Singly dispersed metal

For this reason, these catalysts are also known under the name of supported hydrogen-bonded (SHB) catalysts and, in conjunction with Pd° particles on the same support material, have contributed to generate active heterogeneous systems for the hydrogenation of benzenes in aprotic solvents. Irrespective of the substrate, the combined single-site/dispersed-metal catalyst RhI-Pd0/SiO2 shown in Figure 16.5a was from four- to six-fold more active than supported palladium... 

Subsequent to these early studies, several techniques have been developed to produce well-dispersed metal (hydrous) oxides of different chemical compositions (single or biphasic), consisting of uniform particles in a variety of shapes, including spheres. These efforts are certainly justified in view of the ever-increasing recognition of the importance of such materials in numerous applications in various areas of modern technology and medicine. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

Because the adsorption properties of highly dispersed metals may differ significantly from those of single-crystal surfaces, especially if the metal particles are very small 397), CO adsorption on platinum nanoparticles supported on Si02 (particle diameter > 6nm) is contrasted in the following section with that on Pt(l 11), with particular emphasis on the SFG signal intensities 132). 

While this work did show that a number of different types of single atom sites can be present on the surface of dispersed metal catalysts, the possibility was... 

As discussed previously, this reaction was also run under these same conditions over the series of specifically cleaved platinum single erystals shown in Fig. 3.2. 3 The results of these experiments show that it was the corner atoms on these crystals that promoted C-H bond breaking. Thus, the saturation sites on the dispersed metal catalysts are also comer atoms. Since this saturation site description agrees with that proposed on the basis of the butene deuteration described previously,5 -62 it is likely that the isomerization sites, M, are edge atoms and the hydrogenation inactive sites, M, are face atoms. A similar approach can be used to determine the nature of the active sites responsible for promoting almost any type of reaction. 5.70... 

Catalytic reactions can be run over large, massive metal particles as well as the much smaller, dispersed metal crystallites. The massive metal catalysts can be the single crystal catalysts such as those shown in Fig. 3.2 or polycrystalline forms of bulk metal such as wires, foils or ribbons. These latter materials were used somewhat routinely in the early catalytic research efforts that were involved with developing the mechanisms of vapor phase catalytic processes. These materials were considered to be analogs of the supported catalysts in which the effect of the support, if any, was eliminated. 

Much has been learned in recent years about the structures of metal surfaces. which do not always parallel the crystallography of the bulk material. Well dehned single-crystal surfaces provide us with an atomic view that is helpful in deciphering similar structures existing on dispersed metals. ... 

A possibility to consider polyciystalline platinum surface as some combination of low index surfaces or as some disordered single crystalline surface surely caimot be immediately concluded from the values of zero charge potentials exclusively. Experiments with some intermediate model systems more or less reduceable to simple additive combinations of several planes, or of terraces and steps, are of increasing interest. Microfacetted electrodes, various types of nanoparticles prepared by precise non-electrochemical techniques, non-coalesced electrodeposited particles, " and single platinum microspheres deposited on micro-electrodes, as well as highly ordered templated electrdeposits can be considered as most ordered real platinum materials helpful to discover stractural effects at atomic level. However they all are still too simple to compare with, to say, platinized platinum, and attempts to electrodeposit the dispersed metallic multilayers of more and more ordered type ° are also relevant. 

With respect to the metal recovery for reuse, the study of Lee et al deserves attention. These authors used the oil-soluble compounds of Mo, W, Ni and Co as the precursors for dispersed metal-sulfide catalysts. For single metals, the best performance was observed for the Mo catalyst. The combination of Co -1- Mo gave the most active catalyst for HDS, whereas Ni + Mo was best for HCR. In this study, the fixed bed of extmdates made either of the microporous AC or of y-Al203 was placed downstream of the reaction zone with the aim to remove metals from the product streams. For the former, the overall conversion increased with time on stream. This was attributed to the accumulation of metals on AC. Thus, the metal-deposited AC exhibited catalytic activity. It was noted that the efficiency of the metal removal using the AC extrudates was rather high. It is believed that there are a number of methods that are suitable for the recovery of metals that were trapped by the AC. For example, combustion of the AC will leave behind ash with a high concentration of metals. In this study, an AR containing 26 ppm of V + Ni was used as the feed. 

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