Combined Experimental and Computational Approaches To Elucidate

Article pubs.acs.org/JPCC

Combined Experimental and Computational Approaches To Elucidate the Structures of Silver Clusters inside the ZSM‑5 Cavity Takashi Yumura,*,† Akira Oda,‡ Hiroe Torigoe,‡ Atsushi Itadani,‡ Yasushige Kuroda,‡ Takashi Wakasugi,† and Hisayoshi Kobayashi† †

Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan

‡

S Supporting Information *

ABSTRACT: A combination of experimental and computational analyses suggested the presence of Ag3 or Ag4 clusters inside a nanometer-sized cavity in Ag−ZSM-5 zeolites, which were formed from H−ZSM-5 using the conventional ion-exchange method in an aqueous silver nitrate solution. During the experimental analyses, we investigated the structural and absorption properties of Ag−ZSM-5 through UV−vis diffuse reflectance and X-ray absorption fine structure (XAFS) measurements. The results from the extended XAFS (EXAFS) analysis indicated that clusters contained in the ZSM-5 cavity have Ag−Ag separations of approximately 2.6 Å. The UV−vis measurements indicated that the clusters located in the cavity present three prominent absorption bands centered at approximately 255, 287, and 331 nm for the sample treated at 473 K. The Ag−ZSM-5 treated at 573 or 673 K presents new UV−vis bands at approximately 303 and 319 nm. The experimental results regarding the structural and absorption properties of Ag−ZSM-5 could be well-reproduced by DFT calculations when a model large enough to represent a 10-membered ring of ZSM-5 was used. DFT optimization indicated that the ZSM-5 cavity can accommodate a triangular Ag3 cluster and a butterfly Ag4 cluster whose Ag−Ag separations range from 2.7 to 2.9 Å. According to time-dependent DFT calculations, these clusters have electronic transitions from a completely symmetric 5s-based orbital to a 5s-based orbital with one node. The electronic excitations between the 5s-based orbitals are modulated by the ZSM-5 encapsulation through the resulting deformation of the cluster and interactions between the cluster and framework oxygen atoms. The electronic transitions between the 5s-based orbitals that appropriately explain the UV−vis absorption properties would become fingerprints for identifying the shapes and sizes of clusters inside a zeolite cavity. Our multidisciplinary analyses conclusively determined the origin of the absorption peaks in Ag−ZSM-5 and successfully obtained atomistic information about the states of silver clusters inside a ZSM-5 cavity. The findings of this study will provide useful information for elucidating the structures of active sites in Ag-ZSM-5 and their important role in catalytic reactions such as C−H bond activation in hydrocarbons.

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INTRODUCTION Small silver clusters have unique electronic properties that cannot be observed in their bulk counterparts1 because the clusters have discrete energy levels. A key factor that modulates these properties is the size of the clusters because the size of a cluster determines the energy levels in the frontier orbital regions.2,3 Although larger clusters are usually formed, smallsized clusters are desirable for designing unique optoelectronic devices. Thus, a technique is needed for controlling the sizes of clusters. To date, DNA,4,5 proteins,6 thiols,7−12 polymers,13−15 metal−organic-frameworks (MOFs),16−18 glasses,19 tryptophan,20−22 and zeolites23−64 have been used as templates for stabilizing nanoclusters against agglomeration. In particular, using a cavity of zeolites, which are aluminosilicate-based porous materials65−68 is a promising approach for stabilizing relatively small clusters. The rigid zeolite frameworks repel larger clusters on the inside of the cavity. Consequently, the inner larger clusters are unstable inside the cavity and are deformed and sometimes dissociated into smaller © 2014 American Chemical Society

clusters. The other characteristic is that zeolites possess some Al3+ atoms substituted for Si4+ atoms in the SiO2 frameworks. The Si → Al substitution plays an important role in the formation of smaller silver clusters inside a zeolite cavity. One factor is that the negative charges on the framework oxygen atoms, which are generated by the substitution, fix positively charged clusters through electrostatic interactions. The electrostatic interactions between a surface and a metal cluster would prevent adjacent clusters from coalescing with each other. The other factor is that the positive charges of a cluster that result from the Si → Al substitution due to the requirement of charge balance determine the strength of Ag−Ag bonds through orbital interactions to stabilize the structure of a cluster. Experimentally, silver clusters containing 2−8 atoms can be stabilized by the unique environment of zeolite cavities.23−64 Received: August 11, 2014 Revised: September 25, 2014 Published: September 25, 2014 23874

dx.doi.org/10.1021/jp508150w | J. Phys. Chem. C 2014, 118, 23874−23887

The Journal of Physical Chemistry C

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ment. The powdered sample was placed in a reflectance cell composed of fused silica under vacuum. Spectralon (Labsphere) was used as the reference material. The XAFS spectra were recorded at beamline BL-9C of the KEK-PF facility (Tsukuba, Japan), which was equipped with a double-crystal Si(111) monochromator, under ring operating conditions of 2.5 GeV and 300 mA. The XAFS analysis was performed using the software developed by Professor H. Maeda in ref 69.

We have a special interest in ZSM-5 zeolites with nanometersized cavities because these zeolites provide the ability to stabilize silver clusters with nuclearities of less than 4 through use of the restricted environment. As is well-known, Ag−ZSM-5 can catalyze the decomposition of NOx44−54 and the activation of C−H bonds in hydrocarbons.55−60 With respect to the catalytic reactions in ZSM-5, many experimental reports have suggested that the small silver clusters are responsible for the C−H bond activation55−60 by Ag−ZSM-5 prepared with different treatments (oxidation and reduction by H2 treatment or by photoirradiation).58 According to ref 58, various treatments of Ag−ZSM-5 result in silver clusters with different sizes: the clusters formed in H2-reduced Ag−ZSM-5 are larger than those formed in photoreduced Ag−ZSM-5. Regarding the different sizes of clusters, photoreduced Ag−ZSM-5s transform CH4 into C2H6 under photoirradiation, whereas H2-reduced Ag−ZSM-5s can catalyze the conversion of CH4 into C2H4 via heat treatment.58 Because of the relationships between the interesting properties and cluster sizes or shapes, determining the states of silver atoms inside a ZSM-5 cavity is of importance. For this purpose, numerous experimental attempts, particularly UV−vis diffuse reflectance (UV−vis DR) spectroscopy measurements, have been made over the past decades.45−60 However, to the best of our knowledge, the relationship between the positions of the observed UV−vis bands, as well as their intensities, and the formed cluster-ion species has never been accurately and unambiguously identified from a fundamental perspective. To obtain a better understanding of the dependence of the electronic structures of Ag-ZSM-5 on the structures of the clusters contained within cavities, the present study determined the assignments of the observed bands through a combination of qualitative theoretical results obtained using density functional theory (DFT) calculations and experimental data obtained from X-ray absorption fine structure (XAFS: EXAFS and XANES) measurements.

