ESR study of the formation of O2- radical anions on oxidized CeO2 and CeO2/ZrO2 adsorbing a CO + O2 mixture

Article abstract of DOI:10.1007/s10975-005-0091-y

It is demonstrated by ESR measurements that O ₂ ^- (CO + O₂) radical anions result from CO + O₂ adsorption on the oxidized surface of CeO₂. These radical anions are stabilized in the coordination sphere of Ce⁴⁺ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior determined by CO adsorption. The variation of O ₂ ^- (CO + O₂) concentration with CO adsorption temperature suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5- 10.0)%CeO₂/ZrO₂ system, O ₂ ^- forms on supported CeO₂ and is stabilized on Ce⁴⁺ and Zr ⁴⁺ cations. The stability of O ₂ ^- -Ce ⁴⁺ complexes is lower on supported CeO₂ than on unsupported CeO₂, indicating a strong interaction between the cerium cations and the support.

Full text of DOI:10.1007/s10975-005-0091-y

Kinetics and Catalysis, Vol. 46, No. 3, 2005, pp. 396–402. Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 423–429.
Original Russian Text Copyright © 2005 by Il’ichev, Kuli-zade, Korchak.
ESR Study of the Formation of O–2 Radical Anions on Oxidized
CeO2 and CeO2/ZrO2 Adsorbing a CO + O2 Mixture
A. N. Il’ichev, A. M. Kuli-zade, and V. N. Korchak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
Received January 26, 2004
Abstract—It is demonstrated by ESR measurements that O– (ëé + é2) radical anions result from CO + O2
2
adsorption on the oxidized surface of ëÂé2. These radical anions are stabilized in the coordination sphere of
Ce4+ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior
determined by CO adsorption. The variation of O– (ëé + é2) concentration with CO adsorption temperature
2
suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5–10.0)%ëÂé2/ZrO2
system, O–2 forms on supported ëÂé2 and is stabilized on Ce4+ and Zr4+ cations. The stability of O–2 –ëÂ4+
complexes is lower on supported ëÂé2 than on unsupported ëÂé2, indicating a strong interaction between the
cerium cations and the support.
INTRODUCTION
CO + é2 adsorption. Here, we report on this reaction,
for it is important for understanding the mechanism of
CO oxidation and O2– formation upon NOx + O2 adsorp-
Catalytic systems based on ëÂé2, ZrO2, and ëÂé2–
ZrO2 are promising for heterogeneous selective cata-
lytic reduction of NOx (x = 1, 2) with hydrocarbons in tion on ëÂé2, ZrO2, and CeO2–ZrO2.
excess oxygen and for neutralization of NOx, CO, and
We studied the formation of O–2 radical anions upon
hydrocarbons [1–3]. Although these reactions have
been the subject of numerous studies, their mechanism
remains disputable [1, 4].
CO + é2 adsorption on oxidized CeO2 (O–2 (ëé + é2))
at sample oxidation temperatures of 300–700°ë, under
different adsorption conditions, and for different
amounts of ëÂé2 supported on ZrO2. The interaction of
Assuming that NOx and é2 can be activated on
ëÂé2 and ZrO2, we studied the adsorption of NOx + O2
O–2 with molecules NO, NH3, and CO was also studied.
mixtures on these oxides and found radical anions O2–
on their surface. These radical anions do not form dur-
ing é2 adsorption in the absence of NOx [5–7]. Investi-
EXPERIMENTAL
gation of the mechanism of O–2 (NO + O2) formation on
ëÂé2 and ZrO2 demonstrated that NO interacts with
Cerium and zirconium oxides with a specific surface
area of Ssp = 50 m2/g were obtained by decomposing
Ce(NO3)4 · 6H2O and ZrO(NO3)2 in air at 500°ë for
2 h. The specific surface area was determined by the
BET method from low-temperature argon adsorption
data.
å4+–é2– (å4+ = Zr4+, Ce4+) sites to form å4+–NO22–
adsorption complexes in which the cation charge then
changes from 4+ to 3+ through electron transfer from
NO22– to M4+. Subsequent interaction between M3+ and
an é2 molecule gives O–2 on the M4+ cation [6, 7].
