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Oxygen Release from Cationic Niobium−Vanadium Oxide Clusters, NbnVmOk+, Revealed by Gas Phase Thermal Desorption Spectrometry and Density Functional Theory Calculations Daigo Masuzaki, Toshiaki Nagata, and Fumitaka Mafuné* Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Thermal dissociation of the cationic niobium− vanadium oxide clusters, NbnVmOk+ (n + m = 2−8), was investigated by gas phase thermal desorption spectrometry. The oxygen-rich NbnVmOk+ released O and O2 for odd and even values of n + m, respectively. Substitution of more than one Nb atom in NbnOk+ by V drastically lowered the desorption temperature of O2 for even values of n + m, whereas the substitution of more than two Nb atoms opened a new desorption path involving the release of O2 for odd values of n + m. The substitution effects can be explained by the fact that Nb atoms display the +5 state, whereas V atoms can exist in either the +4 or +5 states. The geometrical structures of selected NbnVmOk+ clusters were optimized and the energetics of the release of O/O2 from the clusters was discussed on the basis of the results of DFT calculations.

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INTRODUCTION A number of chemical reactions proceed with the aid of catalysts comprising transition metals, their oxides, carbides, nitrides, and sulfides.1−3 For catalysts of group V elements, vanadium oxide is known to oxidize SO2 industrially.4 In addition, metal oxides mixed with niobium and tantalum oxides are used to enhance their catalytic activity and selectivity and to prolong catalyst life.5−7 Gas phase oxide clusters of Nb and V, NbnOk+ and VnOk+, exhibit size-dependent properties with even/odd values of n; clusters with even values of n exhibit a similar nature, whereas those with odd values of n exhibit a different set of properties.8−18 Indeed, Castleman and co-workers studied the reactivity of cationic vanadium oxide clusters with ethylene, finding that ethylene was oxidized by V2O5+ and V4O10+, whereas it was simply adsorbed on V3O7+.8−10 He and coworkers investigated the reactivity of neutral vanadium oxide clusters with alkenes and alkynes, revealing that only VO3(V2O5)n can cleave the CC bond; VO3(V2O5)n reacted with C 2 H 4 , C 3 H 6 , C 4 H 8 , and C 4 H 6 and produced VO2(V2O5)nCH2, VO2(V2O5)nC2H4, VO2(V2O5)nC3H6, and VO2(V2O5)nC3H4, respectively.11 Fielicke and co-workers investigated cationic niobium oxide clusters by IR multiple photon dissociation spectroscopy, and reported that Nb2O6+ dissociated into Nb2O4+ and O2, whereas Nb3O8+ released the O atom according to Nb3O8+ → Nb3O7+ + O.12 In our previous studies, cationic niobium oxide clusters [NbnOk+ (n = 2−10)] and vanadium oxide clusters [VnOk+ (n = 2−10)] were synthesized in the gas phase.19,20 The thermal desorption of oxygen from the near-stoichiometric (n:k = 2:5) © XXXX American Chemical Society

clusters was investigated using mass spectrometry in combination with gas phase thermal desorption spectrometry (TDS).19−22 For niobium oxide clusters and vanadium oxide clusters with even values of n, MnO(5/2)n+1+ (M = Nb, V; n = 2, 4, 6, ...), an O2 molecule was released upon heating. The vanadium oxide clusters released O2 readily at a lower temperature than the niobium oxide clusters, implying lower desorption energies of O2 from the clusters. For niobium oxide clusters with odd n (n = 3, 5, ...), release of an O atom was observed with the formation of fully stoichiometric clusters, whereas vanadium oxide clusters with a similar composition released an O2 molecule to become more oxygen-deficient clusters. These experimental results can be explained by the tendency of Nb atoms to favor the +5 oxidation state, whereas V atoms can be in either the +4 or +5 states.20 This raised questions regarding the release of oxygen from mixed clusters of vanadium and niobium oxides as to whether it is released as O atoms or molecules. In this context, it is also interesting to clarify if the oxidation state will continue to determine patterns of thermal dissociation. Hence, in this study, we prepared mixed oxide clusters of niobium and vanadium, NbnVmOk+, in the gas phase. The oxygen release processes were observed using TDS and the mixing effects were investigated at an atomic level. The geometrical structures and energetics were obtained from DFT calculations. Received: February 28, 2017 Revised: April 26, 2017 Published: May 2, 2017 A

