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.

Based on periodic Density Functional Theory (DFT) calculations, carried out using a standard generalized gradient approximation type exchange-correlation functional including or not van der Waals dispersive forces, the ability of the cubic d-MoC(001) surface to capture methane at room temperature is suggested. Adsorption on the orthorhombic b-Mo 2 C(001) surfaces, with two possible terminations, has been also considered and, in each case, several molecular orientations have been tested with one, two, or three hydrogen atoms pointing towards the surface on all high-symmetry adsorption sites. The DFT results indicate that the d-MoC(001) surface shows a better affinity towards CH 4 than b-Mo 2 C(001). The calculated adsorption energy values on d-MoC(001) surfaces are larger, and hence better, than on other methane capturing materials such as metal organic frameworks. Besides, the theoretical desorption temperature values estimated from the Redhead equation indicate that methane would desorb at 330 K when adsorbed on the d-MoC(001) surface, whereas this temperature is lower than 150 K when the adsorption involves b-Mo 2 C(001). Despite this, adsorbed methane presents a very similar structure compared to the isolated molecule, due to a weak molecular interaction between the adsorbate and the surface. Therefore, the activation of methane molecules is not observed, so these surfaces are, in principle, not recommended as possible methane dry reforming catalysts.

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