Reactivity Control of Rhodium Cluster Ions by Alloying with Tantalum

Jan 22, 2016 - Matthias P. Klein , Amelie A. Ehrhard , Jennifer Mohrbach , Sebastian Dillinger , Gereon Niedner-Schatteburg. Topics in Catalysis 2017 ...
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Reactivity Control of Rhodium Cluster Ions by Alloying with Tantalum Atoms 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: Gas phase, bielement rhodium and tantalum clusters, RhnTam+ (n + m = 6), were prepared by the double laser ablation of Rh and Ta rods in He carrier gas. The clusters were introduced into a reaction gas cell filled with nitric oxide (NO) diluted with He and were subjected to collisions with NO and He at room temperature. The product species were observed by mass spectrometry, demonstrating that the NO molecules were sequentially adsorbed on the RhnTam+ clusters to form RhnTam+NxOx (x = 1, 2, 3, ...) species. In addition, oxide clusters, RhnTam+O2, were also observed, suggesting that the NO molecules were dissociatively adsorbed on the cluster, the N atoms migrated on the surface to form N2, and the N2 molecules were released from RhnTam+N2O2. The reactivity, leading to oxide formation, was composition dependent: oxide clusters were dominantly formed for the bielement clusters containing both Rh and Ta atoms, whereas such clusters were hardly formed for the single-element Rhn+ and Tam+ clusters. DFT calculations indicated that the Ta atoms induce dissociation of NO on the clusters by lowering the dissociation energy, whereas the Rh atoms enable release of N2 by lowering the binding energy of the N atoms on the clusters.

1. INTRODUCTION Rh is a rare element that acts as a catalyst in the removal of harmful gases, such as NO from gas mixtures by chemical reduction, and has been used in a number of applications such as three-way catalytic converters in automobiles.1 Although these catalysts have been developed well, there is still potential need for developing more effective catalysts that, for instance, operate at lower temperature. In this regard, the mechanism of NO reduction has been the subject of intensive research over the past few decades.2−11 The use of gas-phase clusters is a cutting-edge approach for investigative studies, as all atoms and molecules involved in the reaction are limited.12−14 Mackenzie and co-workers observed the reaction of NO molecules with isolated cationic and anionic Rh clusters, Rhn±, in the gas phase.15−18 After the adsorption of two NO molecules on the cluster, N2 was released, thereby furnishing the corresponding dioxide cluster, as observed via mass spectrometric analysis of Rhn+ (n < 17). These results were interpreted in terms of the dissociative adsorption of NO, which generated N atoms that migrated across the surface of the cluster. In contrast, no evidence of N2 production was observed for clusters of n = 13 or n > 16, where simple sequential NO adsorption dominated the chemistry. The cluster size dependence as well as the reaction time dependence suggested that the reaction is driven by heat generated upon NO adsorption on the isolated Rhn± in vacuum; the internal available energy derived from the exothermic adsorption process that remained in the isolated clusters without quenching plausibly induced dissociation of NO. © 2016 American Chemical Society

Indeed, in one of our previous studies, we investigated the adsorption of NO molecules on Rhn+ in thermal equilibrium at 300 K.19 The process involved the introduction of the clusters into a reaction gas cell filled with He. Collisions between the clusters and He resulted in satisfactory thermal equilibrium with the reactor wall at 300 K. In addition, Rhn+ with adsorbed NO molecules was introduced into an extension tube heated to temperatures in the range of 300−1000 K. NO molecules adsorbed molecularly on Rhn+ at 300 K were found to dissociate at higher temperatures. The experimental results were consistent with density functional theory (DFT) studies. The potential energy surface for the adsorption and dissociation of NO molecules on Rh7+ indicated that the NO molecules are adsorbed on a hollow (μ3) Rh7+ site in molecular form, with an adsorption energy of −2.0 eV.20 The most stable state where both N and O atoms were separately adsorbed on bridge (μ2) sites was located −2.4 eV from the initial state of Rh7+ and NO. In addition, transition from molecular adsorption to the dissociation of NO occurred with an activation barrier of +1.7 eV. Based on this energy barrier, the NO molecule was considered to be molecularly adsorbed on Rhn+ at 300 K. Here, we question whether the energy barrier for the dissociation of NO can be lowered to the level where the NO molecule dissociates in thermal equilibrium at 300 K on the Received: December 5, 2015 Revised: January 9, 2016 Published: January 22, 2016 861

