Oxygen Reduction Reaction on Ruthenium and Rhodium Nanoparticles Modified with Selenium and Sulfur

Oxygen reduction reaction (ORR) was carried out in a sulfuric acid solution saturated with oxygen on Ru and Rh nanoparticles chemically modified with Se or S. Among the four chalcogen-modified specimens examined, the modification of Ru with Se shows the highest activity. P. Zelenay et al., U.S. Pat. Appl. filed Dec. 5, 2005. Therefore, the highlight of this study is the synthesis and use of the Ru/Se catalyst vs the cluster-type RuxSey catalysts investigated before, and on providing evidence toward similar reactivity functions between Ru/Se and RuxSey. On the nanoparticle Ru/Se electrode, ORR commences at 0.9 V and the diffusion-limiting reduction current is attained at approx 0.4 V vs a reference hydrogen electrode. ORR activity does not decrease in the presence of methanol, showing a full methanol tolerance at methanol concentrations investigated in this study. It is proposed that surface metallic Ru atoms embedded in Se matrices are the catalytic active sites to sustain ORR at a high level. However, the smooth Ru disk surface modified by selenium displays lower activity than a “clean” Ru disk (without Se) and a noticeably lower activity than the nanoparticle Ru/Se. Possible reasons for this behavior are discussed. Finally, a Ru/S catalyst is slightly more active than a H2-reduced Ru black catalyst but is much less active than Ru/Se and RuxSey, and is also methanol-tolerant at 0.1 M methanol concentrations. Both Se and S reduce ORR rates on Rh.