A novel procedure for accurate estimations of the lattice parameter of supported nanoparticles from the analysis of plan view HREM images: Application to the structural investigation of Pd/CeO2 catalysts

摘要

A new procedure to determine, with very high accuracy, the lattice parameter of individual supported nanoparticles of interest in catalytic system is presented, checked and applied to ceria supported Palladium catalysts. The procedure is based on the analysis of double diffraction effects present in High Resolution Electron Microscopy (HREM) images recorded in top view conditions. For nanoparticles about 5 nm in diameter accuracies in the order of ±0.2% are demonstrated by using a method in which intensity profiles of Digital Diffraction Patterns obtained from experimental HREM images are fitted to a set of Lorentzian-type functions. The new method allows characterizing the distribution of lattice parameter of nanoparticles in systems which are out of range for XRD, as it is the case of catalysts with very low loadings or those in which the particle size distribution is dominated by very small (<5 nm) nanoparticles. In the Pd/CeO2 catalysts investigated, evidences of phenomena of large interest to understand the catalytic behaviour like Pd-Ce alloying or Pd nanoparticle encapsulation have been evidenced by measurements of Pd lattice parameters performed using the new method on catalyst reduced at high temperatures (>773 K). Likewise, in Pd/CeC2 catalysts reduced at low temperatures (<773 K), structural accommodation phenomena of the metal nanoparticles on the ceria support have been revealed whose intensity depends both on the particle diameter and on reduction temperature.