Studies of Metal Oxide Clusters: Elucidating Reactive Sites

Spectroscopic and Theoretical Studies of Transition Metal Oxides and Dioxygen Complexes. Yu Gong and Mingfei Zhou , Lester Andrews. Chemical Reviews 0...
0 downloads 0 Views 178KB Size
Download PDF
6136

J. Phys. Chem. B 2002, 106, 6136-6148

FEATURE ARTICLE Studies of Metal Oxide Clusters: Elucidating Reactive Sites Responsible for the Activity of Transition Metal Oxide Catalysts K. A. Zemski,† D. R. Justes, and A. W. Castleman, Jr.* Departments of Chemistry and Physics, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: NoVember 15, 2001; In Final Form: March 4, 2002

Transition metal oxides are widely used as both catalysts and catalytic supports in industrial processes. However, the mechanisms by which these materials function as catalysts and the structure-reactivity relationships are not well understood. In particular, there is a paucity of information on the specific sites responsible for the catalytic activity of bulk surfaces. A valuable approach to identifying the active sites of transition metal oxides is to study the chemistry of gas phase metal oxide clusters. A comprehensive program is underway in our laboratory in which reactivity studies of transition metal oxide clusters are carried out using a tandem mass spectrometer system coupled to a laser vaporization source. The desired transition metal oxide cluster cations are mass-selected and then injected into a reaction cell, whereby reactions with various organic molecules are investigated. The advantage of this technique is that the reactivity of specific clusters can be probed independently of other clusters, thereby providing insight into the intermediates and mechanisms at the active sites present on transition metal oxide catalysts of which the gas-phase clusters are representative models. Hence by employing gas-phase techniques, the effect of varying the composition, stoichiometry, oxidation state, charge state, degree of coordinative saturation, and size of the metal oxide clusters on the reactivity is determined. The findings provide valuable information about reaction intermediates, reaction mechanisms, and structure-reactivity relationships. Therefore, these gas-phase studies provide an understanding of the function of transition metal oxide catalysts at the molecular level that is expected to provide knowledge that will find use in the design of more efficient catalysts. This article provides an overview of findings derived in our laboratory for reactions of group V transition metal oxide clusters, with particular emphasis on the mechanism of oxygen transfer to small organic molecules.

Introduction The potential effectiveness of transition metal oxides, both as catalysts and catalytic supports, has undergone increased scrutiny in recent years due to their usefulness in a wide range of applications.1 Attention has been focused on transition metal oxides because of their ability to influence oxidation-reduction reactions, while being cost efficient and in many cases environmentally benign. In the past, development of new catalysts has typically relied on methods that involve empirical studies of the influence of various catalysts and supports on the overall course and efficiency of reactions. Currently, there is growing interest in using a more systematic scientific approach for catalyst design because there is little doubt that attaining the ability to design catalysts of choice would have an enormous economic impact. Surface experimental techniques that provide information on the atomic structure and composition of surfaces, often present limitations in their ability to directly probe structure-reactivity relationships due to the variety of coordination sites present on surfaces.2 The surface structure of bulk transition metal oxides † Current address: National Jewish Medical Center, 1400 Jackson St., K926, Murphy Laboratory, Denver, CO 80206.

can be envisioned as a collection of clusters of varying sizes and structures. Somorjai and co-workers have studied surfaces extensively and found the binding molecules to be “clusterlike”. This allows for localized bonding models to be used to study surfaces.3,4 Witko and co-workers have performed calculations on vanadium oxide clusters with the intention of describing the local contributions of the active site to aid in explaining factors governing catalytic surface reactions.5 Studies have also shown that using a minimum energy coordinate of charge sensitivity analysis is a useful tool in determining the efficacy of using small clusters as models of catalytic surfaces.6 As a complementary approach, the difficulty of surface inhomogeneity in understanding surface-reactivity relationships can be overcome by examining the reactivity of gas-phase transition metal oxide clusters. Metal oxide surfaces are characterized as possessing charged adsorbed species, occurring by heterolytic dissociative adsorption, and the presence of cationic and anionic vacancies.7 Therefore, gas phase metal oxide cluster ions are regarded as the simplest model for the interaction of these types of active sites on a transition metal oxide catalyst with organic molecules and comparison of gas phase results with condensed phase reactions leads to a better understanding of the interaction of organic molecules with catalytically active sites on surfaces.

10.1021/jp0142334 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/17/2002

Feature Article The effect of composition, stoichiometry, size, charge state, oxidation state, and degree of coordinative saturation, which are among the most important factors affecting catalytic properties, can be examined by employing a gas phase cluster technique such as that employed in the studies reported herein. Additionally, the gas phase reaction products provide valuable information about reaction intermediates, reaction mechanisms, and the relationship between cluster structure and reactivity. Therefore, through gas phase studies an understanding of transition metal oxide catalysts can be achieved at the molecular level, yielding knowledge which can ultimately find use in designing more effective catalysts. Recognition of the value of cluster science in the field of catalysis has grown in recent years with the expectations that a greater understanding of gas-phase clusters will serve to further elucidate the physical and chemical properties of condensed phase catalysts. Developing methods to activate C-C and C-H bonds of organic molecules and oxygenation of hydrocarbons are some of the most important research efforts in catalytic chemistry. For this reason there have been many gas-phase studies on the reactions of bare transition metal ions (M+), monomeric metal oxide ions (MO+), and metal dioxide ions with organic molecules.8-11 However, there have been few gasphase studies that have been directed toward elucidating the mechanisms involved in catalytic chemistry of larger metal oxide clusters. In our laboratory the reactions of group V transition metal oxide cluster ions with various organic molecules have been examined using a tandem mass spectrometer coupled to a laser vaporization source. The desired metal oxide cluster cations are mass-selected using a quadrupole and then injected into a reaction cell, where reactions with organic molecules are investigated. The reactions of vanadium oxide cluster cations with C4 hydrocarbons and halogenated organic molecules revealed that the oxidation state of the vanadium atoms, the size of the cluster, and the degree of coordinative saturation play a large role in the reactivity observed.12-15 Additionally, the reactions of niobium and tantalum oxide cluster ions with n-butane have been probed and the importance of the charge center, composition, size, and degree of coordinative saturation of the cluster on reactivity has been elucidated.16 Due to the paucity of information currently available on the catalytic properties of bulk tantalum oxides, reactions of tantalum oxide cluster cations with 1,3-butadiene, 1-butene, and benzene became the focus of attention as well.17,18 Finally, the reactions of group V transition metal oxide cluster ions with ethylene and ethane have been examined.19 It was determined during these experiments that the identity of the metal, the charge state, cluster stoichiometry, and geometric structure strongly influence the ability of the metal oxide cluster to transfer an oxygen atom to the neutral C2 hydrocarbon. Other evidence of the prospects in gaining new insights into catalysis from studies of gas-phase clusters has been revealed by the investigations of Zamaraev and co-workers.20 In particular, they have shown that reactions of MoxOy+ ions with methanol display similarities with the reactions of methanol over heterogeneous and homogeneous catalysts containing molybdenum-oxygen sites. Last, Rademann and co-workers have demonstrated that mass-selected bismuth oxide cluster cations in the gas phase can be reduced by propene in a manner similar to heterogeneous bismuth oxide catalysts.21 Findings from these gas phase investigations can provide insight into catalytically active centers present on metal oxide surfaces, which can ultimately aid tailoring the design of more effective catalysts.

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6137

Figure 1. Mass distributions of (a) vanadium, (b) niobium, and (c) tantalum oxide cluster cations.

