increase of
intensity of diffraction lines with increasing calcination tempera-
ture, as a consequence of increasing of the crystallite size. The
crystallite size was calculated from the diffraction line extension
using the Scherrer eqn (6) and the data are summarized in Table 1
(second column):
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Table 1 Crystallite size, BET and total pore volume of prepared
samples by calcination at various temperatures
samples, calculated by the multi-point Brunauer–Emmett–
Teller (BET) method ranges from 6 to 102 m2 gꢁ1 and total pore
volume was in range 0.04–0.07 cm3 gꢁ1 (Table 1). Barrett–Joy-
ner–Halenda pore-size distribution plot and nitrogen
adsorption/desorption isotherms (inset) of the as-prepared
representative sample annealed at 500 ꢀC is shown in
Fig. 2(b). According to IUPAC notation,30 microporous materials
have pore diameters of less than 2 nm and macroporous
materials have pore diameters of greater than 50 nm; the
mesoporous category thus lies in the middle. The mean pore
size is around ꢄ3–4 nm and the pore size distribution being
relatively narrow. All samples have a type IV isotherm, which is
characteristic of mesoporous materials with type H2 hysteresis,
which is a characteristic of large-pore mesoporous materials
and can be ascribed to capillary condensation in mesopores.
Sample (annealing
temperature, ꢀC)
Crystallite size
(nm)
BET
Total pore volume
(m2 gꢁ1
)
(cm3 gꢁ1
)
CeO2_200
CeO2_500
CeO2_800
CeO2_1100
12
28
43
51
102
85
18
6
0.05
0.07
0.04
0.05
irregular shapes with a characteristic diameter of several
micrometers. During calcination, the plates were broken down
into smaller submicron particles that remained assembled in
larger clusters. The specic surface area of the as-prepared
Fig. 2 SEM images of the prepared samples (a) carbonate precursor; (b) particle-size distribution curve with pore diameter about 3–4 nm and
nitrogen adsorption–desorption pattern (inset) of sample CeO2 annealed at 500 ꢀC; (c) cerium carbonate annealed at 200 ꢀC; (d) cerium
carbonate annealed at 500 ꢀC; (e) cerium carbonate annealed at 800 ꢀC; and (f) cerium carbonate annealed at 1100 ꢀC.
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The high steepness of the hysteresis indicates the high order of
mesoporosity.
The time dependence of the TPP degradation was measured
and the experimental data were tted to the Guldberg–Waage
Fig. 3 shows the FTIR spectra of the ceria nanopowders cal- form equations for describing of consecutive reactions more
cinated at various temperatures. The absorption peak at around than a single reaction (i.e., a reaction network):39
1300–1400 cmꢁ1 band was identied by its intense vibrations
qTPP ¼ q0TPP exp(ꢁk1s)
(7)
NO (1384.23 cmꢁ1) due to the presence of unreacted nitrate.31
The broad absorption peak at 3000–3600 cmꢁ1 and peak at
around 1630 cmꢁ1 correspond to hydroxyl groups y(O–H) of
water on the surface of the sorbent. In the as-prepared and
annealed samples the residual water and hydroxyl groups were
detected and further heat treatment is causing their elimina-
tion. FTIR spectra of cerium carbonate and annealed sample
k1q0TPP
qDPP
¼
½expðꢁk1sÞ ꢁ expðꢁk2sÞꢅ
(8)
k2 ꢁ k1
k2q0TPP
qPh
¼
½1 ꢁ expð ꢁðk1 þ k2ÞsÞꢅ
(9)
k2 þ k1
where the symbol qTPP represents amount adsorbed at time s;
q0TPP is the fraction of the reactant (TPP) undergoing reaction
with more and less active parts of the surface of a nano-
dispersed cerium dioxide, respectively; k1 and k2 stand for rate
constants of corresponding sub-processes, qDPP is the amount
of partial decompose intermediate DPP at time s; qPh denotes
the complete decompose of the residual amount of TPP (DPP
respectively) at the end of the reaction, if the destructive
capacity of powdered cerium dioxide is sufficient to complete
the decompose of all dosed agents. Results of the stoichiometric
degradations of TPP and degree of conversion are introduced in
Table 2. Examples of the degradation curves are shown in Fig. 4.
