Experimental and Theoretical Study of the Reactions between

Article pubs.acs.org/JPCA

Experimental and Theoretical Study of the Reactions between Vanadium Oxide Cluster Cations and Water Jia-Bi Ma,†,‡ Yan-Xia Zhao,† Sheng-Gui He,†,* and Xun-Lei Ding*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China ABSTRACT: Vanadium oxide cluster cations VxOy+ (x = 2− 6) are prepared by laser ablation and are reacted with D2O in a fast flow reactor under room temperature conditions. A timeof-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Observation of the products (V2O5)1−3D+ indicates the deuterium atom abstraction reaction (V2O5)1−3+ + D2O → (V2O5)1−3D+ + OD. In addition, significant association products (V2O5)1−3D2O+ are also observed in the experiments. Density functional theory calculations are performed to study the reaction mechanisms of V4O10+ with H2O. The calculated results are in agreement with the experimental observations and indicate that H2O is dissociatively rather than molecularly adsorbed in V4O10H2O+ complex.

1. INTRODUCTION Water is the most abundant compound on earth. Water plays an important role in many catalytic processes because water vapor is usually present in the feed gas and often acts as a product in the reactions. Water vapor can also affect the extent of surface hydroxylation, the ratio of Brønsted/Lewis surface acid sites, or the structure of metal oxide surfaces.1,2 An excellent example is that the presence of water can have an effect on molecular structure of supported vanadium oxide catalysts.3,4 Vanadium oxides are one of the most important transition-metal catalysts used both in industry and in the laboratory.5,6 The well-known processes using vanadium oxide based catalysts include oxidation of SO2 to SO3, partial oxidation of methane to formaldehyde, selective reduction of nitric oxide by ammonia, and so on. Therefore, study on the interaction between vanadium oxides and water is important and may serve as an example for investigating the hydration effects on other metal oxides. Due to the complexity of the “real-life situation” and the concept that chemical bond dissociation and formation on a surface can occur at catalytically active sites,7,8 gas-phase clusters are considered to be ideal models of real surface species or active sites over the condensed-phase surface.9−20 There are many studies on reactions of metal oxide clusters with water,21−35 and various reaction channels have been reported. The Schwarz group21,22 reported the degenerate 16O/18O exchange reactions of FeO+ with H218O. Jarrold and co-workers reported the oxidation of tungsten and molybdenum oxide cluster anions by water that is accompanied with H 2 release.29−33 UO+ may react with H2O to release H2 or H atom, which was found by the Duckworth and Russo groups.26,27 Interestingly, radical containing alkaline-earth metal monoxide cations were found to be able to abstract © 2012 American Chemical Society

one H atom from H2O by Bohme, Schrö der, and coworkers.28,36 MO•+ + H2O → MOH+ + OH• (M = Mg, Ca, Sr, and Ba)

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In contrast to the above-mentioned studies on metal oxide clusters with H2O and extensive studies on the reactivity of vanadium oxide clusters toward hydrocarbons,15,37−49 studies on the reactions of vanadium oxide clusters with water are very limited. The only reported work on this topic was conducted by the Schwarz group,24,25 in which they reported the association and degenerate 16O/18O exchange reactions for some vanadium oxide cluster cations. They also found that the loosely bounded dioxygen ligands in oxygen-rich vanadium oxide clusters may be replaced by H2O. Considering the work of Bohme in which hydrogen atom abstraction (HAA) reactions may happen on mononuclear oxide cations,28 here we focused on the possibility of HAA reactions of water on polynuclear vanadium oxide cluster cations VxOy+ (x = 2−6).

2. METHODS 2.1. Experimental Methods. The vanadium oxide cluster cations are pulsed generated by laser ablation of the vanadium disk in the presence of 1% O2 seeded in a He carrier gas with backing pressure of 300 kPa. A 532 nm (second harmonic of Nb3+:yttrium aluminum garnetYAG) laser with energy of 5− 8 mJ/pulse and repetition rate of 10 Hz is used. The cluster formed in a gas channel (2 mm diameter × 25 mm length) is expanded and reacted with pulsed D2O vapor in a fast flow Received: January 10, 2012 Revised: February 7, 2012 Published: February 8, 2012 2049

dx.doi.org/10.1021/jp300279u | J. Phys. Chem. A 2012, 116, 2049−2054

The Journal of Physical Chemistry A

Article

Figure 1. TOF mass spectra for reactions of V2,4,6Oy+ with no (panel a, for reference) and increasing D2O exposure (panels b and c) in the fast flow reactor. Numbers x, y denote VxOy+ and x,yX denote VxOyX+, in which X = D and D2O, D2OD, and (D2O)2.

