Interaction between NO x and the surface of supported heteropoly compounds: In situ IR spectroscopic data

Article abstract of DOI:10.1134/S002315841103013X

The mechanism of activation of nitrogen oxides on unsupported heteropoly compounds and the composition, location, stability, and interconversion mechanisms of adsorption complexes on supported heteropoly compounds have been investigated by in situ IR spectroscopy under thermal desorption conditions. Supporting a small amount of a heteropoly compounds (1% or below) increases NO _x adsorption relative to the adsorption observed for the pure support. This effect is most pronounced for CeO₂ and least pronounced for ZrO₂. The increase in NO _x adsorption is due to NO oxidation to NO₂ on the supported heteropoly compound. The main adsorption species are nitrite and nitrate complexes, which are located on the support. As the temperature is raised, the nitrite complexes turn into the nitrate complexes. The presence of variable-valence ions in the Keggin anion reduces the nitrate complex-surface binding strength. The ions that are not components of the Keggin anion increase the binding strength. The changes in the nitrate complex-support surface binding strength are due to the support modification taking place via the destruction of part of the supported heteropoly compound.

Full text of DOI:10.1134/S002315841103013X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 3, pp. 409–417. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.A. Matyshak, N.V. Konokhov, V.F. Tret’yakov, Yu.P. Tyulenin, O.N. Sil’chenkova, V.N. Korchak, Rui Wong, 2011, published in Kinetika i Kataliz,
2011, Vol. 52, No. 3, pp. 419–428.
Interaction between NOx and the Surface of Supported Heteropoly
Compounds: In Situ IR Spectroscopic Data
V. A. Matyshaka, N. V. Konokhova, V. F. Tret’yakovb, Yu. P. Tyulenina,
O. N. Sil’chenkovaa, V. N. Korchaka, and Rui Wongc
a Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 119991 Russia
c Shandong University, Jinan City, Shandong Province, China
eꢀmail: matyshak@chph.ras.ru
Received April 27, 2010
Abstract—The mechanism of activation of nitrogen oxides on unsupported heteropoly compounds and the
composition, location, stability, and interconversion mechanisms of adsorption complexes on supported hetꢀ
eropoly compounds have been investigated by in situ IR spectroscopy under thermal desorption conditions.
Supporting a small amount of a heteropoly compounds (1% or below) increases NOx adsorption relative to
the adsorption observed for the pure support. This effect is most pronounced for CeO2 and least pronounced
for ZrO2. The increase in NOx adsorption is due to NO oxidation to NO2 on the supported heteropoly comꢀ
pound. The main adsorption species are nitrite and nitrate complexes, which are located on the support. As
the temperature is raised, the nitrite complexes turn into the nitrate complexes. The presence of variableꢀ
valence ions in the Keggin anion reduces the nitrate complex–surface binding strength. The ions that are not
components of the Keggin anion increase the binding strength. The changes in the nitrate complex–support
surface binding strength are due to the support modification taking place via the destruction of part of the
supported heteropoly compound.
DOI: 10.1134/S002315841103013X
One of the central presentꢀday environmental
Catalytic converters generally contain NOxꢀadsorbꢀ
problems is that of reducing the NOх emissions from ing components (alkali metal and alkalineꢀearth metal
oxides), noble metals (usually Pt or Rh), and a support
with a large specific surface area (usually ꢀAl2O3
industrial facilities. This problem extends to gas turꢀ
bines, heat power plants, and automotive internal
combustion engines consuming diesel fuel. A promising
way of solving this problem is catalytic reduction of nitroꢀ
gen oxides under oxidative conditions. Two variants of
this process have been considered in the literature.
γ
)
[10]. In recent years, there have been publications on
catalysts supported on mixed oxides consisting of
TiO2, ZrO2, Al2O3, and CeO2 [11, 12] or on hydrotalꢀ
cite [13]. There has been a comparative study of the
effect of the support (MgO, Al2O3, ZrO2, SiO2) on the
formation of adsorbed nitrates and on the rate of their
reduction with hydrogen [12].
The first variant needs effective catalysts for NOx
reduction to N2 with hydrocarbons in the presence of
oxygen. The existing approaches to creating exhaust gas
treatment catalysts are based on the known data conꢀ
cerning the mechanism of the selective reduction of
nitrogen oxides in the presence of hydrocarbons [1–6].