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COMPUTATION We performed DFT calculations to investigate the structures of silver clusters inside a zeolite cavity. Similar to our previous studies,70−76 we used an aluminum-free ZSM-5 model (silicalite1) that consists of a Si92O151 framework whose terminal Si atoms are saturated with H atoms (Si92O151H66). This model includes a 10-membered ring (10-MR) cavity, as shown in Figure S1 (Supporting Information). To construct models of ZSM-5 containing silver clusters, we placed a silver cluster inside of a 10MR cavity in the Si92O151H66 model while simultaneously substituting some Al atoms for Si atoms. In the present study, the number of substituted Al atoms (n) varied from 1 to 3. We will use the notation Ag x −ZSM-5(Al n ) to represent Ag x − AlnSi92−nO151H66. Using DFT methods, we fully optimized the Agx−ZSM-5(Aln) structures. Although some theoretical works on silver-containing zeolites have been reported,77−82 the present study is the first to investigate silver clusters in a ZSM5 model that is large enough to represent the 10-membered ring cavity in the real system. Similar to a previous study,76 we used the B3PW91 functional83 in this study rather than the popular B3LYP functional84−88 because the results obtained using the B3PW91 functional can appropriately reproduce those obtained from more reliable calculations using coupled-cluster singlet double (CCSD)89 methods in terms of the stability of neutral Ag3 isomers.76 During the optimization using the Gaussian 03 code90 and partially the Gaussian 09 code,91 we used the CEP-121G basis set for extraframework Ag atoms,92,93 the 6-31G basis set for substituted Al atoms and for the two O atoms that are bound to a substituted Al atom,94−96 and the 3-21G basis set for the other atoms97−102 due to the limitations of computational resources. For example, the calculations of Ag4−ZSM-5(Al1), Ag4−ZSM-5(Al2), and Ag4−ZSM-5(Al3) involve 2795, 2810, and 2825 contracted basis functions. In a Agx−ZSM-5(Aln) structure with x − n being odd (even), there is (not) one unpaired electron. Thus, we considered the Agx−ZSM-5(Aln) structure with x − n being even (odd) in the singlet (doublet) state during the B3PW91 optimization. In relevant experiments, different splitting patterns of electronic paramagnetic resonance (EPR) spectra were obtained in silver clusters inside zeolites with different Si/Al ratios.40−43 Because our Agx−ZSM-5(Aln) models have different Si/Al ratios, it is interesting to investigate their electronic and magnetic properties for the purpose of giving a better understanding of the experimental data.

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EXPERIMENTAL SECTION The procedure for preparing Ag−ZSM-5s, which followed that of ref 60, is briefly detailed as follows. An original sodium-type ZSM-5 zeolite (Na−ZSM-5) provided by the Tosoh Co., Japan, was converted into an NH4-type zeolite (NH4−ZSM-5) by ion exchange in an aqueous ammonium nitrate solution at 313 K for 10 min. This procedure was repeated 5 times. The NH4−ZSM-5 sample was evacuated at 773 K for 5 h to obtain an H-type zeolite (H−ZSM-5). The prepared H−ZSM-5 sample was then ionexchanged with Ag+ ions using the conventional ion-exchange method, which consisted of submerging the sample in an aqueous silver nitrate solution (0.02 M) at room temperature for 24 h to afford Ag−ZSM-5-64. The last number corresponds to the ion-exchange capacity, which was evaluated by assuming a one-to-one ion exchange between H+ and Ag+. To investigate the structural and electronic properties of Ag−ZSM-5, we employed various experimental approaches, such as UV−vis DR and X-ray absorption fine structure (XAFS: EXAFS and XANES). All samples used were first evacuated at 300 K by utilizing an in situ cell developed by our laboratory, followed by the measurements of spectra without any exposure to an atmospheric gas. Measurements of the spectra were also carried out in the respective stages after evacuation at increasing temperatures. UV−vis DR spectra of the samples were recorded at 300 K over the wavelength range 200−600 nm using a spectrophotometer (JASCO V-570) equipped with an integrating sphere attach-

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RESULTS AND DISCUSSION I. Properties of Ag−ZSM-5 Obtained from Experimental Approaches. UV−vis DR spectroscopy was used to characterize the states of silver ions and of silver cluster ions formed during the respective evacuation stages of the Ag−ZSM5−64 sample. According to spectra 1 and 2 shown in Figure 1, the Ag−ZSM-5-64 sample exhibited bands in the range between 200 and 250 nm after evacuation at 300 or 373 K. These bands are 23875



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bands have larger values, which clearly explains the observed trend.103 The spectrum of the sample evacuated at 473 K shows three distinct bands centered at approximately 255, 287, and 331 nm, indicating the formation of small clusters. We also found the band at 405 nm, which may be caused by the formation of a relatively larger cluster ion, according to ref 104. In these stages, the EXAFS band that is assignable to Ag−Ag species, which is clearly observed in the spectrum of Ag foil, is rarely observed in this sample, although the XANES data show a faint different nature for the sample evacuated at 473 K: the observation of the band at approximately 25.53 keV, which is on the lower energy side compared with both the sample evacuated at 300 or 373 K and the reference sample, Ag2O. After evacuation at 573 K, the UV−vis DR spectrum exhibits very strong absorption behavior, especially around 303 and 319 nm. In addition, another prominent feature is observed in the band: the absorption spreads out toward the longer wavelength region, indicating the formation of a relatively larger cluster ion. This result is also clearly supported by the EXAFS data, in which a distinct band appears at 2.61 Å (without correction for phase shift) due to the back scattering from Ag−Ag pair species. The spectrum pattern and intensities of the respective bands of this sample are minimally affected by the evacuation at 673 K. Evacuation at higher temperatures of 773 or 873 K caused a decrease in intensities in the entire region, accompanied by a marked band broadening toward the longer wavelength side. This phenomenon is well-explained by considering that further aggregation occurred. Qualitatively, both the EXAFS and XANES data also justify the previously described consideration; the increase in the band intensity observed at approximately 2.61 Å (no phase-shift correction) due to Ag−Ag pair species and the distinct appearance of a band at approximately 25.57 keV. There are several obvious components in the UV−vis spectrum with increasing temperatures. However, detailed assignments for these bands have not explicitly been provided. To date, further detailed assignments for the Ag−ZSM-5 samples treated at higher temperatures have been considerably difficult. II. Computational Analyses of Ag−ZSM-5 Zeolites. To accurately assign obtained UV−vis bands as well as obtain information on how these bands are related with structures of the formed clusters, we further performed density functional theory calculations by using a realistic zeolite model with 10-membered ring cavity that can contain silver clusters. II-1. Space Available for Encapsulating Silver Clusters in a ZSM-5 Cavity Clusters in a ZSM-5 Cavity. To increase our understanding of the experimental data for silver-containing ZSM-5 zeolites, particularly their UV−vis spectra, we performed DFT calculations using the B3PW91 functional. Because of the

Figure 1. (a) UV−vis DR spectra of the Ag-ZSM-5-64 sample evacuated at various temperatures: (1) 300, (2) 373, (3) 473, (4) 573, (5) 673, (6) 773, and (7) 873 K. (b) XANES and (c) EXAFS spectra of the Ag-ZSM5-64 sample evacuated at various temperatures: (1) 300, (2) 373, (3) 473, (4) 573, (5) 673, (6) 773, and (7) 873 K. In these figures, respective spectra of (8) Ag foil and (9) Ag2O are also provided for reference.