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multiple initial geometries. For RhnTam+(NO), RhnTam+(NO)2, dissociatively adsorbed RhnTam+N2O2, and RhnTam+O2, structures with respective molecules and atoms randomly attached to the preoptimized RhnTam+ unit were adopted as initial structures, and the geometrical structures were then optimized. The transition states for NO dissociation and NO migration on Ta6+ were located using the STQN method implemented in Gaussian09 to estimate the reaction barriers between the molecularly and the dissociatively adsorbed clusters.21 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.

cluster, given that the dissociation is one of the bottleneck reactions in the reduction of NO. It is well-known that Ta has higher affinity for N atoms, as evidenced by nitride formation.21 We, therefore, examined the reaction of the bielement clusters, RhnTam+. Specifically, we investigate the reactivity, structures, and energetics of RhnTam+ with n + m = 6 experimentally and by DFT calculations.

2. EXPERIMENTAL SECTION The reactivity of bielement rhodium and tantalum cluster ions, RhnTam+, was investigated using time-of-flight (TOF) mass spectrometry in combination with thermal desorption spectrometry (TDS).22,23 The RhnTam+ cluster ions were prepared by the double pulse laser ablation in a cluster source. Each metal rod was vaporized using the focused second harmonic of a Nd:YAG pulse laser at a repetition rate of 10 Hz. The bielement cluster ions were formed by mixing the vapors in a flow of helium from a first pulsed valve with a stagnation pressure of 0.8 MPa. The timing and the pulse energy of each laser shot to the Rh and the Ta rods were tuned to optimize formation of the clusters. The cluster ions were passed through a reaction gas cell (60 mm length, 2 mm inner diameter), where the reactant NO gas (>99.95%) diluted with He (0.1 MPa) was injected using a second pulsed valve. The concentration of NO in He in the second valve was varied between 0−5% using mass flow and pressure controllers. In addition, the partial pressure of NO inside the vacuum chamber was monitored using a residual gas analyzer (MKS e-Vision 2). After passing the cluster ions through the reaction gas cell, they were introduced into an extension tube (120 mm length, 4 mm inner diameter) before expansion in a vacuum chamber to observe the thermal responses of the cluster ions. The extension tube was heated to 300−1000 K using a resistive heater and 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 ∼1017 molecules cm−3, respectively. Thermal equilibrium of the clusters was, therefore, achieved through collisions with the He carrier gas before expansion in the vacuum.22−25 For the TOF mass analysis, the cluster ions were accelerated to a kinetic energy of 3.5 keV in the acceleration region. After flying through a 2 m flight length including a reflectron, 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 O and N atoms appearing in the mass spectra.

4. RESULTS Figure 1 shows mass spectra of the bielement RhnTam+ cluster ions before and after reaction with NO in He at 300 K. Plot (a)

Figure 1. Mass spectra of RhnTam+ before and after reaction with NO in He at 300 K. Plot (a) shows that RhnTam+ (Rh6+, Rh5Ta+, Rh7+, Rh6Ta+) appeared as the nascent ions before the reaction. Plots (b) and (c) show the mass spectra after reaction with NO at the concentration of 0.5% and 1.1%, respectively. The mass spectra were recorded under the condition that RhnTam+ (m = 0, 1, and 2) was abundantly formed.

