Andrew M. Beale

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).

Implementation of a combined SAXS/WAXS/QEXAFS set-up for time-resolved in situ experiments

It has previously been shown that there are many benefits to be obtained in combining several techniques in one in situ set-up to study chemical processes in action. Many of these combined set-ups make use of two techniques, but in some cases it is possible and useful to combine even more. A set-up has recently been developed that combines three X-ray-based techniques, small- and wide-angle X-ray scattering (SAXS/WAXS) and quick-scanning EXAFS (QEXAFS), for the study of dynamical chemical processes. The set-up is able to probe the same part of the sample during the synthesis process and is thus able to follow changes at the nanometre to micrometre scale during, for example, materials self-assembly, with a time resolution of the order of a few minutes. The practicality of this kind of experiment has been illustrated by studying zeotype crystallization processes and revealed important new insights into the interplay of the various stages of ZnAPO-34 formation. The flexibility of this set-up for studying other processes and for incorporating other additional non-X-ray-based experimental techniques has also been explored and demonstrated for studying the stability/activity of iron molybdate catalysts for the anaerobic decomposition of methanol.

High performing and stable supported nano-alloys for the catalytic hydrogenation of levulinic acid to γ-valerolactone.

The catalytic hydrogenation of levulinic acid, a key platform molecule in many biorefinery schemes, into γ-valerolactone is considered as one of the pivotal reactions to convert lignocellulose-based biomass into renewable fuels and chemicals. Here we report on the development of highly active, selective and stable supported metal catalysts for this reaction and on the beneficial effects of metal nano-alloying. Bimetallic random alloys of gold-palladium and ruthenium-palladium supported on titanium dioxide are prepared with a modified metal impregnation method. Gold-palladium/titanium dioxide shows a marked,~27-fold increase in activity (that is, turnover frequency of 0.1 s−1) compared with its monometallic counterparts. Although ruthenium-palladium/titanium dioxide is not only exceptionally active (that is, turnover frequency of 0.6 s−1), it shows excellent, sustained selectivity to γ-valerolactone (99%). The dilution and isolation of ruthenium by palladium is thought to be responsible for this superior catalytic performance. Alloying, furthermore, greatly improves the stability of both supported nano-alloy catalysts.

Dynamic X‐Ray Diffraction Computed Tomography Reveals Real‐Time Insight into Catalyst Active Phase Evolution

Metals and metal oxides anchored to porous support materials are widely used as heterogeneous catalysts in a number of important industrial chemical processes. These catalysts owe their activity to the formation of unique metal/metal oxide support interactions, typically resulting in highly dispersed actives stabilized in a particular electronic or coordination state. They are employed in fixed-bed reactors as extruded or pelletized millimeter-sized “catalyst bodies” minimizing pressure drops along the length of the reactor. Since the efficiency of the whole catalytic system depends on the behavior and efficiency of the catalyst body per se, its design has very great importance. Crucial to this design is an understanding of the factors which influence the distribution and nature of the active phase during preparation. The type of desired distribution is very much dependant on catalytic process and required products; for example, an egg-shell distribution (as opposed to uniform, egg-white, or egg-yolk), where the active phase is located at the edges of the catalyst body, can be favored if the product forms readily.

Determining the storage, availability and reactivity of NH3 within Cu-Chabazite-based Ammonia Selective Catalytic Reduction systems

Three different types of NH3 species can be simultaneously present on Cu(2+)-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Reduction (NH3-SCR) systems. These include ammonium ions (NH4(+)), formed on the Brønsted acid sites, [Cu(NH3)4](2+) complexes, resulting from NH3 coordination with the Cu(2+) Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4(+) ions react very slowly in comparison to NH3 coordinated to Cu(2+) ions and are likely to contribute little to the standard NH3-SCR process, with the Brønsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4(+) ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu(2+), accompanied by a remarkable enhancement in the low temperature activity. Moreover, the presence of EFAl species could also have a positive influence on the reaction rate of the available NH4(+) ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.

Active phase evolution in single Ni/Al2O3 methanation catalyst bodies studied in real time using combined μ-XRD-CT and μ-absorption-CT

