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

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