Spectral investigation of the interaction of carbon monoxide and dioxygen with low-temperature sublimates of Co(II) meso-tetraphenylporphyrinate: Evidences for oxidative catalysis

Article abstract of DOI:10.1023/A:1010433905648

The interaction of carbon monoxide and dioxygen with low-temperature (T = 77 K) sublimates of cobalt(II) meso-tetraphenylporphyrinate (CoTPP) is studied by IR spectroscopy using isotopically labeled compounds (¹³CO and ¹⁸O₂). The obtained data confirm the coordination of CO and O₂ with the formation of axial complexes CoTPP · L (L = O₂, CO) and the mixed complex O₂ · CoTPP · CO. The heating of the complex containing CO and O₂ to 150 K leads to the appearance of lines in the IR spectrum corresponding to valence asymmetric (V_asOCO) and degenerated deformation (δOCO) vibrations of carbon dioxide. This is evidence for the oxidation of CO. The evacuation of CO₂ at a higher, but still low, temperature and the addition of a new portion of CO and O₂ mixture lead to the formation of new portions of CO₂. This demonstrates the catalytic character of the process. Only low-temperature sublimates have catalytic properties. Sublimates of CoTPP obtained on the surface at room or higher temperature demonstrate neither catalytic activity nor the ability to coordinate any amount of CO and O₂ sufficient for the detection by spectroscopy.

Full text of DOI:10.1023/A:1010433905648

Kinetics and Catalysis, Vol. 42, No. 2, 2001, pp. 281–288. Translated from Kinetika i Kataliz, Vol. 42, No. 2, 2001, pp. 312–319.
Original Russian Text Copyright © 2001 by Kurtikyan, Martirosyan, Akopyan.
Spectral Investigation of the Interaction of Carbon Monoxide
and Dioxygen with Low-Temperature Sublimates
of Co(II) meso-Tetraphenylporphyrinate:
Evidences for Oxidative Catalysis
T. S. Kurtikyan1, 2, G. G. Martirosyan1, and M. E. Akopyan2
1Armenian Institute of Applied Chemistry, National Academy of Sciences of Armenia, Erevan, Armenia
2Center for Molecular Structure Studies, National Academy of Sciences of Armenia, Erevan, Armenia
Received December 22, 1999
Abstract—The interaction of carbon monoxide and dioxygen with low-temperature (T = 77 K) sublimates of
cobalt(II) meso-tetraphenylporphyrinate (CoTPP) is studied by IR spectroscopy using isotopically labeled com-
pounds (13CO and 18O2). The obtained data confirm the coordination of CO and O2 with the formation of axial
complexes CoTPP · L (L = O2, CO) and the mixed complex O2 · CoTPP · CO. The heating of the complex con-
taining CO and O2 to 150 K leads to the appearance of lines in the IR spectrum corresponding to valence asym-
metric (νasOCO) and degenerated deformation (δOCO) vibrations of carbon dioxide. This is evidence for the
oxidation of CO. The evacuation of CO2 at a higher, but still low, temperature and the addition of a new portion
of CO and O2 mixture lead to the formation of new portions of CO2. This demonstrates the catalytic character
of the process. Only low-temperature sublimates have catalytic properties. Sublimates of CoTPP obtained on
the surface at room or higher temperature demonstrate neither catalytic activity nor the ability to coordinate any
amount of CO and O2 sufficient for the detection by spectroscopy.
INTRODUCTION
Low-temperature oxidation of carbon monoxide
was also found in [7–9] on phthalocyanines, which are
similar to porphyrins. It is important that not only phth-
alocyanines of metals monomolecularly dispersed in
zeolite matrices, but also the complexes themselves
without any matrix exhibited a catalytic activity. In this
case, phthalocyanines were carefully crushed in an
agate mortar and evacuated at 280°C for 10 h to obtain
a stronger catalytic effect.
The most important and useful property of metal
porphyrins is their ability to act as catalysts in various
chemical, electrochemical, and photochemical pro-
cesses. By now, considerable information on various
catalytic processes with porphyrins has accumulated.
The results of the investigations in this field are described
in many monographs and review articles [1–3].
Low-temperature studies of oxidative catalysis are
of particular interest for this work. From this point of
view, the most interesting is a series of works [4–6] on
the catalytic oxidation of CO by molecular oxygen. In
these studies, Co(II) meso-tetraphenylporphyrinate
(CoTPP) deposited on titanium dioxide was used.
