Kinetic studies as a method to differentiate between oxygen species involved in the oxidation of propene

Article abstract of DOI:10.1006/jcat.1999.2764

The kinetic equations of propylene oxidation along the nondestructive (nucleophilic oxidation to allylic products) and the destructive (electrophilic oxidation to degraded oxygenated and total oxidation products) pathways were obtained for the oxide catalysts: Bi₂O₂/MoO₃, giving only allylic products, Co₃O₄, showing only total oxidation, and SnO₂/MoO₃, giving both types of products. In all cases, the reaction order of the nucleophilic oxidation was 1 and 0 for propylene and oxygen, respectively, while that of electrophilic oxidation was close to one with regard to oxygen. The surface interactions model, in which propylene reacted either with surface lattice oxide ions to give nucleophilic oxidation products or with transient surface oxygen species to give electrophilic oxidation, was discussed. These transients resulted from the dynamic equilibrium between nonstoichiometric transition metal oxides and gas phase oxygen. The rate constants of the homomolecular isotopic exchange of oxygen were taken as a measure of the surface concentration of the transient oxygen species.

Full text of DOI:10.1006/jcat.1999.2764

Journal of Catalysis 190, 320–326 (2000)
doi:10.1006/jcat.1999.2764, available online at http://www.idealibrary.com on
Kinetic Studies as a Method to Differentiate between Oxygen Species
Involved in the Oxidation of Propene
Jerzy Haber�,1
and Wincenty Turek†
�
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland; and †Department of Physical Chemistry
and Technology of Polymers, Silesian Technical University, Gliwice, Poland
Received July 26, 1999; revised November 16, 1999; accepted November 19, 1999
derivatives, and finally to total oxidation (4–6). It can be ex-
Kinetic equations of propene oxidation along the nondestruc- pected that the rates of these two reaction pathways would
tive (nucleophilic oxidation to allylic products) and the destructive follow different kinetic equations and that it should be
(
electrophilic oxidation to degraded oxygenated and total oxidation
possible to discriminate between these two processes on
the basis of kinetic studies. Unfortunately, in the literature
only the kinetic equations describing the overall reaction
rate are available, which do not permit conclusions to be
drawn about the contribution of different pathways. Thus,
it seemed interesting to determine the rate equations of dif-
ferent elementary steps in the selective and the total oxida-
tion, to discriminate between the participation of different
products) pathways have been determined for three types of oxide
catalysts: Bi2O3/MoO3, giving only allylic products, Co3O4, showing
only total oxidation, and SnO2/MoO3, giving both types of products.
In all cases the reaction order of the nucleophilic oxidation was 1
with respect to propene and 0 with respect to oxygen, whereas that
of electrophilic oxidation was close to 1 with respect to oxygen.
The model of surface interactions is discussed in which propene
reacts either with surface lattice oxide ions to give nucleophilic ox-
idation products or with transient surface oxygen species to give oxygen species and elucidate the reaction mechanism.
electrophilic oxidation. These transients result from the dynamic
The reaction orders of the oxidation of propene to dif-
equilibrium between nonstoichiometric transition metal oxides and ferent products of the selective and the total oxidation
gas phase oxygen, so that the two kinetic equations are coupled by
the equation expressing the equilibrium of surface oxygen vacan-
cies. The rate constants of the homomolecular isotopic exchange of
have been determined. Three catalysts were used for the
kinetic studies, carried out in the temperature range 600–
7
00 K: Bi2O3/MoO3, which is known to give acrolein as
oxygen may be taken as a measure of the surface concentration of
the transient oxygen species.
the product of the allylic oxidation with very high selectiv-
ity, Co3O4, which is known as the total oxidation catalyst,
and SnO2/MoO3, which gives products of both selective and
total oxidation.
