Structures of cobalt oxide cluster cations studied by ion mobility mass spectrometry

Chemical Physics Letters 588 (2013) 63–67

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Structures of cobalt oxide cluster cations studied by ion mobility mass spectrometry Kosuke Ota, Kiichirou Koyasu, Keijiro Ohshimo, Fuminori Misaizu ⇑ Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan

a r t i c l e

i n f o

Article history: Received 27 August 2013 In final form 10 October 2013 Available online 18 October 2013

a b s t r a c t Structures of cobalt oxide cluster cations have been investigated by ion mobility mass spectrometry. þ When the ions were injected into a drift cell with 250 eV kinetic energy, ðCoOÞþ n and Con On1 were þ observed. Collision cross sections were experimentally determined for ðCoOÞþ 27 and Con On1 (n = 5–7). Orientation-averaged collision cross sections were calculated for optimized structures of these ions obtained in quantum chemical calculations. By comparison between experimental and theoretical cross þ sections, ðCoOÞþ 35 ions are found to have monocyclic-ring structures. By contrast, ðCoOÞ6;7 have compact tower structures. Therefore, structural transition from the ring to compact structures occurs at ðCoOÞþ 6. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Cobalt oxide compounds are now widely used for various materials such as catalyst, semiconductors, and so on. For example, recent studies have shown that cobalt oxide catalysts are highly active for oxidation reaction of CO at ambient temperature [1,2]. In order to study reaction mechanisms including the cobalt oxide catalyst, it is important to know the microscopic physical and chemical properties of this compound. Recently, structures and reactivity of cobalt oxide clusters have been studied by gas-phase spectroscopic techniques and theoretical calculations in order to obtain information on the catalysis activity from the microscopic point of view. Quantum chemical calculations of neutral cobalt oxide clusters, (CoO)n, showed that monocyclic-ring structures and tower-type structures are stable for n 6 4 and n P 6, respectively [3]. These cluster structures are totally different from that of bulk cobalt(II) oxide (CoO) solid crystal, which is known as cubic rock-salt type structure. Mass spectrometric studies of cobalt oxide cluster cations in the gas phase were also reported in order to reveal the size-dependent stability of these clusters [4–6]. In addition, photoionization mass spectrometry of neutral cobalt oxide clusters was performed in order to investigate the reactions of clusters with small molecules [7]. Most of the discussions about structures of stable clusters relied on theoretical calculations in such studies because it is in general difficult to obtain experimental evidence for cluster structures only from the mass spectrometric methods. Ion mobility spectrometry (IMS) is known as a method for characterizing geometrical structures of gas-phase ions from the

⇑ Corresponding author. Fax: +81 22 795 6580. E-mail address: [email protected] (F. Misaizu). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.10.030

measurement of a drift velocity, vd, of ions in an electric field. In the IMS experiment, a swarm of ions is injected into a gas cell (ion-drift cell) in which an electric field is applied so as to accelerate ions downstream. Due to a balance of acceleration by the electric field, E, and deceleration by collisions with buffer gas in the cell, vd becomes a constant value proportional to E, i.e.,

v d ¼ KE;

ð1Þ

in which the coefficient K is called to be an ion mobility [8]. In order to facilitate the comparison, the mobility K is usually converted to a reduced mobility, K0, which is defined by

K0 ¼

p 273:15 K: 760 T

ð2Þ

Here p is the gas pressure in Torr and T is the gas temperature in kelvin. An equation of the ion mobility K of thermalized ions drifting through the buffer gas in the electric field was given from the kinetic theory as:

K¼

 1=2 3e 2p 1 ; 16N kB lT eff Xð1;1Þ

ð3Þ

where e is the elementary charge, N is the number density of the buffer gas, kB is the Boltzmann constant, l is the reduced mass, and X(1,1) is a collision integral [9]. When we treat the ion as a hard-sphere, the collision integral reduces to the hard-sphere collision cross section, X. The term Teff is the effective temperature of the ions, and is given by TBG + mBvd2/3kB, where TBG is the buffer gas temperature and mB is the mass of buffer gas. From Eqs. (1)– (3), the time the ion spends in the ion-drift cell is inversely proportional to the ion mobility and directly proportional to the collision cross section [9]. Therefore, the collision cross section of the ion can be evaluated by measuring the mobility in the ion-drift cell.