DOI: 10.1021/acs.jpca.7b01961 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

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EXPERIMENTAL SECTION The thermal dissociation of the niobium−vanadium oxide clusters (NbnVmOk+) was studied by mass spectrometry in combination with gas phase thermal desorption spectrometry (Figure S1).19−22 The gas phase NbnVmOk+ clusters (n + m = 2−8) were synthesized by pulsed laser ablation inside a cluster source. Here, Nb and V metal rods were vaporized by the focused third and second harmonics of separate Nd:YAG pulsed lasers at 355 and 532 nm, respectively. The cluster ions were generated in flowing He gas that contained O2, from a pulsed valve at a stagnation pressure of 0.7 MPa. The concentration of O2 in the gas flow was tuned to 0.02−12%, using mass flow and pressure controllers. By modulating the pulse energies of the lasers, the V-rich and Nb-rich clusters were selectively formed at 310 K (Figure S2). The NbnVmOk+ clusters, formed in the cluster source, passed through a copper extension tube before expansion into a vacuum. The tube (length 120 mm, inner diameter 4 mm) was heated from 300 to 1000 K using a resistive heater, and the temperature was monitored using a thermocouple installed inside it. The residence time of the cluster ions and the density of the He gas in the tube were estimated to be ∼100 μs and ∼1018 molecules cm−3, respectively. Hence, the clusters were considered to achieve thermal equilibrium with the tube, by collisions with the He carrier gas. Indeed, the mass spectrum observed at 1000 K was totally different from that recorded at 310 K (Figure S2); the intensities of oxygen-rich clusters decreased whereas those of other oxygen-deficient clusters increased. TDS plots were obtained by measuring the intensities of the cluster ions using mass spectrometry as a function of temperature. The temperature of the tube was elevated or lowered carefully, at a rate of 7 K min−1. After expansion into the vacuum, the cluster ions were accelerated by a pulsed electric field in a direction perpendicular to the cluster beam, to gain a kinetic energy of ∼3.5 keV, for mass analysis by time-of-flight mass spectrometry. The ions were reversed at the reflectron after passing through a 1 m fieldfree flight tube and detected using a Hamamatsu doublemicrochannel plate detector. The signals were amplified by a 350 MHz preamplifier (Stanford Research Systems, SR445A) and digitized using an oscilloscope (LeCroy LT344L). The mass resolution was high enough (∼1000 at m/z = 1000) to unambiguously assign niobium−vanadium oxide clusters in the mass spectra.

calculated by Asmis et al. from infrared multiple photon dissociation spectroscopy were also referenced.12,29,31,32 The most stable geometries had singlet spin multiplicity. The vibrational frequencies were calculated for all obtained structures to confirm that they corresponded to the energy minima. Zero-point vibration-corrected energies were used to estimate the binding energies of the O2 molecule and the O atom. Natural bond orbital (NBO) analyses were also performed to calculate the natural charge distribution.33,34 In the NBO analyses, the spin was determined by the difference of charge between the α and β spins.

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RESULTS AND DISCUSSION O Atom Release from NbnVmOk+ with Odd Values of n + m. Figure 1 shows the TDS profiles of NbnVmOk+ with n + m

Figure 1. Gas phase TDS profiles of NbnVmOk+ (n + m = 3, 5, and 7) in the temperature range 300−1000 K.

