Synthesis of cerium orthophosphates with monazite and rhabdophane structure from phosphoric acid solutions in the presence of hydrogen peroxide

Article abstract of DOI:10.1134/S0036023616100181

It has been proposed to conduct the synthesis of cerium(III) orthophosphates by reacting cerium(IV) compounds with hydrogen peroxide in the presence of concentrated orthophosphoric acid at ambient temperature. It has been shown that the reaction of H

Full text of DOI:10.1134/S0036023616100181

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2016, Vol. 61, No. 10, pp. 1219–1224. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © L.S. Skogareva, T.O. Shekunova, A.E. Baranchikov, A.D. Yapryntsev, A.A. Sadovnikov, M.A. Ryumin, N.A. Minaeva, V.K. Ivanov, 2016, published in
Zhurnal Neorganicheskoi Khimii, 2016, Vol. 61, No. 10, pp. 1276–1281.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis of Cerium Orthophosphates with Monazite
and Rhabdophane Structure from Phosphoric Acid Solutions
in the Presence of Hydrogen Peroxide
a,
a, b
a, b
a
a
L. S. Skogareva *, T. O. Shekunova , A. E. Baranchikov , A. D. Yapryntsev , A. A. Sadovnikov ,
a
a
a, c
M. A. Ryumin , N. A. Minaeva , and V. K. Ivanov
a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
b
Moscow State University, Moscow, 119991 Russia
Tomsk State University, Lenin Prospekt 36, Tomsk, 634050 Russia
c
*
e-mail: skog@igic.ras.ru
Received February 18, 2016
Abstract―It has been proposed to conduct the synthesis of cerium(III) orthophosphates by reacting
cerium(IV) compounds with hydrogen peroxide in the presence of concentrated orthophosphoric acid at
ambient temperature. It has been shown that the reaction of H O with CeO suspensions in H PO medium
2
2
2
3
4
produces CePO ⋅ xH O (rhabdophane structure), while that with CeO solutions in concentrated H PO
4
2
2
3
4
results in CePO (monazite structure).
4
DOI: 10.1134/S0036023616100181
Orthophosphates of rare earth elements exhibit a
Some other cerium phosphates can be prepared by
number of unique properties that allows their use as soft chemistry methods from solutions of cerium(IV)
optoelectronic and luminescent materials [1–3], UV compounds in phosphoric acid. In particular, the
protectors [4], matrices for nuclear fuel immobiliza- hydrothermal treatment of cerium(IV) compounds in
tion and radionuclide sorbents [5], thermally stable orthophosphoric acid can result in formation of
coatings, etc.
cerium pyrophosphate [11] and different cerium(IV)
orthophosphates and hydrogen phosphates [15–20].
Cerium(III) orthophosphates exist in two forms:
CePO ⋅ xH O, rhabdophane (hexagonal structure)
We supposed that cerium(III) orthophosphates
4
2
and CePO , monazite (monoclinic structure). Rhab- can be synthesized using soft chemistry approach by
4
reduction of cerium(IV) compounds with hydrogen
peroxide in orthophosphoric acid medium.
dophane is usually obtained by mixing aqueous solu-
tions of cerium(III) salts with 1–20 wt % orthophos-
phoric acid or solutions of sodium or ammonium
orthophosphates [6–8]. Nanocrystalline CePO ⋅
4
EXPERIMENTAL
xH O is prepared in the presence of different modifi-
2
Initial chemicals used were 85 wt % aqueous solu-
ers (citric acid, EDTA, urea, biuret, etc.) [9–11],
under hydrothermal treatment conditions at 150–
3
tion of H PO (analytical grade, ρ = 1.685 g/cm ),
3
4
4
8 wt % aqueous solution of hydrogen peroxide (ana-
3
00°C [10, 11], and by the thermal treatment of
lytical grade), Ce(NO ) ⋅ 6H O (analytical grade),
cerium-containing phosphoric acid gels [12]. Anhy-
3 3
2
2
5 wt % aqueous ammonia solution (analytical grade),
drous CePO is usually obtained by high-temperature
4
and CeCl ⋅ 7H O (reagent grade).
treatment of cerium rhabdophane at about 1000°C.
3
2
Cerium(III) orthophosphate of monazite structure
Diluted H
2
O solutions were prepared using CO -
2 2
can be also obtained by precipitation from solutions of free distilled water. Glassware was passivated in all
cerium(III) compounds in non-aqueous solvents at cases by sequential treatment with aqueous solutions:
low temperature [13]. It was shown in [14] that hydro- 10 wt % KOH, 2 N HNO , and 30 wt % H O .
