Hydrogen Sorption in Magnesium Nanoparticles: Size- and Surface-related Phenomena
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1216-W05-04
Hydrogen Sorption in Magnesium Nanoparticles: Size- and Surface-Related Phenomena Luca Pasquini1, Elsa Callini1, Emanuela Piscopiello2, Amelia Montone3, Torben R. Jensen4, Marco Vittori Antisari3, Ennio Bonetti1 1
Department of Physics and CNISM, University of Bologna, Bologna, I 40127, Italy 2 ENEA, C.R. Brindisi, Brindisi, I 72100, Italy 3 ENEA, C.R. Casaccia, Rome, I 00123, Italy 4 Department of Chemistry and iNANO, University of Aarhus, Aarhus, DK-8000, Denmark ABSTRACT Mg nanoparticles with metal-oxide core-shell morphology were synthesized by inert-gas condensation and decorated by in-situ deposition of transition metals: Pd, Ni, Ti. The effects of nanoparticle size on the hydrogen sorption kinetics and the specific action of the different catalysts are presented and discussed in connection with structural and functional analysis. INTRODUCTION Materials engineering of nanostructures with a large specific area of properly functionalized surfaces is presently a highly pursued strategy in the development of hydrogen (H) sorption materials, including carbon nanostructures [1,2,3], metal organic frameworks [4], and metal-hydride based systems such as multilayers [5] and mechanically alloyed nanocomposites [6]. Nanoparticles (NPs) of light metal hydrides are appealing in this respect because their reduced size and large surface area might lead to radically improved H-sorption kinetics over coarse-grained counterparts, while significant changes in the thermodynamics are predicted only at extremely small NP sizes [7]. In order to avoid the agglomeration of NPs upon hydrogen cycling and the subsequent degradation of their kinetic performances, incorporation in porous hosts [8,9] or coating by a native oxide shell [10] have emerged as feasible solutions. In two previous papers we have studied the dependence of the metal-hydride-transformation kinetics on NPs size [10] and surface decoration with Pd as catalytic transition metal (TM) [11]. In this work, we test the influence of different Pd amounts and report on the effect of other TM catalysts, namely Ni and Ti. The interest in the comparison between these three TMs, besides their different atomic masses, lies in their different behavior towards Mg. In fact, Pd and Ni form stable intermetallic phases with Mg. Mg2Ni further exhibits interesting hydrogen storage properties which are not paralleled by any Mg-Pd phase. On the other hand, Ti does not form compounds with Mg. We will show that these characteristics are reflected in different behaviors as concerns the gravimetric capacity and the rate of H-sorption.
EXPERIMENT Mg-based NPs were synthesized in an inert gas condensation (IGC) system by Thermionics, equipped with two thermal evaporation sources placed at the same distance from a rotating collection cylinder cooled by liquid nitrogen. The size of the NPs was tuned by varying the He pressure in the IGC chamber: 2.5 Pa and 250 Pa for small (S) and large (L) NPs, respectively. After warming the cylinder to room temperature, air was admitted into the ch
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