= 3, 5, and 7. Above 850 K, it is seen that the intensity of Nb5O13+ decreases whereas the intensity of Nb5O12+ increases (Figure 1(b0)). The concomitant change of the intensities evidently indicates the release of an O atom from Nb5O13+ upon heating, to form Nb5O12+. Similar changes were observed for Nb4VO12+ and Nb4VO13+ (Figure 1(b1)). Generally, the release of an O atom is energetically unfavorable because a chemical bond is ruptured without the formation of a new bond. Nevertheless, this process occurs because the product species, Nb5O12+ and Nb4VO12+, are quite stable. The oxidation states of Nb and O are +5 and −2, respectively; hence the total charge on the Nb5O12+ cluster is (+5) × 5 + (−2) × 12 = +1. Thus, the cluster is regarded as fully stoichiometric in terms of the oxidation state. In contrast, above 800 K, the intensity of Nb3V2O11+ increases whereas that of Nb3V2O13+ decreases (Figure 1(b2)), suggesting that one O2 molecule is released from Nb3V2O13+. Our previous study revealed that the vanadium oxide clusters, V3O8+, V5O13+, and V7O18+, released an O2 molecule and not an O atom, when heated to 800 K. Hence, the observed O2 release is characteristic to vanadium oxide

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COMPUTATIONAL SECTION DFT calculations were carried out to determine the stable structures of NbnVmOk+ clusters (n + m = 2, 3, and 4) using the Gaussian09 program, and to investigate the binding energies of the clusters.23 Becke’s three-parameter hybrid density functional with the Lee−Yang−Parr correlation functional (B3LYP) was used for all the calculations.24,25 The Stuttgart/Dresden effective core potential (SDD) was used for the Nb and V atoms, where 10 electrons of V and 28 electrons of Nb were treated as the core, whereas for O the 6-311+G(d) basis set was used.26,27 For n + m = 2 clusters, previously reported structures of Nb2O5+ were referenced.10,14,28−30 The initial geometries of the clusters of interest were set by adding one O atom to and removing one O atom from possible sites. The most stable geometries had doublet spin multiplicity. For n + m = 3 clusters, the structures of Nb3O7+ and V3O7+ elucidated by Misaizu et al. from their ion mobility measurements and those B


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The Journal of Physical Chemistry A clusters and could be due to the increase in the ratio of V atoms in the cluster. However, a close examination of the TDS plots shows that the substitution of two Nb atoms by V changes the reaction scheme for n + m = 3, 5, and 7. Indeed, the branching ratio of O2 release to the release of O and O2 (Figure 2), which

Figure 2. Branching ratios of the release of O2 to the release of O and O2, from NbnVmOk+ with n + m = 3, 5, and 7. In all cases, the ratio abruptly increases at m = 2.

Figure 3. Gas phase TDS profiles of NbnVmOk+ (n + m = 2, 4, 6, and 8) in the temperature range 300−1000 K.

extent. In addition, the intensities of Nb4V2O16+ and Nb5VO16+ appear to decrease in two steps, the first drop is in the range 300−600 K and the second drop is in the range 700−1000 K (Figure S3). Most likely, the two steps originate from two structural isomers, which is discussed later. Figure 4 shows the

is almost zero at m = 0 and 1, abruptly increases at m = 2. It is known that V atoms can display the oxidation state of +4, in addition to the +5 state. If the three Nb atoms are in the +5 and the two V atoms in the +4 oxidation states, in Nb3V2O11+, this cluster is fully stoichiometric as (+5) × 3 + (+4) × 2 + (−2) × 11 = +1. The substitution by two V atoms is important because it reduces the net charge by +2, which is equivalent to the charge of one O atom. If V atoms could attain the +3 oxidation state, substitution by one V atom would be sufficient. Hence, the experimental findings suggest that V atoms do not exist in the +3 oxidation state. O2 Molecule Release from NbnVmOk+ with Even Values of n + m. Figure 3 shows the TDS profiles of NbnVmOk+ with even values of n + m ((a0)−(a2), n + m = 2; (b0)−(b2), n + m = 4; (c0)−(c2), n + m = 6; (d0)−(d2), n + m = 8). Above 800 K, it is seen that the intensity of Nb6O17+ decreases whereas the intensity of Nb6O15+ increases (Figure 3(c0)). In addition, the intensity of Nb6 O16+ decreases above 900 K, with a corresponding increase in the intensity of Nb6O14+ (Figure 3(c0)). These changes indicate the release of an O2 molecule. The product species at 1000 K, Nb6O14+ and Nb6O15+, are considered to be “near” stoichiometric, according to the discussion above; in Nb6O14+, as six Nb atoms have an oxidation state of +5, the total charge would be (+5) × 6 + (−2) × 14 = +2. If five Nb atoms have an oxidation state of +5, and one Nb atom has +4, the total charge would be (+5) × 5 + (+4) × 1 + (−2) × 14 = +1. However, the +4 oxidation state is not favorable for Nb atoms, and hence the O2 is released at high temperatures. Drastic changes were observed when one Nb atom is replaced by V. As shown in Figure 3(c1), the release of an O2 molecule from Nb5VO16+ occurs above 400 K. The decrease in temperature upon the substitution of one Nb atom was observed for n + m = 2, 4, 6, and 8. In addition, the TDS plot for Nb4V2Ok+ (Figure 3(c2)) resembles that of Nb5VOk+ (Figure 3(c1)), suggesting that increasing the number of V atoms would not reduce the reaction temperature to a great

