Studying Fischer–Tropsch catalysts using transmission electron microscopy and model systems of nanoparticles on planar supports

Abstract

Nanoparticle model systems on planar supports form a versatile platform for studying morphological and compositional changes of catalysts due to exposure to realistic reaction conditions. We review examples from our work on iron and cobalt catalysts, which can undergo significant rearrangement in the reactive environment of the Fischer–Tropsch synthesis. The use of specially designed, silicon based supports with thin film SiO₂ enables the application of transmission electron microscopy, which has furnished important insight into e.g. the mechanisms of catalyst regeneration.

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P. C. Thüne

Peter Thüne is an assistant professor at the Eindhoven University of Technology. He studied chemistry at the Rheinische Friedich Wilhelms Universität, in Bonn, Germany, and did his PhD in polymerization model catalysts at Eindhoven. His research interests are model catalysts, electron microscopy, X-ray absorption and photoemission, which he preferably applies in the field of polymerization catalysis and nanotube synthesis.

C. J. Weststrate

Kees-Jan Weststrate is a post-doctoral researcher in the group of Prof. Dr J.W. Niemantsverdriet. He has a background in surface science, doing his PhD in Leiden (The Netherlands) and a post-doctoral project at the synchrotron in Lund (Sweden). He works on Fischer–Tropsch-related topics, in collaboration with Sasol Technology (South Africa). In his research he uses both single crystal surfaces and flat model systems to answer fundamental questions related to the Fischer–Tropsch synthesis.

P. Moodley

Prabashini Moodley is a scientist in the Fischer–Tropsch Catalysis and Engineering Research department at Sasol Technology R&D. She studied chemistry at the University of Durban Westville and obtained the MSc degree in 2002. She joined Sasol in 2003, and was seconded to the Eindhoven University of Technology to pursue a PhD degree, which she obtained in 2010 under the supervision of Prof. Hans Niemantsverdriet.

A. M. Saib

Abdool Saib is a Principal Scientist in the Fischer–Tropsch Catalysis and Engineering Research department at Sasol Technology R&D. He completed his honours in chemistry in 1998 at the University of Kwa-Zulu Natal. He then went onto complete his Masters in applied science at the University of Cape Town. He joined Sasol in 2000 and worked on the commercialisation of Sasol's proprietary Co Fischer–Tropsch synthesis catalyst. In 2001 he was seconded to Eindhoven University of Technology where he completed his PhD under the supervision of Prof. Hans Niemantsverdriet. His main areas of interest are cobalt catalyst deactivation and regeneration.

J. van de Loosdrecht

Jan van de Loosdrecht is a Fischer–Tropsch catalysis expert at Sasol Technology, South Africa. He obtained his PhD at Utrecht University, The Netherlands, in 1995 and joined Sasol shortly afterwards. He is co-author of over 25 articles and co-inventor of over 15 patents. He was closely involved in the design, scale-up and commercialization of Sasol's cobalt based Fischer–Tropsch catalyst for the Oryx GTL plant in Qatar. Since 2010 he has been also an industrial fellow at Eindhoven University of Technology.

J. T. Miller

Jeff Miller joined the heterogeneous catalysis group in the Chemical Sciences and Engineering Division at Argonne National Laboratory (ANL) in 2008 after retiring from the BP refining and petrochemicals research department. He received his PhD in chemistry from Oregon State University in 1980. His research interests include catalyst synthesis and kinetics of metallic and alloy nano-particles. His research team also used X-ray absorption techniques at the Advanced Photon Source at ANL to determine the electronic and structural properties of these catalysts under realistic conditions.

J. W. Niemantsverdriet

Hans Niemantsverdriet is Professor Physical Chemistry of Surfaces in the Department of Chemical Engineering and Chemistry at the Eindhoven University of Technology. He studied physics at the Free University in Amsterdam, and obtained his PhD at Delft in 1983. After postdocs in München and Berlin he became an associate professor at the Eindhoven in 1989, and a full professor in 1999. He is the author of over 200 articles and three books, and has been an editor of the Journal of Catalysis since 1996. His main research interests are surface science and model catalysts, in areas such as the Fischer–Tropsch synthesis.

