Nitrogen Molecule Adsorption on Cationic ... - ACS Publications

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Nitrogen Molecule Adsorption on Cationic Tantalum Clusters and Rhodium Clusters and Desorption from Their Nitride Clusters Studied by Thermal Desorption Spectrometry Fumitaka Mafuné,* Yuki Tawaraya, and Satoshi Kudoh Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Adsorption and desorption of N2 molecules onto cationic Ta and Rh clusters in the gas phase were investigated in the temperature range of 300−1000 K by using thermal desorption spectrometry in combination with density functional theory (DFT) calculations. For Ta6+, the first N2 molecule was found to adsorb dissociatively, and it remained adsorbed when Ta6+N2 was heated to 1000 K. In contrast, the second and the subsequent N2 molecules adsorbed weakly as a molecular form and were released into the gas phase when heated to 600 K. The difference can be explained in terms of the activation barrier between the molecular and dissociative forms. On the other hand, when Ta clusters were generated in the presence of N2 gas by the laser ablation of a Ta rod, isomeric clusters, TanNm+, having heat resistivity were formed. For Rh6+, N2 adsorbed molecularly at 300 K and desorbed totally at 450 K. These results were consistent with the DFT calculations, indicating that the dissociative adsorption of N2 is endothermic.

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INTRODUCTION Rh nanoparticles have been used as practical catalysts to remove environmentally unfriendly gases, including NOx.1−6 For NO, the standard Gibbs free energy of formation is +87.60 kJ mol−1; hence, the disproportionation of NO to form N2 and O2 is energetically favorable.7 Nevertheless, the energy barrier for the direct disproportionation of NO is so high that the reaction is inhibited. A catalyst provides an effective reaction field to lower the energy barrier. In one of our previous studies, the adsorption chemistry and mechanism of NO reduction on a small Rh cluster surface was investigated experimentally and theoretically.8 For small clusters (n = 4 and 5), NO adsorbed onto the surface in molecular form because the energy barrier between the molecular form and the dissociative form was sufficiently high. At higher temperatures (700 K) and for larger clusters (n ≥ 7), NO molecules started to dissociate by overcoming the energy barrier, and N2 was released into the gas phase after migration of N atoms onto the surface.9,10 In addition, the adsorption chemistry and mechanism were found to be modified by alloying with Ta atoms.11 Dissociative adsorption became quite stable, lowering the activation barrier for the dissociation. As a result, NO molecules dissociated by passing the energy barrier, causing NO reduction at room temperature. These results for the gas-phase clusters suggest practical application for real catalysts. However, in a practical catalyst, such as three-way catalytic converters in automobiles, the catalyst operates in an atmosphere in which N2 is dominant. It © XXXX American Chemical Society

is possible that N2 adsorbs onto the active sites of the catalyst, deteriorating its reactivity. Nevertheless, the interaction of metal clusters with N2 has not been investigated intensively. Along with a negative behavior such as poisoning of catalysts, interaction of the clusters with N2 would provide important information such as nitrogen fixation by the catalysts. Industrial nitrogen fixation plays important roles in the supply of food and chemical products. For nitrogen fixation, N2 is required to react with other molecules to form chemically reactive substances (e.g., ammonia). However, because the bond dissociation energy of N2 is quite high, it is not easy to cleave the N−N bond, and high temperature is required. To achieve low energy consumption, numerous studies have been conducted.12 In the cluster science, the activation and cleavage of the N−N bond caused by adsorption on transition-metal clusters have been reported.13,14 In the present study, we investigated the interaction between cationic Ta clusters and N2 in the thermal energy region, especially in terms of the adsorption forms, molecular or dissociative, and we compared the results with those of cationic Rh clusters. Reactions of neutral Ta clusters, Tan, with N2 were observed by Hamrick and Morse using a fast-flow reactor at 320 K.15 They measured the rate constants of adsorption of a N2 molecule as a function of the cluster size. The variation of the Received: April 5, 2016 Revised: May 19, 2016

