Isotopic Study of NOxDecomposition over Cu- or Co-Exchanged ZSM-5 Zeolite Catalysts

Abstract

The decomposition of NOxover Cu2+- and Co2+-exchanged ZSM-5 zeolites was investigated using isotope labeled15N18O and temperature-programmed desorption (TPD) technique. We found that formation of NO2intermediates is responsible for enhancing the rate of NOxdecomposition when the catalysts were treated in both15N18O and O2. Co-adsorption of15N18O and O2led to a dramatic increase in the amount of NOxand O2adsorbed. The surface NOxspecies decomposed or desorbed to form NO, O2, N2, and NO2during the TPD. This results in formation of equal amounts of15N16O and15N18O in NO products and 50%15N16O18O, 25%15N16O2, and 25%15N18O2in NO2products. In the absence of O2, formation of N2O was observed at <300°C but not NO2. For Cu–NaZSM-5, its N2O products are exclusively15N218O, suggesting that lattice oxygen is not involved in the formation of N2O (it is derived solely from15N18O) and the isotope exchange between15N218O with the zeolite lattice oxygen (16O) is not significant. However, for Co–ZSM-5, formation of either N2O or NO2is negligible with15N18O treatment. Treatment in both15N18O and O2resulted in some formation of N2O, consisting of equal amounts of15N216O and15N218O, 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 N2formation for Cu–NaZSM-5 treated in15N18O alone is due to formation of N2O, the majority of activity for N2formation is due to formation of NO2intermediates. 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°C) while Co–NaZSM-5 showed little. A TOF of 2.88×10−4s−1·Cu−1for Cu–NaZSM-5 at 122°C and at 0.1 vol.%15N18O and 0.4 vol.% O2is even higher than ∼2×10−4s−1·Cu−1reported in the literature at >350°C, but at a much lower temperature. In terms of N2formation, Co–NaZSM-5 is about 2–3 times more active than Cu–NaZSM-5 at low temperatures (<200°C). For Cu–NaZSM-5, its activity for NO2decomposition is approximately 4 times higher than that for NO at 320–370°C. Co–NaZSM-5 does not show any significant activity for NO decomposition but its activity for NO2decomposition is comparable to Cu–NaZSM-5.