Stability of Catalyzed Magnesium Hydride Nanocrystalline During Hydrogen Cycling. Part II: Microstructure Evolution Chengshang Zhou, † Zhigang Zak Fang,* ,† Robert C. Bowman, Jr., † Yang Xia, † Jun Lu, ‡ Xiangyi Luo, ‡ and Yang Ren § † Department of Metallurgical Engineering, The University of Utah, 135 South 1460 East, Room 412, Salt Lake City, Utah 84112-0114, United States ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South, Cass Avenue, Argonne, Illinois 60439, United States § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South, Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: In Part I, the cyclic stabilities of the kinetics of catalyzed MgH 2 systems including MgH 2 -TiH 2 , MgH 2 - TiMn 2 , and MgH 2 -VTiCr were investigated, showing stable kinetics at 300 °C but deteriorations of the hydrogenation kinetics at temperatures below 150 °C. The present Part II describes the characterization of uncycled and cycled catalyzed MgH 2 by X-ray di ffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. XRD analysis shows the crystallite sizes of the Mg and MgH 2 significantly increased after the cycling. The mean crystallite sizes of the catalysts (TiH 2 and VTiCr) increased moderately after the cycling. SEM and TEM imaging were used to compare the microstructures of uncycled (as-milled) and cycled materials, revealing a drastic change of the microstructure after 100 cycles. In particular, results from energy-dispersive spectroscopy (EDS) mapping show that a change of distribution of the catalyst particles in the Mg and MgH 2 phase occurred during the cycling. 1. INTRODUCTION In Part I, 1 the cyclic stability of the kinetic behavior of catalyzed MgH 2 was investigated using the pressure-composition- isothermal (PCI) method. The cycling measurements demon- strated that at high temperature (300 °C) the hydrogenation and dehydrogenation kinetics were stable after the hydrogen cycling for 100 times. Results also showed that the low- temperature (25-150 °C) hydrogenation kinetics suffered a severe degradation after the cycling. For the systems doped with different catalysts (TiH 2 , TiMn 2 , and VTiCr), the degradations of room-temperature hydrogenation kinetics are similar. Among the three systems, the MgH 2 -VTiCr exhibited slightly better cyclic properties. Comprehensive analysis from Part I indicates that the low-temperature kinetic degradation is mainly attributed to the extended hydrogenation-dehydrogen- ation reactions. The underlying mechanism of the kinetics degradation, however, could not be deduced from these experiments alone. To understand the changes of the kinetics during the cycling, it is necessary to first review the prior hypothesis of the catalytic effect on MgH 2 . According to the reported literature, 2-4 hydrogenation of magnesium involves several steps taking place in sequence: (1) hydrogen molecule absorption and dissocia- tion on the surface of particles, (2) H diffusion in the metal, and (3) nucleation and growth of the hydride phase. Since the diffusion rate of hydrogen through Mg metal is sufficiently high, 2,5,6 the rate-limiting step for hydrogenation is likely to be the step (1) or (3). When magnesium hydride desorbs hydrogen, a nearly reverse order can be defined, 6 namely: (1) nucleation of Mg and transformation from the hydride to the metal phase, (2) movement of H atoms to the surface by diffusion, and (3) association of hydrogen atoms to gaseous hydrogen molecules. For the dehydrogenation process, kinetics and modeling studies suggested that the reaction rate is controlled by three-dimensional growth of Mg nuclei, as the best fitting provided by the use of the Johnson-Mehl-Avrami (JMA) model. 7-11 Although the role of the catalyst is still unclear and under debate, it is considered that catalysts can enhance the hydrogen dissociation on the surface 12 and/or reduce the activation energy of nucleation and growth of nuclei. 13 In order to achieve a fast kinetic rate, a nanostructure MgH 2 -catalyst composite having effective catalytic nanoparticles homogeneously dis- persed is favorable. 14-20 In order to clarify the mechanism of the cycling behavior of the kinetics, a structure analysis of the MgH 2 material in terms of size effect, catalyst location, morphology, and other factors is necessary. In this paper, TEM, SEM, and XRD were used to analyze milled and cycled products of the catalyzed MgH 2 systems. Received: June 28, 2015 Revised: September 4, 2015 Published: September 11, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 22272 DOI: 10.1021/acs.jpcc.5b06192 J. Phys. Chem. C 2015, 119, 22272-22280