Stable Stoichiometry of Gas-Phase Manganese Oxide Cluster Ions

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The Journal of Physical Chemistry

Stable Stoichiometry of Gas-Phase Manganese Oxide Cluster Ions Revealed by Temperature-Programmed Desorption Kohei Koyama, Satoshi Kudoh, Ken Miyajima, and Fumitaka Mafuné* Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, JAPAN

Received: Revised:

ABSTRACT:

Temperature-programmed

desorption

(TPD)

experiments

were

performed on gas-phase manganese oxide cluster ions, namely MnnOm+ (n = 3–20) and MnnOm− (n = 3–18). These cluster ions were prepared by laser ablation of a manganese rod in the presence of oxygen gas, and their composition was investigated using mass spectrometry. The composition of MnnOm+/− distribution lies above the m = (4/3)n line. When the cluster ions were heated up to 1000 K, MnnOm+ (m = (4/3)n +  with  = −1, 0) and MnnOm− (m = (4/3)n +  with = 0, 1) were found to be the predominant species, formed by thermal dissociation. These experimental findings indicate that the nascent manganese oxide clusters comprise robust MnnOm+/− (m/n ≈ 4/3) and weakly bound excess oxygen atoms. Based on the TPD experiments, the oxygen-molecule release was identified as the main dissociation channel. The temperature dependence of O2 desorption was found to be similar among the clusters with the same oxygen excess or deficiency regardless of the number of Mn atoms. The threshold energy of O2 desorption was estimated for Mn4Om+ (m = 6−11) and compared with bond dissociation energies calculated by DFT.

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1. Introduction In recent years, major efforts have been made to obtain a molecular-level understanding of catalytic processes. In addition to conventional surface science studies, gas-phase or well-defined deposited clusters have been used as an ideal model to study in detail the relationship between structure and reactivity.1,2 The mechanism of oxidation of CO and small organic molecules has been explored by reactivity measurements of transition metal oxide clusters containing radical oxygen centers, showing that the mechanism depends on their size, composition, and charge state.3-9 Temperature-programmed desorption (TPD) is a standard technique employed in surface science that provides critical information on the binding energies of atoms and molecules on a solid surface.10 TPD experiments and temperature-programmed reactions (TPR) are powerful tools for investigating the chemical properties of supported catalysts and clusters on surfaces.11,12 The temperature of desorption typically reflects the energy barriers of desorption, which may vary with the nature of the adsorbing species. For CeO2, for instance, O2 molecules desorb from the surface at 400 K, followed by a further O2 desorption at 800 K;13,14 these different desorption temperatures indicate that the weakly bound O2 molecules on the surface are released at lower temperatures, whereas the oxygen atoms of CeO2 desorb at higher temperatures. Hence, measurements of the desorption temperature can be employed to elucidate the mechanism of atoms/molecules binding to the chemical system of interest. Manganese oxides are characterized by a variety of stable stoichiometries, as they have several oxidation states.15-19 In industrial applications, manganese oxide is used as a CO oxidation catalyst (which works at room temperature)20,21 or a catalyst for the selective reduction of NO with NH3.22 The thermochemistry of MnO2 is known to


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be complex, and a series of decomposition steps can be observed, as follows:23-25 723–823 K MnO2

→

~1223 K

Mn2O3

1573 K

→

Mn3O4

→

MnO

(1)

Since each reduction step accompanies the removal of oxygen (a degradation product), the partial pressure of oxygen significantly changes as a function of temperature at each step of decomposition. Under certain conditions, e.g., low oxygen partial pressure or inert atmosphere, the intermediate phase Mn5O8 is produced prior to the completion of the first step shown in eq 1. Ferrandon et al. reported that in a strongly reductive atmosphere (10% H2/Ar), the reduction of manganese oxide occurs in three steps at 463 K, 601 K, and 697 K.26 Nonstoichiometric manganese oxides have been also chemically synthesized. For instance, Iablokov et al. investigated the catalytic activity of MnOx nanocrystals during the oxidation of CO.27 They prepared manganese oxides of various compositions by thermal decomposition of Mn-oxylate precipitation in the presence of oxygen. Nonstoichiometric MnO1.61–1.67 was obtained by annealing at 633 K; this demonstrates its superior CO oxidation activity over stoichiometric Mn2O3 and Mn3O4. Frey et al. prepared MnOx by temperature-programmed oxidation of Mn-oxylates; the Mn oxidation state was found to be 3.4 ± 0.1 by micro-X-ray absorption near-edge structure (micro-XANES) and X-ray photoelectron spectra (XPS).19 Moreover, small clusters in the gas phase are likely to adopt different stoichiometries, as all manganese and oxygen atoms are exposed to the free space, thereby forming a surface. For instance, Zhang et al. reported the direct speciation of the solid samples of MnO2 and MnO by pulsed glow discharge time-of-flight mass spectrometry.28 They found that Mn2O3+ is produced only from the MnO2 sample.



