Further insights into methane and higher hydrocarbons formation over cobalt-based catalysts with γ-Al2O3, α-Al2O3 and TiO2 as support materials

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.

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).

Cobalt-Nickel Nanoparticles Supported on Reducible Oxides as Fischer-Tropsch Catalysts

Efficient and more sustainable production of transportation fuels is key to fulfill the ever-increasing global demand. In order to achieve this, progress in the development of highly active and selective catalysts is fundamental. The combination of bimetallic nanoparticles and reactive support materials offers unique and complex interactions that can be exploited for improved catalyst performance. Here, we report on cobalt-nickel nanoparticles on reducible metal oxides as support material for enhanced performance in the Fischer-Tropsch synthesis. For this, different cobalt to nickel ratios (Ni/(Ni + Co): 0.0, 0.25, 0.50, 0.75, or 1.0 atom/atom) supported on reducible (TiO₂ and Nb₂O₅) or nonreducible (α-Al₂O₃) oxides were studied. At 1 bar, Co-Ni nanoparticles supported on TiO₂ and Nb₂O₅ showed stable catalytic performance, high activities and remarkably high selectivities for long-chain hydrocarbons (C₅₊, ∼80 wt %). In contrast, catalysts supported on α-Al₂O₃ independently of the metal composition showed lower activities, high methane production, and considerable deactivation throughout the experiment. At 20 bar, the combination of cobalt and nickel supported on reducible oxides allowed for 25-50% cobalt substitution by nickel with increased Fischer-Tropsch activity and without sacrificing much C₅₊ selectivity. STEM-EDX and IR of adsorbed CO pointed to a cobalt enrichment of the nanoparticle's surface and a weaker adsorption of CO in Co-Ni supported on TiO₂ and Nb₂O₅ and not on α-Al₂O₃, modifying the rate-determining step and the catalytic performance. Overall, we show the strong effect and potential of reducible metal oxides as support materials for bimetallic nanoparticles for enhanced catalytic performance.

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.