Supported metal clusters

High metal dispersion results in exposure of metal sites of enhanced catalytic activity. I. Stranski et al. used the Gibbs-Wulff principle  

E((ThkiAhki) = Minimum [a = specific surface energy A = surface area) (1) 

The difference between the close-packed crystal faces, such as (110) in bcc or (111) in fee crystals, and the open faces that prevail in the surface of nano particles, is of relevance to heterogeneous catalysis, because most adsorbates are more strongly bonded to the coordinatively unsaturated metal atoms in the open faces. 

Sabatier and Balandin had predicted a relationship between catal)dic activity and heat of adsorption. If a solid adsorbs the reactants only weakly, it will be a poor catalyst, but if it holds reactants, intermediates or products too strongly, it wiU again perform poorly. The ideal catalyst for a given reaction was predicted to be a compromise between too weak and too strong chemisorption. Balandin transformed this concept to a semiquantitative theory by predicting that a plot of the reaction rate of a catal)Tic reaction as a function of the heat of adsorption of the reactant should have a sharp maximum. He called these plots volcano-shaped curvesl This prediction was confirmed by Fahrenfort et al. An example of their volcano-shaped curve is reproduced in Fig. 9.1. They chose the catalytic decomposition of formic acid 

FIGURE9.t. Volcano Plot of formic acid decomposition. Abscissa Calculated A HadsofHCOOH Ordinate Temperature at which rate of HCOOH decomposition reaches the same value for all metals. 

Heterogeneous Catalysis by Metal Clusters. - Present activity in this area reflects the view that supported metal clusters could form the basis of a new generation of heterogeneous catalysts. Characterization of such materials, especially under reaction conditions, will be difficult. Such catalysts will be valuable if they exhibit activities and selectivities that differ from those afforded by conventionally prepared catalysts unfortunately, it is a deficiency of much of the work so far reported that such comparisons have not been made. However, evidence has recently become available that shows that cluster-derived heterogeneous catalysts may exhibit distinctive behaviour. 

Catalysis by Supported Metal Ousters. Reactions that have received most attention are two highlighted previouslynamely carbon monoxide hydrogenation and the activation of carbon-carbon and carbon-hydrogen bonds of saturated hydrocarbons. 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 Rh6(CO)ig [Rh7(CO)i6]  

Some very recent work in carbon monoxide hydrogenation by Basset, 

Piacenti, and M. Bianchi, in Organic Syntheses via Metal Carbonyls , ed. I. Wender and P. Pino, Wiley, New York, 1977, Vol. 2, p. 43. 

A discussion of the electronic structure and properties of clusters, naked or ligated, would not be complete without addressing the problem of supported metal clus- 

Ideally, the substrate should be sufficiently inert to guarantee that the metal-substrate interaction will not affect the properties of the metal particle. Often, however, a relatively strong interaction can occur between substrate and metal species, in particular for small clusters. In a sense, the oxide surface can be considered to be a very special case of a ligand environment stabilizing the metal cluster. The metal electronic states are, however, always perturbed by interaction with the substrate, just as for proper ligands. 

Very little is known theoretically about the metal-oxide interface although a rather limited number of first principles theoretical studies have dealt with the general problem of metal-ceramic interaction. The aim of this section is to discuss some fundamental aspects of the bonding of metal clusters deposited on the surface of simple binary oxides, in particular MgO and AI2O3. 

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A.Y. Stakheev, and L.M. Kustov, Effects of the support on the morphology and electronic properties of supported metal clusters modem concepts and progress in 1990s, Appl. Catal. A 188, 3-35 (1999). 

Gates BC (1995) Supported metal clusters Synthesis, stracture, and catalysis. Chem Rev 95 511-522... 

The extended fine structure (EXAFS) was used to determine bond distances, coordination number and disorder. The near edge (XANES) was used as an Indication of electronic state. Significant results Include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Keywords Clusters Clusters, metal Clusters, supported metal Clusters, ligands bonded to Clusters reactivities and catalytic activities... 

