Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster

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Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry Fuminori Misaizu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on January 6, 2015

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Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Journal:


Manuscript ID:

jp-2014-115674.R1

Manuscript Type:

Special Issue Article

Date Submitted by the Author: Complete List of Authors:

28-Dec-2014 Ohshimo, Keijiro; Tohoku University, Chemistry Azuma, Shohei; Tohoku University, Chemistry Komukai, Tatsuya; Tohoku University, Chemistry Moriyama, Ryoichi; Tohoku University, Chemistry Misaizu, Fuminori; Tohoku University, Chemistry

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Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Keijiro Ohshimo, Shohei Azuma, Tatsuya Komukai, Ryoichi Moriyama, and Fuminori Misaizu* Department of Chemistry, Graduate School of Science, Tohoku University, 6–3 Aoba, Aramaki, Aoba-ku, Sendai 980–8578, Japan

* Corresponding author. Tel.: +81 22 795 6577, Fax: +81 22 795 6580 E-mail address: [email protected] (F. Misaizu)

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Abstract Structures and CO-adsorption reactivities of nickel oxide cluster cations were investigated by ion mobility mass spectrometry.

The series of NinOn-2+, NinOn-1+ and

NinOn+ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into an ion-drift cell.

From the arrival time distributions of

NinOn+ and NinOn-1+ in the ion mobility spectrometry, structural transition from two-dimensional (2D) ring to three-dimensional (3D) compact structures was found at n = 5.

In addition, 2D and 3D structural isomers were found to coexist for Ni5O5+,

Ni6O5+ and Ni7O6+.

By adding CO gas to buffer gas in the ion-drift cell, Ni4O3+ and

Ni5O4+ cluster cations were found to be more reactive for the CO adsorption reactions than Ni4O4+ and Ni5O5+.

Under the pseudo-first-order approximation, rate constants

for CO-adsorption were determined to be (8.4 ± 0.7) × 10-11 cm3 molecule-1 s-1 for Ni4O3+ and (9.6 ± 0.8) × 10-11 cm3 molecule-1 s-1 for Ni5O4+.

These rate constants are

two orders of magnitude faster than those for Ni4O4+ and Ni5O5+ which have reported previously.

These differences of rate constants can be originated in the structures of

the nickel oxide cluster ions. KEYWORDS:

metal oxide cluster, collision cross sections, structural isomers, CO

adsorption, rate constants


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1. Introduction Nickel oxide compounds are widely used as catalysts of valuable reactions such as oxidation of carbon monoxide (CO).1,2

Recently, geometrical structures and

stability of nickel oxide clusters have been studied by gas-phase mass spectrometric experiments3-7 and quantum-chemical calculations.8,9

The study of gas-phase metal

oxide clusters has been an active research field because the clusters are ideal model systems to obtain atomic level insight into the energetics and kinetics of metal-mediated catalytic reactions.10 In bulk phase, nickel oxide is expressed by a chemical formula of NiO. In a previous mass spectrometric study of nickel oxide cluster cations, oxygen-equivalent clusters (NinOn+) and oxygen-rich clusters (NinOn+1+) were observed.3

Ultraviolet

multiphoton dissociation experiments showed that oxygen-deficient clusters (NinOn-1+) were produced as major photofragments from several parent ions.4

A previous study

on the reactivity of NinOm+ cluster cations (n = 1-2, m = 1-4) with CO showed that CO adsorption on the cluster ions preferentially occurs accompanied by the loss of either O2 or nickel oxide units.7

Also in a mass spectrometric study of reactivities of larger

nickel oxide cluster ions (NinOn+x+, n = 4-10, x = -1-+3), attachment of a CO molecule to NinOn+x+ was readily observed for all of the cluster ions with different stoichiometries,



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except for Ni7O6+.5

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A quantum chemical calculation showed that the optimized

structure of neutral Ni6O6 cluster is the NaCl rock-salt type geometry which is similar to the structure of bulk phase.8

Although the understanding of the correlation between

structures and reactivities is important, experimental studies were hardly reported on the structures of nickel oxide cluster cations so far. Ion mobility mass spectrometry (IM-MS), which is a combination of ion mobility spectrometry (IMS) and mass spectrometry (MS), is a powerful method to elucidate the geometrical structures of gas-phase ions.11-13

In IMS, a packet of ions is

injected into a gas cell (ion-drift cell) in which an electric field is applied.

