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Providing fundamental and applied insights into Fischer-Tropsch catalysis: Sasol - Eindhoven University of Technology collaboration Jan van de Loosdrecht, Ionel Mugurel Ciobîc#, Philip Gibson, Nilenindran Sundra (Gregory) Govender, Denzil James Moodley, Abdool Muthalib Saib, Kees-Jan Weststrate, and J. W. (Hans) Niemantsverdriet ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00595 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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Providing fundamental and applied insights into Fischer-Tropsch catalysis: Sasol - Eindhoven University of Technology collaboration Jan van de Loosdrecht1)*, Ionel M. Ciobîcă2), Philip Gibson1), N.S. (Gregory) Govender1), Denzil J. Moodley1), Abdool M. Saib1), C.J. (Kees-Jan) Weststrate3)4), J.W.(Hans) Niemantsverdriet3)4) 1)

Sasol, Group Technology, 1 Klasie Havenga street, Sasolburg, 1947, South Africa Sasol Technology Netherlands BV, Vlierstraat 111, 7544 GG, Enschede, The Netherlands 3) Laboratory for Physical Chemistry of Surfaces, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands 4) Present address: SynCat@DIFFER, Syngaschem BV, PO Box 6336, 5600 HH Eindhoven, The Netherlands * corresponding author; [email protected] 2)

Abstract Although the 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. Fischer-Tropsch synthesis 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 modelling, 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 characterisation tools as well as the application of flat model catalyst systems has led to significantly improved knowledge of deactivation mechanisms. This improved knowledge has assisted in the understanding of new catalysts systems and regeneration processes. Finally, the success of the collaboration has been due to a number of factors. It has been beneficial to both parties to have had a long term collaboration, in which important fundamental catalysis topics were investigated that often took a substantial period of time. The access to high quality modelling and characterisation tools and fundamental understanding as well as industrially relevant supported catalysts operated at realistic conditions has proved vital in our contribution towards the advancement of Fischer-Tropsch Science and Technology. Keywords: Fischer-Tropsch, collaboration, cobalt, iron, surface science

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Introduction Although the 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. The last decade and a half has seen the construction of three world scale GTL (Gas-to-Liquids) plants in Qatar and Nigeria utilizing cobalt based FTS at the heart of the technology to convert natural gas into synthetic fuel. Small scale GTL and BTL (Biomass-toLiquids) microchannel reactors were developed, also using cobalt FTS catalysts, which are currently being commercialized. At the same time, a number of CTL (Coal-to-Liquids) demonstration plants were commissioned in China using iron based FTS. During the same period numerous strides have been made on a fundamental scientific level to understand topics that were previously not well understood including cobalt catalyst deactivation, structure sensitivity, and the FTS mechanism. A large part of the advancement of FischerTropsch Science and Technology can be ascribed to collaboration or partnership between academia and industry due to mutual interest in the field. The academic partner often has a more fundamental understanding of the underlying science, while the industrial partner will have more insight in the practical application and importance. A collaboration between Eindhoven University of Technology and Sasol was initiated in 1999 on the topic of Fischer-Tropsch catalyst deactivation. The expertise at Eindhoven included computational chemistry, surface science on single crystals and flat model catalysts, facilitating access to characterisation techniques like AFM (Atomic Force Microscopy), XPS (X-ray Photoelectron Spectroscopy), electron microscopy, and synchrotron XANES (X-ray Absorption Near Edge Structure) / EXAFS (Extended X-ray Absorption Fine Structure) / XPS. Sasol is a world leader in the application of Fischer-Tropsch technology and had decades of experience with industrial iron-based low- and high-temperature Fischer-Tropsch plants in South Africa as well as newly developed commercial alumina-supported cobalt catalysts, earmarked for factories in Qatar and Nigeria. They have also world class catalyst testing and piloting facilities and access to real industrial catalysts operating in large scale (100bbl/day) demonstration reactors at relevant conditions. After the successful execution of an initial feasibility study on the preparation of flat model catalysts, Sasol seconded the first employee to Eindhoven in 2001 to perform a Ph.D. study entitled “Towards a cobalt Fischer-Tropsch synthesis catalysts with enhanced stability: A combined approach”1. The success of this first joint Ph.D. study lead to seven more Sasol employees obtaining their Ph.D. degrees from Eindhoven University2,3,4,5,6,7,8 as well as to a Sasol sponsored student9. The achievements of this collaboration also lead to the appointment of two permanent Sasol employees for the periods 2001-2015 and 2008-2015 respectively. The outcome of the research was published in about 40 peer-reviewed journal papers10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50. The Eindhoven University of Technology provided academic guidance by Professors Hans Niemantsverdriet, Rutger van Santen and Jaap Schouten.


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The collaboration also spilled over into the South African academic environment and into the South African national science programs, from which several students and young academics visited Eindhoven for periods between several weeks and a full year51,52,53,54,55,56,57. The research subjects investigated dealt with both cobalt and iron based Fischer-Tropsch synthesis. For cobalt based catalysts we studied its deactivation in depth, focussing on oxidation, carbon deposition, sintering and surface reconstruction. Furthermore, we investigated fundamental aspects related to the FT mechanism and the kinetics for both cobalt and iron catalysts. These subjects will be highlighted in this paper, showing the beneficial results for both research parties as well as the synergistic effects for the joint research. Research highlight 1 – Fe FT mechanism Iron-based catalysts are used extensively in Sasol’s South African high and low temperature Fischer-Tropsch plants to produce a variety of synthetic hydrocarbon products including chemicals and fuel feedstocks41. The first collaboration on kinetic and selectivity modelling of industrial iron catalysts between Eindhoven University of Technology and Sasol resulted in a Ph.D. thesis in 20083. The catalyst used in this study was a precipitated (Ruhrchemie-type) iron catalyst which was tested at low temperatures (typically 240°C) in a well- mixed slurry reactor. Kinetic models were proposed for both the Fischer-Tropsch (FT) and Water-Gas-Shift (WGS) reactions as well as a hydrocarbon selectivity model in this dissertation3. A critical review of the open literature3,58 revealed that the inhibition by water and CO2 in the FT rate models was fundamentally incorrect. The observed influence of water on the FT rate could be explained as an indirect effect in that water actually affects the WGS rate which then influences the gas phase partial pressures of CO and H2 and thereby the FT rate. The newly proposed model (Equation 1) differs from the more traditional models for iron in that there is no dependence on the water partial pressure. This FT rate model3,59 was tested on newly measured kinetic data which was carefully designed to ensure a wide range of water partial pressures. The data was also measured by returning the catalyst to reference conditions which ensured that catalyst deactivation was not influenced and therefore all responses could be attributed to the chemical kinetics. 0.5 𝑃𝐻 𝑃𝐶𝑂

𝑟𝐹𝑇 = 𝐴 (1+𝑘 2

2 𝐶𝑂 𝑃𝐶𝑂 )

, with 𝑘𝐶𝑂 = 0.09 bar-1

[1]

The proposed WGS rate model3,60 (Equation 2) was derived assuming a formate intermediate. This specific model provided better predictions at higher pressures in comparison to the other models which were also derived from the formate mechanism. Moreover, this model did not show any systematic errors with the approach to the thermodynamic equilibrium of the WGS reaction which was shown to be the case for models based on first order kinetics in CO.


