Supported metals, small particles compounds

It is also possible to prepare supported metal catalysts from casy-to-rcduce elements. For example, [ Supp -0]Rh( /3-C3H5)2 (with supp - Al, Si, or Ti) reacts with H2 at room temperature to give rhodium metal particles (1.5 nm) [54, 58]. Diallyl compounds of Group 10 elements (Ni, Pd, Pt), forming species singly anchored onto silica or alumina, arc transformed into small metal particles ([Pg.175]

In addition to planar (two-dimensional) faces, the surfaces of supported metal particles consist of edges between surfaces as well as steps and kinks on otherwise planar surfaces (2J). These sites are thought to be highly active for the adsorption and bond dissociation of molecules. Analogies have been drawn between the binding of small molecules (CO, H2, and CH4) to coordi-natively unsaturated sites of metal surfaces and the binding of molecules and atoms (C, N, and S) in clefts of metal clusters. The simplest examples are the four-metal butterfly cluster compounds (Fig. 4), where the cluster geometry... 

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. 

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. 

A number of different methods for the preparation of supported metal (oxide) catalysts are dealt with in this book. In this chapter we discuss deposition precipitation as a generic method to emplace metals, metal oxides, metal sulfides, or metal hydroxides as small particles onto an existing support material. The deposition of the metal (compound) is brought about by a chemical reaction in the liquid phase. This chemical reaction leads to formation of a metal compound with low solubility in the solvent in question. The precipitation that follows is steered to take place exclusively at the surface of a suspended support material. The precipitation of metal hydroxides from an aqueous solution, such as... 

Specific activities of Pd, Pt and Rh catalysts in propene oxidation are reported in Table 1.7. Contrary to what was observed in alkane oxidation, propene oxidation is not very sensitive to the nature of the metal. Quite similar TOP were measured over unsupported metals, while Pt and Pd seemed to be shghtly more active than Rh when supported on alumina. Propene oxidation is not very sensitive to metal particle size. However, intrinsic activity would be rather higher on small particles. As TOP are higher or much higher on unsupported metals, it seems that alumina could play a negative role in propene oxidation. The intermediary formation of partially oxidized compounds (acrolein, alcohols,...) is not excluded. Alumina might store and stabilize these intermediates, slowing down the total oxidation. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

These questions lead on to further fundamental questions concerning the shapes and properties of small metal particles. For example, what is the stable shape for a small metal particle How is this affected by size, method of preparation, temperature, gaseous environment, precursor compound, support morphology, etc. Do small metal particles have different electronic properties from bulk metal Do surface electronic properties depend on particle size, and if so, do they vary in the same way as bulk electronic properties When, indeed, is a particle small enough to have unusual properties ... 

Supported bimetallic catalysts can be made by adsorption of a bimetallic precursor such as molecular cluster compounds, colloidal particles or dendrimer-stabilised particles. In several cases, homogeneous bimetallic particles have been found where the compositions lie within the miscibility gap of the bulk alloy (e.g. with PtAu particles). This suggests that when the particles are small enough and do not possess metallic properties, the normal rules do not apply. 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

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