The mechanism of low-temperature ammonia oxidation on metal oxides according to the data of spectrokinetic measurements

Article abstract of DOI:10.1023/A:1016057818280

Low temperature NH₃ oxidation on MoO₃, Fe₂O₃, Cr₂O₃, and ZnO was studied by the spectrokinetic method. The following adsorbed species were intermediates in this reaction: NH₃ and N₂O on Fe₂O₃ and ZnO; NH₃, N₂O, and NO on Cr₂O₃. The following absorption bands were observed in the IR spectra obtained in the transmittance and diffuse-reflectance regimes under the reaction conditions: 2200/cm (ZnO and Fe₂O₃ and 2080/cm (Cr₂O₃). Taking into account that surface complexes with bands at 2200 and 2080/cm were formed in hydrazine oxidation, it was assumed that a hydrazine-like structure participates in NH₃ oxidation. The presence of readsorption was associated with the high adsorption ability of NH₃ (the characteristic time during which the stationary value of NH₃ absorption band is settled in several seconds), the low flow rate of feed supply (50 mL/min), and the relatively small amount of the catalyst used (in the diffuse-reflectance method, ～1 cc of the catalyst was used). Under these conditions, the measurements of the rate constant of adsorbed NH₃ consumption resulted in the underestimation of the values (for Cr₂O₃, the measured value was k₆ = 3.5 × 10^-3/sec, the calculated value, k₆ = 0.2/sec).

Full text of DOI:10.1023/A:1016057818280

Kinetics and Catalysis, Vol. 43, No. 3, 2002, pp. 363–371. Translated from Kinetika i Kataliz, Vol. 43, No. 3, 2002, pp. 394–402.
Original Russian Text Copyright © 2002 by Sil’chenkova, Korchak, Matyshak.
The Mechanism of Low-Temperature Ammonia Oxidation
on Metal Oxides According to the Data
of Spectrokinetic Measurements
O. N. Sil’chenkova, V. N. Korchak, and V. A. Matyshak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 117977 Russia
Received December 23, 1999
Abstract—Low temperature ammonia oxidation on MoO3, Fe2O3, Cr2O3, and ZnO is studied by the spectro-
kinetic method. It is shown that the following adsorbed species are intermediates in this reaction: NH3 and N2O
on Fe2O3 and ZnO; NH3, N2O, and NO on Cr2O3. All of the detected intermediates are used to construct the
mechanism of the process. In the framework of the proposed mechanism, stationary and nonstationary spectral
and kinetic data are quantitatively processed. The dependence of the rate constants of the same steps on differ-
ent oxides on their physicochemical properties is discussed.
INTRODUCTION
The ratio between different forms of adsorbed oxygen
depends on the nature of the oxides.
The selective catalytic low-temperature oxidation of
ammonia to nitrogen is one of the main processes that
make it possible to decrease the concentration of
ammonia in exhaust gases in industry. Note that the
selectivity of this process is decreased because of the
parallel formation of NO and N2O. Therefore, the
mechanistic study of ammonia oxidation is of both the-
oretical and substantial practical interest from the
standpoint of increasing catalytic activity and process
selectivity in low-temperature oxidation.
Substantial information is currently available on the
surface species formed in NH3 and O2 adsorption on the
catalyst surfaces [1–3] and on the mechanism of this
process [4–6].
Less information is available on the nature of com-
plexes formed under reaction conditions.
Table 1 shows data from the literature on surface
species formed in the reaction of ammonia oxidation.
Note that information on intermediates was obtained in
many studies using spectral measurements before and
after the reaction. Therefore, it has not been proven that
complexes found in the reaction participate in it. We
rated the reliability of such information as low. The
reliability of data obtained using measurements under
conditions of a catalytic reaction is estimated as
medium. When the reaction rate was compared with the
rates of transformations of compounds observed in situ,
the reliability was high. Analysis of data presented in
Table 1 shows that highly reliable data are virtually
absent.
Specifically, it is known that ammonia adsorbs in
the form of NH+4 on Brønsted acid sites (I) and in the
form of coordinated ammonia NH3 ads on Lewis acid
sites (II).