shows the appearance of RhnTam+ (Rh6+, Rh5Ta+, Rh7+, Rh6Ta+) as nascent ions before the reaction. It should be noted that other clusters assignable as RhnTam+, where n = 0− 10 and m = 0−6, were also formed, but only a selected mass region is shown in the figure for illustration. In plots (b) and (c), which show the mass spectra after reaction with NO, ion peaks corresponding to Rh6+NxOy and Rh5Ta+NxOy ((x, y) = (1, 1), (2, 2), (3, 3), (4, 4)) are clearly observed. The contribution of these ions increased with an increase in the concentration of NO. In addition, ion peaks of Rh5Ta+NxOy ((x, y) = (0, 2), (1, 3), (2, 4)) were also apparent, although no such oxygen-rich ions were appreciably formed for the Rh6+NxOy series. Thus, formation of the oxygen rich species is deduced to be dependent on the composition of the clusters. Figure 2 shows maps for each RhnTam+ (n + m = 6) moiety, displaying the intensities of the product ions, RhnTam+NxOy (x = 0, 1, 2, ... ; y = 0, 1, 2, ...), after reaction with NO in He at 300 K. In this context, the molecular formula RhnTam+NxOy does not necessarily indicate that NO is adsorbed dissociatively, as

3. COMPUTATIONAL SECTION To estimate the adsorption energies of molecularly or dissociatively adsorbed NO molecules on RhnTam+ (n + m = 6), DFT calculations were performed using the Gaussian09 program.26 The LANL2DZ effective core potential and basis set were used to describe the Rh and Ta atoms,27 while the 631G(d) basis set was used to describe the N and O atoms.28 Becke’s three-parameter hybrid density functional29 with the Lee−Yang−Parr correlation functional (B3LYP) was used for all calculations.30 The structures of RhnTam+ (n + m = 6) were obtained by optimizing a totally random geometry of atoms as initial structures. Optimization was complete when the structure with the lowest energy was obtained from the 862

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Figure 2. Maps for each RhnTam+ (n + m = 6) displaying intensities of product ions, RhnTam+NxOy (x = 0, 1, 2, ... ; y = 0, 1, 2, ...), after reaction with NO in He at 300 K. A map for Rh3Ta3+ is not shown here because intensities of ion peaks are not precisely estimated due to overlaps of the mass peaks such as Rh3Ta3+O2 and Rh8+N2O2.

Figure 3. Maps for each RhnTam+ (n + m = 6) displaying intensities of product ions, RhnTam+NxOy (x = 0, 1, 2, ... ; y = 0, 1, 2, ...), after heating to 1000 K.

observed after the reaction of NO. As the ion peaks were also observed in the absence of NO, these clusters were interpreted as resulting from generation by laser ablation of preadsorbed NO on the Ta rod. In addition, it is highly likely that the fragments, TaN and TaO, were released upon reaction of NO with the clusters. As discussed below, the reaction with NO results in generation of excess energy that cannot be fully dissipated by the collision of surrounding He atoms. In contrast, the Rh-rich clusters generate a moderate amount of excess energy upon reaction with NO, showing no significant fragmentation. To examine the nature of adsorption of NO, all cluster ions, formed by the reaction with NO, were fed into the heated extension tube. The mass spectrometric responses were observed at 1000 K. Figure 3 shows maps for each RhnTam+ (n + m = 6) species, displaying the intensities of the product ions after heating to 1000 K. In the case of Rh6+, the distribution was found to be narrowed along the diagonal line, indicating desorption of certain molecules (most likely NO) from the clusters upon heating.19 This thermal desorption indicates that the atoms and molecules were weakly bound to the clusters. In contrast, for Ta6+, the distribution of the diagonal and off-diagonal components changed only slightly

no direct information was obtained regarding the nature of the adsorption. Diagonal lines were apparent at y = x, suggesting that the same numbers of N and O atoms were accommodated in the clusters after the reactions. The diagonal line corresponding to Rh6+NkOk (k = 0, 1, 2, ...) for Rh6+ was particularly prominent. It is thus plausible that NO was adsorbed on Rhn+ without any further reactions. In contrast, for Rh5Ta+, off-diagonal components, especially oxygen richspecies such as Rh5Ta+O2 and Rh5Ta+NO3 were produced in high abundance in addition to Rh5Ta+NkOk. These oxygen-rich clusters are considered to be formed via bond dissociation of NO on the cluster surface, followed by N2 release from the cluster, as discussed below. Formation of the oxygen-rich clusters was observed for Rh5Ta+ and Rh4Ta2+, whereas a slight contribution from the off-diagonal components was observed for Ta6+, suggesting that the reaction of NO on the cluster surface was hindered. In other words, oxygen-rich clusters were dominantly formed for the bielement clusters containing both Rh and Ta atoms, whereas these oxygen-rich clusters were not formed in high abundance for the single-element Rh6+ and Ta6+ clusters. For the Ta-rich clusters (such as Rh2Ta4+, RhTa5+, and Ta6+), one O atom-rich or one N atom-rich species (for instance, Ta6+O, Ta6+NO2, Ta6+N, Ta6+N2O, or Ta6+N3O2) was 863

