Exoemission accompanying electron interactions in complex copper-cerium oxide catalysts and synergism phenomena

Article abstract of DOI:10.1023/A:1024806606577

The formation of the electronic structure of the surface of complex copper-cerium oxide catalysts with different copper concentrations was studied using the exoemission methods at different stages of preparation. The introduction of copper enhances the emissivity of CeO₂, and the number of charges emitted from the catalyst surface exceeds the emission activity of the starting CeO₂ and CuO components. The synergism phenomenon in exoemission is compared with synergism in the catalysis of CO oxidation by these systems. The problem of electron interactions between the components of the complex oxides is discussed. The electron interactions are caused by the electron transitions at the interface and result in an increase in both the emission of weakly bound electrons and catalytic activity in the oxidation of CO.

Full text of DOI:10.1023/A:1024806606577

Russian Chemical Bulletin, International Edition, Vol. 52, No. 6, pp. 1289—1295, June, 2003
1289
Exoemission accompanying electron interactions in complex
copperꢀcerium oxide catalysts and synergism phenomena
I. V. Krylova
Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (095) 932 8846. Еꢀmail: krylova@kge.msu.ru
The formation of the electronic structure of the surface of complex copperꢀcerium oxide
catalysts with different copper concentrations was studied using the exoemission methods at
different stages of preparation. The introduction of copper enhances the emissivity of СеО2,
and the number of charges emitted from the catalyst surface exceeds the emission activity of the
starting СеО and СuO components. The synergism phenomenon in exoemission is compared
2
with synergism in the catalysis of CO oxidation by these systems. The problem of electron
interactions between the components of the complex oxides is discussed. The electron interacꢀ
tions are caused by the electron transitions at the interface and result in an increase in both the
emission of weakly bound electrons and catalytic activity in the oxidation of CO.
Key words: exoemission of catalysts, complex oxides, synergism, catalytic oxidation, carꢀ
bon monoxide.
A great attention has recently been given to the synerꢀ
gism and promotion phenomena in catalysis.1 The inꢀ
fluence of the electron transfer between the active phase
and the support on the catalytic activity has been disꢀ
enhances the ability of СеО to absorb oxygen. However,
2
—5
the nature of this interaction remains unclear. As menꢀ
1
tioned, electron interactions can occur in the comꢀ
plex copperꢀcerium oxide catalysts involving the finest
6
11
cussed long ago. The works using the modern methods of
Schottky transitions at the interface of the oxides. The
analysis of the surface, viz., IR spectroscopy, ESR specꢀ
electron interactions in the metal—oxide system, resultꢀ
troscopy, XPS, and temperatureꢀprogrammed desorption
ing in the promotion effect of the СеО ꢀsupported metꢀ
als, have been considered. However, the electronic propꢀ
2
5
(
TPD), made it possible to conclude with a more cerꢀ
tainty about the metal—oxide electron interaction. Ceꢀ
rium dioxide is used as a reservoir for oxygen storage in
the threeꢀway catalysts for purification of automobile exꢀ
hausts. The interaction of СеО2 with noble metals (Pt,
Pd, Rh) and its effect on the catalytic activity in oxidation
erties of the catalyst surface were not directly studied in
the published works.5 It is very difficult to find a direct
relationship between the electronic properties of the cataꢀ
,11
1
lysts and their catalytic activity. In present work we used
electron and ion exoemission to study the formation of
the electronic structure of the surface and electron interꢀ
actions in the copperꢀcerium oxide catalysts for oxidation.
Exoemission (EE) is the lowꢀtemperature emission of
lowꢀenergy electrons and ions. It occurs from the surface
of solids (metals, semiconductors, dielectrics) after they
were affected by different ways ("excitation") and posꢀ
sesses a significant sluggishness (afterꢀemission, AE). The
excitation of solids occurs due to the radiation or meꢀ
chanical (thermomechanical) action, phase transformaꢀ
tions, or physicochemical processes that occur on the
surface (adsorption, desorption, oxidation, corrosion, hetꢀ
7
,8
have been studied in detail. The recent studies have
shown that the catalytic activity of the ceriumꢀcontaining
catalysts in oxidation is substantially enhanced when tranꢀ
1
,3,4
sition metals, including copper, are introduced.
