Steam reforming of ethylene over manganese-chromium spinel oxides.

• A variety of Mn-Cr spinels catalyze ethylene steam reforming at 873 K. • Excess Mn3+ substitutes for Cr3+ in the spinel structure. • Excess Cr3+ phase separates from the spinel to form Cr 2 O 3. • The reforming kinetics are consistent with a modified Mars van Krevelen mechanism. • Mn and Cr oxides are not effective catalysts for the steam reforming of ethylene. A series of Mn-Cr-O spinel catalysts of different stoichiometry was synthesized using a sol-gel method and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, N 2 and H 2 O adsorption, and H 2 -temperature programmed reduction. The introduction of extra Mn to stoichiometric MnCr 2 O 4 led to Mn3+ substitution in the spinel lattice of Mn 1.5 Cr 1.5 O 4 whereas excess Cr in Mn 0.5 Cr 2.5 O 4 formed Cr 2 O 3. Ethylene steam reforming at atmospheric total pressure and 873 K over the Mn-Cr-O spinel catalysts was nearly first order in ethylene, negative order in water and zero order in excess dihydrogen. The reaction kinetics were consistent with a Mars-van Krevelen type mechanism. Whereas Cr 2 O 3 was nearly inactive for ethylene steam reforming, Mn 3 O 4 was substantially more active than the spinel catalysts, but Mn 3 O 4 deactivated significantly upon in situ reduction to MnO. The spinel catalysts were therefore more active and structurally stable for ethylene steam reforming than either of the pure metal oxides. [ABSTRACT FROM AUTHOR]