Hydrogen isotope dynamic effects on partially reduced paramagnetic six-atom Ag clusters in low-symmetry cage of zeolite A

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

Contents lists available at ScienceDirect

HOSTED BY

Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi

Original Research

Hydrogen isotope dynamic effects on partially reduced paramagnetic sixatom Ag clusters in low-symmetry cage of zeolite A Amgalanbaatar Baldansuren1 Photon Science Institute, EPSRC National EPR Facility, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom

A R T I C L E I N F O

A BS T RAC T

Keywords: Reduced Ag clusters H/D isotope exchange and desorption EPR HYSCORE Zeolite A

A well-defined, monodisperse Ag+6 cluster was prepared by mild chemical treatments including aqueous ionexchange, dehydration, oxygen calcination at 673 K and hydrogen reduction 293 K, rather than autoreduction and irradiations with γ-ray and X-ray. H2 reduction was proved as a crucial step to form the nanosize cluster with six equivalent silver atoms. Hydrogen isotope exchange and dynamics were probed by EPR and HYSCORE to provide information relevant to the cluster geometry, size, charge state and spin state. Desorption experiments result in the deuterium desorption energy of 0.78 eV from the cluster, exceeding the experimental value of 0.38 eV for the single crystal Ag(111) surface. These experiments indicate that the EPR-active clusters are in delicate equilibrium with EPR-silent clusters.

1. Introduction The ultimate aim of the modern cluster science is the development of cluster systems in the nanometer range, exhibiting well-controlled properties suitable for particular applications. To achieve the particular task successfully, it requires the development of effective physical and chemical methods to synthesize the cluster systems with a great stability and homogeneity of size and shape distributions. The use of the micro-porous zeolite supports meets the requirements for obtaining such metal clusters with controlled size. The size of metal clusters is constrained along one or more dimensions of zeolite supports. With a few exceptions, such constraints usually render significant changes in the physical, magnetic, and catalytic properties of the clusters. The zeolite cages provide a practical means of preventing the cluster cohesion, because small metal clusters have a strong tendency to form larger particles (d > 10 nm ) driven by surface energy minimization. In addition, the chemical methods are totally sufficient and are even not very complicated to prepare small metal clusters in the pores of the zeolite supports [1]. Zeolite supported metal clusters feature prominently as catalysts in different branches of chemistry. In nanometer range, the reduced Ag clusters are often paramagnetic [2–11] and appear to provide a bridge between the limits of the isolated atom and the bulk. The hyperfine spectra of these clusters were usually very complicated to interpret due to the coexistence of many different structures. The complication stems from the fact that an alternative reduction using irradiation with X- and γ-rays created many

1

defect centers in the support framework, which lead to difficulties in characterizing the reduced Ag clusters as a single small species. Furthermore, such clusters had a limited lifetime of only a few hours under isolated conditions from initial in-situ reductions [7–11]. Therefore, these particular disadvantages imposed the restrictions on a better understanding of the physical, magnetic and chemical properties to date. These motivations are still fundamental and a main driving force to study a formation and particular properties of reduced Ag clusters in the pores of zeolite A, better known as NaA. It was successful to prepare the single, well defined, paramagnetic atomic Ag0, Ag3n+ , Ag n+4 and Ag+6 clusters by “mild chemical treatments” including an aqueous ion-exchange, oxygen calcination and hydrogen reduction in Ag/NaA zeolite with different metal loadings [12–18]. Only Ag atoms exhibit hyperfine anisotropy alone, while Ag3n+ , Ag n+ and Ag+6 clusters are isotropic, thus demonstrating that all the 4 silver atoms are close to equivalent at the cluster surface. These reduced clusters are completely stable in a broad range of temperatures, especially the reduced Ag+6 cluster is spectroscopically observable up to 298 K. This is considered as a sigificant progress in a research field of nanoscale silver clusters and a step toward a complete understanding of unprecedented physical, electronic, magnetic and chemical properties, which fundamentally differ from the bulk Ag. Hydrogen about silver cluster surfaces is of great interest and investigated extensively because of its basic relevance to understanding of a formation, paramagnetism, and elementary steps of catalytic activities for gas storage and adsorption. For example, the hydrogen reduced Ag+6

E-mail address: [email protected]. Pervious address: Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.

http://dx.doi.org/10.1016/j.pnsc.2016.11.004 Received 2 March 2016; Received in revised form 10 November 2016; Accepted 10 November 2016 1002-0071/ © 2016 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Baldansuren, A., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.11.004

