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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]

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: 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]

• Coverage-dependent CO hydrogenation was computed and compared. • At 0.25 reasonable coverage CO dissociation prefers H-assisted path leading to CH formation. • Apparent barrier of methanation is in close agreement with experimental value. • Ketene is the intermediate from CH 2 and CO coupling for the first C-C bond formation. • CH 3 C is the homologue of CH and starts the cycle for chain propagation. For the first time the full catalytic cycle of CO activation, methanation and first C-C bond formation on Fe(110) at high coverage [0.25 ML, p (2 × 2)] have been computed by density functional theory. Under thermodynamic equilibrium, H-assisted CHO formation and CHO dissociation into CH+O are preferred thermodynamically and kinetically. Then CH can be transformed into methane via successive hydrogenation; and the first C-C bond can be formed by CH 2 and CO coupling resulting in ketene (CH 2 CO), which can be hydrogenated to acetyl (CH 3 CO). The next C-O dissociation of CH 3 CO results in CH 3 C (higher homologue CH), and this starts C-C chain growth and catalytic cycle. Both methanation and chain growth have the same rate-determining step as CHO dissociation. The computed apparent barrier of methanation is much close to experimental value. These high coverage results clearly explain the experimentally observed formation of methane as well as high hydrocarbons of synthesis gas conversion on metallic Fe surface. The computed vibrational frequencies of the proposed surface intermediates agree with the experimental data. This high coverage strategy should be applicable to other heterogeneous catalytic reactions having coverage preferences. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We systematically investigate the structural, electronic and magnetic properties, bonding analysis, as well as surface morphology of transition metal monoxides (FeO, CoO and NiO) utilizing density functional theory (DFT) with various functionals across from GGA to GGA+U and hybrid functional. Our results reveal that the screened hybrid functional (HSE) could give a reasonable description on structural and electronic properties of these strongly correlated systems when compared to experiments. Local approximation GGA functional fails to yield the accuracy band gaps. GGA+U approximation are rational when given a reasonable U, but the U which reproduce a reasonable band gap often yield a significant error in describing other properties such as lattice constant or magnetic moment. The computed Crystal Orbital Overlap Population (COOP) indicates that cubic FeO and CoO are metastable phase, while the distortion occurs to enhance stability. Based on the reliable functional, the surface morphology of FeO, CoO and NiO are predicted using Wulff construction approach. [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]

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]

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: Density functional theory was employed to investigate the formation and properties of the oxygen vacancies on the rutile TiO2(110) surface. It is found that the formation of the positively charged bridging-oxygen vacancy (BOV+, 4.2eV) is the most favored one, followed by the positively charged in-plane-oxygen vacancy (POV+, 4.5eV). In contrast, the formation of the neutral bridging-oxygen and in-plane-oxygen vacancies (BOV and POV), and their dication oxygen vacancies (BOV2+ and POV2+) needs much higher energies (7.9 and 8.3 vs. 8.1 and 8.6eV), respectively. [Copyright &y& Elsevier]

• On (2 1 0) surface, the hydrogen dissociation is spontaneous. • The chemical bonding analysis is employed to elucidate the interaction between adsorbate and substrate. • At high coverage, the hydrogen adsorption configuration is surface-dependent. • On (2 1 0) surface, the surface sulfur atoms are easily to be removed by hydrogen. Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is of great importance in rationally modulating the catalytic performance of hydrogenation processes. Herein, we have investigated the interaction between hydrogen and four typical surfaces, (1 0 0), (2 1 0), (2 1 1), and (3 1 1) of pyrite FeS 2 through density functional theory calculations. On (2 1 0) surface, the hydrogen dissociative adsorption on unsaturated-coordination sulfur atoms is favorable both in thermodynamics and kinetics. The hydrogen activation barrier is 0.83 eV with slight exothermic of 0.12 eV on (3 1 1). While on (1 0 0) and (2 1 1) surface, the hydrogen dissociation is unfavorable due to the high activation barriers and remarkable positive reaction energies. For high adsorption coverage, the pure molecule adsorption mode is favorable on (1 0 0) facet, opposed to the other surfaces which have temperature and pressure dependence. The saturated coverage sequence is (1 0 0) > (2 1 0) > (2 1 1) > (3 1 1) for a wide range of temperature and pressure. The remove of sulfur atoms most likely occurs on (2 1 0) surface. Our atomistic insights might be useful in engineering hydrogen-involved processes catalyzed by iron sulfide. [ABSTRACT FROM AUTHOR]

A theoretical chemistry approach combining Wulff construction, density functional theory, and microkinetics enables revealing the nature of facet-dependent key elementary reactions on single-phase iron carbide at single particle level, complementary to current limited understanding of this issue in experimental exploration of iron-catalyzed Fischer-Tropsch Synthesis. • Intrinsic catalysis of a single-phase iron carbide particle in FTS was studied. • Neither too strong nor too weak H adsorption is favorable for CH 4 formation. • Coupling of two H-deficient hydrocarbon intermediates is more facile. • The surface (111) and (10 1 -) are kinetically more viable for C 2 formation. • CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). Elucidation of intrinsic working principle of single-phase iron carbide and its facet-dependent catalytic behavior remains a substantial challenge in iron-catalyzed Fischer-Tropsch synthesis. Here, we provided in-depth understanding of the iron carbide phase-dependent and facet-dependent properties on theoretically established Fe 5 C 2 particle model through an approach combining Wulff construction, density functional theory, and microkinetics. We studied two key probe reactions, C 2 formation via C 1 +C 1 coupling and CH 4 formation, by monitoring surface-dependent thermodynamics and microkinetics. Integration of results thereby allows us to assess the macroscopic catalytic properties at single particle level and evaluate contribution from individual exposed surface. The surface (111) and (10 1 -) are kinetically more viable for C 2 formation, whereas CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). This study enriches the knowledge of the intrinsic working mechanism for single-phase iron carbide and provides fundamental insights into rational design of improved iron-based Fischer-Tropsch synthesis catalysts. [ABSTRACT FROM AUTHOR]

• The boron chemical potential were calculated as a function of temperature, pressure and gas composition. • The bulk and surface stabilities of six iron borides are determined by the boron chemical potential. • The evolution of surface structure and morphology under dynamic conditions were predicted. • CO adsorption is weakened by surface doping of boron atom due to its electronegativity. Surface feature and its variation along with complex atmosphere are of fundamental significance to understanding the functionality of applied materials especially in heterogeneous catalysis and corrosion prevention. Here we performed a unified theoretical study on the surface structure and morphology of iron borides and their evolution under dynamic gaseous conditions by combination of density functional theory, ab initio atomic thermodynamics and Wulff construction. In particular, thermodynamic stability of iron borides and corresponding surfaces varied from the boron chemical potential (Δ μ B ) of certain atmosphere, which increases with decreasing pressure and increasing temperature and concentration of boron source. The stability of boron-rich surfaces has been improved with increasing Δ μ B , while all the Fe-rich facets of iron borides are favorable at low Δ μ B condition. Accordingly, the crystallite morphology of iron borides undergoes significant evolution upon dynamic condition. Finally, the surface properties of iron borides are carefully tested by CO adsorption which indicated the activation ability of CO is closely connected with boron triggered surface charge transfer between Fe and CO. This work was expected not only to help understand the surface structure and morphology of iron borides under realistic condition, but also provides fundamental insights into rational design of corrosion resistant and catalytic materials. [ABSTRACT FROM AUTHOR]

• 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]