Journal Article•
Kinetics of Fischer-Tropsch synthesis on titania-supported cobalt
01 Jan 1999-Preprints-American Chemical Society Division of Petroleum Chemistry (American Chemical Society)-Vol. 44, Iss: 1, pp 69-72
TL;DR: In this paper, the authors obtained kinetic data on a titania-supported catalyst under commercially-representative reaction conditions, and the effects of water partial pressure on the activity of Co/titania were also considered.
Abstract: The objective of this work was to obtain kinetic data on a well-characterized, titania-supported catalyst under commercially-representative reaction conditions. Effects of water partial pressure on the activity of Co/titania were also considered.
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TL;DR: In this paper, the authors provide experimental and theoretical evidence for hydrogen-assisted CO activation as the predominant kinetically relevant step on Fe and Co catalysts at conditions typical of FTS practice.
492 citations
TL;DR: It will be shown how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options.
Abstract: The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer–Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer–Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer–Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C–O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer–Tropsch catalysis.
226 citations
TL;DR: In this paper, the Fischer-Tropsch (FT) reaction is simulated on stepped Ru surfaces with CH as the inserting monomer, and three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal.
Abstract: Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.
194 citations
TL;DR: In this article, the authors compared three types of choices in the system configuration: the initial solar to chemical conversion, separations, and the final product, and found that the choice of separations can yield considerable system benefits; for example, splitting both CO2 and H2O, and separating CO from the produced CO2/CO mix, the system efficiency increases by 10% relative to option C for a resource-efficient full system solar to liquid fuel energy efficiency of 12.9%.
Abstract: Sunshine to Petrol (S2P) is a technology framework using a concentrated solar energy source and energy depleted CO2 and water feedstocks for producing liquid hydrocarbon fuels as sustainable alternatives to vulnerable and limited supplies of conventional petroleum. S2P encompasses numerous design configurations that integrate several unit operations to thermochemically convert CO2 and water to a final energized marketable product. In an earlier paper, hereafter referred to as Paper I, we established both a baseline system design and a methodology for evaluating system efficiencies, economics, and lifecycle impacts. Therein we demonstrated that design details of the balance of system following the initial solar to chemical conversion could have significant impact on full system efficiencies, which largely determine both economics and the lifecycle. Here we assess and compare results from three types of choices in the system configuration: the initial solar to chemical conversion, separations, and the final product. Each design option begins with CO2 capture. Options A–C differ in the initial solar splitting: (A) splitting CO2, (B) splitting H2O and (C) splitting both CO2 and H2O. Significantly, we find that splitting both has notable advantages over splitting just one, in efficiency and consequently in derived minimum selling price (MSP) of a methanol product. Option D splits both but replaces the methanol end-product with likely higher value Fischer Tropsch (FT) liquids. The production of the FT end product comes with a small decrease in solar to fuel energy efficiency (∼3.5% relative decrease from option C) and a small relative increase in the energy equivalent MSP (∼5%). Importantly, we find that in all options, the primary contributor to MSP is the cost of capital for the solar thermochemical sub-system (including the solar collectors) and not in the balance of system components or operating costs. The advantages of options C and D, over the baseline A, stem primarily from the decrease in CO2 to recover and recycle, motivating changing the separation component and replacing conventional and mature MEA-based CO2 separations with a technology to recover the minor component, CO. Of significance, we find that the choice of separations can yield considerable system benefits; for example in option F, splitting both CO2 and H2O, and separating CO from the produced CO2/CO mix, the system efficiency increases by 10% relative to option C for a resource-efficient full system solar to liquid fuel energy efficiency of 12.9%, and the MSP decreased by 18%. Motivated to determine if attractive economics are plausible and to identify the largest opportunities to reduce cost, we show results of a sensitivity analysis for key system and economic parameters. Finally, we construct alternate scenarios that consist of reductions in the most sensitive parameters, which include: solar utility prices, the solar dish–CR5 price, and the interest rate. The most optimistic but plausible parameter set yields an encouraging MSP for methanol of USD 4.24 per GGE (gallon of gasoline energy equivalent). The promising configuration splits both CO2 and H2O, separates the minority component CO, reduces solar derived utility costs an anticipated 60%, achieves 20% reductions in estimated manufacturing cost of the dish/CR5 component, and obtains a favorable 6.4% interest rate.
177 citations
TL;DR: In this paper, carbon isotope transients at reaction steady state are used to examine the effect of water vapor on the amount and reactivity of the surface carbon intermediates involved in the Fischer-Tropsch synthesis on both a supported and an unsupported cobalt catalyst.
156 citations
References
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Book•
30 Jun 1972TL;DR: An overview of Chemical Reaction Engineering is presented, followed by an introduction to Reactor Design, and a discussion of the Dispersion Model.
Abstract: Partial table of contents: Overview of Chemical Reaction Engineering. HOMOGENEOUS REACTIONS IN IDEAL REACTORS. Introduction to Reactor Design. Design for Single Reactions. Design for Parallel Reactions. Potpourri of Multiple Reactions. NON IDEAL FLOW. Compartment Models. The Dispersion Model. The Tank--in--Series Model. REACTIONS CATALYZED BY SOLIDS. Solid Catalyzed Reactions. The Packed Bed Catalytic Reactor. Deactivating Catalysts. HETEROGENEOUS REACTIONS. Fluid--Fluid Reactions: Kinetics. Fluid--Particle Reactions: Design. BIOCHEMICAL REACTIONS. Enzyme Fermentation. Substrate Limiting Microbial Fermentation. Product Limiting Microbial Fermentation. Appendix. Index.
8,257 citations
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 paper, metal dispersion and support effects on Fischer-Tropsch synthesis rate and selectivity were studied at conditions that favor the information of C5+ hydrocarbons (> 80% selectivity).
524 citations
TL;DR: In this article, specific activity and selectivity of unsupported cobalt and cobalt supported on alumina, silica, titania, carbon, and magnesia carriers for CO hydrogenation were measured in a single-pass differential reactor at low conversions, 1 atm, and 175-350 °C.
446 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