Received: May 27, 2007

In Final Form: September 24, 2007

Abstract:

The interactions of CH₄, CO, and CH₄/O₂ with Ni/Al₂O₃ catalysts were examined by the pulse reaction technique and the transient response technique. The adsorption and dissociation of CH₄, CO, CH₄/CO₂, and CH₄/O₂ on nickel alumina catalysts were also extensively investigated by temperature-programmed hydrogenation (TPH) and temperature-programmed desorption (TPD). The phase structure and nickel oxidation states of Ni/Al₂O₃ samples were examined by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. XRD, TPR, and XPS results demonstrated that nickel is mainly present as NiO and NiAl₂O₄ in the as-prepared catalyst, while around 83%-90% nickel is Ni⁰ after the catalyst is reduced at 700 C. Methane pulse reactions and transient response analysis demonstrate that the methane oxidation mechanism changes as the nickel oxidation state changes over Ni/Al₂O₃ when there is no gaseous oxygen present. CH₄ is efficiently oxidized into CO and H₂ via a direct oxidation mechanism when Ni/Al₂O₃ is prereduced, while CH₄ may be converted by a nonselective oxidation process over an oxidized Ni/Al₂O₃. CO pulse reaction results over the prereduced catalyst suggest that the CO disproportionation reaction occurs over Ni/Al₂O₃ catalyst under operation conditions, while CH₄ is generated through the hydrogenation of the surface carbidic species from CO disproportionation. TPD and TPH studies show that the decomposition of methane results in the formation of at least three kinds of surface carbon species on supported nickel catalysts. TPD and TPH results also prove CO is converted to CO₂ and surface carbidic C through the disproportionation reaction.

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