Single-Atom Catalysts Based on O-Functionalized MXenes

Abstract

Single-atom catalysts (SACs) are the maximal expression of nanostructuring, of vital importance to maximize the efficiency of late, scarce, and expensive transition metals (TMs), regularly used as catalysts for a large variety of industrial chemical applications. SACs are fundamentally determined by their stability, where proper substrates that disperse them are key. Focusing on these, two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, have gained much interest due to their inherent 2D nature, which suggests them as a suitable platform for disperse TM adatoms. Here we study, by density functional theory (DFT), the TM single-atom adsorption energy and stability for 4d and 5d TMs on nine O-functionalized MXenes with stoichiometry M2CO2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W), reliable models according to experimental evidences. The TM adsorption sites and heights are identified, analyzing adsorption energy trends along the d series, as well the MXene groups and series, identifying that the adsorption strengths decay along the d series, and are highly influenced by the TM stability, where especially stable d5 and d10 configurations show the smallest adsorption energies. Aside, the adsorption strength increases along the MXene series, but decays down the MXene groups. SACs are found to be energetically favorable for early and late TMs, and specially enhanced on V2CO2, Cr2CO2, Mo2CO2, and W2CO2 MXenes, although dispersion and/or aggregation can be affected by the TM adatom diffusion energy barriers