Impact of thermal and engine ageing on surface morphology and performance of a fully formulated Lean NOx Trap

Dihya  Adouane, Universite Pierre et Marie Curie

Lean NOx traps (LNTs) have shown a great deal of promise for reducing NOx in the highly lean exhausts from diesel and gasoline lean burn engines. LNT is formulated with a storage component, usually alkali or alkaline earth metals such as barium and precious metals such as Pt dispersed over a high surface area gamma-alumina washcoat. These catalysts typically operate in cyclic conditions alternating between lean and rich environment leading to high NOx reduction efficiency and CO, HC conversion. However, performance degradation of catalysts is observed over engine lifetime due to high temperatures exposure and sulfur poisoning.

In this work, we study the influence of thermal ageing on fully formulated Lean NOx Trap and the impact of variation of catalyst composition (PGM/ Ba/ Ce loading) on both thermal ageing and catalyst performances using laboratory and engine bench ageing. Our objective was to begin developing a model for characterizing the main features of LNT performance versus its mileage ageing. However, characterizations are relevant only if they can be used to predict observed trends in engine experiments.

The catalyst used in this study is a commercial Lean NOx-Trap dedicated to Diesel vehicles similar to the production LNT used on some vehicles in Europe today.
Ageing in oven occurs in presence of 10% of H2O and 90% of air. The catalysts were aged in monolithic form for 2h750C, 5h750C, 10h750C and 5h800C.
The ageing process was also carried out in engine bench. Diesel motor was equipped in system of sensor for temperature, pressure and lambda control and connection with Horiba analyzer for pollution measurements. The ageing was carried out in the following conditions of engine: 3000 rpm, 290 Nm for 2h750C, 5h750C, 10h750C and 5h800C.
Then the effect of thermal ageing on surface morphology of the LNT was investigated using TEM (Transmission electron microscopy) and SEM (Scanning electron microscopy), TPR/TPO (Temperature programmed reduction/Oxidation), XRD, N2 physisorption.
To further more understand the influence of each compound on catalyst performances and thermal ageing, a variation of the initial formulation of NOx Trap was studied as follows: without metal, monometallic (Pt), monometallic (Pd), bimetallic(Pt/Pd), Ce/2, Ba/2. The performances of the different catalysts were evaluated on synthetic gas bench in monolithic form for oxidation capacity, NOx storage capacity, oxygen storage capacity, reduction and conversion efficiency for a temperature range between 160C and 450C. The gas mixture was chosen as representative of the real exhaust gas.

NO oxidation efficiency was found to be more important on monometallic (Pt) than monometallic (Pd). The results show also that oxidation efficiency is higher on bimetallic catalyst (Pt/Pd) than monometallic (Pt) alone which means that presence of Pd increases the ability of Pt to oxidize NO to NO2 due to probably a synergic effect between Pt and Pd. It was observed that exposure of catalysts to high temperatures causes the growth of platinum particles size which seems to be higher in case of engine bench than in oven.

It was also observed that NO oxidation efficiency increase after thermal ageing. This could be explained by the fact that Pt is mainly present as a mixture of Pt/Ptx+(oxidative state) on the fresh catalyst and as Pt (metallic state) on the aged catalysts.
Compared to the fresh catalyst, the TPR profiles are deeply modified by the ageing treatments especially for ceria. The ceria seems to sinter more than alumina, particularly after thermal ageing on engine bench.

The results will be used to develop a numerical model including impact of thermal ageing on catalyst performances simulating real emissions certification test cycle.