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

A. Baldansuren

cluster proved enhanced catalytic activities against some molecular gas adsorbates, especially C2H4 and NO [14–17]. Electron paramagnetic resonance (EPR) is a spectroscopic method for determining the structure, dynamics, and the spatial distribution of paramagnetic species. Such species possess at least one unpaired electron are often expected chemically reactive. The unpaired electrons lead to a non-vanishing spin of a particle which can be used as a spectroscopic probe. The transitions between electron spin states can be induced by on-resonant electromagnetic radiation, which is chemically nondestructive, and the energies of the electron spin states depend on a number of structure related parameters. Therefore, EPR is used to provide an unambiguous determination of the existence of mono-disperse Ag clusters supported on Ag/NaA. Furthermore, the modern pulse spectroscopies have the added advantage of being a powerful tool to investigate the hidden hyperfine interactions of weakly coupled nuclei with the unpaired electrons depending upon a choice of the electron spin-echo detection method [19].

temperatures for different time intervals during the evacuation, and then the spectra were collected by X-band EPR at 20 K. The Ag+6 cluster containing hydrogen reduced 12% (wt.) Ag/NaA zeolite sample was exposed to 200 mbar of 16O2 (Westfalen AG, 99.995%) at room temperature. The sample was kept under 16O2 for 1 h, and the whole procedure of adsorption was repeated six times. However, the spectrum was taken separately after evacuating gas residues by each of adsorption on the sample. 250 mbar of 17O2 (Westfalen AG, 99.7%) was adsorbed separately on the Ag+6 cluster in the H2 reduced Ag/NaA sample in the quartz tube, and the sample was kept under a partial pressure of gas at room temperature for 1 h and then evacuated for 30 min. Preparations were also performed with zeolite Y (CU Chemie Uetikon AG, Si/Al=2.7), and all treatments were performed in the same way as for Ag/NaA prepared for the purpose of paramagnetic Ag clusters.

2. Experimental

3. Results and discussion

Zeolite A (Si/Al=1) zeolite was purchased from CU Chemie Uetikon AG in Switzerland. Zeolite samples were heated up in air at a rate of 0.5 K min−1 to 773 K where they were kept for 14 h in order to burn off any organic impurities. Subsequently, 7 g of the heated sample was washed by stirring in 150 ml bi-distilled water containing 40 ml NaCl (10%) solution and 2.76 g of Na 2S 2O3·5 H2O salt. The washing processes were repeated at least nine times. The washed sample was dried in air at 353 K for 24 h. Ag/NaA samples were prepared in a flask containing 2.25 g of pretreated zeolite by aqueous ion-exchange with 50 ml 50 mM AgNO3 solution (ChemPur GmbH in Germany, 99.998%) by stirring at 343 K in the dark for 24 h. The ion-exchanged sample was filtered and rinsed with deionized water several times, and dried in air at 353 K overnight. Chemical analysis by atomic absorption spectroscopy (AAS) demonstrated that the ion-exchange reaction leads to a silver loading of ca. 12% (wt.). A silver loading of 9% and 6% was also prepared separately. Oxidation was performed under a gas stream of O2 (Westfalen AG in Germany, 99.999%) with a flow rate of 17 ml min−1 g−1 from room temperature up to 673 K using a heating rate of 1.25 K min−1 where it was kept for an additional hour. While the sample was held at the final temperature, the residual O2 gas in the reactor was purged by N2 (Westfalen AG, 99.999%) gas for 1 h. Subsequently, the sample was sealed and kept at 673 K overnight. After cooling the sample, reduction was performed in a flow of H2 gas (Westfalen AG, 99.999%, 16 ml min−1 g−1) at room temperature or below for 20 min, which leads to the stabilization of the paramagnetic Ag+6 cluster. Alternatively, D2 (Westfalen AG, 99.0%) reduction was carried out under a static gas pressure of 500 mbar. The reduced sample was transferred into EPR quartz tubes (outer diameter about 4 mm) under nitrogen or argon gas in a glove box. The tubes were sealed with stopcocks for vacuum treatment and gas admission. The sample containing tube was evacuated for 30 min prior to each EPR measurement. The prepared sample can be handled in daylight because this is not photo/light sensitive. For hydrogen isotope exchange, the deuterium gas was filled into an evacuated EPR quartz tube containing about 120 mg of hydrogen reduced 12% (wt.) Ag/NaA sample. The gas was kept for 20 min at a D2 partial pressure of 500 mbar at room temperature. After H/D exchange, the residual D2 gas was pumped off and the sample tube was sealed for measurements. D2 desorption experiments were preformed using a turbo-molecular pump apparatus at temperatures of 383, 408 and 423 K. An EPR quartz tube containing 120 mg deuterium reduced 12% (wt.) Ag/NaA powder sample was connected by a glass adapter valve to the turbomolecular pump apparatus. The vacuum system can achieve pressures as low as 10−5 − 10−6 mbar. The sample was heated to different

