Nb-Gateway for Hydrogen Desorption in Nb₂O₅ Catalyzed MgH₂ Nanocomposite

Abstract

The state of Nb-contained catalysts in MgH 2 nanocomposites was investigated during the full cycle. X-ray diffraction (XRD) results showed that Nb 2 O 5 and Nb reacted with MgH 2 during ball-milling, forming NbH 2 and NbH, respectively. In the following dehydrogenation, the (di)hydride decomposed, and Nb was produced. Then NbH was generated in both samples after rehydrogenation. Similar composition in both samples suggests that the catalytic effect follows the same mechanism, the Nb-gateway model, in which Nb facilitates the hydrogen transportation from MgH 2 to the outside. By contrast, NbO remained during the full cycle. Scanning and transmission electron microscopy (SEM and TEM) observations revealed that the Nb 2 O 5 -doped sample tended to be refined in size, compared to the Nb-doped and NbO-doped ones. Nb crystals in the Nb 2 O 5 -doped sample were observed to be highly dispersed in the sample, with 10−20 nm in size. Given all that, tiny Nb crystals distributed in the composites worked as the gateway facilitating hydrogen transportation and improving dehydrogenation properties. ■ INTRODUCTIONHydrogen is a promising alternative energy carrier because of its prominent advantages such as high energy density, great variety of potential sources, light weight, and environmental friendliness; thus it has been highly regarded. As a medium for hydrogen storage, magnesium hydride has been well studied in the past decades. Having notable advantages such as high capacity (7.6 mass %), light weight, and low cost, it is considered as a promising candidate for hydrogen storage materials.1 However, the absorption and desorption processes require high temperatures of 300−400°C, and the reactions are slow. By ball-milling with some transition metals, the kinetics of the reactions can be improved.2−4 Huot et al. showed the result that MgH 2 nanoparticles catalyzed by 5 mol % Nb completed full desorption within 300 s at 300°C. 4 They confirmed the formation of a short-lived metastable NbH x (x ≈ 0.6) phase during dehydrogenation, thus concluded that it acted as a gateway through which hydrogen from MgH 2 released. 5 A similar mechanism was claimed later by Li et al. in a density functional theory calculation, in which the substitution of Nb at the Mg site followed by the clustering of H around Nb was a likely pathway for hydrogen desorption. 6 On the other hand, Mg−Nb−O perovskite phase was also found in the dehydrogenated composite and believed to help with hydrogen transportation. Transition-metal oxides also show the catalytic effect on facilitating hydrogen absorption and desorption in nanostructured MgH 2 .8−13 Among those catalysts studied so far, Nb 2 O 5 showed a superior catalytic effect. Barkhordarian et al. reported that MgH 2 catalyzed by 0.5 mol % Nb 2 O 5 and ballmilled for 100 h finished desorption within 90 s at 300°C. 10Hanada et al. reported that the composite, MgH 2 and 1 mol % Nb 2 O 5 milled for 20 h, was able to absorb ∼4.5 wt % of hydrogen after full desorption, under a pressure of 1.0 MPa wit...

Formation of Mg–Nb–O rock salt structures in a series of mechanochemically activated MgH2 + nNb2O5 (n = 0.083–1.50) mixtures

“…Ma et al [28] reported the presence of finely dispersed Nb nanocrystals with a typical size of~10e20 nm in a MgH 2 sample ball milled together with Nb 2 O 5 . It was suggested that Nb nanocrystals existing at the surface of MgH 2 particles behave as a gateway for the flow of hydrogen in and out of Mg/MgH 2 .…”

Synthesis of catalytically active rock salt structured Mg x Nb 1−x O nanoparticles for MgH 2 system

“…The XPS study of Friedrichs suggested that Nb 2 O 5 reacts with MgH 2 and gradually undergoes reduction to form catalytically active compounds which in turn offer effective hydrogen diffusion pathways [13]. Recently Ma et al have identified the reduction of Nb 2 O 5 , resulting in the formation of 10e20 nm Nb nanocrystals in a ball milled MgH 2 eNb 2 O 5 mixture [14]. Based upon this finding they suggested that Nb nanocrystals may act as a gateway for the desorption of hydrogen which apparently leads to better system performance.…”

“…In these studies, the formation reaction of MgeNbeO phase occurred in-situ, a situation where excess Mg is always present. As for example, references [7,14], and [15], respectively used the Nb 2 O 5 concentrations 2, 10 and 8 mol%. For a short note on the choice of the concentration of Nb 2 O 5 , see the Supplementary Information (note #1).…”

Microscopic Study on Hydrogenation Mechanism of MgH₂ Catalyzed by Nb₂O₅

“…Instead, the number density of small MgH 2 crystallites increases in the hydrogenation process. According to the Nb-gateway model proposed in our previous work, 11) highly-dispersed tiny Nb in the sample works as a gateway for hydrogen transportation, facilitating hydrogen dissociation and recombination during the process. MgH 2 could rapidly nucleate near the site where the catalyst exists.…”

“…11) On the other hand, we have been pursuing for the direct observation of the process using transmission electron microscopy (TEM). In the previous observation, the dehydrogenation process of MgH 2 catalyzed with 10 mol% Nb 2 O 5 was monitored via high resolution transmission electron microscope (HRTEM).…”