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 CO 2 formation during CO exposure of oxidized CaO surfaces, while the latter was demonstrated by N 2 and/or N 2 O transient formation during NO exposure of a prereduced CaO surface. It was shown that at low temperatures (between room temperature and 500 °C) both N 2 and N 2 O were formed, but at temperatures above 500 °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 N 2 O formation. The importance of an N 2 O 2 - or N 2 O 2 2- intermediate in the formation of N 2 O will be discussed, and the importance of N 2 O decomposition in forming N 2 at temperatures above 500 °C will be compared with a N surface diffusion mechanism.