Mechanism of the formation of O2 - Radical anions on CeO2 and (0.5-10)%CeO2/ZrO2 during the adsorption of an NO-O2 mixture

Article abstract of DOI:10.1007/s10975-005-0090-z

It is established by ESR that the adsorption of an NO + O₂ mixture at 20°C on oxidized CeO₂ (O₂, T = 400-700°C) produces radical anions O ₂ ^- located both on isolated Ce⁴⁺ cations (O ₂ ^- (1)) and in associated anionic vacancies (O ₂ ^- (2)). These species differ in thermal stability. For example, O ₂ ^- (2) decomposes at 20°C, while O ₂ ^- (1) decomposes at 50°C. Only O ₂ ^- (1) species are observed at -196°C in ZrO₂-supported CeO₂. In the case of NO + O₂ adsorption at 20°C, O ₂ ^- is stabilized on Zr⁴⁺ cations and decomposes at 270°C. Increasing the cerium oxide content of the ZrO₂ surface from 0.5 to 10% only partially inhibits the formation of O ₂ ^- -Zr⁴⁺. The Zr⁴⁺ cation is shown to possess a higher Lewis acidity than the Ce⁴⁺ cation, and the ionic bond in O ₂ ^- -Zr⁴⁺ complexes is stronger than that in O ₂ ^- -Ce⁴⁺ complexes. ESR, temperature-programmed desorption, and IR spectroscopic data for various adsorption complexes of NO on CeO₂ suggest that, in the key step of O ₂ ^- formation, free electrons appear on the surface owing to the conversion of adsorbed NO molecules into nitrito chelates on coordinately unsaturated ion pairs Ce⁴⁺-O ₂ ^- .

Full text of DOI:10.1007/s10975-005-0090-z

Kinetics and Catalysis, Vol. 46, No. 3, 2005, pp. 387–395. Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 414–422.
Original Russian Text Copyright © 2005 by Il’ichev, Shibanova, Ukharskii, Kuli-zade, Korchak.
–
2
Mechanism of the Formation of O Radical Anions on CeO
2
and (0.5–10)%CeO /ZrO during the Adsorption
2
2
of an NO–O Mixture
2
A. N. Il’ichev, M. D. Shibanova, A. A. Ukharskii, A. M. Kuli-zade, and V. N. Korchak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
Received November 11, 2003
Abstract—It is established by ESR that the adsorption of an NO + O mixture at 20°C on oxidized ëÂé (é ,
2
2
2
–
2
–
2
4
+
T = 400–700°ë) produces radical anions O located both on isolated Ce cations (O (1)) and in associated
–
2
–
2
anionic vacancies (O (2)). These species differ in thermal stability. For example, O (2) decomposes at 20°ë,
–
–
while O (1) decomposes at 50°ë. Only O (1) species are observed at –196°C in ZrO -supported CeO . In the
case of NO + O adsorption at 20°ë, O is stabilized on Zr cations and decomposes at 270°ë. Increasing the
2
2
2
2
–
4
+
2
2
–
2
4
+
cerium oxide content of the ZrO surface from 0.5 to 10% only partially inhibits the formation of O –Zr . The
2
–
2
Zr4+ cation is shown to possess a higher Lewis acidity than the Ce4+ cation, and the ionic bond in O –Zr com-
4+
–
2
4
+
plexes is stronger than that in O –ë complexes. ESR, temperature-programmed desorption, and IR spec-
–
troscopic data for various adsorption complexes of NO on ëÂé suggest that, in the key step of O formation,
2
2
free electrons appear on the surface owing to the conversion of adsorbed NO molecules into nitrito chelates on
–
2
4
+
coordinately unsaturated ion pairs Ce –O .
–
2
INTRODUCTION
electrons are formed during the formation of NO and
–
NO ions through the adsorption of nitrogen oxides on
Catalytic systems containing ëÂé , ZrO , and
3
2
2
n+
2–
ëÂé –ZrO are promising for heterogeneous neutral- å –é ion pairs.
