Kinetic and thermodynamic investigation of hydrogen absorption by ternary LaNi 4Al and LaNi 4Al-Pd Alloys

Abstract

The process of hydrogen absorption by ternary LaNi 4Al and LaNi 4Al-Pd alloys was investigated by a differential scanning calorimetry method in an atmosphere of hydrogen flow over the temperature range from room temperature to 600°C as well as by X-ray analysis. It was shown that the exothermic process of hydrogen absorption by the LaNi 4Al alloy takes place within two temperature regions, at about 100 and 450°C, respectively. In cases of multiple absorptions and desorptions (as a consequence of activation) the process of absorption is shifted towards lower temperatures. The first exothermic peak corresponds to a first-order reaction in three stages for which kinetic and thermodynamic parameters were determined. The absorption of hydrogen considerably changes (damages) the crystal lattice of the alloy. The addition of small amounts of palladium (LaNi 4Al-Pd alloy) results in a much higher affinity and capacity for hydrogen absorption as shown by the kinetic and thermodynamic parameter, i.e. a lower activation energy, a greater rate constant, a smaller frequency factor and a greater absorption capacity. It was assumed that the role of added palladium consists of producing a spill-over effect thus facilitating the dissociation of the hydrogen molecule whereby the absorption of atomic hydrogen is accelerated. A subsequent addition of palladium to LaNi 4Al alloy increases its absorption capacity.