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

Abstract

A Au(0.85 wt %)/YSZ catalyst was prepared through adeposition-precipitation method and afterward modified with theaddition of a very slight amount of CeO 2 (3.7 wt %) by incipientwetness impregnation. A prior electron microscopy characterizationpoints out that both catalysts, Au(0.85 wt %)/YSZ and CeO 2(3.7 wt%)/Au(0.85 wt %)/YSZ, exhibit a similar Au nanoparticle distributionwith most particles below 5 nm. The CO oxidation reaction was testedover these catalysts in a heating−cooling cycles experiment, whichevidenced a much better stability of the CeO2-modified sample againstdeactivation under very harsh temperature conditions. The character-ization of the catalysts after reaction indicates that the sintering effect ofthe Au nanoparticles is quite similar in both cases, suggesting the key role of specific interactions between Au and CeO2 on theperformance of the surface modified catalyst. An in-depth aberration correction electron microscopy study, combining imagingand analytical techniques, allowed us to characterize the details of the spatial distribution and structure at the atomic level ofCeO 2. The formation of atomically thin CeO2 layers extending on the surface of the YSZ crystallites was detected, particularly inthe form of coherent monolayers epitaxially growth on YSZ(111), which guaranteed an interaction between ceria and thesupported metal phase. Image simulation and density functional theory calculations carried out further confirm the electronmicroscopy observations. A comparison, in terms of stability, to the results observed on a CeO2-modified Au/TiO 2 catalyst ofsimilar composition reveals both a much better performance of the catalyst supported on YSZ and neat differences in the natureof the interactions between CeO2 and the support as well as between Au and CeO2. The structural coherence between CeO 2and the cubic YSZ support triggers specific interaction mechanisms which differentiate the behavior of CeO2 /Au/YSZ catalystsfrom that of CeO 2/Au/TiO2. The whole set of results evidence not only the key role played by highly dispersed and ultrathinceria 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.