Stoichiometry of oxidation reactions involving α-oxygen on FeZSM-5 zeolite

Article abstract of DOI:10.1023/A:1010461217034

The stoichiometry of the low-temperature reaction between surface α-oxygen formed by decomposing N₂O over Fe-containing ZSM-5 zeolite and methane, hydrogen (deuterium), and CO was studied. Methane and hydrogen reacted with α-oxygen in stoichiometric ratios of 1:1.8 and 1:1.6, respectively. The observed stoichiometry was due to the mechanisms of the corresponding reactions. According to a mechanism proposed for the interaction of α-oxygen with methane and hydrogen, this reaction was accompanied by the dissociation of CH₄ and H₂ molecules. α-Oxygen reacted with CO in ratio of 1:1 to form CO₂ in amounts equal to those of α-oxygen on the surface. In contrast to the oxidation of methane and hydrogen, the product of CO oxidation by α-oxygen was weakly bound to the zeolite surface and easily desorbed in the gas phase upon heating. Thus, the effect of blocking could be entirely limited by the performance of the reaction at a higher temperature and by the additional removal of the product by freezing.

Full text of DOI:10.1023/A:1010461217034

Kinetics and Catalysis, Vol. 42, No. 2, 2001, pp. 205–211. Translated from Kinetika i Kataliz, Vol. 42, No. 2, 2001, pp. 230–236.
Original Russian Text Copyright © 2001 by Dubkov, Paukshtis, Panov.
Stoichiometry of Oxidation Reactions Involving a-Oxygen
on FeZSM-5 Zeolite1
K. A. Dubkov, E. A. Paukshtis, and G. I. Panov
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received July 20, 2000
Abstract—The stoichiometry of the low-temperature reaction between surface α-oxygen formed by decom-
posing N2O over Fe-containing ZSM-5 zeolite and methane, hydrogen (deuterium), and carbon monoxide is
studied. Methane and hydrogen react with α-oxygen in stoichiometric ratios of 1 : 1.8 and 1 : 1.6, respectively.
The observed stoichiometry is due to the mechanisms of the corresponding reactions. According to a mecha-
nism proposed for the interaction of α-oxygen with methane and hydrogen, this reaction is accompanied by the
dissociation of CH4 and H2 molecules. For hydrogen, such a mechanism is supported by IR spectroscopic stud-
ies of resulting surface compounds, namely, of new hydroxyl groups that were formed on the zeolite surface in
the course of the reaction. α-Oxygen reacts with CO in the ratio of 1 : 1 to form CO2 in amounts equal to those
of α-oxygen on the surface.
1
INTRODUCTION
into account the fact that α-sites are localized inside the
micropore space, we assumed that the observed depen-
dence of the stoichiometry of the reaction on the con-
centration of α-oxygen may be due to partial site block-
ing by the reaction product firmly bound to the zeolite
surface inside micropores [7]. In this case, part of
α-oxygen may be inaccessible to reagents and, there-
fore, will fail to participate in the reaction. Because the
pore space of ZSM-5 zeolite has a three-dimensional
structure, the formation of the reaction product in small
amounts must not hinder the accessibility of α-sites.
However, when their concentration exceeds some criti-
cal value, they can be blocked. It is this circumstance
that explains the dependence of the fraction of reacting
α-oxygen on its concentration on the zeolite surface.
To verify this assumption and to elucidate the actual
factors responsible for the observed stoichiometry of
reactions involving α-oxygen, in the present work, we
also studied the interaction of CO with α-oxygen,
along with the oxidation of methane and hydrogen
(deuterium). It is obvious that the blocking of a part of
the zeolite pore space by the reaction product should
decrease the accessible volume of micropores. To
reveal this effect, we determined the micropore volume
of FeZSM-5 zeolite before and after the interaction
between methane and α-oxygen.
It is known that the decomposition of N2O over
FeZSM-5 zeolites at temperatures below 300°C is
accompanied by the formation of a new form of surface
oxygen (α-oxygen) [1–3]:
N2O + ( )α = N2 + (O)α.
