Ray Keith G, Klebanoff Leonard E, Lee Jonathan R I, Stavila Vitalie, Heo Tae Wook, Shea Patrick, Baker Alexander A, Kang Shinyoung, Bagge-Hansen Michael, Liu Yi-Sheng, White James L, Wood Brandon C
Lawrence Livermore National Laboratory, Livermore, California, USA.
Sandia National Laboratories, Livermore, California, USA.
Phys Chem Chem Phys. 2017 Aug 30;19(34):22646-22658. doi: 10.1039/c7cp03709k.
Mg(BH) is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB to Mg(BH) complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H dissociation, likely at Mg-terminated MgB surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH) complexes without persistent BH intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB are discussed.
Mg(BH₄)₂是一种很有前景的固态储氢材料，转化为MgB₂时可释放14.9 wt%的氢。尽管已经提出了几种脱氢途径，但氢化过程却不太为人所理解。在此，我们开展了一项实验与理论相结合的研究，阐明了与MgB₂初始吸氢阶段相关的关键原子机制。傅里叶变换红外光谱、X射线吸收光谱和X射线发射光谱与光谱模拟相结合，以表明氢化最初可通过MgB₂直接转化为Mg(BH₄)₂络合物进行。通过将从头算计算与从实验吸氢曲线中提取的势垒相结合，绘制出相关的能量图景，并据此构建了动力学模型。动力学模型的结果表明，初始氢化通过多步过程发生：分子H解离，可能发生在Mg端接的MgB₂表面，随后原子氢迁移至有缺陷的硼位点，并最终在该位点形成稳定的B-H键，直接生成Mg(BH₄)₂络合物，而无持久的BH中间体。文中讨论了对理解MgB₂氢化时化学、结构和电子变化的意义。