The
effect of СеО2 promotion with copper in the catalytic
oxidation processes is in accordance with synergism in
9
the redox properties of this system. This is a consequence
of the interaction between the components. Due to the
reactions in the CuO—CeO system, the processes of oxiꢀ
2
dation with oxygen and reduction with carbon monoxide
occur with a higher efficiency than those in the case of the
starting components. The results obtained in ESR and
TPD studies indicate the simultaneous involvement of
1
2,13
erogeneous catalytic reactions
). The majority of metꢀ
als, semiconductors and their oxides, and hydrophilic
materials of different nature are characterized by the
CuO and СеО in the redox processes. Carbon monoxide
2
–
–
is assumed to be oxidized on the sites localized at the
interface of the oxides.1,9 It has been established that
the strong interaction between transition metals and СеО2
"spontaneous" emission of negative ions (О2 , ОН ,
10
–
СО2 ) and electrons from the surface, which is due to the
technological treatment and storage in air or in aggressive
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1219—1225, June, 2003.
066ꢀ5285/03/5206ꢀ1289 $25.00 © 2003 Plenum Publishing Corporation
1

1
290
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
I. V. Krylova
media. The subsequent heating in the linear regime reꢀ
sults in the thermostimulated emission (TSE) of electrons
and ions or "thermal desorption" of charged particles.
Irradiation in the region of wavelengths exceeding the
photoeffect boundary leads to photostimulated emission
tion of the electronic structure of the catalyst surface, TSE was
measured for the samples heated to 50 °C and calcined in air for
3
h at 650 °С (the last stage of preparation of the copperꢀcerium
oxide catalysts for CO oxidation carried out according to the
1
previously described procedure ).
The catalytic activity of the СеО , CuO, and Cu—Ce—O
2
(
(
PSE). Exoelectrons (ions) are emitted from external
≤10 nm) surface layers and provide an information on
(
25% Cu) samples in CO oxidation was determined using a
microcatalytic setup in the pulse regime. Xꢀray patterns were
obtained with a DRONꢀ2 diffractometer for the СuO and CeO2
samples and Cu—Ce—O (25% Cu) catalyst. The main reflecꢀ
tions in the Xꢀray diffraction patterns of the catalyst and starting
oxides coincided, although some lines of CuO were absent in the
pattern of the coprecipitated sample. The halfꢀwidth of the peaks
corresponding to CuO in the catalyst is ∼ 2ꢀfold larger than that
in the patterns of the starting oxides, indicating an increase in its
the energy spectrum of the localization levels (or the actiꢀ
vation energy of charge desorption) on the surface. The
–
17
–19
exoemission currents are small (10 —10
A), so that
exoemission is detected in the pulse regime using a gasꢀ
flow Geiger counter or in vacuo with a secondary electron
multiplier (SEM). Such a detection ensures a high sensiꢀ
–
9
tivity of the EE methods (∼ 10 of the total number of
surface atoms), which exceeds by several orders of magniꢀ
tude the sensitivity of the traditional methods for studying
the surface (Auger spectroscopy, XPS, XRD, and others).
The purpose of this work is to study the electron interꢀ
actions in the Сu—Се—О system as the most probable
reason for synergism using the EE methods. Exoemission
was measured for the samples prepared according to the
deficiency in the presence of СеО . However, the degree of
2
dispersion cannot be estimated unambiguously using these data.
Results and Discussion
Afterꢀemission and photostimulated emission. After the
starting samples were placed in the working chamber of
the counter and a positive potential was applied to the
anode, an enhanced "background" was observed, that is,
the emission of negative charges descending in time (AE),
whose intensity depends on the copper concentration in
the samples. After AE decayed to the level of the natural
background, the spectra of PSE excitation were detected:
the plots of the EE intensity vs. wavelength of the incident
light. The excitation spectra of PSE for the CeO2 and
CuO samples and two copperꢀcontaining catalysts in the
longꢀwave spectral region are presented in Fig. 1. Analyꢀ
sis of the data shows that PSE for the samples under study
is the selective photoeffect with λmax = 550 nm.
1
procedure described elsewhere, taking into account the
results of this comprehensive study.
Experimental
Catalysts were prepared using the coprecipitation method,14
which coincided completely with a commonly accepted proceꢀ
dure described in our previous work1 in detail, and contained 5,
5
1
0, and 25 wt.% copper. Starting 10% solutions of nitrates were
precipitated with sodium hydrocarbonate at 80 °С. After the
precipitate aged, it was washed with distilled water to remove
+
–
Na and CО3 ions and dried at 50 °С. In subsequent experiꢀ
ments, the samples were calcined in air at 650 °С. The starting
The PSE intensity (I ) for the copperꢀcerium oxide
catalysts is higher than IPSE for СеО and CuO, indicatꢀ
СеО and CuO oxides were prepared using the same procedure.