Numerous studies have been aimed at elucidating
the detailed mechanism of NOx accumulation and
reduction [14–24]. The effects of СО2 and SO2 on
NOx adsorption and reductions were reported [25, 26].
Note that the greater part of these data refers to
Pt/Ba–Al2O3 catalysts. It is conventionally believed
that the first stage of NOx removal is NO oxidation to
NOx on Pt sites. The nitrogen oxide NO2 is desorbed
as nitrites and nitrates on alkali or alkalineꢀearth metal
oxides and then undergoes spillover to the support.
Note that, although there have been numerous
studies on various types of catalytic systems and reducꢀ
tants, the problem is still unsolved. This is largely due
to the high oxygen concentration in diesel exhaust,
which makes NOx reduction to nitrogen much more
difficult to carry out.
A possible way out is suggested by catalytic convertꢀ
ers in which NOх is trapped and, as soon as the trap is
filled, fuel is injected for a short time and desorbed
A literature survey demonstrated that search for
new, efficient NОx neutralization catalysts, as well as
the investigation of the mechanism of this process,
NOх is reduced over a threeꢀway catalyst—second should be continued. Here, we report the mechanism
variant of the process [7–9]. of NOx adsorption and activation on supported hetꢀ
409

410
MATYSHAK et al.
eropoly compounds (HPCs), a new class of catalysts. A loaded with 1% KCu0,25PMo10VWOx was practically
specific feature of HPCs is that they have an ultramiꢀ equal to that of the initial support.
croporous structure with a mean pore size of 1–2 nm
(depending on the heteropolyanion composition and
cation type), which forms upon the removal of the
In Situ IR Spectroscopy
water of crystallization from the initial heteropoly acid
(HPA). By replacing metal atoms in heteropolyanꢀ
ions, one can obtain mixed HPCs containing different
metal atoms. It is thus possible to vary the oxidation
potential and acidity of HPCs in rather wide ranges.
When supported on a solid, these structures can actiꢀ
vate nitrogen oxide adsorption at low temperatures
and favor their reduction by hydrocarbons at elevated
temperatures.
IR transmission spectra were obtained on a Specꢀ
trum RX I FTꢀIR System spectrometer (PerkinElmer)
at 4 cm–1 resolution with a oneꢀscan time of 4.2 s. Use
of quartz reactor cells with spectroscopic ZnS winꢀ
dows made it possible to record IR spectra in situ at
elevated temperatures of up to 500
The samples to be characterized by IR transmission
spectroscopy were pressed into pellets at
= 5000 kg/cm2.
°С [31].
Р
The thickness of a catalyst pellet was equivalent to 20–
70 mg/cm2. The structure and properties of the surface
complexes resulting from NOx entrapment were studꢀ
ied by temperatureꢀprogrammed desorption (TPD).
Before being tested, the sample, placed in the reactor
cell, was subjected to standard heat treatment in flowꢀ
The HPCs were supported on
ZrO2. These materials were chosen for the reason that
ꢀalumina doped with transition metal ions is unstable
θꢀAl2O3, CeO2, or
γ
under hydrothermal treatment conditions and recrysꢀ
tallizes and transition metal cations migrate into the latꢀ
ing N2 at 350°С for 30 min. After the sample cooled to
tice [27]. The highꢀtemperature alumina phase ꢀAl2O3
θ
room temperature, NOx was adsorbed onto it from a
1000 ppm NO + N2 mixture (nitrogen contained
approximately 1000 ppm О2 as an impurity). Thereafꢀ
ter, the cell was purged with a inert gas for removing
weakly bound species and the sample was heated to
suffers from this drawback to a much lesser extent.
Cerium and zirconium dioxides are widely used as
supports and structural and electronic promoters for
enhancing the activity, selectivity, and thermal stability
of catalysts. Ceriumꢀcontaining materials have found
widest use in redox processes owing to the capability of
the fluorite structure to withstand high oxygen deficiency
levels without changing its structural type [28, 29].