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Figure 4. TDS plots showing relative ion intensities of Rh6+NxOx, Rh5Ta+NxOx, and Rh2Ta4+NxOx as a function of temperature.

recombine and evaporate off the cluster surface to form oxygenrich species as represented in eq 2.

when the clusters were heated. Namely, the adatoms are considered to be tightly bound to the clusters. Figure 4 shows TDS plots of the relative ion intensities of Rh6+NxOx, Rh5Ta+NxOx, and Rh4Ta2+NxOx as a function of temperature. For Rh6+, the intensity of the Rh6+N3O3 peak decreased gradually, whereas the peaks of Rh6+N1O1 and Rh6+N2O2 increased slightly and then decreased with increased temperature. Moreover, the increase in the intensity of the Rh6+ peak corresponded directly to the decrease in the intensity of the peaks of the other clusters with increasing temperature. These characteristic changes of the intensities are ascribed to stepwise desorption of the NO molecules as indicated in eq 1.

Rh nTa m+Nx Oy → Rh nTa m+Nx − 2Oy + N2

(2)

Nature of NO Adsorption. The different forms of the NO molecules adsorbed on RhnTam+ (n + m = 6) were evaluated by the theoretical calculations. Figure 5 shows the energy diagram

Rh6+N3O3 → Rh6+N2O2 + NO → Rh6+N1O1 + 2NO → Rh6+ + 3NO

(1)

In contrast, for Rh2Ta4+, the intensities of the Rh2Ta4+NxOx peaks did not change significantly over the entire evaluated temperature range, suggesting that no desorption of molecules occurred at the high temperatures. For Rh5Ta+, Rh5Ta+N2O2 decreased gradually and Rh5Ta+ increases in their relative intensity with an increase in the temperature. In contrast, the intensity of Rh5Ta+NO appeared almost unchanged. As the peaks of Rh5Ta+O and Rh5Ta+O2 increased evidently (see Figure S1), it is highly likely that several reaction pathways open at higher temperature including sequential release of two NO molecules, release of a N2O molecule, and release of a N2 molecule.

Figure 5. Energy diagram of clusters, RhnTam+ (n + m = 6), estimated by DFT calculation, along a reaction coordinate, corresponding to molecular adsorption of two NO molecules, dissociative adsorption of two N and two O atoms, N2 and oxide formation, and N2 release.

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of the clusters, estimated by DFT calculation, along a reaction coordinate corresponding to molecular adsorption of two NO molecules, dissociative adsorption of two N and two O atoms, and oxide formation with N2 release. For instance, two NO molecules are molecularly adsorbed on Rh6+ with a total adsorption energy of −326 kJ mol−1. These NO molecules may be dissociatively adsorbed on Rh6+ with an adsorption energy of −229 kJ mol−1; however, the dissociation of NO on the cluster surface is endothermic. In essence, the Rh atom has lower affinity for the N atom, whereas the Ta atom exhibits higher affinity for both the O and N atoms, as evidenced by oxide and nitride formation.21 Hence, the energy of either molecular or dissociative adsorption is lowered by alloying with Ta atoms. In addition, the height of the activation barrier between molecular and dissociative adsorption is inferred to be reduced to such a level that the activation energy is located below that of the initial state before the reaction. In fact, the barrier between the molecularly and dissociatively adsorbed Ta6+(NO) clusters was estimated to be

