Correlation between Electronic Shell Structure and Inertness of Cun+

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Article

Correlation between Electronic Shell Structure and Inertness of Cu toward O Adsorption at n = 15, 21, 41, and 49 n+

2

Keijiro Ohshimo, Kengo Akimoto, Masato Ogawa, Wataru Iwasaki, Hiroaki Yamamoto, Masahide Tona, Keizo Tsukamoto, Motoyoshi Nakano, and Fuminori Misaizu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00246 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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The Journal of Physical Chemistry

Correlation between Electronic Shell Structure and Inertness of Cun+ toward O2 Adsorption at n = 15, 21, 41, and 49 Keijiro Ohshimo1, Kengo Akimoto1, Masato Ogawa1, Wataru Iwasaki1, Hiroaki Yamamoto2, Masahide Tona2, Keizo Tsukamoto1,2, Motoyoshi Nakano1,3, Fuminori Misaizu1*

1. Department of Chemistry, Graduate School of Science, Tohoku University, 6–3 Aoba, Aramaki, Aoba-ku, Sendai 980–8578, Japan 2. Ayabo Corporation, 1 Hosogute, Fukamacho, Anjo, 446-0052, Japan 3. Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai, 980-8576, Japan

*

Corresponding author: [email protected] (F. Misaizu)

1 ACS Paragon Plus Environment

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Abstract The inertness of metal clusters in air is important for their application to novel materials and catalysts. The adsorption reactivity of copper clusters with O2 has been discussed in connection with the electronic structure of clusters because of its importance in electron transfer from the cluster to O2. Mass spectrometry was used to observe the reaction of Cun+ + O2 (n = 13-60) in the gas phase. For O2 adsorption on Cun+, the relative rate constants of the n = 15, 21, 41, and 49 clusters were clearly lower than those with other n. Theoretical calculations indicated that the inertness of Cu15+ with 14 valence electrons was related to the large HOMO-LUMO gap predicted for the oblate Cu15+ structure. The Clemenger-Nilsson model was used to predict that the electronic subshell of oblate Cu49+ with 48 electrons was closed. This electronic shell closing of Cu49+ corresponds to the inertness for O2 adsorption.


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1. INTRODUCTION The reactivity of metal clusters with O2 molecules has been studied extensively to understand the stability of clusters, and the reactivity of catalysts.1-9 Several sizes of metal clusters (e.g., Al, Cu, Ag, and Au) that have low reactivity with O2 have been identified as magic species.1,2,4-9 O2 adsorption on a metal cluster proceeds by electron transfer from the cluster to O2. Therefore, the electronic structure of the cluster is important for discussion of the size-dependence of the O2 adsorption reactivity. For, example, previous experiments on the reactivity of Aln− cluster anions with O2 indicated inertness at n = 13, 23, and 37.9 The reactivity of metal clusters is explained by the spherical jellium model, in which the clusters are considered to be uniformly positively charged spheres with electrons confined in the potential.10,11 An Al atom has three valence electrons; therefore, the number of free electrons in Aln− is 3n+1. The inertness of Al13−, Al23−, and Al37− can thus be attributed to electronic shell closing at 40, 70, and 112 electrons in the spherical jellium model using a harmonic oscillator potential. For a cluster of Cu atoms, which has one valence electron, inertness with respect to reactions with O2 was also observed at specific sizes of electronic shell closing. Neutral Cun clusters exhibited low reactivities with O2 at n = 20, 34, 40, 48, and 58.1,2 Among these Cun clusters, the inertness of the n = 20, 34, 40, and 58 clusters was



explained by electronic shell closing in the spherical jellium model using a square-well potential.10 The reaction of Cun with O2 involves partial electron transfer from the HOMO of Cun to the antibonding 1πg* orbital of O2.2,12 However, the spherical jellium model cannot explain why the n = 48 cluster is inert, because the electronic shells are not closed in this electron system. In an adsorption study of O2 on mass-selected Cun+ (n = 3-25) cluster ions, a small adsorption cross section was observed for Cu21+ due to the electronic shell closing of 20 electrons.13 In their study, the dominant product was CunO2+ in O2 adsorption for n ≥ 17. Hirabayashi and coworkers concluded that the reactivity of Cun+ toward O2 is mainly ascribed to the partial electron transfer from Cun+ to O2.13 Previous studies have shown that the reaction of metal clusters with O2 includes a spin excitation of the cluster that causes reduced reactivity of clusters with a singlet ground state and large HOMO-LUMO gaps.7,8 These effects of the HOMO-LUMO gap originate in the triplet spin multiplicity of the ground state O2 molecule.3 Bréchignac and coworkers investigated the O2 adsorption reactivity of Agn+ (n = 1-27), and observed low reactivity with Ag15+.4,5 Khanna and coworkers theoretically studied the inertness of Ag15+ with O2.14 In their calculations, the most stable geometrical structure of Ag15+ was oblate with 14 valence electrons. The spherical


