Adsorbents carbonaceous

In the step-feed system used for nitrification and denitrification the feed is introduced along the length of the basins into anoxic ones formed at these entry points (Figure 4). This system has the advantage that a large portion of the mixed liquor is retained in the first number of sections, which allows the breakdown of adsorbed carbonaceous matter and ensures that the nitrifiers are not washed out of the system. However, since a portion of the influent organic carbon and ammonia enters near the end of the tank, some will wash... 

Davis, S.M., Zaera, F. and Somorjai, G.A. (1982) The reactivity and composition of strongly adsorbed carbonaceous deposits on platinum. Model of the working hydrocarbon conversion catalyst. J. Catal., 77, 439. 

Figure 9 shows rapid KDN deactivation of Co-Mo catalysts on both alumina and on carbon. The expectation that the carbon catalyst would deactivate quickly because it has a larger median pore diameter was observed. However, deactivation of the Co-Mo on alumina catalyst in Figure 9 was much faster than the Ni-Mo on alumina catalyst in Figure 8. An explanation for these differences may involve both the chemical coanposition of the catalyst surface as well as the diffusion path length. However, the deactivation of the 3.2 mm Co-Mo on alumina catalyst in Figure 9 was much faster than the 3.2 mm Ni-Mo on almnina catalyst in Figure 8. Since Ni-Mo is often considered to be a better hydrogenation catalyst more hydrogenation of adsorbed carbonaceous species, less coke formation, and less deactivation might be expected.

CH4/CD4 SSITKA experiments were carried out at 1013 K after 40 min on stream in the DRIFT cell. Fig. 3A shows the spectra obtained a) initially under CH4 flow and b) under CD4 flow, 5 min after the switch when the spectrum was not changing anymore significantly. A very large part of the OH bands at 3715 cm and 3583 cm" was shifted to OD bands at 2737 and 2641 cm" when the deuterated methane (C-D bands at 2257 cm" ) was substituted to light methane (C-H bands at 3061 cm ). Note that C-H and C-D dbrations of adsorbed carbonaceous species could be hidden under the relatively large bands of the methane gas phase. 

S.M. Davis, F. Zaera, and G.A. Somorjai. The Reactivity and Composition of Strongly Adsorbed Carbonaceous Deposits on Platinum. Model of the Working Hydrocarbon Conversion Catalyst. J. Catal. ll A >9 (1982). 

A besetting problem with the study of alkene hydrogenation is deactivation of the catalyst by strongly-adsorbed carbonaceous deposits - or acetylenic residues that form even well below room temperature the base metals of Groups 8 and 10 are especially prone to this, but even the noble metals are not immune, and even the most fundamental studies are necessarily made on equilibrium surfaces, much of which is permanently inactivated. This raises the vexed question of whether such poisoned surfaces truly reflect the character of the metal, and indeed whether it is not on the carbonaceous overlayer that the catalysis occurs. Another idea that has been seriously suggested is that reaction actually occurs, in... 

In this class of reaction, ° an initial dissociative chemisorption step occurs in which the molecule is split, usually into adsorbed H and adsorbed carbonaceous fragments, which are subsequently electrochemically oxidized either directly, as with H (MH M + + e), or indirectly, as with the... 

Most other simple organic substances react in an analogous way by chemisorptive dehydrogenation giving an adsorbed carbonaceous residue, e.g., in the cases of oxidation of formic acid, methanol, ethanol, olefins, and paraffins. Oxidation of hydrocarbons will be treated in Section 13.1. 

Coking on the metal is proposed to occur through a series of fragmentation and dehydrogenation reactions, and the products of these reactions combine to form graphitic coke (19). An additional suggested mechanism is polymerization of adsorbed carbonaceous deposits on the metal surface (18,19). 

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. 

Subsequently it decreases with U between 0.9 V and 1.1 V. The coverage QoJsQox with adsorbed carbonaceous species (see section 4 of chapter DC) and the oxygen coverage were also determined [60] under the same voltammetric conditions. Both coverages are plotted as a function of potential in Fig. 36 b. The coverage with carbonaceous species decreases... 

The effect of adsorbed carbonaceous species on hydrogen adsorption was already discussed in section 6 of chapter VI. The quantity... 

Group a includes dehydrogenation reactions [1—4], reactions between adsorbed carbonaceous species and OHa, [4—6] or 0 [7 —10], preelectrochemical steps [11, 12], and desorption steps [13, 14] as rate-determining steps. Examples are given below ... 

Neither the reactant-pair mechanism [19] nor the water discharge mechanism [25] are in agreement with the pH dependence for the oxidation of type I species. The pH dependence may be explained as suggested by Frumkin and Podlovchenko [4] in their discussion of the mechanism of ethanol oxidation. A chemical reaction between adsorbed carbonaceous species and OH d is supposed rate-determining on a heterogeneous surface of the Temkin type [34]. The discussion is the same as that for the oxidation mechanism of chemisorbed carbonaceous species in section 11 of chapter IX. 

It was found that the amount of adsorbed carbonaceous species was too small to be detected by pulse techniques. 

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