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

- CO2 is a mild oxidant in the oxidation of oxygen defects of ceria with and without noble metal presence.
- CO2 was not able to clean up the deposited carbon over the ceria surface which created by fuel pre-treatment.
- Compared to CO2, NO was a much stronger oxidant. The deposited carbon can be oxidized during the NO exposure.
- Traces of NO were able to compete with an excess of CO2 for oxygen vacancies.
- NO is selectively converted to N2 in the presence of an excess of CO2.

Oxygen defects in reduced ceria are the catalytic sites for the NO reduction into N2 in the Toyota Di-Air DeNOx 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.

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