Catalytic decomposition of nitrous oxide over Ru-exchanged zeolites |
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Authors: | Y -F Chang J G McCarty E D Wachsman and V L Wong |
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Affiliation: | Materials Research Center, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025, USA |
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Abstract: | We report that ultrastable faujasite-based ruthenium zeolites are highly active catalysts for N2O decomposition at low temperature (120–200°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 × 10?4 s?1 Ru?1 of Ru-HNaUSY at 200°C versus 5.609 × 10?4 s?1 Ru?1 of Ru-NaZSM-5 at 300°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°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=itk(PN2O) (PO2)?0.5 for Ru-HNaUSY, ?dN2O/dt=k(PN2O) (PO2)?0.1 for Ru-NaZSM-5, and ?dN2O/dt=k(PN2O)?0.2 (PO2)?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°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 (Ea: 46 kJ mol?1) than on Ru-NaZSM-5 (Ea: 220 kJ mol?1). |
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Keywords: | Catalyst preparation (ion exchange) Zeolites Ruthenium zeolites Faujasites Nitrous oxide decomposition Hexammineruthenium Ru-HNaUSY (ultrastable Y) Oxygen effect |
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