Marcus Hilgendorff

Influence of selenium on the catalytic properties of ruthenium-based cluster catalysts for oxygen reduction

The favourable influence of selenium on the catalytic properties of Ru-based catalysts for the oxygen reduction reaction in acid electrolytes has been investigated by rotating disk electrode measurements. Compared to the oxygen reduction of selenium-free Ru-based catalysts, the overpotential at low current densities (ca. 10 μA cm−2) is not affected by the presence of selenium whereas selenium-containing catalysts show higher current densities under fuel cell relevant conditions. The kinetically controlled current density at 0.6 V versus SHE increases 4–5 fold with increasing selenium content. A maximum value is obtained at about 15 mol% Se. This effect is tentatively explained by a modification of the catalytic active centre, which is assumed to consist of RuCCO complexes. IR spectroscopic investigations indicate a reaction of selenium with these complexes. This model is also supported by the study of the electrooxidation of CO. In contrast to the selenium-free catalyst, no CO oxidation is observed on the selenium-containing catalyst. Additional effects of selenium are an enhanced stability towards electrochemical oxidation and a lower amount of Ru oxides formed during synthesis, as evidenced from XRD investigations. Direct four electron oxygen reduction to water is efficient and H2O2 production of these catalysts is small (about 5% at potentials <0.3 V vs. SHE ).

Catalysts for oxygen reduction from heat-treated carbon-supported iron phenantroline complexes

Oxygen reduction catalysts were prepared by heat treatment of carbon supported iron phenantroline complexes in Ar or NH3. The optimum carbon black loading with iron was found to be 2%, the optimum heat treatment temperature was about 800 °C. X-ray diffractogramms and TEM showed the occurrence of crystalline species at higher catalyst loadings; however, these species seem not to contribute significantly to the catalytic activity. From the slope of the Koutecky–Levich plot, an average number of 3.7 electrons transferred per oxygen molecule was calculated, which is consistent with RRDE data. A Tafel slope of about 120 mV (decade)−1 indicates that the first electron transfer is rate determining.

Methanol-resistant cathodic oxygen reduction catalysts for methanol fuel cells

Efforts were made to simplify the structure of Ru-based catalysts, and to tailor industrially practicable methanol insensitive oxygen reduction catalysts both by thermolysis of Ru-carbonyls in organic solvents and by modified preparation techniques of Ru colloids. Selective catalysis was found to be essentially independent of the chalcogene (Se) used which, however, is a crucial factor for facilitating efficient electron transfer. All preparations contained Ru-metal particles of nm size, the surfaces of which were modified by carbonyl and carbido-carbonyl complexes or carbon compounds. The role of carbon as ligand to Ru clusters stabilizing the Ru interface against oxidation and in promoting catalytic electron exchange via nonbonding Ru d-states is theoretically analysed in a model calculation. An analogy is drawn to a biological Fe – only hydrogenase centre in order to discuss projected key experiments for optimizing reduction catalysis: the stabilization of small, inherently unstable catalytic metal clusters by CO or CN and their linking via electron bridges such as S and Se to electron reservoirs (metal colloids).

Carbon supported catalysts for oxygen reduction in acidic media prepared by thermolysis of Ru3(CO)12

Abstract Carbon supported catalysts for oxygen reduction in acidic media based on ruthenium and selenium have been prepared by thermolysis of Ru 3 (CO) 12 in organic solvents. The mass specific activity of these catalysts is higher than that of the unsupported samples. The optimum loading of the supporting carbon with catalyst has been found to be about 10%; higher loadings lead to only a slight increase in activity. As is evident from TEM, agglomeration of catalyst particles occurs, however, these agglomerates are homogeneously distributed over the supporting carbon. The activity is compared with commercial high surface area Pt/carbon and Ru/carbon catalysts. Characterisation of the catalyst with XPS and XRD indicates that it consists of a ruthenium core, which is surrounded by an amorphous shell containing Ru in various oxidation states, selenium and oxygen as well as high amounts of carbon. The real structure of the shell is unknown. After heat treatment, the activity of the catalyst towards oxygen reduction is slightly enhanced.