Roles of NO and O 2 on coke deposition and removal over Cu-ZSM-5

Abstract

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 CO 2 peaks. Therefore, in the presence of NO, CO 2 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.