(I)
The catalytic properties of zeolites in this reaction are
determined by the presence of active sites (α-sites),
which, according to the data of Mössbauer spectros-
copy, are the non-framework complexes of reduced
iron (Fe2+) [4]. α-Oxygen exhibits an extremely high
reactivity: it participates in oxygen isotope exchange
even at room temperature and oxidizes CO, ç2, and
different hydrocarbons including benzene and meth-
ane [5–7]. The stoichiometric reaction between
α-oxygen and methane results in the selective forma-
tion of methanol, which can be extracted from the
zeolite surface.
The previous studies showed that the fraction of
α-oxygen that participated in the reaction with methane
significantly depended on its concentration on the zeo-
lite surface [7]. It was 100% at a concentration of
α-oxygen of ~4 × 1018 O atom/g and almost halved at a
concentration of 2 × 1019 O atom/g. Therefore, the sto-
ichiometry of the reaction or the ratio of the amount of
reacting methane to that of Oα present on the surface
changed from 1 : 1 to 1 : 2 depending on the concentra-
tion of α-oxygen. A similar result was obtained in the
oxidation of hydrogen (deuterium) by α-oxygen [7].
EXPERIMENTAL
We prepared FeZSM-5 zeolite (SiO2/Al2O3 = 72)
containing 0.53 wt % Fe by hydrothermal synthesis
introducing iron as FeCl3 in the initial gel [9]. This sam-
ple was analogous to that described previously in [7].
The sample was transformed into the H-form by
exchanging with an ammonia buffer solution and then
calcined in air at 550°C. The degree of crystallinity of
The system of ZSM-5 zeolite micropores involves
two types of intersecting channels with a cross-section
of ~5.5 Å [8] comparable with molecule sizes. Taking
1
Proceedings of the II All-Russian Workshop on Highly Organized
Catalytic Systems.
0023-1584/01/4202-0205$25.00 © 2001 MAIK “Nauka /Interperiodica”

206
DUBKOV et al.
the zeolite determined from the XRD analysis was α-oxygen will enter into the reaction, and the amount
more than 95%, the specific surface area of the sample of methane or deuterium reacted with it will increase.
was ~400 m2/g. To increase the concentration of active For this purpose, experiments were performed at
sites, the sample was additionally calcined at 900°C, 100°ë, and the condensed products were additionally
whereupon the concentration reached 1.8 × 1019 site/g. frozen out of the gas phase.
The decomposition of N2O and the oxidation of
Figure 1 presents the results for the interaction of
methane, D2, and CO were carried out in a static vac-
methane with α-oxygen on the surface of FeZSM-5.
uum setup at pressures of at least 10–7 torr. A zeolite
Before the experiment, α-oxygen in a concentration of
1.8 × 1019 O atom/g was generated on the zeolite sur-
face by decomposing N2O (0.41 torr) at 250°ë. The
sample was cooled to 100°ë, the reactor was closed,
and ëç4 was admitted into the reaction volume of the
setup until the pressure reached 0.6 torr. It can be seen
from Fig. 1 that methane was significantly consumed at
the moment Ä when reactor is opened. Because meth-
ane is not adsorbed by zeolite at 100°ë [7], this con-
sumption resulted from the interaction between CH4
and α-oxygen. The amount of reacted methane can be
determined from the experimental data. It follows from
Table 1 that the amount of reacted methane was only
almost half the total amount of α-oxygen present on the
surface; that is, the ratio between reacted CH4 and α-
oxygen was 1 : 1.8.
sample (0.5 g of a 0.5–1-mm fraction) was placed in a
quartz microreactor and subjected to standard treat-
ment that included alternate 1-h oxidative and vacuum
treatments at 550°ë with the exposure of the sample to
oxygen at a pressure of 1 torr at the end. The isolated
volume of the microreactor (~5 cm3) comprised an
insignificant part of the total reaction volume of the
setup (620 cm3). The use of such a microreactor
allowed, in particular, considerable enhancement in the
accuracy of adsorption and kinetic measurements. In
thermal desorption experiments, the reactor was heated
from 25 to 550°ë at a linear rate of 6 K/min. The com-
position of the gas phase was analyzed by mass spec-
trometry.
