FTIR, XPS and TPR studies of N2O decomposition over Cu-ZSM-5

Abstract

Composition of N2O on Cu-ZSM-5 was studied by temperature-programmed reaction (TPR), in situ Fourier transform infrared spectroscopy (FTIR) and XPS. It is found that the decomposed intermediates of N2O - dinitrogen and oxygen ion - show adsorption bands at 2161 cm-1 and 910 cm-1 in the IR spectrum, which can be assigned respectively to as N-N stretching vibration and a T-O stretching vibration perturbed by Cu2+. Both bands increase in intensity with temperature in the range 25-250 °C. Unlike the band at 2161 cm-1, whose intensity decreases sharply above 250 °C, the band at 910 cm-1 still persists at higher temperatures. The IR results agree with the TPR profiles of N2 and O2:N2 starts to desorb at 250 °C whereas O2 remains until 310 °C, where N2O decomposes. An oscillation in the O2 signal is observed between 400 and 550 °C, along with a more evident but opposite oscillation in the N2O signal. The oscillation of N2O decomposition and the release rates of O2 are found to correlate with the oxidation-reduction of copper sites by XPS. Based on this evidence, Cu+ is proposed to be the active centre for dissociative adsorption of N2O. The removal of adsorbed oxygen ions through recombination as O2 or through interaction with protons trapped in the zeolite cavities may preserve Cu in the +1 oxidation state, which enables continuous decomposition of N2O.