Desorption of Oxygen from Cationic Niobium Oxide Clusters Revealed

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Desorption of Oxygen from Cationic Niobium Oxide Clusters 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 cationic niobium oxide clusters (NbnOm+) was investigated by gas phase thermal desorption spectrometry. The dominant species formed at 300 K were NbnO(5/2)n+p+ (n = 2, 4, 6, ...; p = 0, 1, 2, ...) and NbnO((5/2)n−1/2)+q+ (n = 3, 5, ...; q = 0, 1, 2, ...). At higher temperatures, the more oxygen-rich clusters were observed to release O2. However, the desorption of O2 from NbnOm+ was found to be insignificant in comparison with VnOm+ because Nb tends to have a +5 oxidation state exclusively, whereas V can have both +4 and +5 oxidation states. The propensity for the release of O atoms was manifested in the formation of NbnO(5/2)n−1/2+ from NbnO((5/2)n−1/2)+1+ for odd values of n, whereas VnO((5/2)n−1/2)+1+ released O2 molecules instead. The energetics of the O and O2 release from the Nb and V oxide clusters, respectively, was consistent with the results of DFT calculations.

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INTRODUCTION Transition metals, as well as their oxides, carbides, nitrides, and sulfides, function as catalysts for a variety of chemical reactions.1−3 Among compounds of group V elements applied as catalysts, vanadium oxide has been used in industry to oxidize SO2.4 In contrast, niobium and tantalum oxides are often mixed with other metal oxides to enhance their catalytic activity and selectivity and to prolong catalyst life.5−7 The gas-phase cluster is a good model to understand catalytic activity on an atomic and molecular level,8 and niobium oxide clusters have been investigated both theoretically and experimentally.9−14 Castleman et al. studied the reactivity of cationic niobium and tantalum oxide clusters with n-butane.9 They found that certain cationic niobium and tantalum oxide clusters could activate the C−C bond of n-butane, whereas vanadium oxide clusters transferred an O atom to n-butane.15 He et al. found that niobium oxide and vanadium oxide clusters could dehydrogenate methane and determined the most stable structures of (Nb2O5)n+ clusters.10 They showed that niobium oxide clusters had higher reactivity than vanadium oxide clusters and that larger niobium oxide clusters are less reactive. Misaizu et al. determined the structure of vanadium oxide clusters by ion mobility mass spectrometry.16 They found that a systematic structural growth pattern occurred for clusters with an even number of vanadium atoms, in which the clusters formed polygonal prisms as the cluster size increased. The clusters with odd numbers of vanadium atoms had an additional VO group compared to the even clusters, which formed a bridge or a pyramidal structure. The systematic growth pattern of anions varied at a larger size (n ≥ 12). These studies focused on the structure and reactivity of the clusters, but they also relate © XXXX American Chemical Society

strongly to their stability. The relative intensity of the clusters is indicative of their stability, and more abundant clusters have been regarded as stable species.17,18 However, abundant species are not always stable because experimental conditions can affect the abundance. The formation and identification of stable metal oxide clusters are difficult.11 Bernstein et al. identified stable compositions of neutral clusters by soft X-ray ionization.12 The key to the experiment was the use of high-energy photons for single-photon ionization, which allowed all neutral clusters to be ionized without significant fragmentation. The compositions of the neutral clusters were obtained after ionization. However, they also claimed that this stability was related to the experimental conditions. Duncan et al. solved this conundrum: massselected clusters were subjected to photodissociation by irradiation of a pulsed laser and the product ions were analyzed.11 The clusters that remained intact or were newly formed after the release of the neutral fragments were stable. These experiments revealed the stable compositions of both cationic and neutral group V clusters to be near-stoichiometric (metal:oxygen = 2:5). There was a slight difference among the V, Nb, and Ta oxide clusters: stable vanadium oxide clusters tend to have fewer oxygen atoms than the niobium/tantalum oxide clusters. However, the strength of the interactions that bind oxygen atoms to the clusters has not been quantified. In our previous research, we evaluated the desorption energy of O2 from near-stoichiometric vanadium oxide clusters (V:O = 2:5; the initial state is O-rich, and the product state is O-deficient).19 Received: December 15, 2016 Revised: January 23, 2017 Published: February 16, 2017 A

