Spectrokinetic study of the mechanism of NOx reduction with propylene over ZrO2 in excess oxygen

Article abstract of DOI:10.1134/S0023158417020082

It has been demonstrated by quantitative spectrokinetic measurements that, on the surface of zirconia stabilized as a tetragonal phase, the rate-limiting step of the selective catalytic reduction of nitrogen oxides (SCR of NO_x) with propylene is the interaction of surface nitrates with C₃H₆ yielding organic nitro compounds. It is hypothesized that propylene reacts not with the nitrates themselves but with the activated complex NO₂^ads whose structure is intermediate between the structures of the monodentate NO₃^? and NO₂ species. Deep C₃H₆ oxidation exerts an adverse effect on the rate of the SCR of NO_x with propylene, and the interaction between O₂ and NO, which yields NO₂ and NO₃^? stimulates further nitrogen reduction to N₂. The effect of the reaction between oxygen and O₂N?C_nH_m on the NO_x reduction rate is variable and is determined by the C₃H₆/NO_x ratio. A generalized scheme of the SCR of NO_x with propylene on the surface of ZrO₂ partially stabilized as a tetragonal phase has been developed by comparing experimental data of this study and data available from the literature.

Full text of DOI:10.1134/S0023158417020082

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 2, pp. 198–210. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © V.A. Matyshak, A.N. Il’ichev, O.N. Sil’chenkova, V.A. Sadykov, V.N. Korchak, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 2, pp. 209–223.
Spectrokinetic Study of the Mechanism of NO Reduction
x
with Propylene over ZrO in Excess Oxygen
V. A. Matyshak *, A. N. Il’ichev , O. N. Sil’chenkova , V. A. Sadykov , and V. N. Korchak
2
a,
a
a
b, c
a
a
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
b
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
c
Novosibirsk State University, Novosibirsk, 630090 Russia
*
e-mail: matyshak@polimer.chph.ras.ru
Received June 2, 2016
Abstract—It has been demonstrated by quantitative spectrokinetic measurements that, on the surface of zir-
conia stabilized as a tetragonal phase, the rate-limiting step of the selective catalytic reduction of nitrogen
oxides (SCR of NO ) with propylene is the interaction of surface nitrates with С Н yielding organic nitro
x
3
6
compounds. It is hypothesized that propylene reacts not with the nitrates themselves but with the activated
ads
−
complex NO2 , whose structure is intermediate between the structures of the monodentate NO and NO
3
2
species. Deep С Н oxidation exerts an adverse effect on the rate of the SCR of NO with propylene, and the
3
6
x
−
3
interaction between О and NO, which yields NO and NO , stimulates further nitrogen reduction to N .
2
2
2
The effect of the reaction between oxygen and O N−С Н on the NO reduction rate is variable and is deter-
2
n
m
x
mined by the С Н /NO ratio. A generalized scheme of the SCR of NO with propylene on the surface of
3
6
x
x
ZrO partially stabilized as a tetragonal phase has been developed by comparing experimental data of this
2
study and data available from the literature.
Keywords: selective catalytic reduction of NO , zirconia catalysts, adsorption of NO , С Н , and their mix-
x
x
3
6
tures with О , properties of surface complexes
2
DOI: 10.1134/S0023158417020082
INTRODUCTION
reduction with hydrocarbons are nitrogen-containing
organic compounds [22–25]. To see which mecha-
nism is real, it is necessary to have information about
hypothetical intermediates forming just under the
reaction conditions. Of particular importance is gain-
ing these data for systems that are considered for use in
new, efficient SCR catalysts.
The discovery of the selective catalytic reduction of
nitrogen oxides (SCR of NO ) with hydrocarbons in
x
excess oxygen over copper-containing zeolite ZSM-5
[
1, 2] was of great significance for proving the possibil-
ity of this reaction taking place. The mechanism of this
process over various catalytic systems is widely dis-
cussed in the literature. An understanding of this
mechanism is necessary for developing an efficient
The literature contains many publications and pat-
ents concerning use of ZrO in the synthesis of cata-
2
lysts for the SCR of NO with hydrocarbons (see, e.g.,
catalyst for the SCR of NO . The following basic
x
x
[
26–39]). Promising catalysts are those based on pil-
schemes of the mechanism are suggested:
lared clays, in which zirconia is in the form of nano-
clusters, or pillars [39–43]. However, the mechanism
of the SCR of NO with hydrocarbons over ZrO and
(
1) NO oxidizes to NO or nitrite–nitrate struc-
2
tures, which then react with the hydrocarbon [3–14];
2) under the reaction conditions, hydrocarbons
form some intermediates [15–18] or even coke [19,
0], which then react with NO ;
x
2
(
pillared clays has practically not been investigated.
Earlier [44–46], using temperature-programmed
2
x
desorption (TPD), electron paramagnetic resonance
(
3) redox mechanism in which the catalytic surface
(
EPR) spectroscopy, and in situ IR spectroscopy, we
is oxidized by NО and is reduced by hydrocarbons
х
discovered that, at the temperatures at which the SCR
[
19, 21].
of NO with propylene occurs, the most abundant
x
In recent years, most researchers have been hold- nitrogen–oxygen complexes on the zirconia surface
ing to the opinion that the intermediates in NO
are nitrates.
98
x
1

SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
199
x
2
2
In this study, the mechanism of NO reduction 5000 kg/cm . The thickness of a pellet was 20 mg/cm .
x
with propylene over ZrO in excess О was investigated The weight of catalyst sample was 29–58 mg (in trans-
2
2
by TPD and in situ Fourier-transform IR spectros- mission spectroscopy) or 0.3 g (in diffuse reflectance
copy. Special attention was paid to the formation of spectroscopy). The IR spectra obtained were
N-containing organic compounds and to the reactiv- computer-processed using the Spectrum program
−
(PerkinElmer, United States).
ity of NO , a hypothetical intermediate in the SCR of
3
−
NO with hydrocarbons.
x
The nitrate concentration [NO ] on the ZrO sur-
3
2
The efficiency of spectral methods depends face was determined using the coefficient of propor-
directly on the amount of adsorbed complexes formed tionality (А , IR transmission coefficient) relating
0
on the catalyst surface. For this reason, we examined a
−
3
[
1
NO ] with the integral absorbance A in the 1170–
320 cm range:
ZrO sample partially stabilized as a tetragonal phase,
2
–1
since the latter has a higher adsorption capacity than
monoclinic ZrO [44]. In addition, tetragonal zirconia
−
3
2
⎡
⎣
NO ⎤ = A
A0 .
