Metal cluster reactivity

Figure 5. A bar graph of metal cluster reactivities with CO on a linear scale. Cluster size increases going into the page and metal types across. Once beyond a few atoms in size most all clusters react at rates within an order of magnitude of each other.

M. Andersson, Metal Cluster Reactivity Adsorption of small molecules in bimolecular collisions, Thesis Chalmers University of Technology, Goteborg, 1995. 

Fig. 1.29. Schematic sketch of the collision cell method for the study of metal cluster reactivity. The supersonic laser vaporization source is depicted on the right hand side. The clusters subsequently pass two collision cells in which reactions can take place. Finally, laser ionization mass spectrometry serves to detect the neutral reaction products [3]...

Ion cyclotron resonance (ICR) mass spectrometry and also penning trap mass spectrometry have been demonstrated by several groups to be a powerful tool to investigate metal cluster reactivity in the gas phase under single collision conditions [24,177-183]. From the schematics in Fig. 1.33, it can be seen that... 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Figure Cl. 1.3. Relative reactivity of transition-metal clusters with H2 (full curves, log scale) and tire promotion...

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Concelcao J, Laaksonen R T, Wang L S, Guo T, Nordlander P and Smalley R E 1995 Photoelectron spectroscopy of transition metal clusters correlation of valence electronic structure to reactivity Rhys. Rev. B 51 4668... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Synthesis, molecular dynamics and reactivity of mixed-metal clusters. G. L. Geoffrey, Acc. Chem. Res., 1980,13,469-476 (29). 

The premise of this review is that synthetic procedures for very mixed"-metal clusters are comparatively well understood, but that reactivity and physical properties are less well studied. Metal core transformations (modifications of a preexisting cluster) fall into both the synthesis and reactivity categories. A summary is presented here, but as they have been reviewed elsewhere (see Refs. 4, 107-109), the account below is necessarily brief. Section lI.E. 1. considers core transformations where the cluster core nuclearity is pre.served, whereas Section 11.E.2. summarizes reactions involving a change in core size. 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

Preliminary indications are quite clear that coordin-atively unsaturated transition metal clusters are reactive, and, importantly, reactive in a controlled way. This chemistry is likely to include RC-H insertion, arom-atization etc., with the chemistry tending to produce... 

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... 

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. 

Cheeseman, M.A. and Eyler, J.R. (1992) Ionization potentials and reactivity of coinage metal clusters. The Journal of Physical Chemistry, 96, 1082-1087. 

Small Co metal clusters Cora (ra = 2-8) react with CO, with sequential addition leading to the saturated Co species [Co2(CO)7], [Co3(CO)i0], [Co4(CO)i2], [Co5(CO)13] and [Co6(CO)i5]-.71 This points towards one of the features of low-valent Co carbonyls a tendency to form stable clusters. Reactivity of Co with 02 is higher but leads to cluster fragmentation, whereas N2 is essentially unreactive. Entry into carbonyl chemistry of low-valent Co is frequently via the well-known dimer Co2(CO)8. A range of reactions commencing with this compound has been developed, as follows. 

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