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jellium model was not appropriate for the oblate structure. The Ag15+ ion was shown to have a closed electronic shell with large HOMO-LUMO gap by application of the oblate jellium model, in which molecular orbitals are split because Ag15+ is distorted from a spherical structure. For coinage metals, the reactions of Aun+ cluster cations with O2 have been studied.15,16 Small Aun+ (n = 2-15) cluster cations are known to be inert toward O2 under thermal reaction conditions.15 However, preadsorption of H2 to Aun+ (n = 2, 4, and 6) is found to promote the O2 adsorption to Aun+.16 This promotion of O2 adsorption is related to the charge transfer from H2 to Aun+. Therefore, electronic structures of Aun+ cluster cations are known to determine chemical reactivity in O2 adsorption. In the present study, we performed a mass spectrometric study of gas-phase reactions between an O2 molecule and Cun+ (n = 13-60) formed by a modulated pulsed power magnetron sputtering (MPPMS) cluster ion source. The observed cluster size was extended to larger Cun+ cluster ions than our previous study17 by applying a higher discharge power for sputtering. The observed size-dependent reactivity of Cun+ with O2 is discussed with respect to the electronic shell closing using the jellium model of clusters. Quantum chemical calculations were also employed to calculate the HOMO-LUMO gaps of Cun+ (n = 13-17) to discuss the observed inertness of Cu15+ with



O2 .

2. EXPERIMENTAL METHOD The present experiments were performed using a vacuum apparatus composed of a metal cluster ion source and a reflectron time-of-flight (TOF) mass spectrometer.17 Copper cluster cations, Cun+, were formed by a MPPMS cluster ion source (Ayabo Corporation, nanojima®) which is similar to the previous studies.17,18 This cluster ion source consists of a combination of a magnetron sputtering source (Angstrom Sciences, ONYX-2 DC MAGII) and a gas aggregation cell. From our previous study,17 we replaced the magnetron sputtering source in order to avoid the arc discharge in the high power sputtering for the production of large cluster ions. A 2-inch diameter copper disk was used as the sputtering target. The aggregation cell was cooled to 100 K by liquid nitrogen. Ar gas was introduced continuously around the target at flow rate of 340 sccm. Discharge occurred by application of a pulsed high voltage to the target. The pulsed high voltage (1.5 ms width) was generated by a pulsed power supply (Zpulser LLC, AxiaTM150) with repetition rate of 150 Hz. The pulse power were 360 W and 1080 W for measurements of small Cun+ (n = 11-25) and large Cun+ (n = 20-75), respectively. Metal ions and atoms formed by sputtering with Ar+ ions were aggregated to metal


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cluster ions by collisions with Ar buffer gas in the cell. The gas pressure in the cell was ~14 Pa. The distance between the magnetron sputtering source and the exit aperture of the cell (12 mm diameter) was 330 mm. After exiting the cell, metal cluster ions were collimated by another aperture (2 mm diameter) which was located 53 mm downstream from the exit aperture of the cell. After passing through the collimating aperture, the metal cluster ions were accelerated to 1.5 keV by pulsed electric fields in an acceleration region of the reflectron TOF mass spectrometer. Cluster ions, which were mass separated in the reflectron mass spectrometer, were detected by a dual microchannel plate. To observe the reactions between Cun+ and O2, pure O2 gas was introduced effusively to the vacuum chamber between the two apertures. The partial pressure of O2 gas (∆ ) was monitored at the source chamber. The vacuum pressure of the source chamber was 2.3×10−1 Pa with ion source operation. The number of collisions between Cun+ and O2 molecule was difficult to estimate quantitatively because we could not measure the vacuum pressure in the beam-crossing reaction region directly. If we assume that the local pressure in the reaction region was at least ~10 times higher than the pressure measured by vacuum gauge, the experiment was performed under the condition of multiple collisions. The collision energy between Cun+ and O2 was estimated to be ~0.04 eV. In this estimation, we assumed that the Cun+ ions



were thermalized by many collisions with Ar buffer gas in the aggregation cell at 100 K. Reaction time t was estimated to be ~10 ms because the group velocity of Cun+ cluster bunch can be assumed to be 5 m/s (Ref. 19) in the reaction region.

