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[Display omitted] • Cr 2 O 3 shows the highest rate for propylene reforming but deactivates rapidly. • Mn 1.5 Cr 1.5 O 4 is the most active and stable catalyst for aromatic steam reforming. • Reforming kinetics are consistent with a Mars-van Krevelen type mechanism. • Of the hydrocarbons tested, only the toluene reforming rate is −1 order in H 2. • DRIFTS suggests H 2 inhibits oxidation of the methyl side group of toluene. Catalytic steam reforming of propylene, benzene and toluene was investigated over Mn-Cr-O spinel oxides with different stoichiometry. The catalysts were structurally stable during reforming conditions except for Mn 3 O 4 , which reduced in situ to MnO. The Cr 2 O 3 catalyst showed the highest initial rate for propylene reforming, whereas the Mn-rich Mn 1.5 Cr 1.5 O 4 spinel was the most active and stable catalyst for aromatic reforming. Reforming rates of olefins and aromatics were comparable under 0.05 bar hydrocarbon and 0.40 bar steam at 873 K. The reforming rate of olefins and aromatics was first order in hydrocarbon and slightly inhibited by H 2 O, which is consistent with a Mars-van Krevelen type mechanism. Of the hydrocarbons investigated, only the rate of toluene reforming was negative first order in excess H 2. Results from diffuse reflectance infrared Fourier transform spectroscopy of adsorbed toluene on a spinel catalyst suggest that H 2 inhibits the rapid oxidation of the methyl group of toluene. [ABSTRACT FROM AUTHOR]

• 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]

• Both NiAl 2 O 4 and NiCr 2 O 4 were stable during ethylene steam reforming at 873 K. • Both Fe and Ni in the NiFe 2 O 4 spinel catalyst reduced during steam reforming. • The NiFe 2 O 4 spinel catalyst severely coked during ethylene steam reforming. • The reforming rate increased after oxidation of the spent NiAl 2 O 4 and NiCr 2 O 4. Spinel oxides containing Ni cations can be present in the barrier layers of ethane steam cracker tubes. As Ni is known to catalyze steam reforming side reactions as well as coke deposition, the catalytic performance of Ni spinel catalysts was investigated. A series of NiM 2 O 4 spinel catalysts (M = Al, Cr, Fe) was synthesized from a sol-gel method and thoroughly characterized. The NiAl 2 O 4 and NiCr 2 O 4 catalysts were active for ethylene steam reforming at 873 K without degradation of the spinel structure. The NiFe 2 O 4 catalyst was the least stable because of partial reduction of Fe3+ to form Fe 3 O 4 , which facilitated Ni-Fe alloy particle formation and severe coking. The reforming activity and the coke deposited on the NiAl 2 O 4 and NiCr 2 O 4 catalyst both increased after oxidative regeneration at 873 K, suggesting a slow reduction of Ni2+ to Ni metal particles during steam reforming that subsequently sinter into larger NiO particles during oxidative regeneration. [ABSTRACT FROM AUTHOR]