Highly active multivalent multielement catalysts derived from hierarchical porous TiNb 2 O 7 nanospheres for the reversi
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Highly active multivalent multielement catalysts derived from hierarchical porous TiNb2O7 nanospheres for the reversible hydrogen storage of MgH2 Lingchao Zhang1,§, Ke Wang1,§, Yongfeng Liu1 (), Xin Zhang1, Jianjiang Hu2, Mingxia Gao1, and Hongge Pan1 1
State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China § Lingchao Zhang and Ke Wang contributed equally to this work. 2
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 14 June 2020 / Revised: 30 July 2020 / Accepted: 17 August 2020
ABSTRACT Critical limitations in applying MgH2 as a hydrogen-storage medium include the high H2 desorption temperature and slow reaction kinetics. In this study, we synthesized hierarchical porous TiNb2O7 spheres in micrometer scale built with 20–50 nm nanospheres, which showed stable activity to catalyze hydrogen storage in MgH2 as precursors. The addition of 7 wt.% TiNb2O7 in MgH2 reduced the dehydrogenation onset temperature from 300 to 177 °C. At 250 °C, approximately 5.5 wt.% H2 was rapidly released in 10 min. Hydrogen uptake was detected even at room temperature under 50 bar hydrogen; 4.5 wt.% H2 was absorbed in 3 min at 150 °C, exhibiting a superior low-temperature hydrogenation performance. Moreover, nearly constant capacity was observed from the second cycle onward, demonstrating stable cyclability. During the ball milling and initial de/hydrogenation process, the high-valent Ti and Nb of TiNb2O7 were reduced to the lower-valent species or even zero-valent metal, which in situ created multivalent multielement catalytic surroundings. A strong synergistic effect was obtained for hybrid oxides of Nb and Ti by density functional theory (DFT) calculations, which largely weakens the Mg–H bonding and results in a large reduction in kinetic barriers for hydrogen storage reactions of MgH2. Our findings may guide the further design and development of high-performance complex catalysts for the reversible hydrogen storage of hydrides.
KEYWORDS hydrogen storage, magnesium hydride, transition metal catalysts, nanospheres, hydrogenation
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Introduction
Hydrogen has great potential to fuel a future clean and sustainable society because of its high energy density, abundant sources, light weight, and low environmental impact [1–3]. However, in practice, a challenge remains to achieve the storage of hydrogen in a safe, efficient, and economic manner [4]. Metal hydrides, especially lightweight metal hydrides, are better hydrogen storages than pressurized H2 and other types of hydrogen storage in terms of gravimetric and volumetric storage capacities and safety [5, 6]. For example, MgH2 with 7.6 wt.% of hydrogen capacity and 110 g·L−1 of volumetric density has aroused extensive attention as the most promising hydrogen storage candidate [7, 8]. However, the desorption of hydrogen from MgH2 requires high temperature (> 300 °C) due
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