Thermal Desorption and Reaction of NO Adsorbed on Rhodium

Jul 9, 2015 - Cationic rhodium clusters, Rhn+ (n = 4–8), were prepared in the gas phase by the laser ablation of a Rh rod. The Rhn+ clusters were in...
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Thermal Desorption and Reaction of NO Adsorbed on Rhodium Cluster Ions Studied by Thermal Desorption Spectroscopy Yuki Tawaraya, Satoshi Kudoh, Ken Miyajima, 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: Cationic rhodium clusters, Rhn+ (n = 4−8), were prepared in the gas phase by the laser ablation of a Rh rod. The Rhn+ clusters were introduced into a reaction gas cell filled with nitric oxide (NO) diluted with He, where they were subjected to collisions with NO and He in a thermal equilibrium at 300 K. The NO molecules were found to adsorb sequentially on the Rhn+ clusters forming Rhn+(NO)m. To examine the adsorption form and the reaction of NO, we heated Rhn+(NO)m in an extension tube located after the reaction gas cell and the thermal response of the clusters, desorption of the fragments, was recorded as a function of temperature (300−1000 K). The desorption of NO molecules was predominantly observed below 500 K, giving either Rhn+(NO)n+1 or Rhn+(NO)n+2, which indicates that there were NO molecules loosely adsorbed on the Rhn+ clusters. Further desorption was found to proceed at higher temperatures (500−1000 K), whereby NO was released from the smaller clusters, Rhn+ (n ≤ 5). In contrast, for the larger clusters (n ≥ 6), N2 release was clearly observed at high temperatures (>800 K). Thus, the reduction of NO occurred for larger clusters at higher temperatures.

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INTRODUCTION Rhodium is an important metal for use in the removal of harmful gases such as NO and CO and thus has been used in a number of applications such as three-way catalytic converters in automobiles.1 The mechanism of NO reduction has been the subject of intensive research over the past few decades, and it has been found that its surface structure is particularly sensitive to temperature, surface coverage, and the properties of the adsorbate. The most stable structure of NO on Rh(111) corresponds to hollow (μ3) site molecules at 0.5 ML coverage, which changes to one atop (μ1) site and two hollow site adsorbates for a coverage of 0.75 ML.2 In the presence of Rh(111) containing preadsorbed CO, the adsorption of NO on the hollow site is inhibited by CO on the atop site.3 In addition, NO has been found to adsorb molecularly on all Rh surfaces at low temperatures ( 16, where simple sequential NO adsorption dominated the chemistry. The cluster size and NO density dependences suggested that the reaction is driven by heat generated upon NO adsorption to the isolated Rhn± in vacuo. The dissociative adsorption of NO and the formation of N2 were supported by density functional theory (DFT) studies. Xie and coworkers obtained a potential energy surface for the adsorption and dissociation of an NO molecule on the Rh7+ cluster, as shown in Figure 1.15 They found that the NO molecule first adsorbs on a hollow Rh7+ site in molecular form, with an adsorption energy of −2.0 eV. In addition, it was found that a transition state exists for the dissociation of NO, with an energy barrier of 1.7 eV. The most stable state where both N Received: May 2, 2015 Revised: July 9, 2015

