Synthesis and Decomposition of Nd2–yCayCo1–xNixO4: The Effect of Resynthesis on the Catalytic Performance of Decomposition Products in the Partial Oxidation of Methane

In order to study the nanocomposite catalysts of the partial oxidation of methane (POM), a series of complex oxide precursors Nd2–yCayCo1–xNixO4 (y = 0.5–1; x = 0–1) were obtained by a freeze-drying synthesis method. It was found that the substitutions of Co with Ni and Ca with Nd in NdCaCoO4 could result in the formation of complex oxides with a K2NiF4 structure within the limited range of x and y. Distortions of the K2NiF4 structure caused by the complete substitution of Co with Ni promoted a change of space group from I4/mmm for NdCaCoO4 to Fmmm for Nd1.5Ca0.5NiO4. Analysis of the oxygen stoichiometry and valence states of transition metals revealed that Co(III) is substituted by the mixture of Ni(II) and Ni(III). The reduction of these complex oxides at 900 °C in hydrogen resulted in the formation of metal oxide nanocomposites consisted of 25–30 nm (Co,Ni) nanospheres at the surface of dense agglomerates of nanosized CaO and Nd2O3. All these composites demonstrated a significant catalytic activity in the POM reaction. The substitution of Co with Ni promoted the appearance of catalytic properties at lower temperatures up to T = 750 °C. The catalytic properties of Co-containing nanocomposites at T < 900 °C were severely affected by the resynthesis of NdCoO3 and Nd2–yCayCo1–xNixO4 in the POM environment. The formation of complex oxides from metal oxide nanocomposites occurred at reduced temperatures and caused a significant decrease of their particle size compared to traditional wet chemical synthesis methods.