Chun-Fang Huo

Insight into CH4 Formation in Iron-Catalyzed Fischer-Tropsch Synthesis

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.

Surface structures of Fe3O4 (111), (110), and (001): A density functional theory study

Abstract The surface structures of Fe 3 O 4 (111), (110), and (001) have been studied at the level of density functional theory. It is found that there are two most stable Fe 3 O 4 (111) surfaces in close energy and terminated with the exposed tetrahedral and octahedral iron layers. Both Fe 3 O 4 (110) and Fe 3 O 4 (001) surfaces have two surface terminations in close energy. The computed results agree well with the experiments and explain reasonably the observed diversity and complexity of the experiments. The computed surface free energies indicate that (111) is less favorable thermodynamically than (110) and (001), and the formation of (111) should be kinetic controlled.

Determining surface structure and stability of ε-Fe2C, χ-Fe5C2, θ-Fe3C and Fe4C phases under carburization environment from combined DFT and atomistic thermodynamic studies

Abstract The chemical–physical environment around iron based FTS catalysts under working conditions is used to estimate the influences of carbon containing gases on the surface structures and stability of ε-Fe2C, χ-Fe5C2, θ-Fe3C and Fe4C from combined density functional theory and atomistic–thermodynamic studies. Higher carbon content gas has higher carburization ability; while higher temperature and lower pressure as well as higher H2/CO ratio can suppress carburization ability. Under wide ranging gas environment, ε-Fe2C, χ-Fe5C2 and θ-Fe3C have different morphologies, and the most stable non-stoichiometric termination changes from carbon-poor to carbon-rich (varying surface Fe/C ratio) upon the increase in ΔμC. The most stable surfaces of these carbides have similar surface bonding pattern, and their surface properties are related to some common phenomena of iron based catalysts. For these facets, χ-Fe5C2-(100)-2.25 is most favored for CO adsorption and CH4 formation, followed by θ-Fe3C-(010)-2.33, ε-Fe2C-(121)-2.00 and Fe4C-(100)-3.00, in line with surface work function and the charge of the surface carbon atoms.

CO adsorption, CO dissociation, and C-C coupling on Cu monolayer-covered Fe(100)

Abstract CO adsorption, CO dissociation and C-H and C-C bond-making reactions on Cu-covered Fe(100) have been computed within the framework of periodic density functional theory. Compared to the clean Fe(100) surface with strong CO activation, Cu monolayer-covered Fe(100) surface has weaker CO adsorption energy, lower CO activation degree; and ultra higher CO dissociation barrier (2.4 eV). Under H co-adsorption, CO dissociation is favored kinetically on Fe(100). On the Cu-monolayer covered Fe(100), co-adsorbed H largely facilitates CO dissociation via the formation of CHO. The energy barrier decreases to as low as 0.92 eV. It is also found that C-H and C-C bond-making reactions of Cu monolayer-covered Fe(100) surface have lower activation barriers and are more exothermic than on the clean Fe(100) surface.

Structures and energetics of H2O adsorption on the Fe3O4(111)surface

Water adsorption on the Fe(subscript tet1)-terminated and Fe(subscript oct2)-terminated surfaces of Fe3O4 (111) has been calculated at the level of density functional theory (GGA/PBE). On the Fe(subscript tet1)-terminated surface at 1/5 monolayer (ML), the molecular adsorption mode with a hydrogen bond and the heterolytically dissociative mode show the highest stability, whereas the hydronium-ion-like structure OH3(superscript +)-OH becomes possible at 2/5 ML, followed by the hydrogen-bonded water aggregate. These results agree well with the available experimental observations. For Fe(subscript oct2)-terminated surface, the molecular water prefers to adsorb on the surface Fe(subscript oct2) atom at 1/6 ML, whereas other adsorption modes become possible and may coexist at 1/3 ML. The Fe(subscript tet1)-terminated surface is more favorable than the Fe(subscript oct2)-terminated surface for water adsorption. The adsorption mechanism has been analyzed on the basis of the calculated local density of state.

