Nitrogen-containing carbon support and Palladium-loaded Tungsten carbide catalyst in electrochemical hydrogen oxidation

Abstract

Platinum metal loaded on carbon has been widely used as a catalyst in proton exchange membrane fuel cell. However, durability and cost problems delay the commercialization of this system. To solve these problems, my researches have been focused on the catalyst and support materials. Nitrogen-containing carbon materials were prepared by acetonitrile pyrolysis on carbon black and used as a support for a Pt catalyst. The Pt particles on N-containing carbon exhibited increased activity and stability in electrochemical hydrogen oxidation relative to Pt on pristine carbon black. The N-doped carbon had a graphitic structure and contained pyridinic and quaternary nitrogen species. The Pt nanoparticles were better-dispersed because of increased hydrophilicity induced by the nitrogen species. And the Pt/N-containing carbon showed higher stability in a potential cycling test than Pt/C, because of an increased metal-support interaction. Using XPS and EELS mapping, I demonstrated that the metal-support interaction became stronger and more specific by adding nitrogen into carbon.Although the carbon oxidation problem could be somewhat relieved by N-doping, carbon black has micropores mostly. Thus it is not adequate for a reaction, because the existence of mesopores is important to make triple-phase boundary for fuel cell reaction. Thus I synthesized mesoporous carbon and N-containing mesoporous carbon using resorcinol, formaldehyde, cetyltrimethylammonium bromide, Ludox and urea. And then loaded 20 wt % Pt on them by polyol method using ethylene glycol. TEM images show that Pt particles loaded on N-containg mesoporous carbon are more dispersed than mesoporous carbon like N-containing carbon black. Although they showed a little higher hydrogen oxidation activity to the commercial Pt/C, both of them were inadequate to replace the commercial Pt/C. Therefore, I tried to find another catalysts. Tungsten carbide/mesoporous carbon composite material was synthesized by using resorcinol, formaldehyde, cetyltrimethylammonium bromide and Ludox HS-40 and used as a support for Pd catalyst. Non-modified Pd (Pd) particles were loaded on the carbon black, mesoporous carbon and tungsten carbide/mesoporous carbon composite by using NH3-mediated polyol reduction method and phosphorus-modified Pd (Pd(P)) particles were also loaded on the supports by the same reduction method with the aid of sodium hypophosphite. The sizes of Pd and Pd(P) particles loaded on the mesoporous carbon were smaller than those of Pd and Pd(P) particles loaded on the carbon black because of the high surface area of the mesoporous carbon and thus showed larger hydrogen oxidation activity. In the case of tungsten carbide, the sizes of Pd and Pd(P) particles loaded on the tungsten carbide/mesoporous carbon composite were similar and a little smaller than those of Pd and Pd(P) loaded on the mesoporous carbon but the hydrogen oxidation activity was largely increased by the strong metal - support interaction between the Pd and the WC. Especially, Pd(P) loaded on the tungsten carbide/mesoporous carbon composite showed larger hydrogen oxidation activity than the commercial Pt/C catalyst. Through XRD, TEM-EDX and XPS, I demonstrated that the phosphorus existed mostly as a phosphate group on the supports. Thus the attractive interaction between the Pd precursor, [Pd(NH3)4]2+ and the phosphate group, PO43- makes small Pd(P) nanoparticles and thus maximizes the Pd(P) particles dispersion on the supports compared to the Pd particles. Through the strong metal - support interaction and the phosphorus modification, Pd(P) loaded on the tungsten carbode/mesoporous carbon composite showed a possibility for replacing commercial Pt/C catalyst in the anode of proton exchange membrane fuel cells.