• PDF / 2,178,399 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 32 Downloads / 272 Views

DOWNLOAD

REPORT

---

1216-W03-19

In-situ TEM observation for reaction mechanism in MgH2 hydrogen storage material Akifumi Ono†*, Shigehito Isobe†, Yongming Wang†, Naoyuki Hashimoto†, Somei Ohnuki† † Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan ABSTRACT In-situ observation on the catalytic effect of Nb2O5 in MgH2 was carried out by using transmission electron microscopy (TEM). We prepared two kinds of samples, because we tried to observe the reaction from two kinds of viewpoints. MgH2 catalyzed with 1 mol% of Nb2O5 was prepared for an overall viewpoint on the desorption process of MgH2 with catalyst by conventional TEM. The dehydrogenation of the 1 mol% sample started at 150 °C and Mg nanosize particles were formed. However, Nb2O5 was not confirmed in diffraction patterns and images, because it was highly dispersed by ball-milled. So MgH2 catalyzed with 10 mol% of Nb2O5 was prepared for local viewpoint to focus the boundary between the catalyst and the Mg phase by high voltage electron micro scope (HVEM). The sample mixed in mortar was prepared for this, because it was difficult to find the boundary in the sample ball-milled. The high resolution images of the 10 mol% sample revealed that the dehydrogenation started from the interface of MgH2 and Nb2O5. The result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH2 phase to the interface between Mg and Nb2O5. INTRODUCTION Mg has been regarded as a promising candidate for a base metal of hydrogen storage alloys, which have large capacity for hydrogen storage [1-3]. However, the reaction speed of hydrogen absorption/desorption is too slow. Recently, it has been suggested that oxide catalyst is effective for improving the hydrogenation and dehydrogenation kinetics of Mg. Barkhordarian et al. reported that Nb2O5 had the best effect on the hydrogen storage properties of Mg and MgH2 as an oxide catalysts [4-6]. Hanada et al. made some composite materials of MgH2 including transition metals and oxides by ball milling method, and reported that MgH2 with 1mol%Nb2O5 has superior dehydrogenation characteristics [7-9]. However, the detail of the catalytic reaction has never been understood. In this study, we observed the change of the high resolution image before and after dehydrogenation with transmission electron microscope (TEM) to examine the details of the catalytic reaction. EXPERIMENT In this study, two kinds of samples including of Nb2O5 1 mol% and 10 mol% were prepared. For 1 mol% sample, MgH2 powder (Alfa Aesar) and Nb2O5 powder (Meso-porous: Sigma-Aldrich) were ball-milled under hydrogen atmosphere of 1.0 MPa for 20 hours. For 10 mol% sample, MgH2 powder and Nb2O5 powder (Single-crystalline: Kojundo chemical Lab.) were mixed by agate mortar in a glove box filled with Ar gas. The samples were handled without exposure to air in whole process. The sample powders were placed on a micro grid, and then, the grid was set into a heating specimen holder in the glove box under Ar atmosphere. The heating specimen holder was pu