Supported metals uniform

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

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... 

In the last few years remarkable progress has been made in the preparation of supported metal catalysts. Entirely new methods have been developed, comprising precipitation of the metal as an insoluble salt or hydroxide on the support under controlled conditions, or loading the support with the metal by means of ion exchange. A feature of catalysts prepared according to the former method (I, 2) is that, after reduction, they have a high metal content (50% by weight, or more), while the metal crystals are still small (20-40 A) and distributed very uniformly over the support. The latter approach yields catalysts with metal crystallites of approximately 10 A however, the metal content is rather low [about 2% (3-5)]. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

This section of this chapter includes a brief review of methods of preparation and properties of supported metal nanoclusters only catalysts that have been relatively well characterized and found to be nearly uniform are considered. The nanoclusters described here lack the structural definition... 

The methods of structure determination of supported nanoclusters are essentially the same as those mentioned previously for supported metal complexes. EXAFS spectroscopy plays a more dominant role for the metal clusters than for the complexes because it provides good evidence of metal-metal bonds. Combined with density functional theory, EXAFS spectroscopy has provided much of the structural foundation for investigation of supported metal clusters. EXAFS spectroscopy provides accurate determinations of metal-metal distances ( 1-2%), but it gives only average structural information and relatively imprecise values of coordination numbers. EXAFS spectroscopy provides structure data that are most precise when the clusters are extremely small (containing about six or fewer atoms) and nearly uniform (Alexeev and Gates, 2000). 

Supported metal catalysts generally show an increase in catalytic activity compared to the pure oxide or metal. Yet these systems are not well characterised, owing to the fact that such catalysts typically consist of a range of different supported metal sites, from small clusters to monolayer islands, all with non-uniform distributions in size and shape. One way to begin to understand such complex systems is to attempt to capture some essential part of the full system by developing model catalysts experimentally or using computer modelling techniques. This chapter concentrates on the latter but in the context of the relevant experimental data. 

The experimental approach needed to understand supported metal cluster catalysts, is to prepare new catalysts based on systems of high uniformity e.g. supported clusters with identical size and composition. There are a number of ways of attaining... 

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

Surface atoms on real catalysts reside in a variety of coordination environments depending on the exposed crystal plane (see Figure 5.1.3) and may exhibit different catalytic activities in a given reaction. Thus, a turnover frequency based on [ ]q will be an average value of the catalytic activity. In fact, the calculated turnover frequency is a lower bound to the true activity because only a fraction of the total number of surface atoms may contribute to the reaction rate. Nevertheless, the concept of a turnover frequency on a uniform surface has proven to be very useful in relating reaction rates determined on metal single crystals, metal foils, and supported metal particles. 

The supported clusters described in this chapter have strong connections to industrial supported metal catalysts, although the clusters or particles in industrial catalysts are much less uniform and less susceptible to incisive structural characterization than those considered here. Development and further investigation of structurally simple oxide- and zeolite-supported molecular catalysts will provide fundamental understanding of structure-activity relationships that will direct design of novel and improved industrial supported metal catalysts. 

Zeolites have been used for years as supports for metal catalysts [1-5]. Such catalysts are typically made by impregnation of the zeolite with an aqueous solution of a metal salt, followed by calcination and reduction in hydrogen. Because the metal particles in such catalysts are typically extremely small and nonuniform in size and shape, often being present both inside and outside the zeolite pore structure, their structures are not well understood. This structural complexity provides a fundamental motivation for preparing and investigating structurally simple zeolite-supported metals, those that are so small and uniform as to be nearly molecular in character and located almost entirely within the zeolite pores investigations of well-defined... 

It must be assumed that the samples listed in Table 2 have distributions of cluster sizes, although the available methods do not provide good evidence of the distributions. It seems likely that zeolite-supported metal clusters made from metal carbonyl clusters (Table 1) incorporate more nearly uniform clusters than samples made by conventional methods from metal salts, but this suggestion is not yet tested. 

In summary, zeolite-supported metal clusters have now been prepared that are so small and apparently nearly uniform in size that they are regarded as nearly molecular. Preparations with metal carbonyl cluster precursors are the best known for making nearly uniform and thus nearly molecular supported clusters, but it is clear that conventional preparation methods based... 

We have shown that small uniform ruthenium particles can be applied on activated CNFs in a reproducible manner when the HDP method is used with RuN0(N03)3 as catalyst precursor. A very uniform distribution of 1-2 nm sized ruthenium particles at an appreciable loading has been obtained. This high dispersion remained almost unchanged upon heating in inert to 973 K. These results clearly demonstrate the applicability of the HDP technique for the preparation of CNF supported metal catalysts, though no surface compound between precursor and support material can be formed. 

Metals or metal oxides are usually supported on the supports such as silica and alumina when they are used as catalysts. The deposition of metal species on the supports often results in the improvement of catalytic activity and selectivity, and/or in the inhibition of their sintering at high temperatures due to the chemical interaction between the metal species and the supports. Because the supported metal or metal oxides are prepared conventionally with an impregnation method, the metal species are mainly supported on the surface of supports. On the other hand, we have prepared the supported metal catalysts by using microemulsion systems [1,2]. The preparation methods can produce the metal or metal oxide particles uniformly covered with silica layers. Thus, the metal species interact strongly with silica. In addition, because metals or metal oxides which work as active sites for the catalytic reactions are covered with silica layers with porous structures, new functions such as shape selectivity could be expected in the catalytic reaction over them. 

Figure 15.4 Transmission electron micrograph of palladium on activated carbon catalyst (a) the metal crystallites are located on the edge of the carbon support (b) uniform distribution of the crystallites. In both cases the crystallites are 2 to 3 nm in size.

Conventional preparation techniques result in non-uniform supported metal species, which are difficult to characterize structurally. Therefore, Gates developed a strategy [266,267] to prepare nearly uniform metal clusters, either on metal oxide support or encapsulated in zeolite cavities. Starting from supported metal carbonyl clusters as precursors, the synthesis proceeded with decarbonyla-tion aiming at a minimal perturbation of the metal frame. However, fragmentation or aggregation of metal clusters may occur. 

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