Effects of Ir-doping on the transition from oxidative coupling to partial oxidation of methane in La2O3–CeO2 nanofiber catalysts: spatial concentration and temperature profiles

Abstract

Spatially resolved temperature and species concentration profiles determined via microprobe sampling and online gas chromatography were used to examine the transition from oxidative coupling of methane (OCM) to catalytic partial oxidation (CPO) as a function of Ir-doping of a La₂O₃–CeO₂ nanofiber fabric catalyst. Experiments were performed at atmospheric pressure at a furnace temperature of 750 °C, feed gas CH₄/O₂ molar ratio of 5 and at a nominal space time of about 60 ms. Reaction products exhibited a dramatic shift from ethane and ethylene (OCM, C₂ selectivity of 54%), in the case of the un-doped catalyst, to exclusively synthesis gas (CPO, CO and H₂ selectivity 90+%), at 1% Ir doping. Species concentration profiles obtained in the latter case indicate the formation of H₂O, H₂ and CO first, followed by the steam reforming of CH₄ to produce the balance of the syngas concomitant with a sharp decrease in H₂O levels. These findings suggest a CPO mechanism that is different from the literature, calling for the revision of detailed chemical kinetic mechanisms proposed before.