Cobalt Fischer–Tropsch Catalyst Regeneration: The Crucial Role of the Kirkendall Effect for Cobalt Redispersion

In situ STEM study on the morphological evolution of copper-based nanoparticles during high-temperature redox reactions

Despite the broad relevance of copper nanoparticles in industrial applications, the fundamental understanding of oxidation and reduction of copper at the nanoscale is still a matter of debate and remains within the realm of bulk or thin film-based systems. Moreover, the reported studies on nanoparticles vary widely in terms of experimental parameters and are predominantly carried out using either ex situ observation or environmental transmission electron microscopy in a gaseous atmosphere at low pressure. Hence, dedicated studies in regards to the morphological transformations and structural transitions of copper-based nanoparticles at a wider range of temperatures and under industrially relevant pressure would provide valuable insights to improve the application-specific material design. In this paper, copper nanoparticles are studied using in situ Scanning Transmission Electron Microscopy to discern the transformation of the nanoparticles induced by oxidative and reductive environments at high temperatures. The nanoparticles were subjected to a temperature of 150 °C to 900 °C at 0.5 atm partial pressure of the reactive gas, which resulted in different modes of copper mobility both within the individual nanoparticles and on the surface of the support. Oxidation at an incremental temperature revealed the dependency of the nanoparticles' morphological evolution on their initial size as well as reaction temperature. After the formation of an initial thin layer of oxide, the nanoparticles evolved to form hollow oxide shells. The kinetics of formation of hollow particles were simulated using a reaction-diffusion model to determine the activation energy of diffusion and temperature-dependent diffusion coefficient of copper in copper oxide. Upon further temperature increase, the hollow shell collapsed to form compact and facetted nanoparticles. Reduction of copper oxide was carried out at different temperatures starting from various oxide phase morphologies. A reduction mechanism is proposed based on the dynamic of the reduction-induced fragmentation of the oxide phase. In a broader perspective, this study offers insights into the mobility of the copper phase during its oxidation-reduction process in terms of microstructural evolution as a function of nanoparticle size, reaction gas, and temperature.

Oxidation of Benzyl Alcohol Using Cobalt Oxide Supported Inside and Outside Hollow Carbon Spheres

Cobalt oxide nanoparticles (6 nm) supported both inside and outside of hollow carbon spheres (HCSs) were synthesized by using two different polymer templates. The oxidation of benzyl alcohol was used as a model reaction to evaluate the catalysts. PXRD studies indicated that the Co oxidation state varied for the different catalysts due to reduction of the Co by the carbon, and a metal oxidation step prior to the benzyl alcohol oxidation enhanced the catalytic activity. The metal loading influenced the catalytic efficiency, and the activity decreased with increasing metal loading, possibly due to pore filling effects. The catalysts showed similar activity and selectivity (to benzaldehyde) whether placed inside or outside the HCS (63 % selectivity at 50 % conversion). No poisoning was observed due to product build up in the HCS.

High-Temperature Pulse Method for Nanoparticle Redispersion

Nanoparticles suffer from aggregation and poisoning issues (e.g., oxidation) that severely hinder their long-term applications. However, current redispersion approaches, such as continuous heating in oxidizing and reducing environments, face challenges including grain growth effects induced by long heating times as well as complex procedures. Herein, we report a facile and efficient redispersion process that enables us to directly transform large aggregated particles into nanoscale materials. In this method, a piece of carbon nanofiber film was used as a heater and high treatment temperature (∼1500-2000 K) is rapidly elevated and maintained for a very short period of time (100 ms), followed by fast quenching back to room temperature at a cooling rate of 10⁵ K/s to inhibit sintering. With these conditions we demonstrate the redispersion of large aggregated metal oxide particles into metallic nanoparticles just ∼10 nm in size, uniformly distributed on the substrate. Furthermore, the metallic states of the nanoparticles are renewed during the heat treatment through reduction. The redispersion process removes impurities and poisoning elements, yet is able to maintain the integrity of the substrate because of the ultrashort heating pulse time. This method is also significantly faster (ca. milliseconds) compared to conventional redispersion treatments (ca. hours), providing a pragmatic strategy to redisperse degraded particles for a variety of applications.

