Reduction requirements for Ru/(K)Fe2O3 catalytic activity in water-gas shift reaction

Article abstract of DOI:10.1007/s10975-005-0016-9

This paper is focused upon the influence of potassium on the reduction behavior and catalytic properties of Fe₂O₃, Ru/Fe ₂O₃ and Ru/(K)Fe₂O₃ catalysts for the water gas shift (WGS) reaction. The effect of promotion by potassium is attributed to stabilization of a highly dispersed ruthenium phase on the iron oxide surface. The hydrogen reduction behavior of Fe₂O₃ catalysts is strongly influenced by time-pressure dependent processes and comprises two or three heavily overlapped TPR peaks which can be ascribed to the following stages of the iron(III) oxide reduction 3Fe₂O₃ → 2Fe₃O₄ → 6FeO → 6Fe. The appearance of FeO as an intermediate phase was confirmed by XRD. The presence of ruthenium(IV) oxide substantially changes the kinetics of the reduction process. In the case of potassium-doped catalysts, the reduction of Fe₂O₃ is substantially different and is assigned to the reduction phase of KFeO ₂. Both ruthenium and potassium have a promoting effect on the catalytic activity for the WGS reaction.

Full text of DOI:10.1007/s10975-005-0016-9

Kinetics and Catalysis, Vol. 46, No. 1, 2005, pp. 107–113. Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 115–121.
Original Russian Text Copyright © 2005 by Malyshev, Paukshtis, Malysheva.
Interaction of N2 with the Acid Sites of Oxides
M. E. Malyshev, E. A. Paukshtis, and L. V. Malysheva
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received June 19, 2003
Abstract—Nitrogen adsorption on SiO2, γ-Al2O3, TiO2, and sulphated zircona (SO2– /ZrO2) is studied by Fou-
4
rier transform IR spectroscopy. Integrated absorption coefficients for the bands due to the N–N vibrations in
nitrogen complexes with Brønsted and Lewis acid sites are determined. A general correlation between inte-
grated absorption coefficients and the positions of N–N bands of nitrogen interacting with the above sites in
zeolites and oxides is discussed. The orientation of a nitrogen molecule relative to Brønsted and Lewis acid sites
is calculated ab initio using a 6-31G** basis set.
INTRODUCTION
groups to form H-complexes. The shift of the absorp-
tion bands of hydrogen-bonded OH groups in com-
plexes with nitrogen and CO is proportional to the
strength of proton sites. The integrated absorption coef-
ficients for the bands due to N–N vibrations in the com-
plexes with bridging OH groups in the HNaY and
HZSM-5 zeolites were calculated. We established a
correspondence between the absorption bands of CO
and nitrogen in the complexes and Lewis sites. The
integrated absorption coefficient for the 2350 cm–1
band due to N–N vibrations in the complexes with
Lewis sites of the HNaY zeolite was calculated. The
integrated absorption coefficient data were correlated
with the N−N band positions for adsorbed nitrogen.
IR spectroscopy of adsorbed molecules with the use
of the CO molecule as a probe is widely employed to
determine the strength and concentration of various
acid sites in zeolites and oxides [1, 2]. In recent years,
small nonpolar molecules, for example, nitrogen, have
been used as probes along with CO [3–7]. Nitrogen has
been found to form donor–acceptor complexes with the
strong Brønsted and Lewis acid sites of zeolites. There
have also been works devoted to the interaction
between the nitrogen and the acid sites of oxides, which
are weaker than sites on the zeolite surface.
It has been shown [8] that nitrogen interacts with
surface Si–OH groups to form H-complexes when it is
adsorbed on silica at –196°ë. In the IR spectrum, the
intensity of the 3749 cm–1 absorption band decreases
and a band at 3715 cm–1 appears. The latter was
assigned to the vibrations of the Si–OH groups in the
nitrogen H-complexes.
Here, we report the adsorption of nitrogen and CO
on oxides with Brønsted and Lewis sites that differ in
nature and strength from the acid sites of zeolites.
EXPERIMENTAL
Upon the interaction of N2 with η-Al2O3 [9], the
spectrum of adsorbed nitrogen shows only an absorp-
tion band with a maximum at 2360 cm–1. This band was
assigned to the vibrations of nitrogen in its complexes
with surface aluminum atoms.