The specific features of pathways to N2 and N2O are
also discussed in the literature. Specifically, the authors
of [18–20] assumed that the NH2 species participate in
the formation of nitrogen, whereas HNO participates in
the formation of N2O. On the other hand, Pery and Sie-
bers [21] argued that N2O is formed via the NH species.
H
H N H
H
H
H N H
O Me O
II
O
We have studied ammonia oxidation on MoO3,
Fe2O3, Cr2O3, and ZnO [14–17] using the spectroki-
netic method, which consists in simultaneous measure-
ments of the rates of surface species transformations
using in situ IR spectroscopy and the rate of product
formation (see Table 1). Information on actual interme-
diates in low-temperature ammonia oxidation is also
summarized in Table 1. Based on these data, the multiple-
step mechanism of the process was proposed [16–17].
Earlier, the hypothesis was advanced [22] that the reac-
tion mechanism is the same over different catalysts if
the adsorbed reactants are the same. Unfortunately, for
O Me O
I
The presence of coordinated ammonia was found on
almost all metal oxide surfaces from which admixtures
are removed studied to date [1].
It is also known that [1, 6, 7] oxygen adsorbs on
transition metal oxides both in molecular and atomic
forms:
2–
O .
ads
O2 gas
O–2 ads
O–ads
0023-1584/02/4303-0363$27.00 © 2002 MAIK “Nauka /Interperiodica”

364
SIL’CHENKOVA et al.
Table 1. Surface species in ammonia oxidation according to IR spectroscopic data
Catalyst Surface complex
Reliability*
Reference
MgO
NH2
L
L
[8]
[9]
Fe2O3/SiO2
Si–NH2, Fe–NH2
NH2, N2H4, NH, HNO, NH+
V2O5, V2O5/TiO2, (V2O5–WO3/TiO2)
L
M
M
L
[10]
[11]
[12]
[13]
[14]
4
NH2, N2H4, NH, HNO, N– , N–
CuO/TiO2
CoO–MgO
MnOx/Al2O3
MoO3
2
3
NO–2 , NO–3
NH3, NH+ , NH2
4
NH+4
H
ZnO, Fe2O3
NH3, H2O
H
H
[15, 16]
[17]
Cr2O3
NH3, NH2, NO
* L = low, M = medium, and H = high.
any specific sample, it is not possible to obtain data for out at temperatures ranging from 150 to 350°C in sta-
all intermediate species observed on different catalysts, tionary and nonstationary regimes.
but the possibility of unobserved species formation and
The intensities of absorption bands were measured
participation in the reaction cannot be excluded.
in units of extinction (D) in the transmittance spectra
Indeed, in a stationary multiple-step process, the con-
and in Kubelka–Munk units in diffuse-reflectance spec-
centration of the most reactive intermediate species can
tra [24],
be below the sensitivity threshold of a spectral method.
f (R, R0) = F(R0) – F(R) = 2εC/s,
Therefore, it was interesting to consider the multiple-
step mechanism of the process, including the steps of
formation and consumption of all adsorbed species
identified in the process of ammonia oxidation on the
cited catalysts, and compare the model calculations of
the process behavior based on the quantitative analysis
of this mechanism with experimental data.
where F(R0) = (1 – R0)2/2R0; F(R) = (1 – R)2/2R; R0 and
R are the reflectance coefficients (albedo) of the sample
at a given frequency before and after molecule adsorp-
tion, respectively; ε is the extinction coefficient of the
vibration; C is the concentration of adsorbed mole-
cules; and s is the scattering ability of a unit volume.
Intermediate species of ammonia oxidation were stud-
ied on the Cr2O3, Fe2O3, and ZnO catalysts (Table 2).
