IN-SITU XANES CHARACTERIZATION OF THE CU OXIDATION-STATE IN CU-ZSM-5 DURING NO DECOMPOSITION CATALYSIS

The oxidation state of Cu in Cu-ZSM-5 has been investigated by the X-ray absorption near-edge structure (XANES) spectroscopic method during NO decomposition catalysis. We designed an in situ reactor system with which we can measure the relative NO decomposition rate while taking XANES spectra. We observed that the 1s --> 4p electronic transition of Cu(I) in Cu-ZSM-5 appears as a narrow, intense peak which is an effective measure of changes in the population of copper oxidation states. This transition is quite intense after Cu-ZSM-5 is activated in inert gas flow. However, its intensity decreases but by no means disappears after the admission of a NO/N2 gas mixture. We conducted the reaction in a temperature cycle around the optimum conversion temperature of 773 K and recorded the XANES at each temperature. We observed that the integrated intensity of the Cu(I) 1s --> 4p transition, which is proportional to the cuprous ion concentration in Cu-ZSM-5, was well correlated with the NO decomposition rate. This finding supports the conjecture that Cu(I) participates in a redox mechanism during catalyzed NO decomposition in Cu-ZSM-5 at elevated temperature.