Catalytic autoignition of higher alkane partial oxidation on Rh-coated foams

Abstract

Through online mass spectrometry it is demonstrated that steady-state production of syngas (CO and H 2) can be attained within 5 s after admitting large alkanes ( i-octane, n-octane, n-decane, or n-hexadecane) and air into a short-contact-time reactor by using an automotive fuel injector and initially preheating the Rh-coated catalyst above the respective catalytic autoignition temperature for each fuel. Minimum catalytic autoignition temperatures on Rh were ∼260 °C for n-octane and 240 °C for i-octane and n-decane. In contrast, catalytic autoignition of n-hexadecane indirectly occurred at temperatures (>220 °C) lower than those of the other fuels investigated because of exothermic homogeneous chemistry that preheated the catalyst (30–60 °C) to a temperature (∼280 °C) sufficient for surface lightoff. Additionally, the ignition kinetics for the large alkanes were determined and compared with those of methane. The step controlling surface ignition possessed an apparent activation energy of ∼78 kJ/mol that was not significantly different between fuels ( p > 0.05). However, a significant difference was found between the ignition preexponential for methane, O(10 4 s −1), and the other large alkanes, O(10 6 −1). The dominant energetic step for large alkane surface ignition is hypothesized to be oxygen desorption at saturation coverage as has been suggested for methane.