Due to a

balance of acceleration of ions by the electric field, E, and deceleration by collisions with buffer gas in the cell, a drift velocity of the ions, vd, becomes a constant value proportional to E, i.e., vd = KE,

(1)

in which the coefficient K is called to be an ion mobility.14

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:

K=

3e 16 N

 2π   k B µ Teff

  

12

1 Ω (1,1)

,

(2)

where e is the elementary charge, N is the number density of the buffer gas, kB is the 4 ACS Paragon Plus Environment

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Boltzmann constant, µ is the reduced mass of the ion and the buffer gas atom, and Ω(1,1) is a collision integral representing an average over collision energy, orientations and inelastic collisions.14

When we treat the ion and neutral as hard sphere without

internal states, the collision integral reduces to the hard-sphere collision cross section, Ω. Although the eq 2 has originally been derived for the collisions of atomic ions with atoms, this equation has been widely applied to the system of polyatomic ions in the buffer gas molecules. The term Teff, the effective temperature of the ions, is given by TBG + mBvd2/3kB, where TBG is the buffer gas temperature and mB is the mass of buffer gas.

The time that the ion spends in the ion-drift cell is thus inversely proportional to

K and directly proportional to Ω, as is known from eqs 1 and 2.15

Therefore, the

collision cross section of the ion can be evaluated by measuring the mobility of the ion in the ion-drift cell.

Recently, by using the home-built IM-MS apparatus, we

investigated the structures of metal oxide cluster cations, (ZnO)n+ (ref 16), (CoO)n+ (ref 17) and (FeO)n+ (ref 18).

Structural transitions from two-dimensional (2D) rings to

three-dimensional (3D) compact structures were commonly observed in these clusters. By using the ion-drift cell as a reaction cell, reactivity of gas-phase atomic, molecular and cluster ions can also be investigated experimentally.19,20

For example, the

chemical reactivity of the silicon cluster cations with ethylene has been studied by



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adding ethylene to helium buffer gas.19 In this study, we have investigated the structures and reactivities of nickel oxide cluster ions, NinOn-1+ and NinOn+ by IM-MS.

Experimental collision cross

sections of these cluster ions were obtained by the measurements of ion mobility.

In

addition, theoretical collision cross sections were calculated by the MOBCAL program21 for optimized structures calculated by quantum-chemical calculations.

By

comparison between experimental and theoretical cross sections, we have determined structures of nickel oxide cluster cations.

By adding CO gas to helium buffer gas in

the ion-drift cell, reactivities and rate constants of nickel oxide cluster cations in CO adsorption reactions have been investigated.

2. Experimental Method IM-MS experiments were performed using a home-built vacuum apparatus composed of a cluster ion source, an ion-drift cell, and a reflectron time-of-flight (TOF) mass spectrometer.

Details of the experimental setup and procedures for IM-MS were

already reported elsewhere.16-18

Nickel oxide cluster cations, NinOm+, were generated

by a combination of laser vaporization of a nickel rod and supersonic expansion of 5 % O2/He mixture gas.

Stagnation pressure of the O2/He mixture gas was 3 atm.


The

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generated cluster ions were injected into the ion-drift cell with kinetic energies of 50-400 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 0.60-0.80 Torr. field in the cell was E = 10 V/cm.

The drift electric

After running through the cell, the ions were

re-accelerated to ~1.8 keV by another pulsed electric fields in an acceleration region of the TOF mass spectrometer at a given time later from the first pulse, t = t0 + ∆t. We hereafter denote this delay time, ∆t, as “arrival time”.

This arrival time consists of

TOFs in three regions: (i) between the first acceleration region and the entrance of the cell, (ii) inside of the cell, and (iii) between the exit of the cell and the acceleration region of TOF mass spectrometer.

By evaluating the drift velocity for satisfying the

measured arrival time, we have calculated these TOFs in three regions by solving the equations of motion of cluster ions.

Therefore, we can obtain the time that an ion

spends in the cell from the measured arrival time.

The time that an ion spends in the

cell depends on the collision cross sections between ions and He atoms.

Therefore,

cluster ions with different cross sections reach the acceleration region of the TOF mass spectrometer at different arrival times. reflectron TOF mass spectrometer.

Finally the ions were mass-analyzed by the

In the IM-MS measurement, we obtained a series

of TOF mass spectra sequentially by scanning automatically the arrival time with the



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LabVIEW program.

