Atomic and electronic structure of molybdenum carbide phases: bulk and low Miller-index surfaces

The geometric and electronic structure of catalytically relevant molybdenum carbide phases (cubic delta-MoC, hexagonal alpha-MoC, and orthorhombic beta-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 beta-Mo2C has a stronger metallic character than the other two polymorphs, both beta-Mo2C and d-MoC have a large ionic contribution, and delta- and alpha-MoC exhibit the strongest covalent character. Among the various surfaces explored, the calculations reveal the high stability of the delta-MoC(001) nonpolar surface, Mo- and C-terminated (001) polar surfaces of alpha-MoC, and the nonpolar (011) surface of beta-Mo2C. A substantially low work function of only 3.4 eV is predicted for beta-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.