ëÂé2/ZrO2 samples were prepared by impregnating
ZrO2 with cerium nitrate solutions with different salt con-
centrations (towards a ëÂé2 content of (0.5–10.0) mol %)
followed by drying at 100°ë and calcination at 350°ë
in air for 5 h. On the assumption that supported cerium
ions are uniformly distributed over the ZrO2 surface in
the (0.5–10.0)%ëÂé2/ZrO2 samples, the ratio of the
number of cerium ions to the number of surface zirco-
nium ions is from 0.15 to 3.
Radical anions O–2 are known to be intermediates in
the low-temperature oxidation of CO on cerium oxide
and cerium-containing catalysts [8, 9]. This allowed us
to assume that O–2 (Né + é2) is involved in CO oxida-
tion upon adsorption of a CO + NO + é2 mixture on
ëÂé2. To confirm this assumption, we studied the influ-
ence of preliminarily adsorbed CO on the formation of
O–2 (NO + é2) on ëÂé2 and found that CO adsorption
The formation of O–2 (ëO + O2) on oxidized ëÂé2
and ëÂé2/ZrO2 was studied by ESR. A 50-mg sample
increases the amount of O–2 by a factor of ~3. This
effect turned out to be due to the formation of O–2 upon
was placed in an ESR tube, pumped to 10–4 Pa, heated
in vacuum at a preset temperature for 1 h, and oxidized
0023-1584/05/4603-0396 © 2005 MAIK “Nauka /Interperiodica”

ESR STUDY OF THE FORMATION OF O–2 RADICAL ANIONS
397
in é2 (PO = 103 Pa) for 20 min. Next, the sample was
2
cooled to 20°ë, pumped, and brought in contact with
the gas. The amount of molecules adsorbed on the sam-
ple was determined from the variation of the gas pres-
sure in a closed reactor during the adsorption or desorp-
tion of preliminarily adsorbed gas using a Pirani gage
[10]. After adsorption, ESR spectra of adsorption com-
plexes in the X range were recorded at 20 or −196°ë on
an EPR-V spectrometer with a Diapazon temperature
attachment. The amount of paramagnetic species was
determined from twice integrated ESR spectra using
ëuSO4 · 5H2O as the standard. The accuracy of these
×3.3
×1.6
1
2
2.015
2.029
2.037
2.045
2.052
DPPH
3
measurements was 20%.
CO, NO, and O2 were obtained in a vacuum using
standard procedures [11].
H
50 G
RESULTS AND DISCUSSION
1. Formation of O2– on ëÂé2 upon CO + é2 Adsorption
After CO or é2(20°ë) is admitted to oxidized
ëÂé2(400–700°ë), no ESR signal is observed. A sig-
nal appears after é2 adsorption (P = 10 Pa) on a sample
that was preliminarily exposed to CO (P = 103 Pa, t =
1–3 min) and then pumped at 20°ë. The ESR signal
from the ëÂé2(400°ë) sample falls in a g-factor range
of 2.037–2.009 (Fig. 1, curve 1). As the ëÂé2(500–
600°ë) oxidation temperature increases, the intensity of
the ESR signal increases and a new line appears at g =
2.045 (Fig. 1, curve 2). For better resolution, the ESR
spectrum of ëÂé2(600°ë) was recorded at –196°ë in
vacuum (Fig. 1, curve 3). The g factors derived from
spectrum 3 are characteristic of the ESR spectrum of
2.009
Fig. 1. (1, 2) ESR spectra recorded at 20°ë for ëÂé
2.011
2
preaged in é at (1) 400 and (2) 600°ë after CO adsorption
2
3
(P = 10 Pa) for 3 min, pumping, and admission of é (P =
2
10 Pa) at 20°ë. (3) Spectrum recorded after sample 2 is
cooled to –196°ë and pumped.
