Nico Fischer

Size‐Dependent Phase Transformation of Catalytically Active Nanoparticles Captured In Situ

The utilization of metal nanoparticles traverses across disciplines and we continue to explore the intrinsic size-dependent properties that make them so unique. Ideal nanoparticle formulation to improve a processs efficiency is classically presented as exposing a greater surface area to volume ratio through decreasing the nanoparticle size. Although, the physiochemical characteristics of the nanoparticles, such as phase, structure, or behavior, may be influenced by the nature of the environment in which the nanoparticles are subjected1, 2 and, in some cases, could potentially lead to unwanted side effects. The degree of this influence on the particle properties can be size-dependent, which is seldom highlighted in research. Herein we reveal such an effect in an industrially valuable cobalt Fischer-Tropsch synthesis (FTS) catalyst using novel in situ characterization. We expose a direct correlation that exists between the cobalt nanoparticles size and a phase transformation, which ultimately leads to catalyst deactivation.

Metal Support Interactions in Co 3 O 4 /Al 2 O 3 Catalysts Prepared from w/o Microemulsions

To obtain nano-sized metal and metal salt crystallites with a narrow size distribution synthesis methods utilizing water in oil (w/o) microemulsions, i.e. reverse micelles, have been widely applied and reported in literature. In this study we show the effect of support addition at different stages of the reverse micelle based preparation of cobalt oxide on alumina model catalysts. All catalysts were characterized with X-ray powder diffraction and Raman spectroscopy indicating the presence of Co3O4 on the Al2O3 support. Studies of the reduction behaviour and X-ray photoelectron spectroscopy however revealed the presence of difficult to reduce cobalt aluminate species in the samples where the support was added during or shortly after the precipitation step in the synthesis process. It can therefore be assumed that if the alumina support is added to the reverse micelle solution unprecipitated Co2+ ions and partially dissolved Al3+ combine and form cobalt aluminates. In the preparations where the solid cobalt precipitates are recovered from the microemulsion and then supported on the carrier, no metal-aluminate formation could be observed. This study therefore gives important information how metal-support interaction can be affected during catalyst preparation using reverse micelles.Graphical Abstract

Tri‐cobalt Carboxylate as a Catalyst and Catalyst Precursor in the Fischer–Tropsch Synthesis

In recent years, several groups have invested significant research capacity to investigate the effect of the metal crystallite size on the various primary and secondary reactions taking place during the Fischer–Tropsch synthesis. Most publications agree that with decreasing crystallite size (dcryst<10 nm) the surface‐specific CO hydrogenation activity (turnover frequency) decreases. In parallel, an increased selectivity to the undesired methane was observed. In the present study we combined and extended previously published data on alumina‐supported cobalt nanocrystallites with observations on an alumina‐supported cobalt carbonyl complex ((CO)9Co3CCOOH) and its decomposition products under Fischer–Tropsch reaction conditions. We could show that with an increased degree of sintering of the complex, that is, a larger cobalt crystallite size, the turnover frequency increased and the tendency to form methane decreased. The results connect seamlessly with those obtained for the nanocrystallites suggesting that the catalyst performance features extend into the sub‐2 nm range.

Oxygenate formation over K/β-Mo2C catalysts in the Fischer–Tropsch synthesis

The Fischer–Tropsch (FT) process, producing long chained waxes and transportation fuels, is competing with fuels derived from crude oils and its profitability is therefore dependent on the global oil price. However, increasing the value of synthesized products could render the profitability of the FTS independent of the common fluctuations in the commodity price (which are mostly due to global political trends and only to a lesser extent due to market requirements). One way to achieve this, is to target the more valuable products of the Fischer–Tropsch spectrum, for example oxygenates. This study investigates the effect of synthesis protocols on the surface characteristics of molybdenum carbide and the use of potassium promoted Mo2C as a catalyst for higher oxygenate (C2+ oxygenates) synthesis in CO hydrogenation. A graphitic surface layer was observed with TEM, XPS and Raman analysis for Mo2C samples carburized at ≥760 °C. The graphitic carbon, blocking active sites and therefore significantly lowering catalytic activity, could be partially removed by means of a temperature programmed hydrogenation, forming methane. An unpromoted β-Mo2C catalyst, carburized at 630 °C, reached CO conversions up to ±40% at the conditions applied. Initial 6.2 wt% K/Mo promotion of the catalyst with potassium showed a significant drop in catalyst activity, however, an increase in potassium content did not further decrease catalyst activity. The selectivity towards oxygenates was enhanced, yet it has a certain optimum with regards to promotor concentration. Simultaneously, the oxygenate distribution shifted towards higher alcohols. The initial methanol content in the total oxygenate product was around 60 C% and decreased to approximately 20 C% upon potassium promotion.

Co3O4 morphology in the preferential oxidation of CO

The preferential oxidation (PrOx) of carbon monoxide is an effective process for the removal of trace amounts of CO in a hydrogen-rich gas stream originating from steam reforming or gasification processes. CO can act as catalyst poison in various downstream processes such as the ammonia synthesis or PEM fuel cells for power generation. The effect on activity and selectivity of different cobalt oxide morphologies (cubes, sheets and belts) in Co3O4/SiO2 model catalysts was studied against conventional near-spherical nanoparticles. With a combination of offline and specialized in situ characterisation techniques the stability and catalytic performance of the model catalysts was monitored. With TEM and XRD, the prepared nanosheets and nanobelts were identified as superstructures constituted by small crystallites with similar catalytic activity to conventional nanoparticles. The nanocubes however, consisting of single crystals or at least large crystalline domains, display a superior surface specific CO oxidation activity which is attributed to the preferential exposure of {001} planes. Catalytic sites on these plains seem to support the formation of the Co3+/2+ redox pair required for the underlying Mars-van Krevelen mechanism.

Designing new catalysts: synthesis of new active structures: general discussion

Philip Davies opened the discussion of the introductory lecture by Avelino Corma: Themetal nanoparticles inside the zeolites are in a different environment from those outside. Is there any difference in their chemistry and their catalytic behaviour? Avelino Corma answered: We were not able to determine the differences in reactivity, other than the accessibility of reactants with different sizes. It should be said that we did not use molecules specifically suited for showing potential electronic–entropic differences. I agree that this is an important point to be considered. What we clearly observed is that the clusters inside the channels were stable towards sintering. Cynthia Friend asked: Have you considered the possible effect of ligands bound to your clusters? Have van der Waals’ interactions been explicitly included? Avelino Corma replied: The theoretical calculation of the interactions of nitrobenzene with the nanoparticles includes the interactions with the support and van der Waals’ interactions. In the case of clusters, H2 dissociation has been carried out on isolated Pt, Au and Au-Pt clusters and van der Waals’ interactions were not considered. Graham Hutchings continued: You rightly point out in your design strategy that the adsorption of the reactant is the key factor, and that adsorption occurs.