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• 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]

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: Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe5C2 under CO and synthesis gas (H2/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μ C) by considering temperature, pressure, and H2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H2 added into CO leads to a large decrease in μ C and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets. [Copyright &y& Elsevier]

• 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: A first-principles calculation based on the density functional theory (DFT) was used to investigate the energetic and electronic properties of Si-doped anatase TiO2 with various silicon concentrations. The theoretical calculations showed that with Si-doping the valence band and conduction band of TiO2 became hybrid ones with large dispersion, which could benefit the mobility of the photo-generated carriers. This result is in agreement with the experimental reports. At lower doping levels, the band gap of Si-doped anatase TiO2 decreases about 0.2eV. With the increase of silicon concentration, the band gap increases gradually and larger formation energies are required during the synthesis of Si-doped TiO2. [Copyright &y& Elsevier]