CO, CO₂, and H₂Interactions with (0001) and (001) Tungsten Carbide Surfaces: Importance of Carbon and Metal Sites

Abstract

In this work, a systematic study on the adsorption of atomic and molecular hydrogen and carbon oxides on cubic (001) and hexagonal (0001) WC surfaces by periodical density functional theory is reported. Calculations have been performed by employing the Perdew–Burke–Ernzerhof exchange correlation functional with van der Waals corrections to account for the dispersive force term. In addition, dipole corrections were applied for W- and C-terminated hexagonal WC(0001) surfaces. Good agreement is found between calculated and reported data for representative bulk properties. Regarding surface properties, our results indicate that atomic hydrogen adsorbs quite strongly while H2 does, in general, dissociatively on the studied surfaces, with very small energy barriers (<0.35 eV) for the cleavage of the H–H bonds. The C sites of the carbide play an essential role in the binding of H atoms and the cleavage of H–H bonds. Studies examining the interaction of tungsten carbide with CO and CO2 also evidence the importance of C sites. The reactivity of C- and W-terminated (0001) hexagonal WC surfaces significantly differs. Atomic hydrogen, carbon monoxide, and CO2 are more stable on a C- than on a W-terminated surface, and only this latter termination is able to cleave spontaneously a C–O bond of the CO2 molecule. This difference in reactivity may open a number of possibilities for fine-tuning the selectivity of the resulting material or designing compounds catalytically active for specific reactions by carefully adjusting the proportion of C, W, and mixed terminations during the synthesis procedure.

Supported Molybdenum Carbide Nanoparticles as Hot Hydrogen Reservoirs for Catalytic Applications

“…Hydrogenation reactions are common in many industrial processes, e.g., in the petrochemical plastic generation, the saturation of vegetable oils polyunsaturated fat acids in food chemistry, or in environmental chemistry, converting greenhouse carbon dioxide (CO 2 ) into greener chemicals . Hitherto, hydrogenations have been mostly catalyzed using scarce and expensive Pt-group metals. , During the last decades, transition Metal Carbides (TMCs) have emerged as an alternative, often displaying superior activities and selectivities for a wide variety of relevant hydrogenation reactions. − …”

Concepts, models, and methods in computational heterogeneous catalysis illustrated throughCO2conversion

“…These materials are normally regarded as available and economic potential substitutes to catalysts based on scarce and expensive Pt‐group late transition metals, displaying similar or even superior catalytic activities and selectivities 218 . One appealing aspect of TMCs concerning CO 2 conversion is that such materials display significantly strong CO 2 adsorption energies, 219 e.g., ranging from −0.61 eV for the C‐terminated orthorhombic β‐Mo 2 C (001) surface, to −3.27 eV for Mo‐terminated β‐Mo 2 C (001) surface, 220 as estimated by DFT means using slab supercell models and the PBE density functional 132 . Regardless of the TMC crystal structure, a common feature is that CO 2 gets not only strongly adsorbed, but also significantly activated; this is, with a bent geometry, see Figure 5(a), and displaying a negative Bader charge 204…”

“…233 Indeed, the theoretical modeling and calculated reaction free energy profile at 600 K with equal pressures of 0.2 bar of both CO 2 and H 2 reactants, see Figure 5(b), revealed that all surfaces are well capable of adsorbing, activating, and easily breaking CO 2 , with energy barriers below 0.59 eV, and specially low for Mo-terminated surfaces, in line with previous results on tungsten carbides with barriers as low as 0.71 eV for Wterminated surfaces. 219 Similarly, H 2 easily dissociates on all the explored TMCs 219,234 indicating that they may be useful for CO 2 conversion to methanol, formic acid, or CO in an H-assisted mechanism.…”

Activation of Gold on Metal Carbides: Novel Catalysts for C1 Chemistry

“…When the metal/carbon ratio is close to one, CO and methanol are the main products for the hydrogenation of CO 2 over a carbide catalyst (Dubois et al, 1992;Xu et al, 2014;Posada-Pérez et al, 2016). Theoretical calculations have shown that, in general, CO 2 binds well on MC(001) surfaces (M = Ti, Mo, Zr, Hf, Nb, Ta, Hf, and W) (Vidal et al, 2012;Posada-Perez et al, 2014;Posada-Pérez et al, 2016;Kunkel et al, 2016;Dixit et al, 2017;Koverga et al, 2019). Figure 11 shows results of DF calculations for the bonding geometry of the CO 2 molecule on plain TiC(001) (Vidal et al, 2012).…”