(1)
(1170−1320)
is an analogue of the structurally stressed elements of
⎦
pillared clays (ZrO pillars). Therefore, data of this
2
The values of these coefficients were determined by
study concerning the mechanism of the SCR reaction
combining in situ IR spectroscopy and thermal NO
x
may be useful in the investigation of NO reduction
x
desorption methods. NO adsorption on the catalyst
x
over pillared clays.
was carried out at 300°C, a temperature at which the
decomposition of surface nitrates and the formation of
−
3
EXPERIMENTAL.
any nitrogen–oxygen complexes other than NO can
be neglected [44]. Thereafter, the cell was purged with
nitrogen for 20 min, an IR spectrum of surface nitrates
was recorded, and NO was desorbed. The value of А
The ZrO sample partially stabilized as a tetragonal
2
phase was obtained from ZrO(NO ) (analytical
3
2
x
0
grade) by precipitating zirconium oxide hydrate with
–1
(
cm /molecule) was calculated from the number of
washed with a 0.1 M NH OH solution, dried at 110°С, desorbed molecules of nitrogen oxides, (N(NO ))
and calcined at 500°С. A detailed description of the and the intensity of the absorption bands of the surface
aqueous ammonia. The resulting ZrO ⋅ nH O was
2
2
des
x
4
preparation, phase composition, and surface structure nitrates:
of this sample is presented in our earlier work [44].
des
А0 = A
1170–1320
N (NOx ).
( )
(2)
Spectrokinetic measurements in the SCR of NOx
with propylene over ZrO were carried out in a flow-
2
−
3
The NO concentration on the catalyst surface
through quartz IR cell reactor (catalyst bed volume of
3
under conditions of the SCR of NO with propylene
x
1
cm ) while lowering the temperature stepwise after
was determined using the same samples as in the
heat treatment of the sample at Т = 500°C in flowing
determination of А .
N for 20 min. To elucidate the effect of the reaction
0
2
mixture composition on the SCR of NO with propyl-
The unsteady-state spectrokinetic studies of the
x
ene, the concentrations of the main components were
varied in the following ranges: 0.038–0.2% NO, 0.1–
−
3
reactivity of NO were performed under isothermal
conditions in the temperature range from 370 to
0
.5% C H , and 1–20% O .
3 6 2
4
40°C, in which it was possible to record IR spectra of
The gas hourly space velocity in all experiments
−
3
surface compounds and to measure the NO and NO
–1
x
was 9000 h . All gases were purified via standard pro-
cedures using anhydrone and ascarite traps. The gases
entering and leaving the reactor were analyzed using a
conversions. Prior to performing spectrokinetic stud-
ies, the sample was heat-treated at 440°C in flowing
nitrogen.
Beckman-951A chemiluminescent NO/NO analyzer
x
As the sample was heat-treated, the state of its sur-
face was continuously monitored by in situ IR spec-
troscopy. This thermal pretreatment was terminated
once the IR spectrum had stopped changing (in
and a Beckman-590 infrared HC/СО analyzer (Beck-
man Coulte, United States).
IR transmission and diffuse reflectance spectra were
recorded on a Spectrum RX I FT-IR System spec-
trometer (PerkinElmer, United States) at a spectral
resolution of 4 cm and a scan time of 4.2 s. Use of
original quartz IR cell reactors with ZnS optical win-
dows made it possible to acquire in situ IR spectra at
~
60 min). After the heat treatment of the sample in
N , the IR cell reactor was shut off and was cooled at
–1
2
the preset temperature.
Simultaneously, the 0.46% NO + 2.5% O /N
2 2
elevated temperatures of up to 600°C. For obtaining a mixture to be adsorbed on the catalyst surface was
diffuse reflectance spectrum, we selected a fraction blown through the pipelines. Before admitting the
with a particle size of ≈1 mm. Transmission spectra mixture into the IR cell, we recorded the IR spectrum
were recorded for samples pressed into pellets at Р = of the sample, which was then subtracted from the
KINETICS AND CATALYSIS Vol. 58 No. 2 2017

2
00
C H conversion, %
MATYSHAK et al.
the reactor volume and V is the volumetric gas flow
gas
rate), we calculated the logarithms of the effective rate
constants of propylene conversion, kС3Н6. A linear
3
6
1
00
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
dependence of ln(k
) on inverse temperature was
4
3
2
1
C3H6
observed in a wide temperature range (400–500°C).
This enabled us to determine the activation energy val-
gas
ues for propylene oxidation (E
) at [NO ] = 0.2%
x
C3H6
and in case there is no nitrogen oxides in the reaction
mixture. These values turned out to be equal within the
experimental error and were found to be ~130 kJ/mol
1
0
(±10%).
0
100
200
300
400
500
600
gas
The NO conversion at [NO ] = 0.038–0.1% is
Temperature, °С
x
x
practically invariable; therefore, the order of the SCR
Fig. 1. Temperature dependence of the propylene conver-
sion at different NO concentrations in the gas phase:
1) 0%, (2) 0.038%, (3) 0.1% ppm, and (4) 0.2%. Initial
reactant concentrations: С Н , 0.2%; О , 2.5%. The bal-
gas
x
of NO with propylene with respect to NO is close
x
x
(
to unity in this range of nitrogen oxide concentrations.
3
6
2
The decrease in the NO conversion as a result of
x
–
1
ance gas is N . V = 150 mL/min (9000 h ).
2
gas
gas
x
increasing [NO ] from 0.1 to 0.2% indicates a
decrease in the order of the reaction with respect to
which may be due to the saturation of the ZrO
2
surface with intermediates of the SCR of NO with
gas
spectra of the sample obtained subsequently at the
given temperature.
After the formation of NO on the catalyst surface, propylene.
the IR cell reactor was shut off and the gas-supplying
pipelines were purged with nitrogen or with the reac-
tion mixture whose interaction with the nitrate com-
plexes was to be studied. After the gas composition in
the gas pipelines became constant, the reaction mix-
ture was admitted to the sample.
NO ,
x
x
−
3
Since the SCR of NO with propylene is a first-
x
gas
x
gas
x
order reaction with respect to NO in the [NO ] =
0
.038–0.1% range, the logarithms of the effective rate
constants of the SCR of NO with propylene (k ) were
x
eff
calculated via Eq. (3). From the slope of the tangent to
The time-dependent intensities of the absorption the ln k versus 1/(RT) curve at 330°C, we estimated
eff
−
3
bands of NO , obtained by in situ IR spectroscopy, the activation energy for the SCR of NO with propyl-
x
were used to calculate the effective formation and con- ene (Е
= 50 kJ/mol) for an initial NO concen-
SCR NOx
x
sumption constants of in N , 0.2% C H /N , and 0.2% tration of 0.1%.