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However, a limited number of experimental and theoretical studies have been proposed on the gas-phase or unsupported manganese oxide clusters.29-31 Ziemann et al. produced manganese oxide clusters by a gas-aggregation technique and observed the formation of (MnO)x+ (x = 1–13) and (MnO)xO+ (x = 4–22) clusters by photoionization at 248 nm;29 the mass-spectral abundance distribution suggested that the most stable structures for the stoichiometric clusters are stacks of rings of (MnO)3 units and that the oxygen-rich clusters prefer structures with a single Mn atom vacancy. Yin et al. studied the reactivity of neutral manganese oxide clusters MnnOm (n = 2–13), which were generated by laser ablation of a Mn foil into 5% O2/He carrier gas and photoionized by a 118-nm laser radiation.30 Mei-Ye et al. experimentally and theoretically studied the reactions between manganese oxide cluster anions and H2S.31,32 However, there have been no reports of the thermal stability of manganese oxide clusters in the gas phase. In the present study, we investigate the TPD processes of cationic and anionic manganese oxide clusters of different composition, both stoichiometric and nonstoichiometric, in a molecular beam (the gas phase) at temperatures higher than room temperature. In addition, we analyze the changes in the cluster abundance as a function of the temperature and thermally determine the chemical formula of the stable manganese oxide clusters.

2. Experimental section The stability of manganese oxide cluster ions was investigated using a time-of-flight (TOF) mass spectrometer in combination with a post-heating method (Figure S1).33,34 Briefly, manganese oxide cluster ions were prepared by laser ablation in a cluster source. A manganese metal rod (Kojundo chemical laboratory Co., Ltd.,


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99.9%) was vaporized using the focused second harmonic (532 nm; 10 Hz) of a Nd3+:YAG laser at a typical pulse energy of 10 mJ in the presence of oxygen diluted in He (total stagnation pressure 0.8 MPa; purity of He and O2 >99.99995% and 99.9%, respectively; pulse duration ca. 400 µs). The oxygen concentration in helium gas was kept constant (typically 0.1% or 5%) using mass-flow controllers (Bronkhorst). The produced cluster ions were then introduced into an extension tube (4 mm in diameter, 120 mm in length) before expansion into a vacuum chamber (post-heating). The temperature of the extension tube was controlled to be in the range 298–1000 K by using a resistive heater and monitored using thermocouples. The residence time of the cluster ions and the number density of the He gas in the extension tube were estimated to be higher than ~100 s and 1018 cm-3, respectively. Hence, the thermal equilibrium of the clusters was achieved by collisions with the He carrier gas well before the expansion into the vacuum. The TPD profiles were obtained by gradually scanning the temperature at 7 K min-1. During the mass analysis, the cluster ions gained kinetic energy of 3.5 keV in the acceleration region. The ions were steered and focused by a set of vertical and horizontal deflectors and an einzel lens. After traveling in a 1-m field-free region, the ions

were

reversed

by the

reflectron

and

detected

using

a

Hamamatsu

double-microchannel plate detector. Signals from the detector were amplified with a preamplifier (Stanford Research Systems SR445A) and digitized using an oscilloscope (LeCroy LT374). The averaged TOF spectra (typically 500 sweeps) were sent to a computer for analysis. The mass resolution (m/Δm) was sufficiently large (>1000) to distinguish an O atom from a H2O molecule that may appear in the mass spectrum as an impurity.



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3. Computational section In order to obtain the bond dissociation energies of Mn4Om+ (m = 4−10), the DFT calculations were performed using the Gaussian09 program with the 6-311+G* basis set. Becke's three-parameter hybrid density functional with the Lee-Yang-Parr correlation functional (B3LYP) was used for all calculations. In particular, the structural isomers of Mn4O6+ with slightly different formation energies were investigated intensively, and the motifs of Mn4O6+ were used for the calculations of Mn4O7-10+ as the initial structure. The vibrational frequencies were calculated for the lowest electronic energy structure of each cluster to obtain zero-point vibrational energy (ZPVE) and check the optimized structure. All the optimized structures had zero imaginary frequencies, suggesting that these structures corresponded to the equilibrium structures. All the calculated energies described below included the ZPVE correction. The B3LYP/6-311+G* method was successfully used to study the reactivity of MnnOm+/- toward H2S,

31,32

and the interpretation of vibrational spectra of manganese

oxides.35 The natural bond orbital (NBO) analyses 36-37 were also performed to calculate the natural charge.

4. Results 4.1 Mass spectra of manganese oxide clusters and their temperature dependence Figure 1a shows a mass spectrum of manganese oxide cluster cations produced by the laser ablation of a manganese rod in the presence of 0.1% oxygen gas in helium at room temperature. When the concentration of oxygen gas in He is lower than 0.1%, cluster ions are not sufficiently abundant for a stable observation by mass spectrometry.


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Ion peaks assignable to MnnOm+ clusters appear in the mass spectra. More specifically, our data showed that Mn3O7–8+, Mn4O6–11+, Mn5O7–12+, Mn6O8–14+, Mn7O9–13+, Mn8O11– 12,14

+

, Mn9O11–17+, and Mn10O13–17+ are abundantly formed. Figure 1b shows the mass

spectra of the manganese oxide clusters after being heated in the extension tube at 1000 K. The number of ion peaks in the mass spectra is reduced, suggesting that specific cluster ions are formed after the thermal dissociation caused by the post-heating. In particular, MnnOm+ (m/n ≈ 4/3) clusters were predominantly produced by dissociation. For instance, Mn3O3–4+, Mn4O4–7+, Mn5O6–8+, Mn6O6–9+, Mn7O8–11+, Mn8O9–12+, Mn9O9– + 13 ,

and Mn10O12–15+ were formed. It is worth to note that, in addition to these

compositions, MnnOm+ clusters with m/n ≥ 4/3 are also observed at 5% O2. Similar composition changes caused by the post-heating were also observed for anionic manganese oxide clusters (Figure 1c, 1d). The anionic clusters typically contained 1–3 more oxygen atoms than the cationic clusters did, when they were prepared in 0.1% O2 in He gas. The composition distributions of cationic and anionic cluster ions formed at room temperature and at 1000 K are illustrated more clearly as a bubble plot in Figure 1e, 1f. For the pristine cationic clusters of MnnOm+, the number of oxygen atoms, m, is equal to (4/3)n + with = 0–2 at room temperature. The distribution narrows above m/n = 4/3 by the heat treatment indicating that the manganese oxide cluster ions that contain a smaller number of oxygen atoms are formed at 1000 K. This finding suggests that oxygen is likely to be released from the cluster ions when they are heated. For anionic clusters, the distribution follows a similar trend; it is above m/n = 4/3 with post-heating, while it is m/n = (4/3)n + with = 1–6 without post-heating. Therefore, at room temperature, the negatively charged clusters possess 1–4 (about 3 on average)



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more oxygen atoms than the positively charged clusters with the same number of Mn atoms (Figures 1e, 1f).

