Catalysis by Metal Nanoparticles

Catalysis using metal derivatives (complexes, salts or surfaces) is a field of chemistry that spans several specialties, including coordination chemistry, physical chemistry, organic chemistry and, for applications, process chemistry.

Homogeneous catalysis is characterized by the fact that the substrate(s) are present in the same phase as the catalyst, usually a liquid phase (solvent). The ability to stabilize highly reactive intermediate species by solubilization, combined with the electronic properties of transition metals in general and noble metals in particular (Pd, Pt, Au, Rh, Ir), has enabled the development of hundreds of highly sophisticated reactions, particularly in the field of fine chemistry, leading to high value-added products (active pharmaceutical ingredients, phytosanitary products, cosmetics, ingredients for perfumery, food flavors, precursors for high-tech materials, etc.). In these cases, the catalyst has to be separated from the products at the end of the reaction, which requires an additional step that is not always efficient. These reactions are characterized by high selectivity and fairly well-defined reaction mechanisms, but scaling them up to industrial scale proves complex and sometimes too costly. Most of the highly efficient reactions developed on a laboratory scale only overcome this limitation in the case of very high value-added products, such as active pharmaceutical ingredients.

On the other hand, heterogeneous catalysis was initially developed in an industrial context for the refining of petroleum derivatives (cracking, steam cracking, catalytic reforming, isomerization...) and is characterized by the fact that the substrate(s) are present in a different phase from the catalyst, most often a fluid phase (liquid or gas) while the catalyst is solid. This phase difference makes it easy to separate the products from the catalyst at the end of the reaction, making the latter highly recyclable. However, bringing the substrate(s) into contact with the catalyst is less efficient and subject to diffusional barriers. Continuous processes are typically used in this case. In this type of catalysis, reaction mechanisms are more difficult to understand, due to the complexity of the catalytic material, with the presence of numerous potential catalytic sites, and the difficulty of observing these by spectroscopic methods, for example. As a result, selectivity is not always achieved, and this type of catalysis has therefore been developed for applications where selectivity is not required, i.e. either for refining mixtures, or for simple, e.g. monofunctional, substrates. Most heterogeneous catalysis reactions involve the production of large volumes of small molecules with low added value (large intermediates and commodities).

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