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.