(511d) A Kinetic Model for NH3 SCR On Cu-BEA Zeolite Using Micro Calorimetry Data

Introduction

Diesel engine exhausts contain NO_x, among other pollutants, and excess oxygen. A standard 3-way catalyst cannot reduce NO_x efficiently under above conditions, due to oxygen poisoning of the noble metal sites. Selective catalytic reduction (SCR) of NO_x by urea is currently being practiced by several diesel engine companies. The ammonia (NH₃), formed from urea, reacts selectively with NO_x 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, NO_x reacts with adsorbed ammonia NH₃ leading to nitrogen and water as products.  

Among several factors, NO_x 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.

Experimental

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. NO_x 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. NO_x 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 NO_x conversion at 200°C. The observed trends in NO_x 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)

Conclusions

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 NO_x conversion and ammonia slip under reactor conditions.

Acknowledgements

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.