Hendrik Schulenburg

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 ).

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.

Platinum-nanoparticles on different types of carbon supports: Correlation of electrocatalytic activity with carrier morphology

The electrocatalytic activity of Pt-nanoparticles used in fuel cells increases by 34% upon going from the usual Pt/Vulcan XC72 to support systems such as Pt/Printex XE2 which have a relatively rough surface strucure.

Characterization of catalytic supports by means of high-resolution TEM and SEM in combination with shadow-casting and decoration procedures

In view of the well-known fact that the nature of a solid support can have a profound influence on the performance of a heterogeneous catalyst, it is of specific interest to understand the correlation between structural details of the support, and size and distribution of the supported metal particles as well as their catalytic activity [1]. With respect to better understanding of this correlation we applied electrochemical studies and highresolution TEM and SEM investigations in combination with shadow casting and decoration procedures on Pt/C catalysts with various carbon supports. In order to study the influence of the surface structure on the preservation of the particle size and distribution, tungsten was evaporated under a flat angle on different types of carbon supports. A special electron impact evaporator developed by us allows the reproducible evaporation of tungsten [2]. This vacuum deposited tungsten layer formed a uniform film at a thickness of 0.2 0.3 nm and showed sufficiently contrast for imaging the finest surface structural details. The tungsten deposited catalysts were studied by means of HRTEM (Hitachi HF 2000) and HRSEM (Hitachi S-5200). Results are summarized in HRSEM micrographs of Fig. 1. The surface roughness depends on the preparation of the carbon black supports. The Vulcan XC72 (Carbot Corp.) reveals a quite smooth surface whereas the Printex XE2 (Degussa) shows a rough texture. In order to proof the existence of differences in the surface roughness between the carbon blacks we carried out gold-decoration experiments. For this purpose the carbon blacks were evaporated with gold under temperature controlled conditions. The gold formed island structures in thickness of 0.3 1 nm. The size, density and distribution of gold cluster on the surface depends on the temperature and is highly influenced by the roughness of the surface. After evaporation procedure the carbon black surfaces are decorated by Au clusters as shown in Fig. 2. The smallest Au cluster and the highest particle density were achieved in case of Printex XE2. We observed that the surface roughness influences the particle size, distribution and preservation of carbon supported Pt nanoparticles. The HRTEM studies of the various carbon blacks before and after platinumoxide loading and after electrochemical reduction to platinum confirmed the results obtained by the roughness studies [3]: Under operation conditions undesired sintering of Pt nanoparticles in case of Printex XE2 support is much less pronounced if compared to the Vulcan XE72 support because Printex XE2 has a much rougher surface (Fig. 3). In our investigations the Pt/Printex XE2 material proves to be the best electrocatalyst for the oxygen reduction reaction [3, 4].