reactor (6 mm diameter × 60 mm length). The D2O vapor is supplied with liquid D2O (99.9%) in a glass container. After reacting in the fast flow reactor, the reactant and product ions exiting from the reactor are skimmed into a vacuum system of a time-of-flight mass spectrometer (TOF-MS) for mass (to charge ratio) measurement. Ion signals are generated by a dual microchannel plate detector and recorded with a digital oscilloscope (LeCroy WaveSurfer 62Xs) by averaging 500−1000 traces of independent mass spectra (each corresponds to one laser shot). The mass resolution is about 400−500 (M/ΔM) with the current experimental setup. More details of experiments can be found in our previous work.50−52 2.2. Theoretical Methods. Density functional theory (DFT) calculations using the Gaussian 03 program53 have been employed to study the reaction mechanisms of V4O10+ + H2O. The hybrid B3LYP functional54,55 and the TZVP basis sets56,57 are used. The justification of the adopted functional and basis sets for vanadium oxides and related species can be found in previous studies.24,58−62 Geometry optimizations with full relaxation of all atoms are performed. Vibrational frequency calculations are performed to check that the reaction intermediates and transition state (TS) species have zero and one imaginary frequency, respectively. The calculated energies reported herein are the relative Gibbs free energies (ΔG298K) under standard conditions (298.15 K, 1 atm) and zero-point vibration corrected energies (ΔH0K). Structures and vibrational frequencies for all of the optimized structures are available upon request.

more intense, which suggests that the vanadium oxide cations abstract one D atom from D2O: (V2O5)1 − 3+ + D2 O → (V2O5)1 − 3 D+ + OD

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In addition to the above deuterium atom abstraction (DAA) reactions, association products (V2O5)1−3D2O+ are also observed after the interaction with D2O: (V2O5)1 − 3+ + D2 O → (V2O5)1 − 3 D2 O+

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+

The intensities of (V2O5)1−3D2O are generally higher than those of (V2O5)1−3D+, indicating that the association may be the main reaction channel. The [(V2O5)1−3D2OD]+ products that may be due to the secondary reaction of (V2O5)1−3D2O+ with D2O or (V2O5)1−3D+ with D2O are also generated. Except for (V2O5)1−3+, no DAA but only association reactions can be identified for other VxOy+ (x ≤ 6) clusters, including V3Oy+ and V5Oy+ (not shown in Figure 1). For example, the association products for (V2O5)0−2V2O4+ with one and two D2O are found in the spectra. The rate of reaction for VxOy+ + D2O is very fast. A rough estimation indicates that VxOy+ can pick up D2O under single collision condition. 3.2. Computational Results. We have recently proposed that it is useful to define a value (Δ) for an oxide cluster MxOyq to clarify the oxygen-richness or poorness: Δ  2y − nx + q, in which q is the charge number and n counts the highest oxidation state of element M (for V, n = 5).19,20,63 It is clear that the reactive clusters found in our experiments, (V2O5)1−3+, are all with Δ = +1. The lowest energy structures of (V2O5)1−3+ have been reported, and all of them contain the mononuclear oxygen-centered radical terminally bonded in the clusters. Such oxygen atom is denoted as Ot¯•, which is the active site in reactions toward small molecules such as hydrocarbons.19,37−40,42−44,49,64,65 To gain insight into the details of the reactions of H2O on (V2O5)1−3+ clusters, reaction mechanisms of V4O10+ + H2O are studied by DFT calculations, and the results are shown in Figure 2. In path I, water molecule approaches V4O10+ and is adsorbed on V atom on which the Ot¯• atom is bonded. There is an encounter complex I1 with binding energy of 1.04 eV (ΔH0K).

3. RESULTS AND DISCUSSION 3.1. Experimental Results. The TOF mass spectra for reactions of VxOy+ (x = 2, 4, and 6) with D2O are plotted in Figure 1. As the relative concentration of D2O increases, the relative intensities of V2O5+ (182 amu), V4O10+ (364 amu), and V6O15+ (546 amu) decrease while those of the peaks with two additional mass units (i.e., 184, 366, and 548 amu) become 2050



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Figure 2. DFT-calculated potential energy profiles (ΔG298K) for reactions of V4O10+ and H2O. The energies with respect to V4O10+ + H2O are given as ΔG298K/ΔH0K in electronvolts. The structures of the reaction intermediates and transition states are given. Some critical bond lengths are given in picometers. Mulliken spin density values are given in parentheses (in μB) for O atoms.

