Effective Factor on Catalysis of Niobium Oxide for Magnesium

Abstract

Magnesium is a promising hydrogen storage material but requires an efficient catalyst to enhance the sluggish kinetics of its hydrogen desorption/absorption reactions. Niobium catalysts have been shown to accomplish this, but the effective factors for catalysis on hydrogen desorption/absorption of Mg are not well understood. In order to investigate these aspects, various types of Nb oxides were synthesized and mixed with Mg, and their catalytic properties were investigated. The spray pyrolysis synthesis of Nb oxides at different temperatures produced homogeneous spherical particles with different degrees of crystallinity, while Nb oxide particles synthesized by simple calcination of ammonium niobium oxalate were nonuniform. These Nb oxides show significant catalytic activities for the hydrogen desorption/absorption of Mg, with amorphous oxides being more effective catalyst precursors than crystalline precursors. Metastable, amorphous Nb oxide is more easily converted to the reduced state, which is the catalytically active state for the reactions. In addition, Nb in the deactivated sample is in the oxidized state compared with the initially activated sample, and the catalytically active (reduced) state is recovered by reactions with hydrogen. Based on these findings, it is concluded that the chemical state of Nb is an important factor in catalyzing the desorption/absorption of hydrogen by Mg, and the catalytically active state can be preserved without further treatments.

Catalytic Activities of Various Niobium Oxides for Hydrogen Absorption/Desorption Reactions of Magnesium

“…It has been reported that amorphous Nb 2 O 5 with a small particle size has higher catalytic activity than that with a large particle size. 14 …”

“…It was found that, amorphous Nb 2 O 5 , which is metastable, can be easily converted to an active state compared with crystalline Nb 2 O 5 . 14 In a recent work, when a Nb oxide was synthesized by the hydrolysis of niobium(V) ethoxide, Nb(OC 2 H 5 ) 5 , the −OH functional group remained in the synthesized Nb oxide. The −OH group inhibits the formation of Nb–O–Nb linkages, and the Nb oxide without heat treatment shows catalytic effects comparable to those of Mg ball-milled with mesoporous Nb 2 O 5 for 20 h for hydrogen absorption/desorption of Mg even in a short-time dispersion process (2 h ball-milling).…”

The Catalytic Role of D-block Elements and Their Compounds for Improving Sorption Kinetics of Hydride Materials: A Review

“…Several compounds such as metal oxides, halides and carbides have been used to improve the sorption kinetics of almost all the families of hydrides, which will be reviewed in this section. Recently, several reports have been published seeking to improve the hydrogen sorption properties of MgH 2 by using binary transition metals as catalysts such as NiCu [112], ZrH 2 [113], HfCl 4 [114], Nb 2 O 5 [115], [116], Mn 3 O 4 [117], FeCo [118], TiO 2 [119], Ti 3 C 2 [120], FeCl 3 [121], Nb(V) ethoxide [122], ZrCo [123], Fe 2 O 3 [124], YH 2 [125], TiH 2 [54] and many more. Zhang et al used NiCu catalyst with different molar ratios and found that among them 5h milled MgH 2 /Ni-50%Cu system desorbed hydrogen at 205.8 • C, which is 96.9 • C lower than the pure MgH 2 as shown in Figure 12 [112].…”

Computational Investigation of MgH2/NbOx for Hydrogen Storage

“…Interestingly, the morphology of Nb 2 O 5 additives can also affect the performance. , Most recently, hollow Nb 2 O 5 has been employed as active catalysts (10 wt %), promoting MgH 2 dehydrogenation with the onset temperature down to 195 °C . Tailoring the electronic state of Nb is also promising to optimize the catalysis performance, such as Nb 2 O 5 -Nb mixing with different ratios, or combining with N-modified Nb 2 O 5 (10 wt %). , To understand these superior performances associated with Nb 2 O 5 , various active phases have been proposed, including ultrasmall NbO particles, reduced Nb 2 O 5 , ,− ultrasmall MgH 2 or NbOx, − new phases (Mg-Nb-O , ), etc. Even so, however, the catalysis mechanism has not been fully understood and several key questions need better clarification, including (i) whether bulk-doped and interface-distributed Nb performs differently; (ii) whether oxygen contributes to the promotion effect, and if so, is there any synergetic effect between Nb and O; and (iii) what the essential role is played by Nb/O in the process.…”

“…A typical challenge is that a reliable comparison between different samples is hardly approachable, under which theoretical calculations have been employed to examine the mechanism associated with size effect, metal oxide additive, and new phases . In principle, Mg–H bonding may be weakened if Nb 2 O 5 can compete with Mg as electron donor, under which reduced Nb 2 O 5 , showing as Nb or NbOx, can be particularly relevant for studying the impact of the cations. − In addition, recent N-Nb 2 O 5 catalyst vividly demonstrates the synergetic role between Nb and N, highlighting the importance of anions. , Experimentally, it had been reported that ppm-level N 2 or O 2 can affect hydrogen adsorption and desorption of MgH 2 samples . Using oxygen as an example, Mg–O bonding (85.6 kcal/mol) is much stronger than Nb–H (31.6 kcal/mol), meaning that O or N dopants can remarkably change local charges of Mg and H. Such an anion-associated mechanism had been employed earlier, − using B/N dopants to reduce the dehydrogenation temperature.…”