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𝑟𝑊𝐺𝑆 = 𝐴

𝑃𝐶𝑂 𝑃𝐻2 𝑂 −𝐾

1

𝑊𝐺𝑆

𝑃𝐻2 𝑃𝐶𝑂2

𝑃𝐻 𝑂 2 ) (1+𝑘𝐻2 𝑂 𝑃𝐻2 𝑂 +𝑘𝑂𝐻 0.5 𝑃𝐻 2

2

, with 𝑘𝐻2 𝑂 = 1.1 bar-1 and 𝑘𝑂𝐻 = 6.3 bar-0.5

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[2]

The denominators in the equations for the FT and WGS models differ from which it was concluded that these reactions occur on different catalytic sites. It can also be concluded that the FT sites are mostly covered with CO (and possibly C1 intermediates) whilst the WGS sites are occupied by water and/or hydroxyl species. The rate expressions for Fe-based catalysts developed at lower temperatures have shown to have limitations at higher temperatures. One possible reason could be that the elementary reactions, which are usually ignored at the lower temperatures due to negligible rate coefficients, become significant at higher temperatures. Another reason could be that the kinetic equations were based on a mechanism which fails to describe the full product distribution of Fischer-Tropsch synthesis at higher temperatures. For these reasons, the Steady-State Isotopic Transient Kinetic Analysis (SSITKA) technique was now used to elucidate mechanistic pathways for the iron catalyst at High Temperature Fischer-Tropsch (HTFT) conditions (300-350 °C)5. Previously, this technique has been successfully utilized at Eindhoven University to develop a mechanistic pathway for cobalt Fischer-Tropsch catalysts61,62,63,64. A more recent review65 highlights the power of the SSITKA technique in mechanistic and kinetics studies in heterogeneous catalysis. A mechanistic pathway for methane formation was proposed19 which was based on modelling 13 CH4 transients obtained from 13CO-SSITKA data. It was shown that two distinct carbon pools (C and C) exist which are both active for methane formation. The reactor modelling results also showed that these carbon pools differ in reactivity and both pools also contribute to C-C coupling reactions. This latter conclusion was verified with modelling of the C2 and C3 hydrocarbon transients44. In this paper44, it was concluded the olefins and paraffins grow on different catalytic sites or do not share the same surface intermediate. Moreover, it was shown that the re-adsorbed olefins follow a more direct pathway to the corresponding paraffin and not via the olefin surface intermediate. The insights into the reactivities of the surface carbon intermediates were obtained by combining the 13CO-SSITKA experiments with isothermal and temperature programmed hydrogenations as well as H/D exchange experiments26. Here is was shown that carbon (13C) deposited via the Boudouard reaction is a reactive surface intermediate as it was detected in the C2+ hydrocarbons. H2/D2 isotopic flushing experiments showed the presence of CsCH surface intermediate which validated surface carbon (Cs) as one of the reactive species for iron at HTFT conditions. The CsCH surface intermediate was also shown to be dominant intermediate on the carbided iron catalysts which is regarded as the active phase for iron at HTFT conditions. The transients observed at the start of the Fischer–Tropsch synthesis were distinctively different on a carbided catalyst compared to a freshly reduced catalyst as shown in Figure 1 for the ethene transients. It was also shown26 that on both the fresh and carbided 4 ACS Paragon Plus Environment

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catalysts, carbon deposition still occurs to the same extent but water formation and methane formation are faster on the carbided catalyst.

Figure 1

Normalised transient responses for ethene from the H/D exchange experiments on a fresh and carbided catalyst (adapted from 26)

The main conclusions from this PhD study5 led to the preferred mechanism for the FischerTropsch synthesis on an iron-based catalyst based as illustrated in Figure 2. This mechanism shows the kinetically relevant carbon pathway reactions. To address the oxygen pathways and specifically the formation of oxygenates, follow up research8 was performed at higher pressures and additional C18O-SSITKA experiments were executed.

ka

C2H4

CH4

CO

C2H6

C3H6

C3H8

kd

CsO

Cs

CsH

CsCsH ki

Figure 2

CsH2CH3,ads

CHCsCHads

kp

CsH2CH2CH3,ads kp

Preferred mechanism for the Fischer-Tropsch synthesis using an iron-based catalyst; adapted from5.

Both 13CO and C18O-SSITKA experiments8 were performed to study the oxygen pathways on iron catalysts at HTFT conditions. The CO2 transients, presented in Figure 3, show the transients for 13CO2 eluting faster than C18O2 and moreover whilst 13C fully replaces 12C on 5 ACS Paragon Plus Environment

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the surface, 18O did not replace all the 16O as evident by the presence of C16O18O after all C16O had been replaced in the gas phase. This suggests that lattice oxygen contributes to the formation of CO2 via a type of Mars-van Krevelen (MvK) redox mechanism20. This MvK type mechanism has also recently been proposed by Ozbek and Niemantsverdriet66 explaining the formation of the monomer and C1 products on iron carbide surfaces. The C18O SSITKA experiments in combination with co-fed oxygenates (ethanol, 1-propanol, 2propanol and 2-butanone) proved to be powerful in elucidating the oxygenate mechanisms. Firstly, the equilibrium reactions proposed in literature67,68,69 between the 1-alcohols and aldehydes as well as the ketones and 2-alcohols of the same carbon number were confirmed. Secondly, the isotopic compositions of ethanol and ethanal from Figure 4 as well as a negligible formation of hydrocarbons during oxygenate co-feeding indicated that; i) primary alcohols / aldehydes form from the same alkyl species and ii) interconversion of these oxygenates takes place via MvK type of mechanism comprising a carboxylate intermediate and thus a C-O bond cleavage step.

Figure 3

Normalised transients for CO2 obtained from 13CO and C18O SSITKA experiments.


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Figure 4

Pseudo stable isotopic carbon and oxygen compositions for ethanol and ethanal following SSITKA switches from 12CO/H2 to 13CO/H2 and C16O/H2 to C18O/H2

The fate of atomic oxygen produced by CO dissociation was explored further36. The removal of oxygen from the Fe(100) surface was found to be a difficult process. Activation barriers for both the first and second hydrogen additions are in the order of 1.1 eV, and the overall process is endothermic by ~2 eV. For the second hydrogenation step the reaction between two hydroxyl groups to form water and surface oxygen has a significantly lower barrier than the reaction between surface hydrogen and OH. The reverse reaction, dissociation of water, is very easy, with a barrier of 0.16 eV for the reaction to form OH and H and 0.03 eV in the presence of surface oxygen (forming 2 OH groups). Thus, the barriers for removal of oxygen from the iron surface are significant, but in particular the high energy input of 2 eV required making water from adsorbed oxygen makes the removal of oxygen a difficult, e.g. very slow process. The same authors also studied the methanation reaction34 on Fe(100), starting from atomic carbon. The activation barriers for the stepwise hydrogenation of atomic carbon are modest, in the order of 0.8 eV, and similarly for water formation, the reaction is found to be strongly endothermic by around 1.45 eV. These results are in line with the isotopic tracing work26 which showed that water and methane formation is slower on a fresh catalyst which is typically more metallic in nature. The importance of using zero-point energy corrections was highlighted34, as the reaction energy without ZPE corrections is only 0.77 eV. For the hydrocarbonaceous species that feature in the FTS mechanism, the C1Hx species being the simplest class, the ZPE correction is dominated by the contribution of the C-H stretching vibrations, in the order of 2500-3000 cm-1. The equivalent ZPE correction of ½ hν then becomes 0.2-0.3 eV. Particularly in reactions where additional C-H bonds are formed the ZPE correction becomes significant. The formation of C2 hydrocarbons, starting from adsorbed atomic hydrogen, carbon and hydrogenated C1 adsorbates on Fe(100) was also published39. Here it was shown that there could be three viable reaction pathways towards ethyl, the principal intermediate to produce either ethane or ethylene. These main results highlight the complexity of describing the Fischer-Tropsch mechanism using DFT calculations only. Yang et al.70 combined DFT calculations with transient and


