Density functional study on the resistance to sulfur poisoning of Ptx (x = 0, 1, 4 and 8) modified α-Mo2C(0001) surfaces

The tolerance of sulfur poisoning of α-Mo₂C(0001) surfaces with different Pt coverages is investigated combining the density functional theory (DFT) results with thermodynamics data using the ab initio atomistic thermodynamic method. It is found that on Mo₂C(0001), Pt clusters tend to form two dimensional planar structures instead of aggregating. The clean Mo₂C(0001) surface interacts with sulfides very strongly and is susceptible to sulfur poisoning. With increasing the coverage of Pt on the Mo₂C surface, the interaction between sulfur and substrate is weakened. The sulfur tolerance ability increases in the order of Mo₂C ≈ Pt₁/Mo₂C < Pt₄/Mo₂C < Pt₈/Mo₂C, where the coverage of Pt on the Mo₂C plays a very effective role. The results provide theoretical guidance for designing Mo₂C based catalysts with high activity and high sulfur resistance.

Monitoring the carburization of molybdenum bimetallic nitrides and oxynitrides with CH4/H2/Ar mixtures: identification of a new carbonitride

A new carbonitride Ni2Mo3(CxNy) has been synthesized by temperature-programmed carburization of the Ni2Mo3N precursor with a CH4/H2/Ar gas mixture at 923 K. This compound has been characterized by X-ray diffraction, elemental analysis, Auger electron spectroscopy, laser Raman spectroscopy, thermogravimetric analysis and field-emission scanning electron microscopy. Ni2Mo3(CxNy) crystallizes in the cubic space group P4(1)32, with a lattice parameter of a=6.64575(3) A, corresponding to the unusual filled beta-Mn structure. Its formation occurs by partial substitution of N by C via a topotactic and pseudomorphic reaction and its stability in air is higher that of Ni2Mo3N. A two-phase mixture with a cubic structure has always been obtained by the same synthetic method applied to the V2Mo(OxNy) precursor. One of these would be of carbidic nature formed at 1153 K following a reaction similar to that proposed for Ni2Mo3(CxNy).

Thiophene Adsorption and Activation on MoP(001), γ-Mo2N(100), and Ni2P(001): Density Functional Theory Studies

The adsorption and dissociation of thiophene on the MoP(001), gamma-Mo(2)N(100), and Ni(2)P(001) surfaces have been computed by using the density functional theory method. It is found that thiophene adsorbs dissociatively on MoP(001), while nondissociatively on gamma-Mo(2)N(100) and Ni(2)P(001). On MoP(001), the dissociation of the C-S bonds is favored both thermodynamically and kinetically, while the break of the first C-S bond on gamma-Mo(2)N(100) has an energy barrier of 1.58 eV and is endothermic by 0.73 eV. On Ni(2)P(001) there are Ni(3)P(2)- and Ni(3)P-terminated surfaces. On the Ni(3)P(2)-terminated surface, the dissociation of the C-S bonds of adsorbed thiophene is endothermic, while it is exothermic on the Ni(3)P-terminated surface.

From nitrides to carbides: topotactic synthesis of the η-carbides Fe3Mo3C and Co3Mo3C

The molybdenum bimetallic interstitial carbides Fe(3)Mo(3)C and Co(3)Mo(3)C have been synthesized by temperature-programmed reaction (TPR) between the molybdenum bimetallic interstitial nitrides Fe(3)Mo(3)N and Co(3)Mo(3)N and a flowing mixture of CH(4) and H(2) diluted in Ar. These compounds have been characterized by X-ray diffraction, laser Raman spectroscopy, elemental analysis, energy dispersive analysis of X rays, thermal analysis (in air) and scanning electron microscopy (field emission). Their structures have been refined from X-ray powder diffraction data. These carbides crystallize in the cubic system, space group Fd3m[a= 11.11376(6) and 11.0697(3)[Angstrom] for Fe and Co compounds, respectively].