The influence of CO₂ on NO reduction into N₂ over reduced ceria-based catalyst

Oxygen defects in reduced ceria are the catalytic sites for the NO reduction into N₂ in the Toyota Di-Air DeNO_x abatement technology. Traces of NO (several hundred ppm) have to compete with the excess amount of other oxidants, e.g., 5% CO₂ and 5% O₂, in an exhaust gas of a lean burn (diesel) engine. The reactivities of CO₂ and NO over a reduced ceria and noble metal loaded reduced ceria have been investigated under ultra-high vacuum system in TAP and under atmosphere pressure in in-situ Raman and flow reactor set-up. The results showed that CO₂ was a mild oxidant which was able to oxidise the oxygen defects, but hardly oxidised deposited carbon over both ceria and noble metal loaded ceria. NO was a stronger oxidant and more efficient in refilling the oxygen defects and able to convert the deposited carbon, which acted as buffer reductant to extend the NO reduction time interval. NO was selectively and completely converted into N₂. The presence of excess CO₂ hardly affected the NO reduction process into N₂.