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 nitrate. The NO-carbon reaction was studied in a fixed-bed now reactor at atmospheric pressure, using two types of experiments: (i) a temperature-programmed reaction (TPR) in a NO/He mixture; and (ii) an isothermal reaction at 300-600 degrees C. The products of the reactions were monitored allowing detailed oxygen and nitrogen balances. All the metals used catalyze the NO-carbon reaction, causing an important decrease in the activation energy and a substantial shift-of the NO reduction curves toward lower temperatures. The TPR curves, the isothermal reactivity data, and the distribution of the products can be explained by means of an oxidation/reduction mechanism that implies a different oxidation 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 temperatures the activity is larger for cobalt and copper, metals whose oxides are reduced by carbon at a lower temperature. This order of activity seems to indicate that, at low temperatures (T < 400 degrees C), the controlling step in the process is the NO chemisorption, whereas at high temperatures (T > 500 degrees C), the reduction of the intermediate oxidized metal species is the rate-limiting step.