EXPERIMENTAL
The setup for spectrokinetic measurements con-
sisted of an IR spectrometer (UR-20 modified for high-
temperature spectral measurements, Bruker IFS 45), a
flow-type heated cell/reactor, and a unit for chromato-
graphic analysis. The cell for registering the transmis-
sion spectra was described in [23]. The cell for register-
ing the diffuse-reflectance IR spectra was a quartz disk
with a rectangular slot (25 × 8 × 3 mm). The open side
was covered with a polished window made of zinc sul-
fide. A heating element, the cell, and the window were
clamped up with springs in a special holder. The reac-
tion mixture entered and left through the outlet and inlet
on a side wall of the cell. The cell was mounted in the
cell compartment of the spectrometer. The outflow gas
was analyzed to determine the concentrations of reac-
tants and products of the reaction. The concentrations
of reactants and products of the reaction were measured
RESULTS
The following absorption bands are observed in the
IR spectra obtained in the transmittance and diffuse-
reflectance regimes under the reaction conditions:
2200 cm–1 (ZnO and Fe2O3) and 2080 cm–1 (Cr2O3).
These bands were assigned to the ν(N≡N) and ν(N–O)
vibrations in surface N2Oδ– and NOδ+ compounds,
respectively. In addition to these bands, we observed a
band at 1560 cm–1 on Cr2O3 assigned to the δas vibra-
tions in the surface NH2 complex. Also, a band at
1620 cm–1 was observed on all catalysts and assigned to
the δas vibrations in NH3 coordinatively bound to the
metal cation. In the latter case, the reaction products
contained N2O and N2.
We studied the dependences of intensities of absorp-
in volume fractions (vol. fr.). The overall flow rate v tion bands at 1620, 2080, and 2200 cm–1 and the rates
was 55 cm3/min. The concentration of NH3 was varied of N2 and N2O formation on the concentrations of oxy-
from 0 to 30 vol %, and the concentration of O2 was gen and ammonia and on temperature in stationary
varied from 0 to 70 vol %. Experiments were carried measurements. Typical results of spectrokinetic mea-
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

THE MECHANISM OF LOW-TEMPERATURE AMMONIA OXIDATION
365
ëpr × 104, vol. fr.
(a)
surements are shown in Figs. 1 and 2. It is seen that the
rates of product (N2 and N2O) formation on all the cat-
alysts studied grow with an increase in the ammonia
concentration in the reaction mixture from 0.20 to
0.25 vol. fr. (Fig. 1a) at a constant concentration of oxy-
gen (0.40 vol. fr.). With a further increase in the con-
centration of ammonia in the mixture, the rates of prod-
uct formation stop changing. The rate of N2O formation
changes in a similar way depending on the concentra-
tion of oxygen in the mixture (Fig. 2a). The concentra-
tion of ammonium in these experiments was constant
and equal to 0.30 vol. fr. On the Cr2O3 and Fe2O3 cata-
lysts, the rate of nitrogen formation passed through a max-
imum at an oxygen concentration of 0.20–0.25 vol. fr. and
steadily increased on ZnO over the whole range of mea-
surements.
The band intensities at 1620 and 2200 cm–1 (on ZnO
and Fe2O3) and at 2080 cm–1 (Cr2O3) decrease with an
increase in the concentration of oxygen in the reaction
mixture and increase with an increase in the concentra-
tion of ammonia (Figs. 1b and 2b).
In the surface ammonia oxidation, the authors of
[10, 11] found bands characteristic of adsorbed hydra-
zine N2H4. To elucidate the nature of possible interme-
diates in the low-temperature oxidation of ammonia,
we carried out experiments on hydrazine oxidation on
Cr2O3, and the results are shown in Fig. 3. It is seen that,
during hydrazine oxidation, absorption bands at 2200
and 2080 cm–1 appear in the spectra. These are analo-
gous to those observed under conditions of ammonia
oxidation on the catalysts studied. Our data show that
hydrazine N2H4 ads can be the precursor of compounds
characterized by bands at 2200 and 2080 cm–1.
16
20
18
16
14
12
10
8
6
4
2
0
14
12
10
8
1
2
3
4
5
6
6
4
2
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
D2200
0.06
(b)
ΘN O
2
0.06
0.05
0.04
0.03
0.02
0.01
0.05
0.04
0.03
0.02
2
1
0.01
0
0
0.05 0.10 0.15 0.20 0.25 0.30
ëNH3, vol. fr.
