Contamination, degradation catalysts

Some general applications of TG-FTIR are evolved gas analysis, identification of polymeric materials, additive analysis, determination of residual solvents, degradation of polymers, sulphur components from oil shale and rubber, contaminants in catalysts, hydrocarbons in source rock, nitrogen species from waste oil, aldehydes in wood and lignins, nicotine in tobacco and related products, moisture in pharmaceuticals, characterisation of minerals and coal, determination of kinetic parameters and solid fuel analysis. 

The activities of fresh, supported platinum and base metal oxidation catalysts are evaluated in vehicle tests. Two catalysts of each type were tested by the 1975 FTP in four 600-4300 cm3 catalytic converters installed on a vehicle equipped with exhaust manifold air injection. As converter size decreased, base metal conversions of HC and CO decreased monotonically. In contrast, the platinum catalysts maintained very high 1975 FTP CO conversions (> 90% ) at all converter sizes HC conversions remained constant 70% ) at volumes down to 1300 cm3. Performance of the base metal catalysts with the 4300-cm3 converter nearly equalled that of the platinum catalysts. However, platinum catalysts have a reserve activity with very high conversions attained at the smallest converter volumes, which makes them more tolerant of thermal and contaminant degradation. 

First, this reserve activity could be utilized to offset thermal and contaminant degradation of catalyst activity. In simplistic terms, the 4300-cm3 production prototype converter could lose 70% of its catalytic activity (with a remaining catalyst volume equivalent to 1300 cm3) without suffering degradation in the overall control of exhaust HC and CO emissions. 

All three major components of an ME A, membrane, catalyst, and diffusion medium, can be contaminated by foreign species during PEMFC operation. Contamination of the diffusion medium usually lowers its hydrophobicity, which leads to water transport problems and can potentially create a continuous liquid path from the membrane to the gas flow channel. Foreign ionic contaminants can directly migrate to the membrane via this liquid path. Consequently, faster contamination may occur under this condition. Since diffusion medium contamination does not directly cause more severe membrane degradation, the discussion of the contamination issue will focus on two aspects membrane contamination and catalyst contamination. 

Heterogeneous catalysts are more common. However, they degrade and need replacement. If contaminants in the feed material or recycle shorten catalyst life, then extra separation to remove these contaminants before the feed enters the reactor might be justified. If the cataylst is sensitive to extreme conditions, such as high temperature, then some measures can help to avoid local hot spots and extend catalyst life ... 

Catalyst lifetimes are long in the absence of misoperation and are limited primarily by losses to fines, which are removed by periodic sieving. Excessive operating temperatures can cause degradation of the support and loss of surface area. Accumulation of refractory dusts and chemical poisons, such as compounds of lead and mercury, can result in catalyst deactivation. Usually, much of such contaminants are removed during sieving. The vanadium in these catalysts may be extracted and recycled when economic conditions permit. 

Bimetallic nanomaterials such as Pd/Fe, Ni/Fe, and Pd/Au are also active catalysts for the degradation of organic contaminants, including halogenated pesticides, nitroaromatics, polychlorinated biphenyls, and halogenated aliphatics (ethenes and methanes) [151]. 

As an example, when automotive catalytic mufflers and converters were introduced many years ago, the automobile industry required the petrochemical industry to eliminate lead from gasoline since lead degraded and reduced the effectiveness of the catalyst and caused the destruction of the gasoline. One set of industrial compounds that can harm catalysts are halogens, a family of compounds that include chlorine, bromine, iodine, and fluorine. Bromine, while not prevalent in industry, is present in chemical plants. Freons are fluorine compounds. Silicone is another compound that is deleterious to catalysts. It is used as a slip agent, or a lubricant, in many industrial processes. Phosphorous, heavy metals (zinc, lead), sulfur compounds, and any particulate can result in shortening the life of the catalyst. It is necessary to estimate the volume or the amount of each of those contaminants, to assess the viability of catalytic technologies for the application. 

Photocatalytic degradation of environmental pollutants by solar energy is a very attractive technology for the remediation of contaminated water [253,323], In some variants of this process, solar UV radiation is absorbed by semiconductor catalyst particles suspended in water. TiOz photocatalytic particles are the most widely used for these applications. 

Asymmetric induction using catalytic amounts of quininium or A-methyl-ephedrinium salts for the Darzen s reaction of aldehydes and ketones with phenacyl halides and chloromethylsulphones produces oxiranes of low optical purity [3, 24, 25]. The chiral catalyst appears to have little more effect than non-chiral catalysts (Section 12.1). Similarly, the catalysed reaction of sodium cyanide with a-bromo-ketones produces epoxynitriles of only low optical purity [3]. The claimed 67% ee for the phenyloxirane derived from the reaction of benzaldehyde with trimethylsul-phonium iodide under basic conditions [26] in the presence of A,A-dimethyle-phedrinium chloride was later retracted [27] the product was contaminated with the 2-methyl-3-phenyloxirane from the degradation of the catalyst. 

Landtreat is a silicate-based inorganic polymer catalyst used for the ex situ treatment of contaminated soils. The vendor claims that it acts as a catalyst to degrade halogenated compounds and organic compounds containing nitrogen and sulfur. 

In summary, extrapolation of reactions determined in homogeneous solutions to the soil system may not result in accurate predictions of possible products. Unanticipated abiotic reactions which occur during bioremediation may influence the products, causing altered degradative pathways of the contaminants. These pathways may be site specific because of differences in abiotic catalysts. Abiotic reactions may occur to both the parent compound or to intermediates formed during biotic alteration of the compound. Bioremediation efforts should include the possibility of site specific abiotic reactions. 

Watts et al. (1994) studied sorption and degradation of hexachlorobenzene on geothite as the iron catalyst and concluded that mineral-catalyzed, Fen-ton-like reactions are controlled by desorption, FI202 concentration, and contaminant structure. 

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