350°C at a constant rate in flowing N2. IR spectra were
recorded at each stage of this procedure. During NOx
TPD, IR spectra were recorded continuously. The
time required for recording one spectrum was 4 min
(60 scans). The concentration of surface complexes
EXPERIMENTAL
was estimated either in units of absorbance (
А) or from
the integral intensity (
А*
, cm–1) of absorption bands.
Catalyst Preparation
All spectroscopic data presented in this work are conꢀ
verted to a pellet thickness of 20 mg/cm2.
Heteropoly compounds based on phosphomolybꢀ
dic acid were obtained by solidꢀphase synthesis [30].
The reaction temperature was controlled with a
MINITERMꢀ300.31 programmable temperature regꢀ
ulator. The deviation of the temperature from the preꢀ
set value was <5°С. The composition of the gas phase
before and after the reactor was determined using a
Supported catalysts were prepared by impregnating
a support with an HPA or HPC solution (1, 5, 10, or
70% of the support weight) followed by drying at
120°С for 3 h and by stepwise calcination at 150, 180,
200, 220, and 250
ture point. The products were thoroughly ground and
were calcined again at 350 for 1.5–2 h.
ꢀAl2O3 was obtained by calcination of commercial
ꢀalumina with a specific surface area of 217 m2/g in
air at 300 for 1 h and then at 800, 900, and 1100
for 3 h at each temperature point. The specific surface
area of the resulting
ꢀAl2O3 was 116 m2/g.
°С for 30–40 min at each temperaꢀ
Beckmanꢀ951A chemiluminescent NO/NOx analyzer.
°С
RESULTS
θ
NOx Adsorption on Unsupported HPCs
γ
°
С
°С
Experiments were performed on H3PMo12O40 and on
the following salts of this acid: K0.5PMo11Sb0.5W0.5Ox,
K0.5Nb0.025PMo11Sb0.5W0.5Ox, K0.5Nb1PMo11Sb0.5W0.5Ox,
θ
Cerium dioxide (Ssp = 79 m2/g) was obtained by
heating Ce(NO3)3 · 6H2O in air at a rate of 7 K/min to
500°С and by holding it at this temperature for 2 h.
KCu0.25PMo10SbWOx, and KCu0.25PMo10VWOx
.
Figure 1 presents the IR spectra recorded after NОх
adsorption on some of these compounds. It is evident
Zirconium dioxide was obtained by calcination of that the unsupported HPCs almost do not adsorb
zirconium nitrate in air at 500°С for 3 h. According to NOx. At the same time, the spectra show an absorption
Xꢀray diffraction data, the resulting ZrO2 consisted of doublet around 1618 cm–1. This absorption characterꢀ
approximately equal amounts of monoclinic and tetꢀ izes the rotational–vibrational band contour of the
ragonal phases. The specific surface area of the oxide NO2 molecule. Therefore, the unsupported HPCs are
was 98 m2/g. The specific surface area of any support capable of oxidizing NO to NO2 at room temperature.
KINETICS AND CATALYSIS Vol. 52
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2011

INTERACTION BETWEEN NOx AND THE SURFACE
411
A
2
0.025
0.020
0.015
0.010
0.005
0
2
1
1
3
1617
3
2300
2000 1900 1800 1700 1600 1500 1400 1300 1200 1100
cm–1
Fig. 1. IR spectra recorded after NО adsorption at room temperature on the heteropoly compounds (1) K Cu PMo SbWO ,
0.5 0.25 10 x
x
K
(2)
K
PMo Sb
W
11 0.5 0.5
O
, and (
3)
PMo Sb W
11 0.5 0.5
Nb
O .
0.025 x
0.5
x
0.5
NOx Adsorption on Supports
Figure 2 shows the spectra recorded after NOx
adsorption on ꢀAl2O3, CeO2, and ZrO2 at room temꢀ
perature (Fig. 2a) and after heating the samples to
280 C in flowing nitrogen (Fig. 2b).
Hereafter, the spectra are presented as the differꢀ
ence between the spectrum recorded after NOx
adsorption and the spectrum taken of the initial solid complexes with cisꢀ and trans
the absorption bands of the nitrate complexes in the
region of the highꢀfrequency component of the ν3
vibration (1500–1700 cm–1).
