Fe2O3-Li2O catalysts for ammonia oxidation

Article abstract of DOI:10.1023/A:1012718016470

The catalytic properties of the Fe₂O₃-Li₂O system in ammonia oxidation were studied in the high-temperature region. The influence of the phase composition of the system on the physicochemical and catalytic properties of th

Full text of DOI:10.1023/A:1012718016470

Russian Journal of Applied Chemistry, Vol. 74, No. 2, 2001, pp. 229 234. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 2, 2001,
pp. 226 231.
Original Russian Text Copyright
2001 by Zakharchenko.
CATALYSIS
Fe2O3 Li2O Catalysts for Ammonia Oxidation
N. I. Zakharchenko
Zhukovskii State Aerospace University, Kharkov, Ukraine
Received June 13, 2000; in final form, October 2000
Abstract The catalytic properties of the Fe2O3 Li2O system in ammonia oxidation were studied in the high-
temperature region. The influence of the phase composition of the system on the physicochemical and catalytic
properties of the catalysts were revealed. The catalytic properties of lithium ferrite, which is the most active
and selective component of the Fe2O3 Li2O system, were studied. The mechanism of lithium ferrite deac-
tivation at 1273 K is considered.
Catalytic oxidation of ammonia to nitrogen(II)
oxide is the basic reaction for industrial production of
nitric acid [1]. Owing to the high cost, deficiency, and
irrecoverable loss of industrial catalysts (platinum,
rhodium, and palladium alloys) in technological pro-
cesses, a search for new efficient non-platinum cat-
alysts (NPC) is urgent. Iron(III) oxide is a promising
support for NPC, used industrially as a component in
the second stage of a combined system for ammonia
oxidation [1, 2]. To preserve the high activity and
selectivity with respect to nitrogen(II) oxide and to en-
hance the thermal and chemical resistance of iron(III)
oxide, various modifying additives are used, and metal
catalytic properties of the systems is of practical and
theoretical interest for the development of scientif-
ically substantiated methods for creating catalysts
with prescribed properties.
Lithium oxide (Li2O) is used as modifying additive
to iron oxide containing catalysts, but the system
Fe2O3 Li2O has not been studied in a wide composi-
tion range [1, 3, 5 6]. The present work is concerned
with the catalytic and physicochemical properties of
the Fe2O3 Li2O system in ammonia oxidation.
The catalyst was prepared by thermal decomposi-
tion in air of ferric nitrate hydrate Fe(NO3)3 9H2O
oxides in particular [1 6]. The available data on cat- and lithium nitrate hydrate Li(NO3) 3H2O (both
alysts for ammonia oxidation are empirical, because
the modern theory of catalysis allows no unambiguous
prediction of a set of properties for catalysts with
various compositions, preparation procedures, and
chemical prehistories. Accumulation of data on the
analytically pure) mixed in a calculated ratio [7].
An X-ray phase analysis was performed on a URS-50I
diffractometer in FeK -radiation. The phase composi-
tion of the Fe2O3 Li2O catalytic system is presented
in Table 1.
