Theoretical and experimental consideration of the reactions between VxOy+ and ethylene

Article abstract of DOI:10.1021/ja021349k

We present joint theoretical and experimental results which provide evidence for the selectivity of V_xO_y⁺ clusters in reactions toward ethylene due to the charge and different oxidation states of vanadium for different cluster sizes. Density functional calculations were performed on the reactions between V_xO_y⁺ and ethylene, allowing us to identify the structure-reactivity relationship and to corroborate the experimental results obtained by Castleman and co-workers (Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 10237). The lowest-energy structures for the V₂O_2-6⁺ and V₄O_8-10⁺ clusters and the V₂O_3-6⁺-C₂H₄ and V₄O₁₀⁺-C₂H₄ complexes, as well as the energetics for reactions between ethylene and V₂O_-6⁺ and V₄O₁₀⁺ are presented here. The oxygen transfer reaction pathway was determined to be the most energetically favorable one available to V₂O₅⁺ and V₄O₁₀⁺ via a radical-cation mechanism. The association and replacement reaction pathways were found to be the optimal channels for V₂O₄⁺ and V₂O₆⁺, respectively. These results are in agreement with the experimental results reported previously. Experiments were also conducted for the reactions between V₂O₅⁺ and ethylene to include an energetic analysis at increasing pressures. It was found that the addition of energy depleted the production of V₂O₄⁺, confirming that a more involved reaction rather than a collisional process is responsible for the observed phenomenon. In this contribution we show that investigation of reactions involving gas-phase cationic vanadium oxide clusters with small hydrocarbons is suitable for the identification of reactive centers responsible for selectivity in heterogeneous catalysis.

Full text of DOI:10.1021/ja021349k

Published on Web 04/29/2003
Theoretical and Experimental Consideration of the Reactions
+
between V
x
O
y
and Ethylene
†
‡
†
,‡
Dina R. Justes, Roland Mitri c´ , Nelly A. Moore, Vlasta Bona cˇ i c´ -Kouteck y´ ,* and
,
†
A. Welford Castleman, Jr.*
Contribution from the Departments of Chemistry and Physics, PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802, and Humboldt UniVersit a¨ t zu Berlin, Institut f u¨ r Chemie,
Brook-Taylor-Strasse 2, D-12489 Berlin, Germany
Received November 13, 2002; E-mail: vbk@chemie.hu-berlin.de; awc@psu.edu
Abstract: We present joint theoretical and experimental results which provide evidence for the selectivity
+
of V
x
O
y
clusters in reactions toward ethylene due to the charge and different oxidation states of vanadium
+
x y
for different cluster sizes. Density functional calculations were performed on the reactions between V O
and ethylene, allowing us to identify the structure-reactivity relationship and to corroborate the experimental
results obtained by Castleman and co-workers (Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J.
+
+
Phys. Chem. A 2001, 105, 10237). The lowest-energy structures for the V
and the V and V complexes, as well as the energetics for reactions between
ethylene and V
to be the most energetically favorable one available to V
2 4
O2-6 and V O8-10 clusters
2
O
3-6+-C
2
+
H
4
4
+
O
10+-C
H
2 4
2
O4-6 and V
4
O
10 are presented here. The oxygen transfer reaction pathway was determined
10 via a radical-cation mechanism.
The association and replacement reaction pathways were found to be the optimal channels for V
+
+
2
O
5
and V
4
O
+
2
O
4
and
+
V
2
O
6
, respectively. These results are in agreement with the experimental results reported previously.
+
Experiments were also conducted for the reactions between V
2
O
5
and ethylene to include an energetic
+,
analysis at increasing pressures. It was found that the addition of energy depleted the production of V
2 4
O
confirming that a more involved reaction rather than a collisional process is responsible for the observed
phenomenon. In this contribution we show that investigation of reactions involving gas-phase cationic
vanadium oxide clusters with small hydrocarbons is suitable for the identification of reactive centers
responsible for selectivity in heterogeneous catalysis.
Introduction
surface science techniques, alternative means of investigating
the reaction mechanisms have been sought. It has been suggested
The ability to selectively oxidize hydrocarbons is important
to so many industrial processes,2,3 and the development of
that a metal oxide surface may be considered as a collection of
metal oxide clusters of a variety of compositions and sizes,4
and the study of gas-phase clusters has gained acceptance as
one approach to model the active sites on catalytic surfaces.6
As an example, molybdenum oxide surfaces may assume several
spatial arrangements ranging in size from trimolybdates to hepta-
and octamolybdates, depending on the experimental conditions.7
Experiments on gas-phase clusters provide an appropriate
environment to investigate the possible influences of composi-
tion, stoichiometry, charge and oxidation states, size, and degree
of coordinative saturation on their reactivity. Studying the
effect these characteristics have on the reactivity of the cluster
provides insight into the nature of active sites responsible for a
particular reaction. The guided ion beam mass spectrometer used
in the studies reported herein is capable of mass-selecting metal
oxide clusters of varying size and stoichiometry, providing
information on the structure-reactivity relationship for the
,5
catalysts that will efficiently accomplish this task continues to
stimulate research in this field. The success in generating an
effective catalyst, maximizing activity without sacrificing
selectivity to the desired product, lies primarily in the greater
understanding of the active sites responsible, and in identifying
the mechanisms which govern the reaction. Without a better
knowledge of the active sites responsible for effecting a
particular reaction, information on a mechanism is also beyond
comprehension. Elucidating the structure-reactivity relationship
of surfaces is important in understanding catalytic selectivity
4
and activity. In view of the difficulty to identify specific active
sites on a catalyst surface with the use of currently available
†
Departments of Chemistry and Physics, Pennsylvania State University.
Humboldt Universit a¨ t zu Berlin, Institut f u¨ r Chemie.
‡
(
(
(
(
1) Zemski, K. A.; Justes, D. R.; Castleman, Jr., A. W. J. Phys. Chem. A 2001,
1
05, 10237.