The concentration of α-oxygen on the zeolite surface
(CO ) after N2O decomposition at 250°ë (reaction (I))
α
At time B, the reaction volume was connected to the
trap cooled with liquid nitrogen. The reactor was closed
after 30 min, the trap was thawed out (at time C), and
the composition of the gas phase was analyzed. As fol-
lows from Fig. 1, once the trap was thawed, the gas
phase contained no additional products of desorption
from the zeolite surface. Contrary to the expectations,
the amount of reacted methane remained unchanged as
the gas phase was cooled. The product of interaction
between CH4 and α-oxygen is impossible to remove
from the zeolite surface even by raising the reaction
temperature to 100°ë, because it is apparently bound
firmly to the zeolite surface. This conclusion was sup-
ported by the results of the control experiment on the
programmed heating of a sample after the oxidation of
methane by α-oxygen at 100°ë (Fig. 2). Before heating
the sample, unreacted methane was evacuated from the
gas phase. Figure 2 shows that, even at 200°ë, the reac-
tion product did not desorb into the gas phase. At tem-
peratures higher than 250°ë, the decomposition of the
reaction product and the liberation of CO and ëé2 in
the gas phase took place. The amount of CO and CO2
corresponded to that of reacted methane (Table 1).
was calculated either by measuring the amount of liber-
ated N2 (or that of consumed N2O) or by the isotope
exchange with 18é2 [7]. The values of CO found by
α
these two methods were in good agreement.
DRIFT spectra were recorded at room temperature
at 2100–6000 cm–1 using a Shumadzu 8300 spectrom-
eter with a resolution of 2 cm–1 and with the accumula-
tion of 100 scans. The reflectance spectra were plotted
in Kubelka–Munk coordinates.
RESULTS AND DISCUSSION
Oxidation of Methane and Deuterium
The reactions of methane and deuterium with α-oxy-
gen were carried out in the following steps:
(1) The standard treatment of zeolite at 550°ë;
(2) The formation of surface α-oxygen by decom-
posing N2O at 250°ë;
(3) The interaction of α-oxygen with ëç4 or D2 at
100°ë;
(4) The freezing of condensed products in a trap at
Figure 3 demonstrates the results of a similar exper-
iment in which the interaction of deuterium with
α-oxygen at 100°C was studied. Note that, at this tem-
perature, H–D exchange with the zeolite surface was
–196°ë for 30 min; and
(5) Thawing out the trap and analysing the compo-
sition of the gas phase.
The idea of the experiments was as follows. Assum- not observed. Figure 3 shows that, at the instant the
ing that the blocking of some part of the zeolite reactor was opened (A), D2 rapidly reacted with α-oxy-
micropore space with the oxidation products of meth- gen. The amount of deuterium that entered the reaction
ane and deuterium is responsible for a decrease in the was somewhat higher than that of reacted methane
stoichiometric ratios in these reactions, we proposed to (compare Tables 1 and 2) and comprised ~63% of the
eliminate this effect by removing the reaction products total amount of α-oxygen present on the zeolite sur-
from the zeolite surface in the course of the reaction. In face. The ratio between the amounts of reacted D2 and
this case, it might be expected that all of the surface α-oxygen was 1 : 1.6 (Table 2).