Fig. 1. Dependences of the concentrations of products (C )
and surface species on the concentration of ammonium in
the reaction mixture at CO = 0.40 vol. fr., T = 240°C
pr
2
(Fe O and Cr O ) and 340°C (ZnO): (a) Experimental val-
In nonstationary measurements, we studied the con-
sumption of surface species absorbing at 2080 and
2200 cm–1 (Fig. 4). The spectrometer was set to the
desired frequency and at the moment t = 0, ammonia
was excluded from the flow of the reaction mixture. As
a result of this experiment, we obtained the dependence
of the surface species concentration on time. Experi-
ments carried out at different temperatures make it pos-
sible to determine the dependence of the rate of surface
species transformation on temperature and the activa-
tion energy of this process. The resulting values of the
activation energies are listed in Table 3.
2
3
2 3
ues of the concentrations (1–3) N and (4–6) N O for (1, 4)
2
2
ZnO, (2, 5) Fe O , and (3, 6) Cr O ; (b) (1) experimental
2
3
2 3
–1
values of the intensity of the absorption band at 2200 cm
and (2) the calculated values of N O coverage on
D
2200
2 ads
the Fe O surface (ΘN O ).
2
3
2
ammonia oxidation on ZnO and Fe2O3 [15, 16], and the
surface complexes with bands at 2080 and 1620 cm–1 are
intermediates in the reaction on Cr2O3 [17]. Taking into
account that surface complexes with bands at 2200 and
It was shown using quantitative spectrokinetic mea- 2080 cm–1 are formed in hydrazine oxidation, we con-
surements that the surface compound with a band at jecture that a hydrazine-like structure participates in
2200 cm–1 is an intermediate product in the reaction of ammonia oxidation.
Table 2. Characteristics of the samples
Catalyst
Preparation procedure
Phase composition
Surface area, m2/g
Cr2O3
Fe2O3
Decomposition of (NH4)2Cr2O3
α-Cr2O3
α-Fe2O3
40
37
Decomposition of FeO(OH) obtained
by the reaction between NH4OH and Fe2(C2O4)3
ZnO
Extra purity great reagent
ZnO
9
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

366
Cpr × 104, vol. fr.
SIL’CHENKOVA et al.
CÔ × 103, vol. fr.
(13) OHZ + OHZ
H2O + OZ + Z.
(a)
18
16
14
12
10
8
6
4
2
Here, Z is the free site on the oxide surface. Oxides
on which the surface species were observed experimen-
tally are presented in parentheses. The absorption
bands of water were only observed in the spectra at
temperatures below 100°C. The spectra obtained at the
reaction temperature did not contain these bands.
Therefore, the step of water adsorption was not
included in the reaction scheme.
20
15
1
2
3
4
5
6
10
5
0
Based on the above scheme, we calculated the con-
centrations of products and surface coverages depend-
ing on the composition of the reaction mixture. The set
of differential equations for the changes in the concen-
trations of species in the gas phase corresponding to
this scheme was solved using a standard program in the
approximation of a well-stirred reactor. Estimates of
the rate constants obtained from nonstationary spectro-
kinetic measurements were used as reference values.
We obtained effective values for k12 (0.012 s–1 on
ZnO and 0.020 s–1 on Fe2O3). The effective values of k6
(1 × 10–3 s–1) and k10 (2 × 10–3 s–1) were estimated for
Cr2O3 in [17].
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
D2200
ΘN O
(b)
2
0.7
0.12
0.10
0.08
0.06
0.04
0.02
0.6
0.5
0.4
0.3
0.2
2
1
0.1
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
The reverse problem was solved using the experi-
CO , vol. fr.
