| Supported catalysts are among the most important examples of nanostructured materials because of their extensive use in energy, chemical and environmental processes. Many times the preparation of supported catalysts has been referred to as 'an art rather than a science'. Over the last decades, however, great strides have been made to develop a science base for catalyst preparation. New or improved models for metal ion adsorption, deposition precipitation, and chemical vapour deposition are amongst these strides. In this proposal we focus on the important but very complex field of catalyst preparation via impregnation and drying. The preparation of supported catalysts mostly comprises the application of a precursor of the active component onto support materials such as silica, alumina or carbon. The preparation of supported catalysts often is carried out by so-called impregnation and drying that involves the use of an aqueous solution of a metal salt that is contacted with the porous support followed by drying to bring about crystallization of the metal salt. In case of concentrated acidic solutions (such as those of metal nitrates) the extent of interaction between the metal cations and the support is limited. Drying then gives rise to crystallization of the metal nitrate in question and redistribution of liquid and salt due to capillary flow may be extensive. From previous work in our laboratory it appeared that next to redistribution one of the key problems of impregnation may be that only partial wetting occurs of the porous solid by the impregnation solution. In order to facilitate fundamental studies on catalyst preparation we propose to make use of nanostructured support materials, more specifically ordered mesoporous materials (OMM) as model supports. In the first sub-project of this proposal, wetting of silica and alumina supports in the form of OMM of variable pore diameter (2-10 nm) will be studied using differential scanning calorimetry (DSC). The extent of wetting can be derived from freeze point depression and heat of crystallization of the solution that has entered the mesopores of the support. The extent of wetting will be related to the surface properties of the support material in particular the hydroxyl group density and location, on the one hand, and the nature of the solution (pH, concentration, viscosity, surface tension), on the other hand. The nature of the confined liquid in the support mesopores will be studied using EXAFS and UV-VIS and compared to bulk (unconfined) solutions. It is expected that, e.g., coordination numbers of the metal ions will be affected by confinement. In the second sub-project, crystallization from the impregnated solution upon drying will be studied. The effects of the method of drying will be studied by comparison of conventional drying and freeze drying. The latter method will restrict mobility of solvent/salt during drying whereas conventional drying may give rise to redistribution of the active component. The crystallization of the metal salt in question will be studied in detail using in situ XRD and Electron Tomography. The latter technique is used to obtain quantitative data on the metal salt dispersion and distribution in 3D over the support material. Relationships between the nature of the confined impregnation solution and the crystallization will be established. Finally, in a case study of the preparation of alumina or silica supported cobalt catalysts prepared via impregnation and drying we will apply above knowledge to arrive at predetermined dispersions and distributions of the active component and demonstrate their effects on the Fischer Tropsch reaction. Since the preparation of supported catalysts often largely determines their performance (activity, selectivity and stability) next to its scientific relevance, the proposed study may give rise to new and improved procedures for industrial catalyst preparation. |
