Direct NO and N2O decomposition and NO-assisted N2O decomposition over Cu-zeolites:: Elucidating the influence of the Cu-Cu distance on oxygen migration

Several zeolites of varying topology and with different Cu/Al ratios were investigated in the catalytic decomposition of NO, N2O and the NO-assisted N2O decomposition. The highest activity in the direct NO and N2O decomposition was found for bis(mu-oxo)dicopper cores in Cu-ZSM-5 followed by the EPR silent Cu in MOR, FER and BEA, while almost no activity was observed over the isolated, EPR detectable Cu sites. This sequence of decreasing activity follows the increasing average distance between Cu sites. An average volume of 35 angstrom(3) per Cu atom (average Cu-Cu distance 4.1 angstrom) appears to be a threshold value separating high and low activity. The high activity for catalysts with small Cu-cu distances is explained by facile oxygen migration over the Cu sites, enabling recombination into gaseous O-2. When the distance between the Cu centers is increased, oxygen migration is hampered. Adding NO results in the scavenging of the deposited O atoms, thereby transporting them into the gas phase. Hence an alternative oxygen migration pathway is created that has the greatest impact on activity of the isolated EPR-detectable Cu sites and a negative effect on the bis(mu-oxo)dicopper cores in Cu-ZSM-5. (c) 2006 Elsevier Inc. All rights reserved.