NATURE AND STRUCTURE OF CALCIUM DISPERSED ON CARBON

A pure phenol-formaldehyde polymer carbon has been oxidized by HNO3, air, and H2O2, to introduce different amounts and types of oxygen surface groups. The HNO3-oxidized carbon has been loaded with different calcium contents by both the ion-exchange and impregnation processes from calcium acetate solution. A maximum value of 3.7% calcium content is found to be ion exchanged. A thermal programmed desorption, mass spectrometry technique (TPD-MS) has been used to investigate the initial state of the calcium species after ion exchange or impregnation. Information about the nature of the calcium species and its distribution in the carbon matrix, as a function of the calcium loading, has been obtained by comparing the TPD profiles of the raw and oxidized carbons with those obtained for the calcium-carbon samples. Two different hypothetical structures for the ion-exchanged calcium have been proposed. The structure depends on the calcium to carbon carboxylic group ratio. For low calcium loading, the ion-exchanged calcium completes its coordination sphere with H2O and CO2 molecules. For high calcium loadings its coordination becomes similar to that of calcium acetate because each Ca2+ ion acts as a nucleation site where a crystal of acetate grows. The TPD interpretation is consistent with reactivity measurements obtained as a function of calcium content. © 1990, American Chemical Society. All rights reserved.