However, for FeSA-N5-C, only O2 molecules form bonds with it, while the other three molecules do not form bonds with it. In addition, the adsorption energy of four gas molecules is highest on the surface of FeSA-N3-C and lowest on the surface of FeSA-N5-C, indicating that adsorption capacity of FeSA-N3-C is strongest and that of FeSA-N5-C is weakest (Figure ). The adsorption energy of O2 is -3.13 eV (FeSA-N3-C), indicating that FeSA-N3-C has a strong adsorption capacity for O2 and that the adsorbed O2 can be used as active species to catalyze the formaldehyde reaction. CO2 and H2O molecules are desorbed from FeSA-N5-C surface with relative ease, which facilitates the oxidation reaction cycle of HCHO. In terms of adsorption energy of H2O molecules (-0.16 eV), the adsorption of H2O molecules, which belongs to physical adsorption, is weakest for FeSA-N5-C, indicating that FeSA-N5-C has excellent water resistance. According to the CeO2 (1 1 1) surface studied by Teng et al.