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was vaporized using the focused second harmonic of an Nd:YAG pulse laser at a typical pulse energy of 10 mJ and a repetition rate of 10 Hz. The cluster ions were formed under a flow of helium from a first pulsed valve with a stagnation pressure of 0.8 MPa. 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 an additional second pulsed valve. The concentration of NO in He in the second valve was varied between 0 and 10% using mass flow and pressure controllers. In addition, the pressure of NO inside the vacuum chamber was monitored using a residual gas analyzer (MKS e-Vision 2). The typical gas pressure inside the reaction gas cell, mainly due to contributions from He, was monitored using a pressure gauge and was found to rise to almost 3 × 103 Pa during pulsing. The number density of He was estimated to be ∼7 × 1017 molecules cm−3, giving a collision frequency of 1.2 × 108 s−1. Because the residence time of the cluster ions in the reaction gas cell was estimated to be ∼70 μs, the clusters were subjected to approximately 8.4 × 103 collisions during that time. 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. The extension tube was heated to 298−1000 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. The thermal equilibrium of the clusters was therefore achieved through collisions with the He carrier gas before expansion into the vacuum.20−22 Both the residence times in the reaction gas cell and in the extension tube were estimated from the measured flight time between the cluster source and the probe region, and their accuracy was ±20%. The gas cell and the tube were thermally isolated by a ceramics, and the outside of the gas cell was additionally cooled by N2 gas. Temperature-dependent changes in the cluster ions were monitored using mass spectrometry. Peak intensities of the clusters were measured as a function of temperature to determine quantitatively clusters that increased or decreased by thermal desorption with a rise in temperature. The intensity curves provide information similar temperature-programmed desorption (TPD) curves for the adsorbates on a solid surface. Furthermore, the cluster ions remaining after thermal desorption were monitored, and the desorption of neutral species from the cluster ions was not detected. In addition, nascent cluster ions were constantly supplied to the temperature-controlled extension tube, and thus the resulting TPD curves are given as integrals.23 The cluster ions were accelerated to a kinetic energy of 3.5 keV in the acceleration region. The ions were detected using a Hamamatsu double-microchannel plate detector, signals of which were amplified with a preamplifier and digitized using an oscilloscope. In addition, the mass resolution (m/Δm) was sufficiently high (>1000 at m = 1000) to distinguish O and N atoms appearing in the mass spectra.

Figure 1. Potential energy surface for the (I) adsorption and (II) dissociation of the first NO molecule on Rh7+.15

and O atoms were separately adsorbed on bridge (μ2) sites was observed 2.4 eV below the initial state of Rh7+ and NO. Because the transition state was found to be 0.3 eV lower in energy than the initial state, NO was able to adsorb dissociatively on Rh7+. In addition, two possible reaction pathways were proposed for the formation of N2 following the adsorption of a second molecule of NO. Taking into account these previous studies, we wished to determine whether N2 could be formed upon the coadsorption of two NO molecules on Rhn+ in a thermal equilibrium at 300 K. According to DFT calculations carried out by Xie and coworkers,15 molecularly adsorbed NO should not dissociate on the Rh7+ cluster upon removal of the adsorption energy; therefore, the formation of N2 should not be observed. We therefore chose to examine the reaction of Rhn+ with NO in the thermal equilibrium at 300 K. This process involves the introduction of the clusters into a reaction gas cell filled with He.16,17 Collisions between the clusters and He are expected to result in a satisfactory thermal equilibrium with the reactor wall at 300 K. In addition, Rhn+ with adsorbed molecules will be introduced to an extension tube elevated at a temperature of 300−1000 K. We expect that the adsorption of NO will be observed at 300 K, while a thermal desorption processes will be observed at higher temperatures. Reactions will be monitored by mass spectrometry.

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EXPERIMENTAL SECTION The reactivity of the rhodium cluster ions, Rhn+, was investigated using a time-of-flight (TOF) mass spectrometer in combination with thermal desorption spectroscopy, as shown in Figure 2.18,19 Rhn+ cluster ions were prepared using pulse laser ablation in a cluster source. A Rh (99.9%) metal rod

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COMPUTATIONAL SECTION To estimate the adsorption energies of N and O atoms and of an NO molecule to Rh6+, we performed DFT calculations using the Gaussian09 program.24 The LANL2DZ effective core potential and basis set were used to describe Rh atoms,25 while the 6-311+G(d) basis set was used to describe N and O

Figure 2. Schematic diagram of the experimental setup used in this study. B

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The Journal of Physical Chemistry A atoms.26 Becke’s three-parameter hybrid density functional27 with the Lee−Yang−Parr correlation functional (B3LYP) were used for all calculations.28 The structures of the molecularly adsorbed Rh6(NO)+ and the dissociatively adsorbed Rh6NO+ reported by Torres and coworkers29 were adopted as initial structures and were reoptimized. The transition states for NO dissociation and NO migration on Rh6+ were located using the STQN method implemented in Gaussian 09 to estimate the reaction barriers between the molecularly and the dissociatively adsorbed clusters.30 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.

