Synergistic Effect of Nitrogen Dopants on Carbon Nanotubes on the Catalytic Selective Epoxidation of Styrene

Nitrogen dopants on carbon materials play vital roles in metal-free carbon catalysis, while the role of pyridinic nitrogen (N_P) or graphitic nitrogen (N_G) is still on debate. In this work, N-doped carbon nanotubes were employed to catalyze the oxidation of styrene (SOR) with tert-butyl hydroperoxide as oxidant. We observed a significant enhancement of SOR activity caused by the incorporation of nitrogen dopants when the N content was lower than 3.51% but a decline of activity at higher N contents. At N content up to 7.51%, even worse performance than that on N-doped carbon nanotubes with 0.98% N content was observed. This behavior was rationalized by thorough density functional theory calculations. Different catalytic functions of N_P and N_G were revealed. More significantly, a synergistic effect between N_P and N_G was demonstrated when they are quite close, which is unbeneficial for the catalytic activity due to the very high energy barriers. The finding may pave a way to the rational design of high-performance metal-free carbon catalysts.