Selective Catalytic Reduction of NOx with NH3 - Kinetic Modeling and Experimental Studies using Zeolite Based Catalysts

Doctoral thesis, 2009

Emissions from the combustion of fossil fuel contain several pollutants which can be converted over a catalyst to less harmful products. An efficient way to reduce nitrogen oxides in a lean environment is to apply selective catalytic reduction of these oxides with ammonia (NH3 SCR). Metal ion exchanged zeolite catalysts have proven to be active and selective for NH3 SCR. In this thesis, iron and copper zeolite catalysts are evaluated, and the activity for NOx reduction is studied at various temperatures and feed concentrations. DRIFT spectroscopy is applied to investigate the surface species that are formed during NH3 SCR conditions, and kinetic models are developed that describe the catalytic activity of the catalysts. The Cu-ZSM-5 catalyst is active in a wide temperature range, but the NO conversion decreases at high temperature due to ammonia oxidation. During NO oxidation, species such as nitrite or nitrate can be identified on the surface and are believed to be involved in the SCR mechanism. High concentrations of ammonia inhibit the NO reduction at 175ºC, possibly due to competition for sites that are needed also in the formation of nitrite or nitrate species. The NOx conversion is enhanced by the introduction of about equal amounts of NO and NO2. The NOx reduction over the Fe-zeolite catalyst is similar to the reduction over the Cu-ZSM-5 catalyst. The iron catalyst is, however, less active for NO reduction at low temperature, but more active in the high temperature range. It is also more sensitive to ammonia inhibition, has a lower NH3 storage capacity, produces less N2O at low NO2 to NOx ratios, and is less active for NO and NH3 oxidation. Two models are developed in this work. A detailed model which describes the experimental observations made for the Cu-ZSM-5 catalyst, and a global model which describes the activity over a supplier Fe-zeolite catalyst. Both models account for a broad range of experimental conditions at a wide temperature range. The models are validated using experimental conditions not included in the parameter estimation and predict these new conditions adequately. The poisoning effect of hydrocarbons is investigated and the results show that the presence of hydrocarbons inhibits the NOx conversion over the catalyst (Fe-beta). The effect is greater in presence of n-octane than in presence of propylene, and the inhibition increases with increased concentration of hydrocarbons. Propylene and n-octane reduce the conversion of NO as well as the conversion of a mixture of NO and NO2.