Over a Cu-ZSM-5 catalyst with a quantified amount of the active Cu2+-dimers (Cu2+-O2.-Cu2+), the kinetics of the catalytic NO decomposition to N2 and O2 was derived on the basis of the proposed reaction mechanism, and such thermodynamic data as adsorption enthalpies of NO and O2 onto the Cu ion dimer sites were evaluated. It was revealed that the enthalpy of the adsorption of NO (ΔH=.34.1 kcal/mol) onto a reduced Cu+-dimer, as the initiating step of NO decomposition catalysis, was higher than that (ΔH=.27.8 kcal/mol) onto an oxidized Cu2+-dimer, or that (ΔH=.27.4 kcal/mol) of the dissociative adsorption of O2 onto the two reduced Cu+-dimers in neighbor. The strong inhibition effect of gas phase oxygen on the kinetic rate of NO decomposition at 400-600 °C could be explained by the thermodynamic predominance of the oxidized Cu2+-dimers against the active reduced Cu+-dimers on the catalyst even at high temperature and under the low partial pressure of oxygen. It was also found that the maximum catalytic activity at temperatures around 500 °C, which was commonly observed in the Cu-ZSM-5 catalyzed NO decomposition reaction, was attributed to the relatively large enthalpy of NO adsorption onto the reduced Cu+-dimers as compared to that of the reaction activation energy (=19.5 kcal/mol), resulting in less favored NO adsorption at the higher temperatures than 500 °C.

NO Decomposition Cu-ZSM-5 Cu-dimers Mechanism Kinetics Enthalpy of Adsorption Activation Energy Thermodynamics

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