Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4â7, 12

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Structures of Rhodium Oxide Cluster Cations Rh7Om (m = 4-7, 12, 14) Revealed by Infrared Multiple Photon Dissociation Spectroscopy Fumitaka Mafune, Kohei Koyama, Toshiaki Nagata, Satoshi Kudoh, Tomokazu Yasuike, Ken Miyajima, Douwe M. M. Huitema, Valeriy Chernyy, and Joost M Bakker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11068 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Physical Chemistry

Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4−7, 12, 14) Revealed by Infrared Multiple Photon Dissociation Spectroscopy Fumitaka Mafuné1*, Kohei Koyama1, Toshiaki Nagata1†, Satoshi Kudoh1, Tomokazu Yasuike2, Ken Miyajima1, Douwe M. M. Huitema3, Valeriy Chernyy3, Joost M. Bakker3 1

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

Komaba, Meguro, Tokyo 153-8902, Japan 2

Department of Liberal Arts, The Open University of Japan, Chiba 261-8502, Japan and

Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 615-8520 Kyoto, Japan 3

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

Toernooiveld 7c, 6525 ED Nijmegen, the Netherlands

ABSTRACT: Vibrational spectra of Rh7Om+ (m = 4−7, 12, 14) were measured in the 300−1300 cm−1 range via infrared multiple photon dissociation (IRMPD) spectroscopy. For the oxygen poor cluster sizes, Rh7Om+ (m = 4−7), IRMPD spectra were recorded through photodissociation of Rh7Om+-Ar complexes. IR spectra for Rh7Om+ (m = 12, 14) were recorded via the release of an O2 molecule from Rh7Om+ producing Rh7Om−2+; no O2 loss was observed from Rh7Om+ (m = 4−7, 8, 10). By comparison with calculated vibrational spectra of several stable isomers obtained using density functional theory (DFT), these IR spectra are assigned to geometrical structures. For m = 4–7, all O atoms are bound to Rh atoms only, with a transition of the Rh core from octahedral to prismatic between m = 5 and m = 6. For the oxygen-rich Rh7O12+ and Rh7O14+ molecular O2 is adsorbed on a bridge site between two Rh atoms. The frequencies of the bands observed signal that the O2 molecules are activated, indicating that rhodium oxide clusters with 1 ACS Paragon Plus Environment

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Rh2O3 composition are still capable to donate electrons to activate O-O bonds in adsorbed O2.

1. Introduction

Rhodium is well known to function as an automobile catalytic converter for reducing nitric oxide (NO) in automobile exhaust.1–5 The mechanism of NO reduction by Rh has been studied for many years using isolated gas-phase clusters, as the use of clusters enables to understand the behavior of N and O atoms at the atomic and molecular levels.6−16 For instance, Mackenzie et al. investigated the reduction of NO on rhodium clusters by collision-induced dissociation (CID) experiments.6−11 Later, Hirabayashi and Ichihashi observed a similar reaction of NO on pure Rh clusters and alloy metal clusters involving Rh by CID experiments.12 Both experiments showed that N2 was desorbed, leaving behind O atoms after two NO molecules were adsorbed on the clusters. For Rh6+, four NO molecules were reduced, yielding two N2 molecules, while further NO molecules were only adsorbed onto Rh6O4+ clusters. In these studies, it was suggested that the remaining O atoms could deteriorate the reduction of further NO molecules by occupying the reactive sites. In a previous study, we carried out a further analysis of the fate of the remaining O atoms. For this purpose, we measured the vibrational spectra of Rh6Om+ clusters using infrared multiple photon dissociation (IRMPD) spectroscopy and determined their geometrical structures by comparing these spectra with those calculated for optimized stable isomers using density functional theory (DFT).17 It was concluded that Rh6O4+ has a capped-square pyramidal Rh geometry with three bridging O atoms and one O atom in 2 ACS Paragon Plus Environment