4.2 TPD of manganese oxide clusters Figure 2 shows the intensity ratio of cationic manganese oxide cluster ions for each n as a function of the temperature representing TPD curves of the clusters. The curves with the same tendency are drawn by using same colors. The detail explanation will be given in section 4.3. To compensate for the fluctuation of signal intensity and to consistently analyze the dissociation paths, the intensity of MnnOm+ was normalized so that the total intensity of nnOm+,  I Mn m

nOm

+

, equals one at each temperature. For n =

7, the relative intensity of Mn7O11+ decreases in the temperature range 300–500 K; in the same temperature range, the relative intensity of Mn7O9+ increases. The intensity changes of Mn7O9+ and Mn7O11+ can be explained in terms of oxygen-molecule release, as follows: Mn7O11+ → Mn7O9+ + O2

(2)

A similar change in the ion intensities was observed for n = 8, i.e., the relative intensity of Mn8O14+ decreases at 300–400 K, while that of Mn8O12+ increases. Interestingly, for temperatures higher than 380 K, the intensity trends of Mn8O12+ and Mn8O10+ invert. The intensity of Mn8O10+ shows no significant changes in the range 500–700 K. The intensity changes of Mn8O10,12,14+ can be explained in terms of sequential oxygen-molecule release, as follows: Mn8O14+ → Mn8O12+ + O2 → Mn8O10+ + 2O2

(3)

The analysis of the TPD plots show that the oxygen-molecule release from MnnOm+ is the main thermal dissociation channel. O2 release also occurs in anionic clusters, as


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illustrated in Figure 3. A full set of TPD profiles for cationic and anionic clusters obtained in this work is displayed in Figures S2 and S3, respectively.

5. Discussion 5.1 Stable stoichiometry of cationic manganese oxide clusters Manganese oxide cluster ions, MnnOm+, were produced by the laser ablation of a manganese metal rod in the presence of 0.1% oxygen gas in helium at room temperature. The composition of nascent MnnOm+ is distributed above m/n ≈ 4/3; however, after post-heating at 1000 K, the distribution of compositions was found to be close to the value m/n ≈ 4/3. In other to visualize how the composition m/n changes along the temperature increase, averaged oxygen atom number m/n was examined for each n. Figure 4a shows the intensity weighted average of oxygen atoms to manganese atoms expressed as

 ( m / n) I m

Mn nO m +

/  I Mn O + , for each n, as a function of the temperature. m

n

m

The average ratio steeply decreases when the temperature is increased to 600 K; above 600 K, it decreases slowly to a value of 4/3 (≈ 1.3). These findings suggest that the nascent manganese oxide clusters comprise robust MnnOm+ (m/n ≈ 4/3) and excess oxygen atoms. At room temperature, the excess atoms are so weakly bound to Mn nOm+ that they can dissociate after moderate heating. This is consistent with other experimental findings, i.e., at room temperature, the number of oxygen atoms in the cluster ions increased with increasing the concentration of oxygen gas in He and MnnOm+ clusters with m/n ≥ 4/3 were formed at 5% O2 (Figure S4b), however, excess oxygen atoms were found to be completely released from the clusters by a heat treatment, thereby forming MnnOm+ (m ≥ (4/3)n). In other words, the mass distributions after heat treatment did not change significantly for the two different oxygen



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concentrations. The stability of MnnOm+ (m/n = 4/3) is an important point to explore. According to the thermochemistry of bulk manganese oxides, the formation of less oxidized manganese oxides is more likely at higher temperatures at a relatively low partial pressure of oxygen;38 for instance, Mn3O4 and MnO are known to be produced at temperatures higher than 1223 K and 1573 K, respectively, in a bulk material under atmospheric conditions (see eq. (1)). According to our experimental procedure, manganese oxide clusters were heated in a molecular beam of 0.1% oxygen gas diluted in He, i.e., to estimate the effective partial pressure of oxygen surrounding the manganese oxide clusters is not trivial. Nevertheless, Mn3O4 is formed and the average ratio m/n (Figure 4a) exhibits a decreasing trend from m/n = 4/3 at higher temperature; this is consistent with the fact that MnO may be preferentially formed over Mn3O4 at higher temperatures.

