Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces
Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces
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DOI：
10.1021/acsami.1c07510
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发表时间：
2021-07-18
影响因子：
9.5
通讯作者：
Greeley, Jeffrey P.
中科院分区：
文献类型：
--
作者：
Gao, Junxian;Sawant, Kaustubh J.;Greeley, Jeffrey P.
过渡金属上超薄氧化膜的结构和化学转变是多相催化中许多复杂现象的核心，例如强金属-载体相互作用（SMSI）。然而，对这些转变的原子尺度理解有限，特别是对于 ZnO 等不可还原氧化物。在这里，通过结合密度泛函理论计算和表面科学技术，包括扫描隧道显微镜、X射线光电子能谱、高分辨率电子能量损失谱和低能电子衍射，我们研究了Pd(111)上明确的超薄ZnOxHy薄膜在不同气相条件[超高真空(UHV)、5 x 10(-7) mbar O-2和D-2/O-2混合物]下的界面相互作用，揭示了不可还原氧化物的 SMSI 效应。在 550 K 下，在 D-2/O-2 混合物 (1:4) 中对亚单层氧化锌薄膜进行连续处理，引发了从双层到单层以及进一步到 Pd-Zn 近表面合金的可逆结构转变，表明氧化锌作为一种不可还原的氧化物，可以在金属表面上铺展，并在氢气存在下表现出类似 SMSI 的行为。开发了混合正则-大正则相图，以弥合 UHV 条件和真正的 SMSI 环境之间的差距，表明除了表面合金形成之外，某些具有本体不存在的化学计量的 ZnOxHy 薄膜在氢存在下通过 Pd 稳定。基于理论和实验观察相结合，我们提出，根据环境条件，ZnO 等不可还原氧化物载体的 SMSI 金属纳米颗粒包封涉及表面（羟基）氧化物和表面合金的形成。
Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, andlow-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions[ultrahigh vacuum (UHV), 5 x 10(-7) mbar of O-2, and a D-2/O-2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D-2/O-2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.