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RESULTS Figure 3 shows the evolution of Rhn+ mass spectra before and after reaction with NO gas. Rhodium cluster ions, Rhn+ (n = 6

Figure 4. Maps showing intensities of the product ions after reaction with 10% NO in He at room temperature. The product species are aligned diagonally, indicating that equal numbers of N and O atoms attach to the Rhn+ species.

clusters rich in single O atoms or single N atoms were produced, as previously mentioned. Furthermore, for n = 7, O2rich clusters, (Rh7+(NO)mO2) are identifiable on the map, albeit of lower intensity, suggesting that N2 release occurred at room temperature. The number of NO molecules adsorbed on the clusters and the contribution of the off-diagonal components therefore varied according to cluster size, n, while basic features of the reactions at room temperature were common to all values of n investigated herein. Figure 5 shows the relative intensities of the Rh6+(NO)m (m = 0−9) species after the reactions as a function of NO

Figure 3. (a) Mass spectrum obtained from pristine Rh6+ and Rh7+ clusters produced by laser ablation of a Rh rod in He (0.8 MPa). (b− d) Mass spectra of the clusters after reaction with NO in He gas at room temperature. The concentrations of NO in He were 0.16, 0.2, and 1% in panels b−d, respectively.

and 7), appeared as the nascent ions before the reaction. It should be noted that Rhn+ clusters where n = 4−8 were also observed; however, only selected ions are shown in the Figure for illustration. Plots b−d show the mass spectra after reaction with NO, where the ion peaks corresponding to Rh6+(NO)m (m = 1, 2, 3, and 4) can be clearly seen. From these plots, it can also be seen that the number of NO molecules adsorbed on Rh6+ (m) gradually increased with an increase in NO concentration in the reaction gas cell. Figure 3 also shows minor ion peaks between the adjacent peaks of Rh6+(NO)m and Rh6+(NO)m+1, which can be unambiguously assigned to Rh6+O, Rh6+N, and their NO adducts. As Rhn+O and Rhn+N were observed in the absence of NO, it is concluded that these clusters were generated by the laser ablation of preadsorbed NO on the Rh rod. When the concentration of NO was increased, further NO adsorption to Rhn+(NO)m was observed. Figure 4 shows maps for n = 4−7 illustrating the intensities of the product ions after reaction with NO in He at 300 K. It can be seen that the product species are predominantly aligned in a diagonal line, indicating that equal numbers of N and O atoms were attached to the Rhn+ species. In addition, minor contributions were observed in the off-diagonal sections of the map, indicating that

Figure 5. Relative intensities of Rh6(NO)m+ (m = 0−9) as a function of the concentration of NO in He at room temperature. Solid lines are the result of least-squares fitting to the stepwise adsorption scheme: Rh6+ → Rh6(NO)+ → Rh6(NO)2+ → etc.

concentration in the reaction gas cell. It can be clearly seen that the number of adsorbed NO molecules on Rh6+ increased gradually with an increase in the NO concentration, indicating that Rh6+(NO)m was generated by the sequential adsorption of NO molecules to the naked cluster according to eq 1 C

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The Journal of Physical Chemistry A Rh 6+ → Rh6+(NO) → Rh6+(NO)2 → Rh 6+(NO)3 → Rh6+(NO)4 → ...

(1)

Rh6+(NO)7

In addition, it can be seen that the species became dominant when the concentration of NO exceeded 5%, suggesting that the adsorption reaction to give Rh6+(NO)8 from Rh6+(NO)7 was slow. For n = 4, 5, and 7, the stepwise adsorption of NO did not appear to depend significantly on the number of adsorbed NO molecules, m, as can be seen by the cluster intensities shifting gradually with concentration. (See Figure S1 in the SI.) The concentration dependences were analyzed based on reaction 1, and it was found that the intensity changes were reproduced well by the models. The majority of Rhn+ reactions could be explained in terms of the simple adsorption of NO. Moreover, the pseudo-first-order rate constant, kn,m, for the formation of Rhn+(NO)m from Rhn+(NO)m−1 and NO was obtained by the least-squares fitting method. The rate constants for NinOm+ with NO have been reported by Vann et al.31 Hence, the rate constants for Rhn+ with NO were calibrated by measuring the rate constant of Ni7O8+ with NO in the same experiment. As shown in Figure 6,