Table 1. Phase composition of the Fe2O3 Li2O catalytic system
Properties of system components
C
Li O, wt %
Phase composition
2
crystal structure
unit cell parameter a, nm
0
-Fe2O3
-Fe2O3 + -Li0.5Fe2.5O4
-Li0.5Fe2.5O4
-Li0.5Fe2.5O4 + -LiFeO2
-LiFeO2
Trigonal -Al2O3
Cubic MgAl2O4
Cubic NaCl
0.5430
0.8330
0.4156
0.9218
0.4628
0 3.5
3.6
3.7 15.7
15.8
15.9 48.2
48.3
48.4 99.9
100.0
-LiFeO2 + Li5FeO4
Li5FeO4
Li5FeO4 + Li2O
Li2O
Rhombic Na2O
Cubic antifluorite CaF2
1070-4272/01/7402-0229 $25.00 2001 MAIK Nauka/Interperiodica

230
ZAKHARCHENKO
Table 2. Properties of the Fe2O3 Li2O catalytic system
3
3
CLi O
,
Ssp,
SNO, %
X
10 ,
Y,
CLi O
,
Ssp,
SNO, %
X
10 ,
Y,
2
2
1
2
1
2
wt % m2 g
( = 6.77 10 2 s) m3 h 1 m
rel. %
wt %
m2 g
( = 6.77 10 2 s) m3 h 1 m
rel. %
30.0
35.0
40.0
45.0
48.3
50.0
53.0
56.0
60.0
65.0
70.0
80.0
90.0
100.0
2.2
1.9
1.7
1.6
1.5
1.4
1.3
1.2
1.0
0.9
0.8
0.6
0.5
0.4
24.3
19.1
15.5
12.2
11.2
9.2
6.8
5.5
3.8
2.6
2.42
2.09
1.87
1.76
1.65
1.54
1.43
1.32
1.10
0.99
0.88
0.67
0.55
0.44
1.33
1.41
1.48
1.50
1.53
1.55
1.58
1.64
1.71
1.73
1.78
1.87
1.92
1.97
0
2.0
3.0
3.6
4.0
5.0
6.0
8.0
10.0
13.0
15.0
15.8
18.0
20.0
25.0
5.9
6.4
6.8
7.1
6.9
6.6
6.3
5.7
5.0
4.2
3.6
3.5
3.1
2.9
2.5
94.7
95.0
95.3
95.5
92.9
85.5
78.6
70.2
62.9
55.7
52.0
51.0
42.1
37.7
30.2
6.50
7.05
7.49
7.82
7.60
7.27
6.94
6.28
5.50
4.63
3.97
3.86
3.42
3.19
2.75
0.30
0.26
0.24
0.21
0.23
0.25
0.27
0.40
0.55
0.75
0.91
0.93
1.02
1.10
1.22
1.9
1.1
0.7
0.4
Note: X is the limiting AAM load, Y is ammonia breakthrough under the critical process conditions, and S
is selectivity
NO
by NO.
The specific surface area Ssp of the catalysts was
measured by low-temperature adsorption of nitrogen
and calculated by the BET equation (Table 2).
gas mixture for NH3, O2, N2, NO, and N2O before and
after catalyst introduction, as in [9]. The sensitivity
of analysis was (in 10 vol %): 3.0 for NH3, 3.5 for
3
NO, and 5.0 for O2, N2, and N2O.
The catalyst selectivity by NO was determined
on a flow-through installation with a quartz reactor
The limiting AAM load was determined by a pro-
cedure [3] consisting in that the catalyst is loaded to
a critical state (dying), with the heat balance of a cat-
alyst disturbed when the reaction passes from the dif-
fusion to the kinetic region.
2
2
10 m in diameter [8]. The catalyst bed (4 12)
2
3
10 m in height was composed of (2 3) 10 -m
granules; the ammonia concentration in the ammonia
air mixture (AAM) was about 10 vol %; the time
2
of contact was 6.77 10 s, and the optimal pres-
The results obtained in studying the catalytic prop-
erties of the Fe2O3 Li2O system are presented in
the Fig. 1 and Table 2.
sure, according to previously obtained data [4], was
0.101 MPa. The test temperature corresponded to
the maximum selectivity of a single-component iron
oxide catalyst (1053 K) [7]. The compositions of
the products of ammonia oxidation and thermal de-
composition of nitrogen(II) oxide on the catalysts
were determined by chromatographic analysis of the
Only two nitrogen compounds, N2 and NO, were
found among the reaction products of ammonia oxida-
tion on the catalysts studied; under conditions far
from critical no ammonia breakthrough occurred.
Thus, the overall conversion of the initial substance
is 100% and the only variable parameter of the cat-
alytic process is the ratio between nitrogen(II) oxide
and molecular nitrogen, i.e., the selectivity by NO
(or by nitrogen). Thermal dissociation of NO may
diminish the observed selectivity by nitrogen(II) oxide:
2NO = N2 + O2.
(1)
Experimental data on the degree of thermal dis-
sociation of nitrogen(II) oxide on the catalysts of
the system studied are presented in Table 3.
Fig. 1. Catalyst selectivity S
vs. composition of the
NO
Fe O Li O system.
