The impact of urea on the performance of metal-exchanged zeolites for the selective catalytic reduction of NOx-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 NO, 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 degrees 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 degrees C Upon steaming at 500 degrees 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 (27) AI and Si-29 NMR experiments clearly show that no detrimental changes of the Si/Al zeolite framework were induced by urea-impregnation followed by steaming at 750 degrees 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 (C) 2010 Elsevier B.V All rights reserved