The release of NOx stored over LNT catalysts: the first step of the reduction process

The reduction of NOx stored on Lean NOx Traps (LNT) has received a great attention in the last years. It is generally believed that the reduction of stored NOx over NSR catalysts implies at first the release of NOx from the catalyst surface in the gas-phase, followed by the fast reduction of the released NOx to N2 or other products. The mechanisms leading to the release of NOx in the gas phase and their subsequent reduction is still under debate. Three main reasons can be individuated to cause the NOx release: i) the heat generated by the exothermic reactions upon switching to the regenerating gases (thermal release); ii) the decrease in the equilibrium stability of the stored nitrates due to the decrease in the partial pressure of NO and /or oxygen (thermodynamic release); iii) the establishment of a net reducing environment (chemical release). Previous studies from our group pointed out that the last one is a Pt-catalyzed process involving surface species that are reduced prior their thermal desorption. Investigations on the processes leading to the release of NOx are rather scarce in the literature; besides, the NOx release can be hardly separated from the subsequent reduction steps since in the presence of a reductant the released NOx are readily reduced to N2 and other species. Accordingly it is difficult to investigate the NOx release process decoupled from the subsequent reduction step. This is the goal of this work, where 15NO isotopic labeling experiments have been carried out as a technique to study the NOx release decoupled from the NOx reduction. Accordingly NOx have been stored on the catalyst surface starting from unlabelled NO and then exchanged with gas-phase labelled NO. The reactivity of different surface species (nitrites and nitrates) has been investigated, and the role of the noble metal (Pt) and of the presence of reductants have been addressed as well. The results obtained in these experiments have been compared with those collected during the thermal decomposition of the same species