Understanding the chemisorption-based activation mechanism of the oxygen reduction reaction on nitrogen-doped graphitic materials

To optimize nitrogen-doped graphitic materials as metal-free catalysts for the oxygen reduction reaction mechanisms have to be better understood. Here, the role played by pyridinic nitrogen-dopants in the chemisorption-based activation of the target reaction is revealed. The study is centered on the monodentate chemisorption of molecular oxygen as the first step of the process. Several configurations of unclustrered nitrogen dopants in which there was always a nitrogen dopant in the edge of the material were tested using DFT. A clearly favorable chemisorbed state for molecular oxygen was found when the pyridinic nitrogen-dopant is hydrogenated and located at an armchair edge. The found chemisorbed state is further favored by additional available charge. By contrast, the chemisorbed state of oxygen is much less favorable when the hydrogenated pyridinic nitrogen-dopants are located at zigzag edges. Moreover, it was found that the charge involved in the hydrogenation of pyridinic nitrogen-dopants remains segregated, becoming available for reduction processes. Detailed reasons for the described facts are given, and an integrated model for the target activation mechanism is proposed including graphitic nitrogen-dopants effects.