Loss of carbon support

Loss of carbon support will disconnect platinum particles from the electron conducting path thus making them electrochemically inactive. They might also combine with other particles reducing the electrochemically active surface area. 

Shao et al. produced high-surface-area-modified tungsten carbide (WC) by TPR without significant loss of carbon support [46], This method can also be used for scale up synthesis of kilogram metal carbide nanoparticles with particle size in the range of 10-50 nm. The Pt catalyst supported on tungsten carbide on Ketjenblack (KB) showed a better oxygen reduction reaction activity compared to conventional catalyst, Pt/KB. 

Expected catalyst life is the same as for magnetite, and is determined by loss of carbon support. 

For the ruthenium catalyst supported on activation carbon, the methanation reaction of the carbon support can also be catalyzed by ruthenium. This is another shortcoming of ruthenium catalyst which results in the loss of carbon support and impact on the catalyst lifetime. However, researchers have found that methanation reaction occurs at higher temperatures than ammonia synthesis reaction, so that the loss of carbon via methanation might be avoided when the reaction is performed at relatively low temperatures. 

Area losses of carbon-supported platinum were determined [87] by electrochemical measurements and by X-ray diffraction. These losses cannot be due to adsorption of impurities since they were detected in roughly the same measure by both techniques. Recrystallization of the supported platinum was suggested [87] as the cause of the reduction of the surface area with time. 

Pt particle agglomeration is due to carbon support corrosion. Electrochemical carbon corrosion is known to occur above 0.9 V. It has been suggested that loss of carbon causes Pt particle agglomeration and electrical isolation, leading to loss in activity. 

The reaction of iV-(2,4-dinitrophenyl)amino acids with base in aqueous dioxane has been shown to give benzimidazole iV-oxides (7). The rate-determining step is likely to be formation of an iV-alkylidene-2-nitrosoaniline intermediate (6), which is followed by rapid cyclization and decarboxylation.19 The loss of carbon dioxide from perbenzoate anions has been investigated by mass spectrometry and electronic structure calculations. The results, including isotopic labelling experiments, support a mechanism involving initial intramolecular nucleophilic attack at either the ortho- or ipso-ring positions. They also indicate that epoxides may be intermediates en route to the phenoxide products.20 There has also been a theoretical study of the formation of trichlorinated dibenzo-/ -dioxins by reaction of 2,4,5-trichlorophenolate ions with 2,4-dichlorophenol.21... 

This finds some support in a comparison of solution viscosities with polymerization time of a few isolated cases of Diels-Alder polymerization reactions. In the polymerization of 2,5-dimethylene-3,4-diphenylcyclo-pentadieneone with N.N -hexamethylene-fo s-maleimide, the reduced viscosity of the polymer increases from 0.97 after 1 hr to 1.20 after six hours (7). It is necessary to assume that the rate controlling step in this reaction is neither the formation of the initial adduct nor the loss of carbon monoxide. The inherent viscosity of the l,6-6is-(cyclopenta-dienyl)hexane-quinone copolymer increases from 0.10 after sixteen hours to 0.12 after twenty four hours reaction time in refluxing benzene (14). 

Recent reports [22, 23] have demonstrated better CO tolerance with higher loadings (1-2 mg/cm ) PtRu catalysts in PEFC anodes, particularly at cell current densities lower than 200 mA/cm. In contrast, a thin-fihn anode catalyst of very low PtRu loading, prepared as a composite of carbon-supported PtRu (0.15 mg/cm ) and recast ionomer [14], did not exhibit lower losses when 5-20 ppm CO was introduced into the hydrogen feed stream [21]. The same PtRu catalyst was successful, however, in... 

The FAB-MS/MS of chlorogenic acid resulted in daughter ions of 191, 179, 161, and 135 m/z. Chlorogenic acid is a quinic acid ester of caffeic acid, thus, one would expect the loss of a caffeoyl unit or caffeic acid. The ion at 191 m/z represents the loss of caffeic acid from chlorogenic acid to give quinic acid. The presence of the 179, 161, and 135 m/z ions are related to the caffeic residue. Minor ions at 309 and 147 m/z are indicative of a loss of carbon dioxide (44 m/z units) from chlorogenic acid and quinic acid, respectively. For further discussion on MS analysis of CAD, see Facino et al. (1993). Using the same rationale, MS data from other CAD can be interpreted and used to support other analytical data. 