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Introduction

Heterogeneous catalysts are usually nano-sized particles on high surface area supports. Characterizing such particles as to structure, composition and morphology in the smallest possible detail that is relevant, i.e. the atomic level, and preferably under the conditions that they have to do their work, is the ultimate aim of catalyst characterization. Often this requires that one studies idealized ‘look alikes’, i.e. model systems, as the real catalysts are often too complex to obtain meaningful results.

Over the past 15 years we have developed planar surface science models of supported catalysts to the stage that preparation, characterization and testing are now possible for a number of catalytic reactions.¹ Several groups are active in the field of planar models of supported catalysts, such as those of Freund,^2,3 Goodman,⁴ Kasemo,⁵ Henry,⁶ Somorjai,⁷ and Besenbacher,⁸ and their coworkers. The unique approach of our group in this area is that we concentrate on realistic models prepared via wet impregnation, and on catalytic reactions beyond those that can be done in typical surface science experiments (notably oxidation of CO). Olefin polymerization,^9,10 selective hydrogenation,¹¹ hydrodesulfurization,^12–14carbon nanotube growth,¹⁵ and Fischer–Tropsch synthesis^16–18 form examples from our recent work.

Model catalysts, preparation, characterization and testing

Silicon wafers provide a convenient platform for model catalysts, as these are perfectly single crystalline, flat and inexpensive. Simple recipes exist to make SiO₂ layers of the desired thickness with fully hydroxylated surfaces, so that these SiO₂/Si(100) systems are realistic models for a silica support.¹ Moreover, it is relatively straightforward to apply titania or alumina layers on top e.g. by wet chemical reactions using titanium alkoxide, or by evaporating or sputtering aluminium oxide onto the surface. These thin oxide films on a semi-conducting substrate possess sufficient conductivity to prevent charging in characterization techniques based on electrons or ions, such as X-ray photoelectron spectroscopy (XPS) or secondary ion mass spectrometry (SIMS).¹

Deposition of the active phase on the model support is possible via a range of techniques, among which wet chemical impregnation by spin-coating mimics industrial practices best.¹⁹ However, chemical vapour/atomic layer deposition, grafting,^20,21 electrostatic adsorption,²² and deposition of preformed particles^15,16,18 have all been demonstrated to work also. The latter method, deposition of preformed colloidal clusters, provides an excellent way to prepare uniform systems with monodispersed particles.

Fig. 1 illustrates the procedure for preparing a flat model of a supported catalyst on which the active precursor is deposited by spincoating.

Fig. 1 Left: preparation of a supported model catalyst by spin coat impregnation starting from a single crystal silicon wafer. Right: schematic cross-section through a silica TEM grid. The 200 × 200 μm² electron transparent membrane consists of a 15 nm thick silicon nitride film, which is oxidized at 750 °C to produce a 3 nm thick silicon dioxide surface layer. For spin coating impregnation a 6 × 6 array of fused TEM grids is stabilized by a 2 mm wide silicon frame.

Testing planar model catalysts with a total surface area of a few cm² at most is very well possible if one uses batch reactors. In this way we have been able to make a detailed kinetic study of thiophene hydrodesulfurization^14,23 and also of the selective hydrogenation of croton aldehyde.¹¹ In special cases as polymerization^24,25 and carbon nanotube synthesis,¹⁵ products stay on the catalyst and analysis is straightforward.

A range of highly informative techniques that cannot easily be applied to high surface area supported catalysts works very well on planar model systems. X-Ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, Rutherford backscattering, and infrared spectroscopy in attenuated reflection mode are all applied in almost routine fashion. Synchrotron methods such as XANES/NEXAFS have been demonstrated to work in grazing incidence or fluorescence mode.^11,26

For morphological studies of nanoparticles, transmission electron microscopy (TEM) is the indicated method, but as the silicon wafer of the support is not transparent to the electron beam, a special model support is needed. The right part of Fig. 1 shows a Si₃N₄/Si(100) system, where the central part of the silicon has been etched away to create a ‘membrane’ window (typical dimensions 100 × 100 μm²) through which the electron beam can pass, enabling imaging of the particles on a sub-nanometre length scale.^15,18 After calcination in dry air the Si₃N₄ forms a 3 nm thick surface layer of silica. In order to disperse the active phase evenly on the flat silica support during spincoating impregnation a minimal wafer size of about 2 × 2 cm² is required. Therefore 36 individual TEM grids are arranged into a square pattern and stabilized by a silicon frame. These silica membranes are robust against chemical treatments in diverse gas mixtures and temperatures of up to 800 °C. This allows us to follow the effect of chemical treatments on the chemical state and morphology of supported phases such as catalyst nanoparticles. Monitoring the state of individual particles in the various stages of their catalytic life is quite possible. We present several examples of this approach in this paper. As almost all cases have been taken from our research on Fischer–Tropsch catalysts, we first give some background on this process.

