The Metathesis of Propylene Catalyzed by Model Oxides Studied Using a High-Pressure Reactor Incorporated into an Ultrahigh Vacuum Chamber

Article abstract of DOI:10.1006/jcat.1997.1585

The olefin metathesis activity of a series of model catalysts consisting of metallic molybdenum, MoO₂, or MoO₃ is investigated using a combined high-pressure reactor/ultrahigh vacuum apparatus. The model catalysts are analyzed using X-ray and ultraviolet photoelectron, Raman, and Auger spectroscopies and it is shown that MoO₂ is the most active above ～650 K. Reaction in this temperature regime proceeds with a high activation energy (～60 kcal/mol). Another, lower-activation-energy (～6 kcal/mol) regime is encountered for reaction below ～650 K and it is suggested that both reactions proceed simultaneously. In addition, the absolute rate and selectivity for the reaction on MoO₂ below 650 K are similar to those found for high loadings of supported molybdenum oxide, suggesting that an oxide layer provides a good model catalyst for this reaction. It is also found that reaction proceeds in the presence of a thick carbonaceous layer where the thickness of this layer decreases with increasing oxidation state. A Raman analysis of the surface of the catalyst following reaction indicates that this layer consists predominantly of hydrocarbon species, although a small amount of graphitic carbon is detected.

Full text of DOI:10.1006/jcat.1997.1585

JOURNAL OF CATALYSIS 167, 470–481 (1997)
ARTICLE NO. CA971585
The Metathesis of Propylene Catalyzed by Model Oxides Studied
Using a High-Pressure Reactor Incorporated into an Ultrahigh
Vacuum Chamber
1
B. F. Bartlett, H. Molero, and W. T. Tysoe
Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53211
Received June 13, 1996; revised December 16, 1996; accepted December 18, 1996
an active catalyst, it has been demonstrated that metallic
The olefin metathesis activity of a series of model catalysts con- molybdenum can catalyze this reaction at sufficiently high
sisting of metallic molybdenum, MoO2, or MoO3 is investigated us- temperatures (>650 K) (23). In this case, the reaction ac-
ing a combined high-pressure reactor/ultrahigh vacuum apparatus.
The model catalysts are analyzed using X-ray and ultraviolet pho-
toelectron, Raman, and Auger spectroscopies and it is shown that
tivation energy is � 65 kcal/mol, substantially higher than
values found for supported molybdenum oxides. This paper
addresses the effect of oxidation on the activity of molyb-
denum for olefin (specifically propylene) metathesis using
MoO2 is the most active above � 650 K. Reaction in this temperature
regime proceeds with a high activation energy (� 60 kcal/mol). An-
a high-pressure reactor incorporated into an ultrahigh vac-
other, lower-activation-energy (� 6 kcal/mol) regime is encountered
uum chamber. This strategy has the advantage that sam-
ples can be prepared and characterized and their catalytic
activity measured without intervening exposure to air. Fur-
for reaction below � 650 K and it is suggested that both reactions
proceed simultaneously. In addition, the absolute rate and selectiv-
ity for the reaction on MoO2 below 650 K are similar to those found
for high loadings of supported molybdenum oxide, suggesting that thermore, this approach allows the nature of the catalyst
an oxide layer provides a good model catalyst for this reaction. It surface to be monitored merely by removing the sample
is also found that reaction proceeds in the presence of a thick car- from the high-pressure reactor into ultrahigh vacuum. This
bonaceous layer where the thickness of this layer decreases with
increasing oxidation state. A Raman analysis of the surface of the
catalyst following reaction indicates that this layer consists pre-
dominantly of hydrocarbon species, although a small amount of
strategy reveals that the surface of the catalyst is covered
by carbonaceous deposits, and restart reactions (see below)
show that the reaction proceeds in the presence of such a
layer. Ex situ Raman analysis of the catalyst surface reveals
that this layer consists primarily of strongly bound hydro-
carbons as well as a small amount of graphitic carbon.
graphitic carbon is detected.
�c 1997 Academic Press
INTRODUCTION
EXPERIMENTAL
An active heterogeneous catalyst for the metathesis of
propylene,
The apparatus that was used for these experiments has
been described in detail elsewhere (24, 25). Briefly, how-
ever, it consists of a bakeable, stainless-steel, ultrahigh vac-
uum chamber operating at a base pressure of 1 � 10
2
C3H6 → C2H4 + C4H8,
�
10
was discovered in 1964 by Banks and Bailey (1) and con-
sists of alumina-support molybdenum oxide. It was soon
found that the reaction could also be catalyzed in homo-
geneous phase (2) and it has been suggested, in this case,
that the reaction proceeds via the initial formation of a car-
bene which acts as the “active site” for catalysis. This is
proposed to react further with an alkene forming a metal-
lacycle which can thermally decompose via the reverse of
its formation pathway to yield metathesis products (3–22).
While metallic molybdenum is not usually thought of as
Torr following bakeout. It is equipped with a quadrupole
mass analyzer for residual gas analysis, leak testing, and
temperature-programmed desorption studies. It also con-
tains a four-grid retarding-field analyzer (RFA) which can
be used for low-energy electron diffraction (LEED) exper-
iments or for collecting Auger spectra. It is also equipped
with a rotatable hemispherical analyzer which was also used
for Auger measurements by collecting the n(E) spectrum
from electrons excited using a 3-kV incident electron beam,
and the conventional dn(E)/dE spectrum is obtained by dif-
ferentiating these data using a Savitzky–Golay algorithm.
