Remarkable enhancement in dehydrogenation of MgH2 by a nano-coating of multi-valence Ti-based catalysts

Abstract

A Ti-based multi-valence catalyst was coated on the surface of ball milled Mg powders (∼1 μm in diameter), aiming to decrease the desorption temperature and increase the kinetics of hydrogen release from MgH₂ by its catalytic effect on thermodynamics. The catalysis coating was prepared by the chemical reaction between Mg powders and TiCl₃ in THF solution, which is ∼10 nm in thickness and contains multiple valences in the form of Ti (0), TiH₂ (+2), TiCl₃ (+3) and TiO₂ (+4). It is believed that the easier electron transfer among these different Ti valences plays a key role in enhancing the hydrogen recombination for the formation of a hydrogen molecule (e.g.). This recombination is generally regarded as the key barrier for hydrogen desorption of MgH₂. Experimentally, temperature-programmed desorption (TPD) and isothermal dehydrogenation analysis demonstrate that the MgH₂ – coated Ti based system (denoted as Mg–Ti) has excellent dehydrogenation properties, which can start to release H₂ at about 175 °C and release 5 wt% H₂ within 15 min at 250 °C. The dehydrogenation reaction entropy (ΔS) of the system is changed from 130.5 J K⁻¹ mol⁻¹ H₂ to 136.1 J K⁻¹ mol⁻¹ H₂, which reduces the T_plateau to 279 °C at an equilibrium pressure of 1 bar. A new mechanism has been proposed that multiple valence Ti sites act as the intermediate for electron transfers between Mg²⁺ and H⁻, which makes the recombination of H₂ on Ti (in forms of compounds) surfaces much easier.