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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]

Abstract: Density functional theory calculations have been carried out on the reaction of ketene hydrogenation on Fe5C2(001) for the understanding of the Fischer–Tropsch synthesis mechanism. The main reaction pathway of ethene formation is CsH2CO→CCsH2 →CHCsH2 →CH2CsH2, and ethane formation follows CsH2CO→CCsH2 →CCsH3 →CCsH3 →CHCsH3 →CH2CsH3 →CH3CsH3, while that of ethanol is CsH2CO→[CsH3CO and/or CsH2CHO]→CsH3CHO→CsH3CH2O→CsH3CH2OH. Detailed comparison shows clearly that ketene dissociation with the formation of hydrocarbons is more favorable than the stepwise hydrogenation with the formation of ethanol. The expected product should be hydrocarbons rather than ethanol, in agreement with the experimental observation. [Copyright &y& Elsevier]

Abstract: Removal of bridging sulfur from the Mo edge of a Mo20S43 cluster has been investigated at the level of density functional theory. It is found that the reductive removal of bridging sulfur is not favored energetically but becomes favorable under CO adsorption. Due to the strong adsorption, CO can help the reductive removal of surface sulfur, and in turn, the reduced Mo edge can favor and facilitate the hydrogenation process of CO. [Copyright &y& Elsevier]

Abstract: The detailed transformation mechanism of methacrolein with methanol to methyl methacrylate over Pd was investigated by density functional theory (DFT) calculation as an attempt to gain more insight into the oxidative esterification reaction. The entire catalytic cycle and reaction energies are computed for various possible paths over Pd20 cluster. It is found that the hydrogen elimination of MAL following acyl group reaction with methoxyl appears to be an energetically favorable pathway in catalyst cycle. The formation of unsaturated alcohol by H addition, and that of saturated aldehyde through the Cn by methoxyl and H are also competitive. The formation of hemiacetal intermediate is not necessary for the production of MMA. [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 adsorption of isopropanol on the clean and hydrated γ-Al2O3 (100) and (110) surfaces was investigated at the level of density functional theory. It is found that isopropanol interacts with the γ-Al2O3 surface via its −OH group. The most stable adsorption site on the (100) and (110) surfaces is Al(3) and Al(4), respectively, with the −OH group of isopropanol orientated to surface oxygen atom. The computed adsorption energies correlate well with the energy level of the surface Lewis sites. On the (100) surface with OH coverage of 8.8, Al(5) is the most stable adsorption site. On the (110) surface with OH coverages of 8.9 and 11.8, Al(2) and Al(1) are the available sites for adsorption. It is to note that water has much larger adsorption energies than isopropanol on both surfaces. [Copyright &y& Elsevier]

Abstract: Hydrogen adsorption on the Fetet1- and Feoct2-terminated Fe3O4(111) surfaces has been computed at the level of density functional theory. At 2/5 monolayer (ML), the most favored hydrogen adsorption on the Fetet1-terminated surface is homolytic and dissociative on surface O atoms, while other adsorption modes become possible at higher coverages. On the Feoct2-terminated surface at 1/3 ML, hydrogen prefers to adsorb heterolytically and dissociatively on surface Fe and O atoms, while other adsorption modes become in close energy at 2/3 and 1 ML. The Fetet1-terminated surface is more favored than the Feoct2-terminated surface for hydrogen adsorption. The adsorption mechanism has been analyzed on the basis of the calculated local density of state. [Copyright &y& Elsevier]

Abstract: Spin-polarized density functional theory has been used to characterize hydrogen adsorption on the Fe3C(100), Fe3C(001) and Fe3C(010) surfaces. It is found that hydrogen adsorbs dissociatively on the three surfaces. On the Fe3C(100) surface, the most stable surface species is CH at 1/3 or 2/3 monolayer, and CH4 at 1 monolayer; in contrast on the Fe3C(001) surface the most stable surface intermediates are 3-fold H at 1/3 monolayer, and CH at 2/3 monolayer, and CH4 at 1 monolayer. On the metallic Fe3C(010) surface, hydrogen adsorbs at 2-fold or 3-fold sites. The computed energetic order of hydrogen adsorption is Fe3C(100)>Fe3C(010)>Fe3C(001), and this differs from that of CO adsorption (Fe3C(010)>Fe3C(100)>Fe3C(001)). [Copyright &y& Elsevier]

