Crossover between multipole Coulomb and Kubas interactions in hydrogen adsorption on metal-graphene complexes

The hydrogen adsorption on alkaline-earth metal dispersed in doped graphenes was studied through ab initio calculations. Substitutional doping in graphenes is explored to control the ionic state of the metal atoms that plays a crucial role for dispersion and hydrogen adsorption. It was found that the adsorption behavior, particularly in Ca-dispersed graphene complexes, exhibits a crossover between the multipole Coulomb and Kubas-type (or orbital) interactions as the ionic state of Ca and the number of adsorbed hydrogen molecules change. The level exchange in s and d orbitals of Ca is responsible for the crossover. This finding enables the optimization of hydrogen adsorption and metal dispersion in graphitic materials, which is useful for developing solid hydrogen storage and efficient catalysts.