2) Warren, B. K.; Oyama, S. T. Heterogeneous Hydrocarbon Oxidation;
American Chemical Society: Washington, DC, 1996.
3) Oyama, S. T.; Hightower, J. W. Catalytic SelectiVe Oxidation; American
(5) Muetterties, E. L. Science 1977, 196, 839.
(6) Lai, X.; Goodman, D. W. J. Mol. Catal. A 2000, 162, 33.
(7) Biela n´ ski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York,
1991.
Chemical Society: Washington, DC, 1993.
4) Witko, M.; Hermann, K.; Tokarz, R. J. Electron. Spectrosc. Relat. Phenom.
1
994, 69, 89.
10.1021/ja021349k CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 6289-6299
9
6289

A R T I C L E S
Justes et al.
clusters under investigation, in particular when combined with
the benefits afforded by the density functional methods reported
here.
Another important issue that can be addressed through
modeling active sites with ionic clusters is unraveling the role
of charge and charge density. It has been proposed that OH
any other VxOy+ cluster.
+
+
V O + C H f V O + (C H O)
(1)
2
5
2
4
2
4
2
4
The proposed product, C2H4O, is shown in parentheses since
neutral species cannot be detected in the mass spectrometer used.
The reaction shown above was proposed to be an oxygen
transfer reaction from the metal oxide cluster to the hydrocarbon
rather than oxygen loss because oxygen loss was not observed
during low-energy collision-induced dissociation (CID) experi-
2
-
7
and O groups exist on γ-Al2O3 surfaces. Also, Kung and
co-workers have suggested that the active sites responsible for
CO oxidation on a gold surface consist of a combination of
-
8
Au - OH and metallic Au atoms. Kiely and co-workers have
proposed a cationic model involving Au as an integral aspect
x+
21
ments conducted previously. Rademann and Fielicke have
investigated the interactions between the VxOy clusters with
of the mechanism for CO oxidation on a gold/iron oxide
+
9
catalyst. Significant relationships in reactivities have been
alkenes and alkanes and found a distinct reactivity of the
observed between the gas and condensed phases. Zamaraev and
co-workers have found similarities in the reactions of methanol
with MoxOy and those over heterogeneous catalysts.10 Heiz,
Landman et al. investigated experimentally and theoretically the
reactivity of small gold clusters supported on magnesia and
found Au8 to be a particularly reactive species to CO combus-
tion.11 The reactivity observed experimentally is likely due to
+
22
(
V2O5)n toward the hydrocarbons studied. In addition,
Schwarz and co-workers performed studies on the reactivity of
+
+
+
VO2 toward ethylene and found VO was produced, yielding
23
acetaldehyde as the most likely neutral product. To understand
these observations, a combined experimental and theoretical
effort was undertaken which allowed us to address the questions
regarding the mechanism and energetic pathways for the oxygen
transfer phenomenon observed for the (V2O5)n clusters with
ethylene.
In this work we present the results of density functional
calculations, which provide key information on the structure-
reactivity relationship for the reactions between the VxOy
1
1
an F-center defect which anchors the octamer to the surface.
Coadsorption of CO and O2 on small gold cluster anions has
+
12
been also studied theoretically by H a¨ kkinen and Landman and
experimentally by Bernhardt et al.13 and Wallace and Whetten.
14
15
Theoretical methods, including charge sensitivity analysis and
+
1
6
ab initio calculations, have been used to aid in the pursuit to
identify and understand the active sites responsible for catalytic
reactions.
clusters and ethylene. The results include the optimized
+
+
structures of the V2O2-6 and V4O8-10 clusters, of the
+
+
V2O3-6 -C2H4 and V4O10 -C2H4 complexes and the reaction
In view of their ability to selectively oxidize hydrocarbons,
+
+
energetics for the reactions between the V2O4-6 and V4O10
17-19
vanadium oxides in particular are of interest to researchers.
clusters with ethylene including molecular dynamics (MD)
simulations along the interesting reaction pathways. Our results
allow for the proposal of a mechanism for the selective oxygen
transfer due to the presence of the reactive centers for a given
cluster size and stoichiometry, and they offer a clear explanation
why different cluster sizes favor different processes. In light of
the calculations conducted for this study, the issue whether the
clusters, and hence the reactions themselves, involved interac-
tions at well-defined thermal energies had become increasingly
important. Therefore, an expanded experimental study of the
size-specific reactions with additional energy was undertaken,
Other research groups have actively pursued the active sites on
vanadia catalysts, and some of these are reviewed in reference
2
0. The work reported in the present study was stimulated by
+
the results obtained from the reactions between VxOy and
ethylene and ethane carried out by the Castleman group at Penn
1
State University. The experiments showed significant and
highly cluster-size-specific reaction pathways, which prompted
further investigation to unravel the mechanism responsible for
+
the observed phenomenon. Specifically, when (V2O5)n , where
+
n ) 1-3, was reacted with ethane and ethylene, (V2O5)n-1V2O4
was identified as a very dominant reaction pathway. This
+
whereby the products resulting from the V2O5 and C2H4
reaction is outlined below using V2O5+ as the example although
reactions were monitored at various energies and pressures. The
experimental results reported here support the proposal that
oxygen transfer is due to an exothermic reaction and not due to
a collisional process resulting in oxygen loss from the cluster.
In this contribution, after a brief description of computational
and experimental methods, we present the results of the joint
theoretical and experimental work and discussion.
+
+
the reaction was also observed with V4O10 and V6O15 .
Importantly, this reactive behavior attributable to oxygen
transfer was not observed to any appreciable extent with
(
8) Costello, C. K.; Kung, M. C.; Oh, H.-S.; Wang, Y.; Kung, H.-H. Appl.
