Metal Clusters and Particles as Catalyst Precursors and Catalysts

Abstract

Bulk metals are used in only a few processes as catalysts where a reasonably high surface area is stabilized by so-called structural promoters. The most prominent example of this class of catalysts is undoubtedly the NH3 synthesis catalyst.1,2 Since metals, in particular noble metals, have high surface free energies, they tend to aggregate unless small particles can be stabilized by placing them on the surface of suitable supports. This class of supported metal catalysts is technologically extremely important. They are efficiently used in selective hydrogenations,3,4 the control of motor-vehicle exhaust gases (three-way catalyst)5 and in catalytic reforming.6–8 High metal surface areas can be obtained when small metal particles are stabilized on support surfaces in high dispersion.9,10 Dispersion is defined as the fraction of metal atoms being exposed. For example, a Pt particle with a diameter of about 1 nm contains less than 50 atoms und has a dispersion close to 100%. This is particularly important for the catalytically highly active, but very expensive noble metals. For economic reasons one has to achieve optimal catalytic efficiency per unit mass of noble metal, and hence, small particles providing high dispersion are demanded. Platinum is in fact one of the most widely used noble metals in catalysis. It finds application in car exhaust catalysts, 5 in hydrogenations 3,4 and catalytic reforming.6-8