Graphitic, nitrogen atoms

While the improvement of the catalytic activity observed by Ozkan and collaborators when their CNx nanostructures were grown on Fe-impregnated supports was interpreted in terms of an increased formation of graphitic edges stabilized by pyridinic nitrogen atoms (and perhaps also by graphitic nitrogen atoms), an alternative explanation to their ORR activity improvement may be the... 

In 2009 [103] and 2010 [104], Takasu and his collaborators used carbonized silk to perform ORR in acid medium (0.5 M H2SO4). The catalysts were derived from a Bombix mori silk fibroin containing 18 different amino acids. The silk was first carbonized at 500 °C in N2 and then ball milled and heat treated in N2 at various temperatures between 500 and 1,500 °C. The resulting materials were finally steam activated at 850 °C. The best results were obtained for a heat treatment of the carbonized silk at 900 °C in N2. This catalyst had a surface area of 588 m /g and an N content of 6 at.% with 29 % of pyridinic and 59 % of graphitic nitrogen atoms. Its Fonset was 0.69 V vs. RHE [103] or 0.73 V vs. RHE [104]. By comparison, the Fonset for Vulcan was 0.20 V vs. RHE. The catalyst was also characterized, for potentials... 

In 2011, Woo and collaborators prepared ORR catalysts from the pyrolysis at 700, 800, and 900 °C of a mixture of iron oxide supported on Vulcan and dicyandiamide (C2H4N4 a dimer of cyanamide) [111]. TEM of the catalysts revealed that at 700 °C metal particles were encapsulated with a carbon layer, while they were mostly in carbon tubes at 900 °C. The total N content was 2.2,3.5, and 6.6 at.% for the catalysts heat treated at 700,800, and 900 °C, respectively. This N content was broken down as 54 % pyridinic and 0 % graphitic nitrogen atoms at 700 °C, while it was 61.4 % pyridinic and 10.7 % graphitic at 900 C. The Fe content was also measured in these catalysts and also for the catalyst iron... 

While pyridinic nitrogen atoms rally contribute one electron to the Ji-electron system of the carbon, pyrrolic and graphitic nitrogen atoms increase the Jt-electron... 

Figure 11.16 The (a) graphitic, (b) cubic and (c) pseudocubic forms of C3N4. Carbon atoms are dark and nitrogen atoms light.

Any element consists of only one kind of atom. Gold consists of only gold atoms, a flask of gaseous nitrogen consists of only nitrogen atoms, and the carbon of a graphite pencil consists of only carbon atoms. 

Figure 8. Center-of-mass trajectories obtained from a simulation of a patch of nitrogen molecules adsorbed on the basal plane of graphite. (Carbon atoms are shown by the small dots.) In part (a) for T=36.9 K, the molecules are commensurate with theVSxVS lattice and vibrate around the site centers except at the edges of the patch. In part (b), 7M4.0 K and the patch has mehed to a 2D fluid that is characterized by chaotic trajectories in the him. (At longer simulation times, the molecules appear to fill the surfece as a 2D gas.) From Ref. [38], Mol. Phys. 55 (1985) 999-1016.

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