Mechansim Understanding of NOx storage, release and reduction on Pt/doped ceria catalysts

Due to the strengthening of emission legislation both in Europe and North America, there is a need for further optimisation of existing emission after-treatment catalytic converters for automotive applications. High surface area ceria is successfully employed as excellent support of metals (Pd, Ru, Pt, etc.) in commercial catalytic systems for the oxidations of CO and propane and automotive emission control.
Ceria is a unique material with a rich and complex chemistry which shows marked structure sensitive properties than can be assessed through shape-controlled synthesis. It possesses the high oxygen storage capacity (OSC), a unique redox property by the cycle of Ce4+/Ce3+ redox pairs and it can increase the migration of lattice oxygen. The catalytic activity of ceria can be further enhanced by the use of dopants.
Among all, Pt doped ceria catalysts show an enhanced NOx storage at low temperature, together with an improved CO/HC light off. It is not clear whether this is due to the presence of higher number of active sites (dopants create a more favourable environment for NOx storage), same number of sites but intrinsically more active, or enhanced NO oxidation activity (rate determining step).
It has been proposed that the dopants increase the concentration of surface vacancies which affect the ionic conductivity, oxygen mobility and oxygen storage capacity of the CeO2. It can be speculated that all these properties are responsible for the enhanced oxidation activity (CO, HC, NO) by promoting oxygen diffusion and the formation of more "reactive oxygen" species. Moreover, the oxygen vacancies could play a role in the mechanism of the reaction, favoring the NOx storage.
It has also been found that the purge efficiency of doped systems is lower compared to the un-doped catalyst. This is related to the different surface intermediates species observed during preliminary NOx storage IR experiments. However, it is unknown how this relates to the presence of dopants (modification of the rate of the redox reaction, different electronic environment which allows stronger NOx adsorption, ...).
Additionally, presence of dopants modifies the Pt reducibility and Pt - CeO2 interaction, allowing more readily activation during rich purge and increased stability to lean deactivation. This is related to:
Improved reducibility after a rich activation treatment
Pt is not fully re-oxidised after the lean treatment and can be easily re-activated when exposed to the reductant present in the gas feed again
Further characterisation is necessary to probe the enhanced promoting effect, especially after the reducing pre-treatment and greater stability to oxidising condition.
Aim of project/expected outcome:
1. Improve the understanding of Pt doped ceria NOx storage mechanism together with the mechanism of rich purge.
2. Gain more knowledge of the rich activation/lean deactivation mechanisms.
3. Determine the structure of the active sites under reaction conditions and differentiate between active species and spectators by transient methods.
4. Development of a global kinetic model.