2
3
6
2
C H + 2.5% O /N flows.
3
6
2
2
gas
The Е
value at [NO ] = 0.2% was deter-
SCR NOx
x
mined under the assumption that the reaction is either
RESULTS
gas
x
first- or zeroth-order with respect to NO . In the for-
Effect of the Reaction Mixture Composition
mer case, Е
was obtained from the derivative of
SCR NOx
on the SCR of NO with Propylene
x
the logarithm of the rate constant of the SCR of NO
x
gas
x
Varying the NO concentration ([NO ]) in the with propylene as a function of inverse temperature.
x
gas phase demonstrated that, as the concentration is When it was assumed that the reaction is zeroth-order
changed in the 0.038–0.1% range, the propylene con- with respect to NO , the derivative was calculated for
x
version remains invariable (Fig. 1) and the tempera- the curve representing the temperature dependence of
ture dependence of the С Н conversion in the SCR
3
6
the nitrogen oxides conversion (X
). Irrespective of
NOx
process is the same as is observed when there is no
what was assumed—first or zeroth order of the reac-
gas
x
NO in the gas phase. Raising [NO ] to 0.2% lead to tion with respect to NO —it was found that Е
≈
x
x
SCR NOx
an increase in the onset temperature of propylene 160 ± 20 kJ/mol.
combustion.
gas
Assuming that deep oxidation of propylene by oxy-
The study of the C H concentration effect on the
3 6
gen (С Н + О → Н О + СО ) is a first-order reac- SCR reaction demonstrated that the effective rate
3
6
2
2
х
gas
constant of NO reduction with propylene increases
x
with an increasing proportion of the reductant in the
tion with respect to C H6 ] and using the equation
3
–
ln(1 – Х) = k τ,
(3) reaction mixture. It was discovered that the SCR of
eff
NO with propylene is a first-order reaction with
x
where Х is the conversion, k is the effective rate con-
stant of the reaction, and τ = V /V = 1/150 min (V is respect to C H .
eff
gas
6
r
gas
r
3
KINETICS AND CATALYSIS
Vol. 58
No. 2
2017

SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
201
x
The propylene conversion decreases with an
increasing С Н concentration in the reaction mix-
ture. However, while at 440°C a decrease in the pro-
pylene conversion is observed throughout the concen-
tration range examined, at 560°C a deviation of the
propylene conversion from 100% takes place only
(а)
3
6
C H conversion, %
NO concentration, %
3
6
2
1
00
0.10
0.09
0.08
9
8
7
6
5
4
0
0
0
0
0
0
0.07
0.06
gas
1
when the initial C H is above 0.2%. Note that, at
3
6
4
40°C, almost no nitrogen dioxide is present in the gas
phase leaving the reactor, while at 560°C and an initial
C H concentration of ≤0.2%, the gas phase leaving
the reactor contains ~30 ppm NO .
0.05
0.04
2
gas
6
3
30
0.03
3
2
20
0.02
0.01
0
The dependence of the reactant conversions on the
10
gas
O2 concentration is plotted in Fig. 2. Clearly, at
0
5
10
15
gas
4
40°C (Fig. 2a) an increase in [O2 ] leads to an
O concentration, %
2
increase in the С Н conversion (curve 1) and to a
3
6
slight increase in the NO conversion (curve 2). At the
(b)
x
same time, at 560°C (Fig. 2b) the NO conversion
C H conversion, %
NO concentration, %
x
3
6
2
gas
(
curve 2) passes through a maximum at [O2 ] = 1.5%.
100
90
80
1
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
Thus, the presence of oxygen in the gas phase can both
accelerate the reduction of nitrogen oxides with pro-
pylene and decrease the NO conversion.
70
x
6
5
4
3
2
0
0
0
0
0
Increasing the initial oxygen concentration above
some critical value (7% at 440°C and 2% at 560°C)
causes the appearance of nitrogen dioxide in the gas
phase leaving the reactor (Fig. 3, curve 3). While at
gas
2
4
40°C the appearance of NO2 takes place simultane-
3
10
ously with the stabilization of the NO conversion
x
(
Fig. 2a), at 560°C nitrogen dioxide in the gas phase is
0
1
2
3
4
5
6
7
observed at 100% С Н conversion (Fig. 2b).
3
6
O concentration, %
2
gas
2
Fig. 2. (1) C H and (2) NO conversions and (3) NO
3
6
x
In Situ IR Spectroscopy
concentration at the reactor outlet as a function of the
under Steady-State SCR Conditions
gas
2
O
concentration at (a) 440 and (b) 560°C. Initial reac-
tant concentrations: NO , 0.2%; С Н , 0.2%. The balance
x 3 6
Aimed at detecting intermediates in the selective
reduction of nitrogen oxides with propylene, we car-
ried out an in situ diffuse reflectance IR spectroscopic
study of the formation of adsorption complexes on the
–1
gas is N . V = 150 mL/min (9000 h ).
2
gas
ZrO surface under conditions of the SCR of NO at
2
x
acting with NO + O and the IR spectrum arising from
2
3
00–500°С (Fig. 3).
the interaction between the catalyst and С Н + О .
3
6
2
The interaction between zirconia and the С Н + О
2
However, the steady-state absorption band intensities
3
6
mixture gives rise to intense absorption bands at 1400– observed in the catalytic reduction of NО
are smaller
470 and 1500–1590 cm in the diffuse reflectance than those observed in the interaction between the
spectrum (Fig. 3, spectrum 1). The appearance of catalyst and NO + O or С + О . In addition, along
weak absorption bands with maxima at 2884 and with the absorption bands arising from the interaction
x
–1
1
Н
2
3
6
2
–1
2
967 cm is also observed. At the same time, as was of ZrO with NO + O and С Н + О , strong absorp-
2 x 2 3 6 2
–1
demonstrated in earlier works [44, 47], the main prod- tion in the 1550–1590 cm range with a maximum at
1565 cm– was observed in the SCR of NO (Fig. 3,
1
ucts of the interaction between ZrO and NO at SCR
2
x
x
temperatures are surface nitrates, which are character-
ized by adsorption bands at 1180–1280 and 1500–
spectra 3, 4).
1
615 cm–1 (Fig. 3, spectrum 2).
Reactivity of Surface Complexes
The IR spectrum recorded under conditions of the
SCR of NO with propylene in excess О (Fig. 3, spec-
trum 3) can be viewed as the superposition of two
In order to elucidate the role of surface nitrates in
the SCR of NO with propylene, we compared the
x
2
x
−
3
spectra, namely, the IR spectrum of the sample inter- rates of consumption of NO
in flowing N (W ),
2 1
KINETICS AND CATALYSIS Vol. 58 No. 2 2017

2
02
MATYSHAK et al.