�c 2000 Academic Press
INTRODUCTION
EXPERIMENTAL
Interaction of a hydrocarbon molecule with oxygen at
the surface of an oxide catalyst results in a network of par-
allel and consecutive elementary steps, in which hydrogen
is abstracted, nucleophilic oxygen is inserted, electrophilic
Determination of Catalytic Activity
The tests of the kinetics of propene oxidation were per-
oxygen reacts with �-electrons, and C–C bonds are cleaved formed in a glass reactor with an inside diameter of 18 mm
1–3). For the catalyst to be selective, it must accelerate (7). The kinetics measurements were carried out using the
(
those consecutive steps which lead to the desired product differential method. The propene conversion degree was
and depress all others. In the case of olefins, in the hetero- 5–15% . Samples of the catalyst of � 3 g each and with grain
geneous oxidation by gas phase oxygen on oxide catalysts, diameters up to 0.75 mm were used. The catalyst bed height
abstraction followed by the nucleophilic addition of lattice in the reactor was lower than its diameter in all cases. The
oxygen results in the selective allylic oxidation, whereas in- reagents, propene and oxygen, were diluted with nitrogen
teraction with the electrophilic oxygen leads to the cleavage to ensure their appropriate concentrations in the reaction
of the C == C bond, to the formation of oxygenated alkane mixture. A constant flow rate of the reaction mixture was
maintained throughout the entire experiment at the value
3
of 15.0 dm /h.
1
To whom correspondence should be addressed. Institute of Catalysis
The reaction products were analysed by gas chromatog-
and Surface Chemistry, Polish Academy of Sciences, Ul. Niezapominajek,
3
0-239 Krak o´ w, Poland. Fax: +48 12 425 19 23. E-mail: nchaber@cyf-kr. raphy. Organic compounds were separated in a steel co-
edu.pl.
lumn of 3-mm diameter and 3-m length, packed with 4%
320
0021-9517/00 $35.00
Copyright �c 2000 by Academic Press
All rights of reproduction in any form reserved.

OXYGEN SPECIES IN THE OXIDATION OF PROPENE
321
of Carbowax 20 M on Chromosorb G, AW, DMCS, 80/ max Molybdenum Co. (surface area determined by the
00 mesh, and detected by FID; CO and CO2 were sep- BET method from low-temperature krypton adsorption
1
2
3+
4+
arated in a steel column of 3-mm diameter and 2-m length, was 2.59 m /g), with Bi and Sn ions from the appro-
packed with activated charcoal, 60/80 mesh, and detected priate nitrate solutions. As indicated by the XPS analysis
by TCD.
(8), the Bi3 ions are located at the surface of MoO3 crystal-
+
Kinetics of the reaction is usually determined using the lites. The concentrations of depositing ions were chosen so
Ostwald method with one of the reagents being taken in a that the solution, taken to prepare the slurry, contained the
very high excess. In such conditions, however, a different amount of metal ions required to produce the surface cover-
reaction mechanism may occur as compared to the reac- age of MoO3 of 1 monolayer and to obtain 1.0Bi2O3/MoO3
tion mixture, containing both reagents in comparable con- and 1.0SnO2/MoO3. The slurry was first slowly dried at
centrations. This may affect also the form of the kinetic room temperature with continuous stirring and then dried
equation obtained. Therefore, in the present study, the mea- at 413 K for 10 h. Finally, the samples were calcinated at
surements were carried out in such a way that compara- 773 K for 5 h in air. Co3O4 was obtained as the result of
ble reagent concentrations (the ratio of concentrations of decomposition of Co(OH)2, carried at 833 K for 6 h in air.
oxygen to propene was kept within the limits 0.78–2.83) Cobalt hydroxide was obtained through precipitation from
were used when determining the reaction orders. From the the cobalt nitrate solution by sodium hydroxide.
values of the rates of formation of different products, the
order with respect to propene and oxygen has been de-
termined separately for the reaction of the allylic (nucleo-
RESULTS
philic) oxidation and the destructive (electrophilic) oxida- (1) Bi O /MoO Catalyst
2
3
3
tion.
Selectivities of propene oxidation over the investigated
catalysts are compared in the Table 1.
In the case of MoO3-based catalysts, measurements were
carried out at constant temperature in two series: (I) at
constant oxygen concentration with varying propene con-
centration and (II) at constant propene concentration with
varying oxygen concentration.
Oxidation of propene over the 1.0Bi2O3/MoO3 catalyst
at 617 K resulted in the formation of acrolein with a selec-
tivity of 97.2% at conversion ranging from 5 to 15% . The
only other detectable product was acetaldehyde formed
with a selectivity of 2.8% . The total oxidation to CO2 was
negligible.