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Structures of ions can be identified by comparison of cross sections measured by IMS experiments with those obtained from structures calculated by quantum chemical calculations. In order to identify structures of cluster ions, IMS has been applied to various cluster ions, such as carbon clusters [10–15], silicon clusters [16], alkali halide clusters [17,18], gold clusters [19], and boron clusters [20]. In this study, we have investigated the structures of cobalt oxide cluster cations, ðCoOÞþ n , by ion mobility spectrometry combined with mass spectrometry, that is, ion mobility mass spectrometry (IM-MS). From mass spectra of cobalt oxide cluster cations, the observed series of clusters were discussed by comparison with the previous collision-induced dissociation (CID) [5] and photodissociation studies [6]. By using our home-built IM-MS experimental apparatus, collision cross sections of ðCoOÞþ n (n = 2– 7) and Con Oþ n1 (n = 5–7) were obtained from the measurements of ion mobility. Collision cross sections of these clusters were also calculated by using GAUSSIAN 09 [21] and MOBCAL programs [22]. By comparison between experimental cross sections and theoretical ones, we determined structures of cobalt oxide cluster cations.

2. Experimental and computational methods IM-MS experiments were performed using a home-built vacuum apparatus composed of a cluster ion source, an ion-drift cell for IMS [23], and a reflectron time-of-flight mass spectrometer (TOFMS) [24]. Details of the experimental setup and procedures for IM-MS were already reported elsewhere [25–27]. Here, we briefly describe the experimental setup and conditions for IM-MS. Cobalt oxide cluster cations, Com Oþ n , were generated by combination of laser vaporization and supersonic expansion. The second harmonic of a Nd:YAG laser was focused onto a rotating and translating cobalt rod for laser vaporization. The produced microplasma containing cobalt vapor was cooled in a channel (40 mm long and 3 mm diameter) by a mixture gas of O2 and He expanded from a pulsed valve (General Valve, Series 9). Stagnation pressure of the mixed gas was 2 atm. The O2 mixing ratio was typically 5% for generating Com Oþ n cluster cations. The generated cluster ions were injected into the ion-drift cell with kinetic energies of 50–250 eV by a pulsed electric field at a given time (t = t0). The cell was 100-mm long and was filled with He buffer gas with a pressure of 0.90 Torr, and a drift electric field of 9.95 V/cm was applied in order to guide the ions downstream. The cell temperature was cooled down to 190 K by liquid N2 circulation. After running through the cell, the ions were re-accelerated by pulsed electric fields in an acceleration region of a TOFMS at a given time later from the first pulse, t = t0 + Dt. We hereafter denote this delay time, Dt, as ‘arrival time’. The time spend in the cell depends on the collision cross sections between ions and He atoms in the cell. Therefore, cluster ions with different structures reach the acceleration region of TOFMS at different arrival times. We obtained TOF mass spectra sequentially by scanning the arrival time between the injection pulse and the ion acceleration pulse. As a result, different structure ions of clusters were separately detected at different arrival times in a two-dimensional plot of TOF vs. arrival time. We also obtained a plot of arrival time distribution (ATD), in which the total ion intensity of a certain TOF peak was shown as a function of the arrival time. In IMS, the ratio of the drift electric field, E, to the number density of buffer gas, N, is an important parameter [9]. Typical other experiments on isomer-separation of carbon clusters, the E/N values were 1.5–10 Td (1 Td = 1017 V cm2) [14,22,28]. It is desirable to keep E/N low for obtaining sufficient collision frequency between the cluster ions and the buffer gas to separate different structure ions. On the other hand, the amount of the cluster ions after the separation decreases at low E/N conditions due to

