Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts
TL;DR: In this paper, different supports (e.g. Al2O3, TiO2, SiO2 and ZrO2 modified SiO 2 or Al2 O3) and a variety of promoters, including noble metals and metal cations, were examined.
Abstract: Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al2O3, TiO2, SiO2, and ZrO2 modified SiO2 or Al2O3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al2O3>TiO2>SiO2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al2O3 and SiO2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al2O3.
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TL;DR: In this article, the authors proposed a method to use the key scientific project of Fujian Province to improve the performance of the Key Scientific Project (KSP) of the National Natural Science Foundation of China (NNSF).
Abstract: National Natural Science Foundation of China [20625310, 20923004]; National Basic Research Program of China [2005CB221408, 2010CB732303]; Research Fund for the Doctoral Program of Higher Education [20090121110007]; Key Scientific Project of Fujian Province [2009HZ0002-1]
664 citations
TL;DR: In this paper, the main causes of catalyst deactivation in cobalt-based Fischer-Tropsch synthesis are poisoning, reoxidation of cobalt active sites, formation of surface carbon species, carbidization, surface reconstruction, sintering of crystallites, metal support solid state reactions and attrition.
514 citations
TL;DR: The influence of cobalt loading (10 −40 wt% Co), cobalt precursor, and promoters (Re, Mn) on the physico-chemical and catalytic properties of mesoporous Co/SBA-15 catalysts for the Fischer-Tropsch Synthesis (FTS) reaction (T=493 K, P=20 bar, H2/CO=2) has been investigated as discussed by the authors.
434 citations
TL;DR: In this paper, the effect of γ -Al 2 O 3 support variables on Fischer-Tropsch synthesis activity and selectivity was carried out at industrially relevant conditions (T = 483 K, P = 20 bar, H 2 / CO = 2.1 ).
431 citations
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TL;DR: In this article, Fischer-Tropsch synthesis (FTS) catalysts with high cobalt concentration and site density are used for the synthesis of hydrocarbons from CO/H2 mixtures.
Abstract: Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.
1,366 citations
TL;DR: In this article, high temperature hydrogen adsorption and conventional 298 K adsorptions of hydrogen and carbon monoxide were characterized by unsupported cobalt and cobalt supported on silica, alumina, titania, magnesia, and carbon.
490 citations
TL;DR: In this article, Ru atoms at the surface of Co crystallites increase the rate of removal of carbon and oxygen species during reaction and during regeneration of deactivated Co catalysts, leading to higher Co site density and enhanced readsorption of α-olefins.
398 citations
TL;DR: In this paper, the effect of water on Co/Al 2 O 3 and CoRe/Al O 3 catalysts has been studied by adding water to the synthesis gas feed and by model studies exposing the catalysts to H 2 O/H 2 feeds using several characterization techniques such as TPR, gravimetry, XPS, TPD and pulse adsorption.
Abstract: The effect of water on Co/Al 2 O 3 and CoRe/Al 2 O 3 catalysts has been studied by adding water to the synthesis gas feed and by model studies exposing the catalysts to H 2 O/H 2 feeds using several characterization techniques such as TPR, gravimetry, XPS, TPD and pulse adsorption. It was found that the CoRe/Al 2 O 3 catalyst deactivates when water is added during Fischer–Tropsch synthesis and model-studies showed that this catalyst oxidizes in H 2 O/H 2 mixtures with a ratio much lower than expected for oxidation of bulk cobalt. The reoxidation increases with increasing H 2 O partial pressure and H 2 O/H 2 ratio. TPR and gravimetry showed only small amounts of bulk reoxidation, while pulse adsorption and TPD indicated large decreases in Co-surface metal. It is suggested that oxidation of highly dispersed phases or surface oxidation are the cause for the observed deactivation. Significant differences in behavior of the Co/Al 2 O 3 and the CoRe/Al 2 O 3 catalyst when exposed to H 2 O/H 2 /He were found by gravimetry, TPR, pulse adsorption and XPS. The CoRe/Al 2 O 3 catalyst was reoxidized more easily in H 2 O/H 2 /He mixtures than the Co/Al 2 O 3 catalyst. This is probably a result of the higher dispersion of the CoRe/Al 2 O 3 catalyst, but a direct influence of Re on the reoxidation cannot be excluded. A phase interacting strongly with the alumina support was found in both catalysts after H 2 O/He exposure, but also another oxide phase was formed. This second phase is reduced at lower temperature for the CoRe/Al 2 O 3 catalyst than for the Co/Al 2 O 3 catalyst.
248 citations
TL;DR: In this article, it was found that the reactivity of the alumina support changes the surface structure and chemical composition of catalysts significantly when the metal−support interaction is weak, and the cobalt content on the surface decreases upon the increase in support reactivity.
Abstract: Co/Al2O3 catalysts have been prepared with conventional impregnation and sol−gel methods to vary the chemical reactivity of the alumina support. The material system has been investigated with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance Fourier transform spectroscopy (DRIFT), Brunauer−Emmett−Teller (BET) method, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) (with thermogravimetric analysis (TGA) and differential thermal analysis (DTA)), and gas chromotography (GC) for the catalytic oxidation of CO. It had been found that the reactivity of the support changes the surface structure and chemical composition of catalysts significantly. When the metal−support interaction is weak, Co3O4 is a predominant surface phase (which is interfaced by a “cobalt surface phase”). With an increase in support reactivity, CoO and CoAl2O4 are found to be present on the surface. The cobalt content on the surface decreases upon the increase in suppor...
201 citations