In the determination of the reaction order of propene
oxidation over Co3O4, the difficulty arose in ensuring a
constant temperature on varying the gas phase composi-
tion due to the high exothermicity when propene was oxi-
dized mainly to CO2. Therefore, a method had to be used
in which the temperature dependence of the reaction rate
was determined for three different oxygen : propene molar
ratios from which reaction orders with respect to propene
and oxygen at a selected temperature could be calculated.
Such an approach was adopted because even in the case
of a strongly exothermic reaction it permits measurements
of kinetics to be performed with high precision at several
freely chosen and easily measurable temperatures.
Figure 1 shows the results of the determination of the
propene oxidation rate at 617 K at constant oxygen concen-
tration in the steady state, amounting to xO = 0.0232 mole
2
fraction as a function of propene concentration and Fig. 2
showsthe resultsofthe determination ofthe rate ofpropene
oxidation at constant propene concentration in the steady
state, amounting to xpr = 0.0188 as a function of the con-
centration of oxygen. In all cases the concentrations of the
reagentsgiven on the abscissa were expressed bymolar frac-
tions and taken as arithmetic average values at the inlet and
outlet of the reactor. On the basis of the log r = f (log xpr)
relationship, shown in Fig. 1, the reaction order of the oxi-
dation of propene to acrolein with respect to propene was
Materials
The Bi2O3/MoO3 and SnO2/MoO3 catalysts were ob- calculated to be �1 = 1, while using the relationship log
tained by the impregnation of MoO3, supplied by Cli- r = f (log xO ), shown in Fig. 2, the order of this reaction
2
TABLE 1
Selectivity of Propene Oxidation
Selectivity (% )
Temp.
(K)
Acetic
aldehyde
Ethyl
acetate
Acrylic
acid
Acetic
acid
Total
oxidation
Catalyst
Acrolein
1
.0Bi2O3/MoO3
617
521
624
97.2
0.2
31.1
2.8
0.4
5.2
—
—
1.6
—
—
2.2
—
—
3.9
—
99.4
56.0
Co3O4
.0SnO2/MoO3
1

3
22
HABER AND TUREK
gen : propene molar ratios. The logarithm of the rate was
plotted as a function of the reciprocal temperature. Par-
allel straight lines obtained indicated that the Arrhenius
relationship is well obeyed, and the activation energy, and
hence the reaction mechanism, does not change when the
composition of the reaction mixture is changed. From these
data the dependence of the reaction rate on the concentra-
tion of propene at a constant concentration of oxygen, and
the dependence of the rate on the concentration of oxygen
at a constant concentration of propene, could be obtained
for any selected temperature and the reaction orders cal-
culated. In the first series of measurements, performed at
a constant starting concentration of oxygen, xO = 0.0280
2
(
mole fraction), in the reagents mixture, three various start-
ing propene concentrations (in mole fractions) were used:
xpr = 0.0178, 0.0140, and 0.0099. In the second measure-
ment series, performed at a constant propene concentra-
tion, xpr = 0.0140 (mole fraction), in the reagents mixture,
three various starting oxygen concentrations were used:
FIG. 1. The rate of oxidation of propene to acrolein over 1.0Bi2O3/
MoO3 at 617 K as a function of the propene mole fraction xpr, at a con-
stant oxygen mole fraction, xO2 = 0.0232. Order of reaction with respect
to propene is �1 = 1.
xO = 0.0347, 0.0280, and 0.0214. The reaction rates for each
2
constant starting composition of reagents were measured
at several temperatures, ranging from 495 to 524 K. The
results obtained for mixtures with varying starting concen-
trations of propene and constant oxygen concentration are
shown in Fig. 3, and those for varying starting concentra-
tions of oxygen and constant propene concentration are
shown in Fig. 4. The linear relationships of the function
log r = f (1/T ) shown in Figs. 3 and 4 are indicative of the
reaction occuring in the kinetic range. The calculations of
the reaction orders with respect to propene �1 and with
respect to oxygen �2 were performed taking into account
the reaction rates as read from the diagrams, for a selected
with respect to oxygen was calculated to be �2 = 0. As
oxidation of propene on this catalyst occurs virtually along
the nondestructive, nucleophilic pathway with the selectiv-
ity to acrolein higher than 97% , it can be assumed that the
reaction orders with respect to propene and oxygen, which
have been determined, represent the orders of the nucle-
ophilic propene oxidation pathway. The kinetics along this
reaction pathway may thus be described by the following
equation:
r = k � pC H .
