摘要
用共沉淀法和浸渍法制备了ZnFe_2O_4、ZnCrFeO_4以及它们在γ-Al_2O_3,TiO_2,SiO_2小球上的担载催化剂.用TPR.XRD.ESR,MBS,脉冲反应等技术对这些催化剂进行了考察.结果表明,载体的引入在不同程度上改变了催化剂的氧化还原性能.在不同的载体上形成尖晶石结构的趋势不同.铁酸盐尖晶石相无论是非载的还是担载的对于多次交替的氧化还原处理不敏感.然而氧化铁相则较不稳定,而且在不同载体上稳定性相差较大.加Cr组份有助于担体上形成正尖晶石相.在某些较难形成铁酸盐尖晶石相的担载催化剂中,其相的组成较为复杂.脉冲反应的结果表明,担载的单一正尖晶石铁酸盐以及γ-Al_2O_3上的氧化铁对丁烯氧化脱氢均有一定的活性,对于既含较多量正尖晶石铁酸盐又含一定量氧化铁的催化剂,其较好的催化性能暗示在这两相之间可能存在某种协同效应.
ZnFe2O4, ZnCrFeO4and their supported catalysts (on TiO2, SiO2and γ-Al2O3) are prepared by precipitation and impregnation respectively. The loading of supported components is all about 25%.Several techniques(TPR,XRD,ESR,MBSand pulse reaction)are used to characterize these catalysts. The results show that supports can change the oxidation-reduction properties of catalysts to a greater or lesser degree. It seems that unsupported ZnFe2O4catalyst is deeply reduced and more difficultly to be reoxidized as compared with the corresponding supported catalysts . The oxidation-reduction properties of the active phase of γ-Al2O3are quite good compared with that of active phase on SiO2, whereas TiO2only has a slight effect on this aspect.The spinel phase whether is on the support or not is not very sensitive to reduction-reoxidation treatment for several times. Because of the interaction between the support and supported component, the trend of forming the spinel structure on the supports is TiO2 > SiO2 > γ-A12O3. Supported ZnO component reacts mainly with y- Al2O3to produce ZnAl2O4instead of ZnFe2O4, so the amount of ZnFe2O4or ZnCrFeO4phase on γ-Al2O3is small. Similarly, the interaction between SiO2and supported iron oxide hinders the formation of ZnFe2O4phase on SiO2. On the contrary, the ZnO and iron oxide on TiO2react with each other completely to form almost pure spinel phase on the support .The iron oxide on different supports has a different stability .The iron oxide on SiO2is more stable than that on γ-Al2O3but it generally turns into some amount of ferromagnetic substance when treated. The addition of Cr component to the catalyst is favourable to forming the spinel structure on supports and improves the oxidation-reduction properties of catalysts. When Cr is added, only pure spinel phase exists on supports except γ-A12O3. Some supported catalysts on which the spinel structure cannot be formed easily have complex composition. Most of the active phase of these catalysts is iron oxide with various forms, including α-Fe2O3, γ-Fe2O3and small particles of Fe3+. The number of these small particles is much more on SiO2than that on γ-Al2O3. They exist not only on the surface but also in the bulk. There are much more num-ber of such small particles on the surface of SiO2 compared with that on the surface of γ-Al2O3. Maybe these small particles containing Fe3+ are quite stable and not sensitive to the oxidation-reduction treatment. The results of pulse reaction show that either the pure spinel phase which exists on TiO2and SiO2or the iron oxide which exists on γ-A12O3 are active for the oxidative dehydrogenation of 2- C4H8to C4H6without oxygen supply. However, the catalytic activity of these catalysts all decreases quickly compared with that of catalyst containing predominantly spinel phase and some amount of iron oxide . Under the reaction condition without oxygen supply, the lattice oxygen of spinel phase is not very active and only few layers near the surface is consumed.On the other hand, the lattice oxygen of iron oxide on γ-Al2O3is highly reactive, and its consumption is fast which maybe make the catalysts lose their activity quickly. When ZnFe2O4is supported on SiO2, Fe3+in the catalyst is difficult to be reduced, so the reaction of selective oxidation cannot be carried out easily which results in its low catalytic activity. When the lattice oxygen at surface with suitable activity is consumed and Fe3+is reduced to Fe2+with successive pulse number of 2-C4H8, the activity of catalyst to selective oxidation decreases and the isomerization of 2-C4H8to l-C4H8becomes notable. It implies that maybe there is a relationship between the spinel phase and iron oxide for the reaction. It is possible that the catalytic activity and selectivity of supported catalysts can be improved by controlling the relative amount of supported oxide component in order to form predominantly spinel phase and some amount of iron oxide phase on certain supports.
出处
《分子催化》
EI
CAS
CSCD
1989年第1期10-19,共10页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金资助课题
