NO oxidation over activated carbon fiber (ACF). Part 1. Extended kinetics over a pitch based ACF of very large surface area

Kinetics on the oxidation of NO was studied over a pitch based activated carbon fiber of very large surface area (pitch ACF) by changing NO and O₂ concentrations in a flow reactor. NO was initially removed through adsorption until saturation, increasing gradually its outlet concentration. In the mean time, NO₂ was found in the gas phase after an induction period to increase its concentration very nearly to the stationary one. It is noted that a maximum NO concentration was observed just before reaching the stationary concentration, where the adsorption of NO was almost saturated. High concentrations of both O₂ and NO increased the amount of saturated adsorption and the stationary conversion of NO, Freundlich equation being well fitted to the observed adsorption and conversion. The reaction order larger than unity in NO suggests multi-molecular intermediate over the ACF surface for oxidation as the dimer has been proposed in the gas phase oxidation of NO. The major adsorbed species found was NO₂ over the ACF surface in the stationary state, although some NO was also found in TPD profile of adsorbed species even in the stationary state. Some reduction of surface NO₂ at its desorption is suggested. A peculiar peak of the maximum NO concentration observed before the stationary conversion may reflect participation of NO to the multi-molecular intermediate which requires a large coverage of adsorbed NO₂ before rapidly releasing NO₂ from the surface to the gas phase.