3. RESULTS AND DISCUSSION Figures 1a and 1c show typical time-of-flight (TOF) mass spectra of copper cluster cations formed by the MPPMS cluster ion source. A series of Cun+ clusters was observed with n = 11-75. Along with the Cun+ cluster series, a series of Ar-tagged Cun+ cluster cations were also observed, as shown in Figures 1a and 1c. This observation of Cun+Ar indicates that the cluster cations were formed at low temperature in the present MPPMS source. The aggregation cell in the present cluster ion source was cooled to 100 K, whereby cold cluster ions tend to be formed in the cell. The size distribution of the ion intensity of Cun+Ar is discussed in the later of this section. By introducing O2 gas effusively into the region between the exit aperture of the cell and the collimating aperture, the ion intensities of O2-adsorbed Cun+ cluster cations, CunO2+, were increased in the mass spectra (Figures 1b and 1d). Because of the lower collision energy (0.04 eV) in the present experiment, it is expected that the following reaction (1) proceeds:


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Cun+ + O2 → CunO2+.

(1)

As shown in Figures 1b and 1d, the ion intensities of the CunO2+ (n = 15, 21, 41, and 49) clusters were quite low compared with the other n. A measure of the relative reactivity (Rn) of Cun+ toward O2 adsorption was estimated to discuss the size-dependent reactivity of Cun+ with O2. For this estimation, a large excess of O2 molecules compared with Cun+ was assumed, so that the former did not change before and after the reactions, although the O2 concentration was carefully controlled to avoid two or more molecules did not adsorb on Cun+. Equation (2) was then used to estimate the Rn values as a measure of the reactivity under the pseudo-first-order approximation: = O = − ln

Cu , (2) Cu

where [Cun+]0 and [Cun+] are the number densities of Cun+ cluster cations before and after the reaction, respectively. [O2] represents the number density of O2 molecules, and t is the reaction time (ca. 10 ms). The [O2] and t values were also assumed to be independent of the Cun+ cluster size, and thus the reactivity Rn was a relative rate constant proportional to kn. [Cun+]0 was calculated by the sum of [Cun+] and [CunO2+] under the assumption that the evaporation of Cu atoms from the cluster ions did not occur after the reaction. The Rn values for n = 20-60 deduced from the ion intensities in Figure 1d are plotted in Figure 2. For n = 13-25, the Rn values were deduced from the



mass spectrum measured under lower power condition for discharge (Figure 1b). These Rn values deduced from the mass spectrum were connected by overlapping at n = 20-25. In this plot, Rn was normalized at n = 16 and is denoted as relative reactivity. The relative reactivities of the n = 15, 21, 41, and 49 clusters were clearly lower than those of the other n clusters, as shown in Figure 2. In the previous study on the reaction between Cun+ (n = 3-25) and O2, a small O2 adsorption cross section was only observed for Cu21+.13


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Figure 1. Typical TOF mass spectra of copper cluster cations formed in the ion source and those reacted with O2. (a) ∆ = 0 Pa, and (b) ∆ = 4×10-3 Pa with a pulse power of 360 W. (c) ∆ = 0 Pa, and (d) ∆ = 7×10-3 Pa with the pulse power of 1080 W. The blue circle corresponds to the Ar-tagged Cun+ cluster cations, Cun+Ar.



In the previous study on the reaction of Cun+ with O2, two reaction products, CunO2+ and Cun-1O2+, were observed for n = 3-25 at a collision energy of 0.2 eV in single collision condition.13 The total cross sections for these reactions showed an even-odd alternation; high cross sections for even n, and low cross sections for odd n, where n = 3-16.13 The Cun-1O2+ ions formed by release of a Cu atom were dominant reaction products, especially for even-sized n between n = 6-16, with the same even-odd alternation. However, the even-odd alternation was not observed for n = 13-16 in the present study (Figure 2). Therefore, the Cun+ ions probably reacted with O2 without the release of a Cu atom in the present experiments, although it is difficult to discuss the reaction pathway because the parent ions were not mass-selected before the reactions. In the present experimental setup, multiple collisions between cluster ions and background gas can occur in the reaction region. Therefore, the CunO2+ product ions may lose its excess energy, which comes from the O2 adsorption energy. This simple adsorption reaction without the elimination of Cu was also rationalized by the low collision energy (0.04 eV) and the low temperature of the cluster ions. This presumption is consistent with the reaction of Cu16+ with O2, where the reaction cross section for the formation of Cu16O2+ gradually increased as the collision energy was decreased from 2.0 to 0.2 eV.12


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Figure 2. Plots of relative reactivity for O2 adsorption on Cun+ (n = 13-60). The relative reactivities were normalized with respect to that at n = 16.