[1]
3
6
(
2) Co3O4 Catalyst
3
temperature of 513 K (10 /T = 1.95), the same for both se-
As already mentioned, the temperature dependence ries of measurements. The mean values were �1 = 0.71 and
of the reaction rate was determined for different oxy- �2 = 0.90. The kinetics of the reaction of the total oxidation
FIG. 2. The rate of oxidation of propene to acrolein over 1.0Bi2O3/
3 4
FIG. 3. The kinetics of complete propene oxidation over the Co O
MoO3 at 617 K as a function of the oxygen mole fraction xO , at a constant
catalyst at a constant oxygen concentration, xO2 = 0.0280 (mole fraction)
in the substrate mixture and at three different propene concentrations:
(1) xpr = 0.0178; (2) xpr = 0.0140; (3) xpr = 0.0099.
2
propene mole fraction, xpr = 0.0188. Order of reaction with respect to
oxygen is �2 = 0.

OXYGEN SPECIES IN THE OXIDATION OF PROPENE
323
FIG. 4. The kinetics of complete propene oxidation over the Co3O4
catalyst at a constant propene concentration, xpr = 0.0140 (mole fraction)
in the substrate mixture and at three different oxygen concentrations: (1)
xO2 = 0.0347; (2) xO2 = 0.0280; (3) xO2 = 0.0214.
FIG. 6. The rate of propene reaction over the 1.0SnO2/MoO3 catalyst
at 624 K as a function of the oxygen mole fraction xO , at a constant
2
propene mole fraction, xpr = 0.0192. Order of reaction with respect to
oxygen is �2 = 0.52.
of propene (electrophilic oxidation), as determined for the ratio, as a function of oxygen concentration, are plotted in
Co3O4 catalyst, can thus be described by the equation
Fig. 6. On the basis of these results the orders of the over-
all reaction of propene oxidation with respect to propene
and oxygen were calculated to be �1 = 0.37 and �2 = 0.52,
respectively. Kinetics of the overall reaction of propene ox-
idation on the surface of the SnO /MoO catalyst may thus
0
.71
0.90
r = k � p
� pO2
.
[2]
C3H6
(
3) SnO2/MoO3 Catalyst
2
3
be described by the formula
Measurements of the reaction rate over SnO2/MoO3
were carried out at 624 K. The results of the determinations
of the oxidation rate at a constant oxygen concentration in
0
.37
0.52
� pO2
r = k � p
[3]
C3H6
the steady state, equal to xO = 0.0205 molar ratio, as a func-
2
tion of the concentration of propene are plotted in Fig. 5, Then, for the same catalyst, the rates of oxidation of
whereas the results obtained at constant propene concen- propene to the products of the nondestructive oxidation
traton in the steady state, amounting to xpr = 0.0188 molar (nucleophilic oxidation)
CH2 == CH–CH3 → CH2 == CH–CHO → CH2 == CH–COOH
and the products of the destructive oxidation (electrophilic
oxidation)
CH2 == CH–CH3 → HCHO + CH3–CHO
→
HCOOH + CH3–COOH → CO2 + H2O
obtained at constant oxygen pressure were plotted sepa-
rately as a function of propene concentration xpr (Fig. 7).
The calculated reaction orders with respect to propene
sel
tot
amounted to �1 = 0.86 and �1 = 0.24 for nucleophilic
and electrophilic oxidation, respectively. Similarly, these
rates obtained at constant propene pressure were plot-
ted separately as a function of oxygen concentration, xO2
(
Fig. 8). The calculated reaction orders with respect to oxy-
FIG. 5. The rate of propene reaction over the 1.0SnO2/MoO3 catalyst
at 624 K as a function of the propene mole fraction xpr, at a constant oxygen
mole fraction, xO2 = 0.0205. Order of reaction with respect to propene is
sel
tot
gen amounted to �2 = 0.07 and �2 = 0.87, respectively.