As a result, cluster ions with different cross sections 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. For the reactivity study of cluster ions, we have observed reactions of the cluster ions with CO by adding 0.1-2.5 % CO gas to He buffer gas in the ion-drift cell. In IMS, the ratio of the drift electric field, E, to the number density of buffer gas, N, is an important parameter.14

Typical other IMS experiments, the E/N values

were 1.5-10 Td (1 Td = 10-17 V cm2).22,23

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, conditions were

optimized with E/N = 24 Td when buffer-gas pressure and temperature in the ion-drift cell were 0.80 Torr and 190 K, respectively.

Although we have not measured the E/N

dependence of the collision cross sections of the present nickel oxide cluster ions, we have measured the collision cross sections of C60+ ions in the range of E/N = 11-25 Td.


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Consequently, collision cross sections of C60+ were measured to be 120-122 Å2 in this E/N range.

These collision cross section values are almost equal to the collision cross

section (124 Å2) derived from the reduced mobility measurement with E/N = 1.4 Td.23 Therefore, the E/N dependence of ion mobility was not considered in the present study.

3. Computational Method Quantum chemical calculations for the geometry optimization of nickel oxide cluster cations, NinOn-1+ (n = 3-7) and NinOn+ (n = 2-9), were performed in order to calculate the collision cross sections of the cluster ions.

Present calculations were

carried out by using B3LYP/6-31+G(d) level in Gaussian 09.24

In a benchmark

calculation, the bond length of NiO molecule (X3Σ-) calculated by B3LYP/6-31+G(d) level (1.6264 Å) reproduced the experimental value (1.6271 Å).25 Charge distributions in optimized structures of cluster ions were estimated by a natural population analysis. In this analysis of Ni4O3+ linear isomer, atomic charge on Ni and O atoms were estimated to be about +1.1 and -1.1, respectively.

Similarly it was found that all

cluster cations in this study are formed by ionic bonds. Orientation-averaged collision cross sections of the cluster ions were calculated by using the projection approximation26 which is included in the MOBCAL program.21



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Although the trajectory method in the MOBCAL program is the most reliable to calculate collision cross sections, the trajectory method requires parameters for Lennard-Jones interaction potentials.

Because it is difficult to determine these

parameters for interaction potentials including nickel atoms, we used the simplest projection approximation method in the present study.

With a quantum-chemically

calculated structure, only adjustable parameters in this approximation are the hard sphere atomic radii of the constituent nickel, oxygen and helium atoms. In the present calculations of collision cross sections, we used atomic radii of 0.86, 1.30 and 1.15 Å for nickel, oxygen and helium atoms, respectively.26

These radii of nickel and oxygen

were determined by slight scaling of crystal radii of Ni2+ and O2− ions (0.83 and 1.26 Å, respectively),27 so as to reproduce the experimental cross section of Ni4O4+ ring isomer. 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 (0.83 Å / 1.26 Å = 0.66).

Similar scaling

procedure was already applied to Aun+ and Bn+ cluster cations by Kappes and his coworkers,28,29 and also to (ZnO)n+, (CoO)n+, and (FeO)n+ by the authors' group.16-18

4. Results and Discussion 4.1. Mass spectra of cluster ions exited from the ion-drift cell


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Figures 1a-d show typical mass spectra of nickel oxide cluster cations which exited from the ion-drift cell at four different ion injection energies.

In the present

apparatus, after injection into the ion-drift cell by a pulsed electric field, cluster ions diffuse spatially before reaching the re-acceleration region of TOF mass spectrometer. Therefore, the TOF mass spectra shown in Figure 1 were obtained by summing up all the TOF spectra measured at every arrival time because of the spread of arrival time distribution of an ion packet. In the mass spectrum obtained at an ion-injection energy (Einj) of 50 eV (Figure 1a), a series of oxygen-equivalent NinOn+ cluster cations was observed for n ≤ 9, in addition to one oxygen-deficient nickel oxide cluster cations NinOn-1+ and a variety of oxygen-rich nickel oxide cluster cations, NinOn+m+.

These stoichiometries were also

observed in the mass spectra of nickel oxide cluster cations generated by laser vaporization in previous studies.3-5

In the mass spectrum at Einj = 150 eV (Figure 1b),

the series of NinOn-1+, NinOn+, and NinOn+1+ cluster cations were observed at n = 2-10. By contrast, at higher Einj of 250 eV (Figure 1c), NinOn-2+, NinOn-1+, and NinOn+ were observed, whereas NinOn+1+ was not observed.