Upon the adsorption of nitrogen (14N–14N) on the
surface of titania [10], two absorption bands were
observed in the spectrum in the region of N–N vibra-
tions, namely, a strong band at 2337 cm–1 and a weak
band at 2345 cm–1. These bands were assigned to nitro-
gen complexes with titania Lewis sites. The integrated
absorption coefficient for the band at 2337 cm–1 was
determined. It is shown in this work that bands at 2260
and 2268 cm–1 appear in the spectrum after the adsorp-
tion of nitrogen (15N–15N).
Substances. We used mesoporous chromatographic
SiO2 (Ssp = 500 m2/g), γ-Al2O3 (brand A1, produced by
Ryazan refinery; Ssp = 200 m2/g), and TiO2 (prepared
from TiCl4 by precipitation with aqueous ammonia and
calcined at 500°ë; Ssp = 200 m2/g). An SO42– /ZrO2 sam-
ple was prepared from zirconia by incipient-wetness
impregnation with a sulfuric acid solution. It had an
SO42– content of 5 wt % and Ssp = 80 m2/g.
High-purity nitrogen and CO were used for adsorp-
tion. Nitrogen was additionally purified over an Ni/Cr
catalyst and then dried with CaA zeolite. Carbon mon-
oxide was also additionally dried.
Before adsorption, samples of SiO2, TiO2, and
SO42–/ZrO2 were calcined at a temperature of 500°ë,
whereas γ-Al2O3 was calcined at 500 or 700°ë
(hereafter, Al2O3500 and Al2O3700, respectively).
Previously [11], we studied samples of Y-zeolites
(with various degrees of exchange that were calcined at
various temperatures) and HZSM-5. Nitrogen was
shown to interact with both weak and strong acid OH Next, the samples were pumped at 400°ë and a pressure
0023-1584/05/4601-0107 © 2005 MAIK “Nauka /Interperiodica”

108
MALYSHEV et al.
of <10–4 Torr for 1 h, and then the sample temperature
was decreased to –196°ë with the use of liquid nitro-
gen. The experiments were carried out according to a
protocol described in [11].
types of Ti–OH group are present on the surface, giving
rise to absorption bands at 3728, 3673, and 3639 cm–1.
Carbon monoxide and N2 form H-complexes that
include Ti–OH groups as well as Al–OH groups. The
intensities of the initial Ti–OH groups in the IR spectra
decrease, and, simultaneously, absorption bands due to
hydrogen-bonded Ti–OH groups with maxima at 3610,
3550, and 3465 cm–1 (Fig.1h, curve 2) or at 3683, 3630,
and 3585 cm–1 (Fig. 1g, curve 2) appear in the lower
frequency region.
The IR spectra of the original OH groups in the sul-
phated zirconia sample calcined at 500°C are presented
in Figs. 1i and 1j (curves 1). A typical absorption band
at 3642 cm–1 is present in the spectrum, indicating the
presence of Zr–O–S–OH groups on the surface.
When CO or N2 interacts with the Zr–O–S–OH
groups, H-complexes are formed, the band due to the
original OH groups weakens, and bands due to the
hydrogen-bonded Zr–O–S–OH groups at 3480 and
3574 cm–1 appear (Figs. 1i and 1j, curves 2).
As was mentioned in our earlier work [11], the shift
of the absorption bands of the surface OH groups in
zeolites upon the formation of H-complexes with a
probe molecule is determined by both the proton affin-
ity (PA) of the acid site and the basicity of the probe
molecule [1]. It can be concluded from the above data
that this assumption is also true for oxides and that the
RESULTS AND DISCUSSION
It is well known that only Si–OH groups are present
on the silica surface. Figures 1a and 1b (curves 1) show
the IR spectrum of the original OH groups in the SiO2
sample calcined at 500°ë. A narrow absorption band
with a maximum at 3740 cm–1 is observed, which is
typical of silica and is assigned to vibrations of the
Si−OH groups.
H-complexes are formed upon the interaction of CO
with Si–OH groups, and the intensity of the band at
3740 cm–1 decreases. A band at 3656 cm–1 appears
simultaneously, which is assignable to the hydrogen-
bonded OH groups (Fig. 1b, curve 2).