Abstract: The effects of reaction conditions on the performance of an Fe-Mn catalyst for Fischer-Tropsch synthesis (FTS) have been investigated in a continuous spinning basket reactor. Experiments were carried out under a wide range of industrially relevant reaction conditions including reaction temperature of 533–573K, pressure of 0.93–2.96MPa, inlet H2/CO molar ratio of 0.80–2.50 and space velocity of (0.46–1.85)×10−3 Nm3 kgcat −1 s−1. The selectivities to low molecular weight hydrocarbons basically increased with the increase of reaction temperature, total pressure and inlet H2/CO ratio, except for ethylene. The FTS reaction rate and the overall oxygenates formation rate increased with increasing reaction temperature, pressure, space velocity and H2/CO ratio on the whole. It was found that the catalyst has relatively high activity and selectivity in producing light alkenes. It also exhibited excellent stability during the 700h run at H2/CO ratio of 1.00, which indicates that the catalyst has good longevity and is suitable for syngas derived from coal with low H2/CO ratio to produce light olefins and high-quality liquid fuels. Meanwhile, it may have potential promising applications in advanced stirred tank slurry reactors. In addition, the detailed investigation of FTS reaction performance for the Fe-Mn catalyst can provide full and accurate basic information for industrial scale-up. [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: An industrial iron-based catalyst (100Fe/5Cu/6K/16SiO2, by weight) was characterized after reduction at different temperatures and after Fischer–Tropsch synthesis (FTS) in a stirred tank slurry reactor (STSR). The BET surface area and pore volume of the catalyst decreases with increasing reduction temperature, and the contrary trend was found for pore size. The iron phase compositions of catalysts reduced with syngas were strongly dependent on pretreatment conditions employed. Pretreatment with syngas at lower temperature prevents iron catalyst activation. Carburization was intensified with the increase in reduction temperature. The formation of iron carbides in reduced catalyst was necessary for obtaining stable high FTS activity. The relationship between the amount of CO2 in tail gas during activation and the Fe3+ (spm) content in the reduced catalyst was observed. The rapid carburization at high reduction temperature resulted in the formation of a superparamagnetic Fe3+ core and an iron carbide layer of the reduced catalyst. FTS activity decreased with the increase in the reduction temperature, but the stability distinctly improved. It was found that the working catalyst loss in the heavier waxy products resulted in higher deactivation rate of the catalyst reduced at lower temperature. With the increase in the reduction temperature, the product distribution shifted towards the lower molecular weight products. [Copyright &y& Elsevier]

Abstract: The effect of adding SiO2 to a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated using N2 physical adsorption, H2 differential thermogravimetric analysis, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer spectroscopy. The FTS performances of the catalysts with or without SiO2 were compared in a fixed bed reactor. The characterization results indicated that SiO2 facilitates the high dispersion of Fe2O3 and significantly influences the Fe/Cu and Fe/K contacts, which play an important role in the surface basicity, reduction and carburization behaviors, as well as the FTS performances. The incorporation of SiO2 enhances the Fe/Cu contact, further enlarges the H2 adsorption and promotes the reduction of Fe2O3 →FeO x , while the transformation of FeO x →Fe is suppressed probably due to the strong Fe–SiO2 interaction. SiO2 indirectly weakens the surface basicity and severely suppresses the carburization and CO adsorption of the catalyst. In the FTS reaction, it was found that SiO2 decreases the FTS initial activity but improves the catalyst stability. Due to the lower surface basicity than the catalyst without SiO2, the catalyst incorporated with SiO2 has higher selectivity to light hydrocarbons and methane and decreased selectivity to the olefins and heavy hydrocarbons. [Copyright &y& Elsevier]

Abstract: A 24T model containing a complete two-layered 12-membered ring connected by an 8-membered side pocket was used to represent the channel of the MOR framework. On the basis of this model, the structures of the acid sites in the main channels of Li- and Na-MOR as well as their interaction with pyridine probe molecule were investigated using the ONIOM2 (QM/QM; B3LYP/6-31G(d,p):HF/3-21G) and ONIOM2 (QM/MM; B3LYP/6-31G(d,p):UFF) methods. There are two stable structures for the Li-MOR cluster; one has Li+ interacting with two oxygen atoms around Al, and the other has Li+ interacting with four oxygen atoms belonging to a five-membered ring. This four-fold Li+ coordination is consistent with Na+ in Na-MOR, and is the same as the Na+ coordination in the intersection model of Na-ZSM-5. The ONIOM2 (QM/MM) method overestimates the adsorption energies of pyridine (195.0/192.8 and 170.6kJ/mol for Li-MOR and Na-MOR, respectively), while the ONIOM2 (QM/QM) method gives reasonable results (171.6/154.7 and 131.3kJ/mol, respectively) compared to the available experimental values (153–195 and 120kJ/mol, respectively). The structural parameters of the adsorption complexes showed that coordination of the alkali ion to the nitrogen atom of pyridine dominates the overall interaction between the zeolite and pyridine, and the adsorption complexes were further stabilized by hydrogen bonding between the hydrogen atoms of pyridine and the lattice oxygen atoms of the zeolite framework. [Copyright &y& Elsevier]

