Search

Highlights: [•] Two 4000bbl/d demonstration scale projects using the MTFT technology are being successfully operated in China. [•] DFT studies on adsorption and reaction mechanisms on iron phases can provide guidance to development of F–T catalysts. [•] Kinetics based on theoretical and experimental results can be very important tools for F–T reactor and process development. [ABSTRACT FROM AUTHOR]

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: The direct and H-mediated dissociation of CO2 on Ni(211) were investigated at the level of density functional theory. Although formate (HCOO) formation via CO2 hydrogenation was widely reported for CO2 adsorption on metal surfaces, it is found that on Ni(211) HCOO dissociation into CHO and O is much difficult, while direct dissociation of adsorbed CO2 into CO and O is more favorable. It is also found that the degree of electron transfer from surface to adsorbed CO2 correlates with the elongation of C–O bond lengths and the reduction of the CO2 dissociation barrier. [Copyright &y& Elsevier]

On the basis of spin-polarized periodic density functional theory including the latest dispersion correction (PBE-D3), the mechanisms of H 2 , O 2 , H 2 O, CO 2 , CO, CH 4 and C 2 H x dissociative adsorption on the hexagonal Mo 2 C surface have been computed. In our study we used the metallic Mo 2 C(001) surface as well as the Mo 2 C(101) surface with Mo/C = 1/1 ratio. It is found that the dissociative adsorptions of these small molecules are exothermic and have low barriers; and the Mo 2 C(001) surface has much stronger dissociative adsorption than the Mo 2 C(101) surface. In contrast to the Mo 2 C(001) surface, OH + H and O + 2H can form equilibrium on Mo 2 C(101) surface. For C 2 H x dissociative adsorption on the Mo 2 C(001) surface, C H bond dissociation is kinetically much more favorable than the C C bond dissociation, and the optimum C 2 H 6 dissociation route follows the order of C 2 H 6 → CH 3 CH 2 + H → C 2 H 4 + 2H → CH 2 CH + 3H → C 2 H 2 + 4H → C 2 H + 5H→ C 2 + 6H. Due to the very strong dissociative adsorption energies, both surfaces can be oxidized easily by using H 2 O; and high oxygen coverage can be expected. The Mo 2 C(001) surface can uptake more surface O atoms than the Mo 2 C(101) surface. These surfaces can also be carburized by using CH 4 , albeit in less extent. On the surface with co-adsorbed CO 2 + 4H; CO 2 dissociation (CO 2 → CO + O → C + 2O) is more favorable than the hydrogenation of CO 2 (CO 2 + H → HCOO or COOH) and CO (CO + H → HCO or COH). It is noted that CO 2 hydrogenation towards CH 4 formation is unlikely on the Mo 2 C(001) surface, while the effective barrier of surface C hydrogenation on the Mo 2 C(101) surface can be reduced by 2O and 2OH pre-covered surfaces. [ABSTRACT FROM AUTHOR]

Abstract: The structure and energy of iron promoted γ-Al2O3 surface have been investigated in the framework of spin-polarized density functional theory using periodic slab model. It is found that the substitution of surface Al3+ by Fe3+ is thermodynamically accessible, and the promoted surfaces become metallic in nature. Full substitution of Al3+ by Fe3+ forms monolayer Fe2O3 surface. H2O adsorption leads to reconstruction of surface structures. On the basis of the computed H2O adsorption energies, the hydroxylated Fe2O3 oxide layer surface is found to be more easily dehydrated than the hydroxylated γ-Al2O3 surface. [Copyright &y& Elsevier]

Abstract: The adsorption and interaction of alkali metals (Li, Na, K, Rb and Cs) with the Fetet1-terminated Fe3O4(111) surface have been computed at the level of density functional theory. At low coverage, adsorption of alkali metals on site-1 (Oa–Oc–Od) is energetically more favorable than on site-2 (Oa–Oc–Od). Li has the strongest adsorption energy, followed by K, Rb, Cs and Na. The computed net charges show that the alkali metals can donate electrons to surface Fe and O atoms in the order of Li

