Coke formation over zeolites and CeO2-zeolites and its influence on selective catalytic reduction of NOx

Selective catalytic reduction, various possible reasons of coke formation, and temperature programmed oxidation of coke deposits are studied over HFER, HZSM-5 and 15 wt% CeO2-Hzeolites. The materials are characterised by TGA, NH3-TPD and in-situ FTIR measurements. HFER based catalysts showed superior NOx (NO + NO2) conversion in SCR with propene compared with HZSM-5 based catalysts. It is found that NO2 (formed by the oxidation of NO) is not the only important intermediate in determining the extent of NO2, conversion. The topology and acidity of the zeolites play an important role in selective activation of propene and its reaction with NO2. Over HZSM-5 based catalysts the rate of deposition of carbonaceous compounds is higher than the rate of reaction of activated propene with NO2, leading to unselective reduction to NO. The nature and the amount of the carbonaceous products deposited over the zeolites are found to depend on the acidity, structure of the zeolite and reaction conditions (inert or oxidative atmosphere). Coke deposition rate is enhanced in the presence of oxygen and most of the coke is retained by the zeolite which is detrimental for NOx reduction. in-situ IR studies show that hydrocarbon deposits are more heterogeneous and carbon rich over HZSM-5 compared with HFER. TPO studies show that only a negligible fraction of hydrocarbon deposits are active in NOx conversion. (c) 2005 Elsevier B.V. All rights reserved.