Highly Dispersed Supported Metal Catalysts

Production of highly dispersed supported metal catalysts... 

A related approach to the preparation of highly dispersed supported bimetalhc catalysts involves the reaction of metal complexes with supported metal clusters or particles. The method is based on the idea that by careful choice of the metal complex and control of the reaction parameters it may be possible to cause the metal complex to react selectively with the supported metal but not with the support surface [13]. Because this approximation to the subject is the main focus of this chapter, it is thoroughly developed in the following sections. 

Passivation often involves the controlled exposure of the catalyst to air at ambient temperature. Rapid exothermic reactions are prevented while forming stable layers which inhibit further rapid reaction upon air exposure. Similar exposure to other passivating reagents would also lead to air stable surface layers on the metallic surfaces. A typical example is the passivation of Ni catalysts which would oxidize catastrophically upon exposure to air, and of highly-dispersed supported Pt catalysts. Many methods or techniques are available ... 

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Supported metal catalysts are used in a large number of commercially important processes for chemical and pharmaceutical production, pollution control and abatement, and energy production. In order to maximize catalytic activity it is necessary in most cases to synthesize small metal crystallites, typically less than about 1 to 10 nm, anchored to a thermally stable, high-surface-area support such as alumina, silica, or carbon. The efficiency of metal utilization is commonly defined as dispersion, which is the fraction of metal atoms at the surface of a metal particle (and thus available to interact with adsorbing reaction intermediates), divided by the total number of metal atoms. Metal dispersion and crystallite size are inversely proportional nanoparticles about 1 nm in diameter or smaller have dispersions of 100%, that is, every metal atom on the support is available for catalytic reaction, whereas particles of diameter 10 nm have dispersions of about 10%, with 90% of the metal unavailable for the reaction. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

In supported metallic catalysts, the metals are usually from Groups VIII and VB of the Periodic Table. For highly dispersed metallic catalysts, the support or the carrier is usually a ceramic oxide (silica or alumina) or carbon with a high surface area, as described in chapter 2. Supported metallic catalysts can be prepared in a number of ways as described by Anderson (1975). A description of some of the methods used to prepare representative model (thin film) and practical (technological) powder systems follows. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

In addition, the same studies that were carried out on the Pt(lll) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart s hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible.within 10%, are within a factor of two identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. 

Catalysts were some of the first nanostructured materials applied in industry, and many of the most important catalysts used today are nanomaterials. These are usually dispersed on the surfaces of supports (carriers), which are often nearly inert platforms for the catalytically active structures. These structures include metal complexes as well as clusters, particles, or layers of metal, metal oxide, or metal sulfide. The solid supports usually incorporate nanopores and a large number of catalytic nanoparticles per unit volume on a high-area internal surface (typically hundreds of square meters per cubic centimeter). A benefit of the high dispersion of a catalyst is that it is used effectively, because a large part of it is at a surface and accessible to reactants. There are other potential benefits of high dispersion as well— nanostructured catalysts have properties different from those of the bulk material, possibly including unique catalytic activities and selectivities. 

We focus here on supported metallic catalysts, since metals are widely used as catalysts. However, the metal catalyst is often expensive. Then, it is usually applied in a finely dispersed form on a high surface area support, since in this circumstance, a large fraction of the metal atoms are exposed to the reactant molecules. 

The formation of highly dispersed particles or crystallites in the synthesis process of, for example, a supported metal catalyst, is governed by nucleation and growth mechanisms (vide supra) that have been described in the literature [15, 16, 21-23]. For sintering or redispersion (spreading and film formation) to occur, particles or atoms, molecules or clusters of the active... 

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