Nitrogen also interacts with Si–OH groups upon
adsorption on SiO2 to form H-complexes. The intensity
of the absorption band at 3740 cm–1 decreases, and
simultaneously a weak absorption band peaking at
3714 cm–1 appears due to the presence of hydrogen-
bonded Si–OH groups (Fig. 1a, curve 2). For conve-
nience, the 3650–3800 cm–1 region is enlarged in
Fig. 1a.
The IR spectrum of the original OH groups in the
γ-Al2O3 sample calcined at 500°ë is presented in
Figs. 1c and 1d (curves 1). The spectrum is a compli-
cated contour in which one can distinguish absorption
bands peaking at 3745, 3722, 3710, and 3685 cm–1 that
are due to vibrations of the Al–OH groups [1].
When CO interacts with the Al–OH groups, H-com-
plexes are formed, the intensities of the bands due to the
initial OH groups decrease, and overlapping bands at
3450–3675 cm–1 appear simultaneously (Fig. 1d,
curve 2). A detailed analysis of these bands was per-
formed in [2]. When nitrogen is adsorbed on the surface
of the Al2O3500 sample, H-complexes with Al–OH
groups are also formed. However, it is impossible to
make out from the IR spectrum which Al–OH groups
interact with N2 and what bands appear as a result,
because these bands are weak (Fig. 1c, curve 2).
The IR spectrum of the original OH groups in the
Al2O3700 sample is shown in Figs. 1e and 1f (curves 1).
This spectrum, as in the case of Al2O3500, is a compli-
cated contour containing the same set of absorption
bands due to vibrations of the OH groups, but these
bands are weaker.
When CO or N2 interacts with the Al–OH groups of
Al2O3700 (Fig. 1f and 1e, curves 2), the same features
are observed in the IR spectra as in the case of the
Al2O3500 sample.
ratio ∆ν(éç)ëé/∆ν(éç)N is 2.3 for both oxides and
zeolites.
2
Figure 2 presents the IR spectra of adsorbed nitro-
gen and CO at various pressures. As can be seen in
Fig. 2b, absorption bands at 2154 and 2170 cm–1 are
observed in the spectrum of CO adsorbed on SiO2. The
band at 2154 cm–1 is assigned to CO vibrations in com-
plexes with Si–OH groups [1]. The 2170 cm–1 band is
usually assigned to CO vibrations in complexes with
bridging Al–OH–Si groups [1]. A weak band at
2196 cm–1 is also observed, which is assigned to CO
vibrations in the complexes with Lewis sites [1]. The
bands at 2170 and 2196 cm–1 in the spectrum of
adsorbed CO are in our case most likely due to traces of
alumina in the sample.
An intense band at 2321 cm–1 in the region of N–N
vibrations is seen in the spectrum of nitrogen adsorbed
on SiO2 (Fig. 2a). It can be assigned to N–N vibrations
in complexes with Si–OH groups, since it appears
together with the band due to hydrogen-bonded Si–OH
groups at 3715 cm–1. A weak shoulder at 2330 cm–1,
which is assigned to N–N vibrations in the complexes
with bridging Al–OH–Si groups [11], as well as the
appearance of a band at 2170 cm–1 in the spectrum of
adsorbed CO, indicate the presence of traces of alumina
in the SiO2 sample. No bands assignable to N–N vibra-
tions in the complexes with alumina Lewis sites, which
could have occurred at a higher frequency, were found.
The spectra of the original OH groups of the titania
sample calcined at 500°C are shown in Figs. 1g and 1 h
The spectrum of CO adsorbed on Al2O3500 is
(curves 1). As follows from this spectrum, three main shown in Fig. 2d. Absorption bands peaking at 2150
KINETICS AND CATALYSIS Vol. 46 No. 1 2005

INTERACTION OF N2 WITH THE ACID SITES OF OXIDES
109
Absorbance, cm2/g
N2
CO
150
100
50
10
(a)
(b)
0
–10
–20
3700 3750 3800
1
1
2
2
0
(c)
(d)
40
20
0
1
2
1
2
30
20
10
0
(e)
(f)
1
2
1
2
12
8
(g)
(h)
1
2
4
1
2
0
–4
(i)
(j)
12
8
1
2
1
4
0
2
–4
3200
3400
3600
3800 3200
3400
3600
3800
ν, cm–1
Fig. 1. IR spectra of the OH groups of (a, b) SiO , (c, d) γ-Al O 500, (e, f) γ-Al O 700, (g, h) TiO and (i, j) SO2– /ZrO : (1) original
2
2
3
2
3
2
2
4
samples and (2) differential spectra after adsorption of (a, c, e, g, i) N and (b, d, f, h, j) CO. P = 32 Torr, P = 3 Torr.