CoTPP/TiO2 system preliminarily treated in a vacuum
at an elevated temperature demonstrates a high cata-
lytic activity at –79°C (0.3 mol CO2 /(mol CoTPP min)).
In the above works, the activation of CO by CoTPP was
proposed. The sites for O2 adsorption were not dis-
cussed there, but it was suggested that these sites are
located on TiO2 surface near CoTPP.
Unfortunately, in the cited papers, vibrational spec-
troscopy was not applied despite its successful applica-
tion to catalytic processes [10, 11].
Earlier, we showed that the low-temperature subli-
mates of CoTPP can coordinate molecular oxygen [12].
The formation of CO2 was found by IR spectroscopy
[14] when we investigated the ability of low-tempera-
ture sublimates of CoTPP to form a mixed carbonyl–
oxygen extra-complex, which is analogous to an isoni-
trile–oxygen complex [13]. This phenomenon can be
related to the low-temperature oxidation of CO by
molecular oxygen catalyzed by CoTPP. Therefore, in
this work, we carried out an experiment to study this
process by IR spectroscopy.
The role of titanium dioxide in the catalytic process
is not completely clear from data presented in [4–6],
but the authors propose that porphyrin is modified as a
Most IR spectrophotometers have low sensitivity at
result of the thermal and vacuum treatments of 2300–2400 cm–1 due to the absorption of IR radiation
CoTPP/TiO2. However, titanium dioxide may only play by atmospheric CO2. When 13CO and O2 gases were
18
the role of porous support with a large surface area for used in this work, the vibrational frequencies νasCO2 of
adsorbed porphyrin molecules.
CO2 molecules formed in the process and containing
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282
KURTIKYAN et al.
dioxide temperatures was maintained by the addition of
the small portions of liquid nitrogen to a cryostat. The
mass of copper holder supporting the support permitted
us to maintain a desired temperature within a precision
of 2–3 K during recording the IR spectrum.
In the second type of experiments, CO, O2, or their
mixture was supplied to CoTPP layer sublimated
beforehand. The gases were supplied from a chamber
equipped with a gauge enabling the measurement of
pressure in the cryostat. The microporous structure of
low-temperature sublimates [17] of meso-tetraphe-
nylporphyrinates of metals (MTPP) enabled the inter-
action of porphyrin with the ligand in total sublimated
layer thickness.
IR spectra were recorded using a Specord M-80 spec-
trophotometer. The spectral broadness of slit was 4 cm–1.
RESULTS AND DISCUSSION
2100
2000 ν, cm–1
Carbonyl Extra-complex CoTPP
12
Fig. 1. IR spectra of co-condensate of CoTPP with CO
(solid line) and with CO (dashed line) at 80 K.
The co-condensation of sublimated porphyrin and
13
CO (13CO) supplied onto the support at a low flow rate
(~30 µmol/h) leads to the appearance of broad (∆ν1/2
Ӎ
30 cm–1) absorption bands at 2066 (2019) cm–1 in the
IR spectrum (Fig. 1). These bands are absent from the
CoTPP spectrum. The isotopic shift of these bands is in
ideal agreement with calculations carried out for CO
molecules in the approximation of harmonic oscillator.
The frequencies of these bands are close to those
observed earlier [18] for the CoTPP + CO system in Ar
matrix. There is no doubt that these bands correspond
to the vibrations νCO(13CO) of the coordinated carbo-
nyl group.
these isotopes fall out of this frequency range. This
makes the results of measurements more reliable.
EXPERIMENTAL
CoTPP was obtained using the method described in
[15]. Before the experiment, CoTPP was additionally
purified by passing through a column filled with dry
alumina of the 2nd activity grade (Brokeman). Gases
were dried using P2O5 and purified from traces of CO2
using a high-vacuum low-temperature technique. Com-
mercially available samples of 13CO (88.4% enrich-
ment) and 18O2 (81.7% enrichment) were purified using
the same method. The method for the preparation of subli-
mated CoTPP layers was described in detail in [13].
