NO Oxidation by Activated Carbon Catalysts: Impact of Carbon Characteristics, Pressure, and the Presence of Water.

Nitric oxide can be removed from flue gas by catalytic oxidation of NO to NO2, followed by dissolution of NO2 in water. The work presented here includes catalytic NO oxidation by activated carbons (ACs) at atmospheric and elevated pressures under dry and wet conditions at ambient temperature. The AC samples had different physicochemical characteristics including surface areas of ∼400-1600 m2/g and micropore volumes of ∼0.2-0.6 cm3/g while having different surface chemistries. Dry tests indicated that introducing nitrogen functionalities or coating with pyrolytic carbon could enhance the catalytic activity of AC for NO oxidation. Nitric oxide concentration profiles from the oxidation experiments under dry conditions showed maximum values after 5-15.5 h of testing and a steady-state condition after ∼12-30 h and that a major release of NO2 began after reaching the maximum values in the NO concentration. Adsorption profiles showed a high rate of NO x adsorption during the early hours of these experiments, and this rate decreased almost exponentially to a near-zero value. A near-complete catalytic conversion was achieved for NO oxidation at 120 psig under dry conditions, substantially higher than the 62% value of the noncatalytic NO oxidation at 217 psig. The wet trickle-bed experiments revealed that an inert packing material with a high external surface was a more suitable option than the ACs for NO oxidation in a wet trickle-bed system, even for ACs that exhibited high catalytic reactivity under dry conditions. Noncatalytic NO oxidation in the trickle-bed system was enhanced by the higher gas-liquid contact surface of the packing material for NO2 dissolution in water. Complete wetting of the hydrophilic AC or the presence of water vapor in the gas in contact with the surface of the superhydrophobic AC could eliminate or drastically reduce the catalytic activity of the AC for NO oxidation.