scattering by many collisions with buffer gas. Therefore, we searched for the highest possible E/N conditions for determining the structures of cluster ions. In the present experiments, condition was optimized with E/N = 22 Td. Although this E/N condition does not appear to be in the low field limit, the deviation of the obtained ion mobility from the low field limit is expected to be too small to affect the discussion. For example, the reduced mobility K0 of oxygen atomic anion (O) in 22 Td is 3% larger than that in a weak electric field (E/N < 20 Td) [29]. The E/N dependence of ion mobility was not considered in the present study. Quantum chemical calculations for the geometry optimization þ of cobalt oxide cluster ions, ðCoOÞþ n (n = 2–7) and Con On1 (n = 5– 7), were performed to evaluate the collision cross sections of the cluster ions. Present calculations were carried out by using the density functional theory (DFT) program of GAUSSIAN 09 [21]. The 6-31+G(d) basis set and the B3LYP functional were used in these calculations. The bond length of CoO+ molecule obtained by the present calculation (1.634 Å) is well reproduced the experimental (1.65 ± 0.01 Å) [30], and theoretical (1.63 Å with PW86/DZVP level) [4] values. Charge distributions in optimized structures of cluster ions were estimated by a natural population analysis [31]. Orientation averaged collision cross sections of the cluster ions were calculated by using the projection approximation [15] which is included in the MOBCAL program [22]. In the MOBCAL program, there are three methods to calculate ion mobilities and collision cross sections: projection approximation, exact hard sphere scattering method, and trajectory method. It is known that the trajectory method is the most reliable to calculate collision cross sections. However, the method requires more parameters (e.g. parameters for Lennard–Jones potentials), than other methods. Because it is difficult to determine these parameters for interaction potentials including cobalt atoms, we used the simplest projection approximation method for the present study. With a quantum-chemically calculated structure, only adjustable parameters in this approximation are the hard sphere atomic radii of the constituent cobalt and oxygen atoms. In the present calculations of collision cross sections, we used atomic radii of 1.51 and 2.41 Å for cobalt and oxygen atoms, respectively. These radii were determined by scaling of crystal radii of Co2+ and O2 ions (0.79 and 1.26 Å, respectively) [32], so as to reproduce the experimental cross section of ðCoOÞþ 3 ring isomers. Similar scaling proþ cedure was already applied to Auþ n and Bn cluster cations [19,20]. In order to decrease flexibility of our scaling, we fixed the ratio of hard sphere atomic radii at the ratio of the known crystal radii (0.79 Å/1.26 Å = 0.63).

3. Results and discussion Figure 1a shows a typical TOF mass spectrum of cobalt oxide cluster cations obtained at an ion-injection energy of 50 eV. This TOF mass spectrum of the cluster cations which exited from the ion-drift cell was obtained by summing up all the TOF spectra measured at every arrival time. Temperature and He-pressure in the ion-drift cell was 190 K and 0.90 Torr, respectively. A series of oxygen-equivalent ðCoOÞþ n cluster cations was observed for 2 6 n 6 6. Along with the series of ðCoOÞþ n , oxygen-rich and -poor cobalt oxide þ þ cluster cations, Con Oþ , Co O n nþ1 nþ2 and Con On1 , were also observed. On the other hand, in the accumulated mass spectrum obtained at an ion-injection energy of 250 eV (Figure 1b), we predominantly þ observed ðCoOÞþ n cluster cations in addition to Con On1 . Oxygenþ rich cobalt oxide cluster cations, Con Oþ and Co O , n nþ2 were not obnþ1 served in Figure 1b. Hence, when the ion-injection energy was high, the intensities of the oxygen-rich clusters decrease relative to those of the oxygen-equivalent and –poor clusters. This tendency is observed in the CIDs of cobalt oxide cluster cations, which

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þ Figure 2. Two-dimensional plot of TOF vs. arrival time of ðCoOÞþ n and Con On1 (N) cluster cations at an ion-injection energy of 250 eV.

Figure 1. Mass spectra of cobalt oxide cluster cations with the ion-injection energy: (a) 50 eV and (b) 250 eV.