Fischer–Tropsch synthesis

Invented in the 1920s by Franz Fischer and Hans Tropsch, this process stands for the conversion of synthesis gas (CO + H₂) to paraffins, olefins, and oxygenated products.^27–30 Used extensively in Germany and Japan during the 2nd world war, and in South Africa after the 1950s, it represents the process of choice for converting natural gas, biomass and coal via syngas into synthetic fuels. The process nowadays runs in South Africa (Sasol; PetroSA), Malaysia (Shell) and Qatar (Sasol/Qatar Petroleum; Shell), with a lot of potential for future use in several other places in the world.

The Fischer–Tropsch synthesis can be done with cobalt or with iron catalysts.³¹Cobalt has a high activity, is stable enough that it has a life time of many months or years, and has a good selectivity towards hydrocarbons in the wax region. These waxes are then hydrocracked to mostly diesel. If the syngas has a composition of approximately H₂/CO ≈ 2, cobalt is the catalyst of choice.³²Cobalt catalysts are normally supported on materials like silica, titania, or alumina.

Starting from coal, the syngas made by gasification or reforming has a lower hydrogen content. As the iron catalyst also possesses water-gas shift activity, it adjusts the composition, but at the penalty of more CO₂ formation. Two variations of the Fischer–Tropsch process with iron catalysts exist: the Low-Temperature version (LTFT) runs at about 230–250 °C and produces long chain hydrocarbons, while the High-Temperature version (HTFT) produces a broader and lighter product spectrum which contains small olefins, oxygenates and a fair share of hydrocarbons in the gasoline range. The chemicals, in particular the small olefins, are of interest as a feedstock for petrochemistry, and there is considerable interest in Fischer–Tropsch just because of this reason.^27,29,33

The HTFT iron Fischer–Tropsch catalyst is an unsupported, fused iron oxide, which is difficult to study, particularly with respect to surface composition and morphology changes. The same holds for the LTFT catalyst, which is a precipitated mixture containing structural and chemical promoters. The metallic iron in the catalyst converts to a complex mixture of carbides and oxides.^28,34,35Fig. 2 illustrates how reduced iron particles completely change morphology as a result of exposure to synthesis gas.¹⁸ As the environment in FTS changes from reducing at low conversion (mostly syngas) to oxidizing (water and CO₂) at high conversion, the phase composition of the catalysts depends on the CO conversion levels. Using model systems in which all the particles have the same size, and interact in more or less the same way with the support is one way to address the problem such that at least some of the complicating factors are circumvented. We show some examples hereafter.

Fig. 2 Iron nanoparticles changing morphology 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. Owing to a marker system, particles can be found back after off-line treatments.

Preparation of size-selected supported iron oxide nanoparticles

Preparing model catalysts by spin-coat impregnation of iron precursors (mainly iron(III) chloride) in water or alcohol gives only limited control over particle size and uniformity. It turned out that we obtain a smooth film of an amorphous iron hydroxide that transforms into a film of needle-like goethite crystals upon calcination (500 °C in dry air) and breaks up into individual particles during reduction, which appear as magnetite (Fe₃O₄) particles after exposure to air. These supported iron oxide nanoparticles are characterized by a broad size distribution centered around 20–40 nm in diameter, which is only mildly dependent on the initial iron loading. Fig. 3a–c illustrates the morphology of these classically impregnated iron catalysts in different stages of the preparation.

Fig. 3 Top: due to hydrolysis spin coating impregnation of iron precursors tends to yield closed layers of amorphous iron hydroxides on the flat silica substrate. These films break up to form particles with a relatively broad particle size distribution upon further oxidation/reduction treatments. Bottom: spin coating colloidal solutions of monodisperse iron oxide particles offers a facile way to prepare planar iron-based model catalysts with excellent control over particle size and coverage.

