PREPARATION AND CHARACTERIZATION OF TITANIA-SUPPORTED IRON AND ALKALI-PROMOTED TITANIA-SUPPORTED IRON CATALYSTS FOR AMMONIA SYNTHESIS (OSTWALD RIPENING, CRYSTALLITES, PROMOTER, PHASE TRANSITION, HYDRAZINE).

摘要

Small metallic crystallites supported on titanium dioxide experience strong interaction with the support when reduced to the metallic state in flowing hydrogen at temperatures near 770 K. In this study, this strong metal-support interaction (SMSI) was investigated. It was found that at temperatures near those at which the metal-support interaction becomes operative, titania undergoes a phase transition from the anatase (elongated tetragonal) to the rutile (tetragonal) crystal forms. The phase transition, which is accompanied by growth of titania crystals, is accelerated in reducing environments and is catalyzed by iron. The phase transition and grain growth are thought to be initiated by Ti('3+) defect centers in the titania lattice which are formed by reduction of the support. The properties of small iron crystallites reduced to the metallic state at various temperatures in the range 643-873 K were investigated by carbon monoxide chemisorption and X-ray diffraction. A strong correlation between the chemical changes of the support and the extent of metal-support interaction suggests that SMSI results from covering of iron crystallites by a highly mobile titanium-oxygen moiety.; The kinetics of the synthesis of ammonia from its elements was investigated over iron and alkali-promoted iron supported on titania. It was found that pretreatment of a catalyst with 100% hydrazine hydrate increases the turnover number for ammonia production by a factor of ten over the untreated sample. The ammonia synthesis over the hydrazine-treated catalyst appears more characteristic of iron uninfluenced by SMSI, as indicated by the kinetic parameters in the Temkin-Pyzhev model. It was concluded that hydrazine treatment suppresses onset of SMSI either by creation of iron particles in a TiN matrix, or some other modification of the support. Promotion with alkali metal not only increased turnover numbers and decreased activation energy, but acted to stabilize supported iron particles against growth by Ostwald ripening. By incorporation of a model describing surface area decay in supported catalysts due to particle ripening into the statistical analysis of the reaction rate data, a significant improvement in the accuracy and precision of the kinetic parameters was realized.