DISCUSSION Adsorption of NO on Clusters. Mass spectrometric analysis of the reaction of Rhn+ with NO suggests that NO molecules were simply adsorbed onto the cluster to form Rhn+(NO)k in He at 300 K and that NO molecules were desorbed from Rhn+(NO)k upon heating. The moderate adsorption and desorption of the NO molecules indicate that the NO molecules were molecularly rather than dissociatively adsorbed on the cluster. This is consistent with the fact that NO is adsorbed molecularly on Rh(111) surfaces at 273 K and desorbs readily at 490 K.3 In contrast, formation of oxygen-rich species by the reaction of RhnTam+ (n + m = 6; m = 1 and 2) with NO molecules and the strong adsorption of the atoms on the clusters despite heating are interpreted as resulting from dissociative chemisorption of NO. In fact, the formation of oxygen-rich species can be explained in terms of migration of the N atoms produced via dissociative chemisorptions; the N atoms may 864

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The Journal of Physical Chemistry A 8 kJ mol−1 from DFT calculation (see Figure 6), whereas that of Rh6+(NO) was 249 kJ mol−1.19

In the case of Rh5Ta+, the energy of dissociative adsorption and the barrier height are both lower, increasing the probability of dissociative adsorption. In addition, the oxide, Rh5Ta+O2, is more stable than Rh5Ta+N2O2, causing N2 release from the cluster. Hence, the NO molecules were dissociatively adsorbed on the cluster, releasing N2 to form Rh5Ta+O2. The barrier height is further lowered for Rh3Ta3+, which further increases the probability of dissociative adsorption. However, the oxide, Rh3Ta3+O2, is less stable than Rh3Ta3+N2O2. The energy of the dissociative adsorption of NO on Rh2Ta4+N2O2 is markedly lower than that of the oxide, Rh2Ta4+O2, suggesting an increase in the probability of dissociative adsorption, but N2 release should be prevented. Nevertheless, mass spectrometric analysis clearly shows that Rh2Ta4+O2 was actually formed although it was less abundant (see Figure 2). It is likely that exothermic NO adsorption increased the internal energy to such an extent that N2 was released prior to thermal quenching of the cluster by the He atoms. The adatoms remained on the cluster upon heating as they were strongly bound (see Figure 4). Thus, the Ta atoms that have higher affinity for N as well as O atoms adequately stabilize NO molecules on the cluster. Alloying the Rh clusters with Ta atoms was found to lower the activation barrier, leading to dissociative adsorption, increasing the ratio of dissociatively adsorbed NO molecules. However, alloying with Ta atoms also lowered the energy level of dissociative adsorption significantly, preventing recombination of the N atoms to form N2. Structures of Rh−Ta Clusters. The lowest-energy geometrical structures of RhnTam+ (n + m = 6, m = 0−6) are shown in Figure 7. The Ta6+ cluster has an octahedral structure as

Figure 6. Energy diagram of Rh6+ and Ta6+ reacting with NO. The energy barrier at the transition state (TS) from the intermediate (IM) to the dissociative adsorption form is calculated to be 8 kJ mol−1 for Ta6+.