5.2 Anionic manganese oxide clusters For the sake of comparison, we also analyzed the anionic clusters, MnnOm-, produced under conditions identical to those of the cationic clusters. In particular, oxygen-rich pristine clusters were prepared at room temperature and subjected to post-heating (Figures 1c, 1d, 1f, and S4c). As mentioned above, the compositions of abundant anionic clusters are also expressed by m = (4/3)n + δ, in which δ is slightly higher than the cationic clusters. Let us consider the origin of this difference. The difference in the number of oxygen atoms of positively and negatively charged stable clusters at ~1000 K having the same number Mn atoms is about 2, except for the small clusters plotted as green lines in Figures 1e and 1f (n = 4 and 5). More specifically,


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Mn8O10,11+ and Mn8O12,13,14−, Mn12O14,15,16,17+ and Mn12O17,18−, and Mn16O21,22+ and Mn16O22,23,24− are abundantly observed after post-heating. Since an oxygen adopts −2 as a charge state, the anions could be produced by the participation of one additional oxygen atom to cation clusters (formally, XnOm+ + O2− = XnOm+1−). In fact, cooper oxide clusters, Cu7O5,6− were produced by the addition to O2− ion toward Cu7O4,5+.34 However as the anionic manganese clusters were two oxygen atoms richer than cationic cluster, the discussion above is not applicable. A close analysis of the stoichiometry of clusters with different sizes (Figures S2 and S3) reveals that the abundant clusters observed after post-heating are Mn10O13+ and Mn9O13−, Mn12O15+ and Mn11O15−, and Mn16O21+ and Mn15O21−. These findings suggest that, for n > 10, the stable cationic Mnn+1Om+ clusters possess the same number of oxygen atoms as the anionic MnnOm− clusters. Notably, some exceptions to this behavior were observed for n = 13 and 19. According to the charge balance, a Mn2+ ion may cause an inversion of the charge (formally, Mn11O15− + Mn2+ = Mn12O15+). Based on the inference, the average m/n ratio of anionic MnnOm− is calculated actually by using n+1 instead of n. In Figure 4b, average m/(n+1) ratios for anionic clusters ranged above 4/3 at room temperature; m/(n+1) approached the value 4/3 at 973 K. On the other hand, for n < 10, the average ratio significantly deviates from m/(n+1) > 4/3, even at high temperatures. The difference in this behavior is likely due to the size effect.

5.3 Analysis of TPD processes For Mn9Om+, nine species, namely Mn9O9–17+, participated in the thermal desorption processes. As oxygen is released from a cluster ion as a molecule rather than atom by atom, the number of oxygen atoms in the clusters is reduced by 2. Hence, two



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series of reaction paths can be considered for the clusters with an even or odd number of oxygen atoms: Mn9O17+ → Mn9O15+ → Mn9O13+ → Mn9O11+ → Mn9O9+

(4)

Mn9O16+ → Mn9O14+ → Mn9O12+ → Mn9O10+

(5)

Each step of the sequential oxygen release can be labeled on the basis of the shape and onset temperature of the TPD curves for all values of n. For example, the curve of Mn9O12+ in the plot in Figure 2f which starts from a low intensity ratio, 0.10, and increases at ~500 K is in red. As the TPD curves for Mn4O5+, Mn5O6+, Mn6O7+, Mn7O9+and Mn8O10+ resemble Mn9O12+ in the shape and onset temperature (Figure 2), they are all drawn in the same color in Figure 2, in this case red. The curve of Mn9O14+ which starts from a high intensity ratio, 0.15, and decreases at ~500 K in the plot in Figure 2f is in blue. The curves of the clusters with an intensity ratio that follows the same trend of Mn9O14+ are also depicted in blue. The curve of Mn9O13+ which remains high in the medium temperature range studied is in green: The curves of the clusters, Mn4O6+, Mn5O7+, Mn6O8+, Mn7O10+, and Mn8O11+, which exhibit similar features, are in green. The curve of Mn9O11+ which gradually increases from close to zero as the temperature is increased, is in black. The black curves changes with the temperature in an opposite manner to the green ones. Thus, the cluster ions with the same values of n can be labeled with ten different colors (Figures 2 and 3, and Scheme 1). Based on this analysis, the labeled clusters can be divided into three categories (see Scheme 1): Mn9O14–17+, Mn9O12,13+, and Mn9O9-11+. Oxygen-rich Mn9O14–17+ clusters exhibit successive desorptions of oxygen molecules at temperatures lower than 400 K; these consist of weakly bound oxygen atoms and a rigid skeleton. Mn9O12,13+ appears in the TPD plot in a wide temperature range (500–1000 K), confirming its


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stability. Oxygen-poor Mn9O9-11+ is formed gradually by heating. It is very likely that the oxygen atoms are so strongly bound to the Mn-O network that higher energy is required to break these bonds. In order to draw general conclusions from the TPD analysis of the clusters, the clusters are displayed in Figure 5 by using a color code. The color patterns of the cationic and anionic clusters align along the slope of m/n = 4/3. A superimposition of the charts (Figure 5a and 5b) reveals that the best match is obtained when the charts are shifted by one Mn atom, i.e., MnnOm+ and Mnn−1Om−, which show similar behavior, as discussed in the section 4.2. In addition, a series of cells in red are located just below the line of m/n = 4/3 for cations and on the line for anions, except for n < 10. Similarly, cells in green are located on the line of m/n = 4/3 for cations and just above the line for anions, except for n < 10. Here, these cells represent thermally durable clusters. However, these lines are not smooth. For instance, red cells for n = 14, 15 are located in the same region of m = 19 of cationic clusters. Assuming that the rule of composition ratio for Mn3O4 is strictly observed, a cluster has to grow by the addition of MnO and MnO2. Thus, a small deviation from the composition ratio m/n ≈ 4/3 may be related to the different degree of structural cage closing for each cluster composition (see Figures S6, S7, and S8). Therefore, we can conclude that the thermally durable clusters are located on the line m = (4/3)n + with being equal to −1, 0 and 0, 1 for cationic and anionic manganese oxide clusters, respectively.

