Nanostructural evolution under reducing conditions of a Pt/CeTbOx catalyst:: A new alternative system as a TWC component

By using a combination of high resolution electron microscopy (HREM) and nanoanalytical electron microscopy (EELS-STEM) techniques, the nanoscale evolution of a 5% Pt/Ce0.8Tb0.2O2-x catalyst submitted to reduction in pure hydrogen within the 423-1173 K range has been investigated. Fine details about the platinum catalytic particles, such as their bulk and surface chemical state, size, shape, or structural relationships with the mixed-oxide support, have been established and their change with reduction temperature monitored. Results indicate that at low reduction temperatures (<773 K) platinum is present in this catalyst in the form of well-faceted metallic particles with clean surfaces. At higher reduction temperatures drastic transformations have been detected. Thus after 1-h reduction at 973 K the decoration of the metal particle surfaces by the support takes place, while after reduction at 1173 K the formation of an intermetallic compound, with LnPt(5) (Ln = Ce, Tb) stoichiometry, occurs. The interpretation of HREM images by image simulation has allowed us to determine the morphology of the intermetallic crystals as being of the beryl-type. Specific orientation relationships between the mixed-oxide support and the intermetallic have also been established by comparing experimental and simulated HREM images, as well as from the analysis of selected area diffraction patterns. Electron energy loss spectra confirm that in the mixed-oxide used as support, cerium and terbium are homogeneously distributed. No phase segregation was observed either in the nonreduced catalysts or in the catalysts resulting from reduction treatments in the whole range of T-red here studied. In the sample reduced at 1173 K, the analysis of the fine structure of EELS spectra recorded across the interface between the support and the supported particles confirms the formation of an intermetallic phase and reveals a preferential incorporation of Ce in these particles. The oxidation state of Ce in the intermetallic is markedly lower than that observed in the support.