Kinetics of the oxidation of propene along the two reaction
pathways is thus described by two different formulas:
�
1 = 0.37.

3
24
HABER AND TUREK
Bi2O3/MoO3 (Eq. [1]), whereas the reaction along the to-
tal oxidation pathway (Eq. [5]) has the order, with respect
to oxygen, similar to the total oxidation observed on the
Co3O4 catalyst.
DISCUSSION
Transition metal oxides are nonstoichiometric com-
pounds, with the composition depending on the equilil-
brium between the lattice and its constituents in the gas
phase. In the equilibrium state, the chemical potential of
oxygen in the gas phase is equal to the chemical potential
of oxide ions in the lattice:
�
O = � 2�
.
[6]
2
O
Chemical potential of oxygen in the gas phase may be ex-
FIG. 7. The rate of propene reaction over the 1.0SnO2/MoO3 catalyst
at 624 K as a function of the propene mole fraction xpr, at a constant pressed as a function of its pressure,
oxygen mole fraction, xO2 = 0.0205. (1) Rate of the reaction leading to
o
the products of the nondestructive oxidation; (2) rate of the destructive
oxidation reactions. The orders of reaction with respect to propene in these
�
= � + RT ln pO2
;
[7]
O2
O
2
sel
tot
reactions are �1 = 0.86 and �1 = 0.24, respectively.
from Eqs. [6] and [7] it follows that the chemical potential
of lattice oxide ions is also a function of oxygen pressure,
the nondestructive (nucleophilic) oxidation,
o
O
�
2� = � + RT ln pO2 .
[8]
O
2
0
.86
0.07
rsel = ksel � p
� pO2
;
[4]
[5]
C3H6
Changes of the chemical potential of oxide ions in the crys-
tal lattice are effected by the generation of lattice defects,
which results in the changes of stoichiometry of the oxide.
Hence, the nonstoichiometry is a function of oxygen pres-
sure:
the total (electrophillic) oxidation,
0
.24
0.87
rtot = ktot � p
� pO2
.
C3H6
It can be noticed that within the limits of experimental er-
ror the kinetics of propene oxidation along the selective
oxidation pathway is described by a rate equation (Eq. [4])
similar to the case of highly selective oxidation catalyst
(
metal : oxygen)oxide = f (pO ).
2
The equilibrium between the oxide lattice and the gas
phase is a dynamic equilibrium, in which the rate of dis-
sociation of the oxide lattice and evolution of oxygen in
the form of O2 gas is equal to the rate of its incorporation
from the gas phase. In the process of dissociation the oxide
ions must be extracted from the surface, electrons injected
into the solid, oxygen atoms recombine to form molecules,
and desorb as dioxygen. The reverse series of elementary
steps must take place upon incorporation. These elemen-
tary steps result in surface equilibria,
O2 *ꢀ O�
�
+ VO *ꢀ O2�
*ꢀ O
�
*ꢀ O2 ads *ꢀ O2 gas,
chem
2 chem
2 chem
so that the surface is always covered by different oxygen
species, asillustrated in Fig. 9. The surface coverage bythese
species depends on the oxygen pressure in the gas phase, the
rate constants of adsorption and chemisorption (transfer
of electrons between adsorbed oxygen molecules and the
solid), the rate of incorporation, and the dissociation pres-
sure ofthe oxide. At lowtempearturesonlythe surface layer
of the oxide may be equilibriated with the gas phase; at suffi-
ciently high temperatures the defects created at the surface
diffuse into the bulk and equilibrium is established between
FIG. 8. The rate of propene reaction over the 1.0SnO2/MoO3 catalyst
at 624 K as a function of the oxygen mole fraction xO , at a constant
2
propene mole fraction, xpr = 0.0192. (1) Rate of the reaction leading to
the products of the nondestructive oxidation; (2) rate of the destructive
oxidation reactions. The orders of reaction with respect to oxygen in these
sel
tot
reactions are �2 = 0.07 and �2 = 0.87, respectively.

