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Surface Activity of Early Transition-Metal Oxycarbides: CO₂ Adsorption Case Study

Kunkel, Christian, Viñes, Francesc, Illas, Francesc
Journal of physical chemistry 2019 v.123 no.6 pp. 3664-3671
adsorption, air, carbides, carbon dioxide, carbon sequestration, case studies, density functional theory, desorption, models, molybdenum, oxidation, oxygen, temperature, zirconium
Theoretical studies and experiments have suggested that transition-metal carbides (TMCs) can be useful materials for carbon capture and storage or usage technologies from air sources. However, TMCs are known to become easily oxidized in the presence of molecular oxygen, and their properties jeopardized while being transformed into transition-metal oxycarbides (TMOCs), which can affect the TMCs’ chemical activity, for example, towards CO₂. Here, by means of density functional theory (DFT) based calculations including dispersion, we address the possible effect of oxycarbide formation in the CO₂ capture course. A careful analysis of different models shows that for group 4 TMCs (TM = Ti, Zr, Hf), their oxidation into TMOCs involves a negligible structural distortion of the outermost oxide surface layer, whereas severe rumplings are predicted for group 5 and 6 TMOCs (TM = V, Nb, Ta, Mo). The large surface distortion in the latter TMOCs results in a weak interaction with CO₂, with adsorption energies below −0.27 eV. On the contrary, in group 4 TMOC surfaces, CO₂ adsorption becomes stronger, with the adsorption values increased by 0.44–1.2 eV, which, according to adsorption/desorption rate estimates, increments the air CO₂ capture temperature window by 175–400 K. The present DFT results point to group 4 TMCs, TiC in particular, as promising materials for air CO₂ capture and storage/conversion, even in the presence of oxygen and the possible formation of TMOCs.