CO, CO ₂ , and H ₂ Interactions with (0001) and (001) Tungsten Carbide Surfaces: Importance of Carbon and Metal Sites

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 H ₂ 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 CO ₂ 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 CO ₂ 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 CO ₂ 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.