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steady-state kinetic modelling to elucidate the reaction mechanism. This study supported the two carbon pool mechanism for methane formation5,26. The calculations using an open iron surface provided a number of important fundamental insights, as discussed in the previous paragraphs, but they also show that metallic iron is most likely too reactive, e.g. it binds oxygen and carbon to strongly making their subsequent hydrogenation very slow. This result fits in well with the different projects on the industrial iron catalyst3,5,8,26,60 in which it also concluded that the formation of hydrocarbons, oxygenates, CO2 and carbon probably occur on different catalytic sites. Research highlight 2 – Deactivation of iron based FT catalysts As mentioned above, iron-based catalysts are an important part of Sasol’s suite of FischerTropsch technologies41. Although Fe catalysts are cheaper than their cobalt counterparts, catalyst stability in commercial reactors is still an important issue. The ideal catalyst should maintain constant activity and a corresponding stable selectivity during time on stream. Commercial reactors and product workup sections are designed for a very narrow set of optimum process conditions. The deactivation of Fe catalysts seems to be as a result of an interplay of various deactivation mechanisms, including phase changes, poisoning, fouling and sintering41. It may be argued that until recently71, relatively little fundamental information has been gathered about these deactivation mechanisms on iron catalysts compared to their cobalt counterparts and this prompted us to investigate this aspect further through our joint collaboration. The synthesis gas feed streams used commercially for iron catalysts contains impurities that can impact the performance of the catalyst. One such contaminant is sulphur and understanding the impact of sulphur therefore is an important objective. It is generally believed that phase transformation of the FTS active Fe carbides into Fe oxides due to oxidation by product water is responsible for some of the deactivation and increased watergas shift behaviour observed during FTS as depicted in Figure 541. A further topic of both fundamental and industrial interest is the activation of the catalyst, formation of iron carbide during FTS and its oxidation as well as chemical promotion of the Fe catalyst. Despite the industrial relevance of sulphur poisoning on FTS, there was a limited amount of fundamental information regarding sulphur. An earlier study by Kritzinger72 indicated that sulphur could have both positive and negative effects on the commercial catalyst depending on the sulphur concentration in the syngas and it was postulated that one sulphur atom could influence around 10 sites. There was little known about which elementary steps are poisoned by sulphur. Density Functional Theory (DFT) calculations provide insights into elementary steps on metallic surfaces73 and we used this tool to investigate the impact of sulphur on the CO dissociation reaction on metallic Fe surfaces 11. In Fe FTS catalysis, CO dissociation is a key step as it determines the rate of formation of the active carbide phase but influences the rate of the FTS. It was calculated that the activation energy of CO dissociation on Fe(100)-Sp(2 x 2), is very similar to the activation energy of CO dissociation on the sulfur-free Fe(100) 8 ACS Paragon Plus Environment

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surface. However, the sign of the reaction changes by the presence of sulphur. This implies that CO dissociation is highly exothermic on the sulfur-free Fe(100) surface, whereas on the Fe(100)-S-p(2 x 2) surface, it becomes slightly endothermic. It was also found that the influence of sulphur in CO dissociation seems to be short-ranged. It could be concluded that sulphur clearly hinders the CO dissociation. These results indicated that sulphur influences the CO adsorption and dissociation on Fe(100) electronically, by changing the relative energy of the reactants, products, and transition-states levels, as well as sterically, by blocking the diffusion of oxygen on the surface, which is mostly responsible for the large exothermicity of the CO dissociation reaction on the sulfur-free Fe(100). Haematite Fe2O3

Magnetite H2

Fe3O4

Carbide H2 (CO)

FexCy

H2 O

WGS

Fischer-Tropsch

(a)

(b) Figure 5

(a) The formation of magnetite from iron carbide by water formation (b) The effect of this phase transition on FTS and WGS activity as a function of time on stream

In order to study the impact of carburization and oxidation of Fe FTS catalysts, use was made of model catalysts supported on TEM silica wafers that were developed at Eindhoven29. These silica wafers contain a Si(100)/Si3N4 /SiO2-membrane with a 15 nm thickness which allows the passage of the electron beam enabling high resolution TEM imaging. The silica membranes are robust against chemical treatments in diverse gas mixtures and high temperatures. Preformed mono-dispersed iron nanoparticles of different sizes where prepared via a colloidal route and spin coated on the TEM wafers to produce the flat model Fe FTS catalysts25. A clear advantage of these systems is that the model systems could be treated in different atmospheres and the exact area could be imaged before and after treatment to study its effect. In the initial studies, these could be used to observe the carburization of Fe as a function of crystallite size with XPS and TEM25. The changes in morphology and chemistry of the model 9 ACS Paragon Plus Environment

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catalysts after calcination, reduction and synthesis gas exposure were followed. Figure 6 clearly shows the morphology changes that occur to the same set of particles when the Fe model catalyst is exposed to synthesis gas and becomes carburized. In the left, the calcined particles are confirmed to be Fe2O3 by both electron diffraction and XPS25. After reduction the Fe2O3 is converted to Fe3O4 and exposure to synthesis gas at 270°C results in carburization and the particle breaking apart to a large extent. The volumetric changes in the transition from oxide to carbide might cause strain which results in the particle break-up74. These images indicate that a large scale restructuring of the iron catalyst may occur during exposure to synthesis gas.

Figure 6

The morphology of the same set of iron nanoparticles after different treatments. Left: iron oxide particles deposited on a SiO2/Si(100) wafer, middle: after reduction (and passivation by exposure to air), and right: after treatment in synthesis gas at 270°C. The preformed iron oxide particles were spin coated on a planar wafer with an etched window to allow TEM measurements; adapted from25.

In literature it is claimed that chemical and structural promoters can influence the stability of the Fe FTS catalyst75,76. Recently, monodisperse Fe and FeMn flat model systems were prepared using hydrothermal decomposition of Fe and Mn –oleate complexes, spincoated on the TEM silica wafers46. It was shown that after reduction the manganese promotion affects the size and dispersion of Fe oxides and retards their sintering to an extent. Elemental mapping of the FeMn catalyst surface after three different treatments is shown in Figure 7. For clarity, the Fe (red) and Mn (green) distributions are superimposed. In the calcined catalyst Mn is predominantly situated at the catalyst's surface (Figure 7, left). Subsequent reduction in H2 at 400°C for 1h results in a homogeneous mixture of Fe and Mn, whereas the particle size appeared a bit larger (Figure 7, middle). However, the Mn is also at the surface of the particle in this case. After syngas treatment for 1h (Figure 7, right), Mn is still predominantly present at the surface of the particles, however a portion the Mn appears to have segregated out of the particles, forming domains of pure MnO on the outside of the ironrich particles, probably at the interface with the support. It was postulated that such an arrangement would help to stabilise the iron particles against agglomeration or disintegration


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during carburisation as in the case of Figure 6. Therefore these small MnO crystallites stabilize the iron carbide particles under synthesis gas.

