A Kinetic Study of NO decomposition on Cu-ZSM5

The mechanism of NO decomposition has been investigated by carrying out a kinetic study on over-exchanged Cu-ZSM5 zeolite at Si/Al = 80. NO decomposition tests has been performed in experimental conditions close to those interesting in the applications. In order to test several rate equations in fitting the experimental data, it has been developed a specific numerical code based on a derivative-free minimisation algorithm. The kinetic model results in steady-state mass balance equations in the gas and solid phase for an isothermal plug flow reactor. The generation term in the differential mass balance equations is constituted by the decomposition rate of NO, and the contemporary presence of NO oxidation, producing NO2 in thermodynamic equilibrium with NO and O2 has been also taken into account. An intrinsic second order dependence on NO partial pressure was found together with an inhibiting effect of O2. Among different mechanistic rate equations, the best fit was obtained by an overall decomposition kinetics, generated by a five-step reaction mechanism based on the reversible adsorption of NO on the catalyst, the consequent partial reduction to N2O and copper sites oxidation, N2O fast decomposition and oxygen reversible desorption. The parametric identification gives a correct dependence of the estimated parameters on temperature: the kinetic constants are monotonously dependent on temperature, even in the presence of a maximum in NO decomposition rate, evidenced by the experimental results and several literature references.