The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces.

The adsorption and activation of a CO2 molecule on cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces have been investigated by means of periodic density functional theory based calculations using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic β-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a β-Mo2C(001) C-terminated polar surface or on a δ-MoC(001) nonpolar surface the CO2 molecule is activated yet the C-O bond prevails. Experimental tests showed that Mo-terminated β-Mo2C(001) easily adsorbs and decomposes the CO2 molecule. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissociate C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2 + 3H2 → CH3OH + H2O reaction. Our theoretical and experimental results illustrate the tremendous impact that the carbon/metal ratio has on the chemical and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.