Figure 4. Temperature at which the intensity of NbnVmO(5/2)(n+m)+1+ equals to the intensity of NbnVmO(5/2)(n+m)−1+ (n + m = 2, 4, 6, and 8). The temperature reduces drastically at m = 1. This temperature was not seen for m = 0 in the range 300−1000 K (up arrow).

temperature at which half of the main isomer, NbnVmO(5/2)(n+m)+1+ (the contribution of the minor isomer was neglected) was converted into NbnVmO(5/2)(n+m)−1+. The temperature decreases for m = 1 and levels off for higher values of m. This indicates that the substitution of one Nb atom by V is quite intrinsic. By this substitution, the product species, Nb5VO14+, composed of five Nb atoms in the +5 and one V atom with +4 oxidation states, is regarded as fully stoichiometric [(+5) × 5 + (+4) × 1 + (−2) × 14 = +1]. Thus, the release of oxygen from niobium−vanadium oxide C


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The Journal of Physical Chemistry A clusters can be explained by the combination of the oxidation states of the atoms. Oxygen Release from V-Rich Clusters. Figure 5b shows the TDS plots of NbnVmOk+ for n + m = 6, with a larger number

Figure 6. Structures of NbnVmO4+ and NbnVmO6+ with n + m = 2, obtained by geometrical optimization from DFT calculations.

Figure 7. Structures of NbnVmO9+ and NbnVmO11+ with n + m = 4, obtained by geometrical optimization from DFT calculations.

same direction. For NbnVmO6+ (n + m = 2), O2 is attached to one of the Nb/V atoms of NbnVmO4+. In the case of NbVO6+, the O−O bond length and the Wiberg bond index were 1.31 Å and 1.25, respectively, and the spin of each O atom was about 0.5. Thus, O2 was attached to the cluster as a superoxide, O2−, considering that the O−O bond length and Wiberg bond index of isolated O2− (or isolated O2) in the gas phase were calculated to be 1.35 Å and 1.30 (or 1.21 Å and 1.51), respectively. The formation of the superoxide is consistent with the discussion on stable stoichiometry using the oxidation states. For NbnVmO6+ (n + m = 2), the four O atoms and one superoxide in the cluster have oxidation states of −2 and −1, respectively, which suggests that the oxidation states of the Nb/ V atoms should be expressed as (+5)(+5). The binding energies (ΔE) of O2 with NbnVmO4+ (n + m = 2) relating to the O2 desorption,

Figure 5. Gas phase TDS profiles of NbnVmOk+ (n + m = 5, 6; n ≤ m) in the temperature range 300−1000 K.