was previously studied by using a tandem mass spectrometry with 8-keV ion kinetic energy [5]. In their observation, a major CID pathway for the oxygen-equivalent ðCoOÞþ n clusters was the loss of a CoO moiety to form ðCoOÞþ n1 fragments. Also in CIDs of the þ oxygen-poor Con Oþ n1 , the loss of a Co atom to yield ðCoOÞn1 fragments was a major CID fragmentation channel. Therefore, it is suggested that the CIDs of the cluster cations occur just after the entering of the ion-drift cell in high ion-injection energy conditions (Figure 1b). Although there is no study of CID of oxygen-rich þ cobalt oxide cluster cations, Con Oþ nþ1 and Con Onþ2 , multiphoton dissociation of these cluster cations has been studied by using the third harmonic of a Nd:YAG laser [6]. In their study, Con Oþ nþ1 and Con Oþ nþ2 dissociate preferentially through the loss of two oxyþ þ gen atoms and the formation of Con On1 and ðCoOÞn cluster cations. þ Therefore, in this study, it is concluded that Con Oþ n1 and ðCoOÞn fragments are formed by CIDs of oxygen-poor, -equivalent, and -rich cobalt oxide cluster cations. Figure 2 shows a two-dimensional plot of TOF vs. arrival time for cobalt oxide cluster cations at the ion-injection energy of 250 eV. At least two series of arrival time distributions, (A) and (B), were observed for ðCoOÞþ n : The series with longer arrival time (A) was observed for 3 6 n 6 5, and another series (B) was for n P 6. The time an ion spends in the drift cell is directly proportional to the collision cross section, X, of the ion [9]. An ion with a large collision cross section exits the cell at a later time than ion with a small cross section [10]. Therefore, a bulky ion has a longer arrival time distribution. Experimental collision cross sections, Xexp, of ðCoOÞþ n cluster cations determined from the arrival time distributions are plotted in Figure 3. In order to evaluate collision cross sections theoretically, optimized structures of ðCoOÞþ n cluster cations have been calculated by quantum chemical calculations with B3LYP/6-31+G(d) level. As shown in Figure 4, the structures of ðCoOÞþ n were obtained to be linear for n = 2 and 3, ring for n = 2–7, cube and tower structures

Figure 3. Collision cross sections of ðCoOÞþ n cluster cations obtained by experiments (Xexp) and theoretical calculations (Xcalc). Structures used in the theoretical calculations are also indicated.

for n = 4–7. Some of these structures were already reported in previous theoretical studies for (CoO)n neutrals [3] and ðCoOÞþ n cations [5]. By the natural population analysis of ðCoOÞþ 4 , atomic charges on Co and O atoms are estimated to be about +1.4 and 1.2, respectively. It is found that all cluster cations in this study are formed by ionic bonds. Table 1 shows the comparison of calculated energies of these isomers. For ðCoOÞþ 2 , the linear isomer is calculated to be 1.11 eV lower in energy than the ring isomer. On the other hand, for ðCoOÞþ n (n = 3–6), the ring isomers are calculated to be more stable than the linear, cube and tower isomers. For ðCoOÞþ 7 , the tower isomer is calculated to be more stable than the ring isomer. We calculated collision cross sections, Xcalc, by using the projection approximation [15] which is included in MOBCAL program [22]. The atomic radii of cobalt and oxygen atoms which were used in the present calculations of Xcalc were scaled in order to reproduce Xexp of ðCoOÞþ3 as noted in the previous section. The calculated Xcalc of these cluster cations are plotted in Figure 3. The values of Xexp and Xcalc of ðCoOÞþ n cluster cations are summarized in Table 1. In Figure 3, Xcalc of ring isomers are found to be in good agreement with Xexp of ðCoOÞþ n (n = 4 and 5). However, for ðCoOÞþ , X of ring isomers are found to be quite larger than Xexp calc 6;7 of n = 6 and 7. Xexp of ðCoOÞþ n (n = 6 and 7) are well reproduced by Xcalc of both tower and cube isomers. It is difficult to distinguish between tower and cube structures for ðCoOÞþ 6 because Xcalc of

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K. Ota et al. / Chemical Physics Letters 588 (2013) 63–67

Figure 4. Examples of optimized structures and natural charges (in parentheses) of ðCoOÞþ n (n = 2–7) cluster cations calculated with B3LYP/6-31+G(d) level. Bond lengths are shown in angstroms. Among several structures with different spin multiplicity, structures with the largest spin multiplicity are shown.