Catal. A 2002, 232, 159.; Oh, H.-S.; Costello, C. K.; Cheung, C.; Kung,
H. H.; Kung, M. C. Stud. Surf. Sci. Catal. 2001, 139, 375.
9) Hodge, N. A.; Kiely, C. J.; Whyman, R.; Siddiqui, M. R. H.; Hutchings,
G. J.; Pankhurst, Q. A.; Wagner, F. E.; Rajaram, R. R.; Golunski, S. E.
Catal. Today 2002, 72, 133.
(
Computational Methods
(
10) Fialko, E. F.; Kikhtenko, A. V.; Goncharov, V. B.; Zamaraev, K. I. J. Phys.
Chem. B 1997, 101, 5772.
11) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; H a¨ kkinen, H.; Barnett,
R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573.
Knowledge about the structural properties and energetics of neutral
and anionic vanadium oxide clusters24 as well as of cationic clusters,
is available from previous density functional studies. The findings are
based on Becke’s hybrid three parameter nonlocal exchange functional
combined with the Lee-Yang-Parr gradient corrected correlation
25
(
(
12) H a¨ kkinen, H.; Landman, U. J. Am. Chem. Soc. 2001, 123, 9704.
13) Hagen, J. Socaciu, L. D.; Elijazyfer, M.; Heiz, U.; Bernhardt, T. M.; W o¨ ste,
L. Phys. Chem. Chem. Phys. 2002, 4, 1707.
(
(
(
(
14) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124, 7499.
15) Nalewajski, R. F.; Korchowiec, J. Comput. Chem. 1995, 19, 217.
16) Pacchioni, G.; Ferrari, A. M.; Giamello, E. Chem. Phys. Lett. 1996, 255,
(21) Bell, R. C.; Zemski, K. A.; Kerns, K. P.; Deng, H. T.; Castleman, A. W.,
Jr. J. Phys. Chem. A 1998, 102, 1733.
5
8.
(
17) Ruth, K.; Burch, R.; Kieffer, R. J. Catal. 1998, 175, 27.
(22) Fielicke, A.; Rademann, K. Phys. Chem. Chem. Phys.. 2002, 4, 2621.
(23) Harvey, J. N.; Diefenbach, M.; Schr o¨ der, D.; Schwarz, H. Int. J. Mass
Spectrom. 1999, 182/183, 85.
(
18) Escribano, V. S.; Busca, G.; Lorenzelli, V. J. Phys. Chem. 1990, 94,
8
945.
(
19) Oyama, S. T.; Somorjai, G. A. J. Phys. Chem. 1990, 94, 5022.
20) Grzybowska- SÄ wierkosz, B. Appl. Catal. A 1997, 157, 409.
(24) Vyboishchikov, S. F.; Sauer, J. J. Phys. Chem. A 2000, 104, 10913.
(25) Calatayud, M.; Andres, J.; Beltran, A. J. Phys. Chem. A 2001, 105, 9760.
(
6290 J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003

+
Reactions between V
x
O
y
and Ethylene
A R T I C L E S
functional (B3LYP),26 where either the all-electron triple-ú valence plus
polarization basis set (TZVP) developed by Ahlrichs and co-workers27
or the standard double-ú plus polarization AO basis set (DZP) was
supersonic expansion. This type of source creates a turbulent environ-
ment during cluster formation, which causes better thermalization of
the clusters producing a cold ion beam.
2
5
employed. To determine the appropriate basis set for calculations
undertaken in the present study, we employed both sets and compared
the results with experimentally determined dissociation energies for
The ion of interest is mass selected in the first quadrupole and
focused into the octopole reaction cell through a set of electrostatic
lenses. In this chamber, ethylene is introduced and maintained at a
constant pressure while the energy of the octopole is increased
incrementally ranging from 0 to 20 V, lab frame. These experiments
were repeated for pressures 0.1 to 0.6 mTorr of ethylene in the collision
cell. The products are then mass analyzed in the second quadrupole
and detected using a channeltron electron multiplier.
+
+
VO and VO . Since the use of the DZP AO basis set does not provide
2
reliable dissociation energies, we employ the TZVP AO basis sets
throughout the work presented in this contribution. The calculated
+
+
dissociation energies for VO and VO
2
are 5.52 and 3.80 eV, using
the TZVP AO basis sets, while the corresponding experimental values
2
8
are D ) 6.09 ( 0.28 and 3.50 (0.36 eV, respectively. Considering
e
the large error bars which result when determining dissociation
thresholds which also include the barrier, the calculated values are in
acceptable agreement with the experimental findings. Gradient based
minimization methods were used in order to determine structures;
stationary points were characterized using vibrational frequency
analysis. Transition states involved in the mechanism of the oxygen
Results and Discussion
+
Structures. The optimized structures for the V2O2-6 and
+
the V4O9,10 clusters are shown in Figure 1, where, atoms
designated by an arrow indicate the location of the unpaired
electron. In general agreement with the literature, the double-
+
+
+
transfer reaction from V
2
O
5
and V
4
O
10 to ethylene have been
bridged structures for the V O
stable structures.
are calculated to be the most
2
2-6
optimized using the synchronous transit guided quasi-Newton (STQN)
25,33
+
For V2O4 , two isomers with the terminal
2
9
method developed by Schlegel and co-workers, and barriers for the
oxygen atoms occupying the trans and cis positions have been
identified with an energy difference of 0.19 eV, the trans isomer
3
0-32
reaction steps were determined.