Activity enhancement of cobalt catalysts by tuning metal-support interactions

Interactions between metal nanoparticles and support materials can strongly influence the performance of catalysts. In particular, reducible oxidic supports can form suboxides that can decorate metal nanoparticles and enhance catalytic performance or block active sites. Therefore, tuning this metal-support interaction is essential for catalyst design. Here, we investigate reduction-oxidation-reduction (ROR) treatments as a method to affect metal-support interactions and related catalytic performance. Controlled oxidation of pre-reduced cobalt on reducible (TiO₂ and Nb₂O₅) and irreducible (α-Al₂O₃) supports leads to the formation of hollow cobalt oxide particles. The second reduction results in a twofold increase in cobalt surface area only on reducible oxides and proportionally enhances the cobalt-based catalytic activity during Fischer-Tropsch synthesis at industrially relevant conditions. Such activities are usually only obtained by noble metal promotion of cobalt catalysts. ROR proves an effective approach to tune the interaction between metallic nanoparticles and reducible oxidic supports, leading to improved catalytic performance.

Tuning metal-support interaction can strongly influence the performance of a catalyst, and is thus essential for catalyst design. Here, the authors investigate reduction-oxidation-reduction treatments as a method to affect metal-support interactions of cobalt-based catalysts in Fischer-Tropsch synthesis.

Reactivity and structural evolution of urchin-like Co nanostructures under controlled environments

In situ transmission electron microscopy (TEM) of samples in a controlled gas environment allows for the real time study of the dynamical changes in nanomaterials at high temperatures and pressures up to the ambient pressure (10⁵ Pa) with a spatial resolution close to the atomic scale. In the field of catalysis, the implementation and quantitative use of in situ procedures are fundamental for a better understanding of the behaviour of catalysts in their environments and operating conditions. By using a microelectromechanical systems (MEMS)-based atmospheric gas cell, we have studied the thermal stability and the reactivity of crystalline cobalt nanostructures with initial 'urchin-like' morphologies sustained by native surface ligands that result from their synthesis reaction. We have evidenced various behaviors of the Co nanostructures that depend on the environment used during the observations. At high temperature under vacuum or in an inert atmosphere, the migration of Co atoms towards the core of the particles is activated and leads to the formation of carbon nanostructures using as a template the initial multipods morphology. In the case of reactive environments, for example, pure oxygen, our investigation allowed to directly monitor the voids formation through the Kirkendall effect. Once the nanostructures were oxidised, it was possible to reduce them back to the metallic phase using a dihydrogen flux. Under a pure hydrogen atmosphere, the sintering of the whole structure occurred, which illustrates the high reactivity of such structures as well as the fundamental role of the present ligands as morphology stabilisers. The last type of environmental study under pure CO and syngas (i.e. a mixture of H₂ :CO = 2:1) revealed the metal particles carburisation at high temperature.

Size dependent stability of cobalt nanoparticles on silica under high conversion Fischer–Tropsch environment

Highly monodisperse cobalt crystallites, supported on Stöber silica spheres, as model catalysts for the Fischer–Tropsch synthesis were exposed to simulated high conversion environments in the presence and absence of CO utilising an in house developedin situmagnetometer. The catalyst comprising the smallest crystallites in the metallic state (average diameter of 3.2 nm) experienced pronounced oxidation whilst the ratio of H₂O to H₂was increased stepwise to simulate CO conversions from 26% up to complete conversion. Direct exposure of this freshly reduced catalyst to a high conversion Fischer–Tropsch environment resulted in almost spontaneous oxidation of 40% of the metallic cobalt. In contrast, a model catalyst with cobalt crystallites of 5.3 nm only oxidised to a small extent even when exposed to a simulated conversion of over 99%. The largest cobalt crystallites were rather stable and only experienced measurable oxidation when subjected to H₂O in the absence of H₂. This size dependency of the stability is in qualitative accordance with reported thermodynamic calculations. However, the cobalt crystallites showed an unexpected low susceptibility to oxidation,i.e.only relatively high ratios of H₂O to H₂partial pressure caused oxidation. Similar experiments in the presence of CO revealed the significance of the actual Fischer–Tropsch synthesis on the metallic surface as the dissociation of CO, an elementary step in the Fischer–Tropsch mechanism, was shown to be a prerequisite for oxidation. Direct oxidation of cobalt to CoO by H₂O seems to be kinetically hindered. Thus, H₂O may only be capable of indirect oxidation,i.e.high concentrations prevent the removal of adsorbed oxygen species on the cobalt surface leading to oxidation. However, a spontaneous direct oxidation of cobalt at the interface between the support and the crystallites by H₂O forming presumably cobalt silicate type species was observed in the presence and absence of CO. The formation of these metal–support compounds is in accordance with conducted thermodynamic predictions. None of the extreme Fischer–Tropsch conditions initiated hydrothermal sintering. Seemingly, the formation of metal–support compounds stabilised the metallic crystallites and/or higher partial pressures of CO are required to increase the concentration of mobile, cobalt oxide-type species on the metallic surface.