KINETICS AND CATALYSIS Vol. 42 No. 2
2001

STOICHIOMETRY OF OXIDATION REACTIONS
207
Neither the freezing of the trap that began at time Ç
nor the subsequent thawing of the trap (at time C)
changed the composition of the gas phase or the
amount of reacted D2. Thus, as in the case of methane,
the product of the reaction between D2 and α-oxygen
did not desorb in the gas phase, because it was firmly
bound to the zeolite surface. For this reason, the
repeated loading of α-oxygen from N2O on the sample
surface required the additional treatment of zeolite at a
high temperature after the oxidation of methane or deu-
terium. The results obtained generally agree with data
reported in [7] and do not contradict the assumption
concerning the possible blocking of a part of the zeolite
micropore space with the reaction product. It is of inter-
est to examine how the stoichiometry of the reaction
change if the reaction product is weakly bound to the
zeolite surface. With this goal, we studied the oxidation
of CO to CO2 by α-oxygen.
A
B
C
0.6
0.4
0.2
0
0
10
20
30
40
Time, min
Fig. 1. Oxidation of CH by α-oxygen over the FeZSM-5
4
zeolite surface at 100°C (A is the instant of opening the
microreactor; B is the beginning of freezing; and C is the
beginning of thawing the trap).
CO Oxidation
Time, min
First, we carried out a control experiment on the
adsorption of CO2 on FeZSM-5 zeolite that was prelim-
inarily treated in a standard way and the subsequent
thermal desorption of CO2 during programmed heating
from room temperature (Fig. 4). CO2 adsorbed signifi-
cantly upon admitting it at room temperature. With an
increase in temperature, ~70% CO2 desorbed at even
100°C, and it regained its initial pressure at 200°C. In a
similar experiment on the thermal desorption of CO
from the zeolite sample, only ~20% carbon monoxide
remained on the surface at 100°C, and it desorbed com-
pletely at ~175°C (Fig. 5).
0
20
40
60
80
0.3
0.2
0.1
0
A
CO
CO2
500
Thus, in contrast to the oxidation of methane and
hydrogen, the product of CO oxidation by α-oxygen
was weakly bound to the zeolite surface and easily des-
orbed in the gas phase upon heating. Therefore, in this
case, the effect of blocking can be entirely eliminated
by the performance of the reaction at a higher tempera-
ture and by the additional removal of the product by
freezing. The experiments on CO oxidation were car-
ried out in the same way as reactions with methane and
deuterium.
100
200
300
400
600
T, °C
Fig. 2. Temperature-programmed desorption of the product
of the oxidation of CH by α-oxygen (A is the beginning of
4
thawing).
CO2 was liberated in the gas phase. This pointed to the
oxidation of CO with α-oxygen. The freezing of the
product in the trap cooled by liquid nitrogen began at
instant B and resulted in a drop in the CO2 pressure to
zero, while the pressure of CO remained unchanged.
Figure 6 presents the results on the interaction
between CO and the zeolite surface at 100°C after load-
ing α-oxygen in a concentration of 1.8 × 1019 O atom/g.
At instant Ä, admitted CO was quickly consumed and After thawing the trap (beginning with instant C), CO2
Table 1. Stoichiometry of the reaction between CH4 and α-oxygen over FeZSM-5 zeolite at 100°C
CH4 : Oα stoichiometric ratio
Amount of Oα on
the surface, O atom/g
Amount of reacting CH4,
molecule/g
Amount of desorbed
CO and CO2*, molecule/g
No.
found from found from CO
CH4 amount and CO2 amount
1
2
1.8 × 1019
1.9 × 1019
1.01 × 1019
1.05 × 1019
1.05 × 1019
1.06 × 1019
1 : 1.8
1 : 1.8
1 : 1.7
1 : 1.8
* Found by decomposing the reaction product at temperatures up to 550°C.
KINETICS AND CATALYSIS Vol. 42 No. 2 2001

208
DUBKOV et al.
Note that the above procedure allowed the complete
Ä
B
C
removal of α-oxygen with the product of the stoichio-
metric reaction at a moderate temperature. If, after the
oxidation of CO by α-oxygen and the subsequent evac-
uation of the sample at 100°C, N2O was repeatedly
decomposed at 250°ë, α-oxygen was formed again in
0.6
0.4
0.2
0
a concentration close to CO after the standard treat-
α
ment of the sample.
