Adsorption of Multiple NO Molecules on Rh n+(n= 6, 7) Investigated by

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C: Surfaces, Interfaces, Porous Materials, and Catalysis n+

Adsorption of Multiple NO Molecules on Rh (n = 6, 7) Investigated by Infrared Multiple Photon Dissociation Spectroscopy Toshiaki Nagata, Satoshi Kudoh, Ken Miyajima, Joost M. Bakker, and Fumitaka Mafune J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04729 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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The Journal of Physical Chemistry

Adsorption of Multiple NO Molecules on Rhn+ (n = 6, 7) Investigated by Infrared Multiple Photon Dissociation Spectroscopy Toshiaki Nagata1§, Satoshi Kudoh1, Ken Miyajima1, Joost M. Bakker2, Fumitaka Mafuné1* 1

Department of Basic Science, School of Arts and Sciences, The University of Tokyo,

Komaba, Meguro, Tokyo 153-8902, Japan 2

Radboud University, Institute for Molecules and Materials, FELIX Laboratory,

Toernooiveld 7c, 6525 ED Nijmegen, the Netherlands

ABSTRACT: The adsorption of multiple nitric oxide molecules on rhodium cluster cations, Rhn+ (n = 6 and 7), was studied by IRMPD spectroscopy in combination with DFT calculations. For n = 6, the first NO molecule adsorbs molecularly on an on-top site of octahedral Rh6+. The second NO molecule adsorbs either molecularly on another on-top site or dissociates into N and O atoms, with a ca. 1:0.4 propensity. The adsorption of a third NO molecule results in forming Rh6+(NO)3 with two molecularly adsorbed NO on on-top sites and one dissociatively adsorbed NO. For n = 7, the first NO molecule adsorbs molecularly either on an on-top site or on a bridge site of Rh7+, followed by partial dissociation into an O atom on the bridge site and the N atom on a hollow site. When a second NO molecule adsorbs on Rh7+(NO), a Rh7+(NO)2 structure is formed with one NO molecule adsorbed on an on-top site and the other NO molecule adsorbed dissociatively. Adsorption of a third NO molecule results in forming Rh7+(NO)3 with one NO molecule adsorbed on an on-top site, one O atom on a bridge site, and the remaining N and O atoms on hollow sites. The adsorption forms of NO molecules on Rhn+ are consistent with the 1 ACS Paragon Plus Environment

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reaction mechanism of NO reduction upon heating of Rhn+(NO)m revealed in our previous study.

1. Introduction Rhodium is used in automobile exhaust gas purification systems, known as three-way catalytic converters, for NO reduction and hence is considered as an essential element in catalysts.1 For a few decades, the mechanism of NO reduction has been intensively investigated: the adsorption form of NO molecules on the Rh surface is found to vary depending on temperature and surface coverage.2-4 Gas-phase clusters are used for studies of reaction mechanisms, in which all involved atoms and molecules are countable by mass spectrometry.5-8 NO decomposition by Rhn± cluster ions isolated in the gas phase was found using mass spectrometry under single-collision conditions by Mackenzie and coworkers:9-11 when two NO molecules adsorbed on the Rhn± cluster, one N2 molecule was released leaving the RhnO2±cluster. When four NO molecules are adsorbed, two N2 molecules were released, leaving the RhnO4 ± cluster. A similar decomposition was observed recently by Ichihashi and Hirabayashi for small bimetallic RhnM+ (2 ≤ n ≤ 6; M = Al, V, Co) clusters.12 These results were explained as resulting from dissociatively adsorbed NO molecules; the generated N atoms subsequently migrate over the cluster surface and encounter to form N2. The interaction of closed shell N2 with the dioxide cluster is so weak that N2 is released from the cluster. The observed reactions are likely to be promoted by heat of adsorption on the clusters isolated in the gas phase. If the clusters were surrounded by a buffer gas, the heat would be taken away readily by collisions with the gas molecules, and hence, the