2
CO
N2
and 2180 cm–1 are present in this spectrum. The band at
In the region of N–N vibrations in the spectrum
2150 cm–1 arises from CO vibrations in the hydrogen- of nitrogen adsorbed on Al2O3500, there are absorp-
bonded complexes with Al–OH groups, whereas the tion bands peaking at 2325 and ~2340 cm–1
band at 2180 cm–1 is due to CO vibrations in the com- (Fig. 2c). The band at 2325 cm–1, which appears as
plexes with weak Lewis sites of alumina [1].
a shoulder, can be assigned to N–N vibrations in the
KINETICS AND CATALYSIS Vol. 46 No. 1 2005

110
MALYSHEV et al.
CO
Absorbance, cm2/g
N2
0.4
(a)
(b)
20
15
10
5
0.3
0.2
0.1
×10
0
0
1.2
60
40
20
0
(c)
(d)
0.8
0.4
0
(e)
(g)
(i)
(f)
(h)
(j)
30
20
10
0
2
1
0
6
50
40
30
20
10
0
4
2
0
15
30
20
10
5
10
0
0
2300
2350
2050
2100
2150
2200
2250
ν, cm–1
Fig. 2. IR spectra of the (a, c, e, g, i) N and (b, d, f, h, j) CO adsorbed on (a, b) SiO , (c, d) γ-Al O 500, (e, f) γ-Al O 700, (g, h)
2
2
2
3
2 3
TiO , and (i, j) SO2– /ZrO . The spectra were recorded while increasing the adsorbate pressure to 32 Torr for N and 3 Torr for CO.
2
2
2
The spectra at P 4= 0.01 Torr are shown in the inset at ×10 magnification.
CO
H-complexes with Al–OH groups [11]. The band at
One can distinguish the following absorption bands
2340 cm–1 corresponds to N–N vibrations in the at 2152, 2183, and 2194 cm–1 in the region of C–O
complexes with alumina Lewis sites, and the band at vibrations in the spectrum of CO adsorbed on Al2O3700
2180 cm–1 is due to C–O vibrations in adsorbed CO. (Fig. 2f). The band at 2152 cm–1 is assigned to CO
KINETICS AND CATALYSIS Vol. 46 No. 1 2005

INTERACTION OF N2 WITH THE ACID SITES OF OXIDES
111
vibrations in the H-complexes with Al–OH groups,
logA0 [cm/µmol]
0
while the bands at 2183 and 2194 cm–1 are assigned to
CO vibrations in the complexes with two types of alu-
mina Lewis site [1].
II
23
–0.2
21
I
10
22
20
9
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
8
19
The spectrum of N2 adsorbed on Al2O3700 shows
the same absorption bands with the same assignments
as the spectrum of Al2O3500 (Fig. 2e). In contrast to the
spectrum of CO adsorbed on Al2O3700, the band due to
the interaction of nitrogen with the second type of
Lewis site cannot be distinguished here.
Absorption bands at 2152, 2160, and 2175 cm–1 are
seen in the IR spectrum of CO adsorbed on TiO2
(Fig. 2e). The first two bands can be assigned to CO
vibrations in the H-complexes with Ti–OH groups, and
the band at 2175 cm–1 is assigned to CO vibrations in
the complexes with weak Lewis sites of titania [1].
In the region of N–N vibrations in the spectrum of
nitrogen adsorbed on TiO2 (Fig. 2g), a band at
2333 cm–1 is observed, which corresponds to N–N
vibrations in the complexes with Lewis sites of titania,
like the band at 2175 cm–1 in the spectrum of adsorbed
CO. A weak shoulder near 2325 cm–1 can be assigned
to N–N vibrations in the H-complexes with Ti–OH
groups.
7
11
12
13
6
5
4
3
18
17
1
2
16
14
15
2310
2320
2330
2340
2350
2360
ν, cm–1
Fig. 3. Correlation between the integrated absorption coef-
ficient and the N–N band position for nitrogen interacting
with acid sites of (1, 5) HZSM-5 500 [11], (2) SiO 500, (3,
2
15) γ-Al O 500, (4, 12, 19, 22) H-β [6], (6) HY(1)400 [11],
2
3
(7, 21) HY(2)700 [11], (8) HY(1)500 [11], (9, 20)
HY(2)600 [11], (10) H-GaZSM-5 [3], (11) H-FER [7], (13)
H-MOR [4], (14) TiO , (17) γ-Al O 700, (18) SO2– /ZrO ,
2
2
3
2
4
and (23) η-Al O [9].
