Potassium/calcium/nickel catalysts for oxidative coupling of methane

Group VIII transition metals and some post-transition metals (e.g, Sn), with alkali and/or alkaline earth additives, are promising materials for OCM. The relationship between these and alkali molybdates is unclear, but there are XPS and other indications of multiple oxidation states (e.g., Ni²⁺/Ni³⁺) and involvement of lattice oxygen in the workings of these catalysts, as with alkali molybdates. It has been reported that inclusion of >60% steam in the feed greatly increases the C₂₊ selectivity of such catalysts at approximately 600°C, by an unknown mechanism. In this paper the authors compare these systems, where the promoters may exist as substitutional impurities in the cubic MgO lattice, to systems based on alkali/alkaline earth nickelates. The latter materials exist as layer-type oxyhydroxides (CsCl₂l structure) with NiO₆ octahedral layers separated by a disordered layer of alkali and alkaline earth ions coordinated to OH^- and water.