At the beginning of this research, the synthesis of reduced silver clusters in Ag/NaY (Si/Al=2.7) was attempted to form stable paramagnetic species. A hyperfine structure of paramagnetic silver clusters was not observed, only the signal observed at g ≈ 2.00 was a superposition of the two axial g species instead. The numerical spectrum simulation with corresponding g values is displayed in Fig. 1a. It is apparent that the axial g signals are assigned neither to silver clusters nor to silver atomic species. To conclude that these signals are due to a specific site and/or defect center within the framework of Ag/NaY zeolite. In Ag/NaY, atomic Ag species normally exhibit well-resolved two doublets in the hyperfine splitting due to the silver isotopes 107Ag and 109 Ag with the large isotropic hyperfine coupling constant of aiso = 590 − 700 G [20–23]. However, the hyperfine spectra of those atomic species are dissimilar to the current anisotropic signal. A conduction electron signal of metallic silver particles was also observed in Ag/NaY, exhibiting the EPR parameters of g=1.960 and ΔHpp = 70 G [24]. Such a singlet spectrum arises from the relatively larger particles with size of ≈5 nm [7]. This suggests that a formation of paramagnetic Ag clusters is sensitive to the inner symmetry of the zeolite cage, and the stabilized clusters will exhibit molecular character rather than metallic character that is only due to conduction electrons [25]. In Ag/NaA (Si/Al=1), the hyperfine spectrum of the paramagnetic Ag+6 cluster appears only after performing appropriate dehydration, oxygen calcination and nitrogen purging at 673 K, and hydrogen reduction at 278 or 298 K. The seven-line splitting with giso ≈ 2.028 Ag ≈ 67 G arises from the isotropic coupling of the unpaired and aiso electron with six equivalent silver nuclei (Fig. 1b). The isotropic giso deviates positively from the free electron ge value, indicating admixture a

b

Ag (g ~ 2.028)

2.061

**

2.009

2.090

3000

3200

3400

Magnetic Field [G]

3600

3000

3200

3400

3600

3800

Magnetic Field [G]

Fig. 1. a) X-band EPR spectrum of the H2 reduced 12% (wt.) Ag/NaY recorded at 20 K and its numerical simulation (red) constituting the superposition (1:1 weighting), where the signal from one species with g ≈ 2.090 and g⊥ ≈ 2.009 , while g ≈ 2.061 and g⊥ ≈ 2.004 from another species. b) Experimental spectrum of the H2 reduced Ag+ 6 cluster (black) in the 12% (wt.) Ag/NaA stabilized after performing reduction at 278 K for 20 min. Its numerically simulated spectrum (red) is based on the spin-Hamiltonian parameters of 6 equivalent Ag nuclei, taking into account the statistical distribution of Ag isotopes.

2

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

A. Baldansuren

Fig. 2b. When completing desorption, the samples were exposed to D2 gas with 500 mbar for 20 min that restored the spectrum almost completely with regard to the intensity. Similar to the final H/D exchange, there is a baseline drift at ∼3600 G . The hyperfine spectrum shows a rather more asymmetry, and consequently the numerical simulation requires a small anisotropy for both g and A tensors, i.e. g ≈ 2.027; g⊥ ≈ 2.028 and A ≈ 66 G ; A⊥ ≈ 68 G, and the fit with six equivalent nuclei still provides a good match for the intensity distribution of the Ag+6 cluster splitting. These tensors are still coaxial, and a small anisotropy corresponds to a distortion of the electronic wave function due to the cluster proximity to the specific cation coordinating site. It apparently indicates that the silver clusters exist in different sites, for example, the α-cages share 6- and 8-membered rings with the β-cages. When deuterium desorbs more easily from the first few Ag+6 clusters, they subsequently convert into the diamagnetic ones, while the other diamagnetic clusters in different sites convert into the paramagnetic ones, since the overall EPR signal does not disappear by continuous desorption. Therefore, the number of paramagnetic clusters is always calibrated on average due to hydrogen isotope mobility. Importantly, mobile deuterium easily penetrates through the six ring with a small diameter into and/or from the β-cages in comparison to other gas molecules like O2 [30]. Desorption is faster at higher temperatures, and so its activation energy can be determined. This is fully reversible experimentally by readsorption of D2 with 500 mbar at room temperature (Fig. 2b), even though the latter process might not be completely activated, and the activation energy equals the desorption energy. The signal intensity decay curves at different desorption temperatures are displayed as a function of different desorption time intervals in Fig. 3a. Hydrogen isotope desorption supposedly involves a recombination of two atoms, which will give rise to second-order kinetics. Nevertheless, the experimental data fit very well with exponential decay curves, indicating that this is compatible with first-order kinetics so that the rate-determining step is desorption rather than recombination. Using the desorption time constants derived from Fig. 3a, a value of Ea = 0.78 eV