Group V Transition Metal Oxide Cluster Ions: Formation and Composition The transition metal clusters employed in the studies conducted in our laboratory are generated in a laser vaporization source coupled with a reactor. A typical cluster cation distribution resulting from laser plasma reactions of vanadium, niobium, and tantalum with molecular oxygen under similar source conditions is shown in Figure 1. Each set of peaks corresponds to clusters having a particular number of metal atoms and the difference in mass between each peak of a set corresponds to one oxygen atom. The first prominent peak in the vanadium oxide cluster cation mass distribution shown in Figure 1a is VO+. The major peaks that are present in the VxOy+ mass distribution are VO1,2+, V2O4-6+, V3O6-8+, V4O8-12+, V5O11-13+, V6O13-16+, and V7O16-18+. By comparison, it is seen that both the niobium and tantalum oxide cluster cation mass distributions begin with MO2+. Overall, the major peaks observed in the cationic niobium oxide mass distribution in Figure 1b are

6138 J. Phys. Chem. B, Vol. 106, No. 24, 2002 NbO2+, Nb2O4-6+, Nb3O7,8+, Nb4O9-11+, Nb5O12,13+, Nb6O14-16+, and Nb7O17+. Additionally, the major peaks observed in the cationic tantalum oxide mass distribution in Figure 1c are TaO2+, Ta2O4-13+, Ta3O7-14+, Ta4O9-13+, and Ta5O12-17+. It is apparent from Figure 1 that reactions of metal oxide clusters can be examined by varying the composition, stoichiometry, size, degree of coordinative saturation, and oxidation state of the metal atoms of the cluster. Additionally, the effect of charge state on a reaction can be examined by investigating group V transition metal oxide cluster anions (MxOy-). One of the first steps in understanding the reactivities of group V transition metal oxide cluster ions is to gain insight into the formation mechanisms and structures of the metal oxide clusters. This can be achieved in part by performing collision-induced dissociation (CID) experiments with an inert gas to gain information on the relative stability and bonding properties of mass-selected group V transition metal oxide cluster ions.12,22,23 The collision-induced dissociation results of group V transition metal oxide cluster cations indicate that MO2+ and M3O7+ are the favored ionic CID products, while MO2, MO3, and M2O5 are the favored neutral products.10,21 Since the neutral CID products are not detected in our experimental apparatus, the neutral loss products mentioned above are assigned based on the difference between the selected cluster and the ionic CID fragment. Furthermore, the collision-induced dissociation results of group V transition metal oxide cluster anions indicate that MO3- and M3O8- are the favored ionic CID products, while MO2, MO3, and M2O5 are the favored neutral products.22,23 Since the clusters MO2+, MO3-, M3O7+, M3O8-, MO2, MO3, and M2O5 are the major products observed during the collisioninduced dissociation of ionic group V transition metal oxide clusters, it is thought that these species are the building blocks of VxOy(, NbxOy(, and TaxOy(. It is worth noting that most of these building blocks, both neutral and ionic, have all of the metal atoms in the +5 oxidation state, which is the most stable oxidation state of the group V transition metals. By combining the collision-induced dissociation results and the cationic mass distributions shown in Figure 1, we conclude that the cationic group V transition metal oxide cluster stoichiometry can be represented as (MO2)x(MO3)y(O2)z+.12,23 Additionally, the collision-induced dissociation results and the anionic mass distributions reveal that the anionic group V transition metal oxide cluster stoichiometry can be represented as (MO2)x(MO3)y(O)0-1-.22,23 This stoichiometry that describes the ionic group V transition metal oxide clusters corresponds well to the bulk phase stoichiometry of group V transition metal oxides, which is MO2 and M2O5.24 Additionally, the guided ion beam mass spectrometer employed in our laboratory allows us to obtain bond dissociation thresholds, the amount of energy required to break a bond, of mass-selected MxOy(. Recently, the CID cross section of VO3- with xenon as a function of cluster ion kinetic energy, which was used to determine the O2V--O bond dissociation threshold of ∆E(O2V--O) ) 5.43 ( 0.31 eV.22 The bond dissociation thresholds for V+-O and OV+-O were also determined from energy dependent CID cross sections yielding values of 6.09 ( 0.28 eV and 3.51 ( 0.36 eV, respectively.22 These values are found to be in good agreement with the values reported in the literature.11,25-27 The group V transition metal oxide clusters that are produced in the laser vaporization source can be classified into one of two categories, either clusters which are stoichiometric or alternatively ones which are oxygen-rich clusters. The vanadium oxide cluster cations, V2O4,5+, V3O6,7+, V4O8,9+, and V5O11,12+, and the niobium and tantalum oxide cluster cations, M2O4,5+,

Zemski et al. M3O7,8+, M4O9,10+, and M5O12,13+, are referred to as stoichiometric clusters, where fragmentation of the cluster occurs only after the addition of center-of-mass energy, ECM > 2 eV. The clusters of higher oxygen content, such as M3O9+, are designated as oxygen-rich clusters because molecular oxygen is lost near thermal energy and under single-collision conditions during collision-induced dissociation experiments. It is believed that oxygen-rich clusters are comprised of the more stable stoichiometric clusters that have molecular oxygen physisorbed or weakly chemisorbed onto the cluster. For example, the M3O9+ can be thought of as M3O7+ with an O2 molecule chemisorbed onto the cluster surface. This is further evidenced during reactions of oxygen-rich group V transition metal oxide cluster cations with a reactant gas. Typically, the major reaction channel of most of these oxygen-rich metal oxide clusters with organic molecules involves a replacement reaction, where the organic molecule replaces an oxygen molecule in accordance with reaction 1.

MxOy(O2)+ + reactant f MxOy(reactant)+ + O2

(1)

It should be mentioned that a higher percentage of oxygen in the mixing tank will preferentially generate oxygen-rich clusters over production of the stoichiometric clusters and that laser power affects the cluster distribution, yielding more of the smaller clusters at higher laser power. Additionally, oxygenrich group V transition metal oxide cluster anions are not produced in our laser vaporization source under the same experimental conditions as the stoichiometric cluster ions. Reactions of Group V Transition Metal Oxide Ions with C2 and C4 Hydrocarbons Hydrocarbons are one of the most abundant and inexpensive raw materials in the chemical industry and provide the starting materials for many useful products. Therefore, devising methods to enhance the chemical conversion of hydrocarbons to more valuable, functionalized products, represents an extremely important field of modern chemistry. However, due to the inert nature of hydrocarbons, selective activation of C-H and C-C bonds is quite a challenge and their use as starting materials in chemical processes is rather limited.28 Cleavage of C-C bonds by heterogeneous catalysts is particularly important to the petroleum industry since it is the central step in the breakdown of hydrocarbons into smaller molecules. However, the mechanism by which this process occurs is still under some debate and the low product selectivity observed with the current heterogeneous catalytic system is a significant drawback.29 Additionally, there is also considerable interest in the selective oxidation of hydrocarbons over transition metal oxide catalysts because these reactions are the basis of numerous industrial processes yielding organic oxides, such as aldehydes and organic acids.30 The unambiguous identification of active sites and reaction mechanisms is essential for understanding the selectivity and activity of an oxidation catalyst. However, the nature of the catalytically active sites and the mechanism of selective oxidation are still in question, which results in only a limited number of effective hydrocarbon oxidation catalysts. Our own research has focused on providing a scientific basis for ultimately tailoring the design of transition metal oxide catalysts, which will be effective in the selective activation of hydrocarbon C-C and C-H bonds and in promoting oxygen-transfer reactions to hydrocarbons. Reactions of mass-selected group V transition metal oxide cluster ions (VxOy(, NbxOy(, and TaxOy() with ethane (C2H6)

Feature Article

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6139

and ethylene (C2H4) have been investigated.19 The major products observed during reactions of MxOy+ with ethane and ethylene are association of the C2 hydrocarbon, MxOyC2Hz+, and atomic oxygen loss, MxOy-1+. The association channel, MxOyC2Hz+, was found to be common to most of the group V transition metal oxide cluster cations examined. However, a significant atomic oxygen loss channel, MxOy-1+, was only observed during the course of reactions of (V2O5)n+, where n ) 1, 2, or 3, with ethylene and ethane, according to reaction 2.