The apparatus also includes a coaxial high-pressure re-
actor which can be sealed and filled to 1 atmosphere while
1
To whom correspondence should be addressed. Fax: (414) 229-5530.
E-mail: wtt@alpha2.csd.uwm.edu.
470
0021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

METATHESIS OF PROPYLENE
471
maintaining ultrahigh vacuum (� 2 � 10 10) in the rest of tron spectra and this yielded an overall spectral resolution
the apparatus. This is attached to a recirculating loop and of 0.15 eV. Photoelectron spectra were excited using 55-eV
the gas recirculated by a pump. The pressure is monitored radiation.
�
by means of a capacitance manometer and the gas com-
The propylene used for these experiments (Linde, CP
position analyzed by diverting aliquots of the gas mixture Grade) was transferred from the cylinder to a glass bottle
to a gas chromatograph. The reaction rate is measured di- and further purified by repeated bulb-to-bulb distillations,
rectly from the product accumulation curve for low (<1% ) and its cleanliness monitored either mass spectroscopically
conversions.
or using the gas chromatograph. In all experiments, the
The molybdenum foil sample is attached to the end of high-pressure reactor was ballasted to 1 atmosphere using
the sample manipulator and can be resistively heated to nitrogen to ensure that the gas circulation rate was identical
�
2000 K or cooled to 80 K by thermal contact to a liquid- for all experiments.
nitrogen-filled reservoir. The sample is cleaned using a
standard procedure which consists of heating at 1450 K
RESULTS
�
7
in 2.0 � 10 Torr of oxygen and the annealing at 2000 K
in vacuo to remove any remaining oxygen. The sample is
judged clean when no impurities (predominantly carbon)
are detected using Auger spectroscopy.
Sample Preparation and Characterization
The model catalysts used to probe variations in catalytic
activity for olefin metathesis as a function of molybdenum
Raman spectra were collected using a spectrometer com- oxidation state consisted of metallic molybdenum, MoO2,
prising a Spex double monochromator where photons are and MoO3. Metallic molybdenum can be obtained rather
detected using a Peltier-effect cooled photomultiplier op- straightforwardly in ultrahigh vacuum using a conventional
erating in pulse-counting mode. Spectra were excited using cleaning procedure as described above, which consists of
either the 514.5- or 488.0-nm lines of an Argon ion laser heating in oxygen to remove any surface carbon and then
and light was back scattered from the sample. The collec- annealing in vacuo to remove any remaining oxygen (26–
tion time for each scan was approximately 20 min, and sev- 28). The sample cleanliness is monitored, in this case, from
eral scans were averaged together to get each spectrum. the Auger spectrum of the sample which is judged clean
The similarity of each scan was examined before averag- when no oxygen or carbon signals are detected. The synthe-
ing to eliminate the possibility of laser-induced alteration sis of MoO2 by the controlled oxidation of metallic molyb-
of the sample. The laser light was steered to the sample via denum has been well documented in the literature and
two turning prisms and passed through an iris to remove these procedures were followed in this work (29–41). Ox-
plasma lines. However, due to the low signal intensity from idation was carried out in the ultrahigh vacuum portion
�
5
the sample, these were still evident in some of the spectra of the chamber at a pressure 3 � 10 Torr of oxygen with
and the laser light was then changed to prevent interference the sample heated to 1130 K for a period of at least 70 s, al-
between Raman and plasma lines.
though longer reaction times generally lead to thicker films.
X-ray photoelectron spectra were collected using a VG Figure 1 displays the Mo 3d photoelectron spectrum of a
EscaLab spectrometer and were excited using Mg K� radi- molybdenum foil that had been treated in this way as a
�
5
ation, and the photoemitted electrons were analyzed using function of oxidation time in 3 � 10 Torr of O2 at a sam-
a 100-mm radius hemispherical analyzer operating at a pass ple temperature of 1130 K. Metallic molybdenum exhibits
energy of 25 eV, yielding an overall spectral resolution of a spin-orbit coupling split doublet with a 3d5/2 peak at 228.2
0
.8 eV.
eV binding energy (BE) with a corresponding 2p3/2 peak
Ultraviolet photoelectron spectra were obtained at the at 231.3 eV. As oxidation proceeds, this peak attenuates in
Wisconsin Synchrotron Radiation Center usingthe Aladdin intensity so that after heating for 185 s the spectrum now
storage ring. The stainless-steel, ultrahigh vacuum cham- exhibits peaks at 229.4 and 232.5 eV BE. The presence of
ber used for these experiments operated at a base pressure the peak at 229.4-eV binding energy confirms the formation
�
10
of � 1 � 10
Torr following bakeout and was attached to of molybdenum dioxide (42–46), although there is a very
the end of a Mark V Grasshopper monochromator. The small feature present at 231.0 eV suggesting the formation
chamber was equipped with a quadrupole mass analyzer of a very small amount of MoO3. The results of annealing
for residual gas analysis and to test gas purities. It also this film in vacuo are also documented in the data of Fig. 2.
