Temperature programmed reduction studies of nickel manganite spinels

Abstract

Temperature programmed reduction (TPR) of manganese, nickel monometallic oxides (Mn 3O 4, Mn 2O 3, MnO 2 and NiO) and nickel manganite oxides (spinels Ni x Mn 3− x O 4 with 0 < x ≤ 1 and ilmenite NiMnO 3), under hydrogen, was investigated. Final and transient products produced were characterised by XRD. Manganese oxides are successively reduced to less oxidised oxide (MnO 2 gives Mn 2O 3 which gives Mn 3O 4 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 Ni 2+ 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 Mn 2+, 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.