Hydrogen adsorption and dissociation on the TM-doped (TM=Ti,Nb) Mg55 nanoclusters: A DFT study

The slow hydrogenation kinetics and high reaction temperature of Mg primarily limit its application for mobile hydrogen storage. H₂ adsorption and dissociation on the pure and TM-doped (TM = Ti, Nb) Mg₅₅ nanoclusters are systematically studied by using density functional theory (DFT) calculations. It is found that the introduction of Ti and Nb atoms into Mg₅₅ nanocluster can greatly modify the electronic structure of Mg₅₅ nanocluster and enhance the stability of system. Through the analyses of results from the climbing image nudged elastic band (CI-NEB) and reaction rate constant, we also find that the energy barriers of H₂ dissociation on TM-doped Mg₅₅ nanoclusters can be significantly decreased due to the addition of Ti and Nb. Adding Ti and Nb atoms can dramatically improve the rate constant of H₂ dissociation, especially for H₂ dissociation on Mg₅₄TM2 (TM atom replacing the inner shell position), Mg₅₄TM3 (TM atom replacing the outermost vertex) and Mg₅₄TM4 (TM atom replacing the outermost edge position) nanoclusters. Moreover, compared with the Ti dopant, the Nb will generate a lower activation barrier for H₂ dissociation on TM-doped Mg₅₅ nanoclusters. We also suggest that the subsurface and surface positions (Mg₅₄TM2, Mg₅₄TM3, Mg₅₄TM4) are the ideal substitutional sites for TMs.