Electron microscopy studies of activation mechanisms in hydrotreating catalysis

Research output: Book/anthology/dissertation/reportPh.D. thesisResearch

  • Christian Dahl-Petersen
The aim of this work is to remedy the limited fundamental insight that exists in terms of the activation and formation of hydrotreating catalysts utilized in industrial oil refining of crude oil. This is done through numerous studies of the conversion of industrially relevant molybdenum oxide materials into molybdenum disulfide (MoS2) utilized as hydrotreating catalyst. Specifically, the work revolves around the use of in situ transmission electron microscopy (TEM) which allows for detailed studies of the reaction dynamics in formation of MoS2 at the atomic scale.

TEM is a powerful visualization technique with a time resolution on the second-scale and spatial resolution capable of visualizing single atoms making it ideal to study fundamental aspects of catalysis. In combination with in situ capabilities it has become an indispensable tool in studying dynamic nanoscale phenomena, whereby it is an excellent choice for studies of the formation of hydrotreating catalysts.

Through in situ TEM sulfidation of molybdenum dioxide nanoparticles, it is shown that the particle surface is converted into MoS2 by exposure to H2S and H2O at low temperature. Increasing the temperature leads to an increasingly larger amount of formed MoS2. MoS2 forms in one of two conformations of the two-dimensional atomic structure that is either in an orientation parallel or perpendicular to the nanoparticle surface. Both the initial growth of MoS2 and the subsequent formation of multi-layered structures is addressed. This shows that initial growth tends to form an apparent bond between the MoO2 surface and the MoS2 edge and that the layer size increases through coalescence. For multi-layered structures, it is found that MoS2 layers grow through a layer-under-layer mechanism, where defects in the outer layers enable radial transport towards the particle center. Transport to the growth front is shown to be enabled by intercalation of sulfur species between the MoS2 layers. This mechanism reveals a pathway for the anisotropic Kirkendall effect in nanostructured materials and explains the observations of parallel growth of MoS2.

The perpendicular conformation of MoS2 is interesting as the active site of MoS2 catalysts coincide with the edge of the two-dimensional layers. By orienting the MoS2 layer perpendicular to the nanoparticle surface, the edges will point directly away from the particle and thus the active sites are easily accessible for the reaction species. It is found that a crystallographic relationship is present between the MoO2 and MoS2 and that a topotactic conversion from the oxidic to the sulfuric phase enables orientational control. In addition, density functional theory studies shows that the topotactic growth is surface dependent and controlled by oxygen to sulfur exchange reactions and surface reconstruction which enable the formulation of an atomic growth mechanism and captures the observations made by in situ TEM.

The effect of the reaction conditions on the growth mechanism for MoS2 from MoO3 is discussed and it is found that the degree of conversion is related to the reaction temperature. Additionally, it is found that changing the sulfidation gas mixture impacts the number, length and orientation of formed MoS2 layers. Specifically, a high partial pressure of H2S in the sulfiding gas yields significantly higher conversion at medium high temperatures than a mixture containing less H2S. Interestingly, the formation of perpendicular MoS2 is observed in the high H2S partial pressure reaction whereas not in the low partial pressure equivalent

Finally, a new approach to increase the number of active sites of MoS2 materials is studied, in which water vapor is used to anisotropically etch grooves into the basal plane of MoS2 layers. Through the use of in situ TEM and post mortem AFM characterization, a reaction mechanism for steam etching is proposed. This mechanism is controlled by the H2O adsorption energy and the formation energy of low surface energy edge sites of MoS2. This method is found to increase the number of edge sites providing a means to enhance the catalyst activity.

In summary, the work presented here elucidates some of the fundamental and governing principles in activation and growth of industrially relevant MoS2 materials. These findings could potentially enable the engineering of improved hydrotreating catalysts and offers new opportunities for preparing catalysts and other nanostructures that exploit unique morphologically controlled properties.
Original languageEnglish
PublisherAarhus Universiet
Number of pages178
Publication statusPublished - 21 Dec 2016

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