NOx reduction in the Di-Air system over noble metal promoted ceria

In this study, the role of the noble metals Pt and Rh (0.5 wt.%) for the selective reduction of NO into N-2 is evaluated by the transient TAP technique and in-situ spectroscopy using a commercial stable ceria support (denoted as CZ) and applying isotopically labelled (NO)-N-15 and O-18(2). The transient operation was mimicked by multi-pulse oxidation (using O-2 or NO) and reduction cycles (using CO, H-2, C3H6 and C3H8), while following quantitatively the catalyst and reactants response. Pt and Rh significantly lowered the temperature of CZ reduction. CO and H-2 only reduce the surface of CZ, while a 2.5 times deeper reduction was achieved by the hydrocarbons C3H6 and C3H8, removing also lattice oxygen. Pt and Rh also promoted carbon deposition after surface reduction. Rh was a more active promoter than Pt, while propene was more reactive than propane over both metals. During the NO reduction the pre-reduced CZ support became gradually re-oxidised and after filling 70-80% of the oxygen vacancies the NO started to appear in the product mixture. In the presence of carbon deposits the lattice oxygen of the CZ reacted with the carbon keeping the CZ in a reduced state, extending the NO decomposition process as long as the carbon was present. The reduction of NO over pre-reduced noble metal/CZ showed a selective formation N-2, while N2O and NO2 were never observed. During the NO reduction process some unidentified N-species remained on the catalyst, the amount depending on the type of catalyst, but finally all nitrogen was released as N-2. The presence of the noble metal led less unidentified N-species on the CZ surface and to a faster N-2 formation rate than that over the bare CZ.