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reactivity could be different from clusters isolated in vacuum. Indeed, when the thermal stability of multiple NO adsorbed onto Rhn+ (n ≥ 7) clusters was studied under nonadiabatic thermalized conditions, the NO decomposition was not observed at 300 K, but evidently observed at temperatures over 800 K, signaled by the release of N2.13 According to DFT calculations, the adsorbed NO is more stable in the dissociative form than in the molecular form for Rhn+ (n = 6, 8, 9), suggesting that the two adsorption forms are separated by an activation barrier, which was considered to hinder NO dissociation.14 Indeed, Xie and coworkers predicted that the energy of molecular adsorption of NO on Rh7+ is −2.0 eV with an energy barrier of +1.7 eV towards dissociation of NO.15 As the energy level of the transition state is lower by 0.3 eV than that of the initial state, NO could be dissociated upon the adsorption on Rh7+ if the adsorption energy is not dissipated by, e.g., collisions with a buffer gas. All these findings are consistent with results from our previous study, which indicate that most of the NO molecules adsorb molecularly on Rhn+ (n = 6−16) in a helium carrier gas, which thermalizes the clusters after reaction with NO, at 173−263 K: the adsorption of NO on the clusters was investigated by infrared multiple photon dissociation (IRMPD) spectroscopy combined with DFT calculations.14 The IRMPD spectra showed that for all n studied (n = 6–16) the NO molecule adsorbs nondissociatively on an on-top site of Rhn+ clusters, while for n = 7, 12, 13, and 14, a bridging adsorption site was clearly found as a second adsorption site. Part of the NO is dissociated upon the adsorption on Rhn+ clusters, and the dissociation ratio is dependent on n. It is now of interest to find whether the adsorption of more NO molecules may induce dissociation of already molecularly adsorbed NO. To examine this, Rhn+(NO)2,3 clusters were prepared in thermalized conditions in a He carrier gas cooled at 173−263 K, 3 ACS Paragon Plus Environment


and their vibrational spectra were recorded using IRMPD spectroscopy. Their geometrical structures were determined from comparison of the experimental and theoretical spectra, a similar method to preceding studies.14, 16-23

2. Experimental methods The adsorption forms of the second and the third NO molecules on cationic rhodium clusters, Rhn+ (n = 6, 7), were studied by IRMPD spectroscopy, performed with a molecular beam instrument for gas-phase cluster studies coupled to an infrared free electron laser facility, FELIX.24 The experimental details are similar to our previous study,14 therefore, here only brief descriptions are given below. The Rhn+ clusters were generated in the gas phase by ablation of a Rh rod (Furuya Metal Co., Ltd.; 99.9%) with a pulsed laser (532 nm, 20 Hz, typically 25 mJ pulse−1) in He carrier gas that was introduced via a first pulsed valve at a stagnation pressure of 0.7 MPa. A NO–He mixture gas (~0.5% NO, total stagnation pressure of 0.1 MPa) was introduced through a second pulsed valve into a reaction gas cell located downstream from the cluster growth channel. The Rhn+ clusters react with NO molecules in the reaction gas cell to form Rhn+(NO)m in thermalized conditions, which is actualized by frequent collisions with He atoms in the buffer gas. The tagging method was used to record IRMPD spectra for Rhn+(NO)m clusters with Ar atoms as a messenger, which is weakly bound to the clusters and readily released upon the photoabsorption.14 The IRMPD spectra were obtained as action spectra by probing loss of the messenger Ar atoms from the Ar-tagged clusters (Rhn+(NO)mAr) by mass spectrometry. To form the Artagged clusters, Ar gas was added into the carrier gas at the concentration of 0.3%–4.0%, and the reaction gas cell was cooled to 173–263 K. In the present experiments, all the gas4 ACS Paragon Plus Environment