DOI: 10.1021/acs.jpca.6b12645 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

After expansion into the vacuum, the cluster ions were accelerated by a pulsed electric field to gain a kinetic energy of 3.5 keV for mass analysis by time-of-flight mass spectrometry (TOF-MS). The ions were reversed at the reflectron after passing through a 1 m field-free flight tube and were detected using a Hamamatsu double-microchannel plate detector. The signals were amplified with a 350 MHz preamplifier (Stanford Research Systems, SR445A) and were digitized using an oscilloscope (LeCroy LT344L). The mass resolution was sufficiently high (>1000 at m/z = 1000) to distinguish niobium oxide clusters in the mass spectra.

The even−odd alternation of the desorption energy of O2 with respect to the number of vanadium atoms indicated that the +5 oxidation state was favored for V atoms. In this study, we conducted thermal desorption spectrometry (TDS) of niobium oxide clusters in the gas phase and investigated the desorption of oxygen in the thermal region. The release of O2 molecules and O atoms could be distinguished by this experiment, and both cases were observed. Moreover, the desorption energies, which have not been quantified in any other experiments on group V clusters, were determined by experiments and compared with DFT calculations. O2 molecules were found to be released from Nb oxide clusters at a higher temperature than from V oxide clusters, and O atom release was also observed. These findings indicated that the fully oxidized state of niobium is more stable than that of vanadium.

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COMPUTATIONAL SECTION DFT calculations were performed to investigate the energetics and determine the stable structures of niobium oxide clusters (Nb2O4,6+, Nb3O6,7,8+, and Nb4O9,11+) and vanadium oxide clusters (V2O4,6+, V3O6,7,8+, and V4O9,11+) using the Gaussian09 program.23 Becke’s three-parameter hybrid density functional with the Lee−Yang−Parr correlation functional (B3LYP) was used for all calculations. Other functionals (BPW91, M06, and so on) were tested, and B3LYP was confirmed as being suitable for optimizing stable structures and determining energies (Supporting Information). The Stuttgart/Dresden effective core potential (SDD) was used for Nb and V atoms, where 10 electrons of V and 28 electrons of Nb were treated as the core, whereas for O, the 6-311G(d) basis set was used. For even n clusters, previously reported structures10,11,15,16,24 of Nb2O5+, Nb4O10+, V2O5+, and V4O10+ were referenced. 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. The structures elucidated by Misaizu et al. for odd n clusters of Nb3O7+ and V3O7+ from their ion mobility measurements and those calculated by Asmis et al. for infrared multiple photon dissociation spectroscopy were referenced.13,16,25 The most stable geometries had singlet spin multiplicity, except for V3O6+ in the triplet state. The vibrational frequencies were calculated for all obtained structures to confirm whether they corresponded to 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. In the NBO analyses, the spin was determined by the difference of the charge of the α and β spins. The feasibility of the NBO analysis for O2 moiety using the 6-311G(d) basis set for O atoms was confirmed by changing the basis set for the O atoms (Supporting Information).

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EXPERIMENTAL SECTION Thermal dissociation of gas-phase niobium oxide clusters (NbnOm+) was investigated using mass spectrometry in combination with thermal desorption spectrometry (TDS).19−22 The NbnOm+ clusters (n = 2−10) were formed using pulsed laser ablation inside a cluster source. A Nb metal rod was vaporized using the focused third harmonic of the Nd:YAG pulsed laser at 355 nm. The cluster ions were generated in the He gas flow containing O2 from a pulsed valve at a stagnation pressure of 0.8 MPa. The concentration of O2 in the gas flow was finely tuned at 0.08% using mass flow and pressure controllers. The prepared NbnOm+ clusters were introduced into a copper extension tube before expansion into a vacuum chamber (Figure 1). The tube was 120 mm long and 6 mm in inner