5.4 Estimation of threshold energy of O2 release from TPD curve Since the O2 desorption



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Mn4O11+ → Mn4O9+ + O2 → Mn4O7+ + 2O2→ Mn4O5+ + 3O2

Page 14 of 42

(6)

is a unimolecular reaction, the rate equation for the first step is given by

[Mn 4O11 ] = [Mn 4O11 ]0 exp( kt ) , +

+

(7)

where the rate constant is defined as k, and the initial and current number densities of Mn4Om+ as [Mn4Om+]0 and [Mn4Om+], respectively, and t is the reaction time in the extension tube. The number density of the product after the reaction varies with the temperature, because the rate constant depends on the temperature. According to the Arrhenius equation, k is given by   Ea k = A exp  k BT

  

(8)

where A, Ea, kB, and T are the pre-exponential factor, the threshold energy, the Boltzmann constant, and the temperature, respectively. By combining equations (7) and (8), we obtain the following equation

[Mn 4 O11+ ] = [Mn 4 O11+ ]0 exp(  A exp(

 Ea )t) k BT

(9)

Similarly, for the sequential oxygen release of reaction (6), the rate equation is given by +

[Mn 4O9 ] = [Mn 4O11 + ]0 ( [Mn 4O 7 + ] = [Mn 4O11+ ]0 (

k1 )(exp(k1t )  exp( k 2t )) k 2  k1

(10)

 k1k 2   k1  k2 k 2  k1 ) exp( t )  exp( t) + exp k3t  k 2  k1  k3  k1 k3  k 2 (k3  k1 )(k3  k 2 ) 

(11)

[Mn 4O5 ] = [Mn 4O11 + ]0  ([Mn 4O11 ] + [Mn 4O9 ] + [Mn 4O7 + ]) +

+

+

(12)

where the rate constants of the first, second and third steps of the reaction (6) are defined as k1, k2 and k3, respectively. As the rate constants are given by


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  Eai k i=Ai exp  k BT

  i  1, 　 2, 　 3, 

(13)

where Ai is the pre-exponential factor, and Eai is the threshold energy, we obtain the following equation

    Ea1     A1exp       k T        E  E + +  B  a1 a2  exp  A1exp t   exp  A2 exp t    [Mn 4 O9 ] = [Mn 4 O11 ]0         k BT    k BT        A2 exp  Ea2   A1exp  Ea1    kT   kT     B   B    (14)

E E    A1exp( a1 ) A2exp( a2 )  kBT kBT  [Mn 4O7 + ] = [Mn 4O11+ ]0     Ea2  Ea1  A exp( )  A exp( ) 1  2 kBT kBT   E E   A1exp( a1 )t A1exp( a1 )t   k T k T B B  exp(    exp(      Ea3  Ea1  Ea3  Ea1 A3exp( )  A1exp( ) A3exp( )  A1exp( )   kBT kBT kBT kBT      Ea2  Ea1 A2exp( )  A1exp( )   k T k T B B    ( A exp(  Ea3 ) - A exp(  Ea1 ))( A exp(  Ea3 )  A exp(  Ea2 ))exp( A exp(  Ea3 )t )  1 3 2 3  3  kBT kBT kBT kBT kBT   (15)

Using eqs. (9), (14), and (15), the threshold energy for each reaction step has been obtained (see Figure 6). The threshold energies range in 0.1−0.4 and 0.9 eV for Mn4O7-11+ and Mn4O6+, respectively. As the signal of the oxygen release from Mn4O6+ appears at a higher temperature, the value may contain a substantial error. However, there is a distinct difference in the threshold energy between Mn4O6+ and Mn4O8+, which will be discussed in the following section based on the DFT calculations.



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5.5 Structures of Mn4Om+ In relation to the threshold energy estimated from the TPD experiments, the most stable structure of Mn4O2−11+ was optimized by the DFT calculations, and the bond dissociation energy of an oxygen molecule was obtained from their formation energy (see Figure 6). The calculated bond dissociation energy did not finely coincide the value of the experiments, but reproduced the size dependence well. Hence, the variation of the energy with the oxygen number should be considered here. As shown in Figure 7, for Mn4O6+, the adamantane-type structure composed of manganese atoms and bridging oxygen atoms was found to be stable. This highly-symmetric structure is also common to other transition metal oxides such as Fe4O6+, Sc4O6+, and Nb4O10+.39-41 The bond dissociation energy of O2 from Mn4O6+ was calculated from the energy difference between Mn4O6+ and Mn4O4+ to be 1.8 eV. As Mn4O4+ was reported to have the ring structure, where Mn and O atoms were alternatingly-bridged, rupturing the bonds of bridging oxygen from Mn4O6+ was considered to require higher energy.7 Other structural isomers of Mn4O6+ were calculated, finding that they were less stable than the adamantine-type structure (see Figure S9). For Mn4O7−10+, the stable structures were the ones with terminal oxygen atom(s) attaching to Mn4O6+. The bond dissociation energy was similarly calculated to be 0.91 eV for Mn4O7+ and be in the range of 0.16−0.60 eV for Mn4O8−11+, meaning that the oxygen molecule was more readily released from Mn4O7−11+ than Mn4O6+. The lower bond dissociation energy from Mn4O7−11+ was likely due to the terminal oxygen atoms which were able to be released without rupturing multiple covalent bonds. Thus, Mn4Om+ (m≥7) having excess oxygen atoms as the terminal oxygen existed at room temperature. The excess oxygen atoms were to be released from Mn4Om+ by moderate heating until adamantane-type Mn4O6+ was formed.


