Selective catalytic reduction of NO 2 with urea in nanocrystalline NaY zeolite

Abstract

In this study, the selective catalytic reduction (SCR) of NO 2 with urea in nanocrystalline NaY zeolite was investigated with in situ transmission Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance spectroscopy. At T = 473 K , the reaction rate for urea-SCR of NO 2 in nanocrystalline NaY zeolite was significantly greater than that in commercial NaY zeolite with a larger crystal size. In addition, a dramatic decrease in the concentration of undesirable surface species, including biuret and cyanuric acid, was observed in nanocrystalline NaY compared with commercial NaY after urea-SCR of NO 2 at T = 473 K . The increased reactivity for urea-SCR of NO 2 was attributed to silanol groups and extra-framework aluminum species located on the external surface of nanocrystalline NaY. Specifically, NO x storage as nitrate and nitrite on the internal zeolite surface was coupled to reactive deNO x sites on the external surface. Isotopic labeling combined with IR analysis suggest that N N bond formation involved both an N-atom originating from NO 2 and an N-atom originating from urea. This is the first clear example demonstrating that the increased external surface area (up to ∼40% of total surface area) of nanocrystalline zeolites can be used as a reactive surface with unique active sites for catalysis.