Abstract: Density functional theory computations have been carried on the hydrogen adsorption mechanism on a Mo27S54 single layer cluster, which has a size (15–20Å) close to that of real catalysts. For one molecule of H2 adsorption, the most stable adsorption form (E ads =−27.2kcal/mol) is the homolytic dissociation on the S edge with the hydrogen atoms keeping away from the plane consisting of all Mo atoms, followed by the heterolytic dissociation (E ads =−26.1kcal/mol) on the intersection of S and Mo edges with the formation of Moc–H and Sc–H bonds. At high coverage with two and three H2, however, dissociated hydrogen adsorption on the Mo sites are much more favored thermodynamically than on the S sites. Moreover, the corner sites are more favored thermodynamically for hydrogen adsorption and formation of coordinatively unsaturated sites than the edge sites. In addition, the activation energy of H2 dissociation and hydrogen transfer processes have been computed to be 2.7–19.2kcal/mol, and these rather low barriers indicate the enhanced mobility of the adsorbed hydrogen on the surface. [Copyright &y& Elsevier]

Abstract: The structure and adsorption of Mo27S(54−x) (x =1–6) clusters have been investigated using density functional theory method. It was found that considerable relaxation occurred at the () edge. The activity has been analyzed on the basis of frontier molecular orbital properties. The results suggest that the possible catalytic sites might be situated between two vicinal 4-fold coordinative unsaturated site CUS. The formation of Mo27S(54−x) (x =4–6) from Mo27S54 is easy in the presence of atomic hydrogen, but difficult under molecular hydrogen. But the situation is different from the case of Mo27S(54−x) formation (x =1–3) that is facile under both the atomic and molecular hydrogen reagent (under the normal HYD/HDS condition). The adsorption of thiophene at various vacancies on the () edge of MoS2 represents that thiophene is unstable to flatly adsorb at the vacancy of Mo27S50. Although uprightly adsorbed thiophene is adsorbed weakly, the flatly adsorbed thiohene is strongly activated when adsorbed between two vicinal 4-fold CUS (9, 10). On the basis of Hirshfeld charge analysis, the donation and back-donation between the thiophene and the substrate might account for the activation of thiophene. [Copyright &y& Elsevier]

Abstract: The reactivity of furfural and its reaction intermediates is separately investigated in a fixed-bed reactor over a commercial catalyst (C1: Cu/Zn/Al/Ca/Na=59:33:6:1:1, atomic ratio) and a self-made multicomponent one (C2: Cu/Cr/Ni/Zn/Fe=43:45:8:3:1, atomic ratio). Some interesting results are obtained in this work. Firstly, both furfural and furfuryl alcohol can eliminate carbon monoxide to produce furan. Secondly, tetrahydrofuran and 2-methyltetrahydrofuran are all very stable, and n-butanol mainly derives from furan rather than tetrahydrofuran. Thirdly, 2-pentanone, 2-methyltetrahydrofuran, 1-pentanol and 2-pentanol are all the hydrogenation products of 2-methylfuran. Finally, with C2 catalyst containing the Ni element, the main reaction product of tetrahydrofurfuryl alcohol is tetrahydofuran, while δ-valerolactone is the main product with C1 one. This work has unified some conflicting mechanisms and discussed the origin of some interesting products, which is important to understand the mechanism and reaction pathway of furfural hydrogenation, and to provide an instruction for the design of new catalytic formulations. [Copyright &y& Elsevier]

Abstract: The two-layered ONIOM method (B3LYP/6-31G(d,p):HF/3-21G) is used to study the interaction of amines (NH3, MeNH2, Me2NH and Me3N) with H-[Ga]MOR. The optimization of the local structure of H-[Ga]MOR cluster leads to two stable bridging hydroxyl sites (O10H and O2H) in the zeolite framework, being different from that of H-[Al]MOR. In the adsorption complexes, all amines are protonated by the acidic proton of H-[Ga]MOR, and the protonated amines (HNR3 +) are stabilized by hydrogen bonds between the negatively charged zeolite oxygen atoms and the hydrogen atoms of the Ne:glyph name="sbnd" />H bonds in the adsorbates. This interaction is confirmed by the structure of the adsorption complexes as well as the calculated IR stretching frequencies. The calculated adsorption energies of amines agree reasonably with the available experimental data. It is found that NH3 prefers to adsorb at the O2H Brønsted site, while Me2NH and Me3N prefer to adsorb at the O10H site, and MeNH2 can be in equilibrium between O2H and O10H. The relative order of the basicity of amines on the basis of the computed adsorption energies agrees well with the experiments, but differs from those in the gas phase (proton affinity) and in solvents (pK a). [Copyright &y& Elsevier]