NO Reduction by Activated Carbons. 5. Catalytic Effect of Iron

The effect of iron as catalyst of the NO-carbon reaction has been investigated. A coal-derived carbon was loaded with iron using different methods and different precursors. A brief exploratory study was also conducted with pitch-derived carbon fibers. The iron-loaded and/or parent carbons were characterized by physical adsorption of CO₂ (at 0 °C) and N₂ (at -196 °C), X-ray absorption fine structure spectroscopy (XAFS), and chemisorption of CO at 25 °C. The NO-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure using two types of experiments: (i) temperature-programmed reaction (TPR) in a NO/He mixture, and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Iron was found to catalyze NO reduction by carbon through an oxidation/reduction (redox) mechanism similar to that reported previously for potassium- and calcium-catalyzed reaction. Nevertheless, the iron species present on the carbon surface before NO reduction (Fe_xO_y or FeO) are less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer less oxygen to the carbon active sites. The results show also that the nature of the catalyst precursor, the catalyst preparation conditions and the reducibility of the catalyst by the carbon determine the chemical state of the catalyst, its dispersion and catalyst/substrate contact, and hence control the catalytic activity of iron in NO reduction by carbon.