Iris Dorbandt

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.

Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

In this work Fe–N–C catalysts were prepared by the oxalate-supported pyrolysis of FeTMPPCl or H2TMPP either in the presence or absence of sulfur. The well-known enhancing effect of sulfur-addition on the oxygen reduction activity was confirmed for these porphyrin precursors. The pyrolysis process was monitored in situ by high-temperature X-ray diffraction under synchrotron radiation (HT-XRD) and thermogravimetry coupled with mass-spectroscopy (TG-MS). It was found that the beneficial effect of sulfur could be attributed to the prevention of iron-carbide formation during the heat-treatment process. In the case of pyrolysis of the sulfur-free precursors an excessive iron-carbide formation leads to disintegration of FeN4-centers, hence limiting the number of ORR active sites on the final catalyst. Physical characterization of the catalysts by bulk elemental analysis, X-ray diffraction (XRD), Raman and 57Fe Mosbauer spectroscopy confirmed the outcome from HT-XRD and TG-MS. It could be shown that the avoidance of carbide formation during pyrolysis represents a promising way to enhance the density of ORR active sites on those catalysts. This can be done either by sulfur-addition or the performance of an intermediate acid leaching. As iron carbide is often found as a by-product in the preparation of Fe–N–C catalysts this work gives some general strategies for enhancing the density of active sites enabling higher current densities.

Quantitative structural assessment of heterogeneous catalysts by electron tomography.

We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium particles to their support and their freely accessible surface area. The shape analysis shows the ruthenium particles deviate significantly from spherical symmetry which increases their surface to volume ratio. The morphological studies help to understand the structure formation mechanisms during the fabrication as well as the high effectiveness of these catalysts in the oxygen reduction reaction at the cathode side of fuel cells.

On the Influence of Sulphur on the Pyrolysis Process of FeTMPP-Cl-based Electro- Catalysts with Respect to Oxygen Reduction Reaction (ORR) in Acidic Media

Pyrolysis of chloroiron-tetramethoxyphenyl-porphyrin (FeTMPP-Cl) in the presence of iron oxalate (± sulphur) leads to the formation of higly porous and active catalysts for the oxygen reduction reaction (ORR). In order to clarify the influence of sulphur the pyrolysis process is analyzed by thermogravimetry (TG) and by high-temperature X-ray diffraction (HT-XRD). In the absence of sulphur iron carbide (FexC) is formed which catalyses the proceeding graphitisation of the pyrolysis products. As a result catalytic active centres are decomposed by this reaction. This can be avoided by the addition of sulphur because iron monosulphide (FeS; troilite) is formed instead of FexC. Furthermore, FeS can easily be removed in a successive etching step so that nearly all inactive by-products can be removed. The results are in accordance with the higher electrochemical performance of the sulphur containing catalysts.

Structural Investigation of Carbon Supported Ru-Se Based Catalysts using Anomalous Small Angle X-Ray Scattering

Currently the commercial applicability of proton exchange membrane fuel cells (PEMFC) is almost exclusively bound to the employment of expensive and rare platinum. However as, recently successfully demonstrated, selenium modified ruthenium based catalysts also exhibit high catalytic activity for the oxygen reduction reaction in fuel cells [1]. One advantage of ruthenium is that it has only one-tenth the costs of platinum. Furthermore, in direct methanol fuel cell (DMFC) the undesired crossover of methanol through the membrane from the anode space into the cathode compartment is still an unsolved issue. Therefore, state of the art platinum cathode catalysts suffer from an activity loss under these conditions due to methanol oxidation. As alternative, carbon supported ruthenium nano-particles modified with selenium were suggested featuring absolute methanol tolerance. Although intense studies of these catalysts were performed during the last decade, no definite conclusion with respect to the nature of the catalytically active sites and the constitution of the RuSe nano-particles could be drawn. We performed an Anomalous Small Angle X-ray Scattering (ASAXS) experiment to clarify the structural and chemical features of these catalytically active Semodified ruthenium nano-particles, such materials were prepared by thermolysis of Ru3(CO)12 in an organic solvent with and without the presence of dissolved selenium. The results are due to be discussed considering data from XRD, XPS, and EXAFS measurements, as well as TEM images [2,3]. Particularly, data about the selenium distribution over the catalysts surface are aspired, because catalytic efficiency depends strongly on the selenium content of the catalysts. Investigating RuSe electrocatalysts we will show that the ASAXS method represents a powerful tool to determine size distributions and volume fraction of structures on a nano scale domain. Furthermore, it becomes a sensitive method on chemical composition fluctuations by taking into account the so-called anomalous or resonant behaviour of the atomic scattering amplitude of an element, containing in the sample, near its x-ray absorption edge. We investigated with ASAXS a complete set of samples, including the final working RuSe catalyst supported on a commercial carbon black, and some intermediate preparation states like non-modified (selenium free) ruthenium nano-particles or the bare carbon support only. We will discuss scattering curves taken in the vicinity of the selenium and the ruthenium K absorption edge, respectively. Figure 1. shows the large Q range of two scattering curves taken at 5eV below the selenium K absorption edge. The sample which contains selenium (red circles) shows a small hump at Q ~ 7 nm while the sample without selenium (blue triangle) does not. Thus, selenium within the sample generates structural features clearly detectable and analysable by SAXS. Taken into account the energy dependences (the anomalous dispersion effect) of the scattering intensity of all measured samples, a structure model of the catalytically active metallic nano-particles has been deduced, suggesting a nearly spherical Ru particle, with a mean diameter of 2.3 nm decorated with Se aggregates. The selenium structure onto the ruthenium nanocrystallites features a diameter less than 0.5 nm. The real shape of these ruthenium supported selenium clusters is not yet finally clarified. However, it is suggested to represent a symmetric ring like observed for free selenium clusters. [4] We investigated the same systems with small angle neutron scattering (SANS) confirming the structure model obtained from ASAXS. The comparison of the volume size distribution of the particles of both methods will be discussed. We also confirmed that the selenium modified ruthenium nano-particles are extremely resistant against particle growth. Even after annealing at 800°C the average particle diameter of the ruthenium particles was found to remain below 2.5 nm.

Charakterisierung von Katalysatormaterialien für Brennstoffzellen mittels Elektronentomografie

Kurzfassung Zur Optimierung moderner Katalysatoren für Brennstoffzellen werden diese elektronen-tomografisch charakterisiert. Die Elektronentomografie ermöglicht einzigartige Einblicke in die Nanometer-Strukturen der metallischen Katalysatorpartikel, die auf einem elektrisch leitenden, inerten Kohlenstoffträger abgeschieden werden. Die dreidimensional bildgebende Methode ermöglicht über qualitative Untersuchungen hinaus detaillierte quantitative Form- und Strukturanalysen der Katalysatormaterialien. So werden beispielsweise die Positionen der Katalysatorpartikel relativ zum Trägermaterial analysiert. Ihre Form und Einbettung in den Träger, welche die für die katalytische Reaktion maßgebliche „freie Oberfläche“ definieren, werden bestimmt. Die Elektronentomografie ermöglicht somit quantitative Vergleiche zwischen verschiedenen Katalysatormaterialien und Herstellungsverfahren. Sie erweitert die Möglichkeiten der Korrelation gewünschter elektrochemischer Eigenschaften mit der Nanostruktur dieser Materialien und macht so weitere Optimierungen der Katalysatormaterialien möglich.