At first glance, the data on the oxidation of CO,
CH4, and D2 by α-oxygen confirm the assumption of
the possible blocking of the zeolite pore space with the
reaction product firmly bound to the zeolite surface.
However, one should take into account the property of
a CO molecule that distinguishes it from CH4 and H2
(or D2) molecules. Carbon monoxide can chemically
bind only one oxygen atom, whereas, in the oxidation
of methane and hydrogen, the possibility of their inter-
action with two α-oxygen atoms cannot be excluded.
Such an interaction may, for example, involve the
abstraction of a H atom from the oxidized molecule
with the subsequent formation of surface OH or CH3
groups. In this case, the stoichiometric ratio between
the reagents and α-oxygen must be 1 : 2, which is close
to the experimental value. This prevents the obtained
results from being considered as the ultimate proof of
the above-mentioned assumption. The strongest evi-
dence for blocking could be a decrease in the accessible
micropore volume of the zeolite after the stoichiomet-
ric reaction involving α-oxygen.
0
10
20
30
40
Time, min
Fig. 3. Oxidation of D by α-oxygen over the FeZSM-5 zeo-
2
lite surface at 100°C (A is the instant of opening the
microreactor; B is the beginning of freezing; and C is the
beginning of thawing the trap).
Time, min
40 50
0
10
B
20
30
Ä
..................................................................
0.4
0.3
0.2
0.1
0
Determination of the FeZSM-5
Zeolite Micropore Volume
The volume of zeolite micropores was determined
by argon adsorption at the temperature of liquid nitro-
gen. Note that the radius of theAr atom (r = 3.84 Å) is close
to the sizes of reacting molecules of CH4 (r = 4.25 Å) and
H2 (r = 3.24 Å) [10]. Therefore, it might be expected
that, in the case of micropore space blocking by the
product of the stoichiometric reaction, it will be inac-
cessible not only to reactants but to argon molecules as
well. To elucidate whether or not the microporous vol-
ume decreased after the stoichiometric reaction involv-
ing α-oxygen, two experiments were carried out at the
temperature of liquid nitrogen. In the first experiment,
argon was adsorbed by FeZSM-5 zeolite subjected to
standard treatment. In the second experiment, before
argon adsorption, methane was oxidized by α-oxygen at
room temperature. The measurements were carried out at
25
100
175
250
T, °ë
Fig. 4. Temperature-programmed desorption of CO from
2
the FeZSM-5 zeolite surface (A is the instant of opening the
microreactor and admitting CO and B is the beginning of
2
thawing).
desorbed. It turned out in this case that the reaction
product can be completely removed from the zeolite
surface. The amount of CO2 formed was equal to that
of α-oxygen on the surface (Table 3). Thus, in the oxi-
dation of CO, all α-oxygen participated in the reaction
and zeolite micropores were not blocked.
Table 2. Stoichiometry of the reaction between D2 and α-oxygen over FeZSM-5 zeolite at 100°C
No. Amount of Oα on the surface, O atom/g
Amount of reacting D2, molecule/g
D2 : Oα stoichiometric ratio
1
2
1.8 × 1019
1.9 × 1019
1.14 × 1019
1.20 × 1019
1 : 1.6
1 : 1.6
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STOICHIOMETRY OF OXIDATION REACTIONS
209
pressures when P/P0 was 0.3, which, according to the data
Time, min
reported in [11], corresponded to the filling of micropores
of ZSM-5 zeolite. The volume of micropores was calcu-
lated from the amount of adsorbed argon assuming that
the adsorbate density equaled the density of liquid argon
(dAr = 1.419 g/cm3) [11].
0
Ä
10
20
30
40
50
60
70
B
.................................................................
