Determination of the number of active oxygen species on the surface of Cr₂O₃ catalysts

The number of active oxygen species (O_s) on the surface of Cr₂O₃ catalysts was determined by using the rectangular pulse technique coupled with the reaction of NH₃ with a preoxidized catalyst, i.e., 2 NH₃ + 3O_s → N₂ + 3H₂O and 2NH₃ + 4O_s → N₂O + 3H₂O. The observed number of active oxygen species was found not to change with experimental variables such as temperature, carrier gas flow rate, weight of catalyst, and pulse width. The area occupied by a site, which is defined as the BET surface area divided by the number of active oxygen species, was almost constant for three kinds of Cr₂O₃ catalysts prepared by different methods, and it was close to literature data.