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.