Understanding the role of Ti in reversible hydrogen storage as sodium alanate: a combined experimental and density functional theoretical approach.
Understanding the role of Ti in reversible hydrogen storage as sodium alanate: a combined experimental and density functional theoretical approach.
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DOI：
10.1021/ja060437s
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发表时间：
2006-08
影响因子：
15
通讯作者：
S. Chaudhuri;J. Graetz;A. Ignatov;J. Reilly;J. Muckerman
中科院分区：
文献类型：
--
作者：
S. Chaudhuri;J. Graetz;A. Ignatov;J. Reilly;J. Muckerman
本文报道了铝酸钠（NaAlH（4））贮氢的实验和理论研究结果。这种材料中的可逆储氢取决于2- 4%Ti掺杂剂的存在。我们的综合研究表明，Ti的作用可能完全与脱氢NaAlH（4）中存在的金属Al相中的含Ti活性催化位点有关。EXAFS数据表明，脱氢样品中含有高度无序的Ti-Al距离分布，在第二配位层之外没有长程有序。我们已经使用密度泛函理论技术来计算可能的Ti安排的Al（001）表面上的Ti覆盖范围从0.125至0.5单层（ML）的化学势，并确定了那些可以通过自发或只有适度激活的途径化学吸附分子氢。化学吸附过程表现出低势垒位点的特征节点对称性：初始掺杂的表面-H（2）加合物的最高占据分子轨道（HOMO）结合了氢的σ反键分子轨道，允许电荷密度从表面转移以解离分子氢。这项工作还提出了一个合理的机制，为运输的氢化铝物种回到NaH晶格，这是支持的Car-Parrinello分子动力学（CPMD）模拟的稳定性和流动性的铝簇（alanes）在Al（001）。作为对钛的作用和铝烷的后续扩散的实验验证，我们实验证明了AlH（3）与NaH反应形成NaAlH（4），而不需要任何催化剂或氢气超压。
We report the results of an experimental and theoretical study of hydrogen storage in sodium alanate (NaAlH(4)). Reversible hydrogen storage in this material is dependent on the presence of 2-4% Ti dopant. Our combined study shows that the role of Ti may be linked entirely to Ti-containing active catalytic sites in the metallic Al phase present in the dehydrogenated NaAlH(4). The EXAFS data presented here show that dehydrogenated samples contain a highly disordered distribution of Ti-Al distances with no long-range order beyond the second coordination sphere. We have used density functional theory techniques to calculate the chemical potential of possible Ti arrangements on an Al(001) surface for Ti coverages ranging from 0.125 to 0.5 monolayer (ML) and have identified those that can chemisorb molecular hydrogen via spontaneous or only moderately activated pathways. The chemisorption process exhibits a characteristic nodal symmetry property for the low-barrier sites: the incipient doped surface-H(2) adduct's highest occupied molecular orbital (HOMO) incorporates the sigma antibonding molecular orbital of hydrogen, allowing the transfer of charge density from the surface to dissociate the molecular hydrogen. This work also proposes a plausible mechanism for the transport of an aluminum hydride species back into the NaH lattice that is supported by Car-Parrinello molecular dynamics (CPMD) simulations of the stability and mobility of aluminum clusters (alanes) on Al(001). As an experimental validation of the proposed role of titanium and the subsequent diffusion of alanes, we demonstrate experimentally that AlH(3) reacts with NaH to form NaAlH(4) without any requirement of a catalyst or hydrogen overpressure.