lower than that of the reactants although the relative barriers with respect to I4 are quite large (0.70 and 0.89 eV, respectively). So both the transfers of H1 atom to O1 (a bridgingly bonded oxygen atom, denoted as Ob) and to O2 (an Ot atom) are kinetically favorable in gas phase to form the products P2 and P3 with two OH moieties (i.e., V4O9(OH)2+). On the other hand, both the transfers of H1 to O3 and H2 to O4 are kinetically forbidden (the paths are not shown in Figure 2) because the transition states in these processes are higher in energy than the reactants (ΔG298K = 0.18 and 0.45 eV, respectively), although the products (IS5 and IS2 in Figure 3), in which two OH moieties bonded on the same V atom, are quite stable. In P2 and P3, the Mulliken spin densities are located in two Ot atoms (about 0.9 and 0.5 μB), which is similar to the spin density distribution of V2O5+.63,65 Thus, P2 or P3 may react with another H2O molecule following the similar mechanisms of HAA reaction on V2O5+ and V4O10+, which rationalizes the observation of [V4O10D2OD]+ in the experiments can be due to the reaction of V4O9(OD)2+ + D2O → V4O9(OD)2D+ + OD, in addition to the other possible reaction V4O10D+ + D2O → (V4O10D)D2O+. According to the energy profile of ΔG298K for reactions of V4O10+ and H2O, the rate-limiting steps are determined to be the transfer of H atom from the H2O moiety to the other O atom. The free energies of the transition states (TS1, TS4, and TS5) are close to each other and are all below that of the reactants, which is consistent with the experiments that both of the signals of DAA and association products are found. Note that the ΔH0K value of P1 is much higher than those of TS1, TS4, and TS5, while the free energy of P1 is close to those of TS1, TS4, and TS5. The observation of both reactions 2 and 3 for V4O10+ system indicates that entropy (S) contributions (−ΔS × T, in which T is the temperature) to free energy (ΔG = ΔH −ΔS × T) should be taken into account; otherwise, reaction 2 may not be able to compete with reaction 3.

After this step, a hydrogen atom is transferred from water molecule to the Ot¯• atom to form the intermediate I2 by surmounting a small free energy barrier of 0.24 eV. In I2 the OH moiety is bonded to the H atom and can be released to form the products V4O10H+ and OH (P1, ΔG298K/ΔH0K = −0.40/−0.43 eV). Thus, the HAA reaction of H2O on V4O10+ is favorable both thermodynamically and kinetically, which is consistent with the observation of V4O10D+ in the experiment (reaction 2). To the best of our knowledge, this is among the first to report HAA reaction for H2O on polynuclear oxide clusters, which may provide new insights into related surface reactions under moist conditions. Similarly, we calculated the reaction pathway of V2O5+ + H2O → V2O5H+ + OH and found that the reaction is also exothermic (ΔG298K/ΔH0K = −0.25/− 0.29 eV) with a surmountable barrier (ΔG298K/ΔH0K = −0.14/ −0.51 eV for the transition state). Association products (V2O5)1−3D2O+ are also found in the experiments (reaction 3). The mechanisms of water association reactions on V4O10+ are studied, and the results are shown as path II in Figure 2. H2O molecule can be adsorbed directly onto a V atom on which a terminal-oxygen (Ot) is doubly bonded (i.e., H2O is adsorbed on the VOt2− site, while in I1 of path I the H2O molecule is adsorbed on the V−Ot¯• site), and an encounter complex I3 is formed. Note that I1 and I3 have very close energies, and they can be transferred to each other easily with a transition state TS2 (ΔG298K = −0.38 eV). I3 can be transformed to I4 by surmounting a tiny barrier of 0.03 eV (TS3). In this process (I3 → TS3 → I4), the H2O approaches closer to the V atom, and the cage structure of V4O10+ is broken. Then one hydrogen atom in the H2O moiety of I4 may be transferred to the nearby oxygen atoms to form structures with two OH moieties. When the hydrogen atom H1 (see Figure 2 for the labels) is transferred to O1 (I4 → TS4 → P2) or to O2 (I4 → TS5 → P3), the free energies of the transition states (−0.50 eV for TS4 and −0.31 eV for TS5) are 2051



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Figure 3. DFT-calculated structural isomers of [V4O10H2O]+ cluster. The point group, electronic state, and energy relative to V4O10+ + H2O (ΔH0K in eV) are given under each isomer. Some critical bond lengths are given in picometers. Mulliken spin density values are given in parentheses (in μB) for O atoms unless specified for V.

as V3). H2O can be molecularly adsorbed on the V3 site as in IS11 and I4. Alternatively, H2O is dissociated into H and OH, and OH is bonded on the V3 atom and H is bonded on an O atom to form the second OH moiety, as in IS1−IS10, P2, and P3. Structures of the cage-broken type are similar to the ground state structure of V4O11− 65−67 with two additional H+ ions.

