Mechanistic Study of NO Oxidation on Copper Zeolites
Abstract
The objective of this work was to identify the nature of the active intermediate for the NO oxidation reaction on copper (Cu) exchanged ZSM-5 zeolite. A combination of Diffuse Reflectance (DRIFTS) and Operando Transmission Fourier Transform Infrared Spectroscopy (FTIR) was used for this purpose. In situ DRIFTS studies revealed that three types of nitrates, 1626 cm-1, 1600 cm-1 and 1565 cm -1, were formed only on the surfaces of those catalysts that were active for NO oxidation. Inactive samples did not show any nitrate formation. The total amount of stored nitrates and the NO oxidation reaction rate, both followed similar trends with varying amounts of copper loading and water content in the feed stream. Hence, nitrates were important intermediates for the NO oxidation reaction. In addition, diamond dust was shown to be a good scatterer for future experiments with dark coloured samples that absorb infrared radiation. A custom-designed operando FTIR reactor with very small internal volume was used for transmission infrared spectroscopy experiments. NO oxidation was used as a simple probe reaction to validate the fast switching FTIR - MS system. The apparent activation energies measured in this novel FTIR reactor and in a separate plug flow reactor (PFR) were very similar. Space velocity was varied by changing the total flow rate and the amount of sample loaded into the reactor, to prove that there were no external mass transfer limitations. These results confirmed that the chemistry for the NO oxidation reaction was consistent in both reactors, thus allowing correlation of results from the two. This setup was used for steady state kinetic measurements along with transient isotope switching experiments. The latter allowed us to follow the isotopic label simultaneously on the catalyst surface using FTIR, and in the effluent phase using a mass spectrometer (MS), furnishing important information about the reaction mechanism. 14N 16O→15N18O switching experiments on multiple Cu/ZSM-5 samples showed that the peak area of the bridged/bidentate nitrate observed at 1626 cm-1 tracked with the 15N containing NO2 product. Thus, it was shown that nitrates are relevant intermediates, for the NO oxidation reaction and that the bridged/bidentate nitrate at 1626 cm -1 is a likely intermediate.
Degree
M.S.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
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