Ni/(RO,CaO) Nanocomposites Produced by the Exsolution of RCaNiO Nickelates (R = Nd, Sm, Eu): Rare Earth Effect on the Catalytic Performance in the Dry Reforming and Partial Oxidation of Methane.

In order to clarify the role of RO in the metal-oxide catalysts derived from complex oxide precursors, a series of RCaNiO (R = Nd, Sm, Eu) complex oxides was obtained. A significant systematic increase in the orthorhombic distortion of the RCaNiO structure (KNiF type, Cmce) from Nd to Eu correlates with a corresponding decrease in their ionic radii. A reduction of RCaNiO in the Ar/H gas mixture at 800 °C causes a formation of dense agglomerates of CaO and RO coated with spherical 25-30 nm particles of Ni metal. The size of metal particles and oxide agglomerates is similar in all Ni/(RO,CaO) composites in the study. Their morphology is rather similar to the products of redox exsolution obtained by the partial reduction of complex oxides. All obtained composites demonstrated a significant catalytic activity in the dry reforming (DRM) and partial oxidation (POM) of methane at 700-800 °C. A systematic decrease in the DRM catalytic activity of composites from Nd to Eu could be attributed to the basicity reduction of RO components of the composite catalysts. The maximum CH conversion in POM reaction was observed for Ni/(SmO,CaO), while the maximum selectivity was demonstrated by NdO-based composite. The possible reasons for the observed difference are discussed.