Christine E. Kliewer

Characterizing Intrinsic Deactivation in Cobalt-Catalyzed Fischer-Tropsch Synthesis

Three intrinsic deactivation modes observed in experimental cobalt Fischer-Tropsch synthesis (Co FTS) catalysts are investigated via ex-situ TEM [1]. These include: cobalt oxidation reversible by mild hydrogen treatment, cobalt agglomeration, and cobalt-support mixed oxide formation. All three mechanisms involve redox transformation of the catalytically active cobalt metal. Earlier studies [2] have shown that (a) Co 0 is the catalytically active entity in Co FTS and (b) the site activity is not influenced by the support or the metal particle size in catalysts containing metal particles larger than ~5 nm. Steady-state isotope transient studies with similar catalysts [3] confirmed these conclusions. Consequently, transformations that reduce Co 0 surface by agglomeration or by oxidizing Co particles will reduce catalytic activity. Bulk thermodynamic data [4,5] suggests that CoO would not form under typical Co FTS conditions. However, reactor studies and TEM observations prove that the behavior of nanoscale metal particles deviates from that of the bulk and that cobalt oxidation does occur during commercially relevant Co FTS conditions. The exact nature of the oxidized species is not clear, but it is shown that the process is enhanced by higher CO conversion (higher H2O/H2 ratio) and smaller metal particle size. This oxidation is fully reversed by hydrogen reduction at Co FTS temperature and pressure. Once the cobalt metal is oxidized, it wets the support surface and thus allows nearby oxidized cobalt particles to contact each other. Subsequent reduction can lead metal agglomeration. Since this growth mechanism starts with metal oxidation, it is also enhanced by CO conversion and smaller particle size. However, due to the spatial proximity requirement, metal particle distances on the support surface also play a role. In this regard, the reduction-oxidation-reduction (ROR) treatment of Co FTS catalysts is also instructive. ROR treatment is known to boost Co FTS activity [6] and increase Co dispersion [7,8]. This work shows that the ROR process involves the formation of hollow oxide domes that break up into smaller Co particles upon re-reduction. Thermodynamic calculations suggest that cobalt-support mixed oxide formation with SiO2, Al2O3, and TiO2 is favored at ~70% or even lower conversion of stoichiometric syngas in Co FTS. This path is kinetically open as soon as Co is converted to Co 2+ via byproduct water. Indeed, evidence for various degrees of mixed oxide formation is presented. In general, this solid-state chemistry is slow, although in the case of Co/SiO2, crystalline mixed oxide needles of well-defined stoichiometry formed rapidly at high CO conversions [9].

Ex-Situ TEM Sulfidation Study of Supported Copper Particles

However, certain processes occur in highly corrosive (sulfur-containing) environments, and it is often not desirable to flow such gas mixtures into the TEM for in-situ studies or even through an ex-situ system due to contamination issues. Consequently, a dedicated ex-situ treatment facility has been developed for these studies. This new facility was designed after our existing ex-situ treatment system but has its own separate reaction cell, welded feed line for sulfur-containing gases, and outlet discharge to an aqueous-based potassium hydroxide scrubber solution (Figure 1). The non-pristine system is, however, designed such that it shares a high temperature furnace with our pristine facility.

Ex-situ TEM: Gaining Fundamental Insights into the Reduction-Oxidation-Reduction (ROR) Process in Small, Bimetallic Particles

Air regeneration of bimetallic catalysts remains an important but poorly understood area of science. The effects of the reduction-oxidation-reduction (ROR) steps on a multi-component materials nanostructure are often unknown. However, with ex-situ electron microscopy-based techniques, new insights into these structure/process variable relationships are possible. Earlier TEM studies conducted using ExxonMobil’s ex-situ treatment facility revealed that the air oxidation of solid Cu particles resulted in the development of Cu oxide hollow domes (torus structures). However, the effect of alloying the Cu was not examined.

Sulfidation of Metal Oxide Crystallites: An Ex-Situ TEM Study

The concentration of sulfur in diesel is becoming regulated to increasingly lower levels. Thus, new catalytic systems are necessary to effectively address this challenge. To effectively develop these new materials, it is important to better understand the complex nanostructures existing in the current hydrodesulfurization (HDS) catalysts. This study looks into that issue by using exsitu TEM time-temperature-transformation (T-T-T) data to follow the development of the active phase in a Ni-promoted Mo-rich HDS catalyst.

Sulfidation of Molybdenum Oxide and Tungsten Oxide Crystallites: an Ex-Situ TEM Study

With environmental considerations mandating ever-increasingly lower sulfur levels in fuels, it is necessary to develop more robust hydrodesulfurization (HDS catalysts). 1-3 In order to improve upon the current commercial catalytic systems, it is critical to first develop an understanding of the morphological transformations that occurs during activation (sulfidation) of the catalyst. 4-8 Because commercial HDS catalysts are highly complex systems containing promoters and supports, 4-8 this initial work follows the sulfidation of two simple (unpromoted, and unsupported) systems (molybdenum oxide and tungsten oxide) to gain an overall, fundamental understanding of the transformations that occur within these materials.

Understanding Bimetallic Catalysts Via Ex Situ TEM Studies

Bimetallic catalysts remain an interesting but poorly understood area of science. The effects of sequential reduction oxidation reduction (ROR) treatments on the nanostructures of these materials are often unknown. However, with the use of exsitu TEM techniques 2 recent studies have begun to explore these effects. Previous ex-situ TEM studies revealed that a simple RO treatment of small, Cu metal particles created torus structures of Cu oxide. The evolution of this Cu oxide structure was related to differences in Cu and O ion diffusion rates. Similar but more extensive exsitu TEM investigations are now being conducted on bimetallic particles.

Electron Microscopy and Imaging

This chapter gives a concise overview of various zeolite morphologies and continues with a general discussion regarding the historical development of electron microscopes and the integration of associated elemental analysis instrumentation. Various specialized forms of electron microscopy (e.g., high resolution imaging, electron tomography, etc.) are reviewed with respect to zeolites. Zeolite-specific sample preparation techniques for scanning electron microscope (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) are discussed in detail. General SEM, TEM and high angle annular dark field STEM imaging protocols for zeolite examination are reviewed with an emphasis on obtaining general microstructural information, pore/channel morphology, and small metal particle dispersion information. Elemental analysis of zeolites using energy dispersive spectrometry and electron energy loss spectrometry is examined.