These observed series were also found

in the mass spectrum at Einj = 400 eV (Figure 1d).

For example, above Einj dependence

was clearly observed in the cluster of n = 4: Oxygen-rich Ni4O4+m+ (m = 0-3) cluster



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cations were observed at Einj = 50 eV, whereas oxygen-deficient Ni4O4-m+ (m = 0-2) cluster cations were predominant at Einj = 250 and 400 eV. Previously, multiphoton dissociation of nickel oxide cluster cations was investigated using the third harmonic of a Nd:YAG laser by Duncan and coworkers.4 In their study, oxygen-deficient fragments with stoichiometries of NinOn-1+ were preferentially formed by photodissociation of NinOn+m+ cluster cations.

Therefore, in

the present IM-MS experiments, oxygen-deficient NinOn-m+ (m = 0-2) cluster cations were most likely formed by collision-induced dissociations (CIDs) of the oxygen-rich NinOn+m+ cluster cations just after entering into the ion-drift cell at high ion-injection energies. In the previous IM-MS study of sodium fluoride cluster cations by the authors’ group, it was confirmed that stable magic-number cluster ions survive during CIDs of other cluster ions in the ion-drift cell.30

Therefore, oxygen-deficient NinOn-m+ (m =

0-2) cluster cations are probably the stable stoichiometries in the nickel oxide cluster cations.

It is worth noting that these stable stoichiometries in the mass spectrum of

nickel oxide cluster cations showed marked contrast with those of other transition-metal (Fe and Co) oxide cluster cations: FenOn+, FenOn+1+, ConOn-1+ and ConOn+ were predominantly observed in the mass spectra.17,18

This metal dependence of stable


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stoichiometry is probably related to the preferable oxidation number of each transition metal atom.



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Figure 1.

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Mass spectra of nickel oxide cluster cations: (a) ion-injection energy (Einj)

was 50 eV, (b) Einj = 150 eV, (c) Einj = 250 eV, and (d) Einj = 400 eV. pressure and temperature in the cell were 0.80 Torr and 190 K, respectively.


Buffer-gas

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4.2. Structures of cluster ions assigned from ion mobility measurement 4.2.1. NinOn-1+ cluster cations Figure 2 shows arrival time distributions (ATDs) of NinOn-1+ (n = 3-9) at Einj = 150 eV.

An ATD of an ion packet depends on the spatial diffusion that occurs during

the traveling of ions in the drift cell.

In the case of relatively slow drift velocity (vd ~

1000 m/s), the measured ATD could be fitted by a Gaussian function.18 We thus analyzed the experimental ATD plot by fitting with Gaussian functions, and as a result, two Gaussian functions were found to be necessary for the fitting of ATDs of Ni4O3+, Ni6O5+ and Ni7O6+. fractions.

In particular, the two ATD components in Ni7O6+ had comparable

In general, observed bands of ATDs were gradually shifted to longer arrival

times with increasing cluster size.

However, the arrival time of Ni5O4+ was almost the

same value with that of the slower component of Ni4O3+.

Also, the arrival time of the

faster component in the ATD of Ni7O6+ was almost the same with that of the slower one in the ATD of Ni6O5+. Experimental collision cross sections, Ωexp, of NinOn-1+ cluster cations determined from these ATDs are summarized in Figure 3.

The errors in Ωexp

were estimated from standard deviations of the experimental values obtained by three independent measurements.



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In order to evaluate the collision cross sections theoretically, optimized structures of NinOn-1+ cluster cations have first been calculated by quantum-chemical calculations with B3LYP/6-31+G(d) level. were obtained for n = 3-7. shown in Figure 3.

As shown in Figure 3, various isomers

Calculated relative energies (∆E) of these isomers are also

For these optimized structures, we calculated collision cross

sections, Ωcalc, by using the MOBCAL program.21

This calculation was based on the

projection approximation method which is included in MOBCAL. of NinOn-1+ cluster cations are also summarized in Figure 3.

The values of Ωcalc

In principle, all possible

spin multiplicities should be examined in obtaining the most stable structures.