of a transition metal with a more than half-filled d orbital into spin density distribution [26]. The isotropic coupling reveals that the 5 s orbital contribution to the unpaired electron molecular wave function is about 10% per Ag nucleus, indicating a strong spin density delocalization on the cluster surface. For noble metal clusters, the electronic structure is dominated by the number of valence electrons that are delocalized [27]. A spin calibration amounts to a small fraction ∼0.044% of all exchanged silver atoms to carry an unpaired electron so that EPR-active clusters with S = 1/2 are about 0.26% of all clusters. This means that most of silver atoms is EPR-silent, forming diamagnetic and/or high spin clusters. Our extended X-ray absorption fine structure (EXAFS) results showed that the paramagnetic and EPRsilent clusters are about the same atomic size [14]. The overall symmetry is cubic for the unit cell of zeolite A, consisting of eight αcages. The cage possesses an inner symmetry of an octahedron, better known as the sodalite cage or pseudo unit cell [28]. The symmetry of Ag both giso and aiso is reflected on the cluster accommodating β-cage symmetry so that it provides an isotropic surrounding for the electronic wave function after hydrogen reduction [14,15]. On continuous reduction, the hyperfine splitting was distorted with a hint of charge decreases to diamagnetic clusters, but was fully recoverable by a partial pressure of O2 at 298 K [16]. As reported, a conduction electron signal evolves from bigger crystallites with a size of ≥1 nm formed in the αcage when neutral cluster atoms become mobile leaving the β-cage following continuous reductions [7,8]. Hydrogen isotope exchange on the reduced Ag+6 cluster was performed at room temperature by filling D2 with 500 mbar into the cluster sample in the EPR quartz tube. The sample was kept under a deuterium partial pressure for 20 min and evacuated before each measurement at 20 K. The exchange process was repeated for a couple of times, and the signal intensity increased initially (Fig. 2a). This indicates that the other diamagnetic clusters located in different cationic sites of the Si/Al framework are converted into the paramagnetic ones. It was reported that predominant silver clusters in treated zeolite A are not paramagnetic [9,29]. No change was observed in the hyperfine splitting, indicating that there are no other structures with six equivalent Ag nuclei, e.g. a planar hexagon. After the third exchange, the baseline of the first-derivative signal slightly drifts at a high field region of ∼3600 G and then exhibits a small asymmetry of the hyperfine splitting. The signal intensity decreased finally suggests only the partially reduced clusters contribute to the isotropic signal (symmetric) of EPR-active clusters. Furthermore, the experiments were performed on the deuterium desorption from the D2 reduced Ag+6 cluster by evacuating the samples at different elevated temperatures of 383, 403, and 423 K. Desorption is related to the deuterium coverage around the silver cluster because of the relative intensity loss. The signal intensity decreased continuously with desorption times, and the highest temperature led to the fastest desorption. After 180 min at 423 K, the cluster structure is still intact and the corresponding hyperfine spectrum is displayed in

EPR signal intensity [a.u.]

a 2400 2000 1600 1200

100 200 300 400 Integrated desorption time [min]

reads D

6.5

H/D

180'

6.0

2800

3200

3600

Magnetic Field [G]

-1

ln[k/min ]

H/D

55' D

H

423 K

400

b

H/D

403 K

800

7.0 a

383 K

Ea = 0.78 eV for Ag cluster

5.5 5.0

2800

3200

Ea = 0.38 eV for Ag(111)

4.5

3600

Magnetic Field [G]

4.0

Fig. 2. a) X-band EPR spectra of the reduced Ag+ 6 cluster in the 12% (wt.) Ag/NaA

b 2.35

2.40

2.45

2.50 -1

2.55

2.60

2.65

-1

1000 T [K ]

collected following isotope exchange H/D reactions at 298 K. b) EPR spectra of the D2 reduced Ag+ 6 cluster collected under continuous desorption at 423 K for different time

Fig. 3. a) EPR signal intensity decreases as a function of desorption time at different temperatures. At each desorption temperature, the initial intensity is as it is (not normalized). b) Arrhenius plot of the relative rate constants for D2 desorption.

intervals. The signal intensity decreases continuously, while no changes in the hyperfine splitting. All measurements were performed at 20 K.

