Synthesis and functionalization of zeolites for NH3-SCR applications
In the strive towards decreasing carbon dioxide emissions from the growing transport sector, the interest for more fuel-efficient combustion engines operating under lean (oxygen excess) conditions has increased. The combustion products, however, formed in such engines contribute considerably to air pollution. Nitrogen oxides (NOx) and particulate matter (PM) are the major toxic components together with carbon monoxide (CO) and hydrocarbons (HC) that need to be regulated. Selective catalytic reduction with ammonia (NH3-SCR) is an efficient method to reduce NOx under lean conditions. Metal-exchanged zeolites are proven to be active catalysts for this process. One of the major requirements for the practical application of zeolites for NOx reduction, which is not yet sufficient, is their durability under hydrothermal conditions. Another problem of SCR systems for vehicles is the relatively low catalytic activity at low temperatures leading to that most of the emitted NOx origins from cold low-load start-up, and short travelling distances. Therefore, further development of the currently used NH3-SCR catalysts is of high practical as well as scientific interest.
In the present work, different strategies to improve ion-exchanged zeolites as NH3-SCR catalysts are investigated. Special attention is paid to modifying the zeolite synthesis conditions, choice of metal for ion-exchange with and effect of method for zeolite functionalization. In particular, zeolites with MFI, CHA and *BEA framework structures have been ion-exchanged with iron and copper by aqueous ion-exchange (AIE), wet impregnation (WI), solid state ion-exchange in air (SSIE) and solid state ion-exchange facilitated by NH3 and NO ([NH3+NO]-SSIE). The catalytic properties of the prepared samples were tested for various reactions relevant to NH3-SCR conditions, namely NH3-SCR, NH3 oxidation, and NO oxidation. Moreover, the physicochemical properties of the samples were characterized by XRD, SEM, TEM, XPS, N2 sorption, ICP-AES, XRF, UV-Vis, NH3-TPD, SSNMR and DRIFTS.
The synthesis conditions as well as choice of the metal and method of functionalization were all shown to be important for the structural and catalytic properties of the functionalized zeolites. Among the methods for functionalization, [NH3+NO]-SSIE generally leads to the most active catalysts. Moreover, the zeolite functionalization with transition metal ions was shown to significantly enhance the NH3-SCR activity. Cu-exchanged zeolites show high NH3-SCR activity over a broad temperature range, especially at low temperatures, while Fe-exchanged zeolites are more active for NH3-SCR at high temperatures. Additionally, sequential ion-exchange with both iron and copper leads to catalysts with high activity over broad temperature range. Furthermore, it was found that Fe2+ species rather than Fe3+ species, and similar, Cu+ species rather than Cu2+ species are beneficial for NOx reduction in NH3-SCR.