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 (NP) or graphitic nitrogen (NG) 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 NP and NG were revealed. More significantly, a synergistic effect between NP and NG 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.