contained a double-pass cylindrical mirror analyzer which These data were collected by heating the sample for a pe-
was used to collect both Auger and photoelectron spectra. riod of 120 s to the temperature indicated adjacent to each
Auger spectra were excited using a 3-keV electron beam spectrum and allowing the sample to cool to room temper-
and the spectra collected by pulse counting the scattered ature, following which the spectrum was recorded. These
electrons and the dn(E)/dE Auger spectrum obtained by results indicate that MoO2 is thermally stable in vacuo up
numerically differentiating the n(E) signal. The analyzer to an annealing temperature of � 1200 K but is reduced to
was operated at a pass energy of 25 eV to collect photoelec- metallic molybdenum by heating to 1420 K. Note that while

4
72
BARTLETT, MOLERO, AND TYSOE
FIG. 1. X-ray photoelectron spectra of a molybdenum foil oxidized
in 3 � 10� Torr of oxygen at 1130 K for various times. The reaction time
5
FIG. 3. Raman spectra of the MoO2 film formed by oxidizing a molyb-
denum foil in 3 � 10 Torr of oxygen at 1130 K for 185 s.
�
5
is marked adjacent to each of the spectra.
the molybdenum detected in XPSismetallicafter heatingto Raman spectrum of a film formed by similarly oxidizing a
this temperature, both Auger and ultraviolet photoelectron metallic molybdenum foil in the ultrahigh vacuum portion
spectroscopy reveal the presence of oxygen on the surface of the apparatus. The spectra were obtained ex situ by re-
which is, in this case, present as an atomic, chemisorbed movingthe sample from the manipulator and transportingit
overlayer which is not completely thermally removed until in an evacuated desiccator to the Raman spectrometer and
the sample is heated to � 2000 K. Also shown in Fig. 3 is a collecting the spectrum. This spectrum exhibits features at
�
1
4
61, 495, 571, 589, ad 744 cm which have previously been
shown to be characteristic of MoO2 (42, 43, 46–48). Figure 4
FIG. 4. Ultraviolet photoelectron spectrum of a “thick” MoO2 film
�
5
FIG. 2. X-ray photoelectron spectra of a molybdenum foil oxidized
in 3 � 10� Torr of oxygen at 1130 K for 185 s (top spectrum) and then
annealed to various temperatures. The annealing temperature is marked
adjacent to each of the spectra.
obtained by oxidizing a molybdenum foil in 3 � 10 Torr of oxygen at
5
1130K for 185sand a “thin” MoO2 film obtained byheatinga molybdenum
foil in 3 � 10 Torr of oxygen at 1130 K for 70 s obtained using 55-eV
�
5
photons.

METATHESIS OF PROPYLENE
473
using the same spectrometer. Both of these spectra are in
good agreement with each other as well as with previously
published spectra of MoO3 (42, 43), confirming that the
in situ treatment of metallic molybdenum leads to the for-
mation of molybdenum trioxide. The origin of the feature
�
1
at 952 cm has been assigned to the Mo == O mode of an
amorphous phase of MoO3 (50). The corresponding Mo2O
�
1
and Mo3O modes should be at 868 and 700 cm , respec-
�
1
tively. The shoulder seen on the 822 cm peak in Fig. 5
between 850 and 880 cm may be the 868 cm mode. The
00 cm mode is too weak to see. The structure of this
�
1
� 1
�
1
7
MoO3 phase is not entirely clear, but is thought to be an
octahedrally coordinated hydrated molybdate species. The
three modes for this species are analogous to the 1002, 822,
�
1
and 670 cm modes of crystalline MoO3 (50, 80–82). A
�
1
peak near 940 cm has also been observed for an octa-
hedrally coordinated polymolybdate species supported on
AlO3 under dehydrated conditions; however, the associ-
�
1
ated peak at 590 cm for this phase cannot be resolved
FIG. 5. Raman spectra of the MoO3 film formed by oxidizing a molyb- (83). As a final check, comparison of the O/Mo Auger ra-
denum foil in 200 Torr of oxygen in the high-pressure reactor at 550 K for
tios of both samples corresponds very well to their relative
stoichiometries, which also served as a convenient method
for periodically checking the nature of the model catalysts.
4
h. Shown for comparison is a Raman spectrum of MoO3 powder.
displays the ultraviolet photoelectron spectrum of both a
Catalytic Activity of Model Catalysts
“
thin” and a “thick” film of MoO2 obtained by oxidation of
�
5
a molybdenum sample in ultrahigh vacuum using 3 � 10
The rate of propylene metathesis was measured from the
Torr of oxygen with the sample heated to 1130 K. The thin slope of the product accumulation curve for the various
sample was made by heating for 70 s (where the XPS data of model oxides. Note that an induction period was found in
Fig. 1showmetallicmolybdenum), and the thick sample was the rate of olefin metathesis, as illustrated in Fig. 6, which
made by oxidation for 185 s (where the data of Fig. 1 reveals displays the concentration of metathesis products as a func-
only MoO2). Both of these spectra are essentially identical, tion of time for reaction of 450 Torr of propylene at a
showing peaks at 22.7-eV BE due to emission from the O 2s
orbital and a broad feature centered at � 6.3-eV BE due to
emission from the O 2p levels. The features within � 4 eV of
the Fermi level (Ef) are due to emission from molybdenum
d bands of the oxide. The sampling depth of photoelectron
spectroscopy depends primarily on the kinetic energy of
the photoemitted electron (49). In the case of XPS (Fig. 1),
this is � 1000 eV, and for UPS (Fig. 4) this is � 30 eV. The
mean free path for � 1-kV electrons (� 20 A˚ ) is substantially
larger than that for � 30-eV electrons (� 3 A˚ ). The similar-
ity between the UPS spectra for the thick and thin films
in Fig. 4 indicate that a uniform MoO2 film grows over the
metallic molybdenum and the metallic peaks evident in the
data in Fig. 1 are just due to the underlying metal detected
because of the larger electron mean-free path in this case.