Figure 7

TEM elemental mapping images of FeMn model catalysts for Fe (red) and Mn (green) after calcination at 350°C for 30 min (a); H2 reduction for 1 h (b) and subsequent syngas treatment; (H2/CO= 1) at 350°C for 1 h (c); adapted from46

Another attractive aspect of these flat model catalysts is that they contain a sufficient amount of active particles, enabling investigations in situ with for example X-ray absorption spectroscopy (XANES, X-ray absorption near-edge spectroscopy). We used XAS to shed light on the activation of the Fe catalysts and role of moisture in this process which is of particular importance from a commercial perspective35. TEM images of the starting model catalyst containing 16nm iron oxide particles are shown in Figure 8a. Figure 8b shows typical Fe K-edge spectra obtained upon treatment of the iron oxide precursor in dry hydrogen, dry syngas (H2/CO = 2). It was demonstrated that the initial Fe2O3 nanoparticles convert into Fe3O4 at 250−300 °C. These particles convert to metallic iron at 350 °C in dry hydrogen. The reaction proceeds via a wustite (FeO(1−x)) phase. In wet hydrogen (Figure 8c), the magnetite nanoparticles are much more stable and are converted to wustite only (not metallic Fe) at high temperatures. The presence of water (2.5 vol% H2O) inhibits the reduction severely, as expected. In this case, Fe3O4 remains stable up to 450°C and at 550°C the reduction proceeds only to form wustite. Further studies were conducted using activation in synthesis gas to produce iron carbide and the carbide oxidation with water was then followed by in-situ XAS. It was also shown that the formation of the surface Fe(II)O layer may play an important role in regulating the relative rates of FT and WGS reactions, thereby controlling the local concentration of water at the active surface and thus stabilizing the iron carbide against further oxidation. In this section we have shown that Fe catalyst models systems can be fabricated to represent industrial catalysts. These can be treated at relevant temperatures and different gas atmospheres and particle chemistry and morphology can be followed. This has given us insights into morphological changes that occur on the catalyst during carburisation and well as the effects of moisture on the activation procedures. The model systems can also help one 11 ACS Paragon Plus Environment

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to understand the location and impact of chemical and structural promotion in stabilising Fe FTS catalysts.

Figure 8

(a) (b) (c) TEM image of monodisperse 16nm Fe2O3 model catalyst (a). Normalized Fe K-edge spectra of the model catalyst during an in situ treatment with dry hydrogen (b) and wet hydrogen (c); adapted from35.

Research highlight 3 – Carbon as a deactivation mechanism for cobalt FT catalysts The performance of cobalt Fischer-Tropsch synthesis catalysts is critical to the economics of gas-to-liquids (GTL) processes as the catalyst precursors are expensive and the overall catalyst cost contributes significantly to the operating cost of the GTL facility. To support the commercialisation of Sasol’s cobalt FT catalysts, current and future, Sasol identified cobalt catalyst deactivation as an important topic to pursue in-depth understanding of together with Eindhoven University of Technology. An alumina supported cobalt catalyst was tested in a demonstration scale slurry bubble column reactor with a diameter of 0.9 meter using synthesis gas that was generated from natural gas. Clean synthesis gas was used ruling out significant deactivation due to poisons in the feed. The catalyst was tested at commercially relevant FTS conditions (230 oC, 20 bar, H2+CO conversion of 50-70%, feed gas composition of 50-60 vol% H2 and 30-40 vol % CO). Figure 9 shows that the activity of the cobalt catalyst decreases during the first 30-40 days on stream, after which it starts levelling off. Slurry samples were taken directly from the demonstration reactor under a nitrogen blanket and was characterised extensively to study the deactivation behaviour.


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(b)

(a) 1.0 0.8

Normalised Activity

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0.6 0.4 0.2 0.0 0

Figure 9

10

20 30 40 Time on line (days)

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60

(a) Sasol’s demonstration scale integrated catalyst and reactor test facility (Sasolburg, South Africa). (b) Normalized activity for a Co/Pt/Al2O3 catalyst during realistic Fischer-Tropsch synthesis in the demonstration scale slurry bubble column reactor14.

At the start of the collaboration oxidation was proposed as a major deactivation mechanism for Sasol’s Co FTS catalyst. There was no consensus in the open literature on the impact of oxidation on the deactivation of cobalt based FTS catalysts15. However, jointly with Eindhoven University, Sasol concluded using a host of characterisation techniques which included Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Absorption Near Edge Spectroscopy (XANES), Magnetic measurements, X-ray Diffraction (XRD) and Computational Chemistry, that oxidation of cobalt by the by-product water could be prevented by tuning the Co crystallite size and the PH2/PH2O ratio during FTS and is not at play for Sasol’s Co FTS catalyst1,12,13,14,15,27. This finding helped reconcile some of the apparent differences in the open literature on the role of oxidation during FTS and was supported by subsequent publications77,78,79 ,80. Thereafter carbon and sintering were proposed as key deactivation mechanisms that were studied in detail further. Deactivation of cobalt based FTS catalysts by carbon is widely postulated in the open literature81,82,83,84,85. This mechanism, out of all the proposed deactivation mechanisms of Co FTS catalysts, is probably the most challenging to prove due to the heavy wax product formed during realistic FTS conditions together with the build-up of inert carbon on the support. The Sasol-TU/e collaboration set out to prove that carbon is a primary deactivation mechanism during FTS as well as to provide mechanistic insights to explain carbon deposition. Temperature programmed techniques are the most commonly employed technique to study carbon deposition and is often easier to perform at industrial research labs. The wax from the spent slurry samples were extracted using a suitable solvent. Thereafter temperature programmed hydrogenation was used to react of the different types of carbon 13 ACS Paragon Plus Environment

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species on the spent catalyst2,17,22. TPH of the used catalysts showed the presence of three broad types of carbonaceous species (Figure 10). The least reactive species toward hydrogen (peak 3, ~430oC) was identified as polymeric carbon based on previous literature82 and measurements of reference carburized compounds as was found to increase with time on line (Figure 11). In order to conclude that this polymeric carbon is deleterious it was crucial to identify the location of the carbon with respect to the catalyst. Using a combination of hydrogen chemisorption, carbon and cobalt mapping with energy-filtered transmission electron microscopy (EFTEM), and low-energy ion scattering (LEIS), it was shown that the polymeric carbon was located on both cobalt and the alumina support. Therefore, polymeric carbon was postulated as one of the causes of activity decline in the extended FTS run.