Figure 7. Maps showing intensities of the product ions for n = 6 after heating in the extension tube at a range of temperatures. The distribution shifts to a lower range at 500 K, showing that NO molecules were desorbed from the clusters upon heating.

Figure 6. Pseudo-first-order rate constants, kn,m (n = 4−8, m = 1−12), for the adsorption of NO on Rhn+(NO)m−1: Rhn+(NO)m−1 + NO → Rhn+(NO)m.

the rate constant tends to increase for small values of m, and decrease for large values of m with an increase in m. (See Table S1 in the SI.) In fact, kn,m increased until either Rhn+(NO)n+1 or Rhn+(NO)n+2 was formed, after which point a sharp decrease was observed. The decrease is considered to relate to the occupation of an active adsorption site for NO so that further molecules need to adsorb on more inactive site, which implies a weaker interaction and concomitantly a reduced heat of absorption/binding energy. To investigate the nature of adsorption of NO on the clusters, we fed all cluster ions prepared by the reaction with NO in He at 300 K into the heated extension tube. The mass spectrometric responses were observed as a function of the temperature of the extension tube. Figure 7 shows the maps for n = 6 where the abundances of the product ions after heating at a range of temperatures are given. The distribution was found to decrease along the diagonal at 500 K, showing that NO molecules desorbed from the clusters upon heating. The offdiagonal components, especially on the oxygen rich side, began to appear when the clusters were heated to 700 K, due to N2 release from the clusters. It could therefore be concluded that the composition of the product species was temperaturedependent. Figure 8a shows the relative ion intensity of

Figure 8. (a) Relative ion intensity of Rh6+(NO)m (m = 0−9) as a function of the temperature of the extension tube. (b) Differential form of plot a.

Rh6+(NO)m as a function of temperature. It can be seen from this plot that the relative ion intensity of Rh6+(NO)m≥8 decreases upon heating above room temperature, suggesting D

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The B3LYP calculations showed that the NO molecule first adsorbed on an atop site of the Rh6+ cluster through N coordination, with an adsorption energy of −1.5 eV. In the dissociation reaction pathway, a transition state formed from the molecularly adsorbed state with an energy barrier of 2.6 eV. This transition state then falls into a more stable state, 1.2 eV lower than the initial state, where the N and O atoms were separately adsorbed on hollow sites on the cluster.30 When an NO molecule was adsorbed onto an atop site of the Rh6+ cluster, the adsorption energy was distributed to the vibrational degrees of freedom of the cluster, causing the temperature of the clusters to rise. The heat of adsorption allows for swift NO dissociation under adiabatic conditions. The present NO adsorption experiment is conducted, however, within the flow of the buffer gas that acts as an effective heat bath that swiftly dissipates heat of adsorption of NO. Thus, molecular adsorption becomes enhanced. The estimated collision frequency (2 × 108 s−1) for this process corresponded to collisions with an average interval of 5 ns. It was therefore clear that adsorption of NO occurred in thermal equilibrium with the reactor wall at room temperature, although quantification of the cooling time was problematic. Hence, it is possible to infer that NO was trapped in the stable molecular state. The second NO molecule adsorbed on Rh6+(NO) increased the internal energy once again, thus enhancing the possibility of NO dissociation; however, once again, the dissociation of NO competed with collisional cooling. Harding et al. found different structural isomers of Rh6+ considering a large range of spin multiplicities by PBE calculations.32 There was an equilibrium between the most stable square bipyramid structure and less stable trigonal prism structure, and the energy barrier of the forward reaction ranged in 0.4 to 0.7 eV. In addition, the reaction path of Rh6+(NO) with molecularly adsorbed NO leading to dissociatively adsorbed atoms was calculated, showing that the trigonal prism structure presented quite low energy barrier of 0.23 to 0.36 eV. Hence, under the adiabatic conditions, the heat of adsorption enhanced swift dissociation of NO. It should be noted that even in the molecular form the binding energies of the first NO molecule to Rhn+ exceed 1.5 and 2.0 eV for n = 6 and 7, respectively.15,30 In contrast, the TPD curves in Figure 8a show that the intensities of Rh6+(NO)8,9 readily decrease with a rise in temperature, suggesting that the NO molecules in these clusters were very loosely bound and were, most likely, physisorbed. From the TPD curve, the threshold energy for the desorption of NO from Rh6+(NO)7 can be calculated. In the experiments reported herein, all cluster ions that passed through the extension tube were observed by mass spectrometry. As the intensities of Rh6+(NO)8,9 were negligible, the intensity of Rh6+(NO)7 was considered to decrease according to eq 3