For 2D

and 3D isomers of Ni7O6+, we have calculated geometrical structures with spin multiplicities of 2S+1 = 2-16 (S; total spin).

Collision cross sections of cluster ions

with all spin multiplicities were calculated to be almost the same values within ±0.6 % errors.

Because spin contaminations were found to be small for high spin multiplicity,

we have calculated the structures with high spin multiplicity for estimating Ωcalc. For Ni3O2+, Ωcalc of the one-dimensional (1D) linear isomer (38 Å2) was found to be in good agreement with Ωexp (40 ± 1 Å2). assigned to the linear isomer.

Therefore, the structure of Ni3O2+ was

For Ni4O3+, two values of Ωexp (43 ± 1 Å2 and 49 ± 1

Å2) were estimated from two Gaussian functions in the ATD.


The Ωcalc values for 1D

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and 2D isomers of Ni4O3+ were calculated to be 51 Å2 and 43-44 Å2, respectively. Therefore, 1D linear and 2D planar isomers were assigned to structures of Ni4O3+.

In

the ATD of Ni4O3+, the slower band assignable to the 1D isomer had larger intensity than the faster band which was assigned to the 2D isomer.

However, in the present

quantum-chemical calculations by B3LYP/6-31+G(d) level, the 1D isomer was less stable than the 2D isomer.

This result suggests that calculations with larger basis set

are indispensable in order to determine the most stable isomer of the cluster ions containing transition metal atoms. Because it was difficult to perform the calculations of larger cluster ions with larger basis set, we assigned the observed isomers on the basis of comparison between observed collision cross sections and theoretical ones. Ni5O4+, Ωcalc of one of the 3D isomers was almost equal to Ωexp (50 ± 1 Å2).

For

The two

Ωexp values for Ni6O5+ (56 ± 2 and 60 ± 2 Å2) coincide with Ωcalc of 2D and 3D isomers. Also for Ni7O6+, slower and faster ATD bands were assignable to 2D and 3D isomers, respectively.

For Ni8O7+, the arrival time of the observed band in ATD was faster than

that of the slower band in ATD of Ni7O6+.

Because the slower band in ATD of Ni7O6+

was assignable to the 2D isomer, the ATD band in Ni8O7+ can be assigned to compact 3D isomer, although no calculation was done for NinOn-1+ with n ≥ 8.



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Figure 2.

Arrival time distributions of NinOn-1+ (n = 3-9) measured at Einj = 150 eV.

Red solid curves are Gaussian functions which are used for fitting the experimental plots (black circles).

Blue solid curves are the sum of two Gaussian functions.


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Figure 3.

Stable structures of NinOn-1+ (n = 3-7) cluster cations calculated with

B3LYP/6-31+G(d) level.

Collision cross section calculated by the MOBCAL

program21 (Ωcalc in Å2) is shown for each structure. stable structures (∆E in eV) are also shown.

Relative energies from the most

Experimental collision cross section (Ωexp

in Å2) of each n is also summarized for comparison.

Spin multiplicities (2S+1) used in

the calculations were also shown.



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4.2.2. NinOn+ cluster cations Figure 4 shows ATDs of NinOn+ (n = 2-9) at Einj = 150 eV.

As with ATDs of

NinOn-1+, observed bands of ATDs were gradually shifted to longer arrival times with increasing cluster size.

In addition, for the fitting of ATDs of Ni5O5+, Ni6O6+, and

Ni7O7+, two Gaussian functions were necessary.

Experimental collision cross sections,

Ωexp, of NinOn+ cluster cations determined from these ATDs are summarized in Figure 5. Optimized structures of NinOn+ cluster cations calculated by quantum-chemical calculations with B3LYP/6-31+G(d) level are also shown in Figure 5.

For Ni2O2+,

Ωcalc of the 1D isomer (35 Å2) almost coincides with Ωexp (36 ± 1 Å2).

On the other

hand, Ωcalc of the 1D isomer for Ni3O3+ (47 Å2) was found to be significantly larger than Ωexp (40 ± 1 Å2).

For NinOn+ with n = 3-5, Ωcalc of the 2D ring isomers are equal to

Ωexp of these cluster cations.

However, Ωcalc of the 2D isomers for NinOn+ with n = 6

and 7 were found to be significantly larger than Ωexp of these cluster cations again.

For

n = 5-9, Ωcalc of the 3D isomers are almost equal to Ωexp of these cluster cations. Therefore, 2D and 3D isomers coexist for n = 5.

