Hydrogen Generation from Ammonia-Borane over Ni–B Amorphous Alloys Prepared from Aqueous Solution Based on Thermodynamic Prediction of Hidden Metastable of State

Abstract

Amorphous alloys are advantageous to obtain excellent catalytic activity from a disordered atomic arrangement and more dangling bonds. The compositional range of the Ni–B amorphous alloy was thermodynamically predicted. The function of the Gibbs energy of mixing, , and the activities of the Ni and B components, , and, , of the liquid phase was extrapolated to room temperature. The negative was found from X_Ni = 0.45 to 0.92, indicating the existence of the liquid phase as metastable state, i.e. formation of amorphous alloy. The small and , that is, the strong attractive interaction between the Ni and B components suggested the liquid phase structure was stable. The Ni–B amorphous alloys were prepared from aqueous solution using reducing agent consistent well with the thermodynamic prediction. The preparation method from aqueous solution was useful to prevent from thermal migration of the atoms toward the equilibrium state. The quasi equilibrium oxygen pressures, (Ni_{in amor.}), and, (B_{in amor.}), of Ni and B components in the amorphous alloys were evaluated from and . (B_{in amor.}) was found to be smaller than (Ni_{in amor.}), revealing that B components in amorphous alloy were preferentially oxidized and inhibited Ni oxidation. Not only a characteristic disordered atomic arrangement but also chemical alloying effect of B results in the excellent catalytic activity of the Ni–B alloy.