2
3
The IR spectrum of CO adsorbed on SO42– /ZrO2 is
due to N–N vibrations in the complexes with Lewis
sites of alumina (2337 and 2338 cm–1), titania
(2333 cm–1), and zirconia (2340 cm–1) are 0.02, 0.049,
0.045, and 0.123 cm/µmol, respectively.
Figure 3 plots, as dots, the integrated absorption
coefficient as a function of the position of the N–N band
due to nitrogen interacting with Brønsted and Lewis
acid sites in silica, alumina, titania, sulphated zirconia,
HZSM-5 zeolite, and HNaY zeolites differing in the
degree of exchange and calcination temperature [11].
Furthermore, we plot data that we calculated, by a pro-
cedure reported in [11], from earlier reported spectra of
nitrogen adsorbed on the zeolites HGaZSM-5 [3],
H-MOR [4], H-β [6], and H-ferrerite (H-FER) [7].
As can be seen in Fig. 3, data points are arranged
near the two curves. Points 1–13 in the region of nitro-
gen vibrations at 2318–2332 cm–1 refer to the interac-
tion of nitrogen with proton sites (curve I), and points
14–23 at 2330–2360 cm–1 refer to the formation of
nitrogen complexes with aprotic sites (curve II). The
integrated absorption coefficient for both types of com-
plex increased with increasing frequency of N–N vibra-
tions.
shown in Fig. 2j. One can see absorption bands at 2169
and 2198 cm–1 and a shoulder at 2189 cm–1. The first
band was assigned to vibrations caused by CO interac-
tion with (Zr–O–S)–OH groups. The bands at 2189 and
2198 cm–1 can be assigned to CO vibrations in the com-
plexes with two types of the Lewis acid site of zirconia.
In the spectrum of nitrogen adsorbed on SO42– /ZrO2
(Fig. 2i), an asymmetrical contour with a peak at
2340 cm–1 is observed in the region of N–N vibrations.
This band can be assigned to N–N vibrations in the
complexes with Lewis sites of zirconia. The asymmet-
rical shape of the curve suggests the presence of a weak
band near 2330 cm–1 due to N–N vibrations in the com-
plexes with (Zr–O–S)–OH groups.
The integrated absorption coefficient A0(N–N) for the
complexes with acid sites was estimated as described in
[11]. The integrated absorption coefficient for Al–OH
and Si–OH bands in the region of the vibrations of OH
groups is 3.0 cm/µmol; it was determined on the basis
of the correlations described in [1] from the decay of
bands in the 3650–3750 cm–1 region and from the aver-
age integrated absorption coefficient of the 2152 cm–1
band in the region of the vibrations of adsorbed CO (the
latter is equal to 2.6). The A0(N–N) values for N–N bands
at 2321 and 2325 cm–1 for the SiO2 and Al2O3500 sam-
ples were calculated to be 0.066 and 0.113 cm/µmol,
respectively.
Gaseous dinitrogen is known to give no IR absorp-
tion bands, because it has a symmetrical structure and,
accordingly, a dipole moment invariable upon absorp-
tion. In Raman spectra, the frequency of N–N vibra-
tions is 2329.9 in the gas-phase, 2327.1 in the solid
The integrated absorption coefficient A0(L…CO) for state, and 2325.9, 2324.3, and 2322.0 cm–1 in the argon,
bands of CO in the complexes with Lewis sites of alu- krypton, and xenon matrices, respectively [12]. As can
mina, titania, and zirconia were taken from [1]. The cal- be seen in Fig. 3, the frequencies of N–N vibrations in
culated integrated absorption coefficients for the bands the complexes with Brønsted sites are lower than the
KINETICS AND CATALYSIS Vol. 46 No. 1 2005

112
MALYSHEV et al.
(‡)
(‡)
N
N
153°
H
O
Al
Si
N2
–16.87 eV
N–N
(b)
N
92° N
Al
O
(b)
Fig. 5. Models of the nitrogen complexes with (a) a Brøn-
sted and (b) a Lewis acid site.