2
mental values of the rate constants k12 = 0.012 s–1 for
ZnO, k12 = 0.02 s–1 for Fe2O3, and k10 = 2 × 10–3 s–1 for
Cr2O3 by optimizing the values of other constants so
that the difference between the calculated and experi-
mentally measured values of the rates of product for-
mation was at most 20%. Note that we failed to ade-
quately describe the results with any other combina-
tions of the rate constants when we used the
experimental value of k6 for Cr2O3. It is likely that the
reason for this is that the experimental value of k6 is
underestimated because of ammonia readsorption. The
presence of readsorption is associated with the high
adsorption ability of ammonia (the characteristic time
during which the stationary value of ammonia absorp-
tion band is settled is several seconds), the low flow rate
of feed supply (50 ml/min), and the relatively small
amount of the catalyst used (in the diffuse-reflectance
method, ~1 cm3 of the catalyst is used). Under these
conditions, the measurements of the rate constant of
adsorbed ammonia consumption results in the underes-
timation of the values (for Cr2O3, the measured value is
Fig. 2. Dependences of the concentrations of products (C )
and surface species on the concentration of oxygen in the
pr
reaction mixture at CNH = 0.30 vol. fr., T = 240°C (Fe O
2
3
3
and Cr O ) and 340°C (ZnO): (a) Experimental values of
2
3
the concentrations (1–3) N and (4–6) N O for (1, 4) ZnO,
2
2
(2, 5) Fe O , and (3, 6) Cr O ; (b) (1) experimental values
2
3
2 3
–1
of the intensity of the absorption band at 2200 cm
D
2200
and (2) the calculated values of N O coverage on the
2
ads
Fe O surface (ΘN O ).
2
3
2
Assuming that all the intermediates found partici-
pate in the reaction on the oxides studied, we propose
the following scheme to describe the process of low-
temperature ammonia oxidation on metal oxides:
(1) NH3(gas) + Z
NH3Z
(ZnO, Cr2O3, Fe2O3–1620 cm–1)
(2) NH3Z
(3) O2 + Z
(4) O2Z
NH3(gas) + Z
O2Z
O2 + Z
k6 = 3.5 × 10–3 s–1, the calculated value, k6 = 0.2 s–1).
(5) O2Z + Z
(6) NH3Z + OZ
(7) 2NH2Z
2OZ (Cr2O3–1010 and 990 cm–1)
NH2Z + OHZ (Cr2O3–1560 cm–1)
N2H4Z + Z
For the convenience of calculations, the rate con-
stants were expressed in reciprocal seconds. For that,
the surface coverages were expressed in monolayer
fractions, and the concentrations of reactants and prod-
ucts in the gas phase were expressed in volume frac-
tions. That is, these values were dimensionless. In this
case, the equation for concentration changes of, for
instance, nitrogen takes the following form:
(8) N2H4Z + O2Z
N2(gas) + 2H2O + 2Z
NOZ + H2O + Z
(9)NH2Z + O2Z
(Cr2O3–2080 cm–1)
(10) NOZ + OZ
NO2bZ + Z
N2OZ + Z
(11) NH2Z + NO2Z
(ZnO, Fe2O3–2200 cm–1)
d[N2]/dt = B1wN – B2[N2].
(12) N2OZ
N2O(gas) + Z
2
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

THE MECHANISM OF LOW-TEMPERATURE AMMONIA OXIDATION
367
1
2
3
4
2300
2100
1900
1700
1500
1300
1100
ν, cm–1
Fig. 3. IR spectra obtained in the process of hydrazine oxidation on Cr O at T, °C: (1) 80, (2) 120, (3) 240, and (4) 310.
2
3
Here, wN is the rate of nitrogen formation, wN
=
2
results assumed well-stirred reactor conditions,
although the well-stirred reactor model does not accu-
rately describe the cell/reactor for the registration of
diffuse-reflectance spectra. Data presented in Fig. 6
were obtained in this cell/reactor. The values of the rate
constants of steps at which the coincidence is observed
are shown in Table 4.
Using the values of the rate constant k10 = 0.5 ×
10–18 cm2 molecule–1 s–1 (Table 4) and the activation
energy Ea = 98 kJ/mol for step 10 (Table 3) for Cr2O3,
we can estimate the value of the preexponential fac-
tor. It is equal to 5 × 10–8 cm2 molecule–1 s–1. This
value is within the range of the preexponential factors
for the elementary heterogeneous reactions (10–1–
10−8 cm2 molecule–1 s–1) [25].