of V atoms; the intensities of Nb3V3O16+ decreased and Nb3V3O14+ increased, with an increase in the temperature. The temperature does not change significantly with the ratio of V atoms (Figure 4). As discussed above, the release of an O2 molecule requires only one reduction site, namely one V atom, and hence, other Nb/V atoms do not interfere in this process. Figure 5a shows the TDS plots of NbnVmOk+ for n + m = 5, with a larger number of V atoms. The increase in the intensities of Nb2V3O11+ and Nb2V3O12+ is seen with an increase in the temperature. As the number of V atoms increases in the cluster, the contribution of NbnVmO11+ becomes dominant, and formation of NbnVmO12+ becomes recessive (Figure 1(b0)− (b2), Figure 2), indicating that in V-rich clusters, the release of an O2 molecule takes place more readily than that of an O atom. In spite of the gradual change in the branching ratio, the temperature of release of the O atoms and O2 molecules does not change with an increase in the number of the V atoms. It is likely that the number of two-combinations from V atoms increases with the number of the V atoms in the cluster, enhancing the probability of O2 molecule release. The smooth changes in the branching ratio and the temperature for odd and even numbers of n + m suggest that the vanadium-rich clusters gradually approach pure vanadium oxide clusters in terms of their thermal dissociation behavior. Structure and Energetics of NbnVmOk+ (n + m = 2 and 4). Geometrical structures of NbnVmO4+ and NbnVmO6+ for n + m = 2, and those of NbnVmO9+ and NbnVmO11+ for n + m = 4, were obtained from the DFT calculations (Figures 6 and 7). For NbnVmO4+ (n + m = 2), two O atoms form a bridge between two Nb/V atoms and two other O atoms occupy the terminal sites. The geometrical isomer of NbVO4+, the formation energy of which is +0.17 eV higher than the most stable NbVO4+, has two terminal O atoms protruding in the

NbnVmO6+ → NbnVmO4 + + O2

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were calculated from the difference of the formation energies of NbnVmO6+, NbnVmO4+, and O2 (Table 1). Evidently, substitution of one Nb atom by V in Nb2O6+ resulted in a drastic reduction of the binding energy. For NbnVmO9+ (n + m = 4), there are six bridging O atoms between four Nb/V atoms and three other O atoms in the terminal sites, leaving one open site. The most stable geometries show that a V atom in NbnVmO9+ tends to be unoccupied, providing the open terminal site, because V atoms can attain the lower +4 oxidation state. The O2 can attach itself to the open site of the Nb/V atoms of NbnVmO9+ to form Nb n V m O 11 + . However, the most stable geometries of NbnVmO11+ are those with the O2 adsorbed on the Nb atom and are more stable by ∼0.5 eV than geometries with O2 adsorbed on the V atom. The binding energy of O2 with Nb2V2O9+ obtained by the difference of the formation energies of the most stable Nb2V2O9+, O2, and Nb2V2O11+ is 1.23 eV. In practice, the release of O2 from the most stable geometry forming the most stable Nb2V2O9+ requires complicated processes. One O atom in the terminal site is removed along with one O atom in O2 that is adsorbed on the Nb atom, or one D


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Table 1 also shows the binding energies of NbnVmO9+ (n + m = 4) with O2, assuming the low-energy isomer of NbnVmO11+. The binding energy is highest for Nb4O9+, and drastically reduces when one Nb atom is substituted by V. Further substitution of Nb by V does not change the binding energy significantly. The dependence of binding energy on the stoichiometry is consistent with the results of the desorption temperature analyses (Figure 4). Structure and Energetics of NbnVmOk+ (n + m = 3). The geometrical structures of the stoichiometric clusters, Nb3O7+ and V3O7+, have been intensively discussed and unambiguously determined.29−32 Using the connectivity similar to that of the initial structure, the geometrical structures of NbnVmO6,7,8+ for (n + m = 3) were obtained by DFT calculations (Figure 9). In

Table 1. Binding Energy of O2 Attached to Nb or V of NbnVmO(5/2)(n+m)+1+ (n + m = 2, 4), as Obtained from DFT Calculations n+m

Nb or Va

reaction

binding energy/ eV

2

Nb Nb V V Nb Nb V Nb V Nb V V

Nb2O6+ → Nb2O4+ + O2 NbVO6+ → NbVO4+ + O2 NbVO6+ → NbVO4+ + O2 V2O6+ → V2O4+ + O2 Nb4O11+ → Nb4O9+ + O2 Nb3VO11+ → Nb3VO9+ + O2 Nb3VO11+ → Nb3VO9+ + O2 Nb2V2O11+ → Nb2V2O9+ + O2 Nb2V2O11+ → Nb2V2O9+ + O2 NbV3O11+ → NbV3O9+ + O2 NbV3O11+ → NbV3O9+ + O2 V4O11+ → V4O9+ + O2

+2.12 +1.11 +1.03 +0.98 +1.89 +1.23 +0.74 +1.23 +0.73 +1.22 +0.74 +0.74

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a There are isomers in NbnVO(5/2)(n+m)+1+ that have an O2 unit on a Nb atom or on a V atom (Figure 7). The notation “Nb” shows the cluster with the O2 unit on Nb, whereas “V” shows the cluster with the O2 unit on V.