A combination of synchrotron μ-XRD-CT and μ-absorption-CT (CT = computed tomography) is demonstrated, providing a unique insight into the solid state changes occurring from within crystalline materials. Specifically, we examine here the solid state changes that occur in a millimetre-sized Ni/γ-Al2O3 catalyst body in both 2D and 3D during calcination and CO methanation for the first time. The combination provides a unique insight into the spatial phase distribution of these materials and how these evolve via a series of solid state transformation processes. For example, initially, two Ni-ethylenediamine (en) complexes were observed on the impregnated and dried body; a hydrated and non-hydrated form, which 2D scans reveal possess an egg-shell and egg-yolk distribution, respectively. Furthermore, the μ-XRD data were of sufficient quality so as to be able to reveal that the particles within the ‘egg-shell’ were larger (∼35 nm) than those of the ‘egg-yolk’ (∼19 nm) and that there were more of them. On calcination, both precursors collapsed, yielding metallic fcc Ni particles with a surprisingly uniform average size distribution over the catalyst (∼4 nm). However, a comparison of the scattering at different stages of the experiment suggested that the crystalline structure of some of the Ni remained diffraction ‘silent’. Calcination in oxygen lead to both Ni oxidation and particle sintering, mainly at the exterior, which on pre-reaction reduction (in H2) yielded again fcc Ni particles (∼4 nm interior, ∼6 nm exterior) with a significant reduction in the amorphous Ni component. The catalyst proved active for CO methanation and, during 2 h time on-stream, no change in the structure composition or shape was observed, leading us to conclude that nano-sized fcc Ni particles on γ-Al2O3 are the active component in CO methanation. This work therefore demonstrates both the power of spatially resolved μ-XRD-CT/μ-absorption-CT measurement of catalytic systems and its advantage over more ‘traditional’ single point studies on small sieve fractions.

Tomographic Energy Dispersive Diffraction Imaging To Study the Genesis of Ni Nanoparticles in 3D within γ-Al2O3 Catalyst Bodies

Tomographic energy dispersive diffraction imaging (TEDDI) is a recently developed synchrotron-based characterization technique used to obtain spatially resolved X-ray diffraction and fluorescence information in a noninvasive manner. With the use of a synchrotron beam, three-dimensional (3D) information can be conveniently obtained on the elemental composition and related crystalline phases of the interior of a material. In this work, we show for the first time its application to characterize the structure of a heterogeneous catalyst body in situ during thermal treatment. Ni/gamma-Al(2)O(3) hydrogenation catalyst bodies have been chosen as the system of study. As a first example, the heat treatment in N(2) of a [Ni(en)(3)](NO(3))(2)/gamma-Al(2)O(3) catalyst body has been studied. In this case, the crystalline [Ni(en)(3)](NO(3))(2) precursor was detected in an egg-shell distribution, and its decomposition to form metallic Ni crystallites of around 5 nm was imaged. In the second example, the heat treatment in N(2) of a [Ni(en)(H(2)O)(4)]Cl(2)/gamma-Al(2)O(3) catalyst body was followed. The initial [Ni(en)(H(2)O)(4)]Cl(2) precursor was uniformly distributed within the catalyst body as an amorphous material and was decomposed to form metallic Ni crystallites of around 30 nm with a uniform distribution. TEDDI also revealed that the decomposition of [Ni(en)(H(2)O)(4)]Cl(2) takes place via two intermediate crystalline structures. The first one, which appears at around 180 degrees C, is related to the restructuring of the Ni precursor on the alumina surface; the second one, assigned to the formation of a limited amount of Ni(3)C, is observed at 290 degrees C.

Pair distribution function computed tomography

An emerging theme of modern composites and devices is the coupling of nanostructural properties of materials with their targeted arrangement at the microscale. Of the imaging techniques developed that provide insight into such designer materials and devices, those based on diffraction are particularly useful. However, to date, these have been heavily restrictive, providing information only on materials that exhibit high crystallographic ordering. Here we describe a method that uses a combination of X-ray atomic pair distribution function analysis and computed tomography to overcome this limitation. It allows the structure of nanocrystalline and amorphous materials to be identified, quantified and mapped. We demonstrate the method with a phantom object and subsequently apply it to resolving, in situ, the physicochemical states of a heterogeneous catalyst system. The method may have potential impact across a range of disciplines from materials science, biomaterials, geology, environmental science, palaeontology and cultural heritage to health.

Correlating Metal Poisoning with Zeolite Deactivation in an Individual Catalyst Particle by Chemical and Phase-Sensitive X-ray Microscopy

Fluid catalytic cracking (FCC) is the main conversion process used in oil refineries. An X-ray microscopy method is used to show that metal poisoning and related structural changes in the zeolite active material lead to a non-uniform core–shell deactivation of FCC catalyst particles. The study links the detrimental effect of V and Ni poisoning with zeolite destruction and dealumination in a spatial manner within a single FCC catalyst particle.

Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM-5 as Revealed by Operando X-Ray Methods

Abstract Combined high‐resolution fluorescence detection X‐ray absorption near‐edge spectroscopy, X‐ray diffraction, and X‐ray emission spectroscopy have been employed under operando conditions to obtain detailed new insight into the nature of the Mo species on zeolite ZSM‐5 during methane dehydroaromatization. The results show that isolated Mo–oxo species present after calcination are converted by CH4 into metastable MoCxOy species, which are primarily responsible for C2Hx/C3Hx formation. Further carburization leads to MoC3 clusters, whose presence coincides with benzene formation. Both sintering of MoC3 and accumulation of large hydrocarbons on the external surface, evidenced by fluorescence‐lifetime imaging microscopy, are principally responsible for the decrease in catalytic performance. These results show the importance of controlling Mo speciation to achieve the desired product formation, which has important implications for realizing the impact of CH4 as a source for platform chemicals.