PEO-b-PS Block Copolymer Templated Mesoporous Carbons: A Comparative Study of Nitrogen and Sulfur Doping in the Oxygen Reduction Reaction to Hydrogen Peroxide.

Carbon materials slightly doped with heteroatoms such as nitrogen (N-RFC) or sulfur (S-RFC) are investigated as active catalysts for the electrochemical bielectronic oxygen reduction reaction (ORR) to H2 O2 . Mesoporous carbons with wide, accessible pores were prepared by pyrolysis of a resorcinol-formaldehyde resin using a PEO-b-PS block copolymer as a sacrificial templating agent and the nitrogen and sulfur doping were accomplished in a second thermal treatment employing 1,10-phenanthroline and dibenzothiophene as nitrogen and sulfur precursors, respectively. The synthetic strategy allowed to obtain carbon materials with very high surface area and mesopore volume without any further physicochemical post treatment. Voltammetric rotating ring-disk measurements in combination with potentiostatic and galvanostatic bulk electrolysis measurements in 0.5 m H2 SO4 demonstrated a pronounced effect of heteroatom doping and mesopores volume on the catalytic activity and selectivity for H2 O2 . N-RFC electrode was employed as electrode material in a 45 h electrolysis showing a constant H2 O2 production of 298 mmol g-1 h-1 (millimoles of H2 O2 divided by mass of catalyst and electrolysis time), with a faradic efficiency (FE) up to 61 % and without any clear evidence of degradation. The undoped carbon RFC showed a lower production rate (218 mmol g-1 h-1 ) but a higher FE of 76 %, while the performances drastically dropped when S-RFC (production rate 11 mmol g-1 h-1 and FE=39 %) was used.

Metal-support interaction in platinum and palladium nanoparticles loaded on nitrogen-doped mesoporous carbon for oxygen reduction reaction.

Mesoporous carbons are highly porous materials, which show large surface area, chemical inertness and electrochemical performances superior to traditional carbon material. In this study, we report the preparation of nitrogen-doped and undoped mesoporous carbons by an optimized hard template procedure employing silica as template, sucrose and ammonia as carbon and nitrogen source, respectively. Surface area measurements assert a value of 900 and 600 m(2) g(-1) for the best doped and undoped samples, respectively. Such supports were then thoroughly characterized by surface science and electron microscopy tools. Afterward, they were decorated with Pt and Pd nanoparticles, and it was found that the presence of nitrogen defects plays a significant role in improving the metal particles dimension and dispersion. In fact, when doped supports are used, the resulting metal nanoparticles are smaller (2-4 nm) and less prone to aggregation. Photoemission measurements give evidence of a binding energy shift, which is consistent with the presence of an electronic interaction between nitrogen atoms and the metal nanoparticles, especially in the case of Pd. The catalytic properties of electrodes decorated with such catalyst/support systems were investigated by linear sweep voltammetry and by rotating disk electrode measurements, revealing excellent stability and good activity toward oxygen reduction reaction (ORR). In particular, although Pd nanoparticles always result in lower activity than Pt ones, both Pt and Pd electrodes based on the N-doped supports show an increased activity toward ORR with respect to the undoped ones. At the same mass loading, the Tafel slope and the stability test of the Pt@N-doped electrocatalysts indicate superior performances to that of a commercial Pt@C catalysts (30 wt % Pt on Vulcan XC-72, Johnson Matthey).