Table 1 Experimental and theoretical collision cross sections (Xexp and Xcalc), spin multiplicities (2S + 1) and relative energy from the most stable isomers (DE) of ðCoOÞþ n (n = 2–7) calculated at B3LYP/6-31+G(d) level. Cluster ions

Xexp/Å2

ðCoOÞþ 2

33.6 ± 0.5

ðCoOÞþ 3

37.0 ± 1.3

2S + 1

Xcalc/Å2

DE/eV

Linear Ring

2 2

31.4 28.0

0.00 1.11

Ring

1 3 1 3

37.3 37.0 41.2 43.9

4.32 0.00 6.38 1.88

2 4 2 4

47.3 46.4 39.2 39.4

0.00 0.78 3.10 3.09

1 3 5 1 3 5

53.6 58.4 57.5 44.8 46.5 46.8

8.23 0.00 1.49 7.13 3.17 1.29

2 4 6 2 4 6 2 4 6

51.5 51.1 51.1 50.7 49.9 50.6 69.4 69.1 69.0

2.27 3.03 3.11 1.04 3.31 2.21 0.00 0.89 1.72

1 1

57.1 75.2

0.00 4.03

Linear ðCoOÞþ 4

46.3 ± 1.5

Ring Cube

ðCoOÞþ 5

53.9 ± 2.0

Ring

Cube

ðCoOÞþ 6

51.6 ± 1.0

Tower

Cube

Ring

ðCoOÞþ 7

57.6 ± 2.0

Tower Ring

tower isomers is almost the same value with that of cube isomers. From these results, we conclude that the structures of ðCoOÞþ n are ring for n = 3–5 and tower and/or cube for n = 6. However, in the present quantum chemical calculations, the ring isomer was the

Figure 5. Optimized structures of Con Oþ n1 (n = 5–7) cluster cations calculated with B3LYP/6-31+G(d) level. Spin multiplicities of these cluster ions are singlet (n = 5 and 7) and doublet (n = 6).

most stable isomers in ðCoOÞþ 6 . This is not consistent with the experimental result; the ring isomer of ðCoOÞþ 6 was not observed at least at high injection energies in the present experiment. This tendency suggests that higher level and larger basis set are needed in order to determine the most stable isomer of the cluster ions containing transition metal atoms. It is known that density-functional calculations sometimes fail to predict the most stable isomer of transition metal oxide clusters [33].

K. Ota et al. / Chemical Physics Letters 588 (2013) 63–67

As shown in Figure 2, arrival time distributions of oxygen-poor Con Oþ n1 cluster cations have almost the same tendency with those of oxygen-equivalent ðCoOÞþ n cluster cations at most of the cluster sizes. However, the arrival time of oxygen-poor Co5 Oþ 4 is about 20 ls shorter than that of oxygen-equivalent ðCoOÞþ 5 . This implies þ that Co5 Oþ 4 has more compact structure than ðCoOÞ5 ring isomers. Figure 5 shows optimized structures of oxygen-poor Con Oþ n1 (n = 5–7) cluster cations by B3LYP/6-31+G(d) level. By using these structures, theoretical collision cross sections Xcalc are estimated to be 44.3, 48.3 and 50.7 Å2 for n = 5–7, respectively. These Xcalc values are almost equal to the experimentally determined collision cross sections (46.9 ± 0.6 (n = 5), 49.7 ± 1.0 (n = 6), and 54.5 ± 0.8 Å2 (n = 7)). Therefore, it is concluded that the structure of Co5 Oþ 4 is the ladder-like structure with one excess cobalt atom (Figure 5). In order to form ring structures in the cobalt oxide cluster cations, all chemical bonds are Co+–O ionic bonds as shown in ðCoOÞþ 4 (Figure 4). Therefore, it is probable that the extra cobalt atom prevents the formation of the stable monocyclic-ring isomer of Co5 Oþ 4. 4. Conclusion Cobalt oxide cluster cations, Com Oþ n , were generated by combination of laser vaporization and supersonic expansion of O2/He mixed gas. Structures of these cluster cations are investigated by using ion mobility mass spectrometry. With the high injection enþ ergy (250 eV), ðCoOÞþ n and Con On1 cluster cations were predominantly observed in the mass spectrum. In the two-dimensional plot of the ðCoOÞþ n , two series of arrival time distributions were observed. By comparison of experimentally determined collision cross sections with theoretical ones, it is concluded that the two series of arrival time distribution are attributed to the monocyclic-ring and compact (tower and/or cube) structures, respectively. Therefore, the structural transition from ring to compact structures þ occurs at n = 6 for ðCoOÞþ n cluster cations. For oxygen-poor Co5 O4 cluster cations, the structure is the ladder-like structure rather than the ring structure. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), and in part by the Research Seeds Quest Program (JST), The

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