Iron-based Fischer-Tropsch Synthesis : new insights from in-situ (micro)spectroscopy, diffraction and theory

Abstract

The Fe-based Fischer-Tropsch synthesis (FTS) catalyst, which converts CO and H2 into longer hydrocarbon chains through a catalytic surface polymerization reaction, is one of the oldest and perhaps most studied systems known in heterogeneous catalysis. However, even though the different Fe bulk phases present during FTS have been identified in the earliest studies, precise structural data of these phases, especially iron carbides, and their role in the activation and deactivation of Fe-based FTS catalyst materials are either unavailable or highly disputed in literature. Related to this, the sensitivity of Fe-based catalysts to oxidation by air is well known. This disqualifies ex-situ structure-performance correlations where the catalyst is exposed to air before characterization, and advocates the use of characterization methods that can be applied to monitor the catalyst during catalytic reaction (i.e. in situ or operando). Therefore, the goal of the work described in this thesis was to contribute to the understanding of the role of the many different Fe phases (metallic, oxidic, carbidic) present in Fe-based catalysts in the activation and deactivation behavior in the low temperature FTS reaction. Special attention was given to the development of in-situ spectroscopic characterization of Fe-based catalyst system at the microscopic, bulk phase and surface chemistry level, under catalytically relevant reaction conditions. Chapter 2 gives an introduction into the proposed Fe-based FTS catalyst reaction mechanisms, phase chemistry and deactivation behavior by reviewing research that has been conducted on Fe-based FTS catalysts since the invention of the synthesis in the 1920s. Chapters 3, 4 and 5, investigate the bulk composition of the Fe-based FTS catalyst under atmospheric pressure reaction conditions. Catalyst systems of increasing complexity, containing different amounts of Cu, K and SiO2 were the subject of study for long and local range ordering in Fe-based catalysts in chapter 3 by the combined application of X-ray diffraction and X-ray Absorption Spectroscopy. Chapters 4 and 5 focus on the nanoscale chemical imaging of the most complex catalyst of this series, a fully promoted, industrially relevant Fe-based FTS catalyst by in-situ Scanning Transmission X-ray Microscopy, a novel nanoscale chemical imaging technique. Chapter 6 explores the theoretical thermodynamic stability of bulk carbide phases as determined from ab initio methods and combines this with state-of-the-art in-situ characterization of the Fe-based FTS catalyst under high pressure reaction conditions using X-ray Absorption Spectroscopy, X-ray Diffraction and Raman Spectroscopy. Chapters 7 and 8 concentrate on the surface chemistry of Fe-based FTS catalysts. In both chapters, in-situ X-ray Photoelectron Spectroscopy was applied to study the catalyst surface in reactive gas atmospheres. In chapter 7, in-situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy, in combination with conventional laboratory based characterization techniques were used to study surface and bulk properties as a function of catalyst formulation. Chapter 8 investigates the differences in surface chemistry of nanosized and bulk iron oxides in reactive gas atmospheres. Ab initio atomistic thermodynamic calculations form a basis for explaining the observed surface phases at different reaction conditions. The thesis concludes with a summary of the most notable findings and concluding remarks in chapter 9