NO REDUCTION BY ACTIVATED CARBONS .5. CATALYTIC EFFECT OF IRON

The effect of iron as catalyst of the NO-carbon reaction has been inve stigated. A coal-derived carbon was loaded with iron using different m ethods and different precursors. A brief exploratory study was also co nducted with pitch-derived carbon fibers. The iron-loaded and/or paren t carbons were characterized by physical adsorption of CO2 (at 0 degre es C) and N-2 (at -196 degrees C), X-ray absorption fine structure spe ctroscopy (XAFS), and chemisorption of CO at 25 degrees C. The NO-carb on reaction was studied in a fixed-bed flow reactor at atmospheric pre ssure using two types of experiments: (i) temperature-programmed react ion (TPR) in a NO/He mixture, and (ii) isothermal reaction at 300-600 degrees C. The reaction products were monitored in both cases, thus al lowing detailed oxygen and nitrogen balances to be determined. Iron wa s found to catalyze NO reduction by carbon through an oxidation/reduct ion (redox) mechanism similar to that reported previously: for potassi um- and calcium-catalyzed reaction. Nevertheless, the iron species pre sent on the carbon surface before NO reduction (FexOy or FeO) are less effective than the potassium species (elemental potassium or potassiu m suboxide) in chemisorbing NO, as a result of which they transfer les s oxygen to the carbon active sites. The results show also that the na ture of the catalyst precursor, the catalyst preparation conditions an d the reducibility of the catalyst by the carbon determine the chemica l state of the catalyst, its dispersion and catalyst/substrate contact , and hence control the catalytic activity of iron in NO reduction by carbon.