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RESULTS Figure 2 shows the mass spectra of the cationic clusters produced after passing through the extension tube at the indicated temperatures. The spectra show ions that are attributed to niobium oxide clusters, NbnOm+ (n = 2−10). Examination of the number of oxygen atoms present in the clusters suggested that the dominant clusters produced were NbnO(5/2)n+p+ (p = 0, 1, 2, ...) for even values of n and NbnO((5/2)n−1/2)+q+ (q = 0, 1, 2, ...) for odd values of n. Also, the intensity distribution of the oxide clusters showed an even−odd alternation with respect to n, as shown in Figure 2a: NbnO(5/2)n−1/2+ was the most intense signal for odd n clusters, whereas NbnO(5/2)n+1+ was the most intense for even n clusters. This even−odd alternation occurs because of the oxidation states of the Nb and O atoms, which

Figure 1. Schematic representation of the experimental apparatus used in this study.

diameter and was heated to 300−1000 K using a resistive heater; the temperature was monitored using a thermocouple installed in the extension tube. 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 by collisions with the He carrier gas in the tube before expansion into the vacuum. The temperature of the tube was elevated or lowered carefully at a rate of 7 K min−1. The intensities of the cluster ions were monitored as a function of temperature using mass spectrometry, and TDS plots were thus obtained. B


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the clusters. Here, the intensities of the clusters decreased on heating regardless of their size and composition, as shown in Figure 2b, because the bunch of the cluster ions in the beam expanded spatially with respect to the volume probed for mass spectrometry. The intensities of the clusters were plotted after normalization, which set the total intensity for the clusters of each n as unity. Figure 3 shows the TDS plots thus obtained for each cluster, in which the intensities of Nb8O22+ and Nb8O21+ decrease and the intensities of Nb8O20+ and Nb8O19+ increase by the same extent as the temperature increases. The concomitant changes in the intensities of NbnO(5/2)n+p+ and NbnO(5/2)n+p−2+ for even n and NbnO((5/2)n−1/2)+q+ and NbnO((5/2)n−1/2)+q−2+ for odd n are suggested to be caused by the desorption of one O2 molecule from the cluster: NbnO(5/2)n + p+ → NbnO(5/2)n + p − 2+ + O2

Figure 2. Mass spectra of cationic niobium oxide clusters produced by laser ablation of a Nb rod in a carrier gas consisting of 0.08% O2, diluted in He (a) at 300 K and (b) at 1000 K. (a) Ion peaks attributed to NbnO(5/2)n+p+ (n = 2, 4, 6, 8, 10; 0 ≤ p ≤ 6) and NbnO((5/2)n−1/2)+q+ (n = 3, 5, 7, 9; 0 ≤ q ≤ 6) are observed in the spectrum. (b) Ion peaks attributed to NbnO(5/2)n+p+ (n = 2, 4, 6, 8, 10; −1 ≤ p ≤ 2) and NbnO((5/2)n−1/2)+q+ (n = 3, 5, 7, 9; q = 0, 1) are observed in the spectrum.

for even n (1)

NbnO((5/2)n − 1/2) + q+ → NbnO((5/2)n − 1/2) + q − 2+ + O2 (2)

for odd n

The temperature at which the desorption of the O2 molecule occurs is dependent on the cluster size, n. At 800 K, the intensity of Nb7O18+ and Nb7O17+ decreased and increased, respectively, by the same amount. The variations in the intensities of NbnO((5/2)n−1/2)+1+ and NbnO(5/2)n−1/2+ for odd n indicates the release of an O atom from the cluster, as shown below:

are discussed further in the next section. As NbnOm+ tends to associate with H2O, several peaks that were assigned to NbnOmHk+ species were also observed. However, the intensity of these peaks was found to be minimized when H2O, which remained in the vacuum chamber as an impurity, was removed by intensive pumping. Figure 2b shows how the mass spectra changed when the cluster ions were heated in the extension tube to 1000 K. The intensity of Nb6O17+ (p = 2), which formed abundantly at 300 K, was reduced at 1000 K, and Nb6O15+ (p = 0) instead became one of the dominant peaks. These variations in the peak intensity suggest the occurrence of heat-induced reactions. The relative intensity of each cluster ion after heating was measured as a function of temperature to elucidate the reaction paths of

