Enhancement of the catalytic activity of activated carbons in oxidation reactions by thermal treatment with ammonia or hydrogen cyanide and observation of a superoxide species as a possible intermediate

The catalytic activity of activated carbons in oxidation reactions with O2 is enhanced after treatment with NH3 or HCN at elevated temperatures (600–900°C). The oxidation of dilute aqueous H2SO3 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 N2 to 900°C. The catalytic activity was significantly increased after heating in H2 or N2 to 900°C although the N content was considerably reduced. The carbon is partly gasified in the reaction with NH3 at 900°C leading to an increase of the pore volume in the micropore range. Thermodesorption experiments with H2O, adsorbed at 300 K and N2 and O2 adsorbed at ca. 80 K gave evidence of the presence of very narrow micropores of molecular dimensions in the NH3-activated carbon. With O2, a desorption peak for a reversibly chemisorbed O2 species was observed at 190–210 K with measurable desorption extending to T > 300 K. This desorption peak was also observed with O2 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 O2− ions. The NH3-activated carbon catalyzed H2O formation from H2 and O2 at T < 170 K. Further chemical evidence for the presence of O2 is described. In addition, it was found that also synthetic graphite of fine particle size catalyzed H2SO3 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.