OXYGEN SPECIES IN THE OXIDATION OF PROPENE
325
[9]
the equation
rn = kn � pC H (1 � �).
3
6
When the reaction is carried out in an excess of oxygen in
the gas phase, the catalyst surface is fully oxidized, � = 0,
and the rate of the reaction is expressed by the formula
rn = kn � pC H .
[10]
3
6
This equation describes well not only the reaction at the sur-
face of the Bi2O3/MoO3 catalyst, where only nucleophilic
FIG. 9. The mechanism of the olefins oxidation reaction over oxide oxidation takes place but also the reaction along the nu-
catalysts.
cleophilic oxidation pathway in the case of the SnO2/MoO3
catalyst, when electrophilic oxidation is competing in a par-
allel pathway.
In the case of the electrophilic oxidation pathway the
rate of the reaction is proportional to propene pressure
the gas phase and the solid oxide. O2� ions, exposed at the
surface, are nucleophilic reagents, whereas O2 chem, O
�
�
,
chem
and O2 ads are electrophilic species. When such a surface and to surface coverage with electrophilic oxygen species
�
�
is contacted with propene molecules, electrophilic oxygen
species attack the �-bond of propene to form epoxy or per-
oxy complexes, which react further by cleavage of the C–C
bonds and form acetaldehyde, formaldehyde, the corre-
sponding acids, esters, and CO2 (scheme 1). In the parallel
O
, O
, and O2 ads:
chem
2 chem
re = ke � pC H � �O .
[11]
3
6
The surface coverage with electrophilic oxygen in the sta-
tionary state of the reaction is the result of its supply by ad-
sorption and its removal through the reaction with propene
molecules and the competing incorporation into the surface
vacancies,
reaction pathway, nucleophilic oxide ions O2 react with ac-
tivated propene molecules to form acrolein (for simplicity
we shall consider only the overall reaction of the oxidation
of propene to acrolein and acrylic acid).
�
Thus, different oxygen species, present at the oxide sur-
face, compete for propene molecules. Moreover, a compe-
kads � pO = kinc � � � �O + ke � pC H � �O ,
[12]
2
3
6
tition also exists between propene molecules and surface and
�
oxide vacancies for the electrophilic O
species, which
chem
kads � pO
can either be captured by propene to form products of elec-
trophilic oxidation (CO2 + H2O in Fig. 9) or be incorpo-
rated into the surface by reaction with oxygen vacancies
VO. The rate of the reaction along the nucleophilic oxida-
tion pathway is proportional to the pressure of propene and
the number of lattice oxide ions exposed at the surface. The
latter is equal to the total number of lattice anionic surface
sites, which is constant, minus the surface concentration of
2
�
O =
.
[13]
kinc � � + ke � pC H
3
6
The rate of electrophilic oxidation can thus be expressed
by the equation
ke � kads � pC H � pO
3
6
2
re =
.
[14]
kinc � � + ke � pC H
3
6
vacancies, which can be expressed by the degree of reduc- The reaction should be of the first order with respect to oxy-
tion �. Hence, the rate of nucleophilic oxidation is given by gen, which has indeed been observed in the case of the total
oxidation catalyst Co3O4 and has also been found to hold
along the electrophilic oxidation pathway in the case of the
SnO2/MoO3 catalyst. The fractional order with respect to
propene depends on the relative importance of the incor-
poration of oxygen into surface vacancies as compared to
its reaction with propene.