To exert simultaneous control over particle size and catalyst loading we turned towards colloidal solutions of preformed iron oxide nanoparticles. These colloidal solutions can be prepared with controlled and narrow particle size distribution (typically better than ±10%) by a number of well documented and reliable methods.³⁶Iron oxide nanoparticles ranging from 4.5 nm to 27 nm in diameters were impregnated (e.g. by spincoating) on the flat, electron-transparent silica membranes shown in Fig. 1. The TEM images (Fig. 3d–h) reveal that these nanoparticles form fractional two-dimensional monolayers on the silica surface with only very limited degree of stacking into the third dimension. Calcination in dry air (20% O₂ in Ar; typically at 500 °C) removes the organic surfactant from the iron oxide surface and establishes direct contact of the iron particles with the silica surface. Fig. 4 illustrates the effect of the calcination on particle morphology for a set of 9 nm and 16 nm particles (top and bottom, respectively). For both sets of particles we observe that individual particles are separated from each other by at least 1.5 nm after impregnation owing to the surfactant shell on the iron oxide surface. After calcination the relative position of the iron oxide nanoparticles remains unchanged, however, the diameter of the 16 nm particles has increased significantly (by about 4 nm in average) due to a surface wetting of the silica surface by the iron oxide. This spreading out results in the direct contact of neighboring particles. In contrast, the 9 nm iron oxide nanoparticles remain separated after calcination as their average diameter increases only by 0.5 nm. In this special case the smaller particles are more stable versussintering than the larger ones.

Fig. 4 A series of iron oxide nanoparticles with 9 nm (top) and 16 nm (bottom) in average diameter supported on SiO₂/SiN_x/SiO₂ membranes after impregnation (left) and after calcination at 500 °C in 20% O₂ in Ar (right).

We observe a similar trend for the stability against sintering upon reduction to metallic iron (Fig. 5). Reduction at 400 °C in hydrogen yields metallic iron nanoparticles, which in the TEM images are surrounded by a thin layer of amorphous oxide formed during sample transfer (through glovebox ambient). Again we observe no sign of sintering for the 9 nm iron oxide particles. In the case of 16 nm particles, however, some neighboring particles have fused together during the reduction treatment (Fig. 5).

Fig. 5 A series of iron oxide nanoparticles with 9 nm (top) and 16 nm (bottom) in average diameter supported on SiO₂/SiN_x/SiO₂ membranes after impregnation (left) and after reduction at 400 °C in H₂ (right). Note: spectra after reduction do not depict the same set of particles as those before reduction.

In situ studies by X-ray absorption spectroscopy

The chemical state of supported iron nanoparticles after chemical treatments is difficult to measure ex situ due to the inevitable reoxidation of the highly reactive metallic iron. Fortunately, flat model catalysts can also be investigated in situ with X-ray absorption spectroscopy (XANES, X-ray absorption near-edge spectroscopy). When applied in grazing incidence mode with detection of the fluorescent X-rays, high quality spectra can be obtained within minutes. As all iron nanoparticles are equally accessible to the reactive gases, chemical transformations prove to be uniform and typically only one phase (oxide, metal or carbide) is stable under given reaction conditions. Fig. 6a shows typical Fe K-edge spectra obtained upon treatment of the iron oxide precursor in dry hydrogen, dry syngas (H₂ : CO = 2 : 1) and wet syngas. With these treatments the iron oxide precursor, which is maghemite (γ-Fe₂O₃), can be converted quantitatively into magnetite (Fe₃O₄), wüstite (FeO), iron carbide (FeC_x, probably Fe₃C of Fe₅C₂) or metallic iron. The reduction of magnetite to metallic iron in hydrogen proves to be very sensitive to the water content of the hydrogen feed (Fig. 6b and c). In dry hydrogen (<200 ppm H₂O) the maghemite precursor transforms into magnetite at about 250 °C and to metallic iron at about 350 °C. During this reduction wüstite (FeO) is clearly present as an intermediate phase. The presence of water (2.5 vol% H₂O) inhibits the reduction severely, as expected. In this case, magnetite remains stable up to 450 °C and at 550 °C the reduction proceeds only to form wüstite. As water is the inevitable product of iron oxide reduction by hydrogen and also of the Fischer–Tropsch reaction, the precise water content of the reactive gas at the catalyst surface is very difficult to control with conventional high surface area catalysts. With iron oxide nanoparticles supported on flat silica the dynamics behavior of the catalyst particles in the reactant environment can be followed with great precision. The experiments demonstrate the sensitivity of iron catalysts to their gaseous environment.

