Department of Physics, Umeå University, S-901 87 Umeå, Sweden.
ACS Nano. 2012 Oct 23;6(10):8904-12. doi: 10.1021/nn302906r. Epub 2012 Oct 8.
Heat treating nitrogen-doped multiwalled carbon nanotubes containing up to six different types of nitrogen functionalities transforms particular nitrogen functionalities into other types which are more catalytically active toward oxygen reduction reactions (ORR). In the first stage, the unstable pyrrolic functionalities transform into pyridinic functionalities followed by an immediate transition into quaternary center and valley nitrogen functionalities. By measuring the electrocatalytic oxidation reduction current for the different samples, we achieve information on the catalytic activity connected to each type of nitrogen functionality. Through this, we conclude that quaternary nitrogen valley sites, N-Q(valley), are the most active sites for ORR in N-CNTs. The number of electrons transferred in the ORR is determined from ring disk electrode and rotating ring disk electrode measurements. Our measurements indicate that the ORR processes proceed by a direct four-electron pathway for the N-Q(valley) and the pyridinic sites while it proceeds by an indirect two-electron pathway via hydrogen peroxide at the N-Q(center) sites. Our study gives both insights on the mechanism of ORR on different nitrogen functionalities in nitrogen-doped carbon nanostructures and it proposes how to treat samples to maximize the catalytic efficiency of such samples.
将含多达六种不同类型氮官能团的氮掺杂多壁碳纳米管进行热处理会将特定的氮官能团转化为对氧还原反应（ORR）更具催化活性的其他类型。在第一阶段，不稳定的吡咯官能团转化为吡啶官能团，随后立即转化为季氮中心和谷氮官能团。通过测量不同样品的电催化氧化还原电流，我们获得了与每种氮官能团相关的催化活性信息。通过这种方式，我们得出结论，N-CNTs 中最活跃的 ORR 活性位是季氮谷位，N-Q(valley)。ORR 中转移的电子数通过环盘电极和旋转环盘电极测量确定。我们的测量表明，对于 N-Q(valley)和吡啶位，ORR 过程通过直接的四电子途径进行，而对于 N-Q(center)位，通过过氧化氢的间接两电子途径进行。我们的研究不仅深入了解了氮掺杂碳纳米结构中不同氮官能团上的 ORR 机制，还提出了如何处理样品以最大限度地提高此类样品的催化效率。