2
2
ization of nitrogen oxides NO (NO and NO ) and oxi-
–
2
x
2
To confirm the above view of the mechanism of O
formation during the adsorption of an NO + O mixture
exerts its promoting effect through supply of é to the (O (NO + O )), we studied this reaction on pure ëÂé
dation of hydrocarbons and CO in automobile and plant
exhaust. It is assumed that ëÂé in these catalysts
2
2
–
2
2
2
2
reaction [1–3]. ZrO improves the thermal stability of
2
and on (0.5–10)%ëÂé /ZrO samples, varying thermal
2 2
the catalyst and the redox properties of ëÂé [4, 5]. It
2
oxidation condition for the catalysts. NO adsorption
has recently been shown [6] that ëÂé and ëÂé –ZrO
2
2
2
species and their interaction with oxygen on ëÂé were
2
are active in the selective catalytic reduction of NOx
with hydrocarbons in excess oxygen, but their role in
the reactions is poorly understood.
also studied by ESR, temperature-programmed desorp-
tion (TPD), and IR spectroscopy.
Electron transfer between adsorbed reactants can
EXPERIMENTAL
play a significant role in NO reduction [7]. Study of
x
Cerium and zirconium oxides with a specific surface
this process on MgO showed that the coadsorption of
2
area of S = 50 m /g were obtained by decomposition
oxygen and hydrogen or weak acids (H–X) [8–12] is
sp
–
of Ce(NO ) · 6H O and ZrO(NO ) in air at 500°ë for
3
4
2
3 2
accompanied by the formation of O radical anions. In
2
2
h. The specific surface area was determined by the BET
these reactions, the first step is the dissociative adsorp-
method from low-temperature argon adsorption data.
2
surf
–
–
–
tion of H–X (X–H + O
X + OH ) and the sec-
ëÂé /ZrO samples were prepared by impregnating
2
2
–
–
+
ZrO with cerium nitrate solutions with different salt con-
ond step is electron transfer (X + é
X + O ).
2
2
2
centrations (towards a ëÂé content of (0.5–10) mol %)
2
Earlier, we studied the coadsorption of O with NO and
2
followed by drying at 100°ë and calcination at 350°ë
NO on ZrO and found that adsorbed nitrogen oxides
2
2
in air for 5 h. On the assumption that supported cerium
–
2
also initiate the formation of O on ZrO [13]. The
2
ions are uniformly distributed over the ZrO surface in
2
reaction mechanism proposed in [14] assumes that free the (0.5–10)%ëÂé /ZrO samples, the ratio of the
2
2
0
023-1584/05/4603-0387 © 2005 MAIK “Nauka /Interperiodica”

3
88
IL’ICHEV et al.
2
.018
of adsorption complexes were recorded at 20 or –196°ë
in the X range on an EPR-V spectrometer with a Dia-
pazon temperature attachment. The number of para-
magnetic species was determined from twice-inte-
grated ESR spectra using ëuSO · 5H O as the standard.
2
.035
2
.043
4
2
The accuracy of these measurements was 20%.
2
.053
Thermal desorption was measured for a sample
placed in an ESR tube. The sample was preoxidized at
DPPH
6
00°ë, and the gas was adsorbed on the sample at 20°ë
for 5 min. After adsorption, the sample was pumped for
1
1
0 min at room temperature and heated at a rate of
0 K/min under continuous pumping. Thermal desorp-
tion spectra as temperature dependences of the varia-
tion of desorbed gas pressure (w ) were recorded using
P
a Pirani gage [15]. The composition of the desorbed gas
was monitored using an MX-7304 mass spectrometer.
The amount of gas desorbed was determined as the
change in pressure by extra TPD runs in a closed reac-
tor.
2
.005
Diffuse reflectance IR spectra were recorded on a
Perkin-Elmer RX FT-IR spectrophotometer after
H
5
0 G
2.008
64 scans. For this purpose, a ëÂé powder (100 mg)
2
was placed in a quartz vacuum cell with ë‡F windows.
2
The sample was heat-treated in the same way as in the
TPD runs.
