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Abstract: The chain growth mechanisms of Fischer–Tropsch synthesis on Fe5C2(001) were investigated at the levels of density functional theory. On the H2 and CO co-adsorbed surface, the formation of CH and CCO is the most favored initial steps. The subsequent steps of CCO coupling with C into CCCO and CCO hydrogenation into CCH2 and CHCH are competitive. Furthermore, CCH from CCH2 and CHCH dehydrogenation can couple with C to form CCCH. Since chain initiation from CO insertion obeys insertion mechanism, and chain propagation from CCH coupling obeys carbide mechanism, both mechanisms are operative and co-operative in Fischer–Tropsch synthesis. The carburized active surfaces can be regenerated and maintained by CO adsorption on the vacancy site, followed by hydrogenation into surface formyl (CHO) and successive dissociation into surface CH and O. In addition surface O can be hydrogenated into surface OH, and H2O formation from surface OH disproponation is energetically more favored than surface OH hydrogenation. [Copyright &y& Elsevier]

An industrial manganese-promoted ultrafine particle (UFP) iron catalyst for slurry phase Fischer–Tropsch synthesis was examined in a 1 dm3 stirred tank slurry reactor under various reaction conditions as a representative of industrial practice (533–573 K, 1.5–2.5 MPa, 1.0–2.5 l (STP)/g-cat h, H2/CO=0.65–2.0). The Fe–Mn UFP catalyst showed high CO conversion and gave high yields of C2–C4 alkenes, partially attributed to less secondary hydrogenation of alkenes during the slurry reaction. The catalyst displayed a slow deactivation rate with time on stream. The effects of reaction conditions on the catalytic activity, hydrocarbon product distribution and the ratio of alkene/alkane were discussed. [Copyright &y& Elsevier]

The structure and stability of seventeen facets of the Hägg carbide phase (χ-Fe 5 C 2 ) under the consideration of K 2 O promotion have been analyzed utilizing density functional theory approach and ab initio atomistic thermodynamics. On the basis of different interaction strengths of these facets with K 2 O promoter, the morphology of the χ-Fe 5 C 2 phase under the variation of K 2 O content has been predicated. At the surface Fe/K atomic ratio of 30, the morphology of the χ-Fe 5 C 2 phase can be modified apparently. Among the most exposed (1 1 1), (1 0 0), ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets, the ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets have more open surface structures as well as provide more surface Fe atoms and less carbon atoms. However, all these mostly exposed facets do not favor CO direct dissociation at low CO coverage. [ABSTRACT FROM AUTHOR]

Abstract: In Fischer–Tropsch synthesis (FTS), the prepared catalyst hematite (α-Fe2O3) originates from goethite (α-FeOOH) dehydration in calcinations process. The structure of α-FeOOH and its topotactic transformation into α-Fe2O3 under 300–900K have been modeled within the framework of density functional theory. Molecular dynamics simulations show that the transformation begins preferentially on the surface and the dehydration is associated with the formation of voids. The loss of H2O in α-FeOOH does not modify the morphology extremely with respect to that of the original α-FeOOH. The transformation process involves hydrogen migration with the formation of adsorbed water, followed by desorption of the water molecule. The calculated barriers for hydrogen migration (118kJ/mol) and water desorption (82kJ/mol) suggest a direct reaction path without the formation of intermediates. [Copyright &y& Elsevier]

Abstract: This paper designs four cases to investigate the performances of the polygeneration processes, which depend on the commercially ready technology to convert coal to liquid fuels (CTL) and electricity with CO2 sequestration. With Excel-Aspen Plus based models, mass and energy conversion are calculated in detail. The simulation shows that the thermal efficiency is down with the synfuels yield decrease though the electricity generation is increased. It also suggests that the largest low heat value (LHV) loss of coal occurs in the gasification unit. From the comparison of the four cases, prominent differences of coal energy transition appear in water–gas shift (WGS) units, Fischer–Tropsch (FT) synthesis and combined cycle processes. CO2 capture and vent are discussed and the results show that the vent amount of CO2 increases with the increase of percentage of the syngas going to produce electricity. The results also show that the ratio of carbon captured to total carbon increases from 58% to 93% which is an important contribution to cutting down the greenhouse gas vent. [Copyright &y& Elsevier]

