Mechanistic insight in NO trapping on Pd/Chabazite systems for the low-temperature NOx removal from Diesel exhausts
Passive NOx Adsorbers (PNA) are a novel technology designed to reduce NOx emissions during the Diesel engine cold start period by adsorbing them at low temperature and thermally desorbing them once the downstream SCR catalyst becomes active. Despite the growing industrial interest in this technology, the mechanisms behind the NOx adsorption and the effects of the operative conditions still need to be fully elucidated. With this aim, transient response methods were applied to study a well characterized 1% Pd-CHA material for PNA applications. In particular we have investigated the NO adsorption/desorption features of the system, carrying out NO adsorption + TPD tests under different operating conditions and analysing the related mechanistic implications. Specifically, both the NO uptake and the apparent thermal stability of the stored NOx species were studied at different adsorption temperatures, addressing also the role of the main exhaust components (O2, H2O, CO). The experiments have been realized in an experimental rig where a powder catalyst sample was loaded in the test reactor and diluted with cordierite. The reactants were fed to the reactor by pulse valves to simulate step changes in the feed mixture composition. Temporal evolution of the species at the reactor outlet was followed with a mass spectrometer (QGA Hiden Analytical) and a UV analyzer (ABB LIMAS 11 HW) + IR analyzer (ABB Uras 14) arranged in a parallel configuration. The Mass Spectrometer monitored all the species involved in the NOx control processes, such as NH3, H2O, N2, NO, O2, Ar, N2O, NO2, CO and CO2. Instead, the UV analyzer provided an accurate continuous measurement of ammonia, NO, NO2 while the dedicated IR analyzer detected CO and CO2.