Absorbance
Combining in situ IR spectroscopy with NO TPD,
x
we found that the integral absorbance in the 1170–
1
440
422
410
–1
1
320 cm range is linearly correlated with the number
1
1544
1355
of NO molecules desorbed as a result of the decom-
1
x
−
3
position of surface NO (Fig. 4). From these data, we
1
calculated the coefficient of proportionality (А =
0
1
226
–
19
–1
2
.5 × 10 cm ) relating the amount of nitrates on
the ZrO surface (58 mg) to the integral absorbance in
1
190
2
1550
−
3
the 1170–1320 cm–1 range (А(1170–1320) = А N(NO ).
0
1580
Using the experimental А
value obtained
(
1170–1320)
2
3
during the interaction between ZrO and NO + C H +
2
3
6
1604
−
3
O at 400°С, we calculated [NO ] for the SCR of NO
2
x
−
3
20
–1
with propylene: [NO ] = 2.4 × 10 g (±10%).
1544
−
3
1565
[NO ] = A (A m
)
0
ZrO2
(4)
−
19
20
=
3.6 (2.5 ×10 × 0.058) = 2.4 ×10 .
Figure 5 shows how the integral absorbance in the
1
380
–1
1
170–1320 cm range observed by in situ IR spectros-
copy at 400°С varies during the interaction of the sur-
face nitrates with flowing nitrogen (curve 1), С Н
4
3
6
(
curve 2), and С Н + О (curve 3). As is clear from
3 6 2
Fig. 4, the decrease in А
depends exponen-
(
1170–1320)
−
tially on the duration of NO conversion; that is, the
3
decomposition of the surface nitrates in nitrogen and
their interaction with С Н /С Н + О are described
3
6
3
6
2
−
3
1
600
1500
1400
1300
1200
ν, cm–1
well by first-order equations with respect to NO .
−
3
The rate constants of NO conversion in flowing
N , С Н , and С Н + О were used to determine the
Fig. 3. Diffuse reflectance IR spectra recorded in the
course of the interaction of ZrO2 with the (1) C H
0.2%) + O (2.5%), (2) NO (0.2%) + O (2.5%), and
3, 4) NO (0.2%) + C H (0.2%) + O (2.5%) mixtures.
T = (1–3) 500 and (4) 340°С.
2
3
6
3
6
2
rate of conversion of the surface nitrates (Table 1). The
3
6
(
(
−
3
2
2
NO conversion rates were calculated via the equations
3
6
2
−
3
W = k [NO ],
−
(5)
(6)
1
1
W = k ⎡NO ⎤, where k = f
(
C H6
)
,
2
2
3
2
3
⎣
⎦
C H (W ), and С Н + О (W ) with the rate of SCR
3
6
2
3
6
2
3
−
⎤
⎦
of NO with propylene at 400°C (WSCR NOx).
⎡
⎣
x
W = k NO , where k = f'
(
С Н ;О .
)
(7)
3
3
3
3
3
6
2
The partial overlapping of the absorption bands The rate of the interaction between the surface nitrates
observed in the interaction between zirconia and the
С Н + О mixture and the absorption bands of the
and propylene (W
−
) was determined under the
NO3 +C3H3
3
6
2
−
3
–1
assumption that, in the reaction between NO
and
surface nitrates (1500–1590 cm ) makes it impossible
to involve, in spectrokinetic studies, all high-fre-
quency absorption bands of surface nitrates but the
С Н /N or (С Н + О )/N , the surface nitrates are
3
6
2
3
6
2
2
converted via two parallel reactions:
604 cm–1 band, which is due to vibrations of biden-
−
3
gas
x
−
⎤
1
⎡
⎣
NO → NO , W = k NO ,
(8)
(9)
1
1
3
⎦
tate nitrates [44]. Use of low-frequency components
−
−
3
NO + С Н → O N – С Н ,
3 3 6 2 n m
ν of the vibrations of NO in recording diffuse reflec-
3
W
−
= W –W or W
−
= W – W .
3 1
NO3 +С3Н6
tance spectra seems impossible because of the intense
2
1
NO3 +С3Н6
absorption due to vibrations of ZrO itself in this spec-
2
For comparing the results of the spectrokinetic
tral range and because of the low nitrate concentration
studies with the results of the SCR of NO with pro-
x
under the SCR conditions. In view of the foregoing,
pylene in excess О , in Table 1 we present the rate and
2
−
3
the spectrokinetic study of the reactivity of NO was effective rate constant of NO
reduction calculated
x
carried out by IR transmission spectroscopy.
from the NO conversion at 400°С.
x
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SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
203
x
Absorbance(1170 – 1320), cm–1
Absorbance(1170 – 1320), cm–1
6
14
1
2
5
4
3
2
1
A = 5.4121e–0.008 t
1
10
2
8
6
4
2
A = 5.4617e–0.0309
t
3
A = 5.4132e–0.0401
t
0
1
2
3
4
5
6
0
5
10
15
20
25
30
35
40
t, min
Amount of NO dx es, ×10–19 molecule
Fig. 5. Variation of the integral intensity of the absorption
Fig. 4. Integral absorbance in the 1170–1320 cm–1 range as
−
bands of NO in the course of nitrate conversion in flow-
3
a function of the amount of NO desorbed in the decom-
x
ing (1) N , (2) 0.2% С Н , and (3) 0.2% С Н + 2.5% О .
2
3
6
3
6
2
−
3
position of NO . The IR spectra were recorded in the
The IR spectra were recorded in the transmission mode.
Vgas = 150 mL/min.
transmission mode.
Additionally, using IR transmission spectroscopy,
In the spectrokinetic experiments involving IR
transmission spectroscopy, the low weight of the ZrO
we determined the rate constants of the conversion of
2
gas
−
3
sample (58 mg) did not allow us to monitor the NOx
NO in N and С Н at 370 and 440°С. This made it
2
3
6
−
3
possible to estimate the activation energy for the release dynamics during the conversion of NO . For
decomposition of the nitrates (~190 kJ/mol) and for acquiring the missing data, the same experiments were
−
carried out on a 400-mg ZrO powder sample. Figure 6
2
the interaction between NO3 and propylene
present the results of an analysis of the gas phase leav-
(
~164 kJ/mol). The activation energies were esti-
ing the reactor during the unsteady-state kinetic study
mated from the slopes of the rectilinear Arrhenius
−
3
of the reactivity of NO .
plots of the respective constants (k and k
−
);
1
NO3 +C3H6
−
After the admission of the gas flow (N , С Н , or
the rate constant from the interaction between NO3
and propylene was calculated from the equation
2
3
6
−
3
С Н + О ) to the sample with NO formed on its sur-
3
6
2
face, an intensive release of nitrogen oxides was
observed in the first 2 min, which was due to the pres-
k
−
= k – k .