Once the N atom undergoes dissociation on the cluster, these atoms are likely to migrate and may recombine to form N2. Here, the energy level of the product, i.e., oxide cluster + N2, is comparable with the energy of dissociative adsorption. In the case of Rh6+, as the energy level of the oxide Rh6+O2 + N2 is lower than that of the dissociatively adsorbed form, thus this reaction step is exothermic. Alloying with three Ta atoms lowers the energy level of the dissociatively adsorbed species relative to that of the oxide cluster plus a gas phase N2 molecule, given that the affinity of Ta for N atoms is higher than that of Rh for N. In addition, the Ta6+ cluster significantly stabilizes two NO molecules in the dissociative form, and hence, the energy level is far below that of Ta6+O2 + N2. Reactions of NO on the Cluster in He Gas. For the isolated clusters in vacuum, the excess energy generated during the reactions can only be dissipated by a slow radiative process. Hence, the internal energy mostly remains in the clusters after the reactions. For an exothermic reaction, the reaction is considered to proceed readily, if there is no reaction barrier in the reaction pathway, and the energy of the barrier is higher than that of the initial state. In contrast, the present experiment was conducted within the flow of the buffer gas that acted as an effective heat bath, which swiftly quenched the internal energy generated during the reactions.19 Hence, even if the reaction of interest is totally exothermic and the highest activation barrier is lower than the initial state, the reaction may not proceed due to the reaction barrier. The theoretical calculations presented above can be used to explain the experimental results as follow: For Rh6+, there is a high energy barrier between molecular and dissociative adsorption, hindering dissociation of the NO molecules on the clusters. Hence, the NO molecules are molecularly adsorbed onto Rh6+ in He at 300 K. Figure 2 shows that the Rh6+ cluster accommodated about five NO molecules at 300 K and released the NO molecules upon heating to 1000 K. Note that the energies of desorption of one NO molecule from Rh6+(NO)2 is 172 kJ mol−1 and from Rh6+(NO) is 154 kJ mol−1.

Figure 7. Geometrical structures of Rh−Ta cluster ions. The spin multiplicity is given in parentheses.

shown in Figure 7g, which is essentially the same as previously reported.31 Rh5Ta+ has a structure in which one Rh atom of Rh6+ is replaced by a Ta atom. In contrast, RhTa5+ has a structure in which one Rh atom is attached to a trigonal bipyramidal Ta5+ structure. This can be explained by the difference in the binding energy of Rh−Rh, Rh−Ta, and Ta− Ta. The energy change resulting from the substitution reaction of one Rh atom in Rh6+ by one Ta atom is exothermic (−372 kJ), while substitution of one Ta atom in Ta6+ by one Rh atom is endothermic (+98 kJ). This tendency indicates that the binding energy of the Rh−Ta pair is higher than that of the Rh−Rh couple and is lower than that of the Ta−Ta pair. Since the binding energy of the Rh−Ta pair is lower than that of the Ta−Ta pair, the Rh atom cannot be embedded into Ta5+. The other clusters also have structures in which the Rh atoms are attached to the Ta cluster cores for the same reason. 865

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The Journal of Physical Chemistry A The lowest-energy structures of selected RhnTam+(NO)2 species are shown in Figure 8. In the case of molecularly

clusters were dominantly formed for the bielement clusters containing both Rh and Ta atoms, whereas oxide clusters were only formed in low abundance for the single-element Rhn+ and Tam+ clusters. The DFT calculations indicate that alloying with Ta atoms lowers either the energy level of molecular or dissociative adsorption. In addition, the height of the barrier between molecular and dissociative adsorption is also lowered to such an extent that the energy is below that of the initial state before the reaction. Hence, the Ta atoms enhance dissociation of NO upon adsorption onto the cluster. In contrast, the energy level of dissociative adsorption is lowered by alloying with three Ta atoms relative to that of the oxide cluster + N2, given that Ta atoms have higher affinity for N atoms. In addition, the Ta6+ cluster significantly stabilizes two NO molecules in the dissociated form, and hence, the energy level is far below that of Ta6+O2 + N2. Thus, the Ta atoms prevent N2 release from the cluster.

Figure 8. Geometrical structures of (a) molecularly adsorbed Rh5Ta+(NO)2, (b) dissociatively adsorbed Rh5Ta+N2O2, (c) molecularly adsorbed Rh 4 Ta 2 + (NO) 2 , (d) dissociatively adsorbed Rh4Ta2+N2O2, (e) molecularly adsorbed RhTa5+(NO)2, and (f) dissociatively adsorbed RhTa5+N2O2. The spin multiplicity is given in parentheses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11898. TDS plots showing relative ion intensities of Rh5Ta+NxOx (x = 0 and 2) and Rh5Ta+Ox (x = 1 and 2) as a function of temperature (PDF)

