Silver 3-alumina

One gram of silver /3-alumina (see above) is placed into a fused quartz test tube about 2 cm in diameter and about 14 cm long. Five grams of lithium chloride is added. It is important that the lithium chloride used have a very low content of other alkali metal impurities, except Cs, since the ion exchange equilibria greatly favor the presence of the other alkali metals in the /3-alumina crystals over lithium. Essentially all of the impurity ends up in the crystals. The fused-quartz test tube is heated to 650° in a furnace. For crystals 1-cm in diameter the time to reach 99% equilibrium is approximately 16 hours. The molten salt is decanted and the crystals are allowed to cool to room temperature. Methyl alcohol containing about 10% propylamine or ethylenediamine is used to wash the product and thereby remove the silver chloride and residual lithium salts. The sample is dried at 400° and stored in a dessicator. The lithium /3-alumina crystals contain less than 0.05% Ag. If the lithium chloride used contains a trace of sodium or potassium, it can be prepurified by treatment with silver /3-alumina at 650°. Each gram of silver /3-alumina will remove about 30 mg of sodium from the melt. The molten lithium chloride, after decantation from the pretreatment silver /3-alumina, can be used to prepare the product, lithium 0-alumina. 

The preparation of this compound from silver (3-alumina is similar to the preparation of lithium /3-alumina. The melt consists of 10 g of potassium chloride. The exchange temperature is 800°. For crystals with diameters of 1 cm it takes about 16 hours to reach 99% of equilibrium. The potassium salts used should contain less than 0.1 wt % sodium. After decantation of the melt the crystals are washed with water containing 2% propylamine or ethylenediamine to remove residual potassium salts and silver chloride. The sample is dried at 200°. The potassium 0-alumina contains less than 0.05% silver. 

One gram of silver )3-alumina crystals is placed in a Vycor tube with a 1-cm i.d. and 20 cm length and closed at one end. Three grams of gallium and 4 g of iodine are added, and the Vycor tube is necked down near the middle in preparation for sealing off. The tube is evacuated with a mechanical pump to... 

Meunier, F.C., Breen, J.P., Zuzaniuk, V. et al. (1999) Mechanistic aspects of the selective reduction of NO by propene over alumina and silver-alumina catalysts, J. Catal., 187, 493. 

Wichterlova, B., Sazama, P., Breen, J.P. et al. (2005) An in situ UV-vis and FTIR spectroscopy study of the effect of H2 and CO during the selective catalytic reduction of nitrogen oxides over a silver alumina catalyst, J. Catal. 235, 195. 

Eranen, K., Lindfors, L.E., Klingdted, F. et al. (2003) Continuous reduction of NO with octane over a silver/alumina catalyst in oxygen-rich exhaust gases combined heterogeneous and surface-mediated homogeneous reactions, J. Catal. 219, 25. 

Li Y.S., Wang Y., Chemically prepared silver alumina substrate for surface- enhanced Raman-scattering, 4/ /)/. Spectrosc 1992 46 142-146. 

Ethylene can be oxidized to ethylene oxide over a silver-alumina catalyst. Experimental data were obtained at 260 C and atmospheric pressure (Wan, Ind Eng Chem 45 234, 1951). Selected data are tabulated. Inlet... 

Jen, H.-W. Study of nitric oxide reduction over silver/alumina catalysts imder lean conditions Effects of reaction conditions and support, Catal Today, 1998, Volume 42, Issues 1-2, 37-44. 

Lack of understanding of the above mentioned issues has led to intense study of not only what is happening on the atomic level, but also the design of new systems that have both higher selectivity and rates of conversion. Three main systems were studied thus far silver-alumina type catalysts, silver-modified manganese species, and silver-modified ceria (Ce02) systems. 

