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Thermal Analysis of Hydrated Gold Cluster Cations in the Gas Phase Toshiaki Nagata, and Fumitaka Mafune J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04119 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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The Journal of Physical Chemistry

Thermal Analysis of Hydrated Gold Cluster Cations in the Gas Phase Toshiaki Nagata†, Fumitaka Mafuné* Department of Basic Science, School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan

Abstract: The binding forms and desorption processes of H2O molecules bound in hydrated gold clusters, Aun(H2O)m+ (n = 2–8, m = 1–4), were investigated by gas-phase thermal analyses under chemical equilibrium conditions and by quantum chemical calculations. The gas-phase clusters were prepared by the laser ablation of a gold target in the presence of water vapor, and detected by mass spectrometry. Aun(H2O)m+ clusters were found to release H2O upon heating, eventually losing all H2O molecules to generate Aun+ at temperatures below 1000 K. In sufficiently concentrated H2O vapor, adsorption and desorption of H2O to/from a given cluster approached equilibrium during heating, and the temperature dependence of this equilibrium provided binding energies of H2O to Aun(H2O)m+. The experimental and computational results suggest two types of H2O binding forms, one in which the H2O molecules are directly bound to the Aun+ core, and the other in which H2O molecules are bound to other H2O molecules through hydrogen bonding. The binding energy of directly bound H2O decreases as the cluster size increases; this size dependence most likely originates from the changes in the charge density of the Aun+ core.

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1. Introduction Supported gold nanoparticles are used as catalysts for various reactions, such as the lowtemperature oxidation of CO, H2, and hydrocarbons.1-6 For practical use under ambient conditions, it is important to understand the effect of moisture on catalytic performance. As a characteristic advantage of nano-gold catalysts, their catalytic activities are enhanced by the coexistence of water.7-11 Nano-gold catalysts have been investigated both experimentally and theoretically using atomically well-defined supported gold nanoclusters.12-17 H2O adsorbed on the surfaces of gold nanoparticles were revealed to strongly interact with co-adsorbed O2, activating the O–O bond; this is considered to be the most plausible reason for the enhanced catalytic oxidation performance observed for these catalysts over those without adsorbed H2O.10, 11, 13

In addition to catalytic reactions enhanced by water, reactions that involve water on gold

catalysts, such as water–gas shifts, are also known.18, 19 In summary, H2O molecules adsorbed on gold surfaces are highly active species that can react with or activate other co-adsorbed reactants. In parallel with the supported gold nanoparticles mentioned above, gas-phase gold clusters have also been investigated. In gas-phase clusters, the limited and strictly defined number of atoms is advantageous for analyzing chemical reactions at the atomic level, and therefore, gasphase metal clusters are useful for modeling catalytic reactions.20-23 In relation to gold catalysis, the interactions of gold clusters with O2, CO, and H2, among others, have been intensively studied.24 For instance, O2 molecules can adsorb onto Aun+/0/− clusters such that the O–O bond is activated through the formation of superoxide or peroxide species, which is a key step during catalytic oxidation.25-33 The effect of co-adsorption has also been reported; for example H2 molecules adsorbed on Aun+ clusters promote the activation of co-adsorbed O2.34 To understand the nature of catalytic CO oxidation on gold catalysts, competitive and cooperative adsorptions


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of O2 and CO on gas-phase gold clusters have been investigated; this study found that clusters such as Au2− catalytically oxidize CO.35-42 The effect of moisture on gas-phase gold clusters was also investigated. The binding of water to gold clusters was found not to be particularly strong; H2O molecules are often defeated in competitive adsorption by other reactants such as CH3Cl and CO.43,

44

H2O promotes the

adsorption of CO on cationic and anionic gold clusters. Pre-adsorbed H2O molecules may possibly relax the adsorption energy of CO and stabilize the CO–cluster complex by removing excess energy from the cluster.45 Adsorbed H2O molecules have also been proposed to lower the energy barrier for adsorption of CO by affecting the electronic and geometrical structures of the gold clusters.46, 47 Recently, we developed a thermal analysis method for gas-phase clusters in which the observation of the thermal desorption of gaseous molecules provides an estimation of the activation energy.48, 49 In particular, interactions between clusters and adsorbed molecules can be investigated quantitatively.33, 50-53 In the present study, the nature of H2O binding to gold clusters was investigated by applying the thermal analysis method to hydrated gold cluster cations. The adsorption and desorption of H2O molecules were found to be reversible, although mainly irreversible desorption was targeted in our previous quantitative estimation of reaction energetics by thermal analysis. By adapting the analytical method to reversible reactions, the binding energies of H2O molecules in Aun(H2O)m+ clusters were obtained. Quantum chemical calculations were also performed to help interpret the experimental results.

