摘要
采用(NH_4)_3[Fe(C_2O_4)_3]·XH_2O的水溶液含浸,Fe(NO_3)_3·9H_2O水溶液饱和浸渍以及Fe(AcAc)_3的甲苯溶液热吸附三种方法制备了在γ-Al_2O_3上分散的氧化铁体系。用多种物理化学手段对用不同方法制备的分散体系进行了细致的表征和比较,结果表明用第一和第三种方法制备的样品中Fe^(3+)是充分分散或单层分散的,但用饱和浸渍法制备时即使在低的载量时(3wt％Fe_2O_3)仍有氧化铁微晶。对于Fe_2O_3/γ-Al_2O_3,分散的结构状态还强烈地依赖于制备条件(参数)的选择。单层分散的Fe^(3+)处于高自旋态,具有正交畸变的局部配位对称性。单层分散的Fe^(3+)在氢气氛中即使升温至973K也只能被还原为Fe^(2+)状态。与单层MoO_x,VO_x,WO_x物种不同,单层分散的FeO_x物种既不能用XRD也很难被FTIR-DRS以及LRS所检测,丁烯脉冲反应表明单层的Fe^(3+)已可满足反应要求,但与晶相α-Fe_2O_3具有不同的选择性。CO脉冲实验表明与充分分散Fe^(3+)相关的晶格氧数目是少的而且较不活泼,流动态的CO+H_2反应结果表明,对于甲烷及其它低碳烃的生成主要取决于铁的价态而与分散的结构变化关系较小。在合成气中的程序升温反应结果进一步证实了这一结论。
Three methods, dipping of support into an excess of aqueous solution of (NH4)3[Fe(C2O4)3]·xH2O, incipient wetness impregnation of Fe(NO3)3 solution and batch adsorption of Fe(acac)3 , are adopted to prepare the well- dispersed or monolayer type ferric oxide on γ- Al2O3. Extensive characterization has been done on the physico-chemical properties of the system. The results indicated that the supported component is monolayer dispersed on the support surface when the samples are prepared by the first and thrid methods. However, even though the loading of supported component is quite low (3 wt. % Fe2O3) the microcrystals of ferric oxide exist in the samples prepared by incipient wetness impregnation method. For the system studied, the structures of dispersion are strongly dependent on the preparation conditions used. It seems that the best calcination temperature is 773 K for the dispersion. As the temperature is much higher than 773 K the diffusion of the ferric ions into the bulk of the support becomes noteable and the solid solution may be formed. On the other hand, if the temperature is lower than 773 K, the dispersion of ferric oxide becomes unfavourable. The monolayer dispersed ferric ions are at high spin state and located in a site with a rhombic or distorted rhombic symmetry. This kind of monolayer dispersed ferric ions cannot be reduced beyond the ferrous state even when the reduction temperature is increased up to 993 K. The stablity against the reduction is thought to be the formation of spinel-like surface compound, FeAl2O4. The interaction between the supported component and the carrier weakened markedly with an increase of the Fe loading. The reduction properties of the large crystals of the ferric oxide existing in the supported samples are almost the same as that of unsupported α-Fe2O3. Different from the MoOx, WOxand VOX monolayer species, the monolayer dispersed FeOx species on γ-Al2O3 is difficult to be detected by FTIR-DRS, LRS as well as XRD. The pulse reaction of 2-C4H8 showed that the monol
出处
《分子催化》
EI
CAS
CSCD
1992年第2期81-93,共13页
Journal of Molecular Catalysis(China)
关键词
氧化铁
氧化铝
分散状态
反应性能
Fe2O3, Al2O3, Dispersion state, Reaction performance.