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Deconvolution of the methane profiled obtained from TPH of a wax-extracted Co/Pt/ Al2O3 catalyst from the extended FTS run in the slurry bubble column. Peak 3 was identified as a polymeric type of carbon22. 1.8

hydrogen resistant carbon (wt%)

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Residual polymeric carbon obtained from TPO experiments following TPH which represents carbon resistant to hydrogen at 350 °C22. 14 ACS Paragon Plus Environment

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It has been reported in literature that oxygenates produced during FTS could serve as precursors for polymeric carbon formation on the catalyst8687. A detailed study was undertaken to understand and verify the role of carboxylic acids during FTS on polymeric carbon formation. IR measurements on spent Co FTS catalyst from the long demonstration run (Figure 9) confirmed the presence of carboxylic acids on the catalyst surface47 From model experiments on the industrial catalyst and support it was deducted that the carboxylic acids were located on the alumina support and not the cobalt metal. The presence of the cobalt metal was found to decompose the acetic acid resulting in atomic carbon. This was supported by single crystal experiments on cobalt where it was shown that acetic acid was thermally unstable at FTS temperatures. Density functional theory (DFT) calculations confirmed the strong adsorption of acetic acid on both the γ-Al2O3 (110) and (100) surfaces. To verify the impact of the carbon produced from carboxylic acids on catalyst deactivation, acetic acid was co-fed with the feed gas at levels of 20-200 ppm into a continuous stirredtank reactor operated at 230°C, ~20 bar, (H2+CO) conversion between 50-70% and feed gas composition of ~50 vol% H2, 30 vol% CO and 20% inert gas (Ar). No significant impact of the co-fed acetic acid on catalyst deactivation and methane selectivity was observed over a 24 day period. Therefore, under typical FTS condition carboxylic acids can be considered to have a negligible impact on polymeric carbon formation and catalyst deactivation. To further understand basic aspects of the role of carbon in catalyst deactivation a molecular modelling and surface science approach was taken, where a close-packed Co surface served as a model catalyst. During Fischer-Tropsch synthesis carbon is ultimately derived from the CO molecule, but as the close-packed surface is not active for CO dissociation ethylene was used as a carbon source instead. A combination of experimental techniques, in particular thermal desorption spectroscopy, scanning tunnelling microscopy and high resolution core level spectroscopy were employed to determine the nature, concentration as well as the morphology of surface carbon37. Table 1 lists how different forms of surface carbon can be prepared. Table 1:

Overview of experimental procedures to produce different types of surface carbon on Co(0001) using ethylene as a carbon source. Adapted from37. Experimental Carbon atom coverage Morphology/structure procedure and chemical nature C2H4 saturation dose at 0.2 ML, atomic C (√3x√3)R30o islands + clean surface + 100 K, heat 630 K disordered atomic C Cycles of C2H4 ↑ with number of Two cycles: 0.5 ML atomic C (clock), saturation dose at 100 cycles further cycles: (√3x√3)R30o islands + K, heat to 630 K graphene C2H4 saturation dose at 0.4 ML, graphene 20% graphene, 80% clean surface 360 K, heat to 630 K Cycles of C2H4 ↑ with number of Graphene islands increase in size, clean saturation dose at 360 cycles surface, (√3x√3)R30o islands K, heat to 630 K C2H4 saturation dose at 0.5 ML, atomic C Reconstructed (clock) 630 K 15 ACS Paragon Plus Environment

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Having established the experimental approach to prepare a pure, atomic carbon saturated surface this information was used to systematically explore how the presence of a welldefined carbon layer influences adsorption of CO and hydrogen the FTS reactants. Using thermal desorption spectroscopy it was found that a saturated layer of atomic carbon, equivalent to 0.5 ML, significantly affects the adsorption of carbon monoxide and hydrogen. A ~50 K downward shift of the desorption temperature was observed for both adsorbates, indicative of a weakening of the adsorption due to the presence of surface carbon. In addition, the adsorption capacity of both adsorbates is suppressed by ~40%. Graphene, on the other hand, completely covers the cobalt surface, leaving no space for either CO or H2 to adsorb. Translated to catalysis this means that in particular aromatic forms of carbon are expected to be a catalyst poison, and, as adsorption of hydrogen is suppressed, it is probably also difficult to remove in-situ once formed on the catalyst surface.

Figure 12 Carbon adsorption on the unreconstructed (111) surface (the unit cell was doubled to put together the missing rows) (left side); on the (111) to (100) surface reconstruction (middle); clock reconstruction of the (111) surface (right side).16 In the often cited STM work by Wilson and de Groot88 it was reported that exposure of the Co(0001) to realistic conditions in terms of pressure and temperature leads to significant rearrangement of the surface structure. DFT calculations16 (figure 12) indicate that carbon can induce a surface reconstruction of the hexagonal cobalt surface, known as the ‘clock’ reconstruction89’90. STM images of the cobalt surface after depositing 0.5 ML atomic carbon, figure 13(a) and (b), confirm that the Co(0001) surface indeed reconstructs as predicted by theory. Although this reconstruction corresponds to a lowering of the cobalt surface atom density of around 10% the excess atoms do not appear to form ad-islands such as those reported by Wilson and de Groot88, and instead are most likely accommodated at existing step edges.


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Figure 13: STM images of different carbon-induced surface structures. (a) large scale image showing five of the six possible domains of the carbon-induced ‘clock’ reconstruction on Co(0001), and (b) an atomic scale image, showing the square arrangement of the surface carbon atoms in the reconstructed surface. (c) Graphene island (light grey) where the bright protrusions are cobalt atoms or clusters encapsulated by the graphene island. Adapted from reference37. Instead, graphene was found to affect the cobalt surface morphology, by ‘capturing’ mobile cobalt ad-atoms during the graphene growth phase. A typical graphene island, such as shown in figure 13(c), shows very specific defects which appear as protrusions with a height of 0.2 nm with respect to the underlying cobalt surface. These are assigned to encapsulated cobalt atoms which originate from step edges, which, at the graphene growth temperature, constantly emit and adsorb single atoms. These mobile ad-atoms are then trapped at the growing edges of graphene, after which graphene continues to grow and encapsulate the cobalt atom. Thus, graphitic forms of carbon not only block the adsorption of CO and hydrogen, FTS reactants, but also stabilize surface roughness. The experiments also provide information about the formation mechanism of graphitic forms of carbon on cobalt, the type of carbon that has the strongest effect on adsorption of the FTS reactants. Coupling of atomic carbon to form polymeric or graphitic types of carbon was for example found to be difficult. Even at 630 K, annealing of a surface covered with 0.5 ML atomic carbon did not produce polymeric forms of carbon. Instead, when a high concentration of acetylene is present on the surface polymeric forms of carbon, in particular graphene, could be formed on the surface already at low temperature91. Translated to FischerTropsch synthesis conditions, this indicates that coupling of unsaturated hydrogen-lean FTS chain growth intermediates are the most likely source of polymeric, deactivating forms of surface carbon during FTS, rather than coupling of surface carbon atoms. In particular acetylenic intermediates, which were identified as the most stable CxHy intermediates on Co(0001), readily react, either via cyclo-polymerization92,93,94 to form aromatic carbon or hydrogen-lean oligomers.


