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Metal oxides, noble, catalysts

Permanent retention of the scavenger elements Cl and Br, as such, on noble metal oxidation catalysts is usually insignificant because of their volatility. In spite of this fact, it has been demonstrated [(66) and references therein] that scavengers by themselves can suppress the oxidation activity of Pt and Pd. [Pg.350]

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... [Pg.356]

Shelef, M., Dalla Betta, R. A., Larson, J. A., Otto, K., and Yao, H. C., Poisoning of Monolithic Noble Metal Oxidation Catalysts in Automotive Exhaust Environment, Am. Inst. Chem. Eng., New Orleans Meet. (1973). [Pg.362]

An alternative way to achieve the photodissociation of water consists in the use of aqueous suspensions of powdered or colloidal semiconductors, in general loaded with noble-metal and/or noble-metal-oxide catalysts which act as short-circuited photoelectrolysis cells. Titanium dioxide was certainly (and is still being) the semiconductor most frequently employed in such systems. [Pg.4]

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. [Pg.53]

Since the pollutants from automobiles have greatly increased with the rapid increase of automobiles in recent years in China, the Ministry of Environmental Protection of China decreed the first regulation controlling pollutants from automobiles in 1983. Successes have been scored in some large cities in the implementation of the regulation in their battle against pollution. The catalysts currently used for this purpose are mainly those containing noble metals of Pt, Pd, Rh etc. But it would be valuable to develop non-noble metal oxide catalysts in view of the rich resources and low cost in China. [Pg.395]

Comparison of Phosphorus and Lead as Poisons. In order to assess the relative toxicities of the two major poisons of noble metal oxidation catalysts, simulated tests were run with the loss of catalyst activity for HC conversion measured as a function of the TBP and the TEL in the feed, these being representative of additives in commercial fuels. [Pg.63]

Use of simulated rigs enabled study of the mode of catalyst poisoning since evaluation of poisoning deactivation curves provides insight into the mechanism by which noble metal oxidation catalysts become deactivated by the acquisition of poisons from the gas phase. [Pg.69]

In general, much less research has been reported on catalytic oxidation at conditions of interest herein on noble metals than has been reported on metal oxides, especially research dealing with the mechanism of complete oxidation. Although supported noble metal oxidation catalysts... [Pg.168]

The activity of the molecular complexes is much higher than that of conventional noble metal oxide catalysts such as Pt02 or RUO2, by one to two orders of magnitude based on one molecule and one repeating unit. [Pg.581]

The development of noble metal catalysts and transition metal oxides for catalytic oxidation of VOCs has been widely reported in the literature. " The review paper published in 1987 by Spivey presents a good overview of catalytic combustion of VOCs. More recent reviews, focusing on the catalytic combustion of a wide range of VOCs by a wide variety of catalysts and on chlorinated VOCs, were published in 2004. In the last two years, two more reviews have been published. These reviews focused on the development of non-noble metal oxide catalysts for catalytic combustion of VOCs and on catalytic combustion catalysts for the removal of polycyclic aromatic hydrocarbons. This review is not intended to be an exhaustive account, but should provide an overview of the current state of research for catalysts used for alkane and aromatic total oxidation. The aim is also to identify the types of catalysts that are likely to be of use in the future, and the obstacles that must be overcome to produce viable catalysts. The development of a catalyst that may be used for the combustion of all classes of compounds under the general term VOC presents a major challenge for future research, as this has not yet been achieved. [Pg.52]

Lin, R., Ding, Y., Gong, L., et al. (2009). Oxidative Bromination of Methane on SUiea-Supported Non-Noble Metal Oxide Catalysts, Appl. Catal. A Gen., 353, pp. 87-92. [Pg.834]

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. [Pg.253]

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). [Pg.503]

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. [Pg.79]

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. [Pg.53]

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]. [Pg.483]

Saalfrank JW, Maier WF. 2004. Directed evolution of noble-metal-free catalysts for the oxidation of CO at room temperature. Angew Chem Int Ed 43 2028-2031. [Pg.91]

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. [Pg.166]

The main classes of materials employed as catalysts are metals (generally transition and noble metals), oxides (including transition-metal oxides), transition-metal sulfides and zeolites. In the following sections, we discuss some of the more common structures and chemistry exhibited by catalytic systems. [Pg.13]

A variety of metal oxides (e.g. V2O5) have been employed for oxidation reactions, besides noble metals (e.g. Pt and Ag). Auto-exhaust catalysts employ metals such as Rh, Pd and Pt besides Ce02 and other oxides. The use of metal oxide catalysts for oxidation reactions has been discussed quite widely in the literature (Grasselli Brazdil, 1985). Perovskite oxides of the type CaMn03 and Laj A jM03 (A = Ca, Sr M = Co, Mn) are excellent candidates as oxidation catalysts. The 14-electron oxidation of butane to maleic anhydride is effectively carried out over phosphorus vanadium oxide catalysts of the type VOPO4 (Centi et al., 1988). [Pg.523]


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See also in sourсe #XX -- [ Pg.184 ]




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Catalysts noble metal

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Metal oxide catalysts

Metal oxides, catalysts oxidation

Metals noble

Noble catalysts

Noble metal oxides

Oxidation noble metal

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