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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: 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]

Abstract: The catalytic hydrodesulfurization (HDS) reactions of COS and CS2 with Mo3S9 cluster have been systematically investigated as a part of HDS of industrial feedstocks by DFT-based method. A detailed description of mechanisms is given, and different pathways are discussed and compared. It is found that the S in COS and CS2 will be hydrogenated, and the reactions will lead to formations of H2S, CO and CS. Comparing with COS, CS2 reacts with lower activation free energy and shows more reaction activity. [Copyright &y& Elsevier]

Abstract: The reaction mechanism of thiophene hydrodesulfurization (HDS) on a Mo3S9 model catalyst has been investigated at the GGA level of density functional theory. On the basis of the computed energetic data, 2,5-dihydrothiophene has been identified as the most important intermediate of HDS processes, and it is responsible for the direct formation of butadiene as the principal product. Neither the stepwise formation of cis-2-butene nor that of butane is competitive. However, butene (or butane) can be obtained from the hydrogenation of the formed butadiene. [Copyright &y& Elsevier]

Abstract: The entire catalytic hydrodesulfurization (HDS) cycle of benzothiophene over Mo3S9 cluster has been investigated at the level of density functional theory. It is found that the hydrogenation and hydrogenolysis route with dihydrobenzothiophene (DHBT) as intermediate is more favored than the hydrogenolysis and hydrogenation route with styrene as intermediate. The stepwise hydrogenolysis of SB–C9 (C(aryl)–S) bond in DHBT leading to the formation of 2-phenylethanethiol is more favored than that of SB–C2 (C(alkyl)–S) bond with 2-ethylthiophenol as intermediate. The computed activation free energy and rate constant of benzothiophene hydrogenation to DHBT agree well with the available experimental data. The difference in HDS reactivity between benzothiophene and thiophene has been discussed. [Copyright &y& Elsevier]

Highlights: [•] Surface terminations of orthorhombic Mo2C catalysts have been computed. [•] (110)-Mo and (100)-Mo terminations are stable at low carbon chemical potential. [•] (111)-Mo/C and (110)-Mo/C terminations are stable at high carbon chemical potential. [•] CO activation on Mo terminations is dissociation controlled zero-order kinetic. [•] CO activation on Mo/C terminations is adsorption controlled first-order kinetic. [ABSTRACT FROM AUTHOR]

Abstract: Spin-polarized density functional theory calculations have been performed to investigate carbon deposition and carbide formation on Fe(100) and Fe(110) at different carbon coverage. On Fe(100) with increasing carbon coverage, the most stable carbon adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and surface carbon clustering is not favored thermodynamically. However, surface carbon clustering is more favored kinetically than carbon diffusion; and carbon diffusion into subsurface and surface carbon clustering become competitive. On Fe(110) with increasing surface carbon coverage, the most stable adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and this process is favored both kinetically and thermodynamically, and surface carbon clustering is neither favored nor competitive. Surface carbon deposition might form on Fe(100), while carbide formation might be found on Fe(110). [Copyright &y& Elsevier]

Abstract: The reactivity of surface OH in CH4 reforming reactions was investigated by using density functional theory calculation. The key reaction pathway from CH4 into syngas by surface OH follows CH4 →CH→CHOH→CHO→CO, which is similar with the pathway induced by surface O in CO2 reforming of CH4 (CH4 →CH→CHO→CO). Surface OH decreases the possibility of CH dehydrogenation into surface carbon. Compared to surface O and OH, surface H can eliminate surface carbon deposition more efficiently. [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: Spin-polarized density functional theory calculations were performed to investigate CO dissociation on clean and hydrogen precovered Fe(111) at 1/3 monolayer coverage. On clean Fe(111), the adsorbed CO first diffuses from the shallow-hollow site to the bridge-like site by elevating 0.20 eV in energy, and then dissociates into C and O atoms by overcoming a barrier of 1.53 eV. Interestingly, the CO dissociation process is accelerated in the presence of H2 via intermediate CHOads (COads + 2Hads → CHOads + Hads → CHads + Oads + Hads). This stepwise path is kinetically more favored with the lowest barrier of 1.17 eV. In contrast, the previously suggested paths, COads + 2Hads → Cads + Oads + 2Hads and COads + 2Hads → Cads + OHads + Hads, are not competitive due to higher barriers (1.76 and 1.79 eV, respectively). The activity of different low-index Fe surfaces toward CO is also compared. [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]