NO, ç , and O gases were obtained in vacuum
2
2
using standard procedures [16].
RESULTS
ESR Data
–
2
Formation of O during adsorption of an NO +
O mixture on oxidized ëÂO . No ESR signal is
2
2
observed upon the adsorption of NO or é (P
=
2
O2
1
02 Pa, PNO = 10 Pa, and T = 20°ë) on oxidized
ëÂO (é , 400–700°ë). An ESR signal appears after
2
2
NO and then é or their mixture NO + O is admitted
2
2
to the sample. The ESR spectrum recorded at –196°ë
2
.011
for ëÂO (é , 400°ë) after NO + O adsorption is pre-
2
2
2
sented in Fig. 1. This spectrum is similar to the spec-
–
trum of O formed on the reduced ëÂé (ç , 400°ë)
2
2
2
Fig. 1. ESR spectrum of ëÂé (é , 400°ë) after NO + O
2
2
2
sample upon oxygen adsorption. Therefore, the adsorp-
tion of an NO + O mixture on oxidized ëÂé produces
adsorption at T = 20°ë. The spectrum is recorded at –196°ë
in vacuum.
2
2
–
2
–
2
radical anions O . The ESR spectra of O on reduced
ëÂé after é adsorption were studied in detail [17].
2
2
number of cerium ions to the number of surface zirco-
nium ions is from 0.15 to 3.
Comparison of these spectra with the spectrum shown
in Fig. 1 suggests that the line with g = 2.035 and g =
1
2
–
2
.011 and the line with g = 2.035, g = 2.018, and g =
1 2 3
The formation of O (NO + O ) was studied by
2
2
–
4+
2.011 are due to O located on Ce cations in isolated
2
ESR. A 50-mg sample was placed in an ESR tube,
–
–
4
pumped to 10 Pa, heated in vacuum at a prescribed anionic vacancies (O
(1)). The line with g = 2.053,
1
2
2
temperature for 1 h, and oxidized in é (P = 10 Pa) g = 2.008, and g = 2.005 and the line with g = 2.043,
2
O2
2
3
1
–
2
3
g = 2.011, and g = 2.008 can be assigned to O on
or reduced in ç (P = 10 Pa) at this temperature for
2 3
2
H2
4
+
Ce cations belonging to aggregated anionic vacancies
O (2)).
2
0 min. Next, the sample was cooled to 20°ë, pumped,
–
2
(
and exposed to the gas. After adsorption, ESR spectra
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

–
2
MECHANISM OF THE FORMATION OF O RADICAL ANIONS ON CeO2
389
NO2 × 10–16, m–2
6
2
.009
2
.036
3
2
5
4
3
2
1
0
2
2
.032
1
2
.042
DPPH
1
3
2
.052
3
50
450
550
650
750
T, °ë
–
2
Fig. 3. Amount of O formed (1) upon NO + O adsorption
2
on oxidized ëÂé and (2, 3) upon é adsorption on ëÂé
2
2
2
that has been reduced in (2) hydrogen and (3) vacuum ver-
sus oxidation or reduction temperature.
H
5
0 G
–
the amount of O decreases monotonically from ~6 ×
2
1
6
15 –2
1
0 to 6 × 10 m as the temperature is increased from
2
.009
400 to 650°ë. In the case of heat treatment in vacuum
curve 3), O appears on ëÂé starting at a heat-treat-
–
2
(
2
Fig. 2. ESR spectra of ëÂé (é , 400°ë) after (1) admis-
2
2
ment temperature T > 470°ë.
sion of an NO + O mixture and (2, 3) subsequent pumping
2
for (2) 10 and (3) 30 min at T = 20°ë. The spectra are
recorded at 20°ë.