O–2 , which is detected upon oxygen adsorption on
reduced ëÂé2 preaged in vacuum at T = 450°C for 1 h
or in ç2 at P = 103 Pa and T = 400°ë for 10 min.
According to published data [12], the line with g1 =
2.029, g2 = 2.016, and g3 = 2.011 and the line with g1 =
2.037 and g2 = 2.011 in spectrum 3 can be assigned to
radical anions O–2 located in the coordination sphere of
is invariable in time. However, the total concentration
of O–2 located on both isolated and associated Ce4+ cat-
ions decreases to 2.0 × 1016 m–2 as the sample is
pumped for 50 min. Readsorption of é2 or CO + é2
does not increase the amount of O–2 . O2– radical anions
Ce4+ cations in isolated anionic vacancies. The line with
decompose completely upon heating of the sample in
vacuum at 100°ë for 10 min. ëÂé2 regains its activity
g1 = 2.052, g2 = 2.009, and g3 = 2.006 and the line with
in the formation of O2– (ëé + é2) on being oxidized at
T ≥ 400°ë.
g1 = 2.045, g2 = 2.011, and g3 = 2.009 are due to O–2 rad-
ical anions adsorbed on cations in associated anionic
vacancies with an increased electron density. It is
The amount of O–2 (ëé + é2) as a function of ëÂé2
assumed that O–2 can be stabilized on Ce3+ cations.
oxidation temperature was studied as follows. First,
ëÂé2 was oxidized at a chosen temperature. Next, CO
Thus, we believe that the adsorption of CO + é2 on
oxidized ëÂé2 produces O–2 (ëé + é2) radical anions
stabilized on isolated and associated anionic vacancies.
Associated sites are observed on the surface after ëÂé2
oxidation in oxygen at T > 400°C.
O–2 (ëé + é2) radical anions on ëÂé2(600°ë) are
stable at –196°ë. Their concentration on the surface in
an oxygen medium at 10 Pa and 20°ë (5.5 × 1016 m–2)
(P = 103 Pa) was adsorbed onto the sample over 3 min,
the sample was pumped, é2 (10 Pa) was let in, and the
ESR spectrum of O–2 was recorded at 20°ë. Next, the
ëÂé2 oxidation temperature was increased and O–2 was
formed on the CeO2 sample under the same CO and é2
adsorption conditions. The plot of the amount of
O–2 (ëé + é2) versus ëÂé2 oxidation temperature is
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

398
IL’ICHEV et al.
N(O2–) × 10–18, g–1
concentration is observed as the é2 pressure is
3.5
increased from 1 to 10 Pa at Pëé = 1 × 103 Pa.
The amount of O–2 (ëé + é2) on ëÂé2 depends on
adsorption temperature. For example, upon successive
adsorption of CO and é2 in amounts of (5 × 1019)–
(5 × 1020) molecule/g at −196°ë, O–2 is not observed on
3.0
2.5
2.0
1.5
1.0
0.5
0
ëÂé2(600°ë). However, O–2 radical anions do form
when CO is adsorbed onto the sample at 20°ë prior to
oxygen adsorption at –196°ë.
The variation of O–2 concentration on ëÂé2(600°ë)
as a function of CO adsorption temperature (TCO)
between 20 and 200°C was studied as follows. First,
CO (P = 1 × 103 Pa) was adsorbed onto a preoxidized
sample at a chosen temperature Tëé for 5 min. After
adsorption, the sample was cooled in CO to 20°ë and
pumped. Next, é2 (P = 10 Pa) was admitted to the sam-
300
400
500
600
700
T, °C
Fig. 2. O– (CO + O ) concentration as a function of ëÂé
2
2
2
oxidation temperature. For O–2 formation conditions, see
the text.
