On the Availability of Active Sites for the Hydrogen Peroxide and Oxygen Reduction Reactions on Highly Dispersed Platinum Nanoparticles

Carbon-supported platinum nanoparticles (Pt/C) were synthesized by the carbonyl chemical route. The obtained Pt/C catalysts with different Pt mass loadings, varying from 5 to 50 wt%, showed constant morphology with particle sizes of about 2 nm and a dispersion of around 50%. The electrochemical surface area, determined from CO stripping and hydrogen underpotential deposition, increased with the Pt mass loading, which confirmed that agglomeration was negligible. The oxygen reduction reaction (ORR) half-wave potential was positively shifted with an increase in the Pt mass loading, and the ORR kinetic current was (40 +/- 10) mu Acm(-2) Pt. The data collected on a rotating ring-disk electrode for the ORR pathway revealed a two-electron serial mechanism (hydrogen peroxide), instead of the four-electron transfer (water). The determined turnover frequency factor indicated good selectivity of the Pt active sites for the ORR with 1 or 2mm H2O2 in the electrolyte for samples with mass loadings ranging from 15 to 40 wt%. The ORR was markedly affected by the mass loading (5-10 wt% Pt/C) in the presence of 2 mm H2O2.