X-ray absorption spectroscopic study of the alumina supported Fe and FeMo catalysts for methane dehydrogenation

Abstract

Nanoscale Fe (5%Fe/Al2O3) and FeMo (4.5%Fe-0.5%Mo/Al2O3) catalysts are prepared by precipitating metal components onto alumina and then reducing in hydrogen at 700 °C for 2 hrs. The catalysts are characterized before and after their exposures to methane at 700 °C, employing ex-situ X-ray absorption spectroscopy (XAS) and X-ray diffraction techniques. XAS data of the as-prepared catalysts shows that, iron precipitates onto alumina as 6L-ferrihydrite, and Mo is present as molybdate (Mo(VI)O42-). The molybdate, which binds the alumina in the as-prepared FeMo catalyst, is partially reduced to MoO2. The reduction of iron to metallic state is incomplete because the ferrous iron binds the alumina, forming hercynite. After several hrs of methane exposure to both the catalysts at 700 °C, a major Fe3C and a graphite phase are observed only in the FeMo catalyst. The observation confirms that the Mo promotes more iron to an active Fe metal, a part of that metal converts to Fe3C and austenite (FexC). Concurrently, the MoO2 converts to Mo-oxycarbide (MoOxCy), the latter then carburize to Mo2C in the FeMo catalyst. The Fe metal, Mo-oxycarbide and metal carbides are mainly responsible for the conversion of methane to H2 and carbon. At reaction temperature > 900 °C, the Mo carbide particles agglomerate, and the excess carbon deposits on this agglomerate leading to catalyst deactivation. The deactivated catalyst is regenerated with CO2 treatment at 1000 °C to restore its initial oxide structure.