Water vapor adsorption on chars and active carbons–oxygen sensors prepared from a tropical tree wood

Carbonizates and active carbons (AC) are useful as sensors of oxygen. Adsorption of water vapor on such materials, prepared from the wood of a tropical tree, Peltogyne from Surinam, was investigated. Reproducible and reversible water vapor isotherms were obtained of preliminarily unheated samples, as well as of heated samples after several adsorption–desorption cycles. Water vapor adsorption revealed very fine (ultramicropores) structure in the investigated materials. Such ultramicropores can not be detected by traditional adsorption of nitrogen at 77 K because of a molecular sieve blocking effect. Several carbonizates at high water vapor pressure exhibited large weight reduction. This weight loss is the result of expulsion of organic impurities from the porous structure under the influence of adsorbed water. These data were confirmed by mass-spectrometric thermodesorption analysis. X-ray data and water vapor adsorption isotherms indicated that the strongly bound water stabilizes the carbonizate structure. When heated at 623 K, the AC exhibited desorption of strongly bound water and modified AC structure.