Abstract: NO probe adsorption on the triangular molybdenum sulfide cluster (Mo28S60) has been computed at the density functional level of theory. At the corner of Mo28S60, dinitrosyl adsorption is energetically more favored than twice the mononitrosyl adsorption, while mononitrosyl adsorption at the edges of hexagonal MoS x clusters is more favored than dinitrosyl adsorption. [Copyright &y& Elsevier]

Abstract: CH4 dissociation on Ni surfaces, which is important in CH4 reforming reactions, was discussed by using density functional theory. It was found that the CH x species were changed to anions after chemisorption. The site preference of CH x (x =0–3) species on Ni(111), Ni(100) and Ni(110) was located on the basis of the computed chemisorption energies. Ni(100) is the most preferred surface for CH4 dissociation, compared to Ni(110) and the widely investigated Ni(111). [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]

• The structure–activity relationship of Fe nanoparticles in the CO activation process was investigated. • Structures includes four aspects: morphologies, sizes, defects, and hetero atoms. • Line dislocation and vacancies suggest an effective way to tune the CO dissociation rate. • CO 2 formation: Eley–Rideal vs. Langmuir–Hinshelwood mechanism. The structure–activity relationship is crucial in catalytic performance and material design but still largely obscure due to the complexity of heterogeneous catalytic systems. CO activation occurs widely in Fischer–Tropsch reactions and pyrometallurgy, and it is a key to understanding carburization. Here, we investigate the structure–activity relationship in Fe nanoparticles by reactive molecular dynamics simulations. We focus on two activities, the adsorption and dissociation of CO, and four structural characteristics, morphologies, sizes, defects, and heteroatoms. The results show that CO adsorption and dissociation varies with the change of nanoparticles. Line dislocation and vacancies can strikingly boost CO dissociation, suggesting an effective way to tune the CO dissociation rate. Further analysis shows that the Eley–Rideal mechanism possibly works in the early periods, followed by the Langmuir–Hinshelwood mechanism in the later periods for CO 2 formation. Our results shed light on the mechanism and possible optimization of the carburization of iron. [ABSTRACT FROM AUTHOR]

[Display omitted] • Temperature-dependent ethylene dissociative adsorption on ruthenium was computed quantitatively. • Revised PBE can best quantify the adsorption energy of ethylene, H 2 and CO. • Computed activation barriers of the dissociation of CH 2 CH 2 * and CH 3 C* agree quantitively with experiments. • CO co-adsorption stabilizes CH 3 C* by blocking surface sites and raising the dissociation barrier. • BEP relationship is found for the formation of CH 3 C* and HCC under the co-adsorbed CO and hydrogen. The dissociative adsorption of ethylene onto Ru(1 1 1) at different temperatures was computed systematically at the first time. At 105 K, ethylene dissociative adsorption has the co-adsorbed CH 2 C*+2H* and CH 3 C*+H* as the first and second stable surface intermediates. At over 330 K, CH 3 C*+H* is converted back into CH 2 C* accompanied by H 2 desorption and the subsequent dissociation of CH 2 C* into HCC*, HC*+C* and 2C*. The computed Arrhenius activation barriers of the dissociation of CH 2 CH 2 (0.18 vs. 0.22 ± 0.04 eV) and CH 3 C (0.54 vs. 0.52 ± 0.04 eV) agree perfectly with the available experimental values, and CH 3 C* represents the most stable surface species. Under CO co-adsorption, the most stable surface species are the co-adsorbed CH 3 C*+H*+3CO*. It is found that CO co-adsorption promotes H 2 desorption and stabilizes CH 3 C* by blocking the surface sites for dissociation and raises the dissociation barrier compared to the clean surface (0.78 vs 0.54 eV). Brønsted–Evans–Polanyi relationship between the activation Gibbs free energy barrier and reaction Gibbs free energy is found for CH 2 C*+2H*+ n CO* = CH 3 C*+H*+ n CO* and CH 2 C*+2H*+ n CO* = HCC*+3H*+ n CO* (n = 0–3). Ethylene adsorption has di-σ and π adsorption configurations in very close energy, and H 2 has adsorption energy of about 0.90 eV. [ABSTRACT FROM AUTHOR]

