Activated oxides with carriers

Reactions of Activated Oxides with Carriers—Catalytic Properties... 

A key issue is the improvement of solar light harvesting. New active materials with high optical absorption in the visible and good photostability are needed. Implementation of carrier multiplication through impact ionization in quantum dots arrays could mitigate the losses related to carrier thermalization. The alternative approach is the development of vertically stacked tandem systems of increasing band gap active materials, which effect H+ reduction and water oxidation on opposite sides. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

Further spectroscopic research has been carried out by Krylov [183, 184] on adsorption of oxygen on M0O3, W03, V2Os and CuO, supported on A1203, MgO and BeO. In all cases, 02 radicals were formed. Extra stabilization occurs when the catalyst is reduced with hydrogen. On a number of active oxides, the 02 intensity is increased drastically by simultaneous adsorption of propene. It is suggested that 02 is attached to the carrier cation. The electron transfer with simultaneous adsorption is then supposed to be... 

A second report of organic carbonate production from epoxide and C02 utilizes copper(I) cyanoacetate, Cu(02CCH2CN), as a carrier of activated C02 (158). Reaction of propylene oxide with Cu(02CCH2CN) at 130°C for 10 hours yields propylene carbonate in 83% yield, based on the... 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

Yoshida et a/.110 concluded from experiments with copper vanadate that the NO reduction on a pre-reduced catalyst proceeds by a mechanism involving adsorbed NO, adsorbed NH3, and adsorbed 02. Shikada et al.nl have shown that by using a complex Si02 —Ti02 oxide as carrier a very active and stable catalyst is obtained which is highly resistant to S02 for long periods. 

The Irradiated uranium soln 1b added to a standardized arsenlte carrier In dilute HgS04, oxidized to arsenate (V) with KBt03 and reduced again vlth potassium metahlsulflte (This oxidation-reduction procedure is carried out to Insure the complete exchange of As-activ ity with the trlvalent As carrier.)... 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

The adsorption of biomolecules onto carriers that are insoluble in water is the simplest method of immobilization. An aqueous solution of the biomolecules is contacted with the active carrier material for a defined period of time. Thereafter the molecules that are not adsorbed are removed by washing. Anionic and cationic ion exchange resins, active charcoal, silica gel, clay, aluminum oxide, porous glass, and ceramics are being currently used as active material. The carrier should exhibit high affinity and capacity for the biomolecule and the latter must remain active in the adsorbed state. The carrier should adsorb neither reaction products nor inhibitors of the biocatalyst. 

A promising class of nonionic surfactants are the pluoronics, which are the block co-polymers of ethylene and propylene oxides with molecular weights ranging from 2,000 to 20,000 g mol 1. The solubility and surface activity of these compounds is determined by the ratio of the lengths of polyoxypropylene (carrier of hydrophobicity) to polyoxyethylene chains (carrier of hydrophilicity). 

The problems encountered with catalysts for auto emission purification are of the same type but of a totally different magnitude. Superior mechanical properties and, specifically, exceptional resistance to attrition are required. Furthermore, these properties should be unaffected by exposure to 1000°C and even 1100°C. Furthermore, a surface area of 50-80 m2/g and porosity of 0.6-0.8 cm3/g must remain after heating. Naturally the carrier must also withstand thermal shocks, offer the least possible diffusional limitation to reagents, and react as little as possible —or at least without unfavorable results—with the catalytically active oxides and metals deposited on its surface. 

To meet the future legislation requirements (such as SULEV in California), modem TWC technologies have recently taken the lead with different washcoat architectures to enhance the activity including inorganic carriers able to resist temperatures as high as 1100°C. Highly thermally-stable ternary (Ce/Zr/X) and even quaternary (Ce/Zr/X/Y) mixed oxides are emerging [10-12]. 

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