(V2O5)n+ + C2Hz f (V2O5)n-1V2O4+ + (C2HzO)

(2)

It is proposed that this reaction pathway is an oxygen transfer from the mass-selected metal oxide cluster cation to the neutral hydrocarbon, because clusters of the stoichiometry (V2O5)n+ do not lose a single oxygen atom upon collision with an inert gas at thermal energy. It should be mentioned that the neutral products cannot be determined in our experimental apparatus because only ionic species are detected. Although we do not specifically identify the neutral product in parentheses, the indicated one is known to be formed in analogous reaction on V2O5 surfaces.31 Hence, it is only our intention to illustrate that a single oxygen atom is most likely transferred from the massselected metal oxide cluster to the C2 hydrocarbon. A significant finding is that the oxygen transfer channel is the major reaction channel during reactions of V2O5+, V4O10+, and V6O15+ with ethane and ethylene, but this reaction pathway is minor or nonexistent for the reactions ethane and ethylene with stoichiometrically equivalent niobium and tantalum oxide cluster cations. These experiments establish that the identity of the metal, the cluster stoichiometry, and the geometric structure strongly influence the ability of the metal oxide cluster to transfer an oxygen atom to ethane or ethylene. The reactions of group V transition metal oxide cluster cations with n-butane (C4H10), 1-butene (C4H8), and 1,3-butadiene (C4H6) were examined in our laboratory.12,16,18,32 The reaction pathways observed include association of the hydrocarbon (MxOyC4Hz+), C-C activation (MxOyC2H4+), dehydration (MxOy-1C4Hz-2+), and atomic oxygen loss (MxOy-1+). One of the principle reaction pathways observed during reactions of V2O5+ and V5O12+ with 1,3-butadiene, V3O7+ with 1,3butadiene and 1-butene, and V2O4+ with all of the C4 hydrocarbons is dehydration, which is shown in reaction 3.

VxOy+ + C4Hz f VxOy-1C4Hz-2+ + H2O

(3)

The dominant reaction pathway involved in the reactions of V2O5+, V4O10+, and V6O15+ with C4 hydrocarbons is atomic oxygen loss, which occurs via reaction 4.

VxOy+ + C4Hz f VxOy-1+ + (C4HzO)

(4)

It is significant to note that V2O5+, V4O10+, and V6O15+ do not lose a single oxygen atom during collision-induced dissociation studies at thermal energy. Therefore, we believe that the atomic oxygen loss channel shown in reaction 4 is due to a reaction with the hydrocarbon and not due to a collision process. In the case of reactions of niobium and tantalum oxide cluster cations with C4 hydrocarbons, the major reaction pathway involves C-C activation (MxOyC2H4+), not dehydration or atomic oxygen loss. Cleavage of the C2-C3 bond of the hydrocarbon to form MxOyC2H4+, shown in reaction 5, was the major reaction product observed during reactions of Nb2O4,5+, Ta2O4,5+, Nb3O7+, and Ta3O7+ with C4 hydrocarbons.

MxOy+ + C4Hz f MxOyC2H4+ + C2Hz-4

(5)

A minor C2-C3 activation channel was observed during reactions of V3O6+ and V5O11+ with n-butane, V3O7+ and V5O12+ with 1-butene and n-butane, and V2O4,5+ with C4 hydrocarbons. Additional C-C activation products, TaxOyCH3,4+ and TaxOyC3H5,6+, were also observed during the reactions of Ta2O4,5+ and Ta3O7+ with n-butane, 1-butene, and 1,3-butadiene. An atomic oxygen loss channel was observed during the reactions of (NbO3)0-1(Nb2O5)1-2+ with C4 hydrocarbons. However, an atomic oxygen loss channel was not observed during the course of reactions of tantalum oxide cluster cations with C4 hydrocarbons. The findings establish the influence of the identity of the metal on the product distribution for reactions of group V transition metal oxide cluster ions with C4 hydrocarbons. Reactions of VxOy+ with Halogen-Containing Organic Molecules The field of halo-organic chemistry continues to grow and there is extensive interest in understanding the basic chemical principles by which fluoro- and chlorocarbons interact with catalytic surfaces. To provide further insight into the mechanisms of the reaction, studies of vanadium oxide cluster cations interacting with hexafluoroethane (C2F6), 1,1,1-trifluoroethane (CH3CF3), difluoromethane (CH2F2), and carbon tetrachloride (CCl4) were undertaken in our laboratory.13-15 Within the range of experimental conditions explored, the clusters V2O7+, V3O6-9+, V4O8-11+, V5O11-13+, V6O13-16+, and V7O16-18+ were found to be inert toward reaction with hexafluoroethane.13 However, the smaller clusters, V2O4-6+, demonstrate a minor association channel, V2OyC2F6+. The vanadium oxide cluster cation reactions with 1,1,1-trifluoroethane display several different reaction pathways.11 Under single collision conditions, all the vanadium oxide clusters examined, except V2O7+, displayed a HF elimination product, VxOyHF+, which arose by reaction 6.

VxOy+ + CH3CF3 f VxOyHF+ + CH2CF2

(6)

V2O5+ is the only cluster that was able to activate the C-C bond of 1,1,1-trifluoroethane to form V2O5CF3+. Additionally, this cluster also demonstrated a reaction channel for the abstraction of two fluorine atoms with transfer of an oxygen atom to the neutral reactant molecule, producing V2O4F2+. Reactions of vanadium oxide cluster cations with difluoromethane were also examined.14 The clusters V2O4+, V3O6,7+, V4O8,9+, and V5O11,12+ displayed an association channel, VxOyCH2F2+, during reactions with difluoromethane. Under single collision conditions, the clusters V2O4+, V3O6,7+, V4O9+, and V5O12+ all displayed a channel for the transfer of a single oxygen atom to the neutral reactant, along with the abstraction of two fluorine atoms to form the product VxOy-1F2+, which is shown in reaction 7.

VxOy+ + CH2F2 f VxOy-1F2+ + CH2O

(7)

At higher pressures, V2O4+ and V3O7+ were able to undergo a second oxygen transfer reaction, accompanied by dual fluorine abstraction to form the products V2O2F4+ and V3O5F4+, respectively. The reactions of vanadium oxide cluster cations with carbon tetrachloride were also investigated.15 In accordance with reaction 8, the chloride ion transfer reaction is observed for all

6140 J. Phys. Chem. B, Vol. 106, No. 24, 2002

Zemski et al.

the clusters, but is most prominent for the lower mass clusters with diminishing intensity as the cluster size increases.

VxOy+ + CCl4 f VxOyCl + CCl3+

(8)

With the exception of the cluster V4O8+, the dominant reaction channel for the stoichiometric vanadium oxide clusters, excluding the CCl3+ product, is the production of neutral phosgene (COCl2) by reaction 9.

VxOy+ + CCl4 f VxOy-1Cl2+ + COCl2

(9)

V4O8+ favors the oxidative halogen transfer reaction to form the product V4O8Cl+, according to reaction 10.

VxOy+ + CCl4 f VxOyCl+ + CCl3

(10)

Many of the clusters display minor channels for the oxidative halogen transfer reaction, while V2O4+, V3O6+, and V4O8+ are the only clusters to demonstrate a channel for the abstraction of two chlorine atoms under single collision conditions as shown in reaction 11.