2. Methods


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2.1. Experimental methods Hydrated gold clusters were prepared by laser ablation of a gold rod (Furuya Metal Co., Ltd., 99.95%) using a 532 nm pulse laser (Nd:YAG laser, second harmonic, 10–20 mJ pulse−1) in the presence of water vapor (0.05–0.25%), diluted by helium (0.5 MPa stagnation pressure; Japan Fine Products Co., Ltd., >99.99995%), and injected as the carrier gas into the cluster source via a pulsed valve. After passing through a room-temperature tube (2 mm diameter, 60 mm length), the clusters were conveyed by the carrier gas into a heating tube (300–1000 K, 4 mm diameter, 120 mm length) for gas-phase thermal analysis. The temperature of the heating tube was controlled by a programmable proportional-integral-derivative (PID) controller (Omron, E5CN-HT) connected to a resistive heater and a K-type thermocouple. During the experiments, the temperature was altered by a rate of 9 K min−1. The cluster-ion residence times and the molecular density of the carrier gas in the heating tube were estimated to be ~100 µs and ~1018 cm−3, respectively. The hydrated gold clusters were heated in the heating tube by collisions with the surrounding carrier gas, which mediates heat transfer between the heating tube and the clusters. After heating, the cluster ions were expanded into a vacuum and then introduced into a differently pumped chamber via a skimmer (2 mm diameter). For the TOF-MS measurements, cationic clusters were accelerated to kinetic energies of 3.5 keV by a series of accelerating electrodes. After traveling through a 1 m field-free region, the ions were reversed by a dual-stage reflectron and detected by a Hamamatsu double-microchannel plate detector. Signals from the detector were amplified with a 350 MHz preamplifier (Stanford Research Systems, SR445A) and digitized using an oscilloscope (LeCroy LT344L). Averaged TOF spectra (typically 500 sweeps)


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were sent to a computer for analysis. The mass resolution (m/Δm) was ~1000, sufficient to assign the ion peaks of interest.

2.2. Computational methods B3LYP54,

55

and MP256-60 calculations were performed to investigate the structures and

energetics of hydrated gold cluster cations, Aun(H2O)m+ (n = 2–4, m = 0–4), using the Gaussian 09 program suite.61 First, initial geometries were generated by randomly setting the positions of the Au atoms and H2O molecules using a Visual Basic code. The number of trial initial geometries depended on the cluster size; for example there were 200 initial geometries set for Au4(H2O)4+. First-stage geometrical optimizations were performed by the B3LYP method using the LanL2DZ basis set for the Au atoms62 and the D95V basis set for the H and O atoms.63 Several low-energy geometries were further optimized using the B3LYP and MP2 methods with the SDD basis set for Au64 and the 6-311++G(d,p) basis set for H and O.65,

66

For B3LYP

calculations, vibrational frequencies were computed for several low-energy structures to confirm that they corresponded to energy minima and to obtain zero-point vibrationally corrected energies.

3. Results 3.1. Thermal dissociation of hydrated gold clusters Figure 1a shows a representative mass spectrum of hydrated gold cluster cations, Aun(H2O)m+, produced by laser ablation of a gold rod in the presence of 0.25% H2O vapor diluted in He carrier gas. Irrespective of the number of Au atoms (n) in the cluster, ions containing three or four H2O molecules were most abundant under these conditions. Almost all


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of the H2O adducts disappeared when the clusters were heated to 1000 K, and only naked gold clusters, Aun+, were observed as prominent peaks (Figure 1b). Figure 2 depicts ion intensities as functions of the heating temperature. The number of H2O molecules incorporated in each cluster decreased upon heating. For instance, in the Au5(H2O)m+ series, in which the Au5(H2O)5+ and Au5(H2O)4+ were the main species observed at room temperature, the number of H2O molecules decreased upon heating from 300 K to ~700 K, and water-free Au5+ clusters, were exclusively observed at temperatures over ~700 K. This temperature dependence indicates that H2O molecules are evidently desorbed from Aun(H2O)m+ clusters according to Δ Aun(H2O)m+ → Aun(H2O)m−1+ + H2O.