2
2
The sample loading for EE measurements was 30 mg. The DTA
and DTG methods have previously14 been used for analysis of
structural phase transitions during the formation of the complex
copperꢀcerium oxides studied in this work.
ing a synergism in the electronic properties of the surface
characterized by PSE.
The results of detection of the starting AE values (the
first reading of I ) and the starting IPSE values at different
compositions of the samples at each wavelength of the
incident light used are presented in Table 1. It can be seen
that the I values of both AE and PSE depend on the
copper content in the samples. The synergism phenomꢀ
enon is observed: the AE and PSE intensities from the
catalysts exceed I of the emission from the starting
samples. This regularity is violated on going to PSE in the
region of the UV action: under UV irradiation with λ =
Exoemission was detected with the gasꢀflow Geiger counter
1
6
of a Protoka industrial instrument (Russia) modified by us for
measurements of TSE and PSE of negative charges. Methane
was the working gas of the counter. Series of narrowꢀband filters
in the 578—257 nm wavelength interval were used to detect PSE
under irradiation with a DRTꢀ230 quartz mercury lamp. The
TSE intensity was measured on heating the samples with a rate
–
1
of 5 deg min in the 20—360 °С temperature interval. After the
sample was placed in the working chamber of the counter and
purged with the working gas (methane), the emission of negative
charges (AE) was detected, which descended in time to the
background level of 1—3 pulse min– . After AE due to the presꢀ
ence of "biographical" charges on the surface decayed, the specꢀ
trum of PSE excitation was obtained, which represented the plot
of the EE intensity vs. wavelength of the incident light. Then the
sample was heated to 360 °С with the simultaneous detection
of TSE. After the sample was cooled, the spectral plot of PSE
was measured again. In order to determine the stage of formaꢀ
2
57 nm, where the surface layer can be decomposed phoꢀ
tochemically (reduced), the PSE intensity increases draꢀ
matically (by one—two orders of magnitude). After the
samples were heated to 360 °С (during the detection of
TSE), the PSE intensity decreased sharply.
Thermostimulated emission from the samples. The reꢀ
sults of detection of TSE from the samples with different
copper concentrations heated after coprecipitation to
1

Synergism in exoemission and catalysis
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1291
I/pulse s–1
I/pulse s–1
1
1
10
00
2
6
4
0
0
8
6
4
2
0
0
0
0
1
4
3
1
00
200
300
T/°С
I/pulse s–1
1
00
4
2
0
2
1
80
6
4
2
0
0
0
3
5
00
600
λ/nm
1
00
200
300
T/°С
Fig. 1. Excitation spectra of PSE in the longꢀwave region for the
samples of CеО (1), CuO (2), and Сu—Сe—О with the copper
content 5 (3) and 10 wt.% (4).
2
Fig. 2. Thermostimulated emission from the starting oxides
and Сu—Ce—O samples heated at 50 °С: 10 wt.% Cu (1),
2
5 wt.% Cu (2), СеО2 (3), and CuO (4). Dotted line correꢀ
5
0 °С are presented in Fig. 2. For the samples containing
sponds to background.
copper in CеО (curves 1 and 2), the main most intense
2
TSE peak is localized in the 120—180 °C temperature
interval. Many jumps and special points in the TSE curves
are due to the diversity of charged forms of oxygen and
on the surface and are retained with different bonding
1
2,13
energies.
Then the Cu—Ce—O samples were calcined
for 3 h at 650 °С in air, which corresponds to the last stage
of catalyst preparation. The results of TSE detection are
–
ОН groups, which are coordinated in different manners
Table 1. Afterꢀemission and photostimulated emission (I/pulse s–1) from the starting Cu—Ce—O samples heated at 50 °С (1) and
below T = 360 °С during TSE (2)*
Sample**
AE
PSE at different λ***/nm
2
57
313
365
405
436
546
578
1
2
1
2
1
2
1
2
1
2
1
2
1
2
CeO2
5
500
5
40
Backꢀ
ground
Backꢀ 20
ground
Backꢀ 20
ground
Backꢀ 20
ground
Backꢀ 30
ground
5
Backꢀ
ground
9
70
Backꢀ 18
ground
10
13
Backꢀ
ground
Backꢀ 22
ground
8
10
Cu—Ce—O (5)
20
9
6
1
2
60
60
25
25
50
55
40
40
5
25
10
20
10
36
42
20
20
6
6
7
4
Cu—Ce—O (10) 60
Cu—Ce—O (25) 45
7
5
Backꢀ 30
ground
Backꢀ 20
ground
Backꢀ 20
ground
5
5
8
35
15
15
11
16
5
CuO
20
338
Backꢀ
ground
Backꢀ
ground
*
*
*
Taking into account transmission of the filters.
