Untersuchung des Verkokungsverhaltens nickelbasierter Katalysatoren in der Methanspaltung
Helmin, Marta; Palkovits, Regina (Thesis advisor); Wasserscheid, Peter (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2018
Hydrogen is one of the most promising energy carriers. One approach, that enables a low-emission production of pure hydrogen and elemental carbon based on fossil fuels, is the metal-catalyzed decomposition of methane. Industrial operation requires a long lifetime of the used catalysts. However, due to the inevitable formation of carbon in form of carbon nanofibers, catalysts suffer strong coking and deactivation in the course of the decomposition reaction. To realise a continuous process, the deactivation behavior of heterogeneous catalysts in methane decomposition was investigated. Various nickel/metal oxide based catalysts were used to examine the influence of material properties (porosity, crystal structure, metal particle size) and process parameters on catalyst stability. The experiments were conducted in fixed bed reactors. The results show a strong negative correlation between the catalyst activity and stability, i.e. any parameter that increases the reaction rate also increases the deactivation rate. Certainly, there was no correlation between carbon capacity (carbon quantity which is accumulated on catalyst surface until complete deactivation) and deactivation rate. Electron micrographs of the used catalyst indicate that catalyst deactivation takes place as a result of the formation of a graphitic layer encapsulating the catalytically active metal particle. Furthermore, a significant dependency between the carbon morphology and catalyst activity could be observed. An increase in catalyst activity is accompanied by a higher graphitization degree of the formed carbon. In long-term experiments, carbon removal shows no negative impact on catalyst activity or the reaction process. Finally, high pressure experiments allowed the exploitation of an uninvestigated field in methane decomposition. An increase in pressure leads to a significant increase in catalyst productivity and stability.