(1)

3.2. Optimized structures of hydrated gold clusters Figure 3 shows the optimized structures of Aun(H2O)m+ (n = 2–4, m = 0–4) obtained by calculations using the B3LYP hybrid functional. The water-free gold clusters were calculated to be linear (Au2+), triangular (Au3+), and rhombohedral (Au4+), and are consistent with the results obtained from other calculations, as well as ion-mobility studies.67 The attachments of H2O molecules onto the gold clusters was found not to change the Aun skeleton using these computational methods. For the Au2+ structure, the first H2O molecule is adsorbed at one of the Au atoms, with the second H2O adsorbed at the other Au atom. As the third H2O cannot become adsorbed at a Au atom, it forms a hydrogen bond with a pre-adsorbed H2O molecule instead. Similarly, the fourth H2O is adsorbed through hydrogen bonding. For the Au3+ structure, three H2O molecules can adsorb onto Au atoms, whereas the forth becomes adsorbed at a H2O molecule through a hydrogen bond. For the Au4+ structure, three H2O molecules adsorb at Au atoms, and the fourth


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is adsorbed at a H2O molecule by a hydrogen bond, leaving one unoccupied Au atom. An isomer of Au4(H2O)4+ in which all the four H2O molecules are bound to Au atoms was found at 0.03 eV above the most stable one (Supporting Information). The structure of Au4(H2O)+ is consistent with a previous report.68 In addition MP2 calculations were also performed. Except for Au4(H2O)3+ and Au4(H2O)4+, the most stable structures were essentially the same as those obtained by the B3LYP method. The most stable MP2-calculated structures for Au4(H2O)3+ and Au4(H2O)4+ contain tetrahedral Au4+ skeletons with three and four adsorbed H2O molecules, respectively (Supporting Information).

4. Discussion 4.1. Analysis of thermal desorption Methods for the thermal analysis of gas-phase clusters by probing their temperaturedependent unimolecular dissociation processes have been developed and are referred to as “gasphase temperature-programmed desorption” (TPD) or “gas-phase thermal desorption spectrometry” (TDS). Unimolecular dissociation reactions can general be expressed by XL → X + L,

(2)

where XL and X are the parent and product species, respectively, and L is the species that leaves. The activation energy for the desorption of L from XL is obtained by determining the temperature dependence of the desorption rate constant. In most cases, the leaving molecules are pre-added into the cluster source or reactor in order to generate the parent clusters; hence they are likely to remain at non-negligible concentrations during thermal dissociation in the gas phase, and are possibly involved in the reverse reaction:


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X + L → XL.

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(3)

This reverse reaction complicates the quantitative analysis of the experimental data by increasing the number of parameters that need to be considered. Therefore, the use of a minimal amount of leaving molecule during cluster generation is a practical strategy; in this manner the effect of the reverse reaction can be neglected. For instance, the thermal desorption of O2 from CenO2n+x+ clusters was measured in the presence of O2 that was seeded in the carrier gas to generate the CenO2n+x+ clusters.48 In this case, the desorption temperature was found to be almost constant, even when the O2 concentration was reduced, suggesting that the effect of the reverse reaction had been fully eliminated. Ideally these clusters would be prepared without the leaving molecule as the reactant, although this is not always possible. For instance, during the thermal desorption of NO2 from CenO2n(NO)+ clusters (CenO2n(NO)+ → CenO2n−1+ + NO2), the parent clusters were prepared using O2 in the carrier gas and NO as a reactant, but not NO2.52 In this case, the reverse reaction, the adsorption of NO2 onto CenO2n−1+, can be neglected because the concentration of NO2 in the gas phase is very low. The temperature dependence of the rate constant can be modeled by the Arrhenius equation: 𝐸𝐸