O atom in O2 adsorbed on the Nb atom moves to the V atom, followed by the release of O2 from the V atom. In either case, the desorption energy would be >1.23 eV, which is higher than the binding energy of O2 with V4O11+. In contrast, if a highenergy isomer (+0.5 eV) with O2 adsorbed on the V atom is exclusively formed, the binding energy would be 0.73 eV (Figure 8). As the simple release of O2 may not require a high-

Figure 9. Structures of NbnVmO6+, NbnVmO7+, and NbnVmO8+ with n + m = 3 obtained by geometrical optimization from DFT calculations.

the same way as n + m = 2, and 4, the binding energies of O2 (or O) with NbnVmO6+ (or NbnVmO7+) were determined (Table 2). In Nb3O6+, three O atoms form a bridge between Table 2. Binding Energy of O or O2 with NbnVmO((5/2)(n+m)−1/2)+1+ (n + m = 3), as Obtained from DFT Calculations

Figure 8. Reaction scheme of Nb2V2O11+ → Nb2V2O9+ + O2. Figure 4 and Table 1 imply that the reaction path shown by the dotted line is dominant.

energy barrier, the desorption energy is considered comparable to the binding energy, which is consistent with the observed value. The formation of the high-energy isomer can be explained by supposing that the most stable Nb2V2O9+ is first formed, and then Nb2V2O11+ is produced by the direct attachment of O2. Indeed, TDS curves in Figure 3 suggest that Nb2V2O11+ comprises two isomers, the one exhibiting O2 desorption at lower temperature and the other exhibiting O2 desorption at high temperature. The latter isomer can be the most stable structure of Nb2V2O11+, and the desorption would be associated with the reaction from the most stable Nb2V2O11+ to the most stable Nb2V2O9+ (Figure 8). The inference is supported by the experimental results that the ratio of the most stable isomer with O2 attached to Nb atom became higher when the ratio of Nb atoms in NbnVmOk+ increased (Figure S4).

n+m

reaction

binding energy/eV

3

Nb3O8+ → Nb3O7+ + O Nb3O8+ → Nb3O6+ + O2 Nb2VO8+ → Nb2VO7+ + O Nb2VO8+ → Nb2VO6+ + O2 NbV2O8+ → NbV2O7+ + O NbV2O8+ → NbV2O6+ + O2 V3O8+ → V3O7+ + O V3O8+ → V3O6+ + O2

+2.21 +2.79 +1.94 +2.79 +2.29 +1.53 +2.03 +1.11

the totally equivalent Nb atoms and the other three O atoms are present at the terminal sites. In contrast, the structure of V3O6+ reveals four bridging O atoms between the V atoms, while two O atoms are present at the terminal sites. Regarding the oxidation states, two Nb/V atoms ought to exhibit the +4 state, while the third atom is in the +5 state, in NbnVmO6+ (n + m = 3). Hence, the geometrical structure of V3O6+ can be explained considering that the central V atom (coordinated by four O atoms) is in the +5 state, whereas the V atoms on the side (coordinated by three O atoms,) are in the +4 state. This is E


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The Journal of Physical Chemistry A also the case for NbV2O6+, where the central Nb atom coordinated by four O atoms has an oxidation state of +5, and the other V atoms are in the +4 state. In Nb2VO6+, the V atom coordinated by three O atoms has an oxidation state of +3, while the other two Nb atoms coordinated by the four O atoms exhibit the +5 state. Indeed, DFT calculations suggest that, for NbV2O6+, the bond length of all the bridging V−O bonds between the V and Nb atoms is 1.88 Å, and that of the two V− O bonds in the terminal sites is 1.55 Å. Each V atom in NbV2O6+ (triplet) has 1.10 spins. In contrast, Nb2VO6+ exhibits two bridging V−O bonds (1.80 Å) and one V−O bond between the V atom and O atom located in the center (1.96 Å). The V atom in Nb2VO6+ has 1.99 spins. For comparison, Nb2VO7+ (which is fully stoichiometric and a singlet) has two bridging V−O bonds (1.78 Å), a hollow V−O bond (1.91 Å), and a terminal V−O bond (1.55 Å). For NbnVmO8+ (n + m = 3), the most stable geometries have O2 adsorbed onto the Nb atom, like in NbnVmO9+ for n + m = 4. The structural isomer of Nb2VO8+ with O2 adsorbed on the V atom is less stable, and the formation energy is +0.27 eV higher than the most stable form. The binding energy of an O atom with Nb2VO7+ was calculated to be Nb2 VO8+ → Nb2 VO7+ + O

