Abstract

The catalytic activity of activated carbons in oxidation reactions with O ₂ is enhanced after treatment with NH ³ or HCN at elevated temperatures (600-900°C). The oxidation of dilute aqueous H ₂SO ₃ was measured as a test of catalytic activity. Nitrogen is chemisorbed in the treatment and XPS showed two N 1s signals with binding energies of 401-400 eV and 399-398 eV which are assigned to amine groups and nitrile and/or pyridine-like nitrogen, respectively. In the case of HCN treatment, the N content was higher on the surface than in the bulk and the accessible micropore volume was reduced. It could be reopened by heating in N ₂ to 900°C. The catalytic activity was significantly increased after heating in H ₂ or N ₂ to 900°C although the N content was considerably reduced.

The carbon is partly gasified in the reaction with NH ³ at 900°C leading to an increase of the pore volume in the micropore range. Thermodesorption experiments with H ₂O, adsorbed at 300 K and N ₂ and O ₂ adsorbed at ca. 80 K gave evidence of the presence of very narrow micropores of molecular dimensions in the NH ³-activated carbon. With O ₂, a desorption peak for a reversibly chemisorbed O ₂ species was observed at 190-210 K with measurable desorption extending to T > 300 K. This desorption peak was also observed with O ₂ adsorbed on HOPG. XPS measurements at T < 170 K showed a signal at an O 1s b.e. of 533 eV which is assigned to O ₂ ^- ions. The NH ₃-activated carbon catalyzed H ₂O formation from H ₂ and O ₂ at T < 170 K. Further chemical evidence for the presence of O ₂ is described. In addition, it was found that also synthetic graphite of fine particle size catalyzed H ₂SO ₃ oxidation. The catalytic activity seems to be enhanced by chemisorbed nitrogen atoms, but the concentration of the active species is below the limits of XPS detection.