ple and the ESR spectrum of O–2 was recorded. The
sample was then pumped and heated in vacuum (with-
presented in Fig. 2. As the temperature increases from out pumping) to 600°ë, and the amount of desorbed
300 to 650°ë, the amount of O–2 increases from zero to
gas was determined. After desorption, the sample was
oxidized in é2 at 600°ë and CO and é2 adsorption was
repeated in a similar way but at another Tëé. It can be
seen from the data for CeO2(600°C) in Table 1 that, as
Tëé increases from 20 to 200°ë, the concentration of
O–2 increases from 5 × 1016 to 10 × 1016 m–2, accompa-
nied by an increase in the amount of gas desorbed after
O–2 (ëé + é2) formation (n(CO + O2)) and in the num-
3 × 1018 g–1, indicating an increase in the number of
active sites involved in O–2 formation. As the oxidation
temperature is further raised to 700°ë, this number
decreases to 1.5 × 1018 g–1. It does not regain its initial
value after the preaging temperature is again lowered to
600°ë. This irreversible decrease in the amount of O2–
is probably caused by a decrease in the surface area of
the sample at T ≥ 650°ë [13].
ber of desorbed CO molecules. The latter quantity was
measured after CO was adsorbed onto ëÂé2(600°ë) at
P = 103 Pa and Tëé = 20–200°ë for 5 min.
The amount of O–2 on ëÂé2 depends on the pressure
Thus, the formation of O– (ëé + é2) shows an acti-
of adsorbable gases (CO and é2). For example, at
2
20°ë, raising the CO pressure from 10 to 5 × 102 Pa at
vation behavior determined by the temperature depen-
dence of CO adsorption rather than by O–2 formation
upon é2 adsorption on ëÂé2. The finding that the con-
centrations of O–2 and adsorbed CO increase with
increasing CO adsorption temperature indicates that
CO adsorption complexes formed at 20–200°ë are
involved in the reaction.
a constant oxygen pressure of 10 Pa increases the con-
centration of O–2 radical anions on ëÂé2 (600°ë) from
1.5 × 1016 to 6.0 × 1016 m–2. A similar increase in O2–
Table 1. O– (CO + O2) concentration as a function of CO
2
adsorption temperature for CeO2 (600°C)
2. Interaction of O–2 (ëé + é2)
with é2, NO, NH3, and CO Molecules
[O2] × 10–16
,
n(CO + O2)* ×
n(CO)** ×
T, °C
m–2
10–16, m–2
10–16, m–2
Upon admission of oxygen (P = 103 Pa) into the
20
100
200
5.0
7.7
7.1
11.0
31.0
3
5.6
7.0
ëÂé2(600°ë) sample, the ESR signal of O–2 (ëé + é2)
decreases by a factor of 1.5 and transforms into a sin-
glet line at ∆Hm‡ı = 34 G. Pumping the sample for 2–
3 min recovers the initial shape and intensity of this sig-
nal. These reversible changes in the ESR spectrum of
10.0
16.0
Note: Experimental conditions: P = 10 Pa; P = 10 Pa; O
CO
2
O2
adsorption temperature, 20°C.
O–2 are evidence of a dipole–dipole interaction between
oxygen molecules and the radical anions.
* Concentration of gas desorbed from the CeO sample at
2
600°C after the formation of O– (CO + O ) on the surface.
2
2
Upon admission of NO (P = 10 Pa, T = 20°ë), the
** Concentration of gas desorbed from the CeO sample at
2
ESR signal of O–2 disappears completely and is not
600°C after CO adsorption in the absence of oxygen.
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

ESR STUDY OF THE FORMATION OF O–2 RADICAL ANIONS
399
2.050
2.040
DPPH
3
2.064
2
1
2.016
2.029
2.037
2.045
2
1
2.029
2.022
2.037
2.050
2.052
H
50 G
H
50 G
2.009
2.010
Fig. 3. ESR spectra of O2– on ëÂé at 20°ë (1) in vacuum
2.011
2
Fig. 4. ESR spectra of O2– on ëÂé at –196°ë (1) in vac-
18
18
2
and (2, 3) after the adsorption of (2) 2 × 10 and (3) 7 × 10
18 –2
–2
uum and (2) after the adsorption of 1 × 10
m
CO.
m
of NH .
