Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH3 in the presence of O-2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458 < T-R < 513 K, 250 < NO (ppm) < 3000, 1000 < NH3 (PPM) < 4000, 1 < O-2 (%) < 4. The kinetic orders were 0.8-1 with respect to NO, 0.5-1 with respect to O-2, and essentially 0 with respect to NH3. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH3 was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu-I is oxidized by O-2 to "Cu-II-oxo", (ii) "Cu-II-oxo" reacts with NO to yield "Cu-II-NxOy", and (iii) finally "Cu-III-NxOy" is reduced by NH3 to give N-2, H2O, and the regeneration of Cu-I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu-I or "Cu-II-oxo" species constitute the rate-determining active center. (C) 2004 Elsevier B.V. All rights reserved.