Enhancement in hydrogenation properties of ball-milled AB5-type hydrogen storage alloy through catalyst

Abstract

Ball-milling of the hydrogen storage alloy has been shown to improve many hydrogenation properties under specific ball-milling parameters. The present investigation is based on further improvement in the hydrogenation properties through the addition of a catalyst. Catalysts are known to increase the rate of reaction. In the present investigation, rigorous studies have been performed on the various factors affecting the hydrogenation reaction through the addition of catalyst during ball-milling. The basic idea of the mechanism of catalyst is the availability of the free valencies behaving like active centres for promoting the reaction rate. Free valencies are more crowded at corners, peaks and cracks on the catalyst. The rough surface of the catalyst and finely dispersed catalyst are more effective in promoting the reaction rate. In the present investigation, various factors like spillover effect; defects introduced to the catalyst and hydrogen storage alloy during ball-milling; elasticity of metal hydride matrix; nature, size, hardness, number of unpaired electrons and variable valency of catalyst; the electronegative difference between catalyst and metal hydride; electron distribution and orbital structure; the ability of the catalyst to transfer stable diatomic hydrogen into the desired ionic configuration; binding between H atoms and catalyst and lowering of activation energy due to catalyst have been discussed in terms of their effect on hydrogenation properties. It has been found that all these factors help in lowering the activation energy for the hydrogenation process and improve the hydrogenation properties.