adsorbed Rh5Ta+(NO)2, one NO is adsorbed on the on-top site of the Ta atom, and another NO is adsorbed on the on-top site of the Rh atom on the opposite side, and the binding energy is close to that of molecularly adsorbed Rh6+(NO)2. In contrast, for molecularly adsorbed Rh4Ta2+(NO)2, one NO is adsorbed on the on-top site of the Ta atom and the other one bridges two Ta atoms: the binding energy is higher than that of molecularly adsorbed Rh6+(NO)2. For Rh5Ta+N2O2 with two dissociatively adsorbed NO units, the two N atoms are bonded to the hollow site surrounded by three Rh atoms, whereas for Rh4Ta2+N2O2, the two N units are bonded to the hollow site surrounded by one Rh atom and two Ta atoms. Since Ta metal generally has higher affinity for nitrogen than Rh metal, as described above, the adsorption energy of the two dissociatively adsorbed NO molecules on Rh4Ta2+ (−901 kJ mol−1) is higher than that for adsorption on Rh5Ta (−624 kJ mol−1), as shown in Figure 5. For RhTa5N2O2+ with two dissociatively adsorbed NO units, the two N atoms are bonded to the hollow site surrounded by three Ta atoms, and the two O atoms are bonded to the bridge site between two Ta atoms. Evidently, the N and O atoms prefer interacting with Ta atoms to Rh atoms.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (no. 25248004) and a Grant-in-Aid for Exploratory Research (no. 26620002) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT), and by the Genesis Research Institute, Inc., for the cluster research.

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REFERENCES

(1) Shimokawabe, M.; Umeda, N. Selective Catalytic Reduction of NO by CO over Supported Iridium and Rhodium Catalysts. Chem. Lett. 2004, 33, 534−535. (2) Loffreda, D.; Simon, D.; Sautet, P. Vibrational Frequency and Chemisorption Site: a DFT-periodic Study of NO on Pd(111) and Rh(111) Surfaces. Chem. Phys. Lett. 1998, 291, 15−23. (3) Nakamura, I.; Kobayashi, Y.; Hamada, H.; Fujitani, T. Adsorption Behavior and Reaction Properties of NO and CO on Rh(111). Surf. Sci. 2006, 600, 3235−3242. (4) Castner, D. G.; Sexton, B. A.; Somorjay, G. A. Leed and Thermal Desorption Studies of Small Molecules (H2, O2, CO, CO2, NO, C2H4, C2H2, and C) Chemisorbed on Rhodium (111) and (100) Surfaces. Surf. Sci. 1978, 71, 519−540. (5) Villarrubia, J. S.; Ho, W. Nitric Oxide Adsorption, Decomposition, and Desorption on Rh(100). J. Chem. Phys. 1987, 87, 750− 64. (6) Kao, C.-T.; Blackman, G. S.; Van Hove, M. A.; Somorjai, G. A.; Chan, C. − M. The Surface Structure and Chemical Reactivity of Rh(111)-(2 × 2)-3NO by HREELS and Dynamical LEED Analysis. Surf. Sci. 1989, 224, 77−96.

CONCLUSIONS Gas phase, bielement rhodium and tantalum clusters, RhnTam+ (n + m = 6), were prepared and subsequently introduced into a reaction gas cell filled with nitric oxide (NO) diluted with He, where they were subjected to collisions with NO and He at room temperature. The product species generated after the collisions with NO and He were observed by mass spectrometry. Analysis of the reaction of Rh6+ with NO suggests that the NO molecules were molecularly adsorbed onto the cluster to form Rhn+(NO)k in He and that NO molecules were desorbed from Rhn+(NO)k upon heating. In contrast, NO molecules were dissociatively adsorbed on RhnTam+ (n + m = 6; m = 1−5) and were strongly bound to the cluster. Due to migration of the N atoms on the cluster, the N atoms could recombine and evaporate off the cluster surface to form oxygen rich species such as RhnTam+O2. Thus, reactivity leading to oxide formation was composition dependent: oxide 866

DOI: 10.1021/acs.jpca.5b11898 J. Phys. Chem. A 2016, 120, 861−867

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DOI: 10.1021/acs.jpca.5b11898 J. Phys. Chem. A 2016, 120, 861−867