VxOy+ + CCl4 f VxOyCl2+ + CCl2

(11)

These findings show the influence of cluster size, degree of coordinative saturation of the cluster, and the oxidation state of the vanadium atoms in the cluster on the reactions of VxOy+ with hexafluoroethane, 1,1,1-trifluoroethane, difluoromethane, and carbon tetrachloride. Influence of Cluster Properties on Reactivity There are many factors which influence the reactivity of group V transition metal oxide cluster ions with hydrocarbons and halogen-containing organic molecules, such as composition, charge state, size, degree of coordinative saturation, stoichiometry, and oxidation state. The remainder of this discussion focuses on how these particular factors influence the reactivity of MxOy( with hydrocarbons and halogen-containing organic molecules. Dependence of Cluster Composition on Reactivity. The reactivity and product distribution of group V transition metal oxide cations, MxOy+, with hydrocarbons are significantly affected by the identity of the transition metal.12,16,18,19,32 Since vanadium, niobium, and tantalum occupy the same group of the periodic table, similar reactivities with hydrocarbons may be anticipated, but the product distribution observed during these reactions are quite different. Figure 2 displays the variation in reactivity of the mass-selected cluster, M2O5+, with n-butane as the identity of M is changed from V to Nb to Ta. It is apparent from this figure that the main reaction product of V2O5+ with n-butane is V2O4+, which is most likely the product of an oxygen transfer reaction. Prior collision-induced dissociation (CID) results of VxOy+ using xenon as the target gas demonstrate that V2O5+ clusters do not exhibit loss of atomic oxygen under single collision conditions at thermal energy.12 Therefore, it is proposed that this reaction is due to oxygen transfer from the mass-selected cluster, V2O5+, to n-butane, resulting in a single oxygen loss product, V2O4+, and a neutral oxygenated C4 hydrocarbon. In contrast, the primary reaction channel of Nb2O5+ and Ta2O5+ with n-butane is M2O5C2H4+, which is a C-C activation channel. Figure 3 shows the variation in reactivity of the mass-selected cluster, M3O7+, with 1,3butadiene as the identity of the metal is changed from V to Nb to Ta. It is apparent from Figure 3 that the major reaction

Figure 2. Relative product branching ratios of (a) V2O5+, (b) Nb2O5+, and (c) Ta2O5+ with n-butane.

pathway, other than association, observed during the reactions of V3O7+ with 1,3-butadiene is dehydration, where at least one V+-O bond must be broken. In contrast, Figure 3 shows that the primary reaction channel of Ta3O7+ with 1,3-butadiene after association is C2-C3 activation and that only the association channel is observed during reactions of Nb3O7+ with 1,3butadiene. It should be mentioned that similar reaction trends are observed during the reactions of group V transition metal oxide cluster cations with C2 hydrocarbons.19 Possible reasons for the anomalous reactivity in reactions of group V transition metal oxide clusters with hydrocarbons are considered below. The availability of a variety of stable oxidation states is one of the key factors that affects oxidation reactions.7 All of the group V transition metals have a strong tendency to be in the +5 oxidation state, but vanadium can also form stable complexes with V2+, V3+, and V4+, as is evident in Figure 1a. The first major peak in the VxOy+ mass distribution is VO+, which has the vanadium atom in the +3 oxidation state. In contrast, the first significant peak in the NbxOy+ and TaxOy+ mass

Feature Article

Figure 3. Relative product branching ratios of (a) V3O7+, (b) Nb3O7+, and (c) Ta3O7+ with 1,3-butadiene.

distribution is MO2+, which has the metal atom in the +5 oxidation state. This trend is also observed for the larger metal oxide cations because clusters, such as V3O6+, V4O8+, and V5O11+, are present in the VxOy+ mass distribution, but not in the NbxOy+ or TaxOy+ mass distribution. Since oxidation states lower than V5+ are stable for vanadium, this generally results in vanadium complexes that are better oxidizing agents than those of niobium or tantalum. In order for an oxygen transfer product to be formed, the metal oxide cluster must be reduced while the hydrocarbon is oxidized. For this reason, it is likely that the availability of accessible oxidation states plays a role during the reactions of group V transition metal oxide cluster cations with hydrocarbons. Therefore, it is possible that oxygen transfer is the major reaction pathway during reactions of certain VxOy+ with hydrocarbons because of the availability of various stable oxidation states. Likewise, it is possible that the oxygen transfer product is not the major reaction product during reactions of NbxOy+ and TaxOy+ with hydrocarbons because

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6141 these clusters are not as easily reduced due to the inaccessibility of a variety of stable oxidation states to niobium and tantalum. Another factor that plays a role in oxygen transfer and dehydration reactions is the strength of the metal-oxygen bond, because in order for these reactions to occur, at least one M-O bond in the cluster must be broken. It has been reported in the literature that the bond strength of the V+-O bond is 135 kcal/ mol, the Nb+-O strength is 164 kcal/mol, and the strength of the Ta+-O is 188 kcal/mol.33 It is understood that the M+-O bond strength in the group V transition metal oxide clusters examined in our investigations are not identical to the values mentioned above because the bond strength changes depending on the number of metal and oxygen atoms present in a cluster. For example, the value of the OTa+-O bond is 140 kcal/mol, which is about 48 kcal/mol weaker than the Ta+-O bond.7 Therefore, it is highly probable that V+-O bonds will be weaker than the stoichiometrically equivalent Nb+-O and Ta+-O bonds of the metal oxide clusters. Since V-O bonds are much weaker than Nb-O or Ta-O bonds, it is possible that vanadium oxide clusters exhibit oxygen transfer and dehydration channels during reactions with hydrocarbons, where at least one V+-O bond would be broken. Furthermore, it is also possible that the oxygen transfer and dehydration channels are not the major reaction pathways observed during reactions of niobium and tantalum oxide cluster cations with hydrocarbons because of the stronger M+-O bonds present in these clusters. The C-C activation channel, MxOyC2H4+, is the favored reaction pathway of certain stoichiometric niobium and tantalum oxide clusters with C4 hydrocarbons, but as shown in Figures 2 and 3, this C-C activation product is a minor reaction channel for the case of reactions of vanadium oxide cluster cations with C4 hydrocarbons. In order for a reaction to proceed favorably, the bonds formed during the reaction must be stronger than the bonds broken during the reaction. Accordingly, the thermochemistry of C-C insertion depends on the strength of the M-C bonds, the C-C binding energy of the hydrocarbon, and any energy barrier present in the reaction channel. It is thought that the reactions of group V transition metal oxide clusters with hydrocarbons could be metal-mediated reactions. Density functional theory calculations by Sambrano et al. on niobium oxide cluster cations have indicated that the positive charge center is located on the niobium atoms of the cluster.34 The positive charge center located on the metal atoms of group V transition metal oxide cluster cations and can enhance the M-C bond formation process. Therefore, the strength of the M-C bonds might be an important factor in the reaction product distribution observed during the reactions of group V transition metal oxide cluster cations with hydrocarbons. Typically, the second- and third-row transition metals are preferred for alkane activation because of the strong M-C bonds that are formed.35 The lanthanide contraction causes the size of the s and d orbitals of the second- and third-row transition metal atoms to be quite similar, which leads to better overlap of the s and d orbitals.36 This results in the general trend of the second and third row transition metals possessing stronger M-ligand bonds than first row transition metals, which might drive the C-C activation reaction. To the best of our knowledge, the only M-C bond dissociation energy that is reported in the literature for all of the group V transition metals is the M+-CH2 bond. The trend of the M+-CH2 bond dissociation energy values as reported in the literature show that the V+-CH2 is the weakest bond, while the Ta+-CH2 is the strongest bond.37 The strength of the M-C bond will govern the exothermicity of the reaction and allow the C-C bonds of hydrocarbons to break. Conse-