To discuss the low reactivity of Cu15+ with O2, equilibrium structures (EQs) of Cun+ (n = 13-17) were first calculated at the M06-2X/def2-SVP level using the Gaussian 09 package.20 The GRRM14 program21-23 was used to search several EQs without intuition at the HF/6-31G level. Structures of the obtained EQs were re-optimized with the M06-2X/def2-SVP level. More than a few EQs were obtained for Cun+ by this procedure; the numbers of obtained EQs were 13, 13, 13, 9, and 6 for n = 13-17, respectively. Among these EQs, the most stable structures of Cun+ are shown in Figure 3. These structures have C2 symmetry for n =13, Cs for n =14, D3 for n =15, C1 for n =16, and

C1 for n

=

17.

In

previous

density

functional theory calculations

(GGA/PBE/[7s5p4d] level) of Cun+ (n = 2-20), the most stable structures were slightly



different from the present results, with Cs, C2, C1, C1, and C1 symmetries for n = 13-17, respectively.24 Optimized structures of Cun+ (n = 13-17) were also calculated with the BPW91/LanL2DZ level.13 However, to the best of our knowledge, a structure of Cu15+ with high symmetry (D3, Figure 3) has not yet been determined.

Figure 3. The most stable structures of Cun+ (n = 13-17) calculated by using the GRRM program with the M06-2X/def2-SVP level. Symmetry is shown in parentheses. Only high Mulliken atomic charge (q ≥ +0.15) is shown in blue parentheses. Spin multiplicity is doublet and singlet for even and odd n, respectively. For n = 15-17, (a) top, and (b) side views are shown. 14 ACS Paragon Plus Environment

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The most stable structure of Cu15+ is an oblate bilayer structure, which is similar to the Ag15+ structure calculated by Khanna and coworkers.14 The electronic shells of these cluster ions are not closed within the spherical jellium model, because these clusters have 14 valence electrons. However, for the oblate structures of Cu15+ and Ag15+, the spherical jellium model is not appropriate because the superatomic 1P and 1D orbitals are split in the oblate structures. As a result, the electronic shells are closed with 14 electrons in Cu15+ and Ag15+ using the oblate jellium model.14 The energy level of orbitals in the DFT calculation is consistent with the Clemenger-Nilsson model as explained in the Supporting Information. The reactivity of free metal clusters with O2 is shown to be low in clusters with filled electronic shells and large HOMO-LUMO gaps.25 For the calculated HOMO-LUMO gaps of the most stable structures of Cun+ (n = 13-17), the Cu15+ ion has the largest energy among these ions, as shown in Table 1. The large HOMO-LUMO gap of Cu15+ is consistent with both the electronic shell closing and the low reactivity with O2. On the other hand, Cu13+ and Cu17+ have larger HOMO-LUMO gaps than Cu14+ and Cu16+, whereas the former two cluster ions have higher reactivities than the latter ions in Figure 2. This inversed correlation between the HOMO-LUMO gap and reactivity is difficult to explain so far.



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Table 1. Energies of HOMO, LUMO, and HOMO-LUMO gaps of Cun+ (n = 13-17) calculated at the M06-2X/def2-SVP level. HOMO level / eV

LUMO level / eV

HOMO-LUMO gap / eV

Cu13

+

-8.50

-5.66

2.84

Cu14

+

-8.10

-5.72

2.38

Cu15

+

-8.63

-5.19

3.44

Cu16

+

-7.98

-5.61

2.37

Cu17

+

-8.16

-5.40

2.76

Without introducing the reactant O2 gas, Ar-tagged Cun+ ions, Cun+Ar, were observed for n ≤ 30 in the mass spectrum (Figure 4). The size distribution of the integrated ion intensity for Cun+Ar is shown in Figure 5. The ion intensities of Cun+Ar (n = 13, 14, and 16-19) were relatively higher than those of n = 15 and n ≥ 20. By using the Mulliken population analysis, we estimated the charge (q) on each Cu atom in the most stable structures of Cun+ (n = 13-17). There are some atoms with high atomic charges (q ≥ +0.15) in Cun+ except Cu15+. Atomic charges of all atoms were less than +0.10 in Cu15+ due to high D3 symmetry. Therefore, high reactivity of Cun+ (n = 13, 14, 16, and 17) in the Ar-tagging is related with the interaction between high atomic charge on Cu and induced dipole on Ar.


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Figure 4. Typical TOF mass spectrum of copper cluster cations (Cun+) and Ar-tagged Cun+ (Cun+Ar) formed in the ion source with the pulse power of 1080 W.

Figure 5. Size distribution of integrated ion intensities in the mass spectrum (Figure 4) of Cun+Ar.

