Mechanically activated metathesis reaction in NaNH2\u2013MgH2 powder mixtures

Sebastiano Garroni,Francesco Delogu,Santiago Cuesta-Lopez,Christian Bonatto Minella,Claudio Pistidda

doi:10.1007/s10853-017-1220-5

Abstract

The present work addresses the kinetics of chemical transformations activated by the mechanical processing of powder by ball milling. In particular, attention focuses on the reaction between NaNH2 and MgH2, specific case studies suitably chosen to throw light on the kinetic features emerging in connection with the exchange of anionic ligands induced by mechanical activation. Experimental findings indicate that the mechanical treatment of NaNH2–MgH2 powder mixtures induces a simple metathetic reaction with formation of NaH and Mg(NH2)2 phases. Chemical conversion data obtained by X-ray diffraction analysis have been interpreted using a kinetic model incorporating the statistical character of the mechanical processing by ball milling. The apparent rate constant measuring the reaction rate is related to the volume of powder effectively processed during individual collisions, and tentatively connected with the transfer of mechanical energy across the network formed by the points of contact between the powder particles trapped during collisions.

Highlights

Mechanical processing of powders by ball milling (BM) is based on the transfer of mechanical energy from milling tools to solid reactants [1,2,3]
Attention focuses on the reaction between NaNH2 and MgH2, specific case studies suitably chosen to throw light on the kinetic features emerging in connection with the exchange of anionic ligands induced by mechanical activation
The statistical description of transformation kinetics was originally applied to the case of mechanical alloying of elemental metals, by far the most intensely studied transformation promoted by BM [22]

Summary

Introduction

Mechanical processing of powders by ball milling (BM) is based on the transfer of mechanical energy from milling tools to solid reactants [1,2,3]. The statistical description of transformation kinetics was originally applied to the case of mechanical alloying of elemental metals, by far the most intensely studied transformation promoted by BM [22] It is a gradual mechanochemical transformation involving the intimate mixing of two, or more, metallic species consequent to the mechanical activation of the powder mixture [19, 22]. This choice stems from the interest attracted by amide–hydride mixtures in the field of hydrogen storage Within such context, the reaction between NaNH2 and MgH2 exhibits a peculiarity that makes it suitable for investigating the effects of mechanical activation on the kinetics of transformation processes involving inorganic phase.

Experimental methods

Results

Conclusions

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