Microemulsion-Templated Synthesis of Highly Active High-Temperature Stable Partial Oxidation Catalysts
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Microemulsion-Templated Synthesis of Highly Active High-Temperature Stable Partial Oxidation Catalysts Mark Kirchhoff1, Ullrich Specht1, and Götz Veser1,2,* 1 Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany 2 Department of Chemical Engineering, University of Pittsburgh, Pittsburgh PA. ABSTRACT Highly active catalysts for high-temperature partial oxidation reactions have been synthesized based on a microemulsion-templated sol-gel synthesis. The catalysts were tested with the direct catalytic oxidation of methane to synthesis gas and showed excellent selectitivites towards syngas combined with very high activity and low ignition temperatures. Furthermore, a surprisingly high long term stability was observed at these high-temperature conditions of T > 900oC. The catalyst therefore seem very promising candidates for high-temperature partial oxidation and hydrogen production from hydrocarbon fuels. INTRODUCTION Micelle-templated syntheses have found wide-spread application in materials research during the past ten years, since the self-assembling properties of surfactants offer a unique environment for the engineering of nanostructured materials [1-3]. Beyond allowing the template-directed synthesis of pore morphologies for nano- and mesoporous materials, the nanometer-sized droplets in reverse (water-in-oil) microemulsions also constitute a controlled environment for chemical reactions, in which single micelles act as individual ’nanoreactors’ [4]. This has been used in recent years to produce materials with large, high-temperature stable surface areas, either as support or catalyst materials, for example through sol-gel syntheses in microemulsions [5, 6]. In this contribution, we report on the synthesis of novel high-temperature stable catalysts for partial oxidation of hydrocarbons through a microemulsion-templated synthesis route. Starting point were so-called hexa-aluminate catalysts, i.e. a group of high-temperature stabilized aluminas which have already been shown to be rather effective catalysts for high-temperature combustion processes, i.e. for total oxidation of hydrocarbons [7, 8]. We tested the suitability of microemulsion-templated hexaaluminate catalysts for the partial oxidation of hydrocarbons. However, in contrast to their high combustion activity, the synthesised catalysts showed only very poor conversions and selectivities under the fuel-rich conditions of partial oxidation. Therefore, the aim of the study was to combine the known high selectivity of noble metal components towards partial oxidation with the high-temperature stabilility of the hexaaluminate support and the large surface area attainable through a microemulsion-templated synthesis route.
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CATALYST SYNTHESIS The steps in the catalyst synthesis are summarized in figure 1. Starting with an inverse (i.e. water-in-oil) microemulsion by combining water, iso-octane and one of several different nonionic surfactants (Neodol, Lutensol, and others), an aqueous solution of the respective metal salt was added
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