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Two Co–Cr–O spinel catalysts with different stoichiometry were synthesized and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption and temperature-programmed reduction in H2. Excess Co in Co1.5Cr1.5O4 existed as Co3+ that substituted for Cr3+ in the octahedral sites of the spinel lattice. High temperature treatment of the spinel catalysts in H2 resulted in stepwise reduction of Co3+ to Co2+ and eventually to Co metal. Both CoCr2O4 and Co1.5Cr1.5O4 catalysts were active and structurally stable in ethylene steam reforming under differential reaction conditions at 873 K, with the areal reforming rate over Co1.5Cr1.5O4 being one order-of-magnitude greater than that over CoCr2O4. The steady state reforming rate after oxidation was comparable to that over the fresh catalyst for both CoCr2O4 and Co1.5Cr1.5O4, indicating the stability of the spinel structure against reduction under steam reforming conditions. [ABSTRACT FROM AUTHOR]

A CeO2/α‐Al2O3 bilayer was coated on a high temperature alloy (Incoloy 800H) by sol–gel dip‐coating and was evaluated for its potential as an anticoking barrier and coke oxidation catalyst. The bilayer effectively functioned as a barrier to metal surface catalyzed coking. The film prevented filamentous catalytic coking via blocking surface active metallic sites on the Incoloy substrate. Furthermore, the bilayer reduced the oxidation temperature of pyrolytic coke deposited on the film surface as compared to a bare oxidized Incoloy substrate, mostly owing to the oxidation catalytic activity of the CeO2 layer. Finally, it is demonstrated that the presence of the α‐Al2O3 buffer layer is critically important to the overall performance. Without the α‐Al2O3 layer, a CeO2 layer nearly completely lost both its barrier and oxidation catalytic functions. It is presumed that metallic species migrating from the substrate during high temperature treatments are responsible for the CeO2 deactivation, likely by blocking catalytic sites on the CeO2 surface. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4019–4026, 2018 [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]