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

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Progress in Natural Science: Materials International

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Original Research isotope dynamic effects on partially reduced paramagnetic six- atom 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

ARTICLE INFO ABSTRACT

fi + Keywords: A well-de ned, monodisperse Ag6 cluster was prepared by mild chemical treatments including aqueous ion- Reduced Ag clusters exchange, dehydration, calcination at 673 K and hydrogen reduction 293 K, rather than autoreduction H/D isotope exchange and desorption and irradiations with γ-ray and X-ray. H2 reduction was proved as a crucial step to form the nanosize cluster EPR with six equivalent silver atoms. Hydrogen isotope exchange and dynamics were probed by EPR and HYSCORE HYSCORE to provide information relevant to the cluster geometry, size, charge state and spin state. Desorption Zeolite A experiments result in the deuterium desorption 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 defect centers in the support framework, which lead to difficulties in characterizing the reduced Ag clusters as a single small species. The ultimate aim of the modern cluster science is the development Furthermore, such clusters had a limited lifetime of only a few hours of cluster systems in the nanometer range, exhibiting well-controlled under isolated conditions from initial in-situ reductions [7–11]. properties suitable for particular applications. To achieve the particular Therefore, these particular disadvantages imposed the restrictions on task successfully, it requires the development of effective physical and a better understanding of the physical, magnetic and chemical proper- chemical methods to synthesize the cluster systems with a great ties to date. These motivations are still fundamental and a main driving stability and homogeneity of size and shape distributions. The use of force to study a formation and particular properties of reduced Ag the micro-porous zeolite supports meets the requirements for obtain- clusters in the pores of zeolite A, better known as NaA. ing such metal clusters with controlled size. The size of metal clusters is It was successful to prepare the single, well defined, paramagnetic 0 n+ n + “ constrained along one or more dimensions of zeolite supports. With a atomic Ag , Ag3 , Ag+4 and Ag6 clusters by mild chemical treat- few exceptions, such constraints usually render significant changes in ments” including an aqueous ion-exchange, oxygen calcination and the physical, magnetic, and catalytic properties of the clusters. The hydrogen reduction in Ag/NaA zeolite with different metal loadings – fi n+ zeolite cages provide a practical means of preventing the cluster [12 18]. Only Ag atoms exhibit hyper ne anisotropy alone, while Ag3 , n+ + cohesion, because small metal clusters have a strong tendency to form Ag4 and Ag6 clusters are isotropic, thus demonstrating that all the larger particles (d >10nm) driven by surface energy minimization. In silver atoms are close to equivalent at the cluster surface. These addition, the chemical methods are totally sufficient and are even not reduced clusters are completely stable in a broad range of tempera- + very complicated to prepare small metal clusters in the pores of the tures, especially the reduced Ag6 cluster is spectroscopically observable zeolite supports [1]. Zeolite supported metal clusters feature promi- up to 298 K. This is considered as a sigificant progress in a research nently as catalysts in different branches of chemistry. field of nanoscale silver clusters and a step toward a complete under- In nanometer range, the reduced Ag clusters are often paramag- standing of unprecedented physical, electronic, magnetic and chemical netic [2–11] and appear to provide a bridge between the limits of the properties, which fundamentally differ from the bulk Ag. Hydrogen isolated atom and the bulk. The hyperfine spectra of these clusters were about silver cluster surfaces is of great interest and investigated usually very complicated to interpret due to the coexistence of many extensively because of its basic relevance to understanding of a different structures. The complication stems from the fact that an formation, paramagnetism, and elementary steps of catalytic activities γ + alternative reduction using irradiation with X- and -rays created many for storage and adsorption. For example, the hydrogen reduced Ag6

