Cobalt catalyst characterization and modification by atomic layer deposition for Fischer-Tropsch synthesis
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Kemian tekniikan korkeakoulu |
Master's thesis
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Authors
Date
2019-06-18
Department
Major/Subject
Functional Materials
Mcode
CHEM3025
Degree programme
Master's Programme in Chemical, Biochemical and Materials Engineering
Language
en
Pages
54 + 2
Series
Abstract
Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed process which produces hydrocarbons from carbon monoxide and hydrogen. FTS is a promising stepping stone to more effectively use available gas resources both fossil and renewable, or to produce petroleum substitutes from industrial sidestreams and captured carbon dioxide converted to carbon monoxide. From the catalysts used in FTS, cobalt-based catalysts are of interest due to their higher activity, heavier product fractions, and natural selectivity towards paraffins as compared to iron-based catalysts. The tailoring of catalysts produced by commonly used impregnation methods is a means to develop more resilient and selective catalysts. Atomic layer deposition (ALD) is capable of coating very conformal layers in porous catalysts, becoming a useful tool in modifying supported catalysts very precisely. Top-coating by ALD alumina and promotion by ALD platinum are the modifications investigated in this thesis to improve resilience and applying a highly dispersed effective promoter. A roster of characterization methods including X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, X-ray fluorescence, among others are discussed and implemented in this work. The catalysts studied are composed of silicon doped gamma alumina supported cobalt catalysts with ~30 wt% Co and 0.1 wt% Pt for the unmodified sample and ALD alumina modified samples. A similar catalyst without Pt promotion is used as base for ALD Pt modified sample. Around a gram of each catalysts was ran in a micro-reactor from 40 to 70 h. The conversions, selectivity and productivity were quantified to determine the practical qualities of the catalysts. Average crystallite size was determined from XRD to observe possible sintering. ALD alumina showed the desired effect in reducing average particle size after the run when compared to unmodified catalyst from approx. 20 nm to as low as 14 nm for 40 ALD cycles. As an active component ALD alumina at 5 cycles increased activity and methanation, while at 20 cycles the properties were similar to the unmodified catalyst. ALD Pt coated catalyst showed product selec-tivity towards lighter products. The hydrogenating effect of platina was strongly present, demon-strating the effectiveness of only 5 ALD Pt cycles. This thesis gave some indication on the effectiveness of ALD modifications for Fischer-Tropsch cobalt catalysts and the synergy between some of the characterization methods used in FTS.Description
Supervisor
Franssila, SamiThesis advisor
Keskiväli, LauraHeikkinen, Niko
Keywords
fischer-tropsch, characterization, cobalt catalyst, atomic layer deposition, top-coating