ΔE = +2.31 eV

states: Nb atoms exist in the +5 state, whereas V atoms can have either the +4 or +5 oxidation states. For odd values of n + m, NbnVmO(5/2)(n+m)−1/2+ is fully stoichiometric and stable with both Nb and V atoms in the +5 oxidation state, whereas NbnVmO(5/2)(n+m)−3/2+ becomes fully stoichiometric, when two V atoms are in the +4 oxidation state. For even values of n + m, NbnVmO(5/2)(n+m)−1+ becomes stoichiometric if one of the Nb/V atoms is in the +4 oxidation state. Substitution of more than one Nb atom by V satisfies the condition. The geometrical structures of selected NbnVmOk+ clusters were optimized and the energetics of the release of O/O2 from the clusters was discussed on the basis of the results of DFT calculations. The binding energies of the O2 molecule to the clusters were found to decrease upon the substitution of the Nb atoms by the V.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b01961. Schematic diagram of experimental apparatus; mass spectra of clusters; plot of temperature vs number of V atoms; plot of isomer ratios; complete author list for ref 23 (PDF)

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whereas the binding energy of an O2 molecule with Nb2VO6+ was found to be slightly lower: Nb2 VO8+ → Nb2 VO6+ + O2

ΔE = +1.94 eV

ΔE = +2.29 eV

*F. Mafuné. E-mail: [email protected].

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ORCID

Fumitaka Mafuné: 0000-0001-8860-6354 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Exploratory Research (No. 26620002) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT). The computations were partially performed using Research Center for Computational Science, Okazaki, Japan. The authors thank Dr. Ryuzo Nakanishi, Dr. Satoshi Takahashi, and Dr. Ken Miyajima for helpful discussion.

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whereas the binding energy of an O2 molecule with NbV2O6+ was NbV2O8+ → NbV2O6+ + O2

ΔE = +1.53 eV

AUTHOR INFORMATION

Corresponding Author

Nevertheless, only O atoms were released from Nb2VO8+, suggesting that the actual desorption energy is higher than the binding energy, as the release of O2 requires a significant deformation of the cluster. In contrast, the binding energy of an O atom with NbV2O7+ was NbV2O8+ → NbV2O7+ + O

ASSOCIATED CONTENT

S Supporting Information *

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Thus, although the binding energies with an O atom are comparable for Nb2VO8+ and NbV2O8+, the binding energy of the O2 molecule drastically decreases when two Nb atoms are replaced by V, which is consistent with our experimental observations.

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CONCLUSION The thermal stability of the cationic niobium−vanadium oxide clusters, NbnVmOk+ (n + m = 2−8), was investigated by gas phase thermal desorption spectrometry to understand the effects of mixing the two group V elements. It was found that one oxygen atom was released from NbnVmO(5/2)(n+m)+1/2+ to form NbnVmO(5/2)(n+m)−1/2+ for odd values of n + m. The oxygen atom release is prominent in the pure niobium oxide clusters and substitution of more than two Nb atoms by V opened a new desorption path for the release of O2 by the formation of NbnVmO(5/2)(n+m)−3/2+. In contrast, one oxygen molecule was released from NbnVmO(5/2)(n+m)+1+ to form NbnVmO(5/2)(n+m)−1+ for even values of n + m. In comparison with the pure niobium oxide clusters, substitution of more than one Nb atom by V drastically lowered the desorption energy of O2. These effects can be explained in terms of the oxidation F


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