Shen Lu, Dadras Jonny, Alexandrova Anastassia N
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
Phys Chem Chem Phys. 2014 Dec 28;16(48):26436-42. doi: 10.1039/c4cp01877j.
Pure and doped sub-nanoclusters can exhibit superb catalytic activity, which, however, strongly depends on their size, shape, composition, and the nature of the support. This work is about surface-deposited sub-nano Pt-based clusters, which are promising catalysts for the reactions of dehydrogenation. Using density functional theory and ab initio calculations, and an ab initio genetic algorithm for finding the global minima of clusters, we found a peculiar effect that Pt5 and Pt4Zn clusters exhibit upon deposition on MgO(100). Both of them change shapes from the gas phase 3-D form to a planar form, and they stand upright on the support. Several reasons are responsible for this behaviour. In part, clusters go flat due to the electron transfer from the support. Indeed, the anionic Pt5(-) and Pt4Zn(-) species are flat also in the gas phase. Charging induces the second-order Jahn-Teller effect (or partial covalency) facilitated by the recruitment of the higher-energy 6p atomic orbitals on Pt into the valence manifold, and that is the reason for the planarization of the anions. Secondly, clusters maximize interactions with the surface O atoms (resulting in further favouring of 2-D structures over 3-D), and avoid contacts with surface Mg atoms (resulting in upright morphologies).
纯的和掺杂的亚纳米团簇可表现出卓越的催化活性，然而，其催化活性强烈依赖于团簇的尺寸、形状、组成以及载体的性质。这项工作聚焦于表面沉积的亚纳米铂基团簇，它们是脱氢反应中很有前景的催化剂。利用密度泛函理论和从头算计算方法，以及一种用于寻找团簇全局极小值的从头算遗传算法，我们发现了Pt5和Pt4Zn团簇在沉积于MgO(100)上时呈现出的一种特殊效应。它们二者均从气相三维形态转变为平面形态，并直立于载体上。这种行为有几个原因。部分原因是，由于载体的电子转移，团簇变平。实际上，阴离子型的Pt5(-)和Pt4Zn(-)物种在气相中也是平面的。电荷诱导了二阶 Jahn - Teller效应（或部分共价性），这是由Pt上高能6p原子轨道进入价层所促成的，这就是阴离子平面化的原因。其次，团簇与表面O原子的相互作用最大化（导致二维结构比三维结构更受青睐），并避免与表面Mg原子接触（导致直立形态）。
