Supported metal-particle catalysis

We first describe the structural proprties of small metal particles. 

---

Application of transmission electron microscopy (TEM) techniques on heterogeneous catalysis covers a wide range of solid catalysts, including supported metal particles, transition metal oxides, zeolites and carbon nanotubes and nanofibers etc. 

Lefebvre, F., Candy, J.P. and Basset, J.M. (1999) Synthesis with Supported Metal Particles hy Use of Surface Organometallic Chemistry Characterization and Some Applications in Catalysis, Vol. 2, Chap 2.7, Wiley-VCH Verlag GmbH, Weinheim. 

Although HRTEM is a very powerful technique for the study of small particles, imaging metal-particle catalysts on supports such as alumina, silica or carbon presents challenges. In order to understand the structure and contrast of very small (<5 nm) supported metal particles which are thought to be the active species in catalysis, key computations of supported small metallic catalysts have been reported by Gai et al (1986). Image computations for supported particles, carried... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

In conclusion, the doping of oxide materials opens promising new routes to change the morphology and electronic properties of supported metal particles as used in heterogeneous catalysis. Thin oxide films are ideally suited to elucidate such doping effects, as they can be explored by means of conventional surface science techniques at a fundamental level. The identified mechanisms can be transferred to real catalysts later, as the doping approach is not based on specific thin-film effects. 

The final section is concerned with the NMR of supported metal particles, predominantly Pt NMR. The data and their interpretation are given in relation to a number of concepts in phenomenological (NMR spectrum and dispersion NMR spectrum and chemisorption) or theoretical (electron deficiency promoting effect) catalysis. 

Support materials most adapted for stabilizing clusters on surfaces are oxide materials. They reveal a relatively large band gap and thus the characteristic, discrete electronic levels of the clusters are maintained to a certain extent. Furthermore, the interaction of the clusters with oxide surfaces, especially with their defects may be substantial and trapping of the clusters is feasible. Oxide supported metal particles are also relevant in industrial catalysis. 

What is the meaning of nanocatalysis Is it not a pleonasm Indeed, industrial catalysis generally takes place at the nanoscale (or sub-nanoscale) most of the catalysts are made of metal particles of a few nanometers in size and in fine all the elementary reaction steps occur at the atomic (or molecular) scale. Thus, catalysis seems to be intrinsically a nanoscale phenomenon. The word nanocatalysis, in fact, does not apply to the catalytic phenomenon itself but to the intrinsic properties of the catalysts, which may change in the nanoscale. As we will see, some properties of nanometer sized, supported metal particles directly affect their catalytic activities and these properties are, in size range of up to a couple of hundred atoms, not scalable from bulk properties. 

The latter point brings us to an important question in the field of catalysis by supported metal particles to which extent is the chemical reactivity of a (sub-) nanocluster affected by the interaction with the substrate Very few theoretical studies were dedicated to this problem, and most of them are related to the surface of MgO, an oxide which interacts weakly widi the supported particle, as shown above. Still, the knowledge accumulated in the course of the years on the structure of surface defects and morphology of the MgO surface allows one to analyze some of the mechanisms which can modify the chemical properties of a supported cluster as a function of the site where nucleation has occurred. 

Synthesis with Supported Metal Particles by Use of Surface Organometallic Chemistry Characterization and some Applications in Catalysis... 

J3.6 Supported metals and supported organometallics J3.7 From molecular carbonyl clusters to supported metal particles synthesis, characterisation, catalysis... 

Finally, the applications of various spectroscopic and structural probes made possible the investigation of catalyst surfaces at a more microscopic leveL Studies with idealized surfaces such as the faces of single crystals in an ultra-high vacuum apparatus allowed us to investigate the role of the surface in catalysis. This was completed by spectroscopic studies of supported metallic particles allowing us to characterize the size dependence of the electronic system. Specific active sites for hydrocarbon isomerization were evidenced and their appearance was linked to the lowered atomic coordination. 

The first chronological appearance of clusters in catalysis is their use as models for heterogeneous catalysts. More precisely, it was found that polynuclear metal complexes such as transition-metal clusters can act as soluble models for supported metallic particles, that are much more complicated to study. Clusters can be isolated and characterized by the classical methods of preparative chemistry. They show typical characteristics of metal surfaces, such as polycentric ligand-metal bonds and delocalized metal-metal bonds. The use of metal clusters as models for the surface of catalysts was named by Muetterties the duster-surface analogy [10]. The first development in this area of research was mainly structural, and consisted in investigating the interaction... 

The growth of supported metal particles by movement and agglomeration is called sintering. Sintering can occur by two different mechanisms (1) migration and coalescence and (2) dissociation of single atoms and movement of the atoms to other particles. It can be seen that these are the same phenomena discussed prevously for particles on surfaces. However, in catalysis other reactants are present and it is possible for mechanism (2) to operate by chemical reaction of surface metal atoms to yield a volatile compound which can be transported to another cluster, decompose, and deposit the atom. For example, Ni particles are easily sintered in a CO atmosphere ... 

Nowadays, catalytic properties of metal nanoparticles are the subject of extensive studies. It has been found recently [1,2] that not only the particle size but also a distance between supported particles affects the properties of nanostmctured catalytic systems. This effect opened up a new way to improve the catalytic properties of supported metal particles by the formation of coatings with the optinial surface particle density. However, most of the known preparative methods are unsuitable for fabrication of dense nanostmctures because closely located crystalhne particles coagulate into larger aggregates. For this reason, a search for methods that can enhance the coagulation stability of the metallic nanoparticles is the task whose accomplishment governs the development of new promising areas in catalysis. 

The reactivity of supported metal particles as a function of their size has been much debated in the literature of heterogeneous catalysis. However, to know with accuracy how many atoms are required for a particular reaction, we must await further experiments with cluster beams such as those described in the previous section. Indeed supported metal catalysts are not well suited to provide answers to this question, since even the most homogeneous of them still exhibit a large range of nuclearities, and their reactivity can be modified by the support. In the meantime, it is now widely accepted that one or two atoms are required to activate H-H, C-H, or 0-H bonds, whereas the activation of C-C and N-N bonds requires a larger ensemble of atoms. 

DFT is a powerful method for determining reaction mechanisms over metal-oxide systems. We have chosen to review studies that focus on developing catalysts for the water-gas-shift reaction because this is a particularly active research area with numerous examples of DFT application to supported metal-oxide catalysis. The studies first considered herein assess the activity of unsupported gold and copper metal clusters, which can then be compared directly to studies over the analogous oxide-supported systems. The importance of considering particle-support interactions is emphasized, because the oxide support can often play an active role in catalytic mechanisms. 

---