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Further release of O2 requires more energy, because the bonds of bridging oxygen need to be ruptured. The drastic increase of the threshold energy of O2 dissociation from Mn4O6+ can be explained in terms of the geometrical structures. The oxygen atoms located in the terminal sites behave differently from those in the bridging sites. In fact, natural charges of atoms, obtained by the NBO analysis, ranged in −0.4 – −0.8 for the bridging oxygen atoms, whereas they are near zero for the terminal oxygen atoms, suggesting that these terminal atoms interact with the manganese atoms in a lesser extent.

5. Conclusions Manganese oxide cluster ions, MnnOm+/−, were prepared by the laser ablation of a manganese metal rod in the presence of 0.1% oxygen gas in He and their composition was investigated using mass spectrometry. The composition of manganese and oxygen atoms in MnnOm+/− distributed above the value of m = (4/3)n+ at room temperature. When the cluster ions were heated up to 1000 K in the extension tube, the composition became oxygen-poor, with m being equal to (4/3)n+. In order to elucidate the thermal dissociation mechanism, relative intensities of MnnOm+/− were recorded as a function of the temperature in the extension tube and a TPD profiles were obtained. The analysis of the TPD profile indicates that the oxygen-molecule release process from oxygen-rich MnnOm+/− clusters was the main thermal dissociation channel. A classification of the TPD profile curves for different cluster compositions revealed that the thermally durable clusters are located on the line of m = (4/3)n+ with being equal to −1, 0 and 0, 1 for cationic and anionic manganese oxide clusters, respectively. The TPD experiments revealed that the temperature dependence of O2 desorption is similar



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among the clusters with the same oxygen atom excess or deficiency regardless of the number of Mn atoms. More quantitatively, the threshold energy of O2 desorption was estimated from the TPD profile for Mn4Om+ (m = 6−11) and compared with the bond dissociation energies obtained by DFT calculation. Among the clusters formed at room temperature, Mn4Om+ (m≥7) was found to possess excess oxygen atoms as the terminal oxygen. The excess oxygen atoms are released from Mn4Om+ by moderate heating until adamantane-type Mn4O6+ was formed. Further release of O2 require more energy, because the bonds of bridging oxygen had to be ruptured. The drastic increase of the threshold energy of O2 dissociation from Mn4O6+ was explained in terms of the geometrical structures.


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ASSOCIATED CONTENT Supporting Information A schematic diagram of the experimental setup used in the present study (Figure S1). TPD profiles of MnnOm+ (n = 3–20) and MnnOm− (n = 3–18), exhibiting intensity ratios of each cluster ion as a function of the temperature of the extension tube (Figures S2 and S3). Stable stoichiometry distribution of neutral clusters: 2D contour-plot representation of the mass distribution of MnnOm- as a function of temperature of the extension tube (Figure S4). 2D contour-plot representation of mass distribution of anionic manganese oxide clusters, MnnOm+/−, as a function of temperature in the extension tube (Figure S5). Relative abundances of MnnOm+/0/− for different numbers of Mn and O atoms at room temperature and ~1000 K (Figure S6). Relative abundances of MnnOm+/− for different numbers of Mn and O atoms at room temperature and ~1000 K obtained by the 0.1%-O2-doped He carrier gas with color codes (Figure S7); same data depicted on the Y axis of number of excess O atoms,  = m−(4/3)n+1 (Figure S8). Floating bar chart of MnnOm+ (n = 3–19) and MnnOm- (n = 3– 18) clusters as a function of the temperature of the extension tube. The optimized structures and relative energies of isomers of Mn4O6+ clusters (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Address



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F.M.: Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, JAPAN Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by a Grant-in-Aid for Scientific Research (A) (No. 25248004), a Grant-in-Aid for Scientific Research (C) (No. 24550010) and for Exploratory Research (No. 26620002) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT) and by the Genesis Research Institute, Inc., for the cluster research.

References (1) Castleman, A. W., Jr. Cluster Structure and Reactions: Gaining Insights into Catalytic Processes. Catal. Lett. 2011, 141, 1243-1253. (2) Lang, S. M.; Bernhardt, T. M. Gas Phase Metal Cluster Model Systems for Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2012, 14, 9255-9269. (3) Johnson, G. E.; Mitrić, R.; Tyo, E. C.; Bonačić-Koutecký, V.; Castleman Jr, A. W. Stoichiometric Zirconium Oxide Cations as Potential Building Blocks for Cluster Assembled Catalysts. J. Am. Chem. Soc. 2008, 130, 13912-13920. (4) Johnson, G. E.; Mitrić, R.; Nössler, M.; Tyo, E. C.; Bonačić-Koutecký, V.; Castleman, A. W., Jr. Influence of Charge State on Catalytic Oxidation Reactions at Metal Oxide Clusters Containing Radical Oxygen Centers. J. Am. Chem. Soc. 2009, 131, 5460-5470. (5) Zhao, Y.; Ding, X.; Ma, Y.; Wang, Z.; He, S. Transition Metal Oxide Clusters with Character of Oxygen-Centered Radical: A DFT Study. Theor. Chem. Acc. 2010, 127, 449-465. (6) Nößler, M.; Mitrić, R.; Bonačić-Koutecký, V. Binary Neutral Metal Oxide Clusters with Oxygen Radical Centers for Catalytic Oxidation Reactions: From Cluster Models Toward Surfaces. J. Phys. Chem. C 2012, 116, 11570-11574. 20 ACS Paragon Plus Environment