[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]

Abstract The surface energies and the morphologies of three iron oxides (α-Fe 2 O 3 , Fe 3 O 4 , and FeO) are systematically investigated by Density Functional Theory (DFT) employing PBE and PBE+U methods. The surface energies are computed under various O chemical potential which is directly related to atmosphere condition. Compared with available experimental observations, PBE provides reasonable prediction on surface stability of all three oxides while the results of PBE+U are not well consistent with experiments for Fe 2 O 3. Based on the Wulff construction and calculated surface energies, the micro-crystal morphologies of three iron oxides are predicted at various oxygen chemical potential. Subsequently, on basis of the data of surface energies, the effect of surface free energies on phase equilibrium of iron oxides nanoparticles are investigated. These results suggest that particle sizes of nanometer have a significant influence on the thermodynamic equilibrium shifting due to their large surfaces energies contributions. This principle can guide people control the phase transition of iron oxides by adjusting surface energies contributions by changing the sizes of particles. Graphical abstract Unlabelled Image Highlights • PBE provides more reasonable prediction on surface stability of three iron oxides (α-Fe 2 O 3 , Fe 3 O 4 , and FeO) than PBE+U. • O-terminations are generally more stable than the Fe-terminations over a wide range of Δμ O for three iron oxides. • The Wulff morphology of iron oxides is a function of O chemical potential. • Particle sizes of nanometer have a significant influence on the thermodynamic equilibrium shifting. [ABSTRACT FROM AUTHOR]

• Another available source of C 1 species is by HCO → CH + O, apart from CO → C + O. • Thermo stability of CH 2 species is a dominant factor in the CH 4 formation. • Different mechanisms of C 1 + C 1 couplings are found on the Fe surfaces. • Exposition of the Fe facets could tune CH 4 selectivity. To tune CH 4 selectivity of Fe-based Fischer-Tropsch synthesis (FTS) in the initial stage is of prime scientific and industrial importance to further improve the catalyst performance. Herein, distribution of CH 4 selectivity on the metallic Fe nanoparticle is predicted by DFT calculations and micro-kinetics analysis about the competition between C 1 hydrogenations and C 1 + C 1 couplings on abundant Fe surfaces including Fe(1 0 0), Fe(1 1 0), Fe(1 1 1), Fe(2 1 1), and Fe(3 1 0). The results show that HCO mechanism (HCO → CH + O) is an available source of C 1 species apart from CO direct dissociation. These Fe surfaces exhibit high effective barriers for CH 4 formation, which is linearly correlated to the thermal stability of CH 2 species. However, carbon chain prolongation on the more stable surfaces greatly depends on the coupling of C and CH species. On the less stable Fe(1 1 1) surface, the CO + C coupling is the main route for chain prolongation. Utilizing the effective barrier difference between the CH 4 formation and the most feasible C 1 + C 1 coupling as a descriptor of CH 4 selectivity, it is quantified that CH 4 selectivity decreases in sequence of Fe(1 0 0) > Fe(2 1 1) > Fe(1 1 0) > Fe(3 1 0) > Fe(1 1 1). It is revealed that thermal stability of the CH 2 species and exposition of the Fe facets could play essential roles in tuning CH 4 selectivity. Trying to expand the area of Fe(2 1 1), Fe(3 1 0) and especially Fe(1 1 1) surfaces would greatly suppress CH 4 selectivity without a decrease of activity. This work provides new insights and design principles for the Fe-based FTS catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Carbon permeation into iron, a very important initial stage in iron-catalyzed heterogeneous reactions such as Fischer-Tropsch synthesis (FTS), is explored theoretically, to extend our thermodynamic and kinetic understanding of the process. The interaction of C atoms with five model surfaces (Fe (100), (110), (111), (211), (310)) was studied in six distinct ways. In the first, the random deposition of C atoms on the Fe surfaces was simulated by molecular dynamics, with C atoms released gradually. It shows that the early stages of carburization is a C permeation process, without much disturbance to the Fe surfaces. In the second approach, C atoms were approached to the surfaces sequentially. They bind readily (by 7-9 eV per C) to the surfaces, but to a different extent—strongest on Fe (100), and weakest on Fe (111). Addition of further C atoms proceeds with a slightly decreasing magnitude of the chemisorption energy, because of the increasing positive charges on the Fe atoms. At a certain coverage, different on each surface, C atoms prefer in calculation to go subsurface. C2 units formed on some of the surfaces. In a third approach, detailed transition paths of C permeation subsurface were calculated, with associated barriers in the order Fe (100) > (111) > (310) > (211) > (110). Differences in stacking geometries of the Fe layers in these surfaces appear to be the main cause of the variation. Comparing C permeation with surface migration on clean surfaces, the barrier of the former is smaller than that of the latter for most of the surfaces, except Fe (111). At intermediate C coverage, the (100) surface also prefers migration to permeation. In a fourth approach, we calculate that with increasing carbon chemical potential, the surface energies of iron (110), (111), and (211) surfaces decrease, while those of (100) and (310) first decrease, then increase. Based on these surface energies, a Wulff construction of nanoparticle facets is made. In a fifth approach, the position in energy of the d-band centers of the Fe surfaces upon C permeation was studied. For all the surfaces, the d-band centers move away from the Fermi level with increasing C coverage, and start to resemble those of the bulk carbide phases at high C coverage. In the last approach, we show that C permeation not only lowers the barriers of model reactions for CH4 formation and C-C chain propagation, two competing processes in FTS, but also changes the selectivity of the two competing processes. At high C coverage, chain propagation becomes preferred. A general picture emerges of C permeation on Fe surfaces as a stepwise process with opposite thermodynamic and kinetic preferences.Graphical Abstract: [ABSTRACT FROM AUTHOR]

