Recovery with calcium oxide sorbent of conversion products of rhodium catalyst for ammonia oxidation

Article abstract of DOI:10.1023/A:1022259230407

The chemical aspects of recovery with a calcium oxide sorbent of rhodium lost in high-temperature oxidation of ammonia were studied. The physicochemical properties of this compound and conditions of its formation were considered.

Full text of DOI:10.1023/A:1022259230407

Russian Journal of Applied Chemistry, Vol. 75, No. 10, 2002, pp. 1594 1597. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 10, 2002,
pp. 1629 1631.
Original Russian Text Copyright
2002 by Zakharchenko.
SORPTION
AND ION-EXCHANGE PROCESSES
Recovery with Calcium Oxide Sorbent of Conversion Products
of Rhodium Catalyst for Ammonia Oxidation
N. I. Zakharchenko
Zhukovsky National Aerospace University, Kharkov, Ukraine
Received June 4, 2002
Abstract The chemical aspects of recovery with a calcium oxide sorbent of rhodium lost in high-tempera-
ture oxidation of ammonia were studied. The physicochemical properties of this compound and conditions
of its formation were considered.
Catalytic oxidation of ammonia to nitrogen(II) ox-
ide underlies the industrial method for manufacture
of nitric acid [1]. Industrial catalysts (Pt, Rh, and Pd
alloys) undergo physicochemical transformations in
technological processes, with the surface structure
and the chemical composition of the catalysts changed
ture), the sorbent was crashed into grains and a frac-
tion with grain size of 2 3 mm was selected.
Since recovery of lost metals and their compounds
is mainly effected by the surface layer of sorbents
[
1, 7 16, 18 20], the sorbent grains were ground af-
ter every 40 h of operation and subjected to repeated
forming and thermal treatment in an inert medium
[
1 6]. A set of such transformations of platinoid cat-
alysts leads to loss of deficient and expensive metals
of the platinum group. An effective way to diminish
the loss of platinum metals is their partial recovery
by sorption masses mainly based on calcium oxide
(Ar) in accordance with the scheme considered above.
The catalytic process of ammonia oxidation was
carried out in a flow-through installation with quartz
reactor 50 cm in diameter. As a catalyst served rho-
dium in the form of a package of five grids with
thread diameter of 0.06 mm. The 17-cm-high bed
of sorbent grains was placed at a distance of 0.8 cm
from the catalyst grids.
[
1, 7 17].
The mechanism of platinum recovery with calcium-
containing sorbents and the chemical aspect of the
process are rather extensively covered in the scientif-
ic literature [1, 7 20]. At the same time, there is no
evidence concerning the mechanism of recovery of
rhodium, entering into the composition of industrial
catalysts for ammonia oxidation (alloy nos. 1 and 5),
with a calcium oxide sorbent; there are no data, either,
on what kind of chemical compounds are formed in
interaction between the products of conversion of
rhodium catalysts and sorbents [1].
The temperature of ammonia oxidation was 1253 K;
the pressure in the system, 0.101 MPa; the content
of ammonia in the ammonia air mixture, 11.5 vol %;
1
and the linear flow velocity of the mixture, 0.40 m s .
The time of catalyst operation and absorption mass
testing was 600 h. An X-ray phase analysis of the
compounds was made on a Siemens D-500 diffractom-
eter with CuK radiation. A differential-thermal analy-
The present communication reports the results ob-
tained in studying the recovery with a calcium oxide
sorbent of rhodium lost in high-temperature oxidation
of ammonia.
sis was done on an MOM Q-derivatograph (Hungary).
The heating rate was 5 deg min 1 in the temperature
range 293 1773 K.
During reactor shutdowns, the system was purged
with dried argon and isolated from air with moisture
absorbers. After being discharged from the reactor,
the sorbent was stored in hermetically sealed vessels
filled with dried argon.
As a starting compound for preparing the sorbent
was used calcium oxide of chemically pure grade.
The reagent was ground and treated with a 70% aque-
ous solution of ethynol (2 3% relative to the sorbent
mass) and compacted into pellets under pressure of
1
50 MPa. After thermal treatment of the mass in air
The X-ray phase analysis of the sorbent demon-
strated that it contains CaO and a rhodium compound
at 473, 673, 873, and 1253 K (2 h at each tempera-
1
070-4272/02/7510-1594 $27.00 2002 MAIK Nauka/Interperiodica