* The content of Cu (wt.%) is indicated in parentheses.
** Wavelength of the stimulating light.

1292
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
I. V. Krylova
I1/pulse s–1
I2•10–2/pulse s–1
The activation energy of the charge emission, which
was calculated using the TSE curves (see Fig. 3), is ∼ 0.8 eV,
–
while the activation energy of О desorption or decomꢀ
2
8
00
00
00
00
–
–
20
position О2 → О + О is equal to 0.8—1.1 eV, which
agrees satisfactorily with the data on TSE. The energy of
atomic recombination О + О + М (М is the third species,
2
1
0
0
2
1
in our case, it is the recombination site) is 5.17 eV,
whereas the energy of interaction О + О is equal22 to
–
6
4
2
3
4.1 eV. Therefore, the energy of recombination of active
species is sufficient for the emission (desorption) of
charged species. Equations (1)—(3) presented above are
only some of possible surface reactions resulting in
exoemission. The presently accepted scheme of formaꢀ
tion of the sorption layer of oxygen and its desorption (on
2
4
1
2
3
heating or under irradiation) has the following form :
–
(О–)ads
(О2–)lat
(
О )
(О2 )ads
2
ads
(
О2–)ads
,
(4)
where Оlat is lattice oxygen.
Catalytic activity of the samples. The results of meaꢀ
suring the catalytic activity in CO oxidation for the
Cu—Ce—O catalysts and starting oxides (CuO, CeO2)
studied in this work (curves 1—3) and in the previous
1
00
200
300 T/°С
Fig. 3. Thermostimulated emission from the Сu—Сe—О samples
and starting oxides calcined in air at 650 °C: CеО2 (1), 10 (2)
and 25 wt.% Cu (3), CuO (4).
1
paper are presented in Fig. 4. In both cases, the 100%
conversion СО → СО for the copperꢀcontaining cataꢀ
2
lysts is achieved at Т < 100 °С (curves 3—6), while for the
copper oxide it is achieved at T = 170—180 °С (curves 2
and 5), and for СеО2 this temperature is much higher
presented in Fig. 3. For all samples, the most intense TSE
peak is localized at Т ≈ 150 °С corresponding to the deꢀ
•
–
composition of the adsorbed radical ion (О2 )ads acꢀ
companied by the emission of negative charges and TSD.
A comparison of Figs. 2 and 3 shows that the main
TSE peak (Тmax = 150 °С) for the catalysts under study is
manifested already at the first stage of their formation
after the coprecipitation of the precursors and heating to
(
curves 1 and 4).
The most interesting is the synergism phenomenon in
EE of the complex copperꢀcerium oxide catalysts, where
the intensity (I ) or the total number (ΣI ) of emitted
charges of AE, PSE (see Table 1), and TSE (see Figs. 2
and 3) exceeds EE from the starting samples. The synerꢀ
5
∼
0 °С. However, the intensity of this peak increases
5 times for the catalysts calcined in air at 650 °С
CA (%)
and ∼ 25 times for the sample with the highest copper
content (25 wt.%).
1
00
Analysis of numerous published17,18 and our12,13 data
shows that the TSE maximum at Т ≈ 150 °С corresponds
to the following transformations in the adsorbed (weakly
bound) oxygen layer:
8
6
4
2
0
0
0
0
1
2
5
3
(
О2–)ads → О ↑ + Оads,
–
(1)
(2)
(3)
6
2
Оads → О + Qrec,
2
–
(
О ) + Оads + Qrec → О + е ↑.
ads
2
4
According to Eqs. (1)—(3), TSE of electrons and ions
is a consequence of transformations in the layer of weakly
bound (adsorbed) oxygen, surface diffusion, and recomꢀ
bination of adsorbed species (including ions), whose enꢀ
ergy (Qrec) is consumed to yield electrons and ions.
