Catalytic Effect of RTO3 Perovskites on Hydrogen Storage and Hydrolysis Properties of Magnesium Hydride

Abstract

The method of reactive ball milling was used to synthesize MgH₂-based composites adding nanoparticles of complex oxides RTO₃ (R-rare earth and T-transition metals) as catalysts and graphite. All composites contain 5 wt.% of complex oxides Dy_0.5Nd_0.5FeO₃ and TbFe_0.5Cr_0.5O₃ synthesized by the sol-gel method, and some of them additionally contain 3 wt.% of graphite. The oxides have an orthorhombic perovskite structure (GdFeO₃ type) and are characterized by an average particle size of 80–300 nm. The effect of perovskites on the hydrogenation of magnesium during the milling process and the improvement of hydrogen sorption-desorption kinetics is demonstrated. The Mg–Dy_0.5Nd_0.5FeO₃ and Mg–TbFe_0.5Cr_0.5O₃ composites absorbed 6.7 and 6.2 wt.% of hydrogen, respectively. X-ray powder diffraction after ball milling did not reveal any new compounds, except magnesium hydride. Thermal desorption from these composites occurs in two stages at temperatures above 300°C. The activation energy (E_a) of hydrogen desorption was determined by the Kissinger method. For the composite with TbFe_0.5Cr_0.5O₃, E_a is 123 kJ/mol, and for the composite with Dy_0.5Nd_0.5FeO₃ E_a = 147 kJ/mol. These composites were also tested as materials for hydrogen generation by hydrolysis in pure water and MgCl₂ water solutions. In pure water, the hydrogen yield during hydrolysis ranged from 320 to 350 ml per gram. The conversion degree was significantly improved by the addition of MgCl₂. It reached 90% (~1400 ml/g) after 30 min of hydrolysis for the MgH₂–nano-TbFe_0.5Cr_0.5O₃. These characteristics show that the synthesized MgH₂–nano-RTO₃ composites can be used in hydrogen generation systems.