[Display omitted] • Real space evidence of p4g(2 × 2) Fe 2 C and Co 2 C surface carbides. • Tip-sample distance dependent STM imaging of Fe 2 C. • Fe 2 X (X = B, C, N, O) with increasing electrons the reconstruction becomes less favourable. Thin FCC (1 0 0) iron and cobalt carbide films were prepared on Cu(1 0 0) to study the connection between their structure, electronic properties and stability. We present the first detailed, real space experimental confirmation of the C-induced clock reconstruction on the FCC(1 0 0) surfaces of iron and cobalt. Both Fe and Co surface carbides show p4g (2 × 2) surface reconstruction with tetracoordinated square planar carbon and pure FCC (1 0 0) metal layers underneath. Combining tip-sample distance dependent STM imaging with theoretical calculations we present different imaging modes of Fe 2 C. Using a combination of angle-resolved x-ray photoelectron spectroscopy (AR-XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and theoretical calculations we provide detailed electronic and structural models for Fe 2 C and Co 2 C p4g (2 × 2) surface carbides and other 2D Fe 2 X interstitial compound systems. In various Fe 2 X (X = B, C, N, O) surface compounds moving to the right in the periodic table with increasing electrons the reconstruction becomes less favorable, while iron carbide shows the highest stability. [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]

The mechanisms of CO hydrogenation with the formation of C 1 surface species on the clean and K 2 O-doped χ-Fe 5 C 2 (111) surfaces with carbon vacancies have been comparatively elucidated. On both surfaces, CO direct dissociation (CO → C + O) is not favored. On the clean surface, the preferred H-assisted pathway is CO → HCO → CH 2 O → CH 3 O → CH 3 OH. On the K 2 O-doped surface, the preferred paths are HCO → CHOH → CH + OH and HCO → CH 2 O → CH 2 OH → CH 2 + OH due to the barrier decrease of the C O bond cleavage of COH, CHOH and CH 2 OH by K 2 O promotion. These results might explain the K 2 O promoted carburization process and the formation of long-chain hydrocarbons. [ABSTRACT FROM AUTHOR]