E-mail address: [email protected]. 1 Pervious address: Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, . 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 A. Baldansuren Progress in Natural Science: Materials International (xxxx) xxxx–xxxx cluster proved enhanced catalytic activities against some molecular gas temperatures for different time intervals during the evacuation, and adsorbates, especially C2H4 and NO [14–17]. then the spectra were collected by X-band EPR at 20 K. + Electron paramagnetic resonance (EPR) is a spectroscopic method The Ag6 cluster containing hydrogen reduced 12% (wt.) Ag/NaA 16 for determining the structure, dynamics, and the spatial distribution of zeolite sample was exposed to 200 mbar of O2 (Westfalen AG, 16 paramagnetic species. Such species possess at least one unpaired 99.995%) at room temperature. The sample was kept under O2 for electron are often expected chemically reactive. The unpaired electrons 1 h, and the whole procedure of adsorption was repeated six times. lead to a non-vanishing spin of a particle which can be used as a However, the spectrum was taken separately after evacuating gas spectroscopic probe. The transitions between electron spin states can residues by each of adsorption on the sample. 17 be induced by on-resonant electromagnetic radiation, which is chemi- 250 mbar of O2 (Westfalen AG, 99.7%) was adsorbed separately + cally nondestructive, and the of the electron spin states on the Ag6 cluster in the H2 reduced Ag/NaA sample in the quartz tube, depend on a number of structure related parameters. Therefore, EPR and the sample was kept under a partial pressure of gas at room is used to provide an unambiguous determination of the existence of temperature for 1 h and then evacuated for 30 min. mono-disperse Ag clusters supported on Ag/NaA. Furthermore, the Preparations were also performed with zeolite Y (CU Chemie modern pulse spectroscopies have the added advantage of being a Uetikon AG, Si/Al=2.7), and all treatments were performed in the powerful tool to investigate the hidden hyperfine interactions of weakly same way as for Ag/NaA prepared for the purpose of paramagnetic Ag coupled nuclei with the unpaired electrons depending upon a choice of clusters. the electron spin-echo detection method [19].