MoO3 is synthesized at higher pressures using 200 Torr
of oxygen with a sample temperature of 550 K (42, 43).
This procedure has been demonstrated to be sufficiently
vigorous to lead to the complete oxidation of molybdenum
metal to MoO3. This is also illustrated by the Raman spec-
trum displayed in Fig. 5 which exhibits features at 670, 822,
and 1002 cm . Shown for comparison is a Raman spectrum
FIG. 6. Typical product accumulation curve for the formation of
metathesis products using a model MoO3 catalyst for reaction at 880 K
�
1
of powdered MoO3 (Mallinckrodt, 99.9% purity) obtained using a propylene pressure of 450 Torr.

4
74
BARTLETT, MOLERO, AND TYSOE
catalyst temperature of 880 K using a model MoO3 catalyst.
The reaction rate (measured from the slope of the curve)
is initially rather low and increases as a function of time on
stream. Reaction rates presented below are those after the
completion of any induction period. Possible origins for this
induction period will be discussed below. Note that induc-
tion periods in the metathesis of olefins have been noted
previously (51, 52).
The reaction temperatures used in these experiments
are substantially higher than those conventionally used for
olefin metathesis catalyzed by supported oxides, which are
usually <650 K. Reactions were carried out, in this case,
above this temperature for comparison with the data for
metallic molybdenum, which is rather inactive for olefin
metathesis until heated to such high temperatures. In addi-
tion to the formation of metathesis products, hydrogenoly-
sis products (CH4 and C2H4) were also detected in the reac-
tion products. Similar hydrogenolysis products have been
noted for the reaction of metallic molybdenum (23).
The relative activities of each of the model catalysts are
displayed in Fig. 7in histogram form for propylene metathe-
sis on each of the model samples for reaction at 870 K using
FIG. 8. Temperature dependence ofthe rate ofolefin metathesisusing
50 Torr of propylene catalyzed by a model MoO2 catalyst plotted in
Arrhenius form (ln(rate) versus 1/T).
4
4
50 Torr of propylene. Rates are given in reactions/site/s,
where a site is taken to be an exposed molybdenum atom
on the (100) face of molybdenum for metallic molybdenum.
Since the catalyst was, in this case, a foil, this yields only
an approximate value of the number of exposed molybde-
num atoms. In the case of MoO2 and MoO3, the molybde-
num surface concentration was measured from the molyb-
denum Auger signals relative to that for the metal for each
of the model oxide catalysts. These results show that metal-
lic molybdenum is relatively inactive for olefin metathesis,
even at such elevated temperatures. MoO2 is the most ac-
tive catalyst and MoO3 shows some intermediate activity
(
53, 54). Note, however, that it is shown below that MoO3
is partially reduced to MoO2 under the reaction conditions,
so that the activity data for an initially formed molybdenum
trioxide are in fact, for some lower oxidation state oxide.
Molybdenum dioxide is stable under these conditions (see
Refs. 42, 43, and below).
The temperature dependence of the most active MoO2
catalyst was measured and the resulting data are displayed
in Arrhenius form in Fig. 8 and yield two distinct activation
energies. At higher temperatures (>650 K), the activation
energy for the reaction is � 63 � 5 kcal/mol, a value very
similar to that measured on metallic molybdenum (23). At
lower temperatures (<650 K), the activation energy is con-
siderably lower (6 � 2 kcal/mol). This value is typical for
olefin metathesis catalyzed by supported molybdenum ox-
ides (57, 58). It is important to keep in mind that all of
these experiments were performed on the same molybde-
num foil sample. After each reaction the sample is cleaned
and a fresh layer of MoO2 is deposited on the Mo substrate
according to the protocol outlined above.
Displayed in Fig. 9 is the selectivity for olefin metathe-
sis as a function of temperature, where the selectivity S is
defined as
RM
S =
,
FIG. 7. Histogram displaying the relative catalytic activity for olefin
metathesis for model molybdenum metal, MoO2, and MoO3 catalysts for
the reaction of 450 Torr of propylene at 870 K.
RM + RH
where RM is the rate of metathesis and RH the rate of

METATHESIS OF PROPYLENE
475
FIG. 9. Temperature dependence of the selectivity to propylene
metathesis using 450 Torr of propylene catalyzed by a model MoO2
catalyst.
FIG. 10. Histogram displaying the relative catalytic activity for olefin
metathesis for model molybdenum metal, MoO2, and MoO3 catalysts for
the reaction of 450 Torr of propylene at 650 K. Shown for comparison is the
corresponding activity for high loadings on molybdenum oxide supported
on alumina (see text).
hydrogenolysis (ethylene plus methane formation rate).
The metathesis selectivity is constant and relatively high
(
� 87% ) for reaction below � 650 K and decreases linearly
rates agree very well. This suggests that MoO2 (and per-
haps MoO3) are the most active forms of the model cata-
lyst and that a thin film of the oxide grown on a metallic
substrate provides a good model system for this catalyst.
Figure 11 displays a typical product accumulation curve for
olefin metathesis at 600 K using 450 Torr of propylene which
is, in this case, linear and exhibits no induction period as was
noted for reaction at higher temperatures.
for reaction above this temperature. This provides addi-
tional evidence for different reaction pathways that pre-
dominate below and above 650 K.
