On the performance of pure and group 2B transition metal-doped metal oxide nanocages as single-atom catalysts for the hydrogen storage process: a DFT study

For an efficient confrontation of the exhaustion of nonrenewable energy sources issue, the storage of hydrogen as an eco-friendly and renewable alternative energy source has received considerable attention. Herein, the performance of pure and group 2B transition metal-doped metal oxide nanocages (M₁₂O₁₂ and TM-M₁₁O₁₂; where M = Zn, Mg, and Be; TM = Zn, Cd, and Hg) as single-atom catalysts for the hydrogen dissociation reaction (HDR) was investigated using DFT calculations. Regarding step-I of the HDR, all the investigated catalysts exhibited remarkable potentiality to adsorb the H₂ molecule with negative BSSE-corrected adsorption energy values up to -5.22 kcal mol^-1. In step-II, further activation for the H₂ molecule over the surface of the M₁₂O₁₂ and TM-M₁₁O₁₂ catalysts occurred, and hence the transition state (TS) structure was obtained. Upon the energetic results, the Zn₁₂O₁₂-based catalysts exhibited higher performance toward the HDR compared to the Mg₁₂O₁₂- and Be₁₂O₁₂-based candidates. Furthermore, the Cd-Zn₁₁O₁₂ catalyst demonstrated the most promising catalytic activity with an activation energy of 9.58 kcal mol^-1 for the H₂⋯Cd-Zn₁₁O₁₂ complex. In step-III, one of two activated H atoms (H1) shifted to the Zn atom, whereas the other hydrogen atom (H2) migrated to the O atom. Analysis of natural bond orbitals and electron density difference outlined the charge transfer from M/TM atoms to their interacting hydrogen atom (H1) and from the O atom to the corresponding hydrogen atom (H2). Quantum theory of atoms in molecules outcomes demonstrated the partial covalent nature of the interactions within the TS structures, pinpointing the optimum catalytic efficiency. The obtained results will provide a comprehensive picture of the behavior of metal oxide-based SACs for HDR catalysis, and hence their performance for the hydrogen storage process.