The spectrum presented in Fig. 1 contains, in addi-
tion to the main band, weak bands shifted by ~20 cm–1
from the main band to higher frequencies. Earlier, the
monocarbonyl complex CoTPP was discovered and
characterized in a frozen toluene glass by EPR method
[19]. It was shown in [18] that, at a low temperature, the
dicarbonyl complex CoTPP can be formed in addition
the monocarbonyl complex. Experiments were carried
out using the matrix-isolation technique in Ar and Kr
matrices. A rather broad band with a maximum at
2078 cm–1 (2032 cm–1 for 13CO) was found by IR spec-
troscopy in an Ar matrix. This band is split into two
bands at 2073 and 2077 cm–1 in a Kr matrix. The
authors concluded from this observation that the band
at 2078 cm–1 consists of two close bands of mono- and
dicarbonyl complexes. This leads to its large halfwidth.
The frequencies are close, probably because of a weak
overlap of the d-orbitals of metal with the antibonding
orbitals of CO, that is, because of a weak trans-effect.
Two types of experiments were carried out to study
the low-temperature interaction of CO with sublimated
layers of CoTPP.
In the first type of experiments, porphyrin was sub-
limated under dynamic vacuum conditions onto liquid-
nitrogen-cooled CsI support. CO was supplied simulta-
neously using a fine valve and an injector. Because
boiling points of CO and N2 are close [16], CO conden-
sation on liquid-nitrogen-cooled support is impossible.
However, it was found that the simultaneous condensa-
tion of CO and CoTPP results in the capture of CO
leading to the appearance of a new intense line of the IR
spectrum at 2000 cm–1 corresponding to coordinated
CO. Then, we treated the complex with various por-
The band at 2066 (2019) cm–1 (Fig. 1) has large a
tions of O2, and recorded IR spectra at different temper- halfwidth and can consist of two overlapping bands. In
atures of support. Temperature was measured by this case, high-frequency bands at 2086 (2040) cm–1 can
chromel–alumel thermocouple. The end of the thermo- be assigned to the cophased vibrations of carbonyl
couple was inserted into a tight hole in a CsI crystal. ligands in the dicarbonyl complex. According to [18, 19],
Temperature between liquid nitrogen and solid carbon these ligands can be active in this case if twisted coor-
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SPECTRAL INVESTIGATION OF THE INTERACTION OF CARBON MONOXIDE
283
O
O
O
C
O
O
Co
F
Co
<
Co
C
O
N
(A)
(B)
(C)
D
N
=
Co
Co
N
N
Scheme 1.
dination of CO is assumed. However, the results of our resolved, only shoulders on high-frequency wings of
experiments damage this assumption. We did not find the main bands (Fig. 6a).
any change in the ratio of the intensities of the main
The frequency of the monocarbonyl complex in subli-
band and the high-frequency band at a much lower rate
mated layer is lower than in the Ar matrix by ~12 cm–1 and
of CO supply when the probability of the formation of
lower than in the Kr matrix by 7 cm–1 (see Fig. 1). That is,
dicarbonyl complex should be minimal.
surrounding influences considerably the frequency of the
In low-temperature experiments, it was difficult to
exclude the presence of water traces in the system. In
the experiments with matrix-isolated CO, the presence
of two bands separated by ~10 cm–1 lying in the region
corresponding to the vibrations of νCO is explained by
this difficulty in [16]. The high-frequency band of these
two is assigned to CO bound to water by a hydrogen
bond [20]. The direction of the shift and the band shape
are similar to those observed in our experiments.
Therefore, in our experiments, it is also impossible to
exclude the formation of hydrogen bond between the
coordinated CO and trace H2O molecules. We did not
find absorption bands in the region 3700–3100 cm–1
where the stretching vibration of H2O was found, but it
is not excluded that this is related to the low intensity of
these bands. However, in experiments where CO was
additionally mixed with water, we did not find the rela-
tive growth of high-frequency bands. This evidently
contradicts to our assumption.
coordinated diatomic ligand. Analogous conclusions were
drawn in [12] for coordinated dioxygen.
Mixed Carbonyl–Oxygen Extra-complex CoTPP
The addition of oxygen to the sublimated layer con-
taining CoTPP · CO (Scheme 1A) leads to considerable
changes in the spectrum in the region of νCO (Fig. 2).
The intensity of the band at ~2070 cm–1 dramatically
decreases, a new absorption band appears in the high-
frequency region at 2119 cm–1. Experiments with 13CO
(Fig. 3) showed that the new band can be assigned to
the vibrations including CO group. The evacuation of
oxygen from cryostat leads to a decrease in the inten-
sity of new band and to an increase in bands of coordi-
nated carbonyl group.