These results indicate that there are two different re-
action pathways leading to the formation of metathesis
products. One is a high-activation-energy pathway which
predominates at high temperatures, and the other is a sig-
nificantly lower activation energy route which therefore
becomes the most important as the temperature is low-
ered. Note that olefin metathesis using supported catalysts
is usually carried out at temperatures less than 650 K. The
correspondence between activation energies measured for
supported catalysts and for MoO2 in this case suggests that
similar reactions are being probed in both cases. Further
confirmation of this notion comes from Fig. 10, which dis-
plays the relative activities of the model catalysts using 450
Torr of propylene at a reaction temperature of 650 K. In
this case, metallic molybdenum is essentially completely
inactive, while both MoO2 and MoO3 show significant ac-
tivity, with MoO3 apparently being slightly more active than
MoO2. This calculation of absolute rate assumes that an ac-
tive site isformed for each available molybdenum atom, and
therefore represents a lower bound for the true turnover
frequency. The actual identity of the active site for catalysis
is not certain. Also shown for comparison is the reaction
rate extrapolated to similar conditions for high loadings
(
18.6% ) of molybdenum oxide supported on alumina (2).
Not only is the activation energy of the model MoO2 cata-
FIG. 11. Product accumulation curve for olefin metathesis catalyzed
lyst similar to that of supported catalysts, but the absolute by MoO2 for reaction at 600 K using 450 Torr of propylene.

4
76
BARTLETT, MOLERO, AND TYSOE
tion state increases, so that clearly one effect of changing
the oxidation state is to diminish the amount of carbon
on the surface during reaction. It has also been demon-
strated using restart reactions that, in the case of metallic
molybdenum, at least, the catalytic reaction proceeds in
the presence of such a layer (23, 59). In this case, the re-
action rate was measured for an initially clean sample. The
sample was then removed from the high-pressure reactor
into the ultrahigh vacuum portion of the apparatus and the
amount of carbon on the surface measured using Auger
spectroscopy. The sample was then reinserted into the high-
pressure reactor, without any intervening treatment, and
the reaction restarted using fresh propylene. Identical re-
action rates were measure in both cases, suggesting that the
reaction was able to proceed in the presence of such a thick
carbonaceous layer. The results of a similar experiment are
shown in Fig. 14 where product accumulation curves for the
freshly prepared MoO2 catalyst (Fig. 14, ) also display an
induction period. The corresponding curve after the sam-
ple had been removed to ultrahigh vacuum and reinserted
into the high-pressure reactor (Fig. 14, ᭹) displays no in-
duction period and the reaction rate is essentially identical
to that in the initial experiment on clean MoO2 after com-
pletion of the induction period. this similarly indicates that
the olefin metathesis reaction again proceeds in the pres-
ence of a thick carbonaceous layer on the oxide catalyst,
although the layer appears to be somewhat thinner in this
case. These results also indicate that whatever changes that
are associated with the catalytic induction period remain
when the sample is removed from the high-pressure reac-
tor into ultrahigh vacuum and after reinsertion into the cell.
This effect could be either the formation of some surface
species on the catalyst or some change (activation) of the
FIG. 12. Pressure dependence ofolefin metathesiscatalyzed byMoO2
at 875 K plotted as log(rate) versus log(pressure) where this refers to the
propylene pressure.
The pressure dependence of the olefin metathesis rate
for reaction at � 880 K is displayed in Fig. 12 in logarith-
mic form. Measurement of the slope of this curve yields an
order for the reaction of 1.1 � 0.1, so that the reaction is
first order in propylene pressure. These kinetic parameters
are very similar to those measured for metallic molybde-
num (23), implying that similar reaction pathways operate
in both regimes.
Nature of the Catalysis Surface during Reaction
It is possible that the surface of the catalysts may become
modified during reaction. These effects can be probed using
a high-pressure reactor/ultrahigh vacuum system by remov-
ing the sample from the reactor to the vacuum portion of
the apparatus without any intervening exposure to air. It
has been noted previously that molybdenum-catalyzed
hydrocarbon conversion reactions proceed in the presence
of a carbonaceous deposit (23, 59). This has been found at
relatively high temperatures during olefin metathesis (23)
but also at considerably lower temperatures for ethylene
hydrogenation (59). Similar thick layers of carbonaceous
deposit are found on the surfaces on the catalyst after reac-
tion in this case. Plotted in histogram form in Fig. 13 is the
amount of carbon found on metallic molybdenum, MoO2,
and MoO3 model catalysts after reaction in 200 Torr of
propylene at 880 K. In all of these cases, the amount of
carbon on the surface was measured from the C/Mo Auger
ratio which was converted into approximate carbon film
thickness using a method that has been described previ-
ously (23). Clearly there is a large amount of carbon present
on the surface during reaction, although the amount of car-
FIG. 13. Histogram comparing the relative amounts of carbonaceous
deposit on model Mo, MoO2, and MoO3 catalysts following reaction in
bon on the surface decreases substantially as the oxida- 200 Torr of propylene at 880 K.

METATHESIS OF PROPYLENE
477
FIG. 14. Product accumulation curve for the metathesis of 450 Torr
of propylene at 880 K using a model MoO2 catalyst: ( ) freshly prepared
catalyst, (᭹) after removal into ultrahigh vacuum and reinsertion into the
vacuum system.
