Fischer-Tropsch Catalysis Research Articles

Capturing the Genesis of an Active Fischer–Tropsch Synthesis Catalyst with Operando X‐ray Nanospectroscopy

AbstractA state‐of‐the‐art operando spectroscopic technique is applied to Co/TiO2 catalysts, which account for nearly half of the world's transportation fuels produced by Fischer–Tropsch catalysis. This allows determination of, at a spatial resolution of approximately 50 nm, the interdependence of formed hydrocarbon species in the inorganic catalyst. Observed trends show intra‐ and interparticular heterogeneities previously believed not to occur in particles under 200 μm. These heterogeneities are strongly dependent on changes in H2/CO ratio, but also on changes thereby induced on the Co and Ti valence states. We have captured the genesis of an active FTS particle over its propagation to steady‐state operation, in which microgradients lead to the gradual saturation of the Co/TiO2 catalyst surface with long chain hydrocarbons (i.e., organic film formation).

Capturing the Genesis of an Active Fischer-Tropsch Synthesis Catalyst with Operando X-ray Nanospectroscopy.

A state-of-the-art operando spectroscopic technique is applied to Co/TiO2 catalysts, which account for nearly half of the world's transportation fuels produced by Fischer-Tropsch catalysis. This allows determination of, at a spatial resolution of approximately 50 nm, the interdependence of formed hydrocarbon species in the inorganic catalyst. Observed trends show intra- and interparticular heterogeneities previously believed not to occur in particles under 200 μm. These heterogeneities are strongly dependent on changes in H2 /CO ratio, but also on changes thereby induced on the Co and Ti valence states. We have captured the genesis of an active FTS particle over its propagation to steady-state operation, in which microgradients lead to the gradual saturation of the Co/TiO2 catalyst surface with long chain hydrocarbons (i.e., organic film formation).

From hydrophilic to hydrophobic: A promising approach to tackle high CO2 selectivity of Fe-based Fischer-Tropsch microcapsule catalysts

Zeolite supported Fischer-Tropsch (FT) catalysis has attracted extensive attentions in past few years. Capsule catalysts have unique characteristics of tuning product selectivity more precisely and enhancing stability of catalysts during reaction, which makes them to stand out of the crowed. Fe-based FT catalysts have been facing limitations for syngas conversion to hydrocarbons with their associated problems of high CO2 selectivity and rapid catalyst deactivation. A series of ZSM-5 supported Fe-based microcapsule catalysts, composed of Fe/ZSM-5 core and silicalite-1 shell, was prepared and employed for direct synthesis of gasoline range hydrocarbons from syngas. Tuning silicalite-1 shell thickness significantly affected water-gas shift reaction with increasing hydrophobicity of catalyst surface, and greatly decreased CO2 selectivity. The microcapsule catalysts significantly suppressed the CO2 selectivity as compared with supported Fe-based catalyst. Moreover, all zeolite supported Fe-based catalysts, showed significantly high gasoline range hydrocarbons selectivity (about 60%). The proposed catalyst design suggests a promising approach to overcome high CO2 selectivity challenge associated with Fe-based FT catalysts.

Application of Plasma Technology in Fischer-Tropsch Catalysis for the Production of Synthetic Fuels

Application of Plasma Technology in Fischer-Tropsch Catalysis for the Production of Synthetic Fuels

Effect of phase and size on surface sites in cobalt nanoparticles

Molecular dynamics was applied to simulate cobalt nanoparticles to understand the effects of particles size and crystal phases on the distribution of surfaces sites relevant for Fischer-Tropsch catalysis. The results indicate a minimal change (<5%) in the coordination number distribution of surface atoms for particles larger than 4 nm even at 500 K, implying that temperature is not sufficient to drive surface reconstruction. We found that the distribution of B5 sites differ significantly between hcp and fcc particles, suggesting different CO adsorption properties for the two phases. HCP particles offer different types of B5 sites which are sterically less hindered for CO adsorption and maybe the reason for differences in catalytic behaviors of hcp vs. fcc nanoparticles. Finally, the smaller nanoparticles have higher radial contraction on the surface, which could impact the deactivation mechanism of these particles by inhibiting the sub-surface diffusion of adsorbate species.