6142 J. Phys. Chem. B, Vol. 106, No. 24, 2002 quently, the M-C bond strength may be a key reason the favored reaction pathway of certain niobium and tantalum oxide cluster cations with hydrocarbons is C-C activation. However, the weaker V-C bonds may not be strong enough to drive the C-C activation reaction. Another difference in the product distribution of the group V transition metal oxide cations with C4 hydrocarbons is that multiple reaction products have been observed during reactions with tantalum oxide cluster cations.18 However, these multiple association products are not observed during reactions of vanadium and niobium oxide cluster cations with C4 hydrocarbons. One reason that might explain why tantalum oxide clusters exhibit multiple association products could be that the cluster undergoes further reactions under multiple collision conditions, where additional metal-carbon bonds are formed. Tantalum has an atomic radius of 1.34 Å, while the atomic radius of the vanadium atom is 1.22 Å.38 Larger atoms, such as tantalum, have been shown to coordinate more oxygen atoms than the smaller vanadium atoms as can be seen from the mass distributions in Figure 1. Therefore, tantalum oxide clusters could exhibit multiple association products because these clusters are larger and contain more coordination sites, which might allow more ligands to attach to the cluster. In fact, the larger sizes of the elements of the second and third transition series compared to those of the first series leads to a general tendency to exhibit higher coordination numbers.38 Thus coordination numbers of seven, eight, and nine are quite common in compounds of second- and third-row transition metals. This is clearly seen from the total ion mass distributions of vanadium, niobium, and tantalum oxide cations shown in Figure 1, which was obtained under similar source conditions. It is apparent upon comparison of the mass distributions that tantalum oxide clusters can accommodate more oxygen ligands than the vanadium and niobium oxide cluster cations. The final significant cluster in each series of tantalum oxide cluster cations is approximately Ta2O14+, Ta3O14+, Ta4O13+, and Ta5O17+. In contrast, the final significant cluster in each series is approximately V2O6+, V3O8+, V4O11+, V5O13+, V6O16+, and V7O18+ and Nb2O6+, Nb3O9+, Nb4O12+, Nb5O13+, Nb6O16+, and Nb7O17+ for vanadium and niobium oxide cluster cations, respectively. Therefore, it is apparent that tantalum oxide clusters can accommodate more ligands than vanadium and niobium oxide clusters with the same number of metal atoms, which is a possible explanation as to why multiple association products are formed during reactions of TaxOy+ with C4 hydrocarbons. Dependence of Charge State on Reactivity. Metal oxide surfaces have surface defects and there is wide ranging interest in the properties of nonstoichiometric compounds, especially ionic species, in terms of their potential catalytic activity. It is recognized that various oxides can frequently be described as having two types of lattice planes, one involving only anions and the other containing cations and some anions in specific layers.3 It has also been noted that the loss of oxygen ions from oxide catalysts can result in nonstoichiometry through the formation of anion vacancies which can in turn have a profound effect on catalytic activity. Pinnavaia and co-workers have studied the catalytic activity of intercalated positive and negative biomimetic macrocyclic metal. They found the [CoPcTs]4intercalated into magnesium aluminum double hydroxide actually increased the catalytic activity and the [CoTMPy]4+ intercalated into the clay decreased the activity of the catalyst.39 Wan and co-workers have found spectroscopic evidence showing C2H4 is produced when O2- and CH4 react under oxidative coupling of methane conditions. Their results also provide

Zemski et al.

Figure 4. Mass spectra of (a) V2O5+ and (b) V2O5- reacted with 0.3 mTorr of ethane.

evidence to suggest that O2- is at least one of the active oxygen species for this reaction over the catalysts studied.40 Zeolites have become an important aspect of catalysis in the latter half of the last century. They are characterized by ionic sites on their surfaces and studies have been performed to investigate the effect of ion exchange on the reactivities of certain zeolites. One study in particular focused on the Ba2+, Al3+ and La3+ ion exchange of ZSM-5, a crystalline microporous aluminosilicate.41 Pakkanen and co-workers found an increase in the catalytic activity of the Al and La ion-exchanged zeolites, which also produced slightly more C5 and olefin products and less C3 and aromatic products than the unmodified ZSM-5 zeolite.41 In addition, Lu and co-workers have showed that molecular oxygen adsorbs to the Ti3+ sites, associated with anion vacancies on a TiO2 surface. They also found the adsorbed oxygen plays an important role in the photooxidation of CH3Cl on TiO2 (110) surfaces.42 The evidence cited above shows that elucidating the nature of catalysts from the point of view of their ionic nature is very critical in further understanding the physical basis for catalysis involving these systems. In our studies this is accomplished by examining the reactivity of both group V transition metal oxide cations and anions. During the course of our studies we have observed a profound effect of charge state on the reactivity of the cluster ions with various reactant gases. We have investigated the reactions of group V transition metal oxide anions with ethane and ethylene. Figure 4 shows the mass spectra of V2O5( with ethane. This figure illustrates the fact that there are no observable reaction products formed during reactions of V2O5- with ethane, despite the fact that V2O5+ does display two reaction channels. Similar trends are also observed

Feature Article during reactions of NbxOy- and TaxOy- with ethane and ethylene, where MxOy- did not display any observable reaction products, but certain MxOy+ did exhibit oxygen transfer and association channels. This demonstrates that the charge state of the cluster has a dramatic effect on the reactivity of group V transition metal oxide cluster ions with ethane and ethylene. Another instance in which the charge state of the cluster has a dramatic effect on the reactivity of group V transition metal oxide cluster ions is with C4 hydrocarbons. It has been determined that there are no observable reaction products formed during reactions of group V transition metal oxide cluster anions with C4 hydrocarbons, despite the fact that specific group V transition metal oxide cluster cations do display dehydration, C-C activation, atomic oxygen loss, and association channels. The dependence of cluster reactivity on charge state has been explored previously. For example, Bondybey and co-workers established that platinum cluster cations react with methane at collision rates, whereas platinum cluster anions react more than a magnitude slower;43 and they have reported that the overall reactivity of Nbx- is considerably smaller than the overall reactivity of Nbx+ with benzene.44 Likewise, Kaldor and coworkers found that the reactivity of gold clusters with hydrogen, methane, and oxygen depends strongly on cluster charge state.45 Additionally, Schro¨der et al. have indicated their preference for examining cationic metal oxides because neutral and anionic species are less likely to activate hydrocarbons.33 This is because the cationic metal oxides, which behave as Lewis acids, initiate alkane activation by attacking the C-C bonds of the alkane.46 Therefore, the C-C activation reactions that are observed during the reactions of NbxOy+ and TaxOy+ with n-butane may be due to the ability of MxOy+ to polarize the n-butane substrate and activate it more easily than MxOy-. It is thought that the presence of the positive charge of the cation may act to strengthen the interaction with n-butane and enhance the effect of electron withdrawal, resulting in increased activation of the C-C bond. However, the opposite effects are observed for metal oxide anions because the negative charge will reduce electron withdrawing effects and less strongly polarize n-butane. These results indicate that the charge center has a very important effect on the reactivity. Additionally, it should be mentioned that experimental studies of oxidation reactions of hydrocarbons over metal oxide catalysts suggest that sites of anion vacancies are the catalytically active centers. Dependence of Cluster Size on Reactivity. Another factor that influences the reactivity of group V transition metal oxide cluster cations with C4 hydrocarbons and halogen containing organic molecules is the size of the metal oxide cluster. The reactions of group V transition metal oxide cations with n-butane display significant size dependent reactivity.16 Figure 5 shows that the C-C activation channel, NbxOyC2H4+, is the preferred reaction pathway for clusters containing x < 3, but this C-C activation channel is not present in reactions of n-butane with cationic niobium oxide clusters containing x g 3. Instead the association channel, NbxOyC4H10+, becomes the preferred reaction pathway when x g 3. A similar size dependent trend is also observed in the reactions of VxOy+ and TaxOy+ with n-butane. Another example of size dependent reactivity has been observed during the reactions of VxOy+ with difluoromethane (CH2F2), where the smaller clusters were found to be more reactive than the larger clusters toward reaction 7.14 This size dependence is illustrated in Figure 6, which shows that the VxOy-1F2+ channel is favored during the reactions of V2O4,5+ and V3O7+ with difluoromethane. However, this reaction channel is minor during reactions of VxOy+ with CH2F2, where

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6143

Figure 5. Branching ratio of the (a) cracking product, NbxOyC2H4+, and (b) association product, NbxOyC4H10+, with 0.15 mTorr of n-butane. Note the general dependence on cluster size in this distribution.