θ
Note the difference between the spectra of the supꢀ
ports after NОx adsorption. In the case of CeO2 [33–37],
NOx adsorption at room temperature yields surface
2−
°
ꢀ
N
and NО– strucꢀ
2O2
at the same temperature.
tures. These complexes show themselves as absorption
between 1100 and 1300 cm–1. The corresponding
absorption bands complicate the spectrum of the nitrite
and nitrate complexes on the CeO2 surface (Fig. 2a).
Note that, as the temperature is raised, the cisꢀ and
The spectra of all of the supports after NOx adsorpꢀ
tion at room temperature generally show absorption
bands from two nitrite complexes and three nitrate
complexes [11, 32] in different ratios. The positions
and assignment of the observed absorption bands are
presented in Table 1.
transꢀ
N2О2 and NО– structures in the presence of
oxygen turn rapidly into nitrite and then nitrate comꢀ
plexes.
After NOx adsorption at room temperature, the
largest amount of nitrite complexes (smallest amount
of nitrate complexes) is observed for ꢀAl2O3 and the
θ
Among the supports examined, cerium dioxide
possesses the best optical properties. Because of this,
the absorption band at 798 cm–1 arising from NОx
adsorption can be observed only for this support (this
spectral region is not shown in Fig. 2). Since the intenꢀ
sity of this band changes with temperature in the same
way as the intensity of the absorption bands of the
nitrate complexes, Philopp et al. [34] assigned this
smallest amount of nitrite complexes (largest amount
of nitrate complexes) is observed for ZrO2. This is
likely due to the fact that the oxides differ in the propꢀ
erties of their surface oxygen.
As the temperature is raised, the absorption bands
of the nitrite complexes weaken and those of the
nitrate complexes strengthen. Because the temperaꢀ
ture was increased in the absence of NОx in the gas
phase, this trend of the band intensitied indicates the band to one of the vibrations in NO3 ads
conversion of the nitrite complexes into the nitrate
complexes.
.
The adsorption of NOx on ZrO2 yields considerable
amounts of nitrate complexes even at room temperaꢀ
ture. Furthermore, the amount of nitrate complexes
on ZrO2 is much larger than on the other supports
(Fig. 2b), even though the supports have similar speꢀ
The bands of the nitrate complexes are fairly close
together (Fig. 2), so it is difficult to determine the
amount of each particular complex species. For this
reason, the surface concentration of nitrate complexes
was estimated in terms of the total area (
А
, cm–1) of cific surface areas.
KINETICS AND CATALYSIS Vol. 52
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2011

412
MATYSHAK et al.
(a)
A
1290
1.2
1.0
0.8
0.6
0.4
0.2
0
1237
3
1408
3
1528
1
1012
2
1339
1623
1
2
1586
cm–1
1800 1700 1600 1500 1400 1300 1200 1100 1000
A
(b)
2.4
2.2
3
1211
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
3
1291
1565
1625
1
995
2
1
2
1585
cm–1
1800 1700 1600 1500 1400 1300 1200 1100 1000
Fig. 2. IR spectra recorded (a) after NО adsorption on supports at room temperature and (b) after subsequent heating to 280°C
x
in flowing nitrogen: (
1
) ꢀAl O , (2) CeO , and (3) ZrO .
θ
2 3 2 2
Effect of the Supported HPC Concentration
on NOx Adsorption
on the pure support. Therefore, the surface complexes
of the HPCꢀloaded samples are located on the supꢀ
port, not on the supported HPC.
