Abstract
High surface area supported metal (oxide) catalysts are essential for the production of fuels, chemicals, pharmaceuticals and the abatement of environmental pollution. Impregnation of high surface area supports, often silica or alumina, followed by drying, calcination and reduction is one of the most frequently applied methods to prepare these materials
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because of its practical simplicity, cost efficiency and small waste streams. Metal nitrates are often preferred as precursor because of their high solubility in water and low costs. However, often low specific metal surface areas are obtained due to metal nitrate redistribution and agglomeration during drying and calcination. Previous studies in this group revealed that decomposition of nickel or cobalt nitrates supported on silica in a flow of 1% v/v NO/He prevents agglomeration, yielding highly dispersed NiO and Co3O4. The goal of the research described in this thesis is to understand the mechanism by which NO prevents agglomeration of metal nitrates, and the relation between the metal nitrate decomposition pathway and the final metal oxide dispersion. To this end, the phase evolution of a wide range of metal nitrates was studied using advanced in and ex situ characterization techniques, including x-ray diffraction (XRD), diffuse reflectance FTIR spectroscopy (DRIFTS), thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM). In this thesis we have shown that in-depth fundamental studies provide valuable insight into the physico-chemical processes that occur during catalyst preparation. Using this methodology we succeeded in identifying the mobile and immobile metal nitrate phases during decomposition and were able to steer the decomposition pathway towards the phases that yielded high metal oxide dispersions. For silica supported nickel and cobalt nitrate hydrate, mobility and agglomeration is reduced by inducing complete hydrolysis to the anhydrous hydroxynitrate. Nearly complete hydrolysis can be attained by a low temperature (?150 C) thermal treatment in a flow of diluted nitric oxide. Full decomposition (25-350 C) in a flow of diluted nitric oxide further decreases mobility at high temperatures, leading to a further increase in the dispersion. Although good results are obtained for silica supported copper nitrate via this method, similar to better results are obtained by dehydration to anhydrous copper nitrate, which will be able to redisperse via the gas phase. Dehydration occurs via the decomposition in a moderate to fast flow of gas, not containing NO. Thus, although copper, nickel and cobalt nitrates may look similar at first glance, their decomposition and agglomeration behavior is very different
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