Catalytic properties of the FE2O3-Bi2O3 system in ammonia oxidation to nitrogen oxides

Article abstract of DOI:10.1023/A:1014209415066

The catalytic properties of the Fe₂O₃-Bi₂O₃ system in ammonia oxidation are studied at 1073 K. The effect of phase composition on the physicochemical and catalytic properties of the system is determined. The cat

Full text of DOI:10.1023/A:1014209415066

Kinetics and Catalysis, Vol. 43, No. 1, 2002, pp. 95–98. Translated from Kinetika i Kataliz, Vol. 43, No. 1, 2002, pp. 104–107.
Original Russian Text Copyright © 2002 by Zakharchenko.
Catalytic Properties of the Fe2O3–Bi2O3 System
in Ammonia Oxidation to Nitrogen Oxides
N. I. Zakharchenko
Zhukovskii National Aerospace University, Kharkov, Ukraine
Received January 19, 2001
Abstract—The catalytic properties of the Fe2O3–Bi2O3 system in ammonia oxidation are studied at 1073 K.
The effect of phase composition on the physicochemical and catalytic properties of the system is determined.
The catalytic properties of individual components of the system (α-Fe2O3, Bi2Fe4O9, BiFeO3, and δ-Bi2O3) are
studied.
INTRODUCTION
calculated with the BET equation according to the stan-
dard procedure [9].
Catalytic ammonia oxidation to NO forms the basis
for the commercial process of nitric acid manufacturing
[1]. A search for non-platinum catalysts (NPC) remains
a research challenge because of the high cost, defi-
ciency, and unrecoverable loss of commercial catalysts
(platinum, rhodium, and palladium alloys) in techno-
logical processes. Iron(III) oxide is one of the promis-
ing NPC components used at the second stage of the
multistage process of ammonia oxidation [1, 2]. To pre-
serve the high activity and selectivity to NO and to
improve on the thermal and chemical stability of Fe2O3,
various modifying agents are used such as metal oxides
[1–7]. Therefore, the accumulation of experimental
data on the catalytic properties of various systems is of
both practical and theoretical importance for the cre-
ation of the justified methods for the design of catalysts
with desired properties. Bismuth(III) oxide is used as a
modifying additive for iron oxide catalysts, but the
Fe2O3–Bi2O3 system has not been studied over a wide
range of compositions. Published data on the catalytic
properties of the system are scarce and contradictory
[1, 3, 4, 6].
The catalyst selectivities to NO were determined in
a flow-type setup with a quartz reactor (2 cm in diame-
ter) according to the procedure described in [10]. The
height of the bed of catalyst granules (2 × 3 mm) was
4–12 cm. The concentration of ammonia in an ammo-
nia–air mixture was ~10 vol %. The contact time was
τ = 6.59 × 10–2 s (under normal conditions), which is an
optimal value [5]. The pressure was 0.101 MPa. The
temperature of tests was 1073 K, which was close to the
optimum for the one-component iron oxide catalyst [8].
The product composition in ammonia oxidation and the
thermal decomposition of NO on the catalyst was deter-
mined by chromatography according to the known pro-
cedure [11] by analyzing the concentrations of NH3, O2,
N2, NO, and N2O in the gaseous mixture before and
after the catalyst bed. The sensitivity threshold of the
methods was (in vol %): 3.0 × 10–3 for NH3, 3.5 × 10–3
for NO, and 5.0 × 10–3 for O2, N2, and N2O.
The maximal catalyst capacity with respect to
ammonia was determined using the procedure
described in [3], which consisted in an increase in the
catalyst capacity up to the critical state of “quenching,”
that is, thermal balance perturbation in the transition
from diffusion to kinetics control.
This work deals with the catalytic and physico-
chemical properties of the Fe2O3–Bi2O3 system for
ammonia oxidation at high temperatures.
The figure and Table 2 show the results of studies of
the catalytic properties of the Fe2O3–Bi2O3 system.
EXPERIMENTAL
The binding energy of surface oxygen with the indi-
vidual components of the catalytic system were deter-
mined from the temperature dependence of the gas-
phase oxygen pressure in the thermodynamic equilib-
rium with oxygen on oxide surfaces according to the
procedure described in [12].
Catalysts were prepared by the thermal decomposi-
tion of the mixtures of hydrated iron (Fe(NO3)3 · 9H2O)
and bismuth (Bi(NO3)3 · 5H2O) nitrates (analytical
purity grade) in an air atmosphere. Nitrates were taken
in a calculated amount. The preparation procedure was
described earlier [8]. X-ray phase analyses were carried
out using an URS-50I diffractometer with FeKα irradi-
ation. The phase composition of the catalytic system is
shown in Table 1.
RESULTS AND DISCUSSION
We found only two nitrogen-containing compounds
The specific surface areas (Table 2) were measured in the products of ammonia oxidation on the studied
using the low-temperature adsorption of nitrogen and catalysts under conditions that were far from critical:
0023-1584/02/4301-0095$27.00 © 2002 MAIK “Nauka /Interperiodica”