The proposed oxygen transfer mechanism was studied and confirmed
using ab initio molecular dynamics with forces calculated using the RI
+
being the lowest in energy. The V2O5 structure contains an
oxygen-centered radical, evidenced by the elongated bond, 1.78
3
2
(
resolution of identity)-DFT procedure with the BLYP functional. To
Å compared to 1.56 Å for the other two terminal V-O bonds
+
initiate the MD simulation, the activated complex of the V
O
2 5
cluster
+
in the structure. Four isomers were found for the V2O6
with ethylene was used. As initial conditions, the geometry of the stable
complex was used, and initial velocities were generated by randomly
distributing the energy corresponding to the stability of the complex
among all internal degrees of freedom. MD simulations performed in
this way enabled us to follow the reaction mechanism, and to verify
the intermediates and reaction steps, involved in the oxygen transfer
reaction.
cluster: the lowest energy structure has two terminal V-O
bonds and the O2 molecule is adsorbed to the cluster as a
peroxide unit with the V-O bond lengths 1.92 and 1.97 Å and
the O-O bond length 1.29 Å. Isomers II and III are the trans
(II) and cis (III) configurations of the structures which have
two terminal V-O bonds and one O2 molecule weakly adsorbed
to the cluster by bonds with lengths 2.05 and 2.01 Å,
respectively. Isomer III with the cis configuration of oxygen
atoms is higher in energy by 0.17 eV than the trans isomer II.
The fourth isomer, structure IV, shows four terminal V-O bonds
and is higher in energy than the first isomer by 1.25 eV. The
Experimental Section
The reactions between the group V metal oxide clusters, V
x y x y
Nb O , Ta O
systematically investigated by the Castleman group at Penn State.
x
O
y
+,
(x ) 2-6, y ) 4-15), and the C2 hydrocarbons were
+ +
1
Among the products generated, one particular size selective reaction
+
cationic structures of V2O2-6 are closely related to the one
+
was observed between the (V
2
O
5
)
n
(n ) 1-3) and ethylene which
2
5
+
obtained by Calatayud et al. with the exceptions of V2O4
required further scrutiny. These experiments, which prompted the
present study, were performed on a guided ion beam mass spectrometer
coupled to a laser vaporization source described in detail elsewhere.28
Briefly, the source consists of a metal rod ablated by the second
harmonic of a Nd:YAG laser, and at a predetermined time, oxygen
seeded in helium (∼8%) is pulsed over the source. The clusters are
formed through plasma reactions and subsequently cooled using
+
and V2O6 , in which case we found the trans isomers to have
lower energies than the cis configurations and the O molecule
2
+
+
in V2O6 is bound as a peroxide unit. The structure for V4O8 ,
+
+
V4O9 , and V4O10 are shown in Figure 1f-h. They exhibit a
cage structure similar to the one assumed by the P O molecule,
4
10
34
+
suggested previously. In the lowest-energy structure of V4O10 ,
+
as in case of the V2O5 , an oxygen-centered radical is present
(
26) Becke, A. D.; Phys. ReV. A 1988, 98, 3098.; Becke, A. D. J. Chem.
Phys.1993, 98, 5648.; Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1998,
with a longer terminal V-O bond, 1.75 Å versus 1.55 Å for
the other terminal V-O bonds, and the unpaired electron is on
this oxygen atom, again designated by an arrow. The structures
3
7, 785.
(
(
27) Sch a¨ fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
28) Bell, R. C.; Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Chem.
Phys. 2001, 114, 798.
+
+
of the V2O3-6 -C2H4 and V4O10 -C2H4 complexes are shown
in Figure 2.
(
(
(
29) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449.; Peng, C.; Ayala, P.
Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1995, 16, 49.
30) Ahlrichs, R.; B a¨ r, M.; H a¨ ser, M.: Horn, H.; K o¨ lmel, C. M. Chem. Phys.
Lett. 1989, 162, 165. Program TURBOMOLE.
The ∆E values discussed below represent the calculated
energies of formation for each complex. In the most stable
31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
+
+
complexes for V2O3 -C2H4 and V2O4 -C2H4, the ethylene is
bound directly to a three-coordinated vanadium atom with ∆E
)
-0.26 and -1.75 eV, respectively. The complexes with
ethylene bound directly to the terminal oxygen atom have in
both cases considerably higher energies as shown in Figure 2.
+
There are two isomers for the V2O5 -C2H4 complex. One
(33) Johnson, J. R. T.; Panas, I. Inorg. Chem. 2000, 39, 3192.
(34) Berkowitz, J.; Chupka, W. A.; Inghram, M. G. J. Chem. Phys. 1957, 27,
87; Brewer, L. Chem. ReV. 1953, 52, 1.
(32) Eichkorn, K.; Treutler, O.; O¨ hm, H.; H a¨ ser, M.; Ahlrichs, R. Chem. Phys.
Lett. 1995, 109, 47.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003 6291

A R T I C L E S
Justes et al.
+
+
+
+
+
+
+
+
Figure 1. Structures for (a) V2O2 (b) V2O3 , (c-I) trans V2O4 , (c-II) cis V2O4 , (d) V2O5 , (e-I) peroxo V2O6 , (e-II) trans V2O6 , (e-III) cis V2O6 ,
(
+
+
+
+
+
+
e-IV) V2O6 , (f) V4O8 , (g) V4O9 , (h-I) V4O10 , (h-II) V4O10 , and (h-III) V4O10 clusters. The energy differences in eV with respect to the most stable
structures are given in round brackets. I-III label the energy sequence of the isomers. Labels of symmetry group and the ground electronic state are also
+
+
given. Bond distances are in Å. Binding energies per atom (in eV) are defined as Eb/n ) [E(VxOy ) - E(V ) - (x - 1)E(V) - yE(O)]/(x + y).
6
292 J. AM. CHEM. SOC. VOL. 125, NO. 20, 2003
9

+
Reactions between V
x
O
y
and Ethylene
A R T I C L E S
+
+
+
+
+
+
Figure 2. Structures for (a-I) V2O3 -C2H4, (a-II) V2O3 -C2H4, (b-I) V2O4 -C2H4, (b-II) V2O4 -C2H4, (b-III) V2O4 -C2H4, (c-I) V2O5 -C2H4, (c-II)
+
+
+
+
+
+
V2O5 -C2H4, (d-I) V2O6 -C2H4, (d-II) V2O6 -C2H4, (d-III) V2O6 -C2H4, (d-IV) V2O6 -C2H4 and (e) V4O10 -C2H4 complexes. Binding energies per
atom (in eV) are defined as Eb/n ) [E(VxOy + C2H4) - E(V ) - (x - 1)E(V) - yE(O) - 2E(C) - 4E(H)]/(x + y + 6). For labels compare Figure 1.