3 2 2
thermal synthesis of cerium orthophosphates leads to
At the first stage of synthesis, nanocrystalline
CePO ⋅ xH O at 100°C and to CePO at temperature cerium dioxide was obtained by the standard proce-
4
2
4
above 200°C. Thus, the synthesis of cerium monazite dure [21] (CeO particle size determined by the Selya-
2
is conducted either under anhydrous conditions or in kov–Scherrer formula was 4.5–5 nm) by addition of a
hydrothermal media at high temperature.
water–isopropanol solution of cerium(III) nitrate to
1219

1
220
SKOGAREVA et al.
formation of bulky colorless precipitate, which rather
an aqueous ammonia solution, the resultant CeO
2
precipitate was separated and washed with distilled quickly dissolved. The subsequent heating of the solu-
water. The obtained cerium dioxide suspensions were tion at 90°C resulted in oxygen evolution and repeated
separated by filtration through a fritted glass filter formation of bulky colorless precipitate. The mixture
(
ISO4793 P 16) using a water-jet pump (~20 mmHg) was heated until oxygen evolution ceased. The resul-
until filtrate separation ceased, thus obtained wet tant suspension was cooled, the precipitate was sepa-
pastes (according to the gravimetric analysis, water rated by decantation (we failed to separate the pre-
content was 60 wt %) were used for further synthesis.
cipitate by filtration through a fritted glass filter
(
ISO4793 P 16), washed with CO -free distilled water
2
Procedure A. A weighed sample of cerium dioxide
until mother liquor pH 6, and dried in air. A supple-
mentary sample of anhydrous cerium(III) orthophos-
phate (control) with monazite structure was obtained
by the following method.
paste (0.50 g) was placed in 5 mL of doubly diluted
8
5 wt % orthophosphoric acid. After careful magnetic
stirring of the resultant suspension at ambient tem-
perature, 5 mL of 12 wt % aqueous solution of hydro-
gen peroxide was slowly added, the suspension color
changed from light-yellow into orange. This color
change is likely to be caused by the formation of
cerium peroxo compounds on the surface of cerium
dioxide [22, 23]. The mixture was heated for 10 min at
A 1 wt % aqueous solution of H PO was slowly
3
4
added to 0.1 M aqueous solution of CeCl (molar ratio
3
3
−
3+
Ce : PO = 1 : 1) under constant magnetic stirring.
4
The resultant suspension was stirred for 6 h, next a
concentrated ammonia solution was added dropwise
to pH 2–3. The obtained gel-like precipitate was sep-
arated by centrifugation, washed with water until neg-
~
80°C until oxygen evolution ceased and suspension
color was discharged, next the mixture was cooled, the
precipitate was separated by filtration and washed with
distilled water.
3
4
−
–
ative test for Cl and PO ions, dried in air, and
annealed in a muffle furnace at 1000°C for 2 h in air.
The IR spectra of the powders as KBr pellets were
recorded on a Specord M-80 spectrometer in the
Procedure B. A weighed sample of cerium dioxide
paste (0.50 g) was placed in 10 mL of 85 wt % H PO ,
3
4
and the mixture was heated at 90°C for 10 min until
complete dissolution of cerium dioxide. The resulting
citrine solution was cooled to ambient temperature, the
color changed to pale yellow. Five milliliters of 1 wt %
aqueous H O solution (molar ratio CeO : H O =
–1
region 400–4000 cm .
Powder diffractograms were recorded on a Bruker
D8 Advance X-ray diffractometer (CuK radiation,
α
θ–2θ-geometry) within 2θ angle range 10°–80°.
2
2
2
2
2
Thermal analysis (weighed samples ~20 mg) was
1
: 1.3), or 4 wt % (molar ratio CeO : H O = 1 : 5), or
2 2 2
®
performed on a Netzsch STA 449 F1 Jupiter thermal
1
2 wt % H O solution (molar ratio CeO : H O =
2
2
2
2
2
analyzer in the temperature range 20–1300°C (heating
1
: 16) was added, which caused a slight heating of the
rate 20 K/min).
reaction mixture, vigorous evolution of oxygen, and
Powder micromorphology was studied using a Carl
Zeiss NVision 40 scanning electron microscope at
accelerating voltage 1 kV without preliminary coating
of sample surface with conducting materials.
–1
Assignment of absorption band frequencies (cm ) in the
IR spectra of cerium(III) orthophosphate samples prepared
by different methods
CePO ⋅ xH O
CePO4
Procedure А Procedure B*
628 s 1632 w
RESULTS AND DISCUSSION
4
2
Assignment
δ(H–O–H)
According to X-ray powder diffraction study, the
powder obtained by procedure A is a hydrated
cerium(III) orthophosphate with rhabdophane struc-
ture (CePO ⋅ xH O, PDF2 35-614, space group
Control
1615 w
1
1092
4
2
⎧
P6 22) (Fig. 1a). Let us note that the sample contains
1
048 о.s
~1000 vs br ⎪ 1058 vs
2
⎧
⎨
⎩
⎪
⎪
νas(P–O)
νs(P–O)
no initial cerium dioxide. The assessment of particle
1014
1016
⎨
size of CePO ⋅ xH O by the Selyakov-Scherrer for-
4
2
⎪
978 w
950 w
632 s
992
mula (for reflections 100, 101, and 110) gave the value
⎪
of ~25 nm.
