Preparation of supported nano-sized cobalt oxide and fcc cobalt crystallites

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: 3–10 nm) 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 pattern of a face-centred cubic (fcc) crystal phase, while normally mixtures of fcc an hcp cobalt were obtained in previous studies. Furthermore, almost complete reduction of the catalyst could be obtained for all crystallite sizes and no Co-aluminate formation was observed. These model catalyst systems allow the study of structure sensitive reactions with an industrially relevant catalyst system in the absence of the commonly encountered difficulties like the formation of strong metal support interactions, co-existence of different metal crystallite phases, an incomplete reducibility and crystallite growth upon exposure to reduction/reaction conditions.

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► Control sizes of supported nano-sized Co3O4 particles with a narrow size distribution.
► Maintain sizes upon reduction at elevated temperatures in hydrogen.
► Degrees of reduction greater than 90%.
► No detectable formation of strong metal support interactions.
► Obtain a pure fcc cobalt phase.