CATALYTIC DECOMPOSITION OF NITROUS-OXIDE OVER RU-EXCHANGED ZEOLITES

We report that ultrastable faujasite-based ruthenium zeolites are highly active catalysts for N2O decomposition at low temperature (120-200-degrees-C). The faujasite-based ruthenium catalysts showed activity for the decomposition of N2O per Ru3+ cation equivalent to the ZSM-5 based ruthenium catalysts at much lower temperatures (TOF at 0.05 vol.-% N2O: 5.132 x 10(-4) s-1 Ru-1 of Ru-HNaUSY at 200-degrees-C versus 5.609 x 10(-4) s-1 Ru-1 of Ru-NaZSM-5 at 300-degrees-C). The kinetics of decomposition of N2O over a Ru-NaZSM-5 (Ru: 0.99 wt.-%), a Ru-HNaUSY (Ru: 1.45 wt.-%) and a Ru-free, Na-ZSM-5 catalyst were studied over the temperature range from 40 to 700-degrees-C using a temperature-programmed micro-reactor system. With partial pressures of N2O and O2 up to 0.5 vol.-% and 5 vol.-%, respectively, the decomposition rate data are represented by: -dN2O/dt = k(P(N2O)) (P(O2))-0.5 for Ru-HNaUSY, - dN2O/dt = k(P(N2O)) (P(O2))-0.1 for Ru-NaZSM-5, and -dN2O/dt = k(P(N2O)-0.2(P(O2))-0.1 for Na-ZSM-5. Oxygen had a stronger inhibition effect on the Ru-HNaUSY catalyst than on Ru-NaZSM-5. The oxygen inhibition effect was more pronounced at low temperature than at high temperature. We propose that the negative effect of oxygen on the rate of N2O decomposition over Ru-HNaUSY is stronger than Ru-NaZSM-5 because at the lower temperatures (< 200-degrees-C) the desorption of oxygen is a rate-limiting step over the faujasite-based catalyst. The apparent activation energy for N2O decomposition in the absence of oxygen is much lower on Ru-HNaUSY (E(a): 46 kJ mol-1) than on Ru-NaZSM-5 (E(a): 220 kJ mol-1).