Catalytic decomposition of nitric oxide over Cu-zeolites

Abstract

Cu-exchanged zeolites have been under investigation as catalysts for NO decomposition. Among the zeolites tested Cu-ZSM-5 had the highest turnover frequency. The Arrhenius plots for all the catalysts leveled off or inverted near 773 K, and these curves were entirely reversible on decreasing the temperature. The NO decomposition rate was first order in NO pressure and was inhibited by O 2. The kinetics could be described in the Langmuir-Hinshelwood form, with the inhibition being half order in O 2 pressure. The redox capacities of Cu-ZSM-5 measured in microbalance experiments were near 0.5 O/Cu, i.e., 1 ϵ −/Cu. Interestingly, O 2 could be desorbed isothermally from an initially oxidized catalyst upon flushing the catalyst with He. ESR spectra confirmed that the Cu 2+ in the catalysts was being reduced to Cu 1+ during the spontaneous desorption of a corresponding amount of O 2. This spontaneous desorption of O 2 was also observed when pure 02 in the microbalance was replaced by an O 2/He mixture, i.e., when the partial pressure of O 2 over the catalyst was lowered. This phenomenon is regarded as a key requirement for a sustained NO decomposition activity. An interpretation of the kinetics in light of these findings is presented.