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(7) Lang, S. M.; Fleischer, I.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Dimensionality Dependent Water Splitting Mechanisms on Free Manganese Oxide Clusters. Nano Lett. 2013, 13, 5549-5555. (8) Li, X. N.; Yuan, Z.; He, S. G. CO Oxidation Promoted by Gold Atoms Supported on Titanium Oxide Cluster Anions. J. Am. Chem. Soc. 2014, 136, 3617-3623. (9) Yuan, Z.; Li, X. N.; He, S. G. CO Oxidation Promoted by Gold Atoms Loosely Attached in AuFeO3- Cluster Anions. J. Phys. Chem. Lett. 2014, 5, 1585-1590. (10) King, D. A. Thermal Desorption from Metal Surfaces: A Review. Surf. Sci. 1975, 47, 384-402. (11) Falconer, J. L.; Schwarz, J. A. Temperature-Programmed Desorption and Reaction: Applications to Supported Catalysts. Catal. Rev. 1983, 25, 141-227. (12) Bernhardt, T.; Heiz, U.; Landman, U. In Chemical and Catalytic Properties of Size-selected Free and Supported Clusters; Nanocatalysis; Springer: 2007; pp 1-191. (13) Yao, H. C.; Yao, Y. F. Y. Ceria in Automotive Exhaust Catalysts: I. Oxygen Storage. J. Catal. 1984, 86, 254-265. (14) Abanades, S.; Flamant, G. Thermochemical Hydrogen Production from a Two-Step Solar-Driven Water-Splitting Cycle Based on Cerium Oxides. Solar Energy 2006, 80, 1611-1623. (15) Pompe, R. Some Oxidation Properties of Manganese and its Lower Oxides. Acta. Chem. Scand. Phys. Inorg. Chem. 1976, 30, 370-374. (16) Desai, B. D.; Fernandes, J. B.; Dalal, V. Manganese Dioxide—A Review of a Battery Chemical Part II. Solid State and Electrochemical Properties of Manganese Dioxides. J. Power Sources 1985, 16, 1-43. (17) Gonzalez, C.; Gutierrez, J.; Gonzalez-Velasco, J.; Cid, A.; Arranz, A.; Arranz, J. Application of Differential Scanning Calorimetry to the Reduction of Several Manganese Oxides. J. Therm. Anal. Calorim. 1998, 52, 985-989. (18) Grundy, A. N.; Hallstedt, B.; Gauckler, L. J. Assessment of the Mn-O System. J. Phase Equilib. 2003, 24, 21-39. (19) Frey, K.; Iablokov, V.; Sáfrán, G.; Osán, J.; Sajó, I.; Szukiewicz, R.; Chenakin, S.; Kruse, N. Nanostructured MnOx as Highly Active Catalyst for CO Oxidation. J. Catal. 2012, 287, 30-36. (20) Berbenni, V.; Marini, A. Oxidation Behaviour of Mechanically Activated Mn3O4 by TGA/DSC/XRPD. Mater. Res. Bull. 2003, 38, 1859-1866.



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(21) Ramesh, K.; Chen, L.; Chen, F.; Liu, Y.; Wang, Z.; Han, Y. Re-Investigating the CO Oxidation Mechanism over Unsupported MnO, Mn2O3 and MnO2 Catalysts. Catal. Today 2008, 131, 477-482. (22) Kapteijn, F.; Singoredjo, L.; Andreini, A.; Moulijn, J. Activity and Selectivity of Pure Manganese Oxides in the Selective Catalytic Reduction of Nitric Oxide with Ammonia. Appl. Catal. B 1994, 3, 173-189. (23) Williams, R.; Liu, B.; Ray, A.; Thomas, P. The Effect of Sampling Conditions on the Thermal Decomposition of Electrolytic Manganese Dioxide. J. Therm. Anal. Calorim. 2004, 76, 115-122. (24) Liu, B.; Thomas, P.; Ray, A.; Williams, R.; Donne, S. DSC Characterisation of Chemically Reduced Electrolytic Manganese Dioxide. J. Therm. Anal. Calorim. 2007, 88, 177-180. (25) Dose, W. M.; Donne, S. W. Kinetic Analysis of γ-MnO2 Thermal Treatment. J. Therm. Anal. Calorim. 2011, 105, 113-122. (26) Ferrandon, M.; Carnö, J.; Järås, S.; Björnbom, E. Total Oxidation Catalysts Based on Manganese or Copper Oxides and Platinum or Palladium I: Characterisation. Appl. Catal. A 1999, 180, 141-151. (27) Iablokov, V.; Frey, K.; Geszti, O.; Kruse, N. High Catalytic Activity in CO Oxidation over MnOx Nanocrystals. Catal. Lett. 2010, 134, 210-216. (28) Zhang, N.; King, F. L. Direct Manganese (Mn) Speciation in Solid State Materials by Pulsed Glow Discharge Time-of-Flight Mass Spectrometry. J. Anal. At. Spectrom. 2009, 24, 1489-1497. (29) Ziemann, P. J.; Castleman, A. W., Jr. Mass-Spectrometric Investigation of the Stabilities and Structures of Mn-O and Mn-Mg-O Clusters. Phys. Rev. B 1992, 46, 13480. (30) Yin, S.; Wang, Z.; Bernstein, E. R. O-Atom Transport Catalysis by Neutral Manganese Oxide Clusters in the Gas Phase: Reactions with CO, C2H4, NO2, and O2. J. Chem. Phys. 2013, 139, 084307. (31) Jia, M.; Xu, B.; Ding, X.; He, S.; Ge, M. Experimental and Theoretical Study of the Reactions between Manganese Oxide Cluster Anions and Hydrogen Sulfide. J. Chem. Phys. C 2012, 116, 24184-24192. (32) Jia, M.; He, S.; Ge, M. Experimental and Theoretical Study of the Reactions between Manganese Oxide Cluster Cations and Hydrogen Sulfide. Chin. J. Chem. Phys. 2013, 26, 679-686. (33) Sakuma, K.; Miyajima, K.; Mafuné, F. Oxidation of CO by Nickel Oxide Clusters Revealed by Post Heating. J. Phys. Chem. A 2013, 117, 3260-3265.