The frequencies of the band at 2119 cm–1 and its 13C
isotope analog at 2072 cm–1 are close to the frequencies
of CO (13CO) in the argon matrix, that is, to 2138
(2093) cm–1 [18]. Therefore, it is possible to suppose
that oxygen substitutes CO from the coordination
sphere of CoTPP, and it remains in the bulk of the layer
in adsorbed (probably, physically adsorbed) state.
It is necessary to note that the Fermi-resonance
nature of the band is also hardly probable, because this
nature also manifests in the experiments with 13CO.
In this work, the reason for the appearance of high-
frequency bands at 2086 (2040) cm–1 was not deter-
However, there are some arguments against this inter-
mined. Perhaps, it is related to the features of molecule pretation of the nature of the band at 2119 (2072) cm–1. If
packing in the layer and with different surroundings of CO (or 13CO) is supplied to low-temperature sublimate
coordinated CO in the samples obtained by the co-con- of CoTPP, another band at 2136 (2086) cm–1 appears in
densation of reagents. This is confirmed by the experi- the IR spectrum in addition to the band corresponding
ments where CO was supplied onto preliminarily pre- to the coordinated CO. This new band could be
pared layers. In these experiments, the bands are not assigned to the physically adsorbed CO (Fig. 6a). Nev-
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284
KURTIKYAN et al.
layer of CoTPP are different. This difference can lead
to the difference in adsorption sites and CO molecule
vibration frequencies.
1
2
The following objection is more serious. It is possi-
ble to see from Figs. 2 and 3 that the appearance of a
new band in the region of νCO is accompanied with the
appearance of a new band in the region of the vibration
frequencies of coordinated dioxygen (~1198 cm–1).
This last band is shifted by more than 50 cm–1 toward
lower frequencies with respect to νO2 of mono-oxygen
complex CoTPP · O2 (Scheme 1B), which was
observed in the sublimated layer [17]. Here, the picture
is much less clear due to the presence of the absorption
bands of porphyrin itself. However, the experiments
with 18O2 (Fig. 4) where an isotope analogue of this band is
recorded as the low-frequency shoulder of the porphyrin
band at unambiguously support the assignment of this
band to νO2. The intensity of this band correlates with the
intensity of the band at 2119 cm–1, increases with an
increase in the O2 pressure, and decreases when O2 is evac-
uated.
Thus, it is possible to assume that the bands at 2119
and 1198 cm–1 correspond to the vibrations of CO and
O2 in the mixed oxygen–carbonyl complex (Scheme 1C).
This assignment is based on the fact that the substitu-
2200 2100 2000
1300 1200 1100
ν, cm–1
Fig. 2. IR spectra in the region of the vibration of CO and
O molecules at 80 K. (1) co-condensate of CoTPP with CO
2
13
tion of CO for CO in the experiments with oxygen
and (2) the transformation of spectrum 1 after the treatment
of the sublimate with 90 torr of O .
addition does not lead to the shift of the bands in the
2
region of O2 vibration (compare the data presented in
Figs. 1 and 2 at 1300–1100 cm–1). In turn, the bands
corresponding to the vibrations including the carbon
atom (2000–2150 cm–1) do not change when 18O2 is
supplied to the cryostat. This is important, because it is
impossible to exclude some other reactions leading to
1
the formation of new carbon–oxygen bonds. For exam-
.
ple, the CO3 radical was shown to be formed as a result
2
of the low-temperature photolysis of CO2 [21]. The IR
spectrum of this radical contains absorption bands in
the region of 2000 and 1100 cm–1.
The stability of the oxygen–carbonyl complex is
poor. Even at the liquid nitrogen temperature, the evac-
uation of oxygen leads to its slow decomposition.
