In situ observation on dehydrogenation of MgH 2 was performed by using transmission electron microscope ͑TEM͒. 7-9 However, the detail of the catalytic reaction has not been understood yet. In this study, the change in the high resolution image before and after dehydrogenation was observed by using with TEM to examine the details of the catalytic reaction.In this study, two kinds of samples with 1 and 10 mol % of Nb 2 O 5 were prepared. For MgH 2 catalyzed with 1 mol % Nb 2 O 5 , MgH 2 powder ͑Alfa Aesar͒, and Nb 2 O 5 powder ͑me-soporous: Sigma-Aldrich͒ were ball-milled under the hydrogen atmosphere of 1.0 MPa for 20 h. On the other hand, for MgH 2 catalyzed with 10 mol % Nb 2 O 5 , MgH 2 powder, and Nb 2 O 5 powder ͑single-crystalline: Kojundo chemical Laboratory͒ were mixed by agate mortar in a glovebox filled with Ar gas. The samples were handled without exposure to air in whole process. The sample powder was placed on a microgrid and the grid was set into a heating specimen holder in the glove box under Ar atmosphere. The heating specimen holder was put into a plastic bag under Ar atmosphere and loaded into TEM equipment in order to prevent the oxidation. The samples were observed from room temperature to 200°C by using a TEM ͑JEOL-2010; 200 kV͒, and high resolution images of a 10 mol % catalyzed sample were obtained by using high voltage electron microscope ͑HVEM, JEM-ARM1300; 1250 kV͒. With respect to temperature control for in situ TEM observation, the temperature was rapidly raised up and kept at 100, 150, or 200°C. The TEM images were analyzed by the selected area electron diffraction pattern ͑SADP͒, fast Fourier transformation ͑FFT͒, and inverse fast Fourier transformation ͑IFFT͒.At the first step, observation by using a conventional TEM ͑200 kV͒ was carried out for MgH 2 catalyzed with 1 mol % Nb 2 O 5 , which showed a remarkable catalytic effect. APPLIED PHYSICS LETTERS 96, 223109 ͑2010͒

We carried out in-situ observation on the catalytic effect of Nb2O5 in MgH2 by using a high voltage transmission electron microscope (HVEM). We synthesized two kinds of samples, which were prepared by ball milling and by hand mixing. In milled sample, Nb2O5 was not confirmed from high resolution images, however, NbO was identified. As the temperature increased, the decomposition of MgH2 proceeded, while Mg formed and grew. It suggested that NbO had the catalytic effect to promote the dehydrogenation of MgH2. In mixed sample, which was prepared to clearly observe the boundary between the catalyst and Mg phase, it was revealed that the dehydrogenation started from the boundary of MgH2 and Nb2O5. This result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH2 through Mg phase to the boundary.

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 nano-size 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.

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