In Figure 3 we present possible isomeric structures (IS1− IS24) of [V4O10H2O]+ that are not included in paths I and II with energies (ΔH0K) less than −0.50 eV with respect to V4O10+ + H2O. Structures can be classified into two types. In the first type of structure, the cage structure of V4O10 moiety is broken by the breaking of one V−Ob bond (this V is denoted 2052



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The energies of [V4O10H2O]+ isomers with cage-broken type structures are all lower than that of separate V4O10+ + H2O by more than 1.4 eV (ΔH0K). Note that P3 is the most stable isomer of [V4O10H2O]+ shown in Figures 2 and 3. Another type of structure in which the V4O10 moiety maintains the cage structure is also considered. The H2O moiety can be adsorbed on a V site without splitting as in I1 and I3 (Figure 2). The H2O can also split into H and OH, and then H is adsorbed on an O atom while OH can be adsorbed on V (as in IS12, IS13, IS15−IS17, IS19−IS22), O (IS14, IS19, IS21, IS23, and IS24), or H (IS18 and I2). The cage-maintained structures (I1−I3 and IS12−IS24) are all less stable than the cage-broken ones (I4, P2, P3, and IS1−IS11), indicating that the adsorption of H2O tends to break the cage structure of V4O10+. Moreover, H2O can be dissociatively rather than molecularly adsorbed on the cluster. These findings may provide some hints on the modification of water on molecular structures of vanadium oxide surfaces. 3.3. Comparing the Reactions of V4O10+ + H2O and V4O10+ + CH4. Water and methane are isoelectronic species, and both of them are important molecules in various reactions. The reaction of V 4 O 10 + + CH 4 has been reported previously:40,68

4. CONCLUSIONS Reactions of vanadium oxide cluster cations with D2O in a fast flow reactor have been studied. Experimental mass spectra suggest that (V2O5)1−3+ clusters are reactive toward water resulting in (V2O5)1−3D+ and (V2O5)1−3D2O+ through the deuterium atom abstraction and adsorption reactions. DFT calculations on the V4O10+/H2O system confirm that both of the two reaction channels are favorable thermodynamically and kinetically. The hydrogen atom abstraction may take place at the terminal oxygen-centered radical site in V4O10+, and H2O is dissociatively rather than molecularly adsorbed in V4O10H2O+ complex. The new reaction channels of H2O on polynuclear oxide clusters found in this work may provide new insights into reactions on vanadia and other related metal oxide surfaces under moist conditions.

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Corresponding Author

*E-mail [email protected] (S.-G.H.), [email protected] (X.-L.D.); phone +86-10-62536990; fax +86-10-62559373. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Chinese Academy of Sciences (Knowledge Innovation Program No. KJCX2-EW-01, Hundred Talents Fund), the National Natural Science Foundation of China (Nos. 20933008 and 21173233), and the 973 Program (No. 2011CB932302).

V4O10+ + CH 4 → V4O10H+ + CH3(ΔG298K /ΔH0K = − 0.92/− 0.91eV)

AUTHOR INFORMATION

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The reaction of V4O10+ + CH4 proceeds by direct HAA without encounter complex like [V4O10...CH4]+, and no transition state can be located. In contrast, for the V4O10+/H2O system a stable [V4O10...H2O]+ complex (I1 in Figure 2) exists, and a small barrier (TS1) is involved in the HAA reaction. When H2O is far from V4O10+ where the electrostatic interaction is dominant, the interaction between polar molecule H2O and charged ion V4O10+ is larger than that for nonpolar CH4. In addition, the bond energy of O−H in H2O is 4.88 eV (by DFT) which is larger than that of C−H in CH4 (4.41 eV by DFT), indicating that it is more difficult to abstract one H atom from H2O than from CH4. As a result, H2O can form an encounter complex (I1) with binding energy as large as 1.04 eV. Note that the HAA reaction of H2O on the mononuclear oxide cluster CaO+ also has an encounter complex and a transition state.28 Another difference between H2O and CH4 systems is that the intense signal of [V4O10D2O]+ is observed, while no association product [V4O10CH4]+ is observed under similar experimental conditions.68 In the V4O10+/CH4 system, the newly formed methyl group is very loosely coordinated to the hydrogen atom in V4O10H+ (C−H distance: 182 pm), and the nonpolar molecule CH3 is easy to be desorbed. In the V4O10+/ H2O system, the DFT-calculated results (Figures 2 and 3) indicate that H2O can be chemisorbed over V4O10+ cluster (dissociative adsorption). H2O moiety may be bonded to V atom directly (I3 in Figure 2), and the strong V−OH2 interaction can even cause the break of the V4O10 cage (I4). In contrast, CH4 cannot make a strong bond with V4O10+. After the splitting of H2O or CH4, the OH moiety can make a V−OH bond (such as in P2 and P3) which is much stronger than the possible V−CH3 bond. Thus, the strong V−O interaction both in V−(H2O) and in V−(OH) may lead to big differences in the reactions of vanadium oxide clusters with H2O compared with the reactions with hydrocarbon molecules.

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