Due to the new developments [5] in fuel cell technology—the manufacture of carbon supported platinum catalysts and the use of the Nafion membrane—the cost of bipolar electrolyzers has been reduced a lot, and therefore almost all commercial devices are of this type. In this case, stainless steel or nickel cathodes are used together with nickel anodes in 25%-35% of potassium hydroxide at temperatures between 65°C and 90°C. The hydrogen current density reaches 100-300 mA/cm2 at cell potentials of 1.9-2.2 V, denoting a faradaic efficiency of 80% (losses in peripheries). Usually, a pressurized cell is employed to increase their performance and to reduce the size of the bubbles, thus lowering the overpotential associated with the process. This can be done with appropriate membranes and insulators and by using temperatures near 100°C. 

The rate of decarboxylation of 6-nitroindoxazene-3-carboxylic acid is subject to dramatic solvent effects which support the anionic nature of the transition state (38).8,53 The marked acceleration on going from water (rate 7.3 x 10-6sec-1) to a dipolar aprotic solvent (e.g., dimethylformamide, rate 3.7 x 10 sec- ) is interpreted in terms of the different solvation requirements of the carboxylate anion (40), with its comparatively localized charge, and the transition state (38) with its delocalized charge. In protic solvents intermolecular hydrogen bonding with the carboxylate ion inhibits decarboxylation by selectively stabilizing the acid, whereas dipolar aprotic solvents stabilize the transition state (38) and hence accelerate loss of carbon dioxide. 

Quinoxalin-2-ones show carbonyl stretching absorption in the region of 1660-1690 cm both in Nujol mulls and KBr discs. The ultraviolet spectrum of quinoxalin-2-one shows maxima at 343,287,254,250, and 228 nm in aqueous solution at pH 4.0 and is closely similar to that of its 1-methyl derivative. These data indicate that quinoxalin-2-one exist predominately in the cyclic amide form rather than as 2-hydroxyquinoxaline, and this conclusion is further supported by the closely similar pX values of quinoxalin-2-one and its 1-methyl derivative, which are -1.38 and -1.15, respectively. 2-Methoxyquinoxaline is by comparison an appreciably stronger base, with a pKa value of 0.28. ° X-Ray examination of crystals of quinoxalin-2-one confirms that it exists in the cyclic amide form. °° Quinoxalin-2-one fragments in the mass spectrometer, as might be expected, by the successive loss of carbon monoxide and hydrogen cyanide from the molecular ion. ... 

Carbon supports strongly affect fuel cell performance. They may influence the intrinsic catalytic activity and catalyst utilization, but also affect mass transport and ohmic losses. This makes analyses of the role of carbon materials rather complicated. Although numerous studies have been devoted to the carbon support improvement, only a few have attempted to establish relationships between the substructural characteristics of carbon support materials and cell performance. The influence of carbon supports on the intrinsic catalytic activity is the subject of Section 12.6.1. In Section 12.6.2 we consider the influence of support on macrokinetic parameters such as the catalyst utilization, mass transport, and ohmic losses. In Section 12.6.3 we review briefly recent data obtained upon utilization of novel carbon materials as supports for fuel cell electrocatalysts. 

Acetyllactic acid results from an acyloin condensation and loss of carbon dioxide, followed by a very interesting ketol rearrangement, which proceeds in a stereochemically uniform manner. Herein, the hydroxy- and keto-groups are oriented syn-periplanar, so that the methyl group is transferred suprafacially on the (Jle)-side, a mechanism, which is supported by data from NMR spectroscopy on model compounds [260] and by preparative examples. [261, 262] The ketol rearrangement is to some extent related to the benzil-benzilic acid rearrangement. 

Another reaction of significance involving methanol is the direct synthesis of dimethyl carbonate (DMC) by carbonylation of methanol with CO which offers a potentially green chemical replacement for phosgene which is used for polymer production and other processes. The direct synthesis of dimethyl carbonate has been pursued over a variety of carbon supported cuprous chloride catalysts, but these catalysts deactivate due to loss of chloride and as such require reactivation by drying and contact with gaseous HCl. King et discovered that the chloride is not necessary to catal-... 

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