Wang Yicheng, Papanikolaou Konstantinos G, Hannagan Ryan T, Patel Dipna A, Balema Tedros A, Cramer Laura A, Kress Paul L, Stamatakis Michail, Sykes E Charles H
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, USA.
Thomas Young Centre and Department of Chemical Engineering, University College London, Roberts Building, Torrington Place, London WC1E 7JE, United Kingdom.
J Chem Phys. 2020 Dec 28;153(24):244702. doi: 10.1063/5.0034520.
Metal alloys are ubiquitous in many branches of heterogeneous catalysis, and it is now fairly well established that the local atomic structure of an alloy can have a profound influence on its chemical reactivity. While these effects can be difficult to probe in nanoparticle catalysts, model studies using well defined single crystal surfaces alloyed with dopants enable these structure-function correlations to be drawn. The first step in this approach involves understanding the alloying mechanism and the type of ensembles formed. In this study, we examined the atomic structure of RhCu single-atom alloys formed on Cu(111), Cu(100), and Cu(110) surfaces. Our results show a striking difference between Rh atoms alloying in Cu(111) vs the more open Cu(100) and Cu(110) surface facets. Unlike Cu(111) on which Rh atoms preferentially place-exchange with Cu atoms in the local regions above step edges leaving the majority of the Cu surface free of Rh, highly dispersed, homogeneous alloys are formed on the Cu(100) and (110) surfaces. These dramatically different alloying mechanisms are understood by quantifying the energetic barriers for atomic hopping, exchange, swapping, and vacancy filling events for Rh atoms on different Cu surfaces through theoretical calculations. Density functional theory results indicate that the observed differences in the alloying mechanism can be attributed to a faster hopping rate, relatively high atomic exchange barriers, and stronger binding of Rh atoms in the vicinity of step edges on Cu(111) compared to Cu(110) and Cu(100). These model systems will serve as useful platforms for examining structure sensitive chemistry on single-atom alloys.
金属合金在多相催化的许多分支中无处不在，现在已经相当明确的是，合金的局部原子结构对其化学反应性会产生深远影响。虽然在纳米颗粒催化剂中难以探究这些效应，但使用与掺杂剂合金化的定义明确的单晶表面进行模型研究能够得出这些结构 - 功能关系。这种方法的第一步涉及了解合金化机制以及形成的原子团簇类型。在本研究中，我们研究了在Cu(111)、Cu(100)和Cu(110)表面上形成的RhCu单原子合金的原子结构。我们的结果表明，Rh原子在Cu(111)表面合金化与在更开放的Cu(100)和Cu(110)表面晶面合金化之间存在显著差异。与Cu(111)不同，在Cu(111)上Rh原子优先与台阶边缘上方局部区域的Cu原子进行位置交换，使得大部分Cu表面没有Rh，而在Cu(100)和(110)表面上形成了高度分散的均匀合金。通过理论计算量化Rh原子在不同Cu表面上的原子跳跃、交换、置换和空位填充事件的能量势垒，可以理解这些截然不同的合金化机制。密度泛函理论结果表明，观察到的合金化机制差异可归因于与Cu(110)和Cu(100)相比，Rh原子在Cu(111)台阶边缘附近具有更快的跳跃速率、相对较高的原子交换势垒以及更强的结合力。这些模型体系将作为研究单原子合金上结构敏感化学的有用平台。