FIG. 16. Raman spectrum of a model MoO2 catalyst (a) before reac-
tion and (b) after reaction in 200 Torr of propylene at 880 K for 30 min.
trioxide (42, 43). Figure 15b displays the corresponding re-
gion of the spectrum after reaction in 200 Torr of propylene
at 880 K. The peaks due to MoO3 are essentially absent fol-
lowing reaction with propylene and additional features are
present due to the formation of MoO2 (42, 43, 46–48). That
is, the initially formed model molybdenum trioxide catalyst
had been reduced during reaction to form molybdenum
dioxide. These results are in good agreement with the work
of others to examine the reducibility of oxidized molybde-
num catalysts which have similarly shown the reduction of
MoO3 to MoO2 under comparable conditions in hydrogen
catalyst itself. Raman spectroscopy was used to probe the
nature of the oxide since the presence of such a thick car-
bonaceous layer on the catalyst sample following reaction
means that the substrate oxide is rather difficult to follow
using other, more surface-sensitive methods such as X-ray
photoelectron spectroscopy, whereas Raman spectroscopy
probes the whole of the film. The resulting Raman spectra
of the model MoO3 catalyst prior to reaction is displayed
in Fig. 15a showing the characteristic Raman peaks of the
(
42, 43, 55, 56).
The results of a similar set of experiments for metathesis
catalyzed byMoO2 are displayed in Fig. 16;the Raman spec-
trum of MoO2 is shown in Fig. 16a (42, 43, 46–48), confirm-
ing that MoO2 had indeed been formed, and Fig. 16b shows
the corresponding spectrum after reaction. No further re-
duction in the sample is apparent, although the spectrum
is attenuated in intensity, probably due to the presence of
the carbonaceous layer present on the sample. This result
is again in accord with those of previous experiments which
have shown that, under similar conditions, MoO3 reduces
primarily to MoO2 (42, 43, 55, 56).
�
1
The origin of the feature at � 900 cm in Figs. 15 and 16
is unclear. There is evidence in the literature for both tetra-
hedrally and octahedrally coordinated molybdate species
having peaks near this region (80–82). It is possible that
this mode and the other surface molybdate mode observed
�
1
(
950 cm ) may have formed from water contamination
during ex situ Raman analysis. It seems unlikely that hy-
drated molybdate species would form during preparation
in ultrahigh vacuum, and formation during reaction may be
FIG. 15. Raman spectrum of a model MoO3 catalyst (a) before reac-
tion and (b) after reaction in 200 Torr of propylene at 880 K for 30 min.

4
78
BARTLETT, MOLERO, AND TYSOE
signed to vibrational modes of absorbed hydrocarbons (60).
In view of the relatively large amount of carbon detected
on the surface following reaction using Auger spectroscopy,
it is suggested that these features are due to these carbona-
ceous species and indicate that they consist substantially
of adsorbed hydrocarbons. Note that the restart reactions
described above on MoO2 and other such experiments on
metallic molybdenum indicate that catalysis proceeds in the
presence of such a carbonaceous layer. This phenomenon
has been seen previously where, for example, ethylene hy-
drogenation catalyzed by metallic platinum was found to
proceed in the presence of an overlayer of ethylidyne (61).
In addition, a similar hydrocarbon layer was also found for
the same reaction catalyzed by metallic molybdenum, ex-
cept in this case the carbonaceous layer thickness was much
greater for the metallic molybdenum catalyst than for plat-
inum (23, 59).
DISCUSSION
FIG. 17. Raman spectrum of a model MoO3 catalyst displaying the
region of the spectrum between 900 and 1800 cm before (a) and after
b) reaction in 200 Torr of propylene at 880 K for 30 min.
�
1
X-ray photoelectron (Figs. 1 and 2), Raman (Figs. 3 and
(
5
), and ultraviolet photoelectron (Fig. 4) spectroscopies
demonstrate that films of MoO2 and MoO3 can be grown
ruled out since these features appear on catalysts not used by the oxidation of metallic molybdenum. MoO is grown
2
for reaction.
3
by controlled oxidation in ultrahigh vacuum and MoO is
Finally, additional features were found in the Raman synthesized by oxidation at higher pressures using the iso-
spectrum of the MoO2 sample after reaction (which were latable reactor. Metallic molybdenum can be formed rather
not present prior to catalysis). These are displayed in Fig. 17 easily by cleaning metallic molybdenum in ultrahigh vac-
�
1
1
for the region between 900 and 1800 cm and in Fig. 18 for uum using well-established techniques. The MoO layer is
2
�
the region between 2700 and 3300 cm and show features thermally rather stable is ultrahigh vacuum (Fig. 2) and
�
1
� 1
around 3000 cm and around 1600 cm . These are as- can be heated to � 1200 K without thermally decompos-
ing. This thermal stability is maintained even in a reducing
atmosphere at 880 K since the Raman spectrum after reac-
tion under these conditions is essentially identical to that
found prior to reaction (Fig. 6). The same is not true for
MoO3 which is reduced to MoO2 by reaction in propylene
at 880 K (Fig. 15). These results are in accord with those
found in other studies of the reducibility of molybdenum
oxides (42, 43, 55, 56).
