The mixture was then refluxed at 60 °C for 4 h. The ethanol solvent was removed by rotary evaporation and the remaining solids were dried overnight in air and finely ground. The powder was calcinated at 700 °C for 2 h under a nitrogen atmosphere (ramping rate: 2 °C/min). A black solid was obtained. It was stirred in a 1 M HNO3 solution at room temperature for 2 h to remove the MgO support and residual cobalt nanoparticles. The solid was further washed with ultra-pure water until the filtrate became neutral. The solid was dried in air at room temperature overnight (see Scheme S1 for illustration).