Activated carbon surface functional groups

Figure 3(a) is the adsorption effect of modified Activated Carbon on Sb " with increasing pH. The pH was adjusted with HCl, selected room temperature, stirring frequency was lOOr/min, the amount of carbon was 5 g/L and the adsorption time was 1 h. The figure shows that with pH increased, activated carbon adsorption effect on Sb increased first and then decreased. When the pH was 5, the adsorption effect is better, and the remaining Sb " concentration dropped to 0.02 mg/L, removal rate was 99.13%. This is mainly because with the pH value increased, carbon surface functional groups will occur with the dissociation of H, thus exposed a large number of active centers, Sb + occupied the active center and effectively adsorbed, so the adsorption amount increased as the pH increased. However, as the pH increased, the chemical interactions between hydroxyl and the metal ions increased, resulted the relative decline in the amount of adsorption and thus activated carbon adsorption effect on Sb + increased first and then decreased. 

A carbon development program was initiated at the Illinois State Geological Survey (ISGS) and the University of Illinois at Urbana-Champaign (UIUC) to investigate the effects of different carbon types, carbon structures, and carbon surface functional groups on the rate and extent of adsorption of vapor-phase mercury. The results from a study to prepare Illinois coal-based activated carbons are presented. Carbon products were made both in bench- and pilot-scale reactors. 

In this section, we will consider the properties of perhaps the second most well developed of the electrochemical capacitor systems those based on the oxides of the transition and noble metals. In general, a distinction is made between the metal oxide systems and the carbon systems on the basis that the former are redox active materials, i.e., charge is stored by means of changes in the oxidation state of the metal when an ionic species (e.g., a proton) reacts with the surface (or bulk) of the oxide, and the chemisorption is accompanied by the simultaneous injection of an electron into the oxide. The carbon capacitors are supposed to be more purely of the double-layer or space-charge variety. In fact, as was pointed out earlier, the distinction is less clear. In the case of carbon, surface functional groups may be oxidized and reduced in very much the... 

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

Figure 3.27. Functional groups on activated carbon surfaces (Vinke, 1991).

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

Hydroxide and carbonate typically form insoluble precipitates with polyvalent cations in natural waters. The activity of both of these species increases with pH. The presence of surface functional groups that are capable of exchanging a proton creates pH dependent-charge, whereby the ionic character of the surface increases with pH [158,284,285]. 

Several studies have been directed toward examination of the interaction of acids and bases with active carbons (I, 8, 10, 17, 18, 19). Boehm (3), Garten and Weiss (9), and Snoeyink and Weber (21) have presented reviews on the subject. Garten and Weiss (8, 9, 10) have shown that acid and alkali sorption can be related to surface functional groups which form during the preparation of the carbon. Alkali sorption occurs principally on carbons activated at temperatures near 400°C., and is attributed to the presence of phenolic and lactone functional groups on the carbon surface. Carbons which sorb acid usually are activated at temperatures near 1000°C. the acid reaction in this case is assumed to take place with chromene (benzpyran) structures on the surface. 

Garten and Weiss (8) have postulated that strong acids react with chromene functional groups on the active carbon surface in the following manner ... 

Processing of CDC was recently reviewed by Nikitin and Gogotsi [28], Carbon derived from selective etching of metals by halogens has shown great promise as the active material in electrochemical capacitors [15,29-31] because of the structure and properties that can be fine-tuned [21,32] and control of surface functional groups [33],... 

Pirjamali and Kiros [289] examined the relationship between the pH values of the aqueous slurries, [which] provides an indication of the surface functional groups of the carbon blacks and [are] equivalent to the point[s] of zero charge of the materials, and the ORR activities in alkaline electrolyte, and the only conclusion possible, without an explanation, was that increased temperature treatment has [a] more significant effect on better electrode performances than low temperature... 

All of these features considerably improve the sorption capacity of carbons, and their capacity to remove contaminants and pollutants interacting with the surface of carbons in a dispersive way [177], In addition, active carbon contains heteroatoms such as oxygen, and, to a smaller degree, nitrogen and sulfur. These atoms are bound to the activated carbon surface in the form of functional groups, which are acidic or basic, giving the activated carbon surface an acidic or basic character, respectively [173,178], It is as well necessary to state that the chemical heterogeneity of the carbon surface is mostly the result of the presence of heteroatoms [175],... 

Because surface functional groups influence the adsorption properties and the reactivity of activated carbons, many methods, including heat treatment, oxidation, animation, and impregnation with various inorganic compounds, have been developed in order to modify activated carbons [183], These modifications can alter the surface reactivity, as well the structural and chemical properties of the carbon, which can be characterized using various methods, as described in detail elsewhere [176],... 

This apparently simple reaction is of multiple relevance in catalytic carbon chemistry. The reaction is used to activate carbon by the increase of surface area due to hole burning. It is further used to create surface functional groups (Section 2.1.9.5). Finally it is used to remove undesired carbon deposits in regeneration procedures. 

The electronic structures of porous solids have been examined by X-ray photoelectron spectroscopy (XPS). However, the penetration depth of electrons is 1 nm at best and XPS cannot examine electronic structures of inner pore-walls. XPS has been often used for the determination of surface chemical structures such as surface functional groups in activated carbon. Ar etching leads to the depth profile of electronic structures. This depth profile is often effective to evidence the presence of nanoporosity. 

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