assigned to the dominant transition component from the 4d10 state to the 4d95s1 state of the Ag+ ions coordinated with water molecules and with lattice oxygen atoms. The difference between the two spectra after evacuation at the respective temperatures can be interpreted in terms of the decrease in the number of coordinated water molecules after evacuation at 373 K. In these cases, the observed absorbance is relatively weak due to the forbidden nature of the 4d−5s transition. On the basis of symmetry rules, this transition is forbidden; hence, the intensities of these bands should be relatively small. A sudden increase in the absorption intensity in the spectrum was clearly observed after evacuation at 473 K. This phenomenon can be interpreted in terms of the formation of some type of small cluster ions composed of Agxn+. In such cases, the transition has become an allowed transition; the absorption intensities of the respective

Figure 2. (a) Optimized Ag1−ZSM-5(Al1) structure obtained from the B3PW91 calculations. (b) Space available for encapsulating silver clusters in a ZSM-5 cavity, as indicated by the yellow circle estimated from the requirement of separation between Al and Ag atoms in the Ag1−ZSM-5(Al1) structure. (c) A triangular Ag3 cluster can be encapsulated in the ZSM-5(Al1) cavity without deformation of the corresponding bare cluster. Note that the bare cluster has Ag−Ag separations of ∼2.7 Å. 23876



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Figure 3. Bare small silver clusters optimized under symmetry constraints: (a) D∞h-Ag2, (b) D3h-Ag3+, (c) D4h-Ag42+, (d) Td-Ag42+, and (e) D2h-Ag42+ clusters. The optimized Ag−Ag separations are given in Å.

Table 1. Electronic Excitations of a Bare Ag2 Cluster with D∞h Symmetrya Exb

fc

electronic transitionsd

403.9 259.5 259.5 207.0 182.2 174.0 171.9 171.9

0.423 0.479 0.479 0.080 0.193 0.322 0.130 0.130

9 → 20 (−0.10) 19 → 20 (0.58) 16 → 20 (0.25) 19 → 21 (0.59) 17 → 20 (0.25) 19 → 22 (0.59) 9 → 20 (0.64) 16 → 21 (−0.17) 17 → 22 (−0.17) 18 → 23 (0.11) 9 → 20 (0.64) 16 → 21 (0.38) 17 → 22 (0.38) 18 → 23 (−0.34) 16 → 21 (0.22) 17 → 22 (0.22) 18 → 23 (0.60) 19 → 26 (−0.14) 11 → 23 (0.12) 12 → 21 (−0.15) 12 → 22 (0.45) 13 → 21 (−0.45) 13 → 22 (−0.15) 10 → 23 (−0.12) 12 → 21 (0.45) 12 → 22 (0.15) 13 → 21 (−0.15) 13 → 22 (0.45)

a

The optimized geometry is given in Figure 3. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transitions: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals: HOMO − n, 19 − n; HOMO, 19; LUMO, 20; LUMO + m, 20 + m, where n and m are integers. Detailed orbital information is provided in Figure 4. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

Table 2. Electronic Excitations of a Bare Ag3+ Cluster with D3h Symmetrya Exb

fc


318.9 318.9 217.2 203.6 189.7 189.7 166.5 166.5

0.316 0.316 0.311 0.205 0.092 0.092 0.166 0.166

27 → 29 (−0.14) 28 → 30 (0.61) 27 → 30 (0.14) 28 → 29 (0.61) 18 → 30 (−0.26) 19 → 29 (−0.26) 28 → 31 (0.56) 18 → 30 (0.41) 19 → 29 (0.41) 23 → 29 (0.10) 24 → 30 (0.10) 28 → 31 (0.31) 13 → 29 (0.11) 14 → 29 (0.38) 15 → 30 (0.38) 23 → 31 (−0.12) 28 → 33 (0.37) 13 → 30 (−0.11) 14 → 30 (0.38) 15 → 29 (−0.38) 24 → 31 (0.12) 28 → 34 (0.37) 13 → 30 (−0.16) 24 → 31 (0.63) 25 → 32 (0.12) 27 → 33 (−0.10) 13 → 29 (−0.16) 23 → 31 (0.63) 26 → 32 (0.12) 27 → 34 (−0.10)

a The optimized geometry is given in Figure 3. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals: HOMO − n, 28 − n; HOMO, 28; LUMO, 29; LUMO + m, 29 + m, where n and m are integers. Detailed orbital information is provided in Figure 4. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

Table 3. Electronic Excitations of a Bare Ag42+ Cluster with D4h Symmetrya Exb

fc


352.1 352.1 260.5 260.5 211.9 211.9 196.9 194.5 194.5

0.288 0.288 0.094 0.094 0.087 0.087 0.478 0.126 0.126

36 → 38 (0.16) 36 → 39 (0.16) 37 → 38 (0.41) 37 → 39 (−0.41) 36 → 38 (−0.16) 36 → 39 (0.16) 37 → 38 (0.41) 37 → 39 (0.41) 35 → 38 (0.54) 35 → 39 (0.23) 36 → 38 (−0.11) 36 → 39 (−0.27) 37 → 38 (0.10) 35 → 38 (−0.23) 35 → 39 (0.54) 36 → 38 (−0.26) 36 → 39 (0.11) 37 → 39 (−0.10) 17 → 39 (0.10) 21 → 38 (0.50) 21 → 39 (−0.43) 17 → 38 (0.10) 21 → 38 (0.43) 21 → 39 (0.50) 24 → 38 (0.11) 25 → 39 (0.11) 37 → 41 (0.63) 17 → 38 (−0.22) 17 → 39 (0.61) 18 → 38 (−0.11) 21 → 38 (−0.11) 17 → 38 (0.61) 17 → 39 (0.22) 18 → 39 (−0.11) 21 → 39 (−0.11)

a

The optimized geometry is given in Figure 3. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals: HOMO − n, 37 − n; HOMO, 37; LUMO, 38; LUMO + m, 38 + m, where n and m is an integer. Detailed orbital information is provided in Figure 5. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

restricted space in zeolites, the number of atoms that can be contained in embedded clusters is limited. First, we discuss the space available for accommodating silver clusters. For this purpose, we first optimized the structure of Ag1−ZSM-5(Al1), as shown in Figure 2. During the optimization, the single silver cation was bound to two framework oxygen atoms with Ag−O

lengths of 2.373 and 2.347 Å, which are similar to those obtained in ref 81. The silver cation was separated from the substituted Al atom by 3.104 Å. Considering the Ag···Al separation, the space available for accommodating silver clusters is limited; in the figure, this space is roughly estimated by the yellow circle, whose diameter is approximately 4.6 Å. Clusters whose cross sections 23877