2
3
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

Fe2O3 Li2O CATALYSTS FOR AMMONIA OXIDATION
231
At the testing temperature and the optimal time of Table 3. Degree of NO decomposition on Fe2O3 Li2O
2
contact (6.77 10 s), nitrogen(II) oxide undergoes catalysts at 1053 K [Gas mixture composition (vol %):
thermal dissociation in trace amounts (Li2O and a mix- 9.5 NO, 71.3 N2, 4.6 O2, and 14.6 H2O (vapor) (
ture of Li5FeO4 with Li2O) to 1.2% (on Li0.5Fe2.5O4). 6.77 10 )]
=
2
The thermal dissociation of NO on lithium oxide does
*
not virtually occur. At higher reagent flow velocities,
i.e., with the time of contact decreasing to 1.2 10 3 s
(critical conditions of catalyst dying), no thermal dis-
sociation of nitrogen(II) oxide occurs, which is con-
sistent with data on other NPC catalysts for ammonia
oxidation [1, 10].
C
Li O, wt %
Phase composition
,
%
2
0
2.0
3.6
-Fe2O3
-Fe2O3 + -Li0.5Fe2.5O4
-Li0.5Fe2.5O4
-Li0.5Fe2.5O4 + -LiFeO2
-LiFeO2
1.0
1.1
1.2
0.7
0.5
10.0
15.8
35.0
48.3
70.0
100.0
At lithium oxide concentrations of up to 3.6%,
iron(III) oxide with a structure of -Fe2O3 hematite
(reflections with interplanar spacings of 0.3680,
0.2690, 0.2510, 0.2204, 0.1844, 0.1693, 0.1482,
and 0.1452 nm) coexists with lithium ferrite
-Li0.5Fe2.5O4 (main reflections with interplanar spac-
ings of 0.589, 0.478, 0.374, 0.340, 0.2946, 0.2769,
0.2514, 0.2084, 0.1703, 0.1605, 0.1474, 0.1273,
0.1204, and 0.1115 nm) as a binary mixture. At
the stoichiometric ratio Li2O : Fe2O3 = 1 : 5 (3.6 wt %
-LiFeO2 + Li5FeO4
Li5FeO4
Li5FeO4 + Li2O
Li2O
0.2
0.1
<0.1
<<0.1**
*
Degree of NO decomposition.
Virtually absent.
**
the -LiFeO2 Li5FeO4 system drastically decreases
the selectivity of catalysts by NO.
Li2O) lithium ferrite is formed with a structure of
the Fe3+[Li0+.5Fe ]O4 inverted spinel. The specific
3+
1.5
At lithium oxide concentrations exceeding 48.3%,
Li5FeO4 coexists with the phase of lithium oxide as
a mixture of two compounds. The selectivity and
the specific surface area of lithium oxide (0.4% and
1
surface area of lithium ferrite (7.1 m2 g ) slightly
exceeds that of iron(III) oxide (5.9 m2 g ). The selec-
1
tivity of lithium ferrite by nitrogen(II) oxide is 95.5,
i.e., exceeds the similar characteristic for -Fe2O3
(94.7%). When the concentration of lithium ferrite,
possessing the higher selectivity and specific surface
area, is raised, the selectivity of catalysts of the
-Fe2O3 -Li0.5Fe2.5O4 system grows.
0.4 m2 g ) are lower than those of Li5FeO4 and all
1
other system components. Raising the lithium oxide
concentration in the Li5FeO4 Li2O system leads to
a monotonic decrease in the selectivity of catalysts by
nitrogen(II) oxide. Thus, the selectivity of the system
depends on its composition, in particular, on the ratio
of phases in the two-component catalyst.
In the lithium oxide concentration range 3.7 15.7%,
the ferrite phase -Li0.5Fe2.5O4 coexists with the
-modification of lithium metaferrite -LiFeO2 (main
reflections with interplanar spacings of 0.239, 0.2073,
0.1467, 0.1252, 0.1200, 0.1089, 0.0925, 0.0845,
0.0640, and 0.0657 nm) as a binary mixture. The se-
lectivity and the specific surface area of lithium meta-
In the Fe2O3 Li2O catalytic system, the only
component of practical interest is lithium ferrite
-Li0.5Fe2.5O4, the most active and selective com-
pound surpassing even -Fe2O3 in its character-
istics. The other system components are low-active
( -LiFeO2 and Li5FeO4) or virtually inactive (Li2O).