3

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

A. Baldansuren

(75.3 kJ mol−1) for the deuterium desorption energy is calculated from the slope of the Arrhenius plot shown in Fig. 3b [16]. This value is higher than the experimental value of 0.38 eV (36.4 kJ mol−1) for the desorption from a (111) single crystal surface [31]. Along with a “support effect”, this difference relates to a “finite size effect” of the octahedral Ag+6 cluster with the strongly reduced coordination N ≈ 4.0 of surface atoms, which is less than that of the Ag(111) single crystal surface [14]. Most importantly, the desorption energy value is in a close agreement with an activation energy of 63 kJ mol−1 for the initial rate of the cluster formation following in-situ H2 reduction [10]. This was interpreted that the process is governed by the diffusion of the cations. To compare with deuterium results, hydrogen desorption was also performed on the H2 reduced cluster. However, this simply did not follow the continuous decrease in the signal intensity at different temperatures, especially at 383 and 423 K. Desorption exhibited the oscillation in the EPR intensity upon heating for different time intervals, indicating that a coverage cannot be explained as an uniform binding or contact of hydrogen per cluster. It is known that the support ionicity determines the hydrogen coverage, and a decreasing trend correlates with decreasing electron density on the support oxygen atom. Electron-deficient oxygens exist in acidic supports with protons (H+ ) or other partly covalent cations [32]. No paramagnetic clusters were observed in the presence of Cs+ and Ca2+ [4], consistent with the fact that electron-rich oxygens exist in basic supports with large alkaline cations [32]. Therefore, diamagnetic clusters in the oxygenrich framework sites, most likely in α-cages with a free diameter of 11.4 Å, experience more hydrogen coverage from surroundings. That is the reason that the isotope exchange reaction results in a rising EPR intensity of the Ag+6 cluster at first (Fig. 2a). Either complete or partial desorption would shift a narrow range in which EPR-active clusters are in equilibrium with a reservoir of similar but EPR-silent clusters. In comparison to molecular deuteron, hydrogen has a less accessibility to the cluster in the β-cage, because of its larger H2 kinetic diameter of 0.275 nm at 273 K [9]. The paramagnetic Ag 6n+ cluster is located in the β-cage with 6.6 Å [3,7–10]. A mechanism of the cluster formation following both hydrogen and irradiation reductions has been very debatable [2–11]. What well known so far is the charged and neutral clusters with a nuclearity of 6 - 14 reveal absorption bands from 19600 to 22700 cm−1 (510– 440 nm) of the near UV region [9,11,33]. Later, this yellow color was interpreted as charge transfer from zeolite oxygen lone pairs to Ag+ , denoted as Ag+(5 s ) ← O(n) [34]. A transformation from these diamagnetic clusters to EPR-active ones seems to be dependent on many factors, such as a degree of metal loading, dehydration, reduction and annealing, indicating that different mechanisms are involved in the clustering process. The current Ag+6 cluster is formed in a dehydrated yellow colored Ag/NaA after performing H2 reduction for 20 min [14–18]. Assuming that six hydrogen atoms would make a contact with the one cluster on average since it has six equivalent Ag nuclei. In a simple mechanism, each hydrogen atom would require one electron from the Ag6 cluster orbitals in order to bind the surface. As a consequence, a hole in the Ag 4d shell would imply a cluster charge exceeding +6. This seems impossible that highly charged clusters would be unstable because of the strong electrostatic (Coulomb) repulsions among positive charges. The silica and alumina tetrahedra have a single negative charge due to electron deficiency at the alumina-oxide site, which has to be compensated by a positive charge of the reduced Ag+6 cluster. Therefore, a hole trap has to be hydrogen related as well. The initial stage of the reduced cluster formation is possibly developed by trapping the hole (h+), and a simple mechanism can be described by Scheme (1):

