Catalysis on bimetallic nanoparticles

Small aggregates of metal atoms exhibit properties that are rather different from typical `bulk` metal objects as a large fraction of the metal atoms is located at the surface, while only a few are fully surrounded by neighbor atoms in a regular fashion. As a consequence, small metal clusters or nanoparticles show a much higher chemical reactivity than the corresponding bulk metal: Surface atoms are less pleased with their current status and tend to interact more actively with surrounding gas molecules. In many chemical engineering applications, we take advantage of this effect, and combine it with other effects caused by the mixing of several metals or metal oxides. Companies are seeking for the perfect material composition, which allows them to accelerate a certain chemical reaction and to steer it towards the desired product. In this project, we use a new, unconventional technique which allows us to create bimetallic, spherical metal clusters of reasonably well-defined size with a specific core-shell structure, similar to the famous `Mozartkugeln` produced in Austria, only much smaller: Our nanoparticles have diameters in the range of a few nanometers. These nano-Mozartkugeln are then subsequently tested for their ability to catalyze certain reactions. Two basic reactions have been chosen for our initial tests of this novel class of materials. The first reaction, the selective hydrogenation of butadiene, plays a big role in the oil industry, e.g. for the removal of butadiene from mixtures with isobutene. The second reaction, the oxidation of nitric oxide, is a more prominent process which takes place inside the catalytic converters of our cars. The core-shell structures to be tested are copper-gold and copper-vanadium oxide, respectively. It is our goal to analyze the possibility of reducing or even replacing the expensive components of common catalysts such as rare earth and transition metals of low abundance. Copper seems to be a promising alternative in this regard, and is therefore tested in both reactions as the core component of our bimetallic nanoparticles.

Small aggregates of metal atoms exhibit properties that are rather different from typical `bulk` metal objects as a large fraction of the metal atoms is located at the surface, while only a few are fully surrounded by neighbor atoms in a regular fashion in all three dimensions. As a consequence, physical properties such as melting point of electric conductivity might be significantly different from the bulk values. The same can be said about chemical features: Small metal clusters or nanoparticles typically show a much higher chemical reactivity than the corresponding bulk metal, simply because surface atoms are less pleased with their current status and tend to interact more actively with surrounding gas molecules. In many chemical engineering applications, we take advantage of this effect, and combine it with other effects caused by the mixing of several metals or metal oxides. Companies are seeking for the perfect material composition, which allows them to accelerate a certain chemical reaction and to steer it towards the desired product. In this project, we have used a new, unconventional technique which allows us to create bimetallic, spherical metal clusters of reasonably well-defined size with a specific core-shell structure, similar to the famous `Mozartkugeln` produced in Austria, only much smaller (by about a factor of 106): Our nanoparticles have diameters in the range of a few nanometers. These nano-Mozartkugeln are then subsequently tested with regards to their physical and chemical properties. Particularly interesting to us was their stability at increasing temperatures. Typically, these materials are used in a temperature range of several hundred degrees Celsius, and undesired structural changes or even the occurrence of phase transitions would compromise their catalytic activity substantially. We could show that our method allows the synthesis of spherical, layered particles in the Mozartkugel-sense of almost any combination of metals, and that cores formed by more reactive metals can be passivated by a sufficiently thick (about three atomic layers), protective shell of gold. This opens many possibilities, e. g. the ability to save on precious metals such as gold, or the combination of ferromagnetic materials such as iron, which would be fully oxidized (turned to rust) in the blink of an eye at this size, with a biocompatible metal such as gold, e. g. in order to allow a magnetically steered delivery of drugs in the human body. Using electron microscopy, we could investigate the oxidation processes on the cluster surfaces often with atomic resolution and introduced a new way of measuring atomic diffusion in metallic alloys. The last phase of our project was dedicated to a very special transition metal, vanadium, which, in its oxide form, plays an important role in exhaust gas management in combustion engines as they are found in automobiles. Here we could show for the first time, that vanadium(V)oxide remains in a salt-like structure even after sublimation from the bulk, and that a follow-up condensation can be used to grow nanoparticles with the same stoichiometric ratio of O and V a little sensation, which might play out highly useful in future catalyst designs involving this material.