Fe1-xO基氨合成催化剂高活性机理初探

 由Fe1-xO催化剂的发现所引伸的新课题及其中潜在的科学暗示,特别是Fe1-xO基催化剂的高活性机理有待深入研究. 本文根据催化剂的还原性能和表面性质及其与催化活性的关联,从Fe1-xO的化学和物理性质出发,对Fe1-xO催化剂的高活性机理提出了一些初步认识. 催化剂前驱体由Fe3O4改变为Fe1-xO,首先使得催化剂的还原易于进行,还原温度降低,还原速度加快,还原出水量减少; 其次,改变了助催化剂在催化剂中的分布,从而改变了还原后催化剂的表面性质如表面酸覆盖度和碱覆盖度及其相对比例,形成了表面酸碱协同作用,促进了表面重构,降低了N2的脱附活化能,削弱了H2的强化学吸附. 这些变化导致催化剂活性提高,同时这些变化都是由Fe1-xO的化学和物理性质引起的,只有维氏体单独存在于催化剂中催化剂才具有高活性. 提出了H2的强化学吸附与催化剂表面酸碱度有关的观点.

Study on Mechanism of High Activity of Fe1-xO-Based Catalyst for Ammonia Synthesis

The wustite-based catalyst is much more active than the best magnetite-based catalysts. It is clear that this discovery has a strong impact on the consolidated scientific knowledge of ammonia catalysts. Possible reasons for the high activity of the wustite-based catalyst are discussed. When the precursor is changed from magnetite to wustite, the catalyst can be reduced easily (i.e. the reduction temperature is decreased, reduction rate is increased and the water production from reduction is reduced) and the catalyst activity increases after reduction. The variation of the kind and distribution of promoters in the precursor results in changes in surface properties, surface texture and ratio of acid surface area to alkali surface area, which lead to the synergistic effect of the acid center and basic center, promote the surface restructuring of Al 2O 3, decrease the N 2 desorption activation energy and weaken the strong chemical adsorption of H 2. The catalyst activity increases with these changes, which can be attributed to the chemical and physical characteristics of wustite. The high activity is obtained only when the wustite phase is present alone in the catalyst. It is proposed that the strong chemical adsorption of H 2 may be related to the surface acidity and basicity on the catalyst.