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Research highlight 4 – Sintering as a deactivation mechanism for Co FT catalysts Besides the above discussed carbon formation, sintering of small cobalt crystallites could also be an important deactivation mechanism for cobalt based Fischer-Tropsch synthesis. Studying cobalt sintering in alumina supported catalysts is not straightforward, due to the difficulty of distinguishing cobalt and alumina in the bright field mode, as a result of the low mass contrast and the high porosity of the support. Applying High Angle Annual Dark Field (HAADF) electron microscopy on thin (50-80 micron) ultra-microtomed samples is a more successful approach. Cobalt on alumina catalyst samples were taken from a 100 bbl/d Fischer-Tropsch demonstration reactor that was operated at industrially relevant FTS conditions (Figure 9). Comparing the HAADF TEM image of a fresh cobalt on alumina catalyst with a spent catalyst after 20 days of Fischer-Tropsch Synthesis, shows an increase in the cobalt crystallite size and a partial disappearance of the grape-like features (Figure 1495). A cobalt crystallite size distribution can be calculated from a combination of multiple images, and this was used to calculate an average cobalt crystallite size as a function of time on time during FTS (Figure 15). It can clearly be seen that the crystallite size rapidly increases during the first few days of FTS where after it levels off. The area weighted average cobalt crystallite size, as determined by TEM, increases from about 9 nm for the fresh catalyst to about 15 nm of the spent catalyst.

Figure 14 HAADF TEM images of a freshly reduced cobalt on alumina catalyst (left) and of a spent catalyst after 20 days of FTS in a demonstration reactor (right)95. It is thus clear that sintering of cobalt crystallites in an alumina supported catalyst does occur. Understanding the mechanism for sintering in this cobalt catalyst would possibly assist in providing direction for future improved catalysts. This knowledge however, cannot be deduced from the obtained results. It was therefore decided to apply the flat model catalyst approach to this scientific problem, which would simplify the research considerably. In this flat model catalyst study7,45 the sintering of planar Co/SiO2/Si(100) catalysts was studied by means of electron microscopy (TEM) and X-ray spectroscopy (XPS) before and after exposing the flat model catalyst to Fischer-Tropsch conditions. The planar model 18 ACS Paragon Plus Environment

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catalyst was prepared by means of spin coating, using a custom-designed model support containing SiO2/Si3N4 membrane that can be used for electron microscopy29. The advantage of this model support is that you can perform TEM measurements, expose the sample to gas and heat treatment ex-situ, and subsequently return in the microscope to the original position on the model catalyst to study the exact same cobalt crystallites. 20

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18 16 14 12 10 8 6 4 0

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Figure 15 Average surface area weighted cobalt crystallite size as function of average cobalt on alumina catalyst age, as determined by means of TEM/HAADF. Firstly, XPS was used to determine whether cobalt sintering occurred during exposure to hydrogen or Fischer-Tropsch synthesis, using the flat model catalysts. The Co/Si peak intensity ratio was used as a qualitative measure for cobalt dispersion (figure 16). It can clearly be observed that exposure to hydrogen did not change this Co/Si ratio and thus did not cause any sintering. Exposing these model catalysts to Fischer-Tropsch synthesis (at 20 bar, 230 °C and H2/CO of 2.0), however, clearly showed a decrease in the Co/Si ratio, and sintering of the cobalt particles did occur. The presence of carbon covering part of the cobalt was excluded, as the C/Si ratio did not change. Any carbon containing compounds on the cobalt or support after FT, were probably removed during the shutdown procedure of the reaction (i.e. CO was switch off and the sample was exposed to pure hydrogen at 230 °C). From the XPS analyses sintering of cobalt during FT could thus be concluded.


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Figure 16 Comparison of Co/Si XPS peak intensity ratio on planar cobalt on silica catalysts before and after exposure to hydrogen and FTS. 45 TEM images on these planar model catalysts also did not reveal any influence of the reduction step on the cobalt crystallite sizes and its distribution. However, the application of FTS conditions clearly changed the catalyst sample. Some cobalt particles of about 4 nm (corrected metal size) disappeared, while slightly larger ones decreased in size (figure 17). TEM measurements thus confirmed the notion from the XPS work that the cobalt crystallites sintered during FTS. From this TEM work it was furthermore concluded that the particle migration and coalescence mechanism was not at play, as random cobalt particle movement could not be observed. Cobalt particles seem to be fixed on the spot and either (i) remained where they were or (ii) decreased in size and disappeared. This observation can only be explained by the fact that Ostwald ripening is the main sintering mechanism. This conclusion seems to be in contradiction to some recent literature96,97, however they inferred the particle migration and coalescence sintering mechanism from changes in cobalt crystallite size distributions and not from direct observations. The difference in behaviour during reduction and Fischer-Tropsch indicated that carbon monoxide has to be important in the sintering mechanism of silica supported cobalt catalysts. Cobalt subcarbonyls, Co(CO)x, could thus play an important role, which was also proposed by Wilson and de Groot88. The impact of water (as a FTS product) is not taken into account in these flat model catalyst experiments as the water partial pressure was extremely low. It was however recently published for industrially relevant alumina supported cobalt catalysts that sintering by means of Ostwald ripening, catalysed by cobalt subcarbonyls, is not only governed by the presence of carbon monoxide in the gas environment, but is further enhanced by the combination of a high carbon monoxide partial pressure with a high water partial pressure98. The importance of water partial pressures with respect to sintering was also reported for carbon99 and silica88 supported catalysts. Whether the influence of high water partial pressures would change the sintering mechanism might need further investigation.


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Figure 17 Bright field TEM images of planar cobalt model catalysts (a) reduced-passivated and (b) after exposure to FTS conditions (20 bar, H2/CO=2, 230 °C, low conversion) followed by passivation. Red dashed circles indicate particles that have been lost during FTS, and blue dashed circles show particles that have decreased in size. The sample measurement area is 180 x 180 nm.45 Using the data from the TEM experiments above, we also investigated whether only the cobalt crystallite size was important during sintering or also the distance between cobalt particles (figure 18). In figure 18, a combination was plotted of the distance between cobalt crystallites, the size of particles, and the fact whether the particles disappeared or decreased in size. It can clearly be seen that the determining factor for sintering was the cobalt crystallite sizes and not the distance between them. Furthermore, in this same figure, we have plotted the cobalt crystallite size distribution of the above-studied alumina supported cobalt based catalyst, as derived from the TEM HAADF images, showing that a portion of the cobalt particles would be susceptible to sintering. The planar model catalyst work has thus nicely shown that Ostwald ripening is the sintering mechanism that is at play and that the most important factor in sintering is the cobalt crystallite size. When designing future catalysts it would thus be important to eliminate cobalt crystallites below 8 nm, as these crystallites might sinter most. Although for most heterogeneous catalysts smaller crystallites will result in a higher active metal surface area, for cobalt FTS catalysts it has been reported that activity and selectivity is negatively affected when decreasing the cobalt crystallite size too much41. This current research on industrial and planar model cobalt based FTS catalysts has now also shown that very small crystallites are not desirable from a sintering point of view. Furthermore, this research has shown that combining industrial observations with surface science research can pave the way for optimised catalysts.


