Highly dispersed metal oxide catalyst

Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts... 

The deposition-precipitation (DP) method has been scarcely used to prepare platinum supported catalysts [1-5], while it is the preferred method to obtain active gold ones [6-7]. Initially, the deposition-precipitation technique has been developed by Geus and Hermans for the production of highly loaded and highly dispersed metal/oxide catalysts [8]. This technique involves the precipitation of the active phase precursor at fte surface of Ae support and its subsequent deposition whereas nucleation in the solution itself should be avoided. 

I 7 6 Properties, Synthesis and Applications of Highly Dispersed Medal Oxide Catalysts 16.3.1.1 Hydrolysis and Condensation of Metal Alkoxides... 

In summary, the Raman studies have provided a deeper understanding of the molecular structure and reactivity properties of bulk metal oxide catalysts during selective oxidation reactions. However, the fundamental insights have primarily been limited to the bulk properties of the bulk metal oxide catalysts. In order to obtain surface information about metal oxide catalysts with Raman spectroscopy (essentially a bulk characterization technique), it is necessary to look at chemisorbed species on the surface of bulk metal oxides (see Sec. VIII) or highly dispersed metal oxide systems such as supported metal oxide catalysts. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

An efficient, low temperature oxidation catalyst was developed based on highly disperse metal catalyst on nanostructured Ti02 support. Addition of dopants inhibits metal sintering and prevents catalyst deactivation. The nanostructured catalyst was formulated to tolerate common poisons found in environments such as halogen- and sulfur-containing compounds. The nanocatalyst is capable of oxidizing carbon monoxide and common VOCs to carbon dioxide and water at near ambient temperatures (25-50 °C). 

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. 

Many catalysts of commercial importance are highly dispersed metals supported on silicon or aluminum oxides. Hansma, Kaska, and Laine (11) showed that these catalysts could be modeled with tunneling junctions by evaporating very thin... 

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. 

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 ... 

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