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In order to study the role of the crystallite size of an active phase in a catalytic reaction it is of utmost importance to be able to synthesise pure phases of crystallites in the desired size range with a narrow size distribution. In this paper we describe a new method to produce Co₃O₄ crystallites in the nanometer size range (average sizes: 3–10nm) utilising reverse micelles as nano reactors. To prepare suitable model catalysts for studies on effects of crystallite size these crystallites can be deposited onto a variety of carriers, in this work an alumina support was used. It is further shown that the supported cobalt oxide crystallites prepared in this study do not undergo extensive sintering under reductive conditions (H₂ flow and temperatures between 375 and 450°C) so that also a series of model catalysts with metallic cobalt crystallites of varied size could be prepared. The resulting metal phase only shows the diffraction …