Abstract: Kinetic Monte Carlo simulations were carried out for Fischer–Tropsch synthesis in a wide range of industrially relevant reaction conditions. The macroscopic performance of different reaction conditions obtained from simulations quite agrees with experimental trends. In all examined conditions, H and CO are dominant species on catalyst surface. The rest intermediates only exist in small amount, and are controlled by H and CO coverages. Activity and selectivity for Fischer–Tropsch synthesis and water gas shift reaction are determined by surface H/CO ratio. Reaction conditions can directly change surface coverage of H, CO, and vacant sites, and consequently exert an influence on macroscopic performance of iron catalyst. [Copyright &y& Elsevier]

Abstract: Mechanisms and kinetics for the reduction of a precipitated iron-based Fischer–Tropsch catalyst in H2 have been investigated using in situ Mössbauer effect spectroscopy (MES) and thermogravimetric (TG) method in the temperature range of 250–350°C. In situ MES results indicate that the reduction of paramagnetic (PM) α-Fe2O3 (70%) and superparamagnetic (spm) Fe3+ (30%) in the fresh catalyst proceed via different steps. PM α-Fe2O3 is firstly reduced to magnetite and then to metallic iron, while the reduction of spm Fe3+ proceeds in three consecutive steps: it is first reduced to magnetite with a significantly rapid rate, then to non-stoichiometric wüstite, and finally to metallic iron. The reduction of PM α-Fe2O3 to Fe3O4 can be described by a two-dimensional Avrami–Erofe’ ev phase change model (formation and growth of nuclei). However, the corresponding overall reduction, which includes the reduction of PM α-Fe2O3 and spm Fe3+ to Fe3O4, can be described by a three-dimensional phase-boundary-controlled reaction model based on the overall extraction ratio of oxygen. The difference between the two models selected for the PM α-Fe2O3 reduction and the corresponding overall reduction is attributed to the rapid reduction of spm Fe3+ to Fe3O4. For the reduction of PM magnetite to α-Fe and its corresponding overall reduction (including the reduction of PM and spm Fe3O4 to α-Fe), it is found that both of them follow the Avrami–Erofe’ ev phase change model (two-dimensional or three-dimensional). The value of apparent activation energy for the overall reduction has been calculated and compared with the literature data. [Copyright &y& Elsevier]

Abstract: The effect of co-feeding CO2 on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO2 partial pressure in the feed gas. It was found that co-feeding CO2 to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO2 from the feed gas. The addition of CO2 led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C19 + selectivity. It also slightly decreased CO2 formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO2 can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions. [Copyright &y& Elsevier]

Abstract: Density functional theory (DFT) calculations have been carried out on the adsorption of CO on the (100), (001) and (010) surfaces of Fe3C. Both (100) and (001) have surface iron and carbon atoms, while (010) has only surface iron atoms. At 1/5ML on (100), the most stable adsorption configuration has adsorbed CO at a three-fold site (three Fe atoms), followed by adsorbed surface ketenylidene at a four-fold site (three iron atoms and one carbon atom). At 1/6ML on (001), the most stable adsorption configuration has adsorbed CO at a four-fold site (four iron atoms). With increased coverage, adsorption at different sites becomes possible and close in energy. On the metallic (010) surface, both two-fold and three-fold adsorptions are close in energy. The electronic states of the most stable adsorption structures have been analyzed accordingly. [Copyright &y& Elsevier]