Figure 6. Branching ratio of the VxOy-1F2+ product with 0.15 mTorr of CH2F2.

x > 3. The final example of size dependent reactivity that we have observed is during the reactions of vanadium oxide cluster cations with carbon tetrachloride.15 The chloride transfer channel, which produces CCl3+ according to reaction 8, was observed for all the vanadium oxide cluster cations examined. However, it was determined that the extent to which this chloride transfer reaction occurs is dependent on the size of the cluster, a fact which is evident from the data shown in Figure 7. As the cluster size increases, the prominence of this channel decreases, with a notable decline starting with the cluster series V4Oy+, and then continues to gradually decline as the cluster size further increases. Then starting at the series V5Oy+, the prominent

6144 J. Phys. Chem. B, Vol. 106, No. 24, 2002

Figure 7. Mass spectra of the reactions of (a) VO2+ (b) V3O7+ and (c) V5O12+ with 0.25 mTorr of CCl4.

reaction pathway becomes the production of neutral phosgene, COCl2, which is shown in reaction 9. There are several possible factors such as internal cluster temperature, charge density, and cluster structure of the group V transition metal oxide cluster cations that warrant consideration in accounting for the observed variation in reactivity with respect to cluster size.14 In the previous section it has been shown that the charge state of the metal oxide cluster plays a considerable role in the reaction pathways that are observed. If the positive charge is delocalized, the smaller clusters will concentrate the charge onto fewer metal atoms. In contrast, the positive charge would be more delocalized for the larger clusters because they possess more metal atoms. Consequently, this could result in the smaller group V transition metal oxide cluster cations strongly polarizing the bonds of the reactant organic molecule. This delocalization of charge might be one of the reasons why the smaller group V transition metal oxide cluster cations were found to be more reactive toward C4 hydrocarbons and halogen containing organic molecules. However, it is also possible that the charge might be localized on one particular metal site. Therefore, the size-dependent nature of these reactions may depend on the structure of the metal oxide clusters. If the positive charge is localized, then it is possible that the charge center of the larger metal oxide clusters may not be as accessible to the neutral reactant molecule compared to the smaller clusters. Previously it was postulated that branching in the cluster structure begins to occur in larger size vanadium oxide clusters when x > 3.12 Therefore, the charge center of the larger metal oxide clusters might be located in the center with the other vanadium atoms surrounding it via bridging oxygen atoms, which might act to block the reactive site of the cluster from the neutral reactant. Although it might be thought that metal oxide clusters may not be efficiently cooled during the cluster formation process, this is unlikely to be a factor in explaining the findings. Direct

Zemski et al. measurements of the internal energy of our metal oxide clusters have not been conducted, but several observations imply that this factor does not contribute to the size dependence observed during reactions of metal oxide clusters with C4 hydrocarbons and halogen containing organic molecules. Under similar experimental conditions, such as laser fluence, backing pressure, and reactant concentration, clusters of the same size distribution mentioned above have been observed to exit the source with attached N2 and CH4.14 The presence of these weakly bound ligands is evidence that the cluster temperatures are quite low. Therefore, we do not believe that the smaller metal oxide clusters are internally excited. Finally, our current experimental apparatus does not allow us to differentiate between molecularly and dissociatively adsorbed products on a cluster. Therefore, in Figure 5 it is possible that the association product, NbxOyC4H10+, can be either a molecularly or dissociatively adsorbed n-butane molecule. It is possible that larger metal oxide cluster cations are able to activate n-butane, but the larger size of these clusters may also allow for association of the neutral products formed during reactions with n-butane. In other words, it is possible that n-butane is dissociatively adsorbed as a pair of ethylene and ethane adsorbates on the larger clusters. This point is illustrated in Figure 5, where the C-C activation channel, NbxOyC2H4+, is only observed for the clusters NbO2+ and Nb2O5+ and the clusters Nb3O7+ and Nb5O12+ display a strong reaction channel that corresponds to the ion mass of NbxOyC4H10+ compared to the other clusters studied. Therefore, it seems probable that the NbxOyC2H4+ product is not observed for Nb3O7+ and Nb5O12+ because the larger size of the metal oxide clusters allows for the adsorption of all of the reaction products onto the cluster and therefore results in a strong reaction channel with an ion mass corresponding to NbxOyC4H10+, which could be either molecularly or dissociatively adsorbed n-butane. Dependence of Coordinative Saturation on Reactivity. Another factor that influences the reactivity of group V transition metal oxide cluster cations with hydrocarbons and halogen containing organic molecules is the degree of coordinative saturation of the cluster. The degree of coordinative saturation is important because only a limited number of ligands can be within bonding distance of the metal oxide cluster due to steric and electronic reasons. Russell and co-workers have determined that the reactivity of metal carbonyl clusters can be related to the degree of coordinative unsaturation because the more open coordination sites present on a cluster, the higher the rate for ion-molecule reactions.47 Likewise, coordinative unsaturation is an important factor in organometallic and heterogeneous catalysis and it has been determined in these types of studies that C-C bond activation requires a high degree of coordinative unsaturation.48 The effect of coordinative saturation on the reactions of niobium and tantalum oxide cluster cations with n-butane can be observed in Figure 8. It is apparent from Figure 8 that the presence of an extra oxygen atom significantly changes the extent to which the cluster can associate n-butane.16 Typically, coordinatively unsaturated sites are responsible for the adsorption of reactant molecules. Therefore, it is possible that the addition of an oxygen atom onto M5O12+ to form M5O13+ causes the reaction site and charge center of the cluster to be sterically hindered, which alters the reactivity of the cluster. Additionally, reactions of vanadium oxide cluster cations with CH3CF3 exhibited a dependence on the degree of coordinative saturation of the cluster.12 The less coordinated clusters seem able to abstract a second HF more efficiently than can the other clusters.

Feature Article

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6145

+

Figure 8. Relative product branching ratios of (a) Ta5O12 and (b) Ta5O13+ with n-butane.

This is most evident in the branching ratios of the clusters V4O8+ and V4O9+, shown in Figure 9,b, with CH3CF3. Both of these clusters react readily with CH3CF3 to abstract HF, but the second HF abstraction by V4O9+ is negligible, whereas V4O8+ is the most efficient cluster studied for eliminating the second HF. Dependence of Cluster Stoichiometry on Reactivity. The reactions of group V transition metal oxide cluster cations with ethylene are a good example of the dependence of cluster stoichiometry on reactivity. Figure 10 suggests that the (V2O5)n+ clusters, where n ) 1 and 2, contain a reactive center toward oxygen transfer, while the other vanadium oxide clusters examined do not.19 It should be mentioned that V6O15+ also showed a significant oxygen loss channel, similar to Figure 10(b) and 10(h), during reactions with ethylene. Based on these results, it is concluded that cluster stoichiometry is a key factor in the reactivity that is observed. It is thought that the vanadium oxide cluster cations with the stoichiometry (V2O5)n+ might possess similar structural features, which enable these particular clusters to readily transfer an oxygen atom to ethylene. Collision-induced dissociation experiments in our laboratory and density functional calculations by Andre´s and co-workers have shed light on the geometric structures of vanadium oxide cluster cations.12,49 From the collision-induced dissociation experiments, density functional theory calculations, and knowledge of the stable oxidation states of vanadium, two different types of structural features might be present on (V2O5)n+ clusters that could result in the enhanced ability of these specific clusters to transfer an oxygen atom to ethylene. The first type of structural feature that is considered is an oxygen centered radical, which is characterized by an extra electron present on one of the oxygen atoms of the (V2O5)n+ cluster and an elongated vanadium-oxygen bond. The second type of structural feature that is considered is the presence of a peroxo ligand on (V2O5)n+ clusters, which is

Figure 9. Relative product branching ratios of (a) V4O8+ and (b) V4O9+ with CH3CF3.

characterized as an oxygen molecule associatively adsorbed on the cluster as a triangular bidentate. All of the collision-induced dissociation results that are discussed in this section (unless otherwise mentioned) are at single collision conditions, which corresponds to less than 0.10 mTorr of collision gas in the octopole. Prior collision-induced dissociation results of (V2O5)n+ using xenon as the target gas demonstrate that these clusters do not exhibit an atomic oxygen loss channel at thermal energy.12 Collision-induced dissociation of (V2O5)n+, where n ) 2 or 3, has revealed that molecular oxygen is lost from these clusters upon collision with an inert gas at thermal energy, which indicates that these clusters have oxygen-rich character.12 The loss of a single oxygen atom from V4O10+ or V6O15+ is not observed during collision-induced dissociation experiments at thermal energy or at a center-ofmass energy in excess of 3 eV. However, atomic oxygen loss is observed when V2O5+ collides with 0.08 mTorr of Kr at ECM g 1.0 eV, but loss of a single oxygen atom at this pressure is not observed at thermal energy.12 Molecular oxygen loss is also observed during CID experiments on V2O5+ at ECM g 2 eV. It should be mentioned that there is no collision energy added to the octopole rods under the conditions where the loss of a single oxygen atom occurs during reactions of (V2O5)n+ with ethylene. From the collision-induced dissociation performed in our laboratory and the DFT calculations by Andre´s and co-workers, it seems that the favored structure of V2O5+ is an oxygencentered radical.12,49 The collision-induced dissociation results of V4O10+ and V6O15+ do not appear to favor this structure because molecular oxygen is easily lost from these clusters. However, if a peroxo ligand is present on a vanadium oxide cluster, loss of molecular oxygen should occur easily during collision-induced dissociation.50 In fact, the presence of a peroxo ligand on (V2O5)n+ clusters might make them active toward oxygen transfer to ethane and ethylene because it has been found

6146 J. Phys. Chem. B, Vol. 106, No. 24, 2002

Zemski et al.

