Hydrogen Isotope Dynamic Effects on Partially Reduced Paramagnetic Six

Progress in Natural Science: Materials International (xxxx) xxxx–xxxx HOSTED BY Contents lists available at ScienceDirect Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi Original Research Hydrogen 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, oxygen 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 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 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 gas 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, 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 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 energies 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 Switzerland. 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].

Load more