𝑘𝑘 = 𝐴𝐴 exp �− 𝑘𝑘 a𝑇𝑇� = 𝐴𝐴 exp(−𝛽𝛽𝐸𝐸a ), B

(4)

where k, A, Ea, kB, T, and β are the rate constant, pre-exponential factor, activation energy, the Boltzmann constant, temperature, and the thermodynamic beta (β = 1/(kBT)), respectively. For reversible reactions, k, A, and Ea for the forward and reverse processes are indicated by subscripts “1” and “−1”, respectively, as shown: 𝑘𝑘1 ,𝐴𝐴1 ,𝐸𝐸a1

�⎯⎯⎯⎯⎯⎯⎯⎯� XL X+L �⎯⎯⎯⎯⎯⎯⎯⎯�

(5)

𝑘𝑘−1 ,𝐴𝐴−1 ,𝐸𝐸a−1


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The rate equation is presented as a differential equation: d[XL] d𝑡𝑡

= −𝑘𝑘1 [XL] + 𝑘𝑘−1 [X][L]

(6)

where [XL], [X], and [L] are the concentrations of species XL, X, and L, respectively, and t is the reaction time. In gas-phase reactions involving clusters, the densities of the clusters (XL and X) are absolutely lower than that of the gaseous reagent (L), therefore [L] can be treated as a constant. In addition, [XL] + [X] is also constant, as the number of X units remains constant throughout the reaction. Under such conditions, the solution to the differential equation is given by 𝐼𝐼 = 𝐼𝐼0 − �𝐼𝐼0 − 𝑘𝑘

𝑘𝑘−1 [L]

1 +𝑘𝑘−1 [L]

� �1 − exp(−(𝑘𝑘1 + 𝑘𝑘−1 [L])𝑡𝑡)�,

(7)

where I is [XL] / ([XL] + [X]), and I0 is the initial value of I at t = 0. The limit when [L] = 0 gives 𝐼𝐼 = 𝐼𝐼0 exp(−𝑘𝑘1 𝑡𝑡),

(8)

which is same as the rate equation of an irreversible unimolecular dissociation. Combining eq. 8 with the Arrhenius equation (eq. 4), the relationship between I and β can be written as 𝐼𝐼 = 𝐼𝐼0 exp�−𝐴𝐴1 𝑡𝑡 exp(−𝛽𝛽𝐸𝐸a1 )�,

(9)

which is used in our thermal analyses. This condition represents the limit of an irreversible reaction. Equilibrium represents another possible limit. When t = ∞, we can write 𝐼𝐼 = 𝑘𝑘

𝑘𝑘−1 [L]

1 +𝑘𝑘−1 [L]

.

(10)

The equilibrium constant, K, is given by 𝐾𝐾 =

[X][L] [XL]

=

(1−𝐼𝐼)[L] 𝐼𝐼

𝑘𝑘

= 𝑘𝑘 1 , −1

(11)

which depends on the temperature through


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𝐾𝐾 = 𝐴𝐴eq exp(−𝛽𝛽∆𝐸𝐸),

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(12)

where Aeq = A1 / A−1 and ΔE = Ea1 − Ea−1. This formula is same as that derived from the Van’t Hoff equation. Accordingly, the relationship between I and β is 𝐼𝐼 =

1

𝐾𝐾 +1 [L]

= 𝐴𝐴eq [L]

1

exp(−𝛽𝛽∆𝐸𝐸)+1

.

(13)

As mentioned above, quantitative analyses of reversible reactions are more complicated than those of simple irreversible reactions because of the higher number of parameters involved. If either of the two limits, irreversible or equilibrium, apply, the data are much easier to interpret. To consider a reversible reaction by referring to the two limits, the rate equation (eq. 7) can be expressed as 𝐼𝐼

𝑘𝑘−1 [L] � �1 (𝑘𝑘 0 1 +𝑘𝑘−1 [L])

1 − 𝐼𝐼 = �1 − 𝐼𝐼 0

− exp(−(𝑘𝑘1 + 𝑘𝑘−1 [L])𝑡𝑡)�.