On the other hand, slower and faster

bands of ATD observed for both n = 6 and 7 are assignable to bulky and compact 3D isomers, respectively. From these results, we have found that the structural transition


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from 1D linear to 2D ring structures occurs at n = 3, and the transition from 2D ring to 3D compact structures at n = 5.



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Figure 4.

Arrival time distributions of NinOn+ (n = 2-9) measured at Einj = 150 eV.

Red solid curves are Gaussian functions which are used for fitting the experimental plots (black circles).

Blue solid curves are the sum of two Gaussian functions.


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Figure 5.

Stable structures of NinOn+ (n = 2-9) cluster cations calculated with

B3LYP/6-31+G(d) level.

Collision cross section calculated by the MOBCAL

program21 (Ωcalc in Å2) is shown for each structure. stable structures (∆E in eV) are also shown.

Relative energies from the most

Experimental collision cross section (Ωexp

in Å2) of each n is also summarized for comparison.

Spin multiplicities (2S+1) used in

the calculations were also shown.



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4.3. CO adsorption on NinOn-1+ cluster cations The blue line in Figure 6 shows typical mass spectrum of nickel oxide cluster cations which exited from the ion-drift cell filled with 2.5 % CO/He mixture buffer gas. For comparison, mass spectrum with pure He buffer gas is shown in red line in Figure 6. The cell temperature was 290 K, and the buffer-gas pressure in the cell was 0.60 Torr. The CO adsorption reaction was observed only for a few nickel oxide cluster cations in this condition: CO-adsorbed NinOn-1+ cluster cations, NinOn-1(CO)+, were clearly observed for n = 4 and 5.

According to the increase of ion-intensities of NinOn-1(CO)+,

intensities of naked NinOn-1+ decrease for n = 4 and 5.

Therefore, it can be considered

that the following CO adsorption reaction (3) proceeds. NinOn-1+ + CO → NinOn-1(CO)+

(n = 4 and 5)

(3)

Rate constants for the adsorption reaction (3), k, have been estimated by changing the CO concentration in the buffer gas inside the ion-drift cell.

For this

estimation, we have assumed that the number of CO molecules is large excess compared with that of cluster ions, and thus it does not change before and after the reactions. Then, we have used the following eq 4 to estimate rate constants under the pseudo-first-order approximation: [୒୧ ୓శ

]

ln [୒୧ ೙୓శ೙షభ] = −݇[CO]‫ݐ‬୰

(4)

೙ ೙షభ బ


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where [NinOn-1+]0 and [NinOn-1+] are the number density of NinOn-1+ cluster cations before and after the reaction, respectively.

Also, [CO] is the number density of CO in

the ion-drift cell, and tr is the reaction time.

The reaction time in the ion-drift cell was

estimated from the arrival time of the cluster ions.

Figure 7 shows the plots of the

experimental data of ln([NinOn-1+]/[NinOn-1+]0) vs. [CO]tr for n = 4 and 5.

From the

slope of the linear fitting of the plots in Figure 7, rate constants for CO adsorption reaction (3) were determined to be k = (8.4 ± 0.7) × 10-11 cm3 molecule-1 s-1 for Ni4O3+, and (9.6 ± 0.8) × 10-11 cm3 molecule-1 s-1 for Ni5O4+.

These rate constants are almost

two orders of magnitude faster than those for Ni4O4+ and Ni5O5+ (~1 × 10-12 cm3 molecule-1 s-1) which have been determined in the previous study.5

The extremely low

rate constants for the latter two cluster ions are also consistent with the present observations: The CO adsorption reaction was not observed for Ni4O4+ and Ni5O5+ in the present experimental condition. In order to understand the difference of CO-adsorption reactivity between Ni4O3+ and Ni4O4+, we have calculated the optimized structures of Ni4O3(CO)+ and Ni4O4(CO)+ by B3LYP/6-31+G(d) level as shown in Figure 8.

From the ATDs of

cluster ions discussed in section 3.2, the 1D linear structure was assigned for bare Ni4O3+, whereas the Ni4O4+ cluster ions were concluded to have 2D ring structures.


In


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the optimized structures of Ni4O3(CO)+ and Ni4O4(CO)+, the CO molecule is coordinated with an Ni atom from the C atom side.

