Structure and kinetics of iron-catalysts in the initial phase of the Fischer-Tropsch synthesis: Combination of Operando-XRD and studies in a fixed-bed reactor

在费托合成(FTS)中，CO和H2被转化为更高级的碳氢化合物(以及副产物水)。所使用的催化剂通常以钴或铁为基础。特别是在铁基催化剂的情况下，直到今天还不清楚是哪一相(S)催化了FT-反应。这主要是因为实际催化剂是在FTS的初始阶段在合成条件下形成的，催化剂中的固相形成了金属铁、铁氧化物和碳化物的复杂混合物。尽管过去通过X射线衍射仪、穆斯堡尔谱和X射线光电子能谱对铁催化剂的组成进行了非原位研究，但很少有研究将铁催化剂的组成和催化行为的依赖性结合起来。众所周知，铁催化剂在FT反应的初始阶段被激活。伴随着这种活化，形成了Fe-碳化物，由此得出结论，这些碳化物对FTS是活跃的。然而，这一结论，以及，另外，碳化物相是(更多/最)活性的，仍然是有争议的“，因此，即使对新催化剂进行了非常详细的表征，也很难获得关于”工作“催化剂的相结构的信息。同样令人怀疑的是，通常在惰性气氛和室温下进行的旧催化剂或废催化剂的非原位表征是否能提供有关工艺条件下催化剂结构的明确信息。我们的初步研究表明，催化剂中铁相的形成取决于还原前处理、催化剂的组成(助剂)和实际FTS过程中的条件。在拟议的项目中，不同铁相的形成将由在FT反应条件下，即在FT反应条件下的所谓低温FTS(T&lt；250℃)来研究，直到达到稳定状态。还应研究不同铁相的形成如何取决于前处理、促进剂和反应条件(温度、压力、CO、H2和水蒸气的分压)。在相同的反应条件下，在固定床反应器中考察催化剂的活性和选择性。因此，应获得有关结构-反应-关系(“催化剂和动力学相”)的信息，并应识别在FTS中活跃的相(S)。是否可以通过选择合适的促进剂和/或还原前处理来调节最终达到稳定状态的活性和选择性，这也是人们非常感兴趣的问题。

In the Fischer-Tropsch synthesis (FTS) CO an H2 are converted to higher hydrocarbons (and water as by-product). Catalysts employed are usually on the basis of cobalt or iron. Especially in the case of iron based catalysts it is not clear until today, which phase(s) catalyse the FT-reaction. The main reason for this is that the actual catalyst is only formed under synthesis conditions during the initial phase of the FTS and the solid phases in the catalyst form a complex mixture of metalic iron, iron oxides and -carbides. Even though the composition of iron catalysts has been studied ex situ via X-ray diffraction (XRD), Moessbauer- and X-ray photoelectron spectroscopy in the past, only very few studies combine the composition and the dependency of the catalytic behaviour thereof. It is well-known that iron catalysts are activated during the initial stage of the FT-reaction. Along with this activation Fe-carbides are formed, from which it was concluded that these carbides are active for the FTS. Nevertheless, this conclusion and, additionally, which carbide phase is the (more/most) active one, is still disputed"Accordingly, the information on the phase structure of the "working" catalyst can hardly be gained, even with a very detailed characterisation of the fresh catalyst. It is also doubtful if the ex situ characterisation of used or spent catalysts, which typically is done in inert atmosphere and at room temperature, supplies definite information on the structure of the catalyst under process conditions. Our preliminary studies with in operando XRD and kinetic measurements in a fixed bed reactor show, that the formation of Fe-phases in the catalyst depends on the reductive pretreatment, the composition (promoters) of the (fresh) catalyst and on the conditions during actual FTSIn the proposed project the formation of different Fe-phases shall be studied by in operando XRD, i.e. at FT-reaction conditions,he focus is thereby the so-called low temperatur FTS (T < 250°C) until steady state is reached. It shall also be examined, how the formation of different Fe-phases depends on pretreatment, promoters and reaction conditions (temperature, pressure, partial pressures of CO, H2 and water vapour). Complementarily, the activity and selectivity of the catalysts will be examined in a fixed bed reactor under the same reaction conditions. Thus, information on the structure-response-relationship ("phases of catalyst and kinetics") shall be gained and the phase(s), which is/are active in FTS, should be identified. It is also of great interest, whether the activity and selectivity finally reached in steady state can be adjusted a priori by a proper choice of promoters and/or reductive pretreatment.