2. Experimental 3. Results and discussion

Zeolite A (Si/Al=1) zeolite was purchased from CU Chemie Uetikon At the beginning of this research, the synthesis of reduced silver AG in . Zeolite samples were heated up in air at a rate of clusters in Ag/NaY (Si/Al=2.7) was attempted to form stable para- − fi 0.5 K min 1 to 773 K where they were kept for 14 h in order to burn off magnetic species. A hyper ne structure of paramagnetic silver clusters any organic impurities. Subsequently, 7 g of the heated sample was was not observed, only the signal observed at g ≈2.00 was a super- washed by stirring in 150 ml bi-distilled water containing 40 ml NaCl position of the two axial g species instead. The numerical spectrum 2 simulation with corresponding g values is displayed in Fig. 1a. It is (10%) solution and 2.76 g of NaO·5H2S 32O salt. The washing processes were repeated at least nine times. The washed sample was dried in air apparent that the axial g signals are assigned neither to silver clusters at 353 K for 24 h. nor to silver atomic species. To conclude that these signals are due to a Ag/NaA samples were prepared in a flask containing 2.25 g of pre- specific site and/or defect center within the framework of Ag/NaY zeolite. treated zeolite by aqueous ion-exchange with 50 ml 50 mM AgNO3 solution (ChemPur GmbH in Germany, 99.998%) by stirring at 343 K In Ag/NaY, atomic Ag species normally exhibit well-resolved two 107 in the dark for 24 h. The ion-exchanged sample was filtered and rinsed doublets in the hyperfine splitting due to the silver isotopes Ag and 109 with deionized water several times, and dried in air at 353 K overnight. Ag with the large isotropic hyperfine coupling constant of – fi Chemical analysis by atomic absorption spectroscopy (AAS) demon- aiso = 590 − 700 G [20 23]. However, the hyper ne spectra of those strated that the ion-exchange reaction leads to a silver loading of ca. atomic species are dissimilar to the current anisotropic signal. A 12% (wt.). A silver loading of 9% and 6% was also prepared separately. conduction electron signal of metallic silver particles was also observed in Ag/NaY, exhibiting the EPR parameters of g=1.960 and Δ=70H G Oxidation was performed under a gas stream of O2 (Westfalen AG pp in Germany, 99.999%) with a flow rate of 17 ml min−1 g−1 from room [24]. Such a singlet spectrum arises from the relatively larger particles temperature up to 673 K using a heating rate of 1.25 K min−1 where it with size of ≈5 nm [7]. This suggests that a formation of paramagnetic was kept for an additional hour. While the sample was held at the final Ag clusters is sensitive to the inner symmetry of the zeolite cage, and the stabilized clusters will exhibit molecular character rather than temperature, the residual O2 gas in the reactor was purged by N2 (Westfalen AG, 99.999%) gas for 1 h. Subsequently, the sample was metallic character that is only due to conduction electrons [25]. fi sealed and kept at 673 K overnight. In Ag/NaA (Si/Al=1), the hyper ne spectrum of the paramagnetic Ag+ cluster appears only after performing appropriate dehydration, After cooling the sample, reduction was performed in a flow of H2 6 gas (Westfalen AG, 99.999%, 16 ml min−1 g−1) at room temperature or oxygen calcination and purging at 673 K, and hydrogen g below for 20 min, which leads to the stabilization of the paramagnetic reduction at 278 or 298 K. The seven-line splitting with iso ≈ 2.028 + Ag and aiso ≈67G arises from the isotropic coupling of the unpaired Ag6 cluster. Alternatively, D2 (Westfalen AG, 99.0%) reduction was carried out under a static gas pressure of 500 mbar. electron with six equivalent silver nuclei (Fig. 1b). The isotropic giso The reduced sample was transferred into EPR quartz tubes (outer deviates positively from the free electron ge value, indicating admixture diameter about 4 mm) under nitrogen or gas in a glove box. The a b tubes were sealed with stopcocks for vacuum treatment and gas Ag (g ~2.028) admission. The sample containing tube was evacuated for 30 min prior 2.061 ** to each EPR measurement. The prepared sample can be handled in 2.009 2.090 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 3000 3200 3400 3600 3000 3200 3400 3600 3800 exchange, the residual D2 gas was pumped off and the sample tube was Magnetic Field [G] Magnetic Field [G] sealed for measurements. Fig. 1. a) X-band EPR spectrum of the H2 reduced 12% (wt.) Ag/NaY recorded at 20 K D2 desorption experiments were preformed using a turbo-molecu- and its numerical simulation (red) constituting the superposition (1:1 weighting), where lar pump apparatus at temperatures of 383, 408 and 423 K. An EPR the signal from one species with g ≈ 2.090 and g⊥ ≈ 2.009, while g ≈2.061 and g⊥ ≈2.004 quartz tube containing 120 mg deuterium reduced 12% (wt.) Ag/NaA + from another species. b) Experimental spectrum of the H2 reduced Ag6 cluster (black) in powder sample was connected by a glass adapter valve to the turbo- the 12% (wt.) Ag/NaA stabilized after performing reduction at 278 K for 20 min. Its molecular pump apparatus. The vacuum system can achieve pressures numerically simulated spectrum (red) is based on the spin-Hamiltonian parameters of 6 as low as 10−5 − 10 −6 mbar. The sample was heated to different equivalent Ag nuclei, taking into account the statistical distribution of Ag isotopes.