Periodic density functional theory (revised PBE; RPBE) and ab initio atomistic thermodynamics were used to assess the dissociative adsorption of H 2 O and CO 2 on a (5 × 5) Ru(0001) surface model. For H 2 O dissociation on clean Ru(0001) [H 2 O → OH + H → O + 2H], the first step has lower barrier than the second step (0.75 vs. 0.80 eV). On O pre-covered Ru(0001), H 2 O dissociation [H 2 O + O → 2OH] has lower barrier (0.62 eV) and is more exothermic (−0.29 vs. −0.15 eV). CO 2 dissociation on clean Ru(0001) [CO 2 → CO + O] needs low barrier (0.23 eV) and is highly exothermic by 1.47 eV. In turn, CO 2 formation from surface O removal by CO has barrier of 1.70 eV, in agreement with the experimentally detected 1.80 ± 0.15 eV. That the computed CO desorption energy (1.55 eV) is smaller than CO oxidation barrier is in agreement with the experimental finding. The computed desorption temperatures of H 2 O, CO and CO 2 under ultrahigh vacuum conditions agree perfectly with the experiments. In addition, high oxygen and OH pre-coverage do not significantly affect the energetics of the dissociation of H 2 O and CO 2 . The excellent agreement between theory and experiment confirms the applied models and methods, and in turn, gives the reliability and confidence about the further predicted results. [ABSTRACT FROM AUTHOR]

Highlights: [•] H2O dissociative adsorption on the χ-Fe5C2(010) surface was investigated. [•] The χ-Fe5C2(010) surface has both iron and carbon regions. [•] Surface iron region is active for H2O dissociative adsorption and H2 formation. [•] Pre-adsorbed oxygen atoms play a significant role for further H2O adsorption. [•] Surface carbon region is inactive for H2O dissociative adsorption. [ABSTRACT FROM AUTHOR]

Abstract: Fe/SiO2 catalysts with different Fe/Si molar ratios were used to investigate the effects of silica on chemical/structural properties and Fischer–Tropsch synthesis (FTS) performance of iron-based catalysts. In the chemical aspect, silica interacts with Fe species by the formation of Feh name="sbnd" />Si structure, which further transforms into an Fe2SiO4 phase during FTS reaction. The interaction largely disturbs the electronic structure of Fe atoms in iron oxide phases and in turn resists the reduction and activation of catalysts. In the structural aspect, silica increases the dispersion of Fe species and inhibits the aggregation of active iron particles. Addition of silica largely changes the adsorption sites of catalysts, i.e., decreases the number of weak H adsorption sites but improves the adsorption strengths of H, C, and O on reduced or carburized catalysts. With increasing amounts of silica, the chemical and structural effects cause the firstly decrease and then the increase of the initial FTS activity and the selectivities of heavy hydrocarbons and olefins during the Fischer–Tropsch synthesis. In addition, an important finding is that a proper amount of silica apparently suppresses the methane selectivity and stabilizes the iron carbide in the FTS reaction. [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: 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: 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: Reaction pathways of CO2 reforming of CH4 on Ni(111) were investigated by using density functional theory calculation. The computed kinetic parameters agree with the available experimental data, and a new and simplified mechanism was proposed on the basis of computed energy barriers. The first step is CO2 dissociation into surface CO and O (CO2 →CO+O) and CH4 sequentially dissociation into surface CH and H (CH4 →CH3 →CH2 →CH). The second step is CH oxygenation into CHO (CH+O→CHO), which is more favored than its dissociation into C and hydrogen (CH→C+H). The third step is the dissociation of CHO into surface CO and H (CHO→CO+H). Finally, H2 and CO desorb from Ni(111) and form free H2 and CO. The rate-determining step is the CH4 dissociative adsorption, and the key intermediate is surface adsorbed CHO. Parameters, which might modify the proposed mechanism, have been analyzed. In addition, the formation, deposition and elimination of surface carbon have been discussed accordingly. [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: Adsorption of oxygen and CO on Mo- and C-terminated α-Mo2C(0001) has been investigated at the density functional level of theory. On the Mo-terminated surface, the most stable adsorption configuration (1) has atomic oxygen capping the threefold Mo hollow position over a second layer carbon atom (V C, C vacancy site). On the C-terminated surface, the most stable adsorption configuration (8) has atomic oxygen capping the threefold Mo hollow site over a first layer carbon atom (H C, C hollow site). For CO adsorption, the most stable configuration (17) has CO capping structure over carbon vacancy (V C) on the Mo-terminated surface, but atop of the surface carbon with the formation of ketenylidene (Ch name="dbnd" />O) species on the C-terminated surface (21). Analysis of density of states reveals the metallic character of the clean and oxygen doped surfaces and the activation of the surfaces by oxygen adsorption. [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]