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orbitals responsible for the absorption strongly depend on the structures of the clusters and on the number of atoms in the clusters. Consequently, the absorption properties are highly sensitive to the cluster structures, as shown in Tables 1−4. In particular, clear differences are observed between the considered clusters in terms of the excitation energies between the 5s-based orbitals: 403.9 nm for Ag2, 318.9 nm for D3h-Ag3+, 352.1 nm for D4h-Ag42+, 269.3 nm for Td-Ag42+, and 399.5 nm for D2h-Ag42+. These absorptions are fingerprints for identifying the sizes and shapes of the clusters. In addition to the fingerprint absorptions, we obtained absorptions with smaller Ex values from the silver clusters, which originated from the 4d → 5s transitions and the 5s → 5p transitions. In the linear Ag2 cluster, Ex values larger than 190 nm were calculated: 259.5 and 207.0 nm. The absorption at Ex = 259.5 nm originates from the 5s → 5p transition, with a minor contribution from the 4d → 5s transition. For the absorption at Ex = 207.0 nm, an electron in a 4d-based orbital is excited to occupy 5s- or 5p-based orbitals. Similar 5s → 5p and 4d → 5s transitions were also obtained from the D3h-Ag3+ cluster, but the transitions possesses smaller Ex values (217.2 and 203.6 nm). The Ag42+ clusters exhibited 4d → 5s absorption peaks whose Ex values were intermediate: 260.5, 211.9, and 194.5 nm for the D4hAg42+ cluster and 192.6 nm for the Td-Ag42+ cluster. In the D4hAg42+ cluster, the 5s → 5p transition was also observed at 196.9 nm due to its planarity. The calculated Ex values that were larger than 200 nm fell within the range of the experimental data shown in Figure 1. II-3. Properties of Agx−ZSM-5(Aln). The previous section demonstrated that the bare clusters in Figure 3 are potential candidates for explaining the UV−vis spectra of the silvercontaining ZSM-5 in Figure 1. However, the electronic properties of the clusters will be strongly modulated by being confined in a ZSM-5 cavity, analogous to our previous study that investigated molecules inside a carbon nanotube.110−115 We previously studied the structural features of Ag3−ZSM-5(Aln) using DFT calculations and found that bare triangular clusters can be contained without significant cluster deformation. Because the bare triangular cluster (Figure 3) is small enough to be encapsulated inside the yellow circle of a ZSM-5 cavity, the zeolite confinement effect is not critical for determining the inner Ag3 structure (Figure 2c). In this paper, we discuss the structural properties of Ag2− ZSM-5(Aln) and Ag4−ZSM-5(Aln) based on the same level of DFT calculations. After obtaining information about their atomic configurations, we compared Ag2−ZSM-5(Aln), Ag3−ZSM5(Aln), and Ag4−ZSM-5(Aln) in terms of their absorption properties. Considering the ZSM-5 cavity that is available to accommodate silver clusters, the Ag4 clusters are too large, and thus, the clusters must be deformed to fit in the cavity. On the other hand, the Ag2 clusters can be easily accommodated in the cavity without deformation of the cluster. Thus, we expect that ZSM-5 confinement effects in Ag2 structures are not important, whereas those in Ag4 structures are important. The following subsections discuss, using DFT calculations, how the structures of Ag2 and Ag4 clusters are influenced by encapsulation in ZSM-5 cavities with different numbers of Al atoms contained inside the framework. II-3-1. Ag2−ZSM-5(Aln). Figure 6 shows the optimized geometries for Ag2−ZSM-5(Aln), where n is the number of contained Al atoms. As shown in Figure 6, the separation between the silver atoms increases from 2.59 to 3.78 Å as n increases.

are smaller than the yellow circle can be encapsulated in the ZSM-5 cavity without significant deformation of their structures. In contrast, larger clusters must be deformed to be incorporated in the cavity. II-2. Properties of Small Agx Clusters (2 ≤ x ≤ 4). Considering the space that can accommodate silver clusters, we examined the properties of small silver Agxn+ clusters (2 ≤ x ≤ 4), where x is the number of atoms contained in a cluster, and n is charge of the cluster. Following refs 105−109, we considered silver clusters with high symmetry [Ag2(D∞h), Ag3+(D3h), and Ag42+(D4h, Td, and D2h)], where n is x − 2. We optimized the silver clusters with symmetrical constraints, as shown in Figure 3. Note that, in all Agxn+ clusters considered, the totally symmetric orbital is doubly occupied. Figure 3 shows that sizes of the Ag2, Ag3+, and Td-Ag42+ clusters are appropriate for being encapsulated in the cavity, but those of the D4h- and D2h-Ag42+ clusters are not. The DFT calculations showed that the Td-Ag42+ cluster is more stable than the D4h-Ag42+ cluster by 18.9 kcal/mol. Note that the D4h-Ag42+ cluster has two imaginary modes, as shown in Figure S2 (Supporting Information): one B1g mode that corresponds to an antisymmetric vibrational mode and one B2u mode that corresponds to an out-of plane bending mode. This result indicates that the D4h-Ag42+ cluster is not a local minimum without a symmetry constraint, and then it tends to deform to a stable cluster, such as the Td cluster. Next, we examined the relationships between the cluster structures and their absorption properties by performing timedependent DFT (TD-DFT) calculations. Detailed information regarding their absorptions [excitation energy (Ex in nm), oscillatory strength (f), and assignment of a certain electronic transition] is given in Tables 1−4. The orbitals that contribute to Table 4. Electronic Excitations of a Bare Ag42+ Cluster with Td Symmetrya Exb

fc


269.3 269.3 269.3 192.6 192.6 192.6

0.244 0.244 0.244 0.072 0.072 0.072

34 → 39 (0.16) 35 → 40 (−0.16) 37 → 38 (0.61) 35 → 38 (0.16) 36 → 39 (0.16) 37 → 40 (0.61) 34 → 38 (−0.16) 36 → 40 (−0.16) 37 → 39 (0.61) 23 → 39 (0.46) 25 → 38 (0.46) 30 → 40 (0.15) 24 → 39 (0.46) 25 → 40 (0.46) 29 → 38 (0.13) 23 → 40 (0.46) 24 → 38 (0.46) 29 → 39 (−0.13)

a The optimized geometry is given in Figure 3. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals: HOMO − n, 37 − n; HOMO, 37; LUMO, 38; LUMO + m, 38 + m, where n and m are integers. Detailed orbital information is provided in Figure 5. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

the absorption are shown in Figures 4 and 5, which show that the small silver clusters have frontier orbitals composed of their 5s orbitals. The HOMOs do not have nodes, but the LUMOs have one node. Below the HOMO, there are occupied orbitals that originate from 4d orbitals. Unoccupied orbitals coming from the 5p orbitals exist above the LUMO. On the basis of orbital symmetry arguments on oscillator strength, three types of electronic excitations are possible in the silver clusters: a transition from the completely symmetric 5s-based orbital to a 5s-based orbital with one node, that from 5s-based orbitals to 5pbased orbitals, and that from 4d-based orbitals to 5s-based orbitals. As shown in Figures 4 and 5, the energy levels of the 23878



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Figure 4. Orbitals contributing to the electronic transitions in the (a) D∞h-Ag2 cluster and (b) D3h-Ag3+ cluster. These orbitals were categorized into three subgroups: those originating from the 4d orbitals on silver atoms, those originating from 5s orbitals, and those originating from 5p orbitals. The frontier orbitals come from 5s orbitals, the HOMO is completely symmetric, and the LUMOs have one node. The orbital numbers are given. The orbital energies relative to the HOMO are given in eV.