Fig. 6 (a) Normalized in situFe K-edge spectra of 18 nm FeO_x/SiO₂/Si(100) model catalysts under various reaction conditions as indicated in the spectra. (b, c) Normalized in situFe K-edge spectra of 18 nm FeO_x/SiO₂/Si(100) model catalysts under dry (b) and wet (c) hydrogen at temperatures as indicated in the spectra.

Models of supported cobalt catalysts

The SiO₂/Si₃N₄/Si(100) model support in Fig. 1 has extensively been used as a carrier for cobalt particles to mimic cobalt Fischer–Tropsch catalysts. In this case the cobalt phase was deposited by spin coating an aqueous solution of cobalt nitrate. A polymer (polymethylvinylether) was added to the solution to increase its viscosity. After deposition the sample was calcined at 350 °C to remove the polymer, and subsequently reduced (425 °C).

The main catalyst-related challenge in the cobalt-catalyzed Fischer–Tropsch synthesis is deactivation, which is attributed to different deactivation processes, sintering of the metallic cobalt particles and deposition of carbon being the two most important reasons.¹⁶

Initially, oxidation of cobalt due to the water produced under Fischer–Tropsch synthesis conditions was widely held responsible for the deactivation.^16,37,38 However, a detailed investigation of a commercial catalyst after various periods in a Fischer–Tropsch slurry reactor showed that the degree of cobalt reduction increased during time on stream.³⁹ In order to test the sensitivity of cobalt nanoparticles with respect to oxidation, a planar model with 4–5 nm Co metal crystallites was studied under H₂O/H₂ mixtures using an in situ near-edge X-ray absorption fine structure (NEXAFS) at the Co 2p edge. As Fig. 7 shows, no oxidation is seen in mixtures of 50% H₂O and 50% H₂ up to 400 °C, which is actually in agreement with the Co/CoO thermodynamic phase diagram. Only exposure to 100% H₂O leads to oxidation of the cobalt particles.¹⁷ Hence these data confirm that quite extreme conditions such as water at 400 °C are needed to oxidize cobalt nanoparticles, and that hence oxidation of cobalt catalysts in the size range of a few nanometres will not occur under practical Fischer–Tropsch conditions.

Fig. 7 Planar cobalt model system on a flat SiO₂/Si(100) support for studying oxidation sensitivity: (a) AFM image and (b) height profile along the horizontal line in (a) indicating that the particles are 1–5 nm in diameter; (c) NEXAFS of the cobalt particles under mixtures of H₂O and H₂ at different temperatures, showing that cobalt is difficult to oxidize even when present as small nanoparticles (adapted from Saib et al.).¹⁷

Regeneration of cobalt catalysts

Sintering of cobalt particles contributes to loss of activity in the Fischer–Tropsch synthesis over time. Regeneration of the spent catalysts is possible, via an oxidation–reduction process.⁴⁰ During this procedure the cobalt is redispersed. The use of transparent, flat model supports is very powerful for studying this process in detail. Fig. 8 shows some results. A collection of metallic cobalt particles (left) was followed during an oxidation–reduction process. The distribution of particles is rather large, with particles from 4–20 nm being present. A passivation layer of CoO (chemical nature confirmed by XPS) is visible as a shell around the metallic core. The central picture shows the same particles after an oxidation (5 °C min⁻¹, 350 °C, 3 h) treatment. The metallic particles have transformed into hollow oxide particles (Co₃O₄), with a much larger outer diameter than the original metal particle.⁴¹ Detailed comparison with the particles before the treatment shows that the size of the void is equal to the size of the metallic particle before oxidation. Void formation during oxidation of cobalt nanoparticles has been reported before.^41,42 At the basis of this lies the Kirkendall effect,^43,44 which is a consequence of the different diffusivities of cobalt and oxygen. Rapid outward diffusion of Co through the oxide layer creates lattice vacancies in the metal core, which condensate into “Kirkendall voids” close to the metal–oxide interface. In this way the oxidation mechanism leads to transport of cobalt away from its original location.