Abstract: A systematic study of CO adsorption on the Fe(111) surface at different coverage is carried out with density functional theory. At 1/3 and 1/2 monolayer (ML) coverage, the shallow-hollow adsorption is the most stable site, while both shallow-hollow and bridge adsorption can coexist at 1 ML. Energetically, the bridge site rather than the suggested deep-hollow represents the reasonable adsorption configuration. In contrast, bent CO on-top and triply capping adsorptions are the most favored forms at 2 ML. The binding mechanism of CO on the Fe(111) surface is also analyzed. [Copyright &y& Elsevier]

• Coverage dependent hydrogen adsorption on Ir(111), Ir(100) and Ir(110) surfaces. • Change of adsorption site on Ir(100) from low to high coverage. • Surface dependent repulsive interaction on Ir(111). • Surface dependent attractive interaction on Ir(100) and Ir(110). • Regular line-shaped adsorption structures were found on Ir(100) and Ir(110). Hydrogen adsorption on the perfect Ir(111) as well as the metastable and unreconstructed Ir(100) and Ir(110) surfaces up to saturation coverage has been systematically computed using periodic density functional theory and ab initio atomistic thermodynamics for understanding the interaction mechanism of hydrogen on iridium surfaces. On the Ir(111) surface including van der Waals dispersion, hydrogen adsorption prefers the threefold hollow sites at low coverage and the top sites at high coverage; in agreement with the experiments (Phys. Rev. B 60 (1999) 14016). The computed adsorption energy and desorption temperature of hydrogen agree with the experiments [−0.57 (fcc-3H) and −0.53 (hcp-3H) vs. −0.55 eV; 180 and 325 K vs. 190 and 310 K, respectively]. On the Ir (100) surface, the bridge adsorption sites are preferred in the whole coverage range, in agreement with the LEED pattern (Phys. Rev. B 73 (2006) 75430), however, adsorption energy and desorption temperature are slightly overestimated by including van der Waals dispersion (−1.50 vs. −1.02 ± 0.15 eV; 470 vs. 425 – 389 K). On the Ir(110) surface, short-bridge sites are preferred at low coverage and the top sites become dominant at high coverage, and the calculated desorption temperatures are close to experiments by including van der Waals dispersion (210 and 365 K vs. 220 and 375 K). At low coverage, the different configurations of hydrogen adsorption on the Ir(111) have the similar energies, indicating their negligible repulsive interaction, while the Ir(100) and Ir(110) surfaces prefer regular line-shape adsorption configurations due to attractive interaction, and such adsorption configurations have not been observed experimentally. Our results show that differences in adsorption configurations and energies are associated with their differences in surface structures, and in turn explain the need of different methods in computing the adsorption properties on different surfaces. Such surface-dependent properties should also be possible on other metal surfaces. Image, graphical abstract [ABSTRACT FROM AUTHOR]