Ag0 species by EPR in 6% Ag/NaA after hydrogen reduction [12]. Thus, the mechanism is a reversible redox reaction of the Ag+6 cluster and has an one-electron nature. Both precursors were previously proposed via disproportionation reaction of 2Ag+ → Ag0 + Ag 2+ in dehydrated Ag/ NaA regarding the silver cation diffusion [9–11]. The 2D electron spin echo envelope modulation (ESEEM) spectra [19], so-called hyperfine sublevel correlation spectra (HYSCORE) [35], were recorded employing the sequence π /2 − τ − π /2 − t1 − π − t2 − π /2 − τ − echo with mw pulse length of tπ /2 = 16 ns and tπ = 32 ns incremented by Δt1,2 = 4 − 8 ns with starting times of t1,2 = 100 ns, and the first two pulse separation time τ = 140 ns. The intensity of the inverted echo following the fourth pulse is measured with t2 and t1 varied and constant τ. Unwanted features from the experimental echo envelopes were removed by using a four-step phase cycle [36]. In both dimensions 512 data points were collected. The relaxation decay was subtracted using baseline corrections (by fitting polynomials of 3–6 degree) in both time domains, subsequently applying apodization (Hamming window) and zero-filling to 1024 data points in both dimensions. After 2D fast Fourier transformation the absolute value spectra were obtained. The spectral resolution is further increased by suppressing the inhomogeneous broadening in the second dimension [19]. HYSCORE spectra are usually presented as either contour or stacked (3D) plots in Matlab [37]. The HYSCORE experiments were first performed for the reduced Ag+6 cluster at 10 K, revealing the intense diagonal peak coincides with the aluminum nuclear Larmor frequency of 27νAl ≈ 3.9 MHz (Fig. 4). This peak represents the framework 27Al nuclei, and the presence of such a “matrix peak” confirms the close proximity between the zeolite framework site (lattice) and the silver cluster. However, a framework Al is not directly coordinated to the Ag+6 cluster since a small isotropic coupling is of the order of ∼2.0 MHz , proportional to a partial spin density population of ∼5.0 × 10−4 on a 27Al nucleus. Furthermore, there were no cross-peaks about 1νH ≈ 14.5 MHz for the proton nuclear Larmor frequency in the weak coupling ( + , + ) quadrant, indicating that no direct spin density is on 1H about the cluster. It suggests that hydrogen is not adsorbed on the cluster surface. On the other hand, a factor influencing the ESEEM intensity of some nuclear transitions in the corresponding electron spin manifold (mS = ± 1/2 ) is their orientation dependence in a powder type (disordered) sample, and so the absence of transition frequency peaks does not rule out the existence of cross-peaks [19]. It is also well known that peaks of nuclei with shallow modulations can be strongly suppressed by nuclei with deep modulations, so-called a cross suppression effect [38]. This effect explains 1H peaks are often very weak or even undetectable in the presence of strong nuclei with I ≥ 1.

Ag+ + e− → Ag0Ag+ + h+( ≡ Si − O − Al ≡− ) → Ag 2+( ≡ Si − O − Al− ≡ )

(1)

Fig. 4. 3D presentation of the X-band HYSCORE spectrum of the H2 reduced Ag+ 6

This is a reasonable assumption since we observed the precursor

cluster in 12% (wt.) Ag/NaA, measured at 3408 G and 10 K. The intense peak from the nuclear Larmor frequency 27νAl ≈ 3.90 MHz of the framework 27Al nuclei.

4

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

A. Baldansuren

molecules cannot enter the β-cage octahedron [30], thereby settling the oxidized clusters entrapped within the bigger α-cage. A size effect of this six-atom cluster lowers an energy barrier of the initial rate of H2 reduction [10]. This is energetically favorable than autoreduction which involves two electron oxidation (activated extraction of O2) of the lattice producing Lewis-acid site [33]. A temperature activated desorption of D2 is of the order of Ea = 0.78 eV in Ag/NaA in comparison to 0.38 eV of single crystal Ag(111) surface. This change reflects on a finite size effect along with a support effect. Hydrogen desorption provides the EPR intensity oscillation rather than a continuous decrease. Thus, deuterium forms thermodynamically stable clusters and desorption shifts a narrow range where EPR-active clusters are equilibrated with a reservoir of similar but EPR-silent clusters. Support ionicity due to the presence of 1H seems to play a crucial role in observing these active clusters overcoming predominant EPRsilent clusters. From HYSCORE results, no direct spin density is on 1H even though reduction plays an important role in balancing spin density delocalization about the Ag+6 cluster. Hydrogen effect is indirectly probed by additional experiments where 17O2 interaction shifts once again spin density distribution around the EPR-active clusters in the low symmetry β-cage.

