Decomposition and reduction of N_2O over copper catalysts

摘要

The kinetics of N_2O decomposition were investigated over Cu/carbon, Cu/al_2O_3, Cu/SiO_2, and Cu/ZSM-5 catalysts with the intent of monitoring the requirements for achieving stoichiometric catalytic decompotistion over Cu and elucidating the role of the support in the catalytic cycle. The very efficient oxygen scavenging capability of carbon led to higher initial activities and turnover frequencies on Cu/C catalysts compared to Cu/ZSM-5; however, rapid gasification of the carbon support limited the lifetime of these Cu/C catalysts. Very low decomposition activity was exhibited by Cu/Al_2O_3 and Cu/SiO_2 at temperatures below 823 K; however, addition of CO or H_2 to the reactor feed stream significantly lowered the temperatures required to achieve catalytic N_2O reduction. With all catalysts, the temperature at which the product ratio in the effluent stream stabilized near the stoichiometric ratio of O_2/N_2 (or CO_2/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 173 K on these catalysts indicated that both Cu~(+1) and Cu~(+2) species coexist during the active phase, as expected for a redox mechanism requiring a balance between these two sites. In contrast, spectra of deactivated catalysts indicated that the Cu sites exist predominantly as Cu~(+2 species, with only a very small Cu~(+1) fraction thus suggesting that the deactivation observed in theses catalysts at lower temperatures is primarily due to the inability to reduce Cu~(+2) cations back to a Cu~(+1) state. Different kinetic rate expressions, derived from sequences of elementary steps appropriate for each catalyst, fit the data well for N_2O decomposition on Cu/ZSM-5 and Cu/Al_2O_3 as well as N_2O reduction by carbon and CO, and they yielded meaningful values for enthalpies and entropies of adsorption for N_2O, O_2 and CO.