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DOI: 10.1021/acs.jpca.6b03479 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

the reaction gas cell was monitored using a piezoresistive transducer attached on the reaction cell and an oscilloscope, and the pressure was found to rise to almost 3 × 103 Pa during pulsing. The extension tube (120 mm in length and 4 mm in inner diameter) was heated to 300−1000 K using a resistive heater, and the temperature was monitored using thermocouples. The residence time of the cluster ions and the density of the He gas in the extension tube were estimated to be ∼100 μs and ∼1018 molecules cm−3, respectively. The thermal equilibrium of the clusters was achieved through collisions with the He carrier gas before expansion into the vacuum. Temperaturedependent changes in the cluster ions were monitored using mass spectrometry. To investigate reactions between the clusters and the reactant N2 gas in a different manner, N2 gas, diluted by He, was introduced from the first pulsed valve. Hence, the clusters were formed in the gas flow of He in the presence of N2. The clusters thus formed entered the extension tube after the reaction gas cell and were then heated. In either case, the concentration of N2 in the He in the pulsed valve was finely tuned between 0−100% using mass flow and pressure controllers. In addition, the partial pressure of N2 inside the cluster source chamber was monitored using a residual gas analyzer. After the extension tube, the cluster ions were accelerated by the pulsed electric field to gain a kinetic energy of 3.5 keV for mass analysis with a time-of-flight spectrometer. After passing through the 1 m long field-free flight tube, the ions were detected using a Hamamatsu double-microchannel plate detector, and the signals were amplified with a preamplifier and digitized using an oscilloscope. The mass resolution (m/ Δm) was sufficiently high (>1000 at m = 1000) to distinguish Rh, Ta, and N atoms in the mass spectra.

rate constants was minimal, suggesting that the size dependence was insignificant. They considered that N 2 molecules chemisorbed dissociatively onto Tan because N2 would desorb at temperatures much lower than 320 K if the N2 molecules adsorbed molecularly. Yadav and Mookerjee discussed whether the adsorption of N2 is dissociative or not on Tan (n ≤ 4) having low 5d occupation based on density functional theory (DFT) calculations.16 They showed the structures of the optimized conformers and found that the most stable conformer adsorbed N2 dissociatively. The following question thus arises: Does N2 actually dissociate upon adsorption onto the Ta cluster? Is N2 molecularly adsorbed onto the cluster so weakly bound to the cluster that it is released at temperatures much lower than 320 K? There is an activation barrier between the molecularly adsorbed and dissociatively adsorbed forms. If the barrier is too high, the molecularly adsorbed form is generated, even if the dissociatively adsorbed form is energetically stable. In the present study, we examined the adsorption of N2 onto Tan+. This process involved the formation of Tan+ and the reaction with N2 in thermal equilibrium at 300 K. In addition, TanNm+ was introduced into an extension tube with elevated temperatures of 300−1000 K. We then investigated whether TanNm+ released N2 by thermal desorption at higher temperatures. The adsorption form of TanNm+ was compared to that of RhnNm+ because Rh atoms have less affinity to nitrogen.11

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EXPERIMENTAL METHODS The adsorption of nitrogen molecules onto Rhn+ (n = 4−8) and Tan+ (n = 5−8) and desorption of nitrogen molecules from their nitrides were investigated using mass spectrometry in combination with thermal desorption spectrometry, as shown in Figure 1.17−21 The clusters were prepared using pulse laser

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COMPUTATIONAL METHODS

To estimate the binding energies of N2 on Ta6+ and Rh6+, DFT calculations were performed using the Gaussian 09 program.22 The LANL2DZ effective core potential and basis set were used to describe the Ta and Rh atoms,23 whereas the 6-31G(d) basis set was used to describe the N atoms.24,25 Becke’s threeparameter hybrid density functional26 with the Lee−Yang−Parr correlation functional (B3LYP) was used for all calculations.27 For dissociatively adsorbed and molecularly adsorbed Ta6Nm+, those with N atoms or N2 molecules randomly attached to the preoptimized Ta6+, respectively, were adopted as initial structures. For dissociatively adsorbed Ta6Nm+, initial geometries generated by placing all atoms at random were also employed. We adopted the optimized structures having the lowest energies as the most stable isomers for each cluster and adsorption form. The geometry optimizations for the Rh6Nm+ clusters were carried out in the same manner. The transition states for N2 dissociation onto Ta6+ were determined using the two-point scaled hypersphere search (2PSHS) method28 in the GRRM11 program,29−31 which is a powerful tool for finding the saddle point between two isomers32 in order to estimate the reaction barriers between the molecularly and the dissociatively adsorbed clusters. The vibrational frequencies were calculated for the obtained transition-state structures, which had single imaginary frequencies, suggesting that these structures corresponded to the first-order saddle points.

