Nitrogen-doped metal-free carbon catalysts for aerobic oxidation of xanthene

•Nitrogen doping to activated carbon enhances its activity for xanthene oxidation.•Doped nitrogen atoms are in the forms of pyridine-type and pyrrole-/pyridone-type.•NH3–air is more effective than NH3 alone and NO for nitrogen doping.•Pyridine-type nitrogen is suggested to be involved in the active sites.•Nitrogen-doped carbon is more active than a commercial Pd catalyst.

N-doped carbons were prepared from a commercial activated carbon (AC) by heat treatments in pure NH3, an NH3–air mixture, and NO and these were used for a model reaction of aerobic oxidation of xanthene to xanthone (XO). The catalytic activity of AC for the title reaction was greatly enhanced by N-doping and the enhancement in the activity depended on the N source gasses used. The effectiveness of the three gases for both N-doping and enhancing the activity was in an order of NH3–air > NH3 > NO. The temperature of N-doping also affected the catalyst activity but not so significantly. X-ray photoelectron spectroscopy (XPS) indicated the presence of pyridine-type and pyrrole-/pyridone-type N species. The catalytic activity of N-doped carbon was correlated with the ratio of the amount of pyridine-type N against that of ether-type O species. The structures of the active sites were discussed on the basis of the XPS and reaction results. Metal-free N-doped AC was less active than a commercial Ru/AC catalyst but more active than a commercial Pd/AC one. The activity of N-carbon slightly decreased after the first recycling but it remained almost the same during the following two-time recycling.

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