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a hollow site, and that Rh6O5+ exhibits a prismatic Rh geometry with three bridging O atoms and two O atoms in a hollow site. In addition, Rh6O6+ showed three intense bands around 600−750 cm−1, which are diagnostic for a prismatic Rh geometry with four bridging O atoms and two O atoms in a hollow site. Considering that Rh6Om+ (m ≤ 3) adopts octahedral Rh structures,11 the geometry change at m = 4 results in a reduction of the number of triangular hollow sites, where N atoms preferentially bind upon dissociative adsorption of NO, thus lowering the probability for this process. This inference is consistent with the interpretation of CID experiments.6,12 In the present study, we extend our investigation to Rh7Om+ and examine if a similar evolution of the geometrical structures takes place with an increase in the number of O atoms. For 4 ≤ m ≤ 7, we investigate this through photofragmentation of weakly bound Rh7Om+-Ar complexes after absorption of multiple IR photons, yielding wellresolved vibrational spectra. Several stable isomers of Rh7Om+ have been obtained by DFT calculations, and their calculated vibrational spectra are compared with the experimental IRMPD spectra. For m = 12 and 14, it proved impossible to generate Ar tagged Rh7Om+ clusters, however, an O2 molecule is found to be released from Rh7Om+ upon irradiation by the IR laser. The IRMPD spectra for m = 12 and 14 exhibit characteristic vibrational bands at frequencies higher than 1000 cm−1, which are assigned to the vibrations of O2 adsorbed on bridge site of the clusters. The frequencies of O2 stretch vibrations are indicative for the strength of activation, as electron donation in the anti-bonding orbital will weaken the O-O bond, shifting the stretch vibration to the red. Thus, we will be able to judge whether rhodium oxide with higher O-content could still play a role in NO reduction.

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2. Experimental methods The geometrical structures of Rh7Om+ (m = 4–7, 12, 14) were studied by IRMPD spectroscopy using a molecular beam based experimental apparatus coupled to the free electron laser for infrared experiments (FELIX).17−25 Gas-phase RhnOm+ cluster ions were generated by laser ablation of a rhodium rod (99.9%) in a 4 mm diameter channel for cluster growth: The channel was filled with He carrier gas seeded with 0.016% oxygen introduced through a pulsed valve at a total stagnation pressure of 0.7 MPa. The laser ablation was performed using the focused 532 nm light from a pulsed Nd:YAG laser with a typical pulse energy of 25 mJ. Ar-attached clusters, RhnOmArp+, were formed by mixing 0.6% argon gas in the carrier gas and cooling the cluster growth channel to about 200 K using liquid N2.17 The temperature was probed by a thermocouple and kept constant to within 1 K using a resistive heater and an electronic feedback circuit. The concentrations of oxygen and argon gases were well-tuned using mass flow and pressure controllers. After cluster formation, the cluster-carrier gas mixture was expanded into vacuum to form a molecular beam. The beam first passed through a 2 mm diameter skimmer, entering a differentially pumped vacuum chamber, and was then collimated by a 1 mm diameter aperture. The cluster beam was irradiated with a counter-propagating IR laser light: The typical FELIX output consists of a pulse train (macropulse) of ~7 µs length with transform limited, ps duration micropulses at a 1 GHz repetition rate. The macropulse energy of 50–100 mJ was attenuated to 10−20 mJ before use by mesh attenuators. The spectral bandwidth was kept at ~0.5 % (full-width at half maximum, FWHM) of the central frequency. After interaction with the IR laser, all cluster ions were detected using a time-of-flight (TOF) mass spectrometer. The experiment was operated at 10 Hz, twice the FELIX repetition rate of 5 Hz, which allows the recording of reference 4 ACS Paragon Plus Environment

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mass spectra in between successive FELIX pulses. For non-Ar-tagged species, spectra were presented as depletion spectra. They are constructed from the raw mass spectra, using the depletion D(ν)= Ion(ν)/Ioff −1, with Ion(ν) and Ioff, the number of ions with and without IR laser irradiation at frequency ν, respectively. A value of D(ν) < 0 thus implies the loss of ions due to IR induced dissociation, whereas a value of D(ν) > 0 implies an increase in the number of ions detected due to dissociation of higher mass species. For Ar tagged clusters, spectra are presented as IRMPD spectra of the cluster with one Ar attached, that can be compared to calculated linear IR spectra. To account for the presence of multiple Ar attached to the clusters that could result in growth of the number of clusters with one Ar attached, we + + make use of the branching ratio, R(ν) = ∑𝑝𝑝 Rh7 O𝑚𝑚 Ar𝑝𝑝 ⁄∑𝑞𝑞 Rh7 O𝑚𝑚 Ar𝑞𝑞 with p = 1, 2,

and q=0,1,2, of all Ar tagged clusters to the total number of clusters. The IRMPD intensity was then calculated as 1 − R(ν)/R0, with R(ν) and R0 the branching ratios with and without IR irradiation, respectively. IRMPD and depletion spectra were normalized on the IR macropulse energy.