Figure 10. Relative product branching ratios of (a-c) V2O4-6+, (d-f) V3O6-8+, and (g-i) V4O9-11+ with ethylene.

that peroxo complexes of early transition metals are active in olefin oxidation reactions.51 Specifically, peroxovanadium complexes perform a variety of oxidation reactions, such as the transformation of alkenes to epoxides and of alkanes to alcohols.52 Unfortunately, no DFT calculations have been performed to determine the structures of V4O10+ and V6O15+, but the presence of a peroxo ligand on these clusters might explain the molecular oxygen loss observed during CID and the oxygen transfer observed during reactions with ethylene. Dependence of Oxidation State of the Metal Atoms in the Cluster on Reactivity. From the work in our laboratory, we have also found the oxidation state of the metal atoms in the clusters can affect their reactivities with various reactant gases. One example that illustrates this is the tendency for clusters with lower oxidation state vanadium atoms to display dominant oxidative chlorine transfer channels, while clusters with vanadium exclusively in the +5 oxidation state predominantly form phosgene.15 The clusters, V3O7+, V5O12+, and V7O17+, all contain vanadium atoms solely in the +5 oxidation state.12 Their ability to abstract a chlorine atom from CCl4 is a minor pathway whereas this channel is predominant with clusters containing vanadium in lower oxidation states such as V4O8+. This cluster has the lowest vanadium oxidation state of all clusters studied and chlorine abstraction is the dominant reaction pathway. V3O6+ has the second lowest average oxidation state of the clusters investigated and also shows a substantial chlorine abstraction. However, the dominant pathway for V3O7+ is

phosgene production. For all other stoichiometric clusters, the dominant vanadium-containing channel is that of phosgene production. This leads us to conclude that chlorine abstraction occurs more easily on clusters containing vanadium atoms in the lower oxidation states. Another example of the effect of oxidation state seen in our laboratory was observed while studying the reactions of V3Oy+ with 1,3-butadiene.12 The oxidation states of the ions of interest in this discussion, V3O6+ and V3O7+, are (+4, +4, +5) and (+5, +5, +5), respectively. It is important to note that in addition to the oxidation states, the structures may also influence the reactivity of these cluster ions. Figure 11 shows that during the reactions with 1,3-butadiene, the V3O6+ cluster only undergoes association while the V3O7+ cluster displays dehydration as well as association channels. The reactions of V3O6+ are proposed to occur via a metal center but the additional oxygen on the V3O7+ cluster appreciably alters the reactivity toward hydrocarbons; thus, the oxygen most likely plays a significant role in the reactions observed. Comparisons with Condensed Phase Reactivity. One of the goals of our research is to relate gas-phase chemistry to condensed phase chemistry. In a review by Wachs et al. on group V transition metal oxide catalysts, it was revealed that active vanadia surface sites are primarily redox in nature.53 In contrast, the active surface sites on niobia and tantala are primarily Lewis acid sites, which causes them to be less effective toward oxidation reactions. Our gas phase studies discussed

Feature Article

J. Phys. Chem. B, Vol. 106, No. 24, 2002 6147 Future Directions

Figure 11. Mass spectra of the reactions of (a) V3O6+ and (b) V3O7+ with 0.25 mTorr of 1,3-butadiene.

herein are related to the results found on group V transition metal oxide surfaces, which is clearly exemplified in Figure 2. This figure reveals that the main reaction product of V2O5+ with n-butane is V2O4+, which is an oxygen transfer reaction and is comparable to the redox nature of vanadia catalyst surfaces. Conversely, the atomic oxygen loss channel is a minor reaction pathway during reactions of Nb2O5+ with n-butane and is nonexistent during reactions of Ta2O5+ with n-butane. Correlations between reactions of gas-phase metal oxide clusters and reactions over condensed phase metal oxide catalysts have also been observed during the reactions of VxOy+ with CCl4. The products observed during the reactions of vanadium oxide cluster cations with carbon tetrachloride support the mechanisms proposed for the condensed phase degradation of CCl4 on vanadium oxide catalysts.15 The studies performed by Mink and co-workers determined that carbon dioxide is the primary product for the degradation of CCl4 on the vanadium oxide catalyst and, after a short period of time, the production of phosgene is observed.54,55 They proposed a mechanism in which CCl4 forms phosgene and carbon dioxide in a two-step process involving two vanadium centers in the condensed phase. During the gas-phase studies of larger vanadium oxide clusters with CCl4, we found that neutral phosgene production, which is shown in reaction 9, is the favored reaction pathway. Referring to Figure 7, it is seen that the gas-phase reactions of CCl4 with the vanadium oxide cluster ions support the mechanism proposed by Mink and co-workers in view of the fact that the cluster VO2+, which contains only one vanadium atom, does not form phosgene. Additionally, the reactions of vanadium oxide cluster cations with 1,3-butadiene have displayed some similarities to those observed over vanadium oxide catalysts. The reactions of V3O7+ and V5O12+ toward the oxidative dehydrogenation of 1,3butadiene are found to differ significantly from similar clusters, such as V3O6+ and V5O11+, which are inert toward these reaction pathways. These differences are apparently related to the oxidation states of the vanadium atoms within the V3O7+ and V5O12+ clusters and may be indicative of the crucial role played by V5+ in heterogeneous oxidation catalysis.

