Study of the reduction and reoxidation of a CaO surface

The reduction of NO with CO and H-2 is shown to comprise two basic reactions: a surface oxygen abstraction by the reducing agent and a reoxidation of the surface by NO. The former reaction step has been demonstrated by transient CO2 formation during CO exposure of oxidized CaO surfaces, while the latter was demonstrated by N-2 and/or N2O transient formation during NO exposure of a prereduced CaO surface. It was shown that at low temperatures (between room temperature and 500 degrees C) both N-2 and N2O were formed, but at temperatures above 500 degrees C only N-2 was observed. The activation energies of the respective steps have been determined using temperature-programmed reaction experiments. The activation energy of the surface oxygen abstraction was determined to be 25 kcal/mol and is similar to the apparent activation energy of the overall reaction; The activation energy of the NO bond breakage was determined to be maximum 10 kcal/mol as measured by N2O formation. The importance of an N2O2- or N2O22- intermediate in the formation of N2O will be discussed, and the importance of N2O decomposition in forming N-2 at temperatures above 500 degrees C will be compared with a N surface diffusion mechanism.