Figure 5. Orbitals contributing to the electronic transitions in the (a) D4h-Ag42+ cluster and (b) Td-Ag42+ cluster. These orbitals were categorized into three subgroups: those originating from the 4d orbitals on silver atoms, those originating from 5s orbitals, and those originating from 5p orbitals. The frontier orbitals come from 5s orbitals, the HOMO is completely symmetric, and the LUMOs have one node. The orbital numbers are given. The orbital energies relative to the HOMO are given in eV.

orbital region of Ag2−ZSM-5(Aln). When n is equal to zero, the HOMO has bonding interactions between the 5s-based orbitals of the silver atoms, as shown in Figure 7. Due to attractive interactions, the Ag−Ag separation is short. As n increases, the

These changes can be associated with the formal charge of the Ag2 cluster contained inside ZSM-5 with a variable number of Al atoms: the formal charge of the inner cluster is equal to n. Accordingly, the cluster has (2 − n) 5s electrons in the frontier 23879



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Figure 6. Optimized Ag2−ZSM-5(Aln) structures obtained from the B3PW91 calculations: (a) n = 0, (b) n = 1, and (c) n = 2. The optimized Ag−Ag separations are given in Å. Values in parentheses indicate calculated spin densities on silver atoms.

Figure 8. Optimized Ag4−ZSM-5(Aln) structures obtained from the B3PW91 calculations: (a) n = 1, (b) n = 2, and (c) n = 3. There are two types of Ag4 clusters inside a ZSM-5(Aln) cavity: (i) one type is clusters distorted from the Td structure (C3v- and D2d-like clusters when n = 1 and 2, respectively), and (ii) the other type is butterfly clusters distorted from the D4h and D2h structures. The optimized Ag−Ag separations are given in Å. Values in parentheses indicate calculated spin densities on silver atoms.

Figure 7. Frontier orbitals of the optimized structures for (a) Ag2− ZSM-5(Al0) and (b) Ag2−ZSM-5(Al2). The orbital energies relative to the HOMO are given in eV.

ZSM-5(Al3) structure. The DFT findings suggest that the number of Al atoms contained in ZSM-5 zeolite has an impact on modulating magnetic properties of silver-containing zeolites. Comparing Figures 3 and 8 shows that the Ag4 clusters are considerably distorted by being encapsulated in a ZSM-5 cavity, which is in contrast to the cases of Ag2 and Ag3. When n = 1 and 2, there are two types of Ag4 clusters stabilized inside the ZSM-5 cavity. One cluster has a structure distorted from tetragonal symmetry, and the other has a butterfly rhombic structure that consists of two triangles with one shared bond. In clusters distorted from the Td structure, there are two types of distortions depending on n: a C3v-like structure with three elongated Ag−Ag bonds (∼2.9 Å) for n = 1 and a D2d-like structure with two elongated Ag−Ag bonds (∼2.9 Å) for n = 2. These geometrical features are reasonable from arguments based on the Jahn− Teller theorem,116,117 as shown in section S3 (Supporting Information). The other clusters have butterfly structures, which are conceptually created by distortion from the D4h structure via the D2h structure through the two imaginary B1g and B2u modes (Supporting Information Figure S2). We can differentiate the butterfly structures between n = 1 and n = 2 by the dihedral angle (D) between the two connected triangles. Note that the D values indicate a deviation from a planar structure. The D value when n = 1 is 176.5°, and that when n = 2 is 111.9°. These results indicate that Ag4 −ZSM-5(Al 2) has a butterfly structure that is significantly distorted from the D2h structure in comparison to the case of Ag4−ZSM-5(Al1). The cluster distortions resulting from encapsulation in ZSM-5 have a strong impact on the electronic properties of the inner silver clusters. As an example, Figure 9 shows the frontier orbitals of Ag4−ZSM-5(Al2), which are formed by 5s-based orbitals. The orbital energies of a Ag4 cluster inside a ZSM-5 cavity are

attraction resulting from the bonding 5s-orbital interactions is decreased, thereby lengthening the Ag−Ag separation. In fact, a longer Ag−Ag separation was found in the optimized Ag2−ZSM5(Al1) structure where the in-phase 5s-orbital is singly occupied. Note that the Ag2−ZSM-5(Al1) structure has spin densities localized on the silver atoms (Figure 6b), reflecting from the inphase 5s-orbital pattern. In contrast, in Ag2−ZSM-5(Al2) the inphase 5s-orbital is unoccupied. As a result, the attractive orbital interactions do not operate, and thus, the two silver atoms are separate from each other. Instead, the HOMO primarily consists of a 4d-based orbital of a silver atom. The frontier orbitals are responsible for determining the excitation energies of Ag2−ZSM5(Aln), as will be discussed below. II-3-2. Ag4−ZSM-5(Aln). Figure 8 shows the optimized geometries for Ag4−ZSM-5(Aln): (a) n = 1, (b) n = 2, and (c) n = 3. The number of Al atoms (n) is also important for differentiating the Ag4−ZSM-5(Aln) structures because the cluster structures are determined by attractive 5s orbital interactions and by repulsive electrostatic interactions, whose strengths depend on n.76 As shown in Figure 8, there are clear differences in the Ag4−ZSM-5(Al3) structure compared to those of Ag4−ZSM-5(Al1) and Ag4−ZSM-5(Al2) with respect to the Ag−Ag separations. Longer Ag−Ag separations (∼2.9 Å) were found in Ag4−ZSM-5(Al3) than in those in which n = 1 and 2. In Ag4−ZSM-5(Al3), repulsive electrostatic interactions between silver cations would overcome the attractive orbital interactions that originated from the 5s-based orbitals. In contrast to Ag4−ZSM-5(Al3), Ag4 clusters were formed in Ag4−ZSM-5(Al1) and Ag4−ZSM-5(Al2). In Ag4−ZSM-5(Al1) structures, we found spin densities localized on the silver atoms whose distributions are quite different from that in the Ag4− 23880