(10)
2
1
NO3 +С3Н6
Table 1. Comparison between the reaction rate W
(NO , 0.2 vol %; С Н , 0.2 vol %; О , 2.5 vol %; V = 150 mL/min;
NOx
x
3
6
2
gas
−
Т = 400°C) and the NO conversion rate
3
−
NO + 0.2% С Н
6
−
gas
x
−
3
3
Reaction
SCR of NOx
NO → NO
NO + 0.2% С Н
3
3 6
3
+
2.5% O2
k, min–1
WNOx, min–1 g–1
3
3
0.008
–
1.9 × 1018
0.04
–
9.6 × 1018
7.7 × 1018
0.03
–
7.2 × 1018
18
18
3 × 10 –5 × 10
d[NO ]/dt, min g–1
−
–1
–
3
, min–1 g–1
5.6 × 1018
–
–
W(
−
NO3+C3H6)
A dash indicates that the reaction rate cannot be calculated because the species is mot involved in the reaction specified in the header of
corresponding column. The values compared are boldfaced.
KINETICS AND CATALYSIS Vol. 58 No. 2 2017

2
04
MATYSHAK et al.
NO concentration, ppm
were observed as well. However, the IR spectra
x
recorded for the interaction between zirconia and the
С Н + О mixture, as well as the spectra recorded
350
300
250
200
3
6
2
1
2
during the SCR of NO (Fig. 3, spectrum 3), shows
х
intense absorption bands due to the vibrations of car-
–1
boxyl (acetate) groups: ν = 1500–1550 cm and ν =
as
s
–1
1400–1465 cm [48, 49, 51, 52]. The presence of
adsorption complexes containing a hydrocarbon frag-
ment (–СН or –СН – group) on the catalyst surface
3
1
50
3
2
4
is also indicated by the high-frequency absorption
100
5
–1
bands at 2884 and 2967 cm (Fig. 3, Table 2).
As judged from absorbance in the 1200–1250 cm
50
6
–1
range, the nitrate concentration on the ZrO surface
0
2
0
2
4
6
8
10 12 14 16 18 20
under the SCR conditions, when the reaction mixture
t, min
contains propylene, is lower than in the case of ZrO
2
interacting with the NO + O mixture. This effect is
Fig. 6. Variation of the (1, 3, 5) NO and (2, 4, 6) NO
2
x
due either to carboxylates blocking surface sites
concentrations in the course of nitrate conversion on
−
3
ZrO at 400°C in flowing (1, 2) N , (3, 4) 0.2% С Н , and
2
2
3
6
responsible for NO formation or to nitrates reacting
–1
(
5, 6) 0.2% С Н + 2.5% О . V = 150 mL/min (9000 h ).
with products of partial propylene oxidation. The lat-
3
6
2
gas
ter assumption is actually equivalent to the hypothesis
that the nitrates are intermediates in the SCR of NO .
x
gas
ence of NOx in the reactor. After the gas was blown
Special attention is deserved by the presence of an
–1
gas
absorption band at 1565 cm in the diffuse reflec-
through the reactor for 3 min, the NOx concentra-
tance IR spectra recorded during the SCR of NO with
x
tion at the reactor outlet was governed by the reactivity
of the surface nitrates and by the specific features of
their interaction the flowing gases. The following
propylene. The low intensity of absorbance in the
1
180–1280 cm– range indicates that the absorption
1
–1
observations were made in these experiments (Fig. 6): band at 1565 cm is not due to the presence of on
nitrite–nitrate structures on the catalyst surface. A
gas
x
(
1) the NO formation curve for the interaction
comparison of the intensity of the absorption band at
−
–1
between NO and N is symbatic to the same curve for 1565 cm with the intensity of absorbance in the
3
2
–1
the interaction between the nitrate and 0.2% С Н ;
1300–1360 cm range rules out assignment of this
band to surface carbonates as well.
3
6
(
2) the main product released into the gas phase in
1
−
3
The 1555–1575 cm– spectral range is known to
the interaction between NO and С Н /С Н + О is
3
6
3
6
2
accommodate the absorption bands due to the ν
as
−
NO, while the main product of NO decomposition vibrations of O N groups in organic nitro compounds
3
2
gas
[
25, 49, 53–57]. The ν vibrations of the nitro groups
in flowing nitrogen is NO ;
s
2
–1
are typically observed at 1320–1380 cm and are
much less intensive than the asymmetric vibrations. In
our case, the ν band of the O N group overlaps with
gas
−
(
3) NO in the interaction between NO and
x
3
С Н + О is more intensive than in the conversion of
the surface nitrates in flowing N or 0.2% С Н .
3
6
2
s
2
–1
2
3
6
the absorption band at 1355 cm , so it is observed as a
–1
separate band at. 1380 cm only at 340°С, when a suf-
ficiently high O N–C H concentration on the cata-
2
n
m
DISCUSSION
lyst surface is reached (Fig. 3, spectrum 4). However,
as can be seen in Fig. 3 (spectrum 3), even at 500°С
Interpretation of the IR Spectra Observed
in the SCR of NOx
the IR spectra recorded for the SCR of NO show, on
x
–1
the high-frequency side of the 1355 cm band, weak
An analysis of diffuse reflectance spectra recorded
and poorly resolved absorption that can be viewed as a
at temperatures of the SCR of NO with propylene
x
manifestation of the ν vibration of the nitro group.