that loosely bound NO molecules desorbed from the cluster ions under such conditions, according to eq 2 Rh6+(NO)9 → Rh6+(NO)8 + NO → Rh 6+ (NO)7 + 2NO

(2)

The relative ion intensity of Rh6+(NO)5,6,7 also decreased gradually with an increase in temperature, although a small amount of these clusters remained even upon heating to 700 K, suggesting that fifth, sixth, and seventh NO molecules adsorbed more tightly to the clusters than the eighth and ninth NO molecules. The sequential release of NO is more clearly demonstrated in the differential plot (Figure 8b). The negative and positive values indicate that the cluster decreases and increases with an increase in temperature, respectively. In the negative region, m of Rh6+(NO)m decreases with an increase in temperature. In addition, the appearance of the negative signal corresponding to Rh6+(NO)3 is observed at approximately the same temperature (870 K) as the appearance of the positive signal corresponding to Rh6+(NO)2, suggesting that NO is released from Rh6+(NO)3 to form Rh6+(NO)2.

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DISCUSSION Molecular Adsorption of NO at Room Temperature. Mass spectrometric analysis of the reaction of Rhn+ with NO indicates that NO molecules adsorbed sequentially onto the cluster to generate the Rhn+(NO)m species at 300 K and that NO molecules desorbed from this species upon heating. In this context, the molecular formula Rhn+(NO)m does not necessarily indicate that NO was adsorbed molecularly, as no direct information was obtained regarding the nature of the adsorption; however, it is possible to infer the likely nature of the adsorption by considering the desorption process of Rhn+(NO)m upon heating. We therefore interpreted the sequential desorption of NO molecules and the increasing intensity of Rhn+ bearing fewer adsorbed NO molecules upon heating as the desorption of molecularly adsorbed NO molecules on the cluster surface. In addition, if NO was adsorbed dissociatively and the N atoms migrated on surface, upon encounter they have a chance to recombine and evaporate off the cluster surface; however, only a minor contribution of N2 release at 300 K was observed for n = 6. Hence, it is reasonable to infer that the NO molecules adsorbed on the clusters in their molecular form. This is consistent with the fact that NO adsorbs molecularly on Rh(111) surfaces at 273 K and desorbs readily at 490 K.3 Nature of NO Adsorption. DFT calculations also support the prediction that NO adsorbs molecularly on Rh6+ (Figure 9).

[Rh6+(NO)7 ] = [Rh6+(NO)7 ]0 exp(−kt )

(3)

Rh6+(NO)7

where the intensities of before and after the unimolecular dissociation in the extension tube are given by [Rh6+(NO)7]0 and [Rh6+(NO)7], respectively, t is the reaction time in the extension tube (∼100 μs), and k is the rate constant. In this case, [Rh6+(NO)7] was measured as the peak intensity in the mass spectra. The peak intensity was found to vary with the temperature of the extension tube, as the rate constant of the unimolecular reaction depends on the temperature. In addition, the rate constant, k, can be deduced from eq 4

Figure 9. Adsorption and dissociation of an NO molecule on Rh6+ calculated by DFT calculations. E