Figures 3 and 4 show the IR spectra recorded for
the 1, 5, 10, and 70% H3PMo12O40/ ꢀAl2O3 samples at
θ
Supporting an HPC at a low concentration (1%)
different temperatures. Note that the spectrum of the causes a marked increase in the amount of NОx
surface complexes on the HPCꢀloaded samples is adsorbed as nitrite and nitrate complexes (Fig. 3,
practically identical to the spectrum of the complexes 1200–1700 cm–1). The same effect is observed for the
Table 1. Spectral characteristics of nitrite and nitrate complexes on different supports [11, 32] at room temperature
Complex
θ
ꢀAl O
CeO
ZrO
2
2
3
2
Nitrite, bidentate
Nitrite, linear
1317, 1410 cm–1
1500 cm–1
1275, 1320 cm–1
1450 cm–1
1290 cm–1
1530 cm–1
1625 cm–1
1585 cm–1
1555 cm–1
Nitrate, bridging (ν3
Nitrate, bidentate
Nitrate, linear
)
1617 cm–1
1600 cm–1
1590 cm–1
1580 cm–1
1566 cm–1
1545 cm–1
KINETICS AND CATALYSIS Vol. 52
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2011

INTERACTION BETWEEN NOx AND THE SURFACE
413
A
1313
1.2
1.0
0.8
0.6
0.4
0.2
0
1417
3370
1618
2
3
2
4
1
3
4
5
1
5
сm–1
3600 3200 2800 2400 2000 1800 1600 1400 1200
Fig. 3. IR spectra recorded after NО adsorption on H PMo
O
/
3
θ
ꢀAl O samples at room temperature: (
1
) ꢀAl O ,
2 3
θ
θ
x
3
12 40
2
3
(2
) 1% H PMo O /
θ
ꢀAl O , (
3
) 5% H PMo O /
θ
ꢀAl O , (
4
) 10% H PMo O /
θ
ꢀAl O , and (
5
) 70% H PMo O /
ꢀAl O .
2 3
3
12 40
2
3
3
12 40
2
3
12 40
2
3
3
12 40
A
1619.45
1295.40
0.77
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0
1586.75
1565.94
2
1
3504.47
3
2
3
4
5
1259.72
1
–0.05
4000
3000
2000
1500
1000 900
cm–1
Fig. 4. IR spectra recorded after NО adsorption on H PMo O /
θ
ꢀAl O samples at room temperature and subsequent heating to 150 C
°
x
3
12 40
2 3
in flowing nitrogen: (
) 70% H PMo O / ꢀAl O .
2 3
1)
θ
ꢀAl O , (
2
) 1% H PMo O /
θ
ꢀAl O , (
3
) 5% H PMo O /
θ
ꢀAl O ,
(4
) 10% H PMo O / ꢀAl O , and
θ
2
3
3
12 40
2
3
3
12 40
2
3
3
12 40 2 3
(5
θ
3
12 40
CeO2ꢀbased samples. Figure 5 presents the temperaꢀ
ture dependences of the amount of the nitrate comꢀ
plexes resulting from NОx adsorption for CeO2,
An opposite effect was observed for the ZrO2ꢀbased
samples. Supporting any amount of HPC onto ZrO2
(as distinct from ꢀAl2O3 and CeO2) leads to a decrease
θ
1% HnPMo11V1O40/CeO2, and 1% HnPMo9V3O40/CeO2
.
in the amount of nitrate complexes relative to that
adsorbed on the pure support (Table 2). The complex–
surface binding strength remains practically
unchanged.
Supporting an HPC on CeO2 results in a greater
increase in the amount of nitrate complexes that supꢀ
porting the same HPC on ꢀAl2O3. The best results
θ
were obtained with the HPC whose Keggin anion conꢀ
tains three vanadium atoms.
As the supported HPC concentration is increased,
the amount of adsorbed NОx decreases (Figs. 3, 4).
This is likely due to the blocking of part of the support
surface. This effect is most pronounced for ZrO2
(Table 2) and least pronounced for CeO2 (Table 3). In
the latter case, even supporting 5% HPC increases the
amount of nitrate complexes. By contrast, supporting
The increase in the amount of nitrate complexes is
likely due to weakly bound surface species because, at
elevated temperatures (Fig. 4), the amounts of nitrate
complexes on
nearly equal.
θ
ꢀAl2O3 and 1% HPC/ ꢀAl2O3 are
θ
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

414
A*, cm–1
MATYSHAK et al.
face binding strength for the supported HPCs is lower
than for the initial support.
Similar results were obtained for NОx adsorption on
supported KCu0.25PMo10SbWOx, KCu0.25PMo10VWOx,
KCu0.5PMo10VWOx, BaPMo12Ox, and other HPCs (1–
5%). By way of example, we present the data obtained
for KCu0.25PMo10VWOx / ꢀAl2O3 (Fig. 6).
