Distinct hydrolysis kinetics and catalytic mechanism of CaMg2, CaMg2X0.1(X= Ni, Zn, Ti) ternary alloys

Abstract

In this experiment, the as-cast CaMg2, CaMg2Ni0.1, CaMg2Zn0.1, CaMg2Ti0.1 ternary alloys have been successfully developed through vacuum melting induction method. The phase compositions and microstructures of these alloys were characterized using X-ray diffraction and scanning electron microscopy. Subsequently, we investigated the kinetics and microscopic mechanism of hydrolysis in MgCl2 aqueous solution for both as-cast and hydrogenated alloys. The hydrogen production curves were fitted at different temperatures using the nucleation-growth Avrami model, which exhibited a good fit to the hydrolysis kinetics. Our findings revealed that the hydrides obtained through H2 absorption exhibited fast hydrolysis reaction rates. Notably, CaMg2Ti0.1 alloy demonstrated excellent hydrolysis properties, with a yield of 1020 mL/g and an activation energy of 37.77 kJ/mol. The activation energy of H–CaMg2Ti0.1 alloy decreased to 15.66 kJ/mol, attributed to the synergistic catalytic effect of TiH2 phase and Ca4Mg3H12 phase. These hydrides exhibited ease of hydrolysis, releasing H+ ions to generate H2 gas. Furthermore, the hydrolysis properties of the as-cast alloys were observed that ranked as CaMg2Ti0.1 > CaMg2Zn0.1 > CaMg2 > CaMg2Ni0.1. However, the order of hydrogenated alloys changed to CaMg2Ti0.1 > CaMg2Zn0.1 > CaMg2Ni0.1 > CaMg2, indicating the synergistic catalytic effect between the Mg phase and Mg2NiH4. Among the Mg-based alloys, Ca–Mg-based alloy hydrolysis showed a research trend towards improving the kinetic properties through alloying.