Development of Structural Characterisation Tools for Catalysts

Abstract

Because of the diversity of their compositions and structures, and the treatments needed to render them active, heterogeneous catalysts present a major challenge in structural characterisation. Electron microscopy provides textural and structural information at the scale of the individual particle. We have been able to analyse expitaxial relationships between nanometer size particles and their support and to determine which crystal faces are most exposed. Chemical analysis can be carried out on individual particles in a bimetallic catalyst. Limitations of this technique are shown for characterisation of catalysts at the atomic scale or in reactive conditions. Here, global analysis methods based on X-ray absorption and diffraction provide more information. X-ray absorption fine structure analysis has been applied to sub-nanometer size particles in platinum based catalysts to explore interactions between the metal and reactive gases such as hydrocarbons and H2S. Differences observed between monometallic and bimetallic solids lead to structural models to explain differences in catalyst reactivity. X-ray diffraction, combined with electron microscopy and nuclear magnetic resonance spectroscopy, shows the presence of different forms of extra-framework aluminium in steamed zeolites. Quantification of some these forms has been possible and a study of their reactivity towards different dealumination agents has been achieved. Work in progress shows the advantages of a combination of X-ray diffraction and absorption to study decomposition of hydrotalcites to form mixed oxides as well as possibilities in infrared spectroscopy of adsorbed CO to determine surface sites in Fischer-Tropsch catalysts. Use of in situ analysis cells enables a detailed description of catalyst structure in reactive atmospheres and opens the possibility of correlating structure with catalytic activity.