Fig. 5. 3D presentation of the X-band HYSCORE spectrum of the reduced Ag+ 6 cluster exposed to 17O2 at 298 K, measured at 3410 G and 10 K. The intense peak from the nuclear Larmor frequency 27νAl ≈ 3.90 MHz of the framework 27Al nuclei.

The HYSCORE experiments were extended to the adsorption of O2 on the reduced cluster (Fig. 5). The spectrum exhibits the matrix peak as well, but its linewidth is significantly reduced. It implies that a spin density distribution around the 27Al nucleus is disturbed by this interaction. In this case, oxygen interaction increases the support basicity and decreases the support acidity due to 1H [32]. The tetrahedral aluminum Al(4) in the basic aluminum salt had quite small quadrupole coupling constant (e2qQ / h ) of the order of ∼1 MHz , studied by magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy [39]. The quadrupole coupling can impose a dramatic effect on the mixing of nuclear spin eigenfunctions and consequently on the lineshape, especially when nuclear hyperfine and quadrupole couplings are similar in strength. This is in line with the current Al ≈ 1.0 MHz. The previous results HYSCORE spectrum, revealing aiso 17 showed that the interaction of O2 around the reduced Ag+6 cluster causes an instantaneous anisotropy of the hyperfine splitting at 4 and 7 K, which was not observed with 16O2 [14,16]. This indicates that interacted oxygen molecules are not rigidly adsorbed on the cluster surface. Therefore, this effect arises from the distorted electronic wave function about the Ag+6 cluster at a very low temperature. This is fully reversible, and the spectra gradually become isotropic and symmetric as temperature rises from 10 to 20 K. Nuclei such as 17O2 with I=5/2 possess an electrical quadrupole moment that results from a nonspherical charge distribution. A nonzero electric field gradient is expected at the Ag+6 cluster, assuming that Ag-Ag bonds have dissimilar lengths along molecular axes (oblate or prolate) at a low temperature. A continuous interaction of 16O2 (I=0) only leads to the EPR intensity loss [14,16]. This interaction obviously shifted once again the equilibrium to the EPR-active clusters, perhaps the opposite direction of a redox reaction following H2 reduction. 17

Acknowledgments The author was grateful to the Deutsche Forschungsgemeinschaft was generally acknowledged since it awarded the one large funding for the Research Training Group 448 "Advanced Magnetic Resonance Type Methods in Materials Science" at the University of Stuttgart. This unit then enrolled and supported the doctoral students. The author was thankful to Prof. E. Roduner (emeritus) for his helpful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19]

4. Conclusions No active species of Ag clusters were observed in Ag/NaY after performing appropriate treatments of calcination, dehydration and reduction. It may be concluded that a formation of paramagnetic silver clusters is strongly dependent on the support type. Monodisperse six-atom clusters were formed by hydrogen reduction in 12% (wt.) Ag/NaA. A minority of the exchanged silver ions is to form EPR-active clusters (S = 1/2 ), coexisting with high-spin and diamagnetic states [16]. It relates to three cation coordination and/or exchange sites. In zeolite A, the four-ring Na + is first exchanged for Ag+ and followed by the three eight-ring cations, while the eight six-ring cations are exchanged only at the end [40]. From our EXAFS results, the total coordination is N ≈ 6 which corresponds to the cluster size of 13 ± 1 atoms after O2 calcination at 673 K [14]. It implies that oxygen