–
2
Formation of O (NO + O ) on oxidized
2
(
0.5-10)%ëÂé /ZrO . No ESR signal is observed
2
2
upon the adsorption of O or NO (P = 10 Pa, PO2
=
–
2
2
NO
The ESR spectrum of O (NO + O ) at 20°ë is pre-
2
5
0 Pa, T = 20°ë) on (0.5–10)%ëÂé /ZrO (é ,
2 2 2
sented in Fig. 2 (curve 1). Spectrum 1 contains lines
from O (1) and O (2). The spectral lines are well
resolved in the region of g values and overlap strongly
in the region of g and g . As the sample is pumped,
4
00−700°ë) samples. However, a signal appears when
–
–
2
2
an NO + O mixture is adsorbed. This signal can be
2
1
seen in the spectrum of the 2%ëÂé /ZrO (é , 400°ë)
2
2
2
2
3
sample (Fig. 4, curve 1). It is characterized by the fol-
O (1) and O (2) species decompose at different rates lowing components of the g tensor: g = 2.033, g
.007, and g = 2.003. Spectrum 1 remains unchanged
–
2
–
=
2
2
1
2
(
curves 1–3). After the sample is evacuated for 30 min,
3
spectrum 1 transforms into spectrum 3, which corre- upon 1-h pumping and is not observed after the sample
–
is heated at 270°ë for 10 min. The above g values and
sponds to O (1). This species decomposes upon heat-
2
–
–
high thermal stability are characteristic of O radical
2
ing at 50°ë in vacuum for 10 min. Thus, O (2) is less
2
–
2
anions formed on Zr4+ cations (O –Zr ) upon the
4+
–
stable than O (1).
2
adsorption of an NO + O mixture on ZrO [14].
2
2
As the ëÂé oxidation temperature is raised from
00 to 700°ë, the ratio of O (1) to O (2) remains vir-
2 2
2
–
–
An increase in the cerium oxide content of the cata-
lyst from 2 to 10% gives rise to a low-intensity absorp-
4
tually invariable and, as can be seen in Fig. 3 (curve 1),
the total amount of these species increases from 1.3 ×
tion line with g = 2.011 (Fig. 4, curve 2) against the
–
2
4
+
background of the intense signal of O –Zr in the ESR
1
6
16 –2
1
0 to 3 × 10 m in the temperature range 400–600°ë
–
16 –2
spectrum of O . This line can be attributed to the small
and then decreases to 2 × 10
m
at 700°ë. The
2
–
–
4
+
decrease in the amount of O on ëÂé (é , 700°ë) is amount of O radical anions adsorbed on Ce cations.
2
2
2
2
likely to be caused by agglomeration taking place on
–
2
The plots of O (NO + é ) concentration versus the
2
the sample surface.
–
cerium oxide content of the sample for different oxida-
tion temperatures are presented in Fig. 5. As the cerium
oxide content is raised from 0.5 to 2%, the O concen-
tration decreases from 2.8 × 10 to (0.5–1.0) × 10 m
regardless of the oxidation temperature. The O con-
In Fig. 3, we plot the concentration of O on the
2
reduced ëÂé surface after oxygen adsorption (P =
–
2
2
2
1
× 10 Pa, T = 20°ë) versus the temperature at which
1
6
16
–
the oxidized sample has been reduced in hydrogen or
–
2
2
,
vacuum. In the case of hydrogen reduction (curve 2),
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

3
90
IL’ICHEV et al.
–
–16
–2
NO2 × 10 , m
1
2
3
4
3
2
1
2
.033
0
2
4
6
8
10
CeO content, mol %
2
2
.007
1
–
2
Fig. 5. Amount of O on ëÂé /ZrO oxidized at (1) 400,
2
2
2
.011
(2) 500, (3) 600, and (4) 700°C versus the percentage of
supported CeO2.
