Nitrates and nitrous oxide formation during the interaction of nitrogen oxides with Cu-ZSM-5 at low temperature

The interaction of NO with Cu-ZSM-5 has been investigated in a temperature range (25–225 °C) in which no steady-state reactions occur. The study has been conducted by performing adsorption tests at variable exposure times and analysing the phenomena that occur by continuous monitoring of the gaseous effluents as well as by performing a subsequent TPD. The effect of temperature, catalyst pre-treatment and composition of the adsorbing mixture (NO/He, NO/O2/He, N2O/He and N2/He) have been evaluated. In situ FTIR experiments have been also carried out under similar conditions.The main observed phenomena are: (i) reduced Cu+ sites are quickly re-oxidized by NO producing large amounts of N2O; (ii) adsorbed NO slowly reacts with gaseous NO to give surface nitrates and still N2O in the gas phase regardless the copper oxidation state. A mechanism of copper re-oxidation and nitrates formation under the different conditions explored has been proposed on the basis of these findings.

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► Cu-ZSM-5 adsorbs large amounts of NO in the temperature range 25–225 °C which undergo surface transformations.
► Reduced Cu+ sites are quickly re-oxidized by NO producing large amounts of N2O.
► Adsorbed NO slowly reacts with gaseous NO giving surface nitrates and N2O regardless the Cu oxidation state.
► Both mechanisms could involve either Cu monomers or copper pairs.