Acidic surface oxides

It is common practice to classify the surface oxides as basic, neutral, and acidic [66, 67], Acidic surface oxides (carboxylic groups) are formed when the carbon is ex... 

An electrochemical explanation of the basic reaction was suggested by P rumkin 17), while Shilov et al. 22, 23) claimed surface oxides of definite structure to be the only cause of either acidic or basic reaction. Shilov formulated the acidic surface oxides as carboxylic acid anhydrides bound to the edges of the carbon layers. 

Thus, two kinds of surface oxides became known. Basic surface oxides are formed always when a carbon surface is freed from all surface compounds by heating in a vacuum or in an inert atmosphere and comes into contact with oxygen only after cooling to low temperatures. As is now known 24), the irreversible uptake of oxygen starts at ca. —40° there is only reversible, physical adsorption at lower temperatures. Acidic surface oxides are formed when carbon is treated with oxygen at temperatures near its ignition point. King 21) found the maximum... 

Neutralization Behavior of Acidic Surface Oxides on Microcrystalline Carbon"... 

Site of the. acidic surface oxides. The question whether the acidic surface oxides are bound to the periphery of the carbon layei-s or to the basal planes of the crystallites could be resolved by oxidation of a graphitized carbon black (46). The particles of carbon black are, at first approximation, spherical. The graphite-like crystallites show such preferential orientation that their c axis are aligned in a radial direction (64, 65). A schematic representation of this secondary structure is given in Fig. 1. On recrystallization between 2000 and 3000°, many small... 

Reaction of Acidic Surface Oxides with Thionyl Chloride [after Diehl 47)Y... 

To remove possible impurities, the gas black is classified and then densified, pelletized (see Section 4.4.1), or submitted to an oxidative aftertreatment (see Section 4.4.7), depending on its intended use. Since gas blacks are formed in the presence of excess air, their surface is oxidized. Acidic surface oxides are predominant. 

Due to production conditions, only gas blacks (and channel blacks) are covered to a certain extent with acidic surface oxides. Furnace blacks contain only small amounts of oxygen in the form of basic surface oxides. 

Surface Acidity/Basicity. - Van Veen and co-workers were one of the first groups to discuss the importance of surface acidity/basicity as it relates to the interactions that favor decorating metal oxide surfaces with certain metal complexes. We alluded to the acidity/basicity of the surface vide supra) in describing how a metal complex may become subject to a reaction with the surface. Van Veen, et al. were interested in decorating surfaces with a wide range of metal acetylacetonate complexes to include Pt and Pd precursors. They showed that metal complexes that were susceptible to attack by acids were likely to decorate the surface of an acidic surface oxide by loss of a ligand to form a firm attachment with the surface. 

In addition to pore size distribution, the surface chemistry of the activated carbon can have an important influence on the adsorption of certain compounds. As the adsorptive surface of most activated carbons is hydrophobic, they are best suited for the removal of neutral organic molecules, while polar and ionic compounds show much less affinity for adsorption. For the adsorption of polar compounds such as phenol, research has shown that the carbon surface chemistry is more relevant than the total available adsorption capacity or surface area [72-74]. It has been found that the presence of acidic surface oxides, whose concentration can be increased by oxygen adsorption or chemical treatment, leads to a decrease in adsorptive capacity for compounds such as phenols and increases the base adsorption capacity [75 [. 

Active carbon Hypochlorous acid Surface oxide on carbon Hydrochloric acid... 

Figure 1. Model of functional groups m acidic surface oxides of carbon (after ref. 20).

It was interesting to know how other chemisorbed elements affect the catalytic behavior of carbons. When carbons are heated in oxygen or air to 573 to 673 K, acidic surface oxides are formed [4,5]. In these surface oxides one finds carboxyl groups, lactones or lactols, and hydroxyl groups of phenolic character... 

Neutralization of acidic surface oxides on carbon 17, 16.4.2.7.3 CjHsNaOS... 

In addition to pore structure, tlie surface chemistry of the adsorbent may have a strong influence on the adsorption of aromatic compounds. Fig. 7 (from [48]) shows that the Langmuir adsorption capacity of phenol is dependent tai the concentration of acidic surface oxides, as defined by Boehm [49]. Furthermore, for acidic and basic species, the effect of the adsorbent s surface charge combined with that of the pH of the solution is extremely important because it determines the nature of the forces (attractive/repulsive) between the adsorbate and the adsorbent surface. For cationic dyes, adsorption is promoted when the surface charge is negative adsorption capacities are 1.7 to 2.3 times higher at basic pH and adsorption rates are doubled [50]. 

If a highly adsorptive carbon is used, as much as 15 g. of Og can be taken up by 100 g. of the carbon. Under the same conditions, steam produces acid groups whose concentration may reach 700 meq. of H+ ions per 100 g. of the preparation. The material is tested by shaking 0.1 g. of the carbon with 100 ml. of 0.05N alcoholic KOH. The H ions can be replaced by CHg groups through methylatlon with diazomethane. Because of the acidic surface oxides, the carbon is readily wetted by water and poorly by benzene, as contrasted with carbon having no acid surface oxides. Above 500°C, Og is released as CO and COg. 

Other hand, if the pH is lower than the IP or PZC an anionic precursor (i.e. PtClg ) should be selected. Though this work focused on silica and alumina supports, it has been extended to carbon as well [32], and several models exist describing Pt precursor adsorption on carbon [33]. These models claim that basic oxygen groups (or electron rich ir-sites) are stronger anchoring sites than the acidic surface oxides. 

Reaction of Acid Surface Oxides with Ammonia (Activated Charcoal Carboraffin Oxidized with HNO3)... 

The surface of a carbon fiber (or of diamond, graphite, and any other crystalline solid) has been described as an extreme case of lattice defect. " The regular configuration of carbon atoms ends abruptly and the surface atoms have a different coordination with dangling bonds which are able to react with any atom or molecule present on the surface. The result is the formation of compounds such as basic or acidic surface oxides, COj, and... 

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