Figure 1. Schematic diagram of the experimental apparatus used in the present study. Two pulsed valves were attached to the apparatus, and N2 gas was introduced as a carrier gas through the first pulsed valve or as a reactant gas through the second pulsed valve.

ablation in a cluster source. A metal rod, Rh or Ta, was vaporized using the focused second harmonic of a Nd:YAG pulsed laser at a typical pulse energy of 20 mJ with a repetition rate of 10 Hz. The cluster ions were formed in a gas flow of He from the first pulsed valve at a stagnation pressure of 0.8 MPa. The prepared Rhn+ or Tan+ clusters entered a reaction gas cell before an extension tube. A reactant N2 gas diluted by He was introduced in the gas cell from the second pulsed valve at a stagnation pressure of 0.1 MPa, where the clusters reacted with N2 molecules. The temporal change of the gas pressure inside B


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Figure 2. (a) Mass spectrum of Tan+ clusters produced by laser ablation of a Ta rod in He (0.8 MPa). (b, c) Mass spectra of the clusters after reaction with N2 gas at room temperature. The pressure of N2 were 0.4 and 1.4 bar in panels b and c, respectively. (d) Mass spectrum of the clusters prepared similarly as panel c after heating the extension tube at 1000 K. (e) Map showing intensities of Tan+Nm (n = 2−9; m = 0−22) produced by reaction with N2 at pressure of 1.4 bar at room temperature. (f) Map showing intensities of Tan+Nm after heating the extension tube at 1000 K.

Figure 3. (a, b) Relative intensities of Ta6+Nm as a function of temperature of the extension tube (TPD plots) and (c) TPD plots for Rh6+Nm prepared in the reaction gas cell at room temperature. The numbers indicate the number of N atoms, m.

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tube. Figure 2d shows the mass spectrum of Tan+Nm after heating at 1000 K. The amount of Tan+Nm with a number of N atoms (m ≥ 4) significantly decreases. The spectral change is interpreted as resulting from the desorption of nitrogen from Tan+Nm. Panels e and f of Figure 2 show maps of the abundance distributions of Tan+Nm after passing through the extension tube at 300 and 1000 K, respectively, for different n values. For n ≥ 3, more than 10 nitrogen atoms adsorbed onto the clusters at 300 K, most of which were found to be released at 1000 K. Figure 3a,b shows the relative intensities of Ta6+Nm as a function of the temperature of the extension tube (TPD plots) for the different values of m. The intensities of Ta6+N8,10,12 decrease in the range from 300 to 400 K, whereas the intensities of Ta6+N4,6 increase in the same range. Then, the intensities of Ta6+N4,6 decrease and that of Ta6+N2 increases around 500 K. Similarly, the intensity of Ta6+N9 decreases and that of Ta6+N7 increases in the range from 300 to 400 K. Then, the intensity of Ta6+N7 decreases and that of Ta6+N5 increases around 500 K, leading to the formation of Ta6+N3 above 600 K. All the intensity changes can be explained by the pairs of increases and decreases. The concomitant changes holding the parity of m suggest stepwise desorption of a N2 molecule as follows:

RESULTS Figure 2a shows the mass spectrum of the as-prepared cationic Ta clusters. Ions assignable to Tan+ (6 ≤ n ≤ 8) are seen in the spectrum. Small peaks to the right of the main Tan+ peaks are assignable to Tan+O. Most likely, oxygen atoms that had adsorbed onto the surface of the metal Ta rod were incorporated to Tan+ upon formation by laser ablation. However, further investigation is beyond the scope of the present study. Figure 2b displays the mass spectrum of the clusters after reaction with N2 in the reaction gas cell. The spectrum indicates that Tan+Nm (m = 1, 2, 3, ...) clusters did form. The number of N atoms adsorbed onto the clusters increased with increasing concentration of N2 gas in the reaction gas cell, suggesting that the N2 molecules adsorbed sequentially on the clusters as follows: Ta n+Nm + N2 → Ta n+Nm + 2

(m = 0, 2, 4, ...)

