Effect of Surface Oxides on Carbon Surface Modification

It is well known that the adsorption properties of activated carbon filters deteriorate significantly on exposure to humid air (> 40% Relative Humidity), due to a process known as ''ageing''. It is postulated that the formation of a wide variety of oxygen-containing species on the carbon surface as a result of the chemisorption of water vapour, are the cause for this fall in performance. Chemical modification of carbon surfaces by halogens has been well documented. In particular, chlorination at elevated temperatures has been studied by a number of workers as a means of elucidating the chemistry of carbon surfaces, whilst Hall and Holmes have demonstrated that lower temperature treatments with chlorine and phosgene (ca 180°C) reduced the deleterious effects of "ageing" of activated carbon filters, with respect to the removal of hydrophobic gases from contaminated airstreams. It has been suggested that the beneficial effect of the lower temperature chlorination results from the loss of carbon-oxygen surface species through the formation of carbon-halogen bonds, which are then resistant to further ageing effects of humid air. The aim of the present research is to gain a better fundamental understanding of the nature of surface oxides on activated carbons, in order that the role of these surface oxides in the ageing and surface modification processes can be established. This paper considers the effect of the degree of surface oxidation of carbons on the chlorination (180°C) process.