2
k8ΘN H ΘO ; N2 is the concentration of nitrogen in vol-
2
4
2
ume fractions; B1 = GSCL/VRCN, B2 = v/VR, and B1 and
B2 are dimensionless values; G is the catalyst loading
(g); S is the catalyst specific surface area (m2/g), CN =
0.446 × 10–4 mol/cm3 = 1/VM, where VM = 22.4 ×
103 cm3/mol is the molar volume; CL = 1.66 × 10–5 mol/m2
is the number of active sites per unit surface area; vis the
flow rate (cm3/s); and VR is the reactor volume (cm3).
It is easy to show that, if the rates of surface steps are
expressed in units of molecule cm–2 s–1, then the dimen-
sionality of the rate constant can be converted to the
dimensionality corresponding to the second-order reac-
tion (cm2 molecule–1 s–1): the value of the rate constant
should be divided by the value of CL expressed in mol-
ecule cm2 (1015 molecule cm2).
DISCUSSION
Figure 5 shows the concentrations of products and
As can be seen from the data shown in Table 1, the
surface coverages measured experimentally and calcu- main information on intermediate compounds was
lated on the basis of the proposed scheme for Fe2O3. obtained under conditions that are far from the real con-
The coincidence of the calculated and experimental ditions of the low-temperature catalytic oxidation of
dependences is rather good considering the assumption ammonia. Obviously, this information is not very reli-
made. Acceptable agreement was also obtained in the able. Unfortunately, it is this information that forms the
framework of the same mechanism for ZnO and Cr2O3. basis for the proposed multiple-step mechanisms of the
Figure 6 shows the experimental and calculated values reaction. For instance, Bagnasco et al. [11] proposed
of the surface coverage by N2O in the course of the set- the reaction mechanism based on changes in the spectra
tling of the stationary reaction regime on ZnO. It is seen of adsorbed ammonia in the absence of oxygen during
that the general S-like shape of the kinetic curve is heating to 150°C. Changes in the activity in parallel
acceptably described. The differences are probably due experiments showed that the reaction products can be
to the fact that the mathematical description of the detected only at 250°C. Therefore, a reasonable ques-
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

368
SIL’CHENKOVA et al.
ΘN O × 104
ëpr × 104, vol. fr.
(a)
(a)
D2200
2
16
14
12
10
8
0.20
14
2
1
12
10
8
6
4
0.15
0.10
0.05
0
2
6
4
4
3
0
50
100
(b)
150
200
2
ΘNO
D2080
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
ëNH , vol. fr.
3
ëpr × 104, vol. fr.
(b)
16
2
1
14
12
10
8
0.005
0
0
100 200 300 400 500 600
ΘN O × 104
(c)
D2200
2
35
30
25
20
15
10
5
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
200
6
4
4
3
2
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
0
ëO , vol. fr.
2
0
50
100
t, s
150
Fig. 5. Dependence of the (1, 3) experimental and (2, 4) cal-
culated values of the concentrations of N and N O for
2
2
Fe O at 240°C (a) on the concentration of ammonia in the
2
3
Fig. 4. Dependences of experimental values of the intensi-
ties of adsorption bands (D) and the calculated values of the
surface coverages by the corresponding species (Θ) on the
reaction mixture at CO = 0.40 vol. fr. and (b) on the con-
2
centration of oxygen in the reaction mixture at CNH
=
3
time elapsed after excluding NH from the flow of the reac-
3
0.25 vol. fr.
tion mixture on (a) ZnO, (b) Cr O , and (c) Fe O . T =
2
3
2 3
240°C (Fe O and Cr O ) and 340°C (ZnO).
2
3
2 3
Fe2O3; NH3 ads, N2Oads, and NOads on Cr2O3; and NH3 ads
and N2Oads on ZnO.
tion arises:Are the changes observed in the spectrum of
adsorbed ammonia relevant to the reaction? It is clear
that the proposed mechanism is unreliable. We studied
the mechanism of this reaction using the spectrokinetic
method [14–17] and simultaneously recorded the spec-
trum of surface species under the reaction conditions
and the rates of product formation. The procedure of
these measurements was described in detail in [23, 26].
