Xiaoqing Pan
Department of Chemical Engineering and Materials Science, Department of Physics and Astronomy, University of California - Irvine, Irvine, California 92697, USA.
Understanding the atomic structures of catalysts under realistic conditions with atomic precision is crucial to design better materials for challenging transformations. For example, under reducing conditions, certain reducible supports migrate onto supported metallic particles and create strong metal−support states that drastically change the reactivity of the systems. The details of this process are still unclear and preclude its thorough exploitation. In the past decade, most of atomic-scale transmission electron microscopy (TEM) studies involving gas-solid interactions were conducted in an environmental TEM, where the gas pressure is typically limited to less than 1/100 of atmosphere. Recently, it has become possible to overcome this limitation through a MEMS-based, electron-transparent closed cell with a heating stage.
In this talk, I will present our recent results using this device (the Protochips AtmosphereTM system) in selected catalyst systems. In a palladium/titania (Pd/ TiO2) catalyst, we directly observed the formation of the oxide overlayers on the supported Pd particles with atomic resolution under atmospheric pressure and high temperature. It shows that an amorphous reduced titania layer is formed at low temperatures, and that crystallization of the layer into either mono- or bilayer structures is dictated by the reaction environment. This transition occurs in combination with a dramatic reshaping of the metallic surface facets. In-situ TEM observations of a modular Pd-ceria core-shell nanostructured catalyst (Pd@CeO<sub< a="">>2) showed that an unexpected structural transformation occurs upon heating at high temperatures. The system reaches to a stable state with the mixture of nanoparticles with two different sizes, which accounts for the exceptional catalytic properties that have been reported. Using the similar techniques, we also studied the core-shell platinum-metal (Pt-M) nanoparticles which show a catalytic performance in the oxygen reduction reaction (ORR) superior to that of pure Pt nanoparticles. To understand the formation mechanism of the Pt shell, we studied thermally activated core-shell formation in Pt3Co nanoparticles via in-situ electron microscopy with the gas cell. The disordered Pt3Co nanoparticle was found to transform into an ordered intermetallic structure after annealing at high temperature (725 °C) in 760 Torr O2, followed by layer-by-layer Pt shell growth on (100) surfaces at low temperature (300 °C). The apparent ‘anti-oxidation’ phenomenon promoted by the ordered Pt3Co phase is favorable to the ORR catalyst, which operates in an oxidizing environment.