Mechanistic study of palladium-catalyzed telomerization of 1,3-butadiene with methanol

AbstractThe Pd-catalyzed telomerization in the presence of phosphine and carbene ligands has been computed. It is shown that the C–C coupling of the less stable complex A with one trans- and one cis-butadiene in syn orientation forms the most stable intermediate B and is favorable both kinetically and thermodynamically. Protonation of B leads to equilibrium of the two most stable isomers of intermediate C. The overall regioselectivity is favored thermodynamically. FigurePd-Catalyzed Telomerization of 1,3-Butadiene and Methanol

Structure and stability of the crystal Fe2C and low index surfaces

Abstract Spin-polarized density functional theory (DFT) calculations have been performed on the structure and stability of Fe 2 C. It is found that orthorhombic Fe 2 C is more stable than hexagonal Fe 2 C by 0.16 eV on the basis of the computed cohesive energies. The structures and stability of the orthorhombic-Fe 2 C low index surfaces have also been investigated at the same level and the low index surfaces have the decreased stability order of (011) > (110) > (100) > (101) > (001). Comparison of the most stable Fe 3 C, Fe 4 C, and Fe 2 C surfaces shows that there is no linear correlation of surface energy and carbon content. And comparison of their most stable surface with the body-centered cubic Fe shows that these carbide surfaces have lower surface energies than the most stable (110) surface of body-centered cubic Fe, indicating that the surface thermodynamics favor carburization at Fe surfaces.

Controllable deposition of Pt nanoparticles into a KL zeolite by atomic layer deposition for highly efficient reforming of n-heptane to aromatics

Atomic layer deposition (ALD) was applied to deposit Pt into KL zeolite channels. The location of Pt deposition and the interaction between Pt and the KL zeolite have been investigated by various characterization methods as well as DFT simulations. It has been demonstrated that Pt nanoparticles (NPs) with precisely controlled size (∼0.8 nm) and high dispersion have been successfully deposited into the micropores of the KL zeolite. The produced Pt/KL catalysts exhibited highly efficient performance for n-heptane reforming to aromatics with a high toluene selectivity up to 67.3% (toluene/total aromatics = 97.8%) and a low methane selectivity (0.9%), in spite of an ultralow Pt loading (0.21 wt%). It was revealed that the strong interaction between Pt and the KL zeolite resulting in the electron-enriched state of Pt and the confinement of Pt in the micropores of the KL zeolite facilitated the aromatization of n-heptane. The small size and high dispersion of Pt NPs contributed to inhibition of the hydrogenolysis reaction. Prevention of agglomeration of Pt NPs due to confinement inside the micropores of the KL zeolite and the strong interaction led to the high stability of the Pt/KL catalysts.

Density functional theory study of the adsorption and reaction of C2H4 on Fe3C(100)

Abstract Spin-polarized density functional theory (DFT) and a periodic slab model were employed to investigate the adsorption of C2H4 on Fe3C(100), which is an active phase of an Fe-based catalyst for Fischer-Tropsch synthesis. The competition between dehydrogenation and cleavage of C2H4 was analyzed. The μ-bridging adsorption mode is more stable than the π or di-σ adsorption modes. Partial rehybridization of the C atoms of C2H4 (sp2→sp3) caused by the interaction of C2H4 with the Fe3C(100) surface resulted in the C atoms in C2H4 having a quasi-tetrahedron geometry. On Fe3C(100) dehydrogenation of C2H4 occurs, while C-C bond cleavage is not competitive. The calculations indicated that vinylidene (CCH2) and vinyl (CHCH2) species are the most abundant C2 species, which may be the major monomeric forms of C2H4 in the chain growth in Fischer-Tropsch synthesis

First-principles study of the mechanism of carbon deposition on Fe(111) surface and subsurface

Abstract A theoretical study of the carbon atoms adsorption and diffusion on the surface and into the subsurface of Fe (111) is performed using DFT calculations. Before the carbon coverages up to 1 ML, the adsorbed carbons tend to exist in an isolated atomic state and cause a reconstruction of Fe (111) surface. The configurations of “mC2+nC” are energetically favorable on the Fe (111) surface at 1 ML ≤ θC ≤ 2 ML. At a higher coverage, complicated adsorbed patterns such as chains and islands are found, and we predict that these carbon islands can function as the nucleation center of the precipitation of graphite or carbon nanotubes on the Fe(111) surface. In the subsurface region, the carbon atom prefers the octahedral site. The barriers for diffusion on and into the Fe (111) surface and subsurface are 0.45 eV and 0.73 eV, respectively. Actually, C2 formation is thermodynamically favored, whereas C migration into the subsurface region is kinetically feasible.