Oxidation of CH4 in excess of O2 using Fe2O3 catalysts

This study investigates methane (CH 4 ) oxidation under oxygen-rich conditions using a systematic series of bare α-Fe 2 O 3 catalysts. The catalysts were prepared by calcination of goethite-type FeOOH at temperatures between 450 and 800 °C and were comprehensively characterized by N 2 physisorption, X-ray diffraction, 57 Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, scanning electron microscopy, temperature-programmed desorption of NH 3 and temperature-programmed reduction by CO. Methane oxidation was examined in the presence of 5 vol% O 2 and 10 vol% H 2 O, achieving CH 4 conversions up to 40% at 550 °C. Structure–activity relationships revealed that a large BET surface area (> 20 m 2 g -1 ), small crystallite size (< 50 nm), high redox activity and a high density of active iron sites are key properties driving the CH 4 oxidation. Additionally, the availability of reactive surface and subsurface oxygen species was found to be crucial. In-situ diffuse reflectance infrared Fourier transform spectroscopy revealed a sequential hydrogen abstraction mechanism for CH 4 activation. Initial hydrogen abstraction occurred at temperatures as low as 50 °C leading to the formation of methoxy and hydroxyl species. Upon thermal activation, further hydrogen transfer and oxidation of methoxy intermediates proceeded yielding formate, bicarbonate, and carbonate species as key surface intermediates. Evaluation of different kinetic models demonstrated that CH 4 oxidation on Fe 2 O 3 is accurately described by a Hougen–Watson expression based on a Langmuir–Hinshelwood-type mechanism. This model accounts for molecular CH 4 adsorption, dissociative O 2 adsorption and competitive H 2 O adsorption. An apparent activation energy of 131 kJ mol -1 was determined. • A systematic series of bare Fe 2 O 3 catalysts are studied for lean CH 4 oxidation • Activation of CH 4 already starts at 50 °C by abstraction of a hydrogen atom • Hydrogen transfer processes affect progressing CH 4 oxidation • Small crystallite sizes, high surface area and redox activity drive CH 4 oxidation • A Hougen–Watson approach predicts CH 4 oxidation kinetics