Michael Claeys

On the effect of water during Fischer–Tropsch synthesis with a ruthenium catalyst

Abstract The addition of water during Fischer–Tropsch synthesis over a supported ruthenium catalyst led to a significant increase in product formation rates and significant changes in product selectivity, in particular lower methane selectivity and improved chain growth. Upon increasing water partial pressures, the total product distribution shifted from ASF distributions, with typical deviations due to olefin reinsertion, to a much narrower distributions. Such distributions can mechanistically not be explained by sole C1-wise chain growth. An additional product formation route considering combination of adjacent alkyl chains to form paraffins (“reverse hydrogenolysis”) has been proposed. The findings are discussed with regard to the crucial mechanistic role of water as a moderator in the kinetic regime of the Fischer–Tropsch synthesis.

Chapter 8 – Basic studies

Publisher Summary This chapter reviews that the understanding of the fundamental processes taking place on the metal surfaces during the fischer-tropsch (FT) synthesis leads to improved catalyst design and improved macroscopic description of the FT process. Reaction parameters are selected to optimize product formation. Knowing the possible surface species on the catalyst surface during the FT synthesis and their reactivity, enables the formulation of reaction pathways. This insight leads to mechanistic descriptions for the rate of product formation in the FT synthesis. The chapter also focuses on the concept of the FT synthesis. It is a polymerization reaction, in which the monomers are being produced in situ from the gaseous reactants hydrogen and carbon monoxide. Thus, all the reaction pathways proposed in literature have three different reaction sections: generation of the chain initiator, chain growth or propagation, and chain growth termination or desorption. It discusses that huge variety of products of different chain length and different functionality is formed in FT synthesis. The actual composition/product distribution of a FT process depends on many reaction variables, such as reaction conditions, the reactor system used, as well as the catalyst formulation and physical properties of a catalyst.

Experimental approaches to the preparation of supported metal nanoparticles

Supported metal particles play an important role in heterogeneous catalysis. It has been shown lately that the size of the metal crystallites in the supported metal catalysts has a profound effect on the catalytic activity, thus necessitating the need for synthesis methods aimed at a strict control of the metal crystallite size in these catalysts. The classical methods used to synthesize supported metal catalysts typically yield a wide metal crystallite size distribution, and average crystallite sizes which are difficult to control. Suitable techniques have been developed to obtain supported metal catalysts with defined crystallite size distributions, inter alia impregnation of reverse micelle microemulsions, colloid impregnation following reverse micelle precipitation or crystallization, and deposition-precipitation. Using these techniques, a series of supported Ru/γ-Al2O3, Co/SiO2, Fe/γ-Al2O3, Fe/C, and Au/ZnO catalysts have been prepared and characterized.

Size‐Dependent Phase Transformation of Catalytically Active Nanoparticles Captured In Situ

The utilization of metal nanoparticles traverses across disciplines and we continue to explore the intrinsic size-dependent properties that make them so unique. Ideal nanoparticle formulation to improve a processs efficiency is classically presented as exposing a greater surface area to volume ratio through decreasing the nanoparticle size. Although, the physiochemical characteristics of the nanoparticles, such as phase, structure, or behavior, may be influenced by the nature of the environment in which the nanoparticles are subjected1, 2 and, in some cases, could potentially lead to unwanted side effects. The degree of this influence on the particle properties can be size-dependent, which is seldom highlighted in research. Herein we reveal such an effect in an industrially valuable cobalt Fischer-Tropsch synthesis (FTS) catalyst using novel in situ characterization. We expose a direct correlation that exists between the cobalt nanoparticles size and a phase transformation, which ultimately leads to catalyst deactivation.

Re‐dispersion of Cobalt on a Model Fischer–Tropsch Catalyst During Reduction–Oxidation–Reduction Cycles

Reduction–oxidation–reduction treatment is employed to regenerate cobalt Fischer–Tropsch synthesis catalysts. Reduction–oxidation–reduction cycles on a model supported‐cobalt Fischer–Tropsch catalyst showed that hollow‐sphere formation occurs during the oxidation step, followed by the formation of a catalyst with an improved metal dispersion. It is deduced that the metal particles consist of both hexagonal close packed Co and face‐centered cubic Co. High‐pressure oxidation led to smaller cobalt oxide crystallites, resulting in a more facile reduction and an increase in metal surface area. Hence, oxidative regeneration is an attractive method to improve the dispersion of cobalt metal on a sintered deactivated catalyst. However, metallic cobalt crystallites may become too small after regeneration, and the intrinsic activity per unit surface area in the Fischer–Tropsch synthesis may drop.

