First-principles investigation of the WC∕HfO2 interface properties (original) (raw)

First-principles investigation of the WC∕HfO[sub 2] interface properties

Journal of Applied Physics, 2006

The thermodynamic and electronic properties of tungsten carbide surfaces and interfaces with monoclinic hafnia ͑WC/ m-HfO 2 ͒ are investigated through first-principles calculations. We show that oxidation of the WC surface and of the WC/ m-HfO 2 interface is energetically favorable. An oxygen monolayer on the W-terminated WC͑0001͒ surface gives rise to a larger vacuum work function than that for the C-terminated WC͑0001͒ surface, while the opposite result is obtained for the WC͑0001͒ effective work function on hafnia: a carbon intermediate layer results in larger work function than an oxygen intermediate layer. This result is explained by the atomic structure of the intermediate layers neighboring the interface which differ if the interface is O or C rich.

Interactions between hydrogen and tungsten carbide: a first principles study

RSC Adv., 2014

Tungsten carbide has been proposed to be a potential inexpensive alternative to platinum for catalyzing hydrogenation reactions. Using density functional theory (DFT) calculations, we performed a systematic study on the geometries and electronic structures of small (WC) n (n ¼ 1-10) clusters. A cubic-like growth of WC clusters was found to be the preferred pathway. The interactions between the WC clusters and hydrogen were compared to interactions of hydrogen with the WC(0001) surface. Both atomic clusters and the crystalline surface of WC can lead to facile dissociation of H 2 while the activation energy on the former is even lower; the H diffusion barrier on WC clusters was found to be significantly higher than that on the WC surface, yet both of the calculated diffusion barriers are well above the values on the platinum counterpart. The lack of hydrogen mobility may exert a profound influence on the kinetics of hydrogen for participating in reactions on WC. The desorption strength of H on (WC) n (n ¼ 2, 4, 6, 9) at various hydrogen coverages was explored and the hydrogen saturated (WC) 9 cluster was used to rationalize the catalytic formation of hydrogen tungsten bronzes. The study provides insights which may be useful for efforts to replace Pt with WC in catalytic processes involving hydrogen.

Density-functional theory study of the effects of atomic doping on the band edges of monoclinic WO3

Physical Review B, 2008

Reactions of hydrogen sulfide (H 2 S) with Nickel/Ytrria-doped zirconia (Ni/YDZ) anode materials might cause degradation of the performance of solid oxide fuel cells when S containing fuels are used. In this paper, we employ density functional theory to investigate S adsorption on metal (M)-doped and undoped Ni(0 0 1) and Ni surfaces. Based on the performed calculations, we analyze the effects of 12 alloying additions (Ag, Au, Al, Bi, Cd, Co, Cu, Fe, Sn, Sb, V, and Zn) on the temperature of transition between clean (S atoms do not adsorb on the surfaces) and contaminated (S atoms can adsorb on the surfaces spontaneously) M-doped Ni surfaces for different concentrations of H 2 S in the fuel. Predicted results are consistent with many experimental studies relevant to S poisoning of both Ni/YDZ and M-doped Ni/YDZ anode materials. This study is important to understand S poisoning phenomena and to develop new S tolerant anode materials.

Electronic properties of hexagonal tungsten monocarbide (h-WC) with 3d impurities from first-principles calculations

Physica B: Condensed Matter, 2009

First principles FLAPW-GGA calculations have been performed to predict the structural, electronic, cohesive and magnetic properties for hexagonal WC doped with all 3d metals. The optimized lattice parameters, density of states, cohesive and formation energies have been obtained and analyzed for ternary solid solutions with nominal compositions W 0.875 M 0.125 C (where M = Sc, Ti….Ni, Cu). In addition, the magnetic properties of these solid solutions have been examined, and magnetization has been established for W 0.875 Co 0.125 C.

