XAFS and Mössbauer spectroscopy characterization of supported binary catalysts for nonoxidative dehydrogenation of methane

Abstract

The structure and speciation of nanoscale, binary catalysts (0.5% M–4.5% Fe/Al 2O 3, with M=Pd, Mo, or Ni) that are very effective for the nonoxidative dehydrogenation of methane have been investigated by X-ray absorption fine structure (XAFS) and Mössbauer spectroscopy. Results are reported for the catalysts after precipitation from solution onto the alumina support, after pre-reduction at 700 °C in hydrogen, and after several hours of reaction in flowing methane. It is found that all three secondary metal additives enhance the reducibility of the catalysts. The phases identified that are believed to have the greatest effect on activity are hercynite (FeAl 2O 4) and nonmagnetic metallic alloys (Fe 2Mo, Fe–Ni–C austenite, Fe–Pd–C austenite, and possibly Fe–Mo–C austenite). Hercynite is believed to enhance the activity of the catalysts by binding the catalyst particles to the alumina surface preventing demetallization. The nonmagnetic alloys are believed to be the active phases for dehydrogenation of methane with simultaneous formation of carbon nanotubes, which grow away from the surfaces of the bound catalyst nanoparticles, preventing deactivation by coking.