Temperature programmed reduction studies of nickel manganite spinels

Temperature programmed reduction (TPR) of manganese, nickel monometallic oxides (Mn3O4, Mn2O3, MnO2 and NiO) and nickel manganite oxides (spinels NixMn3-xO4 With 0 < x less than or equal to 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. (C) 1997 Elsevier Science B.V.