Reshaping CuO on silica to generate a highly active Cu/SiO2 catalyst

Reduction–oxidation treatment [RO] is effective to reshape CuO within a confined area on SiO₂ support, forming highly dispersed nano CuO particles. The “shape” of CuO can be memorized during the activation process, resulting in the formation of a specific Cu metal and thus demonstrating enhanced catalytic activity.

Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions

It is generally accepted that optimal particle sizes are key for efficient nanocatalysis. Much less attention is paid to the role of morphology and atomic arrangement during catalytic reactions. Here, we unravel the structural, stoichiometric, and morphological evolution of gas-phase produced and partially oxidized cobalt nanoparticles in a broad size range. Particles with diameters between 1.4 and 22 nm generated in cluster sources are size selected and deposited on amorphous alumina (Al2O3) and ultrananocrystalline diamond (UNCD) films. A combination of different techniques is employed to monitor particle properties at the stages of production, exposure to ambient conditions, and catalytic reaction, in this case, the oxidative dehydrogenation of cyclohexane at elevated temperatures. A pronounced size dependence is found, naturally classifying the particles into three size regimes. While small and intermediate clusters essentially retain their compact morphology, large particles transform into hollow spheres due to the nanoscale Kirkendall effect. Depending on the substrate, an isotropic (Al2O3) or anisotropic (UNCD) Kirkendall effect is observed. The latter results in dramatic lateral size changes. Our results shed light on the interplay between chemical reactions and the catalyst's structure and provide an approach to tailor the cobalt oxide phase composition required for specific catalytic schemes.

Control and impact of the nanoscale distribution of supported cobalt particles used in Fischer-Tropsch catalysis

The proximity of nanoparticles may affect the performance, in particular the stability, of supported metal catalysts. Short interparticle distances often arise during catalyst preparation by formation of aggregates. The cause of aggregation of cobalt nanoparticles during the synthesis of highly loaded silica-supported catalysts was found to originate from the drying process after impregnation of the silica grains with an aqueous cobalt nitrate precursor. Maximal spacing of the Co3O4 nanoparticles was obtained by fluid-bed drying at 100 °C in a N2 flow. Below this temperature, redistribution of liquid occurred before and during precipitation of a solid phase, leading to aggregation of the cobalt particles. At higher temperatures, nucleation and growth of Co3O4 occurred during the drying process also giving rise to aggregation. Fischer-Tropsch catalysis performed under industrially relevant conditions for unpromoted and Pt-promoted cobalt catalysts revealed that the size of aggregates (13-80 nm) of Co particles (size ~9 nm) had little effect on activity. Large aggregates exhibited higher selectivities to long chain alkanes, possibly related to higher olefin formation with subsequent readsorption and secondary chain growth. Most importantly, larger aggregates of Co particles gave rise to extensive migration of cobalt (up to 75%) to the external surface of the macroscopic catalyst grains (38-75 μm). Although particle size did not increase inside the silica support grains, migration of cobalt to the external surface partly led to particle growth, thus causing a loss of activity. This cobalt migration over macroscopic length scales was suppressed by maximizing the distance between nanoparticles over the support. Clearly, the nanoscale distribution of particles is an important design parameter of supported catalysts in particular and functional nanomaterials in general.

Size dependent reduction-oxidation-reduction behaviour of cobalt oxide nanocrystals

Morphologically similar cobalt oxide nanoparticles (Co3O4) of four different sizes (3 nm, 6 nm, 11 nm and 29 nm) with narrow size distribution were prepared by subtle variation of synthesis conditions. These nanoparticles were used as model materials to understand the structural and morphological changes that occur to cobalt oxide during sequential reduction, oxidation and further re-reduction process as a function of the initial size of cobalt oxide. On reduction, spherical cobalt nanoparticles were obtained independent of the original size of cobalt oxide. In contrast, subsequent oxidation of the metal particles led to solid spheres, hollow spheres or core-shell structures depending on the size of the initial metal particle. Further re-reduction of the oxidized structures was also observed to be size dependent. The hollow oxide shells formed by the large particles (29 nm) fragmented into smaller particles on reduction, while the hollow shells of the medium sized particles (11 nm) did not re-disperse on further reduction. Similarly, no re-dispersion was observed in the case of the small particles (6 nm). This model study provides useful insights into the size dependent behavior of metal/metal oxide particles during oxidation/reduction. This has important implications in petrochemical industry where cobalt is used as a catalyst in the Fischer-Tropsch process.