Supported catalysts cluster species

Up to this point the paramagnetic species have been considered as isolated entities on the surface or in the bulk phase. It is clear, however, that many supported catalysts are actually clusters of ions. If these ions are paramagnetic, such as Cr203 at temperatures above about 30°, then another term must be included in the spin Hamiltonian to account for the exchange energy. This term is written as... 

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

The XPS technique provides identification of oxidation states of metals in supported catalysts, but the determinations are often inexact and require confirmation by other methods. XPS is especially useful for detecting changes in oxidation states of cluster precursors on various oxide supports. For example, the transformation of Rh4(CO)i2 on silica, alumina, MgO, ZnO, and TiOj to yield different surface species such as a raft of Rh(CO)2(OM) or Rh metal aggregates has been inferred from chemical shifts in XPS data (Fig. 5) 39-41). The XPS technique requires ultrahigh vacuum, and instability of the... 

Structures and reactivities of supported metal clusters have been investigated mostly by spectroscopic and chemical methods, and the available data generally show that the supported species display chemistry that is comparable to that of precursor clusters in solution. These supported molecular catalysts provide information on the surface intermediates that is complementary to information on conventional metal catalysts. 

Similar conclusions have been drawn from a study of the hydrogenolysis and isomerization of cyclopropane over catalysts made up of supported rhenium clusters of differing nuclearity.29 The hydrogenolysis of cyclopropane was found to take place only over those catalysts having at least a Re3 nuclearity while the hydrogenation of propene, an isomerization product of cyclopropane, took place on Re I species (Eqn. 3.1). 

These ligand exchange and association reactions require complexes (catalysts) that are stable during the synthesis. An alternative is the In situ formation of the supported species, illustrated by the formation of a supported tetrairidium cluster, as follows ... 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

The result of Sections 6.3 and 6.4 demonstrated how theory, used in concert with experiment, is helping to define opportunities for progress in research with supported metal clusters. Powerful computational tools will soon be able to routinely help identify reaction intermediates on supported heavy metal catalysts. Because reaction intermediates are inherently unstable, they are beyond the reach of current experimental methods such as in-situ EXAFS spectroscopy, which is likely to give evidence only of stable species. 

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