(511e) Kinetics and Mechanistic Studies of Selective Catalytic Reduction of NOx On Fe Based Zeolite Monolith Catalysts

NOx, emitted from diesel vehicles is highly responsible for number of respiratory diseases and is a major cause of ground-level ozone. Hence NOx emission needs to be controlled. Thus selective catalytic reduction (SCR) of NOx (NO + NO2) using ammonia is gaining a lot of attention for mobile applications. In this study a comprehensive steady-state and transient SCR on zeolitic monolith catalysts was carried out in bench-flow and Temporal Analysis of Products (TAP) reactors with the goal to develop mechanistically-based kinetic models for SCR reactor design and optimization. The selective catalytic reduction of NOx with ammonia as a reductant was studied on Fe-based zeolite monolith catalysts. Fe-ZSM-5 catalyst synthesized for this study was prepared by sequential ion exchange step and the resulting zeolite/alumina slurry was coated onto a cordierite monolith support. The synthesis afforded a systematic variation of Fe loading and washcoat thickness. Adsorption and reaction experiments were performed on these catalysts and compared to a commercial sample. These experiments included NH3 and NOx uptake and temperature-programmed desorption (TPD), NO and NH3 oxidation, standard SCR (NO+NH3), fast SCR (NO+NO2+NH3), and NO2 SCR (NO2+NH3).

An inhibiting effect of NH3 on the SCR reaction was observed due to ammonia blocking the sites required for SCR at low temperatures and ammonia oxidation at higher temperatures At lower temperatures a pathway via ammonium nitrate was confirmed during NO2 SCR due in part to N2O formation during a temperature ramp. The yield of N2O exhibited a maximum versus temperature and increased with the NO2/NOx inlet ratios. NOx removal efficiency was over the complete range of NO2/NOx ratio and is found to be maximum at NO2/NOx=0.5. Transient experiments were performed where NOx with various NO2/NOx inlet ratios is introduced on preadsorbed ammonia on the catalyst. This also confirmed that NO2/NOx=0.5 gives optimum NOx removal efficiency at low temperatures. The kinetics of standard SCR and fast SCR reaction, and NO and NH3 oxidation were studied in the temperature range of 200 ? 300 oC to determine the reaction orders (with respect to individual species taking part in these reactions) and activation energies of these reactions. External mass transfer limitations and washcoat diffusion limitations were ruled out in this temperature range. A proposed reaction mechanism and corresponding kinetic model was developed consistent with the experimental observations. Keywords: Selective Catalytic Reduction, Standard SCR, Fast SCR, Ammonia oxidation, NO Oxidation, Washcoat diffusion, Mass transfer.