Carbonaceous intermediates adsorption

In the above expression, [TOC j is the asymptotic residual organic carbon, which cannot be oxidized (Rcl) further. Agreement between the experimental data for combined thermolysis, catalytic, and non-catalytic WO and the model prediction is shown in Figure 5.5. From this plot one can conclude that the oxidation progress is terminated once the catalyst is deactivated due to the adsorption of carbonaceous intermediates on its surface. However, for practical design purposes one should use the lump kinetic approach based on the triangular reaction scheme such as depicted in Figure 5.1. It is believed that the rate laws can be expressed mostly by a simple power function. 

A unique active carbon having very high surface areas over 2500 m / gm, and extraordinary adsorptive capacities was developed in our laboratories. (1) This paper will describe its development, manufacture, properties, and uses. Until recently, samples of this carbon, which were provided worldwide for research and evaluation, were identified as Amoco Grades PX-21, 22, 23, and 24 in the powdered form and Amoco GX-31 and 32 in granular form. The carbon is made (Figure 1) by a direct chemical activation route in which petroleum coke or other carbonaceous sources are reacted with excess potassium hydroxide, KOH, at 400° to 500°C to an intermediate product that is subsequently pyrolyzed at 800°-900°C to active carbon and potassium salts. The salts are removed by water washing. 

The relatively slow rate of hydrocarbon fuel cell oxidations prompted an intensive examination of the adsorption characteristics of organic reactants in the 1960s. Because of the low potential for the development of hydrocarbon fuel cells, such studies have largely subsided today and no modern surface analysis techniques have been applied to characterize intermediates. Conventional adsorption studies of carbonaceous species have been reviewed repeatedly (7, 9-12, 100 -, therefore, we summarize here only some essential adsorption features for fuel cell and selective electrocatalytic oxidations. 

The majority of easily detected compounds at solid anodes under constant applied potentials are self-stabUized via tt-resonance. Therefore, a desirable characteristic of electrodes in dc amperometry is inert. The electrode serves as a sink to provide and remove electrons with no direct involvement in the reaction mechanism. Since TT-resonance does not exist in polar ahphatic compounds (e.g., carbohydrates), stabilization of reaction intermediates is actively achieved via adsorption at clean noble metal electrodes. Faradaic processes that benefit from electrode surface interactions are described as electrocatalytic. Unfortunately, an undesirable consequence of this apiproach is the accumulation of adsorbed carbonaceous materials, which eventually foul the electrode surface. 

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