3
complexes and in a decrease in the amount of O–2 on the
surface.
recovered by pumping the sample for 1 h. This is likely
to be due to the formation of rather stable nonparamag-
netic (NO–O–2 ) complexes on the surface.
Since the authors of several studies assume that O2–
is involved in CO oxidation, we studied the interaction
between CO and O–2 (ëé + é2) at various tempera-
The interaction of ammonia with O–2 at 20°ë
reduces the ESR signal of O–2 and changes the structure
tures. The ESR spectra of O–2 (ëé + é2) on a
ëÂé2(600°ë) sample at –196°ë in a vacuum (curve 1)
of the spectrum. As can be seen from the data presented
in Fig. 3, as the amount of adsorbed NH3 increases from
and after CO admission (5 × 1019 g–1) at –196°ë
(curve 2) are presented in Fig. 4. Comparing spectra 1
2 × 1018 to 7 × 1018 m–2, the intensity of the ESR lines
of O–2 with g1 = 2.037 and 2.029 decreases and the
and 2 shows that the interaction between O–2 and
adsorbed CO results in a shift of the ESR line of O2–
intensity of the line with g = 2.050 increases (see curves
2 and 3). The fact that the intensity of this line increases
by a factor of 2 as the total concentration of O2–
decreases from 6 × 1016 to 3 × 1016 m–2 apparently indi-
with g1 = 2.052–2.029 to 2.064–2.040. The numbers of
spins determined from spectra 1 and 2 are almost the
same. Heating this sample to –150°ë causes desorp-
cates that the lines with g1 = 2.037 and 2.029 shift to tion of all the CO, and spectrum 2 transforms into
spectrum 1.
The increase in g1 in the ESR spectrum of O–2 upon
higher g factors.
According to the ionic model [14], g1 increases if
ammonia adsorption changes the coordination sphere
of the Ce4+ cations by weakening the crystal field due to
the Ce4+ cation on the adsorbed O–2 radical anion. If this
is the case, the distance between the cation and the oxy-
gen radical anion in the ëÂ4+–O2– complex also elon-
gates, which can result in the decomposition of the
physical adsorption of CO can be due to the effect of
adsorbed CO molecules on the coordination sphere of
the cation in the ëÂ4+–O2– complexes, as in the case of
the interaction between NH3 and ëÂ4+–O2– .
Upon admission of CO (Pëé = 103 Pa) at 20°ë, the
ESR spectrum of O–2 disappears within 1 min. The
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

400
IL’ICHEV et al.
Since the rate of O–2 decomposition at 20°ë in vac-
2.033
2.037
2.007
4
3
2
uum is much lower than the rate of O–2 conversion in a
2.012
2.023
CO atmosphere, we can assume that CO displaces O2–
2.003
from the coordination sphere of the cation or reacts with
it to form ëé2 [8, 9]. The latter assumption is more plau-
sible because the O–2 concentration remains unchanged
after Ar (5 × 102 Pa) is admitted to the sample.
2.029
In the oxidation of CO to ëé2 with O–2 radical
anions, the ëÂé2 surface is reduced and its interaction
with oxygen yields O2– . The decrease in O–2 concentra-
tion resulting from several cycles of CO oxidation with
O–2 radical anions is likely to be due to ëé2 molecules
1
H
50 G
blocking the active sites responsible for O–2 (ëé + é2)
formation. These active sites are regenerated at T ≥
400°ë.
2.003
3. Formation of O–2 (ëé + é2)
on (0.5–10.0)%ëÂé2/ZrO2
2.012
2.007
Fig. 5. ESR spectrum of 10%ëÂé /ZrO (600°ë) (1) after
As in the case of ëÂé2, the adsorption of CO or é2
(20°ë) on oxidized (0.5–10.0)%ëÂé2/ZrO2(500°ë)
produces no ESR signal. A signal appears upon oxygen
adsorption (P = 10 Pa) on specimens with preliminarily
adsorbed CO (P = 103 Pa, t = 3 min). Figure 5 (curve 1)
shows this signal for 10.0%ëÂé2/ZrO2. As this sample
is pumped for 2–3 min, the lines with g = 2.028 and
2.023 disappear and the intensity of the line with g =
2.012 decreases (curve 2). Admission of oxygen recov-
2
2
3
CO adsorption (P = 10 Pa) for 3 min, pumping, and admis-
sion of é (P = 10 Pa) at 20°ë. (2–4) The same spectrum
2
recorded after pumping of é for (2, 3) 3 min at (2) 20 and
2
(3) 50°ë and (4) 10 min at 100°ë.
