Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped Carbon via Pyrolysis of Chitosan/Urea/KOH Mixtures and Its Application as a Support for Formic Acid Dehydrogenation Catalysts

The development of cheap, simple, and green synthetic methods for hierarchically porous nitrogen-doped carbon, especially derived from renewable biomass, such as chitosan, remains a challenging topic. Here, we first synthesized hierarchically porous nitrogen-doped carbon (KIE-8) having graphene-like structure via simple pyrolysis of a chitosan/urea/KOH mixture without any conventional sophisticated treatments, such as freeze-drying, hydrothermal carbonization, and soft or hard templating. On the basis of various analyses of KIE-8, we demonstrated that effect of urea on mesopore formation was insignificant; however, when KOH is used as an activating agent in the presence of urea, a large amount of mesopores can be created along with conventional KOH-derived micropores. In addition, it was revealed that chitosan-derived carbon nanosheets directed by urea are torn into chitosan-derived carbon nanoflakes via KOH activation, and mesopores originate from interstitial voids in aggregates of the carbon nanoflakes, and micropores are derived from in-plane pores in each nanoflake. KIE-8 was used as a catalyst support for formic acid dehydrogenation at room-temperature. Pd(6 wt %)/KIE-8 catalysts provided excellent catalytic activity (TOP = 280.7 mol H-2 mol metal(-1) h(-1)), and we demonstrated that the pore structure and nitrogen structure of KIE-8 are crucial factors to determine the catalytic activity.