Supported metals competitive adsorption effect

Most of electrodecarboxylations have been carried out with partially neutralized carboxylic acid. Alkaline and alkaline earth metal as well as ammonium (pyridinium) carboxylates work efficiently as supporting electrolytes. Some metal ions (such as Fe " and Co " ) are found to favor selectively radical reactions in electrodecarboxylation. Addition of certain salts, such as perchlorate, fluoroborate, sulfate, dihydrogen phosphate, bicarbonate, and fluoride, tends to inhibit the radical reaction and favor the formation of cation intermediates [28-31]. The remarkable effects of the salts are well explained in terms of competitive adsorption between the anions and carboxylates. 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts. 

There is no doubt that specific adsorption of anions and organic species occurs in a wide potential range as the experiments were carried out in 0.1 mol dm HCIO4 supporting electrolyte at low concentrations (10 -10 mol dm ) of the species studied. It was found that the adsorption of anions exerts some influence on the anodic dissolution rate however, this effect is very far from that observed in the case of thiourea. In the latter case, the rate of anodic dissolution was significantly influenced by the presence of thiourea, even at its very low concentration. This behavior was ascribed to the strong interaction of thiourea with the metal ions and the formation of complexes both on the metal surface and in the solution phase. The phenomena presented were considered as a result of the adsorption competition of the species studied with C104 ions. Three fundamental mechanisms were formulated for the active dissolution ... 

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