Polyoxometalate Clusters Supported on Functionalized Graphene Sheets as Nanohybrids for the Catalytic Combustion of Liqu
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Polyoxometalate Clusters Supported on Functionalized Graphene Sheets as Nanohybrids for the Catalytic Combustion of Liquid Fuels Jean-Philippe Tessonnier1, Francis M. Haas2, Daniel M. Dabbs3, Frederick L. Dryer2, Richard A. Yetter4 and Mark A. Barteau1 1 Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, U.S.A. 2 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08540, U.S.A. 3 Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08540, U.S.A. 4 Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16802, U.S.A.
ABSTRACT Catalyzing the combustion of liquid fuels offers interesting prospects for further improvement of the fuel economy and the performance of engines, in particular for jets and advanced propulsion systems. Polyoxometalates are well-known oxidation catalysts which are expected to also catalyze combustion reactions. However, their polarity makes their dispersion in fuels particularly challenging. Herein, functionalized graphene sheets were used as a support due to their high surface area as well as their compatibility with the target reaction. In order to further improve the dispersion of the catalyst in fuels, alkyl chains were grafted to the sheets’ surface. An innovative grafting technique was developed to attach alkyls at a variety of oxygencontaining functionalities already present on reduced graphene oxide, such as hydroxyl and epoxy groups. A phase transfer to the organic phase was observed when dispersing the dry powder in water:toluene mixtures. In addition, the dry alkyl chain-modified graphene sheets readily dispersed in common organic solvents without the assistance of sonication. Polyoxometalates (H3PMo12O40 and H4PMo11VO40) were dispersed on the modified sheets as discrete clusters even at a relatively high loading (20 wt.%). The catalytic activity of these nanostructured materials was demonstrated for the combustion of methylcylcohexane, tested here as a model fuel. INTRODUCTION Petroleum-based transportation fuels are blended and conditioned to improve performance. Most additives used in fuel conditioning help maintain the engine and prevent the formation of carbon build-up in the cylinders and on the valves. Some additives serve to enhance fuel combustion properties like autoignition quality or burning rate. One such example is 2ethylhexyl nitrate (2-EHN), which can be added to diesel fuel to increase the cetane number, a measure of fuel autoignition quality [1]. 2-EHN accelerates the ignition process through
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increased rate of free-radical initiation reactions without significantly altering the principal combustion reaction mechanism [2]. Conventional additives have reached their limits, especially for jet engines (turbofan) and advanced propulsion systems (ramjet, scramjet) where complete combustion of fuel must occur within a few milliseconds [3]. Catalyzing the combustion reaction may lead to major improvements, as t
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