The relative activities of the model catalysts at high re-
action temperatures (� 880 K) show strong variation with
oxidation state (Fig. 7) as has been found previously for
supported molybdenum oxide catalysts (53, 54). Note, how-
ever, that the reactivity for a catalyst initially comprising
MoO3 is reduced under reaction conditions. In addition, an
induction period is found for the reaction on both MoO3
(
Fig. 3) and MoO2 (Fig. 14); it is tempting to associate with
the reduction of the MoO3 sample, and clearly this may
contribute to this effect. However, a similar induction pe-
riod is also evident for the metathesis catalyzed by MoO2
which is not reduced similarly, which suggests that there is
another origin for the induction period. This will be dis-
cussed in greater detail below. The activation energy of the
reaction at these high temperatures is considerably larger
FIG. 18. Raman spectrum of a model MoO3 catalyst displaying the
region of the spectrum between 2700 and 3300 cm before (a) and after
b) reaction 200 Torr of propylene at 880 K for 30 min.
�
1
(

METATHESIS OF PROPYLENE
479
than conventionally encountered for olefin metathesis us- Tropsch synthesis) (62) and in this case, the reaction pro-
ing supported molybdenum oxide catalysts and is close to ceeds via polymerization of adsorbed CHx species (63–70).
that found for metallic molybdenum (23). In contrast, re- In this context, we have recently examined the reaction of
action at lower temperatures (<650 K) proceeds with an ethylene catalyzed by metallic molybdenum at tempera-
activation energy more closely akin to that found for sup- tures above 650 K (71, 72). In this case, the metathesis of
ported catalysts (Fig. 8) (57, 58) and, moreover, the abso- ethylene is degenerate since it should yield only ethylene.
lute reaction rates for both MoO2 ad MoO3 are in good However, higher hydrocarbons up to C6 are also detected in
agreement with high loadings (18.6% ) of molybdenum ox- the reaction products for reaction above � 650 K, suggest-
ide on alumina (2) (Fig. 10). In addition, the reaction se- ingthat carbenescan recombine, forminghigher-molecular-
lectivity is high (� 90% ) and rather constant for reaction weight products in this temperature region. It is thus pro-
below � 650 K but decreases linearly with increasing tem- posed that metathesis takes place at higher temperatures
perature above this value (Fig. 9). These results suggest via recombination of the carbene fragments on the surface
that films of molybdenum oxide provide a good model for and that this reaction proceeds in parallel with the lower
the supported metathesis catalysts at high loadings for re- activation energy route but with an activation energy of
action below � 650 K where the surface of the alumina � 65 kcal/mol. Such a reaction route has been proposed pre-
support is likely completely covered by the oxide. Under viously as one of the removal pathways for carbene species
these conditions, it is clear that metallic molybdenum is from the surface during olefin metathesis (73).
completely inactive for the metathesis of olefins while both
A common feature of catalysis, in the case of both met-
MoO3 and MoO2 show similar activities. This result agrees als and oxides, is the presence of a carbonaceous deposit
with what has been observed previously for supported Mo on the surface following reaction. Restart reactions (Fig. 4)
catalysts, where Mo(0) is found to be essentially inactive indicate that reaction takes place in the presence of these
for metathesis and oxidized molybdenum at high load- carbonaceous deposits. They are sufficiently thick that they
ing exhibits some metathesis activity (83). Note also that can be detected using Raman spectroscopy (Figs. 17 and
no induction period is found for reaction at these lower 18) and exhibit C-H stretching modes in the region 2800
�
1
temperatures. These results suggest that there are two dis- to 3100 cm (60) at 2797, 2923, 2983, 3019, 3076, and
�
1
tinct metathesis pathways which proceed simultaneously: a 3116 cm , indicating that they indeed consist of hydro-
high-activation-energy (� 65 kcal/mol) route that predomi- carbon fragments on the surface rather than carbonaceous
nates above � 650 K and a lower-activation-energy route deposits. The most prominent feature in this region is at
�
1
(
� 6 kcal/mol) that operates below this temperature. As 2923 cm which can be assigned to the asymmetric stretch-
mentioned above, the activation energy, absolute rate, and ing mode of a –CH2- species, suggesting that the adsorbed
selectivity for metathesis in the low-temperature regime are hydrocarbon is predominantly saturated. The correspond-
�
1
akin to those measured for supported oxides. In addition no ing symmetric stretching mode should be at � 2850 cm
induction period is found for the low-temperature reaction and, although there is some intensity in this region, it is
Fig. 11). It has been suggested that the reaction pathway rather weak. The corresponding bending modes for methy-
(
�
1
at these lower temperatures proceeds via the initial forma- lene species are at � 1300 cm and a feature is evident
�
1
� 1
tion of a carbene “active site.” This has been proposed to at � 1280 cm (Fig. 17). The peak at � 2980 cm may
react further with an alkene forming a metallacycle which be due to the asymmetric stretching mode of a methyl
can then decompose via the reverse of this reaction path- group which is much weaker than the corresponding methy-
way to yield metathesis products via a two-step reaction lene mode consistent with a relatively long hydrocarbon
(
6–22). It is likely that this reaction pathway operates at chain. The corresponding symmetric stretch should appear
�
1
lower reaction temperatures (<650 K) but that another, at � 2870 cm and the shoulder on the lower frequency
�
1
much higher-activation-energy route, predominates as the side of the main 2923 cm feature may be due to this mode.
temperature increases. It should be emphasized that both The corresponding bending modes should appear at � 1380
�
1
reactions proceed simultaneously and that it is the vast dif- and 1460 cm and may form part of the broad feature be-
�
1
ferences in activation energies for the two pathways which tween 1250 and 1650 cm in Fig. 17. The possible origin
cause one or the other to predominate in different temper- of these features will be discussed in greater detail below.
ature regimes.
