Investigation of Adsorbate Reactivity during NO Decomposition over Different Levels of Copper Ion-Exchanged ZSM-5 Using in Situ IR Technique

Abstract

In situ infrared (IR) and mass spectroscopy (MS) coupled with temperature-programmed reaction (TPR), isotopic temperature-programmed desorption (TPD), step transient, and pulse transient techniques have been used to study the dynamic behavior of adsorbed species in the NO decomposition reaction on over- and underexchanged Cu−ZSM-5. Overexchanged Cu−ZSM-5 showed higher NO decomposition activity and produced more Cu+ sites at lower temperatures than the underexchanged sample during the TPR study. Furthermore, overexchanged Cu−ZSM-5 allows rapid equilibrium between gaseous NO and Cu+(NO)/Cu2+(NO3-) during NO decomposition. N2 formation accompanied by the presence of Cu+(NO) suggests that Cu+ initiates the NO decomposition process. However, no direct correlation between Cu+(NO)/Cu+(NO)2 intensity and N2 formation was observed. Adsorbed oxygen from dissociated NO changes the oxidation state of Cu+ ion, causing the formation of Cu2+(NO3-). While Cu2+(NO3-) decomposes to N2, N2O, NO2, and O2 during TPD, it is only partially responsible for the formation of O2 during NO decomposition. Isotopic study shows that adsorbed oxygen on Cu−ZSM-5 desorbs during the pulse NO reaction. These results demonstrate the presence of two pathways for O2 formation: oxygen produced from the decomposition of Cu2+(NO3-) and oxygen from the desorption of adsorbed oxygen on Cu−ZSM-5.