Characterisation by TPR, XRD and NOx storage capacity measurements of the ageing by thermal treatment and SO2 poisoning of a Pt/Ba/Al NOx-trap model catalyst

The deactivation of a Pt/Ba/Al2O3 NOx-trap model catalyst submitted to SO2 treatment and/or thermal ageing at 800 degreesC was studied by H-2 temperature programmed reduction (TPR). X-ray diffraction (XRD) and NOx storage capacity measurements. The X-ray diffractogram of the fresh sample exhibits peaks characteristic for barium carbonate. Thermal ageing leads to the decomposition of barium carbonate and to the formation of BaAl2O4. The TPR profile of the sulphated sample shows the presence of (i) surface aluminium sulphates, (ii) surface barium sulphates, (iii) bulk barium sulphates. The exposure to SO, after ageing leads to a small decrease of the surface barium-based sulphates, expected mainly as aluminate barium sulphates. This evolution can be attributed to a sintering of the storage material. TPR experiments also show that thermal treatment at 800 degreesC after the exposure to SO2 involves the decomposition of aluminium surface sulphates to give mainly bulk barium sulphates, also pointed out by XRD. Thus, the thermal treatment at 800 degreesC leads to a stabilization of the sulphates. These results are in accordance with the NOx storage capacity measurements. On non-sulphated catalysts, the treatment at 800 degreesC induces to a decrease of the NO, storage capacity, showing that barium aluminate presents a lower NOx storage capacity than barium carbonate. Sulphation strongly decreases the NOx storage capacity of catalysts. whatever the initial thermal treatment. showing that barium sulphates inhibit the NOx adsorption. Moreover, the platinum activity for the NO to NOx oxidation is lowered by thermal treatments.