Pt/CERIA THIN FILM MODEL CATALYSTS AFTER HIGH TEMPERATURE REDUCTION: A (HR)TEM STUDY

Ceria-supported noble metals are widely used as valuable catalysts in deNOx and combustion technology. However, like most noble metal-reducible oxide combinations they are subjected to strong metal-support interaction upon high temperature reduction, resulting in changing catalytic properties [1] and eventually deactivation. An investigation of these phenomena and of structureactivity correlations is greatly facilitated by model systems with planar thin supports. In the present work, well-facetted Pt particles, obtained by epitaxial growth on (001) NaCl surfaces, were supported by thin films of crystalline ceria. These model catalysts are well suited for characterisation by electron microscopy and provide a defined reference state for studies of structural changes during catalyst treatment and reaction. Catalyst changes due to calcination and reduction have been studied after treating the samples in oxygen (673K, 1h), and after hydrogen exposure at increasing temperatures from 673K to 1073K for 1h, either in a flow system or in a circulating batch reactor. The morphology and composition were followed by (HR)TEM, electron diffraction and EELS. In their initial state, most Pt particles are truncated half octahedra with (001) faces perpendicular to the electron beam, exhibiting square or rectangular shapes at lower magnification [2]. They are surrounded by grains of CeO , partly in epitaxial correlation to the metal. In contrast, after a 2