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

5

W.M.H. Sachtler, Catal. Today 15 (1992) 419–429. J.R. Morton, K.F. Preston, J. Magn. Reson. 68 (1986) 121–128. J.R. Morton, K.F. Preston, Zeolites 7 (1987) 2–4. T. Wasowicz, J. Michalik, Radiat. Phys. Chem. 37 (1991) 427–432. J. Michalik, M. Zamadics, J. Sadlo, L. Kevan, J. Phys. Chem. 97 (1993) 10440–10444. J. Michalik, N. Azuma, J. Sadlo, L. Kevan, J. Phys. Chem. 99 (1995) 4679–4686. D. Hermerschmidt, R. Haul, Ber. Bunsenges. Phys. Chem. 84 (1980) 902–907. P.J. Grobet, R.A. Schoonheydt, Surf. Sci. 156 (1985) 893–898. J. Michalik, L. Kevan, J. Am. Chem. Soc. 108 (1986) 4247–4253. R.A. Schoonheydt, H. Leeman, J. Phys. Chem. 93 (1989) 2048–2053. R.A. Schoonheydt, J. Phys. Chem. Solids 50 (1989) 523–539. A. Baldansuren, E. Roduner, Chem. Phys. Lett. 473 (2009) 135–137. I. Tkach, A. Baldansuren, E. Kalabukhova, S. Lukin, A. Sitnikov, A. Tsvir, M. Ishenko, Yu. Rosentzweig, E. Roduner, Appl. Magn. Reson. 35 (2008) 95–112. A. Baldansuren, H. Dilger, R.-A. Eichel, J.A. van Bokhoven, E. Roduner, J. Phys. Chem. 113 (2009) 19623–19632. A. Baldansuren, R.-A. Eichel, E. Roduner, Phys. Chem. Chem. Phys. 11 (2009) 6664–6675. A. Baldansuren, Small Ag Clusters Supported on an LTA Zeolite Investigated by CW and Pulse EPR Spectroscopy, XAS and SQUID Magnetometry, Ph.D. Thesis, Univeristy of Stuttgart, Stuttgart, Germany, 2009. A. Baldansuren, arXiv:1510.02648, [cond-mat.mtrl-sci], 2015. A. Baldansuren, arXiv:1504.00893, [cond-mat.mtrl-sci], 2015. S.A. Dikanov, Y.D. Tsevtkov, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy, CRC Press, Boca Raton, USA, 1992. A. Abou-Kaïs, J.C. Vedrine, C. Naccache, J. Chem. Soc. Faraday Trans. 2 (74) (1978) 959–967. N. Narayana, L. Kevan, J. Chem. Phys. 76 (1982) 3999–4005. N. Narayana, L. Kevan, J. Chem. Phys. 83 (1985) 2556–2559. D.R. Brown, L. Kevan, J. Phys. Chem. 90 (1986) 1129–1133. F. Blatter, K.W. Blazey, Z. Phys. D 18 (1991) 427–429. M. Danilczuk, A. Lund, J. Sadlo, H. Yamada, J. Michalik, Spectrochimica Acta Part A 63 (2006) 189–191. K. Dyrek, M. Che, Chem. Rev. 97 (1997) 305–331. W.A. de Heer, Rev. Mod. Phys. 65 (1993) 611–676. Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Amsterdam, The Netherlands, 2001. J. Texter, R. Kellerman, T. Gonsiorowski, J. Chem. Phys. 85 (1986) 637–639. P.A. Jacobs, J.B. Uytterhoeven, H.K. Beyer, J. Chem. Soc., Faraday Trans. 1 (75) (1979) 56–64.

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

A. Baldansuren

241–256. [37] M. Lin, A. Baldansuren, R. Hart, R.I. Samoilova, K.V. Narasimhulu, L.L. Yap, S.K. Choi, P.J. O'Maley, R.B. Gennis, S.A. Dikanov, Biochemistry 51 (2012) 3827–3838. [38] S. Stoll, C. Calle, G. Mitrikas, A. Schweiger, J. Magn. Reson. 177 (2005) 93–101. [39] A.C. Kunwar, A.R. Thompson, H.S. Gutowsky, E. Oldfield, J. Magn. Reson. 60 (1984) 467–472. [40] M. Meyer, C. Leiggener, G. Calzaferri, Chem. Phys. Chem. 6 (2005) 1071–1080.

[31] G. Lee, P.T. Sprunger, D.B. Poker, D.M. Zehner, E.W. lummer, J. Vac. Sci. Technol. A, vol. 12, 994, pp. 2119 –2123. [32] Y. Ji, A.M.J. van der Eerden, V. Koot, P.J. Kooyman, J.D. Meeldijk, B.M. Weckhuysen, D.C. Koningsberger, J. Catal. 234 (2005) 376–384. [33] J. Texter, R. Kellerman, T. Gonsiorowski, J. Phys. Chem. 90 (1986) 2118–2124. [34] R. Seifert, R. Rytz, G. Calzaferri, J. Phys. Chem. A 104 (2000) 7473–7483. [35] P. Höfer, A. Grupp, M. Nebenführ, M. Mehring, Chem. Phys. Lett. 132 (1986) 279. [36] C. Gemperle, G. Aebli, A. Schweiger, R.R. Ernst, J. Magn. Reson. 88 (1990)

6