Brian H. Harrison

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.

Characterization of superactivated carbons

Abstract The pore structure of an activated carbon and three “superactivated” carbons has been investigated by gas adsorption, mercury porosimetry, and immersion calorimetry. The latter technique was used to show that the micropores present in the carbons were larger than 10 A in width, a result compatible with pore widths (slit geometry) of 14 to 21 A indicated by gas adsorption. Mercury porosimetry was used to define the transitional and macropore structure and to estimate surface area. The problems encountered in attempts to correlate gas adsorption data with mercury porosimetry data and micropore analysis from gas adsorption data are discussed.

Surface studies on carbon: Immersional energetics of spheron 6 in water

Abstract The heats of immersion of Spheron 6 degassed at temperatures up to 1000°C have been measured in water. The removal of surface complexes as CO, C02, H2 and H20 during the degassing has also been followed quantitatively using a mass spectrometer system. It was found that the changes in the immersional energetics could be accounted for by the removal of oxygen complexes and that complexes desorbing as hydrogen do not appear to affect the heat of immersion in water. The heat of immersion in water of that part of the surface of Spheron 6 covered with oxygen complexes was found to be 140 ergs. cm−2.

Heat of immersion studies on carbon formed from polyvinylidene chloride

Abstract A heat of immersion study has been conducted on a highly microporous polyvinylidene chloride (PVDC) carbon. The effect of chain length on the heat of immersion has been investigated for a series of n-alkanes and n-alkanols. By measuring the heat of immersion of the carbon in hexane, as a function of precoverage with hexane, it has been demonstrated that the heat of immersion is a direct measure of the net heat of adsorption. The surface of the carbon appeared to be reasonably uniform with a net heat of adsorption of 10.25 kcal · mole −1 . Using a series of branched chain hydrocarbons, as molecular probes, it was observed that the carbon had an almost uniform pore size with entrances between 5 A and 6 A in width.

Surface studies on graphite: An estimation of the average polarity of the oxygen complexes

Abstract The average polarity of the surface of graphite covered by oxygen complexes has been evaluated using heats of immersion in a series of n -butyl derivatives of different dipole moment. By assuming that the adsorption on the oxygen free surface of graphite can be described purely by dispersion forces the interaction with the dipoles of the n -butyl derivatives has been related to the surface covered by oxygen complexes. It was found that the average electrostatic field at this surface was 1.1 ± 0.1 × 10 5 esu.