3. Computational methods To obtain stable geometries of Rh7Om+ (m = 4, 5, 6, 7, 12) and simulate their vibrational spectra, DFT calculations were performed using the Gaussian09 program.26 Becke's three-parameter hybrid density functional27 with the Lee-Yang-Parr correlation functional28 (B3LYP) was used for all calculations. Before running calculations using larger basis sets, low energy structures of Rh7Om+ from more than one hundred randomly set initial geometries for all possible spin states (triplet to undecatet for Rh7O4+ and Rh7O5+, singlet to septet for Rh7O10+, and singlet to nonet for Rh7O6+, Rh7O7+, Rh7O12+ 5 ACS Paragon Plus Environment


and Rh7O14+) were obtained at reduced calculational costs using the LanL2DZ basis set for Rh atoms29 and the 6-31G(d) basis sets for O and Ar atoms30,31. Geometries with energies up to +0.3 eV from the most stable one were re-optimized using the Stuttgart/Dresden SDD effective core potential (ECP) and basis set for Rh atoms32 and the aug-cc-pVDZ basis sets for the O and Ar atoms.33 These combinations of functional and basis sets are the same as used in our previous work on RhnOm+ clusters, as well as on Rhn+ and RhnTa+ clusters complexed with NO. While the accuracy of DFT methods in general for such high-spin species are likely not accurate to within 0.1 eV, the methods used have shown some success in elucidating structures and reaction pathways of cationic rhodium clusters.17,34-36 In case calculated vibrational spectra for low-energy structures did not reproduce the experimentally observed IRMPD spectrum well, other geometries with higher formation energies were considered extensively. Harmonic vibrational frequency computations for all clusters were carried out at the same level of theory. The calculated spectra were produced by convoluting the unscaled harmonic vibrations with a Gaussian line shape function with a 15 cm–1 FWHM. The IR intensities of the typical bands are given in the Supporting Information.

4. Results and discussion 4.1. Infrared multiple photon dissociation Rhodium oxide cluster cations complexed with Ar atoms, Rh7Om+Arp (m = 4–7; p = 0, 1, 2), were abundantly formed by controlling the partial pressures of oxygen and argon in the carrier gas. Upon IR irradiation of the cluster beam at a wavenumber resonant with a cluster vibrational mode, the sum of the peak intensities of Rh7Om+Ar1,2 decreases, while the peak intensity of Rh7Om+ increases in the same extent, suggesting that 6 ACS Paragon Plus Environment

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Rh7Om+Ar1,2 clusters release the Ar atom(s) to form Rh7Om+ on absorbing (multiple) IR photons. Since the Ar atoms are weakly bound (typical binding energy calculated: ~0.1 eV), they do not significantly affect the geometrical structures of the clusters, as is also evidenced by DFT calculations. Thus, the vibrational spectra of the clusters, Rh7Om+, can be obtained via infrared multiple photon dissociation of their Ar-tagged complexes.17 Experimentally obtained spectra for Rh7Om+ (m = 4−7) exhibit resonances below 800 cm−1, but none are observed in the 800−1000 cm−1 range. The DFT calculations indicate that an O atom adsorbed at a Rh on-top site would give rise to a band in the 800−1000 cm−1 range, so it is concluded that all oxygen atoms are adsorbed on bridge and/or hollow sites. In addition, for the extended Rh (110) surface, oxygen atoms are known to adsorb on bridge sites.37 The experimental spectrum for each Rh7Om+ is compared with the calculated spectra for the stable structural isomers obtained using DFT. Extensive calculations for Rh7Om+ (m = 4−7) allow us to make tentative assignments. Local structures of oxygen rich Rh7Om+ (m = 12 and 14) were identified from characteristic bands appearing in a higher wavenumber region (1000−1250 cm−1). For other RhnOm+, we present the results of IRMPD spectra in Figures S1 and S2.

