Metal catalytic oxidations over

When specifically labelled compounds are required, direct chemical synthesis may be necessary. The standard techniques of preparative chemistry are used, suitably modified for small-scale work with radioactive materials. The starting material is tritium gas which can be obtained at greater than 98% isotopic abundance. Tritiated water can be made either by catalytic oxidation over palladium or by reduction of a metal oxide ... 

O Connell M, Norman AK, Huttermann CF, Morris MA (1999) Catalytic oxidation over lanthanum-transition metal perovskite materials. Catal Today 47 123-132... 

Cyan-cat A process for destroying gaseous hydrogen cyanide by catalytic oxidation over platinum metal at approximately 300°C. 

Econocat A process for deodorizing gases by catalytic oxidation over manganese dioxide activated with other metals. Developed and sold by Cortaulds Engineering. 

The selective catalytic oxidation over highly dispersed noble metal catalysts in aqueous media is gaining interest (1-5). One of the main problems consists of the deactivation of the catalyst. The role of oxygen is recognized as being crucial in this matter (6,7). 

A second concern was that under some conditions sulfur dioxide in exhaust could be emitted as sulfuric acid as a result of catalytic oxidation over the noble metal catalyst. To answer this concern General Motors conducted a 350-car test designed to simulate sulfate emissions on a busy expressway. The U.S. Environmental Protection Agency, other vehicle manufacturers, and several independent environmental monitoring organizations participated in the experiment. This experiment showed conclusively that ambient levels of sulfuric acid under this worse-case simulated exposure situation were far below threshold levels known to produce adverse health effects. 

As shown by Golodets (p. 289), the formation of the carbonate intermediate also may be interpreted in terms of the more conventional redox or Mars-van Krevelen cycle, which explicitly involves the oxidation and reduction of the metal oxide." The Mars-van Krevelen mechanism of catalytic oxidations over metal oxides is a redox mechanism involving both gas phase and lattice oxygen ... 

Cerium oxide-based technology was then modified for European diesel passenger cars with the addition of a relatively small amount of zeolite as well as additional precious metal to meet European requirements for gaseous CO and HC reductions. The purpose of the zeolite was to adsorb gaseous HC (mostly vm-burned diesel fuel) during cold start conditions and subsequently release them to the catalyst once the latter was sufficiently active to initiate catalytic oxidation on the Pt (138). As the exhaust temperature and catalyst light-off is achieved, the HC desorbing from the zeolite is catalytically oxidized over the Pt. 

In the heterogeneous catalytic system, the reacting molecules, and the metal-surface atoms, can be quite mobile. This leads to locally ordered structures when synchronization or self organization phenomena are present however, disorder in the surface layer prevails when these phenomena are absent. For example, it has been proposed that the catalytic oxidation over mixed metal oxides, discussed in Section 2.3.5, actually occurs in the disordered overlayer that forms at the surface under reaction conditions. 

Methanol is converted into formaldehyde by catalytic vapour phase oxidation over a metal oxide catalyst. In one variation of the process methanol is vaporised, mixed with air and then passed over the catalyst at 300-600°C. The formaldehyde produced is absorbed in water and then fed to a fractionating column. A 37% solution of formaldehyde in water is removed from the bottom of the column with some methanol as a stabiliser whilst excess methanol is taken from the top of the column and recycled. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

It is important to emphasize that the atomic oxygen anion-radical plays a role in catalytic oxidation occnrring on varions oxide surfaces. For instance, O reacts with methane at room temperatnre over varions metal oxides (Lee and Gralowsky 1992). On solid catalysis, Q- is more reactive toward alkanes and alkenes than other ionic oxygen species. Iwamoto and Lnnsford (1980) assumed that O is the active oxygen species oxidizing benzene to phenol on with 70% selectivity at... 

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