Active Sites for Oxygen Reduction Reaction in Tantalum Oxynitride Nanoparticles on Multi-Walled Carbon Nanotubes Synthesized Using Ammonia Pyrolysis

Group 4 or group 5 metal oxide compounds have attracted attention as an oxygen reduction reaction (ORR) catalyst in acidic media where they displayed excellent stability.¹ Because of their short development period, discussions on the active sites in these oxide compounds are lacking compared with those in other non-platinum group metal catalysts, e.g., compounds of iron/cobalt, nitrogen and carbon and these metal-free nitrogen-doped carbons (CN_x). Besides, recent progress in the activity of oxide compounds has attracted suspicious eyes for the active sites as it was achieved primarily by changing the oxide precursor from the metal carbonitrides to oxy-metal phathalocyanine, which could produce both oxides and CN_x. We recently synthesized oxide catalyst containing one of the group 5 metals, tantalum via the decomposition of oxy-tantalum phthalocyanine (TaOPc) in melted salicylic acid powders to control the size of tantalum oxide (TaO_x) particles to the order of nanometers and to connect TaO_x to a support, multi-walled carbon nanotube (MWCNT). Graphitic carbons built from TaOPc successfully provided conductive paths to TaO_x insulation particles.² The activity of TaO_x-MWCNT was far higher than tantalum-free phthalocyanine-adsorbed MWCNTs after pyrolysis under identical conditions. This work thus demonstrated that the active site was TaO_x, not CN_xspecies originating from TaOPc.

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 CN_x 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 CN_x. In this study, two different TaO_x-MWCNT catalysts were synthesized by changing the preparation of TaOPc precursors. Then they were pyrolyzed under ammonia gas at various temperatures to obtain TaO_xN_y-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.

 

References

(1)           A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima and K. Ota, Electrochim. Acta, 55, 8005 (2010).

(2)           A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima and K. Ota, Phys. Chem. Chem. Phys., 17, 7643 (2015).

(3)           M. Chisaka, T. Iijima, T. Yaguchi and Y. Sakurai, Electrochim. Acta, 56, 4581 (2011).

(4)           M. Chisaka, H. Sasaki and H. Muramoto, Phys. Chem. Chem. Phys., 16, 20419 (2014).

Acknowledgment

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.