96
ZAKHARCHENKO
NO and N2. Ammonia breakthrough was not observed.
Thus, the overall conversion of the starting compound
was 100%. We only observed a change in the NO : N2
ratio with time. That is, the selectivities to NO and N2
changed. The thermal dissociation of NO (Table 3)
resulted in a decrease in the apparent selectivity to NO:
S
NO, %
100
96
92
88
84
2NO = N2 + O2.
(I)
At this temperature and at the optimal contact time
(6.59 × 10–2 s), from 1.0 to 1.6% of NO dissociates.
0
20
40
60
80 100
[Bi2O3], wt %
The selectivity to NO drops due to reaction (I) on
individual components as follows (in %):
Dependence of the process selectivity to NO (S ) on the
NO
α-Fe2O3—1.0,
Bi2Fe4O9—1.5,
BiFeO3—0.9,
δ-Bi2O3—1.3.
composition of the Fe O –Bi O system.
2
3
2 3
α-Fe2O3–Bi2Fe4O9 results in a monotonic increase in
the catalyst selectivity to NO (up to 59.3 wt % Bi2O3,
see Table 2).
With an increase in the feed-flow rate, and a
decrease in the contact time to 1.2 × 10–3 s (the critical
conditions of catalyst quenching), NO did not dissoci-
ate thermally (Table 3). This agrees with data for other
NPCs for ammonia oxidation [1].
When the concentration of bismuth is lower than
59.3 wt %, hematite α-Fe2O3 (main signals in X-ray
diffraction patterns with interplanar distances of
0.3680, 0.2690, 0.2510, 0.2204, 0.1844, 0.1693,
0.1482, and 0.1452 nm) coexists with bismuth ferrite
Bi2Fe4O9 (main peaks in X-ray diffraction patterns with
interplanar distances of 0.5988, 0.4218, 0.3731, 0.3451,
0.3319, 0.3163, 0.3082, 0.2996, 0.2895, 0.2654, 0.2442,
0.2395, 0.1932, 0.1842, 0.1762, and 0.1624 nm) in the
form of two compounds. When the reactant ratio is sto-
With a concentrations of bismuth oxide ranging
from 59.4 to 74.4 wt %, Bi2Fe4O9 coexists with another
bismuth ferrite phase (BiFeO3) (the main peaks with
interplanar distances of 0.3945, 0.2808, 0.2781,
0.2273, 0.1975, 0.1774, 0.1626, 0.1612, 0.1607,
0.1314, 0.1248, 0.1103, and 0.1053 nm) in the form of
two compounds. Bismuth orthoferrite BiFeO3 is
formed at a stoichiometric ratio of the components
Fe2O3 : Bi2O3 = 1 : 1 (74.5 wt % Bi2O3). The selectivity
to NO (87.0%) on bismuth orthoferrite and its specific
surface area (5.1 m2/g) are lower than the correspond-
ing characteristics for Bi2Fe4O9. An increase in the con-
centration of bismuth oxide in the Bi2Fe4O9–BiFeO3
results in a drastic decrease in the selectivity to NO.
With a concentrations of Bi2O3 ranging from 74.6 to
ichiometric Fe2O3 : Bi2O3 = 2 : 1 (59.3 wt % Bi2O3), 99.9 wt %, the system contains the phases of BiFeO3
orthorhombic bismuth ferrite is formed. The selectivity and δ-Bi2O3 (the main peaks with interplanar distances
to NO on Bi2Fe4O9 (96.6%) and its specific surface area of 0.3253, 0.2693, 0.2390, 0.1957, 0.1872, 0.1758,
(8.0 m2/g) are better than the analogous characteristics 0.1662 nm) as a mixture of two compounds. The selec-
for α-Fe2O3 (94.4% and 5.6 m2/g, respectively). An tivity to NO on Bi2O3 (92.1%) and the specific surface
increase in the bismuth ferrite concentration in area (7.0 m2/g) are higher than the analogous character-
Table 1. Phase composition of the catalytic system Fe2O3–Bi2O3
Characteristics of system components
[Bi2O3], wt %
0
Phase composition
crystalline structure
lattice parameters, nm
α-Fe2O3
trigonal, α-Al2O3-type
a = 0.5430
–
0–59.2
59.3
α-Fe2O3 + Bi2Fe4O9
Bi2Fe4O9
–
rhombic
a = 0.7881
b = 0.8410
c = 0.6010
–
59.4–74.4
74.5
Bi2Fe4O9 + BiFeO3
BiFeO3
–
cubic with rhombohedral distortion,
perovskite-like
a = 0.3955
α = 89°32′
–
74.6–99.9
100.0
BiFeO3 + Bi2O3
–
α-Bi2O3
cubic, CaF2-type
a = 0.5622
KINETICS AND CATALYSIS Vol. 43 No. 1 2002