⎪⎩ 952 s
The IR spectrum of this compound (Fig. 2a, table)
displays typical for orthophosphate anion absorption
⎧
⎨
⎩
616 s
616 s
575
620 vs
580 s
–1
bands in the region ~1000 and ~600 cm , corre-
5
68 vw
δas(P–O)
ν(Ce–O)
sponding to the stretching and deformational vibra-
⎧
⎪
⎨
⎧
⎪
⎨
3−
552 s
tions, respectively, of PO4 ion, as well as absorption
⎪
⎩
⎪
⎩
bands of water molecules. The data of IR spectroscopy
indicate the presence in rhabdophane structure of
water molecules coordinated to cerium ion, which
provides a possibility to represent its formula as
5
40 s
536
540 s
410 w
*
The sample was obtained using 12 wt % H O .
2
2
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SYNTHESIS OF CERIUM ORTHOPHOSPHATES
1221
(а)
c
b
10
20
30
40
θ, deg
b)
50
60
70
2
(
а
20
30
40
θ, deg
50
60
70
2
Fig. 1. Diffractograms of CePO ⋅ xH O (a) and CePO
4
4
2
(
b) powders prepared by procedures A and B, respectively.
[
Ce(H O) ]PO . Up to now, the data on the crystal
2 x 4
structure of rhabdophane are incomplete. In particu-
lar, two possible space groups were noted in [24] for
LnPO ⋅ xH O (Ln = La, Ce, or Nd): P3 21 and P6 22,
4
2
1
2
the position of water molecules in the crystal structure
was not established. It is generally assumed that water
molecules in rhabdophane structure are located in the
channels formed by bound [Ce-PO ] chains oriented
4
along the c axis. The number of water molecules (x) in
CePO ⋅ xH O is not higher than 0.5 according to the
4
2
data [7, 24], but can be larger according to assessments
performed in [8]. Note that the authors [25] deter-
mined space group C2 for isostructural compound,
hydrated samarium orthophosphate, and showed the
presence in its structure of water molecules directly
coordinated to the ion of the rare earth element, the
number of water molecules per samarium atom was
found to be 0.667.
1800 1600 1400 1200 1000 800 600 400
ν, cm–1
Fig. 2. IR spectra of CePO ⋅ xH O (a) and CePO (b)
4
2
4
powders prepared by procedures A and B (using 12 wt %
hydrogen peroxide solution), respectively, and the control
To make the chemical composition of the obtained
sample more exact, we conducted its thermogravimet-
sample of CePO
(c).
4
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 61 No. 10 2016

1222
SKOGAREVA et al.
(a)
(b)
1
μm
1 μm
(c)
(d)
1
μm
1 μm
Fig. 3. SEM images of samples: CePO ⋅ xH O prepared by procedure A (a), CePO prepared by procedure B using 1 wt % (b),
4
2
4
4
wt % (c), and 12 wt % (d) hydrogen peroxide.
ric analysis. The data of thermal analysis allow us to
Since orthophosphoric acid is not oxidizing or
conclude that weight loss for this sample proceeds in reducing agent, the dissolution of CeO in concen-
2
three stages. Physically adsorbed water is lost at the trated H PO (85 wt %) seems to result in formation of
3
4
first stage (the maximal rate of weight loss is observed
at ~75°C), while water molecules present in the chan-
nels of cerium rhabdophane structure are lost at the
second stage (~200°C). A small weight loss at ~690°C
cerium(IV) complexes containing orthophosphate
2
4
−
anions as ligands, most likely in the form of H PO .
2
Unfortunately, there is no information in the literature
on alike cerium(IV) complexes (similar data on
cerium(III) coordination compounds were obtained
in the works [28, 29]), but taking into account the
(
0.2%, the third stage) can be related to either removal
of residual water and hexagonal → monoclinic CePO
4
phase transition [8] or oxygen evolution on account of
reduction of admixture cerium(IV) phosphates [26].
The estimation of water content in hydrated cerium
acidity constants for orthophosphoric acid (pK
=
a1
2
.15, pK = 7.20, pK = 12.32), one can suppose that
a2 a3
orthophosphate gave x = 0.56, which agrees well with their composition can be described by the formula
IV
the above literature data.