This method makes it possible to determine whether or
not the complex observed is an intermediate of the reac-
tion by comparing the rates of surface compound trans-
formation and the rate of product formation. If these
Results obtained in this work show that, in the
framework of the proposed mechanism, it is possible to
describe the experimental data for the oxides studied
(Figs. 1b, 2b, and 4–6). In our case, we managed to
describe quantitatively the dependences of the rate of
nitrogen and N2O formation on the composition of the
reaction mixture (Fig. 5). The composition of the reac-
tion mixture varied over broad ranges. Moreover, the
mechanism describes the experimental dependences of
surface coverages by intermediate complexes on the
same parameters under stationary conditions and the
dependences of coverages on time when ammonia is
rates coincide, then we have reasons to assume that the excluded from feed flow (Fig. 4) and in the course of the
settling of the stationary regime of the reaction (Fig. 6).
surface compounds observed are intermediates in the
reaction [27]. Thus, we managed to determine the inter-
All these facts allow us to assume that the mecha-
mediate species of this reaction, NH3 ads and N2Oads, on nism of the reaction is the same on the oxides studied.
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

THE MECHANISM OF LOW-TEMPERATURE AMMONIA OXIDATION
369
At the same time, we do not believe that the proposed
D2200
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
ΘN O × 104
2
mechanism completely described the process of low-
temperature ammonia oxidation on oxides, because not
all the intermediates were detected experimentally.
Similarly, not all of the rate constants were measured.
However, it is clear from the above experimental data
that the main features of the process are reflected in the
scheme. This fact allows us to analyze changes in the
rate constants of the same steps when changing the
oxide nature. Analysis of data presented in Table 4
shows that, in the series ZnO, Fe2O3, and Cr2O3, the rate
constants of steps change in two ways: k7 and k10 are
described by a curve with a maximum, whereas the
other constants change steadily in the same series.
However, the rates of increase in these constants differ.
When solving the reverse problem, it was found that the
calculated values are most sensitive to changes in the
values k1–k4 when the experimental data are described
quantitatively. The value of the constant k9 determined
the ratio between the rates of N2 and N2O formation. A
change in the adsorption constant of ammonia k1 corre-
lates with a change in the rate of reaction product for-
mation and with the heat of ammonia adsorption [28].
The corresponding data are presented in Table 5. The
same table shows data from the literature on the activa-
tion energy of oxygen desorption [29].
8
2
1
7
6
5
4
3
2
1
0
0
500
1000
t, s
1500
2000
Fig. 6. Changes in (1) D
(experiment) and (2) Θ
N2O
2200
(calculation) in the process of settling the stationary regime
of NH oxidation on ZnO. The feed composition is 0.2%
3
NH + 0.4% O . T = 240°C.
3
2
The correctness of the calculated constants, specifi-
cally k4, is supported by data from the literature on the
values of the activation energies of oxygen desorption
from the oxides studied (Table 5). This value decreases
in the series ZnO, Fe2O3, and Cr2O3. This corresponds
to an increase in the respective constant (Table 4).
Such a ratio of parameters (the reaction rate, the
constant, and the heat of ammonia adsorption) can be
explained using the Brønsted–Polanyi relationship
The constants k7 and k10 refer to the addition-type
steps, and their values pass through a maximum in the
series ZnO, Fe2O3, and Cr2O3 (Table 4). According to
[33], the distance between the metal atoms and the non-
metal atoms on the oxide surface passes through a min-
imum. In other words, there is a reverse dependence of
the constant and the distance between metal and non-
metal atoms. Possibly, the distance between the stabili-
zation sites of neighboring species is important for
these steps: the shorter this distance, the higher the con-
stant.