CATALYTIC PROPERTIES OF THE Fe2O3–Bi2O3 SYSTEM
97
Table 2. Properties of the catalytic system Fe2O3–Bi2O3
istics for BiFeO3. An increase in the concentration of
bismuth oxide in the BiFeO3–δ-Bi2O3 results in an
increase in the selectivity of the process to NO.
Thus, the selectivity of ammonia oxidation on the
Fe2O3–Bi2O3 system depends on the composition and
on the ratio between the phases of the two-component
catalysts Fe2O3–Bi2Fe4O9, Bi2Fe4O9–BiFeO3, and
BiFeO3–Bi2O3.
[Bi2O3],
wt %
High-temperature ammonia oxidation on the cata-
lysts occurs via two parallel reactions [1, 13]:
0
5.6
5.7
5.9
6.2
6.6
7.0
7.5
7.7
8.0
7.2
6.5
5.5
5.1
5.3
5.5
5.7
5.9
6.2
6.6
7.0
94.4
94.5
94.6
94.9
95.3
95.6
96.1
96.3
96.6
94.4
91.8
88.6
87.0
87.9
88.8
89.5
90.1
90.8
91.5
92.1
6.17
6.28
6.50
6.83
7.27
7.71
8.26
8.48
8.81
7.93
7.16
6.06
5.62
5.84
6.06
6.28
6.50
6.83
7.27
7.71
0.38
0.36
0.30
0.25
0.18
0.13
0.07
0.05
0.03
0.10
0.19
0.41
0.51
0.46
0.41
0.36
0.30
0.25
0.18
0.13
4NH3 + 5O2 = 4NO+6H2O,
4NH3 + 3O2 = 2N2 + 6H2O.
(II)
5.0
(III)
10.0
20.0
30.0
40.0
50.0
55.0
59.3
63.0
67.0
72.0
74.5
78.0
82.0
85.0
88.0
92.0
96.0
100.0
The redox mechanism of the process [13] deter-
mines the relationship between the selectivity and the
binding strength of oxygen on the catalyst surface [13,
14]. The adsorption heat can be a measure of the bind-
ing strength [13, 14]. For the catalysts of the same
nature (metals or metal oxides), there are optimal val-
ues for the energies of oxygen binding to the surface to
obtain the maximal selectivity to NO [13–15]. Devia-
tions from the optimal values of the oxygen binding
strength lead to a decrease in the selectivity to NO and
an increase in the selectivity to N2. The experimental
binding energies of oxygen on the catalysts under study
are 125.0–143.5 kJ/mol é2 (the oxidized state of the
surface).
Probably, the most active and selective component
of the Bi2Fe4O9 system in the steady state is character-
ized by optimal energy characteristics, the maximal
number of active sites under given conditions of the
reaction, including optimal values of the catalyst–oxy-
gen binding energy for obtaining the maximal selectiv-
ity to NO. An increase in the binding energy of
adsorbed oxygen relative to the optimal value results in
a decrease in the selectivity to NO and an increase in
the selectivity to N2, because in the formation of the
product of deeper oxidation (NO), more oxygen–cata-
lyst bonds should be cleaved than in the case of molec-
ular nitrogen formation [13, 15]. A decrease in the
energy of oxygen binding relative to the optimal value
results in an increase in the probability of direct ammo-
nia interaction with the catalyst surface (reaction (III))
to form molecular nitrogen. The selectivity to NO
decreases in this case [13, 15].
The composition of the system under certain reac-
tion conditions (temperature, pressure, and reactant
concentrations) determines a set of energy characteris-
tics of the catalyst surface, its activity, and selectivity.
The maximal values of the catalyst capacity with
respect to ammonia are shown in Table 2. The maximal
capacities of the catalysts increase with an increase in
their specific surface areas: the maximal capacity is
observed for Bi2Fe4O9 (8.81 × 103 m3 NH3 h–1m–2)
and the lowest one is observed for BiFeO3 (5.62 ×
103 m3 NH3 h–1m–2). In terms of the maximal capacity,
the high temperature modification δ-Bi2O3 is the next
best after bismuth ferrite (Bi2Fe4O9). The maximal
–2
* The contact time τ is 6.59 × 10 s.
Table 3. The degree of NO decomposition for different
phase compositions of the catalysts in the Fe2O3–Bi2O3 system
at 1073 K (the composition of the gaseous mixture in vol %:
NO, 9.5; N2, 71.3; O2, 4.6; H2O, 14.6)
Degree of NO decomposition
(in %) at τ = 6.59 × 10–2 s, %*
[Bi2O3], wt %
6.59 × 10–2
0 (α-Fe2O3)
1.1
1.3
1.6
1.5
1.0
1.2
1.4
30.0 (α-Fe2O3 + Bi2Fe4O9)
59.3 (Bi2Fe4O9)
61.0 (Bi2Fe4O9 + BiFeO3)
74.5 (BiFeO3)
92.0 (BiFeO3 + δ-Bi2O3)
100.0 (δ-Bi2O3)
–3
* When the contact time τ is 1.2 × 10 s, the degree of NO decom-
position is equal to zero.
KINETICS AND CATALYSIS Vol. 43 No. 1 2002