Abstract: The structural properties and iron–silica interaction of the precipitated iron catalysts incorporated with or without silica have been investigated by powder X-ray diffraction (XRD), Mössbauer spectroscopy, and temperature-programmed reduction (TPR) with H2 consumption quantitative analysis. The Fischer–Tropsch synthesis (FTS) performances of the catalysts were carried out in a slurry-phase continuously stirred tank reactor (CSTR). It was found that, for as-prepared samples, nano-particles of iron oxide mixtures (α- and/or γ-Fe2O3) formed on the silica-incorporated catalyst, whereas well-crystallized hematite (α-Fe2O3) is the only phase on the silica-free catalyst. After reduction in H2, the observed phase in the silica-free catalyst is α-Fe, whereas wüstite (Fe x O) and iron (II) silicate (Fe2SiO4) are the main phases in the silica-incorporated catalyst. In H2-TPR, the Fe3+ ions in the silica-incorporated catalyst are reduced to Fe (0) via Fe2+ ions as intermediate and the reduction extent of iron oxides obviously decreased comparing with the silica-free catalyst. The differences in structural properties and reduction behavior between the silica-free and the silica-incorporated catalysts may be due to the high dispersion effect of silica and the iron–silica interaction. The iron–silica interaction also decreases the FTS activity and improves the light hydrocarbon selectivity. [Copyright &y& Elsevier]

Abstract: A systematic study was undertaken to investigate the effects of Al2O3/SiO2 ratio on reduction, carburization and catalytic behavior of iron-based Fischer–Tropsch synthesis (FTS) catalysts promoted with potassium and copper. The catalysts were characterized by N2 physical adsorption, CO2 temperature-programmed desorption (TPD), H2 temperature-programmed reduction (TPR) and Mössbauer effect spectroscopy (MES). CO2-TPD indicated that Al2O3 binder has stronger acidity than SiO2 binder and weakens the surface basicity of the catalysts. H2-TPR profiles suggested that the lower Al2O3/SiO2 ratio promotes the reduction of Fe2O3→Fe3O4. With further increasing Al2O3/SiO2 ratio, the transformation of Fe2O3→Fe3O4 shifts to higher temperatures. The MES results showed that the increase of Al2O3/SiO2 ratio leads to the relatively large crystallite size of α-Fe2O3 and inhibits carburization of the catalyst. During reaction tests in a fixed bed reactor it was found that a maximum in catalyst activity is noted at the Al2O3/SiO2 ratio of 5/20 (weight basis). The selectivity to olefins shows a rapid decrease and the formations of methane and light hydrocarbons are promoted with increasing Al2O3/SiO2 ratio. The oxygenate selectivity in total products increases with increasing Al2O3/SiO2 ratio. [Copyright &y& Elsevier]

The kinetic experiments of Fischer–Tropsch synthesis (FTS) over an industrial Fe–Cu–K catalyst are carried out in a micro-fixed-bed reactor under the conditions as follows: temperature of 493–542 K, pressure of 10.9–30.9 bar, H2/CO feed ratio of 0.98–2.99, and space velocity of 4000–10 000 h−1. The effects of secondary reactions of olefins are investigated by co-feeding C2H4 and C3H6. A detailed kinetics model taking into account the increasingly proven evidence of the olefin re-adsorption mechanism is then proposed. In this model, different sites are assumed for FTS reactions and water gas shift (WGS) reaction, respectively. Rate expressions for FTS reactions are based on the carbide polymerisation mechanism, in which olefin re-adsorption is considered to be a reverse step of olefin desorption reaction. Rate expression for WGS reaction is based on the formate mechanism. An integral reactor model considering both FTS and WGS kinetics is used to describe the reaction system, and the simultaneous estimation of kinetic parameters is conducted with non-linear regression procedure. The optimal model shows that the rate determining steps in FTS reactions proceed via the desorption of hydrocarbon products and the adsorption of CO and the slowest step in WGS reaction is the desorption of gaseous carbon dioxide via formate intermediate species. The activation energies of FTS reactions and WGS reaction are in good agreement with literature values. [Copyright &y& Elsevier]