Combined temperature-programmed processes, pulse reactions, and on-line mass spectroscopy study of CH4, CO, and H2 interaction with Ni/Al2O3 catalysts

The interactions of CH4, CO, and CH4/O-2 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/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/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 NiO after the catalyst is reduced at 700 degrees 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 H-2 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-alpha through the disproportionation reaction.