NbnO((5/2)n − 1/2) + 1+ → NbnO(5/2)n − 1/2+ + O (3)

for odd n +

The TDS plots for vanadium oxide clusters, VnOm (n = 2− 10), are also shown in Figure 3 for comparison.19 The observed intensity variations could all be explained by the release of an O2 molecule; i.e., the release of an O atom was not observed for VnOm+. For instance, the intensity of V7O18+ decreased and the intensity of V7O16+ (rather than V7O17+) increased by the

Figure 3. Relative intensities of NbnOm+ and VnOm+ (n = 2−10, m is displayed in the figure for each curve) produced after heating in an extension tube. The curves for NbnOm+ and VnOm+ with the same n are compared in one box, with the TDS curves of NbnOm+ shown above those of VnOm+. C


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Table 1. Desorption Energy of O2 from MnO(5/2)n+1+ (M = Nb, V; n = 2, 4, 6, 8) as Determined by the TDS Curves and Binding Energy of O2 as Obtained from DFT Calculations

same amount. In addition, the O2 molecule release occurred at a lower temperature for VnOm+ than for NbnOm+. Indeed, the intensities of V8O22+ and V8O21+ decreased below 500 K, whereas the intensities of Nb8O22+ and Nb8O21+ decreased only above 800 K. Although both V and Nb belong to group V, the differences in their behavior suggest that vanadium oxide clusters are more readily reduced. Clusters with particular stoichiometries were found to play a major role in the formation of NbnOm+ or VnOm+ clusters at 300 K and desorption of oxygen at 300−1000 K. The main experimentally observed oxygen release processes can be expressed as p = 2 → p = 0 and p = 1 → p = −1 for even n and q = 3 → q = 1 and q = 2 → q = 0 for odd n. Thus, DFT calculations were used to optimize the structures of selected clusters (n = 2, 3, 4; p = −1, +1; q = −1, 0, +1) and to calculate their associated formation energies. Figure 4 shows the

ΔE = +1.14 eV

Nb2O6+ → Nb2O4+ + O2 V2O6+ → V2O4+ + O2 Nb4O11+ → Nb4O9+ + O2 V4O11+ → V4O9+ + O2 Nb6O16+ → Nb6O14+ + O2 V6O16+ → V6O14+ + O2 Nb8O21+ → Nb8O19+ + O2 V8O21+ → V8O19+ + O2

8

desorption energy/ eV

binding energy/ eV

a

+0.94 +2.14 +0.79 +2.21 +0.68 +1.90 +0.22

± ± ± ± ± ± ±

+2.29 +1.14 +2.03 +0.87

0.07b 0.21 0.01b 0.12 0.01b 0.18 0.01b

a

This energy was too high to be determined in the temperature range 300−1000 K. bReference 19.

The metastable isomer of V2O6+ was found to be 0.30 eV higher in energy than the most stable V2O6+, and the O2 in metastable V2O6+ was bound rather weakly to V2O4+ in an endon geometry with a Wiberg bond index of 0.33. The energy of the metastable isomer of Nb2O6+ is 1.57 eV higher than the most stable Nb2O6+, and the Wiberg bond index between O2 and Nb2O4+ is 0.25. In the case of the most stable isomers, the sum of the Wiberg bond index between the V2O4/Nb2O4 unit and the superoxide attached side-on is 1.36/1.13. Electron transfer from the V−O or Nb−O bond (this O atom is a component of the superoxide) to the V or Nb atom results in a weaker bond between O2 and the metastable isomers of V2O4+ or Nb2O4+. This process involves electron transfer from the superoxide to the V2O4 or Nb2O4 unit. The significance of these isomers is that the desorption of O2 from the most stable V2O6+ and Nb2O6+ isomers proceeds via electron transfer, and the energy of the metastable isomers can indicate how readily this occurs.14,26 The proximity of the energy of the metastable isomer of V2O6+ to that of the most stable isomer contrasts with the case for Nb2O6+. Thus, it is highly likely that V2O6+ bears both O2 molecule and superoxide bonding characters, resulting in the lower desorption energy of O2 from V2O6+. For Nb4O9+ (n = 4, p = −1), there are six O atoms bridging between four Nb atoms and other three O atoms in the terminal sites. The O2 is attached to one of the Nb atoms of Nb4O9+ to form Nb4O11+ (n = 4, p = +1). The binding energy of O2 with Nb4O9+ was calculated to be Nb4 O11+ → Nb4 O9+ + O2