+
J. AM. CHEM. SOC.
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A R T I C L E S
Justes et al.
structure depicts the ethylene bonding to the cluster through
both terminal oxygen atoms, ∆E ) -4.44 eV, and the second
in which the ethylene is bound only to one oxygen atom, ∆E
)
-3.53 eV. The former structure has a lower energy than the
3
5
+
latter. There are four isomers for the V2O6 -C2H4 complex.
In isomer I, the lowest-energy structure, the ethylene molecule
is bound directly to the vanadium atom and the O2 molecule is
adsorbed as a peroxo group to the other vanadium atom, ∆E =
-1.74 eV. The second isomer (II) shows the ethylene bound to
one oxygen atom of the adsorbed O2, ∆E = -1.16 eV. Isomer
III differs from isomer I by the O2 molecule bound in the
superoxo form, ∆E = -1.06 eV. Isomer IV is the trans
configuration of isomer II. The differences in energy between
isomer I and isomers II, III, and IV are 0.58, 0.68, and 0.74
+
eV, respectively. In the V4O10 C2H4 complex the ethylene binds
to the oxygen atom with the radical center as shown in Figure
2
.
V2O4+ Reactions. The reactions between the V2O4-6+
clusters and ethylene are summarized in Table 1. Single oxygen
atom loss from V2O4+ is not observed experimentally. The
calculated energy for the single oxygen loss reaction for V2O4+
shows the reaction to be thermodynamically unfavorable with
an energy of +4.75 eV shown in the following reaction.
+
+
3
V O + C H f V O + O + C H4
2
4
2
4
2
2
∆
E ) +4.75 eV (2)
Likewise, the energy for the reaction yielding molecular oxygen
loss was determined to be +9.64 eV and, as expected, not seen
during the experiment.
+
+
2
V O + C H f V O + O + C H
4
2
4
2
4
2
2
2
y
∆
E ) +9.64 eV (3)
x
For the V2O4+ cluster, association is thermodynamically the
most favorable pathway shown in reaction 4.
+
+
V O + C H f V O -C H
4
∆E ) -1.75 eV (4)
2
4
2
4
2
4
2
Experimentally, this is the only reaction channel observed for
+
V2O4 reactions with ethylene. The association product, al-
though present under single-collision conditions, displays an
increase in intensity with increasing ethylene pressure in the
collision cell. This trend is expected due to a stabilizing effect
of the third-body collisions which occur under multiple-collision
conditions.
The oxygen transfer reaction for V2O4+ to yield C2H4O is
marginally thermodynamically favorable with a calculated
energy of -0.02 eV, shown in reaction 5.
+
+
3
V O + C H f V O + C H O
∆E ) -0.02 eV
5)
2
4
2
4
2
2
4
(
Although this reaction is slightly energetically favorable, it was
not observed experimentally. This can be explained by the
course the reaction takes. The oxygen transfer reaction occurs
by first forming the association product between the cluster and
the ethylene molecule. Then, to complete the oxygen transfer
reaction pathway, a strong V-O bond must be broken, requiring
4
.75 eV of energy. Considering that the association reaction
1.
(
35) Justes, D. R.; Castleman, A. W., Jr.; Mitri c´ , R.; Bona cˇ i c´ -Kouteck y´ , V. Eur.
J. Phys. D. In press.
Tbale
6294 J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003

+
Reactions between V
x
O
y
and Ethylene
A R T I C L E S
only achieves a gain of 1.75 eV of energy, oxygen transfer is
not energetically feasible. In addition, the ethylene molecule
binds to a vanadium atom rather than to an oxygen atom, making
the oxygen transfer reaction more complicated than simply
breaking a V-O bond. Considering the steric issues and the
energy barrier that must be overcome to break a V-O bond, it
is very unlikely that this reaction pathway would be observed
during the experiment. The theoretical calculations performed
isomer shown in Figure 2, requiring 4.70 eV, and this is
evidently an insurmountable energy barrier for the reaction.
+
Experimentally, the V2O6 undergoes a replacement reaction,
+
yielding V2O4 -C2H4, as the dominant product, illustrated
below in reaction 10; yet it is slightly less energetically favorable
than the oxygen transfer reaction (-0.66 eV versus -0.71 eV)
shown above in reaction 9, although the energy difference may
be within the limits of accuracy of the calculations. However,
the replacement reaction does not require the cleavage of the
strong O-O bond.
+
for the reactions between V2O4 and ethylene provide an
explanation for the reason that only one reaction pathway,
namely association, is observed. Single and molecular oxygen
losses are highly unfavorable energetically, and oxygen transfer
has a barrier, which is too difficult to overcome under the
experimental conditions.
+
+
V O + C H f V O -C H + O
2
2
6
2
4
2
4
2
4
∆E ) -0.66 eV (10)
+
Although the association product for V2O6+ is energetically the
most favorable channel (1.74 eV), as mentioned above, it is
observed experimentally with a much weaker intensity than the
replacement reaction. Again, due to the need for a third-body
collision to stabilize the association complex, this result is in
accordance with expectations. In the latter case it is possible
that the amount of energy required to cleave the adsorbed O2
V2O6 Reactions. The reaction for single oxygen loss from
V2O6+ to produce V2O5 , is not observed experimentally, and
this can be explained by the calculated results. The single oxygen
loss reaction is energetically unfavorable with ∆E ) +4.06 eV
as shown below.