Organophosphates rapidly degrade in the environment by
hydrolysis. The hydrolysis of organophosphate esters generally
takes place either through a trigonal bipyramidal hydrox-
yphosphorane as intermediate (with the expansion of the
coordination number of phosphorus from four to ve).40 Simi-
larly, on metal oxides surface the organophosphate species can
be adsorbed and chemically decomposed that is oen attrib-
uted to the proportion lot of reactive sites. Therefore, they can
be effectively used as industrial adsorbents,41 catalysts,42 and
several other potential decontamination applications.43 Due to
their high surface area, a large number of highly reactive edges,
corner defects sites, defects in the crystal lattice (Lewis acid
attributed to metal cations) and reactive sites (such as surface
hydroxyl groups), they may chemically degrade toxic
compounds down to non-toxic or only slightly toxic organic
(200 ꢀC) show small absorption due to the carbon–oxygen
double bond (C]O) at about 2500 cmꢁ1
.
It was also iden-
32,33
tied weak absorption band corresponding to the valence
vibration Ce–O (550.55 cmꢁ1).34
3.2 Degradation mechanism of TPP on CeO2 surface
Some transition metal oxides (such as MgO, CaO, Al2O3, etc.)
exhibit unique ability to adsorb and decompose organophos-
phates from the group of organophosphorus pesticides, chem-
ical warfare agents (CWA's) and also aryl organophosphate
ame retardants.35–38
compounds.44 Janos and co-workers (2014) found that in
ˇ
nanocrystalline CeO2 are irregularities in the lattice near the
surface such as Ce3+ that allows adsorption and stoichiometric
degradation of toxic organophosphorus pesticides (e.g. para-
thion methyl, chlorpyrifos, etc.).20 As we shown in this work by
XPS, annealing temperature have negligible effect on Ce3+
Fig. 3 FTIR spectra of annealed CeO2 powders prepared by the
chemical precipitation method: (a) carbonate precursor, (b) 200 ꢀC, (c)
500 ꢀC, (d) 800 ꢀC and (e) 1100 ꢀC.
Table 2 The degree of removal, k1 and k2 for triphenyl phosphate (TPP) and degree of conversion for diphenyl phosphate (DPP) and phenol (Ph),
respectively, on cerium dioxides prepared at various temperatures
Triphenyl phosphate (TPP)
Degree of removal (%)
Diphenyl phosphate (DPP)
Phenol (Ph)
Sample
k1 (minꢁ1
)
k1 (minꢁ1
)
k2 (minꢁ1
)
Degree of conversion (%)
k1 (minꢁ1
)
k2 (minꢁ1
)
CeO2_200
CeO2_500
CeO2_800
CeO2_1100
51.8
70.2
16.7
3.10
0.119
0.459
0.043
0.011
ND
ND
40.9
115.3
5.95
ND
0.056
0.043
1.858
ND
0.102
0.082
1.978
ND
0.036
0.018
ND
0.069
0.133
ND
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Fig. 4 Degradation of TPP on cerium oxide annealed at 200 ꢀC, 500 ꢀC, 800 ꢀC and 1100 ꢀC.
content, and hence Ce3+/Ce4+ ratio. The results suggested that
Ce3+/Ce4+ ratio could not be one of the main parameters
involved in CeO2 reactivity and irregularities, different particles
morphology also participate in reactivity. Zhang et al. showed
that CeO2 nanoparticles with similar morphology, nevertheless
with different oxygen vacancies concentration showed different
catalytic activity in photocatalytic water oxidation.45
Similarly to pesticides, other organophosphates like tri-
phenyl phosphate (TPP) can degrade on the surface of CeO2.
