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

Abstract

The adsorption and activation of a CO 2 molecule on cubic d-MoC(001) and orthorhombic b-Mo 2 C(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 b-Mo 2 C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO 2 and carbon monoxide formation, whereas on a b-Mo 2 C(001) C-terminated polar surface or on a d-MoC (001) nonpolar surface the CO 2 molecule is activated yet the C-O bond prevails. Experimental tests showed that Mo-terminated b-Mo 2 C(001) easily adsorbs and decomposes the CO 2 molecule. This surface is an active catalyst for the hydrogenation of CO 2 to methanol and methane. Although MoC does not dissociate C-O bonds on its own, it binds CO 2 better than transition metal surfaces and is an active and selective catalyst for the CO 2 + 3H 2 -CH 3 OH + H 2 O 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 CO 2 .

Highly Active Au/δ-MoC and Cu/δ-MoC Catalysts for the Conversion of CO2: The Metal/C Ratio as a Key Factor Defining Activity, Selectivity, and Stability

“…This difference in selectivity reflects variations in the bonding modes of CO 2 on the different carbides. 17,28 In general, a decrease in the metal/carbon ratio in a carbide usually reduces the reactivity of the system as a consequence of electronic ⎯a raise in the positive charge on the metal centers⎯ and structural effects⎯ a reduction in the number of metal centers exposed on the carbide surface. 47,48 Theoretical calculations indicate that CO 2 adsorbs molecularly on TiC(001) and δ-MoC(001).…”

“…47,48 Theoretical calculations indicate that CO 2 adsorbs molecularly on TiC(001) and δ-MoC(001). 17,27,28 The cleavage of a C-O bond occurs only after hydrogenation of the molecule and formation of a COOH intermediate. 28 On the other hand, one of the C-O bonds in carbon dioxide dissociates rather easily on β-Mo 2 C(001), and dissociation of the second requires only a relatively small activation barrier.…”

“…28 On the other hand, one of the C-O bonds in carbon dioxide dissociates rather easily on β-Mo 2 C(001), and dissociation of the second requires only a relatively small activation barrier. 17,26,49 The C deposited on this carbide is hydrogenated to produce methane. 17,26 The catalytic performance of metal carbides can be enhanced by adding transition or noble metals to their surfaces.5 ,23,28-31 Au and Cu adatoms undergo electronic perturbations when in contact with TMC(001) surfaces.…”

“…17,26,49 The C deposited on this carbide is hydrogenated to produce methane. 17,26 The catalytic performance of metal carbides can be enhanced by adding transition or noble metals to their surfaces.5 ,23,28-31 Au and Cu adatoms undergo electronic perturbations when in contact with TMC(001) surfaces. 29 Results of scanning tunneling microscopy (STM) indicate that at small coverages (θ < 0.2 ML), Au and Cu grow on TiC(001) forming very small particles, many of them two-dimensional.…”

“…17 As mentioned above, a decrease in the metal/ carbon ratio when going from β-Mo 2 C(001) to δ-MoC(001) reduces the reactivity of the surface due to an increase in the positive charge on the Mo centers and structural changes that lower the number of exposed Mo atoms. 17,47,48 The reduction in the reactivity towards CO 2 is accompanied by a reduction in the binding energy of O adatoms 17 which is crucial for avoiding deactivation by the formation of an oxycarbide on the catalyst surface. Thus, the metal/carbon ratio plays a key role for defining the activity, selectivity and stability of δ-MoC(001) as a catalytic material.…”