STATE OF COPPER IN A WORKING, LOW-CONCENTRATION CUH-ZSM-5 CATALYST FOR EXHAUST-GAS PURIFICATION - IN-SITU ESR MONITORING

A flow cell was designed for in-situ ESR monitoring of the state of co pper ions in a working Cu-ZSM-5 catalyst at high temperatures. The equ ilibrium oxidation state of the copper in the catalyst depends both on the temperature and the composition of the gas mixture. At 500 degree s C in [C3H6 (or C2H5OH) + NO + O-2] with excess oxidant, the conditio n prevailing during selective catalytic reduction (SCR) of NOx, virtua lly all the copper ions preserve the Cu2+ state. At 200-350 degrees C in the same gas mixtures a partial reduction of the Cu2+ ions is obser ved. Steam-aging (650-700 degrees C) or dry calcination (850-900 degre es C) of CuH-ZSM-5 lead to the change in Cu2+ coordination without not iceable agglomeration or encapsulation of the isolated cations. The dr astic loss of catalytic activity due to such treatment is associated w ith the change in the localized coordination of the isolated cupric si tes.