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(4)

where A, Ea, kB, and T are the pre-exponential factor of the Arrhenius equation, the threshold energy for the reaction, the Boltzmann constant, and the temperature, respectively. By combining the rate equation and the Arrhenius eq 4, eq 5 can be deduced [Rh6+(NO)7 ] = [Rh6+(NO)7 ]0 exp(−At exp(−Ea /kBT )) (5)

The threshold energy required for the desorption of NO from Rh6+(NO)7 can therefore be obtained by fitting this model equation to the observed intensity ratio. This yields a value of 0.20 ± 0.02 eV, which is less than the desorption energy of the first NO molecule from Rh6+(NO). It can therefore be considered that the first few NO molecules adsorbed chemically to Rh6+ with a binding energy of >2 eV, while further NO molecules were more weakly bound. In addition, the eighth and ninth NO molecules in Rh6+(NO)8,9 were found to be particularly loosely bound. Taking into consideration the number of NO molecules present on the cluster, they must exist in a second layer. Heat-Driven Reduction of NO. Mackenzie and coworkers revealed that isolated Rhn+ clusters in the gas phase reduced NO efficiently.12−14 The adsorption of two or more NO molecules on Rhn+ led to the exclusive formation of Rhn+O2, thus releasing N2 from the cluster. The efficiency of NO reduction was found to be 100% for n = 7 and 8, and no additional byproducts (e.g., N2O and NO2) were produced. Mackenzie suggested that the formation of N2 was triggered by the internal energy. In addition, in the collision-induced dissociation (CID) experiments with a low collision energy, the adsorption of NO resulted in the clusters gaining an internal energy equivalent to the NO adsorption energy. Hence, within a vacuum, the cluster remained hot, although it was cooled gradually by slow radiative cooling. Once adsorbed on Rh6+, the NO molecule was able to dissociate on the surface. With the similar attachment of a second NO molecule to the Rh6+NO species, the N atoms migrate on surface, and upon encounter they have a chance to recombine and evaporate off the cluster surface, thus resulting in the formation of N2. An alternative reaction mechanism involving the formation of N2O as an intermediate was also expected to operate with relation to the formation of N2, but this process will not be discussed in further detail here9,11 because mass spectrometry was not able to verify it. In contrast, in our He-filled reaction gas cell at room temperature, the clusters cooled rapidly due to collisions with surrounding He atoms and therefore reached a thermal equilibrium with the reaction gas cell. The difference between the various reaction products confirms that the reduction of NO was driven by heat. Indeed, in addition to the thermal desorption of molecularly adsorbed NO on rhodium clusters, the release of N2 was observed when the clusters were heated. Figure 10 shows the total intensity of Rhn+(NO)m summed for all m and the total intensities of both Rhn+(NO)mO2 and Rhn+(NO)mO4 summed for all m as a function of temperature. In this case, Rhn+(NO)mO2 and Rhn+(NO)mO4 were formed by the reduction of NO and subsequent release of N2 according to eqs 6 and 7 Rh n+(NO)m + 2 → Rh n+(NO)m O2 + N2

Figure 10. Total intensity of Rhn+(NO)m (black solid line), total intensities of Rhn+(NO)mO2 and Rhn+(NO)mO4 (red solid line), total intensity of Rhn+(NO)mO (black dashed line), and total intensity of Rhn+(NO)mN (red dashed line) as a function of temperature for n = (a) 4, (b) 5, (c) 6, and (d) 7.