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mixing ratios were controlled by mass flow controllers to optimize conditions. Also, the stability of experimental conditions was checked by monitoring partial pressures of NO and Ar in the cluster source chamber by a residual gas analyzer. Thus-formed clusters were expanded into vacuum with the carrier gas. A molecular beam was formed by the expansion in vacuum and introduced into a differentially pumped vacuum chamber through a skimmer (2 mm diameter). Then the molecular beam was narrowed to a 1 mm diameter by an aperture to match the counterpropagating IR laser beam. The IR laser from FELIX consists of a pulse train (macropulse) of ~7 μs duration with transform limited micropulses at 1 GHz. Energy of the macropulse was typically ~50 mJ pulse−1, and was attenuated to 0.06−20 mJ pulse−1 before use. For the high wavenumber region (>1550 cm−1), the IR laser was attenuated to ~0.1 mJ pulse−1 to prevent saturation of the strongly absorbing NO vibrations; to ascertain that no bands were missed, IRMPD spectra in this region were also measured with more intense, about 2 mJ pulse−1, IR laser light. All cluster ions were analyzed by time-of-flight mass spectrometry (TOF-MS) after interaction with the IR laser. The experiments were operated at the repetition rate of 20 Hz, while the FELIX repetition rate was 10 Hz. Therefore, we can record reference mass spectra between the FELIX macropulses to minimize the effect of source fluctuations. Wavenumbers of the IR laser were calibrated using a grating spectrometer. The spectral bandwidth was 0.2−0.3% root mean square of the central frequency. The IRMPD spectra were obtained from the measured mass spectra as in our previous study.14 First the branching ratio of Rhn(NO)m+Arp (p = 1, 2) to the total number of Rhn(NO)m+Arp (p = 0–2) was calculated.14 Then the IRMPD intensity is defined as a logarithmic ratio, −ln(R(ν)/R0), where R(ν) and R0 are the branching ratios with and 5 ACS Paragon Plus Environment


without irradiation, respectively. The IRMPD intensities were corrected by the IR macropulse energy P(ν), i.e., −ln(R(ν)/R0)/P(ν) is used for the IRMPD spectra in this study. For each IRMPD spectrum, the R(ν)/R0 ratios are shown in Supporting Information (Figure S1).

3. Computational methods Stable structures of Rhn+(NO)m (n = 6, 7; m = 2, 3) and their vibrational spectra were calculated by DFT with the Gaussian09 software suite.25 The B3LYP hybrid functional26,27 was used for all calculations. First, for each cluster, 100 or more initial geometries were generated randomly and optimized using basis sets of LanL2DZ for Rh28 and 6-31G(d) for N and O29, 30 to reduce the computational cost. Subsequently, several structures with low energies were re-optimized using basis sets of SDD for Rh31 and augcc-pVDZ for N and O.32 Harmonic vibrational frequencies were computed at the same level of theory for all obtained structures. All vibrational frequencies provided here are unscaled, meaning that they are generally higher than those of the observed bands. The calculated spectra were made by the convolution of the calculated vibrational modes with a Gaussian-type lineshape, whose full width at half maximum (fwhm) was 15 cm–1, for ease of viewing positions and intensities of expected spectral peaks in figures. The relative energies of isomers are shown after the zero-point vibrational energy correction. Kinetically trapped structures as well as the most stable structure are likely to appear in the experiments because of activation barriers on a pathway to forming the most stable structure. Since the N–O bond dissociation is considered to require a large activation energy, structures with intact NO molecules are considered even when they have high formation energies compared to the most stable isomer. 6 ACS Paragon Plus Environment