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ZAKHARCHENKO
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capacity depends on the rate of the chemical reaction on
the surface, which is determined by the chemical com-
position of the catalyst. The maximal capacity charac-
terizes the catalyst activity and productivity [1, 16].
Under critical conditions of the process (τ = 1.2 ×
10–3 s), the consecutive reaction of NO decomposition
via reaction (I) is not observed (Table 3), but some por-
tion of unreacted ammonia was found after the catalyst
bed; that is, ammonia breakthrough takes place
(Table 2). With an increase in the specific surface area
of the catalyst, ammonia breakthrough declines in
agreement with the higher activity of these catalysts.
The maximal breakthrough is observed for BiFeO3
(0.51%), and the minimal breakthrough is observed for
Bi2Fe4O9 (0.03%).
Thus, of all the components of the Fe2O3–Bi2O3 sys-
tem, bismuth ferrite (Bi2Fe4O9) can be recommended as
an efficient catalyst for ammonia oxidation. Bismuth
orthoferrite (BiFeO3) is insufficiently active and selec-
tive in this process. Both bismuth ferrite and hematite
are superior to the high-temperature modification of
bismuth oxide (δ-Bi2O3) in the selectivity. Also,
δ-Bi2O3 readily sublimates at T > 973 K [17] and melts 12. Sazonov, V.A., Popovskii, V.V., and Boreskov, G.K.,
Kinet. Katal., 1968, vol. 9, p. 307.
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temperature catalyst for ammonia oxidation. Data on 13. Golodets, G.I., Geterogenno-kataliticheskie reaktsii s
uchastiem molekulyarnogo kisloroda (Heterogeneous
Catalytic Reactions Involving Molecular Oxygen), Kiev:
Naukova Dumka, 1977.
the catalytic properties of the system can be used in the
design and use of modified iron oxide catalysts for
ammonia oxidation.
14. Boreskov, G.K., Geterogennyi kataliz (Heterogeneous
Catalysis), Moscow: Nauka, 1986.
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KINETICS AND CATALYSIS Vol. 43 No. 1 2002