The Effect of Accelerated Hydrothermal Aging On NH3-SCR Over Cu-Hbea Catalyst

Introduction

Diesel engines have good fuel efficiency, due to that they operate with excess of oxygen. However, the excess of oxygen that is fed into the engine is also present in the exhaust stream. This disables the selectivity of the standard three-way catalyst towards nitrogen oxides (NO_x). It is crucial to reduce the NO_x, due to environmental problems. One effective solution is to use ammonia (NH₃) selective catalytic reduction (SCR). Urea is sprayed into the exhaust, and the urea decomposes and hydrolyses to form ammonia. The ammonia reacts selectively with the NO_x and produces nitrogen and water. Iron and copper based zeolite catalysts have shown very good performance, however they can be hydrothermally aged. The objective of this work was to examine different reactions involved in the SCR mechanism over Cu-Beta and the effect the hydrothermal aging has on these reactions.

Experimental

The catalyst was prepared from hydrogen beta zeolite (HBEA) from Zeolyst International. Copper was ion exchanged into the HBEA by an aqueous ion exchange method. A total of 4wt% copper was added to the zeolite, which will be referred to as Cu-BEA. The zeolite powder was coated on a monolith using the incipient wetness technique. One of the monoliths was hydrothermally treated at 500°C, 600°C, 700°C, 800°C and 900°C for 3 hours each. After each aging a set of experiments were conducted to evaluate the specific change in performance of the SCR mechanism as well as the oxidation of NH₃ and NO and the adsorption and desorption of NH₃. The investigated temperature range was 150°C-500°C. A total gas flow of 3500 ml/min was used resulting in a space velocity of 30 300h^-1. The gas flow contained 8%O₂, 5%H₂O and 5%CO₂ as well as different levels of NH₃, NO and NO₂. The gas mixture was analyzed with a MKS MultiGas FTIR.

The adsorption and desorption behavior was further investigated using the powder directly in a Setaram Sensys DSC. The sample was hydrothermally treated at 500°C, 600°C, 700°C and 780°C and the coverage dependent heat of adsorption was calculated by performing a stepwise adsorption of NH₃ at different temperatures [1]. The gas stream was analyzed with a Hiden HPR-20 QUI mass spectrometer.

Results and discussion

The influence of hydrothermal aging on the SCR mechanism was studied. The different SCR reactions were investigated separately. In Figure 1 the results for a fast SCR experiment are shown. A gas flow composed of 5%H₂O, 5%CO₂, 8%O₂, 400ppm NH₃, 200ppm NO and 200ppm NO₂ was used. The temperature was increased in steps from 150°C to 500°C. After each experiment the catalyst was hydrothermally treated, which is described in the experimental section. At 150°C a similar conversion of NO_x is observed for aging temperatures up to 800°C. The aging at 900°C caused a significant drop in activity. When the temperature is increased during the experiment the conversion increases as well. At 300°C and 400°C the experiments that were done when the catalyst was aged up to 700°C reach full conversion. For higher aging temperatures full conversion was not observed. When the temperature in the experiment is increased to 500°C a drop in conversion is observed. NH₃ oxidation starts to play a role in that temperature range and therefore not all the NH₃ is available for the SCR reaction. The NH₃ storage capacity decreased with higher aging temperatures as well as the NO and NH₃ oxidation activity.

The coverage depended heat of adsorption of NH₃ was investigated in the calorimeter, using the method described earlier [1]. We could see a trend towards less strong bound ammonia for higher aging temperatures which is in good agreement with TPD experiments that were conducted.

Figure 1. NH₃ SCR over a Cu-BEA catalyst after different hydrothermal treatment. Conditions: 200 ppm NO, 200ppm NO₂, 400 ppm NH₃, 8 % O₂ and 5 % H₂, SV = 30 300h^-1.

Conclusions

Ammonia SCR is an important technique for reducing NO_x from diesel and lean burn gasoline engines. Zeolite based catalysts have been proven to be a good choice for this reaction, however they can be hydrothermally aged. Since reactions on other components, like diesel particulate filter regeneration, might lead to very high exhaust gas temperatures the objective of this study was a detailed study of the catalyst while undergoing hydrothermal aging.

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. One author (Louise Olsson) would also like to acknowledge the Swedish foundation for strategic research (F06-0006) for additional 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.

References

1.      N. Wilken, K. Kamasamudram, N.W. Currier, J. Li, A. Yezerets, L. Olsson, Catalysis Today 151 (2010) 237.