NO REDUCTION BY ACTIVATED CARBON .6. CATALYSIS BY TRANSITION-METALS

Some first series transition metals (Cr, Fe, Co, Ni, and Cu) have been investigated as catalysts of the NO reduction by carbon. An activated carbon was loaded by impregnation from an excess solution of metal ni trate. The NO-carbon reaction was studied in a fixed-bed now reactor a t atmospheric pressure, using two types of experiments: (i) a temperat ure-programmed reaction (TPR) in a NO/He mixture; and (ii) an isotherm al reaction at 300-600 degrees C. The products of the reactions were m onitored allowing detailed oxygen and nitrogen balances. All the metal s used catalyze the NO-carbon reaction, causing an important decrease in the activation energy and a substantial shift-of the NO reduction c urves toward lower temperatures. The TPR curves, the isothermal reacti vity data, and the distribution of the products can be explained by me ans of an oxidation/reduction mechanism that implies a different oxida tion states of the metal and the carbon matrix. The results show that, at low temperatures, iron, cobalt, and nickel are the most effective, as they are metals able to chemisorb NO dissociatively; at high tempe ratures the activity is larger for cobalt and copper, metals whose oxi des are reduced by carbon at a lower temperature. This order of activi ty seems to indicate that, at low temperatures (T < 400 degrees C), th e controlling step in the process is the NO chemisorption, whereas at high temperatures (T > 500 degrees C), the reduction of the intermedia te oxidized metal species is the rate-limiting step.