Effects of O-2 on the reduction of NO over prereduced CaO surfaces: A mechanistic understanding

The effect of O-2 on the reduction of NO over prereduced CaO surfaces is in vestigated. The experimental results suggest the existence of at least thre e different reaction channels, of which two are related to the high-tempera ture reduction of the CaO surfaces and involve the use of extra electrons i n breaking the NO bond. The third reaction channel does not employ extra el ectrons for bond breaking, but the activity is affected by the amount of ad sorbed surface oxygens. The difference between the former two reaction chan nels is found in the temperature needed for an observable activity. The rea ction channel which is already active at low temperatures is described by a model based on F-centers, whereas the one which needs elevated temperature s involves a hole transport through the bulk. The activation energy for thi s transport is determined experimentally using a temperature-programmed rea ction technique as well as theoretically by means of ab initio quantum chem istry calculations. Room-temperature exposure to O-2 is suggested to result in a poisoning of the F-centers, but has only a minor effect on the reacti on channel proposed for high temperatures. Effects on the reduction of NO o f time as well as temperature for the O-2 exposure step are also investigat ed and found to be consistent with an understanding based on the coexistenc e of different reaction channels.