State of copper in a working, low-concentration CuH-ZSM-5 catalyst for exhaust gas purification: in-situ ESR monitoring

摘要

A flow cell was designed for in-situ ESR monitoring of the state of copper ions in a working Cu-ZSM-5 catalyst at high temperatures. The equilibrium oxidation state of the copper in the catalyst depends both on the temperature and the composition of the gas mixture. At 500℃ in [C_3H_6 (or C_2H_5OH) +NO + O_2] with excess oxidant, the condition prevailing during selective catalytic reduction (SCR) of NO_x, virtually all the copper ions preserve the Cu~(2+) state. At 200-350℃ in the same gas mixtures a partial reduction of the Cu~(2+) ions is observed. Steam-aging (650-700℃) or dry calcination (850-900℃) of CuH-ZSM-5 lead to the change in Cu~(2+) coordination without noticeable agglomeration or encapsulation of the isolated cations. The drastic loss of catalytic activity due to such treatment is associated with the change in the localized coordination of the isolated cupric sites.