Effect of type and localization of nitrogen in graphene nanoflake support on structure and catalytic performance of Co-based Fischer-Tropsch catalysts

Is Carbon Heteroatom Doping the Key to Active and Stable Carbon Supported Cobalt Fischer-Tropsch Catalysts?

Carbon supports are an interesting alternative to established oxidic catalyst supports for Co-based Fischer-Tropsch synthesis (FTS) catalysts as they allow high Co reducibility and do not suffer from the formation of Co/support compounds. To optimize Co-based carbon-supported FTS catalysts, significant research has focused on doping carbon supports with heteroatoms, aiming to enhance both catalytic activity and stability. While improvements in FTS performance have been reported for N-doped carbon supports, the exact effects of heteroatom doping are still poorly understood, largely due to difficulties in directly comparing Co FTS catalysts supported on doped versus nondoped carbon materials. In this study, we synthesized a series of highly comparable N-, S-, and P-doped carbon nanofiber (CNF) model supports, which were combined with size-controlled, colloidal Co nanoparticles to create well-defined model FTS catalysts. Comprehensive characterization of these catalysts using in situ X-ray absorption spectroscopy (XAS), in situ X-ray diffraction (XRD), and in situ magnetometry revealed that the presence of dopants significantly altered the structure and properties of the catalytically active Co0 phase, affecting Co coordination numbers, crystal phase composition, and magnetic behavior. Challenging optimistic literature reports, our findings demonstrate that all the studied heteroatoms negatively impact either FTS activity or catalyst stability. Co on N-doped CNFs experienced rapid deactivation due to increased sintering as well as Co phase transformations, which were not observed for Co on nondoped CNFs. Co on S-doped CNF suffered from instability of carbon-bound S species in a hydrogen atmosphere, contributing to low FTS performance by S-poisoning. Finally, Co on P-doped CNFs displayed strong metal-support interactions that improved sintering stability, but FTS activity was hampered by low Co reducibility and the loss of active Co0 due to a complex sequence of cobalt phosphide formation and its subsequent decomposition into phosphorus oxides and cobalt oxide species under FTS conditions.

Graphene Nanoflake- and Carbon Nanotube-Supported Iron-Potassium 3D-Catalysts for Hydrocarbon Synthesis from Syngas

Transformation of carbon oxides into valuable feedstocks is an important challenge nowadays. Carbon oxide hydrogenation to hydrocarbons over iron-based catalysts is one of the possible ways for this transformation to occur. Carbon supports effectively increase the dispersion of such catalysts but possess a very low bulk density, and their powders can be toxic. In this study, spark plasma sintering was used to synthesize new bulk and dense potassium promoted iron-based catalysts, supported on N-doped carbon nanomaterials, for hydrocarbon synthesis from syngas. The sintered catalysts showed high activity of up to 223 μmol_CO/g_Fe/s at 300-340 °C and a selectivity to C₅₊ fraction of ~70% with a high portion of olefins. The promising catalyst performance was ascribed to the high dispersity of iron carbide particles, potassium promotion of iron carbide formation and stabilization of the active sites with nitrogen-based functionalities. As a result, a bulk N-doped carbon-supported iron catalyst with 3D structure was prepared, for the first time, by a fast method, and demonstrated high activity and selectivity in hydrocarbon synthesis. The proposed technique can be used to produce well-shaped carbon-supported catalysts for syngas conversion.

New Composite Contrast Agents Based on Ln and Graphene Matrix for Multi-Energy Computed Tomography

The subject of the current research study is aimed at the development of novel types of contrast agents (CAs) for multi-energy computed tomography (CT) based on Ln-graphene composites, which include Ln (Ln = La, Nd, and Gd) nanoparticles with a size of 2-3 nm, acting as key contrasting elements, and graphene nanoflakes (GNFs) acting as the matrix. The synthesis and surface modifications of the GNFs and the properties of the new CAs are presented herein. The samples have had their characteristics determined using X-ray photoelectron spectroscopy, X-Ray diffraction, transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy. Multi-energy CT images of the La-, Nd-, and Gd-based CAs demonstrating their visualization and discriminative properties, as well as the possibility of a quantitative analysis, are presented.

Preparation of Nitrogen Doped Biochar-Based Iron Catalyst for Enhancing Gasoline-Range Hydrocarbons Production

Developing catalysts to obtain high space time yield (STY) of gasoline-range hydrocarbons via Fischer-Tropsch synthesis (FTS) is a huge challenge due to the restriction of Anderson-Schulz-Flory distribution. Herein, a nitrogen doped biochar-based iron catalyst was synthesized by a one-step method using sugar cane bagasse as carbon precursor, which exhibited an excellent gasoline STY of 8.65 g_C5-12 g_Fe^-1 h^-1, exceeding most reported catalysts. A strong positive relationship between the amount of pyrrolic N and long-chain hydrocarbons selectivity was displayed. The characterization results indicated that pyrrolic N configuration on anchor sites tuned effectively the dispersion of iron species and metal-support interaction as well as CO adsorption, improving the FTS performance.

Jellyfish-like few-layer graphene nanoflakes: high paramagnetic response alongside increased interlayer interaction

Jellyfish-like graphene nanoflakes (GNF), prepared by hydrocarbon pyrolysis, are studied with electron paramagnetic resonance (EPR) method. The results are supported by X-ray photoelectron spectroscopy (XPS) data. Oxidized (GNFox) and N-doped oxidized (N-GNFox) flakes exhibit an extremely high EPR response associated with a large interlayer interaction which is caused by the structure of nanoflakes and layer edges reached by oxygen. The GNFox and N-GNFox provide the localized and mobile paramagnetic centers which are silent in the pristine (GNF p ) and N-doped (N-GNF) samples. The change in the relative intensity of the line corresponding to delocalized electrons is parallel with the number of radicals in the quaternary N-group. The environment of localized and mobile electrons is different. The results can be important in GNF synthesis and for explanation of their features in applications, especially, in devices with high sensitivity to weak electromagnetic field.

Fischer–Tropsch synthesis using a cobalt catalyst supported on graphitic carbon nitride

The nitrogen atoms in a g-C₃N₄ support improved cobalt reduction in a prepared Co/g-C₃N₄ catalyst.