Reduction by Activated Carbons. 3. Influence of Catalyst Loading on the Catalytic Effect of Potassium

The results of an investigation are reported concerning the NO reduction activity of a carbon loaded by impregnation from solution with varying amounts of a potassium catalyst. The samples were characterized by CO₂ chemisorption at 250 °C and NO chemisorption at 60 °C. The catalytic effect of potassium in NO reduction was evaluated at atmospheric pressure in a fixed-bed flow reactor. Two types of experiments were performed: (i) temperature-programmed reaction (TPR) in a NO/He mixture; and (ii) isothermal reaction at 300-600 °C. Both TPR and temperatureprogrammed desorption experiments were also conducted subsequent to NO chemisorption. The reaction products were monitored in all cases, thus allowing detailed oxygen and nitrogen balances to be determined. Three characteristic reactivity regions were found in the TPR profiles: (a) high catalytic activity at low temperatures due to dissociative NO chemisorption, accompanied by N₂ and N₂O evolution and oxygen accumulation on the catalyst surface; (b) severe catalyst deactivation at intermediate temperatures as the catalyst becomes fully oxidized; (c) recovery of high catalytic activity at high temperatures as the catalyst is reduced by the carbon, accompanied by the evolution of CO₂ and CO. An increase in potassium loading was found to affect only the first region, extending the period of high activity. A good correlation was found between steadystate isothermal catalytic activity and the amount of oxygen accumulated on the surface upon NO chemisorption at 60 °C, especially for NO reduction at low temperatures (e.g., 300 °C). Therefore, dissociative NO chemisorption seems to be a promising technique for titrating the active sites in potassium-catalyzed NO reduction by carbon.