2 A. Baldansuren Progress in Natural Science: Materials International (xxxx) xxxx–xxxx of a transition metal with a more than half-filled d orbital into spin Fig. 2b. When completing desorption, the samples were exposed to density distribution [26]. The isotropic coupling reveals that the 5 s D2 gas with 500 mbar for 20 min that restored the spectrum almost orbital contribution to the unpaired electron molecular wave function completely with regard to the intensity. Similar to the final H/D is about 10% per Ag nucleus, indicating a strong spin density exchange, there is a baseline drift at ∼3600 G. The hyperfine spectrum delocalization on the cluster surface. For noble metal clusters, the shows a rather more asymmetry, and consequently the numerical electronic structure is dominated by the number of valence electrons simulation requires a small anisotropy for both g and A tensors, i.e. fi that are delocalized [27]. A spin calibration amounts to a small fraction g ≈2.027; g⊥ ≈2.028 and A ≈66G; A⊥ ≈68 G, and the t with six ∼0.044% of all exchanged silver atoms to carry an unpaired electron so equivalent nuclei still provides a good match for the intensity distribu- + that EPR-active clusters with S =1/2 are about 0.26% of all clusters. tion of the Ag6 cluster splitting. These tensors are still coaxial, and a This means that most of silver atoms is EPR-silent, forming diamag- small anisotropy corresponds to a distortion of the electronic wave netic and/or high spin clusters. Our extended X-ray absorption fine function due to the cluster proximity to the specific cation coordinating structure (EXAFS) results showed that the paramagnetic and EPR- site. It apparently indicates that the silver clusters exist in different silent clusters are about the same atomic size [14]. The overall sites, for example, the α-cages share 6- and 8-membered rings with the α β fi + symmetry is cubic for the unit cell of zeolite A, consisting of eight - -cages. When deuterium desorbs more easily from the rst few Ag6 cages. The cage possesses an inner symmetry of an octahedron, better clusters, they subsequently convert into the diamagnetic ones, while known as the sodalite cage or pseudo unit cell [28]. The symmetry of the other diamagnetic clusters in different sites convert into the Ag fl β paramagnetic ones, since the overall EPR signal does not disappear both giso and aiso is re ected on the cluster accommodating -cage symmetry so that it provides an isotropic surrounding for the electronic by continuous desorption. Therefore, the number of paramagnetic wave function after hydrogen reduction [14,15]. On continuous reduc- clusters is always calibrated on average due to hydrogen isotope tion, the hyperfine splitting was distorted with a hint of charge mobility. Importantly, mobile deuterium easily penetrates through decreases to diamagnetic clusters, but was fully recoverable by a partial the six ring with a small diameter into and/or from the β-cages in pressure of O2 at 298 K [16]. As reported, a conduction electron signal comparison to other gas molecules like O2 [30]. evolves from bigger crystallites with a size of ≥1 nm formed in the α- Desorption is faster at higher temperatures, and so its activation cage when neutral cluster atoms become mobile leaving the β-cage energy can be determined. This is fully reversible experimentally by following continuous reductions [7,8]. readsorption of D2 with 500 mbar