Current work in our laboratory is concentrated on determining the bond dissociation thresholds of VxOy( (where x > 1), NbxOy(, and TaxOy(. From our prior studies reviewed herein, it is evident that the structure, both geometric and electronic, of metal oxide cluster ions plays a key role in their reactivities with various reactant gases. The collision-induced dissociation experiments performed in our laboratory provide some insight into the structures of group V transition metal oxide ions, in particular through comparison with theoretical work on their bonding. The studies provide valuable thermochemical information which assists in evaluating possible reaction channels. To determine the structures of these metal oxide cluster ions, theoretical calculations must be performed to supplement the information obtained from the collision-induced dissociation experiments. In addition, calculations can be performed on reactions of metal oxide cluster ions with reactant gases, which would give insight into the identity of the oxygenated neutrals that are formed during reactions and into the mechanism of the reactions that we observe experimentally. Toward this goal, we have started a collaboration with Professor V. BonacicKoutecky, of the Humboldt University in Berlin, to perform density functional calculations on group V transition metal oxide cluster ions in order to provide some structural insight. Additional calculations could also aid in determining the mechanisms under which metal oxide clusters can invoke oxygen transfer to organic substrates and also aid in determining the identity of the oxygenated neutral hydrocarbons formed during reactions with organic molecules. Prospects for further understanding of the nature of catalytic reactions, and the development of new efficient catalytic materials can be expected to be derived from work of the type described herein. This is certainly a promising approach and we can expect many important developments to be forthcoming. Acknowledgment. The authors would like to gratefully acknowledge DuPont, the Department of Energy, Grant DEFG02-92ER14258, and a Goali grant from the National Science Foundation, Grant No. CHE-9632771, for their financial support. We also thank Dr. Richard Bell for his significant contributions to much of the experimental work described herein. References and Notes (1) Gai-Boyes, P. L. Catal. ReV.-Sci. Eng. 1992, 34, 1. (2) Queeney, K. T.; Friend, C. M. J. Phys. Chem. B 2000, 104, 409. (3) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994; p 402. (4) Somorjai, G. A. J. Phys. Chem. 1990, 94, 1013. (5) Witko, M.; Hermann, K.; Tokarz, R. J. Electron. Spectrosc. Relat. Phenom. 1994, 69, 89. (6) Nalewajski, R. F.; Korchowiec, J. Computers Chem. 1995, 19, 217. (7) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: New York, 1989; pp 169-195. (8) Holthausen, M. C.; Fiedler, A.; Schwarz, H.; Koch, W. J. Phys. Chem. 1996, 100, 6236. (9) Heinemann, C.; Wesendrup, R.; Schwarz, H. Chem. Phys. Lett. 1995, 239, 75. (10) Schro¨der, D.; Schwarz, H. Angew. Chem, Int. Ed. Engl. 1995, 34, 1973. (11) Harvey, J. N.; Diefenbach, M.; Schro¨der, D.; Schwarz, H. Int. J. Mass Spectrom. 1999, 182/183, 85. (12) Bell, R. C.; Zemski, K. A.; Kerns, K. P.; Deng, H. T.; Castleman, A. W., Jr. J. Phys. Chem. A 1998, 102, 1733. (13) Bell, R. C.; Zemski, K. A.; Castleman, A. W., Jr. J. Phys. Chem. A 1998, 102, 8293. (14) Bell, R. C.; Zemski, K. A.; Castleman, A. W., Jr. J. Phys. Chem. A 1999, 103, 2992. (15) Bell, R. C.; Zemski, K. A.; Castleman, A. W., Jr. J. Phys. Chem. A 1999, 103, 1585.

6148 J. Phys. Chem. B, Vol. 106, No. 24, 2002 (16) Zemski, K. A.; Justes, D. R.; Bell, R. C.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 4410. (17) Zemski, K. A.; Bell, R. C.; Castleman, A. W., Jr. Int. J. Mass Spectrom. 1999, 184, 119. (18) Zemski, K. A.; Bell, R. C.; Castleman, A. W., Jr. J. Phys. Chem. A 2000, 104, 5732. (19) Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 10237. (20) Fialko, E. F.; Kikhtenko, A. V.; Goncharov, V. B.; Zamaraev, K. I. J. Phys. Chem. B 1997, 101, 5772. (21) Kinne, M.; Heidenreich, A.; Rademann, K. Angew. Chem. Int. Ed. 1998, 37, 2509. (22) Bell, R. C.; Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Chem. Phys. 2001, 114, 798. (23) Zemski, K. A. PhD Thesis. The Pennsylvania State University, University Park, PA, 2001. (24) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 1st ed.; Pergamon Press: Oxford, 1984. (25) Aristov, N.; Armentrout, P. B. J. Am. Chem. Soc. 1986, 108, 1806. (26) Sievers, M. R.; Armentrout, P. B. J. Chem. Phys. 1995, 102, 754. (27) Dyke, J. M.; Gravenor, B. W.; Hastings, M. P.; Morris, A. J. Phys. Chem. 1985, 89, 4613. (28) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879. (29) Wojciechowski, B. W.; Corma, A. Catalytic Cracking: Catalysts, Chemistry, and Kinetics; Marcel Dekker: New York, 1986; pp 41-98. (30) Warren, B. K.; Oyama, S. T. Heterogeneous Hydrocarbon Oxidation; American Chemical Society: Washington, DC, 1996; p 9. (31) Sanchez Escribano, V.; Busca, G.; Lorenzelli, V. J. Phys. Chem. 1990, 94, 8945. (32) Deng, H. T.; Kerns, K. P.; Castleman, A. W., Jr. J. Phys. Chem. 1996, 100, 13386. (33) Schro¨der, D.; Schwarz, H.; Shaik, S. Struct. Bonding 2000, 97, 91. (34) Sambrano, J. R.; Andre´s, J.; Beltra´n, A.; Sensato, F.; Longo, E. Chem. Phys. Lett. 1998, 287, 620. (35) Whittborn, A. M. C.; Costas, M.; Blomberg, M. R. A.; Siegbahn, P. E. M. J. Chem. Phys. 1997, 107, 4318. (36) Siegbahn, P. E. M.; Blomberg, M. R. A. J. Am. Chem. Soc. 1992, 114, 10548. (37) (a) Aristov, N.; Armentrout, P. B. J. Am. Chem. Soc. 1986, 108, 1806. (b) Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1987, 91, 6178.

Zemski et al. (c) Bauschlicher, C. W., Jr.; Partridge, H.; Sheehy, J. A.; Langhoff, S. R.; Rosi, M. J. Phys. Chem. 1992, 96, 6969. (d) Hettich, R. L.; Freiser, B. S. J. Am. Chem. Soc. 1987, 109, 3543. (e) Sievers, M. R.; Chen, Y. M.; Haynes, C. L.; Armentrout, P. B. Int. J. Mass Spectrom. 2000, 196, 149. (f) Irikura, K. K.; Goddard, W. A. J. Am. Chem. Soc. 1994, 116, 8733. (38) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; John Wiley and Sons: New York, 1988; pp 934-939. (39) Chibwe, M.; Ukrainczyk, L.; Boyd, S. A.; Pinnavaia, T. J. J. Mol. Catal. A. 1996, 113, 249. (40) Wan, H. L.; Zhou, X. P.; Weng, W. Z.; Long, W. R.; Chao, Z. S.; Zhang, W. D.; Chen, M. S.; Luo, J. Z.; Zhou, S. Q. Catal. Today 1999, 51, 161. (41) Tynja¨la¨, P.; Pakkanen, T. T. J. Mol. Catal. A 1996, 110, 153. (42) (a) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 3005. (b) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Chem. Phys. 1995, 102, 4657. (43) Achatz, U.; Berg, C., Joos, S.; Fox, B. S.; Beyer, M. K.; NiednerSchatteburg, G.; Bondybey, V. E. Chem. Phys. Lett. 2000, 320, 53. (44) Berg, C.; Beyer, M.; Achatz, U.; Joos, S.; Niedner-Schatteburg, G.; Bondybey, V. E. J. Chem. Phys. 1998, 108, 5398. (45) Cox, D. M.; Brickman, R.; Creegan, K.; Kaldor, A. Z. Phys. D 1991, 19, 353. (46) Field, F. D. In SelectiVe Hydrocarbon ActiVation: Principles and Progress; Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; VCH: New York, 1990; pp 241-261. (47) Anderson-Fredeen, D. J.; Russell, D. H. J. Am. Chem. Soc. 1985, 107, 3762. (48) Rybtchinski, B.; Milstein, D. J. Am. Chem. Soc. 1999, 121, 4528. (49) Calatayud, M.; Andre´s, J.; Beltra´n, A. J. Phys. Chem. A 2001, 105, 9760. (50) Schro¨der, D.; Jackson, P.; Schwarz, H. Eur. J. Inorg. Chem. 2000, 1171. (51) Yudanov, I. V.; Di Valentin, C.; Gisdakis, P.; Ro¨sch, N. J. Mol. Catal. A 2000, 158, 189. (52) Butler, A.; Clague, M. J.; Meister, G. E. Chem. ReV. 1994, 94, 625. (53) Wachs, I. E.; Briand, L. E.; Gao, X., Catal. Today 2000, 57, 323. (54) Mink, G.; Berto´ti, I.; Battistoni, C.; Sze´kely, T. React. Kinet. Catal. Lett. 1985, 27, 39. (55) Mink, G.; Berto´ti, I.; Battistoni, C.; Sze´kely, T. React. Kinet. Catal. Lett. 1985, 27, 33.