Rh n+(NO)m + 2 O2 → Rh n+(NO)m O4 + N2

(7)

An increase in the total intensities of both Rhn+(NO)mO2 and Rhn+(NO)mO4 summed for all m therefore corresponded to an enhanced reactivity in reactions 6 and 7. Also shown is the total intensities of the cluster ions of Rh n + (NO) m O and Rh n + (NO) m N summed for all m. The formation of Rhn+(NO)mO corresponds to the release of N2O from Rhn+(NO)m+2, while the formation of Rhn+(NO)mN corresponds to the release of NO2 from Rhn+(NO)m+2. These values did not change significantly, and so it was expected that neither NO2 nor N2O was produced from Rhn+ (n = 3−8), even at higher temperatures. For the extended Rh surface, these molecules are known to be produced during NO reduction, especially at higher coverages and higher temperatures.33 In contrast, no formation of NO2 and N2O was observed in the CID processes of Rhn+ and NO.12−14 Hence, the exclusive formation of N2 was considered to be specific to the cluster ions. At >700 K, for n = 7, the intensities of Rhn+(NO)mO2 and Rh n +(NO) m O 4 were found to increase, while that of Rhn+(NO)m decreased, indicating that NO was reduced at higher temperatures. This process involved the molecularly adsorbed NO beginning to dissociate on the surface of the cluster upon heating, followed by the formation of N atoms that migrated on surface and encountered to give molecules of N2. From examination of the temperature dependence of this process, the total intensity at higher temperatures was found to be due to the presence of Rh7+O2, Rh7+(NO)O2, Rh7+(O2)2, Rh7+(NO)O4, and Rh7+(NO)2O2. The NO reduction seems to occur predominantly in Rh7+(NO)m≤5. The reduction of NO at higher temperature became significant for n ≥ 7. In contrast, for n = 4, the total intensities of Rh4+(NO)mO2 and Rh4+(NO)mO4 (m = 1, 2, 3, ...) did not increase with increasing temperature, and the contribution of Rh4+(NO)m remained high over the entire temperature range studied. Hence, the release of N2 was considered the minor reaction pathway in this case. The temperature dependences shown in Figure 10 suggest that reactivity of Rhn+ was clearly sizedependent and that NO was reduced more readily with an

(6) F

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increase in size. At >800 K, where n ≥ 6, the relative intensity corresponding to N2 release was found to increase. In the case of the extended Rh surface, it is known that NO dissociation requires an empty neighbor site, so at higher surface coverage, dissociation is hindered. In addition, small clusters provide for surfaces of large curvature. This enlarges distances among adjacent surface sites and thereby unavoidably increases any barrier of migration. Thus, NO dissociation becomes more demanding on small clusters. Indeed, according to DFT calculations, Rh4+ has a tetrahedral structure with only four faces.34

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (no. 25248004) and 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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CONCLUSIONS Adsorption, desorption, and reactions of gas-phase rhodium clusters, Rhn+, with NO were examined in the thermalized condition. Cationic rhodium clusters, Rhn+ (n = 4−8), were introduced into a reaction gas cell filled with NO diluted by He at room temperature, where they were subjected to collisions with He and NO. It was confirmed by mass spectrometry that the NO molecules adsorbed sequentially on the Rhn+ clusters to give Rhn+(NO)m at 300 K. The resulting Rhn+(NO)m species were then heated in the extension tube, where it was found that the response to heat varied with temperature and size (n). The temperature desorption spectroscopy and DFT calculation indicated adsorption forms of the NO molecules as follows: The first few NO molecules adsorbed molecularly on Rhn+ with the adsorption energy of ∼−2 eV, and the other NO molecules in Rhn+(NO)n+1 adsorbed rather weakly with the adsorption energy of ∼−0.2 eV. In addition, Rhn+(NO)n+1 had more NO molecules, which were adsorbed so loosely that the NO molecules desorbed at 800 K), indicating that NO was reduced on the clusters. Finally, it was also found that neither NO2 nor N2O were produced from Rhn+ where n = 3−8, even at higher temperatures. Because no formation of NO2 and N2O was reported in the CID experiments of the reaction of Rhn+ clusters with NO, the exclusive formation of N2 was concluded to be specific to the cluster ions. ASSOCIATED CONTENT

S Supporting Information *

Relative intensities of Rhn+(NO)m after reaction of Rhn+ with NO at room temperature as a function of NO concentration in the reaction cell. Structure of Rh6+(NO) calculated by DFT calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b04224.

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DOI: 10.1021/acs.jpca.5b04224 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.5b04224 J. Phys. Chem. A XXXX, XXX, XXX−XXX