Carbon deposits effects on the selective catalytic reduction of NO over zeolites using temperature programmed oxidation technique

Abstract

Temperature programmed oxidation (TPO) technique was adapted to investigate selective catalytic reduction (SCR) of NO over various types of zeolites. SCR reactions in this study were separated into two steps: (1) hydrocarbon deposition to form carbonaceous deposits on catalyst surface and (2) oxidation and/or reduction of the carbon deposits. From this technique, it was found that carbon deposits must be involved in the SCR because both N2 and CO2 were detected during oxidation of the hydrocarbon exposed zeolites with NO+O2. It was also observed that Cu-ZSM-5 and H-MOR induced more coke formation than H-ZSM-5 and HY whereas H-ZSM-5 has a higher NO reduction rate than the others. The amount of carbonaceous deposit depends on hydrocarbon type but the NO reduction seemed to be independent of the coke precursor. When NO+O2 was added during exposure of hydrocarbon to Cu-ZSM-5, the carbonaceous structure was not change but the deposition rate is dramatically increased. It is suggested that NO+O2 accelerates hydrocarbon deposition or that coke may be formed in another route involving SCR. In addition, N2 was detected during TPO by O2 of this exposed zeolite. This indicates that nitrogen compounds have to be either part of carbonaceous deposit or they may adsorb on the deposit in the zeolite. Further study of incorporation of nitrogen in the carbon deposit or on zeolite will be carried out.