Elemental base oxide catalysts

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Catalysts used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are largely based on alumina-supported molybdenum or tungsten to which cobalt or nickel is added as a promoter [11]. As the catalysts are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2 (or by exposing the catalyst to the sulfur-containing feed). The function of hydrogen is to prevent the decomposition of the relatively unstable H2S to elemental sulfur, which would otherwise accumulate on the surface of the... 

Catalytic gas detection is based on the principal that oxidation of a combustible gas in air is promoted at the surface of a heated catalyst such as a precious metal. The oxidation reaction results in the generation of heat that provides a direct measure of the concentration of the gas that has been reacted. The sensing element embodying the catalyst is a small bead that is supported with the sensor. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

The design employs filters for particulate removal, a single desiccant wheel coated with both low temperature VOCs oxidation catalyst based on nanostructured catalyst and regenerable VOCs adsorbent made from modified mesoporous silica. The distribution of the active elements along the wheel thickness was optimized and shown in Fig. 12.8-6. The adsorbents are coated along two-thirds of the wheel thickness and all catalysts are concentrated on one... 

In this chapter, a brief summary of studies that made use of calorimetry to characterize compounds comprising group IIIA elements (zeolites, nitrides, and oxides catalysts) was presented. It was demonstrated that adsorption microcalorimetry can be used as an efficient technique to characterize the acid-base strength of different types of materials and to provide information consistent with the catalytic data. 

From a mechanistic viewpoint it is worth noting that the TS-1 catalyst contains the same chemical elements in roughly the same proportions as the Shell amorphous TiIV/Si02 catalyst referred to earlier. However, the former displays a much broader range of activities than the latter. A possible explanation may be that the TS-1 catalyst contains more (or more active) isolated titanyl centres than the amorphous Ti1v/Si02. Based on the quite remarkable results obtained with TS-1 we expect many more examples of redox zeolites, i.e. zeolites, alpos, etc. modified by isomorphous substitution with redox metal ions in the crystal lattice, as selective oxidation catalysts.66... 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

Aside from the recently described Cu/Th02 catalysts, copper on chromia and copper on silica have been reported to catalyze methanol synthesis at low temperatures and pressures in various communications that are neither patents nor refereed publications. It is not feasible to critically review statements unsupported by published data or verifiable examples. However, physical and chemical interactions similar to those documented in the copper-zinc oxide catalysts are possible in several copper-metal oxide systems and the active form of copper may be stabilized by oxides of zinc, thorium, chromium, silicon, and many other elements. At the same time it is doubtful that more active and selective binary copper-based catalysts than... 

Metal oxide catalysts can be classified as oxides of transition elements or as oxides of other typical metals. Typical transition elements include Cr, Fe, Co, Mo, and V, whose oxides catalyze oxidation and reduction reactions by changing the oxidation state of the metal ion. For selective oxidation of hydrocarbons, mixed oxides containing Mo and V are widely used. Oxides of other metals (acidic oxides such as silica and silica-alumina, basic oxides such as CaO and MgO, and amphoteric oxides such as alumina) catalyze acid or base reactions such as alkylation, isomerization, and hydration-dehydration. 

Scheelite oxides are a good example of how structure-property relationships have been used to create a modem catalyst. More detailed descriptions of these and other catalysts are given in Oxide Catalysts. The scheelite stmcture was discussed in Section 3.4.5. Scheelite based oxides are of interest as catalytic materials because of their ability to form with transition elements in high but easily reducible valence states and to form a variety of defects and defect stmctures that provide numerous sites for selective catalytic activity. 

The second approach is based on addition of a precious metal or oxide catalyst (Fig. 12.1b). The design of a catalyst usually requires a large number of experiments because various catalyst materials are available and their concentrations should be optimized. In this respect, the combinatorial method is advantageous. Indeed, there are reports on the design of catalysts or surface doping elements that promote the selective gas sensing reaction.48"50... 

In contrast to the main-group elements, metals of the d-block usually show one-electron separated oxidation states. Thus, the discrimination between metal-centred and ligand-centred electron transfer is not easy and must often be based on several spectroscopic and electrochemical methods. As examples Section 4.4 reports on recent investigations on the nickel-containing cofactor F430 and Section 4.5 on the famous oxidation catalyst system [(Por)Ru(CO)(L)]. Sometimes the peculiar electrochemistry of porphyrins allows unusual oxidation states as shown with the divalent gold species [(Por)Au(II)] in Section 4.6. 

CeOj is an inevitable component in the automobile exhaust purification catalysts in which precious metals are the main active elements. In the wet-oxidation catalysts based on precious metals, CeO also plays a very important role in activating them. 

The paper presents data on development of Mn oxide catalysts for selective oxidation of lean methane mixtures with air to produce CO2 and generate heat. To obtain catalysts, new approaches to the synthesis of polyoxide materials based on Mn were adopted. Catalysts were modified by doping with La, Ce, Ba and Sr nitrates which were deposited from solutions onto the stabilized 2%Ce/0-Al2O3 support (of surface area 100 m /g and pellet diameter 4-5 mm). By varying the components of the impregnation mixture, it was possible to optimize the chemical composition and ratio of elements in the multi-component catalysts (at Ba Sr La Ce Mn = 1 1 1 7 10 ratio). The catalyst composition conformed to the oxide stoichiometry in the perovskite structure. 

Introduction. - Oxides are suitable as catalysts in redox as well as acid-base reactions since they are able to participate in the exchange of electrons, protons or oxide ions. Oxides of the main group elements are used as acid-base catalysts and carriers whereas transition metal oxides often serve as redox catalysts and precursors for preparation of different active phases. One of the most important applications for oxide catalysts is in oxidation reactions. 

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