Double flame spray pyrolysis as a novel technique to synthesize alumina-supported cobalt Fischer–Tropsch catalysts

•Co/Al2O3 Fischer–Tropsch (FT) catalysts were prepared by one-step single (SFSP) and double flame spray pyrolysis (DFSP).•SFSP catalysts were only FT active using uneconically high cobalt loadings.•DFSP catalysts were FT active after adjusting the intersection distance (x) of the two flame reactors.•With increasing x, CoOx and Al2O3 nanoparticles grow independently in each flame resulting in higher Co reducibility.•Good adhesion of both oxides after intersection stabilizes Co-particles against sintering in the FT reaction.

The controlled deposition of a catalytically active material on a support, resulting in defined particle sizes and shapes, as well as control over the specific surface area of the active material and the support are usually limited when applying conventional catalyst preparation techniques. Flame spray pyrolysis offers the potential to overcome this limitation and is tested for the preparation of Fischer–Tropsch catalysts. Conventional single flame spray pyrolysis and, for the first time, double flame spray pyrolysis are compared for the synthesis of alumina supported cobalt catalysts. In the latter process the metal oxide precursors are combusted individually in two opposing nozzles. The key parameter for defining the final material composition is the intersection distance of the flames, which was systematically varied. The Fischer–Tropsch performance of the Co-based catalysts was studied in a fixed bed reactor at 230 °C and 20 bar. The catalytic results are discussed on the basis of structural characterization of the different catalysts by XRD, BET, TPR, UV–vis and TEM/EF-TEM. While catalysts made by single flame spray pyrolysis were inactive in the Fischer–Tropsch reaction regardless of whether cobalt was subsequently mixed with alumina or the supported catalyst was directly prepared in the flame reactor, the double flame sprayed catalysts showed good catalytic activity. Depending on the intersection distance of the two flames, the formation of cobalt oxide and alumina occurred separately in each flame reactor. In spite of the independent particle growth in the two flames, the double flame reactor geometry lead to good adhesion of the two oxides resulting in good stabilization of cobalt nanoparticles on the alumina support during the Fischer–Tropsch reaction.

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