Silver-alumina type catalysts are by far the most widely used, especially since they are the main catalytic source in the epoxidation of ethylene. Therefore, they are readily available and already have undergone extensive studies. Many systems have sought to utilize the presence of NO (another harmful environmental species) in gas feeds. In this case, the NO species would be reduced to N2, causing oxidation of the hydrocarbon with the support of the catalyst. Studies have helped to elucidate the active species on the catalyst surface at varying temperatures and species leading to the desired products (31). Results from a recent study point to the active silver species being a [Ag O Al] bound intermediate that leads to N2 formation (32). If the silver is present in nanoparticle form, it is simply believed to be a spectator. Other work showed mixed results on the benefit of silver-based alumina systems for the oxidation of methane and higher hydrocarbons. The effect is dependent on the type of reactor system prepared (33,34). 

Lindfors, L.-E., Eranen, K., Klingstedt, F. and Murzin, D.Yu. (2004) Silver/alumina catalyst for selective catalytic reduction of NOx to N2 by hydrocarbons in diesel powered vehicles. Top. Catal., 28, 185-189. 

Thomas, J.F., Lewis, S.A., Bunting, G.B., Storey, J.M., Graves, R.L. and Park, P.W. (2005) Hydrocarbon selective catalytic reduction using a silver-alumina catalyst... 

Shimizu, K., Higashimata, T., Tsuzuki, M. and Satsuma, A. (2006) Effect of hydrogen addition on S02-tolerance of silver-alumina for SCR of NO with propane. 

Shimizu, K., Shibata, J., Yoshida, H., Satsuma, A. and Hattori, T. (2001) Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons structure of active sites and reaction mechanism. Appl. Catal. B Environ., 30, 151—162. 

Sazama, P. and Wichterlova, B. (2005) Selective catalytic reduction of NOx by hydrocarbons enhanced by hydrogen peroxide over silver/alumina catalysts. Chem. Commun, 38, 4810-4811. 

The (3-Man residue was joined to the 4-OH of GlcNAc with silver alumina-silicate as promoter. 

Fig. 23.2. Structure and infrared spectra of hydrogen / -alumina. (a) Silver -alumina conduction plane with an AI-OIT defect the filled circle represents the oxygen of the AI-O-AI bridge between spinel blocks, (b) Conduction plane of fi/P"-alumina. (c)...

This material is prepared by allowing silver ) -alumina to exchange with molten [NO] [AICIJ. ... 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

The Kestner-Johnson dissolver is widely used for the preparation of silver nitrate (11). In this process, silver bars are dissolved in 45% nitric acid in a pure oxygen atmosphere. Any nitric oxide, NO, produced is oxidized to nitrogen dioxide, NO2, which in turn reacts with water to form more nitric acid and nitric oxide. The nitric acid is then passed over a bed of granulated silver in the presence of oxygen. Most of the acid reacts. The resulting solution contains silver at ca 840 g/L (12). This solution can be further purified using charcoal (13), alumina (14), and ultraviolet radiation (15). 

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

Hydrogenation of Acetylenes. Complete hydrogenation of acetylenes to the corresponding alkanes, which maybe requited to remove acetylenic species from a mixture, or as a part of a multistep synthesis, may be accompHshed using <5 wt % palladium or platinum on alumina in a nonreactive solvent under very mild conditions, ie, <100°C, <1 MPa (10 atm). Platinum is preferred in those cases where it is desired to avoid isomeri2ation of the intermediate olefin. Silver on alumina also can be used in this appHcation as can unsupported platinum metal. 

Ethylene oxide (qv) was once produced by the chlorohydrin process, but this process was slowly abandoned starting in 1937 when Union Carbide Corp. developed and commercialized the silver-catalyzed air oxidation of ethylene process patented in 1931 (67). Union Carbide Corp. is stiU. the world s largest ethylene oxide producer, but most other manufacturers Hcense either the Shell or Scientific Design process. Shell has the dominant patent position in ethylene oxide catalysts, which is the result of the development of highly effective methods of silver deposition on alumina (29), and the discovery of the importance of estabUshing precise parts per million levels of the higher alkaU metal elements on the catalyst surface (68). The most recent patents describe the addition of trace amounts of rhenium and various Group (VI) elements (69). 

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