The effect of Cu promoter on CH 4 formation from CO hydrogenation on Fe(100) has been investigated using spin-polarized density functional theory computations on periodic slab models. Two pathways of CH 4 formation are considered: (a) direct dissociation of CO followed by successive C hydrogenation and (b) successive hydrogenation of CO followed by deoxygenation of CH x O (x = 1–3). On the clean and Cu doped surfaces, direct dissociation of CO followed by successive C hydrogenation is more favorable kinetically and the overall CH 4 formation is endothermic. Compared with the clean surface, the Cu doped surface can suppress CH 4 formation by raising the barrier from the co-adsorbed CO + 4H. Methanol formation is unfavorable on both surfaces kinetically and thermodynamically. Reversely, methanol decomposition is favorable. It is also noted that Cu doping lowers the adsorption ability of the Fe(100) surface and therefore the adsorption energies of all species on the potential energy surfaces. [ABSTRACT FROM AUTHOR]

High coverage hydrogen adsorption on twenty two terminations of orthorhombic Mo2C has been systematically studied by using density functional theory and ab initio thermodynamics. Hydrogen stable coverage on the surfaces highly depends on temperatures and H2 partial pressure. The estimated hydrogen desorption temperatures under ultra-high vacuum condition on Mo2C are in reasonable agreement with the available temperature-programmed desorption data. Obviously, hydrogen adsorption can affect the surface stability and therefore modify the surface morphology of Mo2C. Upon increasing the chemical potential of hydrogen which can be achieved by increasing the H2 partial pressure and/or decreasing the temperature, the proportions of the (001), (010), (011) and (100) surfaces increase, while those of the (101), (110) and (111) surfaces decrease. Among these surfaces, the (100) surface is most sensitive upon hydrogen adsorption and the (111) surface is most exposed under a wide range of conditions. Our study clearly reveals the role of hydrogen on the morphology of orthorhombic Mo2C catalyst in conjugation with hydro-treating activity. Graphical abstractImage 1Highlights· Coverage dependent H2 adsorption on Mo2C surfaces is systematically calculated. · Hydrogen adsorption is essential in forming active Mo2C catalyst. · (111) is the most exposed Mo2C surface and (100) is the most sensitive to condition. · Hydrogen desorption temperatures on Mo2C can be reasonably predicted. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 Conversion to CH 4 and CH 3 OH on ZrO 2 support Ni catalysis was computed. • Metal-Metal and Metal-support Interaction was analysed. • Reaction Mechanisms, differently from metallic Ni, were identified. • Metal interaction determines structure of support Ni and reaction mechanisms. To better understand the metal-support interaction and structure–activity relationship, we computed the mechanisms of CO 2 hydrogenation to CH 4 and CH 3 OH on ZrO 2 supported Ni catalyst. On a supported irregular Ni 13 cluster, which exhibits a balanced metal–metal and metal-support interaction with surface Zr and O atoms covered, two preferred routes [CO 2 → CO → CHO → CH/CH 3 O → CH 3 → CH 4 /CH 3 OH] are found, and the rate-determining step is the CO 2 direct dissociation. Such mechanisms differ very strongly from those reported on flat metallic Ni(1 1 1) surface and Ni clusters supported on different metal oxides, demonstrating that different supports generate different metal-support interaction that simultaneously changes the metal structures and the reaction mechanisms. [ABSTRACT FROM AUTHOR]

