Ni/(R2O3,CaO) Nanocomposites Produced by the Exsolution of R1.5Ca0.5NiO4 Nickelates (R = Nd, Sm, Eu): Rare Earth Effect on the Catalytic Performance in the Dry Reforming and Partial Oxidation of Methane

Abstract

In order to clarify the role of R2O3 in the metal-oxide catalysts derived from complex oxide precursors, a series of R1.5Ca0.5NiO4 (R = Nd, Sm, Eu) complex oxides was obtained. A significant systematic increase in the orthorhombic distortion of the R1.5Ca0.5NiO4 structure (K2NiF4 type, Cmce) from Nd to Eu correlates with a corresponding decrease in their ionic radii. A reduction of R1.5Ca0.5NiO4 in the Ar/H2 gas mixture at 800 °C causes a formation of dense agglomerates of CaO and R2O3 coated with spherical 25–30 nm particles of Ni metal. The size of metal particles and oxide agglomerates is similar in all Ni/(R2O3,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 R2O3 components of the composite catalysts. The maximum CH4 conversion in POM reaction was observed for Ni/(Sm2O3,CaO), while the maximum selectivity was demonstrated by Nd2O3-based composite. The possible reasons for the observed difference are discussed.

Nd2−xSrxNiO4 Solid Solutions: Synthesis, Structure and Enhanced Catalytic Properties of Their Reduction Products in the Dry Reforming of Methane

“…An increase in the Sr content in the initial complex oxide was accompanied by a systematic increase in the complete reduction temperature. The thermal analysis of the reduction processes of the as-obtained complex oxides in the Ar/H 2 mixture revealed two different stages of this process for all samples in the study (Figure 3A-C), as it was observed before for other K 2 NiF 4 complex oxides [19][20][21]. According to the XRD analysis results of the reduction products, the partial reduction in the complex oxide precursor at the first stage caused its transformation into another complex oxide with an individual crystal structure.…”

“…Due to the similarity of the catalytic processes, the DRM catalysts often demonstrated significant catalytic activ- Taking into account the results of our previous studies, it was expected that the reduction products of Nd 2−x Sr x NiO 4±δ with the morphology described before would demonstrate significant catalytic properties in the DRM process. Due to the similarity of the catalytic processes, the DRM catalysts often demonstrated significant catalytic activity and selectivity in the partial oxidation of the methane (POM) reaction: another widely discussed alternative to the currently applied steam reforming process [2,[19][20][21]. However, the production of syngas by DRM at T = 700-800 • C over the Ni-based catalysts could be more efficient than POM.…”

“…The number of the available Ni-containing complex oxides with the K 2 NiF 4 structure is much larger, which opens a broader selection of the possible metal-oxide compositions that can be obtained by their thermal reduction [18]. Our previous studies demonstrated the efficiency of the metal-oxide nanocomposites obtained by the thermal reduction of (Ln,Ca) 2 (Ni,Co)O 4 as catalysts for the partial oxidation and dry reforming of methane [19][20][21]. The essential morphological feature of as-obtained composites is the formation of the Ni metal nanoparticles at the surface of larger agglomerates of the oxide-based reduction products.…”

Formation of the metal-oxide nanocomposites during the partial reduction of Nd-Sr nickelates

“…Recent studies demonstrated that metal-oxide nanocomposites with morphology rather similar to the products of redox exsolution could be obtained by the complete thermal reduction of complex oxides of rare earth, cobalt, and nickel with K 2 NiF 4 structure [11][12][13]. Another feature of these nanocomposites that makes them similar to the redox exsolution products is their excellent catalytic activity, selectivity, and stability of the catalytic properties in the processes of partial oxidation and dry reforming of methane at T = 750-850 • C.…”