†Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States.
Acc Chem Res. 2015 May 19;48(5):1515-23. doi: 10.1021/acs.accounts.5b00101. Epub 2015 May 1.
Metal oxide films can form on late transition-metal catalysts under sufficiently oxygen-rich conditions, and typically cause significant changes in the catalytic performance of these materials. Several investigations using sensitive in situ surface characterization techniques reveal that the CO oxidation activity of Pd and other late transition-metal catalysts increases abruptly under conditions at which metal oxide structures begin to develop. Findings such as these provide strong motivation for developing atomic-scale descriptions of oxidation catalysis over oxide films of the late transition-metals. Surface oxygen vacancies can play a central role in mediating oxidation catalysis promoted by metal oxides. In general, adsorbed reactants abstract oxygen atoms from the lattice of the oxide surface, thereby creating oxygen vacancies, while gaseous O2 replenishes the reactive surface oxygen atoms and eliminates oxygen vacancies. Oxygen vacancies also represent a distinct type of surface site on which the binding and reactivity of adsorbed species can differ compared with sites on the pristine oxide surface. Detailed characterization of vacancy-mediated rate processes is thus essential for developing reliable mechanistic descriptions of oxidation catalysis over reducible metal oxide films. Careful measurements performed in ultrahigh vacuum (UHV) using well-defined oxide surfaces in combination with molecular simulations afford the capability to isolate and characterize such reaction steps, and thus provide information that is needed for developing mechanistic models of oxidation catalysis over metal oxides. In this Account, we discuss vacancy-mediated processes that are involved in the oxidation of CO on the PdO(101) surface as determined from UHV surface science experiments and density functional theory (DFT) calculations. These studies show that CO binds strongly on Pd atoms that are located next to surface oxygen vacancies, and diffuses rapidly to these sites during reduction of the oxide surface by CO. The enhanced binding also raises the energy barriers for desorption and oxidation of CO, but the difference in these barriers remains nearly identical to that for CO adsorbed on the pristine PdO(101) surface. These recent studies also show that oxygen from the subsurface efficiently eliminates surface oxygen vacancies during CO oxidation at temperatures as low as 400 K, and thereby reveal a facile pathway by which PdO(101) surface domains can be maintained during oxide reduction.
在富氧条件下，晚期过渡金属催化剂上可以形成金属氧化物薄膜，通常会导致这些材料的催化性能发生显著变化。使用几种敏感的原位表面特征技术进行的多项研究表明，在金属氧化物结构开始发展的条件下，Pd 和其他晚期过渡金属催化剂的 CO 氧化活性会突然增加。这些发现为发展晚期过渡金属氧化物薄膜氧化催化的原子尺度描述提供了强有力的动力。表面氧空位在介导金属氧化物促进的氧化催化中可以发挥核心作用。一般来说，吸附的反应物从氧化物表面的晶格中抽取氧原子，从而产生氧空位，而气态 O2 则补充反应性表面氧原子并消除氧空位。氧空位也代表了一种独特的表面位，与原始氧化物表面上的位相比，吸附物种的结合和反应性可能有所不同。因此，详细表征空位介导的速率过程对于开发还原型金属氧化物薄膜氧化催化的可靠机理描述至关重要。在超高真空 (UHV) 中使用定义明确的氧化物表面进行仔细测量，并结合分子模拟，具有隔离和表征这些反应步骤的能力，从而为开发金属氧化物氧化催化的机理模型提供了所需的信息。在本报告中，我们讨论了从 UHV 表面科学实验和密度泛函理论 (DFT) 计算中确定的在 PdO(101)表面上 CO 氧化涉及的空位介导过程。这些研究表明，CO 强烈地吸附在位于表面氧空位附近的 Pd 原子上，并且在氧化物表面被 CO 还原时迅速扩散到这些位置。增强的结合还会提高 CO 的脱附和氧化的能垒，但这些壁垒之间的差异与在原始 PdO(101)表面上吸附的 CO 几乎相同。这些最近的研究还表明，在 400 K 以下的温度下，来自次表面的氧可以有效地消除 CO 氧化过程中的表面氧空位，从而揭示了 PdO(101)表面域在氧化物还原过程中得以维持的简便途径。