This feature is consistent with the

experimental result on structure determination for CO adsorbate on NiO(100) surface by photoelectron diffraction method.31

Binding energies between nickel oxide cluster

ions with CO molecule were calculated to be 1.78 eV and 1.03 eV for Ni4O3(CO)+ and Ni4O4(CO)+, respectively.

This difference of binding energy may be originated from

the different shapes of the nickel oxide cluster ions (1D linear and 2D ring), and this difference may also affect the rate constants.

As for the difference of rate constants

between Ni5O4(CO)+ and Ni5O5(CO)+, it is difficult at present to apply the same discussion because of the presence of plural structural isomers and reactive sites in Ni5O4+ and Ni5O5+.

Thus, further theoretical calculations are necessary to discuss the

rate constants between these ions from the structures of Ni5O4(CO)+ and Ni5O5(CO)+.


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Figure 6.

Mass spectra of nickel oxide cluster cations, NinOm+ (denoted by (n,m)),

which exited from the ion-drift cell filled with pure helium gas (red), 2.5 % CO/He mixture gas (blue). 290 K, respectively.

Buffer-gas pressure and temperature in the cell were 0.60 Torr and The ion-injection energy (Einj) was 150 eV.



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Figure 7.

Plots of the eq 4: (a) Ni4O3+ + CO → Ni4O3(CO)+, (b) Ni5O4+ + CO →

Ni5O4(CO)+.

Buffer-gas pressure and temperature in the cell were 0.60 Torr and 290 K,

respectively.

The concentrations of CO in the mixture gas were between 0.0 % and

1.0 %.


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Figure 8. Optimized structures of Ni4O3(CO)+ and Ni4O4(CO)+ calculated by B3LYP/6-31+G(d) level.

Spin multiplicities were 2S+1 = 4 and 2 for Ni4O3(CO)+ and

Ni4O4(CO)+, respectively.



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5. Conclusion In this study, we have investigated the structures and reactivities of nickel oxide cluster cations by ion mobility mass spectrometry.

In the mass spectra measured

at high ion-injection energies of 250-400 eV, NinOn-2+, NinOn-1+, and NinOn+ cluster cations were predominantly observed.

We have determined the structures of NinOn-1+

and NinOn+ cluster cations by comparison between experimental collision cross sections and theoretical ones.

For NinOn+, the structural transition from 1D linear to 2D ring

structures was observed at n = 3.

Also, the structural transition from 2D ring to 3D

compact structures was observed at n = 5.

In addition, 2D and 3D structural isomers

were found to coexist for Ni5O5+, Ni6O5+ and Ni7O6+. By adding CO to the buffer gas in the ion-drift cell, we have examined the reactivity of CO adsorption on nickel oxide cluster cations.

For Ni4O3+ and Ni5O4+,

cluster cations were found to be reactive for the CO adsorption reactions.

Rate

constants of CO-adsorption reactions were determined to be k = (8.4 ± 0.7) × 10-11 cm3 molecule-1 s-1 and (9.6 ± 0.8) × 10-11 cm3 molecule-1 s-1 for Ni4O3+ and Ni5O4+, respectively.

These rate constants are much faster than those for Ni4O4+ and Ni5O5+

(~1 × 10-12 cm3 molecule-1 s-1) which have been determined in the previous study.5


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The difference of the shapes between Ni4O3+ (1D linear) and Ni4O4+ (2D ring) probably affects the observed difference of rate constants.



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Associated content Supporting Information The complete author list of ref 24.

This material is available free of charge

via the Internet at http://pubs.acs.org.

Author Information Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), the Research Seeds Quest Program (JST), The Murata Science Foundation.

Theoretical calculations were partly

preformed using the Research Center for Computational Science, Okazaki, Japan.


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(22) Rusyniak, M. J.; Ibrahim, Y. M.; Wright, D. L.; Khanna, S. N.; El-Shall, M. S. Gas-Phase Ion Mobilities and Structures of Benzene Cluster Cations (C6H6)n+, n = 2-6. J. Am. Chem. Soc. 2003, 125, 12001-12013. (23) Dugourd, Ph.; Hudgins, R. R.; Clemmer, D. E.; Jarrold, M. F. High-Resolution Ion Mobility Measurements. Rev. Sci. Instrum. 1997, 68, 1122-1129.

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Table of Contents Graphic


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Structures and CO-Adsorption Reactivities of Nickel Oxide Cluster

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