Insight into the structure and morphology of Run clusters on Co(111) and Co(311) surfaces

The adsorption configurations, growth modes and morphology of a Ru promoter under the approximate conditions of cobalt catalyzed Fischer–Tropsch synthesis (FTS) were investigated by density functional theory (DFT) calculations.

Insight into the role of UV-irradiation in photothermal catalytic Fischer–Tropsch synthesis over TiO2 nanotube-supported cobalt nanoparticles

To explore an efficient catalytic system with high activity and selectivity is the key to improve Fischer–Tropsch synthesis (FTS) technology and the main focus in the academic field.

Cobalt catalysts for Fischer–Tropsch synthesis: The effect of support, precipitant and pH value

Abstract In this report, Co-based catalysts supported on ZnO, Al 2 O 3 and ZrO 2 as well as the ZrO 2 derived from different precipitants and different pH values were prepared by co-precipitation method. Their catalytic Fischer–Tropsch synthesis (FTS) performance was investigated in a fixed-bed reactor. The results revealed that Co catalyst supported on ZrO 2 exhibited better FTS catalytic performance than that supported on ZnO or Al 2 O 3 . For the Co/ZrO 2 catalyst, different precipitants showed the following an activity order of NaOH > Na 2 CO 3 > NH 4 OH, and the best pH value is 13. The catalysts were characterized by N 2 adsorption–desorption, XRF, XRD, H 2 -TPR, H 2 -TPD and TEM. It was found that the main factor affecting the CO conversion of the catalyst was the amounts of low-temperature active adsorption sites. Moreover, the selectivity of C 5 + hydrocarbons had a positive relationship with the peak temperature of the weak hydrogen adsorption sites. The higher the peak temperature, the higher the C 5 + selectivity is.

Providing Fundamental and Applied Insights into Fischer–Tropsch Catalysis: Sasol–Eindhoven University of Technology Collaboration

Although Fischer–Tropsch synthesis (FTS) was discovered more than 90 years ago, it remains a fascinating topic, having relevance from both an industrial and academic perspective. FTS based on cobalt and iron catalysts was studied in depth during an extensive 15-year collaboration between Eindhoven University of Technology, The Netherlands, and Sasol, South Africa. The primary objective of the collaboration was to obtain fundamental information that could assist in understanding practical issues in FTS over iron and cobalt catalysts. For iron-based catalysts, industrial slurry reactor work was combined with SSITKA and DFT modeling, resulting in improved clarity, with respect to the kinetics and mechanisms of FTS. This knowledge is important, with respect to designing large-scale industrial processes. In the case of cobalt-based FTS research, the combination of commercially relevant supported cobalt catalysts with sophisticated characterization tools, as well as the application of flat model catalyst system...

On the Role of Water in Heterogeneous Catalysis: A Tribute to Professor M. Wyn Roberts

From the earliest studies of heterogeneous catalysis, it was apparent that water plays a more important role in many systems than simply acting as a solvent. Its wide ranging effects have attracted increasing attention in recent years and was the topic of Prof. M.W. Roberts’ final paper. The present review explores some of the latest work on water in reactions ranging from CO oxidation to Fischer Tropsch catalysis, the different mechanisms proposed for its role are discussed and compared.

Synergistic Promotion of Co/SiO2 Fischer–Tropsch Catalysts by Niobia and Platinum

Transition-metal oxides and noble metals are well-known selectivity and activity promoters in cobalt-based Fischer–Tropsch catalysis. Niobia has been shown as an effective selectivity promoter as support material; however, its low porosity limits the cobalt loading. To combine the selectivity-promoting properties of niobia with a highly porous support, niobia-modified silica was prepared and applied as a support for Co and PtCo catalysts with cobalt loadings up to 21 wt %. Niobia promotion was found to increase the C5+ selectivity at 1 bar; however, it appeared to be ineffective at 20 bar. Promotion of Co/SiO2 by a combination of platinum and niobia yielded an increase of the cobalt-weight normalized activity by a factor of 2–3 in the case of amorphous niobia and by a factor of 3–4 with niobia nanocrystals present, due to both an increased number of active sites and an increased cobalt-surface specific activity (turnover frequency).

