Synergistic Effect of Fe–Co Bimetallic Catalyst on FTS and WGS Activity in the Fischer–Tropsch Process: A Kinetic Study

A DFT study towards dynamic structures of iron and iron carbide and their effects on the activity of the Fischer-Tropsch process

The Fe-based Fischer-Tropsch synthesis (FTS) catalyst shows a rich phase chemistry under pre-treatment and FTS conditions. The exact structural composition of the active site, whether iron or iron carbide (FeCx), is still controversial. Aiming to obtain an insight into the active sites and their role in affecting FTS activity, the swarm intelligence algorithm is implemented to search for the most stable Fe(100), Fe(110), Fe(210) surfaces with different carbon ratios. Then, ab initio atomistic thermodynamics and Wulffman construction were employed to evaluate the stability of these surfaces at different chemical potentials of carbon. Their FTS reactivity and selectivity were later assessed by semi-quantitative micro-kinetic equations. The results show that stability, reactivity, and selectivity of the iron are all affected by the carbonization process when the carbon ratio increases. Formation of the carbide, a rather natural process under experimental conditions, would moderately increase the turnover frequency (TOF), but both iron and iron carbide are active to the reaction.

The normalization of the active surface sites of bimetallic Pd-Pt catalysts, their inhomogeneity, and their roles in methane activation

Multi-metallic catalysts containing Pt species are widely used. As there is no methodology to evaluate the quantity of active surface sites of Pt or other metal species, researchers have only published the total conversion or selectivity of all active surface sites. This study focuses on Pt-Pd bimetallic catalysts and uses both methane reaction kinetics and infrared (IR) spectroscopy to characterize the surface Pd and Pt sites. The surface Pt sites, which were determined from the fitted rate coefficients, were evaluated in the reaction region where the catalyst structure was insensitive to catalytic performance. Another methodology involves IR spectroscopy to normalize the active surface sites. As three typical absorption bands of Pt species were observed during CO chemisorption, spectral deconvolution was conducted to obtain the integrated intensity of the Pd and Pt species, and the quantity of surface Pd and Pt sites was calculated. The two methods have good consistency, and the IR spectra are considered to be more suitable for calculating the quantity of active surface sites. In addition, the IR spectra revealed a correlation between oxidative Pd surface sites and methane reactivity. The ionic Pd sites provide abundant oxygen intermediates in the catalytic reaction and improve the catalytic performance. As for the surface Pd species and bulk Pd species, the XPS results indicate a similar variation in the Pd^δ+/(Pd^δ+ + Pd⁰) ratio vs. Pd/Pt ratio.

Kinetics and Selectivity Study of Fischer–Tropsch Synthesis to C5+ Hydrocarbons: A Review

Fischer–Tropsch synthesis (FTS) is considered as one of the non-oil-based alternatives for liquid fuel production. This gas-to-liquid (GTL) technology converts syngas to a wide range of hydrocarbons using metal (Fe and Co) unsupported and supported catalysts. Effective design of the catalyst plays a significant role in enhancing syngas conversion, selectivity towards C5+ hydrocarbons, and decreasing selectivity towards methane. This work presents a review on catalyst design and the most employed support materials in FTS to synthesize heavier hydrocarbons. Furthermore, in this report, the recent achievements on mechanisms of this reaction will be discussed. Catalyst deactivation is one of the most important challenges during FTS, which will be covered in this work. The selectivity of FTS can be tuned by operational conditions, nature of the catalyst, support, and reactor configuration. The effects of all these parameters will be analyzed within this report. Moreover, zeolites can be employed as a support material of an FTS-based catalyst to direct synthesis of liquid fuels, and the specific character of zeolites will be elaborated further. Furthermore, this paper also includes a review of some of the most employed characterization techniques for Fe- and Co-based FTS catalysts. Kinetic study plays an important role in optimization and simulation of this industrial process. In this review, the recent developed reaction rate models are critically discussed.

Improving thermal diffusivity of supported Fe-based Fischer–Tropsch catalysts to enhance long-chain hydrocarbon production

Fischer Tropsch synthesis (FTS) is highly exothermic so heat removal remains crucial. In this study, a rational procedure is examined to remove heat in the FTS by improving the thermal diffusivity on a series of Fe-based catalysts.

Synthesis and crystal structure of a new heteronuclear complex of Fe(iii)-K designed to produce effective catalysts for CO hydrogenation

A new paramagnetic heteronuclear complex formulated as [K₃Fe(μ-ox)₃(H₂O)₃]_n (1), where ox^2- is oxalate, has been synthesized under hydrothermal condition. The molecular structure of complex 1 was characterized by elemental analysis, Fourier-transform infrared spectroscopy (FT-IR) and single-crystal X-ray diffraction (SCXRD). The results of SC-XRD analysis revealed that complex 1 crystallizes in the centrosymmetric space group P2₁/c of a monoclinic system with cell dimension a = 7.7175 (4) Å, b = 19.8009 (7) Å, c = 10.2623 (5) Å, and β = 107.634 (5)° at 100 K. The thermal behavior of complex 1 was studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The magnetic behavior of complex 1 was studied at room temperature by a vibration sample magnetometer (VSM). Thermal decomposition of the silica and alumina supports of complex 1 at 650 °C resulted in the main catalysts, Fe₂O₃-K₂O/SiO₂ and Fe₂O₃-K₂O/Al₂O₃. The catalytic activity of the main catalysts was evaluated for CO hydrogenation. For comparative purposes, the reference catalysts of Fe₂O₃-K₂O/SiO₂ and Fe₂O₃-K₂O/Al₂O₃ were prepared by the impregnation method. The structure and composition of the catalysts were investigated by FT-IR spectroscopy, powder X-ray diffraction (PXRD), N₂ adsorption-desorption analysis, scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive X-ray analysis (EDX). We tested all catalysts for hydrogenation of CO at 5 bar of pressure in the temperature range of 593-673 K. It was found that the main catalysts have better CO conversion and selectivity to desired products, such as light olefins, than the reference catalysts.

Kinetic Study Based on the Carbide Mechanism of a Co-Pt/γ-Al2O3 Fischer–Tropsch Catalyst Tested in a Laboratory-Scale Tubular Reactor

A Co-Pt/γ-Al2O3 catalyst was manufactured and tested for Fischer–Tropsch applications. Catalyst kinetic experiments were performed using a tubular fixed-bed reactor system. The operative conditions were varied between 478 and 503 K, 15 and 30 bar, H2/CO molar ratio 1.06 and 2.11 at a carbon monoxide conversion level of about 10%. Several kinetic models were derived, and a carbide mechanism model was chosen, taking into account an increasing value of termination energy for α-olefins with increasing carbon numbers. In order to assess catalyst suitability for the determination of reaction kinetics and comparability to similar Fischer–Tropsch Synthesis (FTS) applications, the catalyst was characterized with gas sorption analysis, temperature-programmed reduction (TPR), and X-ray diffraction (XRD) techniques. The kinetic model developed is capable of describing the intrinsic behavior of the catalyst correctly. It accounts for the main deviations from the typical Anderson-Schulz-Flory distribution for Fischer–Tropsch products, with calculated activation energies and adsorption enthalpies in line with values available from the literature. The model suitably predicts the formation rates of methane and ethylene, as well as of the other α-olefins. Furthermore, it properly estimates high molecular weight n-paraffin formation up to carbon number C80.