H [Ce (H PO ) ] (m = 0–2).
m 2 4 n
According to the data of scanning electron micros-
copy, the sample of hydrated cerium orthophosphate
consists of needle-like particles typical for rhab-
dophane [27] (Fig. 3a).
Oxygen evolution observed immediately after
hydrogen peroxide addition to phosphoric acid solu-
tions of cerium(IV) orthophosphate complexes seems
to be caused by the decomposition of H O catalyzed
2
2
X-ray powder diffraction study of the sample pre-
by free orthophosphoric acid present in excess in the
reaction system. It is known that the rate of hydrogen
peroxide decomposition increases in the presence of
perchloric, sulfuric, orthophosphoric, and hydroflu-
oric acids, the catalytic decomposition of H O is
pared by procedure B showed that it is a monophasic
CePO with monazite structure (Fig. 1b). The assess-
4
ment of coherent-scattering domain size for this sam-
ple gave value larger than 100 nm. The diffractograms
of the corresponding samples are virtually identical
2
2
+
when 1, 4, and 12 wt % hydrogen peroxide solutions associated with the formation of H O cation [30].
3
2
were used for precipitation.
After decomposition of H O excess, relatively stable
2 2
The formation of anhydrous cerium orthophos- solutions containing peroxide compounds formed,
phate upon precipitation from aqueous solution at low which evolve oxygen only on heating. The stability of
temperature is rather unusual. We suggest the follow- such solutions can be associated with the fact that
ing explanation of this fact.
cerium(IV) orthophosphate compounds behave as
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SYNTHESIS OF CERIUM ORTHOPHOSPHATES
1223
inhibitors of H O decomposition due to formation in produces aggregates of rather unusual star-like form
2
2
solution of mixed peroxoorthophosphate complexes composed of spindle-shaped particles (Fig. 3b–3d).
IV
m–
The formation of non aggregated spindle-shaped par-
of supposed composition [Ce (H PO ) (O ) ] or,
2
m–
4 x
2 y
IV
ticles in solvothermal synthesis of CePO was
4
more likely, [Ce (H PO ) (HO ) ] .
2
4 x
2 y
observed previously by the authors [37].
The complexes decompose upon heating of their
solutions resulting in the reduction of cerium(IV) to
cerium(III) and evolution of gaseous oxygen. It is the
fact that cerium in cerium(IV) peroxoorthophosphate
complexes does not coordinated to water molecules
seems to cause the formation of anhydrous cerium
orthophosphate upon decomposition of these com-
plexes.
On the other hand, as described above, the reaction
of hydrogen peroxide in orthophosphoric acid
medium with hydrated cerium dioxide leads to forma-
tion of hydrated cerium orthophosphate with rhab-
dophane structure. This reaction seems to proceed
because the initial nanodisperse cerium dioxide con-
tains water molecules and OH groups directly coordi-
nated to cerium ions [31, 32]. Moreover, the reaction
of cerium dioxide with hydrogen peroxide can lead to
formation of brightly colored cerium peroxo com-
plexes [22, 23], whose decomposition also can pro-
duce OH-coordinated cerium ions.
When hydrogen peroxide concentration increases
from 1 to 12%, the size of CePO particles decreases
4
and aggregates faceting is lost. The formation and
growth of star-like aggregates is likely to be associated
with the realization of mechanism of oriented attach-
ment of nanoparticles [38].
Thus, we showed in this work that hydrogen perox-
ide can be used as a reagent in the synthesis of different
modifications of cerium(III) orthophosphate in
orthophosphoric acid medium. The precipitation with
hydrogen peroxide from cerium-containing phos-
phoric acid solutions leads to formation of anhydrous
cerium(III) orthophosphate with monazite structure,
while the reaction of hydrated cerium dioxide with
hydrogen peroxide in orthophosphoric acid medium
results in hydrated cerium orthophosphate with rhab-
dophane structure.
ACKNOWLEDGMENTS
The IR spectra of all samples obtained by proce-
dure B (Fig. 2b, table) agree well with the previously
obtained data [12, 26] and contain strong wide band in
This work was performed with the use of equip-
ment of the Shared Facility Center, Kurnakov Insti-
tute of General and Inorganic Chemistry, Russian
Academy of Sciences, with financial support by the
Presidential Grant Program (MK–8977.2016.3) and
–1
the region 1200–900 cm (ν ) and two split bands in
3
the region 600 cm–1 (ν ), one of which is a doublet,
4
while another is a triplet. The splitting of these bands the Presidium of the RAS (the Program of Basic
provides an opportunity to affirm that PO groups in Research no. 14).
4
monazite structure are directly coordinated to cerium
atoms, which corresponds to the data on the structure
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Translated by I. Kudryavtsev
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