Spin-polarized density functional theory calculations have been performed to investigate the carbon pathways and hydrogenation mechanism for CH(4) formation on Fe(2)C(011), Fe(5)C(2)(010), Fe(3)C(001), and Fe(4)C(100). We find that the surface C atom occupied sites are more active toward CH(4) formation. In Fischer-Tropsch synthesis (FTS), CO direct dissociation is very difficult on perfect Fe(x)C(y) surfaces, while surface C atom hydrogenation could occur easily. With the formation of vacancy sites by C atoms escaping from the Fe(x)C(y) surface, the CO dissociation barrier decreases largely. As a consequence, the active carburized surface is maintained. Based on the calculated reaction energies and effective barriers, CH(4) formation is more favorable on Fe(5)C(2)(010) and Fe(2)C(011), while Fe(4)C(100) and Fe(3)C(001) are inactive toward CH(4) formation. More importantly, it is revealed that the reaction energy and effective barrier of CH(4) formation have a linear relationship with the charge of the surface C atom and the d-band center of the surface, respectively. On the basis of these correlations, one can predict the reactivity of all active surfaces by analyzing their surface properties and further give guides for catalyst design in FTS.

On the basis of density functional theory calculations, the chemisorption of CO 2 on the transition metal surfaces was investigated to find out the key factors controlling its adsorption strength and activation degree. The interaction mechanism of CO 2 with the metal surfaces was discussed by analyzing the density of states. The adsorption strength of CO 2 is controlled by the d-band center of the metal surfaces and also affected by the charge transfer from the metal surfaces to the chemisorbed CO 2 . The degree of CdO bond activation depends on the transferred charge. Therefore, both d-band center of the metal surfaces and the charge transfer should control the chemisorption of CO 2 .

Owing to their outstanding effect on improving catalytic reactivity, alkali-metal promoters have been widely used in industry [1,2] and extensively studied in academia. [3][4][5][6][7][8][9][10][11][12] Potassium-promoted iron catalysts for Fischer-Tropsch synthesis (FTS) and ammonia synthesis are the most representative examples of this effect. Owing to the complexity of real catalytic systems, the microscopic understanding of the alkalimetal promotion effect is still an elusive and challenging subject.In the past decades, most experimental and theoretical studies have focused on the co-adsorption systems involving alkali-metal atoms, for example, K + CO/metal. Five types of alkali-metal-adsorbate interactions were proposed to explain the alkali-metal promotion effect: 1) substrate-mediated charge transfers, [3] 2) direct bonding between alkali metal and adsorbate, [4] 3) electrostatic interactions, [5] 4) alkalimetal-induced molecular polarization, [6] and 5) nonlocal alkali-metal-induced enhancement of the surface electronic polarizability. [7] Although these studies represent excellent surface science, they are not immediately relevant to catalysis, because alkali-metal salts (or oxides) rather than metallic alkalis are used in heterogeneous catalysis. More importantly, previous studies only highlighted the alkali-metal-induced effect on the electronic structure of metallic substrate and coadsorbed molecules, while the more intriguing aspect, the alkali-metal effect on the surface structure of catalysts, has never been taken into account. On the basis of these proposals, the drastic changes in catalytic activity and selectivity caused by very low loadings of alkali-metal promoters cannot be reasonably explained. [8] As metallic iron is the active catalyst in ammonia synthesis, [9] iron-based FTS catalysts show an rich phase chemistry of metal, oxides, and carbides under reaction conditions.

The stability of β-Mo2C surfaces has been computed at the level of density functional theory under the consideration of the temperature, pressure, and molar ratios of CH4 /H2 and CO/CO2 gas mixtures by using ab initio atomistic thermodynamic calculations. It was found that the (001) surface is most stable at low temperature, whereas (101) becomes dominant at high temperature with CH4 as carbon source, and the computed surface stability is supported by the experimental X-ray diffraction pattern and intensity. For CO as carbon source, the (101) surface has the smallest surface Gibbs free energy at temperatures up to 1000 K and is most stable. On the basis of the Wulff-type particle shapes from surface Gibbs free energies the (101) facet represents the largest surface area of β-Mo2C. Our findings are in perfect agreement with the results of high-resolution transmission electron microscopy.

No abstract

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-FeO, FeO, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% "screened" Hartree-Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.

As active phases in low-temperature Fischer−Tropsch synthesis for liquid fuel production, epsilon iron carbides are critically important industrial materials. However, the precise atomic structure of epsilon iron carbides remains unclear, leading to a half-century of debate on the phase assignment of the ε-Fe 2 C and ε′-Fe 2.2 C. Here, we resolve this decades-long question by a combined theoretical and experimental investigation to assign the phases unambiguously. First, we have investigated the equilibrium structures and thermal stabilities of ε-Fe x C (x = 1, 2, 2.2, 3, 4, 6, 8) by first-principles calculations. We have also acquired X-ray diffraction patterns and Mossbauer spectra for these epsilon iron carbides and compared them with the simulated results. These analyses indicate that the unit cell of ε-Fe 2 C contains only one type of chemical environment for Fe atoms, while ε′-Fe 2.2 C has six sets of chemically distinct Fe atoms.

No abstract