0.6
0.4
0.2
0
It turned out that the micropore volume of ZSM-5
zeolite did not decrease after the interaction of methane
with α-oxygen (Table 4). This indicates that the effect
of micropore space blocking was absent after the oxi-
dation of methane by α-oxygen. Thus, the difference in
the stoichiometry of oxidation by α-oxygen for meth-
ane and hydrogen and for carbon monoxide is due to
other reasons, most probably, to the mechanisms of the
corresponding reactions. The assumption that the inter-
action of methane and hydrogen with α-oxygen
involved the dissociation of reacting molecules seems
to be the most probable. To verify this assumption, we
studied the products formed in the reaction of hydrogen
with α-oxygen on the surface of FeZSM-5 zeolite by IR
spectroscopy.
25
100
175
250
T, °C
Fig. 5. Temperature-programmed desorption of CO from the
FeZSM-5 zeolite surface (A is the instant of opening the
microreactor and B is beginning of thawing).
Ä
B
C
Study of the Interaction of H2
with α-Oxygen by IR Spectroscopy
0.6
0.4
0.2
0
Figure 7 presents the IR spectra of FeZSM-5 zeolite
in the region of the stretching vibrations of OH groups.
The initial spectrum of the sample subjected to stan-
dard treatment exhibited three main absorption bands,
which corresponded to vibrations of zeolite OH groups.
According to the published data, absorption bands at
3740 and 3610 cm–1 are related to the terminal silanol
Si−OH [12–15] and bridge Al–OH–Si [14–16] groups,
respectively. A less intense absorption band at
3670 cm–1 is usually assigned to OH groups bound to
extra-framework forms of Al [16, 17]. Because hydro-
gen was weakly adsorbed on the zeolite after standard
treatment, short evacuation was sufficient to com-
pletely remove it, and it gave no new absorption band.
The spectrum also remained unchanged after loading
α-oxygen in a concentration of 1.8 × 1019 O atom/g on
the zeolite surface (Fig. 7, spectrum 2).
CO2
CO
1
2
0
10
20
30
40
Time, min
Fig. 6. Oxidation of CO with α-oxygen over FeZSM-5 zeo-
lite surface at 100°C (A is the instant of opening the
microreactor; B is the beginning of freezing; and C is the
beginning of thawing the trap): (1) the amount of CO and
(2) the amount of CO .
2
A qualitative change in the spectrum was observed
after the interaction of H2 with the zeolite surface on
which α-oxygen was present (Fig. 7, spectrum 3). In
the region of the stretching vibrations of OH groups, an
intense new absorption band at 3674 cm–1 with a shoul-
der at 3635 cm–1 appeared. These bands may be
groups, respectively [18]. They are well seen in the dif-
ference spectrum 4 obtained by subtracting the spec-
trum of the initial sample from spectrum 3. Obviously,
these absorption bands are related to new surface OH
assigned to the terminal Fe–OH and bridge Fe–OH–Si groups that were formed during the oxidation of hydro-
Table 3. Stoichiometry of the reaction between CO and α-oxygen over FeZSM-5 zeolite at 100°C
No. Amount of Oα on the surface, O atom/g Amount of CO2, formed, molecule/g
CO : Oα stoichiometric ratio
1
2
1.8 × 1019
1.8 × 1019
1.76 × 1019
1.73 × 1019
1 : 1.02
1 : 1.04
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2001

210
DUBKOV et al.
should also include the abstraction of H atoms from the
CH4 molecules with the formation of the hydroxy and
methoxy groups on the zeolite surface. Note that the
kinetic isotope effect in the reaction of methane with
α-oxygen demonstrated that the reaction really
involved the rupture of the C–H bond the methane mol-
ecule as a rate-limiting step [20].