(1)

In addition, Tan+Nm having odd numbers of N atoms were observed, the formation of which is not trivial, as the clusters reacted with N2. It is likely that N2 that was adsorbed onto the surface of the Ta metal rod was released upon laser ablation with Ta clusters. Another possibility is fragmentation of the clusters, where the adsorption of N2 onto Tan+ generates available energy that is equivalent to the adsorption energy of N2, which could cause fragmentation, releasing neutral TaN. This issue will be discussed later based on the DFT calculations. To examine how strongly N atoms adsorbed onto the clusters, the product cluster ions were heated in the extension

Ta n+Nm → Ta n+Nm − 2 + N2

(2)

What should be emphasized is that three N atoms remain adsorbed at 1000 K. These findings suggest that a few N atoms are strongly adsorbed onto Tan+ and the rest are weakly adsorbed. C


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The Journal of Physical Chemistry A In comparison, Figure 3c shows the TPD plots for Rh6+Nm. A variety of Rh6+Nm (0 ≤ m ≤ 12) clusters appear at 300 K, similarly to the Tan+ clusters. However, the intensities of Rh6+Nm (m ≥ 2) start decreasing at 300 K and become zero at 450 K, whereas the intensities of Rh6+ and Rh6+N start increasing at 300 K and level off at 450 K. The intensity changes are considered to be caused by the desorption of N2. Rh6+Nm with odd numbers of N atoms are converted to Rh6+N by the release of N2, and no further release of N atoms occurs because the release of N atoms is energetically unfavorable. Analysis of the intensity curves using the Arrhenius equation suggests that the energy barrier for the release of N2 from Rh6+Nm (m = 6, 8) is ∼0.3 eV.20 Figure 4 shows optimized structures of Ta6+, Ta6+N2, and Ta6+N4 by DFT calculations. It was confirmed that Ta6+ had

the molecularly adsorbed form in structure and has an energy +0.99 eV higher than intermediate (IM2). Figure 6 shows the TPD plots of Tan+Nm (n = 4, 5, 7, 8) prepared at 300 K for a series of even numbers of N atoms.

Figure 6. TPD plots of Tan+Nm (n = 4, 5, 7, 8) prepared at 300 K for a series of even numbers of N atoms. The numbers indicate the number of N atoms, m.

Figure 4. Optimized structures of Ta6+, Ta6+N2, and Ta6+N4 by DFT calculations.

Ta4+Nm, clusters with m ≥ 4 disappear at 450 K. The intensity of Ta4+N2 decreases and the intensity of Ta4+ increases with increasing temperature, suggesting that N2 molecules, weakly adsorbed onto Ta4+, desorb sequentially at higher temperatures. It should be noted that some Ta4+N2 remains at 1000 K. Namely, Ta4+N2 is considered to comprise Ta4+NN and Ta4+(N2). Ta5+Nm, clusters with m ≥ 4 disappear similarly at 450 K. The intensity of Ta5+N2 increases to the same extent that Ta5+Nm (m ≥ 4) decreases, suggesting that Ta5+N2 mainly comprises Ta5+NN. For Ta7+ and Ta8+, Ta7 +N4 remains at 1000 K, and the contribution of Tan+N4 is higher for Ta8+N4. Thus, evidently, greater numbers of N2 molecules are able to adsorb dissociatively with increasing cluster size, n.