Fig. 8 Co particles supported on a flat SiO₂ support during different stages of an oxidation–reduction cycle. Left: model catalyst in the metallic state before oxidation; middle: hollow oxide particles formed upon oxidation; right: reduction of the hollow particles, which break up into several smaller metallic particles (adapted from Saib et al.).¹⁶

During reduction (425 °C) the hollow oxide particles break up into a number of smaller particles (Fig. 8, right), located at the position of the original oxide shell. Comparing the images before and after the oxidation–reduction cycle clearly shows that the cobalt phase is redispersed, i.e. large particles break up into smaller ones. The relevance for catalysis is that in this way the catalytically active area has increased. In the case of the particle marked in red the surface area of the particles after oxidation–reduction is 160% of the surface area of the single particle before the treatment.¹⁶

Concluding remarks

The enormous developments in electron microscopy over the past decades have enriched the field of catalyst characterization to an extent that we now almost routinely can study the morphology and behavior of single catalyst particles in reactions.⁴⁵ An attractive way is to do this in situ, in microscopes that offer the opportunity to expose catalysts to reactive gases at low pressures.^46–48 The other possibility is to do this with idealized model systems with a marker system, where the area of interest can be found back after off-line treatments, as we described here. The advantage of this approach is that there are hardly any limitations on the reactive environment under which one wishes to study the catalyst, as these are detached from the microscope.

This is of particular importance for cobalt and iron Fischer–Tropsch catalysts, as their morphology and composition depend critically on the high pressure reaction conditions under which they operate. It is hard to imagine that such conditions can be realized in an in situelectron microscope.

The microscopy examples we discussed in this article all concern morphology changes as a result of exposure to a reactive environment. Information on the overall composition of the particles has been obtained from separate experiments at synchrotrons or in the laboratory. It is, however, also possible to obtain compositional information on the local scale, as for example recently reviewed by Weckhuysen and colleagues.^49,50 We show an example in Fig. 9, where we have used electron energy loss spectroscopy to obtain the local composition of an initially iron oxide nanoparticle model system that was used to grow carbon nanotubes.¹⁵ The iron oxide particles on the TEM support of the Fig. 1 sample were exposed to ethylene at 700 °C for 20 seconds only, in order to observe initial stages of nanotube formation. After reaction the sample was exposed to air and transferred into the microscope, an FEI Titan Krios operating at 300 kV. The TEM–EELS image in Fig. 9 shows iron metal particles (green) that are protected from oxidation because they are encapsulated in carbon (the red shells), but also unprotected iron oxide particles (blue-green). One iron particle in the image started to grow a multi-walled carbon nanotube. This example clearly illustrates the potential of the TEM–EELS method to obtain chemical information on the nanometre scale, and looks particularly attractive to study e.g. the composition of alloy catalysts, or the location of promoters and poisons.⁵¹

Fig. 9 TEM–EELS of an iron model catalyst which just started to grow a carbon nanotube. The picture was taken after exposure to air. Iron metal (green) encapsulated in carbon (red shell) remains metallic but the unprotected particles oxidize in air and show up by their oxygen and iron atoms (blue-green; figure adapted from Moodley et al.).¹⁵

In conclusion, we hope that the cases highlighted in this paper illustrate the versatility of flat model catalysts for addressing fundamentally and industrially relevant issues in catalysis. These systems, consisting of realistically prepared catalyst particles on planar supports with thin windows facilitating transmission electron microscopy, enable one to apply

• spectroscopic characterization techniques such as XPS, XANES, FTIR-ATR;

• microscopic methods such as AFM, SEM and TEM and TEM–EELS or EFTEM;

• diffraction analysis in the electron microscope or at synchrotrons;

• reactivity measurements with the catalysts in batch reactors, such that products accumulate until quantities are reached that can be analyzed with a gas chromatograph; in the case of polymerization reactions or carbon nanotube synthesis the product grows and stays on the catalyst, which makes product analysis straightforward.

As a final important aspect we mention that due to the specific geometry of these model catalysts, notably the absence of pores, all particles are visible for measurement, ensuring that spectroscopic characterization and catalytic testing are always done on the same particles. We consider it particularly rewarding that several industries have shown their interest in this approach, which has led to a considerable list of joint publications with our group. We intend to continue the application of planar model catalysts in Fischer–Tropsch studies and to explore the capabilities illustrated in this on multi-component catalysts in other industrially important reactions.

Notes and references

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