The mixed carbonyl–oxygen complex was also
found in the Ar and Kr matrices [18]. The frequencies
of vibrations νCO and νO2 in this matrix-isolated com-
plex are at 2089 and 1261 cm–1, respectively. Although
the direction of the frequency shift of these vibrations
during the formation of the mixed complex is the same
as in the case of the sublimated layer, an increase in
νCO and a decrease in νO2, the values of these shifts,
are higher. For example, an increase in νCO when car-
bonyl complex is substituted for the mixed oxygen–car-
bonyl complex, is equal to 15 cm–1, and a decrease in
νO2 when the oxygen complex O2 · CoTPP is substi-
tuted with O2 · CoTPP · CO is equal to 17 cm–1. For the
sublimated layer, these shifts reach values of 54 and
53 cm–1, respectively. This difference in the frequencies
2100 2000
1300 1200 1100
ν, cm–1
Fig. 3. IR spectra in the region of the vibration of CO and
O molecules at 80 K. (1) co-condensate of CoTPP with
2
13
CO and (2) the transformation of spectrum 1 after the
treatment of the sublimate with 50 torr of O .
2
ertheless, these arguments cannot be considered strict,
because it is impossible to exclude that the structures of
sublimated layers formed by co-condensation of inter-
acting components (CO and CoTPP) and formed by the
action of CO on the preliminarily formed sublimated of coordinated ligands during the transfer from the iso-
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SPECTRAL INVESTIGATION OF THE INTERACTION OF CARBON MONOXIDE
285
lated state to the associated state can be related to
extremely strong intermolecular interaction in the sub-
limated layer or with the difference in coordination.
1
It was shown in [22–24] that the molecules of tet-
raarylporphyrin compounds can hardly be packed in
the three-dimensional space due to the large size and
hardness of the molecule, as well as due to almost per-
pendicular orientation of aryl rings with respect to por-
phyrin macroring. As a result, they form macroporous
structures with molecular-sized pores. That is why
these structures are called “porphyrin sponges.” It is
important that the pores of these compounds are long
narrow channels located opposite to the axial positions
of the metal. The scheme of the cross section of this
channel is presented in Scheme 2A as a circle. If these
compounds are precipitated from a solution, these
pores are filled with the solvate molecules or with axi-
ally coordinated ligands.
2
3
In low-temperature sublimates of MTPP, these
pores make it possible the diffusion of potential ligands
into the bulk of the layer and the formation of axial
complexes along its full thickness [17]. The analysis of
structural data [22] shows that, in the system under
investigation, one of the ligands can be in tight contact
with another ligand coordinated by the porphyrin mol-
ecule from the layer placed through one layer from the
first molecule. This is shown in Scheme 2B. These con-
tacts are probably a reason for much more pronounced
shifts of stretching vibration frequencies during the for-
mation of the mixed complex in the sublimated layer
compared to the case of matrix isolation.
1300
1200
1100
ν, cm–1
Fig. 4. (1) IR spectrum of co-condensate of CoTPP with CO
in the region of oxygen vibration, (2) the transformation of
spectrum 1 after the supply of O , and (3) the transformation
2
18
of spectrum 1 after the supply of O .
2
We cannot exclude completely the possibility of
ligand coordination from the same side of porphyrin
plane accompanied by the coming of metal ion out of
this plane. The possibility of the coordination of two O2
molecules from the same side of the metal phthalocya-
nine plane was mentioned in [25]. In any case, the fact
that the additional coordination of CO leads to the more
Oxidative Catalysis on Low-Temperature
Sublimates of CoTPP
The heating of the co-condensate containing the
mixed oxygen–carbonyl complex leads to a decrease in
the intensity of νCO and νO2 bands in this complex and
efficient activation of molecular oxygen resulting in a to an increase in the intensity of the bands at 2336
further decrease in νO2 frequency and in the strength (2268) cm–1 (Fig. 5) and 658 (640) cm–1 (in the experi-
13
coefficient of the O–O bond is important.
ments with CO and CO, respectively). These bands
Co
M
O
O
M
M
Co
Co
O
C
M
Co
F
(A)
(B)
<
Scheme 2.
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KURTIKYAN et al.
the adsorption of CO2 in the first case. This adsorption
1
2
probably involves active catalytic sites in the layer.
Indeed, after evacuation at about 160 K of initially
formed CO2 and after the subsequent supply of a new por-
tion of CO + O2 mixture at the liquid-nitrogen temperature,
the formation of a new portion of CO2 takes place. Thus,
the low-temperature oxidation of carbon monoxide is cata-
lytic. The IR spectrum of the sublimated layer of CoTPP
after the removal of the coordinated ligand and CO2 at an
elevated temperature reproduces the spectrum of the initial
CoTPP; that is, the molecules of CoTPP do not change dur-
ing the process of CO oxidation.
