Activities of Pt/C catalysts prepared by low temperature chemical reduction methods

Carbon supported Pt nanoparticles were prepared in aqueous solutions by two methods: (1) room temperature NaBH4 reduction of H2PtCl6 with (Pt-1) or without (Pt-2) sodium citrate as the stabilizing agent and (2) microwave-assisted reduction of H2PtCl6 at boiling temperature using citrate as the reducing agent cum stabilizer (Pt-3 and Pt-4). The catalysts were characterized by X-ray diffraction, energy dispersive X-ray microanalysis and transmission electron microscopy, and their activities in room temperature methanol electrooxidation were evaluated by cyclic voltammetry and benchmarked against a commercial E-TEK catalyst with the same Pt loading. TEM showed smaller Pt particles if citrate was present in the synthesis, with average particle sizes of 4.0 and 2.2 nm for Pt-1 and Pt-3, respectively. While the presence of citrate was generally beneficial to forming smaller and more uniform Pt nanoparticles, a large excess of citrate (H2Cyt−:PtCl62− = 20:1) in the microwave synthesis had led to a decrease in catalytic activity. The microwave-synthesized Pt/C obtained using a smaller excess of citrate (H2Cyt−:PtCl62− = 5:1) demonstrated the best mass activity, surpassing the performance of the commercial catalyst. In addition, a size effect was found in this study: with the decrease in the platinum particle size in the range of 2.2–4 nm, the specific activity of Pt for room temperature methanol oxidation also decreased.