ΔE = +2.03 eV

The vanadium oxide clusters, V4O9+ structures as Nb4O9+ and Nb4O11+. with V4O9+ was calculated to be

(4)

V4O11+ → V4O9+ + O2

(Table 1). The DFT calculations suggested that the vanadium oxide clusters V2O4+ and V2O6+ had a similar geometrical structure. The binding energy of O2 with V2O4+ is V2O6+ → V2O4 + + O2

2

6

optimized structures of even n clusters. For Nb2O4+ (n = 2, p = −1), there are two O atoms bridging between two Nb atoms and the other two O atoms at the terminal sites. The geometrical isomer, the formation energy of which is +0.13 eV higher than the most stable Nb2O4+, has two terminal O atoms protruding in the same direction. For Nb2O6+ (n = 2, p = +1), O2 is attached to one of the Nb atoms of Nb2O4+. The O−O bond length and Wiberg bond index were 1.30 Å and 1.24, respectively, and the spin of each O atom was about 0.5. Thus, the O2 was attached to the cluster as a superoxide, O2−. Note that the O−O bond length and Wiberg bond index of isolated O2− were 1.35 Å and 1.26, respectively (Supporting Information). The binding energy of O2 with Nb2O4+ was calculated from the difference of the formation energy of Nb2O6+, Nb2O4+, and O2 to be ΔE = +2.29 eV

reaction

4

Figure 4. Optimized structures of M2O4+, M2O6+, M4O9+, and M4O11+ (M = Nb (a)−(d), and M = V (e)−(h)). All spin states are doublet.

Nb2O6+ → Nb2O4 + + O2

n

(6)

+

and V4O11 , have the same The binding energy of O2

ΔE = +0.87 eV

(7)

+

which is also much lower than that of Nb4O11 .

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DISCUSSION Oxidation States of Cluster Atoms. The oxidation states of the atoms in the clusters can explain the formation of oxides and the release of oxygen from the oxides. As the oxidation states of niobium and oxygen atoms are +5 and −2, respectively, a NbnOm+ species with n:m = 2:5 can be defined as a stoichiometric composition. More precisely, considering that the total charge of a cationic cluster is +1, the composition NbnO5/2n−1/2+ (q = 0; Nb3O7+, Nb5O12+, Nb7O17+, Nb9O22+) for odd n is fully stoichiometric in terms of the charge state:

(5)

which is much lower than that for Nb2O6+. Metastable structural isomers with a similar structure but a different spin multiplicity (quartet) exist for both V2O6+ and Nb2O6+. The O2 moiety in the metastable isomers with a higher spin state was molecular O2, as indicated by the O−O bond length (1.21, 1.20, and 1.21 Å for V2O6+, Nb2O6+ and an isolated O2 molecule, respectively), Wiberg bond index (1.49, 1.49, and 1.51), and the sum of the O−O spins (1.96, 1.97, and 2.00). D


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The Journal of Physical Chemistry A (+5) × n + (−2) ×

where A, Ea, and kB refer to the pre-exponential factor, desorption energy, and Boltzmann constant, respectively. By combining eqs 8 and 9, we obtain eq 10:

( 25 n − 12 ) = +1. These species were formed

abundantly at 300 K and remained in abundance after heating at 1000 K, suggesting that full stoichiometric clusters are stable in the thermal energy region. We focused on the O2 desorption for clusters with even n, leading to a transition from more oxygen-rich clusters (p = 1) to more oxygen-poor clusters (p = −1). For odd n, the O desorption induced a transition from more oxygen-rich clusters (q = 1) to stoichiometric clusters (q = 0). For even n, the more oxygen-poor clusters (p = −1; Nb2O4+, Nb4O9+, Nb6O14+, Nb8O19+, Nb10O24+) contained mainly fully oxidized Nb with a single reduced Nb atom, the oxidation state of which is expressed hereafter as ... (+5)(+5)(+5)(+5)(+5)(+5)(+4). The oxidation state of the stoichiometric clusters (q = 0) is expressed as ... (+5)(+5)(+5)(+5)(+5)(+5)(+5). Compared with NbnOm+, the TDS curves of VnOm+ are distinguished by two features: the release of an O2 molecule from VnOm+ at a lower temperature than NbnOm+, and the absence of any release of an O atom release from VnOm+. It is likely that these differences are caused by the fact that V(+5) is reduced more easily V(+4) than is Nb(+5). For instance, after the release of an O atom from Nb7O18+, the oxidation state of the resulting Nb7O17+ is expressed as (+5)(+5)(+5)(+5)(+5)(+5)(+5). However, after the release of an O2 molecule from V7O18+, the oxidation state of V7O16+ is expressed as (+5)(+5)(+5)(+5)(+5)(+4)(+4). Generally, the release of an O atom is considered to be energetically unfavorable because a chemical bond must be ruptured without the formation of a new chemical bond. Furthermore, vanadium atoms can have an oxidation state of +4 in addition to the +5 state, resulting in the formation of V7O16+ by the release of O2. In contrast, the release of an O atom proceeds for Nb7O18+ because all the Nb atoms in Nb7O17+ have the oxidation state +5, which is fully stable. For the more oxygen-rich clusters, such as Nb2O6+, the Nb atoms can be found in higher oxidation states than +5. However, our DFT calculations suggest that there is a superoxide O2− moiety in Nb2O6+. Thus, 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 atoms should be expressed as (+5)(+5). In this context, the +5 oxidation state provided by the more oxygen-rich clusters is expected to be favorable for the Nb oxide clusters. Desorption Energies. Quantitative values were extracted from the TDS curves by least-squares fitting with the Arrhenius equation. In our experimental setup, a cluster was assumed to dissociate unimolecularly in the extension tube. Thus, the intensity of a cluster, I(T), decreases exponentially with the reaction time, t, as I (T ) = exp( −k(T )t ) I0

⎛ ⎛ E ⎞⎞ I (T ) = exp⎜⎜ −At exp⎜ − a ⎟⎟⎟ I0 ⎝ kBT ⎠⎠ ⎝

The desorption energies, Ea, were obtained by fitting the experimental TDS curves with eq 10, in which At and Ea were set as variable parameters. Note that the reaction time, t, was estimated to be ∼100 μs from the velocity of the beam and the length of the tube, though this value was not used for the analysis of Ea. Table 1 shows Ea for the desorption of O2 from VnOm+ and NbnOm+ as estimated from the TDS curves and the DFTobtained binding energies of O2 with VnOm−2+ and NbnOm−2+. The experimental value of Ea for NbnOm+ is not available, and the binding energies obtained by the DFT calculation suggest that the energy is too high for the O2 desorption to be estimated experimentally within the range of 300−1000 K. The desorption energies of V2O6+ and V4O11+ are consistent with the binding energies, suggesting that O2 desorption is almost barrierless. Indeed, O2 attaches to one of the V atoms in V2O4+ and V4O9+ in the form of O2, and the O2 is considered to leave into the gas phase without significant structural deformation in the transition state. Release of an O Atom. As discussed in the previous section, the TDS curves for odd n clusters indicate that an O atom is released from NbnO((5/2)n−1/2)+1+ clusters above 800 K. This finding is consistent with the results by Duncan et al.,11 who observed fragmentation of the size-selected clusters following the multiphoton excitation of MnOm+ clusters (M = V, Nb, and Ta) at 532 and 355 nm. The internal energy provided by the absorption of photons caused dissociation, producing several fragments in the gas phase. The release of an O atom was observed for, among others, Nb3O8+, Nb5O13+, and Nb7O18+, which led to the formation Nb3O7+, Nb5O12+, and Nb7O17+, respectively. No significant O atom release was observed for the vanadium oxide clusters. The DFT calculations also supported these results. Figure 5 shows the optimized structures of odd n clusters. For Nb3O7+

(8)

where T, I0, and k(T) refer to the temperature, intensity of a cluster before entering the extension tube, and rate constant for unimolecular dissociation, respectively. In addition, the rate constant is expressed by the Arrhenius equation as ⎛ E ⎞ k(T ) = A exp⎜ − a ⎟ ⎝ kBT ⎠

(10)

Figure 5. Optimized structures of M3O6+ and M3O8+ (M = Nb (a), (b) and M = V (c), (d)). (a), (b), (d) spin states are singlet and (c) spin state is triplet.