4.2. IRMPD spectrum of Rh7O4+ Figure 1(a) shows the IRMPD spectrum for Rh7O4+. The observed signal-tonoise of the spectrum is fair, but not great. Prominent bands are found at 555, 590, 660 and 695 cm−1. In addition, weaker bands likely exist below 550 cm−1, notably one just below 400 cm−1, but their presence is difficult to be certain of, as the signal is comparable to the noise level. In our extensive structure search by DFT calculations, we found a capped octahedron with two O atoms on bridging sites, and two in hollow sites to be the 7 ACS Paragon Plus Environment


most stable Rh7O4+ structure 4A (Figure 1(b)). When looking at this structure from another side, one can also describe it as a Rh bi-capped square pyramid with two capping Rh atoms located at adjacent pyramidal faces (left and front in Figure 1b). The second most stable isomer, 4B, is another bi-capped pyramid, now with the capping Rh atoms on opposite pyramidal faces (left and right in Figure 1c), where two O atoms are in bridge sites and two others in hollow sites. The formation energy of 4B is only +0.08 eV higher than that of 4A. Both structures and their calculated linear IR spectra are shown in Figures 1(b) and (c). The IRMPD spectrum appears mostly consistent with the vibrational spectrum of 4A predicting three intense bands at 598, 684, and 728 cm−1 (unscaled values) and a weak band below 540 cm−1. The predicted prominent peak at 598 cm−1 is composed of an intense band at 598 cm−1 (87.5 km mol−1) and a weaker one at 611 cm−1 (38.8 km mol−1) which appears as a shoulder in the spectrum of 4A. The experimental band at 555 cm−1 could be due to the predicted resonance at 540 cm−1, although the observed intensity appears larger than what is predicted. With the exception of this intensity mismatch, all bands in the IRMPD spectrum are consistent with the presence of structure 4A. Structure 4B has a less favorable agreement with the experimental spectrum: the experimental band at 660 cm−1 may be predicted by a strong band at 683 cm−1, but the 695 cm−1 band is much stronger than the predicted band at 734 cm−1. Further bands at lower frequency give a similar, but not better match than 4A. Thus, we tentatively assign the spectrum for Rh7O4+ to structure 4A, although we think structure 4B has a minor contribution.

4.2. IRMPD spectrum of Rh7O5+ Figure 2(a) shows the IRMPD spectrum of Rh7O5+. Prominent bands appear at 8 ACS Paragon Plus Environment

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420, 455, 600 and 720 cm−1 and some bands are discernable as a shoulder of the 600 cm−1 peak. Among several low-energy structures exhibiting a symmetric pentagonal bipyramid with five bridging O atoms, structure 5A is the most stable isomer (Figure 2(b)). The predicted spectrum for 5A provides a not unreasonable match for the experiment in the spectral region above 600 cm−1, but the fairly strong bands predicted between 500 and 600 cm−1 are not observed. Also, no bands are predicted below 500 cm−1, although there are clearly resonances at 420 and 455 cm−1 in the IRMPD spectrum. As our DFT calculations suggest that bands around 420−455 cm−1 are typical for the vibration of an O atom adsorbed on a hollow site, we must look for such structures. Figure 2(c) shows isomer 5E, one of the stable isomers with three O atoms in hollow sites, with a formation energy of +0.15 eV higher than that of the most stable isomer. Isomer 5E is not the second most stable, and there are again multiple isomers with similar geometrical structures and comparable formation energies (see Figure S3). The vibrational spectrum exhibits several bands below 500 cm−1, which are consistent with the IRMPD spectrum. The spectral resemblance suggests that the Rh7O5+ ought to be assigned to 5E, as the other structural isomers such as 5A do not exhibit bands below 500 cm−1, although we are not able to rule out the possibility that 5A contributes to the IRMPD spectrum. It is of interest to note that 5E can be constructed by adding one O atom on a hollow site of structure 4A.

4.4. IRMPD spectrum of Rh7O6+ Figure 3(a) shows the IRMPD spectrum of Rh7O6+. Prominent bands are found at 535, 620, 650, 690 and 720 cm−1. The calculated most stable isomer 6A has an open, butterfly-like structure with four bridging O atoms and two O atoms in hollow sites 9 ACS Paragon Plus Environment


(Figure 3(b)). In addition, a capped trigonal prism isomer with four bridging O atoms and two O atoms in a hollow site, 6B, is found as second most stable isomer at +0.31 eV higher in formation energy (Figure 3(c)). Given a reasonable similarity between the observed IRMPD spectrum and that calculated for 6A, we tend to assign the spectrum to this isomer. The only bands that are not explained well are the ones at 650 and 690 cm−1. Since structure 6B’s single most prominent band falls right in this spectral region at 711 cm−1, we think isomer 6B also contributes to the IRMPD spectrum. Since the spectrum for Rh7O5+ was assigned to structure 5B, which appears constructed from assigned structure 4A by the addition of one single O, one would suppose that a next addition of an O atom could result in a capped octahedral structure of Rh7O6+. We did find such an isomer 6C on the nonet spin potential energy surface as the third most stable one at +0.35 eV higher in the energy than 6A (see Figure 3(d)). It cannot be ruled out that this isomer does contribute to the spectrum given the absence of strong resonances below 600 cm−1 in isomers 6A and 6B. While the current calculations thermodynamically disfavor the presence of 6C, such a cluster could of course kinetically be favored in the formation process.

