Roles of NO and O2 on coke deposition and removal over Cu-ZSM-5

Toluene and propene are used as model hydrocarbon gases for producing coke over Cu-ZSM-5. Temperature programmed oxidation technique is adapted to investigate roles of NO and O-2 on coke deposition and removal over Cu-ZSM-5 catalyst. It is found that when a gas mixture of NO and O-2 is added during hydrocarbon exposure, the carbonaceous structure is not changed but the deposition rate is dramatically increased. It is suggested that the presence of NO + O-2 accelerates hydrocarbon deposition or that coke may be formed in another route involving SCR. In addition, N-2 is detected during TPO by O-2 of this exposed Cu-ZSM-5. This indicates that nitrogen compounds have to be a part of carbonaceous deposits or they may adsorb on the deposits on the zeolite. By using XRD and ex situ FT-IR, it is found that the structure of coke is polyaromatic, not graphite-like form. It is also found that gasification reactivity of the carbonaceous deposits depends on the oxidizing gas (O-2, NO, NO + O-2) with respect to both composition and concentration. However, the rate of the gasification at each maximum temperature is about similar, independent on the oxidizing agent. Furthermore, N-2 peaks appear to coincide with CO2 peaks. Therefore, in the presence of NO, CO2 appears to be formed preferably by NO reduction rather than coke combustion. NO seems to be more reactive for oxidizing of carbonaceous deposits rather than O-2. Certainly, NO plus O-2 in the TPO oxidizing gas accelerates coke removal. (C) 2004 Elsevier B.V. All rights reserved.