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Figure 18 Plot of the (i) affected cobalt crystallite sizes in the fresh planar model catalyst and the distance of these particles to their nearest non-disappearing neighbour as well as (ii) cobalt crystallite distribution of a fresh industrial alumina supported cobalt catalyst, fitted to a lognormal curve (courtesy of P. van Helden and D. Kistamurthy). Future Perspective Although at the time of writing low oil prices seemingly discourage new developments, we nevertheless anticipate that in the long term synthesis gas and Fischer-Tropsch based technology will continue to feature as selective options in the gas monetization and ultimately the global energy landscape. Currently available renewable energy sources primarily produce electrical energy, and the long term goal to substitute fossil fuels by renewables, also requires efficient use of electricity for the production of chemicals, the so called “electrification of the chemical industry”. Synthesis gas, CO + H2, which can be produced from water and CO2 using electrical energy, is a key intermediate for reactions that produce energy-rich chemicals such as synthetic natural gas (SNG) and liquid fuels (via Fischer-Tropsch synthesis), see Figure 19. This represents an attractive route for efficient large scale and/or long term energy storage, due to the unsurpassed energy content of hydrocarbon fuels per unit of volume, as well as their ease of storage and transport. This technology also has a prominent place in the solar refinery, where solar energy is used as the principal energy source that drives the production of chemical products, replacing fossil sources100.


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Figure 19 Synthesis gas and Fischer-Tropsch synthesis in CTL, GTL, BTL and PTL (coal, gas, biomass and power to liquids, respectively); The combination of electrolysis based on sustainable electricity and syngas technology is expected to play an increasingly important role in both the storage of electricity and the greening of coal to liquids processes in the future. The first step in “Power-to-Liquids” (PtL) is activation of CO2 and H2O to produce hydrogen and CO, which can then be used as input for a Fischer-Tropsch synthesis process to produce liquid fuels and/or chemical feedstock. For this application, other requirements apply than in the large scale CTL and GTL processes. The intermittency of power availability calls for a more flexible and smaller scale of operation, and ideally FTS selectivity patterns without excessive gas and wax production, to avoid extensive post processing. Efficiency improvements in power-to-liquids technology will immediately find application in current energy technology. For example, coal is the primary energy source in China, South Africa and Australia, and will inevitably be used to fulfil the worlds increasing energy demand in other countries as well. Responsible use of coal entails minimizing emissions of hazardous compounds by concentrating coal use at large facilities with sophisticated exhaust gas cleaning systems as well as making use of the concentrated CO2 streams to do chemistry. The coal-to-liquids process is an example of a large-scale facility using coal. The CO2 emissions in this process are inherently high (carbon efficiencies ≤35% are the rule rather than the exception). By supplying green hydrogen or synthesis gas to the process the carbon efficiency can be improved significantly, a useful concept for industrial parties that operate coal-to-liquids processes today. Note that the application of water electrolysis in the context of CTL technology also implies that oxygen can be used in the gasification of coal, which is beneficial as oxygen is in fact a valuable and expensive commodity. Countries with abundant gas resources but relatively undeveloped distribution networks and/or associated industrial infrastructure might still opt to monetize these gas resources via “GTL” processing. The synthetic fuel products are easier to distribute via conventional transport means and such facilities are considered critical seeding mechanisms for sustainable industrial nodes. Countries that come to mind are Uzbekistan, Turkmenistan and the like. Although the recently discovered natural gas fields in East Africa (also an area with relatively undeveloped gas processing and distribution infrastructure) fit some of the criteria above, the 23 ACS Paragon Plus Environment

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gas monetization options are more varied and complex due to regional and geographical influences. LNG, Gas to Power, GTL and CNG will compete on economic, strategic and geopolitical bases. The extraction of FT derived chemical feedstock over and above or as replacement for synthetic fuels has received focussed attention in the past and will continue to do so. Value added products from FT, e.g. olefins, selected oxygenates, speciality wax products and synthetic lubricants all serve to enhance the value proposition of Fischer-Tropsch technology. These developments are typically enabled by new generations of selective catalysts and add on technologies that in themselves rely on catalyst and product work up sections that can effectively convert syncrude into fit for purpose chemicals. The playing field for the development of new generations of FT catalysts are increasingly being defined by progress in the molecular and atomic characterization of materials, especially under in situ or operando conditions. Fundamental understanding of the behaviour of matter under transient conditions and the appreciation of nanostructure selectivity relationships are gradually finding application in the design, production and regeneration of FT catalysts. Fischer-Tropsch synthesis ranks among the most complex reaction mechanisms in catalysis, and it will continue to pose many scientific challenges, even on relatively simple cobalt catalysts, which for example do not contain chemical promotors. At the molecular level, the interplay between the various elementary reaction steps that take place on the surface of the catalyst, e.g. monomer formation, coupling and chain termination, form the basis of the activity and selectivity. Owing to the multitude of surface reactions taking place simultaneously and the difficulty to measure reaction intermediates in-situ under operating conditions the exact molecular details of FTS are still to be resolved. For the relatively simple cases of Co and Ru which are active in their metallic form, different views have been proposed regarding the locus of CO dissociation as well as regarding the surface intermediates responsible for chain growth of the hydrocarbon product chain. Bottom-up approaches using model systems, in particular using a theoretical approach101,102,103 have been used to study individual elementary reaction steps in isolation in order to build up the understanding of the reaction network. Alternatively, co-feeding studies have been used, where isotopically labelled species are used to elucidate mechanistic details104. Experimental studies using single crystal surfaces, which have significantly added to fundamental understanding of reactions such as automotive catalysis105 and ammonia synthesis106 have only been employed to a limited extent in relation to the Fischer-Tropsch synthesis reaction107. Here are definitely opportunities for future study. Available studies on well-defined surfaces have shed light on the structure dependence of direct CO dissociation108,109,110 a step related to catalyst activity. Information on hydrocarbon chemistry, i.e. chain growth and chain termination steps that speak primarily to selectivity, is on the other hand typically only available for C1 and C2 intermediates, products that show atypical selectivity behaviour in applied FTS. In addition, experimental literature covers only a small selection of Ni and Ru surface geometries, very little information is available for cobalt 24 ACS Paragon Plus Environment

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surfaces111,112,113,114,115,116, while experimental studies on iron carbide systems are completely absent. However, the study of hydrocarbon reactions on surfaces has recently demonstrated value for understanding FTS catalysis. Experiments backed up with theoretical calculations indicate that alkylidyne chain growth mechanism for cobalt-based FTS catalysts, shown in figure 20, could very well be a dominant pathway43. Figure 20 Alkylidyne chain growth mechanism proposed for FTS synthesis on cobalt catalysts, derived from experimental studies in combination with theory calculations. (adapted from ref.43)