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(LUMO + 2), which lies above the LUMO + 1 by 0.43 eV, is associated with the b2g orbital in the D4h cluster. A detailed discussion on the relationships between the cluster structures and their frontier orbitals is provided in section S2 (Supporting Information). These unoccupied orbitals are important for determining the Ag4−ZSM-5(Aln) structures in higher spin states, whose structures are shown in Figure S4 (Supporting Information). II-4. Electronic Transitions of Silver-Containing ZSM-5 Zeolites. In the final subsection, we focus on the electronic transitions of Agx−ZSM-5(Aln) through the use of TD-DFT calculations. A considerable amount of memory was required for the TD-DFT calculations of whole Agx−ZSM-5(Aln) models, and therefore, there are limitations in obtaining the number of states generated from the electronic transitions. In fact, we obtained only three excitation states in the whole-model calculations, whose absorption information is tabulated in Table S5 (Supporting Information). Unfortunately, the calculated Ex ranges cannot cover those obtained in the spectra of Figure 1. To overcome the limitations in the TD-DFT calculations, we created two types of smaller models by removing some atoms from the Agx−AlnSi92−nO151H66 (whole model); one is Agx−AlnSi39−nO48H60, and the other is Agx−AlnSi29−nO36H44, as shown in Figure S5 (Supporting Information). Note that the atomic coordinates of the smaller models are the same as those of the remaining atoms in the whole model, and thus, the structures of the silver clusters and their coordination environments remain unchanged. Information regarding the electronic transitions obtained from the smaller models is also given in section S5 (Supporting Information). Roughly speaking, the calculated Ex values in the whole model can be reproduced by the smaller models, as shown in Supporting Information Figure S2. Furthermore, TD-DFT calculations using the smallest model yield Ex ranges that are capable of covering the experimental Ex values. Thus, we will discuss the absorption behaviors of silvercontaining zeolites using the smallest model, whose information is provided in Tables 5−8. Figure 10i shows the electronic transitions in the truncated Ag3−ZSM-5(Al1) and Ag4−ZSM-5(Al2) models, and Figure S6(a) (Supporting Information) shows the transitions in the truncated Ag2−ZSM-5(Al0) model. As shown in Tables 7 and 8,

Figure 9. Frontier orbitals of the optimized Ag4−ZSM-5(Al2) structures with (a) the D2d-like cluster and (b) the butterfly cluster. In both cases, the frontier orbitals consist of 5s orbitals on the contained silver atoms. The orbital energies relative to the HOMO are given in eV.

significantly changed from those in the corresponding bare cluster. For example, in Ag4−ZSM-5(Al2) with the D2d-like cluster, there are no triply degenerate orbitals that can be observed in the Td-Ag42+ cluster. The LUMO, LUMO + 1, and LUMO + 2, which are formed by raising the t2 orbitals, lie above the HOMO by 4.42, 5.00, and 5.25 eV, respectively. Similarly, the degenerate eu orbitals in the D4h-Ag42+ cluster are split as a result of the encapsulation in a ZSM-5(Al2) cavity in which the cluster transforms into the butterfly cluster. The LUMO and LUMO + 1, which originate from the eu orbitals, are 4.20 and 5.04 eV less stable than the HOMO, respectively. Another unoccupied orbital

Table 5. Selected Information on Electronic Excitations of Truncated Ag2−ZSM-5(Al0) Modela Exb Silver Cluster inside ZSM-5 405.2 378.2 341.7 301.6 299.7 Silver Cluster Taken from Cavity 404.0 259.5 259.5

fc


0.223 0.096 0.068 0.088 0.158

388 → 389 (0.70) 388 → 390 (0.70) 388 → 392 (0.66) 388 → 396 (0.56) 388 → 397 (0.15) 388 → 398 (0.23) 388 → 396 (−0.19) 388 → 397 (0.65)

0.423 0.479 0.479

9 → 20 (−0.10) 19 → 20 (0.58) 16 → 20 (0.21) 17 → 20 (0.15) 19 → 21 (0.48) 19 → 22 (0.35) 16 → 20 (−0.15) 17 → 20 (0.21) 19 → 21 (−0.35) 19 → 22 (0.48)

a

The optimized geometry is given in Figure 6. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals in Ag2−ZSM-5(Al0): HOMO, 388; LUMO, 389; and LUMO + n, 389 + n, where n and m are integers. Detailed information is provided in section S7(a) (Supporting Information), and the contributing orbitals are given in Supporting Information Figure S7-a, where only orbitals whose amplitudes are mainly distributed on silver clusters are given. In the cluster taken from the cavity, the labels of the frontier orbitals are HOMO − n, 19 − n; HOMO, 19; LUMO, 20; LUMO + m, 20 + m, where n and m are integers. Because the cluster taken from cavity is similar to the corresponding bare cluster, the orbital numbers are the same as those in the bare cluster in Figure 4. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations. 23881



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Table 6. Selected Information on Electronic Excitations of Truncated Ag3−ZSM-5(Al1) Modela Exb

fc

Silver Cluster inside ZSM-5 303.4 282.7 260.0 257.1 Silver Cluster Taken from Cavity 317.1 315.6 216.3 202.5 188.3 188.1


0.309 0.318 0.078 0.144

397 → 398 (0.69) 397 → 399 (0.68) 397 → 400 (0.68) 397 → 401 (0.68)

0.313 0.303 0.312 0.203 0.090 0.092

27 → 30 (−0.14) 28 → 29 (0.61) 27 → 29 (0.14) 28 → 30 (0.61) 18 → 29 (0.27) 19 → 30 (−0.24) 28 → 31 (0.56) 18 → 29 (−0.39) 19 → 30 (0.43) 28 → 31 (0.31) 14 → 29 (−0.35) 15 → 30 (−0.32) 28 → 33 (0.44) 14 → 30 (0.41) 15 → 29 (−0.29) 24 → 31 (0.12) 28 → 34 (−0.39)

a

The optimized geometry is given in Figure 2. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals in Ag3−ZSM-5(Al1): HOMO, 397; LUMO, 398; LUMO + 1, 399; LUMO + 3, 401. Detailed information is provided in section S7(b) (Supporting Information), and the contributing orbitals are given in Supporting Information Figure S7-b, where only orbitals whose amplitudes are mainly distributed on silver clusters are given. The orbitals of 397, 398, and 399 come from 5s orbitals on contained silver atoms and that of 401 comes from 5p orbitals on the silver atoms. In the cluster taken from the cavity, the labels of the frontier orbitals are HOMO − n, 28 − n; HOMO, 28; LUMO, 29; LUMO + m, 29 + m, where n and m are integers. Because the cluster taken from cavity is similar to the corresponding bare cluster, the orbital numbers are the same as those in the bare cluster in Figure 4. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

Table 7. Selected Information on Electronic Excitations of Truncated Ag4−ZSM-5(Al2) Model with the Butterfly Clustera Exb Silver Cluster inside ZSM-5 324.5 276.3 258.1 247.5 Silver Cluster Taken from Cavity 320.1 295.2 262.6 245.4 245.3

fc


0.285 0.146 0.157 0.130

406 → 407 (0.69) 406 → 408 (0.58) 406 → 409 (0.51) 399 → 407 (−0.22) 403 → 408 (−0.27) 404 → 408 (0.33) 405 → 408 (−0.20) 406 → 409 (0.28)