4.5. IRMPD spectra of Rh7O7+ Figure 4(a) shows the IRMPD spectrum of Rh7O7+, which is dominated by a strong band at 680 cm−1, with shoulders at 640 and 700 cm−1. In addition, weaker bands can be identified at 500 and 545 cm−1, and potentially a 590 cm−1 shoulder to the 640 cm−1 band. The calculated most stable structure 7A is a distorted prism with five bridging O atoms and two in hollow sites, and has a singlet electronic configuration (Figure 4(b)). The predicted spectrum for this structure shows one strong band at 723 10 ACS Paragon Plus Environment

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cm−1. There is an isomer, 7B, with +0.24 eV higher formation energy, which takes the form of a distorted capped prism with three bridging O atoms and four O atoms in hollow sites (Figure 4(c)). The spectrum for this structure shows two strong bands between 640 and 740 cm−1 in the general vicinity of the strongest absorption bands in the experimental spectrum, which could also explain the observed spectrum. Alternatively, the similar structure in a triplet configuration, 7C (the third most stable structure with +0.33 eV higher formation energy), yields a spectrum dominated by one strong band at 710 cm−1 (Figure 4(d)). Thus, the stable isomers adopt distorted capped prism structures and exhibit strong bands near 700 cm−1, which appear to account well for the bands in the experimental spectrum.

4.6. Evolution of the geometrical structure of Rh7Om+ with the number of O atoms The current combined spectroscopic and theoretical investigations indicate that the Rh atoms form a capped octahedron in Rh7Om+ for m = 4 and 5, and a capped prism for m = 6 and 7 besides the butterfly-like open structure 6A of Rh7O6+. The change in geometry of the Rh framework at m = 6 results in a reduction in the number of triangular hollow sites. This trend is similar for Rh6Om+:17 there, the Rh framework Rh6Om+ has a capped pyramidal geometry for m = 4 and a prismatic geometry for m = 5 and 6. Thus, it appears that the cluster geometry is strongly dependent on the number of oxygen atoms that can stabilize alternative Rh skeletal structures. Indeed, Rh7O4+ is formed by adding simply one Rh atom on Rh6O4+ without changing connectivity of the O atoms. In our recent paper, we discussed dissociative adsorption of NO on Rh6Om+. A reduction in the number of a triangular hollow sites was considered to make the dissociation of NO unfavorable, since N atoms prefer to bind to a hollow site when NO adsorbs 11 ACS Paragon Plus Environment


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dissociatively.17 The present results suggest that this is also the case for Rh7Om+.

4.6. Oxygen molecule release from Oxygen rich Rh7Om+ clusters For larger Rh7Om+ (m ≥ 8) clusters, we investigated the optical response without tagging Ar atoms, as Ar atoms hardly attached to the clusters. For these species, intensity changes by IRMPD were plotted as a function of wavenumber (see Figure 5). Rh7O14+ exhibits depletion (loss) peaks over the whole spectral range of 400−1320 cm−1, whereas Rh7O10+ shows growth (gain) peaks. In addition, the spectrum for Rh7O12+ shows a mixture of gain and depletion peaks. The wavenumbers of the gain peaks of Rh7O12+ correspond to the loss peaks of Rh7O14+, whereas the gain peaks of Rh7O10+ correspond to the loss peaks of Rh7O12+. Therefore, we conclude that the IR multiple photon excitation of Rh7Om+ (m = 12, 14) results in the release of an O2 molecule: Rh7Om+ → Rh7Om–2+ + O2

(1)

There are no gain or loss peaks in the spectrum of Rh7O8+, indicating that neither Rh7O10+ nor Rh7O8+ dissociate by releasing O2 molecules upon IR multiple photon excitation. This suggests that upon going from m = 10 to 12, the binding energy of O2 to Rh7Om+ decreases substantially allowing O2 elimination upon absorption of multiple IR photons. This is consistent with the results of gas phase thermal desorption spectrometry, where the threshold energies for the release of O2 molecules from Rh7Om+ were found to be 1.7, 1.8, 0.2 and 0.5 eV for m = 8, 10, 12, and 14.14 Hence, dissociation is not achieved for clusters with a threshold energy as high as 1.7 eV under the current experimental conditions.