Recent experimental studies that try to bridge the pressure and/or materials gap, such as single crystal studies under high pressure conditions117,118,119 or supported nanoparticle systems120,121 show the complexity of the surface under reaction conditions in terms of a high surface coverage of mobile adsorbates, in particular CO122, on a dynamic surface that exposes different types of under-coordinated sites. Such in-situ studies reveal the complexity of surface structure and dynamics on the surface but are already too complex to extract relevant information on the elementary steps. The essential questions to understand selectivity in FTS catalysis are which elementary reaction steps in the overall reaction mechanism, occurring in the presence of a high concentration of, in particular, adsorbed CO, control selectivity and which surface structure is optimal for a highly selective catalyst, in other words: “what happens where?”. A number of studies show that CO, even when present in at a ppm level, has a large impact on hydrogenation reactions. Its presence in large concentrations as a spectator species during reaction is expected to have a significant effect on FT synthesis as well. This typical characteristic of FT, i.e. the reaction takes place on a highly covered surface has received little attention in fundamental studies to date. Experimental efforts should therefore in particular focus on studying elementary reaction steps of hydrocarbon species (i) as a function of surface structure, and in particular, (ii) on the role of CO as a spectator species. While olefins and paraffins constitute by far the majority of the products, small amounts of oxygenates, branched products and even aromatics form part of the product slate as well. Mechanistic research focuses on the straight-chain hydrocarbons, but an understanding how the minority products arise would also be desirable. For example, the notion that alkynes play a role in chain growth immediately suggests a route to aromatics, as for example the trimerization of acetylene to benzene is a known reaction, and coupling of two acetylenes and one propylene might be considered as the route to toluene, which could then grow in a similar fashion as the methylidyne species in Figure 20. This would be an interesting direction for research and could eventually be a stimulus for further development of chemicals from FTS.


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Finally, all Fischer-Tropsch catalysts are prone to deactivation. While trivial causes such as poisoning can be avoided, finding ways to stabilize the supported catalyst particles is a difficult challenge, particularly since Ostwald Ripening appears as the main underlying cause45, with a possible role for inactive carbon formation as well. As the process is governed by the composition of the gas phase needed for Fischer-Tropsch synthesis – notably CO - on the one hand and the surface energetics of the metal particles on the other, it seems that Ostwald-ripening will be virtually impossible to avoid when using cobalt catalysts. Hence, efficient strategies for regeneration will be very relevant. For iron based catalysts, the reasons behind deactivation have not been fully identified, and this would also be an important subject for further study. When studying deactivation, it is paramount to include in-situ studies to ensure that the correct phenomena are addressed. For example, in situ XRD and insitu magnetic measurements are highly suitable98,123,124. We firmly believe that Fischer-Tropsch synthesis will be a subject of long lasting interest, both for scientific research and for practical application. Concluding Remarks During the course of this paper we have presented various research highlights to showcase the highly productive and successful long-term collaboration between Sasol Technology and Eindhoven University of Technology. It can be seen that collaborative research effort between industry and academia if harnessed in the correct manner can yield answers to scientific and technological questions which can be of benefit to both parties. The primary objective of the collaboration was to obtain fundamental information that could assist in understanding practical issues in Fischer-Tropsch synthesis involving iron and cobalt catalysts. For iron based catalysts industrial slurry reactor work was combined with SSITKA and DFT modelling, resulting in improved clarity with respect to the kinetics and mechanisms of iron based Fischer-Tropsch synthesis. This knowledge is important with respect to designing large scale industrial processes. In addition, both in-situ and ex-situ experiments with novel flat model iron based catalyst systems were successfully performed improving the knowledge on catalyst phase changes during catalyst activation, FischerTropsch synthesis, as well as the impact of promoters. In the case of cobalt based FT research, the combination of commercially relevant supported cobalt catalyst samples that were tested in large scale reactors under realistic FT conditions, with sophisticated characterisation tools as well as the application of flat model catalyst systems has led to significantly improved knowledge on deactivation mechanisms. It was shown that oxidation is not relevant as a deactivation mechanism for catalysts tested under industrially relevant conditions, while carbon formation and sintering are very important for the long-term stability of cobalt based FT catalysts. This improved knowledge has assisted in the understanding of new catalysts systems and regeneration processes.


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In this particular collaboration, the impact of this industry-academic collaboration has gone beyond the primary objectives and the collaboration was promoted further with Sasol resulting in other research groups in the non- Fischer-Tropsch related fields also becoming involved. The collaboration went even further than affecting the two research parties, and has had a substantial spill over and impact on the academic community in South Africa. During the period 2000-2010 Sasol used the mechanism of an independent industrial advisory board to assist Sasol Technology in identifying and formulating strategic catalysis related research themes, assessing and improving in-house technical competencies and facilitating specialist training opportunities globally. Professor Rutger van Santen (Eindhoven University of Technology), one of the advisory board members, soon shared his experiences of the Dutch University-Industry partnership model and the ambit of the advisory board was extended to also include Sasol Technology’s partnerships with research teams at selected South African Universities. These efforts were very successful and spawned several lasting international collaborations, notably also outside of the Sasol specific research areas. Funded by the South African Department of Science and Technology (DST) and administered by the National Research Foundation (NRF), the Centres of Excellence (established in 2004) are research hubs that have been set up around South Africa to boost research excellence and capacity development. The centres seek to enable researchers to collaborate across disciplines and between institutions on long term projects that are locally relevant and contribute to international competitiveness. In catalysis, the c*change network, coordinated by Professors Fletcher and Claeys from the University of Cape Town, but including catalysis groups from all South African universities, has fulfilled this role in a particularly successful way. Professor Hans Niemantsverdriet (Eindhoven University of Technology) was nominated to the C*change governing board in 2010 and performs a valuable role in benchmarking the activities and deliverables of the centre with best practices in The Netherlands and elsewhere. This international link has facilitated the participation of the centre’s academic personnel as guest lecturers in several catalysis related global summer and autumn school courses and international catalysis conferences. Sasol Group Technology has funded a corporate program aimed at supporting local universities in selected science and engineering disciplines since 2005. The Physical Chemistry department of the University of the Free State (UFS) under the leadership of Professor Swarts was offered the opportunity to collaborate with Prof Niemantsverdriet of the Laboratory for Physical Chemistry of Surfaces, Eindhoven University of Technology with a view of transferring some of their skills and methodologies used in the preparation and characterization of flat model catalyst systems to South Africa. Four UFS staff members stayed at Eindhoven in about ten visits between 2010 and 2015, which resulted in at least six publications so far51,52,53,54,55,125. This facilitated collaboration blossomed and expanded into research themes beyond the initial expectation. Finally, the success of the collaboration has been due to a number of factors. It has been beneficial to both parties to have had a long term collaboration, in which important 27 ACS Paragon Plus Environment

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fundamental catalysis topics were investigated that often took a substantial period of time. These achievements would not have been possible with for example a single shorter term Ph.D. study. During this prolonged period the communication between the partners remained strong and despite the geographical separation regular project meetings were held on a monthly basis and face to-face workshops were conducted every 18 months, which helped to steer the research program and keep the focus on the right goals. In addition, Sasolemployees where seconded to Eindhoven for various periods during the collaboration. A certain degree of flexibility is also allowed enabling the academic partner to be innovative especially regarding the tools that could be used to tackle the scientific questions. The strong relationship and network building between the industrial and academic partners has led to cross-pollination within the academic scientific environment and has benefitted the wider South African catalysis community. It is clear that both partners brought their own strengths into the collaboration and in the end this proved to be a winning combination. The access to high quality modelling and characterisation tools and fundamental understanding in combination with access to industrially relevant supported catalysts operated at realistic conditions has proved vital in our collaborative contribution towards the advancement of Fischer-Tropsch Science and Technology. References 1

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