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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]

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]

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]