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compoimds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

The longer metal-oxygen distances of about 2.6 A observed by EXAFS spectroscopy for these and related supported metal clusters suggest weak interactions between the metal and surface oxygen atoms these EXAFS contributions are not determined with as much confidence as those characterized by the shorter distances, and the interactions are not well understood. 

Metal-metal distances in supported metal clusters (e.g., It4, nearly 2.70 A) determined by EXAFS spectroscopy essentially match those in coordinatively saturated clusters of the same metal (e.g., Ir4(CO)i2). These distances are about O.2-O.3A greater than the metal-metal distances in the free (gas-phase) clusters (e.g., It4, 2.44 A) [32]. Similar results have been determined for supported OS5C [33] and Rhe [28,29]. 

These comparisons prompted the Rosch group [32,33] to conclude that some Ugands remained on the supported clusters after decarbonylation this conclusion may be quite general—supported metal clusters are highly reactive, and typical oxide and zeoUte supports are not unreactive. Thus, a representation of supported clusters such as tetrairidium on 7-AI2O3 as 4/7-AI2O3 is a simplification that fails to account for the ligands on the cluster. 

The commonly investigated ligands on the supported metal clusters mentioned before include CO, hydride, and hydrocarbons. The evidence of hydrides and hydrocarbons is not as strong as one would wish. 

Much remains to be done to develop the chemistry of organic hgands on supported metal clusters, and substantial progress is to be expected as the samples are well suited to characterization, by IR, NMR, and neutron scattering (F. Li, J. Eckert, and B.C. Gates, unpubhshed results) spectroscopies, as well as density functional theory. 

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

The foregoing results characterizing structurally simple supported metal clusters can be generalized, at least qualitatively, to provide fundamental understanding that pertains to industrial supported metal catalysts, with their larger, nonuniform particles of metal. 

Pacchioni G, Illas F. 2003. Electronic structure and chemisorption properties of supported metal clusters model calculations. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker. 

Supported model catalysts are frequently prepared by thermally evaporating metal atoms onto a planar oxide surface in UHV. The morphology and growth of supported metal clusters depend on a number of factors such as substrate morphology, the deposition rate, and the surface temperature. For a controlled synthesis of supported model catalysts, it is necessary to monitor the growth kinetics of supported metal... 

Because the size regime of n=l-6 atoms is of great practical significance to the spectroscopic, chemical and catalytic properties of supported metal clusters in both weakly and strongly interacting environments (28), it is important to study very small metal clusters in various types of substrate as well as in the gas phase. In this way, one can hope to develop a scale of metal cluster-support effects (guest-host interactions) and evaluate the role that they play in diverse technological phenomena. 

Owing to the sheer number of studies that have been carried out to understand the properties of supported metal clusters, it is impossible to discuss all of them here. Several excellent reviews can provide more information in this area.36-53 64 For the purposes of this discussion, we simply focus on some of the work from our laboratory regarding supported Au clusters. 

A related approach to the preparation of highly dispersed supported bimetalhc catalysts involves the reaction of metal complexes with supported metal clusters or particles. The method is based on the idea that by careful choice of the metal complex and control of the reaction parameters it may be possible to cause the metal complex to react selectively with the supported metal but not with the support surface [13]. Because this approximation to the subject is the main focus of this chapter, it is thoroughly developed in the following sections. 

The accessibility of new techniques such as EXAFS brings researchers a powerful tool for unambiguous determination of the true core metallic framework of such systems. Thus, the relationship between the parent carbonyl precursor, the support and the final metal-supported particles has been studied at the structural atomic level in some cases. This can allow differentiation of the catalytic behavior of supported metal particles with bulk-like properties from that of supported metal clusters, opening the way to understanding the mechanism of metal-catalyzed reactions and extending the concept of sensitive or insensitive structure reactions from metal aggregates to clusters. 

B. C. Gates, Supported metal clusters synthesis, structure, and catalysis. Chem. Rev. 95, 511-522 (1995). 

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