The reaction mechanism, shown in Fig. 6, suggest that the
reactions are similar to hydrolysis reactions proceeding in the
environment. The surface hydroxyls (ꢁOH) and surface phys-
isorbed water can act as strong nucleophiles and attack the
electrophile phosphorus center.46
Using a simplied degradation test (xed reaction time 32
min), the degradation efficiency of the cerium oxides annealed
ꢀ
at the temperatures of 200 to 1100 C was compared with the
Fig. 5 TPP decomposition activity of CeO2 annealed at 200, 500, 800
and 1100 ꢀC compared with other metal oxides (TiO2, NiO, ZnO, and
Fe3O4) after 32 min reaction.
degradation efficiency of some other transition metal oxides. As
can be seen, the other tested oxides are not suitable for
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Fig. 6 Hydrolysis reactions of TPP on the surface of nanostructured CeO2: R1, R2, R3 show phenyl groups.
decomposition of TPP or other organophosphorus compounds.
The cerium oxide samples annealed at 200 and 500 ꢀC exhibits
Conflicts of interest
the highest TPP activity (see Fig. 5). TiO2 has shown relatively
highest activity compared to other transition metal oxides
which could be probably ascribed to the slight inuence of
photocatalytic properties of TiO2 that could be subject for
further research.
There are no conicts to declare.
Acknowledgements
Additional nancial support was obtained from the Internal
In polar media (acetonitrile), triphenyl phosphate (TPP)
leads to the formation of phenol (Ph) as the main degradation
product via intermediate diphenyl phosphate (DPP). It has been
hypothesized that TPP rapidly adsorbs and decomposes
through the phosphoryl oxygen on the surface of CeO2 at Lewis
acid (metal atom) or at Brønsted acid (surface hydroxyl)
sites.47,48 The mechanism of interaction (Fig. 6) on the CeO2
surface, suggested that, at rst, TPP is adsorbed molecularly
through the phosphoryl oxygen to the surface at an acid site
(metal cation Ce4+), followed by elimination of the phenoxy
group. This phenoxy group subsequently combines with surface
hydrogen atoms to release phenol into the solvent as the main
degradation product, which can be immediately analyzed with
chromatography (e.g. HPLC-MS).
The kinetics of triphenyl phosphate (TPP) hydrolysis was
investigated on CeO2 powders prepared at various calcination
temperatures. On the CeO2 (500 and 800 ꢀC) surface were found
hydrolysis products diphenyl-phosphate (DPP) and phenol (Ph).
Further hydrolysis of diphenyl-phosphate probably proceed
much slower and no other intermediate (e.g. monophenyl
phosphate) was not found in these experiments.
Student Grant Agency (Grant No. SPS 984599) of the University
´
ˇ
´
of Jan Evangelista Purkyne in Ustı nad Labem. The nancial
support of the Czech Scientic Foundation (grant no. 19-
ˇ ´
07460S) is acknowledged. Michaela Slusna from the Materials
Chemistry Department, Institute of Inorganic Chemistry AS CR
ˇ
v.v.i., Reˇz, is thanked for measurements of the surface areas and
porosity of samples.
References
¨
1 A. Moller, R. Sturm, Z. Xie, M. Cai, J. He and R. Ebinghaus,
Environ. Sci. Technol., 2012, 46, 3127–3134.
2 C. C. Zhang and F. S. Zhang, Chem. Eng. J., 2014, 240, 10–15.
3 C. Anderson, D. Wischer, A. Schmieder and M. Spiteller,
Chemosphere, 1993, 27, 869–879.
4 K. Lin, Environ. Chem. Lett., 2009, 7, 309–312.
5 S. H. Brandsma, J. De Boer, P. E. G. Leonards, W. P. Cono
and A. Covaci, TrAC, Trends Anal. Chem., 2013, 43, 217–228.
6 H. Meyer, M. Neupert, W. Pump and B. Willenberg, Kunstst.
Ger. Plast., 1993, 83, 3–6.
