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The short residence times available in supersonic combustion require some pre-reaction under mixing-controlledrich conditions and/or methods to lower the reaction temperature for auto-ignition. Adding catalysts could be such an option to achieve this objective. Catalytic ignition of toluene over the surfaces of in situ-generated free metal (Fe and Ni) nanoparticles was investigated experimentally in an aerosol reactor. The metal nanoparticles (Fe and Ni) were generated by decomposition of the corresponding metal carbonyls. Gas-phase (aerosol) size distributions along with transmission electron microscopy used to characterize the morphology of catalyst particles at different temperatures are presented. The effluent gas product and the fuel ignition temperature were determined by mass spectrometry. In comparison to non-catalytic homogeneous ignition, the addition of metal nanoparticles can lower the ignition temperature by as much as 150 °C under rich conditions but had little effect under lean conditions. Iron was found to be a more active catalyst than nickel. Inspection of the catalyst product indicated that sintering was occurring at relatively low temperatures presumably as a result of the exothermic reaction on the particle surface. Turnover frequency as high as 80 s À1 was achieved, implying a greater catalyst efficiency than commonly found for substrate-stabilized catalysts.