As a rule, the temperatures of the TSE peaks and desorpꢀ
tion rate coincide.19
100
200
T/°С
Fig. 4. Catalytic activity in the oxidation of CO on the СеО (1),
CuO (2), and Cu—Сe—О (25 wt.% Cu) samples (3) studied
in this work and the СеО2 (4), СuO (5), and Cu—Сe—О
2
1
2,13
1
(25 wt.% Cu) (6) samples according to the published data.

Synergism in exoemission and catalysis
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1293
1
2
CA/T* –1)•10–3
ΣI•10–3/pulse
(
3
3
2
.0
3
2
.5
.0
2
1
0
20
30
40
50
100
[
Cu] (wt.%)
Fig. 5. Sum of the emitted charges (ΣI) in the interval T = 20—350 °С (1) and the catalytic activity (CA) (2, 3) as functions of the Cu
1
content: published data (2) and results of this work (3); Т* is the temperature of 100% CO oxidation.
gism phenomenon in TSE is manifested at both the first
see Fig. 2) and last (see Fig. 3) stages of preparation of
the catalysts. The plots of the catalytic acitivity (CA) and
the sum of emitted charges (ΣI ) vs. copper content are
presented in Fig. 5.
Thus, according to the results obtained for both the
heterogeneous catalytic oxidation of CO and the study of
EE, synergism phenomena are observed. They are espeꢀ
cially pronounced in Fig. 5, indicating that weakly bound
oxygen is involved in these processes and the nature of the
sites of EE and oxidative catalysis is the same: these are
the active surface sites for which electron exchange is
facilitated.
ena in the catalysis of CO oxidation and in EE, for the
Сu—Се—О catalysts studied. When the coordinatively
unsaturated cerium ions (in the reduced state) are present
(
5
on the surface, СеО is an nꢀtype semiconductor, while
2
the copper oxide is a semiconductor with the holeꢀtype
2
4
conductivity. It is known that the contact of two semiꢀ
conductors with the nꢀ and рꢀtypes of conductivity results
in the charge separation at the boundary (Fig. 6). The
"depleted" layer with the enhanced concentration of elecꢀ
trons in the рꢀsemiconductor and holes in the nꢀsemiꢀ
conductor appears near the boundary. The thickness of
2
4
the boundary layer can reach 10 —10 Å.
Localization of holes in the nearꢀboundary layer of
Nature of synergism phenomena. Based on the analysis
of the presented published and our experimental data,
including XRD, we propose the following scheme of elecꢀ
tron interactions, which induce the synergism phenomꢀ
СеО designates the formation of weakly bound oxygen
(О , О) according to the scheme
2
–
О2–)lat + р → (О–)ads + V*,
(
(
(5)
(6)
–
О ) + р → Оads
,
ads
Concentration of carriers
pꢀType
nꢀType
where V* is the oxygen vacancy.
Nd
The subsequent recombination of these species can
result in the desorption (emission) of the charged and
neutral forms of oxygen, for example, according to Eq. (3).
Emission occurs due to the energy of the electronꢀhole or
other types of recombination of oxygen species accompaꢀ
nied by energy evolution.
Na
x
Depleted
layer
In the region of the depleted (negatively charged) CuО
–
dp
dn
2
+
layer at the boundary of the p—n transition, the Cu
Fig. 6. Scheme of the depleted layer near the contact of the рꢀ
and nꢀsemiconductors. Concentration of the charge carriers in
the depleted layer at different positions of the point of the relaꢀ
tively sharp р—nꢀtransition. Axis х is the distance from the
boundary deep into the semiconductor: N and N are the conꢀ
4
ions, which prevail in the Сu—Се—О catalysts, are reꢀ
+
0
+
duced, adding electrons to form Cu (or Cu ). The Cu
1
ions were found in the Cu—Се—О catalysts using XPS.
Thus, the weakly bound oxygen species (О , О) (see
–
a
d
Eqs. (5) and (6)), oxygen vacancies V*, which are sites of
oxygen adsorption, and Cu+ ions (active sites of CO adꢀ
sorption) are simultaneously present at the CuO/CeO2
centrations of acceptors and donors with the constant value
everywhere, except for the dp and dn points, where they change
jumpwise.24

1294
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
I. V. Krylova
interface. The charge equilibrium in the boundary layer is
corresponds to the TSE intensity from СеО . The maxima
2
violated during CO or О adsorption, and the processes of
of TSE for measurements in vacuo are localized at
T = 90—160 °С, and the more intense peak is localized at
T = 250—300 °C; TSE is accompanied by jumps of gas
evolution marked in Fig. 7. On the repeated heating, the
TSE peaks at Т < 200 °С are not manifested due to the
removal of weakly bound adsorbates, whereas the TSE
peak at 275 °С is reproduced due to the diffusion of
2
О2 and СО adsorption should occur simultaneously to
retain the equilibrium, which is consistent with the previꢀ
4
ously published data.