In petrochemical industry, zeolites as solid acid catalysts play an important role and Y zeolites with high surface area and large pore openings are mainly used for catalytic cracking. Aiming at the problem that the specific positions of Al atoms in zeolites cannot be accurately distinguished experimentally and the acid strength can only be studied qualitatively, we used computational model and method to study the position of Al substitution and the resulted acid strength of Y zeolite at atomic and molecular level. The position of Al substitution with gradually decreasing Si/Al ratio (Si/Al = 47–2.4) and the corresponding Brønsted acid strength of the resulting multiple acid sites of HY zeolites are systematically investigated based on periodic DFT calculation including dispersion correction (GGA-PBE-D3). The thermodynamically more stable structures prefer Al distribution having the next-nearest neighbor of aluminum atom pairs (Al–O–Si–O–Al), and based on these stable configurations, the adsorption of ammonia (NH 3) and pyridine (C 5 H 5 N) has been computed to estimate the Brønsted acid strength. It is found that the adsorption of NH 3 and C 5 H 5 N prefers the formation of ammonium (NH 4 +) and pyridinium (C 5 H 5 NH+) ions at each acid sites and the adsorption enthalpy does not correlate with the Bader charge of the acid proton. Most importantly, the adsorption enthalpy decreases with the decrease of Si/Al ratio, indicating that the more the Al substitution the weaker the Brønsted acid strength. It is noted that the adsorption enthalpy of all individual acid sites at a given Si/Al ratio can differ strongly, despite the formation of NH 4 + and C 5 H 5 NH + ions, indicating their mutual interaction and local confinement effect. The calculated adsorption enthalpies agree fully with the experimentally measured results (Si/Al = 8.6–2.4). Hyroxyl nest, defect sites formed from dealumination, has much weaker acid strength than the Brønsted acid sites. Our results provide a rational basis for further studies on the enhanced catalytic activity of HY zeolites with different Si/Al ratios. [Display omitted] • Structure and stability of Y-Zeolite with multiple Al substitution were computed using DFT. • The stability of Al distribution prefers the next-nearest neighbor of aluminum atom pairs. • The acid strength was computed based on the adsorption enthalpy of ammonia (NH 4 +) and pyridine (C 5 H 5 NH+). • The adsorption enthalpy decreases with the decrease of Si/Al ratio. • The more the Al substitution the weaker the Brønsted acid strength. [ABSTRACT FROM AUTHOR]

Spin-polarized density functional theory computations have been performed to investigate n CO + m H 2 co-adsorption and mutual interaction on the Fe(1 1 0) and Fe(1 1 1) surfaces. It is found that CO pre-coverage affects hydrogen adsorption strongly, while H pre-coverage hardly affects CO adsorption. It is possible to have CO + H 2 co-adsorption on Fe(1 1 0), while very strong preference of CO pre-coverage on Fe(1 1 1) has been found. On Fe(1 1 1), it is also found that each CO molecule blocks on average 2H adsorption sites at all CO pre-coverage. All these findings are consistent with available experimental results. Compared with CO/H 2 ratios in gas phase, CO/H 2 surface ratios are very different and much more sensitive to temperature than pressure. Our results provide the basis for exploring the mechanisms of iron-catalyzed conversion of synthesis gas. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] Most, medium and least stable Fe5C2(111), (−411) and (001) surfaces are used. [•] Adsorption of H2O molecules and O atoms was calculated at high coverage. [•] Surface stability correlates with the adsorption energies of H2O and O. [•] High coverage H2O prefers hydrogen bonding instead of adsorption on iron. [•] The number of surface O atoms depends on temperature and H2O/H2 ratio. [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: 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: 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: 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: The structures and energetics of the chemisorbed CO2, CH x species and H as well as C2H4 on the α-Mo2C(0001) surface have been computed at the GGA-RPBE level of density functional theory. It is found that CO2 adsorbs dissociately into CO and O, in agreement with the experimental finding. The adsorbed O, CH x and H species prefer the site of three surface molybdenum atoms over a second layer carbon atom (V C site). On the basis of the calculated adsorption energies of CH x and H, the sequential dehydrogenation of CH4 and the C/C coupling reaction of CH x have been discussed. [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: 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 structure and stability of the neutral cages (M12O36 and M18O54, M=Mo and W) of the Keggin and Wells-Dawson anions have been investigated at the level of density functional theory. These neutral cages can be viewed topologically as polyhedra containing triangles (f 3) and squares (f 4). The relative stability of these polyhedra is determined by the location and separation of the f 3, and the α-Keggin cage without fused triangles (N33=0) is, therefore, more stable than the β isomer with N33=3. This rule is also applicable to the Wells-Dawson cages. The structures and stabilities of these neutral cages mimic those of fullerenes within the framework of isolated pentagon rule. [Copyright &y& Elsevier]

Abstract: The structure and stability of the α/β-[AlW12O40]5− anions have been investigated with seven density functional based methods. It has been found that the relative stability of these anions are sensitive to the density functionals, and the α isomer is more stable than the β one in both gas phase and simulated aqueous medium. This conclusion has also been supported by further calculations on the relative stability of the highly charged α/β-[XW12O40] y− (X=Fe(III), Co(III), Zn(II)) anions. The BP86 incorrect assignment of the α/β-[XW12O40]5− (X=Al, Ga) stability in gas phase can be attributed to the insufficient functional. [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]