Emiel de Smit

The renaissance of iron-based Fischer–Tropsch synthesis: on the multifaceted catalyst deactivation behaviour

Iron-based Fischer-Tropsch catalysts, which are applied in the conversion of CO and H2 into longer hydrocarbon chains, are historically amongst the most intensively studied systems in heterogeneous catalysis. Despite this, fundamental understanding of the complex and dynamic chemistry of the iron-carbon-oxygen system and its implications for the rapid deactivation of the iron-based catalysts is still a developing field. Fischer-Tropsch catalysis is characterized by its multidisciplinary nature and therefore deals with a wide variety of fundamental chemical and physical problems. This critical review will summarize the current state of knowledge of the underlying mechanisms for the activation and eventual deactivation of iron-based Fischer-Tropsch catalysts and suggest systematic approaches for relating chemical identity to performance in next generation iron-based catalyst systems (210 references).

Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy

The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts’ nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 °C to monitor in situ phase changes in a complex iron-based Fisher–Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 °C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC

The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer-Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer-Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe(3)C, χ-Fe(5)C(2), and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline χ-Fe(5)C(2) was highly susceptible to oxidation during FTS conditions, while a catalyst containing θ-Fe(3)C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the ϵ-χ-θ carbide transformation as a function of the carbon chemical potential μ(C).

In‐situ Scanning Transmission X‐Ray Microscopy of Catalytic Solids and Related Nanomaterials

The present status of in-situ scanning transmission X-ray microscopy (STXM) is reviewed, with an emphasis on the abilities of the STXM technique in comparison with electron microscopy. The experimental aspects and interpretation of X-ray absorption spectroscopy (XAS) are briefly introduced and the experimental boundary conditions that determine the potential applications for in-situ XAS and in-situ STXM studies are discussed. Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron Fischer-Tropsch catalysts as showcases. In the discussion, we critically compare STXM-XAS and STEM-EELS (scanning transmission electron microscopy-electron energy loss spectroscopy) measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials.

Nanoscale Chemical Imaging of the Reduction Behavior of a Single Catalyst Particle

A closer look: Investigation of the reduction properties of a single Fischer-Tropsch catalyst particle, using in situ scanning transmission X-ray microscopy with spatial resolution of 35 nm, reveals a heterogeneous distribution of Fe(0), Fe(2+), and Fe(3+) species. Regions of different reduction properties are defined and explained on the basis of local chemical interactions and catalyst morphology.

X-ray Imaging of Zeolite Particles at the Nanoscale : Influence of Steaming on the State of Aluminum and the Methanol-To-Olefin Reaction

In view of the limited oil reserves the methanol-to-olefin (MTO) process is an interesting catalytic route to provide raw materials for chemical industries. In the last decades, a vast number of studies have been devoted to increase our understanding of this important catalytic reaction leading to a consensus concerning the mechanism.[1–4] Accordingly, MTO is thought to proceed through the so-called “hydrocarbon pool” (HCP) mechanism,[5, 6] in which methanol is added to an organic scaffold present within the zeolite framework. This is followed by elimination of olefinic species in a closed catalytic cycle. Microporous silicoaluminophosphates and aluminosilicates, such as SAPO-34 and ZSM-5, are often used as MTO catalysts because of their unique acidic and structural properties. In the case of ZSM-5 the formation of ethene and propene is governed by two different catalytic routes,[7,8] allowing in principle to control the ethene/propene ratio. Unfortunately, throughout the MTO reaction undesired carbon deposits are formed in the narrow micropore system of ZSM-5, leading to severely restricted diffusion and therefore limited catalytic activity.[9] To overcome these limitations efforts have been made to improve the pore accessibility during synthesis,[10–12] and/or in post-synthetic steps,[13, 14] resulting in significant improvements in the diffusion properties of ZSM-5. In this work, two commercial ZSM-5 zeolites with dimensions of approximately 200–800 nm have been studied by scanning transmission X-ray microscopy (STXM). The first sample, denoted as ZSM-5-C, was calcined for 6 h at 5508C, whereas the second sample, further labeled as ZSM-5-S, was steamed for 3 h at 7008C. Details on the preparation and characteristics of ZSM-5-C and ZSM-5-S can be found in the Supporting Information (Figures S1–S13, Tables S1–S6). We will show how STXM, in combination with bulk characterization techniques, allows investigating the physicochemical properties of ZSM-5 zeolites in a novel way at the nanoscale.[ 15, 16] More specifically, detailed chemical maps, with a spatial resolution of 70 nm, have been obtained of aluminum, oxygen, and carbon, even under realistic reaction conditions.[17–19] In this manner, the influence of steaming on the state of aluminum, that is, the coordination and spatial distribution, as well as on the MTO performance, has been unraveled.

On the Surface Chemistry of Iron Oxides in Reactive Gas Atmospheres

Heterogeneous catalysis is based on the generation and subsequent combination of chemical species retained on the surface of a catalytic solid. Elementary reaction steps, that is, the dissociation of reactants and association to products, take place at the solid–gas or solid–liquid interface. Therefore, maximizing the accessible specific catalytic surface area, by reducing primary particle sizes, increases the (weight based) catalyst activity and results in higher material efficiency. However, surface and electronic properties of solids are often also significantly altered with decreasing particle sizes.[1,2] This results in size-dependent catalytic performance, better known as the particle size effect.[3–5] Although this effect has been well documented for many catalytic reactions, the exact underlying reasons for the different performance are often more difficult to access.

Three-dimensional structure and defects in colloidal photonic crystals revealed by tomographic scanning transmission X-ray microscopy.

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.

Scanning Transmission X‐Ray Microscopy as a Novel Tool to Probe Colloidal and Photonic Crystals

Photonic crystals consisting of nano- to micrometer-sized building blocks, such as multiple sorts of colloids, have recently received widespread attention. It remains a challenge, however, to adequately probe the internal crystal structure and the corresponding deformations that inhibit the proper functioning of such materials. It is shown that scanning transmission X-ray microscopy (STXM) can directly reveal the local structure, orientations, and even deformations in polystyrene and silica colloidal crystals with 30-nm spatial resolution. Moreover, STXM is capable of imaging a diverse range of crystals, including those that are dry and inverted, and provides novel insights complementary to information obtained by benchmark confocal fluorescence and scanning electron microscopy techniques.

In-situ scanning transmission X-ray microscopy of catalytic materials under reaction conditions

In-situ Scanning X-ray Transmission Microscopy (STXM) allows the measurement of the soft X-ray absorption spectra with 10 to 30 nm spatial resolution under realistic reaction conditions. We show that STXM-XAS in combination with a micromachined nanoreactor can image a catalytic system under relevant reaction conditions, and provide detailed information on the morphology and composition of the catalyst material. The nanometer resolution combined with powerful chemical speciation by XAS and the ability to image materials under realistic conditions opens up new opportunities to study many chemical processes.