Clearly, the Cu ions in the HPC increase the amount
of nitrite and nitrate complexes and increase their bindꢀ
140
120
100
80
3
2
1
θ
60
40
20
ing strength relative to that observed for
θꢀAl2O3.
As in the case of ꢀAl2O3, supporting an HPC onto
θ
0
30
150
210
280
CeO2 reduces the nitrate complex–surface binding
strength. The amount of nitrate complexes on the surꢀ
face of the CeO2ꢀsupported samples is much larger
than on the support even for NОх adsorption at room
temperature (Fig. 5). This is likely due to the high
activity of the surface oxygen of CeO2 and the presence
of variableꢀvalence ions in the Keggin anion.
T, °C
Fig. 5. Temperature dependence of the amount of nitrate
complexes for ( CeO , ( ) 1% H PMo V O / CeO
and ( ) 1% H PMo V O /CeO
1)
2
,
2
2
n
11
1
40
3
.
2
n
9
3
40
It follows from the data listed in Table 3 that supꢀ
porting 1% KCu0,25PMo10VWOx increases both the
mount of nitrate complexes and the strength of their
binding to the surface relative to the binnding strength
in the case of adsorption on CeO2. For the 5%
KCu0.25PMo10VWOx/CeO2 sample, this effect is much
weaker. Therefore, variableꢀvalence ions in the Keggin
anion reduce the nitrate complex–binding strength,
while the ions that are not components of the Keggin
anion increase the binding strength. This is true for the
>1% of any HPC onto θꢀAl2O3 leads to a decrease in
NОx adsorption.
Note that NОx adsorption on the HPCꢀloaded
samples (as distinct from NОx adsorption on the supꢀ
port (Fig. 2)) causes a considerable buildup of hydroꢀ
genꢀbonded hydroxyl groups on the surface (Figs. 3, 4)
in the case of ꢀAl2O3 and ZrO2. For the CeO2ꢀbased
θ
samples, NОx adsorption does not change the specꢀ
trum of hydroxyl groups.
CeO2ꢀ and ꢀAl2O3ꢀbased materials and, to a much
θ
lesser extent, for the ZrO2ꢀbased materials. In the latꢀ
ter case, the nitrate complex–surface binding strength
is almost invariable. Note that the amount of nitrate
complexes on ZrO2 surface is large even after NОх
adsorption at room temperature.
Effect of the Composition of the Supported HPC
on NOx Adsorption
Investigation of NОx adsorption on materials conꢀ
taining different HPAs (1%) demonstrated that it is
possible to vary the nitrate complex–surface binding
strength. The data presented in Table 4 show how the
amount of nitrate complexes on the surface of
Al2O3ꢀsupported HPAs with different vanadium conꢀ
tents varies with temperature up to 280оС
At > 150 , the amount of nitrate complexes on
the Vꢀcontaining HPCs is smaller than on the support
(Table 4). This means that the nitrate complex–surꢀ
θ
ꢀ
DISCUSSION
This study demonstrated the following:
The unsupported HPCs oxidize NO to NO2 at
room temperature.
Supporting a small amount of an HPC (no more than
1%) increases NOх adsorption relative to the adsorption
observed for the pure support. This effect is most proꢀ
.
Т
°С
nounced for CeO2 and least pronounced for ZrO2
.
Table 2. Temperatureꢀdependent amounts of nitrate comꢀ
plexes (
A
*) for the ZrO2ꢀbased catalysts
The main adsorption species are nitrite and nitrate
complexes. They are located on the support. As the
temperature is raised, the nitrite complexes turn into
the nitrate complexes.
Supporting a larger amount of HPC (5%) causes a
decrease in NOх adsorption relative to the adsorption
on the pure support because of the blocking of part of
the support surface.
The introduction of vanadium ions into the Keggin
anion increases the amount of nitrate complexes and
decreases the strength of their binding to the surface
relative to what is observed for the pure support. The
Catalyst/ , °C 30 C 150°C 210°C 280°C
T
°
ZrO
32 164 164 159
37 110 110 105
17 106 107 103
2
1% KCu PMo VWOx/ZrO
2
0.5
10
1% Ba KPMo O /ZrO
2
0.5
12 40
5% KCu PMo VWOx/ZrO
2
8
6
52
32
49
33
44
31
0.5
10
5% Ba KPMo O /ZrO
2
0.5
12 40
–1
Note: A* (cm ) is the total area of the absorption bands of nitrate
–1
complexes in the 1500–1700 cm range.
