Role of atomic hydrogen supply on the onset of CO$_2$ methanation over La–Ni based hydrogen storage alloys studied by in-situ approach

Mechanochemical CO$_{2}$ methanation reactions using LaNi$_{5}$ and LaNi$_{4.6}$Al$_{0.4}$ hydrogen storage alloy powders were investigated by the in-situ monitoring of the gas pressure change during ball-milling. Methane generation begins when the H$_{2}$ partial pressure drops due to the H-uptake by the powder. Phase transition occurred in the sample after milling for 15 min and 224 min, with separate metallic Ni, La-oxide and La-hydroxide phases observed. Methane generation continued even after this phase separation. Our results imply that the formation of La-hydroxide at the surface and sub-surface contributed to methane generation during ball-milling. A comparison of LaNi$_{5}$ and LaNi$_{4.6}$Al$_{0.4}$ suggests the amount of hydrogen stored in the hydrogen storage powder dominates the timing of the onset of the methane generation.