EXAFS Study of Ni-Mn Catalyst Structure

The Structure of Ni-Mn bimeallic catalysts used for CO₂ methanation was characterized by EXAFS technique, The EXAFS spectra of Ni and Mn in Ni-Mn/ SiO₂ catalysts obtained with transmission manner were theoretically calculated by the associated Fourier—transform and the least-square fitting. In Ni-Mn/ SiO₂（Ox） calcinated at 450℃, the R_Ni-O（2.05Å）, R（Mn-O）（1.90Å） and the RSFs of Ni and Mn are comparable to those of NiO and MnO₂ Models respectively, which indicates that the Ni and Mn in Ni-Mn/SiO₂（Ox） existed in the form of NiO and MnO₂ , comparing with models, the R_Ni-Ni（2.50Å） and R_Mn-O（2.27Å） of Ni-Mn/ SiO₂（Re） suggests that Ni and Mn have been reduced into metallic nickel and MnO respectively. The coordination number of Ni-Ni （N=7.3） in Ni-Mn/SiO₂（Re） smaller than that of bulk nickel （N=12） resulted from the high dispersion of nicke^l. The very weak and indistinct XRD peaks of Ni-Mn/SiO₂（Re） also prove this conclusion. The increased activity for CO₂ methanation on Ni-Mn/SiO₂（Re） compared with that on monometallic Ni/SiO₂ can be interpreted as the high dispersion of nickel and the strong interaction between Ni and MnO.