Improving the Activity and Stability of YSZ-Supported Gold Powder Catalyst by Means of Ultrathin, Coherent, Ceria Overlayers. Atomic Scale Structural Insights

A Au­(0.85 wt %)/YSZ
catalyst was prepared through a deposition-precipitation
method and afterward modified with the addition of a very slight amount
of CeO2 (3.7 wt %) by incipient wetness impregnation. A
prior electron microscopy characterization points out that both catalysts,
Au­(0.85 wt %)/YSZ and CeO2(3.7 wt %)/Au­(0.85 wt %)/YSZ,
exhibit a similar Au nanoparticle distribution with most particles
below 5 nm. The CO oxidation reaction was tested over these catalysts
in a heating–cooling cycles experiment, which evidenced a much
better stability of the CeO2-modified sample against deactivation
under very harsh temperature conditions. The characterization of the
catalysts after reaction indicates that the sintering effect of the
Au nanoparticles is quite similar in both cases, suggesting the key
role of specific interactions between Au and CeO2 on the
performance of the surface modified catalyst. An in-depth aberration
correction electron microscopy study, combining imaging and analytical
techniques, allowed us to characterize the details of the spatial
distribution and structure at the atomic level of CeO2.
The formation of atomically thin CeO2 layers extending
on the surface of the YSZ crystallites was detected, particularly
in the form of coherent monolayers epitaxially growth on YSZ(111),
which guaranteed an interaction between ceria and the supported metal
phase. Image simulation and density functional theory calculations
carried out further confirm the electron microscopy observations.
A comparison, in terms of stability, to the results observed on a
CeO2-modified Au/TiO2 catalyst of similar composition
reveals both a much better performance of the catalyst supported on
YSZ and neat differences in the nature of the interactions between
CeO2 and the support as well as between Au and CeO2. The structural coherence between CeO2 and the
cubic YSZ support triggers specific interaction mechanisms which differentiate
the behavior of CeO2/Au/YSZ catalysts from that of CeO2/Au/TiO2. The whole set of results evidence not
only the key role played by highly dispersed and ultrathin ceria surface
layers as modifier and stabilizer of the performance of Au-based CO
oxidation catalysts but also how advanced, aberration corrected, electron
microscopy techniques are a requirement to unveil the structure of
such unique nanostructures.

本研究通过沉积沉淀法（deposition-precipitation）制备了Au(0.85 wt%)/YSZ催化剂，随后采用初湿浸渍法（incipient wetness impregnation）引入极少量CeO₂（3.7 wt%）对其进行改性。
前期电子显微镜（electron microscopy）表征结果显示，Au(0.85 wt%)/YSZ与CeO₂(3.7 wt%)/Au(0.85 wt%)/YSZ两种催化剂的金纳米颗粒分布特征相似，绝大多数颗粒粒径小于5 nm。
针对上述两种催化剂开展的一氧化碳（CO）氧化反应性能测试采用升降温循环实验方案，结果表明经CeO₂改性的样品在严苛温度条件下展现出更优异的抗失活稳定性。
反应后催化剂表征结果显示，两种催化剂中金纳米颗粒的烧结（sintering）效应无显著差异，这提示金与CeO₂之间的特异性相互作用对表面改性催化剂的性能起到关键调控作用。
本研究结合成像与分析技术，开展了深入的像差校正电子显微镜（aberration correction electron microscopy）表征，揭示了CeO₂原子级分辨率下的空间分布与结构细节。
研究发现YSZ晶粒表面形成了原子级超薄的CeO₂层，尤其是在YSZ(111)晶面上形成的相干外延单层，这为氧化铈与负载金属相之间的相互作用提供了可靠保障。
图像模拟与密度泛函理论（density functional theory，DFT）计算进一步验证了电子显微镜的观测结果。
将本研究结果与相近组成的CeO₂改性Au/TiO₂催化剂的稳定性测试结果进行对比，发现YSZ负载型催化剂的综合性能显著更优，且CeO₂与载体、金与CeO₂之间的相互作用本质存在明显差异。
CeO₂与立方相YSZ载体之间的结构相干性，催生了特异性相互作用机制，这使得CeO₂/Au/YSZ催化剂的反应行为与CeO₂/Au/TiO₂催化剂截然不同。
整套研究结果不仅证实了高度分散的超薄氧化铈表面层作为Au基CO氧化催化剂的改性剂与稳定剂的关键作用，同时也表明，先进的像差校正电子显微镜技术是揭示这类独特纳米结构的必要手段。