RECOVERY WITH CALCIUM OXIDE SORBENT OF CONVERSION PRODUCTS OF RHODIUM CATALYST 1595
of unknown composition (Fig. 1). To eliminate the
effect of unreacted sorbent on the X-ray diffraction
patterns obtained, the sorbent was dissolved in 25%
HNO of chemically pure grade at 308 K. In doing
3
so, account was taken of the fact that rhodium com-
pounds do not dissolve in HNO of this concentration
3
at moderate temperatures [21]. The liquid phase was
separated from the solid phase and removed. The sol-
id phase was thoroughly and repeatedly washed with
2
+
hot twice-distilled water (antil no Ca and NO ions
3
were contained in washing water) and subjected to
thermal treatment in an inert medium (Ar) at 473
523 K to constant weight.
The X-ray diffraction patterns of the solid residue
contained only lines characteristic of the compound
CaRh O (Fig. 1). The parameters of the X-ray pat-
terns of this compound are presented in the table.
2
4
Fig. 1. X-ray diffraction patterns of the sorbents and prod-
ucts of rhodium recovery. (2 ) Bragg angle. (1) Initial
sorbent, (2) sorbent after 600 h of catalyst operation, and
(3) product of interaction between the sorbent and rhodium
compounds (CaRh O ).
The set of lines in the diffraction patterns (inter-
planar spacings d and intensities I) is in good agree-
ment with data from the ASTM file [22], which in-
dicates that the chemical compound CaRh O , hav-
2
4
2
4
value calculated using Eq. (1), which indicates that
the inert medium rules out any influence of rhodium
sublimation in the form of rhodium(III) oxide [21, 23].
ing rhombic lattice (a = 0.9169, b = 1.0747, c =
.3087 nm) of the CaFe O structural type with Pnam
0
2
4
space group, is formed in the sorbent.
The formation of CaRh O in the sorbent can be
The composition of the CaRh O compound was
confirmed by X-ray phase and chemical analyses [23].
2
4
2
4
accounted for on the basis of a comparative analysis
of the conditions under which the products of rhodium
catalyst conversion are recovered and the conditions
of synthesis of the given compound.
A thermogram of the compound CaRh O is pres-
2
4
ented in Fig. 2.
When the substance is heated in an inert medium
argon) to prevent high-temperature oxidation of Rh
The authors of [24] synthesized CaRh O by
prolonged (for no less than 150 C) heating in air
2
4
(
and its sublimation in the form of rhodium(III) oxide
21, 23] in the temperature range 1713 1763 K, loss
[
Characteristics of X-ray diffraction patterns of the CaRh O
2
4
of mass (15.49%) is observed, associated with endo-
thermic decomposition of the compound by the equa-
tion
compound found in the sorbent
d, nm
I, %
d, nm
I, %
2
CaRh O = 2CaO + 4Rh + 3O2.
(1)
2
4
0.5373
12
24
49
8
1
3
0.1978
0.1850
0.1823
0.1797
0.1711
0.1664
0.1624
0.1546
0.1539
0.1527
0.1508
0.1497
0.1340
0.1277
17
34
25
7
8
10
3
16
9
13
22
11
9
0
.4617
The products of dissociation are calcium oxide,
rhodium metal, and oxygen, which is characteristic
of double oxides including a noble metal [23]. The
0.4515
0.3456
0.3329
0.2972
0.2822
0.2686
X-ray diffraction patterns of products of CaRh O4
2
decomposition, cooled in an inert medium (Ar), con-
tain, in addition to reflections related to calcium ox-
ide (0.276, 0.239, 0.169, 0.1447, 0.1383, 0.1203,
.1072, 0.0980 nm), also reflections from metallic
rhodium (0.2196, 0.1902, 0.1345, 0.1147, 0.1098,
.0951, 0.0872, 0.0850 nm). An endothermic peak
3
49
91
100
43
7
0
0
0
0
0
0
.2626
.2569
.2551
.2308
.2265
.2114
0
0
11
4
appears in the DTA curve at 1743 10 K.
7
For the CaRh O compound, the loss of mass in
0.2001
9
2
4
decomposition virtually did not exceed the expected
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 10 2002

1596
ZAKHARCHENKO
(
T = 1173 1373 K, P = 0.101 MPa) of a mixture
of rhodium(III) oxide (or rhodium metal) with cal-
cium oxide (carbonate). Under these conditions, me-
tallic rhodium is converted into rhodium(III) oxide
[
21, 23, 24].
In high-temperature oxidation of ammonium, the
rhodium catalyst undergoes a number of physico-
chemical transformations [1, 25] in accordance with
the reaction equations
4
Rh(c) + 3O2(g) = 2Rh O3(c),
(2)
(3)
(4)
(5)
2
Rh(c) = Rh(g),
4Rh(c) + 3O2(g) = 2Rh O3(g),
2
4Rh(g) + 3O2(g) = 2Rh O3(g).
2
Fig. 2. Thermogram of CaRh O . Substance mass 100 mg,
2 4
1
heating rate 5 deg min . (T) Temperature and ( m) mass
change.
At a process temperature of 1253 K rhodium subli-
mates predominantly in the form of rhodium(III) ox-
ide [21, 26]. However, in the gas phase too, rhodium
is readily oxidized by the oxygen present in the reac-
tion medium to rhodium(III) oxide. Rhodium(III) ox-
ide dissociates at 1333 K [21], being, therefore, stable
at the reaction temperature.
(2) Data on chemical aspects of recovery with the
calcium oxide sorbent of rhodium lost in high-tem-
perature oxidation of ammonia can be used in devel-
oping theoretical foundations for creation of effective
sorption masses.
The forming products of reactions (2) (5), Rh and
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2
3
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