Both the СеО and Cu O clusters, on which the acꢀ
2
2
9
tive copper sites are localized, have been shown to be
simultaneously reduced (in СО) or oxidized (in О ). The
2
–
high activity and synergism in CO oxidation are attribꢀ
charged oxygen (in the form of О ) from the bulk. Curve 2
4
uted to the high redox ability of the sites at the interface
in Fig. 7 presents the results of measuring TSE from СеО2
containing 25 wt.% Cu. Here the sharp jump of the emisꢀ
sion current is observed near T = 200 °С and has, most
likely, the electric breakdown character. The further heatꢀ
ing to 275 °С, which coincides with Тmax of TSE from
of the oxides, and this agrees with the mechanism of
electron interactions (see reactions (5) and (6)) proposed
by us for the adsorption and catalysis at the interface of
the oxides of electron and hole semiconductors. The
model presented allows one to understand the reason
СеО , results in a sharp increase in the emission current
2
for the easier reduction of СеО in the presence of
and gas evolution followed by the electric breakdown in
the sample—EE detector gap in the chamber for emission
measurements. Strong electric fields appeared at the inꢀ
2
copper.10
Measurements of TSE in a high vacuum. In order to
check experimentally the applicability of the scheme of
electron transitions on contact of the oxides with differꢀ
ent types of conductivity (see Fig. 6), we carried out exꢀ
2
4
6
–1
terface of the р—n oxides can achieve 10 V cm . The
application of a positive potential to the detector under
the conditions of gas evolution results in the breakdown
of the vacuum gap between the catalyst surface and detecꢀ
tor. It is characteristic that the repeated heating of this
sample results in a low level of the emission current,
which is almost independent of the temperature below
T = 275 °С, after which it decreases sharply to the level of
the background: the neutralization of charges upon the
breakdown at the interface of the oxides resulted in the
disappearance of TSE at the temperature exceeding the
breakdown temperature.
–
6
periments on the detection of TSE in vacuo (10 Torr)
using a VEUꢀ6 secondary electron multiplier using
the previously described1 setup. Figure 7 presents the
TSE from the starting СеО2 sample and Cu—Се—О
3
(
25 wt.% Cu) catalyst heated in air at 650 °С. Curve 1
I1/pulse s–1
I2•10–3/pulse s–1
p
The synergism phenomena observed in EE and cataꢀ
lytic CO oxidation on the complex copperꢀcerium oxide
catalysts are explained from the viewpoint of electron
transitions appeared on contact of semiconductors with
8
4
3
2
1
0
0
0
0
different types of conductivity: nꢀtype СеО and pꢀtype
2
2
Сu О. The direct study of the electronic properties of the
2
1
6
4
2
catalyst surface, which is inaccessible for the transitional
methods of studying the surface, using the exoemission
p
p
(
AE, PSE, and TSE) methods confirmed the assumption
p
on possible electron interactions in the complex oxide
catalysts resulting in the synergism phenomena in caꢀ
talysis.
1
It should be noted in conclusion that, despite the inꢀ
tense development of studies of catalysts using the modꢀ
2
2
5
ern (traditional) methods for analysis of the surface, no
data on electronic (ionic) processes in catalysts were obꢀ
3
2
5,26
tained. According to the published data,
the methods
2
of studying catalytic processes, which can fix the stages of
catalyst preparation or reactions with electron or ion transꢀ
fer, are presently not available. Exoemission is the only
efficient method for studying electronic and ionic proꢀ
cesses on the surface accompanied by the formation
1
00
200
300 T/°С
Fig. 7. Thermostimulated emission from the СеО2 (1) and
Cu—Сe—О (25 wt.% Сu) (2, 3) samples calcined at 650 °С and
stored in air, for the first (2) and second measurements of
TSE (3). Measurements were carried out in a deep vacuum:
p↑ are jumps of gas evolution during the detection of TSE
from СеО2.
2
7
of active sites, heterogeneous catalytic processes in
2
8,29,30
situ,
and electron interactions in metallic supported
and complex oxide catalysts.

Synergism in exoemission and catalysis
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 6, June, 2003
1295
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Received April 17, 2002;
in revised form December 4, 2002