+
+
+
5
V O + C H f V O + O + C H4
2
6
2
4
2
2
+
molecule from the complex, 1.09 eV, to yield V2O4 -C2H4, is
∆
E ) +4.06 eV (6)
a relatively facile barrier to overcome, as indicated by MD
simulations. Considering the amount of energy available to the
complex and the ease with which this is accomplished for the
The energy calculated for the molecular oxygen loss for V2O6+
was found to be +1.09 eV and is illustrated in the reaction
below.
+
V2O6 cluster during CID experiments at near thermal energies,
this possibility is very likely. In addition, at elevated reactant
gas pressures, multiple collisions occur, increasing the likelihood
of O2 loss and favoring the replacement reaction. The single
oxygen loss and the oxygen transfer pathways are improbable
due to energetic barriers involved in breaking the O-O bond.
In conclusion, taking into account experimental conditions, as
well as structural and energetic properties of involved species,
we confirm that the replacement reaction, the most dominant
pathway observed experimentally, is energetically the most
+
+
V O + C H f V O + O + C H
4
2
6
2
4
2
4
2
2
∆
E ) +1.09 eV (7)
Even though the reaction is calculated to be endothermic, the
loss of O2 from V2O6 , which is observed during the experiment,
is most likely the result of an ion-molecule impact. An
alternative possibility is that the mass distribution is comprised
of a large component of V2O6 where O2 is weakly adsorbed
+
+
+
to V2O4 . During collision-induced dissociation experiments for
+
viable option for the V2O6 cluster.
+
V2O6 interacting with xenon, molecular oxygen loss, forming
+
V2O5 Reactions. Calculations were also performed on the
+
V2O4 , is observed at near thermal energies, implying that O2
+
35
possible reaction pathways for V2O5 with ethylene. The
results are briefly summarized below. The association reaction
+
21
is not strongly bound to the V2O4 cluster. This can also be
seen from the structure of V2O6 given in Figure 1e, which
shows the O2 molecule bound to the cluster through bonds with
lengths of 1.97 and 1.92 Å.
The association product for V2O6 yielding V2O6 -C2H4
+
+
forming V2O5 -C2H4 was determined to have two stable
isomers, one formed through two terminal V-O bonds and the
second through bonding to a single V-O terminal bond. The
calculated energies were found to be energetically favorable by
+
+
gains 1.74 eV of energy, shown below in reaction 8.
4
.44 and 3.53 eV for the respective isomers as discussed earlier.
+
+
+
6
Atomic and molecular oxygen-loss pathways producing V2O4
and V2O3 , respectively, were calculated. The changes in
V O + C H f V O -C H
∆E ) -1.74 eV (8)
2
6
2
4
2
2
4
+
energies were found to be thermodynamically unfavorable
Association of ethylene to V2O6+ is observed experimentally,
although it is only a minor pathway since a three-body
mechanism is required for stabilization.
The oxygen transfer reaction for the V2O6 cluster to yield
C2H4O is thermodynamically favorable with an energy -0.71
yielding +2.24 and +1.78 eV for atomic and molecular oxygen
+
+
loss, respectively. Experimentally, V2O4 and V2O3 are
observed products during the reactions with ethylene at thermal
energies, yet neither is observed during CID at thermal energies.
However, they are observed after the addition of 1-2 eV center-
of mass frame energy. These observations have led us to believe
another mechanism must be responsible for each reaction.
Therefore, oxygen transfer is proposed to be responsible for
the atomic and molecular losses observed during the experi-
ments. Single oxygen transfer gives a ∆E ) -2.53 eV to give
+
eV, shown in reaction 9.
+
+
5
V O + C H f V O + C H O
∆E ) -0.71 eV
9)
2
6
2
4
2
2
4
(
However, there is no experimental evidence for oxygen transfer
+
to occur between the V2O6 cluster and ethylene. As mentioned
+
V2O4 and acetaldehyde shown in the reaction below.
above, the association reaction produces 1.74 eV of energy for
the complex. However, for the oxygen transfer reaction to occur,
+
+
V O + C H f V O + C H O
∆E ) -2.53 eV
2
5
2
4
2
4
2
4
+
the V-O double bond must be broken in the first V2O6 -C2H4
(11)
J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003 6295

A R T I C L E S
Justes et al.
Molecular oxygen transfer was a very minor product observed
during the experiment and was found to have two possible
mechanisms forming either ethene-diol, ∆E ) -1.05 eV, or
two formaldehyde molecules, ∆E ) -0.65 eV. Despite the less
favorable overall energy for producing two formaldehyde
molecules, this reaction pathway is more likely to occur due to
a high-energy barrier which makes formation of ethene-diol less
probable.
It is important to note that the oxygen transfer phenomenon
+
observed between the (V2O5)n clusters and ethylene occur at
both single- as well as multiple-collision conditions. Since the
reactions occurred under single-collision conditions, there was
no alternative mechanism available to the cluster to release the
energy gained from the association reaction with ethylene.
