摘要
本文用EXAFS方法测定了CO_2加氢甲烷化的镍锰双金属催化剂的结构。Ni-Mn/SiO,催化剂中镍和锰的EXAFS谱采用透射法获得,实验数据经过快速付立叶变换及最小二乘法参数拟合处理后.焙烧的Ni-Mn/SiO_2(Ox)催化剂的R_(Ni-O)=2.05A,R_(Mn-O)=1.90A,Ni和Mn的RSFs图分别与NiO、MnO_2的相近,表明镍、锰主要是以NiO、MnO_2的晶粒分散在SiO_2载体上.还原的Ni-Mn/SiO_2(Re)催化剂的R_(Ni-Ni)=2.50A,R_(Mn-O)=2.27A,说明已经生成了金属镍和MnO,镍的配位数为7.3,表明Ni-Mn/SiO_2(Re)中镍是以高分散的小晶粒存在。XRD方法也只观察到Ni-Mn/SiO_2(Re)有很弱的衍射峰,也证实生成的Ni、MnO晶粒是很小的,与EXAFS有一致的结论。与Ni/SiO_2催化剂相比,Ni-Mn/SiO_2(Re)催化荆的CO_2甲烷化高活性原因,我们认为是Ni与MnO的强相互作用及镍的高分散导致的。
The Structure of Ni-Mn bimeallic catalysts used for CO_2 ethanation was characterized by EXAFS technique, The EXAFS spectra of Ni and Mn in Ni-Mn/ SiO_2 catalysts obtained with transmission manner were theoretically calculated by the associated Fourier—transform and the least-square fitting. In Ni-Mn/ SiO_2(Ox) calcinated at 450℃, the R_(Ni-o)(2.05A), R(Mn-o)(1.90A) and the RSFs of Ni and Mn are comparable to those of NiO and MnO_2 Models respectively, which indicates that the Ni and Mn in Ni-Mn/SiO_2(Ox) existed in the form of NiO and MnO_2 , comparing with models, the R_(Ni_Ni) (2.50A) and R_(Mn_o)(2.27A) of Ni-Mn/ SiO_2(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/SiO_2(Re) smaller than that of bulk nickel (N=12) resulted from the high dispersion of nickel. The very weak and indistinct XRD peaks of Ni-Mn/SiO_2(Re) also prove this conclusion. The increased activity for CO_2 methanation on Ni-Mn/SiO_2(Re) compared with that on monometallic Ni/SiO_2 can be interpreted as the high dispersion of nickel and the strong interaction between Ni and MnO.
