Decomposition and reduction of N2O over copper catalysts

The kinetics of N2O decomposition were investigated over Cu/carbon, Cu/Al2O 3, Cu/SiO2, and Cu/ZSM-5 catalysts with the intent of monitoring the requir ements for achieving stoichiometric catalytic decomposition over Cu and elu cidating the role of the support in the catalytic cycle. The very efficient oxygen scavenging capability of carbon led to higher initial activities an d turnover frequencies on Cu/C catalysts compared to Cu/ZSM-5; however, rap id gasification of the carbon support limited the lifetime of these Cu/C ca talysts. Very low decomposition activity was exhibited by Cu/Al2O3 and Cu/S iO2 at temperatures below 823 K; however, addition of CO or H-2 to the reac tor feed stream significantly lowered the temperatures required to achieve catalytic N2O reduction. With all catalysts, the temperature at which the p roduct ratio in the effluent stream stabilized near the stoichiometric rati o of O-2/N-2 (or CO2/N-2) = 1/2 was found to correspond to that at which O- 2 either desorbs or is removed by carbon. DRIFT spectra of CO adsorbed at 1 73 K on these catalysts indicated that both Cu+1 and Cu+2 species coexist d uring the active phase, as expected for a redox mechanism requiring a balan ce between these two sites. In contrast, spectra of deactivated catalysts i ndicated that the Cu sites exist predominantly as Cu+2 species, with only a very small Cu+1 fraction, thus suggesting that the deactivation observed i n these catalysts at lower temperatures is primarily due to the inability t o reduce Cu+2 cations back to a Cu+1 state. Different kinetic rate expressi ons, derived from sequences of elementary steps appropriate for each cataly st, fit the data well for N2O decomposition on Cu/ZSM-5 and Cu/Al2O3 as wel l as N2O reduction by carbon and CO, and they yielded meaningful values for enthalpies and entropies of adsorption for N2O, O-2 and CO. (C) 1999 Elsev ier Science B.V. All rights reserved.