The size-dependent site composition of FCC cobalt nanocrystals

Cobalt-based Fischer–Tropsch (FT) catalysts typically contain reactive cobalt metal nanoparticles on support materials. Such metal nanoparticles are expected to have properties that are strongly size-dependent. Noting the size-dependence of the Co FT rate for particle diameters below 10nm, we aimed to see whether the FCC Co nanoparticle site composition exhibits a corresponding size-dependence. This aim was realised by: building and optimising realistic and representative close-packed Co nanoparticle models at various sizes; defining, identifying and counting the exposed surface sites; generalising these findings based on the small subset of simulated nanocrystal models. Using this approach, the site composition of five defined sites with increasing size up to 8nm was obtained. The three defined defect/step sites increased in concentration with increasing size, with the B5-A site and the B6 site reaching a stable value at about 4nm, while the B5-B site still increased somewhat even at 8nm. This confirms the size-dependent change in the surface site composition for FCC Co nanocrystals. The shape and site composition ranges were also compared to previously calculated Wulff shapes and showed good agreement. Implications of the size-dependent FCC Co site composition on the FT catalysis process are also discussed.

The influence of CO adsorption on the surface composition of cobalt/palladium alloys

Segregation induced by the adsorption of gas phase species can strongly influence the composition of bimetallic surfaces and can therefore play an important role in influencing heterogeneous catalytic reactions. The addition of palladium to cobalt catalysts has been shown to promote Fischer Tropsch catalysis. We investigate the adsorption of CO onto bimetallic CoPd surfaces on Pd{111} using a combination of reflection absorption infrared spectroscopy and medium energy ion scattering. The vibrational frequency of adsorbed CO provides crucial information on the adsorption sites adopted by CO and medium energy ion scattering probes the surface composition before and after CO exposure. We show that cobalt segregation is induced by CO adsorption and rationalise these observations in terms of the strength of adsorption of CO in various surface adsorption sites.

Characterisation and Catalytic Fischer–Tropsch Activity of Co–Ru and Co–Re Catalysts Supported on γ-Al2O3, TiO2 and SiC

A number of supported cobalt catalysts for Fischer–Tropsch synthesis were considered. Catalysts were prepared by impregnation of the supports with a cobalt precursor resulting in cobalt concentrations of 15 or 20 wt%. The active metal was supported onto aluminum oxide, titanium dioxide and silicon carbide supports while rhenium or ruthenium metals were used as promoters, whose concentrations were 0, 0.2 or 1.0 wt%. The catalysts were characterized by a number of methods including BET, chemisorption, and XRD. Some of the catalysts were tested for catalytic activity and selectivity in the Fischer–Tropsch reaction using a fixed bed reactor. Based on the results, the addition of promoter metals increases the dispersion of the active metal cobalt and correspondingly a decrease in the cobalt metal particles, this effect was most evident for the alumina supported catalysts. Further, increased concentrations of the ruthenium promoter (1.0 wt%) slightly increased the selectivity to CH4 and decreased selectivity to C5+.

A Trimetal Carbene with Reactivity Reminiscent of Fischer–Tropsch Catalysis

Metal-carbenes are common reaction intermediates in chemical catalysis; however, very few catalysts are known with a trimetal-carbene active center. In this work, we describe a computational study into the chemical and catalytic properties of a unique trimetal carbene containing a Ru2Pt-carbene core, [(CpRu)2(μ2-CH2)(μ3-NCH3)Pt(P(CH3)3)2], with and without bound CO. The chemical structure and individual orbital interactions are identified from a natural bond orbital analysis, and the reaction pathways for consecutive intramolecular CH3 and H transfer (or vice versa) are investigated. The studies show that CO binding has dramatic effects on the reaction mechanism and weakens the Ru–H bond strength. The reaction processes are rationalized by valence bond models and thermochemical bond strength studies. The work leads to suggestions on how to improve this chemical system for enhanced alkane synthesis.

