Activated carbon oxidized

The surface oxygen content also affects the resistivity of the carbon particles. Thus active carbon oxidation, as in other powdered carbon materials, enhances, whereas heat treatment in an oxygen-free atmosphere diminishes electrical resis-... 

Comparison of the surface density (pmol/m ) of the functional groups on active carbons oxidized by nitric acid in relation to carbon oxidized in air (see Table 4). shows a significantly higher (about 10 times) concentration of quinoid (carbonyl) structures. The gentler Nernst slope for CWN2—Ox is indicative of reduction with partial ionization of the hydroxyl group, according to the reaction... 

TABLE 3 Effect of Activated Carbon Oxidation on the Adsorption of Cr(III) and Cr(VI)... 

Figure 13.5 Distribution of acidity constants of acidic surface groups on an activated carbon oxidized with nitric acid. (Reprinted from Refs [38] and [43] with permission from Elsevier.)...

Radovic and coworkers [38] recently investigated the effects on the adsorption process of the pH and the nature of the functional groups on the aromatic adsorptive and the adsorbent. For this purpose, they used an as-received activated carbon oxidized with nitric acid and nitrided with ammonia to study the adsorption of aniline and nitrobenzene, which are, respectively, electron-donating and electron-withdrawing groups. Results found by these authors, together vidth... 

Then the performances of the activated carbons oxidized with air to different extents were compared. It was observed that the activity increased with the degree... 

Alkali compoiuids are known to be active carbon oxidation catalysts (as reviewed in [3]). Results for Li, K, Na, and Cs are not reported here as problems were encoiuitered, preparing soot-alkali hydroxide mixtures, following our tiglit and loose contact mode procedure. 

The effects of heat treatments and chemical oxidation on the distribution of surlace groups on carbon materials have been widely studied and discussed based on the results from Boehm titration [14, 16, 44, 204—207]. Table 1 shows as an example, the Boehm titration of a series of activated carbons oxidized with nitric acid at different concentrations and a saturated solution of ammonium peroxodisulfate [207]. 

Oxygen content, point of zero charge and Boehm titration of a series of activated carbons oxidized with nitric acid and ammonium peroxodisulphate [207]. Reprinted with permission from C.O. Ania, J.B. Parra, J.J. Pis, Ads. Sci. Technol., 22 (2004) 337... 

Fig. 27. CO2-TPD profiles from activated carbon oxidized with varims oxidized agents A3N) nitric acid, A3S) ammonium persulfate and A30) Iqidrogien peroxide [327]. Reprinted with permission from S. ffaydar, C. Moreno-Castilla, M.A. Ferro-ftercia, F. Carrasco-Marin, J. Riveia-Utrilla, A. Pcrrard and JJ>. Joly, Carbon, 38 (2000) 1297...

Fig. 28. TPD speeba of SO2 evolution from an activated carbon oxidized with sulfuric acid [12]. Reprinted with permission fhan A.P. Tetzyk, J. Colloid Interf. Sci., 268 (2003) 301...

Among them, diffuse reflectance FTIR spectroscopy (DRIFTS) has been very popular, as is the case in the field of heterogeneous catalysis in general. Changes in surface chemistry of activated carbons, oxidized with different agents (HNO3, H2O2, and ammonium persulfate), were studied by DRIFTS in addition to Boehm titration, potentiometric titration, and water adsorption measurements... 

Preoxidation of bituminous coal is a crucial step in the preparation of activated carbons. Oxidation produces a decrease in the caking properties, or even its total destruction (ref. 12). In fact, an important transformation in the chemical composition and in the porous structure of the coals (ref. 13) was produced. 

Prior to determination of an isotherm, all physisorbed material has to be removed from the surface of the adsorbent. This is best achieved by exposure of the surface to high vacuum, the exact conditions required (temperature and residual pressure) being dependent on the particular gas-solid system. In routine determinations of surface area it is generally advisable not to remove any chemisorbed species which may be present thus, the hydroxylated oxides are usually outgassed at 1S0°C. Microporous adsorbents such as zeolites or active carbons however require higher temperatures (350-400 C, say) for complete removal of physisorbed material from their narrowest pores. An outgassing period of 6-10 hours (e.g. overnight) is usually sufficient to reduce the residual pressure to 10 Torr. 

See Adsorption, LIQUID separation Aluminum compounds, aluminum oxide (alumina) Carbon, activated carbon Ion exchange Molecular sieves Silicon... 

Activated carbons are made by first preparing a carbonaceous char with low surface area followed by controlled oxidation in air, carbon dioxide, or steam. The pore-size distributions of the resulting products are highly dependent on both the raw materials and the conditions used in their manufacture, as maybe seen in Figure 7 (42). 

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

Hollow Fiber with Sorbent Walls. A cellulose sorbent and dialy2ing membrane hoUow fiber was reported in 1977 by Enka Glan2stoff AG (41). This hoUow fiber, with an inside diameter of about 300 p.m, has a double-layer waU. The inner waU consists of Cuprophan ceUulose and is very thin, approximately 8 p.m. The outer waU, which is ca 40-p.m thick, consists mainly of sorbent substance bonded by ceUulose. The advantage of such a fiber is that it combines the principles of hemodialysis with those of hemoperfusion. Two such fibers have been made one with activated carbon in the fiber waU, and one with aluminum oxide, which is a phosphate binder (also see Dialysis). 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

Quantitative Analysis of All llithium Initiator Solutions. Solutions of alkyUithium compounds frequentiy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allyhc and henzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

In past years, metals in dilute sulfuric acid were used to produce the nascent hydrogen reductant (42). Today, the reducing agent is hydrogen in the presence of a catalyst. Nickel, preferably Raney nickel (34), chromium or molybdenum promoted nickel (43), or supported precious metals such as platinum or palladium (35,44) on activated carbon, or the oxides of these metals (36,45), are used as catalysts. Other catalysts have been suggested such as molybdenum and platinum sulfide (46,47), or a platinum—nithenium mixture (48). 

Another method employed is the treatment of aqueous solutions of aminophenols with activated carbon (81,82). During this procedure, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate (82) is added to increase the quaUty and stabiUty of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine (82), or sodium hydrosulfite (83) also is recommended during precipitation or crystallization of aminophenols. 

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