title: Atomic and electronic structure of molybdenum carbide phases: bulk and low Miller-index surfaces.
abstract: The geometric and electronic structure of catalytically relevant molybdenum carbide phases (cubic δ-MoC, hexagonal α-MoC, and orthorhombic β-Mo2C) and their low Miller-index surfaces have been investigated by means of periodic density functional theory (DFT) based calculations with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Comparison to available experimental data indicates that this functional is particularly well suited to study these materials. The calculations reveal that β-Mo2C has a stronger metallic character than the other two polymorphs, both β-Mo2C and δ-MoC have a large ionic contribution, and δ- and α-MoC exhibit the strongest covalent character. Among the various surfaces explored, the calculations reveal the high stability of the δ-MoC(001) nonpolar surface, Mo- and C-terminated (001) polar surfaces of α-MoC, and the nonpolar (011) surface of β-Mo2C. A substantially low work function of only 3.4 eV is predicted for β-Mo2C(011), suggesting that this system is particularly well suited for (electro)catalytic processes where surface → adsorbate electron transfer is essential. The overall implications for heterogeneously catalysed reactions by these molybdenum carbide nanoparticles are also discussed.
authors.affiliation: Departament de Química Física & Institut de Quimica Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Barcelona, Spain.
authors.forename: José Roberto dos Santos
authors.initials: JR
authors.lastname: Politi
authors.forename: Francesc
authors.initials: F
authors.lastname: Viñes
authors.forename: Jose A
authors.initials: JA
authors.lastname: Rodriguez
authors.forename: Francesc
authors.initials: F
authors.lastname: Illas