Some evidence refuting the alkenyl mechanism for chain growth in iron-based Fischer–Tropsch synthesis

Abstract Recently, Maitlis et al. [J. Catal. 167 (1997) 172] proposed an alternative reaction pathway for chain growth in the Fischer–Tropsch synthesis. In this mechanism, chain growth is assumed to occur by methylene insertion into a metal–vinyl bond, forming an allyl species that will subsequently isomerise to a vinyl species. Organo-metallic allyl complexes, Fe{[η 5 -C 5 H 5 ](CO) 2 CH 2 CHCH 2 } and Fe{[η 5 -C 5 (CH 3 ) 5 ](CO) 2 CH 2 CHCH 2 } were synthesised. Under thermal treatment, the decomposition of these complexes was observed, instead of the isomerisation. In a hydrogen atmosphere, the reduction of the iron–carbon bonds and the hydrogenation yielding iron–alkyl species was observed. This clearly shows that the proposed vinyl–allyl isomerisation is unlikely to occur in mono-nuclear iron complexes. Hence, it might be expected that the reaction mechanism proposed by Maitlis et al. [J. Catal. 167 (1997) 172] is unlikely to be the main route for chain growth in the Fischer–Tropsch synthesis.

Hydrocarbons via CO2 Hydrogenation Over Iron Catalysts: The Effect of Potassium on Structure and Performance

We present a study in which the suitability of potassium promoted iron-based Fischer–Tropsch (FT) catalysts for the generation of synthetic natural gas additives via the hydrogenation of carbon dioxide through a combined reverse water gas shift (WGS) and FT reaction is studied. Using novel in situ instrumentation based on XRD and magnetometry techniques the reversible conversion of metallic iron to Hägg carbide under reaction conditions and its decomposition in hydrogen could be monitored. The facilitating effect of potassium in the formation of iron carbide could be exposed as function of time on stream. While the FT reaction was reduced in the presence of high potassium loadings the reverse WGS reaction seemed to be unperturbed. A faster activation of an iron phase obtained via the decomposition of iron carbide, compared to the initial activation of a pristine iron phase obtained via the reduction of iron oxide was witnessed.Graphical Abstract

Metal Support Interactions in Co 3 O 4 /Al 2 O 3 Catalysts Prepared from w/o Microemulsions

To obtain nano-sized metal and metal salt crystallites with a narrow size distribution synthesis methods utilizing water in oil (w/o) microemulsions, i.e. reverse micelles, have been widely applied and reported in literature. In this study we show the effect of support addition at different stages of the reverse micelle based preparation of cobalt oxide on alumina model catalysts. All catalysts were characterized with X-ray powder diffraction and Raman spectroscopy indicating the presence of Co3O4 on the Al2O3 support. Studies of the reduction behaviour and X-ray photoelectron spectroscopy however revealed the presence of difficult to reduce cobalt aluminate species in the samples where the support was added during or shortly after the precipitation step in the synthesis process. It can therefore be assumed that if the alumina support is added to the reverse micelle solution unprecipitated Co2+ ions and partially dissolved Al3+ combine and form cobalt aluminates. In the preparations where the solid cobalt precipitates are recovered from the microemulsion and then supported on the carrier, no metal-aluminate formation could be observed. This study therefore gives important information how metal-support interaction can be affected during catalyst preparation using reverse micelles.Graphical Abstract

Effective Utilization of the Catalytically Active Phase: NH3 Oxidation Over Unsupported and Supported Co3O4

The performance of pellets of unsupported and silica-supported Co3O4 in the ammonia oxidation was investigated as a function of the particle size to investigate the utilization of the catalytically active phase in these materials. The obtained activity in terms of ammonia conversion over the silica-supported Co3O4 is higher compared to the conversion over the unsupported Co3O4, despite a lower cobalt oxide loading and more severe diffusional limitations. The effectiveness factor for the silica-supported catalyst is slightly lower than the effectiveness factor for the unsupported catalyst in the form of pellets of similar size. However, the effective utilization of cobalt within the catalyst is higher for the silica-supported catalyst, mainly due to the higher dispersion of the catalytically active phase.Graphical AbstractIncreased observed rate constant for silica-supported Co3O4 despite lower cobalt loading, and stronger diffusional constraints.

Promoting χ-Fe5C2(100)0.25 with copper – a DFT study

Abstract The role of copper in iron based Fischer–Tropsch catalysts was investigated using DFT with χ-Fe5C2(100)0.25 as a model surface. The presence of atomic copper on the iron-rich χ-Fe5C2(100)0.25-surface is more favorable than its presence in surface. Nevertheless, the segregation of copper from the surface yielding fcc-Cu remains an exergonic process. Carbon monoxide at a coverage of 2.2 CO per nm2 stabilizes atomic copper on this surface. The presence of copper results in the redshift in the stretching frequency of adsorbed CO. The mobility of copper atoms was investigated on χ-Fe5C2(100)0.25 in the presence of CO. The hopping frequency is reduced due to the presence of CO, although never enough to avoid formation of fcc-Cu on a shorter time scale than typically required for the formation of hydrocarbons in the Fischer–Tropsch synthesis.