0.286 0.131 0.119 0.053 0.070

33 → 38 (−0.14) 36 → 39 (−0.12) 37 → 38 (0.60) 36 → 38 (0.45) 37 → 39 (0.51) 28 → 38 (0.22) 36 → 38 (0.48) 37 → 39 (−0.33) 25 → 38 (0.18) 26 → 38 (−0.25) 28 → 38 (0.51) 35 → 39 (0.13) 37 → 40 (0.24) 25 → 38 (−0.13) 26 → 38 (−0.36) 28 → 38 (−0.34) 35 → 39 (0.21) 37 → 40 (0.35)

a

The optimized geometry is given in Figure 8. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals in Ag4−ZSM-5(Al2): HOMO − n, 406 − n; HOMO, 406; LUMO, 407; LUMO + 1, 408; LUMO + 2, 409. Detailed information is provided in section S7(c) (Supporting Information), and the contributing orbitals are given in Supporting Information Figure S7-c-1, where only orbitals whose amplitudes are mainly distributed on silver clusters are given. The orbitals of 406−409 come from 5s orbitals on contained silver atoms, and those in the other listed orbitals comes from 4d orbitals on the silver atoms plus orbitals on the zeolite framework as minor contributions. In the cluster taken from the cavity, the labels of the frontier orbitals are HOMO − n, 37 − n; HOMO, 37; LUMO, 38; LUMO + m, 38 + m, where n and m are integers. Detailed orbital information is provided in Supporting Information Figure S7-c-2. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

see significant differences between parts i and ii in Figure 10 but less significant differences between parts ii and iii in Figure 10, the interactions between a cluster and the ZSM-5 framework play a crucial role in determining the excitation energies; otherwise, the cluster distortion dominates the excitation energies. Figure 10 shows that the factors that are important for determining the Ex values are dependent on the size of the cluster relative to the size of the ZSM-5 cavity. The butterfly Ag4 cluster is large relative to the ZSM-5 cavity, and therefore, the ZSM5(Al2) encapsulation causes a significant distortion of the cluster from the D4h cluster, resulting in a significant change in the frontier orbital energies. The formation of the butterfly cluster in a ZSM-5 cavity results in three fingerprint absorptions at 324.5, 276.3, and 258.1 nm (Table 7),119 which is in contrast to the bare D4h Ag42+ cluster that possesses one fingerprint absorption at 352.1 nm (Table 3). Similar three fingerprint absorptions can be

the oscillatory strengths of the butterfly Ag4 structure inside the ZSM-5(Al2) cavity are larger than those of the encapsulated D2dlike cluster. The results indicate that the encapsulated butterfly cluster is the main source of UV−vis peaks rather than the D2dlike structure, although the two clusters lie close in energy.118 Accordingly, we concentrate our discussion on the Ag4−ZSM5(Al2) structure with the butterfly cluster. For comparison, Figure 10 also displays the electronic excitations of silver clusters taken from the optimized Agx−ZSM-5(Aln) geometries (Figure 10ii) and those of bare symmetric clusters (Figure 10iii). A comparison of Figure 10, parts i−iii, shows that the ZSM-5 encapsulation has two factors for determining the excitation energies (Ex values) in an encapsulated silver cluster. The first factor is distortion of a cluster structure as a result of the ZSM-5 encapsulation, and the other factor is interactions of a silver cluster with oxygen atoms of the ZSM-5 framework. When we 23882



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Table 8. Selected Information on Electronic Excitations of Truncated Ag4−ZSM-5(Al2) Model with the D2d-like Clustera Exb Silver Cluster inside ZSM-5 313.2 281.5 275.9 249.0 245.2 Silver Cluster Taken from Cavity 275.6 273.3 253.5 205.0

fc


0.119 0.130 0.093 0.069 0.093

406 → 407 (0.69) 403 → 407 (0.24) 406 → 408 (0.46) 401 → 407 (0.48) 406 → 408 (−0.36) 401 → 408 (0.40) 406 → 409 (−0.32) 401 → 408 (0.30) 403 → 408 (0.21) 406 → 409 (0.31)

0.237 0.224 0.184 0.069

36 → 39 (−0.20) 37 → 38 (0.60) 36 → 38 (0.24) 37 → 39 (0.60) 34 → 38 (0.21) 35 → 38 (0.13) 35 → 39 (0.19) 37 → 40 (0.58) 25 → 38 (0.49) 34 → 40 (0.27)

a The optimized geometry is given in Figure 8. bEx (nm), excitation energy. cf; oscillatory strength. dElectronic transition: combination of occupied and unoccupied orbitals contributing to a certain electronic transition. Labels of the frontier orbitals in Ag4−ZSM-5(Al2): HOMO − n, 406 − n; HOMO, 406; LUMO, 407; LUMO + 1, 408; LUMO + 2, 409. Detailed information is provided in section S7(d) (Supporting Information), as well as contributing orbitals are given in Supporting Information Figure S7-d-1, where only orbitals whose amplitudes are mainly distributed on silver clusters are given. The orbitals of 406−409 come from 5s orbitals on contained silver atoms, and those in the other listed orbitals come from 4d orbitals on the silver atoms plus orbitals on the zeolite framework. In the cluster taken from the cavity, the labels of the frontier orbitals are HOMO − n, 37 − n; HOMO, 37; LUMO, 38; LUMO + m, 38 + m, where n and m are integers. Detailed orbital information is provided in Supporting Information Figure S7-d-2. The values in parentheses indicate a CI coefficient in a certain excited configuration, as obtained from TD-DFT calculations.

Figure 10. Electronic transitions of truncated Agx−ZSM-5(Aln) structures obtained from time-dependent DFT (TD-DFT) calculations with the B3PW91 functional: (a) Ag3−ZSM-5(Al1), (b) Ag4−ZSM-5(Al2) with the butterfly cluster, and (c) Ag4−ZSM-5(Al2) with the D2d-like cluster. In part i, the oscillatory strengths ( f) of Agx−ZSM-5(Aln) are plotted as a function of the excitation energies (Ex in nm). The electronic transitions of silver clusters taken from the optimized Agx−ZSM-5(Aln) structures are given in part ii, and those of the corresponding bare symmetric clusters (Figure 3) are given in part iii. The blue dots indicate fingerprint absorptions originating from electronic transitions between 5s-based orbitals, from the completely symmetric orbital to an orbital with one node, and the red and black dots indicate absorptions originating from 4d → 5s and 5s → 5p transitions.

In contrast, the ZSM-5 encapsulation does not induce substantial distortions of the Ag2,120 Ag3, and Td-Ag4 clusters because these clusters are small relative to the yellow circle in Figure 2. In fact, similar cluster structures with and without ZSM-

obtained from the butterfly cluster taken from the optimized Ag4−ZSM-5(Al2). Thus, the cluster distortion resulting from the ZSM-5 encapsulation is important for determining the excitation energies. 23883



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approaches are useful for elucidating the structures of the catalytic active sites of Ag−ZSM-5 and for understanding the mechanisms for their catalytic reactions, such as C−H bond activation in hydrocarbons.