Fig. 6 (curve 1). The signal is axially symmetric, with a
hyperfine structure due to the interaction between the
unpaired electron and the nuclear spin of nitrogen in the
NO molecule. The parameters of the g and A tensors for
spectrum 1 are g = 1.998, g = 1.865, A = 30 G, and
×2
2
⊥
||
⊥
A = 0, which are similar to earlier reported data [18,
|
|
1
9]. The concentration of NO molecules calculated
H
17
–2
from spectrum 1 is 0.2 × 10 m . Raising the ëÂé
preoxidation temperature from 300 to 600°ë increases
2
5
0 G
1
7
17
–2
the NO concentration from 0.2 × 10 to 7.0 × 10 m
in this case, the parameters of the g and A tensors
remain unchanged), indicating an increase in the con-
(
4
+
centration of Ce Lewis acid sites on which NO mole-
cules are adsorbed [20]. The difference between the
concentration of adsorbed NO (1.6 × 10 m ) and the NO
concentration deduced from ESR data (7.0 × 10 m ) is,
most likely, due to the formation of nonparamagnetic
dimeric NO adsorption species on the surface.
18
–2
17
–2
Figure 6 also shows the ESR spectra of NO
adsorbed on 10%ëÂé /ZrO (é , 500°ë) (curve 2) and
2
2
2
2
.003
ZrO (é , 500°ë) (curve 3) recorded under similar con-
2
2
ditions. Spectrum 2 has the same hyperfine structure as
spectrum 1. When the cerium content of the samples is
decreased, the hyperfine structure disappears and spec-
trum 2 transforms into spectrum 3. It is most likely that
spectrum 2 is complicated and is the sum of the ESR
Fig. 4. ESR spectra of ëÂé /ZrO (é , 500°ë) samples
2
2
2
with a ëÂé content of (1) 2 and (2) 10% after NO + é
2
2
adsorption and pumping at 20°ë.
4
+
4+
1
6
–2
spectra of NO adsorbed on Ce and Zr cations. Spec-
centration remains in the (0.5–1.0) × 10 m interval
4
+
trum 3 is due to NO molecules adsorbed on Zr cat-
as the CeO content is further increased in the range 5–
2
ions.
1
0%.
The absence of a hyperfine structure in spectrum 3
is not related to line broadening due to the dipole–
dipole interaction between adsorbed NO molecules:
Thus, in the oxidized (0.5–10)%ëÂé /ZrO samples
2
2
–
at 20°ë, O (NO + O ) radical anions are mainly stabi-
2
2
lized in the coordination sphere of Zr4+ cations. Sup- reducing the amount of adsorbed NO by a factor of
ported ëÂé partially inhibits the formation of 10−20 decreases the intensity of the ESR signal of NO
2
–
2
O (NO + O ) on ZrO .
but no hyperfine structure appears [14]. According to
earlier data [20], the presence of a hyperfine structure in
2
2
Paramagnetic NO adsorption species and its
interaction with O on ëÂé and ëÂé /ZrO . The
the ESR spectra of NO adsorbed on ëÂé and
2
4+
2
2
2
2
1
0%ëÂé /ZrO indicates a weak acidity of the Ce
2 2
ESR spectrum of NO adsorbed on ëÂé (é , 300°ë) at
2
2
ions, while its absence in the ESR spectrum of NO on
1
9
4+
–
196°ë (Nads = 6 × 10 molecule/g) is presented in Zré suggests that the Zr ions are strongly acidic.
2
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

–
2
MECHANISM OF THE FORMATION OF O RADICAL ANIONS ON CeO2
391
The ESR spectra of NO adsorbed on ëÂé2,
0%ëÂé /ZrO , and ZrO at –196°ë are stable and
1.998
1
2
2
2
remain unchanged upon pumping of the samples for
h. The intensity of the spectra decreases as the sam-
A =30 G
1
⊥
ples are heated without pumping. The disappearance of
the spectra between –150 and –80°C is accompanied by
NO desorption into the sample bulk. Subsequent cool-
ing of the sample to –196°ë results in NO adsorption,
and the ESR signal of NO is completely recovered.
When oxygen (P = 50 Pa, T = –196°ë) is admitted
to a ëÂé (é , T = 400–600°ë) sample containing
2
2
adsorbed NO, the ESR signal of NO disappears and an
–
2
16
–2
ESR signal from O appears ((1.2–2.5) × 10 m ),
whose spectrum is similar to that presented in Fig. 1.