at room temperature (Fig. 2b), even + Hydrogen isotope exchange on the reduced Ag6 cluster was though the latter process might not be completely activated, and the performed at room temperature by filling D2 with 500 mbar into the activation energy equals the desorption energy. The signal intensity cluster sample in the EPR quartz tube. The sample was kept under a decay curves at different desorption temperatures are displayed as a deuterium partial pressure for 20 min and evacuated before each function of different desorption time intervals in Fig. 3a. Hydrogen measurement at 20 K. The exchange process was repeated for a couple isotope desorption supposedly involves a recombination of two atoms, of times, and the signal intensity increased initially (Fig. 2a). This which will give rise to second-order kinetics. Nevertheless, the experi- indicates that the other diamagnetic clusters located in different mental data fit very well with exponential decay curves, indicating that cationic sites of the Si/Al framework are converted into the paramag- this is compatible with first-order kinetics so that the rate-determining netic ones. It was reported that predominant silver clusters in treated step is desorption rather than recombination. Using the desorption zeolite A are not paramagnetic [9,29]. No change was observed in the time constants derived from Fig. 3a, a value of Ea =0.78eV hyperfine splitting, indicating that there are no other structures with six equivalent Ag nuclei, e.g. a planar hexagon. After the third fi a exchange, the baseline of the rst-derivative signal slightly drifts at a 2400 high field region of ∼3600 G and then exhibits a small asymmetry of the hyperfine splitting. The signal intensity decreased finally suggests only 2000 383 K the partially reduced clusters contribute to the isotropic signal (sym- metric) of EPR-active clusters. 1600 Furthermore, the experiments were performed on the deuterium + 1200 desorption from the D2 reduced Ag6 cluster by evacuating the samples at different elevated temperatures of 383, 403, and 423 K. Desorption 800 403 K

is related to the deuterium coverage around the silver cluster because EPR signal intensity [a.u.] of the relative intensity loss. The signal intensity decreased continu- 400 423 K ously with desorption times, and the highest temperature led to the 100 200 300 400 fastest desorption. After 180 min at 423 K, the cluster structure is still Integrated desorption time [min] intact and the corresponding hyperfine spectrum is displayed in 7.0 E =0.78eVforAgcluster a a b H/D reads D 6.5

180' H/D ] 6.0 -1 55' H/D 5.5 k /min D H ln[ 5.0

4.5 E =0.38eVforAg(111) 2800 3200 3600 2800 3200 3600 a b Magnetic Field [G] Magnetic Field [G] 4.0 2.35 2.40 2.45 2.50 2.55 2.60 2.65 Fig. 2. a) X-band EPR spectra of the reduced Ag+ cluster in the 12% (wt.) Ag/NaA 6 1000 T-1 [K-1] collected following isotope exchange H/D reactions at 298 K. b) EPR spectra of the D2 + ff reduced Ag6 cluster collected under continuous desorption at 423 K for di erent time Fig. 3. a) EPR signal intensity decreases as a function of desorption time at different intervals. The signal intensity decreases continuously, while no changes in the hyperfine temperatures. At each desorption temperature, the initial intensity is as it is (not splitting. All measurements were performed at 20 K. normalized). b) Arrhenius plot of the relative rate constants for D2 desorption.

