Surfaces as Catalysts

In Section 14 we considered crystalline solids and approximated their structures as if the materials were infinite and periodic in all three dimensions. This may be a good approximation when studying the bulk properties of the material, but for some properties the fact that the material is finite is of ultimate importance. Among these are those properties that are related to the existence of surfaces. The fact that the atoms at the surfaces have a lower coordination than those in the interior and accordingly have dangling bonds leads to the existence of surface-specific properties. These include surface reconstructions for which the surface atoms adopt a structure different from that of the interior, as well as a higher reactivity of the surface atoms. The latter is the topic of the present section. 

In studying the surfaces one may take one out of three different approaches. The introduction of a single surface in a three-dimensional crystalline solid 

Alternatively, one may re-introduce the translational symmetry in the third direction by constructing a fictitious system consisting of repeated slabs separated by regions of vacuum. The slabs should be so thick that the central region resembles the inner part of the infinite crystal. Moreover, also here extra molecules will have to be introduced periodically. 

Finally, one may consider a finite cluster as a model system for the surface without or with adsorbants. In this case there may be dangling bonds in all directions, although one may be interested in only those of one of the surfaces. The other dangling bonds may be saturated by, e.g., hydrogen atoms. 

Ultimately an adsorbed molecule may have modified electronic properties compared with the molecule in the gas phase. Thereby it may react more easily with another molecule, either co-adsorbed on the surface or in the gas phase, so that the surface has acted as a catalyst. There has been some work devoted to such studies, but these will not be described further here. 

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Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

Yasseri AA, Sharma S, Jung GY et al (2006) Electroless deposition of Au nanocrystals on Si(lll) surfaces as catalysts for epitaxial growth of Si nanowires. Electrochem Solid-State Lett 9 C185-C188... 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

The molten salts quickly dissolve the metal oxides at high temperatures to form a clean metal surface. Other uses are as catalysts and in fire-retardant formulations (see Flame retardants). 

Catalyst Particle Size. Catalyst activity increases as catalyst particles decrease in size and the ratio of the catalyst s surface area to its volume increases. Small catalyst particles also have a lower resistance to mass transfer within the catalyst pore stmcture. Catalysts are available in a wide range of sizes. Axial flow converters predorninanfly use those in the 6—10 mm range whereas the radial and horizontal designs take advantage of the increased activity of the 1.5—3.0 mm size. 

The soHds used as catalysts are typicady robust porous materials with high internal surface areas, typicady, hundreds of square meters per gram. Reaction occurs on the internal catalyst surface. The typical soHd catalyst used ia iadustry is a composite material with aumerous components and a complex stmcture. 

This is an ion-exchanger like the sulfonated polymer. The siUca surface can also be functionalized with phosphine complexes when combined with rhodium, these give anchored complexes that behave like their soluble and polymer-supported analogues as catalysts for olefin hydrogenation and other reactions ... 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

An apparent first-order specific rate increases with liquid rate as the fraction of wetted surface improves. Catalyst effectiveness of particles 3 to 5 mm (0.12 to 0.20 in) diameter has been found to be about 40 to 60 percent. 

The mesoporous ordered silicas of different type represent the new generation of materials with unique properties. The discovery of these materials became basis for creation of new catalysts, adsorbents, sensors and supporter for other molecules. The most important way of the modifying physical and chemical properties of mesopurous silicas consist in organic components incorporation on the silica surface as part of the silicate walls or their insertion within channels of the mesopores. This ensured that interest in synthesis and study of functionalized mesoporous materials shai ply grew. In spite of it, these materials are studied insufficiently. 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

The autocatalytic hardening of tannins without addition of formaldehyde or another aldehyde as crosslinker is possible, if small traces of alkaline Si02 are present as catalyst and also a high pH is used, or with certain tannins just by the catalytic action induced by the wood surface [152-160]. 

Single-wall tubes. Following the synthesis studies of stuffed nanocapsules, single-wall (SW) tubes were discovered in 1993(9,10]. SW tubes are found in chamber soot when iron[9] and cobalt[10] were used as catalysts, and for nickelfl 1,40] they grow on the surface of the cathode slag. For iron catalyst. 

The most commonly used catalysts are palladized charcoal or calcium carbonate and platinum oxide. For better isotopic purity, the use of platinum oxide may be preferred for certain olefins since the substrate undergoes fewer side reactions while being chemisorbed on the platinum surface as compared to palladium.Suitable solvents are cyclohexane, ethyl acetate, tetrahydrofuran, dioxane or acetic acid-OD with platinum oxide. 

As catalyst for the Rosenmund reaction palladium on a support, e.g. palladium on barium sulfate, is most often used. The palladium has to be made less active in order to avoid further reduction of the aldehyde to the corresponding alcohol. Such a poisoned catalyst is obtained for example by the addition of quinoline and sulfur. Recent reports state that the reactivity of the catalyst is determined by the morphology of the palladium surface." ... 

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