Fe2O3在ZrO2上的分散状态及其对催化性能的影响

用多种物理化学手段, 对用不同方法制备的在ZrO_2上分散的氧化铁体系进行了细致的表征和比较. 对氧化铁在ZrO_2上分散的结构状态及其对催化性能的影响有了较为全面的了解.

DISPERSION STATE OF FERRIC OXIDE ON ZrO_2 AND ITS INFLUENCE ON THE CATALYTIC PERFORMANCE

Three methods, dipping of support into an excess of aqueous solution of (NH_4)_3Fe(C_2O_4)_3]·xH_2O, incipient wetness impregnation of Fe(NO_3)_3 solution and batch adsorption of Fe(AcAc)_3 in toluene were adopted to prepare supported ferric oxide on ZrO_2. Extensive characterization has been done on the physico-chemical properties of the system. The results indicated that monolayer dispersion of the supported component can be realized using the dipping and adsorption methods. It is difficult to obtain a full coverage of FeO_x on ZrO2. In order to get monolayer dispersion, it is importment that the adsorbed particles should be fully washed with pure toluene in adsorption method or the remained solution on the support particles should be carefully removed in dipping method. The monolayer FeO_x, espically the "quasi-isolated" Fe—O polyhedrons are difficultly detected by FTIR-DRS, LRS as well as XRD. TEM and HEED techniques were adopted to further comfirm the observation. The monolayer dispersed Fe~(3+) is at high spin state and located in a site with distorted rhombic symmetry. The monolayer FeO_x on ZrO_2 is easily reduced to ferrous state as compared with the unsupported bulk ferric oxide or even those monolayer FeO_x species on TiO_2(A) and γ-Al_2O_3. However, further reduction of ferrous ion is still difficult. The microcrystal of ferric oxide on ZrO_2 is reduced through an intermediate compound during the reduction process. The results of pulse reaction of 2-C_4H_8 with or without gas oxygen showed that even the monolayer "quasi-isolated" polyhedrons are still effective to catalyse the ODH of the reactant. Different from the bulk ferric oxide, the monolayer catalysts are sensitive to the presence of gas oxygen for the formation of surface residues and CO_x. In addition, the monolayer FeO_x species are not active for the methanation reaction.