The higher frequency features in the C–H stretching re-
�
1
The reaction kinetics(both pressure dependence (Fig. 12) gion (evident at 3019, 3076, and 3116 cm ) may be indica-
and activation energy (Fig. 8)) are similar for the oxides tive of some olefinic bonds in the adsorbed hydrocarbon
and metal for reaction at high temperatures (>650 K) (23). (74–77).
This suggests that the reaction pathway may be identical
The Raman spectrum of small graphitic particles exhibits
�
1
for these catalysts at high temperatures. It should also be broad features at � 1600 and 1350 cm (78, 79). The broad
�
1
noted that metallic molybdenum model catalysts are active feature at � 1600 cm (Fig. 17) may be due to the presence
for hydrocarbon formation from CO and hydrogen (Fisher– of some graphitic species on the surface. The corresponding

4
80
BARTLETT, MOLERO, AND TYSOE
�
1
feature at � 1350 cm has narrower peaks superimposed whereas Raman data and previous work indicate that the
on it which have been discussed above. bulk of the catalyst is not reduced under these conditions.
In view of the fact that reaction products are found due A possible explanation may be that the outer portion of the
to polymerization of carbonaceous fragments on the sur- MoO2 catalyst is reduced during reaction and that the in-
face, these surface hydrocarbons may also be formed by a duction period reflects this reduction of the surface atoms,
similar route. Note that they are present at some equilib- whereas the bulk of the sample remains as an oxide.
rium thickness since restart reactions were carried out after
many turnovers (in excess of 20,000 in the case of the data
shown in Fig. 14) had been completed and the reaction pro-
ceeded at a constant rate when the carbon-covered surface
was reinserted into the reaction cell (Fig. 14). If the amount
of carbon on the surface was continually increasing, this
would likely result in a continuous decrease in the rate of
the catalytic reaction with time on stream. Such a carbona-
ceous layer has also been detected during ethylene hydro-
genation catalyzed byMo(100) and appearsto be a common
feature of transition-metal and also transition-metal-oxide-
catalyzed hydrocarbon reactions. Note that the films found
on molybdenum and molybdenum oxides are considerably
thicker than those found on Group VIII metals, so that, for
example, ethylene hydrogenation proceeds on platinum in
the presence of a monolayer of adsorbed ethylidyne. It is
probable that the presence of such a layer has the effect of
inhibiting catalysis both by blocking sites and by impeding
access to the surface. It is likely that the thicker layer found
for molybdenum catalysts also contributes to their rela-
tively low activity, for example, for ethylene hydrogenation
compared to platinum. It is difficult, however, to extrapo-
late these conclusions regarding the presence of a carbona-
ceous layer to the performance of supported molybdenum
catalysts due to the significant difference between the reac-
tion conditions investigated here and the actual conditions
under which supported catalysts are used.
CONCLUSIONS
Model catalysts can be synthesized that consist of metal-
lic molybdenum, MoO2, and MoO3. Variations in activity
for olefin metathesis are found as a function of sample ox-
idation state where MoO2 is found to be the most active
above � 650 K and both MoO2 and MoO3 are active for re-
action below � 650 K and where the kinetics resemble those
of high loadings (18.6% ) of an oxide on an alumina support.
Catalysts are characterized using ultraviolet and X-ray pho-
toelectron and Raman spectroscopies which confirm the
formation of the model oxides, and Raman spectroscopy
demonstrates that MoO3 is reduced during reaction at high
temperatures.
A thick carbonaceous layer is found on the surface of the
catalysts during reaction at high temperature, and restart
reactions confirm that catalysis proceeds in the presence of
this film. Raman spectra of the film indicate that it consists
of a hydrocarbon anchored to the surface of the catalysts.
An induction period is found in the rate of metathesis at
high temperatures and this is tentatively assigned to a re-
duction of the outermost layer of the catalyst during reac-
tion.
ACKNOWLEDGMENTS
We gratefullyacknowledge support ofthiswork bythe U.S. Department
Finally, we turn our attention now to the nature of the of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences,
under Grant DE-FG02-92ER14289. This work is based upon research
conducted at the Synchrotron Radiation Center, University of Wisconsin–
Madison, which is supported by the NSF under Award DMR-95-31009.
induction period observed at high temperatures (>650 K)
for olefin metathesis (51, 52). Note that no such induction
period is found for reaction at lower temperatures on any of
the oxides, or on the metal surface when used as a metathe-
sis catalyst. This suggests then that the accumulation of car-
bonaceous deposits is not responsible for the induction pe-
riod since such deposits are found on catalysts that do not
exhibit the effect, for example, metallic molybdenum. It
would also be expected that accumulation of carbonaceous
deposits would, in contrast to experimental observations,
lead to a decrease rather than an increase in activity with
time on stream. The other change that has been noted dur-
ing the reaction is a reduction in oxidation state from +6
to +4 for the MoO3 catalyst, and the induction period may
reflect the transformation from an inactive MoO3 to a more
active MoO2 catalyst. This would explain the absence of an
induction period on metallic molybdenum (since this is not
reducible) and at lower reaction temperatures (since the
catalyst is not reduced). However, an induction period was
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