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4. Results and discussion 4.1. Adsorption of two NO molecules on Rh6+ Figure 1a shows the IRMPD spectrum for Rh6+(NO)2. In the low frequency range, it is recorded with a substantially higher IR pulse energy (typically a factor 100 higher) than in the higher frequency range to ensure visibility of all modes. Since the presented spectra are corrected for laser power, as described in Experimental methods section, the spectra in the lower frequency range are multiplied by a factor 100 to make the peaks visible against a similar signal-to-noise ratio as in the high frequency range. The spectrum exhibits a prominent doublet band in the 1750−1900 cm−1 range and at least six weaker bands below 700 cm−1. Bands in the 1500−2000 cm−1 spectral range are readily assigned to the stretching mode of NO, and especially those in the 1800−2000 cm−1 region are known to be characteristic for molecular NO adsorbed on ontop sites of Rh6+.14 Hence, the doublet band suggests that NO molecules adsorb on two different on-top sites of Rh6+. Indeed, the DFT calculations indicate that an octahedral Rh6+ geometry with two NO molecules adsorbed on on-top sites is among the most stable species (isomer 6-2B, Figure 1c). The two bands present in the doublet could potentially be assigned to the symmetric and anti-symmetric combinations of NO stretching modes. These two bands are predicted at 1946 and 1918 cm−1 for the symmetric and antisymmetric modes, respectively, which are higher wavenumbers than those observed. Similar band shifts were reported in our previous study on Rhn+(NO): a possible reason is vibrational anharmonicity which lowers resonant frequencies from the calculated harmonic vibrations.14 However, for structure 6-2B only four bands are predicted below 600 cm−1, for

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instance, at 530 (4.1 km mol−1) and 562 cm−1 (7.3 km mol−1) with intensities of 0.21% and 0.38% of the most intense band at 1918 cm−1 (1912 km mol−1), respectively, and none between 600 and 1000 cm-1. The observation of at least six bands, amongst which at least one band above 600 cm−1, suggests spectral contributions from other structural isomers. As molecularly adsorbed NO generally only gives weak bands below 600 cm−1, isomers with dissociated NO should be considered. The DFT calculations indicate that octahedral Rh6+ with one NO molecule adsorbed on an on-top site and one dissociatively adsorbed NO is the most stable (isomer 6-2A, Figure 1b). In addition, isomers with two dissociatively adsorbed NO (6-2C) and with two molecularly adsorbed NO on bridge sites (6-2D) are also found, but their formation energies are 0.59 eV higher than that for 6-2A. Isomer 6-2D can further be ruled out by the absence of activity in the 1500-1700 spectral range, which also is not apparent when the clusters are irradiated with a factor of 20 higher IR laser powers (Figure 1, red trace). Considering these energetics and the comparison with the predicted spectra in panels b–e, the observed bands at 420, 490, 580, and 640 cm−1 are assigned to geometry 6-2A, and the band at 530 cm−1 to 6-2B. In this region, the most intense band for 6-2A (observed at 640 cm−1, predicted at 665 cm−1) corresponds to a Rh3–N stretching mode of the atomic N, and the most intense band for 6-2B (observed at 530 cm−1, predicted at 562 cm−1) corresponds to a Rh–NO stretching mode. The predicted bands at 350 cm−1 for 6-2A and at 358 and 378 cm−1 for 6-2B may be hidden below 300 cm−1, which is out of range of our observation in the present study. If the observed bands for Rh6+(NO)2 result from the presence of two isomers with among them three intact NO molecules, we would expect to observe three bands around 1800 cm−1: DFT predicts two bands for 6-2B at frequencies 1918 and 1946 cm−1, and one for 6-2A at 1954 cm−1. However, only two bands are visible at 1810 and 1840 8 ACS Paragon Plus Environment

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cm−1, respectively. Since the bands, predicted at 1946 and 1954 cm−1, lie quite close, they might overlap resulting in one single band with a shoulder. Indeed, a fit of the bands in the 1750−1900 cm−1 region to three Lorentzian line shape functions of equal width (21 cm−1 fwhm) results in peaks at 1812, 1838, and 1857 cm−1 with the intensity ratio of 1:0.66:0.27 (Figure S2). The 1812 and 1838 cm−1 peaks are assigned to isomer 6-2B, (anti-symmetric and symmetric modes, respectively; predicted ratio 1:0.73) and the third to 6-2A. If isomers 6-2A and 6-2B existed in the same amounts, the peak intensity ratio would be 1:0.73:0.86. According to this analysis, the relative abundance of 6-2A to 6-2B is only ~0.3; of course this abundance has some uncertainty as it is based on a shoulder. We can also estimate the abundance ratio from the observed band intensities at 420, 490, 590, and 640 cm−1 (assigned to 6-2A) and at 520 cm−1 (assigned to 6-2B) and the calculated IR intensities, yielding a ratio 6-2A to 6-2B of 0.4±0.2. From this, the assignment of the shoulder of the 1840 cm−1 band to 6-2A appears reasonable. Thus, although the isomer for Rh6+ with one molecularly and one dissociatively adsorbed NO (structure 6-2A) is lower in energy than the isomer with two intact NO molecules (6-2B) according to the DFT calculations, the spectral analysis suggests that the abundance of 6-2A is only ~0.4 of that of 6-2B. The abundance ratio is, most likely, related to the formation mechanism, which will be discussed in the following section.