It may thus be concluded that two parallel reaction
pathways may be followed by propene when adsorbed at
the surface of an oxide catalyst: its selective allylic oxida-
tion by surface lattice oxide ions (nucleophilic oxidation)
and its destructive oxidation by reactive oxygen species
�
�
O
, O
, and O2 ads (electrophilic oxidation). These
chem
2 chem
two reaction pathways are described by different rate equa-
tions, but are coupled together by the equation, describing
SCHEME 1

3
26
HABER AND TUREK
TABLE 2
Homomolecular Isotopic Exchange of Oxygen
Activation
energy
Temperature
range (K)
Rate of reaction
�
2
� 1
� 1
(kJ mol )
Oxide
(mol cm
s
)
References
Bi2O3 � MoO3(1 : 1)
MoO3
SnO2
747–773
798–873
723–793
398–523
No exchange
(9), (10), (11)
(12)
5
1.0 � 10
213
113
67
8
4.0 � 10
(12)
(12)
1
3
Co3O4
2.0 � 10
the generation and annihilation of surface oxygen vacan-
cies:
CONCLUSION
At the surface of oxide catalysts propene is oxidized
along two parallel reaction pathways in which two different
types of oxygen take part: one characteristic for the selec-
tive oxidation catalysts, which has been identified earlier
as the lattice oxygen (9), and the second, characteristic of
the total oxidation catalysts, appearing at the surface as the
result of the dynamic equilibrium between the metal oxide
and the gas phase oxygen. The two pathways are described
by different rate equations and can be identified by kinetics
measurements.
2
�
�
�
O
= V + O
.
O
O
chem
The importance of these two pathways depends on the
relative values of kn, ke, kinc, kdiss, kchem, and kads. The
first two rate constants depend on the nature of the hy-
drocarbon molecule, whereas the rate constants describ-
ing the transformations of surface oxygen species kinc, kdiss,
kchem, and kads are characteristic of the oxide. No data exist
which would enable the separate calculation of their val-
ues; one can, however, assume that measurements of the
rate of homomolecular isotopic exchange of oxygen could
permit the comparison of these values for different oxides.
The rate constants of the homomolecular isotopic oxygen
exchange for the oxides studied, as taken from Refs.
REFERENCES
1
2
. Bielanski, A., and Haber, J., “Oxygen in Catalysis.” Marcel Dekker
Inc., New York, 1991.
. Haber, J., “Studies Surface Science Catalysis,” Vol. 72, p. 279. Elsevier,
Amsterdam, 1992.
(
9–12), are summarized in Table 2. The highest rate of
exchange is observed on the surface of Co3O4, which in-
dicates that the surface of this oxide shows the highest
coverage with electrophilic oxygen species. Accordingly,
only the electrophilic oxidation, mainly to the total com-
bustion products, is found. Practically, no exchange is ob-
served in the case of Bi2O3/MoO3. Thus, no chemisorbed
electrophilic oxygen species are present at the surface, only
oxide ions, and the selectivity to allylic oxidation products
3. Grasselli, R. K., and Burrington, J. D., Adv. Catal. 30, 133
(1981).
4
. Libre, J. M., Barbaux, Y., Grzybowska, B., and Bonnelle, J. P., React.
Kinet. Catal. Lett. 20, 249 (1982).
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idation” (B. K. Warren, and S. Ted. Oyama, Eds.), ACS Sympo-
sium Series 638, p. 20. American Chemical Society, Washington D.C.,
1996.
5
6
. Haber, J., Studies Surface Science Catalysis, Vol. 110, p. 1. Elsevier,
(
nucleophilic oxidation) is very high. The rate of exchange
Amsterdam, 1997.
at the surface of SnO2 is 5 orders of magnitude smaller than
that at the surface of Co3O4, but many orders of magnitude
greater than that in the case of the Bi2O3–MoO3 system.
Thus, the surface is much less populated with electrophilic
oxygen species and both—the electrophilic and the nucle-
ophilic oxidation pathways—are followed in similar pro-
portions. The present study leads to the conclusion that the
7
8
. Br u¨ ckman, K., Haber, J., and Turek, W., J. Catal. 114, 169 (1988).
. Br u¨ ckman, K., Haber, J., and Wiltowski, T., J. Catal. 106, 188
(1987).
9. Keulks, G. W., Krenzke, L. D., and Noterman, T. M., Adv. Catal. 27,
83 (1978).
1
1
1
0. Keulks, G. W., J. Catal. 19, 232 (1970).
1. Wragg, R. D., Ashmore, P. G., and Hockey, J. A., J. Catal. 22, 49
(
1971).
two pathways are described by different rate equations and 12. Boreskov, G. K., “Catalysis: Science and Technology,” Vol. 3, p. 39.
Springer Verlag, Berlin, 1982.
can be identified by the kinetic measurements.