Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer-Tropsch Reaction: Evidence of a Two-Site Reaction Model

Fe Nanoparticle Size Control of the Fe-MOF-Derived Catalyst Using a Solvothermal Method: Effect on FTS Activity and Olefin Production

The design of a highly active Fe-supported catalyst with the optimum particle and pore size, dispersion, loading, and stability is essential for obtaining the desired product selectivity. This study employed a solvothermal method to prepare two Fe-MIL-88B metal-organic framework (MOF)-derived catalysts using triethylamine (TEA) or NaOH as deprotonation catalysts. The catalysts were analyzed using X-ray diffraction, N₂-physisorption, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, H₂ temperature-programed reduction, and thermogravimetric analysis and were evaluated for the Fischer-Tropsch synthesis performance. It was evident that the catalyst preparation in the presence of TEA produces a higher MOF yield and smaller crystal size than those produced using NaOH. The pyrolysis of MOFs yielded catalysts with different Fe particle sizes of 6 and 35 nm for the preparation in the presence of TEA and NaOH, respectively. Also, both types of catalysts exhibited a high Fe loading (50%) and good stability after 100 h reaction time. The smaller particle size TEA catalyst showed higher activity and higher olefin yield, with 94% CO conversion and a higher olefin yield of 24% at a lower reaction temperature of 280 °C and 20 bar at H₂/CO = 1. Moreover, the smaller particle size TEA catalyst exhibited higher Fe time yield and CH₄ selectivity but with lower chain growth probability (α) and C₅₊ selectivity.

Effect of Pre-Treatment Conditions on the Activity and Selectivity of Cobalt-Based Catalysts for CO Hydrogenation

We investigated the effect of pre-treatment conditions on the activity and selectivity of cobalt catalysts for Fischer–Tropsch synthesis (FTS) by varying both the reduction atmosphere and the reduction temperature. Catalysts supported on SiO2, Al2O3, and TiO2, prepared via incipient wetness impregnation, were evaluated, and activation temperatures in the range 250–350 °C were considered. Activation with syngas led to a better product selectivity (low CH4, high selectivity to liquid hydrocarbons, and low paraffin to olefin ratio (P/O)) than the catalysts reduced in H2 at lower activation temperatures. The CoxC species suppressed the hydrogenation reaction, and it is hypothesised that this resulted in the high selectivity of olefins observed for the syngas pre-treated catalysts. On the basis of the experimental results, we postulated that a synergistic effect between Co0 and CoxC promotes the production of the long chain hydrocarbons and suppresses the formation of CH4. In addition, for systems aimed at producing lower olefins, syngas activation is recommended, and for the FTS plants that focus on maximising the production of higher molecular weight products, H2 activation might be considered. These results provide insights for the future FTS catalyst design and for target product-driven operations.

Passivation of Co/Al2O3 Catalyst by Atomic Layer Deposition to Reduce Deactivation in the Fischer–Tropsch Synthesis

The present work explores the technical feasibility of passivating a Co/γ-Al2O3 catalyst by atomic layer deposition (ALD) to reduce deactivation rate during Fischer–Tropsch synthesis (FTS). Three samples of the reference catalyst were passivated using different numbers of ALD cycles (3, 6 and 10). Characterization results revealed that a shell of the passivating agent (Al2O3) grew around catalyst particles. This shell did not affect the properties of passivated samples below 10 cycles, in which catalyst reduction was hindered. Catalytic tests at 50% CO conversion evidenced that 3 and 6 ALD cycles increased catalyst stability without significantly affecting the catalytic performance, whereas 10 cycles caused blockage of the active phase that led to a strong decrease of catalytic activity. Catalyst deactivation modelling and tests at 60% CO conversion served to conclude that 3 to 6 ALD cycles reduced Co/γ-Al2O3 deactivation, so that the technical feasibility of this technique was proven in FTS.