To gain further insight into the course of the oxygen transfer
reactions and to obtain additional confirmation about the
proposed mechanism, ab initio MD simulations at constant
energy corresponding to the microcanonical ensemble have been
carried out. The results show that the association of ethylene to
+
V2O5 is followed by a very rapid (60-80 fs) hydrogen transfer
from the carbon atom bound to oxygen toward the terminal
carbon atom. After the hydrogen transfer has taken place, the
V-O bond is broken within the next 100 fs, giving rise to the
+
acetaldehyde and V2O4 as the final products. In addition, the
MD simulations also confirmed that the formation of two
formaldehyde molecules and not the ethene-diol are products
+
of the reaction giving V2O3 as the minor product in the
experiment. In view of the findings that oxygen atom loss can
occur with sufficient collision energies with neutral atoms (Xe),
we performed additional experiments to uncover the effect of
systematically raising the energy of the reaction under conditions
of constant selected pressures of ethylene within the reaction
+
+
+
cell. The relative intensities of V2O5 , V2O4 , and V2O3 at
varying energy are shown in Figure 3 as a function of increasing
pressure, with the laboratory frame energy as the variable
parameter. A common observation at each pressure in the
+
branching ratio is the relative increase of V2O5 and decrease
+
of V2O4 as the energy is increased incrementally. This indicates
that as the energy of the reaction increases, the efficiency of
the reaction decreases, i.e., less oxygen is being transferred to
the ethylene. This is opposite to the trend that would arise if
the oxygen atom loss were merely due to an unreactive,
energetic CID process. The addition of energy has an effect of
increasing the temperature of the system, thereby indicating this
reaction system has a negative temperature dependence on the
reaction efficiency. Barlow and co-workers studied the reaction
Figure 3. Branching ratios for the reaction between V2O5+ and C2H4 as a
+
+
function of pressure showing the relative intensities of (a) V2O5 , (b) V2O4 ,
+
and (c) V2O3 . Each symbol represents a different energy, 5, 10, 15, and
20 V.
38
with increasing temperature up to a certain point. The collision
+
+
rate for the reaction between V2O5 and ethylene should be
between O2 and deuterated methanes and found conditions
where, as the temperature increased, the rate coefficients
decreased.36 This phenomenon has been linked to the formation
-9
3
somewhat higher than 10 cm /s and our preliminary investiga-
tion into the rate constants for this reaction shows the value to
be considerably lower than this. It has been shown that negative
temperature dependencies do occur for slow bimolecular
exchange reactions which do not possess a positive activation
energy in the forward direction toward the production of the
of a complex and the existence of an energy barrier to the
forward reaction pathway that lies below the energy level of
36,37
the reactants.
As can be seen in Figure 4a, the energy barrier
of the transition state (TS) is lower in energy than that of the
initial reactants and the proposed mechanism involves the
formation of a complex. It has also been observed when the
rate constant is significantly lower than the collision rate
constant, a double-well exists, and the rates generally decrease
3
8,39
association complex.
present reaction can be seen from the energetic profile in Figure
a; the initial step in the oxygen transfer reaction is the formation
That such a situation arises in the
4
of a very stable association complex without an activation barrier
to overcome.
(
36) Barlow, S. E.; Van Doren, J. M.; DePuy, C. H.; Bierbaum, V. M.; Dotan,
I.; Ferguson, E. E.; Adams, N. G.; Smith, D.; Rowe, B. R.; Marquette, J.
B.; Dupeyrat, G.; Durup-Ferguson, M. J. Chem. Phys. 1986, 85, 3851.
37) Olmstead, W. N.; Brauman, J. I. J. Am. Chem. Soc. 1977, 99, 4219.
(38) Lindinger, W.; Fehsenfeld, F. C.; Schmeltekopf, A. L.; Ferguson, E. E. J.
Geophys. Res. 1974, 79, 4753.
(39) Meot-ner, M. Gas-Phase Ion Chem. 1979, 1, 197.
(
6296 J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003

+
Reactions between V
x
O
y
and Ethylene
A R T I C L E S
Figure 4. Energetic profile for the reaction between (a) V2O5+ and (b) V4O10+ and C2H4.
The parallels discussed above between our results and
other kinetic studies on reactions involving intermediate com-
plexes and transition states provides further support for our
proposed mechanism for the oxygen transfer reaction to C2H4
reaction products are observed in the mass spectra. The reverse
reaction for the V2O3 reaction is unlikely since two formal-
+
dehyde molecules would need to be present for the reverse
reaction to take place, and this is statistically highly improbable.
Also, similar to the oxygen transfer possibility discussed above,
+
from V2O5 .
+
It is also important to note that if the process occurring
further reactions would not account for the rise in V2O5
+
between V2O5 and C2H4 was a simple collisional process,
intensity observed as the energy in the collision cell is increased.
+
+
resulting in the production of V2O4 , O and C2H4, the intensity
V4O10 Reactions. Considering that the oxygen transfer
+
of V2O4 would increase, not decrease, as energy is added, as
reaction occurred only for clusters with the stoichiometry,
+
would occur in the case of a collision-induced dissociation
reaction. This energetic analysis is also further evidence that
the vanadium oxide cluster ions are thermalized during their
reactions in the collision cell. Another possibility to consider
(V2O5)n with n ) 1-3, the question remained whether a
common structural element is present in each of the three
clusters. By using density functional theory, it was determined
+
that V2O5 was an oxygen-centered radical, and after additional
+
+
is whether further reactions are occurring to yield V2O3 .
calculations were performed, it was found that V4O10 also had
+
However, as can be seen in Figure 3c, the intensity of V2O3
this characteristic. There is an elongated terminal V-O bond
does not increase over the energy range studied, and no other
and an unpaired electron localized at the oxygen atom common
J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003 6297

A R T I C L E S
Justes et al.
V4O10+ are shown in Figure 4, a and b, respectively. They show
+
the proposed pathways, which the reactions between V2O5 and
+
V4O10 with ethylene are most likely to take. The general
mechanism for this reaction is proposed in Figure 5. The
mechanism begins with the formation of a single bond between
the first carbon atom of the ethylene and the oxygen atom. The
radical, initially located on the oxygen atom, is transferred to
the second carbon atom in the ethylene molecule. The radical
is then shifted from the second carbon atom to the first carbon
atom by hydrogen transfer from the first carbon to the second
carbon. Dissociation of the C2H4O product then occurs, leaving
the vanadium atom, which had been bound to the oxygen atom
with the radical center. Comparison of the reaction pathway
+
+
for V2O5 and V4O10 shows that reaction intermediates have
very similar relative stabilities with respect to the reactants and
the products, reflecting common structural properties and the
generality of the proposed mechanism.
Conclusions
In conclusion, the structure-reactivity relationship for VxOy+
clusters toward ethylene has been clearly identified. An overview
of our theoretical and experimental results is given in Table 1.
