Carbon black desorption

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Fig. 2.29 Comparison of nitrogen adsorption at 78 K on a carbon black (Sterling FT) before and after graphitization (a) The amount adsorbed on the ungraphitized sample plotted against the amount x, adsorbed on the graphitized sample, at the same pressure, b) The corresponding isotherms O, adsorption, , desorption on the ungraphitized sample (4 runs) A. adsorption A desorption, on the graphitized sample (4 runs).

Fig. 5.10 The adsorption isotherms of n-hexane (A) and of water (B) on graphitized carbon black.Solid symbols denote desorption. (After...

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

The major concern with carbon black exposure is the simultaneous exposure to polycyclic aromatic hydrocarbons that are strongly adsorbed to the respirable carbon black particles and from which PAHs may be elutriated in vivo under conditions of human exposure. However, in a number of studies, attempts to elutriate PAH with biological fluids have been largely unsuccessful, and prolonged extraction with boiling aromatic solvents is required for quantitative desorption. Carbon black has been... 

PAHs adsorbed on particles of carbon black were also photostabilized (Behymer and Hites, 1988). However, Barofsky and Baum (1976) demonstrated that BaP, anthracene, BaA, and pyrene deposited on carbon microneedle field desorption emitters and exposed to UV radiation were all photooxidized to carbonyl compounds. Similarly, PAHs can photodegrade efficiently in air when adsorbed to substrates of silica gel, alumina, or glass plates (e.g., see Lane and Katz, 1977 Kormacher et al., 1980 Behymer and Hites, 1985 Yokely et al., 1986). 

Contrary to carbon-black-filled conventional rubbers, which form a semi-rigid interface at the carbon black surface, PDMS chain units at the silica surface are not rigidly linked to the silica surface. Two types of dynamic processes are thought to occur at the interface relatively fast anisotropic reorientation of chain units in the interfacial layer and slow adsorption-desorption of chain units (Figure 10.13) [108, 113]. 

MN acetylation is most likely related to the high polarity and bulkiness of the products with limitations in the reaction rate by product desorption. Dealumination would have a positive effect on the acetylation rate because of the decrease in the zeolite hydrophilicity and of the increase in the rate of diffusion of the bulky products owing to elimination of extra-framework A1 species. Curiously, in anisole acetylation, the Si/Al ratio of the HBEA zeolite had practically no effect on the reaction rate. However it is worth noting that most of the tested samples had Si/Al ratios between 11 and 30. Like for 2-MN acetylation,[28,32] the performance of HBEA zeolites in anisole acetylation depends on their crystallite size.[17] This was shown by comparing the activities of samples with large size (0.1-0.4 pm) and of a nanosize sample (0.01-0.02 pm) prepared within the pores of a carbon black matrix. The superior performance of the nanosize sample was ascribed to a decrease in diffusional constraints limiting the desorption of the bulky and polar p-methoxyacetophenone product from the BEA micropores. 

Type IV isotherms are characterized by the presence of a hysteresis loop (i.e., adsorption and desorption branches are not coincident) due to the capillary condensation on the mesopores. They are characteristic of adsorbents that have a wide proportion of mesopores (i.e., compacted carbon blacks under pressure, nanostructured carbons prepared using mesoporous silica as templates, etc.). 

The relative contributions of the different desorption processes changes with the average surface structure. Comparison of the data in Fig. 28 with those of an analogous oxidation experiment with a different carbon substrate (carbon black FW-1) shown in Fig. 29 reveals that the carbon black contains significantly more basic functional groups than the amorphous fullerene black. This can be traced back to a reduced... 

The discussion in the Introduction led to the convincing assumption that the strain-dependent behavior of filled rubbers is due to the break-down of filler networks within the rubber matrix. This conviction will be enhanced in the following sections. However, in contrast to this mechanism, sometimes alternative models have been proposed. Gui et al. theorized that the strain amplitude effect was due to deformation, flow and alignment of the rubber molecules attached to the filler particle [41 ]. Another concept has been developed by Smith [42]. He has indicated that a shell of hard rubber (bound rubber) of definite thickness surrounds the filler and the non-linearity in dynamic mechanical behavior is related to the desorption and reabsorption of the hard absorbed shell around the carbon black. In a similar way, recently Maier and Goritz suggested a Langmuir-type polymer chain adsorption on the filler surface to explain the Payne-effect [43]. 

The service performance of rubber products can be improved by the addition of fine particle size carbon blacks or silicas. The most important effects are improvements in wear resistance of tire treads and in sidewall resistance to tearing and fatigue cracking. This reinforcement varies with the particle size, surface nature, state of agglomeration and amount of the reinforcing agent and the nature of the elastomer. Carbon blacks normally are effective only with hydrocarbon rubbers. It seems likely that the reinforcement phenomenon relies on the physical adsorption of polymer chains on the solid surface and the ability of the elastomer molecules to slip over the filler surface without actual desorption or creation of voids. 

The last Issue to be dealt with Is the apparent irreversibility of the adsorption. One quite often encounters the opinion, especially In the older literature, that polymer adsorption would be an Irreversible phenomenon. These ideas are based on the hysteresis found In the adsorption isotherms desorption Isotherms (obtained by dilution with solvent) do not coincide with adsorption Isotherms (obtained by adding more polymer at given amount of solvent). Qualitatively, this was already discussed in sec. 5.3d. An experimental example Is given in fig. 5.31, for the adsorption of a polydisperse rubber from heptane on two types of carbon black (differing In specific surface area) ). The desorption Isotherms are found to He considerably above the adsorption Isotherms, the extent of desorption being very small. 

Figure 5.31. Adsorption-desorption hysteresis as observed for rubber adsorbing from n-hexane on to two different types of carbon black. Arrows indicate ascending (increasing Cp) and descending (decreasing Cp) branches. Redrawn from ref.

The attainable enrichment and clean-up in SPE depend primarily on the selectivity and affinity of the sorbent for the selected target analyte or analytes, the sample load capacity for the analytes and the rate of mass transfer to and from the binding sites, the latter affecting the minimum desorption volume and thus the enrichment that can be obtained. Other factors of importance are the reproducibility of the recovery yields and the stability and reusability of the sorbent when online procedures are desired. For hydrophobic analytes satisfactory results are usually obtained using standard reversed phase sorbents. Thus hydrophobised silica (C8, Cl8), styrene-divinylbenzene copolymers (PS-DVB) and graphitised carbon black (GCB) are the conventional sorbent materials used in SPE (Fig. 15.2)... 

Detailed proof of this mechanism was achieved by using carbon-14 tagged copolymers of long chain methacrylates with vinyl pyridine. Carbon black dispersions stabilized with the tagged polymer were electrodeposited and then anode and cathode were assayed for carbon-14 in a scintillation counter. The carbon was deposited on the anode, but the clean cathode had the higher C-14 count, showing that the cations in the oil phase were indeed the dispersant polymer. The adsorption-desorption process which allowed dispersants to desorb from particles and to... 

Concerning CO/O2 reaction on perovskite oxide, the "suprafacial" mechanism is assumed, as well as for the carbon black/oxygen reaction. Carbon adsorption on the catalyst surface could be made tlirough the C-C bond or the C-0 surface complexes, assuming that C is bonded to the Mn ion with donation of carbon lone pair into the empty 3dz orbital to form s bond accompanied by back donation of the t2g electrons of Mn ion to anti-bonding i orbital of C-0 or C-C. Moreover, the mechanism begins by simultaneous adsorption of carbon and oxygen, the interaction between adsorbed species causes the CO2 formation, the desorption of which releases the catalytic active sites. 

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