spectrum is not recovered by pumping the sample and
appears only after admission of oxygen (10–102 Pa) but
with a lower intensity as compared to the initial spec- ers these lines. The intensity of these lines depends on
oxygen pressure, and their g values are close to the g
trum. Thrice repeated treatment of the sample with CO
values for O–2 on ëÂé2 (g1 = 2.028, g2 = 2.016, and g3 =
and then é2 reduces the concentration of O–2 radical
2.011); therefore, they can be assigned to O–2 radical
anions from 6 × 1016 to 2 × 1016 m–2, indicating a
decrease in the reactivity of ëÂé2 under these condi-
tions. The reactivity is completely recovered after
10-min oxidation of ëÂé2 in é2 at 400–600°C.
anions located on supported Ce4+ cations. The finding
that this type of O–2 radical anion is less stable than the
O–2 radical anion on ëÂé2 indicates a strong interaction
between the ZrO2 support and the Ce4+ cations [15].
Spectrum 2 in Fig. 5 is complicated and consists of
two ESR signals. The first is characterized by g values
of 2.037 and 2.012. It disappears upon heating of the
sample in vacuum at 100°ë (curves 3, 4). Therefore, it
is due to O–2 radical anions in the coordination sphere
of Ce4+. The second signal, characterized by g1 = 2.033,
g2 = 2.007, and g3 = 2.003, disappears only after heating
of the sample at 270°ë for 10 min and is attributed to
O–2 radical anions stabilized on Zr4+ cations [6, 7].
Table 2. O–2 (CO + O2) concentration (×10–16 m–2) as a func-
tion of the CeO2 content of CeO2/ZrO2(500°C) after adsorp-
tion of CO (P = 103 Pa) and O2 (P = 10 Pa) at 20°C
O–2 concentration
Total concentra-
CeO2 content
of the sample, %
tion of O–2
on Ce4+
0
0
–
0.5
2.0
5.0
10.0
0.02
0.30
1.40
2.0
–
The data presented in Table 2 demonstrate how the
total concentration of O–2 depends on the ëÂé2 content
of ëÂé2/ZrO2(500°ë) obtained by adsorption of CO
(P = 103 Pa, t = 3 min) and é2 (P = 10 Pa) at 20°ë.
O–2 (ëé + é2) does not form on pure Zré2(500°ë). For
0.15
0.80
1.60
5.0
100
5.0
KINETICS AND CATALYSIS Vol. 46 No. 3 2005

ESR STUDY OF THE FORMATION OF O–2 RADICAL ANIONS
401
(0.5–10.0)%ëÂé2/ZrO2, both the total O–2 concentra-
Therefore, we can assume that the free electrons
involved in O–2 (ëé + é2) formation are produced dur-
tion and the concentration of weakly bound O–2 adsorp-
ing the formation of carbonate or carboxylate com-
plexes.
tion species increase with increasing amount of sup-
ported ëÂé2. The finding that the concentration of
weakly bound O–2 species increases as the cerium con-
Evidently, the carbonate and carboxylate complexes
result from the oxidation of CO with surface oxygen
since this process is retarded on reduced ëÂé2 [17].
Furthermore, this process must be accompanied by the
reduction of Ce4+ cations to Ce3+ [18]. Subsequent
interaction between the reduced cations and adsorbed
é2 molecules through electron transfer from Ce3+ to
é2, ads affords oxygen radical anions, which are stabi-
lized in the coordination sphere of coordinately unsat-
urated Ce4+ ions.
tent of a supported sample is increased confirms the
assumption that these species are stabilized on Ce4+ cat-
ions.
