Isotopic study of NOx decomposition over Cu- or Co-exchanged ZSM-5 zeolite catalysts

The decomposition of NOx over Cu2+- and Co2+-exchanged ZSM-5 zeolites was investigated using isotope labeled (NO)-N-15-O-18 and temperature-programmed desorption (TPD) technique. We found that formation of NO2 intermediates is responsible for enhancing the rate of NOx decomposition when the catalysts were treated in both (NO)-N-15-O-18 and O-2. Co-adsorption of (NO)-N-15-O-18 and O-2 led to a dramatic increase in the amount of NOx and O-2 adsorbed. The surface NOx species decomposed or desorbed to form NO, O-2, N-2, and NO2 during the TPD. This results in formation of equal amounts of (NO)-N-15-O-16 and (NO)-N-15-O-18 in NO products and 50% (NOO)-N-15-O-16-O-18, 25% (NO2)-N-15-O-16, and 25% (NO2)-N-15-O-18 in NO2 products. In the absence of O-2, formation of N2O was observed at <300 degrees C but not NO2. For Cu-NaZSM-5, its N2O products are exclusively (N2O)-N-15-O-18, suggesting that lattice oxygen is not involved in the formation of N2O (it is derived solely from (NO)-N-15-O-18) and the isotope exchange between (N2O)-N-15-O-18 With the zeolite lattice oxygen (O-16) is not significant. However, for Co-ZSM-5, formation of either N2O or NO2 is negligible with (NO)-N-15-O-18 treatment. Treatment in both (NO)-N-15-O-18 and O-2 resulted in some formation of N2O, consisting of equal amounts of (N2O)-N-15-O-16 and (N2O)-N-15-O-18, suggesting either isotopic exchange occurred between N2O and lattice oxygen, or between NO and lattice oxygen, or formation of N2O involved lattice oxygen. Except that the low-temperature activity for N-2 formation for Cu-NaZSM-5 treated in (NO)-N-15-O-18 alone is due to formation of N2O, the majority of activity for N-2 formation is due to formation of NO2 intermediates. The main difference between Cu-NaZSM-5 and Co-NaZSM-5 is that for Cu-NaZSM-5, in addition to the low-temperature activity shared by Co-NaZSM-5, it showed significant activity for nitric oxide decomposition at high temperatures (>360 degrees C) while Co-NaZSM-5 showed little. A TOF of 2.88 x 10(-4) s(-1). Cu-1 for Cu-NaZSM-5 at 122 degrees C and at 0.1 vol.% (NO)-N-15-O-18 and 0.4 vol.% O-2 is even higher than similar to 2 x 10(-4) s(-1). Cu-1 reported in the literature at >350 degrees C, but at a much lower temperature. In terms of Nz formation, Co-NaZSM-5 is about 2-3 times more active than Cu-NaZSM-5 at low temperatures (<200 degrees C). For Cu-NaZSM-5, its activity for NO2 decomposition is approximately 4 times higher than that for NO at 320-370 degrees C. Co-NaZSM-5 does not show any significant activity for NO decomposition but its activity for NO2 decomposition is comparable to Cu-NaZSM-5. (C) 1997 Academic Press, Inc.