Hydrocarbon synthesis using Iron and Ruthenium/SiO2 with FISCHER-TROPSCH catalysis

Fe2(CO)9, Fe3(CO)12 and Ru3(CO)12 clusters were used as precursors for silica supported metals. The impregnated silica solids were obtained in organic solvents under inert atmosphere and the adsorbed complexes and reduced metals characterized by FT-IR, SEM EDX and HRTEM. The catalysts showed good Fischer-Tropsch (FT) activity; the main products were alkanes, alkenes and medium and higher alcohols as analyzed by GCMS. The Ru catalysts showed higher alcohols selectivity. HRTEM showed Ru nanoparticle size.

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

During the last years it has been an increasing focus on fundamental studies of the Fischer–Tropsch synthesis (FTS). Steady-state isotopic transient kinetic analysis and first principles investigation have proven to be important methods in studying the reaction mechanism of FTS. The present contribution deals with recent progresses in understanding of the F–T mechanism on Co catalysts based on combined DFT calculations, in situ characterization, steady-state isotopic transient kinetic analysis and microkinetics. A brief outlook into future perspectives of the FTS for converting synthesis gas from different carbon sources to fuels and chemicals is also provided.

Influence of Chemical Composition on the Catalytic Activity of Small Bimetallic FeRu Nanoparticles for Fischer–Tropsch Syntheses

FeRu nanoparticles were prepared according to an organometallic route using {Fe[N(Si(CH3)3)2]2}2 and (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene) ruthenium(0) Ru(COD)(COT) precursors followed by their insertion into a mesoporous MCF-17 support host. The resulting nanoparticles had a uniform size of approximately 2 nm, with a relative Ru amount of up to 33 at.%. Steady-state Fischer–Tropsch catalysis at 6 bar total pressure (H2/CO = 1:1) demonstrated light olefins production with a selectivity close to 50 % (ex. CO2) for catalysts with low Ru content (5 at.%). The selectivity pattern changed to long chain-paraffin production with increasing Ru amounts. These catalysts were also more active than those containing few Ru. X-ray photoelectron spectroscopy showed under-parity Ru amounts to effectively cover the surface of Fe nanoparticles. The nanoparticle distribution inside the MFC-17 host was characterized by microtomia/transmission electron microscopy.

Formation of 2-alkenes as secondary products during Fischer–Tropsch synthesis

The H2/D2 switching experiments showed that there is an inverse isotope effect during iron catalyzed Fischer–Tropsch (FT) reactions. The rate of hydrocarbon production (r) increased when syngas CO/H2 was switched to CO/D2 with the ratio of rH/rD being 0.55–0.58. The inverse isotope effect in every propagation step was calculated to be 0.88, which is a reasonable value for an inverse isotope effect at 270°C that originates from the C–H (D) bond hybridization change from sp2 to sp3. These results are similar to cobalt catalyzed FT reactions that were explained by the modified alkylidene mechanism by which 1-alkenes and n-alkanes are the primary products of the FT reaction and the others are products of secondary. The detailed analysis of products produced during iron catalyzed FT reactions with CO/H2 or CO/D2 as the syngas feeds showed that while the ratios of [1-alkenes]H/[1-alkenes]D are less than 1, the ratios of [2-alkenes]H/[2-alkenes]D are more than 1; this indicates that 2-alkenes are produced through a pathway that is different from 1-alkenes. These results also suggest that there is a normal isotope effect during the formation of 2-alkenes through the corresponding 1-alkenes, which will result in the ratio of [1-alkenes]H/[1-alkenes]D being less than 1. The formation of 2-alkenes is explained by the alkylidene mechanism.

In situ PXRD studies of heterogeneous catalysts and pre-catalysts

During the course of the last couple of years, my collaborators and I have studied a number of catalytic systems using a lab based PXRD facility at our disposal. Of particular interest to us has been the supported catalysts used in Fischer Tropsch catalysis as well as those used in the synthesis of multiwalled carbon nano tubes. These studies have all proven invaluable to the understanding of the often complex phase evolution that is an intricate and inherent part of the heterogeneous processes of interest to us. Selected results will be presented to illustrate the usefulness and value of these studies. For example below is the intensity profile of the diffraction patterns collected during the heat-treatment of the pre-catalyst: A – anatase, R – rutile, S – silicon, H – hematite and P – pseudobrookite