Combined Temperature-Programmed Processes, Pulse Reactions, and On-Line Mass Spectroscopy Study of CH4, CO, and H2Interaction with Ni/Al2O3Catalysts

The interactions of CH 4, CO, and CH 4 /O 2 with Ni/Al 2 O 3 catalysts were examined by the pulse reaction technique and the transient response technique. The adsorption and dissociation of CH 4, CO, CH 4 /CO 2, and CH 4 /O 2 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/Al 2 O 3 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 NiAl 2 O 4 in the as-prepared catalyst, while around 83%−90% nickel is Ni 0 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/Al 2 O 3 when there is no gaseous oxygen present. CH 4 is efficiently oxidized into CO and H 2 via a direct oxidation mechanism when Ni/Al 2 O 3 is prereduced, while CH 4 may be converted by a nonselective oxidation process over an oxidized Ni/Al 2 O 3 . CO pulse reaction results over the prereduced catalyst suggest that the CO disproportionation reaction occurs over Ni/Al 2 O 3 catalyst under operation conditions, while CH 4 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 CO 2 and surface carbidic C α through the disproportionation reaction.