1
Pumping the sample has no effect on the shape or inten-
–
sity of the ESR spectrum of O and does not bring
2
about an ESR spectrum of NO. After subsequent addi-
–
tion of NO, the ESR signal of O disappears and the
2
1.865
ESR signal of NO is completely recovered. After the
2
3
sample is pumped, adding é results in the disappear-
2
–
2
ance of the ESR signal of NO but no ESR signal of O
appears. The adsorption of NO on the pumped sample
again recovers the ESR signal of NO. The reversible
recovery and disappearance of the ESR signal of NO
during the adsorption of NO and O , respectively, with-
2
–
2
out O formation indicate that the paramagnetic NO
H
adsorption species is not involved in the formation of
–
1
00 G
O (NO + O ) on ëÂé . Earlier, we drew a similar con-
2
2
2
clusion for the adsorption of NO and é on ZrO [14].
2
2
1
.935
On the one hand, the disappearance of the NO signal
upon admission of oxygen to the sample is, probably,
due to the displacement of NO molecules by oxygen
from the surface to the gas phase and the removal of NO
Fig. 6. ESR spectra of NO adsorbed at −196°ë on (1)
CÂé (é , 300°ë), (2) 10%ëÂé /ZrO (O , 500°ë), and
2
2
2
2
2
(
3) ZrO (O , 500°ë).
2 2
–
by pumping. On the other hand, the interaction of O2
Because the paramagnetic NO adsorption species is
with NO on the surface probably produces diamagnetic
–
–
inactive in O (NO + O ) formation, NO complexes on
2
2
complexes (NO–O ), which are not observed in the
2
ëÂé were studied by TPD and IR spectroscopy.
2
ESR spectrum.
The interaction between adsorbed NO and é on
2
1
0%ëÂé /ZrO (é , 400°ë) at –196°ë also results in the
IR Spectroscopic Data
2
2
2
disappearance of the signal from NO and in the appear-
ance of the signal presented in Fig. 7 (curve 1). After
the sample is heated to T = 20°ë under continuous
The IR spectra of adsorption complexes on
ëÂé (é , 600°ë) at 20°ë are presented in Fig. 8.
2
2
These spectra were obtained after NO adsorption fol-
pumping, spectrum 1 transforms into spectrum 2,
–
2
lowed by pumping of the sample at different tempera-
4+
which is due to O –Zr . The subtraction of spectrum 2
3
tures for 10 min. For instance, after NO (P = 10 Pa) is
from spectrum 1 gives spectrum 3 with g-tensor param-
eters of g = 2.036 and g = 2.011, which are character-
–1
let in, absorption bands due to N O (2241 and 1252 cm )
2
–
1
2
and chelates of the nitrite ion NO (1275, 1166, and
24 cm ) are observed in the spectrum (curve 1) [18,
19]. After the sample is pumped at 20°ë, the absorption
–
2
4
+
istic of O located on Ce cations in isolated anionic
2
–1
8
–
vacancies. Therefore, at −196°C, O (NO + O ) radical
2
2
–
anions are formed on the 10%ëÂé /ZrO sample, bands of N O disappear while those of NO remain
2
2
2
2
being stabilized both on particles of the ëÂé phase
2
unchanged (curve 2). Heating the sample in vacuum at
100°ë exerts no effect on the spectrum of nitrites
and on the surface of the ZrO support.
2
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

3
92
IL’ICHEV et al.
1
Spectrum 2 illustrates the desorption of NO that has
3
been adsorbed at 20°ë and P = 10 Pa. The low-temper-
ature region (40–180°ë) contains a peak with two max-
1
7
–2
2
ima at Tmax = 70 and 100°ë (NNO = 1.2 × 10 m ).
Between 200 and 520°C, there are two peaks at T
=
max
2
60 and 480°C. The NO concentrations corresponding
2
to these peaks are approximately the same and are equal
1
7
–2
to 3.3 × 10 m . At 400–520°ë, this sample desorbs
the same amount of oxygen as the oxidized sample.