Sustainable recycling of LiCoO2 cathode scrap on the basis of successive peroxymonosulfate activation and recovery of valuable metals

Increasing demand and waste of lithium-ion batteries (LIBs) has adversely affected resources and the environment. Multistage utilization of spent LIBs is essential to their sustainable development. Here, we propose a simple recycling method of LiCoO₂ cathode scrap, based on the first use of the cathode scrap as a catalyst to degrade organic pollutants via peroxymonosulfate activation, and subsequent recovery of valuable metals from the used catalyst. Compared with pristine LiCoO₂, the LiCoO₂ cathode scrap exhibits excellent catalytic performance due to the active sites generated, such as the vacancy generation and electronic structure modulation by the degradation of LiCoO₂ during the continuous lithiation and delithiation processes. The removal efficiency of cathode scrap to the o-phenylphenol exceeds 98% within 60 min, and the degradation efficiency is still above 95% after the 10th use because its unique sandwich and porous structure ensure the stability and recyclability. After multiple catalytic reactions, due to the generation of crack, the separation of the sandwich structure, and further degradation of active materials, the leaching efficiency of transition metals from the cathode scrap in deep eutectic solvent is promoted. 86% of lithium and 95% of cobalt are leached from the used catalyst respectively. This study provides a promising strategy for the sustainable development of LIBs and promotes the utilization of spent LIBs in multiaspect.

On the Cobalt Carbide Formation in a Co/TiO2 Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction

Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO2 Fischer-Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO2 catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.

Fischer–Tropsch Synthesis: Computational Sensitivity Modeling for Series of Cobalt Catalysts

Nearly a century ago, Fischer and Tropsch discovered a means of synthesizing organic compounds ranging from C1 to C70 by reacting carbon monoxide and hydrogen on a catalyst. Fischer–Tropsch synthesis (FTS) is now known as a pseudo-polymerization process taking a mixture of CO as H2 (also known as syngas) to produce a vast array of hydrocarbons, along with various small amounts of oxygenated materials. Despite the decades spent studying this process, it is still considered a black-box reaction with a mechanism that is still under debate. This investigation sought to improve our understanding by taking data from a series of experimental Fischer–Tropsch synthesis runs to build a computational model. The experimental runs were completed in an isothermal continuous stirred-tank reactor, allowing for comparison across a series of completed catalyst tests. Similar catalytic recipes were chosen so that conditional comparisons of pressure, temperature, SV, and CO/H2 could be made. Further, results from the output of the reactor that included the deviations in product selectivity, especially that of methane and CO2, were considered. Cobalt was chosen for these exams for its industrial relevance and respectfully clean process as it does not intrinsically undergo the water–gas shift (WGS). The primary focus of this manuscript was to compare runs using cobalt-based catalysts that varied in two oxide catalyst supports. The results were obtained by creating two differential equations, one for H2 and one for CO, in terms of products or groups of products. These were analyzed using sensitivity analysis (SA) to determine the products or groups that impact the model the most. The results revealed a significant difference in sensitivity between the two catalyst–support combinations. When the model equations for H2 and CO were split, the results indicated that the CO equation was significantly more sensitive to CO2 production than the H2 equation.

Use of Plasma-Synthesized Nano-Catalysts for CO Hydrogenation in Low-Temperature Fischer⁻Tropsch Synthesis: Effect of Catalyst Pre-Treatment

A study was done on the effect of temperature and catalyst pre-treatment on CO hydrogenation over plasma-synthesized catalysts during the Fischer–Tropsch synthesis (FTS). Nanometric Co/C, Fe/C, and 50%Co-50%Fe/C catalysts with BET specific surface area of ~80 m² g^–1 were tested at a 2 MPa pressure and a gas hourly space velocity (GHSV) of 2000 cm³ h⁻¹ g⁻¹ of a catalyst (at STP) in hydrogen-rich FTS feed gas (H₂:CO = 2.2). After pre-treatment in both H₂ and CO, transmission electron microscopy (TEM) showed that the used catalysts shifted from a mono-modal particle-size distribution (mean ~11 nm) to a multi-modal distribution with a substantial increase in the smaller nanoparticles (~5 nm), which was statistically significant. Further characterization was conducted by scanning electron microscopy (SEM with EDX elemental mapping), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The average CO conversion at 500 K was 18% (Co/C), 17% (Fe/C), and 16% (Co-Fe/C); 46%, 37%, and 57% at 520 K; and 85%, 86% and 71% at 540 K respectively. The selectivity of Co/C for C₅₊ was ~98% with 8% gasoline, 61%, diesel and 28% wax (fractions) at 500 K; 22% gasoline, 50% diesel, and 19% wax at 520 K; and 24% gasoline, 34% diesel, and 11% wax at 540 K, besides CO₂ and CH₄ as by-products. Fe-containing catalysts manifested similar trends, with a poor conformity to the Anderson–Schulz–Flory (ASF) product distribution.