Thus, O–2 (ëé + é2) formation on (0.5–10.0)%
ëÂé2/ZrO2 occurs only on supported ëÂé2 and O–2 is
stabilized on Ce4+ and Zr4+ cations. O2– radical anions
can be stabilized on Zr4+ cations when the reaction
occurs at the oxide interface or when there is spillover
of O–2 from the ëÂé2 phase to the Zré2 support.
The coordinately unsaturated Ce4+ ions and surface
oxygen involved in the formation of O–2 (ëé + é2)
Note that, for (0.5–10.0)%ëÂé2/ZrO2, no O–2 radi-
result from the dehydroxylation of ëÂé2 as the sample
is heat-treated in oxygen. The amount of these species
increases with an increase in ëÂé2 preaging tempera-
cal anions were observed on cerium cations located in
associated anionic vacancies. Apparently, they do not
form because the cerium phase on the ZrO2 surface is
finely dispersed even when the concentration of sup-
ported Ce4+ ions is three times higher than the concen-
tration of surface Zr4+ ions, as in the case of
10.0%CeO2/ZrO2.
ture. This is indicated by increasing O–2 (ëO + O2) con-
centration with an increase in ëÂé2 oxidation temper-
ature (Fig. 2).
The concentration of coordinately unsaturated Ce4+
ions on the oxidized ëÂé2(600°ë) surface is 7 × 1017 m–2
[6]. This value is ten times higher than the observed
concentration of O–2 (ëé + é2). This difference proba-
bly arises from the difference between the concentra-
tion of O–2 stabilization centers and the concentration of
4. Mechanism of Formation of O–2 (ëé + é2) on ëÂé2
The activation behavior of O–2 (ëé + é2) formation
on ëÂé2 is determined by the temperature dependence
of the CO adsorption. This is indicated by the fact that
both the O–2 (ëé + é2) concentration and the CO cov-
surface oxygen involved in the formation of electron
donors in CO adsorption.
Oxygen species on the ëÂé2 surface were consid-
ered in earlier works [16, 17]. These include monooxy-
gen ions octahedrally coordinated to cations, bridging
erage of the surface increase as the CO adsorption tem-
perature increases from 20 to 200°ë (Table 1). On the
other hand, it is known [16] that CO is oxidized on
ëÂé2 at T ≥ 400°ë. Therefore, CO adsorption between
20 and 200°ë produces surface complexes that are
active in the formation of O–2 (ëé + é2).
dioxygen adsorption species, and O–2 radical anions. As
is demonstrated above, O2– radical anions are involved
in CO oxidation on ëÂé2 at 20°ë. It is most likely that
this reaction proceeds through the formation of carbon-
ates decomposing at T ≥ 400°ë [17]. We did not
According to IR spectroscopic data [17], CO
adsorption on the oxidized ëÂé2 surface at 20°ë
results in the formation of a linear CO adsorption spe-
cies, bidentate and monodentate carbonates, and inor-
ganic carboxylates. The linear CO adsorption species is
easily removed by pumping. Therefore, it is not
observe O–2 radical anions in the formation of O–2 (ëé
+ é2) on the original oxidized ëÂé2(400–700°ë) spec-
imens. Therefore, the formation of carbonate and car-
boxylate complexes active in this reaction involves
monooxygen ions or bridging dioxygen species.
involved in the formation of O–2 (ëé + é2). The car-
bonates and carboxylates remain on the surface, and
their amount increases as the CO adsorption tempera-
ture is increased from 20 to 200°ë. The complexes
decompose completely at T ≥ 400°ë. This temperature
dependence of the formation and decomposition of car-
bonates and carboxylates correlates well with the way
the O–2 (ëé + é2) concentration on ëÂé2 varies with
increasing CO adsorption temperature (Table 1) and the
fact that ëÂé2 regains its activity in O–2 (ëé + é2) for-
mation upon preaging of the sample at T ≥ 400°ë.
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