Density-functional theory study of the effects of atomic impurity on the band edges of monoclinic WO3

Physical Review B, 2008

Reactions of hydrogen sulfide (H 2 S) with Nickel/Ytrria-doped zirconia (Ni/YDZ) anode materials might cause degradation of the performance of solid oxide fuel cells when S containing fuels are used. In this paper, we employ density functional theory to investigate S adsorption on metal (M)-doped and undoped Ni(0 0 1) and Ni surfaces. Based on the performed calculations, we analyze the effects of 12 alloying additions (Ag, Au, Al, Bi, Cd, Co, Cu, Fe, Sn, Sb, V, and Zn) on the temperature of transition between clean (S atoms do not adsorb on the surfaces) and contaminated (S atoms can adsorb on the surfaces spontaneously) M-doped Ni surfaces for different concentrations of H 2 S in the fuel. Predicted results are consistent with many experimental studies relevant to S poisoning of both Ni/YDZ and M-doped Ni/YDZ anode materials. This study is important to understand S poisoning phenomena and to develop new S tolerant anode materials.

Chloropentafluoroethane Hydrodechlorination over Tungsten Carbides: Influence of Surface Stoichiometry

Journal of Catalysis, 2002

Tungsten carbides were prepared by temperature-programmed reaction of WO 3 with methane/hydrogen mixture and activated according to several thermal treatments by varying the temperature or the feed composition. Modification of these parameters was expected to change the surface composition (C/W). Such surface modifications have been related to acid-base properties observed in the decomposition of isopropanol. Finally, it was found that such acidic properties are probably involved in the dehydrohalogenation of chloropentafluoroethane responsible mainly for the formation of unsaturated compounds. c 2002 Elsevier Science (USA) Key Words: tungsten carbide; catalytic hydrodechlorination; chloropentafluoroethane; X-ray photoelectron spectroscopy; acidic properties of WC; decomposition of isopropanol.

Ab initio study of the fcc-WC(100) surface and its interaction with cobalt monolayers

Applied Surface Science, 2011

The WC(1 0 0) surface has been studied by using ab initio methods of the density functional theory and pseudopotentials. Calculations have shown that surface and undersurface atoms move from their bulk positions. Namely, carbon atoms moved outward, while tungsten atoms moved inward. Five geometric cases for Co/WC(1 0 0) system were compared: (A) Co atoms are above C atoms; (B) Co atoms are above W atoms; (C) Co atoms are in the four-fold sites above WC pairs; (D and E) Co atoms are above the W-W-C and CC -W threefold sites, respectively-and the (A) case has been found to be energetically preferable. In all cases, Co layers have been found to be ferromagnetic. The densities of states for the bulk fcc-WC, the WC(1 0 0) surface, and the WC/Co system were compared.

The effect of vanadium-carbon monolayer on the adsorption of tungsten and carbon atoms on tungsten-carbide (0001) surface

We report a first-principles calculations to study the effect of a vanadium-carbon (VC) monolayer on the adsorption process of tungsten (W) and carbon (C) atoms onto tungsten-carbide (WC) (0001) surface. The essential configuration for the study is a supercell of hexagonal WC with a (0001) surface. When adding the VC monolayer, we employed the lowest energy configuration by examining various configurations. The total energy of the system is computed as a function of the W or C adatoms' height from the surface. The adsorption of a W and C adatom on a clean WC (0001) surface is compared with that of a W and C adatom on a WC (0001) surface with VC monolayer. The calculations show that the adsorption energy increased for both W and C adatoms in presence of the VC monolayer. Our results provide a fundamental understanding that can explain the experimentally observed phenomena of inhibited grain growth during sintering of WC or WC-Co powders in presence of VC.

Ab initio investigation of structural and electronic properties of tungsten dioxide

, Phone: þ375 172 938 869; Fax: þ375 172 938 869 A detailed study of electronic properties of tungsten dioxide (WO 2) has been performed by means of the projector-augmented wave method. Two phases were considered: stable monoclinic, which was characterized as a metallic one, and high-temperature metastable orthorhombic, which was found to be a semiconductor with an energy gap of about 0.6 eV. We also show a possibility that oxygen vacancies in the monoclinic tungsten oxide providing the WO 1.75 stoichiometry can result in the semiconducting behaviour of this compound as well.