revealed that, in the absence of nitrogen oxides (Fig. 3,
s
spectrum 1), the main propylene adsorption species
Thus, the IR spectroscopic data, as well as our EPR
spectroscopic data [46], indicate that N-containing
on ZrO are products of partial С Н oxidation by oxy-
2
3
6
gen (Table 2). The diffuse reflectance IR spectra show organic compounds form on the ZrO surface. It can
2
no bands due to the double bond –С=С–, which, in
be hypothesized that the diamagnetic complexes that
–1
propylene, absorbs light in at 1600–1650 cm [48, 49]. decompose to yield coke and were detected by com-
–1
In the 2975–3090 cm range, which is characteristic bining TPD and EPR spectroscopy are the nitro com-
of the vibrations if the =СН group [48–50], no bands pounds (O N–C H ) observed by in situ IR spectros-
2
2
n
m
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SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
205
x
Table 2. Assignment of the absorption bands observed in the diffuse reflectance IR spectra under conditions of the SCR of
NO with propylene, according to Ref. [46]
x
Reaction mixture composition
Assignment
Absorption
band, cm–1
0.2% NO + 2.5%
0.2% NO
0
.2% С Н + 2.5% О
3
6
2
О2
+
+ 0.2% С Н + 2.5% О
3
6
2
{SO4}2–
1355
+
+
+
(1400–1465)
νs and νas
{–CO2}
–
–
+
+
and (1500–1550)
νs and ν
as
2
884 and 2967
+
–
–
{CH}
N
N
O
O
1180–1200
+
+
+
+
O
O
Zr
ν3
Zr
Zr
1200–1250
NO3‾
ν3
O
1
604
–
–
+
+
+
+
N
O
O
Zr
Zr
ν3
O
N
1580
O
O
O
O
Zr
ν3
N
O
1
550
–
–
+
–
+
+
Zr
1
380
νs and νas
and 1565
{O N–}
2
A dash indicates that the corresponding absorption band was not observed.
copy in the SCR of NO with propylene in excess О
NO with propylene in excess О is equal to the rate of
x 2
x
2
(
Fig. 3, Table 2). In addition, experimental data on the
−
the interaction between NO and С Н calculated via
3
3
6
mechanism of the SCR of NO with hydrocarbons and
x
Eq. (9) as the difference between the rate of nitrate
relevant data available from the literature suggest that
−
3
the N-containing organic compounds are intermedi- conversion in С Н + О (W ) and the rate of NO
3
6
2
3
ates in the SCR of NO with propylene.
x
decomposition in N (W ). The activation energy of
2 1
the SCR of NO with propylene in excess О at
x
2
[
NO ] = 0.2% is also equal to the activation energy of
Reactivity of Nitrates on the ZrO Surface
gas
2
−
the interaction between NO and С Н determined by
within the experimental error the rate of the SCR of spectrokinetic experiments at Т = 370–440°С. These
As can be seen from the data presented in Table 1,
3
3
6
KINETICS AND CATALYSIS Vol. 58 No. 2 2017

2
06
MATYSHAK et al.
Absorbance(1170 – 1320), cm–1
6
−
3
increase in [NO ] during the interaction between
ZrO and NO and the rate of nitrate decomposition.
2
x
−
3
W = d[NO ] dt + W .
(11)
dA/dt = –0.0077t + 0.4082
4
1
5
4
3
2
1
2
R = 0.99
Figure 7 shows how the integral absorbance in the
–1
1
170–1320 cm range at 400°C varies in the course of
the interaction between ZrO (58 mg) and the gas mix-
2
ture containing 0.2% NO and 2.5% O .
2
−
−
3
The rate of the variation of [NO ] and the NO
3
formation rate were calculated from the derivative of
–1
absorbance at the А
= 3.6 cm point (dA/dt =
(1170–1320)
–1
–1
0
.35 cm min ), at which the nitrate coverage of the
−
3
surface corresponded to the NO concentration in the
SCR of NO with propylene in excess О .
x
2
−
0
2
4
6
8
10
12
14
16
t, min
W = d[NO ] dt +W = 1 (A 0.058)dA dt
4 3 1 0
(12)
18
19
+
1. 9 ×10 ≈ 2. 6 ×10 .
Fig. 7. Integral absorbance in the 1170–1320 cm–1 range as
a function of the time of interaction between ZrO and NO
The value obtained for the rate of nitrate formation
in the NO + O flow turned out to be almost 4 times
2
2
1
9
–1 –1
(
0.2%) + O (2.5%)/N at Т = 400°C. The IR spectra were
(by 1.9 × 10 min h ) larger than the rate of nitrate
conversion in the SCR of NO with propylene (Table 1).
2
2
recorded in the transmission mode. Vgas = 150 mL/min;
x
gas
pellet weight, 58 mg.
Therefore, at [NO ] = 0.2% and Т = 400°C in the
SCR of NO with propylene in excess О , the ZrO
2
x
2
surface undergoes saturation with nitrates; that is, the
coincidences demonstrate that, under the chosen con-
ditions (reaction mixture composition, volumetric
flow rate, and temperature), the interaction between
−
−
3
rate of the reaction between NO and С Н is indepen-
3
6
dent of the concentration of nitrogen oxides in the gas
phase. This verifies the above assumption that, at
gas
NO and С Н is the rate-limiting step of the SCR of [NO ] > 0.2%, NO reduction by propylene is a first-
3
3
6
x
x
gas
NO with propylene in excess О . However, in spite of
x
2
order reaction with respect to NO because of the sat-
x
the equality of the reaction rate and activation energy uration of the ZrO surface by intermediates of the SCR
2
values, it seems suspect that the rate constant of the of NO . Thus, for ZrO , as for Al O [58], the interac-
x
2
2
3
−
3
Н is actually the rate-limit-
6
tion of the nitrates with С
ing step of the SCR of NO with propylene in excess О .
3
interaction between NO and propylene (k
−
=
NO3 +C3H6
x
2
–1
0
.022 min ) and the effective rate constant of the
conversion of nitrogen oxides in the steady-state SCR
of NO (k = 33 min ) calculated via Eq. (3) under
Earlier, we discovered by IR and EPR spectroscopy
46] that the interaction of NO /NO + O with propyl-
[
–1
2
2
x
eff
ene yields N-containing organic compounds. Here, it
is reasonable to believe that, when the surface nitrates
are in the activated state with weakened N–O and/or
the assumption that the reaction is first-order with
gas
respect to [NO ] differ by a factor greater than 103.
x
O–Zr bonds, they will react with С Н in the same
This apparent contradiction can be explained by
assuming that, under the conditions of selective NOx
3
6
way, yielding O N–C H . According to the literature,
2
n
m
the rate and pathway of the further conversion of
O N–C H are determined by the NO and О con-
reduction with propylene in excess О , there is a large
2
2
n
m
x
2
−
3
amount of NO because of the high NO oxidation
x
centrations in the gas phase and by the energy barriers
rate. In order to verify this hypothesis, the rate of nitrate of the corresponding reactions.
18
conversion in the C H + O flow (7.2 × 10 mole-
3
6
2
T,°C
−
O N–C H ⎯ ⎯ ⎯ →N + NCO
2
n
m
2
cule/(min g)) was compared with the nitrate formation
(13)
+
[ ]
CO + CO + CH O 48, 57 ,
2 2
rate in the interaction between ZrO and NO + O .
