Active Sites for Oxygen Reduction Reaction in Tantalum Oxynitride Nanoparticles on Multi-Walled Carbon Nanotubes Synthesized Using Ammonia Pyrolysis
Substitution of oxygen atoms in these oxide catalysts by nitrogen is an excellent technique to stabilize oxygen defects which have been acknowledged to be an ORR active sites. The nitrogen-doping has often been performed after oxide particles were supported on carbon supports3 or simaltaneously.4 Therefore, ORR activity of CNx species synthesized by the nitrogen doping process and that of metal-oxynitrides have been carefully compared to discuss active sites.3,4 In any of these previous works,3,4 the oxynitrides showed higher activity than CNx. In this study, two different TaOx-MWCNT catalysts were synthesized by changing the preparation of TaOPc precursors. Then they were pyrolyzed under ammonia gas at various temperatures to obtain TaOxNy-MWCNTs. The ORR activity of these two catalyst types in acidic media have been carefully compared to clarify the active sites. For the first time, slightly oxidized Ta3N5was proposed to be an ORR active site.
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The authors gratefully acknowledge Dainichiseika Color & Chemicals Mfg. Co. and Showa Denko K. K. for supplying TaOPc and MWCNT powders, respectively. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.