Experimental Studies of NOx Catalysts; Storage, Reduction and Sulphur Deactivation
The aim of this thesis work was to investigate how different gaseous compounds, present in an automotive exhaust from lean-tuned combustion, interact with so-called NOx storage catalysts containing Al2O3 as a support, BaO as the NOx storage compound and noble metals to provide oxidation and reduction functions. The investigations included model sample preparations, flow reactor studies with various gas mixtures and spectroscopic experiments (FTIR and XPS).
The results show that several of the exhaust gas components (CO2, NOx, SO2, O2) interact strongly with the catalyst, and both the storage compound (BaO) and the noble metals are affected. Due to the affinity of barium oxide for sulphate formation, and since sulphur is present in significant amounts in gasoline, diesel and lubricants, special attention was paid to sulphur compounds in the exhaust gas. The sulphur studies indicated that deactivation (poisoning) occurs in both net oxidising and net reducing gas mixtures containing SO2. After exposure in oxygen excess significant amounts of sulphates were detected with XPS and FTIR. Sulphates were also detected during net reducing sulphur exposure, but in much smaller amounts.
It is suggested that the mechanism for sulphur deactivation under net oxidising and net reducing conditions are different. During lean conditions sulphate formation on the NOx storage sites on BaO seems to be the dominant mechanism. The results obtained during rich conditions, with SO2 in the gas mixture, suggest a mechanism dominated by deactivation of the NO oxidation and NOx reduction functions of the catalyst, e.g. by formation of (reduced) sulphur species on the noble metal particles.