2
2
NOx
The rate of the variation of the nitrate concentra-
O N–C H ⎯ ⎯ ⎯ →N + CO
2 n m 2
(14)
tion on the ZrO surface can be expressed in terms of
2
+ CO + H O 38, 48 ,
[ ]
2 2
the variation of the intensity of the corresponding IR
−
O2
O N–C H ⎯ ⎯ ⎯→NO + N
absorption bands. The NO formation rate (W4,
2
n
m
x
2
3
(15)
–
1
–1
min g ) will be equal to the sum of the rate of
+ CO + CO + H O
[ ]
25,48 .
2
2
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2017

SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
207
x
Our results and data available from the literature on
The SCR of NO with propylene is accompanied by
x
the SCR of NO with hydrocarbons [22–25, 48, 59–61]
x
deep С Н oxidation, whose activation energy (E
)
3
6
3
C H
6
suggest the following reaction scheme for NO reduction
is invariable throughout the NO gx as concentration
range. This indicates that deep propylene oxidation
proceeds via a mechanism that is practically indepen-
by propylene over ZrO :
2
O
2
−
3
NO ⎯ ⎯ ⎯→NO
dent of the SCR of NO . At the same time, the rate of
x
T,C3H6
64 kJ mol
gas
6
⎯
⎯ ⎯ ⎯ ⎯ →
O N−C H
(16) the SCR of NO depends directly on the C H and,
2
n
m
x
3
1
−
1
absorption bands 1565, 1380 cm
accordingly, on the efficiency of the interaction
between С Н and О . The fractional reaction order of
NOx,O2,T
3
6
2
⎯
⎯ ⎯ ⎯ ⎯ →N + CO + H O.
2 2 2
gas
6
the SCR of NO with propylene with respect to C H
x
3
can be explained by the fact that nitrogen oxides are
reduced not by С Н molecules from the gas phase but
by products of their interaction with the ZrO surface.
Processes Accompanying NO Reduction to N2
3
6
2
The above NO reduction scheme is very simplified
and disregards some processes exerting a strong effect
The increase in the onset temperature of С Н oxi-
3
6
on the rate of NO conversion to N . As is demon-
2
dation at a high NO concentration (0.2%) is due to
x
strated by experimental data of this study, the rate of
the blocking of the catalyst surface by nitrates. With an
increasing temperature, the adsorption complex cov-
erage of the surface decreases (Fig. 3), the nitrates stop
blocking the catalyst surface, and empty sites active in
the SCR of NO with propylene on the ZrO surface is
x
2
largely determined by the concentration of surface
nitrates. The decrease in the NO conversion as a
x
gas
x
deep C H oxidation appear.
result of [NO ] increasing from 0.1 to 0.2%, which
3
6
means a decrease in the order of the reaction with
While an increase in the reductant (С Н ) concen-
3
6
gas
−
3
respect to NO , is due to an increase in the NO for-
x
tration leads to an increase in the NO conversion, the
x
mation rate, which leads to saturation of the ZrO sur-
2
presence of oxygen in the gas phase can both accelerate
face with nitrates. This explanation is confirmed by
the absence of the absorption bands of surface nitrates
in the diffuse reflectance IR spectra recorded under
nitrogen oxide reduction by propylene and diminish the
NO conversion. This is due to the fact that the effect of
x
oxygen on the SCR of NO is determined both by the
x
conditions of the SCR of NO with propylene at
x
rate of deep С Н oxidation, which decreases the reduc-
3
6
gas
[
NOx ] = 0.038–0.1%. In this range of nitrogen oxide tant concentration, and by the involvement of О in the
2
−
−
y
concentrations, the NO formation rate is insufficient oxidation of NO to NO /NO , oxidation of nitrogen-
3
2
−
3
containing organic compounds, coke burning, and par-
tial oxidation of hydrocarbons.
for NO accumulation on the catalyst surface. As a
consequence, the NO reduction reaction is first-order
with respect to NO . At the same time, according to
spectrokinetic data, at [NOx ] = 0.2% and Т ≤ 400°С
the NO formation rate is much higher than the NO
conversion rate; that is, surface saturation with
gas
x
The favorable effect of О is that the presence of
2
gas
oxygen in the gas phase enhances NO oxidation on the
ZrO surface [44], which, as was demonstrated above,
2
−
3
−
3
is a necessary condition for the occurrence of the SCR
of NO via the intermediate formation of N-contain-
x
nitrates takes place.
gas
2
ing organic compounds. The adverse effect of O is
The increase in the apparent activation energy of
that it diminishes the number of hydrocarbon mole-
the SCR of NO (E
) from 50 to 160 kJ/mol
x
SCR NOx
cules capable of reducing nitrogen oxides. For exam-
caused by increasing the NO concentration in the ple, it is clear from Fig. 2b that the increase in the NOx
x
reaction mixture from 0.1 to 0.2% can be explained by conversion at 560°С, which is caused by the increase
−
3
gas
the fact that, with an increasing NO formation rate of [O ], stops once 100% propylene conversion is
2
(
concentration), which leads to a decrease in the reac-
gas
2
reached. The main result of a further increase in [O ]
gas
tion order with respect to NOx , the contribution is a decrease in the proportion of С Н reacting with
3
6
−
3
nitrogen oxides. In case the catalyst surface is satu-
from the heat of NO formation to the experimental
rated with nitrogen–oxygen complexes and there is a
ESCR NOx value decreases. The nonlinearity of the
gas
2
observed dependence of the logarithm of the effective sufficient amount of reductant, an increase in [O ]
rate constant for the SCR of NO with propylene (k ) has no effect on the NO
conversion (Fig. 2a). The
reduction rate reaches some steady-state value
x
x
eff
x
on inverse temperature is another fact that can be NO
explained in terms of the variation of the nitrate cover- that is determined by the adsorption capacity and
age of the surface.
activity of the catalyst.
KINETICS AND CATALYSIS Vol. 58 No. 2 2017

2
08
MATYSHAK et al.
Of particular significance are processes that involve
The possibility of these reactions of NO with С and
2
nitrogen dioxide or lead to NO formation. As we СО is due to the fact that, at Т > 300°C, the N-contain-
2
demonstrated by EPR spectroscopy [46], nitrogen ing organic compounds on the ZrO surface decompose
2
dioxide is more reactive than NO in the conversion of
С Н into N-containing organic compounds. The
to yield coke.
3
6
gas
x
interaction between NO and С Н on the ZrO sur-
2
3
6
2
The finding that the intensity of NO evolution
face occurs even at 20°C, while the nitrates that are
−
3
in the interaction between NO and the С Н +
3
6
intermediates in NO reduction to N react with pro-
2
О /N mixture is higher than in the nitrate conver-
sion in flowing N2 or С Н /N is apparently
2
2
pylene only upon their thermal activation (Т ≥ 300°С).