Furthermore, we calculated the distribution of electron
density in the N2+ and HN2+ species, which model the
N–N
interaction of nitrogen with Brønsted and Lewis acid
sites, respectively. All the bonding orbitals of nitrogen
are filled, whereas the N2+ and HN2+ species have unoc-
N2+
–14.33 eV
cupied low-energy bonding orbitals. As is clear from
Fig. 4a, any interaction with nitrogen is possible only in
the direction perpendicular to the N–N bond and this
can result only in a decrease in the N–N frequency.
From the electron density distribution for the lowest
(c)
unoccupied orbital, one can deduce that the N2+ species
(Fig. 4b) forms a complex via a two-center mechanism
and the HN2+ species (Fig. 4c) forms a complex via a
HN2+
–4.75 eV
HN–N
one-center mechanism. When the N2+ and HN2+ species
interact with an electron-donor site (base), the fre-
quency of N–N vibrations can increase. Hence, one can
assume that the back donation effect, which means that
the adsorbate not only donates electrons but also
accepts them, plays an important role in the interaction
between nitrogen and Lewis and Brønsted acid sites.
The orientations of the nitrogen molecule on the
Brønsted and Lewis acid sites were ab initio calculated
for fragments of the HZSM-5 zeolite (80 atoms) and
γ-Al2O3 (30 atoms) using the HyperChem Pro 6 pack-
age and a 6-31G** basis set (necessary structural data
were taken from Inorganic Crystal Structure Data
Base). The geometry of the nitrogen complexes with
Brønsted and Lewis acid sites is shown in Fig. 5. The
Fig. 4. Spatial probability density function for an electron
occurring in the HOMO of (a) the nitrogen molecule and in
the LUMOs of the (b) N2+ and (c) HN2+ species.
frequency of N–N vibrations in free nitrogen, whereas
the frequencies of N–N vibrations in the complexes
with Lewis sites are higher than the frequency of N−N OHN angle in the H-complex is 174°, and the HNN
angle is 153°, not 180°, as was accepted in previous
works. The AlNN angle in the nitrogen complex with a
Lewis site is 92°; that is, the nitrogen molecule is
aligned with the Al–O bond. The calculated data sug-
gest that the electrons of one atom of an adsorbed nitro-
gen molecule interact with the proton and the oxygen
atom of a bridging OH group; that is, we have a one-
vibrations in free nitrogen. This finding for the Brøn-
sted and Lewis acid sites is, possibly, evidence that the
corresponding nitrogen complexes have different struc-
tures.
To interpret the results obtained, electron density
distribution in the free nitrogen molecule was calcu-
lated ab initio using the HyperChem Pro 6 package. center two-bonded mechanism. When a complex with a
KINETICS AND CATALYSIS Vol. 46 No. 1 2005

INTERACTION OF N2 WITH THE ACID SITES OF OXIDES
113
2. Knözinger, H., Proc. Int. Symp. on Acid–Base Catalysis,
Tanabe, K., Hattori, H., Yamaguchi, T., and Tanaka, T.,
Eds., Tokyo: Kodansha, 1989, p. 147.
Lewis site is formed, the nitrogen molecule interacts
with aluminum through one of its atoms and with the
adjacent oxygen through the other atom; that is, a two-
center mechanism is observed. Hence, as was assumed
above, a back donation mechanism takes place. These
facts account for the two types of relationship for the
nitrogen complexes with Lewis and Brønsted acid sites.
However, more detailed quantum chemical calculations
in combination with the DFT and other more compli-
cated methods are required for any ultimate conclu-
sions.
3. Arean, C.O., Palomino, G.T., Geobaldo, F., and
Zecchina, A., J. Phys. Chem., 1996, vol. 100, p. 6678.
4. Geobaldo, F., Lamberti, C., Ricchiardi, G., Bordiga, S.,
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5. Wakabayashy, F., Kondo, J., Domen, K., and Hirose, C.,
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ACKNOWLEDGMENTS
8. McDonald, R.S., J. Am. Chem. Soc., 1957, vol. 79,
p. 850.
We are grateful to Cand. Sci. I.L. Zilberberg for
assistance in quantum chemical calculations and help-
ful discussion.
9. Zubkov, S.A., Borovkov, V.Yu., Gagarin, S.G., and
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