Theoretical studies of MXene-supported single-atom catalysts: Os-1/Ti2CS2 for low-temperature CO oxidation

We report herein a new sulfur-functionalized MXene Ti2C (Ti2CS2)-supported osmium-metal single-atom catalyst (SAC) Os-1/Ti2CS2 with high low-temperature catalytic activity for CO oxidation. Using periodic density functional theory calculations, the most stable SAC, Os-1/Ti2CS2, has been screened from a series of group 8-11 transition metal SACs M-1/Ti2CS2 (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au). The calculations show that it is favorable for O-2 and CO to be coadsorbed on the Os-1 single atom (SA) of Os-1/Ti2CS2 and the adsorption energy of the first O-2 molecule is slightly higher than that of CO. Moreover, the termolecular co-adsorption of O-2 + 2CO on Os-1 SA is also possible, which is favorable for CO oxidation on Os-1 SA through a novel three-molecule reaction mechanism. Accordingly, four different catalytic mechanisms, the Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), termolecular Langmuir-Hinshelwood-A (TLH-A) and termolecular Langmuir-Hinshelwood-B (TLH-B), are systematically studied for CO oxidation by O-2 on OW Ti2CS2. The theoretical studies indicate that the TLH-B mechanism is the most feasible for CO oxidation with the reaction barrier energy of only 0.74 eV, which is far lower than for L-H, E-R and TLH-A with barrier energies of 1.06, 1.09 and 1.47 eV, respectively. The results provide fundamental understanding to the surface chemistry of MXene and designing new sulfur-functionalized two-dimensional MXene catalytic nanomaterials.