The domination of nitrogen dioxide formation in the
3
6
2
−
3
explained by the oxidation of the N-containing
organic compounds by oxygen present in the reaction
mixture. According to the literature [53, 59–61], the
gas
conversion of NO in flowing N (Fig. 6) is evidence
2
that, in the SCR of NO with propylene, the nitrates are
x
intermediates in NO oxidation to NO . These data sug-
2
presence of O2 prevents some of the O N groups in
gest that, under the SCR conditions, propylene reacts
x
the N-containing compounds from being reduced
not with the nitrates as such but with NO resulting
2
and, as a consequence, these groups are released into
from their decomposition or with the activated complex
the gas phase NO or remain on the catalyst surface.
ads
x
(
NO2 ), whose structure is intermediate between the
structures of monodentate NO and NO :
When the reductant concentration is insufficient and
the catalyst surface is saturated with nitrogen–oxy-
gen complexes, this process leads to a decrease in the
−
3
2
gas
NO conversion. At the same time, at high [C H ],
(17)
In this case, NO desorption will be characterized by
x
3
6
Zr–ONO ꢀZr–OꢁNO ꢀNO .
2
2
2
the oxidation of the O N–С Н hydrocarbon frag-
2
n
m
ments by molecular oxygen makes it possible to avoid
2
the coking of the catalyst surface and the blocking of
Е = 26 kJ/mol, which is equal to the difference active sites of ZrO , thus exerting a favorable effect
a
2
−
3
on the SCR of NOx.
between Е for NO decomposition (190 kJ/mol) and
SCR NOx
a
Е
(164 kJ/mol).
The presence of NO in the gas phase after the
2
CONCLUSIONS
interaction between the reaction mixture and ZrO is
2
explained by the fact that, when the amount of reduc-
Thus, it was found that, on the surface of zirconia
tant is insufficient and/or when the catalyst surface is stabilized as its tetragonal phase, the rate-limiting step
saturated with nitrogen–oxygen complexes, the rate of of the SCR of NO with propylene is the interaction
x
NO oxidation to NO becomes higher than the rate of between the surface nitrates and С Н , which yields
2
3
6
the interaction between NO and C Н .
N-containing organic compounds—hypothetical
2
3
6
intermediates in the reduction of nitrogen oxides to
Of considerable interest are data concerning the
formation of nitrogen oxides in the gas phase in the
N . Apparently, propylene reacts not with the nitrates
2
ads
2
−
but with the activated complex (NO ), whose struc-
interaction of NO3 with flowing N , C H /N , or
2
3
6
2
ture is intermediate between the structures of
C H + O /N (Fig. 6). The similarity of the time
3
6
2
2
−
3
gas
monodentate NO and NO .
2
dependences of NOx evolution for the interaction
between the nitrates and N to the same dependence
2
Deep С Н oxidation exerts an adverse effect on
3 6
for the interaction between the nitrates and С Н /N is
3
6
2
the rate of the SCR of NO with propylene, and the
x
due to the fact that, in both cases, NO evolution into
x
interaction between О and NO, which yields NO
2
2
−
3
gas
the gas phase is a result of NO decomposition. At the
same time, the absence of NOx at the reactor outlet in N . At the same time, the effect of the reaction
−
3
and NO , stimulates further nitrogen reduction to
2
−
3
between oxygen and O N–С Н on the rate of NO
2 n m
reduction is dual and is determined by the С Н /NO
x
the reduction of NO with excess propylene is possibly
due to the high rate of the formation of N-containing
3
6
x
ratio.
organic compounds in the interaction between С Н
3
6
and NO or to the reduction of nitrogen dioxide to NO
2
Having compared our experimental data with those
available from the literature, we suggested a general-
via reactions (18) and (19):
ized reaction scheme for the SCR of NO with propyl-
N O + C → NO + CO,
2
(18)
x
ene on the surface of ZrO partially stabilized as its
2
N O + CO → NO + CO .
(19) tetragonal phase:
2
2
KINETICS AND CATALYSIS
Vol. 58
No. 2
2017

SPECTROKINETIC STUDY OF THE MECHANISM OF NO REDUCTION
209
20)
x
gas
NO2
NO + CO + H O
x
x
2
T
CO,
26 kJ/mol
C,
O2
(
O2
C3H6
x
NO , T
NO3−
ads
NO
NO2
O N C H
N + C + CO + H O
2 x 2
2
n
m
164 kJ/mol
The data obtained for the mechanism of NOx 13. Matyshak, V.A., Il’ichev, A.N., Ukharsky, A.A., and
Korchak, V.N., J. Catal., 1997, vol. 171, p. 245.
reduction with propylene over ZrO suggest that, for
2
1
4. Shimizu Ken-ichi, Kawabata, H., Satsuma, A., and
increasing the activity of zirconia in the SCR of NO
x
Hattori, T., J. Phys. Chem. B, 1999, vol. 103, p. 5240.
with hydrocarbons in the presence of excess О , it is
2
1
5. Bennett, C.J., Bennett, P.S., Golunski, S.E.,
Hages, J.W., and Walker, A.P., Appl. Catal., A, 1992,
vol. 86, p. L1.
necessary to enhance the reactivity of the surface
nitrates. The activation energy for the breaking of
−
3
nitrogen–oxygen bonds in NO and/or oxygen–zir-
16. Teraoka, Y., Ogawa, H., Furukawa, H., and Kagawa, S.,
conium bonds can be reduced by promoting ZrO with
Catal. Lett., 1992, vol. 12, p. 261.
2
3
d elements of noble metals, as is known well from the 17. Iwamoto, M., Yashiro, H., Ooe, H., Banno, K.Y., and
literature [62–64].
Okamoto, F., Shokubai (Catalyst), 1990, vol. 32, p. 91.
1
8. Torikai, Y., Yahiro, H., Mizuno, N., and Iwamoto, M.,
Catal. Lett., 1991, vol. 9, p. 91.
ACKNOWLEDGMENTS
This work was supported by the Russian Founda-
tion for Basic Research, grant no. 14-03-00248.
1
9. Burch, R. and Millingham, P.J., Appl. Catal., B, 1993,
vol. 2, p. 101.
20. Matyshak, V.A., Il’ichev, A.N., Ukharskii, A.A., and
Korchak, V.N., Kinet. Catal., 1999, vol. 40, no. 1, p. 105.
2
1. Inui, T., Kojo, S., Shibata, M., Yoshida, T., and Iwa-
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KINETICS AND CATALYSIS
Vol. 58
No. 2
2017