A new vision on nanocatalysts

Final Report Summary - NANOSONWINGS (A new vision on nanocatalysts)

The project Nanos-on-Wings concerned the synthesis of small metal nanoparticles in a controlled manner with the aim to use them in catalysis. It is known from surface chemistry of heterogeneous catalysts that metal different surfaces may display different catalytic properties. In recent years it has been recognized that giant clusters or small metal nanoparticles (typically 50–500 metal atoms) hold the promise that their catalytic properties may be completely different from those of their bulk analogs or their monometallic complexes, in addition to the fact that a much larger proportion of the metal is at the surface and thus a higher activity per metal is achieved. In our approach we have combined the tools of homogeneous and heterogeneous catalysis to obtain the best of two worlds. The major topic we have studied is the effect that ligands may exert on metal nanoparticles in, preferably heterogeneous catalytic reactions. The ligands serve at least two purposes, they change the catalytic properties of the metal and they stabilize the nanoparticles, in the same sense that ligands stabilize monometallic molecules. The use of ligands has been explored and utilized by all scientists working in this area, because in the absence of stabilizers the small particles will condense to the more stable larger particles, as monometallic particles would do. It is known that the stabilizers, ligands in our terminology, control the size and the shape of the nanoparticles during their formation. In addition the ligands can carry more functional groups that can participate in the catalytic processes. Ligands may also coordinate selectively to specific sites of the nanoparticle thus blocking certain reactions that take place on these sites.
A range of ligands has been investigated for RuNPs and RuPtNPs and indeed the catalytic activity for hydrogenation varied enormously with the nature of the ligand, the substrate and the solvent. New phosphine ligands were designed and synthesized carrying wings that were thought to protect the nanoparticles. Several known carbene ligands carrying wings were applied on NMPs for the first time. As the ligands change during the preparation process it was important to establish their nature on the actual nanoparticle catalyst, also after the catalysis. A variety of state-of-the-art spectroscopic techniques were used to ascertain the bonding mode to the metal and the nature of the ligands, and also the dynamics of the species at the surface.
It was found that for arene hydrogenation the strongest donor ligands gave the fastest catalysts, but unfortunately they are also the least stable. However, the second best systems still are the fastest catalysts known today per gram of metal used. An immobilized version gave unusually high rates and for this one we applied for a patent.
During the hydrogenation studies of organic substrates by ruthenium nanoparticles we found a new reaction to forma pyrazines from diketones, which was found to be a reaction that only took place when nanoparticles were used and not for any other catalyst, be they homogeneous or heterogeneous. The products are useful in medicinal chemistry.
Selective hydrogenations of substrates containing more unsaturated groups were studied and interesting selectivities were found. Amongst these substrates also bio-feedstocks were hydrogenated and remarkable selectivities were observed in some cases. A new gold catalyst was developed using ligands not earlier used in metal nanoparticle research that selectively only catalyses the hydrogenation of aldehydes. However, when electron rich ligands were used, gold returned to its common activity for nitro reduction and the selectivity of the catalyst was lost for substrates that contain both functional groups.
We found an easy and reproducible low temperature process for the tunable synthesis of monodisperse, high quality iron carbide and iron/iron carbides nanocrystals, air-stable after some initial losses and displaying excellent magnetic properties with tunable magnetic anisotropy. These particles have no precedent. Like for the other nanoparticles the synthesis is based on an organometallic approach, unprecedented in this instance. Several new spectroscopic techniques were explored and one of them was entirely new which gave our results an extra push forward.
In summary with new nanoparticles in hand, new synthetic routes developed, state-of-the-art spectroscopy applied, we have made major progress in the field.

Final Report Summary - NANOSONWINGS (A new vision on nanocatalysts)

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