(14)

The right side is the product of two terms that correspond to the equilibrium limit (the first term) and the irreversible limit (the second term). Here, the deviation from those limits is expressed by 𝑘𝑘−1 [L]

𝐼𝐼0 (𝑘𝑘1 +𝑘𝑘−1 [L])

: Deviation from the irreversible limit,

exp(−(𝑘𝑘1 + 𝑘𝑘−1 [L])𝑡𝑡) : Deviation from equilibrium limit.

The irreversible limit can be applied when its deviation is close zero, which is satisfied by a low value of [L], and depends on the values of k1 and k−1. On the other hand, the equilibrium limit can be applied when its deviation is nearly zero, in other words at a high value of (k1 + k−1[L])t. Considering that the reaction time t is determined by the experimental setup, high values of [L] result in a system close to the equilibrium limit, depending on the value of k−1 for the target reverse reaction. Figure 4 shows simulated curves of parent-ion relative intensities for reversible reactions, which are calculated by a combination of eqs. 4 and 7. When the density of the leaving


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molecules is low enough, for example when [L] = 1013 cm−1 as shown in panels a and b, the intensity curves are almost identical to those of the irreversible limit, as exhibited by the [L] = 0 curve in each panel. Conversely, when the density of the leaving molecules is sufficiently high, for example when [L] = 1016 cm−1, the intensity curve is almost identical to that corresponding to the equilibrium limit, as shown in panel c. In these two near-limit cases, the intensity versus temperature curves are sigmoidal. In contrast, when [L] lies between these limit cases, for example when [L] = 1014 or 1015 cm−1, the curves are not simply sigmoidal, and their shapes vary with the temperature dependence of the reverse reaction. If the reverse reaction has an Arrhenius-type temperature dependence (Ea−1 > 0, Figure 4a), in which the reaction rate increases with temperature, the relative intensity I gradually increases with increasing temperature at lower temperatures, corresponding to enhancement of the reverse reaction, and decreases with increasing temperature at higher temperatures, corresponding to enhancement of the forward reaction. If the reverse reaction is of the anti-Arrhenius-type (Ea−1 < 0, Figure 4b), in which the rate decreases with increasing temperature, the relative intensity I gradually decreases with increasing temperature at lower temperatures, corresponding to a disfavoring of the reverse reaction, and quickly decreases with increasing temperature at higher temperatures, corresponding to the promotion of the forward reaction. These simulations indicate how to approach these two limits. If decreasing the density of L changes the intensity curve from nonsigmoidal to sigmoidal, the reaction type is approaching the irreversible limit. On the other hand, if increasing the value of [L] results in a non-sigmoidal to sigmoidal change in the intensity curve, the reaction type is approaching the equilibrium limit. Under irreversible-limit conditions, the experimentally observed temperature dependence provides the activation energy for desorption (Ea1). On the other hand, under equilibrium-limit


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conditions, the observed temperature dependence provides the binding energy (ΔE); in other words the energy difference between the reactant (XL) and products (X + L).

4.2. Application of the equilibrium limit to the experimental water desorption behavior of hydrated gold clusters The results from experiments involving the thermal desorption of H2O from Aun(H2O)m+ can be analyzed based on the discussion in the preceding subsection. Figure 5 displays thermal desorption plots for the Au3(H2O)m+ series under conditions in which the carrier gas has been seeded with different concentrations of H2O. Note that desorption is a sequential process, as shown in Figure 2; in other words −H2 O

⋯ �⎯⎯� Au3 (H2 O)2

+ −H2 O

−H2 O

�⎯⎯� Au3 (H2 O)+ �⎯⎯� Au3 + .