1
2
3
It can be assumed that, for such a dissociative mech-
anism, α-sites in the zeolite matrix should be arranged
in pairs in which two active sites are at a rather close
distance from each other. The dependence of the sto-
ichiometry of methane oxidation on the concentration
of α-oxygen on the zeolite surface [7] may be due to the
behavior of α-oxygen in the filling of such pair sites
during the decomposition of N2O. At a low concentra-
tion of α-oxygen on the surface, single éα atoms are
mainly formed; with an increase in the concentration,
the fraction of pairs of éα atoms increased and
approached 100% when the coverage of α-sites was
maximum. In this case, the interaction of the substrate
molecule with two atoms of éα was made possible, and,
as a result, the stoichiometric ratio approached 1 : 2. In the
reaction involving methane, this value was close to an
experimental value of 1 : 1.8 (Table 1), whereas it was
somewhat lower (1 : 1.6) in the reaction with hydrogen
(Table 2). The reason for underestimating this ratio
remains unclear. We cannot exclude that a small part of
hydrogen reacted in such a way that hydrogen atoms
formed in the dissociation of H2 molecule were bound
not to the nearest éα atoms but to other sites of the sur-
face. It is also possible that some of the H2 molecules
were bound to one rather than two éα atoms. Neverthe-
less, the obtained results leave no doubts that the major
part of hydrogen reacted with α-oxygen via the disso-
ciative mechanism.
4
3000
3200
3400
3600
3800
ν, cm–1
Fig. 7. IR spectra of FeZSM-5 zeolite in the region of OH
vibrations recorded (1) after the standard treatment of the
zeolite, (2) after the formation of α-oxygen on the zeolite
surface; (3) after the reaction between H and α-oxygen;
2
and (4) after subtracting spectrum 1 from spectrum 3 (the
difference spectrum).
gen. Note that the results obtained here qualitatively
agree with data reported in [19].
Thus, the occurrence of the reaction between hydro-
gen and α-oxygen via the dissociation of H2 molecules
was confirmed. The mechanism of the reaction can be
outlined in the following way. The interaction with
α-oxygen was accompanied by the rupture of the bond
in H2 molecules and the formation of HOα groups on
the zeolite surface. The second hydrogen atom reacted
with the nearest α-oxygen atom. This resulted in the
formation of two OH groups per each molecule of
the reactant:
H2 + 2Oα
2HOα.
(II)
If the residual hydrogen was removed from the gas
phase by evacuating after the reaction, the subsequent
heating of the sample at temperatures higher than
350°ë resulted in the desorption of é2 [7]. It can be
assumed that, with an increase in temperature, the OH
groups were removed from the zeolite surface as H2é
and the remaining α-oxygen desorbed as é2:
In contrast to methane and hydrogen, CO reacted
with α-oxygen in a stoichiometric ratio of 1 : 1 to give
CO2, which desorbed into the gas phase and freed the
active site:
Cé + Oα
( )α + ëO2.
(IV)
Thus, the observed difference in the stoichiometry
of the oxidation of methane, hydrogen, and carbon
monoxide reflected the difference in the mechanisms of
the interaction of these molecules with α-oxygen. Our
1
--
2HOα
H2é + O2 + 2( )α .
(III)
2
Because methane reacted with α-oxygen almost in studies gave better insight into the properties of the
the same stoichiometric ratio, it can be assumed that, in active oxygen formed during the decomposition of N2O
this case, the reaction also proceeded via a similar dis- on Fe complexes in FeZSM-5 zeolites and the mecha-
sociative mechanism. The reaction with methane nisms of oxidation reactions involving active oxygen.
Table 4. Micropore volume of FeZSM-5 zeolite before and after the reaction between CH4 and α-oxygen at 25°C
Initial pressure
of Ar, torr
Ar pressure after
adsorption, torr
Amount
of adsorbed Ar, torr
Micropore
No.
Treatment
volume, cm3/g
1
2
3
Vacuum, O2, 550°C
Vacuum, O2, 550°C
CH4 + Oα
43.4
43.4
43.4
6.8
6.5
7.1
36.6
36.9
36.3
0.159
0.160
0.157
KINETICS AND CATALYSIS Vol. 42
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2001

STOICHIOMETRY OF OXIDATION REACTIONS
211
10. Breck, D.W., Zeolite Molecular Sieves, New York: Wil-
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