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

FeTi金属间化合物是最常用的储氢合金之一，自从Reilly和Wiswall首次发现FeTi的这一有趣的特性以来，人们已经做了大量的工作。FeTi由于其相对便宜的材料成本和约H/FeTi = 1.9的储氢容量而在实际应用中作为储氢材料是有吸引力的候选者。众所周知，FeTi的表面在暴露于氢气时表现出非常不活跃的行为。FeTi合金在高温和氢压下的初始活化需要循环氢化-脱氢。然而，MA过程明显降低了储氢能力。对于研磨的Pd粉末，也发现最大储氢容量的类似显著降低。在本研究中，我们采用机械研磨法制备了纳米结构的FeTi样品。并且，我们使用Sieverts型装置测量氢吸收速率，并研究了表面污染的影响（低真空暴露时间：0min 1h）对吸氢动力学的影响，通过测量纳米结构FeTi的初始吸氢速率与压力的关系，发现随着暴露于低真空的时间的增加，表面污染的进行，n从1.0变化到0.5。这意味着速率决定步骤从具有2个相对薄的氧化物层的合金表面上的H分子的解离转变为H原子通过生长的表面氧化物层的渗透。

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.