4.7. Activation of the O-O bond by oxygen rich clusters 12 ACS Paragon Plus Environment

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Taking advantage of the O2 release, the IRMPD spectra of Rh7O12+ and Rh7O14+ were obtained from the gain spectrum of Rh7O10+ and the loss spectrum of Rh7O14+. The IRMPD spectra obtained by a similar protocol followed for other clusters (Rh8O13+, Rh8O14+, Rh9O13+ and Rh9O15+) are shown in Figure S2 of the Supporting Information. In contrast to the spectra for Rh7Om+ (m = 4−7) where only bands below 800 cm−1 were observed, in the Rh7O12+ spectrum a prominent band appears at 1020 cm−1, and two bands appear at 1050 and 1200 cm−1 for Rh7O14+. This pattern is not unique for Rh7Om+ IRMPD spectra: Rh8O14+ and Rh9O15+ both exhibit intense bands near 1000 cm−1 (see Figure S2). Figure 6(a) shows the IRMPD spectrum of Rh7O12+. Bands appear at 450, 490, 610, 660, 710 and 1020 cm−1. The most stable geometry 12A obtained by the DFT calculations is also shown (Figure 6(b)). Its predicted spectrum seems to account for the bands below 800 cm−1, however, the band appearing at 1020 cm−1 in the IRMPD spectrum is not consistent with the predicted spectrum, as this exhibits a band at 1200 cm−1. Figure 6(c) shows another stable isomer with an O2 molecule on a bridge site, its formation energy is +0.58 eV higher than that of 12A. The vibrational spectrum exhibits several bands below 800 cm−1 and an intense band at 1100 cm−1, which is more consistent with the IRMPD spectrum. One could wonder that, for the systems described above, assignments were made with much smaller differences between observed and calculated band positions. One factor certainly different is that those experiments require to overcome the much lower binding energy of Ar to the cluster, rather than that of O2 to the cluster. This higher binding energy requires a substantially larger number of IR photons, and it is well-established that for higher binding energies bands detected using IR-MPD spectroscopy exhibit broadening and red-shifting effects.38 The spectral resemblance suggests that the Rh7O12+ ought to be assigned to 12B. 13 ACS Paragon Plus Environment


Let us discuss the origin of the intense bands based on local structures of O2 on the clusters. In one of the previous studies, we observed release of two O2 molecules from Rh7O14+, when it was heated to 600 K.14 The desorption energies of one O2 molecule from Rh7O14+ and from Rh7O12+ were estimated to be 0.5 and 0.2 eV, suggesting that one and two O2 molecules are weakly bound to Rh7O10+ in Rh7O12+ and Rh7O14+, respectively. Hence, assuming that one and two O2 molecules are weakly bound to the core Rh7O10+ in Rh7O12+ and Rh7O14+, respectively, we obtained several stable geometries of Rh7O10+, and then re-optimized them after attaching O2 in various positions. Separately, several stable geometries of Rh7O12+ were calculated by randomly setting the initial geometries of the atoms. In total, this resulted in a distribution of 44 stable isomers. Figure 7(a) shows a histogram for the calculated highest wavenumbers of vibrational bands for all of these. All vibrations involve motions of the O atoms in the clusters. Among them, there are 26 isomers with intact O2 molecules adsorbed on a bridge site between the two Rh atoms, the vibrations of which appear mainly above 900 cm−1 (Figures 7(b)−(d)). In contrast, there are 18 isomers with no intact O2 and all vibrations of clusters appear below 950 cm−1 (see Figure 7(a)). In addition, very weakly bound O2 molecules exhibit a band in the 1500−1600 cm−1 range, which is close to the vibration of a free O2 molecule at 1551 cm−1 (see Figure 7(d)). We conclude that the bands experimentally observed at 1020, 1050 and 1200 cm−1 are due to O2 molecules adsorbed on a bridge site (see Figure 7(c)). Hence, O2 is able to adsorb on the bridge sites of the Rh7O10+ forming local -Rh-O-O-Rh- structures. According to the natural bond orbital (NBO) analysis, bridging O2 moieties were found to be negatively charged, suggesting that electron density is transferred from the oxide cluster to the unoccupied anti-bonding π* orbital of O2. Indeed, for one of the stable isomers exhibiting a vibration at 1037 cm−1, the O-O bond length is 1.31 Å, which is 14 ACS Paragon Plus Environment

