Selective Oxidation, Reforming and Combustion of Methane over NiO/Ni on Ceria-Zirconia

Significant quantities of methane occur in remote locations where the economy of scale for commercial reformers is not feasible. Selective oxidation of methane offers an attractive approach to convert these stranded methane reserves to condensable alcohols that can be transported to a central processing facility in pipelines. This contribution analyzes the potential of well dispersed NiO clusters on a redox-active ceria zirconia (CZ) support as catalyst for such a process.

Ceria-zirconia (CZ) is capable of stabilizing well-dispersed metal and metal oxide clusters, activating O₂ and storing activated oxygen species. The combination of these properties makes it a promising support for catalysts for methane conversion in oxidative environments. Pyridine adsorption followed by IR spectroscopy and EDX mapping indicate that most NiO clusters only contain a small number of Ni atoms. The synergy between supported NiO clusters and CZ was demonstrated by in-situ XANES experiments. Specifically, CZ is capable of supplying NiO with oxygen until a reservoir of active oxygen species is depleted. Combined with the well-known activity of CZ to activate O₂, these results illustrate CZ can facilitate oxidation reactions of hydrocarbon moieties on the supported NiO clusters

Depending on the pretreatment and reaction conditions, NiO/CZ or Ni/CZ can convert methane by selective oxidation, reforming and combustion. Selective oxidation to alcohols can be achieved at moderate temperatures when nickel is present in NiO clusters and the feed is rich in methane and steam but contains limited amounts of oxygen. In the presence of larger amounts of O₂, complete combustion becomes the dominant reaction, whereas reduced Ni clusters favor reforming reactions. The strong metal-support interactions between the different components of this catalyst stabilize NiO particles that appear to be too small to catalyze the formation of graphitic coke or carbon nanotubes. The dispersion of nickel can be increased further by specific synthesis techniques like strong electrostatic adsorption, co-precipitation, and combustion synthesis.