Characterization of Supported Metal Complexes

In the following we list common methods that have been used to characterize supported metal complexes and show how these techniques can be used to learn more about the surface-metal complex morphologies and chemical properties. 

2 Chemical Analysis of Supported Ionic Metal Complexes. - When the metal complex forms an ionic solid that can participate by complete ion exchange of the anion for surface anionic species, it may be possible to document the 

we suggest that the ion exchange mechanism can be fostered when the ligands within the metal complex show strong Lewis base functions. As a result, we try to use these type of ligands when designing a metal complex to be used as the source of the neutral ion in a supported catalyst. 

The results of Cu loadings observed for samples prepared by ion exchange of silica with [Cu(en)2] appear to be confirmed by the recent data reported by Toupance, et al on their attempts to prepared Cu/silica solids. They reported Cu loadings 2 wt% when the cation was the copper ethylenediamine however, they did not mention the use of a co-solvent. 

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Alexeev, O., and Gates, B. C., EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors, Top. CataL 10,273 (2000). 

Chemists have prepared metal complexes containing metal atoms/ions as a means to understand better the structure, chemical bonding, and properties of metals and metal ions. One of the first efforts to affix these metal complexes to a surface as a means to create a supported catalyst was reported by Ballard followed by reports collected by Yermakov, et al. and Basset et al. We distinguish here between metal complexes that contain zero-valent metals and those that show metal cations and we limit this review to complexes containing metal ions as others have published extensive reviews of zero-valent, metal clusters and their chemistryIn our previous three reviews on the chemistry of supported, polynuclear metal complexes, we described efforts to synthesize and characterize oxide-supported, metal complexes as adsorbents, catalysts and precursors to supported metal oxides. In one application of this technology, efforts were... 

The characterization of supported metal catalysts is a matter of some complexity and supported bimetallic catalysts even more so. Nevertheless the development and application of methods for determining catalyst structure is essential for an understanding of why the performance of a selected combination of metal(s) and support varies as a function of preparative variable, activation procedure, reaction conditions, or time. Although some aspects of catalyst structure can be routinely determined, the basic measurement of absolute metal dispersion by selective chemisorption/gas titration is still the subject of many publications and the necessity of cross-checking by instrumental methods is generally appreciated. The characterization of supported metal catalysts also involves some less accessible properties, e.g., the sites available on crystallites as a function of size, high-temperature... 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

In the last three decades, we have designed and successfully prepared various supported metal complexes on oxide surfaces that exhibit unique catalytic activities and selectivities that are different from those of their homogeneous analogues [3,4,9, 12-15]. With the aid of several sophisticated spectroscopic techniques, the structures and roles of catalytically active species on surfaces have been characterized and identified [3, 4,9,12-25]. Chemical interactions between metal complexes and oxide surfaces can provide new reactivity of metal species by the construction of a spatially controlled reaction environment and the formation of unsaturated active metal species, leading to high catalytic activity, selectivity and durability [21-25]. 

In the following paragraphs, methods of preparation and characterization of structurally simple supported metal complexes are summarized, and examples are presented that illustrate characterization data and support general conclusions about structure, bonding, reactivity, and catalysis. 

In recent years researchers have begun to use density functional theory to model supported metal complexes, representing the support as a cluster (fragment of the bulk support). Examples of supported complexes characterized by both theory and spectroscopy are presented below. 

A limitation of supported metal nanoclusters prepared from molecular metal carbonyl clusters is that, so far, clusters of only several metals (Ru, Rh, Ir, and Os) have been made in high yields (80 to 90%, with the likely impurity species being mononuclear metal complexes). However, this disadvantage is offset by the advantage of the characterizations, which show that some clusters are stable even during catalysis, at least under mild conditions. 

The theoretical parameters characterizing Ir4 in zeolite NaX (Fig. 3) indicate Ir-O distances of about 2.2 A, in good agreement with EXAFS data (Ferrari et al., 1999) and approximately equal to the metal-oxygen bond distances found experimentally and theoretically for supported metal complexes, as discussed above. When the structure of Fig. 3 is rotated 60°, the theory indicates an Ir-O distance of about 2.7 A, in agreement with the longer distances observed by EXAFS spectroscopy (but this agreement may be fortuitous). 

The catalytic activity of zeolites in alkane to olefin reactions, photochemical conversion reactions, Fischer-Tropsch hydrogenation, isocyana-tion, carbonylation, and related chemistry make up the last theme. An important focus of this is to explore the utility of zeolites as selective heterogeneous catalysts for reactions that involve Group VIII metals. The mechanistic nature of some of this chemistry is presented, along with the characterization of supported organometallic transition metal complexes. 

A wealth of detailed evidence on the nature of supported metals can readily be obtained from infrared characterization studies, but correct interpretation of much of this evidence is still far from clear. The surface chemistry of supported metals is generally very complex, and assertions as to the origins of various band shifts and the exact nature of adsorption sites should be taken with some caution at present. Clearly, however, better understanding of the complex nature of supported metal catalysts should contribute greatly to the development of more efficient catalysts for many important industrial processes and to more efficient pretreatment and regeneration procedures. 

The literature was reviewed to describe the newest efforts to synthesize and characterize supported polynuclear metal complexes as adsorbents and catalysts. This review includes our attempts to model the equilibrium structures and properties of the metal complexes, using simple quantum mechanics, as a means to understand better the interactions between the surface and the metal complexes. Special attention is directed towards the characterization of the supported metal complexes before and after ligand removal. We compare these modeling results with observations in the literature so as to understand better the fundamental processes that govern the interactions between the metal complexes and the surfaces. With this enhanced understanding of these governing factors, it should be easier to prepare oxide solids decorated with metal complexes having the desired physico-chemical properties. 

InfraRed Spectroscopy. - Infrared spectroscopy (IR) has been used for over 50 years to examine solid catalysts and the events that accompany their use. Since that time, a large number of publications have chronicled the strengths and weakness for using IR as a means to characterize the surface of solids. Well-defined, metal complexes supported on certain solids are amenable to careful characterizations by IR. For one application of this technique, qualitative aspects of the metal complex structure can be deduced as a function of metal loading on the support, or with changes in the pretreatment, etc. 

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