In Figure 2, the relative reactivities of Cu21+ and Cu41+ with O2 were significantly lower than those of the adjacent sizes. These low reactivities can be explained by the electronic shell closing of 20 and 40 electrons in the Cu21+ and Cu41+ 17 ACS Paragon Plus Environment


cluster ions, respectively. In previous study of reactions of Cun+ (n = 3-25) with O2,13 the reactivity of reaction (1) with Cu21+ was lower than those of the adjacent n. In addition, in the reactions of neutral Cun clusters with O2, Cu20 and Cu40 have low reactivities due to electronic shell closing.1,2 Large HOMO-LUMO gaps were already reported for Cu20 and Cu40 from photoelectron spectroscopy measurements of copper cluster anions, Cun− (n = 6-41).26 The electronic shell closing and large HOMO-LUMO gaps of these clusters are consistent with the inertness of Cu21+ and Cu41+ toward O2. Although the electronic shell closes with 58 electrons, the relative reactivity of Cu59+ was not lower than those of Cu58+ and Cu60+ in Figure 2. On the other hand, neutral Cu58 exhibited low reactivity with O2 which was explained by the closed electronic shell.1,2 The reason of this discrepancy between neutral and cationic system is unclear so far. This question may be unveiled by the study of O2-adsorption on Cun− cluster anions. The relative reactivity of Cu49+ with O2 was also strikingly low, as shown in Figure 2. However, a closed electronic shell is not expected for the cluster with 48 valence electrons within the spherical jellium model. It should be noted that Cu48 also exhibited low reactivity in the neutral Cun + O2 reaction,1,2 and that the ionization potential of neutral Cu49 is lower than those of other clusters,27,28 even though the authors did not offer a reason for this. The present study determined that the 48-electron


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clusters are inert because of the electron shell closing of such axially symmetric distortion systems, as noted in the previous discussion of Cu15+. The Clemenger-Nilsson diagram was developed to discuss electronic structures of axially distorted clusters.29 The Clemenger-Nilsson diagram showed that a degenerated level of electronic shells is split into sublevels due to the axially symmetric distortion.29 In the diagram, sublevels differing only in the sign of Λ are degenerate with each other, making these sublevels (including spin) 4-fold degenerate, where Λ is the projection of the orbital angular momentum along the axis of symmetry. In addition, sublevels with Λ = 0 are 2-fold degenerate. Therefore, 2 and 4 electrons can occupy the Λ = 0 and Λ ≠ 0 sublevels, respectively. The superatomic 1S21P61D102S21F142P6 orbitals are closed with 40 electrons. When the shape of a cluster distorts to oblate, the next 1G orbital is split into five sublevels. In these sublevels, the energetically lower two Λ ≠ 0 sublevels are occupied by 8 electrons. Therefore, the oblate cluster with 48 electrons has a closed shell, i.e. it is implied that Cu49+ has a closed electronic shell with the oblate structure.

4. CONCLUSION In conclusion, copper cluster cations, Cun+, were generated by the combination of modulated pulsed power magnetron sputtering and a gas aggregation cell. The



adsorption reactivity of Cun+ with O2 was observed for n = 13-60 using mass spectrometry. For this reaction, Cu15+ was inert, which is related to the electronic shell closing predicted by the oblate jellium model. In the Cun+ + O2 reactions, the inertness of Cun+ was clearly observed also for the n = 21, 41, and 49. The electronic shell closing of Cu21+ and Cu41+ was predicted using the spherical jellium mode. The Clemenger-Nilsson model was used to predict that the electronic subshell of oblate Cu49+ with 48 electrons was closed. Such electronic shell closing is related to the inertness of size-specific copper cluster cations.

ASSOCIATED CONTENT Supporting Information Geometrical coordinates of structures of Cun+ (n = 13-17). The comparison between the energy level of orbitals of Cu15+ in the DFT calculation and the Clemenger-Nilsson model. The complete author list of ref 20. This material is available free of charge via Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Fax: +81 22 795 6580. Tel.: +81 22 795 6577.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (C) (No. 16K05641) from the Japan Society for the Promotion of Science (JSPS), the Research Seeds Quest Program (Japan Science and Technology Agency), The Murata Science Foundation, and Steel Foundation for Environmental Protection Technology. Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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TOC Graphic



Figure 1 162x272mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2 170x124mm (300 x 300 DPI)



Figure 3 78x116mm (300 x 300 DPI)


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Figure 4 107x74mm (300 x 300 DPI)



Figure 5 140x96mm (300 x 300 DPI)


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65x43mm (300 x 300 DPI)


Correlation between Electronic Shell Structure and Inertness of Cun+

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