Abstract
Catalysts are indispensable for modern day society since they are used in the production of transportation fuels, chemicals and materials. Understanding the structure-activity relation for a catalytic system allows the formulation of catalyst structure specifications that optimizes activity, selectivity and stability. For supported catalysts, size and shape of the catalytic
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nanoparticles and their distribution over the support are of paramount importance to achieve the desired catalytic properties. Nevertheless, catalyst preparation has long been considered an art rather than a science. In this work, catalyst preparation is rationalized from the phase behavior of the precursor salt or salt solution and is mainly performed within the framework of silica supported cobalt catalysts for Fischer-Tropsch synthesis. The Fischer-Tropsch synthesis involves the conversion of CO and H2 gas into longer chain hydrocarbons to produce transportation fuels and lubricants from feedstock other than crude oil, e.g. natural gas, coal or biomass. Therefore, this process has a high industrial and economic relevance. Supported cobalt catalysts require a controlled preparation method as there is an optimum cobalt nanoparticle size of ~6 nm. Furthermore, high metal loadings are needed to obtain high activity of the supported catalysts. At the same time, the nanoparticles should be distributed uniformly over the support such that their nearest neighbor distance is maximized and deactivation due to the coalescence of nanoparticles (sintering) is minimized. Finally, due the large scale of the applications, a convenient preparation method producing little waste is desired. Impregnation and drying is such a convenient preparation method and is commonly applied in industry and academia. However, it suffers from a lack of control over the nanoparticle dispersion and distribution, especially when using low cost transition metal nitrate salts with a high solubility in water. In this thesis we aim to acquire fundamental insight into this preparation method to reach the ultimate goal: equally-sized and maximally-spaced nanoparticles. First, pore filling with the precursor solution and salt by impregnation or melt infiltration, respectively was confirmed using differential scanning calorimetry and the depressed melting points of the confined phases as compared to the extraporous phases. Using transmission electron microscopy the distribution of the solution over the pores of the support was visualized after impregnation and after drying. This showed that freeze-drying led to a homogeneous distribution of the salt and subsequently to a more homogeneous nanoparticle distribution in the final catalyst
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