摘要
采用(NH_4)_3[Fe(C_2O_4)_3]·xH_2O稀水溶液含浸,Fe(NO_3)_3·9H_2O水溶液饱和浸渍以及Fe(AcAc))_3的甲苯溶液热吸附方法制备了在TiO_2(锐钛矿)上分散的氧化铁体系。用多种物理化学手段进行了表征,结果表明由含浸和热吸附法制备的样品中,负载组分FeO_x很容易充分分散或单层分散,这些单层物种很难被XRD,FTIR-DRS或LRS所检测,单层分散的Fe^(3+)处于高自旋态,具有较小的正交畸变配位对称性,但不是孤立四面体的配位结构,没有发现置换Fe^(3+)的存在。实验结果还表明,由XPS的电子结合能数据鉴别Fe^(3+)和TiO_2表面相互作用的强弱是困难的,充分分散的FeO_x表面物种在973K的还原温度下其Fe^(2+)尚不能被进一步还原,在TiO_2(锐钛矿)表面上单层分散的Fe—O多面体仍然是丁烯ODH反应的有效活性位。单层型催化剂的选择性不同于晶相a-Fe_2O_3,且与是否存在气相氧密切相关。
Three methods, dipping of support into an excess of aqueous solution of (NH4)3[Fe(C2O4)3] ?xH2O, batch adsorption of Fe(AcAc)3in toluene and incipient wetness impregnation of Fe(NO3)3, are adopted to prepare the well-dispersed or monolayer dispersed ferric oxide on TiO2(anatase). Extensive characterization has been done on the physico-chemical properties of the system. The results indicated that the supported component is easily well-dispersed or monolayer dispersed when the dipping or adsorption method is used. This kind of monolayer FeOx species is difficutly detected by XRD, FTIR-DRS and LRS. Like the monolayer Fe3+species on γ-Al2O3, the ferric ion on TiO2(a) is also at the high spin state and the site symmetry is distorted rhombic, but the degree of the distoration is less than that of ferric ions on γ-Al2O3. This rhombic distroted site only exist on the anatase. There is no substituted ferric ion can be seen in the samples. It seems that the coordination structure of the monolayer dispersed Fe3+ cannot be tetrahedral. We also found that it is quite difficult to distinguish the degree of the interaction between the surface Fe3+ and the support using the B. E. of the ferric ion obtained in XPS experiment. The monolayer FeOx species cannot be reduced beyond the ferrous state as the reduction temperature is increased up to 973 K. The stability seems to arise from the formation of a two-diminsional surface compound, FeTiO3. For the bulk of ferric oxide, whether it is supported or not, some ferric ions can be reduced to metallic iron in the same temperature range. The monolayer Fe-O polyhedrons on anatase surface are still effective for activation of the reactant. The selectivity of the monolayer catalyst is different from that of bulk of ferric oxide and its difference mainly comes from the contribution of the uncovered support surface. It is also found that the product distribution varied sensitively with the existence of gas oxygen.
出处
《分子催化》
EI
CAS
CSCD
1992年第3期173-182,共10页
Journal of Molecular Catalysis(China)
关键词
氧化铁
氧化钛
分散状态
催化剂
Ferric oxide, Titanium oxide, Dispersion state, Reaction Performance.
