Surface Passivated Air and Moisture Stable Mixed Zirconium Aluminum Metal-Hydride Nanoparticles
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SURFACE PASSIVATED AIR AND MOISTURE STABLE MIXED ZIRCONIUM ALUMINUM METAL-HYDRIDE NANOPARTICLES Albert Epshteyn, Joel B. Miller, Katherine A. Pettigrew, Rhonda M. Stroud, and Andrew P. Purdy Chemistry Division, Naval Research Laboratory, 4555 Overlook Ave., SW, Washington, DC, 20375 ABSTRACT Synthesis of surface passivated Zr-Al mixed metal-hydride nanoparticles was accomplished via a multi-step process. The initial reaction to produce the zirconium aluminum hydride was via decomposition of zirconium tetrahydroaluminate (Zr(AlH4)4) while exposed to ultrasound produced by a bench-top ultrasonic cleaning bath. The particles were surface passivated using carbohydrates and were shown to be stable in air and partially stable in water. TEM imaging suggests the existence of smaller particles made of a Zr-Al alloy that range in size from 1.8 nm to 7.9 nm in diameter and are interspersed with larger particles that range from tens to hundreds of nanometers in diameter. It was also shown that the carbohydrate-derived coating of the nanoparticles is present as an aluminum alkoxide gel surrounding the core particles. INTRODUCTION Metal-hydride nanoparticles have been sought after, for one, due to their potential utility as hydrogen storage materials.1 There have been several patents issued, as well as several publications in recent years for metal-hydride nanoparticle materials.2,3 The authors of a recent review on nano-engineered hydrogen storage materials asserted that nanoparticle materials decrease the active particle size increasing surface area and grain boundaries, in turn improving the adsorption and desorption kinetics, decreasing the enthalpy of formation and thereby decreasing the release temperature of metal hydrides.1 A theoretical investigation by Clark et al.4 of various Zr-Al materials showed such materials as potentially having good hydrogen storage properties. It has recently been reported that NaAlH4 acquires significantly improved hydrogen adsorption and desorption properties when doped with Ti or Zr.5 Larger scale synthesis of air and moisture sensitive metal nanoparticles is a challenge, and poses an obstacle to investigating the physical properties of these materials via traditional techniques that would expose them to air. To obtain materials that do not oxidize when handled in air while still retaining certain useful properties, the surface of the nanoparticles can be protected by a passivating agent. In order to keep the intrinsic properties of the unpassivated material it is important to maximize the active metal content of the material while minimizing the amount of passivator present on the nanoparticle surface. We report a homogeneous solution-based method used to produce well-defined passivated air and moisture stable Zr-Al partial hydride nanoparticle materials on gram scale. Herein we report the initial characterization of these particles by 27Al magic angle spinning (MAS) NMR, SEM, TEM, oxygen bomb calorimetry, and microanalysis. EXPERIMENT General. All air and moisture sensitive m
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