The impact of urea on the performance of metal-exchanged zeolites for the selective catalytic reduction of NO_x-Part II. Catalytic, FTIR, and NMR studies

The selective catalytic reduction (SCR) with urea over metal-exchanged zeolites is one of the most promising contenders for the reduction of NOx emissions in diesel exhaust. However, one major concern – a possible deactivation of the catalyst by urea deposits – has not been investigated sufficiently as yet. The formation of urea deposits on the catalyst after long-term operation was simulated by impregnating Fe–beta zeolite powder with 32.5 wt.% aqueous urea solution followed by hydrothermal aging at temperatures between 250 and 750 °C. The SCR activity was measured after each hydrothermal aging step. It could be shown that Fe–beta can be reversibly deactivated by urea deposits such as cyanuric acid and ammelide (as identified by ATR-FTIR) if the impregnated catalyst was hydrothermally aged at temperatures not higher than 250 °C. Upon steaming at 500 °C a complete regeneration of the SCR activity as well as a total decomposition of urea deposits left on the catalyst was observed. In addition, MAS 27Al and 29Si NMR experiments clearly show that no detrimental changes of the Si/Al zeolite framework were induced by urea-impregnation followed by steaming at 750 °C compared to the dealumination observed for hydrothermal aging alone. Thus, the urea-induced deactivation is fully reversible since no permanent structural changes in the zeolite could be identified