13
18
If the mixture CO + CO + O2 + O2 is used, six
bands at 2334, 2317, 2300, 2268, 2252, and 2233 cm–1
can be found in the IR spectrum. The calculations car-
ried out for the ν3 of the linear triatomic molecule [26]
demonstrate an agreement of the frequencies of these
bands with the anticipated frequencies for the mole-
cules of CO2, OC18O, 18OC18O, 13CO2, O13C18O, and
3
18O13C18O. If the gas content of the cryostat where the
experiment was carried out, is frozen on cold support of
another cryostat, these bands give absorption maxima
at 2346, 2330, 2314, 2280, 2264, and 2247 cm–1. Thus,
the vibration frequencies of solid CO2 and of CO2 in the
2300
2200
2100
2000 ν, cm–1
presence of sublimated layer differ by 12–14 cm–1. This
confirms that CO2 molecules are concentrated inside
the layer, but not on its surface in the form of a frozen
film. Six bands with somewhat lower resolution were
also recorded at 660–630 cm–1 where the bands of
degenerate deformational vibrations νCO2 of various
isotopic modifications of carbon dioxide are reflected.
Fig. 5. (1) IR spectrum of co-condensate of CoTPP with
CO at 80 K, (2) the transformation of spectrum 1 after the
supply of oxygen and heating the layer up to 130 K, and (3)
the transformation of spectrum 1 after the supply of oxygen
and heating the layer up to 150 K.
13
13
In the experiments, we used C16O and C16O. There-
can clearly be assigned to νCO2. An increase in temper-
ature leads to the decomposition of mixed oxygen–car-
bonyl complex, therefore the strict correlation between
a decrease in the intensity of νCO band in this complex
and an increase in the intensity of ν3CO2 band seems to
be impossible. However, it is possible to suppose that
the presence of the mixed complex in the sublimated
layer promotes the catalytic oxidation of CO because
the O–O bond in this complex became weaker.
18
18
fore, the presence of OC18O and O13C18O as final
reaction products demonstrates the possibility for the
formation of a compound with two 18O atoms as reac-
tion intermediate. This could be, for example, the
.
above-mentioned CO3 radical, which may have the
trans-structure O–O–C=O [27] or the structure with a
three-membered ring [21].
It is interesting to note that this reaction does not
take place if the mixture of CO with O2 is supplied to
the cooled sublimate obtained by the deposition at
room temperature or if the low-temperature sublimate
was re-cooled after heating the layer to room tempera-
ture. Without solvate molecules or axially coordinated
additional ligands, this process leads to the contraction
of pores [22]. This contraction must hinder the efficient
diffusion of reactant molecules into the film. This was
confirmed by our experiments with molecular oxygen
CO2 formation was also detected when heating the
sublimated layer of CoTPP onto which CO and O2 were
consecutively or simultaneously supplied at the liquid
nitrogen temperature (Fig. 6). In this case, not only the
bands corresponding to coordinated CO, O2, and the
mixed complex were observed in the IR spectrum, but
also the bands corresponding to adsorbed CO. The
intensity of the band of carbon dioxide increases in this
case up to a certain value and does not increase further,
in spite of the presence of CO and O2 in the cryostat.
Carbon dioxide formed in the first cycle probably [12]. In these experiments, we did not detect O
2 coordi-
inhibits the further process. This effect was also nated by CoTPP by spectroscopy at liquid-nitrogen
observed on phthalocyanines [9]. In this case, the fre- temperature if the pressure of O2 was lower than
quency of CO2 vibration is higher by 12–14 cm–1 than 300 torr and CoTPP layer was prepared by sublimation
the frequency found for solid CO2 obtained by recon- at room temperature or this layer was preliminarily
densation of CO2 onto a cold support of second cryostat heated to room temperature. In our opinion, the main
connected with the first one in a series. This confirms reason for these differences is related to the activity of
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287
Our data show, how strong the catalytic activity of
the sublimated layer depends on its structure, which is
in turn determined by the conditions of its formation.
This dependence was mentioned many times [3, 28] for
organic semiconductor materials. Thus, the supermo-
lecular structure of sublimated layer of CoTPP plays a
decisive role in the catalytic reaction of carbon monox-
ide oxidation by molecular oxygen.
ACKNOWLEDGMENTS
This work was supported by International Science
and Technology Center, grant no. A-21.
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