Stuart S. Barton

Acidic and basic sites on the surface of porous carbon

Abstract Thermal desorption, acid-base depletion, mass titration and calorimetric measurements on a series of porous BPL carbons are interpreted as indicating that acid surface sites are oxygen based while basic sites are associated with the carbon itself. Both acidic and basic sites were found on all carbons whether they were heat treated to 900 °C in nitrogen or oxidized, chemically with nitric acid or electrochemically. For the most extensively oxidized sample, the number of basic sites is approximately zero, while for a heat treated carbon the number of acidic sites is very small. An analysis for the wholly acid carbon and the entirely basic carbon yields p K a and p K b values which are characteristic of molecular keto or hydroxy carboxylic acids and Lewis bases, respectively.

The adsorption of methylene blue by active carbon

Abstract The extent of adsorption of methylene blue from an aqueous solution is a convenient indicator in the evaluation of active carbons. The adsorption process is, however, complicated by factors inherent in the structures of both the methylene blue solution and the active carbon. These factors include the tendency of methylene blue to form molecular aggregates in solution, molecular sieving imposed by the pore size distribution of the carbon and the heterogeneous nature of the energies of adsorption sites. Temperature variation of adsorption and calorimetric experiments show, however, that the results obtained can be rationalized on the basis of these structural factors and the kinetic effects which result from them.

Water and cyclohexane vapour adsorption on oxidized porous carbon

Abstract Changes in cyclohexane and water vapour adsorptions and enthalpies of immersion for a series of progressively oxidized porous carbons have been measured. The adsorption and thermal results are discussed in terms of the pore-filling mechanism of Dubinin and the influence of surface oxides on the water adsorption is examined. It is concluded that the important enthalpic contribution for cyclohexane comes from pore-filling while for water, the hydrogen bonding-like interaction around surface oxide sites plays the major role.

The adsorption of water vapor by porous carbon

Abstract Water vapor adsorption isotherms on active carbon and graphite have been measured and analyzed by means of the two Dubinin-Serpinski equations and a third similar equation, based on the same principles. Surface oxide content and enthalpy of immersion measurements are shown to give support to the validity of these equations.

Acidic surface oxide structures on carbon and graphite—I

Abstract Samples of carbon (Spheron 6) and graphite have been degassed at temperatures up to 800°C and the decomposition of the surface oxides to CO and CO2 followed with a mass-spectrometer. Changes in the concentration of surface groups on the degassed samples was also followed through their reaction with methyl magnesium iodide (Grignard reagent). By considering these results and those of earlier base uptake measurements it has been concluded that cyclic esters are primarily responsible for the surface acidity. Two distinct types of acidic oxide were found on the carbon surface, both appeared to be lactone groups, but only one had an ‘active’ hydrogen associated with it. Only one type of acidic oxide was found on the graphite surface which also had an ‘active’ hydrogen associated with it. Possible structures for these groups are discussed.

Surface studies of carbon: Acidic oxides on spheron 6

Surface oxygen complexes on Spheron 6, which thermally desorb as CO2, appear to be responsible for the acidity of the carbon. The base uptake of samples degassed at various temperatures has been related to the amounts of oxygen complex remaining on the surface. Two types of acidic oxides, both of which desorbed as CO2, were observed. At temperatures around 250°C an oxide which acts as a very weak monobasic acid is decomposed and at about 600°C a second oxide, which is stronger and dibasic is decomposed. The heat of neutralization of this second acid was found to be approximately 12 kcal-mole−1.

Surface studies on carbon: Water adsorption on polyvinylidene chloride carbon

Abstract The adsorption of water on polyvinylidene chloride carbon has been studied, with particular emphasis being placed on the contribution of the surface oxides on the adsorption process. The extent of the surface oxides has been measured by thermal desorption techniques and has been related to the adsorption isotherm and the immersional energetics. It has been found that the surface oxides desorbing as CO are probably the sites for the initial adsorption of water at low relative pressures (0.0–0.2), and are situated within the pores of the carbon. These sites present a relatively uniform surface for water adsorption with a net heat of adsorption of 5.7 kcal/mole. The water molecules adsorbed on these sites then act as centers for further adsorption, in a multilayer type of process with a net heat of adsorption of only 0.40 kcal/mole, until pore filling is complete.

Surface Studies on Graphite: Acidic Surface Oxides,

Abstract The extent of chemically detectable acidic surface oxides on graphite has been compared to the total coverage of the surface with oxide. Evolved gas analysis techniques using a mass spectrometer have been used to determine the composition of the gases evolved (CO, CO 2 , H 2 and H 2 O) on thermal decomposition of the surface species. The thermal stability of the acidic oxides neutralized by sodium hydroxide has been related to the oxide which on thermal decomposition yields CO 2 . It appears that a large fraction of the oxide surface, probably composed of carbonyl groups yielding CO on thermal decomposition, is neutral in character

An investigation of the pore structure and molecular sieve properties of polyvinylidene chloride carbons

Abstract Straight and branched chain alkanes have been used as molecular probes in a study of the molecular sieve properties of carbons obtained by fast and slow pyrolysis of polyvinylidene chloride. The uniformity of micropore size in both types of carbon is demonstrated by the sharp reduction in their capacity to adsorb alkanes having two or more methyl groups as side chains, particularly when two of the methyl groups are attached to the same carbon atom. Heats of immersion of the carbons in the same alkanes show an even sharper reduction for branched chain alkanes, presumably as a consequence of the constraints imposed on molecules adsorbed in narrow pores resulting in lower net integral heats of adsorption. It is concluded that the carbons contain a uniform array of micropores 5.5–6.0Ain width.

Surface studies on graphite: Immersional energetics

Abstract The heats of immersion of graphite degassed at temperatures up to 1000°C have been measured in water, methanol and benzene. The removal of oxygen complexes during degassing has been followed quantitatively using a mass-spectrometer system allowing the changes in immersional energetics to be related to the desorbed species.