IN SITU ESR MONITORING OF CUH-ZSM-5 UP TO 500-DEGREES-C IN FLOWING DRY MIXTURES OF NO(NO2), C3H6(C2H5OH), AND EXCESS O-2

摘要

A flow cell was used for ESR monitoring of the state of copper ions in Cu-ZSM-5 in situ at 20-500 degrees C in a He stream containing C3H6 (or C2H5OH), NO, and O-2. The gas mixtures approximate those in the selective catalytic reduction of NOx, save for the absence of steam. At room temperature, in propene flow, the formation of an oligomeric, carbonaceous material is accompanied by the reduction of a significant part of the Cu2+ cations. Heating of the sample at 200-300 degrees C leads to quantitative reduction of the cupric ions. Ethanol is a less active reductant, and its sorption at 20-100 degrees C results only in a coordination change of the Cu2+ by Octahedral complex formation. The onset of the reduction of the Cu2+ ions in the [C2H5OH + He] flow is at similar to 200 degrees C. The equilibrium oxidation state of the copper in CuH-ZSM-5 depends on both the temperature and the composition of the gas mixture. At 500 degrees C in the presence of propene or ethanol and in an excess of oxidant, the condition prevailing during SCR, virtually all the copper remains in the Cu2+ state. At lower temperatures, in the 200-350 degrees C range, in the same gas mixtures, a partial reduction of the Cu2+ ions takes place. High-temperature oxidation of the propene is accompanied by coke deposition on the outer surface of the zeolitic crystals. (C) 1995 Academic Press, Inc. [References: 25]