3 A. Baldansuren Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

(75.3 kJ mol−1) for the deuterium desorption energy is calculated from Ag0 species by EPR in 6% Ag/NaA after hydrogen reduction [12]. Thus, + the slope of the Arrhenius plot shown in Fig. 3b [16]. This value is the mechanism is a reversible redox reaction of the Ag6 cluster and has higher than the experimental value of 0.38 eV (36.4 kJ mol−1) for the an one-electron nature. Both precursors were previously proposed via desorption from a (111) single crystal surface [31]. Along with a disproportionation reaction of 2Ag+02 → Ag + Ag + in dehydrated Ag/ “support effect”, this difference relates to a “finite size effect” of the NaA regarding the silver cation diffusion [9–11]. + The 2D electron spin echo envelope modulation (ESEEM) spectra octahedral Ag6 cluster with the strongly reduced coordination N ≈4.0 of surface atoms, which is less than that of the Ag(111) single crystal [19], so-called hyperfine sublevel correlation spectra surface [14]. Most importantly, the desorption energy value is in a close (HYSCORE) [35], were recorded employing the sequence −1 agreement with an activation energy of 63 kJ mol for the initial rate πτπtπtπτ/2 − − /2 −12 − − − /2 − − echo with mw pulse length of of the cluster formation following in-situ H2 reduction [10]. This was tπ/2 =16 ns and tπ =32ns incremented by Δ=4−8t1,2 ns with fi interpreted that the process is governed by the diffusion of the cations. starting times of t1,2 = 100 ns, and the rst two pulse To compare with deuterium results, hydrogen desorption was also separation time τ =140 ns. The intensity of the inverted echo following τ performed on the H2 reduced cluster. However, this simply did not the fourth pulse is measured with t2 and t1 varied and constant . follow the continuous decrease in the signal intensity at different Unwanted features from the experimental echo envelopes were re- temperatures, especially at 383 and 423 K. Desorption exhibited the moved by using a four-step phase cycle [36]. In both dimensions 512 oscillation in the EPR intensity upon heating for different time data points were collected. The relaxation decay was subtracted using intervals, indicating that a coverage cannot be explained as an uniform baseline corrections (by fitting polynomials of 3–6 degree) in both time binding or contact of hydrogen per cluster. It is known that the support domains, subsequently applying apodization (Hamming window) and ionicity determines the hydrogen coverage, and a decreasing trend zero-filling to 1024 data points in both dimensions. After 2D fast correlates with decreasing electron density on the support oxygen Fourier transformation the absolute value spectra were obtained. The atom. Electron-deficient exist in acidic supports with protons spectral resolution is further increased by suppressing the inhomoge- (H+) or other partly covalent cations [32]. No paramagnetic clusters neous broadening in the second dimension [19]. HYSCORE spectra are were observed in the presence of Cs+ and Ca2+ [4], consistent with the usually presented as either contour or stacked (3D) plots in Matlab fact that electron-rich oxygens exist in basic supports with large [37]. alkaline cations [32]. Therefore, diamagnetic clusters in the oxygen- The HYSCORE experiments were first performed for the reduced + rich framework sites, most likely in α-cages with a free diameter of Ag6 cluster at 10 K, revealing the intense diagonal peak coincides with Å 27 11.4 , experience more hydrogen coverage from surroundings. That is the aluminum nuclear Larmor frequency of νAl ≈3.9MHz (Fig. 4). the reason that the isotope exchange reaction results in a rising EPR This peak represents the framework 27Al nuclei, and the presence of + fi “ ” fi intensity of the Ag6 cluster at rst (Fig. 2a). Either complete or partial such a matrix peak con rms the close proximity between the zeolite desorption would shift a narrow range in which EPR-active clusters are framework site (lattice) and the silver cluster. However, a framework Al in equilibrium with a reservoir of similar but EPR-silent clusters. In + is not directly coordinated to the Ag6 cluster since a small isotropic comparison to molecular deuteron, hydrogen has a less accessibility to coupling is of the order of ∼2.0 MHz, proportional to a partial spin −4 27 the cluster in the β-cage, because of its larger H2 kinetic diameter of density population of ∼5.0 × 10 on a Al nucleus. Furthermore, there n+ were no cross-peaks about 1ν ≈ 14.5 MHz for the proton nuclear 0.275 nm at 273 K [9]. The paramagnetic Ag6 cluster is located in the H β-cage with 6.6 Å [3,7–10]. Larmor frequency in the weak coupling (+,+) quadrant, indicating A mechanism of the cluster formation following both hydrogen and that no direct spin density is on 1H about the cluster. It suggests that irradiation reductions has been very debatable [2–11]. What well hydrogen is not adsorbed on the cluster surface. On the other hand, a known so far is the charged and neutral clusters with a nuclearity of factor influencing the ESEEM intensity of some nuclear transitions in −1 6 - 14 reveal absorption bands from 19600 to 22700 cm (510– the corresponding electron spin manifold (mS =±1/2) is their orienta- 440 nm) of the near UV region [9,11,33]. Later, this yellow color was tion dependence in a powder type (disordered) sample, and so the interpreted as charge transfer from zeolite oxygen lone pairs to Ag+, absence of transition frequency peaks does not rule out the existence of denoted as Ag(5+ s )←O(n) [34]. A transformation from these diamag- cross-peaks [19]. It is also well known that peaks of nuclei with shallow netic clusters to EPR-active ones seems to be dependent on many modulations can be strongly suppressed by nuclei with deep modula- factors, such as a degree of metal loading, dehydration, reduction and tions, so-called a cross suppression effect [38]. This effect explains 1H annealing, indicating that different mechanisms are involved in the peaks are often very weak or even undetectable in the presence of clustering process. strong nuclei with I ≥ 1. + The current Ag6 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 compen- + sated by a positive charge of the reduced Ag6 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):