1
ferrite (51.0% and 3.5 m2 g ) are considerably lower
than the similar characteristics for -Li0.5Fe2.5O4.
The increase in the concentration of the low-active
phase of lithium metaferrite in the -Li0.5Fe2.5O4
-LiFeO2 system leads to an abrupt monotonic de-
crease in the selectivity of catalysts by nitrogen(II)
oxide.
The high-temperature oxidation of ammonia on cat-
alysts occurs by two parallel reactions [1, 11]:
4NH3 + 5O2 = 4NO + 6H2O,
4NH3 + 3O2 = 2N2 + 6H2O.
(2)
(3)
X-ray phase analysis shows that, at lithium oxide
concentrations of 15.9 48.2%, lithium metaferrite
-LiFeO2 coexists with the Li5FeO4 phase as a binary
mixture. Li5FeO4 ranks far below -LiFeO2, and
The redox mechanism of the reactions [11] is re-
sponsible for the correlation between the selectivity
and strength of binding of chemisorbed oxygen to the
catalysts [12]. The measure of the strength of oxygen
binding to the catalyst is the heat of its chemisorption
especially -Li0.5Fe2.5O4, in selectivity and specific
1
(11.2% and 1.5 m2 g ). Raising the
surface area
concentration of the low-active Li5FeO4 phase in [11, 12]. For catalysts of certain nature (metals and
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

232
ZHAKHARCHENKO
the catalysts, the ammonia breakthrough decreases,
which is consistent with the higher activity of such
catalysts. The ammonia breakthrough is the highest
for lithium oxide (1.97%) and the lowest for lithium
ferrite (0.21%). In this respect, -Fe2O3 is close to
lithium ferrite (0.30%), their specific surface areas
differing insignificantly.
Fig. 2. Selectivity S
of lithium ferrite vs. operation
The kinetic characteristics of the reaction on lithi-
um ferrite, which is the most active and selective
component of the Fe2O3 Li2O system, were deter-
mined using the burning and dying temperatures
of a lithium ferrite pellet, i.e., the temperatures at
the critical points. The reaction rate was calculated
by the method in which the temperature limits of
the external diffusion region were determined using
the dying of a catalyst pellet on lowering the
AAM temperature [16]. The temperature of the cat-
alyst surface was measured with a TPP-2 thermo-
couple inserted into the pellet from the side of AAM
flow. A layer of lithium ferrite granules (2.0 3.0)
NO
1
time t. The linear velocity of AAM flow is 0.8 m s
(operation conditions). Temperature (K): (1) 1073, (2) 1173,
and (3) 1273.
metal oxides) there exists an optimal energy of oxy-
gen binding to the compound surface, ensuring the
maximum selectivity by nitrogen(II) oxide [11 14].
The deviation of the binding energy from the optimal
value impairs the NO selectivity and improves the se-
lectivity by N2. The binding energies of the sur-
face oxygen to the individual components of the cat-
alytic system in the oxidized state were determined
from the temperature dependence of the pressure of
oxygen in thermodynamic equilibrium with the sur-
3
10 m in size was placed between the pellet and
the reactor walls to eliminate the heat loss. The ki-
netic parameters of the process were calculated by
the Buben equation [17] solved for two reaction
rates at a constant oxygen concentration. The equation
has the form
1
face oxygen of the compounds [15] (kJ mol O2):
-Fe2O3 143.5, -Li0.5Fe2.5O4 138.2, -LiFeO2 179.8,
Li5FeO4 224.3, and Li2O 242.5.
For hematite and lithium ferrite, the energies of
oxygen binding to the surface are close and the selec-
tivities differ only slightly. The energy of oxygen
binding to the surface of -LiFeO2, and especially to
the Li5FeO4 and Li2O surfaces, much exceeds that for
-Fe2O3 and lithium ferrite. The product of stronger
ammonia oxidation (i.e., nitrogen(II) oxide) is formed
with breaking of a greater number of oxygen catalyst
bonds, compared with molecular nitrogen. This means
that the selectivity by NO decreases with increasing
binding energy of adsorbed oxygen [11, 13, 14], and
it is this phenomenon that is observed for -LiFeO2,
Li5FeO4, and Li2O.
a
b
a
a
b
(4)
(1 + a) 1 + (m
a = T/ T0
1)
1
= 0,
1, b = Q C0 /( T0),
= RT0 /E,
where m is the reaction order by ammonia, T is the
catalyst surface temperature at the critical point (K),
T0 is the AAM temperature, C0 is the ammonia con-
centration in the flow, and are the mass- and heat-
transfer coefficients, Q is the heat effect of the reac-
tion, and E is its activation energy.