KINETICS AND CATALYSIS Vol. 52
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2011

INTERACTION BETWEEN NOx AND THE SURFACE
415
Table 3. Temperatureꢀdependent amounts of nitrate comꢀ Table 4. Effect of the composition of HPC/
θ
ꢀAl2O3 on the
plexes (
A
*) for the CeO2ꢀbased catalysts
Catalyst/ , °C 30 C 150
27 44
amount of nitrate complexes at different temperatures
T
°
°
C 210
°
C 280
°C
Catalyst/
T
, °C
30
°
C 150
°C 210
°
C 280
°C
CeO2
44
96
39
48
46
41
71
20
36
35
θ
ꢀAl O
3
4
4
5
67
68
69
71
73
68
68
68
66
56
61
63
2
3
1% KCu PMo VWOx/CeO2 36 101
0.25
10
1% H PMo O /θꢀAl O
3
12 40 2 3
5% KCu PMo VWOx/CeO2 55
43
51
55
0.25
10
1% HnPMo V O /θꢀAl O
11
1
40 2 3
5% HnPMo V O /CeO
2
62
62
11
1
40
1% HnPMo V O /θꢀAl O
9
3
40 2 3
5% HnPMo V O /CeO
2
9
3
40
–1
–1
Note: A* (cm ) is the total area of the absorption bands of nitrate
Note: A* (cm ) is the total area of the absorption bands of nitrate
–1
–1
complexes in the 1500–1700 cm range.
complexes in the 1500–1700 cm range.
The amount of nitrate complexes on the surface of
the HPCꢀloaded samples is much larger than on CeO2
even upon NОх adsorption at room temperature. This
is likely due to the high reactivity of the surface oxygen
of CeO2 and the presence o variableꢀvalence ions in
the Keggin anion.
substitution of copper ions for protons in the HPC
strengthens the nitrate complex–surface binding. The
strongest nitrate–surface binding is observed for ZrO2
;
the weakest binding, for ꢀAl2O3
θ
.
NОх adsorption causes a buildup of hydroxyl
groups on the ꢀAl2O3 and ZrO2 surfaces. This effect is
θ
The following scheme of NO interaction with the supꢀ
ported HPA is suggested for explaining the above data:
not observed for CeO2
.
Hn[PMo11V1O40]4a–ds + NO
Hn[PMo11V1NO2O39]4a–ds
IV e
4–
(I)
Hn[PMo11V1 O39]ads + NO2
IV e
4–
Hn[PMo11V1 O39]ads + Os + Hn[PMo11V1O40]4ad–s
,
(II)
(III)
NO2 + Os
NO3 ads
Scheme 1.
The increase in NO adsorption is due to NO oxidaꢀ the high activity of the surface oxygen of ZrO2 for NO
tion to NO2 on the HPC surface, and NO2 forms surꢀ
face nitrate complexes much more readily than NO.
adsorption. In this case, intensive NO adsorption
takes place without NO preoxidation to NО2 as well.
As a consequence, the decrease in the specific surface
area of the support upon the supporting of an HPC at
any concentration leads to a decrease in the amount of
NO adsorbed.
As follows from the experimental data presented
ion Fig. 5, NO is more readily oxidizable into NO2 in
the presence of heteropoly salts containing variableꢀ
valence ions, such as Cu2+. This is apparently due to the
fact that the redox properties of the copper ion favor the
stabilization of the electrons liberated in step (I).
The hydroxyl groups in the HPA show themselves
as broad absorption bands (Fig. 3); it is possible that
they are responsible for the broad peak at 2400 cm–1.
