Graphitic carbon nitride (g-C3N4) shows great application potential in the activation of CO2 due to its basic surface functionalities and highly specific electronic properties. Herein, to improve its catalytic performance, g-C3N4 was activated by protonation using H2SO4. The texture, surface chemistry and electronic properties of the as-prepared g-C3N4 were then studied. The synthesis of cyclic carbonate from the cycloaddition of CO2 and epoxide was selected as a model reaction to investigate the catalytic performance. The protonated g-C3N4 exhibited a much higher catalytic activity than the pristine g-C3N4. The generation of terminal amino and hydroxyl groups due to the hydrolysis of g-C3N4 under acidic conditions as well as the higher specific surface area are responsible for the enhanced catalytic performance.