(15)

In order to compare the experimental curves with the simulated ones (Figure 4), it is useful to focus on each step in the reaction sequence individually. When focusing on such a step, namely Au3(H2O)m+ → Au3(H2O)m−1+ + H2O, ions comprising m or more H2O molecules, Au3(H2O)k+ (k ≥ m), are regarded as parent ions, while ions with less H2O molecules, Au3(H2O)k+ (k ≤ m−1), are regarded as products; so that the quantitative relationship between parent and product species, defined in this manner, change only according to the reaction step of interest. Under low H2O concentration conditions (0.05%), the parent ions were observed to gradually decrease in intensity at lower temperatures and rapidly at higher temperatures (i.e., at 300–500 K and 500–600 K, respectively, for Au3(H2O)2+ → Au3(H2O)+ + H2O). This temperature dependence resembles the simulation curve in Figure 4b, where desorption is considered to involve an anti-Arrhenius-type reverse reaction under intermediate [L] conditions.


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Therefore, the experimental results suggest that the adsorption of water on gold clusters and their hydrates has an anti-Arrhenius temperature dependence. When the concentration of H2O was increased, the desorption curves began to resemble ideal sigmoids. With 0.25% H2O in the carrier gas, the desorption curves exhibit plateaus with relative intensities of unity at the lower temperatures, which simply dropped to zero at higher temperatures. Such characteristics are consistent with the equilibrium limit (Figure 4c). We conclude that under high H2O concentration conditions (0.25%), equilibria between desorption and adsorption for the hydrated gold clusters were almost achieved. Indeed, the experimental results with 0.25% H2O could be fitted by the equilibrium-limit equation (eq. 13) to reproduce the measured values (Figure 2), while the irreversible-limit model did not reproduce the experimental curves at the lower H2O concentrations.

4.3. Binding energies of water in hydrated gold clusters Figure 6 depicts the binding energies (ΔEs) of water molecules to the Aun(H2O)m+ clusters (eq. 1) obtained both experimentally and computationally. The experimental values were estimated by curve fitting of the 0.25% H2O data to the equilibrium-limit simulation (eq. 13), as described in the preceding subsection. The binding energy of H2O in Au3(H2O)+ was reported to be 0.98 ± 0.1 eV,44 as estimated by RRKM theory analysis with the ‘loose’ TS model.42 The present thermal analysis provided an energy of 1.14 eV, which is consistent with the value reported in the literature. The experimental data show that ΔE for the release of H2O from Aun(H2O)+ decreases with increasing n (Figure 6a). It is highly likely that this is due to the charge densities of the cationic Aun+ clusters, namely, larger clusters with the same charge have a


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lower charge density. It is natural that the interaction between a H2O molecule and a Aun+ cluster would be influenced by the charge density. Theoretical values were calculated as energy differences according to ΔE = E(Aun(H2O)m−1+) + E(H2O) – E(Aun(H2O)m+).

(16)

Comparing the results from the B3LYP and MP2 calculations, the MP2-generated ΔE values tend to be higher by 0.1–0.4 eV than those calculated by B3LYP. For the two computational methods, the basis set superposition error (BSSE) in ΔE for Au3(H2O)+ → Au3+ + H2O, for instance, was estimated using the counterpoise method.69, 70 BSSE values of 0.06 and 0.24 eV were estimated at the B3LYP and MP2 levels of theory, respectively. When these two BSSEs are taken into account, the two computational methods provide very close values of ΔE. These geometry optimizations suggest that there are two types of bound H2O molecules in the Aun(H2O)m+ clusters; one involving H2O molecules directly bound to Au atoms, and the other involving H2O molecule bound to adsorbed H2O molecules through hydrogen bonding (Figure 3). The former type corresponds to H2O molecules in the primary coordination spheres of the Aun+ cores, while the latter corresponds to molecules in the secondary coordination spheres. These H2O binding types are similar to those reported for hydrated atomic gold cations, Au(H2O)m+.71-73 Desorption of a H2O molecule requires the cleavage of either a Au–O bond or a hydrogen bond. H2O release from Aun(H2O)2+ (n ≥ 2) showed similar tendencies to H2O release from Aun(H2O)+ (n ≥ 2); ΔE decreased as n increased (Figure 6b). In contrast, ΔE for Aun(H2O)3+ exhibited a different trend; Au2(H2O)3+ (n = 2) released a H2O molecule more easily when compared to the remaining trihydrated clusters (Aun(H2O)3+, n ≥ 3), for which ΔE decreased with increasing n (Figure 6c), as was observed during the desorption of H2O from Aun(H2O)1,2+. In the


