  Active sites and catalytic mechanism of nitrogen-doped graphene in oxygen
reduction reaction (ORR) have been extensively studied but are still
inconclusive, partly due to the lack of an experimental method that can detect
the active sites. It is proposed in this report that the active sites on
nitrogen-doped graphene can be determined via the examination of its chemical
composition change before and after ORR. Synchrotron-based X-ray photoelectron
spectroscopy analyses of three nitrogen-doped multilayer graphene samples
reveal that oxygen reduction intermediate OH(ads) which should chemically
attach to the active sites remains on the carbon atoms neighboring pyridinic
nitrogen after ORR. In addition, a high amount of the OH(ads) attachment after
ORR corresponds to a high catalytic efficiency and vice versa. These pinpoint
that the carbon atoms close to pyridinic nitrogen are the main active sites
among the different nitrogen doping configurations.
