Temperature programmed reduction studies of nickel manganite spinels

Article abstract of DOI:10.1016/s0040-6031(97)00299-2

Temperature programmed reduction (TPR) of manganese, nickel monometallic oxides (Mn₃O₄, Mn₂O₃, MnO₂ and NiO) and nickel manganite oxides (spinels Ni_xMn_{3 - x}O₄ with 0 < x ≤ 1 and ilmenite NiMnO₃), under hydrogen, was investigated. Final and transient products produced were characterised by XRD. Manganese oxides are successively reduced to less oxidised oxide (MnO₂ gives Mn₂O₃ which gives Mn₃O₄ which further gives MnO). Although the position and shape of the peaks corresponding to the different reduction steps depend on experimental conditions and sample granulometry, activation energy is found to be the same for the same kind of transformation. Whatever is the crystalline structure, nickel manganites are reduced in two main steps. During the first step, formation of a solid solution of NiO in MnO matrix arises then reduction of Ni²⁺ cations occur during the last step and the final product of reaction is a mixture of MnO and metallic nickel. Hence, contrary to manganese oxides, Mn cations of mixed compounds undergo direct reduction to Mn²⁺, giving MnO without any other way through less oxidised species of manganese. We think that it should be explained by the promoting effect of nickel.