the form of a tetragonal bipyramid, which is consistent with the structures obtained by other methods and basis sets.33 For Ta6+N2 and Ta6+N4, there are two different forms, molecularly adsorbed nitrogen and dissociatively adsorbed nitrogen. Hereafter, molecularly adsorbed and dissociatively adsorbed forms are expressed as, for instance, Ta 6 + (N 2 ) 2 and Ta6+NNNN, respectively. Considering the formation energy, Ta6+NN is much more stable than Ta6+(N2), and Ta6+NNNN is more stable than Ta6+(N2)2. Hence, dissociatively adsorbed forms can be formed stably. However, the calculations also suggested that there was an activation barrier between the molecularly adsorbed and dissociatively adsorbed forms, as shown in Figure 5a. The barrier leads to the most stable form via one of the dissociatively adsorbed forms, which resembles

Figure 5. Energy diagrams of N2 adsorption on (a) Ta6+ and (b) Ta6+NN. TS and IM stand for transition state and intermediate state, respectively. D


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adsorption energy of N2 onto Ta6+NN as a molecular form is −0.74 eV.

DISCUSSION Adsorption of the First N2 Molecule onto Tan+. The molecular adsorption energy of N2 onto the on-top site of Ta6+ was −0.80 eV. Ta6+ + N2 → Ta6+(N2)

ΔE = −0.80 eV

Ta6+NN + N2 → Ta6+NN(N2)

ΔE N2adTa6 = −5.7 eV

Whereas the adsorption energy of N2 onto Ta6 NN as a dissociative form is −2.5 eV.

(3)

Ta6+NN + N2 → Ta6+NNNN

ΔE = +7.6 eV

ΔE = −2.5 eV

(7)

Because the stabilization energy generated upon dissociative adsorption is moderate, it is highly likely that the second and the subsequent N2 molecules adsorb molecularly onto Ta6+NN, forming Ta6+NN(N2)k (k = 1, 2, 3, ...). This inference is consistent with the 2PSHS calculations on the activation barrier between them. The barrier is 1.33 eV higher than the molecularly adsorbed form (IM1) and 0.78 eV higher than the initial state, Ta6+NN + N2 (see Figure 5b). Because the energy of the dissociative adsorption is not significant, the activation barrier is considered to remain high, and Ta6+NN(N2) is formed before the barrier. Adsorption of N2 Molecule onto Rhn+. The TPD plots in Figure 3c show that Rh6+Nm (m ≥ 2) totally disappears at 450 K and only Rh6+ and Rh6+N remain. The TPD curves suggest that N2 molecules are bound to Rh6+ very weakly. Indeed, the desorption energy, estimated by using the Arrhenius equation, is as low as 0.3 eV. Note that nitrogen is released as a N2 molecule; hence, Rh6+ is formed from Rh6Nm+ with even numbers of N atoms, whereas Rh6+N is formed from Rh6+Nm with odd numbers of N atoms. The release of just one N atom is not likely because it is energetically unfavorable. The desorption energy is consistent with the results of the DFT calculations. The adsorption energy of molecular N2 to Rh6+ is −0.46 eV.

(4)

The required desorption energy is too high for Ta6+NN to release N2 into the gas phase. Hence, the N2 molecule is considered to adsorb onto Ta6+ dissociatively. Now, let us discuss the possibility of fragmentation. According to the DFT calculations, the dissociation energy forming Ta5+N and TaN from Ta6+NN is +7.6 eV. Ta6+NN → Ta5+N + TaN

(6)

+

The activation energy forming Ta6+NN was calculated to be +0.04 eV from the initial state, Ta6+ + N2. Hence, Ta6+NN is possibly produced by overcoming the barrier if the initial energy is conserved. However, in the present experimental setup, the reaction proceeded in the presence of He atoms, which were estimated to collide with a cluster every 5 ns.8 The He atoms may have removed internal energy from the clusters, thermalizing them. When the clusters are totally thermalized, the cluster can be trapped inside one of the wells between the initial state and the activation barrier, leaving Ta6+(N2). Thus, dissociation of N2 onto the cluster competed with the energy dissipation. The TPD plots in Figure 3a,b show that there is a minimum possibility that N2 is released from Ta6+N2 when it is heated to 1000 K, indicating that nitrogen is strongly bound to Ta6+. Because the total vibrational energy of the cluster heated at 1000 K, given by the vibrational degree of freedom and kBT, is ∼1.6 eV, N2 should be released from the cluster if a N2 molecule adsorbs molecularly. In contrast, for the dissociative adsorption onto Ta6+, the adsorption energy of N2 on Ta6+ was calculated to be −5.7 eV. Ta6+ + N2 → Ta6+NN

ΔE = −0.74 eV

Rh6+ + N2 → Rh6+(N2)

ΔE = − 0.46 eV

(8)

It was found that the N2 molecule adsorbs onto the on-top site of Rh6+. For comparison, the energy of dissociative adsorption was also calculated, and it was found to be +0.98 eV.

