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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 electronic structures, stabilities and magnetic properties of the Fe3O4(111), (110) and (001) surfaces have been computed at the level of density functional theory by including the Hubbard parameter (U) for describing the on-site Coulomb interaction of iron 3d electrons. Among the six Fe3O4(111) terminations, the Fetet1 (exposing tetrahedral coordinated iron) and Feoct2 (exposing octahedral coordinated iron) terminations are more stable and have metallic character. For the Fe3O4(110) surface, strong surface distortion has been found; the A-layer termination (exposing tetrahedral coordinated iron) has metallic character, while the B-layer termination (exposing tetrahedral and octahedral coordinated iron) has half-metal character. For the Fe3O4(001) surface, both A-layer (exposing tetrahedral coordinated iron) and B-layer (exposing octahedral coordinated iron) terminations have half-metal character. The surface stability of (111)>(001)>(110) on the basis of the computed surface energies agrees well with the experimental findings, and explains reasonably the observed diversity and complexity of the experiments. [Copyright &y& Elsevier]

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: 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]

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]

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: 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: 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: 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]

• 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]