Cluster-support interactions, types

The ability to conduct EXAFS studies on complexes in non-crystalline phases and in particular on active catalyst systems in situ has been the driving force for much of the development and application of the technique to the analysis of cluster geometry. In the earlier phase of this sort of work, i.e. studies of type (iii), the objective was to identify the cluster species present on the support and the nature of the cluster-support interaction. Three general modes of attachment of cluster complexes to supports have been explored ... 

With oxide-supported cluster complexes, models have been made to mimic the cluster-support interaction, but their reactivity has not been studied. For example, the silica-supported cluster SiCH2CH2PPh20s3H2(CO)io undergoes a decarbonylation reaction, not to give the Os3H2(CO)9(L) species, as occurs in solution, but to give an uncharacterized species which may be of the type 0s3(p2 0)3(C0)x (where M2 0 represents a surface oxygen atom). It should be possible to model the reactivity of such supported complexes. 

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. 

However, these molecular dynamics calculations suffer some limitations the empirical nature of the potential (especially for the metal-support interaction) and the arbitrary separation between the metal-metal and metal-support interactions (the metal-metal potential is probably perturbed near the interface). Indeed, according to the type of potential used, very different results are obtained. In the case of Pd/MgO, a mean dilatation [91] or contraction [92] is observed. For finite-temperature molecular dynamics, the calculations are limited to very short times and it is not sure that the equilibrium shape is reached. As we have seen in the last section the cluster shape can be blocked for a long time on facetted metastable shapes. 

It is apparent from the discussion above that metal-support interactions of the electronic type have been proposed to explain a large variety of catalytic phenomena. The exact nature and mechanism of this interaction depend on the particular catalytic system, i.e., the support material, the nature of the active phase, the size of the metal cluster or particle, the gaseous atmosphere under which the catalyst operates, and possibly other parameters. 

The above classification suggests that under properly chosen condition the subject of this chapter, i.e. metal ion-metal nanocluster ensemble sites (MIMNES) can be formed in most of the above types of catalysts. For instance, from bimetallic clusters of type (i) and (ii) MIMNES can be formed under conditions of mild oxidation. In catalysts type (iii) MIMNES should exist both under oxidative and reductive environment. In catalysts type (iv) any metal-support interaction with the involvement of non-reducible oxide can also be considered as MIMNES. The only requirement for the formation of MIMNES is the atomic closeness of the two types of sites. 

Following the breakthrough research results of Hutchings and Hamta, there has been a dramatic increase in the interest in gold catalysis [1], It has been demonstrated that the physieochemical and catalytic properties of gold catalysts depend mainly on the type of support and the preparation method. Both parameters influence the size of Au clusters [1]. Interaction between gold and the metals localized in the support plays also an important role and can determine the catalytic activity of the catalysts. 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

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