1
It follows from the TPD data that only stable nitrito
complexes are formed on the oxidized ëÂé surface
2
upon NO adsorption at 20°ë. Approximately 50% of
them decompose with NO desorption at ~300°ë, and
the other 50% turn into surface nitrates decomposing at
2
.036
2
.014
~
480°ë. The nature of the NO complexes decomposing
3
at 70–100°ë remains unclear. They may be the two
hypernitrite species that are assumed [19] to be inter-
mediates in the formation of N O. The absorption bands
2
–1
due to these hypernitrites (1350, 1015, and 954 cm for
H
2
2
–
2–
2
–1
cis-N O and 1105 cm for trans-N O ) were not
2
2
observed (Fig. 8), probably because of the low concen-
tration of these species on the surface.
2
.011
DISCUSSION
Fig. 7. ESR spectra of 10%ëÂé /ZrO (é , 500°ë) after
2
2
2
–
the (1) interaction between adsorbed NO and O at –196°ë
Mechanism of O (NO + O )
2
2
2
and (2) subsequent pumping at 20°ë and cooling to
Formation on Oxidized ëÂé2
−
196°ë. (3) Differential spectrum obtained by subtracting
spectrum 2 from spectrum 1.
Oxygen adsorption on ëÂé that has been reduced
2
–
2
in hydrogen or vacuum is known to produce O radical
3
+
anions through electron transfer from Ce to adsorbed
(
curve 3), while heating at 300°ë results in the disap-
pearance of the absorption bands of nitrites and in the
appearance of absorption bands at 1554, 1222, and are not detected by ESR in the reduced ëÂé
3
+
é
[17]. Ce cations are paramagnetic; however, they
and
ëÂé /ZrO samples at –196 and 20°ë, probably
2
2
–
1
1
013 cm , assignable to monodentate or bidentate
2
2
–
because of the short time of spin–lattice relaxation.
Ce cations are also absent from oxidized ëÂé and
nitrates NO (curve 4) [21]. The nitrates decompose in
3
3
+
2
vacuum at 480°ë.
ëÂé /ZrO because é adsorption on these materials
2
2
2
–
2
–
2
produces O . The formation of O (NO + O ) on oxi-
2
TPD Data
dized ëÂé indicates that NO adsorption complexes are
involved in this reaction as electron donors.
Let us consider the mechanism of this reaction in
more detail. We believe that active sites in this reaction
2
Oxygen (curve 1) and NO (curve 2) desorption
spectra for oxidized ëÂé (é , 600°ë) are presented in
2
2
Fig. 9. It follows from the shape of spectrum 1 that,
after this specimen is treated with oxygen at 20°ë and
4+
2–
are coordinately unsaturated ions Ce and O formed
by surface dehydroxylation. This assumption is favored
P = 2 × 10 Pa, é is desorbed at T = 470°ë. A sim- by the fact that the concentration of O (NO + O )
2
–
2
max
2
2
ilar spectrum was recorded when ëÂé (é , 600°ë)
was heated without oxygen treatment. This suggests
that at 20°ë oxygen does not form stable adsorption
2
2
increases with increasing ëÂé oxidation temperature
2
(Fig. 3, curve 1).
Depending on the NO adsorption temperature, dif-
ferent surface compounds form on the coordinately
species on oxidized ëÂé . The é desorption tempera-
2
2
ture is equal to the temperature of ëÂé reduction in
4+
2–
4+
2
unsaturated ions Ce and O : NO–Ce (–196°ë),
–
vacuum (Fig. 3). The amount of desorbed é converted nitrites
, nitrous oxide N O (20°ë), and nitrates
2
2
NO
2
1
7
–2
–
to surface concentration is NO2 = 1 × 10 m and does
NO (300°ë). The participation of the compounds
not exceed 2% of the amount of O ions on the surface. NO–ë and NO in this reaction seems to be improb-
3
–
3
2
–
4+
Apparently, é desorption is due to the removal of
interstitial oxygen from the Ce cations.