The radical center located at one of the peripheral O atoms in
Figure 5. General mechanism for the reaction between (V2O5)+n)1,2 and
C2H4.
to both V2O5+ and V4O10 , designated by an arrow as seen in
+
+
+
V2O5 and V4O10 is responsible for the observed oxygen
transfer in the framework of radical-cation mechanism. In
addition to the structural properties, the strength of the bond
between the transition metal atom and the oxygen atom also
plays an important role in the oxygen transfer. For example,
our preliminary calculations indicate that despite the common
+
Figure 1d and 1h. The oxygen transfer reaction between V4O10
and ethylene is shown below.
+
+
9
V O + C H f V O + C H O
∆E ) -2.01 eV
12)
4
10
2
4
4
2
4
(
+
+
+
structural properties of Nb2O5 and V2O5 , oxygen transfer from
Similar to the reaction for V2O5 , this reaction is also
thermodynamically favorable. The energetic profile for the
+
Nb2O5 to ethylene is observed to a considerably lower extent,
due to the stronger Nb-O bonding.
reaction is shown in Figure 4b. The pathway depicted in Figure
+
Our experimental and theoretical results provide evidence for
4
b for the reaction between V4O10 and ethylene is very similar
+
+
the selectivity of the V Oy clusters due to the charge as well
to the one just described for the reaction with V2O5 shown in
x
34
+
as the capability of vanadium to assume different oxidation states
Figure 4a. However, unlike the V2O5 cluster binding ethylene
+
+
for different cluster sizes. In the case of (V O ) n ) 1,2, the
to two oxygen atoms, the V4O10 cluster initially binds the
2
5 n
oxygen atom can be reversibly bound and released promoting
catalytic oxidation of hydrocarbons, providing evidence that gas-
phase clusters can serve as well suited prototypes for identifica-
tion of reactive centers. In fact, the active sites identified as
being responsible for oxidation are in agreement with indirect
experimental findings of active sites on vanadia/titania surface,41
which were used to explain the formation of products such as
acetaldehyde and formaldehyde. This illustrates that the study
of reactions involving gas-phase cationic vanadium oxide
clusters and small hydrocarbons is suitable for identification of
reactive centers responsible for selectivity in heterogeneous
catalysis.
ethylene molecule through a single oxygen atom. This is due
to the terminal oxygen atoms being too far apart in the V4O10
cluster for the ethylene to bind two terminal O atoms (cf. Figure
+
2
). As can be seen in the energetic profile for the reaction
+
between V4O10 and ethylene in Figure 4b, the transfer of the
hydrogen from the oxygen-bound carbon to the second carbon
is lower in energy than the activated complex by 1.99 eV. The
+
V-O is then cleaved, separating the C2H4O from the V4O9
cluster. It should be pointed out that a weak molecular oxygen
loss channel was also observed experimentally, giving V4O8+
as a product. This, however cannot be due to the most stable
+
structure of V4O10 , as determined through DFT calculations,
but to the higher-energy isomer (cf. Figure 1). It is possible
that in the cluster beam a small amount of V4O8 with adsorbed
There have been several condensed phase studies performed
on the oxidation of ethane and ethylene over vanadia cata-
+
18-20,42-45
molecular oxygen, which can be easily released at thermal
lysts.
Acetaldehyde was an observed oxidation product
+
energies, is present. Experimentally, the production of V4O8
+
(40) Asmis, K. R.; Br u¨ mmer, M.; Kaposta, C.; Santambrogio, G.; von Helden,
from V4O10 is most likely a collisional process since this
G.; Meijer, G.; Rademann, K.; W o¨ ste, L. Phys. Chem. Chem. Phys. 2002,
reaction is observed during collision induced dissociation
4, 1101.
21
(41) Luan, Z.; Meloni, P. A.; Czernuszewicz, R. S.; Kevan, L. J. Phys. Chem.
experiments conducted previously in our laboratory. In support
B 1997, 101, 9046.
+
of this, Asmis et al. observed the V4O8 fragment when
(42) Oyama, S. T.; Middlebrook, A. M.; Somorjai, G. A. J. Phys. Chem. 1990,
94, 5029.
+
+
V4O10 was photodissociated, supporting the V4O8 -O2 struc-
(
43) Linke, D.; Wolf, D.; Baerns, M.; Timpe, O.; Schl o¨ gl, R.; Zeyss, S.;
Dingerdissen, U. J. Catal. 2002, 205, 16.
+
40
ture of V4O10 .
(44) Argyle, M. D.; Chen, K.; Bell, A. T.; Iglesia, E. J. Phys. Chem. B 2002,
Oxygen Transfer Radical-Cation Mechanism. The ener-
106, 5421.
+
getic profiles for the oxygen transfer reactions for V2O5 and
(45) Otsuka, K.; Uragami, Y.; Hatano, M. Catal. Today 1992, 13, 667.
6298 J. AM. CHEM. SOC.
9
VOL. 125, NO. 20, 2003

+
Reactions between V
x
O
y
and Ethylene
A R T I C L E S
of ethane and ethylene over vanadia/silica catalysts.42 In
addition, ethylene is a significant product of the oxidation of
processes can in fact lead to custom-designed catalysts for
specific purposes.
43,44
ethane over vanadia catalysts
and is proposed to be one of
Acknowledgment. We acknowledge the National Science
Foundation, Grant No. CHE-99-06341 for financial support of
the experimental work reported herein and for the collaboration
with the Bona cˇ i c´ -Kouteck y´ research group. Helpful discussions
with Eldon E. Ferguson are gratefully acknowledged.
the pathways ultimately yielding acetaldehyde.35 Finally, Otsuka
and co-workers were able to achieve a higher yield of acetal-
dehyde from the partial oxidation of ethane, which they attribute
to an increased number of the active sites they believe are
responsible for the reaction.45 This shows that a greater
understanding of the active sites which facilitate certain catalytic
JA021349K
J. AM. CHEM. SOC.
9
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