Title : Tuning zeolitic parameters for the trapping and elimination of pollutants and toxic gases - application in automotive cold-start and nuclear safety
The development of new trapping and abatement strategies is key in order to avoid the dissemination of harmful pollutants and toxic substances into the environment. By comparison with physisorption-related processes, irreversible trapping generally involves chemisorption or catalytic reactions of the targeted molecules on the active sites of specifically-designed materials. Well-known applications include for instance the trapping of NOx and SOx or the catalytic elimination of diesel soot in DPF. In these last years, new applications are emerging. A key target to reduce current hydrocarbon (HC) emissions from automotive exhausts is to improve their abatement under cold-start conditions, which mostly arise during the first two minutes of the driving cycle. One of the most promising after-treatment technology is to trap the HC at low temperatures on a sorbent, typically a zeolitic material, which can be combined with an oxidation or a deNOx catalyst, so that the desorbing HC can be easily burnt at higher temperatures or alternatively reduce nitrogen oxides. In that respect, the zeolitic material has to be not selective to the type of HC (i.e. propene, toluene and decane) and the HC desorption temperature has to be high enough to match with the light-off temperature of the oxidation or DeNOx catalyst. Another emerging application for zeolitic materials is their use as iodine trap in the context of a severe nuclear accident (Fukushima-like). In that respect, it has to be ensured that no radioactive iodine could be released from the filter under dynamic flowing conditions over a long period. Stable trapping could in principle be obtained using specifically-tailored silver zeolites thanks to the formation of AgI precipitates involving the catalytic transformation of CH3I and I2 on silver sites. In both applications, the zeolitic filter has to withstand the presence of inhibitors (H2O, COx, NOx…) and maintain its properties over a wide range of temperatures.
In this presentation, we will see how it is possible to finely tune the zeolitic parameters, such as the pore size and connectivity, the Si/Al ratio, the nature and content of compensating cations in order to meet the above-mentioned requirements in both applications. A screening of several zeolitic sorbents will be presented thanks to a quantitative evaluation of multiple breakthrough curves, TPD and TPSR data. Characterization of the materials before and after test, as well as the use of in situ diffuse reflectance spectroscopies (DRIFTS and DRS-UV-Vis) will provide important insight on the nature of active sites and the complex network of reactions associated with the storage and decomposition of trapped molecules under representative conditions.