Er = E0 – α QNH
,
(1)
3
where Er is the activation energy of the reaction, E0 is a
constant term, and QNH is the heat of the formation of
3
a complex between the reactant and a Lewis site (in our
case, the heat of ammonia adsorption). Similar relation-
ships for reactions over Lewis sites were considered in
detail in [30]. According to formula (1), an increase in
the heat of ammonia adsorption should lead to a
decrease in the activation energy of the reaction and,
therefore, to an increase in the reaction rate. The exper-
imental heats of ammonia adsorption [28] increase
when switching from ZnO to Cr2O3. The reaction rate
increases in the same series (Table 5). This fact con-
firms that formula (1) is correct for this reaction, that
the proposed mechanism correctly describes ammonia
oxidation on all the oxides studied, and that ammonia
adsorption plays an important role in determining the
rate of ammonia oxidation.
It is clear that the conclusions drawn here are tenta-
tive. For a more detailed determination of the nature of
the steps, it is necessary to determine the preexponen-
tial factors of the rate constants and the activation ener-
gies. In this work, we failed to do that because the tem-
perature interval where spectral and catalytic studies
could be carried out simultaneously was short (~20°C).
Table 3. Activation energies of surface species consump-
tion in the reaction of NH3 oxidation on the samples studied
The acidic properties of Fe2O3 and Cr2O3 were com-
pared in [30] using the adsorption band δs of (NH3 ads
)
[31]. According to [32], the acidity of the oxides stud-
ied increases in the series ZnO, Fe2O3, and Cr2O3.
Because the constants k1 and k3 are the constants of
reactant adsorption (for ammonia and oxygen) on coor-
dinatively unsaturated surface sites (Lewis acid sites),
the transfer of electrons from the reactant molecules of
the surface sites requires a lower activation energy if an
oxide is less acidic.
Intermediate Absorption
Catalyst
ZnO
Ea, kJ/mol
complex
band, cm–1
N2Oads
N2Oads
NOads
2200
2200
2080
1620
54
104
98
Fe2O3
Cr2O3
Cr2O3
NH3 ads
25
KINETICS AND CATALYSIS Vol. 43 No. 3 2002

370
SIL’CHENKOVA et al.
Table 4. The values of the rate constants for the steps of low-temperature ammonia oxidation at 240°C
ZnO
Fe2O3
Cr2O3
Constant
s–1
cm2 molecule–1 s–1
s–1
cm2 molecule–1 s–1
s–1
cm2 molecule–1 s–1
k1
0.02
0.01
0.01
0.1
2 × 10–17
–
0.01
0.1
1 × 10–17
–
0.1
0.2
0.1
1.5
0.2
2.0
3.0
5.0
0.8
0.005*
0.2
0.1
2.0
1 × 10–16
–
k2
k3
1 × 10–17
0.1
1 × 10–16
1 × 10–16
k4
–
0.2
–
–
k5
0.02
0.2
2 × 10–17
2 × 10–16
3 × 10–16
5 × 10–16
2 × 10–17
1 × 10–17
1 × 10–17
–
0.003
0.2
3 × 10–18
2 × 10–16
5 × 10–15
5 × 10–15
1 × 10–16
1 × 10–16
1 × 10–16
–
2 × 10–16
2 × 10–15
3 × 10–15
5 × 10–15
8 × 10–16
5 × 10–18
2 × 10–17
–
k6
k7
0.3
5.0
k8
0.5
5.0
k9
0.02
0.01
0.01
0.012*
0.1
0.1
k10
k11
k12
k13
0.1
0.1
0.02*
1.0
1 × 10–16
1 × 10–15
2 × 10–15
* Experimentally determined.
In this interval, we were unable to determine the above formations of ammonium ions is currently unavailable.
At the same time, another mechanism of ammonia oxi-
dation is possible that involves the formation of an
ammonium ion on the surface and its further transfor-
mation.
parameters with a high degree of accuracy.
Above, we considered the results of spectrokinetic
measurements on oxides with pronounced Lewis acid-
ity. At the same time, there are some data suggesting
that the surface NH+4 complex is formed on oxides with
Brønsted acidity. Corresponding evidence was
obtained for MoO3 [14] and Cr2O3 [17]. In the latter
case, the formation of Brønsted sites was provided by
the oxidative treatment of the sample. More detailed
information on the steps associated with further trans-
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