(511d) A Kinetic Model for NH3 SCR On Cu-BEA Zeolite Using Micro Calorimetry Data
Diesel engine exhausts contain NOx, among other pollutants, and excess oxygen. A standard 3-way catalyst cannot reduce NOx efficiently under above conditions, due to oxygen poisoning of the noble metal sites. Selective catalytic reduction (SCR) of NOx by urea is currently being practiced by several diesel engine companies. The ammonia (NH3), formed from urea, reacts selectively with NOx to produce N2 and H2O over an SCR catalyst. Among several potential SCR catalysts, copper and iron exchanged zeolites are at the forefront of diesel exhaust cleaning technology. One obvious and also considered as a key function of these zeolite based SCR catalysts is their ammonia adsorption and release properties. Over zeolite catalysts, NOx reacts with adsorbed ammonia NH3 leading to nitrogen and water as products.
Among several factors, NOx conversion and ammonia slip predictions on an SCR catalyst under transient conditions needs a thorough understanding of ammonia adsorption and desorption phenomenon and kinetic models are needed. The objective of this study is to develop a kinetic model for NH3 SCR that includes experimentally determined energetic of the NH3 adsorption and desorption processes using micro calorimetry over a Cu-Beta zeolite model catalyst.
The catalyst powder was prepared by aqueous ion-exchange (IE) of a beta zeolite with a silica to alumina ratio of 38, from Zeolyst International. To introduce controlled amount of copper, the powder zeolite was ion-exchanged with NaNO3 followed by ion-exchange with Cu(CH3COO)2.
Calorimetric measurements were conducted on powdered catalysts using differential scanning calorimeter (DSC, Sensys from Setaram) instrument. Coverage dependent heats of ammonia adsorption were measured with temperature step-response experiments, where ammonia is adsorbed at different temperatures. A monolith coated with Cu-beta zeolite SCR catalyst was prepared and catalyst performance under varying reaction conditions was evaluated to estimate various parameters needed for the kinetic models.
Results and discussion
We have developed a new methodology that enables measurement of the coverage dependent heats of ammonia adsorption at atmospheric pressure during flow conditions. The energies measured by calorimetry were incorporated into the SCR kinetic model. NOx conversion, NH3 and NO oxidation under several reaction conditions such as varying inlet reactants and temperatures are used to estimate the required kinetic parameters for the SCR model and for its validation.
Figure 1 shows an example of concentration changes of outlet reactants and products under temperature transient conditions. Throughout the experiment the inlet gas composition was 400ppm NH3, 400ppm NO, 8%O2, 2%H2O and 2%CO2. NOx conversion and ammonia slip under steady state and temperature transient conditions were followed by increasing the SCR catalyst temperature from 100 to 500°C in 100°C intervals. At the start of the reaction at 100°C, too low temperature for SCR reaction to occur, early NO breakthrough compared to NH3 can be observed. The delay in ammonia evolution, despite insignificant NO conversion, is due to NH3 adsorption on the catalyst surface. As the temperature is increased to 200°C under SCR reaction conditions, ammonia evolution from the catalyst, due to desorption, is observed along with significant NOx conversion at 200°C. The observed trends in NOx and ammonia concentrations in Fig. 1 can be explained by ammonia adsorption / desorption, ammonia oxidation and SCR reactions. The developed model can describe a broad range of different inlet gas concentrations at various temperatures. The model is also validated with transient experiments, which were not used in the model development. The model can describe the different experimental features well.
Figure 1. Steady state concentrations of NH3, NO and NO2. The inlet gas concentration was 8 % O2, 400 ppm NO, 400 ppm NH3, 6 % H2O and 5 % CO2. (if you are discussing model incorporate a trace predicted by model)
A method was developed to measure heats of ammonia adsorption and desorption at various surface catalyst surface coverage levels. These calorimetrically measured energy parameters along with the other kinetic parameters measured with reactor studies are incorporated in the SCR model. The model developed based on the above methodology successfully predicted transient NOx conversion and ammonia slip under reactor conditions.
This work has been performed within the Competence Centre for Catalysis and Cummins Inc. The authors would like to thank Cummins Inc. for the financial support. The financial support for the micro calorimeter from the Swedish Research Council (Contract: 621-2003-4149 and 621-2006-3706) and for the FTIR from Knut and Alice Wallenberg Foundation, Dnr KAW 2005.0055, is gratefully acknowledged.