Noble metal oxidation catalysts

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. 

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). 

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. 

The oxidation of CO at low temperatures was the first reaction discovered as an example of the highly active catalysis by gold [1]. Carbon monoxide is a very toxic gas and its concentration in indoor air is regulated to 10-50 ppm depending on the conditions [61]. An important point is that CO is the only gas that cannot be removed from indoor air by gas adsorption with activated carbon. On the other hand, metal oxides or noble metal catalysts can oxidize CO at room temperature. 

Wet air pollution control (WAPC) devices are used to treat exhaust gases from stainless steel pickling operations, thereby generating wastewater, which are treated using the selective catalytic reduction (SCR) technology in which anhydrous ammonia is injected into the gas stream prior to a catalyst to reduce NO, to nitrogen and water. The most common types of catalysts are a metal oxide, a noble metal, or zeolite. 

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). 

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. 

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... 

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). 

The AFC type was originally created for the Apollo program, after that a modernized version has been developed and is even now in use to provide electrical power for shuttle missions. The electrolyte in this fuel cell is KOH, concentrated (85 wt %) for fuel cells operated at relatively high temperatures, that is, around 250°C, and less concentrated (35-50 wt %) for cells operated at lower temperatures, that is, less than 120°C [6,9,11], In the construction of these fuel cells, the electrolyte is retained in a matrix, typically asbestos, and a wide range of catalysts, for example, Ni, Ag, metal oxides, and noble metals, can be used for both the hydrogen and the oxygen electrodes [8,9],... 

Furfural 69 has been used as a chemical feedstock for the production of furan via two production methods involving the decarbonylation of furfural <2005MI7>. Processes in both the liquid and gas phases were described for the preparation of furan through the decarbonylation of furfural using noble metal and metal oxide catalysts. The results of the study led the authors to state that the research trends for preparing furan based on the decarbonylation of furfural should mainly be concentrated on more effective catalysts and environmentally friendly processes. 

Because of the complex nature of the reactions that take place in the converter, a mixture of catalysts is used. The most effective catalytic materials are transition metal oxides and noble metals such as palladium and platinum. A catalytic converter typically consists of platinum and rhodium particles deposited on a ceramic honeycomb, a configuration that maximizes the contact between the metal particles and the exhaust gases. In studies performed during the last ten years researchers at General Motors have shown that rhodium promotes the dissociation of NO molecules adsorbed on its surface, thereby enhancing the conversion of NO, a serious air pollutant, to N2, a natural component of pure air. 

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. 

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. 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

In principle, LTCC can be carried out over noble metal catalysts or on oxide catalysts. Noble metals such as Pt and Pd have been studied already in the last century. The advantages of noble metals are ... 

Just as for LTCC, for HTCC also both noble-metal and oxide catalysts are important, but they are used in a very different way they are used together simultaneously but separately in different parts of the same reactor or combustor, as will be shown later (in Fig. 7). For HTCC, the interesting metal oxide catalysts are the transition metal oxides, mixed oxides, and complex oxides like spinels and perovskites. In general, high oxidation activity requires metal ions of variable valency, typical redox systems. There is some regularity here, for instance for oxides of metals of the Fourth Period in the Periodic Table in the complete oxidation of various hydrocarbons. C03O4 is the most powerful combustion catalyst here. 

Thermal aging, S02 poisoning, and reaction of the active phase with the support are factors that are known to be important in the deactivation of solid catalysts. Noble metals are usually poisoned by lead, whereas base metal oxide catalysts are more susceptible to poisoning by sulfur (88,286-289). Indeed, the deactivation of oxides when used in oxidation or reduction processes and particularly as catalysts for exhaust gas purification has been attributed to a large extent to S02 (14,174,290). In this section, some aspects of the S02 poisoning effect and the nature of the interactions of S02 with perovskite oxides are reviewed. 

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. 

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

The project goals are to significantly improve both the kinetic performance of the electrocatalyst powder at low noble metal loading and its utilization in the cathode layers through layer structure development. Limitations in the catalyst performance will be addressed through combinatorial discovery of supported catalyst compositions and microstructures. The discovery of these new catalyst formulations will be carried out under conditions that have been scaled for commercial powder production. A large variation of binary, ternary and quaternary noble metal -transition metal alloys and mixed metal-metal oxide catalyst compositions will be screened. To improve the utilization/performance of the catalyst in MEAs,... 

The catalytic partial oxidation (CPO) of methane is an interesting alternative to to the well-established steam reforming (SRM) process for syngas production in small-scale units. However, due to the severe reaction conditions (T = 800-950°C, contact times of few ms) in CPO processes, stable and active catalysts are still required. Several catalytie systems have been used in this process, such as noble metal-based catalysts, metal-based catalysts, metal oxide catalysts and perovskites [1]. In particular, catalysts obtained by the calcination of hydrotalcite-like compounds (HTlcs) have been widely used in the CPO of methane, as they can be easily and cheaply synthesized, with a highly-dispersed... 

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. 

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. 

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