Performances of SOx traps derived from Cu/Al hydrotalcite for the protection of NOx traps from the deactivation by sulphur

The behavior of Cu/Al mixed oxides (Cu/Al ratio in the 1:2–1:5 range) have been studied as novel, noble-metal free SOx traps to protect NOx traps from the deactivation by sulphur. The investigation was made both in a thermobalance apparatus and in a flow reactor, the latter simulating the reaction conditions and space-velocities of exhaust gases from lean-burn or diesel engines. The analysis of the SO2 uptake curves as a function of the Cu/Al ratio and reaction temperature indicates that the reaction mechanism of SO2 uptake depends on two reversible surface processes (the chemisorption of SO2 and its oxidation by copper ions) and a nearly irreversible process (bulk diffusion of the sulphate species). The rate of these processes depends on (i) the Cu/Al ratio and nature of the surface copper species, (ii) the surface area of the catalyst, (iii) the reaction conditions, and (iv) the degree of sulphation. The SOx traps showing the best performances in thermogravimetric tests were found also to show the best behavior in flow reactor tests, confirming the validity of thermogravimetric tests, notwithstanding the different composition of the feed used. The SOx trap having a Cu/Al ratio of 1:2 shows better performances with respect to a reference ‘‘state-of-the-art’’ SOx trap containing 2% Pt.