The effect of Si/Al ratio and copper exchange level on isothermal kinetic rate oscillations for N2O decomposition over Cu-ZSM-5: a transient FTIR study

Kinetic rate oscillations in the decomposition of N2O over Cu-ZSM-5 were studied over a series of catalysts with varying Si/Al ratios and copper exchange levels. Oscillations were observed to occur over all catalysts with Si/Al≥29 and exchange level >100%, including excessively-exchanged catalysts with Cu/Al>1.0. FTIR spectroscopy showed that the same monodentate nitrate species was present under reaction conditions for all catalysts displaying oscillatory behavior, and that the coverage of this species was correlated to the gas phase oscillations. Catalysts with low Si/Al ratios did not show oscillation due to a combination of factors: (1) additional actives sites exist on these catalysts (possibly Cu ion pairs) that stabilize additional nitrate species, (2) the nitrate species desorb at a lower temperature compared to the other zeolite catalysts, and (3) the formation of nitrate on these catalysts was shown to be an order of magnitude slower than on the catalysts which show oscillations. FTIR-based kinetic studies of nitrate formation verified that the most critical reaction occurs between N2O and extra lattice oxygen in order to form NO, which is converted rapidly to surface nitrate. FTIR also identified a possible intermediate in the formation of nitrate at 1537 cm−1 that has been assigned to monodentate nitrite on Cu2+ ions.