Crystal structure catalysts

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

Crystal structure of solids. The a-crystal form of TiCla is an excellent catalyst and has been investigated extensively. In this particular crystal form of TiCla, the titanium ions are located in an octahedral environment of chloride ions. It is believed that the stereoactive titanium ions in this crystal are located at the edges of the crystal, where chloride ion vacancies in the coordination sphere allow coordination with the monomer molecules. 

Three common uses of RBS analysis exist quantitative depth profiling, areal concentration measurements (atoms/cm ), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profilii of polymers, high-T superconductors, optical coatings, and catalyst particles. ... 

The sodium in the E-cat is the sum of sodium added with the feed and sodium on the fresh catalyst. A number of catalyst suppliers report sodium as soda (Na20). Sodium deactivates the catalyst acid sites and causes collapse of the zeolite crystal structure. Sodium can also reduce the gasoline octane, as discussed earlier. 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

Thus zeolite ZSM-5 can be grown (ref. 15) onto a stainless steel metal gauze as shown in Figure 6. Presumably the zeolite crystals are chemically bonded to the (chromium-) oxide surface layer of the gauze. After template removal by calcination and ion exchange with Cu(II) a structured catalyst is obtained with excellent performance (ref. 15) in DeNOx reactions using ammonia as the reductant. 

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

The properties and characteristics of the five HNLs used as catalysts in stereoselective syntheses are listed in Table 2. The crystal structures of these HNLs have been determined during the last decade. From the crystal structures and kinetic measurements, the mechanistic pathways of cyanogenesis could be established. 

Hydrogen is produced when the membrane is illuminated in contact with a NaSH solution, the efficiency being comparable to the best colloidal or powdered CdS preparations. The crystal structure of CdS was found to influence greatly the yield with cubic P-CdS acting as a more active catalyst than hexagonal a-CdS. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

Fig. 5.6. Crystal structure of tetrakis-P, P, P P -(4-methylphenyl)-l,l -binaphthyldi-phosphine-1,2-diphenyl-1,2-ethanediamine ruthenium borohydride catalyst. Reproduced from J. Am. Chem. Soc., 124, 6508 (2002), by permission of the American Chemical Society.

Both mechanistic and computational studies have been used to explore the catalytic process. A crystal structure of the catalysts is available (Figure 5.7).152 The... 

Fig. 5.7. Crystal structure of borane complex of a,a-diphenylprolinol oxazaborolidine catalysts. Reproduced from...

Phosphate ester crystal structures have been determined of zinc 1,5,9-triazacyclononane including an interesting structure containing an oligophosphate bridged zinc unit.450 The zinc complex of 1,5,9-triazacyclododecane was studied as a hydrolysis catalyst for substituted phenyl acetates.451 Kinetic analysis suggested that hydrolysis occurs by a mechanism involving hydroxide attack of a metal-bound carbonyl. 

Juengsuwattananon K, Jaroenworaluck A, Panyathanmapom T, Jinawath S, Supothina S (2007) Effect of water and hydrolysis catalyst on the crystal structure of nanocrystalline Ti02 powders prepared by sol-gel method. Physica status solidi a 204(6) 1751-1756... 

Prior to this 1996 study, there had been no reports of boratabenzene complexes of early transition metals.42 An X-ray crystal structure of the catalyst revealed a C2-symmetric geometry that resembles Cp2Zr-based bent metallocenes. The bond lengths suggest a strong B-N it interaction (rotational barrier measured by NMR 18 kcal/mol) and a very weak Zr B interaction ( t 5 coordination of the boratabenzene ring). 

Unfortunately, the ethylene oligomers produced by the Ph-substituted catalyst are a complex mixture of 1-alkenes, 2-alkyl-1-alkenes, and 2-alkenes (Scheme 25). In contrast, the OEt-substituted catalyst selectively affords 1-alkenes (average number of inserted ethylene units 7), due to its relative reluctance to react with olefins other than ethylene 44 An X-ray crystal structure of the OEt-substituted... 

Adsorption is the preferential concentration of a species at the interface between two phases. Adsorption on solid surfaces is a very complex process and one that is not well understood. The surfaces of most heterogeneous catalysts are not uniform. Variations in energy, crystal structure, and chemical composition will occur as one moves about on the catalyst surface. In spite of this it is generally possible to divide all adsorption phenomena involving solid surfaces into two main classes physical adsorption and chemical adsorption (or chemisorption). Physical adsorption arises from intermolecular forces... 

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