7 K. Suuronen, M. Pesonen, M. L. Henriks-Eckerman and
K. Aalto-Korte, Contact Dermatitis, 2013, 68, 42–49.
8 T. P. Keogh, J. Am. Dent. Assoc., 1999, 130, 474–476.
9 Y. Kawagoshi, S. Nakamura and I. Fukunaga, Chemosphere,
4. Conclusion
2002, 48, 219–225.
ˇ
ˇ
´ˇ
Cerium oxide is very perspective reactive adsorbent for the rapid 10 P. Janos, P. Kuran, M. Kormunda, V. Stengl, T. M. Grygar,
ˇ
ˇ
ˇ ´
´
destruction of aryl organophosphate ame retardants, alterna-
M. Dosek, M. Stastny, J. Ederer, V. Pilarova and L. Vrtoch,
tively, other organophosphates compounds (OPs). CeO2
J. Rare Earths, 2014, 32, 360–370.
samples were synthesized by the simple chemical precipitation 11 Y. Zhou and J. a. Switzer, J. Alloys Compd., 1996, 237, 1–5.
of cerous nitrate with an excess of ammonium bicarbonate in 12 A. Bumajdad, J. Eastoe and A. Mathew, Adv. Colloid Interface
aqueous solution. The results showed that prepared CeO2
samples were effective in the decomposition of triphenyl 13 K. D. Pollard, H. A. Jenkins and R. J. Puddephatt, Chem.
phosphate (TPP) by cleavage of the P–O–aryl bond in the Mater., 2000, 12, 701–710.
retardant molecule in a polar aprotic solvent (acetonitrile). 14 Y. Zhou, R. J. Phillips and J. A. Switzer, J. Am. Ceram. Soc.,
These ndings could lead to the future use of cerium-based 1995, 78, 981–985.
reactive sorbents for practical applications in decontamina- 15 K. S. Lin and S. Chowdhury, Int. J. Mol. Sci., 2010, 11, 3226–
tion nanotechnologies such as polar “water-compatible” 3251.
degradation strategy in connection with the technologies sup- 16 B. M. Gel, G. S. Priya, A. Kanneganti, K. A. Kumar, K. V. Rao
Sci., 2009, 147–148, 56–66.
porting bioremediation techniques for bio-removal of phenolic
compounds.
and S. Bykkam, International Journal of Scientic and Research
Publications, 2014, 4, 1–4.
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Paper
RSC Advances
17 M. Li, Z. Liu, Y. Hu, Z. Shi and H. Li, Colloids Surf., A, 2007, 33 N. Kovac, J. Faganeli, O. Bajt, B. Orel and A. Surca Vuk,
301, 153–157. Mater. Geoenvirnment, 2005, 52, 81–85.
18 M.-Y. Cho, K.-C. Roh, S.-M. Park, H.-J. Choi and J.-W. Lee, 34 A. a. Ansari, P. R. Solanki and B. D. Malhotra, J. Biotechnol.,
Mater. Lett., 2010, 64, 323–326.
2009, 142, 179–184.
ˇ
ˇ
´
19 Y. Zhai, S. Zhang and H. Pang, Mater. Lett., 2007, 61, 1863– 35 V. Stengl, V. Houskova, S. Bakardjieva, N. Murafa,
ˇ´
´
ˇ
ˇ
1866.
M. Marıkova, F. Oplustil and T. Nemec, Mater. Charact.,
ˇ
ˇ
ˇ
´
´
20 P. Janos, T. Hladık, M. Kormunda, J. Ederer and M. Stastny,
2010, 61, 1080–1088.
Adv. Mater. Sci. Eng., 2014, 2014, 1–12.
36 S. Rajagopalan, O. Koper, S. Decker and K. J. Klabunde,
Chem. - Eur. J., 2002, 8, 2602–2607.