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much longer than that of free O2 (1.21 Å), and almost as long as superoxide, O2− (1.33 Å).39,40 The vibrational bands are red-shifted from the free O2, because the O-O bond is weakened. To investigate possible correlations, the O-O bond length and the calculated vibrational wavenumber are plotted as a function of the sum of the natural charges of the two O atoms in Figure 8. Although the natural charge cannot be interpreted as a real charge, it allows for a qualitative understanding of charge transfer. We find that the O-O bond length increases, while the vibrational wavenumber decreases (see Figure 8(b)) with an increase in the negative charge in the O-O moiety (see Figure 8(a)). The vibrational band in 1020−1200 cm−1 then corresponds mostly to a charge donation in the range of −0.4e to −0.2e, and to a 1.33−1.28 Å O-O bond length. In addition, there exists an isomer with the O-O moiety (1.41 Å O-O bond length and −0.69e natural charge) which shows the vibrational band at 884.6 cm−1, although it is not typical. The spectral changes going from Rh7O12+ to Rh7O14+, a blue-shift of the 1020 cm−1 band to 1050 cm−1 and the appearance of the second vibrational band in Rh7O14+ at 1200 cm−1 (see Figure 5), indicate that the donated electron density from Rh7O10+ is shared by the two O2 molecules. Thus, after charge donation to the first O2 moiety by Rh7O10+ resulting in Rh7O12+, the cluster can still afford to donate further electron density for adsorbing a second O2 molecule activating its O–O bond. As mentioned above, similar bands near 1000 cm−1 were observed for Rh8O14+ and Rh9O15+. In other words, Rh8O12+ and Rh9O13+ are able to donate electron density to adsorbing O2: As these species can be described as RhnO(3/2)n+ for even n and RhnO(3/2)n−(1/2)+ for odd n. we tentatively generalize our observation in stating that , rhodium oxide clusters with the Rh2O3 composition can still potentially activate an O–O bond. 15 ACS Paragon Plus Environment


5. Conclusions Vibrational spectra of RhnOm+ were recorded in the 300−1300 cm−1 spectral range using the free-electron laser for infrared experiments (FELIX) through infrared multiple photon dissociation (IRMPD) of RhnOm+ and RhnOm+-Ar complexes. The geometrical structures of Rh7Om+ (m = 4−7) were discussed by comparing the experimental IRMPD spectra with those calculated for several stable isomers obtained by DFT calculations. For Rh7O4+ and Rh7O5+, geometries comprising capped octahedral Rh atoms structures with O atoms in bridge and hollow sites were found. For Rh7O6+ and Rh7O7+, geometries composed of a capped Rh prism with O atoms in bridge and hollow sites contribute the IRMPD spectrum. The change in the Rh atoms geometry in Rh7Om+ at m = 6 results in a decrease in the number of triangular hollow sites, which is quite similar to Rh6Om+. According to our previous study using the gas phase thermal desorption spectrometry, O2 molecule is known to be weakly bound to Rh7Om+ (m = 10, 12) and strongly bound to Rh7Om+ (m = 6, 8). Hence, an O2 molecule was released from Rh7Om+ (m = 12, 14) to form Rh7Om−2+ upon multiple photon excitation, whereas no O2 release was observed from Rh7Om+ (m = 8, 10). The IRMPD spectra of oxygen rich Rh7O12+ and Rh7O14+ exhibit prominent bands at 1020, 1050 and 1200 cm−1 that are assigned to vibrations of O2 adsorbed on a bridge site. The adsorption form was observed specifically for the oxygen-rich clusters. The analysis of the O-O bond length, and the sum of the natural charges of the two O atoms, suggests that Rh7O10+, and more generally oxide cluster with (Rh2O3)n composition, can still afford to donate an electron for adsorbing O2 molecules and 16 ACS Paragon Plus Environment

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potentially activate O-O bonds.

■ ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publication website at DOI:_. IRMPD spectra of Rh5O4+, Rh5O5+, Rh8O5+, Rh9O7+, Rh10O6+, Rh11O6+, Rh11O7+; IRMPD spectra of Rh8O13+, Rh8O14+, Rh9O13+ and Rh9O15+; Isomers of Rh7O5+ with comparable formation energies as the isomer 5B; Vibrational spectra and geometrical structures of stable Rh7O12+ isomers: Vibrational spectra and geometrical structures of Rh7Om+ (m = 4, 5, 6 and 7) obtained by DFT calculations; Vibrational wavenumbers of stable isomers of Rh7Om+ (m = 4, 5, 6 and 7).

■ AUTHOR INFORMATION Corresponding Author *Phone: +81-3-5454-6597. E-mail: [email protected] Tel: +81-3-5454-6597 ORCID Fumitaka Mafuné: 0000-0001-8860-6354 Joost Bakker: 0000-0002-1394-7661 Toshiaki Nagata: 0000-0002-7127-1487 Ken Miyajima: 0000-0002-5385-8911



Present Address †T.N.: Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS We gratefully acknowledge the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) for the support of the FELIX Laboratory, and thank the FELIX staff, particularly Dr. Britta Redlich for her skillful assistance. The calculations were performed in part using the facilities of the Research Center for Computational Science, Okazaki, Japan. T.N. is grateful for a Research Fellowship from the Japan Society for the Promotion of Science (JSPS JP17K14433, JP17J02017).

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Yamaguchi, M.; Kudoh, S.; Miyajima, K.; Lushchikova, O. V.: Bakker, J. M.; Mafuné, F. Tuning the Dissociative Action of Cationic Rh Clusters Towards NO by Substituting a Single Ta Atom, J. Phys. Chem. C in press.

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Figure captions Figure 1. (a) IRMPD spectrum of Rh7O4+and (b−c) DFT calculated vibrational spectra of the most stable Rh7O4+ isomer 4A (b) and the second most stable isomers 4B (c). The gray line indicates the zero level. Calculated spectral intensities are convoluted with a 15 cm−1 Gaussian lineshape function. The calculated formation energy relative to that of the most stable isomer and the spin state are shown in each panel. Red and blue balls represent O and Rh atoms, respectively.

Figure 2. (a) IRMPD spectrum of Rh7O5+ and (b−c) DFT calculated vibrational spectra of the most stable Rh7O5+ isomer 5A (b) and a higher energy isomer 5E (c). Note that this isomer 5E is not the second most stable one. See Supporting information for other stable isomers and caption of Figure 1 for other information.

Figure 3. (a) IRMPD spectrum of Rh7O6+ and (b−d) vibrational spectrum of the most stable (b), the second (c) and the third (d) most stable isomers of Rh7O6+ obtained by the DFT calculations. See caption of Figure 1 for other information.

Figure 4. (a) IRMPD spectrum of Rh7O7+ and (b−d) vibrational spectra of the most stable (b), the second (c) and the third (d) most stable isomers of Rh7O7+ obtained by the DFT calculations. See caption of Figure 1 for other information.

Figure 5. Depletion spectra of Rh7Om+ (m = 8, 10, 12, 14). The intensities have been normalized by the IR macropulse energy. The gray line in the experimental spectra indicates the zero level. 24 ACS Paragon Plus Environment

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Figure 6. (a) IRMPD spectrum of Rh7O12+ and (b) calculated vibrational spectrum of the most stable isomers of Rh7O12+. (c) Vibrational spectrum of a stable isomer of Rh7O12+ reproducing well the IRMPD spectrum. See caption of Figure 1 for other information.

Figure 7. (a) Histogram of the wavenumbers of vibrational bands of 44 stable isomers of Rh7O12+ (bin size 50 cm−1); (b−d) typical adsorption forms of O2 with the vibrational wavenumbers obtained by the DFT calculations.

Figure 8. (a) Correlation of the sum of natural charges in the two O atoms in units of e, with the O-O bond length of molecular O2 in the stable isomers of Rh7O12+ (panel a) and with the wavenumber of the O2 stretch vibration (panel b). The red dashed lines in panel a show the O-O bond lengths of free O2 (1.21 Å) and superoxide, O2− (1.33 Å), those in panel b the vibrational wavenumber of free O2 (1551 cm−1) and that of superoxide, O2− (1090 cm−1).



Figure 1


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Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4â7, 12

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Structures of Rhodium Oxide Cluster Cations Rh7Om+ (m = 4â7, 12