As the HPA content is increased, the absorption intenꢀ
Among the samples examined, ZrO2 and 1%
HnPMo9V3O40/CeO2 contain the largest amounts of
nitrate complexes. This fact can likely be explained by sity in this region remains approximately invariable
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

416
MATYSHAK et al.
and the intensity of the absorption band at 3370 cm–1 a buildup of hydroxyl groups on the support surface (Figs. 3,
increases. To explain the fact that NОх adsorption causes 4), we suggest the following reaction scheme:
Hn[PMo11V1O40]4a–ds + NO
Hn[PMo11V1NO2O39]4a–ds
IV e
4–
(I)
Hn[PMo11V1 O39]ads + NO2
IV e
Hs + Hn – 1[PMo12V1IVO39]a3d–s
,
4–
Hn[PMo11V1 O39]ads
(II)
(III)
Hs + O
2OHs
OHs
(IV)
(V)
H2O + Os
NO2 + Os
NO3 ads
Scheme 2.
The oxidation of NO to NО2 can be accompanied by fies the support, changing the properties of surface
a decrease in the charge of the Keggin anion (step (II) in oxygen and, accordingly, the nitrate complex–support
Scheme 2) and, accordingly, by a decrease in the numꢀ binding strength. Further evidence in favor of this supꢀ
ber of protons bound to this anion [38]. The liberated port surface modification taking place is that the
protons form an additional amount of hydroxyl groups absorption bands of the nitrate complexes on the
(step (III) in Scheme 2). It is due to this circumstance HPCꢀloaded samples are shifted by 1–2 cm–1 relative
that NОx adsorption increases the number of hydroꢀ to those of the same complexes on the pure support.
genꢀbonded hydroxyl groups on the
θ
ꢀAl2O3ꢀ and
It is interesting that the supporting of an HPA or
HPC causes a considerable buildup of hydrogenꢀ
bonded hydroxyl groups on the support (Figs. 3, 4,
3000–3700 cm–1).
According to NMR and IR spectroscopy data [39–
43] and conductivity measurements, the HPA
H3PMo12O40 has two types of protons. One is delocalꢀ
ized, hydrated, highly mobile proton. The other is not
hydrated and is less mobile. This type of proton is
directly bonded to the Keggin anion.
After supporting the HPA on the support, the
hydrated protons localize on the support surface.
Upon heat treatment (dehydration), the protons can
form hydroxyl bonds with surface oxygen. It is also
possible that protons are captured by cationic vacanꢀ
cies on the surface [44, 45]. As a result, supporting the
HPC yields rather large amounts of various kinds of
hydroxyl groups.
ZrO2ꢀsupported heteropoly cid (Figs. 2, 3, 5, 6). By
contrast, IR spectra do not indicate the appearance of
extra hydroxyl groups on CeO2. According to Qi et al.
[37], the surface oxygen of CeO2 is extraordinarily
mobile. Even vacuum treatment generates vacancies
in this material. Owing to the high reactivity of the surꢀ
face oxygen, the resulting hydroxyl groups recombine
rapidly to yield water (step (IV) in Scheme 2).
This mechanism of the interaction of NO with the
sample is likely due to the destruction of the supported
HPA. This destruction of part of the supported HPC is
indirectly indicated by the change in the binding
strength of the nitrate complexes located on the supꢀ
port surface in the presence of a small amount (1%) of
the active component. The decomposed HPC modiꢀ
A*, cm–1
The study of NO adsorption on the supported
HPCs demonstrated that, by varying the HPC amount
and composition, it is possible to control the amount
of adsorbed molecules and the adsorbate–surface
binding strength. The amount of adsorbed molecules
90
2
1
80
70
60
50
40
30
20
10
0
3
correlates with the efficiency of NO oxidation to NО2
;
the binding strength, with the modification of the supꢀ
port surface via the destruction of part of the supꢀ
ported HPC.
30
150
210
280
T, °C
ACKNOWLEDGMENTS
Fig. 6. Temperature dependence of the amount of nitrate
complexes (total area of the absorption bands of the nitrates)
This study was supported in part by the Russian
Foundation for Basic Research, project no. 08ꢀ03ꢀ
92222ꢀGFEN.
ν
3
for (
) 5% KCu PMo VWO / ꢀAl O .
2 3
1)
θ
ꢀAl O , (
2)
1% KCu PMo VWO / ꢀAl O , and
θ
2
3
0.25
θ
10
x 2 3
(3
0.25
10
x
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

INTERACTION BETWEEN NOx AND THE SURFACE
417
22. Nova, I., Lietti, L., and Forzatti, P., Catal. Today, 2008,
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