ˇ
ˇ´ˇ
ˇ
ˇ
´
21 J. Lunacek, O. Zivotsky, P. Janos, M. Dosek, A. Chrobak,
ˇ
ˇ´
´
´
M. Marysko, J. Bursık and Y. Jiraskova, J. Alloys Compd., 37 T. H. Mahato, G. K. Prasad, B. Singh, K. Batra and
2018, 753, 167–175.
K. Ganesan, Microporous Mesoporous Mater., 2010, 132, 15–
21.
ˇ
ˇ
22 P. Janos, J. Ederer and M. Dosek, Nova Biotechnol. Chim.,
2014, 13, 148–161.
23 K. Eguchi, T. Setoguchi, T. Inoue and H. Arai, Solid State
38 G. W. Wagner, O. B. Koper, E. Lucas, S. Decker and
K. J. Klabunde, J. Phys. Chem. B, 2000, 104, 5118–5123.
39 R. W. Missen, C. A. Mims and B. A. Saville, Introduction to
Chemical Reaction Engineering, Wiley, 1998.
Ionics, 1992, 52, 165–172.
ˇ
ˇ
´ˇ
´
24 P. Janos, P. Kuran, J. Ederer, M. Sastny, L. Vrtoch,
ˇ
ˇ
M. Psenicka, J. Henych, K. Mazanec and M. Skoumal, Adv. 40 S. Bondarenko and J. Gan, Environ. Toxicol. Chem., 2004, 23,
Mater. Sci. Eng., 2015, 2015, 1–8. 1809.
25 P. Janos, J. Ederer, V. Pilarova, J. Henych, J. Tolasz, D. Milde 41 S.-Y. Lee and S.-J. Park, J. Ind. Eng. Chem., 2014, 23, 1–11.
ˇ
ˇ ´
and T. Opletal, Wear, 2016, 362–363, 114–120.
26 S. M. Hirst, A. S. Karakoti, R. D. Tyler, N. Sriranganathan,
S. Seal and C. M. Reilly, Small, 2009, 5, 2848–2856.
27 S. Deshpande, S. Patil, S. V. Kuchibhatla and S. Seal, Appl.
Phys. Lett., 2005, 87, 133113.
42 C. L. Carnes and K. J. Klabunde, J. Mol. Catal. A: Chem., 2003,
194, 227–236.
43 T. H. Mahato, G. K. Prasad, B. Singh, J. Acharya,
a. R. Srivastava and R. Vijayaraghavan, J. Hazard. Mater.,
2009, 165, 928–932.
28 L. Minervini, Solid State Ionics, 1999, 116, 339–349.
44 N. Sharma and R. Kakkar, Adv. Mater. Lett., 2013, 4, 508–521.
ˇ
€
ˇ
€
€
29 S. Askrabiac, Z. Dohceviac-Mitrovia, A. Kremenoviac, 45 Y.-C. Zhang, Z. Li, L. Zhang, L. Pan, X. Zhang, L. Wang, Fazal-
€
€
N. Lazareviac, V. Kahlenberg and Z. V. Popoviac, J. Raman
Spectrosc., 2012, 43, 76–81.
e-Aleem and J.-J. Zou, Appl. Catal., B, 2018, 224, 101–108.
46 Y. Rao and D. M. Antonelli, J. Mater. Chem., 2009, 19, 1937.
30 F. Caruso, W. V. Gmbh and W. Isbn, Colloids and Colloid 47 M. B. Mitchell, V. N. Sheinker and E. a Mintz, J. Phys. Chem.
Assemblies, 2004, vol. 14. B, 1997, 101, 11192–11203.
31 M. Chelliah, J. B. B. Rayappan and U. M. Krishnan, J. Appl. 48 E. Lucas, S. Decker, A. Khaleel, A. Seitz, S. Fultz, A. Ponce,
Sci., 2012, 12, 1734–1737.
32 M. Mecozzi, E. Pietrantonio, M. Amici and G. Romanelli,
Analyst, 2001, 126, 144–146.
W. Li, C. Carnes and K. J. Klabunde, Chem. - Eur. J., 2001,
7, 2505–2510.
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