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(34) Morita, K.; Sakuma, K.; Miyajima, K.; Mafuné, F. Thermally and Chemically Stable Mixed Valence Copper Oxide Cluster Ions Revealed by Post Heating. J. Phys. Chem. A 2013, 117, 10145-10150. (35) Gutsev, G. L.; Rao, B. K.; Jena, P. Electronic Structure of the 3d Metal Monocide Anions. J. Phys. Chem. A 2000, 104, 5374-5379. (36) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F.NBO Version 3.1. (37) Reed A.E.; Weinstock R.B.; Weinhold F., Natural Population Analysis. J. Chem. Phys. 1985, 83, 735. (38) Shenouda, F.; Aziz, S. Equilibria and Hysteresis in the System Mn2O3–Mn3O4–O2. J. Appl. Chem. 1967, 17, 258-262. (39) Kirilyuk, A.; Fielicke, A.; Demyk, K.; von Helden, G.; Meijer, G.; Rasing, Th. Ferrimagnetic Cagelike Fe4O6 Cluster: Structure Determination from Infrared Dissociation Spectroscopy. Phys. Rev. B. 2010, 82, 020405. (40) Zhao, X. Y.; Ding, X. L.; Ma, Y. P.; Wang, Z. C.; He, S. G. Transition Metal Oxide Clusters with Character of Oxygen-Centered Radical: A DFT Study. Theor. Chem. Acc. 2010, 127, 449-465. (41) Fielicke, A.; Meijer, G.; von Helden, G. Infrared Spectroscopy of Niobium Oxide Cluster Cations in a Molecule Beam: Identifying the Cluster Structure. J. Am. Chem. Soc. 2003, 125, 3659-3667.



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Figure captions Figure 1.

(a) Mass spectrum of MnnOm+ produced by laser ablation of a

manganese metal rod in the presence of 0.1% oxygen gas in He. (b) Mass spectrum of MnnOm+ after being heated in the extension tube at a controlled temperature of 1000 K. (c) Mass spectrum of MnnOm− produced by laser ablation of a manganese metal rod in the presence of 0.1% oxygen gas in He. (d) Mass spectrum of MnnOm− after heated in the extension tube controlled at 973 K. Relative abundances of MnnOm+/− with a different number of Mn and O atoms at room temperature and ~1000 K. Cationic clusters by (e) 0.1% O2 and (f) anionic clusters by 0.1% O2. The area of the bubbles represent the relative abundances normalized in each column of n. Light blue and light green lines represent the average number of oxygen atoms for each column of n at RT and ~1000 K, respectively.

Figure 2.

TPD profiles of cationic manganese oxide clusters, MnnOm+/- (n = 4–

9), exhibiting intensity ratios of each cluster ion as a function of the temperature of the extension tube. The intensity of each cluster composition is normalized by using the following expression: I Mn nO m +

I m

Mn n O m +

. The oxygen number is color coded, from the

smaller side, in the following order: dark green, purple, black, red, green, blue, cyan, pink, orange, and dark yellow. Solid and dashed lines indicate the number of oxygen atoms are even and odd, respectively.

Figure 3.

TPD profiles of anionic manganese oxide clusters, MnnOm− (n = 4–9),

exhibiting intensity ratios of each cluster ion as a function of the temperature of the extension tube. The intensity of each cluster composition is normalized by using the


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following expression: I Mn n O m -

I

Mn n O m -

. The oxygen number is color coded from the

m

smaller side, in the following order: dark green, purple, black, red, green, blue, cyan, pink, orange, and dark yellow. Solid and dashed lines indicate the number of oxygen atoms are even and odd, respectively.

Figure 4.

(a) Average O/Mn ratios, m/n, of cationic MnnOm+ (n = 3–20) clusters

as a function of temperature. (b) Average O/Mn ratios obtained by m/(n+1) for anionic MnnOm− (n = 3–18) clusters.

Figure 5.

Classification of TPD profile curve types of MnnOm+/− clusters. The

color code represents each curve style of the TPD profile of the studied clusters. The color order is as follows: dark green, purple, black, red, green, blue, cyan, pink, orange, and dark yellow. For instance, red corresponds to the cluster, which rapidly increases by post-heating (see Figures 3 and 4). Dash lines indicate the slope of m/n = 4/3 as a reference.

Figure 6.

Threshold energy for an oxygen molecular release from of Mn4Om+ (m

= 6–11) obtained by the TPD experiments (solid circle). Bond dissociation energies of Mn4Om+ → Mn4Om-2+ + O2 obtained by DFT calculations (open square) are also shown.

Figure 7.

Optimized geometrical structures of Mn4Om+ (m = 2–11) obtained by

the DFT calculations. The spin multiplicity is given in parentheses. The number in the plot show the natural charge of the atom. The charge of the terminal oxygen atoms ranged below 0.1.



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Figure 1. Mafuné et al.


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Figure 3 Mafuné et al.


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Scheme 1. Mafuné et al.



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Stable Stoichiometry of Gas-Phase Manganese Oxide Cluster Ions

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