4.2. Adsorption of a third NO molecule on Rh6+ Clearly, the adsorption of two NO molecules under thermal conditions does not lead to dissociation of both NO, let alone to NO conversion, as observed in the single collision experiments.9,10 Since the surface coverage of Rh by NO plays an important role on extended surfaces, it can be envisaged that the adsorption of a third NO could lead to 9 ACS Paragon Plus Environment


a more efficient dissociation. Figure 2a shows the IRMPD spectrum for Rh6+(NO)3. The spectral region below 700 cm−1 contains at least eleven discernible bands; in the NO stretching region, a prominent band at 1860 cm−1 is observed with a shoulder to the red. A spectral deconvolution using Lorentzian line shape functions yields a frequency of 1820 cm−1 and a relative intensity of 0.4 for the shoulder. When irradiated with a factor of 20 more IR laser fluence (red trace in Figure 2a), the bands at 1840 and 1860 cm−1 merge into a single feature at 1850 cm−1, but no additional features appear. This doublet structure suggests the presence of either one isomer with two molecularly adsorbed NO, or two isomers with at least one molecularly adsorbed NO each. The most stable geometry calculated, 6-3A, comprises two molecularly adsorbed NO on on-top sites and one dissociatively adsorbed NO. The NO stretching vibrations for this isomer are calculated to lie at 1956 and 1972 cm−1, and can be identified as the antisymmetric and symmetric combination of the two NO stretch vibrations. Their spectral separation is smaller than what is observed, and their intensity ratio is reversed. A second geometry, 6-3B, has two molecularly adsorbed NO, one on an on-top site and the other on a bridge site, and one dissociatively adsorbed NO. Three isomers with three molecularly adsorbed NO were found: one (6-3C) has all NO molecules on on-top sites, the other two (Figure S3 of Supporting Information) have all three NO molecules on bridge sites, or only two NO on bridge sites and one on an on-top site. The formation energies of all three isomers are over 1 eV higher than isomer 6-3A. For all isomers with molecular NO on a bridge site, NO stretching vibrational bands are predicted around 1700 cm−1, which were not observed in the present experiment. Therefore, of the three allmolecularly bound NO isomers, only 6-3C is shown in Figure 2. One further isomer with one molecularly adsorbed NO and two dissociatively adsorbed NO was also found (610 ACS Paragon Plus Environment

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3D), however, its formation energy is much higher (by +1.34 eV) than the most stable isomer. Considering the formation energies and the predicted bands, the observed bands at 440, 470, 530, 560, 620, 670 and 1860 cm−1 ought to be assigned to the geometry 63A. The most intense band at 1860 cm−1 corresponds to the anti-symmetric combination of two NO vibrations, predicted at 1956 cm−1. The symmetric combination mode predicted at 1972 cm−1 is likely to appear in the right shoulder of this band. The shoulder at 1840 cm−1 can potentially be assigned to 6-3C as a minor component; its kinetic formation is conceivable, as stated later, although it is thermodynamically unfavorable. It is now worth considering how isomer 6-3A can be formed by sequential adsorption of NO molecules onto bare Rh6+. As observed in our previous work, the first NO molecule adsorbs mainly on an on-top site of octahedral Rh6+.14 There exists an isomer with dissociatively adsorbed NO on Rh6+, but its population is very limited (see Scheme I): the spectral analysis suggested that the abundance of Rh6+ with dissociated NO is

Adsorption of Multiple NO Molecules on Rh n+(n= 6, 7) Investigated by

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