Activated carbons oxygen surface functionalities

Surface Functional Groups The amount and species of surface functional groups is controlled primarily by the manufacturing process. The chemical properties of most commercial activated carbons are due to the incorporation of oxygen into the carbon surface during the activation process. For manufacturing of activated carbon catalysts, surface functional groups play a very important role and as such, influence the performance in the final application, as has been shown for several examples. 

In the case of oxidized activated carbon samples (D-O sample), the carbon-oxygen surface function groups play a dominant role. These functional groups undergo surface ionization and ion-exchange reactions between the H+ on the carbon surface and Cu + ions in the solution. 

Figure 8.3 Oxygen surface functions most commonly found at the surface of activated carbons (1) carboxylic acid, (2) lactone, (3) phenol and (4) carbonyl.

In general, the activated carbons contain a few mmol of elemental oxygen in 1 g of the carbon as surface functionalities. Oxygen-containing surface functionalities are also a significant factor that dominates the capacitance of the activated carbon electrode [15]. Typical oxygen-containing... 

Figure 434. Isotherms of adsorption of H2O at 298 K on activated carbons from peach stones (M-series) and plum stones (R-series) containing increasing amounts of oxygen surface functionality (Rodriguez-Reinoso et at., 1992).

The volume of micropores and their size distributions in activated carbons, are a function of (a) the activation equipment (b) bum-off (c) the starting carbonaceous precursor (d) the activating gas (e) rate of gasification (a function of temperature) (f) rate of diffusion of activating gas(es) into and product gas(es) out of the carbon (g) extent of product inhibition of rate (h) amount and nature of inorganic impurities in char and (i) amounts of oxygen surface complexes present. 

Li YH, Lee CW, Gullet BK. Importance of activated carbon s oxygen surface functional groups on elemental mercury adsorption. Fuel 2003 82(4) 451-457. 

The mesoporous carbon xerogel support (X25) was submitted to an oxidative treatment using HNO3 in order to increase the amonnt of oxygenated surface functions. Such treatments were previously studied using a microporous activated carbon (SX+) supplied by NORIT (Sbet 920 m. g ) [2],... 

HNO3 functionalized carbons are used as supports for the preparation of bimetallic Pd-Bi/C catalysts. The aim is to take advantage of the oxygenated surface functions introduced on the carbon, which are expected to act as anchoring sites. Two neutral coordination complexes were selected to incorporate the two metals onto differently functionalized caibon supports. The grafted samples are then submitted to an activation treatment and characterized by SEM, XRD and XPS. It appears that the order of incorporation strongly influences the surface properties of the catalysts and thus modifies their activity in the oxidation of glucose. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

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. 

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