(5)

Rh6+ + N2 → Rh6+NN

The dissociative adsorption of N 2 accompanying the fragmentation process releasing TaN is endothermic by 1.9 eV. Hence, based on DFT calculations, fragmentation is unlikely, when only a single N2 molecule is involved. Nevertheless, we are unable to rule out the possibility of fragmentation if multiple N2 molecules are involved. With an increasing concentration of reactant N2 gas, multiple molecules adsorb onto the cluster. The available energy generated upon adsorption of the N2 molecules is dissipated by the collision of He atoms; however, the available energy can exceed the dissociation energy, causing fragmentation. Adsorption of Multiple N2 Molecules onto Tan+. The TPD plots in Figure 3a,b show that Ta6+Nm with m ≥ 4 totally disappears at 550 K and that Ta6+N2 increases in intensity instead, suggesting that the second and the subsequent N2 molecules adsorb weakly onto Ta6+NN. The activation energy for the N2 desorption from Ta6+N4 was estimated from the TPD plot by using the Arrhenius equation, and it was found to be as small as 0.26 ± 0.10 eV, the error of which represents the standard deviation of energy values by the multiple measurements. This value can be too low because our TPD experiments sometimes underestimate the bond dissociation energies by 0.5 eV at most for those species desorbing in the temperature range of 300−400 K.18,34 According to the DFT calculations, the

ΔE N2adRh6 = + 0.98 eV

(9)

Evidently, dissociative adsorption is so endothermic that it is not the case for Rh6+. The positive adsorption energy is considered to originate from the high bond dissociation energy of N2 (ΔEN2bond = 9.8 eV, 1Σg+) and a small binding energy of a Rh atom to a N atom. In contrast, the energy for dissociative adsorption is negative for Ta6+ because Ta atoms have a very high affinity to N atoms. Indeed, the binding energy between N and Ta atoms should be as high as 7.7 eV, [(ΔEN2bond − ΔEN2adTa6)/2 = (9.8 + 5.7)/2 eV] considering the energy balance, whereas the binding energy between N and Rh atoms is 4.4 eV, [(ΔEN2bond − ΔEN2adRh6)/2 = (9.8−0.98)/2 eV]. Formation of Isomeric Clusters. Experimentally, Tan+Nm and Rhn+Nm can be prepared differently by introducing N2 gas diluted by He gas from the first valve (see Figure 1). Because N2 molecules exist around laser plasma formed upon laser ablation of the metal rods, a variety of energetic reactions can occur, such as formation of N atoms and the reaction of N2 with atomic Ta, which may produce isomers of clusters having different structures. Figure 7 shows TPD plots for thusprepared Rhn+Nm and Tan+Nm. Evidently, N2 molecules are totally released at 450 K for Rhn+Nm, the same results as those seen in the reaction gas cell. The similarity indicates that N2 E