Ag+e+− →AgAg+h(≡Si−O−Al≡ 0++ −) 2+ − + →Ag (≡Si−O−Al≡) (1) Fig. 4. 3D presentation of the X-band HYSCORE spectrum of the H2 reduced Ag6 cluster in 12% (wt.) Ag/NaA, measured at 3408 G and 10 K. The intense peak from the 27 27 This is a reasonable assumption since we observed the precursor nuclear Larmor frequency νAl ≈3.90MHzof the framework Al nuclei.

4 A. Baldansuren Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

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 extrac-

tion of O2) of the lattice producing Lewis-acid site [33]. A temperature activated desorption of D2 is of the order of Ea =0.78eVin 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 EPR- Fig. 5. 3D presentation of the X-band HYSCORE spectrum of the reduced Ag+ cluster 1 6 silent clusters. From HYSCORE results, no direct spin density is on H 17 exposed to O2 at 298 K, measured at 3410 G and 10 K. The intense peak from the even though reduction plays an important role in balancing spin 27 27 nuclear Larmor frequency νAl ≈3.90MHzof the framework Al nuclei. + ff density delocalization about the Ag6 cluster. Hydrogen e ect is 17 indirectly probed by additional experiments where O2 interaction The HYSCORE experiments were extended to the adsorption of shifts once again spin density distribution around the EPR-active 17 O2 on the reduced cluster (Fig. 5). The spectrum exhibits the matrix clusters in the low symmetry β-cage. peak as well, but its linewidth is significantly reduced. It implies that a 27 spin density distribution around the Al nucleus is disturbed by this Acknowledgments interaction. In this case, oxygen interaction increases the support 1 basicity and decreases the support acidity due to H [32]. The The author was grateful to the Deutsche Forschungsgemeinschaft tetrahedral aluminum Al(4) in the basic aluminum salt had quite small was generally acknowledged since it awarded the one large funding for 2 quadrupole coupling constant (eqQh/ ) of the order of ∼1 MHz, studied the Research Training Group 448 "Advanced Magnetic Resonance Type by magic-angle spinning nuclear magnetic resonance (MAS-NMR) Methods in Materials Science" at the University of Stuttgart. This unit spectroscopy [39]. The quadrupole coupling can impose a dramatic then enrolled and supported the doctoral students. The author was ff e ect on the mixing of nuclear spin eigenfunctions and consequently thankful to Prof. E. Roduner (emeritus) for his helpful discussions. on the lineshape, especially when nuclear hyperfine and quadrupole couplings are similar in strength. This is in line with the current References Al HYSCORE spectrum, revealing aiso ≈1.0MHz. The previous results 17 + showed that the interaction of O2 around the reduced Ag6 cluster [1] W.M.H. Sachtler, Catal. Today 15 (1992) 419–429. causes an instantaneous anisotropy of the hyperfine splitting at 4 and [2] J.R. Morton, K.F. Preston, J. Magn. Reson. 68 (1986) 121–128. 16 – 7 K, which was not observed with O2 [14,16]. This indicates that [3] J.R. Morton, K.F. Preston, Zeolites 7 (1987) 2 4. – interacted oxygen molecules are not rigidly adsorbed on the cluster [4] T. Wasowicz, J. Michalik, Radiat. Phys. Chem. 37 (1991) 427 432. ff [5] J. Michalik, M. Zamadics, J. Sadlo, L. Kevan, J. Phys. Chem. 97 (1993) surface. Therefore, this e ect arises from the distorted electronic wave 10440–10444. + [6] J. Michalik, N. Azuma, J. Sadlo, L. Kevan, J. Phys. Chem. 99 (1995) 4679–4686. function about the Ag6 cluster at a very low temperature. This is fully reversible, and the spectra gradually become isotropic and symmetric [7] D. Hermerschmidt, R. Haul, Ber. Bunsenges. Phys. Chem. 84 (1980) 902–907. 17 [8] P.J. Grobet, R.A. Schoonheydt, Surf. Sci. 156 (1985) 893–898. as temperature rises from 10 to 20 K. Nuclei such as O2 with I=5/2 [9] J. Michalik, L. Kevan, J. Am. Chem. Soc. 108 (1986) 4247–4253. possess an electrical quadrupole moment that results from a non- [10] R.A. Schoonheydt, H. Leeman, J. Phys. Chem. 93 (1989) 2048–2053. spherical charge distribution. A nonzero electric field gradient is [11] R.A. Schoonheydt, J. Phys. Chem. Solids 50 (1989) 523–539. – Ag+ [12] A. Baldansuren, E. Roduner, Chem. Phys. Lett. 473 (2009) 135 137. expected at the 6 cluster, assuming that Ag-Ag bonds have dissimilar [13] I. Tkach, A. Baldansuren, E. Kalabukhova, S. Lukin, A. Sitnikov, A. Tsvir, lengths along molecular axes (oblate or prolate) at a low temperature. A M. Ishenko, Yu. Rosentzweig, E. Roduner, Appl. Magn. Reson. 35 (2008) 95–112. 16 continuous interaction of O2 (I=0) only leads to the EPR intensity [14] A. Baldansuren, H. Dilger, R.-A. Eichel, J.A. van Bokhoven, E. Roduner, J. Phys. – loss [14,16]. This interaction obviously shifted once again the equili- Chem. 113 (2009) 19623 19632. [15] A. Baldansuren, R.-A. Eichel, E. Roduner, Phys. Chem. Chem. Phys. 11 (2009) brium to the EPR-active clusters, perhaps the opposite direction of a 6664–6675. redox reaction following H2 reduction. [16] 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. 4. Conclusions [17] A. Baldansuren, arXiv:1510.02648, [cond-mat.mtrl-sci], 2015. [18] A. Baldansuren, arXiv:1504.00893, [cond-mat.mtrl-sci], 2015. [19] S.A. Dikanov, Y.D. Tsevtkov, Electron Spin Echo Envelope Modulation (ESEEM) No active species of Ag clusters were observed in Ag/NaY after Spectroscopy, CRC Press, Boca Raton, USA, 1992. performing appropriate treatments of calcination, dehydration and [20] A. Abou-Kaïs, J.C. 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