The coefficients
and
were calculated from
the known equations [18].
The limiting ammonia load grows with increasing
specific surface area of the catalyst: it is the highest
The kinetic parameters of ammonia oxidation on
lithium ferrite are as follows: temperature of catalyst
burning, 527 K; ammonia concentration in AAM,
10 vol %; activation energy of the reaction,
2
for lithium ferrite [7.82 103 m3 NH3 h 1 m ] and the
2
lowest for lithium oxide (0.44 103 m3 NH3 h 1 m ]
(Table 2). The limiting load for iron(III) oxide (6.50
2
103 m3 NH3 h 1 m ] is slightly lower than that for
1
9.54 kJ mol NH3; reaction order by ammonia, 0.19;
lithium ferrite. This parameter depends on the rate of
the chemical reaction on the surface, which is, in turn,
determined by the chemical composition of the cat-
and catalyst selectivity by NO (at 1053 K), 95.5%.
A study of the dependence of catalyst selectivity on
operation time confirmed the high stability of lithium
ferrite at 1073 1173 K (Fig. 2).
alyst. Under the critical process conditions (
=
3
1.2 10 s), no decomposition of nitrogen(II) oxide
by pathway (1) is observed, but part of unreacted
ammonia is found after the catalyst (breakthrough)
(Table 2). With increasing specific surface area of
The selectivity of lithium ferrite at 1073 and
1173 K decreases by 0.4 and 0.7%, respectively, after
160 h of operation. The iron(III) oxide selectivity at
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

Fe2O3 Li2O CATALYSTS FOR AMMONIA OXIDATION
233
1073 K decreases by 3.5% after the same period of
time [7]. With the process temperature raised to
1273 K, lithium ferrite undergoes deactivation under
the influence of the elevated temperature and the re-
action medium: the catalyst selectivity after 160 h of
operation decreases by 3.5%. To elucidate the deac-
tivation mechanism, the chemical and phase composi-
tion of the catalyst were studied along with its struc-
ture. X-ray diffraction patterns of the surface layer of
a catalyst that operated at 1273 K contain lithium fer-
rite reflections and reflections with interplanar spac-
ings of 0.485, 0.297, 0.253, 0.242, 0.2101, 0.1712,
0.1614, and 0.1485 nm, characteristic of magnetite
Fe3O4 [19]. This conclusion is confirmed by IR spec-
tral data. The IR spectra of the surface layer of a cat-
alyst after operation contain the magnetite absorption
Table 4. Changes in selected structural and catalytic
properties of lithium ferrite -Li0.5Fe2.5O4 during operation
at 1273 K
Root-mean-square
granule size,
nm
Limiting load by
Ssp,
m2 g
3
t, h
ammonia, A 10 ,
1
2
m3 h 1 m
4
40
60
6.8
6.6
6.4
5.8
4.9
3.3
2.7
200
207
218
240
265
308
340
7.49
7.27
7.05
6.39
5.40
3.63
2.97
80
100
140
160
1
peaks at 407, 427, 480, 557, 673, and 980 cm [20].
Magnetite suppresses the catalyst selectivity, i.e., it is
the low-active phase [7] (the magnetite selectivity at
1073 K is 7%). The intensity of the phase transforma-
tions occurring in the catalyst grows with increasing
reaction temperature, as shown by X-ray diffraction
analysis and IR spectroscopy. The increase in the con-
centration of the low-active phase with growing tem-
perature results in a more abrupt decrease in the cat-
alyst selectivity. In addition, the catalyst operated at
1273 K contains a new, relatively low-active phase
-modification of lithium metaferrite -LiFeO2. The
selectivity of -LiFeO2 at 1053 K is 51%. The appear-
alyst dying) the activity of lithium ferrite decreases
and the limiting load falls from 7.49 103 to
2
2.97 103 m3 NH3 h 1 m ) as a result of recrystal-
lization and a decrease in the specific surface area of
the catalyst.