5 environments were found in these cases. Consequently, we observed similarities between parts ii and iii in Figure 10. In this case, interactions of a smaller cluster with framework oxygen atoms are critical for differentiating the Ex values from the bare cluster cases. For example, Ag3−ZSM-5(Al1) exhibits two fingerprint absorption peaks at 303.4121 and 282.7 nm (Table 6). Although the absorption peak position around 282.7 nm slightly depends on the sizes of the models,122 the Ex values in Ag3−ZSM-5(Al1)123 are different from that in the bare Ag3+ cluster (318.9 nm, Table 2) and that in the cluster taken from the optimized Ag3−ZSM-5(Al1) (317.1 nm, Table 6). Finally, we compared the experimental and computational results in terms of the Ex values for the electronic excitations that originated from silver clusters inside a ZSM-5 cavity (Figures 1 and 10). We found good agreements between the experimental and computational results in terms of the fingerprint absorptions at approximately 303 and 290 nm from the encapsulated Ag3 cluster and in terms of those at approximately 320 and 255 nm from the encapsulated butterfly Ag4 cluster. The agreements suggest that Ag−ZSM-5 contains triangular Ag3 and butterfly Ag4 clusters inside the 10-membered ring cavity. The atomistic information was only obtained through the combination of experimental and computational approaches. Therefore, the multidisciplinary approaches are powerful tools for elucidating the states of silver ions inside a 10-membered cavity of ZSM-5.

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ASSOCIATED CONTENT

* Supporting Information S

Model of ZSM-5 zeolite (S1); orbital changes of deformation of the D4h Ag42+ cluster (S2); Jahn−Teller distortion of Td cluster (S3); higher spin states of Ag4−ZSM-5(Aln) (S4); TD-DFT calculations of Agx−ZSM-5(Aln) dependent on model size (S5); electronic transitions of Ag2−ZSM-5(Al0) with the linear cluster (S6); assignment of electronic transitions of Agx−ZSM-5(Aln) (S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 81-75-724-7571. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Petr. Nachtigall at Charles University (Czech Republic) for valuable discussions on the behaviors of silver species inside a ZSM-5 cavity. T.Y., T.W., and H.K. thank Prof. Kohei Kadono at the Kyoto Institute of Technology for informative discussions on the properties of silver clusters embedded in a glass matrix. The project was partially supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JSPS) (T.Y. at Kyoto Institute of Technology) (No. 26790001). At Okayama University, financial support was provided by Japan Society of Promotion Science (Grants-in-Aid for Scientific Research No. 21655021). XAFS measurements were performed under the proposal Nos. 2011G538 and 2012G597 of the Photon Factory Advisory Committee. A.O. and H.T. each acknowledge financial support from the Japan Society for the Promotion of Science (Research Fellowship for Young Scientists, DC1).

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CONCLUSIONS We obtained new information on the states of silver cations inside a ZSM-5 cavity (Ag−ZSM-5) using a combination of experimental and computational approaches. During the multidisciplinary analyses, we performed UV−vis DR and X-ray absorption fine structure (XAFS) measurements of Ag−ZSM-5 and density functional theory (DFT) calculations using a model large enough to represent a 10-membered ring of ZSM-5. According to the extended XAFS (EXAFS) analyses, Ag−ZSM5, which was generated from H−ZSM-5 using the conventional ion-exchange method in a aqueous silver nitrate solution, contains small clusters with Ag−Ag separations of approximately 2.6 Å. The encapsulated clusters present distinct UV−vis bands centered at approximately 255, 287, and 331 nm. During the heat treatment of Ag−ZSM-5, new UV−vis bands appeared at approximately 303 and 319 nm. With respect to the structural and electronic features, the experimental results are well-reproduced by the DFT-optimized Ag−ZSM-5 structures that contain triangular Ag3 or butterfly Ag4 clusters. The optimized Ag−Ag separations range from 2.7 to 2.9 Å. To better interpret the UV−vis spectra, we performed timedependent DFT calculations to elucidate the electronic excitations of the Ag3 and Ag4 clusters inside a 10-membered ring of the ZSM-5 zeolite. With respect to the calculated electronic excitations, the encapsulated Ag3 and Ag4 clusters have fingerprint absorptions due to transitions from a completely symmetric 5s-based orbital to a 5s-based orbital with one node. The fingerprint absorptions are strongly influenced by the encapsulations because the encapsulations induce cluster distortion and result in interactions between clusters and the framework oxygen atoms. Regarding their excitation energies (Ex), a mixture of triangular Ag3 clusters (Ex = 303 and 290 nm) and butterfly Ag4 clusters (Ex = 320 and 255 nm) can appropriately explain the UV−vis peak positions. Therefore, the combination of experimental and computational approaches demonstrated that triangular Ag3 or butterfly Ag4 clusters exist inside a ZSM-5 cavity. The findings obtained from the combined

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(111) Yumura, T.; Kertesz, M. Cooperative Behaviors in Carbene Additions through Local Modifications of Nanotube Surfaces. Chem. Mater. 2007, 19, 1028−1034. (112) Yumura, T.; Kertesz, M.; Iijima, S. Confinement Effects on SitePreferences for Cycloadditions into Carbon Nanotubes. Chem. Phys. Lett. 2007, 444, 155−160. (113) Yumura, T. Chemically Reactive Species Remain Alive inside Carbon Nanotubes: A Density Functional Theory Study. Phys. Chem. Chem. Phys. 2011, 13, 337−346. (114) Yamashita, H.; Yumura, T. The Role of Weak Bonding in Determining the Structure of Thiophene Oligomers inside Carbon Nanotubes. J. Phys. Chem. C 2012, 116, 9681−9690. (115) Yumura, T.; Yamashita, H. Modulating the Electronic Properties of Multimeric Thiophene Oligomers by Utilizing Carbon Nanotube Confinement. J. Phys. Chem. C 2014, 118, 5510−5522. (116) Jahn, H. A.; Teller, E. Stability of Polyatomic Molecules in Degenerate Electronic States IOrbital Degeneracy. Proc. R. Soc. London, Ser. A 1937, 161, 220−235. (117) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons, Inc.: New York, 1985. (118) The energy difference in Ag4-ZSM-5(Al2) between the butterfly and D2d-like clusters is 0.3 kcal/mol. (119) In addition to the fingerprint electronic transitions, electronic transitions from the 4d-based orbitals to 5s-based orbitals were calculated at 271.9, 266.2, and 247.5 nm, whose Ex values are also quite different from those in the corresponding bare cluster. See Supporting Information Table S7-c-1. (120) In Ag2-ZSM-5(Al0), there is one fingerprint absorption between the 5s-based orbitals at 417.7 nm, from the symmetric orbital to the antisymmetric orbital. Additionally, B3PW91 calculations revealed an electronic excitation from the 4d-based orbitals to 5s-based orbitals at 294.5 nm. (121) The Ex value at 303.4 nm is not sensitive to the size of the ZSM-5 models, as shown in Table S5-2-a (Supporting Information). (122) The second largest Ex value depends on the size of the ZSM-5 models. Table S5-2-a (Supporting Information) shows 295.3 nm for the whole Ag3-ZSM-5(Al1) model, 289.6 nm for the medium model, and 282.7 nm for the small model. (123) Other important transitions, the 4d→5s transitions, were observed at 224.4, 240.1, and 257.1 nm. See Supporting Information Table S7-b-1.

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