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Semiconducting mixed cluster Chevrel compounds (Mo 4 Ru 2 Se 8 ) catalyze the reduction of molecular oxygen in acid medium

Single crystals of Re 6 Se 8 Br 2 and Re 6 S 8 Br 2 were synthesized by chemical vapour transport reactions. The latter was crystallized. X-ray structure analysis reveals that Re 6 S 8 Br 2 is isostructural to Re 6 Se 8 Br 2 . Both semiconducting cluster materials were photoelectrochemically characterized and showed p-type behaviour. From the photocurrent action spectra, their estimated band gaps are 0.84 eV and 1.65 eV for the selenide and sulphide, respectively, with a probably indirect optical transition.

The oxygen reduction reaction (ORR) activity of a Ru 1.92 Mo 0.08 SeO 4 catalyst, a Vulcan XC72-supported Ru catalyst and, for comparison, a Vulcan XC72-supported Pt catalyst was studied with a rotating ring-disk electrode. The very similar reaction characteristics of the two Ru catalysts in pure and CH 3 OH-containing H 2 SO 4 electrolyte, which differ markedly from those of the Pt catalyst, indicate that the reactive centers in both Ru catalysts must be identical. They are highly selective (>95%) toward reduction to H 2 O (four electron pathway), independent of the presence of methanol. In the latter case, they are 100% selective toward the ORR, i.e., completely methanol tolerant, while the ORR on Pt catalysts is accompanied by significant CH 3 OH oxidation. Based on mass specific current densities, however, the Ru catalysts are significantly less active than the standard Pt catalysts. Only at methanol concentrations above 10-30 mM does their methanol tolerance make them more active than Pt/Vulcan. Implications for their use as cathode catalysts in a direct methanol fuel cell are discussed.

We report the results of an investigation of the interaction between Se and Ru in Ru−Se electrocatalysts showing high activity and methanol tolerance in the oxygen reduction reaction (ORR). The res...