(n = 3, q = 0), it is known that three O atoms bridge between three Nb atoms, three O atoms are present in the terminal sites, and one O atom is in the hollow site.16 Hence, Nb3O8+

(9) E



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(n = 3, q = 1) had the same structure as Nb3O7+, with one excess O atom in a terminal site. The binding energy of an O atom with Nb3O7+ was found to be Nb3O8+ → Nb3O7+ + O

ΔE = +2.34 eV

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b12645. Binding energy graph; Wiberg bond index graph; metastable structures; atomic coordinates, natural charges, and Wiberg bond index matrix; complete author list for ref 23 (PDF)

(11)

ΔE = +3.01 eV

(12)

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This finding is consistent with the experimental evidence that Nb3O8+ released an O atom rather than an O2 molecule at higher temperatures. In contrast, the binding energy of an O atom with V3O7+ was +

+

V3O8 → V3O7 + O

ΔE = +2.18 eV

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

(13)

Fumitaka Mafuné: 0000-0001-8860-6354 Notes

ΔE = +1.35 eV

The authors declare no competing financial interest.

(14)

V3O8+

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Table 2. Desorption Energy of O or O2 from MO((5/2)n−1/2)+1+ (M = Nb, V; n = 3, 5, 7, 9) as Determined from TDS Curves and Binding Energy of O or O2 as Obtained from DFT Calculations

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which is consistent with the experimental observation that released an O2 molecule rather than an O atom, and the desorption energy of this reaction agrees well with the calculated result (Table 2).

n

reaction

3

Nb3O8+ → Nb3O7+ + O Nb3O8+ → Nb3O6+ + O2 V3O8+ → V3O7+ + O V3O8+ → V3O6+ + O2 Nb5O13+ → Nb5O12+ + O V5O13+ → V5O11+ + O2 Nb7O18+ → Nb7O17+ + O V7O18+ → V7O16+ + O2 Nb9O23+ → Nb9O22+ + O V9O23+ → V9O21+ + O2

5 7 9

desorption energy/ eV

binding energy/ eV

+1.93 ± 0.22

+2.34 +3.01 +2.18 +1.35

a a

+1.45 +1.70 +1.49 +1.69 +1.30 +1.39 +1.26

± ± ± ± ± ± ±

0.04b 0.05 0.03b 0.13 0.17b 0.15 0.14b

AUTHOR INFORMATION

Corresponding Author

The binding energy of an O2 molecule with V3O6+ was V3O8+ → V3O6+ + O2

ASSOCIATED CONTENT

S Supporting Information *

In contrast, the binding energy of an O2 molecule with Nb3O6+ was found to be higher than for an O atom: Nb3O8+ → Nb3O6+ + O2

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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) and by the Genesis Research Institute, Inc. (cluster research). REFERENCES

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The values for these reactions were calculated rather than experimentally observed. bReference 19.

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CONCLUSION Cationic niobium oxide clusters, NbnOm+ (n = 2−10), were prepared in the gas phase by laser ablation of a Nb rod. The thermal oxygen desorption from the near-stoichiometric (n:m = 2:5) niobium oxide clusters was investigated using TOF-MS and TDS. We compared these results with the TDS of cationic vanadium oxide clusters. For niobium oxide clusters with even values of n, i.e., NbnO(5/2)n+1+ (n = 2, 4, 6, ...), the release of an O2 molecule required a higher energy than for vanadium oxide clusters. For odd n niobium oxide clusters, i.e., NbnO((5/2)n−1/2)+1+ (n = 3, 5, ...), the release of O atoms was observed to be stoichiometric, while vanadium oxide clusters with the same composition released an O2 molecule to become more oxygenpoor clusters. This observation can be explained by the strong tendency of Nb to favor the +5 oxidation state. Herein, we demonstrate that these oxygen desorption energies could be determined experimentally and confirm the accuracy of the measurements with DFT calculations. F


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Desorption of Oxygen from Cationic Niobium Oxide Clusters Revealed

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