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optimized structure of Au2(H2O)3+, the H2O moiety to be released is bound to another H2O molecule by a hydrogen bond, while the leaving H2O molecules are bound to Au atoms in Au2– + 4(H2O) ,

Au2–4(H2O)2+, and Au3,4(H2O)3+ (Figure 3). Therefore, the different trend for

Au2(H2O)3+ is due to the different types of the bond undergoing cleavage, suggesting that the cleavage of a hydrogen bond requires less energy than that for a Au–O bond. For the desorption of H2O from Au2(H2O)3+ and Au2–4(H2O)4+, the computationally obtained ΔE values are larger than those obtained experimentally. According to the B3LYP calculations, these parent clusters (Au2(H2O)3+ and Au2–4(H2O)4+) have one or more H2O molecules bound to other H2O molecules through hydrogen bonds (Figure 3); hence, their dissociation energies correspond to the energy required for the cleavage of a hydrogen bond. In other words, it would appear that the calculations overestimate the hydrogen-bond energies. The binding energies of H2O in Aun(H2O)4+ were low and less size-dependent compared to those of Aun(H2O)1–3+ (Figure 6d). B3LYP calculations suggest that one or two H2O molecules are bound to other H2O molecules in the stable structures of Au2–4(H2O)4+ (Figure 3); in other words, the bonds being cleaved are hydrogen bonds. Considering the discussion in relation to H2O release from Aun(H2O)3+, the types of bonds undergoing cleavage in Au2– 4(H2O)4

+

are consistent with the observed low ΔE. The MP2 calculations for Au4(H2O)4+ gave a

different structure from that calculated by B3LYP; all H2O molecules are directly bound to Au atoms. While MP2 generally provides more accurate results to those from DFT methods, it is difficult to determine which structure is correct at this moment.

5. Conclusions


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The desorption of water molecules from hydrated gas-phase gold clusters (Aun(H2O)m+; n = 2–8, m = 1–4) under chemical equilibrium conditions was investigated by thermal analysis and quantum chemical calculations. The clusters were generated in the gas phase by laser ablation of a gold rod in the presence of H2O seeded in the He carrier gas, and detected by time-of-flight mass spectrometry. The Aun(H2O)m+ clusters were found to release H2O molecules upon heating, resulting in the formation of unhydrated gold clusters (Aun+) below 1000 K. The observed desorption of H2O was found to be reversible; the adsorptions and desorptions of H2O onto/from the Aun(H2O)m+ clusters were almost in equilibria under conditions of sufficiently concentrated H2O vapor. A method for the analysis of reversible dissociation was formulated in the present study. Thermal analyses of the clusters under equilibrium conditions provided binding energies for H2O to Aun(H2O)m+. Experimental and computational results suggest two binding forms for H2O; one in which H2O molecules are directly bound to Au atoms, and one in which they are bound to other H2O molecules by hydrogen bonding. H2O molecules bound by hydrogen bonds are relatively easy to desorb compared to those directly bound to Au atoms. The binding energy of the directly bound H2O decreases as the cluster size (n) increases; this is most likely related to the charge densities of the cationic clusters.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:_ Low-lying isomers of Au4(H2O)4+ calculated by B3LYP; MP2-calculated most stable isomers of Au4(H2O)3+ and Au4(H2O)4+; complete author list for ref 61.


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AUTHOR INFORMATION Corresponding Author *(F.M.) E-mail: [email protected]. Tel: +81-3-5454-6597 Present Address †(T.N.) Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Grants-in-Aid for Exploratory Research (No. 26620002), for Young Scientists (B) (No. 17K14433), and for JSPS Research Fellow (No. 17J02017) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT). The calculations were performed in part using the facilities of the Research Center for Computational Science, Okazaki, Japan. T.N. is grateful for a Research Fellowship from the Japan Society for the Promotion of Science (JSPS). The authors thank Dr. Ryuzo Nakanishi, Dr. Satoshi Takahashi, Dr. Satoshi Kudoh, and Dr. Ken Miyajima for helpful discussions.

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