Dynamic observations of Au catalysts by environmental electron microscopy

Au clusters in the size range (1-8 nm), supported on MgO and TiO2, are studied at the atomic scale by High Resolution Transmission Electron Microscopy (HRTEM), in standard conditions and during cycles of gas treatments by environmental HRTEM. Their morphology, the structure of the interface with the substrate and their adhesion energy are deduced from top and profile views, according to the preparation techniques, by atomic deposition in ultra high vacuum or by deposition precipitation in wet conditions. Under hydrogen atmosphere the gold clusters exhibit a truncated octahedron shape while under oxygen they show a rounding of the shape. These evolution are reversible in hydrogen/oxygen cycles. In the case of the CO oxidation by Au clusters supported on TiO2, the formation of CO2 in the microscope sample holder is evidenced by a differentially pumped mass spectrometer connected to the environmental sample holder, at the same time as the rounding of the clusters. The implication of the shape evolution in CO oxidation is discussed.