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CONCLUSIONS

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ASSOCIATED CONTENT

The adsorption and desorption of N2 molecules with cationic Ta and Rh clusters in the gas phase were investigated in a thermal energy region by using thermal desorption spectrometry in combination with DFT calculations. Clusters formed by the laser ablation of metal rods were heated at 300−1000 K after a reaction with N2 molecules, and the thermal responses were observed by mass spectrometry for differently sized clusters. For Ta6+, the first N2 molecule, adsorbed molecularly, was found to readily dissociate onto the cluster surface by overcoming the activation barrier. Indeed, Ta6+N2 remained adsorbed when it was heated to 1000 K. In contrast, the second and the subsequent N2 molecules adsorbed weakly as molecular forms because the activation barrier was too high for the dissociation to occur. These N2 molecules were released into the gas phase when heated to 600 K. The adsorption chemistry of N2 molecules for Tan+ depended on the cluster size, n, and more N2 molecules were adsorbed dissociatively with increasing n. In contrast, for Rh6+, N2 adsorbed molecularly at 300 K, but it was found to desorb completely at 450 K, indicating that the N2 molecules were very weakly bound to the clusters. The DFT calculations indicated that dissociation of a N2 molecule on the surface was endothermic. When Ta clusters were generated in the presence of N2 gas by the laser ablation of a Ta rod, isomeric clusters, TanNm+, having heat resistivity were formed. The isomers were assigned to tantalum nitride clusters, in which a N atom bridges two Ta atoms and constructs the whole framework. Because N2 molecules exist around the laser plasma formed upon laser ablation of the Ta rod, a variety of energetic reactions can occur, such as formation of N atoms and reactions of N2 with atomic Ta that may produce isomers of clusters having different structures.

Figure 7. TPD plots for (a) Rh6+Nm and (b) Ta6+Nm formed with N2 in the carrier gas from the first pulsed valve. The numbers indicate the number of N atoms in the cluster, m.

molecules are weakly bound to the clusters. As discussed above, the energy of the dissociative adsorption of N2 onto Rh6+ is +4.4 eV; hence, formation of Rh6+NN is not likely because the reaction is endothermic. Even if it is formed inside the energetic plasma, N2 should be released by the reverse reaction. The calculation is thus consistent with the experimental results. In a sharp contrast, desorption of N2 was observed to a lesser extent for Tan+Nm, and N atoms remained in the clusters even at 1000 K: the intensities for Tan+Nm (m = 10 and 12) decreased at 450 K, whereas those for Tan+Nm (m = 2, 4, 6, and 8) remained unchanged above 700 K. The difference in temperature dependence from the one prepared in the reaction gas cell suggests that isomers of clusters stable against heat, most likely the structural isomers of Tan+Nm with N atoms incorporated in the framework, are produced. For instance, the intensity of Ta6+N6 increases with increasing temperature below 450 K and then levels off. The heat resistivity allows us to infer that Ta6+NNNNNN is formed. In contrast, the intensity of Ta6+N4 initially increases and then decreases below 700 K. However, the intensity levels off at temperatures higher than 700 K, but it does not reach zero even at 1000 K, indicating that there are isomers of Ta6+N4, i.e., Ta6+(N2)NN and Ta6+NNNN. The former cluster can release weakly bound N2 even at lower temperatures, generating Ta6+NN, whereas the subsequent cluster remains stable at 1000 K. Figure 8 shows the

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03479. TPD plots for Rhn+Nm (n = 5, 7, and 8) prepared in the reaction gas cell at room temperature and spin multiplicities and atomic coordinates for optimized structures (PDF)

Figure 8. Lowest energy isomers of Ta6+Nm (m = 6, 8, 10; see Figure 4 for m = 2 and 4).

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AUTHOR INFORMATION

Corresponding Author

lowest energy isomers of Ta6+Nm (m = 6, 8, 10; see Figure 4 for m = 2 and 4). For m ≤ 8, a N atom bridges two Ta atoms or sits on a hollow site of a capped trigonal bipyramids. For m = 10, a N atom gets into a Ta core and constructs the framework. Instead, a bond length of Ta−N of N atoms located outside is slightly elongated (0.20 nm), suggesting that the N atoms are weakly bound. Indeed, the desorption energy of N2 from Ta6+N10 was calculated to be 0.35 eV, which is significantly smaller than that of the other clusters (Ta6+N2, 5.67 eV; Ta6+N4, 2.45 eV; Ta6+N6, 3.59 eV; and Ta6+N8, 3.39 eV). The calculated values are consistent with our TPD results.

*E-mail: [email protected]. Tel: +81-3-54546597. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Grants-in-Aid for Scientific Research (A) (25248004) and Exploratory Research (26620002) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT) and by the Genesis Research Institute, Inc. (cluster research). F


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