2
able for the following reasons. For the first compound,
é adsorption at –196°ë is accompanied by the decom-
2
4
+
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

–
2
MECHANISM OF THE FORMATION OF O RADICAL ANIONS ON CeO2
393
Absorbance
1
166
1
2
1
116
1
252
2
241
1405
8
24
1
275
1
368
3
4
1
013
1
554
1
222
2
800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800
–
1
ν, cm
Fig. 8. IR spectra of ëÂé (é , 600°ë) after (1) admission of NO at 20°ë and (2–4) subsequent pumping at (2) 20, (3) 100, and (4)
2
2
3
00°ë.
–
2
–
2
position of NO–ëÂ4+ and by the displacement of NO Since the ESR spectra of O (NO + O ) and O (O ) on
2
2
–
the oxidized and reduced ëÂé samples are similar, it
is evident that the O (NO + O ) species are stabilized
above the O (NO + O ) formation temperature (20°C). in the coordination sphere of Ce cations near nitrito
into the sample bulk without O formation. In the sec-
2
2
–
ond case, nitrates result from nitrites at 300°ë, well
2
2
–
4+
2
2
–
2
Nitrous oxide is inactive in the reaction because no O
–
2
results from the coadsorption of N O and O on ëÂé .
2
2
w , arb. units
P
The appearance of free electrons upon the formation of
chelate nitrites seems to be most probable. According
to an earlier report [19], chelate nitrite results from
interaction between an NO molecule and a coordinately
1
8
1
6
14
4
+
2–
unsaturated ion pair ë –é followed by electron
1
1
2
0
8
6
4
2
2
1
transfer from the NO molecule to the surface with
4
+
3+
reduction of the Ce cation to Ce (reaction (I)). The
reduction of cations upon nitrite formation is indicated
by the appearance of electrons in the conduction band
–
2
[
22]. In this case, O formation is due to adsorbed é
2
molecules capturing electrons from the conduction
band (reaction (II)).
0
200
400
600
T, °ë
–
N
O
O
Ce3+
–
Fig. 9. (1) O and (2) NO temperature-desorption spectra
4
+
2–
2
NO + Ce –O ⇒
,
(I)
for ëÂé (é , 600°ë). Adsorption conditions: 20°C, P =
O2
2
2
3
+
4+
2
3
Ce + O ⇒ CÂ –O .
(II)
2 × 10 Pa, P = 10 Pa.
NO
2
2
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

3
94
IL’ICHEV et al.
–
2
the bond is covalent rather than ionic. This covalent
complexes. Probably, the formation of O (NO + O ) on
2
bond results from the overlap of the occupied π orbital
ZrO proceeds by a similar mechanism [14].
y
2
of oxygen and unoccupied 4f orbitals of the cerium cat-
ion [24].
–
2
–
2
4+
Inhibition of O (NO + O ) Formation
The covalent bond in the O –ë complexes is
2
–
on ZrO by ëÂé
4+
2
2
weaker than the ionic bond in the O –Zr complexes:
2
–
2
4+
the O –ë complexes decompose below 50°ë,
It was demonstrated [23] that 2 mol % MoO sup-
ported on ZrO completely inhibits the formation of whereas the O
O (NO + O ) on ZrO through immobilization of Mo
cations on surface O ions. According to the data pre-
sented in Fig. 5, this reaction is not completely inhib-
ited by ëÂé even when the concentration of supported
Ce ions exceeds the surface concentration of Zr cat-
3
–
2
4+
–Zr complexes decompose at 270°ë.
In this case, we can assume that Zr cations are more
acidic than Ce cations. This assumption is also sug-
gested by the analysis of the hyperfine structures of the
ESR spectra of NO adsorbed on ëÂé and ZrO at
2
–
2
4+
6
+
2
2
–
4+
2
2
2
2
−
196°ë (Fig. 6).
4
+
4+
ions by a factor of 3, as in the 10%ëÂé /ZrO sample.
2
2
We believe that the fact that the activity of ZrO2
decreases as the percentage of supported cerium oxide
increases from 0.5 to 2% is due to single cerium oxo
complexes and dimeric oxide clusters covering the sur-
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e
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2
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