Selective catalytic reduction of NO2 with urea in nanocrystalline NaY zeolite

In this study, the selective catalytic reduction (SCR) of NO2 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 NO2 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 NO2 at T=473 K. The increased reactivity for urea-SCR of NO2 was attributed to silanol groups and extra-framework aluminum species located on the external surface of nanocrystalline NaY. Specifically, NOx storage as nitrate and nitrite on the internal zeolite surface was coupled to reactive deNOx sites on the external surface. Isotopic labeling combined with IR analysis suggest that NN bond formation involved both an N-atom originating from NO2 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.