Thus, a set of chemical and phase transformations
of the catalyst, yielding low-active and low-selective
components (Fe3O4 and -LiFeO2) with rearranged
structure is the reason for lithium ferrite deactivation.
In the temperature range 1073 1173 K, lithium ferrite
is a highly selective and stable catalyst for ammonia
ance of the new, low-active phases of magnetite and oxidation.
-LiFeO2 at elevated temperatures is associated with
the redox mechanism of ammonia oxidation [1, 5, 11]
and is in good agreement with the results of thermo-
dynamic calculations [21]. According to these data
the high probability of lithium ferrite transformation
CONCLUSIONS
(1) The catalytic properties of the Fe2O3 i2O sys-
tem in ammonia oxidation were studied in the com-
ponent concentration range 0 100 wt %.
at T 1273 K (P0 = 0.021 MPa, air) by the reaction
2
(2) The catalytic properties of the individual com-
ponents ( -Fe2O3, Li2O, -Li0.5Fe2.5O4, -LiFeO2,
and Li5FeO4) of the system were determined.
6Li0.5Fe2.5O4 = 3LiFeO2 + 4Fe3O4 + O2
(5)
results in the formation of new phases, found in ex-
perimental studies. Similar transformations during
heat treatment of lithium ferrite were observed in [22].
(3) The influence of the phase composition of
the system on the physicochemical and catalytic prop-
erties of the catalysts, including the activity and se-
lectivity by nitrogen(II) oxide, was revealed. The ac-
tivity and the selectivity of the system depends on
composition and, in particular, on the ratio of the
amount of -Fe2O3 phase to that of the -Li0.5Fe2.5O4
phase of a two-component catalyst. The same is true
for the Li0.5Fe2.5O4 and -LiFeO2, -LiFeO2 and
Li5FeO4, and Li5FeO4 and Li2O phases.
Under the influence of high temperature, the cat-
alyst recrystallizes, with the specific surface area
decreasing (Table 4). After 160 h of operation the
size of lithium ferrite granules increases from 200 to
340 nm. The reaction conditions being far from crit-
ical, i.e., the reaction being limited by ammonia dif-
fusion toward the catalyst surface, the specific sur-
face area does not exert decisive influence on the se-
lectivity of lithium ferrite [1, 3, 6]. This means that
the drop in selectivity at 1273 K is mainly due to
phase and chemical transformations of the catalyst.
Under the critical conditions of the reaction (cat-
(4) The process parameters were determined on
the ferrite catalyst, which is the most active and selec-
tive component of the Fe2O3 Li2O system.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

234
ZHAKHARCHENKO
10. Zhidkov, B.A., Orlova, S.S., Bachenko, G.A., and
(5) A set of chemical and phase transformations of
Plygunov, A.S., Khim. Tekhnol., 1979, no. 1, pp. 5 8.
the Li0.5Fe2.5O4 catalyst, resulting in the formation of
low-active and low-selective components (Fe3O4 and
-LiFeO2) with rearranged structure, is responsible for
the deactivation of lithium ferrite at 1273 K.
11. Golodets, G.I., Geterogenno-kataliticheskie reaktsii
s uchastiem molekulyarnogo kisloroda (Heterogeneous
Catalytic Reactions Involving Molecular Oxygen),
Kiev: Naukova Dumka, 1977.
(6) The obtained data on the catalytic properties
of the Fe2O3 Li2O system can be used in developing
highly efficient modified catalysts for ammonia ox-
idation.
12. Boreskov, G.K., Geterogennyi kataliz (Heterogeneous
Catalysis), Moscow: Nauka, 1986.
13. Golodets, G.I., Kinet. Katal., 1987, vol. 28, no. 2,
pp. 337 341.
14. Il’chenko, N.I., Pyatnitskii, Yu.I., and Pavlenko, N.V.,
Teor. Eksperim. Khim., 1998, vol. 34, no. 5,
pp. 265 281.
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