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

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 Tplateau 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.