Effect of Surface Contamination of the nano-FeTi Hydrogen Storage Alloys on Reaction Rate

FeTi金属间化合物是最传统的储氢合金之一，自从Reilly和Wiswall首次发现FeTi这一有趣的特征以来，人们已经做了大量的工作。FeTi是一种很有吸引力的储氢材料，因为它的材料成本相对较低，储氢容量在H/Fei=1.9左右。众所周知，FeTi表面在氢气中表现出非常不活跃的行为。FeTi在高温和氢气压力下的初始活化需要循环加氢-脱氢。最近报道了机械合金化合成金属间化合物的研究。然而，MA工艺明显降低了储氢容量。磨制的Pd粉的最大储氢量也有类似的显着降低。在本研究中，我们采用机械研磨的方法制备了纳米结构的FeTi样品。用Sieverts‘s型吸氢仪测量了吸氢速率，考察了纳米FeTi样品表面污染(低真空暴露时间：0min 1h)对吸氢动力学的影响，通过测量纳米FeTi样品初始吸氢速率的压力依赖关系，发现随着暴露时间的延长，n从1.0变化到0.5。这意味着，速率控制步骤从具有2个相对较薄的氧化层的合金表面的H分子的解离转变为H原子通过生长的表面氧化层的渗透。

The FeTi intermetallic compound is one of the most conventional hydrogen storage alloys, and much work has been done since the first finding of this interesting feature of FeTi by Reilly and Wiswall. FeTi is an attractive candidate for practical use as a hydrogen storage material because of its relatively inexpensive material costs and a hydrogen storage capacity around H/FeTi =1.9.As is well known, the surface of FeTi exhibits very inactive behaviour on exposure to hydrogen gas. Cyclic hydrogenation-dehydrogenation is needed for the initial activation of FeTi at high temperatures and hydrogen pressures.Recent studies on the synthesis of intermetallic compounds by mechanical alloying (MA) have been reported. However, the MA process pronouncedly reduces the hydrogen storage capacity. A similar marked reduction in the maximum hydrogen storage capacity was also found for milled Pd powder. The maximum hydrogen storage capacity does not dramatically recover for those alloys even after annealing treatment.In this study, we prepared the nano-structured FeTi sample prepared by mechanical grinding (MG). And, we used a Sieverts' type apparatus for the measurement of the rate of hydrogen absorption and-investigated the effect of surface contaminations (low vacuum exposures time : 0min 1h) of the surface of the nano-structured FeTi sample on the hydrogen absorption kinetics.From the measurement of the pressure dependence of the initial hydrogen absorption rate of the nano-structured FeTi, n was found changed from 1.0 to 0.5 as the surface contamination proceeded with increasing exposure time to low vacuum. This means that the rate determining step changed from the dissociation of H molecules on the alloy surface with 2 relatively thin oxide layers to the permeation of H atoms through grown surface oxide layers.